Physical speciation of trace metals in Fe–Mn concretions from a rendzic lithosol developed on Sinemurian limestones (France)

Because of their high content in heavy metals, Fe–Mn concretions present in soils can be a source of release of trace metals in the environment. Metal-rich concretions were isolated from a top horizon of a Rendzic Lithosol developed on limestones in France. The distribution of metallic elements (Cr, Co, Ni, Cu, Zn, Pb) was studied at different scales: field sampling, isolated concretions, and individual particles. Methods of investigation combined bulk chemical analysis, optical microscopy, X-ray diffraction (XRD) and microanalysis using scanning and transmission electron microscopes. The concretions are constituted by concentric rings and contain quartz, feldspars, kaolinite, mica, goethite, and Ti oxide as crystalline phases. A strong heterogeneity in the chemical composition is evidenced at all different scales. The total composition of different populations of Fe–Mn concretions depends on field sampling. Metals are detected at the micrometer and nanometer scales with variable frequencies, occurrences reflecting the abundance of metal-bearing components and the size of metal clusters. Statistical treatments of microanalytical data show marked trends. A strong positive correlation of Mn (and to a lesser extent of Ca) with Ni, Zn, (and to a lesser extent with Co and Cu) indicates trace metals to be associated with poorly crystallized Mn–Ca-rich areas. The correlation of Pb with Fe and P suggests its incorporation in an Fe-rich phosphate component. No correlation of metals with well crystallized silicates, Fe-sesquioxides or sulfate minerals is established. Cr was diffusely distributed.