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.
Nanoscale zero-valent iron (nZVI) can react with molecular oxygen to produce reactive oxygen species, but very low yield limits its widespread use. In this study, the effect of ligand-citric acid (CA) addition on the dissolved oxygen (DO) activation for in-situ generation of ROS and subsequent 2,4-DCP degradation by graphene carbon layer coated nZVI composites (GBC@nZVI) was systematically investigated. It was found that the introduction of CA significantly increased the ROS production and resulted in the generation of OH, O 2 − and 1 O 2 with values of 2.1, 2.3, and 1.5 times higher than that in systems without CA, respectively. Ligand CA could enhance the degradation of adsorbed 2,4-DCP over a wide pH range. The CA addition resulted in a significant increase in the degradation of 2,4-DCP from 40.8 % to 79.9 % and a significant decrease in the adsorption from 51.1 % to 19.1 % compared to the system without CA. This phenomenon can be attributed to the fact that the incorporation of CA promoted the efficient recycling of Fe(II)/Fe(III) in the surface-bound state from the iron oxide film on the GBC@nZVI which enhanced the generation of interfacial ROS using DO, and further facilitated the degradation of adsorbed 2,4-DCP rather than immobilization by adsorption alone. On the other hand, the formation of Fe(Ⅲ)-CA complexes with high stability provides enough soluble Fe(II) to produce more ROS in solution via the oxygen reduction. Furthermore, the graphene carbon layer is able to act as an electron conductor to promote the efficient transfer of electrons except to inhibiting the rapid oxidation of Fe 0 . The study not only reveals the promotion effect of ligand-CA addition on the in-situ generation of ROS from nZVI activated dissolved oxygen, but also provides a simple and green strategy for the removal of electron-rich aromatic pollutants from water pollution.
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