Hydroxyl radical dominated degradation of aquatic sulfamethoxazole by Fe/bisulfite/O: Kinetics, mechanisms, and pathways.

In this study, batch experiments were carried out to investigate the key factors on sulfamethoxazole (SMX) removal kinetics in a new AOPs based on the combination of zero valent iron (Fe) and bisulfite (S(IV)). With the increase of Fe from 0.25 mM to 5 mM, the removal rate of SMX was linearly increased in the Fe/S(IV)/O system by accelerating the activation of S(IV) and Fe corrosion to accelerate. In the first 10 min of reaction, the increasing concentration of S(IV) inhibited SMX removal after since the high S(IV) concentration quenched reactive oxidative species (ROS). Then SMX removal rate was accelerated with the increase of S(IV) concentration after S(IV) were consumed up. The optimal ratio of S(IV) concentrations to Fe concentration for SMX removal in the Fe/S(IV)/O system was 1:1. With SMX concentrations increasing from 1 to 50 μM, SMX removal rate was inhibited for the limitation of ROS yields. Although the presence of SO and OH was confirmed by electron paramagnetic resonance (EPR) spectrum, OH was identified as the dominant ROS in the Fe/S(IV)/O system by chemical quenching experiments. Besides, strong inhibitive effects of 1,10-phenanthroline on SMX degradation kinetics by Fe/S(IV)/O proved that the generation of ROS was rely on the release of Fe(II) and Fe(III). The generation of SO was ascribed to the activation of S(IV) by Fe(II)/Fe(III) recycling and the activation of HSO by Fe(II). And OH was simultaneously transformed from SO and generated by Fe/O. Density functional theory (DFT) calculation was conducted to reveal special reactive sites on SMX for radicals attacking and predicted intermediates. Finally, four possible SMX degradation pathways were accordingly proposed in the Fe/S(IV)/O system based on experimental methods and DFT calculation.