Thiol-functionalized TiO2 microspheres and Fe(0) nanoparticle-loaded polymeric hydrogel for the selective recovery of Cr(iii) and Cr(vi) from the aquatic medium

Abstract

The selective recovery of different oxidation states of chromium from the aquatic medium is considered as reconciliation between fast growing industrialization and clean water demand. Herein, we report strategically designed functionalized nanoparticle-loaded polymeric nanohybrid material as a benchmark chromium sorbent. A facile synthesis of TiO₂ core–shell microspheres were carried out and subsequently the core removed and the hollow shell was decorated with thiol (–SH) functional groups. Eventually, these microspheres were loaded in alginate to form beads [SH-TiO₂-MS-Ca-Alg]. In addition, a polymeric hydrogel material Fe(0) NP mixed with thiol-functionalized TiO₂ microsphere encapsulated in alginate [Fe(0)-SH-TiO₂-MS-Ca-Alg] was also prepared. Under different ambient conditions, Fe(0)-loaded bead was selective toward Cr(VI) and pristine bead was selective toward Cr(III). The role of Fe(0) as a reducing agent for the better recovery of Cr(VI) is discussed in detail. The Fe(0)-SH-TiO₂-MS-Ca-Alg bead offers selectivity for both Cr(VI) and Cr(III) together in a single prescription, which is a significant breakthrough. The experimental observations were validated using density functional theory (DFT) calculations, which unveil the preferred binding site of Cr and structural transformation due to their sorption. The developed materials achieved record sorption capacities of 231.06 mg g⁻¹ and 132.22 mg g⁻¹ for Cr(III) and Cr(VI), respectively. Based on the structural evidence derived from both experimental observations and theoretical studies, a probable mechanistic pathway governing the interaction of Cr(III) and Cr(VI) species with SH-TiO₂MS and Fe(0)-SH-TiO₂-MS is proposed.