Optimization of tin oxyhydroxide-decorated biochar for improved hexavalent chromium uptake from drinking water

Abstract

A new class of inorganic/biobased nanocomposites is introduced for the efficient removal of hexavalent chromium from drinking water in compliance to the new concentration levels described by European Union legislation as safe (25 μg/L). Studied nanocomposites are based on the combination of Sn(II) oxy-hydroxides which serve as a reductant of Cr(VI) to Cr(III), and on biochar, derived by residual hemp stem which provides a high specific surface area substrate. Nanoadsorbents consist of an amorphous biochar network decorated by nanostructured Sn oxy-hydroxide identified as Sn6O4(OH)4 resulted after the partial decomposition of Sn21Cl16(OH)14O6 which takes place during wet blending. Importantly, all synthesis steps were designed according to the principles of sustainable chemistry for low cost and minimum toxicity of reagents, reuse of residuals and low energy consumption. For instance, production of biochar was optimized to receive sufficient specific surface area while minimizing thermal energy consumption. On the other hand, Sn oxy-hydroxide is prepared by the aqueous precipitation of a non-toxic tin salt precursor under acidic conditions followed by the separation of solid to recover used acids in the next batch. Evaluation of adsorption performance demonstrated that the nanocomposite loaded with 20%wt. of Sn oxy-hydroxide succeeds an increase of the obtained adsorption capacity, referring to residual concentration 25 μg/L, by three times (21.6 mg/gSn) in comparison to the pure Sn oxy-hydroxide adsorbent with respect to the Sn active phase content (6.9 mg/gSn). This approach aims to provide a new way to produce low cost Cr(VI) adsorbents contributing to the exploitation of biomass residuals towards added-value products but also to the significant decrease of required inorganic active adsorbent quantities and corresponding reagents to achieve the same adsorption capacity.