Variable Ni isotope fractionation between Fe-oxyhydroxides and implications for the use of Ni isotopes as geochemical tracers

Nickel (Ni) isotopes have recently emerged as a new biogeochemical tracer in marine environments, but our understanding of the mechanisms of Ni isotope fractionation in natural systems with regards to its fractionation by mineral surfaces is incomplete. This study aims to provide experimental constraints on Ni isotope fractionation during adsorption to goethite and 2-line ferrihydrite, two Fe minerals that vary in terms of distinct crystalline properties. We conducted two types of adsorption experiments: one with variable pH (5.0 to 8.0) and constant initial Ni concentration, one at a constant pH of 7.7 and variable initial Ni concentrations. Isotopic measurements were made on both the solid phase and the supernatant solutions in order to determine the Ni isotope fractionation factors (Δ^60/58Ni_min-aq = δ^60/58Ni_min − δ^60/58Ni_aq) between the mineral and aqueous phases. Our results show preferential adsorption of lighter Ni isotopes during adsorption of Ni to Fe oxyhydroxides presumably under conditions of near equilibrium conditions. Adsorption to goethite generates the greatest fractionation, with Δ^60/58Ni_min-aq = −0.77 ± 0.23‰ (n = 14, 2sd), whereas adsorption to 2-line ferrihydrite samples yield Δ^60/58Ni_min-aq = −0.35 ± 0.08‰ (n = 16, 2sd). Using Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, we found that Ni forms an inner-sphere complex and that its coordination environment does not vary significantly with pH nor with surface loading. In addition, we found no evidence of Ni incorporation into the mineral. We suggest that the more than two-fold increase in Ni isotope fractionation in goethite relative to 2-line ferrihydrite is due to the lower Ni-Fe coordination number in the second shell, which results in the formation of a weaker surface complex and thus favors the adsorption of lighter Ni isotopes. These results show that Ni isotope fractionation during sorption by Fe-oxyhydroxides is dependent on mineralogy, which has important implications for the use of Ni isotopes as environmental tracers and the interpretation of their record in sedimentary rocks.