Three Ursolic Acid-Based Amphipathic Supramolecular Hydrogels for Highly Efficient and Reversible Removal of Chromium(VI)

Hexavalent chromium (Cr(VI)) is an extremely poisonous heavy metal ion that poses a major risk to both human health and the environment. In this study, three types of amphipathic supramolecular hydrogels (C2-MUP, C2-MUBP, and C4-MUP) were prepared using ursolic acid (UA) as a substrate and covalently modified with pyridine (Pyr). The synergistic reduction and adsorption performances of these three hydrogels for the removal of Cr(VI) from water were then evaluated. The findings demonstrate that electrostatic attraction is a significant factor in Cr(VI) adsorption on hydrogels. Meanwhile, the Pyr-grafted hydrogels were found to enhance the adsorption and degradation of Cr(VI), with the adsorption capacity of Cr(VI) increasing with concentration. The adsorption behavior of Cr(VI) is more accurately described by the pseudo-second-order kinetic and Langmuir models, which suggests that the process is a monolayer chemical adsorption. The maximum adsorption capacities of C2-MUP, C2-MUBP, and C4-MUP hydrogels for chromium were 258.31, 259.37, and 258.49 mg/g, respectively, which are higher than those of adsorbents. Importantly, an amphipathic supramolecular hydrogel can selectively capture Cr(VI) in the presence of anionic competition and effectively capture Cr(VI) at different pH values. Additionally, the synthesized hydrogel is highly reusable, with no significant decrease in Cr(VI) removal efficiency after six cycles. The newly synthesized ursolic acid amphipathic supermolecular hydrogels show superior recovery performance and ultrahigh capacity for Cr(VI) removal. These properties provide vast application possibilities in the handling of chromium wastewater.