Cadmium Enrichment Mechanism and at the Rock-soil Interface in Carbonate Weathered Soil Profile and Its Environmental Significance

<p indent="0mm">The weathering of carbonate rocks contributes to cadmium (Cd) enrichment in surface soils, a key factor in the high geochemical background of heavy metals in karst regions. The past studies have identified unusual Cd accumulation at the weathering front of soil profiles, the rock-soil interface, although its enrichment mechanism remains poorly understood. This study investigated a typical weathering profile in Zongdi Town, Ziyun Miao and Buyei Autonomous County, Guizhou Province, southwestern China. Mineralogical, physicochemical, and geochemical analyses were performed on bedrock and overlying soil samples to determine total Cd content and its geochemical forms. The results reveal significant Cd enrichment at the rock-soil interface, reaching <sc>31.60 mg/kg.</sc> The surface soil exhibited a pH of 5.38–5.82, with relatively pure limestone as the parent rock. Calcite was present exclusively in the bedrock and at the rock-soil interface, where Cd primarily occured in a weak acid-extractable form. The dissolution of limestone released Cd, which was subsequently adsorbed or co-precipitated with carbonate minerals (calcite) at the rock-soil interface, leading to profound Cd accumulation. Additionally, under acidic conditions in the upper soil profile, Fe-Al oxides decomposed, forming Fe and Al complexes. Mobile Cd fractions bound to these complexes and migrated downward with percolating solutions, accumulating at the rock-soil interface. Thus, the rock-soil interface serves as a sink for most of the mobile Cd transported within the weathering soil profile.