Enhanced removal of antimony by acid birnessite with doped iron ions: Companied by the structural transformation.

In the environment, antimony as a priority control pollutant is mainly associated with Fe- or Mn- related minerals. In this work, acid birnessite (AB) doped with iron was synthesized as the artificial mineral to study the adsorption and oxidation of antimony. As compared to the pristine birnessite, Fe-doping birnessites show a markedly enhanced removal efficiency for both Sb(III) and Sb(V), where 10% Fe exhibited an excellent adsorption capacity of 759 mg/g Sb(III). The removal of Sb(III) clearly underwent a novel kinetic process of adsorption-desorption- (re-adsorption). By monitoring the kinetics with XRD, XPS, and IR, it is demonstrated that the three-stage kinetics were attributed to the strong interaction between Sb(III) and birnessite, including Sb(III) oxidation, followed by destruction of birnessite and then phase transformation into vernadite. Furthermore, the increase of iron content doped into birnessite enhanced the rate of its phase transition, which led to an increased adsorption of the oxidized antimony on the surface of vernadite by substituting iron and manganese associated with hydroxyl group. This work suggested that the strong interactions between heavy metal ions and mineral particles, more than adsorption, are critical to the transformation, mobility and biotoxicity of antimony in nature.