Laboratory Investigations of Enhanced Sulfate Reduction as a Groundwater Arsenic Remediation Strategy

Abstract

Landfills have the potential to mobilize arsenic via induction of reducing conditions in groundwater and subsequent desorption from or dissolution of arsenic-bearing iron phases. Laboratory incubation experiments were conducted with materials from a landfill where such processes are occurring. These experiments explored the potential for induced sulfate reduction to immobilize dissolved arsenic in situ. The native microbial community at this site reduced sulfate in the presence of added acetate. Acetate respiration and sulfate reduction were observed concurrent with dissolved iron concentrations initially increasing from 0.6 μM (0.03 mg L −1 ) to a maximum of 111 μM (6.1 mg L −1 ) and subsequently decreasing to 0.74 μM (0.04 mg L −1 ). Dissolved arsenic concentrations initially covaried with iron but subsequently increased again as sulfide accumulated, consistent with the formation of soluble thioarsenite complexes. Dissolved arsenic concentrations subsequently decreased again from a maximum of 2 μM (148 μg L −1 ) to 0.3 μM (22 μg L −1 ), consistent with formation of sulfide mineral phases or increased arsenic sorption at higher pH values. Disequilibrium processes may also explain this second arsenic peak. The maximum iron and arsenic concentrations observed in the lab represent conditions most equivalent to the in situ conditions. These findings indicate that enhanced sulfate reduction merits further study as a potential in situ groundwater arsenic remediation strategy at landfills and other sites with elevated arsenic in reducing groundwater.

Experimental analysis of arsenic precipitation during microbial sulfate and iron reduction in model aquifer sediment reactors

“…In addition to Farquhar et al (2002) and Wolthers et al (2005b), other experimental studies that examined As sequestration by sulfide minerals include Rittle et al (1995), Castro et al (1999), Jong and Parry (2003), Keimowitz et al (2007). How their results compare to ours is unclear, however, because none of these studies identified the sulfide minerals that formed.…”

“…This observation implies that we may be able to remediate As contamination in situ in reducing aquifers simply by injecting SO 4 2À to stimulate microbial SO 4 2À reduction (Kirk et al, 2004). Other studies have also concluded that enhancing SO 4 2À reduction may be useful for As remediation, including Rittle et al (1995), Castro et al (1999), Jong and Parry (2003), Lee et al (2005), and Saunders et al (2005), Keimowitz et al (2007). In a field test of this strategy at a naturally contaminated site in Bangladesh, Saunders et al (2008) observed a decrease in aqueous As concentration coinciding with stimulated SO 4 2À reduction.…”

Arsenic, iron and sulfur co-diagenesis in lake sediments

“…Model 1 has been used in many studies to calculate As speciation in sulfidic waters and it is assumed in all of them that redox processes are controlled by the couple SO 2À 4 /HS À . (O'Day et al, 2004;Bostick et al, 2005;Haque and Johannesson, 2006;Lee et al, 2006;Keimowitz et al, 2007;Beak et al, 2008;Gallegos et al, 2008). The sub-set of reactions describing As sulfidation in Model 2 (reactions 19-45) was provided by Helz and Tossell (2008); these authors critically reviewed As speciation in previous laboratory studies and used an ab initio quantum chemical method to derive a provisional model of As speciation in sulfidic waters.…”

Microbial sulfidogenesis in ferrihydrite-rich environments: Effects on iron mineralogy and arsenic mobility

“…For these reasons, sulfidogenesis via microbial SO 4 2À -reduction is widely believed to retard As mobility in reducing subsurface environments. Furthermore, it has been suggested that As-contaminated groundwaters might be remediated by the deliberate in-situ stimulation of SO 4 2À -reducing bacteria (Kirk et al, 2004;Keimowitz et al, 2007).…”