Cadmium Mixing Behavior in Estuaries: Redox Controls on Removal and Mobilization

This study investigates various hydrological and redox conditions controlling phase transformation, removal and remobilization of Cd in three different estuaries in Taiwan. It was found that Cd mixing patterns in estuarine waters are controlled mainly by redox conditions and salinity. In a highly urbanized estuary segment low oxygen conditions caused sulfide production and the formation of stable Cd sulfide minerals. In oxygen-replete waters [dissolved oxygen (DO) > 200 mu M], high concentrations of Mn(II), likely from exchange with bottom sediments were oxidized into Mn oxides, on which Cd is adsorbed. However, in the lower estuary regions elevated chloride concentrations are likely able to compete for Cd, with large proportions of Cd becoming complexed with chloride. The stable Cd sulfide minerals, resistant to oxidation in estuaries having short flushing time, are likely settling out and depositing into bottom sediments. Reversible Cd adsorption onto Mn oxides can enhance Cd mobility in lower estuary and coastal regions where Cd chloro-complexes form, resulting in greater Cd transport fluxes into the ocean.