Combination of interfacial reduction of hexavalent chromium and trivalent chromium immobilization on tin-functionalized hydroxyapatite materials

We present an ecofriendly hydroxyapatite (HAP) material functionalized with tin (Sn/HAP) for an efficient interfacial reduction of Cr(VI) to Cr(III). Tin was deposited on HAP at different concentrations (from 0.2 to 1.2 mmol/g) using colloidal or clear acid solutions of SnCl2. The morphological and structural properties of fresh and used Sn/HAP samples were determined (N2 adsorption-desorption, transmission electron microscopy techniques, XRPD, XPS). Tests were performed under various conditions: at different pH 3-7, inert or oxidant atmosphere, sample aging (up to 15 days). Sn/HAP samples prepared using colloidal acid solutions revealed the best performances in the reduction of Cr(VI) to Cr(III), carried out under mild conditions (40 °C in acidified solution) with initial concentration of Cr(VI) in the range from 25 to 50 ppm. Best removal of Cr(VI) (10 mg/g) was obtained by using a Sn-concentration of 0.65 mmol/g with complete adsorption of the formed Cr(III) at HAP surface. This finding was associated with higher Sn-dispersion (surface Sn, 9.4 at.%) compared to samples prepared from clear solutions (surface, Sn 7.7 at.%), as evidenced by HAADF-STEM/EDX and XPS analyses. Remarkably, tin was tightly retained on the HAP surface under reaction conditions (0.7% leaching), confirming the occurrence of Cr(VI) reduction at solid-liquid interphase.