Carbonate rocks as natural buffers: Exploring their environmental impact on heavy metals in sulfide deposits

Carbonate rocks are closely related to the genesis and spatial distribution of polymetallic sulfide deposits. The natural buffering of carbonate rocks can reduce the ecological impact of heavy metals produced by mining and smelting. Ignoring the buffering effect of carbonate rocks on the heavy metals in the mine environment leads to inaccurate ecological risk assessment, wasting land resources and funds. This study investigates Cd, Zn, and Pb distribution and speciation in the water and soil-rice system in the polymetallic sulfide deposit at Daxin, Guangxi. The study aims to reveal the effects of the natural buffering of carbonate rocks on the migration and transformation of heavy metals. The results show that the water Zn and Cd concentrations decreased from 1857.0 to 0.9 mg L^-1 to 0.16 and 0.001 mg L^-1, respectively, from the mining area to 4 km downstream. The natural buffering of carbonate increases the water pH from 2.80 to 7.64, resulting in a tendency for Cd, Zn, and Pb to separate from the aqueous phase and enrich the sediments. Soil Cd content in the mining area reached 110.0 mg kg^-1 (mean value 55.88 mg kg^-1), and rice Cd seriously exceeded the maximum limit. However, the weathering of carbonate reduces the migration ability and bioavailability of Cd. Soil Cd is mainly in the Fe-Mn bound and carbonate-bound fractions, resulting in lower Cd content in downstream soils (mean value 2.73 mg kg^-1). Soil CaO, tFe₂O₃, and Mn hindered the uptake of soil Cd by rice rendering a lower exceedance of Cd in downstream rice. Therefore, this study recommends a farmland management plan under the premise of rice Cd content and integrated soil Cd content, which ensures food safety and fully utilizes farmland resources. This result provides a scientific basis for ecological risk assessment, mine environmental protection, and management in the carbonatite sulfide mine environment.