Visible light-induced hydrogels towards reversible adsorption and desorption based on trivalent chromium in aqueous solution

Abstract

How to detect and recover Cr(III) (Cr3+) from contaminated water through an economical and environmental way is not negligible but significant to solve serious pollution of heavy metal ions. Herein, a spiropyran [1′-(n-butyl)-3′,3′-dimethyl-6-nitrospir[chromene-2,2′-indoline] (SPBU) with negative photochromism is exploited as a visual detector for Cr3+. In contrast to positive photochromism of spiropyran derivatives, the synthesized SPBU can isomerize from colorless form spiropyran (SP) to the corresponding colored merocyanine (MC) form and further complex with Cr3+ under darkness without essential UV irradiation, providing a facile strategy enable visual detection of Cr3+ in aqueous solution. Importantly, we have synthesized SPBU-Polyethylene glyol (SPBU-PEG) hydrogels with negative photochromism via physical blending. Evidence from practical performance confirms that Cr3+ can be captured from aqueous solution under darkness and repelled from the hydrogel effectively upon visible light irradiation to achieve the regeneration of the designed adsorbent with a save-energy manner, avoiding any desorbent and secondary contamination. Additionally, the SPBU-PEG hydrogels have excellent Cr3+ selectivity and reproducibility. This work provides a significant greener way to exploit visible light responsive materials for reversible recovery of heavy metal ions from contaminated water with preferable performance, broadening sustainable development for purification systems.