Impact of controlled redox conditions on nickel in a serpentine soil

Abstract

Serpentine soils exist in many regions around the world; they are naturally enriched with nickel (Ni). An adequate understanding of soil processes determining Ni solubility is a special need particularly since less research has been addressed to Ni behavior under dynamic and controlled redox conditions. Our aim was (1) to characterize the properties of a serpentine soil and (2) to determine the impact of predefined redox windows on the mobility and dynamics of Ni in a serpentine soil. A soil with high geogenic Ni concentrations from Serbia was incubated using an automated biogeochemical microcosm system. Redox windows were created from reducing to oxidizing conditions in predefined steps of approximately 100 mV. Three microcosms were used as replicates; redox potential (E-H) and pH were automatically monitored every 10 min. The samples were centrifuged, and the supernatants were immediately filtered under N-2 atmosphere and analyzed for soluble Ni, iron (Fe), manganese (Mn), dissolved organic carbon (DOC), and sulfate (SO (4) (2-) ). X-ray diffraction was performed to assess mineral composition of the soil grain-size fractions. Nickel binding forms in the bulk soil were determined by the sequential extraction according to Tessier et al. (1979). Total Ni concentration in the bulk soil was 550 mg kg(-1). Quartz, chlorite, serpentine minerals, and secondary minerals were the prevalent minerals. The residual fraction contained the most Ni (91.3%). Nickel percentages of the fractions: Fe/Mn oxides, organic matter, carbonate, and exchangeable were low. Soluble Ni concentrations varied in the range 77-166 mu g L-1, showing a linear decrease with increasing E-H. Soluble Ni was positive correlated with Fe, Mn, and DOC and inversely correlated with SO (4) (2-) . Dissolution and precipitation of Fe/Mn oxides, organic matter transformations, and adsorption on solids are important processes controlling the Ni solubility during redox change. Nickel concentrations at definite redox windows were in the same order of magnitude as the exchangeable Ni determined by the sequential extraction procedure. Our study demonstrates that considerable amounts of Ni can be mobilized during low E-H despite a high Ni retention capacity of the soil. The sequential extraction might provide a reliable estimation of the potential mobile Ni under dynamic redox alterations. The interactions of DOC, pH, Fe, and Mn (hydr)oxides are controlling the dynamics of soluble Ni under changing E-H conditions. It is important to verify the detected dynamics at various scales and in other serpentine soils in the future.