How does the coupled action of freeze-thaw and acidification affect the release of toxic elements from indigenous Zn smelting slags?

Under complicated field conditions, such as the coupled effects of freeze - thaw cycles (FTs) and acidification, the leaching behavior of potentially toxic element (PTEs) from indigenous Zn smelting slags (ZSS) was intricately connected to the mineralogy (e.g., the composition, assemblage, microstructure of mineral particles). In this study, FTs tests were carried out to explore the interactions between PTEs release and ZSS mineralogy. Subsequently, advanced characterization techniques were adopted to quantify the mineralogy and microstructure of ZSS. The results indicated that ZSS were mainly composed of silicate minerals (e.g., quartz, biotite and chlorite) and secondary Fe (III) oxyhydroxides (e.g., magnetite and limonite), accounting for 67.48% and 24.23%, respectively. The occurrence mode analysis revealed that 81.95% of As, 21.31% of Pb and 7.77% of Zn were hosted in limonite. About 37.89%, 59.34% and 34.50% of Cd, Pb and Zn were associated with carbonate bound fractions. Under FTs interacting with different pH conditions, the leaching concentrations of As, Cd, Cu, Pb and Zn did not significantly increase with the increase in FTs and pH. The microstructure damage of mineral particles in ZSS with the higher porosity was caused by both FTs and proton corrosion. More importantly, the geochemical modeling results suggested that the precipitation of hematite and magnetite, as well as the dissolution of arsenolite, might have little impacts on PTEs release under FTs and FTs with acidification. This work would provide a deeper understanding of PTEs release from smelting waste slags under complex physicochemical interactions.