Biogeochemistry of Ni and Pb in a periodically flooded arable soil: Fractionation and redox-induced (im)mobilization

Abstract

The redox-induced (im)mobilization of nickel (Ni) and lead (Pb) under pre-definite redox conditions and their binding forms were studied in a periodically flooded, slightly acidic arable soil enriched with serpentine minerals at the Velika Morava River valley, Serbia. The total contents of Ni and Pb were 152 and 109 mg kg−1, respectively. Geochemical fractionation of Ni, combined with mineralogical analysis, confirmed its geogenic origin in the soil. Potentially mobile fractions were the dominating binding forms of Pb; thus, indicating anthropogenic sources as prevailing. Risk assessment indicated a low risk of Ni and Pb transfer from soil to other environmental constituents. However, the results imply that geogenic metals might pose higher environmental risk than those from anthropogenic origin, in dependence of their total concentrations and contents in the specific solid-phase fractions. Flooding of the soil was simulated in an automated biogeochemical microcosm system, which allows a control and a continuous measurements of redox potential (EH) and pH. Subsequently, the EH was increased in steps of approximately 100 mV from anoxic to oxic conditions. Concurrently, the concentrations of soluble Ni, Pb, iron (Fe), manganese (Mn), dissolved organic carbon (DOC), and sulfates were measured. The EH was brought from low to high values (−220 to 520 mV) and correlated negative with soluble Ni, Pb, Fe, Mn and DOC. Soluble Ni ranged from 125 to 228 μg l−1 while Pb ranged from 3.0 to 21.4 μg l−1. Concentrations of both metals in solution were high at low EH and decreased with increasing EH. Nickel immobilization may be attributed to sorption to or co-precipitation with re-oxidized Fe-Mn (hydr)oxides, whereas Pb, in addition, might be immobilized via precipitation with inorganic ligands, such as carbonates and phosphates. The results imply that Ni and Pb solubility might also be related to the formation of metal-DOC complexes. The detected dynamic and mechanisms might be useful in providing critical information for assessing the potential environmental risk and creating appropriate environmental management strategies for agricultural areas enriched with Ni and Pb.