The adsorption route and stabilization mechanism of Cd and Pb in soil from Pb-Zn smelter using ferrous sulfide coated with alginate.

Natural and synthetic ferrous sulfide (FeS) is commonly used in simultaneous stabilization of multi-heavy metal-contaminated soil due to its high selectivity. The coating of alginate on FeS (FeS@A) can enhance the activity and availability toward heavy metals (HMs). However, the quantified contributions of alginate and inner FeS to the adsorption/stabilization of HMs are still limited, which can provide the key information to stabilizer regulation. Two types of FeS@A with different coating of alginate were synthesized by using co-precipitation method. The enhanced dispersity, antioxidant performance, and activity could be obtained with appropriate coating of alginate on FeS, i.e. FeS@A0.5. The activity of FeS in FeS@A0.5 was accordingly enhanced, leading to higher adsorption capacities of inner FeS in FeS@A0.5 toward cadmium (Cd, 436 mg g-1) and lead (Pb, 951 mg g-1) than FeS (378 and 617 mg g-1). Nearly 85 % of adsorbed Cd on FeS@A0.5 was in the form of (Fe,Cd)S sulfides and the chelation of Cd with alginate groups contributed to another 15 %. In comparison, 98.1 % of adsorbed Pb was existed in PbS sulfides rather than chelation with alginate. The activity of inner FeS in FeS@A could also be achieved in real soil scenario due to high resistance against MnIV. Up to 85 % of Cd and Pb could be stabilized in soil treated with 3 % FeS@A0.5 even under air-sufficient condition whereas the corresponding values were less than 40 % for FeS-treated soil. In addition, some adsorbed Pb and Cd were gradually fixed by the secondary ferric minerals, ensuring the long-term stabilities of Cd and Pb in soil. Thus, FeS@A could be used as suitable alternative to simultaneous stabilization of multi-HMs-contaminated soil through optimizing coating thickness of alginate. The feasibility of FeS@A on the remediation of actual long-term Pb/Cd-contaminated soil to reveal the application potential was testified in this study.