Electrochemical oxidation (EO) effectively degrades phenolic compounds in phenol-containing wastewater, but the chlorinated byproducts generated during the process raised concerns. In this study, degradation efficiency of phenol and formation rules of chlorinated byproducts in the presence of chlorine (Cl − )were systematically and comparatively studied in two EO systems with boron-doped diamond (BDD) and ruthenium-iridium-coated titanium (RuO 2 -IrO 2 /Ti) as anodes. The results showed that Cl − significantly enhanced phenol degradation efficiency of the two anode systems. At the same Cl − concentration, degradation efficiency of BDD-EO was superior to that of RuO 2 -IrO 2 /Ti-EO, it had a higher apparent removal rate for phenolic compounds, including phenol, 4-NP, 4-CP, BPA, and RES, and was less affected by coexisting ions. However, in the BDD-EO system, the total amount of produced chlorinated byproducts (3.593 mM Cl) was approximately 50 times greater than that produced in the RuO 2 -IrO 2 /Ti-EO system (0.0723 mM Cl). These chlorinated byproducts included inorganic chloro-byproducts (ClO 3 − and ClO 4 − ) and organic chloro-byproducts (such as chloroform, trichloroacetic acid, and trichloroacetaldehyde). The results confirmed that HOCl generated by the oxidation of Cl − on BDD and RuO 2 -IrO 2 /Ti anodes through the • OH/direct electron transfer was the key active species for enhancing the phenol degradation and driving the formation of chlorinated byproducts. The toxicity assessment shows that the additive toxicity of inorganic chloro-byproducts in the RuO 2 -IrO 2 /Ti-EO system is approximately 1/57 that in the BDD-EO system, and the toxicity of organic chloro-byproducts is approximately 1/18. Selecting the appropriate anode is therefore critical to balance treatment efficiency with the need to minimize toxic chlorinated byproducts.
Lei Xu,Shaoyu Tang,Dan Li,Xiao Ma,Yunqing Zhu,Jianjiang Lu,Junfeng Niu
The current study presents a comprehensive and novel attempt to degrade aqueous tri(2-chloroethyl) phosphate (TCEP) by metal-oxide-coated Ti anodes for the first time. Six metal-oxide-coated Ti electrodes (Ti/SnO 2 -Sb, Ti/SnO 2 -Sb/PbO 2 , Ti/SnO 2 -Sb/La-PbO 2 , Ti/SnO 2 -Sb/Ce-PbO 2 , Ti/SnO 2 -Sb/Ga-PbO 2 and Ti/SnO 2 -Sb/Zr-PbO 2 ) were prepared and characterized. Ti/SnO 2 -Sb/La-PbO 2 showed the highest pseudo-first-order reaction rate constant (2.1 × 10 −2 min −1 ) and lowest energy consumption (21.35 Wh L −1 ) in the degradation of 1 mg L −1 TCEP at 10 mA cm −2 , mainly because of its highest oxygen evolution potential and OH production. Higher current density and neutral condition favored TCEP degradation, while the presence of Cl − or NO 3 − could suppress the degradation. In the electrochemical degradation of 10 mg L −1 TCEP, mineralization percentage of 48.0% (mineralization current efficiency of 0.30%) and PO 4 3− release percentage of 72.7% were achieved at 150 min. By the attack of OH on the C O and C Cl bonds in TCEP molecular structure, products of chloroethanol, Cl − and PO 4 3− were released, with the major electrocatalytic degradation pathways of dechlorination and hydroxylation. Based on the results of flow cytometry, the relative toxicity to Escherichia coli effectively decreased to 4.93% in TCEP degradation. Compared with the UV-AOPs, electrochemical oxidation process is proved to be feasible and effective for mineralization and detoxification of TCEP in water.
Addressing the ever-growing arsenite pollution in groundwater has become an essential and urgent issue for ensuring the safety of human beings and the environment. It is highly attractive to preoxidize As(III) to relatively low toxic As(V) for further efficient arsenic removal by adsorption, ion exchange, or coagulation. In this study, a novel heterostructure Cu2O@TiO2 nanotube array coated titanium anode (CTNTA) was fabricated by combination of electrochemical anodization and electrodeposition and used for the highly efficient photoelectrocatalytic oxidation conversion of As(III) to As(V) in aqueous solution. The morphology, composition, and photoelectric performances of the CTNTA anode were systematically characterized. The effect of the loading amount of Cu2O nanoparticles on the photoelectrocatalytic oxidation kinetics of As(III) was investigated, and the comparison study among photocatalytic, electrocatalytic, and photoelectrocatalytic oxidations of As(III) was also explored to demonstrate the synergistic effect of As(III) oxidation conversion performance. The heterostructure CTNTAs-1.00 delivered a substantially enhanced photoelectrocatalytic oxidation ratio of As(III) at 95% within 30 min at a current density of 10 mA·cm–2 under visible-light irradiation. The electrochemical deposition amount of Cu2O nanoparticles on TiO2 nanotube arrays (TNTAs) was found to be positively correlated with the achievement of boosted As(III) oxidation efficiency. The high photoelectrocatalytic oxidation performance of the CTNTA was ascribed to the favored separation efficiency of photoinduced carriers and electron–hole pairs under both visible light irradiation and electric potential conditions. The photoelectrocatalytic oxidation reaction mechanism was proven to be ascribed to the active species of holes (h+), superoxide radicals (•O2–), and hydroxyl radicals (•OH). These results highlight the high conversion rate of As(III) by the photoelectrocatalytic oxidation reaction on such a nanostructured anode, which offers a promising methodology for further in-depth oxidation of As(III) in aqueous solution.
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