Fractionation and Extractability of Sulfur, Iron and Trace Elements in Sulfidic Wetland Soils.
Edward Burton, Richard T. Bush, and Leigh A. Sullivan. Southern Cross University, 1 Military road, Lismore, Australia
Pyrite and other reactive iron-sulfide minerals are important to trace element behavior in many wetlands. These minerals provide sinks for potentially toxic trace elements that are introduced from anthropogenic and natural sources. The bioavailability and potential mobility of trace elements in wetland soils is therefore largely determined by the geochemical cycling of key Fe and S fractions. We examined Fe and S fractionation, and the related extractability of selected trace elements (As, Cd, Cr, Cu, Ni, Pb and Zn) in wetland soils adjacent to a former municipal landfill in eastern Australia. The soil profiles were strongly reducing (Eh < -120 mV) and contained moderately high concentrations of pore-water sulfide (200 – 600 umol/L) and acid-volatile sulfide (AVS; 9.9 – 129 umol/g). Pyrite-S concentrations increased with depth, attaining concentrations as high as 251– 669 umol/g. The Degree of Sulfidisation was generally high (> 80%), indicating that Fe may be limiting pyrite accumulation. The ratios of AVS to pyrite-S increased with sediment depth, as expected for the pyritisation of solid-phase AVS. This conversion of AVS to pyrite-S is expected to profoundly influence the fate of AVS-bound trace elements. Cadmium, Pb and Zn extractability in 1 M HCl indicated that these elements are not significantly sequestered during pyritization, whereas sequestration may be important for As, Cu and possibly Ni. Extractability trends for Cr suggest that diagenesis in sulfidic soils may enhance Cr reactivity. Overall, replacement of AVS by pyrite during diagenesis may enhance the reactivity of Cd, Cr, Pb and Zn, whereas As, Cu and possibly Ni may be rendered less reactive.