Rare earth element patterns: A tool for identifying trace metal sources during wetland soil reduction

In wetland soils, several soil phases such as Fe(III)–oxyhydroxides, organic matter (OM) or mixed Fe–OM particles can host trace metals which can be subsequently released during soil reduction. Anoxic and oxic wetland soil incubation experiments, combined with analyses of soil solutions sampled from a natural wetland during a reduction event, are used to test the possibility that rare earth elements (REE) could be used as a tool to identify the soil phases contributing to trace metal solubilization. Significant amounts of trace metals (Cu, Cr, Co, Ni and Pb) and REE are released during anoxic incubation of the wetland soil, concomitantly with the build-up of high concentrations of Fe(II) and dissolved organic matter (DOM). Rare earth element patterns obtained in the soil solution exhibit a middle rare earth elements (MREE) downward concavity. The REE pattern obtained from field samples yields the same feature as developed in the soil solution from an oxic incubation experiment at pH 7 designed to promote soil OM desorption. By contrast, significantly different REE patterns are obtained in incubation experiments designed to promote chemical reduction of Fe–oxyhydroxides in soils. The REE pattern displays a continuous REE enrichment from La to Lu. These distinct and recognizable REE signatures allow us to conclude that (i) soil organic matter is the main source of REE and trace metals during wetland soil reduction; (ii) Fe(II) is provided by the reduction of amorphous Fe(III) nanoparticles embedded within the organic matter, which do not bind REE or other trace metals in significant proportions (REE and trace elements being preferentially complexed to organic matter); and finally (iii) REE provide a reliable and powerful tool, suitable for identifying trace metal sources during wetland soil reduction.