An Integrated Approach for the Recovery of Sn from Used Water Adsorbents

This research examined a scenario for the recovery of a high-capacity Cr(VI) adsorbent, consisting of Sn6O4(OH)4, after reaching its operational lifetime. To accomplish this target, a sequence of processes involving the spent adsorbent’s decomposition, the separation of Sn/Cr ions, and Sn6O4(OH)4 reconstruction was investigated. Characterization of the saturated adsorbent indicated its extended oxidation to SnO2 during its use according to the occurring Cr(VI) to Cr(III) reduction mechanism, which is responsible for the loading of 19 mg Cr/g. To decompose saturated adsorbent, the optimized process involved the dissolution by HCl using a solid concentration of 10 g/L, a solid to acid mass proportion of 1:20, an increase of the temperature at 75 °C. Such conditions brought a dissolution rate of more than 95% and 92.5%, respectively, of the total Sn and Cr in the spent adsorbent. Then, separation of Cr was succeeded by the addition of hydrazine, which acts as a reducing agent for the transformation of Sn(IV) to Sn(II); the precipitation of Cr(III) at pH 3, and the reconstruction of Sn6O4(OH)4 in a second step after increasing pH to 7. The recovered adsorbent stabilized a higher percentage of Sn(II) than the initial material, which explains the improvement of the removal efficiency by 50% in the Cr(VI) adsorption capacity.