Synergistic immobilization of chromium in tannery sludge by ZnO and TiO2 and the oxidation mechanism of Cr(III) under alkaline in high temperature

The disposal of the Cr containing tannery sludge has become a serious environmental problem in China, which has attracted increasing attention. However, experimental and theoretical research is still needed. We developed a feasible route to synergistic immobilization the Cr in tannery sludge into the spinel-based phases by addition of ZnO and TiO2. ZnO and Cr3+ can form stable spinel-based phase, and TiO2 can suppress the formation of Cr6+. The significantly synergistic effect between ZnO and TiO2 promoted the immobilization of Cr in tannery sludge and reduce the leaching of the chromium in the annealed sludge. The oxidation of Cr3+ to Cr6+ in the presence of alkaline substances has been elucidated by density functional theory, and it is revealed that the electrons from the Cr-d orbit jump to the Ca-d and directly transform into the O2.