Micrometer-scale internal structure and element distribution of Fe-Mn nodules in Quaternary red earth of Eastern China

Abstract

Fe-Mn nodules are the common feature of tropical and subtropical soils and contain abundant information of pedogenic processes, palaeoenvironmental changes, and element geochemistry. The main aim of the present study was to determine the internal structure and spatial distribution of elements in the Fe-Mn nodules to better understand the 3D internal structure, enrichment, and dynamics of heavy metals in nodules and to provide more aspects to explore the possible heavy metal sequestration and pedoenvironmental implications of Fe-Mn nodules in soils. The studied Typic Plinthudult was developed on Quaternary red earths in Eastern China. The Fe-Mn nodules in the Bs horizon of soil were separated and classified into four size fractions (5–8, 3–5, 2–3, and 1–2 mm). The 3D microstructure of Fe-Mn nodules was examined by means of synchrotron radiation-based X-ray microcomputed tomography (SR-mCT), and the spatial distribution of Fe and Mn in nodules was studied by scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS). The association of heavy metals with Fe and Mn oxides in nodules was described by selective chemical dissolution, SEM/EDS, and principal component analysis. The SR-mCT images indicated that the 5–8, 3–5, and 2–3 mm nodules exhibited well-defined ring structures, while the 1–2 mm nodule exhibited homogeneous fabric. The internal microstructures of nodules could be divided into four parts: Fe-rich ring, Mn-rich ring, Fe and Mn overlapped ring, and the gap between ring structures. The Fe-Mn nodules were significantly enriched in Mn, Pb, Ni, Cu, and Zn relative to the soil matrix. In particular, the concentrations of Mn and Pb in the nodules were 150 and 90 times greater than those in the soil matrix, respectively. A clear partitioning of heavy metals between Mn and Fe oxide phases was observed in the nodules, indicating that Pb was mainly present in Fe oxides, while Ni, Cu, and Zn were mainly associated with Mn oxide phases. The SR-mCT and SEM-EDS revealed the detailed internal microstructure of the Fe-Mn nodules and geochemical dynamics of heavy metals in the soil system. The Fe-Mn nodules have very high scavenging ability in sequestrating toxic heavy metals in soils, such as Pb and Ni. The microstructure and spatial distribution of Fe and Mn in nodules reflected the cycle of alternating drying and wetting conditions and served as an important basis for inferring the pedogenic processes and pedoenvironmental conditions.