Mercury removal from municipal secondary effluent with hydrous ferric oxide reactive filtration.

This study evaluated the ability of hydrous ferric oxide reactive filtration (HFO-RF) to remove mercury (Hg) from municipal secondary effluent at four study sites. Pilot HFO-RF systems (136 m3 /day) at two sites demonstrated total Hg concentration removal efficiencies of 96% (inflow/outflow mean total Hg: 43.6/1.6  ng/L) and 80% (4.2/0.8 ng/L). A lightly loaded medium-scale HFO-RF system (950 m3 /day) had a concentration removal efficiency of 53% (0.98/0.46 ng/L) and removed 0.52 mg/day of total Hg and 2.2 µg/day of methyl-Hg. A full-scale HFO-RF system (11,400 m3 /day) yielded a total Hg concentration removal efficiency of 97% (87/2.7 ng/L) and removed an estimated 0.36 kg/year of Hg. Results suggest that the quality of secondary effluent, including dissolved organic matter content, affects achievable minimum total Hg concentrations in effluent from HFO-RF systems. Low HFO-RF effluent concentrations (<1 ng/L) can be expected when treating secondary effluent from suspended-growth biological treatment systems. PRACTITIONER POINTS: Trace levels of mercury in municipal secondary effluent can negatively impact receiving waters. Hydrous ferric oxide reactive filtration (HFO-RF) can remove mercury from municipal secondary effluent to levels below the Great Lakes Initiative discharge standard of 1.3 ng/L. Mercury removal to low concentrations (< 1 ng/L) using HFO-RF appears to be associated with secondary effluents with low dissolved organic matter content. HFO-RF can also remove total phosphorus and turbidity to low concentrations.

Unknown risk: co-exposure to lead and other heavy metals among children living in small-scale mining communities in Zamfara State, Nigeria.

The lead poisoning crisis in Zamfara State, Northern Nigeria has been called the worst such case in modern history and it presents unique challenges for risk assessment and management of co-exposure to multiple heavy metals. More than 400 children have died in Zamfara as a result of ongoing lead intoxication since early in 2010. A review of the common toxic endpoints of the major heavy metals advances analysis of co-exposures and their common pathologies. Environmental contamination in Bagega village, examined by X-ray fluorescence of soils, includes lead, mercury, cadmium, arsenic and manganese. Co-exposure risk is explored by scoring common toxic endpoints and hazard indices to calculate a common pathology hazard risk ranking of Pb > As > Hg >> Cd > Mn. Zamfara presents an extreme picture of both lead and multiple heavy metal mortality and morbidity, but similar situations have become increasingly prevalent worldwide.

Biochar-integrated reactive filtration of wastewater for P removal and recovery, micropollutant catalytic oxidation, and negative CO2 e: Life cycle assessment and techno-economic analysis.

Life cycle assessment (LCA) and techno-economic analysis (TEA) models are developed for a tertiary wastewater treatment system that employs a biochar-integrated reactive filtration (RF) approach. This innovative system incorporates the utilization of biochar (BC) either in conjunction with or independently of iron-ozone catalytic oxidation (CatOx)-resulting in two configurations: Fe-CatOx-BC-RF and BC-RF. The technology demonstrates 90%-99% total phosphorus removals, adsorption of phosphorus to biochar for recovery, and >90% destructive removal of observed micropollutants. In this work, we conduct an ISO-compliant LCA of a 49.2 m3 /day (9 gpm) field pilot-scale Fe-CatOx-BC-RF system and a 1130 m3 /day (0.3 MGD) water resource recovery facility (WRRF)-installed RF system, modeled with BC addition at the same rate of 0.45 g/L to quantify their environmental impacts. LCA results indicated that the Fe-CatOx-BC-RF pilot system is a BC dose-dependent carbon-negative technology at -1.21 kg CO2 e/m3 , where biochar addition constitutes a -1.53 kg/m3 CO2 e beneficial impact to the process. For the WRRF-installed RF system, modeled with the same rate of BC addition, the overall process changed from 0.02 kg CO2 e/m3 to a carbon negative -1.41 kg CO2 e/m3 , demonstrating potential as a biochar dose-dependent negative emissions technology. Using the C100 100-year carbon accounting approach rather than Cnet reduces these CO2 e metrics for the process by about 25%. A stochastic TEA for the cost of water treatment using this combinatorial P removal/recovery, micropollutant destructive removal, and disinfection advanced technology shows that at scale, the mean cost for treating 1130 m3 /day (0.3 MGD) WRRF secondary influent water with Fe-CatOx-BC-RF using the C100 metric is US$0.18 ± US$0.01/m3 to achieve overall process carbon neutrality. Using the same BC dose in an estimation of a 3780 m3 /day (1 MGD) Fe-CatOx-BC-RF facility, the carbon neutral cost of treatment is reduced further to US$0.08 ± $0.01 with added BC accounting for US$0.03/m3 . Overall, the results demonstrate the potential of carbon negativity to become a water treatment performance standard as important and attainable as pollutant and pathogen removal. PRACTITIONER POINTS: Life cycle assessment (LCA) of a pilot scale tertiary biochar water treatment process with or without catalytic ozonation at a WRRF shows a carbon negative global warming potential of -1.21-kg CO2e/m3 while removing 90%-99% TP and >90% of detected micropollutants. Biochar-integrated reactive filtration use can aid in long-term carbon sequestration by reducing the carbon footprint of advanced water treatment in a dose-dependent manner, allowing an overall carbon-neutral or carbon-negative process. A companion paper to this work (Yu et al., 2023) presents the details related to the process operation and mechanism and evaluates the pollutant removal performance of this Fe-CatOx-BC-RF process in engineering laboratory pilot research and field WRRF pilot-scale water resource recovery trials. Techno-economic analysis (TEA) of this biochar catalytic oxidation reactive filtration process using Monte Carlo stochastic modeling shows a forecasted carbon-neutral process cost with low P and micropollutant removal as US$0.11/m3 ± 0.01 for a 3780-m3/day (1 MGD) scale installation with BC cost at US$0.03/m3 of that total. The results demonstrate the potential of carbon negativity to become a water treatmentperformance standard as important and attainable as pollutant and pathogen removal.

Biochar integrated reactive filtration of wastewater for P removal and recovery, micropollutant catalytic oxidation, and negative CO2 e: Process operation and mechanism.

Biochar (BC) use in water treatment is a promising approach that can simultaneously help address societal needs of clean water, food security, and climate change mitigation. However, novel BC water treatment technology approaches require operational testing in field pilot-scale scenarios to advance their technology readiness assessment. Therefore, the objective of this study is to evaluate the system performance of BC integrated into hydrous ferric oxide reactive filtration (Fe-BC-RF) with and without catalytic ozonation (CatOx) process in laboratory and field pilot-scale scenarios. For this investigation, Fe-BC-RF and Fe-CatOx-BC-RF pilot-scale trials were conducted on synthetic lake water variants and at three municipal water resource recovery facilities (WRRFs) at process flows of 0.05 and 0.6 L/s, respectively. Three native and two iron-modified BCs were used in these studies. The commercially available reactive filtration process (Fe-RF without BC) had 96%-98% total phosphorus (TP) removal from 0.075- and 0.22-mg/L TP, as orthophosphate process influent in these trials. With BC integration, phosphorus removal yielded 94%-98% with the same process-influent conditions. In WRRF field pilot-scale studies, the Fe-CatOx-BC-RF process removed 84%-99% of influent total phosphorus concentrations that varied from 0.12 to 8.1 mg/L. Nutrient analysis on BC showed that the recovered BC used in the pilot-scale studies had an increase in TP from its native concentration, with the Fe-amended BC showing better P recovery at 110% than its unmodified state, which was 16%. Lastly, the field WRRF Fe-CatOx-BC-RF process studies showed successful destructive removals at >90% for more than 20 detected micropollutants, thus addressing a critical human health and environmental water quality concern. The research demonstrated that integration of BC into Fe-CatOx-RF for micropollutant removal, disinfection, and nutrient recovery is an encouraging tertiary water treatment technology that can address sustainable phosphorus recycling needs and the potential for carbon-negative operation. PRACTITIONER POINTS: A pilot-scale hydrous ferric oxide reactive sand filtration process integrating biochar injection typically yields >90% total phosphorus removal to ultralow levels. Biochar, modified with iron, recovers phosphorus from wastewater, creating a P/N nutrient upcycled soil amendment. Addition of ozone to the process stream enables biochar-iron-ozone catalytic oxidation demonstrating typically excellent (>90%) micropollutant destructive removals for the compounds tested. A companion paper to this work explores life cycle assessment (LCA) and techno-economic analysis (TEA) to explore biochar water treatment integrated reactive filtration impacts, costs, and readiness. Biochar use can aid in long-term carbon sequestration by reducing the carbon footprint of advanced water treatment in a dose-dependent manner, including enabling an overall carbon-negative process.

Microscale Imaging of Thermal Conductivity Suppression at Grain Boundaries.

Grain-boundary engineering is an effective strategy to tune the thermal conductivity of materials, leading to improved performance in thermoelectric, thermal-barrier coatings, and thermal management applications. Despite the central importance to thermal transport, a clear understanding of how grain boundaries modulate the microscale heat flow is missing, owing to the scarcity of local investigations. Here, thermal imaging of individual grain boundaries is demonstrated in thermoelectric SnTe via spatially resolved frequency-domain thermoreflectance. Measurements with microscale resolution reveal local suppressions in thermal conductivity at grain boundaries. Also, the grain-boundary thermal resistance - extracted by employing a Gibbs excess approach - is found to be correlated with the grain-boundary misorientation angle. Extracting thermal properties, including thermal boundary resistances, from microscale imaging can provide comprehensive understanding of how microstructure affects heat transport, crucially impacting the materials design of high-performance thermal-management and energy-conversion devices.

Iron-ozone catalytic oxidation reactive filtration of municipal wastewater at field pilot and full-scale with high-efficiency pollutant removal and potential negative CO2 e with biochar.

Iron-ozone catalytic oxidation (CatOx) shows promise in addressing challenging wastewater pollutants. This study investigates a CatOx reactive filtration (Fe-CatOx-RF) approach with two 0.4 L/s field pilot studies and an 18-month, 18 L/s full-scale municipal wastewater deployment. We apply ozone to leverage common sand filtration and iron metal salts used in water treatment into a next-generation technology. The process combines micropollutant and pathogen destructive removal with high-efficiency phosphorus removal and recycling as a soil amendment, clean water recovery, and the potential for carbon-negative operation with integrated biochar water treatment. A key process innovation is converting a continuously renewed iron oxide coated, moving bed sand filter into a "sacrificial iron" d-orbital catalyst bed after adding O3 to the process stream. Results for the Fe-CatOx-RF pilot studies show >95% removal efficiencies for almost all >5 × LoQ detected micropollutants, with removal rates slightly increasing with biochar addition. Phosphorus removal for the pilot site with the most P-impacted discharge was >98% with serial reactive filters. The long-term, full-scale Fe-CatOx-RF optimization trials showed single reactive filter 90% TP removal and high-efficiency micropollutant removals for most of the compounds detected, but slightly less than the pilot site studies. TP removal decreased to a mean of 86% during the 18 L/s, 12-month continuous operation stability trial, and micropollutant removals remained similar to the optimization trial for many detected compounds but less efficient overall. A >4.4 log reduction of fecal coliforms and E. coli in a field pilot sub-study suggests the ability of this CatOx approach to address infectious disease concerns. Life cycle assessment modeling suggests that integrating biochar water treatment into the Fe-CatOx-RF process for P recovery as a soil amendment makes the overall process carbon-negative at -1.21 kg CO2 e/m3 . Results indicate positive Fe-CatOx-RF process performance and technology readiness in full-scale extended testing. Further work exploring operational variables is essential to establish site-specific water quality limitations and responsive engineering approaches for process optimization. PRACTITIONERS POINTS: Adding ozone to WRRF secondary influent flows into tertiary ferric/ferrous salt dosed sand filtration amplifies a mature reactive filtration technology into a catalytic oxidation process for micropollutant removal and disinfection. Expensive catalysts are not used. Iron oxide compounds used to remove phosphorus and other pollutants act as sacrificial catalysts with ozone, and these rejected iron compounds can be returned upstream to aid in secondary process TP removal. Biochar addition to the CatOx process improves CO2 e sustainability and phosphorus removal/recovery for long-term soil and water health. Short duration field pilot scale and 18-month full-scale operation at three WRRFs with good results demonstrate technology readiness.

CO2 e footprint and eco-impact of ultralow phosphorus removal by hydrous ferric oxide reactive filtration: A municipal wastewater LCA case study.

Dual upflow reactive filtration by a slowly moving sand bed with continuously renewed, hydrous ferric oxide-coated sand is used for removing polluting substances and for meeting the ultralow 0.05 mg/l total phosphorus discharge permit limits at a 1.2 million liters per day (0.32 million gallons per day) water resource recovery facility in Plummer, Idaho, in the United States. A life cycle assessment (LCA) of this reactive filtration installation was carried out to assess the environmental hotspots in the system and analyze alternative system configurations with a focus on CO2 equivalent (CO2 e) global warming potential, freshwater and marine eutrophication, and mineral resource scarcity. "What if" scenarios with alternative inputs for the energy, metal salts, and air compressor optimization show trade-offs between the impact categories. Key results that show a comparative reduction of global warming potential include the use of Fe versus Al metal salts, the use of renewable energy, and the energy efficiency benefit of optimizing process inputs, such as compressor air pressure, to match operational demand. The LCA shows a 2 × 10-2  kg CO2 e footprint per cubic meter of water, with 47% from housing concrete, and an overall freshwater eutrophication impact reduced by 99% versus no treatment. The use of renewable hydropower energy at this site isolates construction concrete as a target for lowering the CO2 e footprint. PRACTITIONER POINTS: The main LCA eco-impact hotspots in this dual reactive filtration tertiary treatment are construction concrete and the ferric sulfate used. Iron salts show smaller impact in global warming, freshwater eutrophication, and mineral resource scarcity than "what if scenario" aluminum salts. The energy mix for this site is predominantly hydropower; other energy mix "what if" scenarios show larger impacts. Operational energy efficiency and thermodynamic analysis show that fine tuning the air compressor helps reduce carbon footprint and energy use. LCA shows a favorable 2 x 10-2 kg CO2e/m3 water impact with 99% reduction of freshwater eutrophication potential versus no treatment.

Chemical speciation and bioavailability of selenium in the rhizosphere of Symphyotrichum eatonii from reclaimed mine soils.

Knowledge of rhizosphere influences on Se speciation and bioavailability is required to predict Se bioavailability to plants. In the present study, plant-availability of Se to aster (Symphyotrichum eatonii (A. Gray) G.L. Nesom) was compared in rhizosphere soils and nonrhizosphere (bulk) soils collected from a reclaimed mine site in southeastern Idaho, U.S. X-ray spectroscopy was used to characterize the oxidation state and elemental distribution of Se in aster roots, rhizosphere soils, and bulk soils. Percent extractable Se in aster rhizosphere soil was greater than extractable Se in corresponding bulk soils in all samples (n = 4, p = 0.042, 0.051, and 0.052 for three extractions). Selenium oxidation state mapping of 28 regions within the samples and X-ray absorption near edge structure (XANES) spectra from 26 points within the samples indicated that the rhizosphere and bulk soil Se species was predominantly reduced Se(-II,0), while in the aster roots, high concentrations of Se(VI) were present. Results show that within the rhizosphere, enhanced Se bioavailability is occurring via oxidation of reduced soil Se to more soluble Se(VI) species.

Effects of dietary selenomethionine on cutthroat trout (Oncorhynchus clarki bouvieri) growth and reproductive performance over a life cycle.

A 2.5-year feeding trial was conducted in which cutthroat trout (Oncorhynchus clarki bouvieri) were fed either a basal diet (1.2 microg Se/g diet) or the basal diet supplemented with 2, 4, 6, 8, or 10 microg Se/g diet as selenomethionine from 1 g weight to maturation [corrected]. After 44 weeks of feeding, a subsample of fish was removed from dietary treatment groups and fed the basal diet for an additional 32 weeks. Concentrations of Se in whole fish and eggs increased in proportion to dietary Se intake, but no differences in growth, feed intake, survival, or egg hatchability were observed among dietary groups. Cranial-facial deformities in second-generation offspring were less than 6% in all treatment groups except for fish fed the diet supplemented with 4 microg Se/g diet as selenomethionine [corrected], where a 9.2% incidence was observed. Fish switched from selenomethionine-supplemented diets to the basal diet lost Se, calculated as microg Se lost/g weight gain, at 1.01, 2.84, 4.42, and 4.42 for dietary treatment groups 3, 4, 5, and 6, respectively. Results suggest no toxicity of dietary selenomethionine up to 10 microg/g supplemented diet and that with total life-cycle exposure, cutthroat trout increase Se excretion to maintain whole-body concentrations below toxic levels.

Legacy lead arsenate soil contamination at childcare centers in the Yakima Valley, Central Washington, USA.

BACKGROUND: From the early 1900s to the 1950s, Yakima Valley orchards were commonly treated with lead arsenate (LA) insecticides. Lead (Pb) and arsenic (As) soil contamination has been identified on former orchard lands throughout Central Washington and pose a threat to human health and the environment. OBJECTIVES: The levels of Pb and As in soil and interior dust at participating childcare centers in the Upper Yakima Valley (Yakima County), Washington were sampled to explore exposure potential for young children. METHODS: Childcare center soils were collected from two soil depths, homogenized, and analyzed in bulk by a field-portable X-ray fluorescence spectrometer (XRF). Interior dust wipes samples were collected from at least two locations in each facility. All soil samples >250 mg/kg Pb and/or >20 As mg/kg were sieved to 250 µm, tested by XRF a second time, and analyzed via acid digestion and inductively coupled plasma mass spectrometry (ICP-MS) analysis. RESULTS: Bulk and sieved XRF results, as well as ICP-MS to XRF results were strongly correlated. Maximum Pb and As XRF results indicated that 4 (21%) and 8 (42%) of the 19 childcare centers surveyed exceeded the regulatory standard for Pb and As, respectively. Historic land use was significantly associated with elevated Pb and As levels. Interior dust loadings were below United States Environmental Protection Agency (EPA) guidelines. CONCLUSIONS: Childcare centers are areas of intensive use for children and when coupled with potential residential exposure in their homes, the total daily exposure is a potential hazard to children.

Micro-spectroscopic investigation of selenium-bearing minerals from the Western US Phosphate Resource Area.

Mining activities in the US Western Phosphate Resource Area (WPRA) have released Se into the environment. Selenium has several different oxidation states and species, each having varying degrees of solubility, reactivity, and bioavailability. In this study we are investigating the speciation of Se in mine-waste rocks. Selenium speciation was determined using bulk and micro-x-ray absorption spectroscopy (XAS), as well as micro-x-ray fluorescence mapping. Rocks used for bulk-XAS were ground into fine powders. Shale used for micro-XAS was broken along depositional planes to expose unweathered surfaces. The near edge region of the XAS spectra (XANES) for the bulk rock samples revealed multiple oxidation states, with peaks indicative of Se(-II), Se(IV), and Se(+VI) species. Micro-XANES analysis of the shale indicated that three unique Se-bearing species were present. Using the XANES data together with ab initio fitting of the extended x-ray absorption fine structure region of the micro-XAS data (micro-EXAFS) the three Se-bearing species were identified as dzharkenite, a di-selenide carbon compound, and Se-substituted pyrite. Results from this research will allow for a better understanding of the biogeochemical cycling of Se in the WPRA.

Effect of ultrasound frequency on antioxidant activity, total phenolic and anthocyanin content of red raspberry puree.

Ultrasound in the 20-1000 kHz range show unique propagation characteristics in fluid media and possess energy that can break down fruit matrices to facilitate the extraction of valuable bioactive compounds. Red raspberries carry significant amounts of specific antioxidants, including ellagitannins and anthocyanins that are important for human health. The objective of this study was to investigate the effects of ultrasound frequencies associated with cavitation (20 kHz) and microstreaming (490 and 986 kHz) on total antioxidant activity (AOA), total phenolics content (TPC), and total monomeric anthocyanin content (ACY) of red raspberry puree prepared from crushed berries. The pureed fruit was subjected to high-intensity (20 kHz) and higher frequency-low intensity (490 and 986 kHz) ultrasound for 30 min. The temperature of treated purees increased to a maximum of 56 °C with 986 kHz. Sonication at 20 and 490 kHz significantly (p<0.05) affected the AOA, ACY, and TPC of red raspberry puree, while 986 kHz had no significant effect on ACY and AOA (p<0.05). In all cases, ultrasound treatment had significant and positive effect on at least one of the measured parameters up to 30 min. Sonication beyond 10 min (and up to 30 min) using 20 kHz either produced no change or caused a drop in AOA and ACY. However, for 986 and 20 kHz, TPC, increased by 10% and 9.5%, respectively after 30 min (p<0.05) compared to the control. At 20 kHz, AOA and ACY increased by 17.3% and 12.6% after 10 min. It was demonstrated that 20 kHz ultrasound treatment, when limited to 10 min, was the most effective for extraction of bioactive compounds in red raspberry compared to 490 and 986 kHz although the effect could be similar at the higher frequencies if different amplitudes are used.

Selenium biogeochemical cycling and fluxes in the hyporheic zone of a mining-impacted stream.

The influence of hyporheic exchange on selenium (Se) biogeochemistry and mobility in sediments is unknown. A multiscale investigation of Se biogeochemistry in the hyporheic zone of East Mill Creek (EMC), southeastern Idaho, USA, was performed using in situ surface water and pore water geochemical measurements, a field-based stream tracer test, and energy-dependent micro synchrotron X-ray fluorescence (mu-SXRF) measurements of Se speciation in sediments. The active hyporheic zone was determined to be 12 +/- 3 cm. Pore water redox profiles indicated that a transition to suboxic conditions begins at approximately 6 cm. Modeling pore water Se and solid phase analysis suggested Se uptake is occurring. Micro-SXRF analysis of sediments showed reduced elemental Se or selenides throughout the profile and selenite in surface sediments. Field geochemical measurements and microscale analysis both support the hypothesis that reduction in the hyporheic zone promotes sequestration of surface water Se.

Macro- and microscale investigation of selenium speciation in Blackfoot river, Idaho sediments.

The transport and bioavailability of selenium in the environment is controlled by its chemical speciation. However, knowledge of the biogeochemistry and speciation of Se in streambed sediment is limited. We investigated the speciation of Se in sediment cores from the Blackfoot River (BFR), Idaho using sequential extractions and synchrotron-based micro-X-ray fluorescence (micro-SXRF). We collected micro-SXRF oxidation state maps of Se in sediments, which had not been done on natural sediment samples. Selective extractions showed that most Se in the sediments is present as either (1) nonextractable Se or (2) base extractable Se. Results from micro-SXRF showed three defined species of Se were present in all four samples: Se(-II,O), Se(IV), and Se(VI). Se(-II,O) was the predominant species in samples from one location, and Se(IV) was the predominant species in samples from a second location. Results from both techniques were consistent, and suggested that the predominant species were Se(-II) species associated with recalcitrant organic matter, and Se(IV) species tightly bound to organic materials. This information can be used to predict the biogeochemical cycling and bioavailability of Se in streambed sediment environments.

Microbial studies of a selenium-contaminated mine site and potential for on-site remediation.

Surface water Selenium (Se) concentrations are above regulatory standards at several active and inactive phosphate mine sites in the US Western Phosphate Resource Area. The focus of the present study was to examine the impacts of the microbial communities on the oxidation state of Se in overburden waste from the Smoky Canyon phosphate mine in Idaho, USA. Microbial populations were found that reduce soluble selenate (SeO (4) (2-) ) to insoluble elemental Se. Microcosm experiments were conducted for molecular genetic analysis of this microbial community by rRNA gene profiling. An acetone pretreatment step was developed to remove interfering pre-petroleum hydrocarbons from the samples prior to extraction. PCR was used to amplify 16S and 18S rRNA genes present in the microbial community DNA. The amplified products were subjected to denaturing gradient gel electrophoresis (DGGE). Isolates and excised DGGE bands were amplified and sequenced for identification to determine the relative importance of culturable isolates to the total microbial population. Analysis of samples from different sites at the mine showed how Se contamination and previous remediation treatments changed the microbial populations across the site. Members of the family Enterobacteriaceae were dominant among the selenate reducing isolates from the site containing high Se levels. In particular, Serratia fonticola was isolated repeatedly from contaminated Smoky Canyon Mine site samples. Packed column studies were performed with seleniferous waste rock fractions from Smoky Canyon Mine. Column amendments consisted of combinations of iron, compost, and whey. Eh, pH, and extractable Se measurements were taken. Tests with infiltrated water showed columns containing an organic amendment combined with iron metal were the most resistant to Se leaching. Iron-based compounds from the corroding metal are thought to strongly bind the Se reduced by microbial activity, thereby stabilizing the Se in an insoluble form. We conclude that long-term stabilization of selenium at contaminated mine sites may require reductive microbial processes combined with abiotic immobilization by iron, either natural or engineered, to stabilize the Se and retard re-oxidation and release. Iron-selenide or iron-selenite compounds are more stable and resistant to leaching, especially when removed from active weathering.

Microscopically focused synchrotron X-ray investigation of selenium speciation in soils developing on reclaimed mine lands.

Chemical speciation determines Se solubility and therefore its bioavailability and potential for transport in the environment. In this study we investigated the speciation of Se in soil developed on reclaimed mine sites in the U.S. Western Phosphate Resource Area (WPRA) using micro-X-ray absorption near-edge structure (micro-XANES) spectroscopy and micro-X-ray fluorescence (micro-XRF) mapping. Selenium was nonuniformly distributed in the soils and positively correlated with Fe, Mn, Cu, Zn, and Ni. Sixteen points of interest (POI) from three soil samples were analyzed with micro-XANES spectroscopy. The XANES data indicated that Se is present in the soils in at least three oxidation states, Se(-II, 0), Se(IV), and Se(VI). Selenides or elemental Se dominated 7 of the 16 POI. Selenate was the dominant species at only one of the POI. The remaining eight POI were composed of both Se(IV) and Se(VI), with minor Se(-II, 0) contributions. The results of this research suggest that the reduced Se species in the soil parent material are oxidizing to Se(VI), one of the more mobile species of Se in the environment. This information can be used to better predict and manage Se availability in soils.