Goethite enhanced the Cr(VI) reduction by gallic acid through the stabilisation of free radicals: spectroscopic evidences and implication for transformation of Cr(VI) in an Oxisol

Abstract

ABSTRACT Produced from root exudation activities and the decomposition of organic matter, gallic acid (GA) has been shown to be widely involved in the electron transfer reactions in soils as well as the transformation of redox-sensitive pollutants, while which was subject to the mediation of solid surfaces with the unknown mechanisms. This study aimed to investigate the catalytic effect of goethite on the Cr(VI) reduction by GA and explore the implication for the transformation of Cr(VI) in an oxisol. The results have shown that the adsorption of GA by goethite followed pseudo-second-order model. The GA was very stable at pH 4.0 in solution, whereas its oxidation by O2wasaccelerated in the presence of goethite as evidenced by the rapid appearance of IR peak of quinones. Similarly, the reduction of Cr(VI) by GA proceeded slowly with semi-quinonic species as the intermediate oxidation products of GA, while which would be significantly enhanced with the addition of goethite or oxisol. The mechanism was ascribed to that surface adsorption promoted the dissociation of –OH groups of GA and the subsequent formation of semiquinone radicals. Most importantly, these free radicals could be stabilised on the goethite surfaces, which was believed to be favourable for the reduction of Cr(VI). The catalysis of aqueous Fe(III) was excluded in this study due to its low concentration in the reaction systems. Furthermore, the majority of produced Cr(III) was adsorbed on the surfaces as the form of octahedron. The findings above indicated the environmental risk of Cr(VI)-polluted acidicsoilswould be alleviated when GA was present.