Mechanistic evaluation of chromium (VI) photoreduction through poly (ethylene glycol)-chromate complex formation

Hexavalent chromium (Cr(VI)) has long been recognized as a significant threat to human health and ecosystems. However, achieving the rapid and efficient reduction of Cr(VI) to the non-toxic Cr(III) by using conventional photocatalysts remains a challenge. While polyethylene glycol (PEG) is widely used as a photoreductant for Cr(VI) reduction, the core factors influencing the Cr(VI) reduction reaction and underlying mechanisms have not been thoroughly explored. In this study, we examined the effects of different factors on the photoreduction of Cr(VI) by PEG. Experimental results reveal that PEG-2000 exhibits the strongest photoreduction capability. Moreover, the optimum pH for photoreduction is 0.5, while the most favorable temperature is 30 °C. Furthermore, the intermediates and photo-induced mechanism of Cr(VI) photoreduction by PEG were analyzed. The photoreduction reaction proceeds following the formation of a complex between PEG and Cr(VI). This complex absorbs light energy, facilitating the rapid transfer of electrons from the excited oxygen clusters to chromic acid. Both ∙O 2 − and ∙OH radicals are involved in the photoreduction process, and radical trapping experiments confirm that electrons are the primary contributors to the reduction. This study provides a comprehensive investigation into the mechanism of Cr(VI) reduction by PEG, offering valuable insights into the practical application of photoreductants in catalytic reduction processes.