Grantee Research Project Results
Influence of Biogeochemical Cycling of Iron and Sulfur on Arsenic SequestrationEPA Grant Number: F5B30313
Title: Influence of Biogeochemical Cycling of Iron and Sulfur on Arsenic Sequestration
Investigators: Saalfield, Samantha L.
Institution: Dartmouth College
EPA Project Officer: Lee, Sonja
Project Period: January 1, 2005 through December 31, 2008
Project Amount: $111,172
RFA: STAR Graduate Fellowships (2005) RFA Text | Recipients Lists
Research Category: Academic Fellowships
Arsenic is a widespread groundwater contaminant, both internationally and within the United States. This research responds to the increasing evidence of arsenic contamination worldwide by investigating biogeochemical mechanisms by which arsenic may be preferentially released from aquifer sediments. Because aqueous (dissolved) arsenic is the primary health risk, it is critical to improve our understanding of the processes controlling arsenic partitioning between the aqueous and solid phases. Specifically, this research focuses on reductive release of arsenic from iron oxides in the presence of sulfide, both produced in situ through biological processes and introduced in dynamic systems. Mechanistic laboratory research is integrated with descriptive field data to establish the dynamics of the characterized processes on environmental scales. We seek to constrain the effect of sulfide in controlling the fate of arsenic in iron oxide-rich systems undergoing reduction.
Arsenic solubility and mobility is closely tied to coupled geochemical cycles occurring in aquifer environments. Sulfur cycling in particular has been correlated with changes in arsenic speciation in many soil environments, and reductive processes involving sulfide could conceivably release toxic levels of arsenic into solution. Related processes, especially iron sulfide precipitation, may also sequester arsenic and maintain low dissolved levels.
Controlled flow-through and batch experiments are used to investigate the role of both abiotic and biogenic sulfide on iron and arsenic speciation in anoxic systems. Experiments begin with either idealized or natural iron oxide-rich sediments containing sorbed arsenic, and sulfide is then introduced either directly or by dissimilatory (microbial) reduction of sulfate. Parameters, including sulfide flux, initial arsenic speciation (As(V) or As(III)), and amount of arsenic loading, are varied in order to explore the dynamic processes of the system and their kinetic controls.
Dissolved concentrations of iron, arsenic, and sulfur are measured using conventional analyses, primarily ICP-OES. Ion-selective electrodes and spectrophotometry are employed in determining dissolved speciation. X-ray adsorption spectroscopy and diffraction is used to probe changes in the arsenic retention mechanisms and the mineralogy of solid phases.
Incorporation of field data will allow further evaluation of the mechanisms observed in the laboratory, and their importance in natural systems.
This research will ultimately provide a mechanistic description of the biogeochemical conditions and processes through which sulfate reduction in natural anoxic systems controls arsenic mobilization and sequestration. Such mechanistic knowledge can be broadly applied to develop remediation strategies, predict environmental fates, and improve water quality in arsenic-impacted areas.