Effects of cyclic changes in pH and salinity on metals release from sediments

The effects of dynamic changes in pH and salinity on metal speciation and release are investigated with sediments posed in a simulated estuarine environment. The release of Zn, Cd, Mn, and Fe was studied using sediment from the Anacostia River (Washington, DC, USA) spiked with freshly precipitated amorphous cadmium sulfide to increase Cd content. The sediment was exposed to salt water (high pH, ionic strength) and freshwater (neutral pH, minimal ionic strength) continuously and alternately (to mimic tidal changes) in small microcosms over 100 d. At the conclusion of the experiments, the vertical profiles of acid volatile sulfide (AVS) and simultaneously extracted metals (SEM) as well as porewater metals and anion concentrations were characterized. Acid volatile sulfide oxidation at the sediment surface led to a commensurate increase in dissolved metal species and metal release that was strongly dependent on the changes in the overlying water characteristics. Total Cd release was substantially higher during exposure to salt water, although, as a result of complexation, predicted dissolved Cd²⁺ concentration in the overlying water was higher during exposure to freshwater. Total Zn release was little changed during exposure to salt water and freshwater, although the predicted dissolved Zn²⁺ concentration was much higher during freshwater exposures. No significant iron was released because of the rapid oxidation of ferrous iron (Fe²⁺) in aerobic surficial sediments and overlying water. The present study suggests that cyclic changes in pH and salinity in the overlying water can dramatically influence metal release from estuarine sediments.