Enhanced immobilization of metal pollutants in sewage sludge ash (SSA)-cement pastes by calcium chloride and nitrate: Experimental and DFT studies

Urbanization, Portland cement production, and urban waste have caused concern about increasing environmental pollution. Sewage sludge ash (SSA) comes from the incinerating plant of sewage sludge, which is a by-product of wastewater treatment. SSA can cause metal pollution if improperly handled. Recycling SSA into cement composite such as concrete (i.e., the most widely used material after water) can obviate landfilling and mitigate CO2 and metal pollutants from cement production and SSA. Chemicals called accelerators, such as CaCl2 and Ca(NO3)2 are used to overcome the relatively slow hydration of SSA cement pastes (CPs). In this study, we found the capability of CaCl2 and Ca(NO3)2 to enhance Mo and Cr immobilization by CPs with SSA (i.e., mixed with Portland and blast furnace types of cement). The mechanistic investigation was performed by employing nitrogen gas adsorption, thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy. The role of the chemical properties rather than physical characteristics of the hydrates was more significant for the immobilization. CaCl2 and Ca(NO3)2 enhanced more formation of aluminate ferrite tri-sulfate (AFt), increasing the host sites for MoO42- and CrO42-. Density functional theory (DFT) simulations showed the likelihood of SO42- in AFt substituted by MoO42- and CrO42-. The unveiled capabilities and mechanism of CaCl2 and Ca(NO3)2 to enhance the MP immobilization of SSA-CPs disclosed new knowledge for the mitigation strategy of pollution from increasingly generated SSA.