Carbonate-hosted sphalerite weathering regulates cadmium mobilization in soils

Cadmium (Cd)-bearing sphalerite occurs in carbonate-hosted zinc (Zn) deposits and can be deposited as particulate matter (PM) on the surrounding soils during mining activities. Weathering of the sphalerite-bearing PM releases Cd, yet the role of associated carbonates in controlling Cd mobility remains unclear. This study investigates Cd mobilization from carbonate-hosted sphalerite ore particles (SP) and Cd distribution between solid and aqueous phases in acidic and alkaline soils. At low Ca/S ratios, sphalerite dissolution led to similar annual Cd mobilization rates in acidic (1.41 μg Cd per g SP per a) and alkaline soils (1.29 μg Cd per g SP per a). However, higher Ca/S ratios significantly reduced Cd mobility due to Cd retention as CdCO₃ in both solid and solution phases. In acidic soils, Cd-bearing sphalerite weathering caused Cd depletion and enrichment in sulfide ore and CaCO₃ phases, respectively. In alkaline soils, CdCO₃ nanomaterials precipitated on zincite due to the incompatibility of Cd with the structure of ZnO and its desorption from the negatively charged Fe (hydr)oxide surfaces. For all characterized samples, nanoparticulate (nano)-Cd showed significant positive correlations with nano-Fe and nano-organic carbon (bulk chemical data) and was sequestrated as CdCO₃ nanomaterial by OM-Fe (hydr)oxide colloids in soil solutions (TEM data). These observations highlight that Cd-bearing nanomaterials control Cd mobilization in carbonate-rich soils affected by sphalerite-bearing PM deposition.