Role of Reduced Sulfur in the Transformation of Cd(II) Immobilized by δ-MnO2.

Mn oxides are the major sinks for Cd(II) in the aquatic environment. At the redox interface, reduced sulfur might affect the fate of sorbed Cd(II) by either reducing Mn oxides or forming strong complexes with Cd(II). Here, we investigated the fate of Cd(II) immobilized on δ-MnO2 affected by reduced sulfur (S2- and cysteine). A low concentration of S2- led to Cd(II) migration from vacant sites to edge sites, while a high concentration of S2- largely converted Cd(II) adsorbed on the surface of δ-MnO2 to CdS. At low pH, the cysteine addition led to the release of Cd(II) initially adsorbed at the δ-MnO2 vacant sites into the solution and caused the migration of a small portion of Cd(II) to the δ-MnO2 edge sites. At high pH, a high concentration of cysteine led to the detachment of Cd(II) from δ-MnO2, Cd(II) readsorption by Mn(III)-bearing minerals, and Cd-cysteine formation. Changes of Cd(II) speciation were caused by δ-MnO2 dissolution induced by reduced sulfur, the competition of generated Mn(II/III) for the adsorption sites, and the precipitation of Cd(II) with reduced sulfur. This study indicates that reduced sulfur is a critical factor controlling the fate of Cd(II) immobilized on Mn oxides in the aquatic environment.