Nickel in a serpentine-enriched Fluvisol: Redox affected dynamics and binding forms

We determined redox-induced (im)mobilization of geogenic nickel (Ni) as well as binding forms of Ni in a Fluvisol at the River Velika Morava valley (Serbia), enriched with serpentine minerals. The selected site is representative for intensive agricultural land use in the area and susceptible to dynamic redox conditions due to periodical flooding. A seven-step sequential extraction, grain-size and light-liquid separation as well as mineralogical analyses were used, first, to assess binding forms of Ni and second, to determine relationships between grain-size fractions, abundance of heavy density minerals, and Ni concentration in the bulk soil. The sequential extraction revealed that the majority of Ni was in the residual fraction, followed by organic matter and Fe oxides. Fine grain-size fraction (<63μm) was the major location of accumulation of Ni in the soil. Minerals which are characteristic for serpentine soils such as serpentine, spinels, hematite, and magnetite were found in silt and in the heavy density fraction (>2.9gml⁻¹) of the medium and fine sand. However, the light minerals quartz, chlorite, micas, and secondary clay minerals dominate the soil mineral composition. Thus, total Ni concentration in the soil is derived from the content of Ni-bearing minerals and diluted by the minerals which are low in Ni.We simulated flooding using an automated biogeochemical microcosm system and determined the release dynamics of Ni at controlled redox potentials (EH) in soil slurries. Pre-defined redox-windows were systematically created in steps of approximately 100mV from reducing to oxidizing conditions while EH and pH were continuously monitored. In parallel, the release dynamics of soluble nickel (Ni), iron (Fe), magnesium (Mg), manganese (Mn), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and sulfate (SO4²⁻) were measured at each EH-window. Our results highlighted that geogenic Ni can be mobilized to a considerable amount during low EH, while elevating EH from reducing to oxidizing conditions generated an immobilization Ni. We suggested that mobilization of Ni has been primarily affected by formation of Ni–DOC complexes at low EH, whereas Ni seems to be immobilized as a result of formation of Fe/Mn (hydro)oxides and the linked co-precipitation of Ni during oxidation. Factor analysis (FA) as multivariate statistical method explained 85.08% of the variance (67.89% and 17.19% component Nos. 1 and 2, respectively). The FA reveals that soluble Ni, Fe, DOC, Mn, and Mg were clustered in one group which indicate that the combined effect of DOC together with the chemistry of Fe, Mn, and Mg might be linked to the redox-induced release dynamics of Ni. The practical perspective of the study was to draw attention to dynamics of soluble Ni in fluctuating conditions for a better ecological risk assessment of floodplain sites under agricultural use. Nevertheless, similar studies should be conducted with further serpentine soils from various sites world-wide to verify the detected dynamics and processes.