Grantee Research Project Results
1997 Progress 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.
Current 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 Period Covered by this Report: December 15, 1996 through December 14, 1997
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 which 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 are to (i) determine rates and underlying mechanisms of the reduction of sorbed arsenate in model systems, contaminated soils and mine tailings; (ii) evaluate the importance of As-sulfide formation under conditions typical of As contaminated soils and mine tailings; and (iii) evaluate the role of reduction of sorbed arsenate on the mobility and transport of As in contaminated soils, mine tailings and aquifers. It is hoped 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.To accomplish these objectives, we have completed several complementary experiments with the overall goal of increasing our understanding of the processes important in controlling As cycling in natural environments. First, we have performed several experiments under abiotic conditions where simple inorganic species such as H2 (g), N2(g) and sulfide are used as electron donors for reduction of As(V). Results from these experiments have been used primarily to contrast the reduction of As(V) in biotic systems using microbial inoculum from several As contaminated sites. Biologically mediated As(V) reduction has been evaluated in systems where As(V) is presented entirely as a soluble species or sorbed to Fe oxides common in natural environments. Finally, we have completed several column experiments using As contaminated soils to evaluate the potential mobility of As after As(V) is reduced to As(III).
Both abiotic and biotic arsenate reduction rates have been evaluated in batch reactors and closed head-space serum vials. Biotic experiments were inoculated with a cell extract from an As contaminated soil collected near Three Forks, MT. One set of As (V) reduction experiments were conducted using the original cell extract (CN-0) and a separate set of experiments using a cell culture enriched repeatedly on glucose and high As levels (CN-8). Reduction rates of soluble As(V) were shown to be first-order in As(V) concentration in both enrichments, however, As(V) reduction was considerably slower in the original cell extract (CN-0). Further, biotic reduction rates are considerably faster than abiotic rates using potential inorganic electron donors. The enrichment process selected for a glucose fermenter capable of reducing As(V) without significant dissimilatory reduction. It appears that the mechanism for As(V) reduction in this particular enrichment culture (CN-8) is detoxification, possibly via an ars gene, which has been described in the literature. Future work will be directed towards evaluation of microbial community structure in cell enrichments and the relative importance of detoxification versus dissimilatory reduction of As(V) in As contaminated soils.
Microbially mediated As(V) reduction occurs relatively rapidly (time scale of hours) in solution. However, we have hypothesized that As reduction will be substantially slower in the presence of Fe oxides for two possible reasons: (i) rate-limited desorption of As(V) from the Fe-oxide surface, and (ii) competitive electron consumption by Fe(III). Our results using serum vials and batch reactors show that indeed, the reduction of As(V) in the presence of Fe-oxides is considerably slower than reduction rates for aqueous As(V). It appears that the reduction of As(V) in the presence of Fe oxides is controlled by rates of reductive dissolution of the Fe oxide phase or via desorption of As(V) from the oxide surface. We have not found evidence for significant direct reduction of sorbed phase As(V). Furture work under this objective will focus on quantification of As(V) and As(III) species on the Fe oxide surface during microbial oxidation of C.
Finally, column experiments have been completed which were designed to observe relationships among microbial C utilization, redox potential, EH, (as measured using a Pt electrode) and the mobilization of As in As-contaminated soils and tailings. To provide a more thorough view of redox potential within the columns, As(III), As(V), Fe(II), Fe(III), and sulfide production were also monitored. During the first year of study, all experiments were conducted with reprocessed mine tailings (RT) (sampled near the Superfund site in Anaconda MT) which contained ~3,000 mg kg-1 total As. Liming of RT resulted in effluent pH exceeding 7.6 and corresponded with significant increases of mobilized As in comparison to unlimed treatments. Both limed and unlimed treatments exhibited similar C utilization rates and correspondingly low EH values. Aeration of limed RT receiving >6mM C resulted in rates of As mobilization and reduction similar to those observed in unaerated/limed treatments. Other researchers have also observed that gross aerobic environments such as surface waters of euthrophic lakes, can support significant concentrations of reduced As (III) (Abdullah et al. 1995, Sohrin et al 1997). We suspect that mechanisms of As(V) reduction may include detoxification which may occur under aerobic conditions as well as anaerobic conditions. This hypothesis will be tested using additional column experiments designed to determine effects of variable C substrate and O2 levels on the rates of As (V) reduction and subsequent obilization of As.
Journal Articles:
No journal articles submitted with this report: View all 12 publications for this projectSupplemental Keywords:
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.