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.