SO 4 − and OH radicals are important oxidative species that play important roles in the oxidation of organic contaminants by peroxomonosulfate (PMS)/persulfate (PS) and Co 3 O 4 under acidic conditions. However, the facet effect of Co 3 O 4 on the catalytic oxidation of phenol by PS under alkaline conditions has not been investigated yet. In this study, we synthesized three Co 3 O 4 nanoparticles with different facets [rod (110), cube (100) and octahedra (111)]. The Co 3 O 4 rods show the highest catalytic reactivity in the oxidation of phenol by PS, followed by Co 3 O 4 cubes and Co 3 O 4 octahedra. Quenching experiments and EPR analysis rule out contributions of SO 4 − , OH, O 2 − and 1 O 2 to phenol oxidation and suggest that H-abstraction from phenol to phenoxyl radicals is an important pathway in all Co 3 O 4 -PS systems. The analysis of total organic carbon (TOC) and solid characterizations reveal that more than 96.6 % phenol has been transformed into polyphenols, instead of being mineralized into smaller organic molecules or CO 2 in the rod-PS-phenol system. The DFT (density functional theory) calculations further confirm that PS is more readily activated via a non-radical pathway on the Co 3 O 4 (110) plane with appropriate Co(II)-Co(II) distances, and the activated O O bonds then abstract H atoms from phenol molecules with the production of phenoxyl radicals, as well as SO 4 − and OH radicals. The produced phenoxyl radicals then link together to form dimers, trimers, oligomers and eventually insoluble polymers. This study not only enriches our understanding on the facet effect of different Co 3 O 4 in the oxidation of phenol, but also gives new insight into turning phenolic wastes into polyphenol "treasures".
Electrode fouling is a problem that commonly occurs during electro-oxidation water purification. This study focused on identifying the fouling behavior of Pt electrode associated with the formation of polymeric layer during electro-oxidation of phenol. The in situ electrochemical measurements and non-destructive observation of the electrode morphology were reported. The results demonstrated that the electrode fouling was highly dependent on thermodynamic process of electrode that was controlled by anode potential. At anode potential lower than 1.0 V vs SHE, the direct electro-oxidation caused the electrode fouling by the formation of polymeric film. The fouling layer decreased the electrochemically active surface area from 8.38 cm 2 to 1.57 cm 2 , indicated by the formation of polymeric film with thickness of 2.3 µm, increase in mass growing at a rate of 3.26 µg/cm/min. The degree to which the anode was fouled was independent of anion in the electrolyte. In comparison, at anode potential higher than 2.7 V vs SHE, the anions (e.g., chloride) could exert a major influence to the behavior of electrode fouling. The presence of chloride was shown to mitigate the fouling of electrode significantly through preventing the formation of polymeric film by active chlorine (e.g., Cl• and Cl 2 ) produced from anodic oxidation of chloride. Since chloride is the most abundant anionic species existing in both natural and engineered water system, this study not only offers a deep insight into the mechanism of electrode fouling, but also suggests strategies for anti-fouling in the presence of chloride in electro-oxidation process.
Jianqiu Gong,Weiwei Zhang,Tapas Sen,Yichen Yu,Yuchen Liu,Jinlong Zhang,Lingzhi Wang
Photocatalysis is a promising technology to treat dilute phenol–Cr(VI) mixture, where photoinduced electrons are commonly thought of as the main active species for Cr(VI) reduction. However, it generally depends on the surface adsorption to achieve efficient electron transfer. Meanwhile, the possible contribution of reductive quinone derivatives oxidized from phenol to Cr(VI) reduction has been rarely explored. The key is to explicitly understand the relation between the phenol oxidation pathway and Cr(VI) reduction. Herein, Ag/AgCl/MIL-101(Fe) prepared by directly loading Ag nanoparticles on MIL-101(Fe) was applied to the joint treatment of phenol and Cr(VI). The role of quinone derivatives in reducing Cr(VI) was revealed by excluding the surface adsorption. During the phenol oxidation, •OH radicals were more consumed in the initial stage for the ring opening of phenol. Meanwhile, 1O2 evolved from •O2– gradually caused the accumulation of reductive quinone intermediates, which dramatically accelerated the Cr(VI) reduction afterward. This study demonstrates the significance of controlling the evolution process of active oxygen species for the joint photocatalytic treatment of phenol–Cr(VI) mixture.
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