Grantee Research Project Results
Final Report: Rates of Arsenic Oxidation-Reduction Reactions in Contaminated Soils: Effects on Arsenic Fate and Mobility
EPA Grant Number: R825403Title: Rates of Arsenic Oxidation-Reduction Reactions in Contaminated Soils: Effects on Arsenic Fate and Mobility
Investigators: Inskeep, William P. , Jones, C. A. , Macur, R. E. , Langner, H. W.
Institution: Montana State University
EPA Project Officer: Chung, Serena
Project Period: December 15, 1996 through December 14, 1999
Project Amount: $329,735
RFA: Environmental Fate and Treatment of Toxics and Hazardous Wastes (1996) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management , Safer Chemicals
Objective:
Arsenic (As) is an important priority pollutant found in soils contaminated by arsenical pesticides, natural geothermal sources, and mine tailings. The chemical and biological processes that control the fate and mobility of As in contaminated soils and mine tailings are complex, primarily due to transformations of numerous As species, which occur under temporally variable oxidation-reduction conditions. The objectives of this project were to: (1) determine rates and underlying mechanisms of the reduction of sorbed arsenate in model systems, contaminated soils, and mine tailings; (2) evaluate the importance of As-sulfide formation under conditions typical of As-contaminated soils and mine tailings; and (3) evaluate the role of reduction of sorbed arsenate on the mobility and transport of As in contaminated soils, mine tailings, and aquifers. It was our goal that results obtained from this study will improve the link between fundamental kinetic processes controlling As speciation and watershed-scale processes such as mobility, transport, and bioavailability.Over the course of this project, we have completed four major initiatives designed to understand chemical and microbial interactions that control rates of arsenate reduction in soils and sediments. Specifically, we have: (1) characterized As reduction rates catalyzed by a fermenting microorganism isolated from an As-contaminated soil, (2) studied the reduction of arsenate in the presence and absence of iron (Fe) oxides using different microbial populations, (3) evaluated the role of arsenate reduction on solubilization of arsenic in contaminated mine tailings under aerobic conditions, and (4) evaluated the reduction of arsenate and subsequent precipitation of As sulfides under constructed wetland environments. This report will summarize experimental progress during Year 3 of a 3-year project.
Summary/Accomplishments (Outputs/Outcomes):
Reduction of Arsenate Under Fermentative Conditions. In attempts to evaluate mechanisms responsible for arsenate reduction in soils from the Madison River Basin, MT, batch experiments were conducted in serum bottles using microorganisms either harvested directly from a contaminated soil, or harvested and then enriched on glucose and arsenate. The reduction of arsenate varied considerably between an unenriched (CN-0) and enriched microbial population (CN-8). In experiments with CN-0, As(V) reduction proceeded more slowly and was not significant until optical densities plateaued at approximately 0.6 after 1 day. Conversely, As(V) reduction in the presence of CN-8 occurred simultaneously with microbial growth, commencing within 0.5 days. Analyses of the fate of glucose C indicated that the CN-8 culture was fermenting glucose, and was not using As(V) as a terminal electron acceptor. These results suggest that CN-8 was not coupling As(V) reduction with oxidation of glucose, but was likely reducing As(V) via a different mechanism such as detoxification. Molecular analysis of these microbial communities has showed that CN-0 is a mixed population, whereas CN-8 is a single organism with a closest Ribosomal Database Project (RDP) relative of Clostridium intestinalis, a strict anaerobe known to ferment glucose. An individual isolate obtained from the CN-8 enrichments shows 100 percent similarity to the CN-8 population based on a 300 base pair region of the 16 S gene. These results are significant in that As(V) reduction apparently was not coupled with respiration, but likely resulted from a detoxification mechanism that may be common in contaminated soil and sediment environments.The reduction of As(V) by CN-8 as a function of initial As concentration was first-order in As(V) from 6-600 M As. At As(V) concentrations greater than 600 M, rates of As(V) reduction were constant and independent of As(V) concentration. Also, we verified that As(V) reduction rates were first-order in microbial number during the growth phase. This work on As reduction kinetics catalyzed by a fermenting microorganism was submitted and accepted in the Soil Microbiology Division of Soil Science Society of America Journal, and should be printed in January 2000.
Reduction of Arsenate in the Presence of Iron Oxides. As shown with CN-8, microbially-mediated reduction of aqueous phase As(V) occurred relatively rapidly within a time scale of hours. We originally hypothesized that As reduction would be substantially slower in the presence of Fe oxides for two possible reasons: (1) rate-limited desorption of As(V) off of the Fe-oxide surface, and/or (2) competitive electron consumption by Fe(III). Previous work stated above (Jones, et al., 2000) indicated that reduction of As(V) in the presence of Fe-oxides was considerably slower than reduction rates for aqueous As(V). A more exhaustive series of experiments were conducted using CN-8 to investigate the effect of ferrihydrite on reduction rates of As(V). Experiments were conducted in serum bottles using glucose as a sole C source at three levels of total As (constant level of ferrihydrite) where the ratio of sorbed to aqueous phase As(V) varied by several orders of magnitude.
Results from these experiments showed that aqueous phase As(V) was reduced within 1-2 days, similar to time scales of As(V) reduction in the absence of ferrihydrite. We also estimated the amount of As(V) and As(III) within the sorbed phase, and found that essentially all of the sorbed phase As was As(V), indicating that CN-8 was not reducing sorbed phase As. Given that CN-8 was fermenting glucose under these conditions, we did not expect significant amounts of Fe(III) reduction and subsequent dissolution of ferrihydrite. Measurements of Fe(II) in solution and extracted from ferrihydrite confirm that significant reduction of Fe was not occurring under these conditions. Consequently, reduction of total As was limited by rates of As(V) desorption from ferrihydrite. These results are significant in that they indicate an additional scenario for As reduction in soils/sediments relative to the common belief that As reduction will be controlled by reductive dissolution of the Fe oxide phase. Our work described under this initiative was recently submitted to Environmental Science and Technology (December 1999), and is still out for review.
Reduction of Arsenate Under Column Conditions Using As-Contaminated Mine Soils. Column experiments were conducted using reprocessed mine tailings (RT) containing ~3,000 mg kg-1 total As, sampled near the Superfund site in Anaconda, MT. Experiments were designed to observe relationships among microbial C utilization, redox potential, EH (as measured using a Pt electrode), and the mobilization of As. Treatments included liming (25.6 g kg-1 60% CaCO3/40% Ca(OH)2 ) and aerated versus nonaerated systems.
Sterile controls demonstrated low As mobilization rates and high redox values near 400 mV. With a low C influent (0.1 mM C), As mobilization rates increased, and after approximately 6 days, the majority of dissolved As was As(III). Interestingly, Pt electrode redox values remained near 400 mV despite the fact the majority of effluent As was As(III). In experiments with higher influent C (6 mM total C: glucose and lactate), Pt redox values dropped within 3 days and eventually stabilized in the ?150 mV range. The As mobilization rates were similar to experiments conducted with low C influent, and again, the majority of effluent As was As(III). The high C treatment was repeated under aerated conditions to prevent Pt redox values from declining, and similar As mobilization rates were observed. In fact, even under aeration, As(III) was the predominant species in column effluent after approximately 5 days. Collectively, these experiments show that under biotic conditions, As(III) is produced under higher redox conditions than would be expected based strictly on thermodynamic calculations. The results also suggest that microbial-mediated As reduction may be occurring by pathways other than dissimilatory reduction.
We have analyzed microbial communities (using denaturing gradient gel electrophoresis, DGGE, of 16 S rDNA fragments) present in As-contaminated soil columns, and have isolated several organisms from these columns that reduce As(V) to As(III) under well aerated conditions. Microbial community analyses of soil column environments demonstrate changes in microbial populations after liming. One of the bacterial isolates obtained from the limed soil treatment was found to be closely related to Sphingomonas yanoikuyae; this band matches bands observed in soil treatments after liming. The ability of the S. yanoikuyae-like isolate to reduce As(V) to As(III) was confirmed under aerobic batch-reactor conditions. Consequently, our results show that As(V) reduction may be fairly common in contaminated soils or sediments at redox values that previously have been considered too high to favor As(III). We believe that the mechanism of As reduction in such cases involves detoxification via reductases as has been noted in the literature with several enteric microorganisms. Results from this work currently are in a draft manuscript that is planned for submission to Geomicrobiology in early 2000.
Reduction of Arsenate in Constructed Wetlands. We also conducted a series of experiments to evaluate the reduction of As(V) in constructed wetland cells designed to treat wastewater rich in C, N, and sulfate. The microbial communities present in these wetland cells are capable of oxidizing the majority of influent C and are actively reducing sulfate to sulfide. Real-time redox potential measurements at various depths within the wetland cells show reducing conditions in the vicinity of ?200 to ?300 mV. The fate of added arsenate was evaluated in several treatments with and without input of wastewater C. Under no C inputs, total soluble As remained at 6 mg L-1 and was predominantly As(V). However, with C inputs, As(V) declined rapidly within 0.5 days, with subsequent formation of As(III). Shortly after, total soluble As?which is now mostly As(III)?declined to < 2 mg L-1. During this same time frame, sulfate was reduced to sulfide in treatments receiving C. The decline in total soluble As in these environments is likely due to the formation of arsenic sulfides, which form after As(V) is reduced to As(III). Solution data from these experiments have been used to calculate saturation indices (log (IAP/Ksp)) relative to amorphous As2S3. In treatments containing C, the saturation indices hover near 0, indicating possible equilibrium with this solid phase. In the absence of C, the saturation indices are many orders of magnitude undersaturated, reflecting the low As(III) and sulfide values for this treatment.
A draft manuscript describing this work currently is under author review. This manuscript will be submitted to Environmental Science and Technology in early 2000. The primary findings suggest that wetland environments conducive to sulfate reduction will result in accumulation of phases; however, these amorphous precipitates are susceptible to rapid oxidation and re-release of As(V).
We plan to continue experiments designed to evaluate the diversity of microbial processes that play a role in As cycling in soils and natural waters. During the last year, we initiated new work on As cycling in thermal waters and soils of Yellowstone National Park (YNP). Geothermal waters in YNP often have elevated levels of As (1-10 ppm range), nearly two to three orders of magnitude higher than the drinking water standard. The geothermal environments contain a diverse group of both procaryotes and eucaryotes that appear adapted to high levels of As. We currently are exploring mechanisms by which these thermal organisms process or metabolize As. We also are investigating the microbial ecology of As-active microorganisms in As- contaminated soils. The results obtained in the current U.S. Environmental Protection Agency grant will be used as a platform to secure additional funding to continue our efforts to document the diversity of organisms involved in As cycling in soils and natural waters.
Journal Articles on this Report : 7 Displayed | Download in RIS Format
Other project views: | All 12 publications | 7 publications in selected types | All 7 journal articles |
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Darland JE, Inskeep WP. Effects of pH and phosphate competition on the transport of arsenate. Journal of Environmental Quality 1997;26(4):1133-1139. |
R825403 (Final) |
not available |
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Darland JE, Inskeep WP. Effects of pore water velocity on the transport of arsenate. Environmental Science & Technology 1997;31(3):704-709. |
R825403 (Final) |
Exit Exit |
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Jones CA, Inskeep WP, Neuman DR. Arsenic transport in contaminated mine tailings following liming. Journal of Environmental Quality 1997;26(2):433-439. |
R825403 (1998) R825403 (Final) |
not available |
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Jones CA, Inskeep WP, Bauder JW, Keith KE. Arsenic solubility and attenuation in soils of the Madison River Basin, Montana: impacts of long-term irrigation. Journal of Environmental Quality 1999;28(4):1314-1320. |
R825403 (1999) R825403 (Final) |
Exit |
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Jones CA, Langner HW, Anderson K, McDermott TR, Inskeep WP. Rates of microbially mediated arsenate reduction and solubilization. Soil Science Society of America Journal 2000;64(2):600-608. |
R825403 (1999) R825403 (Final) |
Exit |
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Langner HW, Inskeep WP. Microbial reduction of arsenate in the presence of ferrihydrite. Environmental Science & Technology 2000;34(15):3131-3136. |
R825403 (1998) R825403 (1999) R825403 (Final) |
Exit |
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Macur RE, Wheeler JT, McDermott TR, Inskeep WP. Microbial populations associated with the reduction and enhanced mobilization of arsenic in mine tailings. Environmental Science & Technology 2001;35(18):3676-3682. |
R825403 (1999) R825403 (Final) R827457E03 (Final) |
Exit |
Supplemental Keywords:
soil, water, leachate, ecological effects, bioavailability, metabolism, effluent, arsenic, aquatic, remediation, environmental chemistry, microbiology, ecology, analytical, molecular analyses, northwest, Montana, MT, EPA Region 8., RFA, Scientific Discipline, INTERNATIONAL COOPERATION, Toxics, Waste, Geographic Area, Water, Ecosystem Protection/Environmental Exposure & Risk, POLLUTANTS/TOXICS, Water & Watershed, Bioavailability, National Recommended Water Quality, Contaminated Sediments, Environmental Chemistry, Geochemistry, State, Arsenic, Fate & Transport, Hazardous Waste, Ecological Risk Assessment, Water Pollutants, Hazardous, fate and transport, hazardous waste treatment, aquatic, contaminated mines, fate, sediment treatment, contaminant transport, redox metabolism, contaminated sediment, mine tailings, sediment transport, transport contaminants, arsenic sulfide, arsenic oxidation, contaminated soil, chemical contaminants, toxicity, mining, watershed influences, aquatic ecosystems, environmental stressors, environmental toxicant, harmful environmental agents, aquifers, redox cycle, aresenic oxidation reduction, arsenic mobility, water quality, Montana , hazardous waste sites, arsenic exposure, exposure assessment, arsenic oxidation reduction, groundwater, mining impacted watershedProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.