Enrichment Mechanisms of Cadmium in Natural Manganese-Rich Nodules from Karst Soils

Natural manganese (Mn)-rich nodules effectively sequester cadmium (Cd) in soils and influence on the geochemical cycling of soil Cd, yet microscale understanding of their enrichment mechanisms remains limited. From a regional survey of 1448 rhizosphere soil-rice sample pairs in karst areas of Guangxi, China, we identified and characterized Mn-rich nodules from representative sites to investigate their role in Cd sequestration. Using chemical extractions combined with laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and X-ray photoelectron spectroscopy (XPS), we revealed that Mn oxides serve as primary Cd carriers in Mn-rich nodules, accounting for approximately 63.6% of total Cd, dominating Cd enrichment through binding mechanisms of Cd-O and Cd-OH bonds characteristic of inner-sphere surface complexation and structural incorporation. The stability of these chemical interactions was confirmed by pH-dependent experiments, showing <5.0% of total Cd release even at pH = 3.0. The high Mn(III) mass ratios (36.0%-40.0%) in nodules facilitated stable inner-sphere complexation, resulting in increased Cd retention. These study findings reveal the exceptional Cd sequestration capacity of natural Mn-rich nodules, and provide insights for the high concentrations and low availabilities of soil Cd in karst, which can aid in developing strategies for managing Cd-polluted karst soils.