Identifying the evolution of primary oxidation mechanisms and pollutant degradation routes in the electro-cocatalytic Fenton-like systems

Mechanistic insight into the degradation of sulfadiazine by electro-Fenton system: Role of different reactive species

Sulfadiazine (SDZ), a widely used effective antibiotic, is resistant to conventional biological treatment, which is concerning since untreated SDZ discharge can pose a significant environmental risk. Electro-Fenton (EF) technology is a promising advanced oxidation technology for efficiently removing SDZ. However, due to the limitations of traditional experimental methods, there is a lack of in-depth study on the mechanism of ·OH-dominated SDZ degradation in EF process. In this study, an EF system was established for SDZ degradation and the transformation products (TPs) were detected by mass spectrometry. Dynamic thermodynamic, kinetic and wave function analysis of reactants, transition states and intermediates were proposed by density functional theory calculations, which was applied to elucidate the underlying mechanism of SDZ degradation. Experimental results showed that amino, benzene, and pyrimidine sites in SDZ were oxidized by ·OH, producing TPs through hydrogen abstraction and addition reactions. ·OH was kinetically more likely to attack SDZ- than SDZ. Fe(IV) dominated the single-electron transfer oxidation reaction of SDZ, and the formed organic radicals can spontaneously generate the de-SO2 product via Smiles rearrangement. Toxicity experiments showed the toxicity of SDZ and TPs can be greatly reduced. The results of this study promote the understanding of SDZ degradation mechanism in-depth. ENVIRONMENTAL IMPLICATION: Sulfadiazine (SDZ) is one of the antibiotics widely used around the world. However, it has posed a significant environmental risk due to its overuse and cannot be efficiently removed by traditional treatment methods. The lack of in-depth study on SDZ degradation mechanism under reactive species limits the improvement of SDZ degradation efficiency. Therefore, this work focused on SDZ degradation mechanism in-depth under electro-Fenton system through reactive species investigation, mass spectrometry analysis, and theoretical calculation. The results in this study can provide a theoretical basis for improving the SDZ degradation efficiency which will contribute to solving SDZ pollution problems.

Highly efficient heterogeneous electro-Fenton reaction for tetracycline degradation by Fe-Ni LDH@ZIF-67 modified carbon cloth cathode: Mechanism and toxicity assessment

In this work, a novel and efficient Fe-Ni LDH@ZIF-67 catalyst modified carbon cloth (CC) cathode was developed for tetracycline (TC) degradation in heterogeneous electro-Fenton (Hetero-EF) process. Compared to Fe-Ni LDH/CC (75.7%), TC degradation rate of Fe-Ni LDH@ZIF-67/CC cathode increased to 95.6% within 60 min. The synergistic effect of hetero-EF and anodic oxidation process accelerated electron transfer, the maximum H2O2 production of Fe-Ni LDH@ZIF-67/CC electrode reached 264 mg/L, improving utilization efficiency of H2O2. The cathode possessing a satisfied TC degradation performance over a wide pH (3-9). Free radical capture experiment revealed the collaboration of ·O2-, ·OH, and 1O2 play a significant role in TC degradation. The 5 cycles experiment and metal ion leaching experiment showed that the proposed Fe-Ni LDH@ZIF-67/CC has good recyclability and stability. In addition, the proposed Fe-Ni LDH@ZIF-67/CC cathode achieved satisfying performance in real water (tap water: 97.3%, lake water: 97.7%), demonstrating the possibility for practical application. TC degradation pathways were proposed by theory analysis and experimental results. The toxicity of TC intermediates was reduced by Hetero-EF degradation according to Toxicity Estimation Software Tool and Escherichia coli growth inhibition experiments. This work provides a novel modified cathode to improve removal efficiency of antibiotics in wastewater.

Mechanistic investigation of the electricity and gallic acid synergistically accelerated Fe(III)/Fe(II) cycle for the degradation of carbamazepine

This study investigated the application of a natural plant polyphenol, gallic acid (GA) to form complex with iron to promote the redox cycle of Fe(III)/Fe(II) under neutral initial pH conditions in the electrochemical (EC) system for activation of peroxymonosulfate (PMS) to efficiently degrade carbamazepine (CBZ). Results demonstrated that the synergistic effects of GA and EC significantly improved the removal efficiency, and the EC/GA/Fe(III)/PMS system effectively removed 100% of CBZ within a wide initial pH range of 3.0-7.0. The optimum stoichiometric ratio of GA to Fe(III) was found as 2:1. Investigations including quenching experiment, chemical probe analysis, and electron paramagnetic resonance (EPR) analysis were conducted to identify the primary reaction radicals as •OH, SO4•-, along with the 1O2 and Fe(IV). In the EC/GA/Fe(III)/PMS system, the synergistic effect of GA and electrochemistry led to a remarkable enhancement in the generation of •OH. Furthermore, the complexation reduction mechanism of GA and Fe(III) was proposed based on experimental and instrumental analyses, which demonstrated that the semi-quinone products of GA were the main substances promoting the Fe(III)/Fe(II) cycle. Mass spectrometry results showed that CBZ generated 27 byproducts during degradation, with formic acid as the main product of GA. The degradation efficiency of the EC/GA/Fe(III)/PMS system remained stable and excellent, exhibiting remarkable performance in the presence of various inorganic anions, including Cl- and NO3-, as well as naturally occurring organic compounds such as fulvic acid (FA). Overall results indicated that the EC/GA/Fe(III)/PMS system can be applied to effectively treat practical wastewater treatment without requirement of pH adjustment.

CuFeS2 supported on dendritic mesoporous silica-titania for persulfate-assisted degradation of sulfamethoxazole under visible light

Sulfamethoxazole (SMX) is a prevalent sulfonamide antibiotic found in the environment, and it has a variety of detrimental effects on environmental sustainability and water safety. Recently, the combination of photocatalysis and sulfate radical-based advanced oxidation processes (SR-AOPs) has attracted a lot of interest as a viable technique for degradation of refractory pollutants. In this study, a visible light active CuFeS2 supported on dendritic mesoporous silica-titania (CuFeS2-DMST) photocatalyst was synthesized to improve the ability of TiO2 to activate persulfate (PS) by introducing CuFeS2 (Fe2+/Fe3+, Cu+/Cu2+ redox cycles). The CuFeS2-DMST/PS/Vis system demonstrated superior SMX degradation efficiency (88.9%, 0.0146 min-1) than TiO2 because of reduced e-/h+ recombination, excellent charge separation and mobility, and a greater surface area than TiO2. Furthermore, after four consecutive photocatalytic cycles, the system demonstrated moderate stability. From chemical quenching tests, O2●-, h+, 1O2, SO4●- and ●OH were found to be the main reactive oxidizing species. The formed intermediates during the degradation process were identified, and degradation mechanisms were proposed. This study proposes a viable technique for activating PS using a low-cost, stable, and high-surface-area TiO2-based photocatalyst, and this concept can be applied to design photocatalysts for water treatment.

An anode and cathode cooperative oxidation system constructed with Ee-GF as anode and CuFe2O4/Cu2O/Cu@EGF as cathode for the efficient removal of sulfamethoxazole

This study aimed to further improve the degradation efficiency of pollutants by electrochemical oxidation system and reduce the consumption of electric energy. A simple method of electrochemical exfoliation was used to modify graphite felt (GF) to prepare an anode material (Ee-GF) with high degradation performance. An anode and cathode cooperative oxidation system was constructed with Ee-GF as the anode and CuFe₂O₄/Cu₂O/Cu@EGF as the cathode to efficiently degrade sulfamethoxazole (SMX). Complete degradation of SMX was achieved within 30 min. Compared with anodic oxidation system alone, the degradation time of SMX was reduced by half and the energy consumption was reduced by 66.8 %. The system displayed excellent performance for the degradation of different concentrations (10-50 mg L^-1) of SMX, different pollutants, and under different water quality conditions. In addition, the system still maintained 91.7 % removal rate of SMX after ten consecutive runs. At least 12 degradation products and seven possible degradation routes of SMX were generated in the degradation process by the combined system. The eco-toxicity of degradation products of SMX was reduced after the proposed treatment. This study provided a theoretical basis for the safe, efficient, and low energy consumption removal of antibiotic wastewater.

Electrochemical activation of periodate with graphite electrodes for water decontamination: Excellent applicability and selective oxidation mechanism

Advanced oxidation technologies based on periodate (PI, IO₄^-) have garnered significant attention in water decontamination. In this work, we found that electrochemical activation using graphite electrodes (E-GP) can significantly accelerate the degradation of micropollutants by PI. The E-GP/PI system achieved almost complete removal of bisphenol A (BPA) within 15 min, exhibited unprecedented pH tolerance ranging from pH 3.0 to 9.0, and showed more than 90% BPA depletion after 20 h of continuous operation. Additionally, the E-GP/PI system can realize the stoichiometric transformation of PI into iodate, dramatically decreasing the formation of iodinated disinfection by-products. Mechanistic studies confirmed that singlet oxygen (¹O₂) is the primary reactive oxygen species in the E-GP/PI system. A comprehensive evaluation of the oxidation kinetics of ¹O₂ with 15 phenolic compounds revealed a dual descriptor model based on quantitative structure-activity relationship (QSAR) analysis. The model corroborates that pollutants exhibiting strong electron-donating capabilities and high pK_a values are more susceptible to attack by ¹O₂ through a proton transfer mechanism. The unique selectivity induced by ¹O₂ in the E-GP/PI system allows it to exhibit strong resistance to aqueous matrices. Thus, this study demonstrates a green system for the sustainable and effective elimination of pollutants, while providing mechanistic insights into the selective oxidation behaviour of ¹O₂.