Visible light photocatalytic reduction of Cr(VI) over polyimide in the presence of small molecule carboxylic acids

This paper documents the visible light induced photocatalytic conversion of highly toxic Cr(VI) by polyimide (PI) in the presence of small molecule carboxylic acids (SMCAs). The formation of PI-SMCA surface complexes resulted in the suppression of electron-hole recombination of PI and the production of more organic radicals (such as RCOO.-), accelerating the reduction of Cr(VI). The findings also indicated that SMCA-promoted conversion of Cr(VI) depends on the structure characteristics of the SMCAs. The removal of Cr(VI) was further boosted by raising light intensity and SMCA content, adding Fe3+ and Cu2+, or decreasing solution pH. Benefit from the acid resistance of PI and the formation of soluble Cr(III)-SMCA complex, only negligible declines in the Cr(VI) removal rate were observed even after 5 cycle tests of PI. Moreover, PI was effective and efficient in the treatment of the actual Cr(VI)-containing tannery wastewater under visible light irradiation, without adding any extra agent.