Environmental context
The use of water contaminated with arsenic for drinking and irrigation is linked to water and food borne diseases throughout the world. Although reducing conditions in soils and sediments are generally viewed as enhancing arsenic mobility in subsurface environments, we show they can actually promote As sequestration in the presence of reduced sulfur species and labile organic matter. We propose that sulfurisation of organic matter and subsequent binding of As to thiol groups may offer an innovative pathway for As remediation.
Abstract
Flow-through reactors (FTRs) were used to assess the mobility of arsenic under sulfate reducing conditions in natural, undisturbed lake sediments. The sediment slices in the FTRs were supplied continuously with inflow solutions containing sulfate and soluble AsIII or AsV and, after 3 weeks, also lactate. The experiment ran for a total of 8 weeks. The dissolved iron concentration, pH, redox potential (Eh), as well as aqueous As and sulfur speciation were monitored in the outflow solutions. In FTRs containing surface sediment enriched in labile organic matter (OM), microbial sulfate reduction led to an accumulation of organically bound S, as evidenced by X-ray absorption spectroscopy. For these FTRs, the inflowing dissolved As concentration of 20μM was lowered by two orders of magnitude, producing outflow concentrations of 0.2μM monothioarsenate and 0.1μM arsenite. In FTRs containing sediment collected at greater depth, sulfide and zero-valent S precipitated as pyrite and elemental S, while steady-state outflow arsenite concentrations remained near 5μM. The observations thus suggest that As sequestration is enhanced when sediment OM buffers the free sulfide and zero-valent S concentrations. An updated conceptual model for the fate of As in the anoxic As–C–S–Fe system is presented based on the results of this study.
Evaluating zinc isotope fractionation under sulfate reducing conditions using a flow-through cell and in situ XAS analysis
