Fixation of trace elements - A positive side effect of nitrate removal from contaminated groundwater

This study presents the binding of trace elements, which can be a positive side effect of adding organic C to remediate nitrate (NO₃^-) contamination in groundwater. The processes responsible for trace element removal are examined via circulation column experiments, with two different types of unconsolidated sandy sediments from aquifers in Denmark (Quaternary) and Germany (Cretaceous). Selected trace elements (As, Co, Ni, V and Zn) were injected into the nitrate-polluted water of the column experiments at a concentration of around 100 μg·L^-1. By addition of ethanol as electron donor, a decrease in trace element concentrations (e.g., Ni 50.2 to 76.2 % and Zn 92.2 to 94.0 %) was observed in both sediments together with the expected denitrification. A six-step sequential extraction procedure was carried out to identify the sinks of the trace elements. Results show sorption of trace elements, but also binding by newly formed oxyhydroxides extractable by oxalate. Due to pH differences resulting from the properties of the sediments (Haltern Formation, Germany: pH approx. 5.5 and meltwater deposits, Denmark: pH approx. 7.8), carbonate precipitation was promoted in the meltwater deposits, in which trace elements likewise bind. Saturation indices from reactive transport modeling support these conclusions. The effectivity of denitrification differs between the two sediments, which is presumably likewise due to the pH and the existing microbiology. Sulfate (SO₄^2-) reduction and binding of trace elements by sulfides was not observed. Nevertheless, a number of emerging questions open up the need for further research. As the trace element concentrations do not increase at the end of the experiments, this method seems to be a positive side effect from mitigating NO₃^- concentrations in groundwater by addition of organic C.