Reductive removal of Cr(VI) by citric acid promoted by ceramsite particles: Kinetics, influential factors, and mechanisms

Natural organic acids can reduce Cr(VI) to Cr(III) in soil and aquatic environments, but that reductive reaction is too slow for practical application in a Cr(VI) removal process. In this study, we prepared ceramsite particles (CPs) and used them in a citric acid-based Cr(VI) reduction process to analyze their catalytic functions and find their optimal reduction conditions under increasing pH and with and without light. CPs increased the apparent reduction rate constants from 16.35 at pH = 2.6 to 44.85 mg L-1 d(-1), and increasing pH decreased the CPs' catalytic effect. Light positively influenced the reduction rate, which more than doubled under illumination compared to the rate in the dark. We identified soluble Fe(III), that had dissolved from CPs in the reaction solution, as the catalyst for Cr(VI) reduction by citric acid. Under light, soluble Fe(III) complexed with citric acid, and those initiated complexes yielded Fe(II)-citrate and reductive organic radicals that promote Cr(VI) reduction. In the dark, Fe(III)-citrate combined with Cr(VI) to form ring complexes that were responsible for rapid Cr(VI) reduction. Our results reveal how CPs containing soluble Fe(III) can efficiently promote Cr(VI) reduction by organic matter in an aqueous solution.