Robert A Root

Interests

Research

geochemistry, metal speciation, EXAFS, XANES, X-ray spectroscopy, weathering, contaminant transport, medical mineralogy

Courses

Scholarly Contributions

Journals/Publications

• Bai, B., Kong, S., Root, R. A., Liu, R., Wei, X., Cai, D., Chen, Y., Chen, J., Yi, Z., & Chorover, J. (2024). Release mechanism and interactions of cadmium and arsenic co-contaminated ferrihydrite by simulated in-vitro digestion assays. Journal of hazardous materials, 467, 133633.
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  Cadmium (Cd) and arsenic (As) co-contamination is widespread and threatens human health, therefore it is important to investigate the bioavailability of Cd and As co-exposure. Currently, the interactions of Cd and As by in vitro assays are unknown. In this work, we studied the concurrent Cd-As release behaviors and interactions with in vitro simulated gastric bio-fluid assays. The studies demonstrated that As bioaccessibility (2.04 to 0.18 ± 0.03%) decreased with Cd addition compared to the As(V) single system, while Cd bioaccessibility (11.02 to 39.08 ± 1.91%) increased with As addition compared to the Cd single system. Release of Cd and As is coupled to proton-promoted and reductive dissolution of ferrihydrite. The As(V) is released and reduced to As(Ⅲ) by pepsin. Pepsin formed soluble complexes with Cd and As. X-ray photoelectron spectroscopy showed that Cd and As formed Fe-As-Cd ternary complexes on ferrihydrite surfaces. The coordination intensity of As-O-Cd is lower than that of As-O-Fe, resulting in more Cd release from Fe-As-Cd ternary complexes. Our study deepens the understanding of health risks from Cd and As interactions during environmental co-exposure of multiple metal(loid)s.
• Bose, M., Root, R. A., Guan, Y., Eaton, J., Wittmann, A., Skrmetti, T., & Desch, S. J. (2024). Evidence of both molecular cloud and fluid chemistry in Ryugu regolith. Science advances, 10(30), eadp3037.
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  The sulfur chemistry of (162173) Ryugu particles can be a powerful tracer of molecular cloud chemistry and small body processes, but it has not been well explored. We report identification of organosulfurs and a sulfate grain in two Ryugu particles, A0070 and A0093. The sulfate grain shows oxygen isotope ratios (δO = -11.0 ± 4.3 per mil, δO = -7.8 ± 2.3 per mil) that are akin to silicates in Ryugu but exhibit mass-independent sulfur isotopic fractionation (ΔS = +5 ± 2 per mil). A methionine-like coating on the sulfate grain is isotopically anomalous (δN = +62 ± 2 per mil). Both the sulfate and organosulfurs can simultaneously form and survive during aqueous alteration within Ryugu's parent body, under reduced conditions, low temperature, and a pH >7 in the presence of N-rich organic molecules. This work extends the heliocentric zone where anomalous sulfur, formed by selective photodissociation of HS gas in the molecular cloud, is found.
• Kong, S., Cai, D., Shao, Y., Wei, X., Yi, Z., Root, R. A., & Chorover, J. (2024). Identification of key factors and mechanism determining arsenic mobilization in paddy soil-porewater-rice system. Journal of hazardous materials, 479, 135684.
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  Arsenic (As) mobilization in paddy fields poses significant health risks, necessitating a thorough understanding of the controlling factors and mechanisms to safeguard human health. We conducted a comprehensive investigation of the soil-porewater-rice system throughout the rice life cycle, focusing on monitoring arsenic distribution and porewater characteristics in typical paddy field plots. Soil pH ranged from 4.79 to 7.98, while porewater pH was weakly alkaline, varying from 7.2 to 7.47. Total arsenic content in paddy soils ranged from 6.8 to 17.2 mg/kg, with arsenic concentrations in porewater during rice growth ranging from 2.97 to 14.85 μg/L. Specifically, arsenite concentrations in porewater ranged from 0.48 to 7.91 μg/L, and arsenate concentrations ranged from 0.73 to 5.83 μg/L. Through principal component analysis (PCA) and analysis of redox factors, we identified that arsenic concentration in porewater is predominantly influenced by the interplay of reduction and desorption processes, contributing 43.5 % collectively. Specifically, the reductive dissolution of iron oxides associated with organic carbon accounted for 23.3 % of arsenic concentration dynamics in porewater. Additionally, arsenic release from the soil followed a sequence starting with nitrate reduction, followed by ferric ion reduction, and subsequently sulfate reduction. Our findings provide valuable insights into the mechanisms governing arsenic mobilization within the paddy soil-porewater-rice system. These insights could inform strategies for irrigation management aimed at mitigating arsenic toxicity and associated health risks.
• Yu, Y., Root, R. A., Castrejon Miranda, R., Sierra Alvarez, M. R., Chorover, J. D., & Field, J. A. (2024). Abiotic reductive trnasformation of 3-nitro-1,2,4-triazol-5one by zero-valent iron. International Journal of Environmental Science and Technology, 21, 25-34. doi:https://doi.org/10.1007/s13762-023-04951-4
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  The military is switching over to insensitive munitions compounds (IMCs) to avoid unintentional detonations during handling and use of explosives. 3-nitro-1,2,4-triazol-5-one (NTO) is an important component of IMCs. NTO may contaminate the subsurface due to its high aqueous solubility. Thus, there is a need to develop remediation technologies for the treatment of NTO-containing (waste) water. This study demonstrated that zero-valent iron (ZVI) reductively transformed NTO to its daughter product, 3-amino-1,2,4-triazol-5-one. The pseudo first-rate constant (k1) of NTO reduction by micron-sized ZVI at pH 3 was 192.6 h−1. Kinetic degradation experiments performed at different pH values showed that ZVI did not effectively reduce NTO at pH 6 (k1 = 0.6 h−1) or higher. The rapid NTO reduction in acidic conditions may be due to dissolution of iron precipitates on the ZVI surface. Additional experiments were conducted to assess the effectiveness of various depassivating pretreatments with deionized water, acetic acid, hydrochloric acid, or bicarbonate. Treatment with 1 M HCl for 15 min was the most effective depassivation method for a ZVI material containing a thick passivating layer (ca. 880 nm), achieving 84.0% NTO removal after 10 min of reaction. On the other hand, a milder treatment involving washing with a diluted bicarbonate solution (60 mM) was sufficient for a ZVI material that was less passivated (estimated thickness of the passivating layer ≈ 300 nm). This study demonstrates that ZVI treatment is a promising approach for the remediation of NTO-contaminated sites or wastewater and provides critical information to optimize this process.
• Liu, R., Kong, S., Shao, Y., Cai, D., Bai, B., Wei, X., Root, R. A., Gao, X., Li, C., & Chorover, J. (2023). Mechanisms and health implications of toxicity increment from arsenate-containing iron minerals through gastrointestinal digestion. Geoderma, 432.
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  Inadvertent oral ingestion is an important exposure pathway of arsenic (As) containing soil and dust. Previous researches evidenced health risk of bioaccessible As from soil and dust, but it is unclear about As mobilization mechanisms in health implications from As exposure. In this study, we investigated As release behaviors and the solid-liquid interface reactions toward As(V)-containing iron minerals in simulated gastrointestinal bio-fluids. The maximum As release amount was 0.57 mg/L from As-containing goethite and 0.82 mg/L from As-containing hematite at 9 h, and the As bioaccessibility was 10.8% and 21.6%, respectively. The higher exposure risk from hematite-sorbed As in gastrointestinal fluid was found even though goethite initially contained more arsenate than hematite. Mechanism analysis revealed that As release was mainly coupled with acid dissolution and reductive dissolution of iron minerals. Proteases enhanced As mobilization and thus increased As bioaccessibility. The As(V) released and simultaneously transformed to high toxic As(III) by gastric pepsin, while As(V) reduction in intestine was triggered by pancreatin and freshly formed Fe(II) in gastric digests. CaCl reduced As bioaccessibility, indicating that calcium-rich food or drugs may be effective dietary strategies to reduce As toxicity. The results deepened our understanding of the As release mechanisms associated with iron minerals in the simulated gastrointestinal tract and supplied a dietary strategy to alleviate the health risk of incidental As intake.
• Liu, Y., Root, R. A., Abramson, N., Fan, L., Sun, J., Liu, C., & Chorover, J. (2023). The effect of biogeochemical redox oscillations on arsenic release from legacy mine tailings. Geochimica et cosmochimica acta, 360, 192-206.
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  Exposed and un-remediated metal(loid)-bearing mine tailings are susceptible to wind and water erosion that disperses toxic elements into the surrounding environment. Compost-assisted phytostabilization has been successfully applied to legacy tailings as an inexpensive, eco-friendly, and sustainable landscape rehabilitation that provides vegetative cover and subsurface scaffolding to inhibit offsite transport of contaminant laden particles. The possibility of augmented metal(loid) mobility from subsurface redox reactions driven by irrigation and organic amendments is known and arsenic (As) is of particular concern because of its high affinity for adsorption to reducible ferric (oxyhydr)oxide surface sites. However, the biogeochemical transformation of As in mine tailings during multiple redox oscillations has not yet been addressed. In the present study, a redox-stat reactor was used to control oscillations between 7 d oxic and 7 d anoxic half-cycles over a three-month period in mine tailings with and without amendment of compost-derived organic matter (OM) solution. Aqueous and solid phase analyses during and after redox oscillations by mass spectrometry and synchrotron X-ray absorption spectroscopy revealed that soluble OM addition stimulated pyrite oxidation, which resulted in accelerated acidification and increased aqueous sulfate activity. Soluble OM in the reactor solution significantly increased mobilization of As under anoxic half-cycles primarily through reductive dissolution of ferrihydrite. Microbially-mediated As reduction was also observed in compost treatments, which increased partitioning to the aqueous phase due to the lower affinity of As(III) for complexation on ferric surface sites, e.g. ferrihydrite. Oxic half-cycles showed As repartitioned to the solid phase concurrent with precipitation of ferrihydrite and jarosite. Multiple redox oscillations increased the crystallinity of Fe minerals in the Treatment reactors with compost solution due to the reductive dissolution of ferrihydrite and precipitation of jarosite. The release of As from tailings gradually decreased after repeated redox oscillations. The high sulfate, ferrous iron, and hydronium activity promoted the precipitation of jarosite, which sequestered arsenic. Our results indicated that redox oscillations under compost-assisted phytostabilization can promote As release that diminishes over time, which should inform remediation assessment and environmental risk assessment of mine site compost-assisted phytostabilization.
• Palawat, K., Root, R. A., Cortez, L. I., Foley, T., Carella, V., Beck, C., & Ramírez-Andreotta, M. D. (2023). Patterns of contamination and burden of lead and arsenic in rooftop harvested rainwater collected in Arizona environmental justice communities. Journal of environmental management, 337, 117747.
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  As climate change exacerbates water scarcity, rainwater harvesting for household irrigation and gardening becomes an increasingly common practice. However, the use and quality of harvested rainwater are not well studied, and the potential pollutant exposures associated with its use are generally unknown. There are currently no federal standards in the United States to assess metal(loid)s in harvested rainwater. Project Harvest, a community science research project, was created to address this knowledge gap and study the quality of harvested rainwater, primarily used for irrigation, in four environmental justice communities in Arizona, USA. Community scientists collected 577 unique rooftop harvested rainwater samples from 2017 to 2020, which were analyzed for metal(loid)s, where arsenic (As) concentrations ranged from 0.108 to 120 μg L and lead (Pb) concentrations ranged from 0.013 to 350 μg L and compared to relevant federal/state standards/recommendations. Community As and Pb concentrations decreased as: Hayden/Winkelman > Tucson > Globe/Miami > Dewey-Humboldt. Linear mixed models were used to analyze rooftop harvested rainwater data and results indicated that concentrations of As and Pb in the summer monsoon were significantly greater than winter; and contamination was significantly greater closer to extractive industrial sites in three of the four study communities (ASARCO Hayden Plant Superfund Alternative site in Hayden/Winkelman, Davis-Monthan United States Air Force Base in Tucson - Pb only, and Freeport McMoRan Copper and Gold Mine in Globe/Miami). Based on models, infrastructure such as proximity to roadway, roof material, presence of a cistern screen, and first-flush systems were not significant with respect to As and Pb when controlling for relevant spatiotemporal variables; whereas, cistern age was associated with Pb concentrations. These results however, indicate that concentrations vary seasonally and by proximity to industrial activity, not by decisions made regarding collection system infrastructures at the individual home level. This study shows that generally, individuals are not responsible for environmental contamination of rooftop harvested rainwater, rather activities and decisions of government and corporate industries control contaminant release.
• Palawat, K., Root, R. A., Cruz, L. I., Foley, T., Carella, V., Beck, C., & Ramírez-Andreotta, M. (2023). Dissolved arsenic and lead concentrations in rooftop harvested rainwater: Community generated dataset. Data in brief, 48, 109255.
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  Here, we detail arsenic (As) and lead (Pb) concentrations in community science generated rooftop harvested rainwater data from Project Harvest (PH), a co-created community science study, and National Atmospheric Deposition Program (NADP) National Trends Network wet-deposition AZ samples as analyzed by Palawat et al. [1]. 577 field samples were collected in PH and 78 field samples were collected by NADP. All samples were analyzed via inductively coupled plasma mass spectrometry (ICP-MS) for dissolved metal(loid)s including As and Pb by the Arizona Laboratory for Emerging Contaminants after 0.45 um filtration and acidification. Method limits of detection (MLOD) were assessed and sample concentrations above MLODs were considered detects. Summary statistics and box and whisker plots were generated to assess variables of interest such as community and sampling window. Finally, As and Pb data is provided for potential reuse; the data can be used to assess contamination of harvested rainwater in AZ and to inform community use of natural resources.
• Shakya, A., Dodson, M., Artiola, J. F., Ramirez-Andreotta, M., Root, R. A., Ding, X., Chorover, J., & Maier, R. M. (2023). Arsenic in Drinking Water and Diabetes. Water, 15(9).
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  Arsenic is ubiquitous in soil and water environments and is consistently at the top of the Agency for Toxic Substances Disease Registry (ATSDR) substance priority list. It has been shown to induce toxicity even at low levels of exposure. One of the major routes of exposure to arsenic is through drinking water. This review presents current information related to the distribution of arsenic in the environment, the resultant impacts on human health, especially related to diabetes, which is one of the most prevalent chronic diseases, regulation of arsenic in drinking water, and approaches for treatment of arsenic in drinking water for both public utilities and private wells. Taken together, this information points out the existing challenges to understanding both the complex health impacts of arsenic and to implementing the treatment strategies needed to effectively reduce arsenic exposure at different scales.
• Yu, Y., Root, R. A., Sierra-Alvarez, R., Chorover, J., & Field, J. A. (2023). Treatment of the insensitive munitions compound, 3-nitro-1,2,4-triazol-5-one (NTO), in flow-through columns packed with zero-valent iron. Environmental science and pollution research international, 30(23), 64606-64616.
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  The need for effective technologies to remediate the insensitive munitions compound 3-nitro-1,2,4-triazol-5-one (NTO) is emerging due to the increasing use by the US Army and environmental concerns about the toxicity and aqueous mobility of NTO. Reductive treatment is essential for the complete degradation of NTO to environmentally safe products. The objective of this study is to investigate the feasibility of applying zero-valent iron (ZVI) in a continuous-flow packed bed reactor as an effective NTO remediation technology. The ZVI-packed columns treated an acidic influent (pH 3.0) or a circumneutral influent (pH 6.0) for 6 months (ca. 11,000 pore volumes, PVs). Both columns effectively reduced NTO to the amine product, 3-amino-1,2,4-triazol-5-one (ATO). The column treating the pH-3.0 influent exhibited prolonged longevity in reducing NTO, treating 11-fold more PVs than the column treating pH-6.0 influent until the breakthrough point (defined as when 85% of NTO was removed). The exhausted columns (defined as when only 10% of NTO was removed) regained the NTO reducing capacity by reactivation using 1 M HCl, fully removing NTO. After the experiment, solid-phase analysis of the packed-bed material showed that ZVI was oxidized to iron (oxyhydr)oxide minerals such as magnetite, lepidocrocite, and goethite during NTO treatment. This is the first report on the reduction of NTO and the concomitant oxidation of ZVI in continuous-flow column experiments. The evidence indicates that treatment in a ZVI-packed bed reactor is an effective approach for the removal of NTO.
• Hammond, C. M., Root, R. A., Maier, R. M., & Chorover, J. (2022). Metal Lability and Mass Transfer Response to Direct-Planting Phytostabilization of Pyritic Mine Tailings. Minerals (Basel, Switzerland), 12(6).
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  Understanding the temporal effects of organic matter input and water influx on metal lability and translocation is critical to evaluate the success of the phytostabilization of metalliferous mine tailings. Trends of metal lability, e.g., V, Cr, Mn, Co, Ni, Cu, Zn, and Pb, were investigated for three years following a direct-planting phytostabilization trial at a Superfund mine tailings site in semi-arid central Arizona, USA. Unamended tailings were characterized by high concentrations (mmol kg) of Fe (2100), S (3100), As (41), Zn (39), and Pb (11), where As and Pb greatly exceeded non-residential soil remediation levels established by Arizona. Phytostabilization treatments included a no-compost control, 100 g kg compost with seed, and 200 g kg compost with and without seed to the top 20 cm of the tailings profile. All plots received supplemental irrigation, effectively doubling the mean annual precipitation. Tailings cores up to 90 cm were Collected at the time of planting and every summer for 3 years. The cores were sub-sectioned at 20 cm increments and analyzed via total digestion and an operationally defined sequential extraction for elemental analysis and the calculation of a mass transfer coefficient normalized to Ti as an assigned immobile element. The results indicate that Pb was recalcitrant and relatively immobile in the tailings environment for both the uncomposted control and composted treatments with a maximum variation in the total concentration of 9-14 mmol kg among all samples. Metal lability and translocation above the redox boundary (. 30 cm depth) was governed by acid generation, where surficial pH was measured as low as 2.7 ± 0.1 in year three and strongly correlated with the increased lability of Mn, Co, Ni, Cu, and Zn. There was no significant pH effect on the lability of V, Cr, or Pb. Translocation to depths was greatest for Mn and Co; however, Zn, Ni, Cr, and Cu were also mobilized. The addition of organic matter enhanced the mobilization of Cr from the near surface to 40-60 cm depth (pH > 6) over the three-year phytostabilization study compared to the control. The increased enrichment of some metals at 60-90 cm indicates that the long-term monitoring of elemental translocation is necessary to assess the efficacy of phytostabilization to contain subsurface metal contaminants and thereby protect the surrounding community from exposure.
• Moses, A., Kilungo, A. P., Mclain, J. E., Root, R. A., Abrell, L. M., Buxner, S. R., & Ramirez, M. D. (2022). Minding the gap: socio‑demographic factors linked to the perceptionof environmental pollution, water harvesting infrastructure,and gardening characteristics. Journal of Environmental Studies and Sciences, 12, 594–610. doi:https://doi.org/10.1007/s13412-022-00769-7
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  With the ongoing need for water conservation, the American Southwest has worked to increase harvested rainwater efforts to meet municipal needs. Concomitantly, environmental pollution is prevalent, leading to concerns regarding the quality of harvested rainwater. Project Harvest, a co-created community science project, was initiated with communities that neighbor sources of pollution. To better understand how a participant’s socio-demographic factors affect home characteristics and rainwater harvesting infrastructure, pinpoint gardening practices, and determine participant perception of environmental pollution, a 145-question “Home Description Survey” was administered to Project Harvest participants (n = 167) by project promotoras (community health workers). Race/ethnicity and community were significantly associated (p < 0.05) with participant responses regarding proximity to potential sources of pollution, roof material, water harvesting device material, harvesting device capacity, harvesting device age, garden amendments, supplemental irrigation, and previous contaminant testing. Further, the study has illuminated the idiosyncratic differences in how underserved communities perceive environmental pollution and historical past land uses in their community. We propose that the collection of such data will inform the field on how to tailor environmental monitoring efforts and results for constituent use, how community members may alter activities to reduce environmental hazard exposure, and how future studies can be designed to meet the needs of environmentally disadvantaged communities.
• Rios-Valenciana, E. E., Menezes, O., Niu, X. Z., Romero, J., Root, R. A., Chorover, J., Sierra-Alvarez, R., & Field, J. A. (2022). Reductive transformation of the insensitive munitions compound nitroguanidine by different iron-based reactive minerals. Environmental Pollution (Barking, Essex : 1987), 309(119788), 119788. doi:https://doi.org/10.1016/j.envpol.2022.119788
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  Nitroguanidine (NQ) is an emerging contaminant being used by the military as a constituent of new insensitive munitions. NQ is also used in rocket propellants, smokeless pyrotechnics, and vehicle restraint systems. Its uncontrolled transformation in the environment can generate toxic and potentially mutagenic products, posing hazards that need to be remediated. NQ transformation has only been investigated to a limited extent. Thus, it is crucial to expand the narrow spectrum of NQ remediation strategies and understand its transformation pathways and end products. Iron-based reactive minerals should be investigated for NQ treatment because they are successfully used in existing technologies, such as permeable reactive barriers, for treating a wide range of organic pollutants. This study tested the ability of micron-sized zero-valent iron (m-ZVI), mackinawite, and commercial FeS, to transform NQ under anoxic conditions. NQ transformation followed pseudo-first-order kinetics. The reaction rate constants decreased as follows: commercial FeS > mackinawite > m-ZVI. For the assessed minerals, the NQ transformation started with the reduction of the nitro group forming nitrosoguanidine (NsoQ). Then, aminoguanidine (AQ) was accumulated during the reaction of NQ with m-ZVI, accounting for 86% of the nitrogen mass recovery. When NQ was reacted with commercial FeS, 45% and 20% of nitrogen were recovered as AQ and guanidine, respectively, after 24 h. Nonetheless, NsoQ persisted, contributing to the N-balance. When mackinawite was present, NsoQ disappeared, but AQ was not detected, and guanidine accounted for 11% of the nitrogen recovery. AQ was ultimately transformed into cyanamide, whose dimerization triggered the formation of cyanoguanidine. Alternatively, NsoQ was transformed into guanidine, which reacted with cyanamide to form biguanide. This is the first report systematically investigating the NQ transformation by different iron-based reactive minerals. The evidence indicates that these minerals are attractive alternatives for developing NQ remediation strategies.
• Root, R. A., & Chorover, J. (2022). Molecular speciation controls arsenic and lead bioaccessibility in fugitive dusts from sulfidic mine tailings. Environmental Science. Processes & Impacts. doi:10.1039/d2em00182a
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  Communities nearby mine wastes in arid and semi-arid regions are potentially exposed to high concentrations of toxic metal(loid)s from fugitive dusts deriving from impoundments. To assess the relation between potentially lofted particles and human health risk, we studied the relationship between pharmacokinetic bioaccessibility and metal(loid) molecular speciation for mine tailings dust particulate matter (PM), with elevated levels of arsenic and lead (up to 59 and 34 mmol kg, respectively), by coupling bioassay (IVBA) with X-ray absorption spectroscopy (XAS). Mine tailing efflorescent salts (PM) and PM from the surface crust (0-1 cm, PM) and near surface (0-25 cm) were isolated to
• Kadoya, W. M., Madeira, C. L., Hoppe-Jones, C., Solsten, T., Snyder, S. A., Root, R. A., Sierra-Alvarez, R., Chorover, J., & Field, J. A. (2021). The Role of Manganese Dioxide in the Natural Formation of Organochlorines. ACS ES&T Water, 1(12), 2523-2530.
• Menezes, O., Yu, Y., Root, R. A., Gavazza, S., Chorover, J., Sierra-Alvarez, R., & Field, J. A. (2021). Iron(II) monosulfide (FeS) minerals reductively transform the insensitive munitions compounds 2,4-dinitroanisole (DNAN) and 3-nitro-1,2,4-triazol-5-one (NTO). Chemosphere, 285, 131409.
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  As military applications of the insensitive munitions compounds (IMCs) 2,4-dinitroanisole (DNAN) and 3-nitro-1,2,4-triazol-5-one (NTO) increase, there is a growing need to understand their environmental fate and to develop remediation strategies to mitigate their impacts. Iron (II) monosulfide (FeS) minerals are abundant in freshwater and marine sediments, marshes, and hydrothermal environments. This study shows that FeS solids can reduce DNAN and NTO to their corresponding amines under anoxic ambient conditions. The reactions between IMCs and the FeS minerals were surface-mediated since they did not occur when only dissolved Fe and S were present. Mackinawite, a tetragonal FeS with a layered structure, reduced DNAN mainly to 2-methoxy-5-nitroaniline (MENA), which in turn was partially reduced to 2-4-diaminoanisole (DAAN). The layered structure of mackinawite provided intercalation sites likely responsible for partial adsorption of MENA and DAAN. Mackinawite entirely reduced NTO to 3-amino-1,2,4-triazol-5-one (ATO). The reduction of IMCs showed concurrent oxidation of mackinawite to goethite and elemental sulfur. A commercial FeS product, composed mainly of pyrrhotite and troilite, reduced DNAN to DAAN and NTO to ATO. At pH 6.5, DNAN and NTO transformation rates were 667 and 912 μmol h m, respectively, on the mackinawite surface and 417 and 1344 μmol h m, respectively, on the commercial FeS surface. This is the first report of the reduction of a nitro-heterocyclic compound (NTO) by FeS minerals. The evidence indicates that DNAN and NTO can be rapidly transformed to their succeeding amines in anoxic subsurface environments and aquatic sediments rich in FeS minerals.
• Moravec, B., Keifer, V., Root, R. A., White, A. M., Wang, Y., Olshansky, Y., Mcintosh, J. C., & Chorover, J. D. (2021). Experimental weathering of a volcaniclastic critical zone profile: Key role of colloidal constituents in aqueous geochemical response. Chemical Geology, 559(5), 119886. doi:10.1016/j.chemgeo.2020.119886
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  Weathering profiles are often complex, extending from more highly transformed materials in the near surface (e.g., mobile soils) to less weathered parent material (e.g., variably porous bedrock) at depth. It is difficult to resolve from field data the impacts of material properties on the short-term rates of mineral weathering when different depths of the profile are reacted with aggressive meteoric waters (i.e., dilute and undersaturated with respect to primary silicates). In the present study, we aimed to measure variation in mineral transformation reactions that occurs under controlled laboratory conditions for samples collected as a function of depth (e.g., spatial distribution of geologic texture, mineral assemblage, and weathering features) across a deep weathering profile in volcaniclastic parent rock. We conducted a series of batch weathering experiments of extracted core materials from two borings to 35 m across a zero-order catchment in the rhyolitic Jemez River Basin Critical Zone Observatory, NM, USA. Upon reaction with aqueous solutions pre-equilibrated with atmospheric CO2, mineral dissolution was not limited to one phase, but included a combination of reactions including (at decreasing weathering rates) calcite > zeolites > phyllosilicates > amorphous SiO2 > feldspar. Mineral transformation rates were dependent on the mineral assemblage, texture, and legacy of hydrothermal alteration. Results also indicated an important role of existing and neoformed colloids in Al, Si, and Fe mobilization and redistribution, especially for materials with evidence of previous hydrothermal alteration. Volcanic breccia, which makes up the top 14 m of the western portion of the catchment, was comprised primarily of weathered lithics, where aqueous solution chemistry was controlled by rapid calcite dissolution/precipitation reactions. Hydrothermally altered tuff, which makes up the top 15 m over most of the catchment, exhibited initial dispersion of colloidal zeolites, which subsequently dissolved, giving rise to smectite precipitation (either in-situ and/or along flowpaths). Solute signatures deriving from water/rock interactions in deep, hydrothermally-altered vesicular tuff were comparable to those in shallow altered tuff, but different from those in deep, unaltered, fracture-dominated tuff. We attribute differences to reactive surface area susceptible to chemical attack by aggressive waters (greater in altered rocks) and primary mineral shielding by Fe and Mn oxides on fracture surfaces in unaltered tuff. This study highlights the use of experimental weathering of extracted cores to help interpret field-based, hydrochemistry with an approach that may be employed in other geologically complex terrains.
• Nakaoa, A., Unoa, S., Yanaia, J., Kuboterab, H., Tanakac, R., Root, R. A., & Kosaki, T. (2021). Distance-dependence from volcano for Asian dust inclusions in Andosols: A key to control soil ability to retain radiocesium. Geoderma, 385, 1-9. doi:doi.org/10.1016/j.geoderma.2020.114889
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  The fine-sized mica in Allophanic Andosols is not originated from volcanic ashes.The δ18O values of fine-quartz in the Andosols is similar to those of Asian dust.The quartz content associatively increased with mica as further away from a volcano.Asian-dust-derived mica is crucial to increase RCs retention ability in Andosols.
• Vázquez-Ortega, A., Perdrial, N., Reinoso-Maset, E., Root, R. A., O'Day, P. A., & Chorover, J. (2021). Phosphate controls uranium release from acidic waste-weathered Hanford sediments. Journal of hazardous materials, 416, 126240.
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  Mineral dissolution and secondary phase precipitation may control the fate of inorganic contaminants introduced to soils and sediments during liquid waste discharges. When the solutions are aggressive enough to induce transformation of native minerals, incorporated contaminants may be released during dissolution due to percolation of meteoric waters. This study evaluated the release of uranium (U) from Hanford sediments that had been previously reacted for 180 or 365 days with liquid waste solutions containing U with and without 3 mM dissolved phosphate at pH 2 and 3. Flow-through column experiments were conducted under continuous saturated flow with a simulated background porewater (BPW; pH ~7) for 22 d. Up to 5% of the total U was released from the sediments reacted under PO-free conditions, attributable to the dissolution of becquerelite and boltwoodite formed during weathering. Contrastingly, negligible U was released from PO-reacted sediments, where meta-ankoleite was identified as the main U-mineral phase. Linear combination fits of U L-edge EXAFS spectra of sediments before and after BPW leaching and thermodynamic calculations suggest that the formed becquerelite and meta-ankoleite transformed into schoepite and a phosphuranylite-type phase, respectively. These results demonstrate the stabilization of U as recalcitrant uranyl minerals formed in sediments and highlight the key role of PO in U release at contaminated sites.
• Wang, D., Root, R. A., & Chorover, J. (2021). Biochar-templated surface precipitation and inner-sphere complexation effectively removes arsenic from acid mine drainage. Environmental science and pollution research international, 28(33), 45519-45533.
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  Treatment of aqueous leachate from acid mine tailings with pristine biochar (BC) resulted in the removal of more than 90% of the dissolved arsenic with an attendant rapid and sustained pH buffering from 3 to 4. Pine forest waste BC was transformed to a highly effective adsorbent for arsenic remediation of acid mine drainage (AMD) because the dissolved iron induced "activation" of BC through accumulation of highly reactive ferric hydroxide surface sites. Physicochemical properties of the BC surface, and molecular mechanisms of Fe, S, and As phase transfer, were investigated using a multi-method, micro-scale approach (SEM, XRD, FTIR, XANES, EXAFS, and STXM). Co-located carbon and iron analysis with STXM indicated preferential iron neo-precipitates at carboxylic BC surface sites. Iron and arsenic X-ray spectroscopy showed an initial precipitation of ferrihydrite on BC, with concurrent adsorption/coprecipitation of arsenate. The molecular mechanism of arsenic removal involved bidentate, binuclear inner-sphere complexation of arsenate at the surfaces of pioneering ferric precipitates. Nucleation and crystal growth of ferrihydrite and goethite were observed after 1 h of reaction. The high sulfate activity in AMD promoted schwertmannite precipitation beginning at 6 h of reaction. At reaction times beyond 6 h, goethite and schwertmannite accumulated at the expense of ferrihydrite. Results indicate that the highly functionalized surface of BC acts as a scaffolding for the precipitation and activation of positively charged ferric hydroxy(sulf)oxide surface sites from iron-rich AMD, which then complex oxyanion arsenate, effectively removing it from porewaters. Graphical abstract.
• Hammond, C. M., Root, R. A., Maier, R. M., & Chorover, J. (2020). Arsenic and iron speciation and mobilization during phytostabilization of pyritic mine tailings. Geochimica et cosmochimica acta, 286, 306-323.
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  Particulate and dissolved metal(loid) release from mine tailings is of concern in (semi-) arid environments where tailings can remain barren of vegetation for decades and, therefore, become highly susceptible to dispersion by wind and water. Erosive weathering of metalliferous tailings can lead to arsenic contamination of adjacent ecosystems and increased risk to public health. Management via phytostabilization with the establishment of a vegetative cap using organic amendments to enhance plant growth has been employed to reduce both physical erosion and leaching. However, prior research suggests that addition of organic matter into the oxic weathering zone of sulfide tailings has the potential to promote the mobilization of arsenate. Therefore, the objective of the current work was to assess the impacts of phytostabilization on the molecular-scale mechanisms controlling arsenic speciation and lability. These impacts, which remain poorly understood, limit our ability to mitigate environmental and human health risks. Here we report on subsurface biogeochemical transformations of arsenic and iron from a three-year phytostabilization field study conducted at a Superfund site in Arizona, USA. Legacy pyritic tailings at this site contain up to 3 g kg arsenic originating from arsenopyrite that has undergone oxidation to form arsenate-ferrihydrite complexes in the top 1 m. Tailings were amended in the top 20 cm with 100, 150, or 200 g kg (300-600 T ha) of composted organic matter and seeded with native halotolerant plant species. Treatments and an unamended control received irrigation of 360 ± 30 mm y in addition to 250 ± 160 mm y of precipitation. Cores to 1 m depth were collected annually for three years and sectioned into 20 cm increments for analysis by synchrotron iron and arsenic X-ray absorption spectroscopy (XAS) coupled with quantitative wet chemical and mass balance methods. Results revealed that > 80% of arsenic exists in ammonium oxalate-extractable and non-extractable phases, including dominantly ferrihydrite and jarosite. Arsenic release during arsenopyrite oxidation resulted in both downward translocation and As attenuation by stable Fe(oxyhydr)oxide and Fe (hydroxy)sulfate minerals over time, highlighting the need for sampling at multiple depths and time points for accurate interpretation of arsenic speciation, lability, and translocation in weathering profiles. Less than 1% of total arsenic was highly-labile, i.e. water-extractable, from all treatments, depths, and years, and more than 99% of arsenate released by arsenopyrite weathering was attenuated by association with secondary minerals. Although downward translocation of both arsenic and iron was detected during phytostabilization by temporal enrichment analysis, a similar trend was measured for the uncomposted control, indicating that organic amendment associated with phytostabilization practices did not significantly increase arsenic mobilization over non-amended controls.
• Manjón, I., Ramírez-Andreotta, M. D., Sáez, A. E., Root, R. A., Hild, J., Janes, M. K., & Alexander-Ozinskas, A. (2020). Environmental monitoring and exposure science dataset to calculate ingestion and inhalation of metal(loid)s through preschool gardening. Data in brief, 29, 105050.
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  Metal(loid) contamination may pose an increased risk of exposure to children residing near legacy and active resource extraction sites. Children may be exposed to arsenic, cadmium, and/or lead by ingestion and/or inhalation while engaging in school or home outdoor activities via environmental media including water, soil, dust, and locally grown produce. It is thus critical to collect site-specific data to best assess these risks. This data article provides gastric and lung bioaccessibility assay (IVBA) data, as well as environmental monitoring data for water, soil, dust, and garden produce collected from preschools (N = 4) in mining communities throughout Nevada County, California in 2018. Arsenic, cadmium, and lead concentrations in the aforementioned media and synthetic gastric and lung fluids were measured by inductively coupled plasma-mass spectrometry (ICP-MS). This dataset provides useful metal(loid) concentrations for future risk assessments for similar settings.
• Ramirez, M. D., Abrell, L. M., Kilungo, A. P., Mclain, J. E., & Root, R. A. (2019). Partnering for action: community monitoring of harvested rainwater in underserved, rural, and urban Arizona communities. Water Resources IMPACT.
• Root, R. A. (2020). Resolving Deep Critical Zone Architecture in Complex Volcanic Terrain. Journal of Geophysical Research: Earth Surface.
• Dinali, G. S., Root, R. A., Amistadi, M. K., Chorover, J., Lopes, G., & Guilherme, L. R. (2019). Rare earth elements (REY) sorption on soils of contrasting mineralogy and texture. Environment international, 128, 279-291.
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  Rare earth elements (REY) are the lanthanide elements (Z = 57-71), which have an ever-growing occurrence in present-day industries, agriculture, and modern life. Consequently, environmental concentrations are expected to increase accordingly as a result of intensified utilization. Soils are an important sink for REY, yet little research has been conducted concerning activity, inputs, and lability in soil systems. This study evaluated the REY (lanthanides + yttrium) sorption and partition coefficients (K) in two broadly representative natural soils (A horizon), with contrasting mineralogy and organic character, formed under distinct environmental conditions: an Oxisol from Brazil and a Mollisol from the USA. Batch reactions of soils suspended in a background electrolyte solution of 5 μmoles kg of Ca(NO) at 1:100 solid to solution were reacted with 80 μmoles kg REY added individually and in multi-REY competitive systems to evaluated adsorption after 3 h and 72 h over a wide pH range (from ca. 2 to 8). Results showed sorption was similar for all REY within each soil type when examined at the natural measured soil pH; Mollisol pH 6.85, Oxisol pH 4.35. However, REY sorption (by K) was nearly two-fold greater in the Mollisol compared to the Oxisol for the single REY experiments. Multi-REY competitive sorption reactions showed a decrease in K for both soils at 3 and 72 h, and to a greater extent for the Mollisol, indicating soil type had a strong effect on the sorption affinity of each REY. It was also observed that REY sorption increased from low to high pH (pH 2-8) in the Oxisol, and increased with pH from 2 up to the point zero charge (PZC) in the Mollisol, then stabilized. The varying REY K values from these two distinct and abundant soils, with and without REY competition, and over a range of pH are explained in terms of soil mineralogy (i.e., 2:1 clays in the Mollisol; oxides in the Oxisol) and organic matter content. Our findings show that soil characteristic controls sorption, precipitation, and cation exchange capacity, which are the key mechanisms for predicting REY fate and transport in the environment.
• Hottenstein, J. D., Neilson, J. W., Gil-Loaiza, J., Root, R. A., White, S. A., Chorover, J., & Maier, R. M. (2019). Soil Microbiome Dynamics During Pyritic Mine Tailing Phytostabilization: Understanding Microbial Bioindicators of Soil Acidification. Frontiers in microbiology, 10, 1211.
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  Challenges to the reclamation of pyritic mine tailings arise from acid generation that severely constrains the growth of natural revegetation. While acid mine drainage (AMD) microbial communities are well-studied under highly acidic conditions, fewer studies document the dynamics of microbial communities that generate acid from pyritic material under less acidic conditions that can allow establishment and support of plant growth. This research characterizes the taxonomic composition dynamics of microbial communities present during a 6-year compost-assisted phytostabilization field study in extremely acidic pyritic mine tailings. A complementary microcosm experiment was performed to identify successional community populations that enable the acidification process across a pH gradient. Taxonomic profiles of the microbial populations in both the field study and microcosms reveal shifts in microbial communities that play pivotal roles in facilitating acidification during the transition between moderately and highly acidic conditions. The potential co-occurrence of organoheterotrophic and lithoautotrophic energy metabolisms during acid generation suggests the importance of both groups in facilitating acidification. Taken together, this research suggests that key microbial populations associated with pH transitions could be used as bioindicators for either sustained future plant growth or for acid generation conditions that inhibit further plant growth.
• Manjón, I., Ramírez-Andreotta, M. D., Sáez, A. E., Root, R. A., Hild, J., Janes, M. K., & Alexander-Ozinskas, A. (2020). Ingestion and inhalation of metal(loid)s through preschool gardening: An exposure and risk assessment in legacy mining communities. The Science of the total environment, 134639.
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  Children residing in mining towns are potentially disproportionately exposed to metal(loid)s via ingestion and dust inhalation, thus, increasing their exposure when engaging in school or home gardening or playing outside. This citizen science study assessed preschool children's potential arsenic (As), cadmium (Cd), and lead (Pb) exposure via locally grown produce, water, incidental soil ingestion, and dust inhalation at four sites. Participants were trained to properly collect water, soil, and vegetable samples from their preschools in Nevada County, California. As, Cd, and Pb concentrations in irrigation sources did not exceed the U.S. EPA's maximum contaminant and action levels. In general, garden and playground As and Pb soil concentrations exceeded the U.S. EPA Regional Screening Level, CalEPA Human Health Screening Level, and California Department of Toxic Substances Control Screening Level. In contrast, all Cd concentrations were below these recommended screening levels. Dust samples (
• Hammond, C. M., Root, R. A., Maier, R. M., & Chorover, J. (2018). Mechanisms of Arsenic Sequestration by Prosopis juliflora during the Phytostabilization of Metalliferous Mine Tailings. Environmental science & technology, 52(3), 1156-1164.
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  Phytostabilization is a cost-effective long-term bioremediation technique for the immobilization of metalliferous mine tailings. However, the biogeochemical processes affecting metal(loid) molecular stabilization and mobility in the root zone remain poorly resolved. The roots of Prosopis juliflora grown for up to 36 months in compost-amended pyritic mine tailings from a federal Superfund site were investigated by microscale and bulk synchrotron X-ray absorption spectroscopy (XAS) and multiple energy micro-X-ray fluorescence imaging to determine iron, arsenic, and sulfur speciation, abundance, and spatial distribution. Whereas ferrihydrite-bound As(V) species predominated in the initial bulk mine tailings, the rhizosphere speciation of arsenic was distinctly different. Root-associated As(V) was immobilized on the root epidermis bound to ferric sulfate precipitates and within root vacuoles as trivalent As(III)-(SR) tris-thiolate complexes. Molar Fe-to-As ratios of root epidermis tissue were two times higher than the 15% compost-amended bulk tailings growth medium. Rhizoplane-associated ferric sulfate phases that showed a high capacity to scavenge As(V) were dissimilar from the bulk-tailings mineralogy as shown by XAS and X-ray diffraction, indicating a root-surface mechanism for their formation or accumulation.
• Huskey, D. A., Curlango-Rivera, G., Root, R. A., Wen, F., Amistadi, M. K., Chorover, J., & Hawes, M. C. (2018). Trapping of lead (Pb) by corn and pea root border cells. PLANT AND SOIL, 430(1-2), 205-217.
• Khatiwada, R., Abrell, L., Li, G., Root, R. A., Sierra-Alvarez, R., Field, J. A., & Chorover, J. (2018). Adsorption and oxidation of 3-nitro-1,2,4-triazole-5-one (NTO) and its transformation product (3-amino-1,2,4-triazole-5-one, ATO) at ferrihydrite and birnessite surfaces. Environmental pollution (Barking, Essex : 1987), 240, 200-208.
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  The emerging insensitive munitions compound (IMC) 3-nitro-1,2,4-triazole-5-one (NTO) is currently being used to replace conventional explosives such as 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), but the environmental fate of this increasingly widespread IMC remains poorly understood. Upon release from unexploded solid phase ordinances, NTO exhibits high aqueous solubility and, hence, potential mobilization to groundwater. Adsorption and abiotic transformation at metal oxide surfaces are possible mechanisms for natural attenuation. Here, the reactions at ferrihydrite and birnessite surfaces of NTO and its biotransformation product, 3-amino-1, 2, 4-triazol-5-one (ATO), were studied in stirred batch reactor systems at controlled pH (7.0). The study was carried out at metal oxide solid to solution ratios (SSR) of 0.15, 1.5 and 15 g kg. The samples were collected at various time intervals up to 3 h after reaction initiation, and analyzed using HPLC with photodiode array and mass spectrometric detection. We found no detectable adsorption or transformation of NTO upon reaction with birnessite, whereas ATO was highly susceptible to oxidation by the same mineral, showing nearly complete transformation within 5 min at 15 g kg SSR to urea, CO and N. The mean surface-area-normalized pseudo-first order rate constant (k) for ATO oxidation by birnessite across all SSRs was 0.05 ± 0.022 h m, and oxidation kinetics were independent of dissolved O concentration. Both NTO and ATO were resistant to oxidation by ferrihydrite. However, NTO showed partial removal from solution upon reaction with ferrihydrite at 0.15 and 1.5 g kg SSR and complete loss at 15 g kg SSR due to strong adsorption. Conversely, ATO adsorption to ferrihydrite was much weaker than that measured for NTO.
• Khatiwada, R., Olivares, C., Abrell, L., Root, R. A., Sierra-Alvarez, R., Field, J. A., & Chorover, J. (2018). Oxidation of reduced daughter products from 2,4-dinitroanisole (DNAN) by Mn(IV) and Fe(III) oxides. Chemosphere, 201, 790-798.
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  Abiotic transformation of anthropogenic compounds by redox-active metal oxides affects contaminant fate in soil. The capacity of birnessite and ferrihydrite to oxidize the insensitive munitions compound, 2,4-dinitroanisol (DNAN), and its amine-containing daughter products, 2-methoxy-5-nitro aniline (MENA) and 2,4-diaminoanisole (DAAN), was studied in stirred reactors at controlled pH (7.0). Aqueous suspensions were reacted at metal oxide solid to solution mass ratios (SSR) of 0.15, 1.5 and 15 g kg and solutions were analyzed after 0-3 h by high performance liquid chromatography coupled with photodiode array or mass spectrometry detection. Results indicate that DNAN was resistant to oxidation by birnessite and ferrihydrite. Ferrihydrite did not oxidize MENA, but MENA was susceptible to rapid oxidation by birnessite, with nitrogen largely mineralized to nitrite. This is the first report on mineralization of nonphenolic aromatics and the release of mineralized N from aromatic amines following reaction with birnessite. DAAN was oxidized by both solids, but ca. ten times higher rate was observed with birnessite as compared to ferrihydrite at an SSR of 1.5 g kg. At 15 g kg SSR, DAAN was removed from solution within 5 min of reaction with birnessite. CO evolution experiments indicate mineralization of 15 and 12% of the carbon associated with MENA and DAAN, respectively, under oxic conditions with birnessite at SSR of 15 g kg. The results taken as a whole indicate that initial reductive (bio)transformation products of DNAN are readily oxidized by birnessite. The oxidizability of the reduced DNAN products was increased with progressive (bio)reduction as reflected by impacts on the oxidation rate.
• Root, R. A. (2018). Immobilization of Rhus vernicifera laccase on sepiolite; effect of chitosan and copper modification on laccase adsorption and activity. Applied Clay Science.
• Root, R. A. (2018). Wet–dry cycles impact DOM retention in subsurface soils. Biogeosciences.
• Thomas, A. N., Root, R. A., Lantz, R. C., Sáez, A. E., & Chorover, J. (2018). Oxidative weathering decreases bioaccessibility of toxic metal(loid)s in PM emissions from sulfide mine tailings. GeoHealth, 2(4), 118-138.
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  Environmental contamination from legacy mine-waste deposits is a persistent problem due to the long history of hard-rock mining. Sulfide ore deposits can contain elevated levels of toxic metal(loid)s that, when mobilized by weathering upon O and HO infusion, can result in groundwater contamination. Dry-climate and lack of vegetative cover result in near-surface pedogenic processes that produce fine-particulate secondary minerals that can be translocated as geo-dusts leading to ingestion or inhalation exposure in nearby communities. In this study, bioassays were combined with synchrotron-based x-ray spectroscopy and diffraction to determine the potential risk for toxic element release from dust (PM) samples into biofluid simulants. PM were isolated from across the oxidative reaction front in the top meter of tailings subjected to 50 years of weathering under semi-arid climate, and introduced to synthetic gastric- and alveolar-fluids. Aqueous concentrations were measured as a function of reaction time to determine release kinetics. X-ray diffraction and absorption spectroscopy analyses were performed to assess associated changes in mineralogy and elemental speciation. bioaccessibility of arsenic and lead was highest in less-weathered tailings samples (80-110 cm) and lowest in samples from the sub-oxic transition zone (40-52 cm). Conversely, zinc release to biofluids was greatest in the highly-weathered near-surface tailings. Results indicate that bioaccessibility of As and Pb was controlled by (i) the solubility of Fe-bearing solids, (ii) the prevalence of soluble SO , and (iii) the presence of poorly-crystalline Fe(III) oxide sorbents, whereas Zn bioaccessibility was controlled by the pH-dependent solubility of the stable solid phase.
• Valentín-Vargas, A., Neilson, J. W., Root, R. A., Chorover, J., & Maier, R. M. (2018). Treatment impacts on temporal microbial community dynamics during phytostabilization of acid-generating mine tailings in semiarid regions. The Science of the total environment, 618, 357-368.
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  Direct revegetation, or phytostabilization, is a containment strategy for contaminant metals associated with mine tailings in semiarid regions. The weathering of sulfide ore-derived tailings frequently drives acidification that inhibits plant establishment resulting in materials prone to wind and water dispersal. The specific objective of this study was to associate pyritic mine waste acidification, characterized through pore-water chemistry analysis, with dynamic changes in microbial community diversity and phylogenetic composition, and to evaluate the influence of different treatment strategies on the control of acidification dynamics. Samples were collected from a highly instrumented one-year mesocosm study that included the following treatments: 1) unamended tailings control; 2) tailings amended with 15% compost; and 3) the 15% compost-amended tailings planted with Atriplex lentiformis. Tailings samples were collected at 0, 3, 6 and 12months and pore water chemistry was monitored as an indicator of acidification and weathering processes. Results confirmed that the acidification process for pyritic mine tailings is associated with a temporal progression of bacterial and archaeal phylotypes from pH sensitive Thiobacillus and Thiomonas to communities dominated by Leptospirillum and Ferroplasma. Pore-water chemistry indicated that weathering rates were highest when Leptospirillum was most abundant. The planted treatment was most successful in disrupting the successional evolution of the Fe/S-oxidizing community. Plant establishment stimulated growth of plant-growth-promoting heterotrophic phylotypes and controlled the proliferation of lithoautotrophic Fe/S-oxidizers. The results suggest the potential for eco-engineering a microbial inoculum to stimulate plant establishment and inhibit proliferation of the most efficient Fe/S-oxidizing phylotypes.
• Honeker, L. K., Neilson, J. W., Root, R. A., Gil-Loaiza, J., Chorover, J., & Maier, R. M. (2017). Bacterial Rhizoplane Colonization Patterns of Buchloe dactyloides Growing in Metalliferous Mine Tailings Reflect Plant Status and Biogeochemical Conditions. Microbial ecology, 74(4), 853-867.
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  Plant establishment during phytostabilization of legacy mine tailings in semiarid regions is challenging due to low pH, low organic carbon, low nutrients, and high toxic metal(loid) concentrations. Plant-associated bacterial communities are particularly important under these harsh conditions because of their beneficial services to plants. We hypothesize that bacterial colonization profiles on rhizoplane surfaces reflect deterministic processes that are governed by plant health and the root environment. The aim of this study was to identify associations between bacterial colonization patterns on buffalo grass (Buchloe dactyloides) rhizoplanes and both plant status (leaf chlorophyll and plant cover) and substrate biogeochemistry (pH, electrical conductivity, total organic carbon, total nitrogen, and rhizosphere microbial community). Buffalo grass plants from mesocosm- and field-scale phytostabilization trials conducted with tailings from the Iron King Mine and Humboldt Smelter Superfund Site in Dewey-Humboldt, Arizona, were analyzed. These tailings are extremely acidic and have arsenic and lead concentrations of 2-4 g kg substrate. Bacterial communities on rhizoplanes and in rhizosphere-associated substrate were characterized using fluorescence in situ hybridization and 16S rRNA gene amplicon sequencing, respectively. The results indicated that the metabolic status of rhizoplane bacterial colonizers is significantly related to plant health. Principal component analysis revealed that root-surface Alphaproteobacteria relative abundance was associated most strongly with substrate pH and Gammaproteobacteria relative abundance associated strongly with substrate pH and plant cover. These factors also affected the phylogenetic profiles of the associated rhizosphere communities. In summary, rhizoplane bacterial colonization patterns are plant specific and influenced by plant status and rhizosphere biogeochemical conditions.
• Root, R. A. (2017). A XANES and Raman investigation of sulfur speciation and structural order in Murchison and Allende meteorites. Meteoritics and Planetary Science.
• Gil-Loaiza, J., White, S. A., Root, R. A., Solís-Dominguez, F. A., Hammond, C. M., Chorover, J., & Maier, R. M. (2016). Phytostabilization of mine tailings using compost-assisted direct planting: Translating greenhouse results to the field. The Science of the total environment, 565, 451-461.
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  Standard practice in reclamation of mine tailings is the emplacement of a 15 to 90cm soil/gravel/rock cap which is then hydro-seeded. In this study we investigate compost-assisted direct planting phytostabilization technology as an alternative to standard cap and plant practices. In phytostabilization the goal is to establish a vegetative cap using native plants that stabilize metals in the root zone with little to no shoot accumulation. The study site is a barren 62-hectare tailings pile characterized by extremely acidic pH as well as lead, arsenic, and zinc each exceeding 2000mgkg(-1). The study objective is to evaluate whether successful greenhouse phytostabilization results are scalable to the field. In May 2010, a 0.27ha study area was established on the Iron King Mine and Humboldt Smelter Superfund (IKMHSS) site with six irrigated treatments; tailings amended with 10, 15, or 20% (w/w) compost seeded with a mix of native plants (buffalo grass, arizona fescue, quailbush, mountain mahogany, mesquite, and catclaw acacia) and controls including composted (15 and 20%) unseeded treatments and an uncomposted unseeded treatment. Canopy cover ranging from 21 to 61% developed after 41 months in the compost-amended planted treatments, a canopy cover similar to that found in the surrounding region. No plants grew on unamended tailings. Neutrophilic heterotrophic bacterial counts were 1.5 to 4 orders of magnitude higher after 41months in planted versus unamended control plots. Shoot tissue accumulation of various metal(loids) was at or below Domestic Animal Toxicity Limits, with some plant specific exceptions in treatments receiving less compost. Parameters including % canopy cover, neutrophilic heterotrophic bacteria counts, and shoot uptake of metal(loids) are promising criteria to use in evaluating reclamation success. In summary, compost amendment and seeding, guided by preliminary greenhouse studies, allowed plant establishment and sustained growth over 4years demonstrating feasibility for this phytostabilization technology.
• Honeker, L. K., Root, R. A., Chorover, J., & Maier, R. M. (2016). Resolving colocalization of bacteria and metal(loid)s on plant root surfaces by combining fluorescence in situ hybridization (FISH) with multiple-energy micro-focused X-ray fluorescence (ME μXRF). Journal of microbiological methods, 131, 23-33.
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  Metal(loid)-contamination of the environment due to anthropogenic activities is a global problem. Understanding the fate of contaminants requires elucidation of biotic and abiotic factors that influence metal(loid) speciation from molecular to field scales. Improved methods are needed to assess micro-scale processes, such as those occurring at biogeochemical interfaces between plant tissues, microbial cells, and metal(loid)s. Here we present an advanced method that combines fluorescence in situ hybridization (FISH) with synchrotron-based multiple-energy micro-focused X-ray fluorescence microprobe imaging (ME μXRF) to examine colocalization of bacteria and metal(loid)s on root surfaces of plants used to phytostabilize metalliferous mine tailings. Bacteria were visualized on a small root section using SytoBC nucleic acid stain and FISH probes targeting the domain Bacteria and a specific group (Alphaproteobacteria, Gammaproteobacteria, or Actinobacteria). The same root region was then analyzed for elemental distribution and metal(loid) speciation of As and Fe using ME μXRF. The FISH and ME μXRF images were aligned using ImageJ software to correlate microbiological and geochemical results. Results from quantitative analysis of colocalization show a significantly higher fraction of As colocalized with Fe-oxide plaques on the root surfaces (fraction of overlap 0.49±0.19) than to bacteria (0.072±0.052) (p
• Ramirez-Andreotta, M. D., Lothrop, N., Wilkinson, S. T., Root, R. A., Artiola, J. F., Klimecki, W., & Loh, M. (2016). Analyzing Patterns of Community Interest at a Legacy Mining Waste Site to Assess and Inform Environmental Health Literacy Efforts. Journal of environmental studies and sciences, 6(3), 543-555.
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  Understanding a community's concerns and informational needs is crucial to conducting and improving environmental health research and literacy initiatives. We hypothesized that analysis of community inquiries over time at a legacy mining site would be an effective method for assessing environmental health literacy efforts and determining whether community concerns were thoroughly addressed. Through a qualitative analysis, we determined community concerns at the time of being listed as a Superfund site. We analyzed how community concerns changed from this starting point over the subsequent years, and whether: 1) communication materials produced by the USEPA and other media were aligned with community concerns; and 2) these changes demonstrated a progression of the community's understanding resulting from community involvement and engaged research efforts. We observed that when the Superfund site was first listed, community members were most concerned with USEPA management, remediation, site-specific issues, health effects, and environmental monitoring efforts related to air/dust and water. Over the next five years, community inquiries shifted significantly to include exposure assessment and reduction methods and issues unrelated to the site, particularly the local public water supply and home water treatment systems. Such documentation of community inquiries over time at contaminated sites is a novel method to assess environmental health literacy efforts and determine whether community concerns were thoroughly addressed.
• Rodriguez-Freire, L., Moore, S. E., Sierra-Alvarez, R., Root, R. A., Chorover, J., & Field, J. A. (2016). Arsenic remediation by formation of arsenic sulfide minerals in a continuous anaerobic bioreactor. Biotechnology and bioengineering, 113(3), 522-30.
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  Arsenic (As) is a highly toxic metalloid that has been identified at high concentrations in groundwater in certain locations around the world. Concurrent microbial reduction of arsenate (As(V) ) and sulfate (SO4 (2-) ) can result in the formation of poorly soluble arsenic sulfide minerals (ASM). The objective of this research was to study As biomineralization in a minimal iron environment for the bioremediation of As-contaminated groundwater using simultaneous As(V) and SO4 (2-) reduction. A continuous-flow anaerobic bioreactor was maintained at slightly acidic pH (6.25-6.50) and fed with As(V) and SO4 (2-) , utilizing ethanol as an electron donor for over 250 d. A second bioreactor running under the same conditions but lacking SO4 (2-) was operated as a control to study the fate of As (without S). The reactor fed with SO4 (2-) removed an average 91.2% of the total soluble As at volumetric rates up to 2.9 mg As/(L · h), while less than 5% removal was observed in the control bioreactor. Soluble S removal occurred with an S to As molar ratio of 1.2, suggesting the formation of a mixture of orpiment- (As2 S3 ) and realgar-like (AsS) solid phases. Solid phase characterization using K-edge X-ray absorption spectroscopy confirmed the formation of a mixture of As2 S3 and AsS. These results indicate that a bioremediation process relying on the addition of a simple, low-cost electron donor offers potential to promote the removal of As from groundwater with naturally occurring or added SO4 (2-) by precipitation of ASM.
• Root, R. A. (2016). Pore water chemistry reveals gradients in mineral transformation across a model basaltic hillslope. Geochemistry, Geophysics, Geosystems.
• Sengupta, A., Wang, Y., Meira Neto, A. A., Matos, K. A., Dontsova, K., Root, R., Neilson, J. W., Maier, R. M., Chorover, J., & Troch, P. A. (2016). Soil Lysimeter Excavation for Coupled Hydrological, Geochemical, and Microbiological Investigations. Journal of visualized experiments : JoVE.
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  Studying co-evolution of hydrological and biogeochemical processes in the subsurface of natural landscapes can enhance the understanding of coupled Earth-system processes. Such knowledge is imperative in improving predictions of hydro-biogeochemical cycles, especially under climate change scenarios. We present an experimental method, designed to capture sub-surface heterogeneity of an initially homogeneous soil system. This method is based on destructive sampling of a soil lysimeter designed to simulate a small-scale hillslope. A weighing lysimeter of one cubic meter capacity was divided into sections (voxels) and was excavated layer-by-layer, with sub samples being collected from each voxel. The excavation procedure was aimed at detecting the incipient heterogeneity of the system by focusing on the spatial assessment of hydrological, geochemical, and microbiological properties of the soil. Representative results of a few physicochemical variables tested show the development of heterogeneity. Additional work to test interactions between hydrological, geochemical, and microbiological signatures is planned to interpret the observed patterns. Our study also demonstrates the possibility of carrying out similar excavations in order to observe and quantify different aspects of soil-development under varying environmental conditions and scale.
• Nelson, K. N., Neilson, J. W., Root, R. A., Chorover, J., & Maier, R. M. (2015). Abundance and Activity of 16S rRNA, AmoA and NifH Bacterial Genes During Assisted Phytostabilization of Mine Tailings. International journal of phytoremediation, 17(1-6), 493-502.
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  Mine tailings in semiarid regions are highly susceptible to erosion and are sources of dust pollution and potential avenues of human exposure to toxic metals. One constraint to revegetation of tailings by phytostabilization is the absence of microbial communities critical for biogeochemical cycling of plant nutrients. The objective of this study was to evaluate specific genes as in situ indicators of biological soil response during phytoremediation. The abundance and activity of 16S rRNA, nifH, and amoA were monitored during a nine month phytostabilization study using buffalo grass and quailbush grown in compost-amended, metalliferous tailings. The compost amendment provided a greater than 5-log increase in bacterial abundance, and survival of this compost-inoculum was more stable in planted treatments. Despite increased abundance, the activity of the introduced community was low, and significant increases were not detected until six and nine months in quailbush, and unplanted compost and buffalo grass treatments, respectively. In addition, increased abundances of nitrogen-fixation (nifH) and ammonia-oxidizing (amoA) genes were observed in rhizospheres of buffalo grass and quailbush, respectively. Thus, plant establishment facilitated the short term stabilization of introduced bacterial biomass and supported the growth of two key nitrogen-cycling populations in compost-amended tailings.
• Root, R. A., Hayes, S. M., Hammond, C. M., Maier, R. M., & Chorover, J. (2015). Toxic metal(loid) speciation during weathering of iron sulfide mine tailings under semi-arid climate. Applied geochemistry : journal of the International Association of Geochemistry and Cosmochemistry, 62, 131-149.
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  Toxic metalliferous mine-tailings pose a significant health risk to ecosystems and neighboring communities from wind and water dispersion of particulates containing high concentrations of toxic metal(loid)s (e.g., Pb, As, Zn). Tailings are particularly vulnerable to erosion before vegetative cover can be reestablished, i.e., decades or longer in semi-arid environments without intervention. Metal(loid) speciation, linked directly to bioaccessibility and lability, is controlled by mineral weathering and is a key consideration when assessing human and environmental health risks associated with mine sites. At the semi-arid Iron King Mine and Humboldt Smelter Superfund site in central Arizona, the mineral assemblage of the top 2 m of tailings has been previously characterized. A distinct redox gradient was observed in the top 0.5 m of the tailings and the mineral assemblage indicates progressive transformation of ferrous iron sulfides to ferrihydrite and gypsum, which, in turn weather to form schwertmannite and then jarosite accompanied by a progressive decrease in pH (7.3 to 2.3). Within the geochemical context of this reaction front, we examined enriched toxic metal(loid)s As, Pb, and Zn with surficial concentrations 41.1, 10.7, 39.3 mM kg (3080, 2200, and 2570 mg kg), respectively. The highest bulk concentrations of As and Zn occur at the redox boundary representing a 1.7 and 4.2 fold enrichment relative to surficial concentrations, respectively, indicating the translocation of toxic elements from the gossan zone to either the underlying redox boundary or the surface crust. Metal speciation was also examined as a function of depth using X-ray absorption spectroscopy (XAS). The deepest sample (180 cm) contains sulfides (e.g., pyrite, arsenopyrite, galena, and sphalerite). Samples from the redox transition zone (25-54 cm) contain a mixture of sulfides, carbonates (siderite, ankerite, cerrusite, and smithsonite) and metal(loid)s sorbed to neoformed secondary Fe phases, principally ferrihydrite. In surface samples (0-35 cm), metal(loid)s are found as sorbed species or incorporated into secondary Fe hydroxysulfate phases, such as schwertmannite and jarosites. Metal-bearing efflorescent salts (e.g., ZnSO·HO) were detected in the surficial sample. Taken together, these data suggest the bioaccessibility and lability of metal(loid)s are altered by mineral weathering, which results in both the downward migration of metal(loid)s to the redox boundary, as well as the precipitation of metal salts at the surface.
• Hayes, S. M., Root, R. A., Perdrial, N., Maier, R., & Chorover, J. (2014). Surficial weathering of iron sulfide mine tailings under semi-arid climate. Geochimica et cosmochimica acta, 141, 240-257.
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  Mine wastes introduce anthropogenic weathering profiles to the critical zone that often remain unvegetated for decades after mining cessation. As such, they are vulnerable to wind and water dispersion of particulate matter to adjacent ecosystems and residential communities. In sulfide-rich ore tailings, propagation to depth of the oxidative weathering front controls the depth-variation in speciation of major and trace elements. Despite the prevalence of surficial mine waste deposits in arid regions of the globe, few prior studies have been conducted to resolve the near-surface profile of sulfide ore tailings weathered under semi-arid climate. We investigated relations between gossan oxidative reaction-front propagation and the molecular speciation of iron and sulfur in tailings subjected to weathering under semi-arid climate at an EPA Superfund Site in semi-arid central Arizona (USA). Here we report a multi-method data set combining wet chemical and synchrotron-based X-ray diffraction (XRD) and X-ray absorption near-edge spectroscopy (XANES) methods to resolve the tight coupling of iron (Fe) and sulfur (S) geochemical changes in the top 2 m of tailings. Despite nearly invariant Fe and S concentration with depth (130-140 and 100-120 g kg, respectively), a sharp redox gradient and distinct morphological change was observed within the top 0.5 m, associated with a progressive oxidative alteration of ferrous sulfides to (oxyhydr)oxides and (hydroxy)sulfates. Transformation is nearly complete in surficial samples. Trends in molecular-scale alteration were co-located with a decrease in pH from 7.3 to 2.3, and shifts in Fe and S lability as measured via chemical extraction. Initial weathering products, ferrihydrite and gypsum, transform to schwertmannite, then jarosite-group minerals with an accompanying decrease in pH. Interestingly, thermodynamically stable phases such as goethite and hematite were not detected in any samples, but ferrihydrite was observed even in the lowest pH samples, indicating its metastable persistence in these semiarid tailings. The resulting sharp geochemical speciation gradients in close proximity to the tailings surface have important implications for plant colonization, as well as mobility and bioavailability of co-associated toxic metal(loid)s.
• Menka, N., Root, R., & Chorover, J. (2014). Bioaccessibility, release kinetics, and molecular speciation of arsenic and lead in geo-dusts from the Iron King Mine Federal Superfund site in Humboldt, Arizona. Reviews on environmental health, 29(1-2), 23-7.
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  Mine tailings contain multiple toxic metal(loid)s that pose a threat to human health via inhalation and ingestion. The goals of this research include understanding the speciation and molecular environment of these toxic metal(loid)s (arsenic and lead) as well as the impacts particle size and residence time have on their bioaccessibilty in simulated gastric and lung fluid. Additionally, future work will include smaller size fractions (PM10 and PM2.5) of surface mine tailings, with the goal of increasing our understanding of multi-metal release from contaminated geo-dusts in simulated bio-fluids. This research is important to environmental human health risk assessment as it increases the accuracy of exposure estimations to toxic metal(loid)s.
• Rodriguez-Freire, L., Sierra-Alvarez, R., Root, R., Chorover, J., & Field, J. A. (2014). Biomineralization of arsenate to arsenic sulfides is greatly enhanced at mildly acidic conditions. Water research, 66, 242-253.
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  Arsenic (As) is an important water contaminant due to its high toxicity and widespread occurrence. Arsenic-sulfide minerals (ASM) are formed during microbial reduction of arsenate (As(V)) and sulfate (SO4(2-)). The objective of this research is to study the effect of the pH on the removal of As due to the formation of ASM in an iron-poor system. A series of batch experiments was used to study the reduction of SO4(2-) and As(V) by an anaerobic biofilm mixed culture in a range of pH conditions (6.1-7.2), using ethanol as the electron donor. Total soluble concentrations and speciation of S and As were monitored. Solid phase speciation of arsenic was characterized by x-ray adsorption spectroscopy (XAS). A marked decrease of the total aqueous concentrations of As and S was observed in the inoculated treatments amended with ethanol, but not in the non-inoculated controls, indicating that the As-removal was biologically mediated. The pH dramatically affected the extent and rate of As removal, as well as the stoichiometric composition of the precipitate. The amount of As removed was 2-fold higher and the rate of the As removal was up to 17-fold greater at pH 6.1 than at pH 7.2. Stoichiometric analysis and XAS results confirmed the precipitate was composed of a mixture of orpiment and realgar, and the proportion of orpiment in the sample increased with increasing pH. The results taken as a whole suggest that ASM formation is greatly enhanced at mildly acidic pH conditions.
• Valentín-Vargas, A., Root, R. A., Neilson, J. W., Chorover, J., & Maier, R. M. (2014). Environmental factors influencing the structural dynamics of soil microbial communities during assisted phytostabilization of acid-generating mine tailings: a mesocosm experiment. The Science of the total environment, 500-501, 314-24.
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  Compost-assisted phytostabilization has recently emerged as a robust alternative for reclamation of metalliferous mine tailings. Previous studies suggest that root-associated microbes may be important for facilitating plant establishment on the tailings, yet little is known about the long-term dynamics of microbial communities during reclamation. A mechanistic understanding of microbial community dynamics in tailings ecosystems undergoing remediation is critical because these dynamics profoundly influence both the biogeochemical weathering of tailings and the sustainability of a plant cover. Here we monitor the dynamics of soil microbial communities (i.e. bacteria, fungi, archaea) during a 12-month mesocosm study that included 4 treatments: 2 unplanted controls (unamended and compost-amended tailings) and 2 compost-amended seeded tailings treatments. Bacterial, fungal and archaeal communities responded distinctively to the revegetation process and concurrent changes in environmental conditions and pore water chemistry. Compost addition significantly increased microbial diversity and had an immediate and relatively long-lasting buffering-effect on pH, allowing plants to germinate and thrive during the early stages of the experiment. However, the compost buffering capacity diminished after six months and acidification took over as the major factor affecting plant survival and microbial community structure. Immediate changes in bacterial communities were observed following plant establishment, whereas fungal communities showed a delayed response that apparently correlated with the pH decline. Fluctuations in cobalt pore water concentrations, in particular, had a significant effect on the structure of all three microbial groups, which may be linked to the role of cobalt in metal detoxification pathways. The present study represents, to our knowledge, the first documentation of the dynamics of the three major microbial groups during revegetation of compost-amended, metalliferous mine tailings.
• Gao, X., Root, R. A., Farrell, J., Ela, W., & Chorover, J. (2013). Effect of silicic acid on arsenate and arsenite retention mechanisms on 6-L ferrihydrite: A spectroscopic and batch adsorption approach. Applied geochemistry : journal of the International Association of Geochemistry and Cosmochemistry, 38, 110-120.
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  The competitive adsorption of arsenate and arsenite with silicic acid at the ferrihydrite-water interface was investigated over a wide pH range using batch sorption experiments, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) modeling. Batch sorption results indicate that the adsorption of arsenate and arsenite on the 6-L ferrihydrite surface exhibits a strong pH-dependence, and the effect of pH on arsenic sorption differs between arsenate and arsenite. Arsenate adsorption decreases consistently with increasing pH; whereas arsenite adsorption initially increases with pH to a sorption maximum at pH 7-9, where after sorption decreases with further increases in pH. Results indicate that competitive adsorption between silicic acid and arsenate is negligible under the experimental conditions; whereas strong competitive adsorption was observed between silicic acid and arsenite, particularly at low and high pH. In-situ, flow-through ATR-FTIR data reveal that in the absence of silicic acid, arsenate forms inner-sphere, , complexes at the ferrihydrite surface across the entire pH range. Silicic acid also forms inner-sphere complexes at ferrihydrite surfaces throughout the entire pH range probed by this study (pH 2.8 - 9.0). The ATR-FTIR data also reveal that silicic acid undergoes polymerization at the ferrihydrite surface under the environmentally-relevant concentrations studied (e.g., 1.0 mM). According to ATR-FTIR data, arsenate complexation mode was not affected by the presence of silicic acid. EXAFS analyses and DFT modeling confirmed that arsenate tetrahedra were bonded to Fe metal centers via binuclear bidentate complexation with average As(V)-Fe bond distance of 3.27 Å. The EXAFS data indicate that arsenite forms both mononuclear bidentate and binuclear bidentate complexes with 6-L ferrihydrite as indicated by two As(III)-Fe bond distances of ~2.92-2.94 and 3.41-3.44 Å, respectively. The As-Fe bond distances in both arsenate and arsenite EXAFS spectra remained unchanged in the presence of Si, suggesting that whereas Si diminishes arsenite adsorption preferentially, it has a negligible effect on As-Fe bonding mechanisms.
• Root, R. A., Fathordoobadi, S., Alday, F., Ela, W., & Chorover, J. (2013). Microscale speciation of arsenic and iron in ferric-based sorbents subjected to simulated landfill conditions. Environmental science & technology, 47(22), 12992-3000.
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  During treatment for potable use, water utilities generate arsenic-bearing ferric wastes that are subsequently dispatched to landfills. The biogeochemical weathering of these residuals in mature landfills affects the potential mobilization of sorbed arsenic species via desorption from solids subjected to phase transformations driven by abundant organic matter and bacterial activity. Such processes are not simulated with the toxicity characteristic leaching procedure (TCLP) currently used to characterize hazard. To examine the effect of sulfate on As retention in landfill leachate, columns of As(V) loaded amorphous ferric hydroxide were reacted biotically at two leachate sulfate concentrations (0.064 mM and 2.1 mM). After 300 days, ferric sorbents were reductively dissolved. Arsenic released to porewaters was partially coprecipitated in mixed-valent secondary iron phases whose speciation was dependent on sulfate concentration. As and Fe XAS showed that, in the low sulfate column, 75-81% of As(V) was reduced to As(III), and 53-68% of the Fe(III) sorbent was transformed, dominantly to siderite and green rust. In the high sulfate column, Fe(III) solids were reduced principally to FeS(am), whereas As(V) was reduced to a polymeric sulfide with local atomic structure of realgar. Multienergy micro-X-ray fluorescence (ME-μXRF) imaging at Fe and As K-edges showed that As formed surface complexes with ferrihydrite > siderite > green rust in the low sulfate column; while discrete realgar-like phases formed in the high sulfate systems. Results indicate that landfill sulfur chemistry exerts strong control over the potential mobilization of As from ferric sorbent residuals by controlling secondary As and Fe sulfide coprecipitate formation.
• Solís-Dominguez, F. A., White, S. A., Hutter, T. B., Amistadi, M. K., Root, R. A., Chorover, J., & Maier, R. M. (2012). Response of key soil parameters during compost-assisted phytostabilization in extremely acidic tailings: effect of plant species. Environmental science & technology, 46(2), 1019-27.
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  Phytostabilization of mine tailings acts to mitigate both eolian dispersion and water erosion events which can disseminate barren tailings over large distances. This technology uses plants to establish a vegetative cover to permanently immobilize contaminants in the rooting zone, often requiring addition of an amendment to assist plant growth. Here we report the results of a greenhouse study that evaluated the ability of six native plant species to grow in extremely acidic (pH ∼ 2.5) metalliferous (As, Pb, Zn: 2000-3000 mg kg(-1)) mine tailings from Iron King Mine Humboldt Smelter Superfund site when amended with a range of compost concentrations. Results revealed that three of the six plant species tested (buffalo grass, mesquite, and catclaw acacia) are good candidates for phytostabilization at an optimum level of 15% compost (w/w) amendment showing good growth and minimal shoot accumulation of metal(loid)s. A fourth candidate, quailbush, also met all criteria except for exceeding the domestic animal toxicity limit for shoot accumulation of zinc. A key finding of this study was that the plant species that grew most successfully on these tailings significantly influenced key tailings parameters; direct correlations between plant biomass and both increased tailings pH and neutrophilic heterotrophic bacterial counts were observed. We also observed decreased iron oxidizer counts and decreased bioavailability of metal(loid)s mainly as a result of compost amendment. Taken together, these results suggest that the phytostabilization process reduced tailings toxicity as well as the potential for metal(loid) mobilization. This study provides practical information on plant and tailings characteristics that is critically needed for successful implementation of assisted phytostabilization on acidic, metalliferous mine tailings sites.
• Root, R. A., Vlassopoulos, D., Rivera, N. A., Rafferty, M. T., Andrews, C., & O'Day, P. A. (2009). Speciation and natural attenuation of arsenic and iron in a tidally influenced shallow aquifer. GEOCHIMICA ET COSMOCHIMICA ACTA, 73(19), 5528-5553.
• Campbell, K. M., Root, R., O'Day, P. A., & Hering, J. G. (2008). A gel probe equilibrium sampler for measuring arsenic porewater profiles and sorption gradients in sediments: I. Laboratory development. Environmental science & technology, 42(2), 497-503.
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  A gel probe equilibrium sampler has been developed to study arsenic (As) geochemistry and sorption behavior in sediment porewater. The gels consist of a hydrated polyacrylamide polymer, which has a 92% water content. Two types of gels were used in this study. Undoped (clear) gels were used to measure concentrations of As and other elements in sediment porewater. The polyacrylamide gel was also doped with hydrous ferric oxide (HFO), an amorphous iron (Fe) oxyhydroxide. When deployed in the field, HFO-doped gels introduce a fresh sorbent into the subsurface thus allowing assessment of in situ sorption. In this study, clear and HFO-doped gels were tested under laboratory conditions to constrain the gel behavior prior to field deployment. Both types of gels were allowed to equilibrate with solutions of varying composition and re-equilibrated in acid for analysis. Clear gels accurately measured solution concentrations (+/-1%), and As was completely recovered from HFO-doped gels (+/-4%). Arsenic speciation was determined in clear gels through chromatographic separation of the re-equilibrated solution. For comparison to speciation in solution, mixtures of As(III) and As(V) adsorbed on HFO embedded in gel were measured in situ using X-ray absorption spectroscopy (XAS). Sorption densities for As(III) and As(V) on HFO embedded in gel were obtained from sorption isotherms at pH 7.1. When As and phosphate were simultaneously equilibrated (in up to 50-fold excess of As) with HFO-doped gels, phosphate inhibited As sorption by up to 85% and had a stronger inhibitory effect on As(V) than As(III). Natural organic matter (>200 ppm) decreased As adsorption by up to 50%, and had similar effects on As(V) and As(III). The laboratory results provide a basis for interpreting results obtained by deploying the gel probe in the field and elucidating the mechanisms controlling As partitioning between solid and dissolved phases in the environment.
• Root, R. A. (2008). A gel probe equilibrium sampler for measuring arsenic porewater profiles and sorption gradients in sediments: II. Field application to Haiwee reservoir sediment. Environmental Science and Technology.
• Root, R. A. (2007). Arsenic sequestration by sorption processes in high-iron sediments. Geochimica et Cosmochimica Acta.
• O'Day, P. A., Vlassopoulos, D., Root, R., & Rivera, N. (2004). The influence of sulfur and iron on dissolved arsenic concentrations in the shallow subsurface under changing redox conditions. Proceedings of the National Academy of Sciences of the United States of America, 101(38), 13703-8.
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  The chemical speciation of arsenic in sediments and porewaters of aquifers is the critical factor that determines whether dissolved arsenic accumulates to potentially toxic levels. Sequestration of arsenic in solid phases, which may occur by adsorption or precipitation processes, controls dissolved concentrations. We present synchrotron x-ray absorption spectra of arsenic in shallow aquifer sediments that indicate the local structure of realgar (AsS) as the primary arsenic-bearing phase in sulfate-reducing conditions at concentrations of 1-3 mmol.kg(-1), which has not previously been verified in sediments at low temperature. Spectroscopic evidence shows that arsenic does not substitute for iron or sulfur in iron sulfide minerals at the molecular scale. A general geochemical model derived from our field and spectroscopic observations show that the ratio of reactive iron to sulfur in the system controls the distribution of solid phases capable of removing arsenic from solution when conditions change from oxidized to reduced, the rate of which is influenced by microbial processes. Because of the difference in solubility of iron versus arsenic sulfides, precipitation of iron sulfide may remove sulfide from solution but not arsenic if precipitation rates are fast. The lack of incorporation of arsenic into iron sulfides may result in the accumulation of dissolved As(III) if adsorption is weak or inhibited. Aquifers particularly at risk for such geochemical conditions are those in which oxidized and reduced waters mix, and where the amount of sulfate available for microbial reduction is limited.
• Root, R. A. (2004). X-ray absorption spectroscopic study of Fe reference compounds for the analysis of natural sediments. American Mineralogist.

Proceedings Publications

• Reynoso, L., Bose, M., Williams, L., Wittman, A., Root, R. A., & Castillo-Rogez, J. (2022, October). Amino Acid Glycine Observed within Sodium Chloride Precipitates: Laboratory Analysis of Ceres Analogues. In Bulletin of the American Astronomical Society, 54.
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  Here, we report laboratory experiments to synthesize sodium chloride (NaCl) salts under equilibrium conditions with the addition of the amino acid glycine (C2H5NO2). We performed microscopy and spectroscopy of samples to assess how the presence of glycine changes the chemical, physical, and optical properties of the salts. These experiments are designed to investigate carbon incorporation within NaCl crystals, likely occurring on Ceres’ surface. Such work on Ceres analog minerals is crucial for the design of a robust sample collection system for sample return at the Occator site, as suggested for a future New Frontiers class mission in the recently released Origins, Worlds, Life — Decadal Strategy for Planetary Science and Astrobiology 2023-2032.
• Root, R. A., Maier, R. M., & Chorover, J. (2020). SPECIATION AND BIOACCESSIBILTY OF PARTICULATE MATTER FROM LEGACY MINE TAILINGS. In Geological Society of America Abstracts with Programs.
• White, A., Root, R. A., Mcintosh, J. C., White, A., Root, R. A., Moravec, B. G., Mcintosh, J. C., Chorover, J., & Carr, B. J. (2019). RESOLVING CRITICAL ZONE STRUCTURE AND WEATHERING PROFILES ACROSS A GEOLOGICALLY COMPLEX SUB-ALPINE WATERSHED. In Geological Society of America Abstracts with Programs.
• Root, R. A. (2018). Organic acid effect on arsenate bioaccessibility in gastric and alveolar simulated biofluid systems. In Environmental Arsenic in a ChangingWorld - 7th International Congress and Exhibition Arsenic in the Environment, 2018.
• Root, R. A., Zhao, L., Sun, A., Root, R. A., Olshansky, Y., Hong, H., Fang, Q., & Chorover, J. (2018). CONTINUOUS MONITORING OF SOIL AND ECOSYSTEM RESPIRATION IN A SEMI-ARID HIGH-ELEVATION CRITICAL ZONE OBSERVATORY. In Geological Society of America Abstracts with Programs.
• Root, R. A., Zhao, L., Sun, A., Root, R. A., Olshansky, Y., Hong, H., Fang, Q., & Chorover, J. (2018). COUPLING BETWEEN SOIL RESPIRATION AND GEOCHEMICAL REACTIONS IN THE JEMEZ RIVER BASIN CRITICAL ZONE OBSERVATORY. In Geological Society of America Abstracts with Programs.
• White, A., Root, R. A., Mcintosh, J. C., White, A., Root, R. A., Moravec, B. G., Mcintosh, J. C., & Chorover, J. (2018). DECONVOLVING LEGACY AND CONTEMPORANEOUS WEATHERING IN A PORPHYRITIC RHYOLITE AND RHYOLITIC TUFF DOMINATED UPLAND CATCHMENT, VALLES CALDERA, NEW MEXICO. In Geological Society of America Abstracts with Programs.
• Root, R. A., Wang, Y., Maier, R. M., Hammond, C. M., & Chorover, J. D. (2015). Biogeochemistry of metalliferous mine tailings during phytostabilization. In AGU.
• Thomas, A., Root, R. A., Kong, S. Q., Chorover, J. D., & Chorover, J. D. (2015). Influence of Organic Matter - Mineral Interfacial Reactions on Metal(loid) Speciation and Bioaccessibility. In AGU.
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  abstract id. B33D-0727
• Wang, Y., Troch, P. A., Troch, P., Sengupta, A., Root, R. A., Neto, A. A., Dontsova, K. M., & Chorover, J. D. (2015). Dissecting the Hydrobiogeochemical Box. In AGU.
• Maier, R. M., Neilson, J. W., Valentin-vargas, A., Root, R. A., Neilson, J. W., Jennings, L. L., Hottenstein, J. D., Honeker, L. K., Hammond, C. M., Gil-loaiza, J., Chorover, J. D., & Maier, R. M. (2014). Microbial Diversity and Metal Speciation Changes in Mine Tailings Following Compost-Assisted Direct Planting: A Four-Year Superfund Site Field Study. In AGU.
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  abstract id. B24C-07

Presentations

• Reynoso, L., Bose, M., & Root, R. A. (2023, Spring). Positive glycine incorporation in Ceres analogue minerals. Lunar and Planetary Science ConferenceLunar and Planetary Institute.
• Bose, M., & Root, R. A. (2023, Spring). Ryugu particles contain sulfur in multiple oxidation states. 54th Lunar and Planetary Science Conference. Woodlands, TX: Lunar and Planetary Institute.
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  Sulfurization of O2-poor organic matter in Ryugu may be a key process occurring in Ryugu’s interior. It reduces reactive functional groups and adds cross links between small unstable molecules and likely converts them into recalcitrant organosulfur coatings on minerals.
• Root, R. A., & Chorover, J. D. (2023, July). The bioaccessibility of arsenic and lead from sulfidic mine tailings is controlled by contaminant and host speciation. Goldschmidt 2023 Conference. Lyon, France: Geochemical Society.
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  Arsenic and lead are toxicants with no beneficial biological function. These chalcophile elements are enriched in sulfide ores and can become problematic when deposited as fine particulate matter (PM) in mine tailings. Communities nearby mine wastes in arid and semi-arid regions are potentially exposed to toxic metal(loid)s from fugitive dusts deriving from these impoundments. To assess the relation between potentially lofted PM and human health risk, we studied the relationship between pharmacokinetic bioaccessibility and metal(loid) molecular speciation for tailings from a Superfund site with arsenic and lead as the contaminants of concern by coupling in vitro bioassay (IVBA) with X-ray absorption spectroscopy (XAS). Tailings PM with arsenic and lead up to 59 and 34 mmol kg-1 were reacted with synthetic gastric and lung fluid for 30 s to 100 h to investigate toxic metal(loid) release kinetics. Bioaccessible (BAc) fractions of arsenic and lead were about 10 and 100 times greater in gastric than in lung fluid simulant, respectively, and 10-100% of the maximum gastric BAc from PM10 and PM150 occurred within 30 s, with parabolic dissolution highly-reactive PM followed by slower release from less soluble sources. Arsenate within jarosite and sorbed to ferrihydrite, and lead from anglesite, were identified by XAS as the principal contaminant sources in near surface tailings. Analysis of pre- and post-IVBA PM indicated the release of arsenic in lung fluid was principally from arsenic-substituted jarosite, whereas in synthetic gastric fluid arsenic complexed on ferrihydrite surfaces was preferentially released and subsequently repartitioned to jarosite-like coordination at extended exposures. Lead dissolved at 30 s was subsequently repartitioned back to the solid phase as pyromorphite in phosphate rich lung fluid. The bioaccessibility of lead in PM was limited due to robust sequestration in plumbojarosite.
• Chorover, J. D., Troch, P. A., Cunningham, C., Wang, Y., & Root, R. A. (2022, December). Colloid Formation and its Contribution to Landscape Chemical Denudation During Weathering of Tephritic Model Hillslopes. AGU Fall Meeting. Chicago, IL: A.
• Root, R. A. (2022, October). Speciation and toxicity: Bioaccessibility of airborne arsenic- and metal-rich particulate matter. USGS SynerGEM Seminar. Reston, VA: USGS.
• Root, R. A., & Chorover, J. D. (2022, May). Measurement and modeling of sulfide tailings diagenesis in controlled experimentation. Implications for mine waste remediation. School of Mining and Mineral Resources: Faculty Engagement. Tucson, AZ: School of Mining and Mineral Resources.
• Ramirez, M. D., Sandhaus, S., Sandoval, F., Kilungo, A. P., Mclain, J. E., Root, R. A., Abrell, L. M., Buxner, S. R., Kaufmann, D. B., Cortez, I., Theresa, F., Margaret, D., Lisa, O., Palawat, K., Norma, V., Annabelle, G., & Tasnim, A. (2021). Building a transdisciplinary, bilingual community science program to advance environmental health in underserved communities. Citizen Science AssociationCitizen Science Association.
• Ramirez, M. D., Root, R. A., & Kunal, P. K. (2020, Fall). Co-created citizen science to understand the quality of harvested rainwater and influential predictors for current and future irrigation use. American Geophysical Union Fall MeetingAGU.
• Thomas, R., Root, R. A., Curlango-Rivera, G., & Mclain, J. E. (2020, December). Heavy metal(loid) contamination of medicinal plants: implications for public health of indigenous people. AGU Fall Meeting. Virtual: American Geophysical Union.
• Chorover, J. D., Moravec, B. G., White, A. M., Root, R. A., Olshansky, Y., & Mcintosh, J. C. (2019, Dec). Connections between shallow and deep structure of an extrusive critical zone influence hydrochemical response. AGU Fall Meeting. San Francisco, CA: AGU.
• Hottenstein, J. D., Neilson, J. W., Gil-Loaiza, J., Root, R. A., Chorover, J. D., & Maier, R. M. (2019, Jan). Soil Microbiome Dynamics during Revegetation of Pyritic Mine Tailings: Understanding Microbial Bioindicators of Soil Acidification. SSSA International Soils Meeting. San Diego, CA: SSSA.
• Mollaneda, J., Wang, Y., Amistadi, M. K., Root, R. A., Troch, P. A., & Chorover, J. D. (2019, Dec). Transport Induced Mineral Dissolution Through Intensive Hydrological Cycles in Incipient Basalt Hillslopes. AGU Fall Meeting. San Francisco, CA: AGU.
• Moravec, B. G., Root, R. A., Carr, B., White, A. M., Mcintosh, J. C., & Chorover, J. D. (2019, Dec). Linking near surface geophysics to critical zone architecture and biogeochemical processes. AGU Fall Meeting. San Francisco, CA: AGU.
• Ramirez, M. D., Abrell, L. M., Mclain, J. E., Kilungo, A. P., & Root, R. A. (2019, April). Project Harvest: A Co-Created Citizen Science Rainwater Harvesting Program in Rural and Urban Arizona Communities. 2nd Cobre Valley Forum on Water. Globe, Gila County, Arizona..
• Ramirez, M. D., Abrell, L. M., Mclain, J. E., Kilungo, A. P., Root, R. A., Sandoval, F., Solis-Leon, J., Moses, A., Davis, L., Villagomez, N., & Kaufmann, D. (2019, March). Project Harvest Be Informed. Grow Smarter. Water Research Resource Center Brown Bag Seminar. Tucson, AZ: Water Research Resource Center.
• Ramirez, M. D., Davis, L., Buxner, S. R., Kilungo, A. P., Root, R. A., Mclain, J. E., Abrell, L. M., & Sandoval, F. (2019, March). Effect of environmental monitoring method on participant self-efficacy for science in underrepresented communities. Citizen Science Conference. Raleigh, North Carolina: Citizen Science Association.
• Ramirez, M. D., Root, R. A., & Solis-Leon, J. (2019, Dec). Metal(loid)s in Rooftop Harvested Rainwater near Hazardous Waste and Toxic Release Sites. AGU Fall Meeting. San Francisco, CA: AGU.
• Chorover, J., Root, R., Hammond, C., & Maier, R. (2017, APR 2). Biogeochemical transformation of metal(loid)s in a disturbed critical zone. ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY.
• Root, R. A. (2017, April). Mineral resource recovery has driven the Anthropocene; now what?. University of Vermont, Department of Geology, Seminar Series. University of Vermont, Burlington VT.
• Root, R., Maier, R., & Chorover, J. (2017, APR 2). Stabilization of metalliferous mine tailings during mesocosm-scale phytostabilization. ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY.
• Root, R. A. (2015, October). Coupling biogeochemical cycles and metal(loid) lability in sediments. LCLS/SSRL Users meeting. SLAC/Stanford University: LCLS/SSRL Users Meeting.
• Chorover, J., Root, R., Hammond, C., Valentin, A., & Maier, R. (2013, SEP 8). Toxic metal(loid) speciation is controlled by iron mineral (bio)weathering inphyto-stabilized mine tailings. ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY.
• Root, R. A., Hammond, C. M., Adel, A., Amistadi, M. K., Maier, R. M., & Chorover, J. (2013, SEP 8). (Bio)geochemical mechanisms of metalloid phytostabilization in arid mine tailings. ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY.
• Vargas, A. V., Nelson, K. N., Root, R. A., Chorover, J., & Maier, R. M. (2013, SEP 8). Analysis of the metabolic potential and phylogenetic composition of rhizosphere microbial communities during the phytostabilization of metalliferous mine tailings. ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY.
• Root, R. A., O'day, P., Hering, J., Campbell, K., & Vlassopoulos, D. (2008, JUL). Predicting arsenic behavior in high-iron subsurface environments. GEOCHIMICA ET COSMOCHIMICA ACTA.

Poster Presentations

• Byars, B., Baudin, N., Root, R. A., Rasmussen, C., & Chorover, J. D. (2023, October). Coccidioides posadasii hot spots: characterization of Valley Fever-ridden Arizona soils. GSA Connects 2023. PIttsburg, PA: GSA.
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  Coccidioidomycosis, or “Valley Fever,” is a disease caused by inhalation of the soil-borne fungal arthroconidia (spores) of Coccidioides spp. More than 20,000 cases of Coccidioidomycosis were reported to the CDC in 2019, with most of the cases occurring in residents of highly endemic areas (California’s San Joaquin Valley [C. immitis] and Arizona’s Sonoran Desert [C. posadasii]). Coccidioides is not evenly distributed throughout these endemic areas, instead it is found in “hotspots”. Factors controlling the geographic range and distribution of Coccidioides are not well defined, although soil, climatic, and environmental properties are expected to be important. Additionally, the ecological niche of Coccidioides is still poorly understood, and it remains uncertain whether these fungi are mainly linked to small mammals, such as burrowing rodents, or if they exist as saprotrophs in soil. This study aims to better understand the habitat of C. posadasii by determining correlations between its presence or absence and soil physicochemical properties. Soils were collected from both burrows and non-burrows at 3 known C. posadasii positive locations in Arizona, across Maricopa, Pima, and Pinal counties. Soil chemistry was evaluated on soil extracts, including pH, electrical conductivity (EC), dissolved organic matter content, and soluble and exchangeable ion compositions. Particle size distribution (PSD) was determined to establish soil texture and aid in the prediction of porosity and reactivity. Physical properties including soil temperature and water content are continuously measured in-situ at the sites. Work is ongoing to determine soil characteristics and conditions that are essential to fungal growth and to constrain C. posadasii habitat. The larger objective of this research is to aid in the development of a predictive geospatial model of C. posadasii incidence in the state of Arizona.
• De Gracia, X. M., Root, R. A., Alghzawi, M., & Chorover, J. D. (2023, October).

  What properties of mine tailings control the release of metal(loid)s to solution? [1^st place Geology and Health Division Graduate Student Poster].
  

  . GSA Connects 2023. Pittsburgh, PA: GSA.
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  Many legacy mines have tailings that have been exposed for hundreds of years to the climate conditions of each site. They contain different minerals and metal(loid)s that can enhance or limit the transport of hazardous elements into the subsurface or transported as airborne particulate matter. These weathering processes can affect the health of the surrounding environment and population. To elucidate the possible hazards, it is essential to characterize these residues' physical and chemical properties by different techniques. Mine tailings from polymetallic sulfide ore from three States in the Western US were characterized: a humid climate in Montana (Keating and Mammoth), a dry climate in Nevada (Tybo), and a moderate climate in Arizona (Iron King). Elemental composition was screened on-site using a portable X-ray fluorescence (PXRF). The specific surface area (SSA) of the tailings was analyzed through the BET N2(g) absorption technique, using Micromeritics Gemini VII to obtain information on tailings surfaces available for reaction and metal(loid)s adsorption. Particle size distribution (PSD) was studied using LS 13-320 laser diffraction particle size analyzer. Knowing the distribution of particle sizes helps to understand the material’s potential porosity and reactivity. X-ray diffraction was used to analyze the mineralogical composition of the tailings. Batch and flow-through experiments were conducted to monitor the developed pH changes and released metal(loid)s in tailings exposed to water and oxygen. Tybo, NV, tailings have the highest concentrations of toxic metal(loid)s studied. Pb was higher at the surface compared to deeper areas (~1 m). At Mammoth in MT, Pb was lowest at the surface and the highest at deeper layers. Based on the calcium concentration of the three sites, Tybo has the highest acid neutralization capacity. This can be seen in the flow-through reactive experiment, where it maintains its neutral pH. From the batch and flow-through reactive experiments, Mammoth becomes acidic faster than the other sites. Mammoth has the highest specific surface area and the smallest particle size of the three sites, meaning it can have the highest reactivity. The tailings from varying climates show weathering depth profiles and products, as well as metal(loid) release, that differs by site.
• Ederer, A., Gallery, R. E., Root, R. A., & Chorover, J. D. (2023, December). Mineralogical Controls on Chemical Depletion Patterns in the Santa Catalina Mountains. AGU 2023. San Francisco, CA: American Geophysical Union.
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  Among several soil forming factors, climate and lithology may be thought of as co-evolving controllers of soil type that act on the system from the top-down and the bottom-up, respectively. The downward hydraulic forcings from precipitation events give rise to dissolution-precipitation reactions and subsequent translocation of secondary minerals, which are typically more fine-grained than the minerals from which they were derived. Chemical enrichment-depletion patterns (a.k.a., “tau plots”) are commonly used to assess the extent of alteration of a geochemical weathering profile, and these enrichment patterns are generally controlled by the distribution of soil minerals. Depletion of large primary mineral particles enriched in mobile elements and accumulation of smaller secondary mineral particles enriched in less mobile elements is commonly observed but few studies have provided direct correlation of geochemical and mineralogical datasets. Varying levels of agreement between mineral assemblage and associated chemical enrichment may convey key process linkages that could be further elucidated with particle size analysis (PSA). In addition to helping define size ranges of well-characterized soil mineral particles, full spectrum PSA, as conducted with laser diffractometry, can reveal spikes along the particle size distribution at specific depths indicative of mineral precipitation patterns, which are hypothesized to have a distinct chemical signature (e.g., Al and Si enrichment). We collected PSA, tau plot, and quantitative mineralogical data for two catenas on the same lithology but subject to different climates in the Santa Catalina Mountains. We postulated that differences in soil development in 2-dimensional cross-sections were dependent on effective precipitation and hillslope hydrology. Differences in particle size down the soil profile and along each catena sequence was found to be largely controlled by variation in moisture content: more moisture (in the toeslope and higher in elevation) resulted in a smaller average particle size. These variations in full spectrum PSA were related to correlated variation in mineralogical composition and chemical enrichment-depletion patterns.
• Borkan, W., Root, R. A., Chorover, J. D., Brusseau, M. L., Ouni, A., & Taffet, M. (2022, December). Sequential Extraction: Measuring Sorption Capacity of Sandstone for Uranium. AGU Fall Meeting. Chicago, IL: AGU.
• Chukwuonye, G. G., Palawat, K., Root, R. A., & Ramirez, M. D. (2022). Metals in rooftop harvesting rainwater from environmental justice communities: Contaminant patterns and stakeholder impacts. . ASA, CSSA, and SSSA Annual MeetingASA, CSSA, and SSSA Annual Meeting.
• Ramirez, M. D., Abrell, L. M., Buxner, S. R., Cortez, I., Davis, L., Dewey, M., Foley, T., Henriquez, P., Jones, M., Kaufmann, D., Kilungo, A. P., Mclain, J. E., Ochoa, L., Root, R. A., Sandhaus, S., & Sandoval, F. (2021). Engaging Diverse Communities for Environmental Health Justice. National Science Foundation’s AISL PI meetingNational Science Foundation.
• Henson, C., Villagomez-Marquez, N., Abrell, L. M., Buxner, S. R., Kilungo, A. P., Mclain, J. E., Root, R. A., Sandoval, F., & Ramirez, M. D. (2020, October). Per-polyfluoroalkyl substances in roof-harvested rainwater. Emerging Contaminants Summit. Westminster, Colorado (Virtual).
• Moses, A. J., Abrell, L. M., Buxner, S. R., Kilungo, A. P., Mclain, J. E., Obergh, V., Root, R. A., Sandoval, F., & Ramirez, M. D. (2020, November). Indicator organism presence in urban and rural Arizona community garden soils watered with harvested rainwater. ASA-CSSA-SSSA International Annual Meetings. Virtual: Agronomy Societies.
• Root, R. A., Ramirez, M. D., & Mendoza, C. (2020). Arsenic Field-Test Kits Provide Rapid and Accurate Results in Harvested Rainwater. ASA, CSSA and SSSA International Annual MeetingASA, CSSA and SSSA International Annual Meeting.
• Sonia, M., Abrell, L. M., Villagomez-Marquez, N., Palawat, K., Ramirez, M. D., Mclain, J. E., Root, R. A., Buxner, S. R., Kilungo, A. P., & Flor, S. (2020, August). What's in your rainwater? Inorganic and organic contaminants measured in roof-harvested rainwater. KEYS Research Internship Showcase. Tucson, Arizona (Virtual): Keep Engaging Youth in Science Internship Program.
• Villagomez-Marquez, N., Abrell, L. M., Buxner, S. R., Kilungo, A. P., Mclain, J. E., Root, R. A., Flor, S., Ramirez, M. D., & Chorover, J. D. (2020, October). Pesticides detected in roof-harvested rainwater in rural and urban Arizona communities. Emerging Contaminants Summit. Westminster, Colorado (Virtual).
• Villagómez-Márquez, N., Abrell, L. M., Buxner, S. R., Kilungo, A. P., Mclain, J. E., Root, R. A., & Ramirez, M. D. (2020, March). Emerging contaminants in Roof-harvested Rainwater. Emerging Contaminants Summit. Westminster, CO: Federallabs.
• Carter, A., Thomas, R., Huskey, D., Curlango-Rivera, G., Root, R. A., Ottman, M. J., Hawes, M. C., & Mclain, J. E. (2019, July). Plant root border cell interactions with uranium and cadmium: a potential phytoremediation tool. Botany 2019: Sky Islands and Desert Seas Conference. Tucson, Arizona: Botanical Society of America, others.
• Minke, A., Cuello, J. L., Root, R. A., & Mclain, J. E. (2018, February). Using freshwater algae to remove Pb from water. University of Arizona Water Resources Research Center 2018 Annual Conference. Tucson, Arizona: University of Arizona Water Resources Research Center.
• Ramirez, M. D., Root, R. A., & Alfaifi, T. (2018, April). Translocation assessment of Al, Zn, As,Pb,Ni and Cd in three culturally relevant crops: sesame (Sesamum indicum), corchorus (Corchorus olitorius), and arugula (Eruca vesicaria). SWESx. The University of Arizona.
• Ramirez, M. D., Sandoval, F., Root, R. A., Mclain, J. E., Kilungo, A. P., Buxner, S. R., Abrell, L. M., Montijo, F., & Villagomez-Marquez, N. (2018, March). Investigating emerging organic contaminants in harvested rainwater via co-created citizen science: what is in your rainwater?. Science of the Environment Earth Day Poster Presentation. Tucson, Arizona: University of Arizona Department of Soil, Water and Environmental Science.
• Ramirez, M. D., Sandoval, F., Root, R. A., Mclain, J. E., Kilungo, A. P., Buxner, S. R., Abrell, L. M., Moses, A., & Solis-Leon, J. (2018, March). Socio-demographics in citizen science: does governance model matter?. Science of the Environment Earth Day Poster Presentation. Tucson, Arizona: University of Arizona Department of Soil, Water and Environmental Science.
• Sandoval, F., Root, R. A., Ramirez, M. D., Mclain, J. E., Kilungo, A. P., Buxner, S. R., Abrell, L. M., Davis, L., Kaufmann, D., & Sandhaus, S. (2018, May). Advancing Informal Environmental Health STEM Learning through Co-Created Citizen Science. Teaching and Learning in the Food-Energy-Water Nexus: Toward a National and Collaborative for Food, Energy and Water Systems (NC-FEW).. Washington, DC.
• Hayes, S. M., Root, R., Perdrial, N., & Chorover, J. (2012, MAR 25). Oxidative mineral weathering of mine tailings: Coupled mineral and toxic metal transformations at the Iron King Superfund Site. ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY.
• Root, R. A., Hayes, S. M., Schowalter, C., & Chorover, J. (2010, JUN). Coupled arsenic and sulfur speciation in semi-arid mine tailings. GEOCHIMICA ET COSMOCHIMICA ACTA.
• Illera, V., O'Day, P. A., Choi, S., Rivera, N. A., Root, R., Rafferty, M., & Vlassopoulos, D. (2006, SEP 10). Immobilization of arsenic in a contaminated soil using ferrous sulfate and type V Portland cement. ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY.

Others

• Root, R. A. (2019). Data_Sheet_1_Soil Microbiome Dynamics During Pyritic Mine Tailing Phytostabilization: Understanding Microbial Bioindicators of Soil Acidification.docx. https://doi.org/10.3389/fmicb.2019.01211.s001
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  Challenges to the reclamation of pyritic mine tailings arise from in situ acid generation that severely constrains the growth of natural revegetation. While acid mine drainage (AMD) microbial communities are well-studied under highly acidic conditions, fewer studies document the dynamics of microbial communities that generate acid from pyritic material under less acidic conditions that can allow establishment and support of plant growth. This research characterizes the taxonomic composition dynamics of microbial communities present during a 6-year compost-assisted phytostabilization field study in extremely acidic pyritic mine tailings. A complementary microcosm experiment was performed to identify successional community populations that enable the acidification process across a pH gradient. Taxonomic profiles of the microbial populations in both the field study and microcosms reveal shifts in microbial communities that play pivotal roles in facilitating acidification during the transition between moderately and highly acidic conditions. The potential co-occurrence of organoheterotrophic and lithoautotrophic energy metabolisms during acid generation suggests the importance of both groups in facilitating acidification. Taken together, this research suggests that key microbial populations associated with pH transitions could be used as bioindicators for either sustained future plant growth or for acid generation conditions that inhibit further plant growth.