Jump to main content or area navigation.

Contact Us

Extramural Research

Final Report: Physiochemical and Microbial Controls on the Speciation and Release of Arsenic into Ground and Surface Waters

EPA Grant Number: R826189
Title: Physiochemical and Microbial Controls on the Speciation and Release of Arsenic into Ground and Surface Waters
Investigators: Hamers, Robert J. , Banfield, Jillian F.
Institution: University of Wisconsin - Madison
EPA Project Officer: Lasat, Mitch
Project Period: January 1, 1998 through December 31, 2000 (Extended to April 14, 2002)
Project Amount: $312,496
RFA: Exploratory Research - Environmental Chemistry (1997)
Research Category: Engineering and Environmental Chemistry



The objectives of this project are to understand the fundamental inorganic and organic (biological) mechanisms that contribute to release of arsenic into the environment from arsenic-bearing sulfide minerals, and to understand the subsequent chemical transformations between different oxidation states.

Summary/Accomplishments (Outputs/Outcomes):

Arsenic in Primary Minerals. In an early phase of this research, we examined arsenic-bearing pyrite via high-resolution transmission electron microscopy, energy dispersive X-ray microanalysis, and synchrotron analysis. Results showed that reduced arsenic (XANES edge position indicated ~ -1 oxidation state, as in arsenopyrite) was heterogeneously distributed throughout the pyrite (FeS2). Concentrations did not correlate with the position of marcasite-like (thus, arsenopyrite-like) stacking faults or other defects.

Arsenic in Acid-Mine Drainage Systems. Our work has involved both laboratory and field components. We have conducted research on arsenic in the environment through field characterization studies in combination with experiments designed to probe the form of arsenic in primary sulfide minerals, the kinetics of arsenic release from sulfide mineral surfaces, and the factors that affect its speciation and fate in solution or solid reaction products. Our work has explicitly included consideration of inorganic and biological factors and the biochemistry of arsenic tolerance. Efforts in the Banfield group have centered on biologically-impacted dissolution kinetics, characterization of microbial populations, and biochemical aspects of arsenic tolerance in order to optimally complement studies by researchers in the Hamers group.

The funding has partially supported five graduate students (Gihring, Edwards, Druschel, McGuire, and Oliphant) and a part of one postdoctoral appointment (Bond). This work also has led to one patent (pending) as an experimental method for improving the safety of arsenic-laden drinking water.

We have studied the geochemistry and microbiology of an acid mine drainage system characterized by very elevated arsenic concentrations (~ 50 ppm arsenic). This has allowed the following experiments: (1) in situ studies of the colonization of precharacterized arsenopyrite (FeAsS) surfaces that were left in the environment for lengths of time ranging up to 1 year.

Surfaces were rapidly colonized by a variety of organisms, indicating high arsenic tolerance in the microbial population; and (2) isolation of organisms able to tolerate very high concentrations of arsenic for use in dissolution kinetics experiments and characterization of the arsenic tolerance mechanism.

Laboratory experiments showed that iron oxidizing prokaryotes increase dissolution rates through production of ferric iron (the primary surface oxidant at low pH). The extent to which microbial iron oxidation impacts arsenic release scales directly with the size of the microbial population. Sulfur-oxidizing organisms have been shown to remove sulfur accumulations formed during arsenopyrite oxidative dissolution; however, this process causes no appreciable increase in arsenopyrite dissolution rates (though it does increase the rate of generation of sulfuric acid).

In parallel with kinetics experiments, a study was undertaken to evaluate the extent to which attached microorganisms cause local pitting on arsenopyrite surfaces. In most cases, sulfur- and iron-oxidizing species did not cause sufficient localized enhancement of the dissolution rate to result in formation of pits detectable by scanning electron microscopy; however, one isolate, Ferroplasma acidarmanus, was able to form distinct local pits at cell attachment sites. This result is of interest because this archaeon is inferred to have iron-oxidizing enzymes directly on the cell surface, thus, potentially in direct contact with the mineral surface. This result verifies that the distribution of reactive surface area on a sulfide mineral can be modified by microbial attachment.

The mechanisms by which microorganisms tolerate arsenic, and the extent to which they are tolerant to arsenic, are highly relevant to analysis of the arsenic geochemical cycle. Consequently, we have characterized the tolerance of the dominant acidophile at our study site, F. acidarmanus, to arsenite and arsenate. Results show that this organism does not oxidize or reduce arsenic, thus, it does not utilize arsenic in metabolic energy generation. F. acidarmanus has been shown to lack plasmids, so we infer its arsenic resistance mechanism is chromosomal. Genome sequencing conducted as part of a separate project has yielded gene sequence data for analysis in this project. The arsenic resistance pathway contains arsARB genes that have homology with chromosomal As resistance genes in archaea and bacteria, and to bacterial As-resistance plasmids. The preliminary genome assembly suggests that arsR and arsB (the regulatory region and the transporter gene) are located separately from arsA (ATPase), which appears to be missing the A2 subunit (as is the case in some other microbes). The function of an arsA containing a single A1 subunit is uncertain; however, given the apparent origin of the A1-A2 subunits from gene duplication (Rensing, et al., 1999) and the existence of one organism with duplicate, unlinked A1 subunits, suggests involvement of arsA in arsenic resistance is probable. arsC (As5+ reductase) does not appear to be present in F. acidarmanus, as expected, based on experiments that show this organism does not reduce As5+; however, F. acidarmanus is tolerant to >1,000 ppm As3+ and As5+ and can grow on FeAsS.

Arsenic in hydrothermal systems has been studied at the Hanna Ranch, adjacent to Mt. Lassen National Park, and at Yellowstone National Park. In the Hanna Ranch work, we isolated a thermophilic bacterium related to Thermus aquaticus that is able to both oxidize and reduce arsenic. Arsenic oxidation kinetics of this and other Thermus spp. have been examined in some detail in the laboratory. To date, there is no evidence that the organism obtains metabolic energy from arsenite oxidation; however, arsenic reduction during anaerobic respiration of organic compounds is a source of metabolic energy. We have documented the arsenic concentration and form along the course of a hot spring channel lined with Thermus sp. bacteria at Yellowstone and demonstrated rapid oxidation of arsenite to arsenate. We interpret this as confirmation of the ability of organisms of this genus to dramatically modify arsenic speciation in the environment.

The biochemical mechanism of arsenite oxidation is under ongoing investigation. This represents the first report of a widely distributed bacterial group to impact arsenic oxidation state. The result is significant because oxidation significantly modifies the bioavailability and toxicity of arsenic and the ability of oxyhydroxides to retard its mobility in the environment. We have examined the partitioning of arsenate between solutions and secondary minerals. As has been demonstrated previously in the laboratory, arsenic is strongly sorbed by iron oxydroxide minerals such as ferrihydrite and goethite, which are abundant in areas peripheral to acid mine drainage sites, soils, and sediments. Arsenic also is partitioned into zinc sulfide formed due to the activity of sulfate-reducing bacteria. The concentration factor in the solid relative to solution is about 106 x.

Influence of Microbes on Surface Composition and Arsenic Release Rate. The effects of different microbial populations on the oxidative dissolution of FeAsS at 37°C and pH 1.5 were examined to better understand the mechanism and kinetics of dissolution under conditions that simulate acid mine drainage. Samples of arsenopyrite were exposed to a sulfur-oxidizing isolate (Thiobacillus caldus), an iron-oxidizing isolate (Ferroplasma acidarmanus), and a mixed enrichment culture containing T. caldus, F. acidarmanus, and Leptospirillum ferrooxidans. An increase in dissolution rate was observed only in the presence of iron-oxidizing microorganisms (i.e., F. acidarmanus or the enrichment culture). The chemical speciation at the mineral surface in the presence of these iron-oxidizing species is indistinguishable from that of abiotic control reactions under the same conditions; both are dominated by elemental sulfur. In contrast, experiments with T. caldus indicate that the quantity of elemental sulfur on the mineral surface is less than 1 percent of the amount observed on samples exposed to the F. acidarmanus culture. Surprisingly, removal of the elemental sulfur from the mineral surface by the sulfur-oxidizing species is not accompanied by an increase in the dissolution rate of the mineral.

This finding suggests that elemental sulfur must be distributed very heterogeneously on the mineral surface. To test this hypothesis, spatially resolved spectroscopic studies of oxidized arsenopyrite surfaces were conducted using an imaging Raman microscope with 3 mm resolution. The resulting chemical maps of the mineral surface reveal that elemental sulfur is present in randomly distributed, isolated patches on the order of tens of microns in size.

Although spectroscopic investigations of oxidized arsenopyrite surfaces revealed information about the speciation and spatial distribution of reaction products, further study of the mechanism of arsenopyrite oxidation required quantitative information about the reaction products. To accomplish this, we developed a new method for the quantitative determination of elemental sulfur. After laboratory oxidation of the mineral sample, elemental sulfur is extracted from the surface in an organic solvent and quantitatively analyzed by high-performance liquid chromatography. Subsequent studies using this method revealed that elemental sulfur accounts for more than half of the total reacted sulfur when arsenopyrite is oxidized by ferric iron. This finding has significant implications for the possible mechanism of arsenopyrite oxidation. If arsenopyrite oxidizes in a scheme analogous to the commonly accepted mechanism for pyrite, whereby the sulfur is initially oxidized to thiosulfate, elemental sulfur could be formed by the decomposition of thiosulfate in the acidic solution. This scheme, however, limits elemental sulfur to no more than 50 percent of the total reacted sulfur. Our quantitative determination of elemental sulfur eliminates the thiosulfate mechanism as a possibility for the oxidation of arsenopyrite. Instead, we propose that arsenopyrite must initially form lower oxidation state products such as polysulfides and elemental sulfur rather than sulfoxy anions.

Perhaps, the most important result from this work is that elemental sulfur is a common intermediate (>50 percent, in some cases) product of the geochemical reactions, but it is not incorporated into most of the existing geomchemical models aimed at predicting metal release rates. Although, under the condition investigated here, we found that as the sulfur deposits are sufficiently thin and porous, they do not have a major effect on the reactant transport to and from the mineral surface, but under certain conditions elemental sulfur intermediates may inhibit diffusion and thereby become rate-controlling.

Mechanism of Catalytic Oxidation of As(III) to As(V) on Mineral Surfaces. We have found that the catalytic oxidation of As(III) to As(V) is limited by an electron-transfer step that is controlled by the electrochemical potential of the solution in contact with the pyrite surface. Thus, the rate of reaction is not proportional to the ferric ion concentration (as would be expected for a simple diffusion-controlled reaction), but is controlled by the ratio of Fe3+/Fe3+. Thus, solutions that contain even moderate amounts of Fe3+ can act as very effective catalytic oxidizers provided that the Fe2+ concentration also is low. Because As(III) is much more toxic than As(V), this information suggests the rapid oxidation of As(III).

Journal Articles on this Report : 11 Displayed | Download in RIS Format

Other project views: All 40 publications 12 publications in selected types All 11 journal articles

Type Citation Project Document Sources
Journal Article Edwards KJ, Schrenk MO, Hamers RJ, Banfield JF. Microbial oxidation of pyrite: experiments using microorganisms from an extreme acidic environment. American Mineralogist 1998;83:1444-1453. R826189 (Final)
not available
Journal Article Edwards KJ, Goebel BM, Rodgers TM, Schrenk MO, McGuire MM, Hamers RJ, Pace NR, Banfield JF. Geomicrobiology of pyrite (FeS2) dissolution:case study at Iron Mountain, California. Geomicrobiology Journal. 1999;16(2):155-179. R826189 (1999)
R826189 (Final)
not available
Journal Article Edwards KJ, Bond PL, Gihring TM, Banfield JF. An archaeal iron-oxidizing extreme acidophile important in acid mine drainage. Science. 2000;287(5459):1796-1799. R826189 (Final)
not available
Journal Article Edwards KJ, Bond PL, Banfield JF. Characteristics of attachment and growth of Thiobacillus caldus on sulfide minerals. Environmental Microbiology 2000;2:324-332. R826189 (1999)
R826189 (Final)
not available
Journal Article Edwards KJ, Bond PL, Druschel GK, McGuire MM, Hamers RJ, Banfield JF. Geochemical and biological aspects of sulfide mineral dissolution: lessons from Iron Mountain, California. Chemical Geology, Volume 169, Issues 3-4, 1 September 2000, Pages 383-397. R826189 (1999)
R826189 (Final)
not available
Journal Article Edwards KJ, Hu B, Hamers RJ, Banfield JF. A new look at microbial leaching patterns on sulfide minerals. FEMS Microbiology Ecology 2001;34:197-206. R826189 (1999)
R826189 (Final)
not available
Journal Article Gihring TM, Druschel GK, McKlesky B, Hamers RJ, Banfield JF. Rapid arsenite oxidation by Thermus aquaticus and Thermus thermophilus: field and laboratory investigations. Environmental Science and Technology 2001, 35, 3857-3862. R826189 (Final)
not available
Journal Article McGuire MM, Hamers RJ. Extraction and quantitative analysis of elemental sulfur from sulfide mineral surfaces by high-performance liquid chromatography. Environmental Science and Technology 2000;34:4651-4655. R826189 (Final)
not available
Journal Article McGuire MM, Banfield JF, Hamers RJ. Quantitative determination of elemental sulfur at the arsenopyrite surface after oxidation by ferric iron: mechanistic implications. Geochemical Transactions 2001;2:25 R826189 (Final)
  • Full-text: Geothermal Transactions
  • Abstract: Geothermal Transactions
  • Other: Geothermal Transactions
  • Journal Article McGuire MM, Jallad KN, Ben-Amotz D, Hamers RJ. Chemical mapping of elemental sulfur on pyrite and arsenopyrite surfaces using near-infrared raman imaging microscopy.Applied Surface Science 2001;178(1-4):105-115. R826189 (Final)
    not available
    Journal Article McGuire MM, Edwards KJ, Banfield JF, Hamers RJ. Kinetics, surface chemistry, and structural evolution of microbially mediated sulfide mineral dissolution. Geochemica et Cosmochimica Acta 2001;65:57-72. R826189 (Final)
    not available
    Supplemental Keywords:

    acid deposition, drinking water, water, watersheds, groundwater, adsorption, leachate, ecological effects, health effects, bioavailability, metabolism, cellular, organism, genetic predisposition, chemicals, toxic substances, acid rain, effluent, discharge, metals, intermediates, toxics, aquatic, remediation, bioremediation, oxidation, decision-making, environmental chemistry, biology, hydrology, geology, genetics, modeling, analytical, measurement methods, northwest, California, CA, Wisconsin, WI., RFA, Scientific Discipline, Toxics, Geographic Area, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, POLLUTANTS/TOXICS, Bioavailability, Ecology, Water & Watershed, Environmental Chemistry, Ecosystem/Assessment/Indicators, Ecosystem Protection, Arsenic, Chemistry, pesticides, State, Microbiology, Ecological Effects - Environmental Exposure & Risk, Bioremediation, Water Pollutants, Drinking Water, Groundwater remediation, Watersheds, fate and transport, health effects, chemical probes, electron microscope, ecological exposure, microbial risk assessment, human health effects, arsenic transformation, physicochemical controls, biodegradation, spectroscopic studies, aquatic restoration, weathering of minerals, physiochemical controls, kinetic studies, chemical transport, surface water, rate of release, arsenic release, aquatic ecosystems, chemical releases, contaminant release, ecosystem, weathering, Wisconsin (WI), pyrite, speciation and release of arsenic, arsenic exposure, California (CA), microbial controls, groundwater, heavy metals, microbial, mining impacted watershed

    Relevant Websites:

    http://hamers.chem.wisc.edu/research/geochem/amd.htm Exit
    http://www.geology.wisc.edu/~jill/gbio.html Exit

    Progress and Final Reports:
    Original Abstract
    1999 Progress Report

    Top of Page

    The 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.

    Jump to main content.