Environmental Geochemical Behavior of Soil Ferromanganese Nodules: A Review

BRIEF REPORTSoil ferromanganese nodules (FMNs) are secondary products of pedogenic processes that differ markedly from the surrounding soil matrix in morphology, microstructure, and geochemical composition[1]. Their genesis is governed by parent material, variations in edaphic conditions (e.g., moisture, redox potential, pH), and biological factors[2]. These FMNs are not only important pedogenic features, rich in information about paleoclimatic conditions and pedogenic environments during their formation but also exhibit a strong enrichment capacity for various trace elements, particularly heavy metals[3-4]. They are considered key to elucidating the apparent paradox of “high total content” versus “low bioavailability” of heavy metals in soils of high geological background areas[11-12].　　Currently, knowledge regarding the formation mechanisms, environmental behaviors, and microscale enrichment processes of FMNs remains limited. Therefore, the complex pedogenic processes governing FMNs genesis and the relative contributions of various controlling factors are systematically synthesized in this review. Adopting the ecological functions of FMNs as a conceptual framework, their role in heavy metal sequestration and environmental modulation is elucidated, their implications for soil health is revealed, novel insights for soil risk assessment in geochemical high-background regions is provided, and priority directions for future research are identified.　　1. Formation mechanisms, evolution, and paleoenvironmental significance of FMNs　　The formation of FMNs results from the combined effects of physical, chemical, and biological factors. Reduced Mn and Fe ions, which are highly soluble in soil solution, can migrate within the soil profile. During seasonal droughts or drops in the water table, the soil becomes oxidizing, and soluble Fe2+ and Mn2+ are oxidized to insoluble Fe3+ and Mn4+ oxides, which precipitate. Conversely, during rainy seasons or waterlogged periods, the soil turns reducing, leading to the reductive dissolution and migration of some Fe-Mn oxides[17-18]. This periodic cycle of “dissolution−migration−oxidation−precipitation” promotes the continuous coagulation and growth of Fe-Mn oxides around a nucleus (e.g., mineral grains, organic fragments, micropores), ultimately forming FMNs[19-20].　　(1) Abiotic factors. The pedogenic development of FMNs within undisturbed soil profiles and their associated biogeochemical cycling are governed by diverse biotic and abiotic factors, notably pH, redox potential (Eh), biological activity, and alternating wetting and drying cycles[2,15,19,21-25]. Among these, Eh (specifically, alternating redox regimes) constitutes the primary driving mechanism, governing redox states and thereby inducing encrustation, metal precipitation and dissolution, sequestration, migration, and the development of concentric ring structures during FMNs biogenesis[2]. The chemical composition and weathering characteristics of the parent material fundamentally govern the availability of precursor elements for FMNs formation. Consequently, variations in pedogenic environments and/or soil development stages result in considerable morphological, structural, and compositional heterogeneity of FMNs across different soil types[1]. Mineralogically, FMNs comprise polymineralic assemblages. Phyllosilicate minerals within these FMNs are predominantly inherited from the surrounding soil matrix[34]. The principal Fe-Mn oxide phases comprise goethite, hematite, birnessite, lithiophorite, and todorokite[35-36]. The specific mineral assemblage is governed by soil pH, Eh, and parent material composition. For example, lithiophorite and birnessite are favored under acidic conditions, whereas todorokite predominates in neutral to alkaline environments[4,37].　　(2) Biotic factor. Microorganisms have been demonstrated to play a pivotal role in FMNs formation[39]. Specifically, Mn-oxidizing bacteria contribute to nodular genesis via biomineralization processes, encompassing biologically induced mineralization (BIM) and biologically controlled mineralization (BCM). For instance, Bacillus spp. facilitate the accumulation of Fe and Mn ions on their cellular surfaces, thereby inducing the nucleation and growth of mineral phases[40]. Furthermore, processes such as root radial oxygen loss can generate micro-scale redox gradients, thereby driving localized Fe and Mn precipitation.　　(3) Paleoenvironmental significance. A defining feature of FMNs is the presence of concentric ring structures. These concentric structures robustly archive pedogenic environmental conditions, thereby serving as reliable proxies for reconstructing paleoclimatic regimes and pedogenic environments. Analysis of banding thickness, elemental ratios (e.g., Fe/Mn), isotopic compositions, and mineralogical phases enables the reconstruction of paleoclimatic conditions (frequency and intensity of wetting-drying cycles), paleohydrological regimes (water table fluctuations), and the chronology of pedogenesis[18-19,24].　　2. Ecological effects of FMNs and mechanisms of heavy metal enrichment　　(1) Ecological impacts. FMNs exhibit remarkable enrichment of heavy metal elements[38,55-56]. Their core ecological effect lies in their ability to significantly reduce the bioavailability of the immobilized heavy metals. On one hand, FMNs can alter the physicochemical characteristics or chemical speciation of heavy metals, thereby changing their potential mobility and bioavailability[59-60]. On the other hand, heavy metals fixed within FMNs exhibit high stability[61-66]. These findings challenge traditional paradigms of soil environmental risk management. The applicability of soil quality standards based solely on total heavy metal content is questionable in karst regions. For a more scientific and accurate assessment of ecological risks in high geological background areas, the portion of heavy metals fixed within FMNs, which has low bioactivity, should be considered[28].　　(2) Sequestration mechanisms. Application of advanced micro-analytical techniques, including X-ray absorption fine structure (XAFS) spectroscopy, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), X-ray diffraction (XRD), and Raman spectroscopy[76-80] , has revealed that enrichment mechanisms in FMNs comprise: (1) direct isomorphous substitution via surface coordination; (2) oxidative substitution; (3) elemental partitioning between δ-MnO2 and FeOOH; (4) combined enrichment involving Mn-phase oxidative sorption and Fe-phase structural sorption; and (5) sorption of specific elements by FeOOH[81].　　Adsorption and complexation: The high metal content of FMNs is manifested in their micro-scale sorption mechanisms and mineralogical configurations. Specifically, metal enrichment at the micro-scale denotes the accumulation of metals within the layered or tunnel components of Fe-Mn coatings and nodules at the atomic level[81]. Manganese oxides (e.g., birnessite, lithiophorite, and todorokite) exhibit substantial sorption capacities for heavy metal cations, arising from their high specific surface areas, abundant surface hydroxyl groups, and permanent structural charges[82-83].　　Coprecipitation (Chelation mechanism): Coprecipitation within FMNs constitutes a complex authigenic process governed by diverse physical, chemical, and biological factors, representing a more stable and long-term immobilization mechanism. Under favorable redox potentials and pH conditions conducive to the presence of potent adsorbents and oxidative carriers, certain metal ions adsorbed onto MnO2 surfaces may undergo further oxidation to form less soluble oxides. Alternatively, other metal ions, by virtue of ionic radii and charges comparable to those of Mn4+ or Fe3+, may be directly incorporated into crystal lattices via isomorphous substitution, thereby becoming permanently sequestered within the nodule matrix[48].　　Synergistic effects: The partitioning of elements between manganese oxides and iron oxy/hydroxide phases is governed by their aqueous speciation and the surface charge characteristics of oxide colloids or particles. Although manganese oxides predominate, iron oxy/hydroxides within nodules contribute significantly[85-86]. These iron oxides also exhibit adsorption capacities and, through the formation of “core-shell” structures or mixed-phase aggregates with manganese oxides, provide additional adsorption sites and more complex microenvironments, thereby enhancing the sequestration and immobilization of heavy metals[15,87].　　3. Influence of FMNs on the migration and transformation of characteristic elements like Cr, As, and P　　(1) Impacts on redox-sensitive elements. The environmental functions of FMNs extend beyond heavy metal cation immobilization; they also significantly influence the mobility and redox speciation of redox-sensitive elements. Specifically, Mn oxides within FMNs can oxidize Cr(Ⅲ) to Cr(Ⅵ) and As(Ⅲ) to As(Ⅴ), thereby altering their geochemical behavior in soils[59-60,91-92]. Moreover, nodules exhibit substantial sorption capacities for oxyanions, including As(Ⅴ), Cr(Ⅵ), and phosphate (PO43−)[27,95].　　(2) The role of FMNs is dualistic. FMNs may function as both “sinks” and potential “sources”. Under drastic changes in environmental conditions, such as extreme soil acidification, prolonged waterlogging that induces strongly reducing conditions, or the introduction of substantial organic chelators, partial dissolution of nodules may occur, which releases sequestered heavy metals and nutrient elements back into the environment, thereby causing secondary contamination[14,55].　　4. Future perspectives　　Soil FMNs have garnered considerable attention for their pivotal role in the biogeochemical cycling of heavy metals. To advance the field, future research should prioritize the following directions: (1) elucidating FMNs formation rates and growth kinetics, along with their real-time coupling with heavy metal immobilization processes, and quantifying the pathways and rates of key geochemical reactions; (2) elucidating the community structure and core functional genes of functional microorganisms, along with their biogenic driving mechanisms in FMNs biomineralization and heavy metal speciation transformation; (3) revealing critical environmental thresholds for FMNs stability and quantifying secondary heavy metal release fluxes under extreme conditions and establishing dynamic early-warning models for risk assessment; (4) developing mechanistic models for quantitative prediction of FMNs formation and heavy metal immobilization capacity across varying environmental conditions, through the integration of physical, chemical, and biological processes.