Environmental Geochemical Behavior of Soil Ferromanganese Nodules: Research Progress

Ferromanganese nodules (FMNs) are by-products of soil formation processes, widely developed in karst geological areas with high background, and are the core to explain the phenomenon of “high total content and low bioavailability” of heavy metals in soils in karst regions. However, there is still a lack of understanding of their micro-mechanisms; the lack of systematic correlation between the dynamic formation and evolution of nodules and their environmental behaviors, as well as the unclear summary of the differences and regularities of nodule geochemical behaviors under different epigenetic environments. This article systematically reviews the latest progress in soil ferromanganese nodule research, including: (1) The formation mechanism, evolutionary process, and paleoenvironmental significance of nodules are summarized: Different physical, chemical, and biological factors jointly control the regular growth of nodules, with the most core driving factor being the redox potential. By comparing the geochemical characteristics of nodules under different epigenetic environments, a possible influence path model is proposed; (2) The ecological effects of nodules and the research on the enrichment mechanisms of heavy metals, as well as the experimental techniques, are summarized: The contents of Cd, Pb, Ni, and other elements in ferromanganese nodules can reach dozens of times that of the corresponding soil, and the maximum release amount in pH leaching experiments is only 17% (As). By extending X-ray absorption fine structure spectroscopy (EXAFS), X-ray absorption near edge structure spectroscopy (XANES), laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS), and other techniques, it is confirmed that nodules strongly fix heavy metals through surface complexation, specific adsorption, and co-precipitation. Nodules can affect the migration and transformation of characteristic elements such as arsenic, chromium, and phosphorus through oxidation or adsorption. Future research urgently needs to establish a dynamic risk warning model based on environmental parameters, strengthen interdisciplinary integration, incorporate the research results of nodule environmental effects into soil environmental quality standards and risk management practices, achieve refined management and safe utilization of land resources, and provide scientific basis for the precise evaluation of soil risks in geological high background areas and the safe utilization of land.