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., CaCl 2 , Ca(NO 3 ) 2 , MgSO 4 , Na 2 SO 4 , Fe(NO 3 ) 3 , AlCl 3 , Zn(NO 3 ) 2 , or Mn(NO 3 ) 2 ). Sn/HAP was able to reduce Cr(VI) with complete Cr 3+ adsorption on HAP surface, except in the presence of Fe 3+ and Al 3+ ions. Some metal ions co-existing in solution, such as Fe 3+ , Al 3+ , Zn 2+ , and Mn 2+ , 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 (k 25°C ) value of ca. 30 M -1  s -1 . The collection of adsorption isotherms of Cr 3+ and Fe 3+ , 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.