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Reactions of Biogenic Hydrogen Sulfide With Arsenic-Doped Ferrihydrite: Pathways of Arsenic Mobilization and SequestrationEPA Grant Number: FP916369
Title: Reactions of Biogenic Hydrogen Sulfide With Arsenic-Doped Ferrihydrite: Pathways of Arsenic Mobilization and Sequestration
Investigators: Kocar, Benjamin D.
Institution: Stanford University
EPA Project Officer: Zambrana, Jose
Project Period: January 1, 2004 through December 31, 2006
Project Amount: $111,688
RFA: STAR Graduate Fellowships (2004) RFA Text | Recipients Lists
Research Category: Academic Fellowships , Engineering and Environmental Chemistry , Fellowship - Environmental Chemistry and Environmental Material Science
Iron (Fe) and sulfur (S) minerals, ubiquitous in soils and aquifers around the world, have a pronounced effect on the geochemical cycling of arsenic (As). The complexities of As cycling are compounded by the presence of microbes capable of transforming (via respiration or detoxification) As, thus changing the oxidation state and mobility of this metalloid. The objective of this research is to examine biotic and abiotic factors governing solid and aqueous phase transformations of As sorbed to a model Fe hydroxide, ferrihydrite, in the presence of reduced S species (dissolved sulfide). I hypothesize that: (1) reductive dissolution of As-bearing ferrihydrite will result in an increase in As mobility; (2) As mobility will increase when poorly crystalline, As-bearing Fe minerals such as ferrihydrite are transformed to lower surface area minerals (e.g., goethite, magnetite); (3) increases in As mobility will be offset by the addition of hydrogen sulfide to the system, which will induce the formation of high surface area sulfide minerals (such as mackinawite, FeS) capable of directly immobilizing As; and (4) previously unexplained mechanisms of Fe and S mineral phase formation, such as magnetite and pyrite, will be discerned by the reaction of dissolved sulfide with ferrihydrite. These mechanisms will allow us to better describe the behavior of As in a complex mineralogical system containing Fe, S, and bacteria.
My experimental approach will consist of three investigations that will probe the effect of dissolved sulfide and microorganisms on the solid and aqueous phase chemistry of As. First, I will investigate As fate in an abiotic column system packed with As(V)-loaded ferrihydrite-coated sand and reacted with dissolved sulfide under dynamic flow. This study will elucidate abiotic pathways of As partitioning and transformation by reduced S, simulating the intrusion of dissolved sulfide on an As-rich soil or sediment. In my second investigation, I will add single species of Fe- and/or S-reducing bacteria, such as Shewanella putrifaciens,Desulfovibrio vulgaris, or Sulfosprillium barseii , to the column system to understand biotically driven As mobilization processes. My third and final investigation will utilize the same column system and As(V)-loaded sand but will be inoculated with an isolated natural microbial community from a wetland where both Fe and S redox transformations occur. This study will yield valuable information regarding mechanisms that govern the fate and mobility of As in subsurface systems.