Simultaneous removal of 2,4-DCP and Cr(VI) in water by gBC@nZVI: Cr(V) mediated ROS formation

In the actual polluted environment, the composite pollution system was usually composed of heavy metals and organic pollutants, especially Cr(VI) and chlorophenols often co-existed in industrial wastewater. In this study, a composite material consisting of multilayer graphene carbon shells coated with nZVI (gBC@nZVI) with various excellent properties was applied to deal with the composite system of Cr(VI) and 2,4-dichlorophenol (2,4-DCP) coexisting in the wastewater, and the intrinsic mechanism for the simultaneous removal of heavy metals and organic pollutants was systematically investigated. The results showed that in the composite pollution system (Cr(VI) = 20 mg/L, 2,4-DCP = 15 mg/L), complete removal of Cr(VI) could be achieved in 60 min, while 2,4-DCP achieved 95.2 % removal, with the contribution of oxidative degradation being 76.4 %. It was found that during the efficient adsorption and reduction of Cr(VI) by gBC@nZVI, the generation of the metastable intermediate Cr(V) was verified by probe compounds, electron spin resonance and UV–visible spectroscopy, and further experiments demonstrated that the generation of •OH was closely related to Cr(V) and promoted the oxidative degradation of the coexisting 2,4-DCP. Quenching experiments with the addition of specific scavengers indicated that •OH was the active species directly dominating the oxidative degradation of 2,4-DCP, whereas Cr(V) was indirectly involved in the degradation of 2,4-DCP by promoting the generation of •OH. In addition, the increase of solution pH negatively affected the degradation of 2,4-DCP in the composite polluted system, mainly due to the hindered generation of •OH in the alkaline environment. This work provided theoretical support for the promising application of gBC@nZVI in real environments contaminated with a combination of Cr(VI) and chlorophenols.