Effect of Short-Term Resuspension Events on Trace Metal Speciation in Polluted Anoxic Sediments

Abstract

The effects of the short-term resuspension of a contaminated anoxic estuarine sediment on solid-phase metal speciation have been studied. Preliminary experiments investigated the oxidation rates of model metal sulfide phases to provide mechanistic information for interpreting the observations on the natural sediment. FeS and MnS model phases were particularly labile and oxidized rapidly in aerated waters. In contrast, CdS, CuS, PbS, and ZnS model phases were kinetically stable over periods of several hours. The oxidation rate of free sulfide (HS-) was significantly slower than that of FeS and MnS. Upon sediment resuspension, the rapid decrease in acid volatile sulfide (AVS) could be accounted for by the oxidation of iron monosulfide phases. Over prolonged resuspension periods (>300 min), AVS decreased to values lower than the simultaneously extracted metals [SEM = ∑Cd, Cu, Ni, Pb, Zn (1 M HCl extraction, 30 min)] concentration, indicating that a significant fraction of trace metal sulfide phases may be oxidized during resuspension events. During 8-h sediment resuspension experiments, SEM(Cu) increased from 0.1 to ca. 2 μmol/g while the SEM measured for the other metals remained constant. The increase in SEM(Cu) was shown to be an artifact of the AVS/SEM analytical procedure. Fe(OH)3, formed through the oxidation of FeS, dissolved upon acidification to produce Fe3+(aq), which subsequently oxidized acid-insoluble copper sulfide mineral phases in the sediment. The implications of these observations for natural systems and for the assessment of metal toxicity using AVS/SEM procedures are discussed.