Organic matter-driven hydroxyl radical generation enhances emerging pollutant removal in plant rhizosphere of constructed wetlands: Coupled direct and indirect effects
Jingyuan Yue, Xiaojin Hu, Huijun Xie, Bo Sun, Zhen Hu, Jian Zhang
Abstract
The hydroxyl radical (•OH) plays a key role in the removal of emerging pollutants (EPs) within redox environments. The rhizosphere is known for its pronounced fluctuations in oxygen levels. However, the mechanisms governing •OH generation and its effects on EPs within the rhizosphere of constructed wetlands (CWs) remain insufficiently explored. In this study, we investigated the generation of •OH and its multiple roles in the removal of carbamazepine (CBZ) in laboratory-scale vertical flow CW mesocosms. The systems were filled with gravel substrate, planted with Juncus effusus, and fed with synthetic wastewater. In situ experiments showed that 0.33 μmol/kg •OH was naturally generated in the rhizosphere. Water-soluble organic matter enriched with phenolic hydroxyl groups (ArOH) dominated sustainable •OH generation, in which ArOH contributed 60.63 ± 23.34 %. Furthermore, •OH contributed to 41.80 ± 8.09 % CBZ removal through radical-mediated direct oxidative pathway and indirect effects. Specifically, key CBZ-degrading enzyme activity decreased 4.95-fold in quenching experiments, indicating that •OH indirectly stimulated CBZ removal by increasing enzyme activity. Besides, the presence of •OH enhanced CBZ co-metabolism through mediating bioavailable intermediate product formation. In sum, the enhancement of CBZ removal mainly through direct •OH oxidation, with the regulatory role of •OH on microbial degradation assisting CBZ removal according to the result of the structural equation model. Our findings highlight the overlooked roles of •OH and provide novel insight into substrate optimization in CWs.