Selective and Efficient Photoextraction of Aqueous Cr(VI) as a Solid-State Polyhydroxy Cr(V) Complex for Environmental Remediation and Resource Recovery

Aqueous hexavalent chromium (Cr­(VI)) treatment and chromium resource recovery toward Cr-containing wastes are of significant importance and necessity to both wastewater remediation and resource recovery. Herein, via mild photoreaction conditions with isopropanol (IPA) as an electron donor, a catalyst-free strategy for aqueous Cr­(VI) extraction to form an insoluble polyhydroxy Cr­(V) complex is developed for the first time. Aqueous Cr­(VI) with concentration from 5 to 150 ppm can be efficiently extracted with high selectivity even in the presence of coexisting ions, and the total Cr concentration in residue solution can be as low as 0.5 ppm. The Cr resource could be efficiently recovered as pure Cr2O3 by calcinating the resulting Cr­(V) precipitate. Outstanding extraction efficiency could be realized with various IPA concentrations (1.3–12.0 mol/L) by coordinately tuning the pH value to promote the formation of Cr­(VI)-IPA ester. The formed ester undergoes intramolecular electron transition under visible light irradiation, resulting in a polyhydroxy solid-state Cr­(V) intermediate complex. The controlled pH value blocks further reduction of Cr­(V) to soluble Cr­(III); thus the insoluble Cr­(V) intermediate complex is stabilized thermodynamically under ambient conditions. Because of its electric neutrality property and the strong intermolecule interaction via hydrogen bonds, a dioxo-bridged di-nuclear Cr­(V) complex {Cr2(μ-O)2(OH)4[OCH­(CH3)2]2} is finally precipitated as the main product. Satisfactory extraction and recovery of Cr from chromium-plating wastewater and discarded stainless steel verify that this approach is ideal for both one-step purification of Cr­(VI)-containing wastewater and selective resource recovery from Cr-containing solid wastes in practical application.