Stabilization/solidification of heavy metals in sludge ceramsite and leachability affected by oxide substances

To investigate stabilization of heavy metals in ceramsite made from wastewater treatment sludge (WWTS) and drinking water treatment sludge (DWTS), leaching tests were conducted to find out the effect of SiO2:Al2O3, acidic oxides (SiO2 and Al2O3), Fe2O3: CaO:MgO, and basic oxides (Fe2O3, CaO, and MgO) on the binding ability of heavy metals. Results show that as ratios of SiO2: Al2O3 decrease, leaching contents of Cu and Pb increase, while leaching contents of Cd and Cr first decrease and then increase; under the variation of Fe2O3:CaO:MgO (Fe2O3 contents decrease), leaching contents of Cd, Cu, and Pb increase, while leaching contents of Cr decrease. Acidic and basic oxide leaching results show that higher contents of Al2O3, Fe2O3, and MgO are advantageous to improve the stability of heavy metals, while the binding capacity for Cd, Cu, and Pb is significantly reduced at higher contents of SiO2 and CaO. The solidifying efficiencies of heavy metals are improved by crystallization, and the main compounds in ceramsite are crocoite, chrome oxide, cadmium silicate, and copper oxide. These results can be considered as a basic understanding for new technologies of stabilization of heavy metals in heavily polluted WWTS.