Heavy Metal Enrichment in Ferromanganese Nodules and Soil Ecological Risk Assessment in the Karst Area with High Geological Background

Ferromanganese nodules exhibit strong capacity for heavy metal immobilization and are thus a crucial contributor to the high geological background in karst areas. Heavy metals sequestered within ferromanganese nodules display low bioavailability, which leads to an overestimation of ecological risk in areas with high geological backgrounds. However, limited attention is given to the enrichment process of heavy metals and the overestimated ecological risk of ferromanganese nodules in karst areas. Here, the surface soils and ferromanganese nodules are collected from a region dominated by carbonate and clastic rocks to investigate the enrichment of heavy metals (Cr, Ni, Cu, Zn, As, Cd, Pb, and Hg), the influence of parent rock, and their ecological implications in Northeastern Guangxi. Results show the following findings: (1) Heavy metals are enriched in ferromanganese nodules, with Cr and As correlating with Fe, and Cd and Pb correlating with Mn. (2) The spatial distribution of each element closely matches parent rock lithology, and high heavy-metal concentrations of both soils and ferromanganese nodules occur in carbonate areas. (3) The proportion of contaminated samples generally decreases after excluding the contribution of ferromanganese nodules, leading to a decline in risk level in carbonate areas, while clastic areas exhibit minimal change.