Roles of calcium alginate coating ferrous sulfide in promoting Pb, Zn, and Cd immobilization in the Pb/Zn smelter...

Alginate-modified ferrous sulfide (FeS) has received much attention in heavy metals (HMs) adsorption. However, whether calcium alginate (CA) could hinder FeS oxidation and promote HMs immobilization in Pb/Zn smelter soil, remains unclear. Herein, CA prolonged half-lives of S(-II) in CA-coated FeS (FeS-CA) by 92.2 % and 46.9 % when coexisting with O₂ and MnO₂, resulting in that S(-II) content in soil treated by FeS-CA was 38.7 % higher than bare FeS. The Pb, Zn, and Cd immobilization efficiencies by FeS within FeS-CA (FeS_FeS-CA) were 79.1 %, 70.2 %, and 80.1 %, 3.3 %-17.6 % higher than that by bare FeS. The enhancement was mainly attributed to that labile HMs reduced by FeS_FeS-CA were 20.6 %-32.2 % higher than bare FeS due to 19.5 %-30.5 % more SOM-bound labile HMs captured by FeS_FeS-CA. The labile Pb was converted into Fe/Mn (hydr)oxides-bound and residual fractions, while labile Zn and Cd were converted into Fe/Mn (hydr)oxides-bound fractions. The CA was stable during soil incubation and leaching. It could immobilize 10.8 %-28.2 % of HMs and promote 12.8 %-16.6 % of the labile HMs conversion due to competitively adsorbing the SOM-bound HMs. The HM-bridging configurations of CA-HM-DOM were responsible for DOM adsorption, accompanied by HMs stabilization with high adsorption energies (>7 eV). The CA-bound HMs were stable under the rainfall conditions. Furthermore, CA could recapture more than 82.2 % of HMs released from solid phases via carboxyl complexation when H⁺ is input, alleviating HM leaching. The results indicate that FeS-CA is an effective and applicable stabilizer for HM immobilization in Pb/Zn smelter soil.