Contamination of Floodplain Soils along the Wupper River, Germany, with As, Co, Cu, Ni, Sb, and Zn and the Impact of Pre-definite Redox Variations on the Mobility of These Elements

Abstract

This study demonstrates that floodplain soils of the River Wupper, Germany, are seriously contaminated with metal(loid)s. We used an automated biogeochemical microcosm system allowing controlled variation of redox potential (EH) to assess the impact of pre-definite redox conditions on the dynamics of arsenic (As), cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), antimony (Sb), and zinc (Zn) in homogenized soil material taken from an acid floodplain soil. The concentrations of Co, Cu, Mn, Ni, Sb, and Zn in soil solution were low at low EH, possibly due to the precipitation of metal sulfides, and increased with rising EH, presumably caused by their association with dissolved organic carbon (DOC). A significant positive correlation between metal/DOC-ratio and EH indicated that the binding of the metals to DOC shifted from stronger to weaker when EH rose. Decreasing As concentrations with increasing EH in soil solution indicated co-precipitation with Fe(hydr)oxides and/or oxidation of more soluble As(III) to less soluble As(V) during oxidation. The other studied elements seemed not to co-precipitate with newly formed Fe(hydr)oxides when EH rose, possibly due to the prevailing low pH. In the future, the specific role of DOC and sulfur chemistry on metal(loid) dynamics should be elucidated more fully, and similar studies should be conducted with additional frequently flooded soils worldwide to verify these results.