Antimony isotope fractionation during Sb(V) and Sb(III) adsorption on secondary Fe-minerals (schwertmannite, ferrihydrite) typical of mine waters

Abstract

The application of antimony (Sb) isotopes as tracers for Sb environmental cycling is currently limited. Indeed, there is a lack of knowledge of isotope fractionation factors associated with key (bio-) geochemical processes controlling its behaviour in surface environments. This study investigated the equilibrium isotope fractionation generated by Sb(V) and Sb(III) sorption on two iron minerals typical of acid mine drainage (AMD) impacted streams, ferrihydrite and schwertmannite, under controlled conditions of pH and solid to liquid ratio. Sorption behaviour and Sb isotope fractionation were similar for the different mineral phases and Sb oxidation degrees, with fractionation factors Δ123Sbsolid-solution of −0.25 ± 0.08 ‰ for Sb(III) and −0.34 ± 0.08 ‰ for Sb(V) adsorbed on ferrihydrite and −0.36 ± 0.06 ‰ for Sb(III) and −0.27 ± 0.03 ‰ for Sb(V) adsorbed on schwertmannite. The pH and initial Fe:Sb ratio did not significantly affect the Δ123Sbsolid-solution value under the experimental conditions. The light 121Sb isotope was preferentially adsorbed on the mineral phases, following an equilibrium closed system between dissolved and adsorbed Sb species. This fractionation may be related to the apparition of iron as the second closest neighbour which distorts the Sb(OH)3 or Sb(OH)6- atomic polyhedron. This study confirms that Sb equilibrium isotope fractionation occurs during sorption of Sb(III) and Sb(V) onto secondary iron minerals and suggests that the pH and redox of Sb do not exert significant effect. pH and redox conditions are important parameters which control Sb mobility in AMD streams, therefore, the study provides data for interpreting Sb isotope signatures in mine waters.