Electrochemically enhanced activation of Co₃O₄/TiO₂ nanotube array anode for persulfate toward high catalytic activity, low energy consumption, and long lifespan performance

Fan Qiu ^a, Luyao Wang ^a, Hongxiang Li ^b, Yanan Pan ^a, Haiou Song ^{b, *}, Junjie Chen ^a, Yang Fan ^a, Shupeng Zhang ^{a, *}

Abstract

Advanced oxidation processes (AOPs) can directly degrade and mineralize organic pollutants (OPs) in water by generating reactive oxygen species with strong oxidizing ability. The development of advanced electrode materials with high catalytic performance, low energy consumption, no secondary pollution, and long lifespan has become a challenge that must be addressed in this field. A heterojunction catalyst loaded with Co₃O₄ on TDNAs (Co₃O₄/RTDNAs) was designed and constructed by a simple and efficient pyrolysis (Co₃O₄/TDNAs) and electrochemical reduction. Co₃O₄ can be uniformly distributed on the inner wall and surface of the TiO₂ nanotubes, enhancing the specific surface area while forming a tight conductive interface with TiO₂. This facilitates rapid transmission of electrons, thereby assisting Co₃O₄ in quickly activating PS to form reactive oxygen species. The Ti³⁺ and Ov generated in Co₃O₄/RTDNAs can significantly improve the electrocatalytic degradation of OPs. Also, the interface formed by Co₃O₄ and RTDNAs will effectively suppress Co²⁺ leakage, thereby reducing the risk of secondary pollution. When the reaction conditions were 1 mM PMS (PDS) and a current density of 5 mA/cm² in the EA-PMS (PDS)/Co₃O₄/RTDNA system, 30 mg/L TC can achieve 83.24% (81.89%) removal in 120 min, with very low cobalt ion leaching, while the energy consumption was reduced significantly. Therefore, EA-PS/Co₃O₄/RTDNA system has strong stability and a high potential for treating the OPs in AOPs.

In summary, we prepared a novel Co₃O₄/RTDNA anode by electrochemical oxidation, room temperature impregnation, high-temperature annealing, and cathodic reduction. The anode was then applied to electrochemically activated persulfate (EA-PS) to degrade tetracycline (TC). The Co₃O₄/RTDNA anode has a unique structure rich in oxygen defects and Ti³⁺. CV, LSV, and EIS results show that Co₃O₄ loading enhances TiO₂ conductivity, while cathodic reduction can further increase the OEP of the electrode (2.34 V vs. SCE). The charge transfer resistance of the electrode was reduced, and the cathode reduction significantly improved the stability of the Co₃O₄/RTDNA electrode, having a service life of 600 h.

When the reaction conditions were 1 mM PMS (PDS) and a current density of 5 mA/cm² in the EA-PMS (PDS)/Co₃O₄/RTDNA system, 30 mg/L TC can achieve 83.24% (81.89%) removal in 120 min, with very low cobalt ion leaching. The TC removal conformed to a pseudo-second-order kinetic model, suggesting that the Co₃O₄/RTDNA anode has excellent catalytic performance for EA-PS. In the EA-PS/Co₃O₄/RTDNA system, TC was degraded through the free radical pathway, and ·OH and SO₄^-· were essential to the process. Due to the different structure and oxidation pathway of persulfate, the proportions of free radicals in EA-PMS/Co₃O₄/RTDNA reaction system and EA- PDS/Co₃O₄/RTDNA reaction system are different. The TC degradation pathways include elimination reaction, hydroxyl addition, amino oxidation, demethylation, dehydration, etc. ECOSAR identified that the acute toxicity of most intermediates reduced during the degradation. Compared with the EA/Co₃O₄/RTDNA system, the COD removal and current efficiency of the EA-PMS(PDS)/Co₃O₄/RTDNA system were significantly improved, while the energy consumption was reduced significantly. Therefore, Co₃O₄/RTDNAs can be used and reused as a new anode for EA-PS OPs degradation; it has strong stability and a high potential for treating the organic matter in wastewater.

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