Cycling of trace metals and rare earth elements (REE) in acid sulfate soils in the Plain of Reeds, Vietnam

Trace metals and rare earth elements (REE) are subject to intensive chemical cycling in acid sulfate soils. The oxidation of pyrite bearing deposits and the subsequent acidification mobilizes a variety of trace metals and REE. In this study solid phase and pore water data of three cores from the Plain of Reeds in the Mekong River Delta, Vietnam, were analyzed. Distinct authigenic enrichments of trace metals and REE were observed in the solid phase of these cores. Thermodynamic modeling was employed to elucidate the controls on trace metal and REE mobility and the governing factors for the formation of these authigenic enrichments.The formation of REE-enrichments in these settings is clearly controlled by pH and the availability of potent sorbents. Under suitable conditions trace metals such as e.g. zinc, nickel and cobalt are likely bound as trace metal monosulfides in the enrichment zones. The solid phase and pore water data and the results of a thermodynamic model suggest a possible sequential precipitation of trace metal sulfides. However, the incorporation of trace metals into iron sulfides cannot be ruled out. In the case of lead and arsenic, the formation of Pb-As-jarosite proved to be a further major sink.These insights provide a better understanding of the controls and governing factors for trace metal and REE mobility in acid sulfate soils and will therefore help to assess impacts on soil and water.