The leaching behaviors of lead, zinc, and sulfate in pyrite ash contaminated soil: mineralogical assessments and environmental implications

Contaminated soil with high mobility of potentially toxic elements (PTEs) can threaten the environment and human health. Precisely quantifying trends in PTEs accumulation in the soil under changing pH conditions is essential to minimize potential exposure. However, this has long been a hard-to-monitor subject experimentally due to the relatively low content of PTEs and the lack of detailed knowledge of the minerals that control PTEs' leaching. Here we profoundly investigate the critical role of soil mineralogy in PTEs release and predict the leaching behavior of PTEs by exploiting the modeling approach. The investigated sample comes from a brownfield site devoted to fertilizers production. Hematite, jarosite, and gypsum are the major mineralogical phases, with zinc sulfate, anglesite, kintoreite, and Pb-bearing jarosite being identified as the dominant Pb and Zn phases. pH-dependent leaching tests in combination with geochemical modeling were used to reveal the potential leaching mechanisms and contaminants solubility-controlling phases at pH ranging from 1 to 12. The experimental and modeling results both demonstrated that Pb and Zn have an amphoteric leaching behavior, with the lowest leached concentrations at the neutral/alkaline region around pH values from 8.0 to 10.0. The calculated saturation indexes suggest that Pb retention is controlled by anglesite, cerussite, and hydrocerussite, while Zn retention is attributed to zinc carbonates and hydroxides. Further, jarosite and ferrihydrite may play a role in Pb and Zn retention. In comparison, the sulfate release increases with pH values, which is governed by the equilibrium of jarosite, gypsum, and anglesite. The overall results highlight the value of converging experimental-geochemical modeling approaches to gain a deeper understanding of PTEs' release and retention, which is difficult to reveal through experiments alone. These advances may be pivotal in the sustainable management and design of remediation strategies.