The coordination complex formed by oxalic acid and chromate and the mechanism for enhanced photoreduction of Cr(Ⅵ)under visible-light irradiation
Oxalic acid,which mainly originates from root exudation and plant residue decomposition,is one of the most widely existiug carboxylic acids,and has a dramatic impact on the reduction of chro-mate in natural waters.In this study,isothermal titration calorimetry(ITC),theoretical calculations,and electrospray ionization high resolution mass spectrometry(ESI-HRMS)analysis revealed that oxalic acid could complex with chromate to form a Cr(Ⅵ)-oxalate complex via an esterification process at pH≤4.The in situ generated HC2CrO-7 complex was a visible-light-active species,and visible light could induce electron transfer from the oxalate unit to the Cr(Ⅵ)within the HC2CrO7-complex via a HOMO-LUMO electronic transition,resulting in Cr(Ⅵ)reduction.Subsequently,we verified the Cr(Ⅵ)reduction intermediates,and the hypotoxic Cr(Ⅲ)species[Cr3+-(C2O42-)3]3-was the final product.Finally,the pathway of Cr(Ⅵ)reduction by oxalic acid under visible-light irradiation was proposed.This study will provide a theoretical basis for efficient photoreduction of Cr(Ⅵ)pollution using oxalic acid.
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