Unraveling the effects of gallic acid on Sb(III) adsorption and oxidation on goethite

Natural organic matter (NOM) profoundly influences the metal(loid)s immobilization by iron (Fe) minerals, which, in turn, are the host phase for Sb that control its mobilization and bioavailability. However, the potential effect of the redox-active phenolic acids derived from NOM on this process has rarely been explored. Therefore, the adsorption of Sb(III) on goethite and Sb(III) oxidation in the presence of a common phenolic acid-gallic acid (GA)-was systematically investigated. The reaction mechanisms were unraveled by the combination of wet chemistry, electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure (XAFS) spectroscopy. Our results showed the addition of GA significantly inhibited Sb adsorption on goethite as the pH increased. Except for the competitive adsorption by GA, the inhibition also resulted from the extensive Sb(III) oxidation in the suspension and the lower Sb(V) sorption capacity of goethite at higher pH. The alkaline condition facilitated the generation of reactive oxygen species (hydrogen peroxide) and quinone species (semiquinone radical (SQ center dot(-)) and oxidized GA), accounting for the significant Sb(III) oxidation in the solution and on the goethite surface. Besides, the edge-sharing inner-sphere complexes were formed between Sb and goethite regardless of the presence of GA. Our findings demonstrated the remarkable role of the phenolic groups in NOM on the adsorption of Sb(III) on goethite.