Micromorphology, mineralogy, and geochemistry of ferromanganese nodules in tropical soils and their impacts on heavy metal sequestration

Abstract

Ferromanganese nodules (FMNs) are important products of soil-forming processes that can reflect the historical pedogenic environment and affect the elemental cycle in the soil. Despite extensive investigations on the morphology, microstructure, and components of FMNs, non-destructive observation of the 3D microstructure and elemental distribution feature of FMNs remains difficult, owing to the limitations of conventional measurement techniques and instrumental accuracy. In this study, the micromorphological, mineralogical and geochemical characteristics of FMNs in basalt-derived soils in tropical China were examined using different techniques, including scanning electron microscopy, X-ray diffraction, chemical extraction, and synchrotron radiation-based X-ray microtomography and microfluorescence mapping. Mn and Fe in FMNs were found to be inversely distributed in concentric rings, with Mn mainly enriched in the interior and Fe mainly distributed at the edge of the nodule. A line scan of the cross-section of FMNs revealed symmetrical distributions of the peak Fe and Mn contents and Fe:Mn ratios, indicating alternating dry/wet conditions and oxidation/reduction reactions during the formation of FMNs. FMNs were significantly enriched in P and various heavy metals, including Ba, Co, Cu, and Ni. The enrichment factors of P, Ba, Co, Cu, and Ni were in the range of 1.20–4.68, 8.34–27.12, 12.92–24.74, 2.51–3.55, and 2.75–3.58, respectively, reflecting the potential of FMNs to influence the soil nutrient cycle and heavy metal sequestration. Factor analysis further verified the adsorption of heavy metals by FMNs and indicated that the formation of FMNs was accompanied by an increase in Mn-bearing mineral and a decrease in phyllosilicate. Our study demonstrates that FMNs provide important information for understanding changes in pedogenic environment and heavy metal behavior in tropical soils.