Electrochemical degradation of tetracycline hydrochloride in sulfate solutions on boron-doped diamond electrode: The accumulation and transformation of persulfate

In this study, a novel electrifying mode (divided power-on and power-off stage) was applied in the system of BDD activate sulfate to degrade tetracycline hydrochloride (TCH). The BDD electrode could activate sulfate and H₂O to generate sulfate radicals (SO₄^•-) and hydroxyl radicals (^•OH) to remove TCH, and SO₄^•- could dimerize to form S₂O₈^2-. Then, the S₂O₈^2- was activated by heat and quinones to generate SO₄^•- for the continuous degradation of TCH during the power-off stage. In addition, the intermittent time has a significant effect on the degradation of TCH. Factors, affecting the accumulation of S₂O₈^2-, were analyzed using a full factorial design, and the accumulation of S₂O₈^2- could reach 16.2 mM in 120 min. The results of electron spin resonance and radical quenching test showed that SO₄^•-, ^•OH, direct electron transfer (DET), and non-radical in the system could effectively degrade TCH, and SO₄^•- was dominated. The intermediate products of TCH were analyzed by HPLC-QTOF-MS/MS, and the TCH mainly underwent hydroxylation, demethylation and ring opening reactions to form small molecules, and finally mineralized. The results of the feasibility analysis revealed that some intermediates have high toxicity, but the system could improve the toxicity. The results of energy consumption indicated that the intermittent electrifying mode could make full use of the persulfate generated during the power-on stage and reduce about 30% energy consumption. In conclusion, this work demonstrated that it was economically feasible to degrade TCH in wastewater by activating sulfate with BDD electrodes with an intermittent electrifying mode.

Electrochemical treatment of aqueous solutions of organic pollutants by electro-Fenton with natural heterogeneous catalysts under pressure using Ti/IrO2-Ta2O5 or BDD anodes

The treatment of toxic organic pollutants by electro-Fenton (EF) presents some drawbacks such as the necessity to work at low pH and the low solubility of oxygen in water contacted with air or oxygen at room pressure that results often in slow and relatively low abatements. Here, the coupled adoption of natural heterogeneous catalysts and of relatively high pressure was proposed in order to improve the performances of EF for the treatment of organic pollutants. Caffeic acid (CA) and 3-chlorophenol were used as model resistant organic pollutants. EF process was performed using both conventional homogeneous FeSO₄ and natural heterogeneous catalysts (pyrite, chalcopyrite, Fe₂O₃ and Fe₃O₄) as iron catalysts and oxygen at various pressures in the absence or in the presence of BDD anode. The effect of the nature of the catalyst, the oxygen pressure, the current density and the catalyst load was widely investigated in order to optimize the process. It was shown that the coupled utilization of a natural heterogeneous catalyst such as chalcopyrite and a relatively high pressure allows to obtain the total removal of CA and a high removal of the TOC (about 75%) in short times (2 h) with relatively high current efficiencies using an Iridium based anode. In the case of 3-chlorophenol, the utilization of a BDD anode was necessary to achieve a high removal of the pollutant and the TOC. It was shown that the removal of 3-chlorophenol can be effectively performed in different water bodies and with different initial concentrations of 3-chlorophenol.