We investigated the concentration of heavy metals in the Fe-Mn nodules in soils derived from the carbonate rocks of typical karst areas in Guangxi, Southwest China. Compared with the soil background values in Guangxi, heavy metals are substantially enriched in the following order: Cd (268.13) > Pb (39.46) > Cr (11.80) > Zn (8.43) > Ni (6.16) > Cu (3.65). CaCl2 extraction results indicate that heavy metals in the nodules are substantially stable, while the proportions of the metals released to the surrounding environment are extremely low (≤ 0.003%). Moreover, pH-static leaching experiments indicate that heavy metals can hardly be dissolved into the surrounding soil environment under natural conditions (pH 6-8). However, once the soil environment is acidified, heavy metals incorporated in the nodules will gradually release into the surrounding environment, thereby causing potential ecological risks.
Keywords: CaCl2 extraction; Ecological risk; Fe–Mn nodules; Heavy metals; Karst area; pH-static leaching experiments.