Addressing the issue of surface mechanisms and competitive effects in Cr(VI) reductive-adsorption on tin-hydroxyapatite in the presence of co-ions

Our group recently proposed an innovative sustainable reductant-adsorbent material, tin(II)-hydroxyapatite (Sn/HAP, ca. 10 wt% Sn) for the interfacial Cr(VI) reductive adsorption process. In this study, Cr(VI) removal capacity was evaluated in multi-component solutions containing representative background ions (i.e., CaCl2, Ca(NO3)2, MgSO4, Na2SO4, Fe(NO3)3, AlCl3, Zn(NO3)2, or Mn(NO3)2). Sn/HAP was able to reduce Cr(VI) with complete Cr3+ adsorption on HAP surface, except in the presence of Fe3+ and Al3+ ions. Some metal ions co-existing in solution, such as Fe3+, Al3+, Zn2+, and Mn2+, were also adsorbed on HAP surface. Reuse experiments of the Sn/HAP sample, up to 7 runs, resulted in a total amount of reduced Cr(VI) of ca. 15–18 mg g−1. Fast kinetics of Cr(VI) reductive adsorption at 25 °C in a multi-metal component solution was observed. The pseudo-second order model was in excellent agreement with the experimental kinetic data, leading to a rate constant (k25°C) value of ca. 30 M−1 s−1. The collection of adsorption isotherms of Cr3+ and Fe3+, together with TEM–EDX analysis permitted the unveiling of competitive adsorption phenomena between metal ions. The obtained results demonstrate that Sn/HAP could be an efficient material for the removal of hexavalent chromium in aqueous solutions containing high concentrations of inorganic impurities.