NSLS-II Friday Lunchtime Seminar

Arsenic (As) groundwater contamination is thought to result from the reductive dissolution of As-bearing iron oxides by microorganisms. Our understanding of the how reactions occur commonly depends on observing the consumption and production of reactants and products respectively. This approach can be insufficient in environmental systems containing complex compositions and phases that can react through multiple and distinct coupled biological and chemical processes. We suggest it is more effective to study environmental processes by considering dissolved and mineral species as reactive intermediates in geochemical cycles where their concentration is controlled by the balance between their production, consumption and transport. Here, we apply this approach to elucidate a critical role of sulfur (S) cycling in iron reduction in sediments affected by As contamination. Arsenic (As) contamination in soil and groundwater is commonly associated with iron reduction because the concentration of dissolved As and Fe(II) increase together as Fe(III) minerals convert to Fe(II) minerals. In contrast, sulfate concentrations are usually low and stable in groundwaters affected by As. This stability is often interpreted as evidence for the minimal role of S cycling in Fe reduction; however, there is mounting biochemical and mineralogical evidence that sulfate reduction is active and critical to both Fe(III) reduction and As mobilization. Sediment iron mineralogies indicate that reactive Fe(III) minerals are present in most sediments and this Fe(III) is particularly susceptible to chemical and biological reduction. These Fe minerals were transformed to Fe(II) carbonate, green rusts, and, rarely, iron sulfides as the sediments became reduced. The microorganisms identified in sediments undergoing reduction were not iron reducers, and often were autotrophs involved in S cycling. This data suggests that sulfate reduction is active, and tightly coupled to the oxidation of sulfide and elem