Grantee Research Project Results
Final Report: Processes Influencing the Mobility of Arsenic and Chromium in Reduced Soils and Sediments
EPA Grant Number: R825399Title: Processes Influencing the Mobility of Arsenic and Chromium in Reduced Soils and Sediments
Investigators: Fendorf, Scott
Institution: Stanford University
EPA Project Officer: Hahn, Intaek
Project Period: January 28, 1997 through January 27, 2000
Project Amount: $293,573
RFA: Exploratory Research - Water Chemistry and Physics (1996) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Safer Chemicals
Objective:
The overall goal of our project was to assess the controlling factors in the reduction of Cr(VI) and As(V) within soils and sediments. The specific questions we wished to address were: (1) What are the contributions of abiotic and biotic pathways in the reduction of arsenate and chromate?; (2) Under what conditions does each mechanism contribute; and (3) What are the reaction mechanisms and resulting products?Accordingly, the objectives of our proposed research were to: (1) determine the potential for Cr(VI) and As(V) reduction processes by inorganic and biological reductants within anaerobic soils and sediments; (2) assess factors controlling the fate of arsenic and chromium under reducing soil conditions; (3) determine potential mobility of products for various reduction pathways of arsenate and chromate.
Our research encompassed both laboratory and field research. The former was used to determine specific reaction mechanisms and kinetic parameters while the latter was conducted to substantiate the cycling of these toxins within natural systems.
Summary/Accomplishments (Outputs/Outcomes):
Inorganic contaminants are a wide-spread problem that jeopardize environmental quality. Unlike organic molecules that may undergo degradation, inorganic ions cannot be transformed into innocuous species. Their longevity in the environment therefore makes them of great concern. As a consequence, a knowledge of trace element reactions in soils and waters is necessary to preserve environmental quality--being necessary both for determining management and remediation strategies of contaminated sites. In particular, understanding reactions of redox-reactive trace elements is important due to large differences in toxicity and bioavailability of different oxidation states. In this study, we will investigate redox and sorption reactions of arsenic and chromium that affect both their hazards and risks in the environment. A primary emphasis of this research is on reduction reactions, biological and chemical, that transform arsenic and chromium species upon the transition from aerobic to anaerobic conditions.Reduction of Cr(VI) to Cr(III) decreases the toxicity and mobility of chromium in soils and water. In addition, the formation of a highly insoluble Cr(III) product would decrease the likelihood of future Cr(III) re-oxidation. Amorphous iron sulfide minerals like mackinawite (FeS1-x) have the potential to reduce large quantities of Cr(VI) and in the process form very stable [Cr, Fe](OH)3 solids. In the first year of this study, we examine the effectiveness of amorphous FeS as a reductant of Cr(VI) by identifying the solution and solid phase products of the reaction between FeS suspensions and chromate. Iron sulfide suspensions at pH 5.0, 7.0 and 8.0 were reacted with a range of Cr(VI) solutions from 50 to 5000 µM in a N2 atmosphere for 3 days. Solutions were analyzed using ICP-AES, IC, and colorimetric methods; solids were analyzed using XRD, TEM, EDS, and XANES spectroscopy. Iron sulfide removes all of the added Cr(VI) from solution for the reaction conditions studied and reduces between 85 percent and 100 percent of the Cr(VI) to Cr(III). Chromate reduction occurs dominantly at the FeS surface and results in [Cr0.75,Fe0.25](OH)3; while less extensive, reduction of Cr(VI) by Fe(II) (aq) also is prevalent and produces a solid with the opposite Cr:Fe ratio, [Cr0.25,Fe0.75](OH)3.
In contrast to the behavior of chromium, reducing environments generally lead to more hazardous conditions with respect to arsenic, increasing both it mobility and toxicity. Thus, defining reactions that promote the reduction of arsenic need to be understood in order to have an accurate knowledge of its hazard. Aqueous sulfide (H2S or HS-) is a strong reductant and often occurs at appreciable concentrations in reduced systems, and consequently it may play an integral part in arsenic redox chemistry. To evaluate this possibility, we investigated the kinetics and reaction pathways of arsenate with sulfide. Arsenate reduction by hydrogen sulfide is rapid and conforms to a second-order kinetic model, having a rate constant, k = 3.2 x 102 M-1 h-1, that is more than 300 times greater at pH 4 than pH 7. However, arsenite is not the direct reaction product. Rather, arsenic-sulfide complexes develop, including the formation of a trimeric species (HxAs3S6x-3), that persist in solution for several days, ultimately dissociating and leading to the production of dissolved arsenite. The precipitation of orpiment is dominant only at high (20:1) S:As ratios, considering the reaction conditions used in this study (133 µM As, pH 4). Hence, models of arsenic behavior in the environment should consider abiotic reduction of arsenate by sulfide, at least under moderately acidic conditions, and the possibility of dissolved As-sulfide complexes.
A paradox exists for the noted elevated dissolved concentrations of arsenic in recently derived anaerobic soils and sediments. Although the reduction of arsenic increases its solubility at circumneutral pH, hydrous ferric oxides (HFO) strongly sorb both As(V) (arsenate) and As(III) (arsenite), the two primary inorganic species. Thus, in the presence of excess HFO, reductive dissolution of iron may be the dominant mechanism by which As is released into solution, NOT arsenate reduction. In this research we observed that the dissimilatory iron-reducing bacterium Shewanella alga strain BrY promoted As mobilization from a crystalline ferric arsenate, as well as from sorption sites within whole sediments. S. alga cells releases arsenate from the mineral scorodite (FeAsO42H2O) as a result of dissimilatory (i.e., respiratory) reduction of Fe(III) to Fe(II). Solid phase analysis with SEM-EDS and XAFS (X-ray absorption fine-structure spectroscopy) revealed that the valence states of Fe and As in the solid phase product were identical to those in solution, i.e. Fe(II) and As(V). Additionally, As(V) sorbed to sediments from Lake Coeur d'Alene, Idaho, a mining-impacted environment enriched in both Fe and As, was solubilized by the activity of S. alga BrY. In neither experiment was As(III) detected. We conclude that arsenic mobility can be enhanced by the activity of dissimilatory iron-reducing bacteria in the absence of arsenic reduction.
It is therefore apparent that the fate of arsenic may be critically tied to
the cycling of iron. Thus, we investigated the correlation between iron and
arsenic in a mine waste-influenced wetland in Northern Idaho (Cataldo Flats, ID)
during seasonal transitions. Iron and As concentrations in the pore waters at
the site were highly variable but were at the highest levels during the spring
and summer months. Using X-ray absorption near edge structure (XANES)
spectroscopy, we confirmed that arsenite, arsenic sulfides, and arsenate species
were present in the soil-solids, all of which varied seasonally within the site.
Additionally, the reactivity of Fe and As solid phases, as measured by selective
sequential extractions, changed seasonally. Iron and As were positively
correlated in the sodium acetate/acetic acid (carbonates), HCl (amorphous
materials), hydroxylamine-hydrochloride/acetic acid (crystalline oxide), and
hydrofluoric acid (silicate) extractable fractions; these pools also comprised
the largest portions of extractable Fe and As. Consequently, arsenate and
arsenite species detected using XANES spectroscopy are likely to be associated
with carbonates in the summer, with iron (hydr)oxides in the fall and winter,
and with silicates in the spring with each of these components playing an
important role in As sequestration and availability within the site. This work
clearly demonstrates the role iron plays in the cycling of arsenic.
Journal Articles on this Report : 5 Displayed | Download in RIS Format
Other project views: | All 15 publications | 5 publications in selected types | All 5 journal articles |
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Type | Citation | ||
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Cummings D, Caccavo F, Fendorf S, Rosenzweig RF. Arsenic mobilization by the dissimilatory Fe(III)-reducing bacterium Shewanella alga BrY. Environmental Science & Technology 1999;33(5):723-729. |
R825399 (Final) |
not available |
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Hansel CM, Fendorf S, Sutton SE, Newville M. Characterization of Fe plaque and associated metals on the roots of mine-waste impacted aquatic plants. Environmental Science & Technology 2001;35(19):3863-3868. |
R825399 (Final) |
not available |
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La Force MJ, Hansel CM, Fendorf S. Arsenic speciation, seasonal transformations, and co-distribution with iron in a mine waste-influenced palustrine emergent wetland. Environmental Science & Technology 2000;34(18):3937-3943. |
R825399 (Final) |
not available |
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La Force MJ, Fendorf S. Solid-phase iron characterization during common selective sequential extractions. Soil Science Society of America Journal 2000;64(5):1608-1615. |
R825399 (Final) |
not available |
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Rochette EA, Li GC, Bostick BC, Fendorf S. Kinetics of arsenate reduction by dissolved sulfide. Environmental Science & Technology 2000;34(22):4714-4720. |
R825399 (Final) |
Exit |
Supplemental Keywords:
chromium, arsenic, reductive stabilization, kinetics, bioavailability, chemicals, water, groundwater, soil, adsorption, chemical transport, heavy metals, bacteria, aquatic, environmental chemistry, ecology, analytical., RFA, Scientific Discipline, Toxics, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, National Recommended Water Quality, Contaminated Sediments, Remediation, Environmental Chemistry, Ecosystem/Assessment/Indicators, Arsenic, Chemistry, Fate & Transport, fate and transport, risk assessment, aquatic ecosystem, fate, arsenic transformation, cellular redox status, human health effects, redox metabolism, colloidal particles, contaminant transport, soil sediment, Chromium, agricultural watershed, contaminated sediment, sediment transport, spectroscopic studies, adverse human health affects, chemical contaminants, kinetic studies, microbial pollution, processes influencing mobility, arsenic mobility, water quality, arsenic exposure, microbial, aquatic biotaRelevant Websites:
http://soils.stanford.edu ExitProgress 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.