Iron and sulfur mineralogy and redox transformations in soils and sediments: Implications for trace metal dynamics

Iron minerals are ubiquitous in near surface environments and often readily sorb or coprecipitate with many trace metals. Transformations of iron phases are, therefore, integral to the fate of trace metals in most near surface aqueous regimes. Aqueous sulfide is an effective reducatant and often occurs contemporaneously with ferric minerals. This work probes the reductive dissolution of ferric (hydr)oxides via sulfidization and the associated impact on toxic trace metal release and sequestration.;Here, we demonstrate the important mechanistic pathways of iron (hydr)oxide sulfidization through a detailed, abiotic, lab-based inquiry. Highly impacted mine-waste sediments were then used in incubation studies amended to select for different components of the natural microbial community. Solution concentrations were measured and extensive synchrotron spectroscopies were used to probe mineralogy at regular time points. A kinetic model of the Fe-S-H2 O system was built and fit to the measured data.;Toxic trace metals repartitioned upon reduction to stable reduced phases; however, significant aqueous metal concentrations were transiently maintained during the conversion. A kinetic balance of iron reducing bacteria and sulfur reducing bacteria maximized arsenic release in the soils. We field tested this relationship via measurement of solution concentrations and synchrotron based speciation analysis on samples from Cambodia, a site of significant natural arsenic contamination to a drinking water supply for over 11 million people. We linked current groundwater arsenic contamination to age of land feature through amount and quality of trapped organic matter (OM) during sedimentation. We show that the OM fuels microbiota at appropriate kinetic activity to induce rates of iron mineralogical transformations that sustain transient arsenic release.