Oxidation mechanism of As(III) in the presence of polyphenols: New insights into the reactive oxygen species

The interaction between humic substances (HS) and arsenic (As(III)) is an important process affecting the fate and transformation of arsenic in the environment. In the present study, batch experiments were conducted to investigate the interactions between As(III) and polyphenols, as simple model compounds of HS in aqueous solutions. The results indicated that As(III) could be oxidized to As(V) in the presence of gallic acid (GA), caffeic acid (CA) and protocatechuic acid (PA) at neutral and alkaline conditions. By using electron paramagnetic resonance (EPR) technique, it was found that the formation of semiquinone radical (SQ(center dot-)) in polyphenol solutions induced the generation of reactive oxygen species (ROS) including hydrogen peroxide (H2O2) and hydroxyl radical ((OH)-O-center dot). Both SQ(center dot-) and ROS were responsible for the oxidation of As(III) to As(V). Free radical quenching studies showed that semiquinone and H2O2 accounted for about 70%, while (OH)-O-center dot accounted for 30% of As(III) oxidation in GA/As(III) system at pH 9.0. Moreover; the As(III) oxidation kinetics process well followed the pseudo-first-order reaction. The increase of pH and dissolved oxygen concentration favored the formation of ROS, and thus promoted the As(III) oxidation. The findings of this study provided some new insights into the mechanism of the interactions between HS and arsenic, and contributed to the better understanding of arsenic geochemical cycles. (C) 2015 Elsevier B.V. All rights reserved.