Direct and mediated anodic oxidation of organic pollutants

Synergetic Oxidation of the Hydroxyl Radical and Superoxide Anion Lowers the Benzoquinone Intermediate Conversion Barrier and Potentiates Effective Aromatic Pollutant Mineralization

Regulation of the free radical types is crucial but challenging in the ubiquitous heterogeneous catalytic oxidation for chemosynthesis, biotherapy, and environmental remediation. Here, using aromatic pollutant (AP) removal as a prototype, we identify the massive accumulation of the benzoquinone (BQ) intermediate in the hydroxyl radical (•OH)-mediated AP degradation process. Theoretical prediction and experiments demonstrate that BQ is both a Lewis acid and base because of its unique molecular and electronic structure caused by the existence of symmetrical carbonyl groups; therefore, it is hard to be electrophilically added by oxidizing •OH as a result of the high reaction energy barrier (ΔG = 1.74 eV). Fortunately, the introduction of the superoxide anion (•O2-) significantly lowers the conversion barrier (ΔG = 0.91 eV) of BQ because •O2- can act as the electron donor and acceptor simultaneously, electrophilically and nucleophilically add to BQ synchronously, and break it down. Subsequently, the breakdown products can then be further oxidized by •OH until completely mineralized. Such synergistic oxidation based on •OH and •O2- timely eliminates BQ, potentiates AP mineralization, and inhibits electrode fouling caused by high-resistance polymeric BQ; more importantly, it effectively reduces toxicity, saves energy and costs, and decreases the environmental footprint, evidenced by the life cycle assessment.

Effect of resistance difference on distribution of antibiotics in bacterial cell and conjugative gene transfer risks during electrochemical flow through reaction

The occurrences and spread of antibiotic resistance (AR) mediated by horizontal gene transfer (HGT) of antibiotic resistance genes (ARGs) in aquatic environment have been aggravated because of the abuse of antibiotics. While the pressure of different antibiotics is known to induce the spread of AR in bacteria, whether distribution of different antibiotics in cell structure could affect HGT risks is not clear. Here, a significant difference between the distribution of tetracycline hydrochloride (Tet) and sulfamethoxazole (Sul) in cell structure during electrochemical flow through reaction (EFTR) process was firstly reported. Meanwhile, EFTR treatment possessed excellent disinfection performance and consequently controlled the HGT risks. The intracellular Tet (iTet) was discharged through efflux pumps to increase the content of extracellular Tet (eTet) due to the resistance of donor E. coli DH5α under the selective pressure of Tet, declining the damage of donor and plasmid RP4. The HGT frequency was 8.18-fold increase compared with that by EFTR treatment alone. While the secretion of intracellular Sul (iSul) was inhibited by blocking the formation of efflux pumps to inactivate the donor under the Sul pressure, and the total content of iSul and adsorbed Sul (aSul) to be 1.36-fold higher than that of eSul. Therefore, the reactive oxygen species (ROS) generation and cell membrane permeability were improved to release ARGs, and •OH attacked plasmid RP4 in the EFTR process, inhibiting the HGT risks. This study advances the awareness of the interaction between distribution of different antibiotics in cell structure and the HGT risks in the EFTR process.

Toxicological aspect of water treated by chlorine-based advanced oxidation processes: A review

As well established in the literature, residual toxicity is an important parameter for evaluating the sanitary and environmental safety of water treatment processes, and this parameter becomes even more crucial when chlorine-based processes are applied for water treatment. Eliminating initial toxicity or preventing its increase after water treatment remains a huge challenge mainly due to the formation of highly toxic disinfection by-products (DBPs) that stem from the degradation of organic contaminants or the interaction of the chlorine-based oxidants with different matrix components. In this review, we present a comprehensive discussion regarding the toxicological aspects of water treated using chlorine-based advanced oxidation processes (AOPs) and the recent findings related to the factors influencing toxicity, and provide directions for future research in the area. The review begins by shedding light on the advances made in the application of free chlorine AOPs and the findings from studies conducted using electrochemical technologies based on free chlorine generation. We then delve into the insights and contributions brought to the fore regarding the application of NH₂Cl- and ClO₂-based treatment processes. Finally, we broaden our discussion by evaluating the toxicological assays and predictive models employed in the study of residual toxicity and provide an overview of the findings reported to date on this subject matter, while giving useful insights and directions for future research on the topic.

Insight into micropollutant abatement during ultraviolet light-emitting diode combined electrochemical process: Reaction mechanism, contributions of reactive species and degradation routes

Electrochemical process coupling with ultraviolet light-emitting diode for micropollutant abatement was evaluated in the treatment of wastewater containing Cl^-. Four representative micropollutants, atrazine, primidone, ibuprofen and carbamazepine, were selected as target compounds. The impacts of operating conditions and water matrix on micropollutant degradation were investigated. Fluorescence excitation-emission matrix spectroscopy spectra and high performance size exclusion chromatography were employed to characterize the transformation of effluent organic matter in treatment. The degradation efficiencies of atrazine, primidone, ibuprofen and carbamazepine are 83.6 %, 80.6 %, 68.7 % and 99.8 % after 15 min treatment, respectively. The increment of current, Cl^- concentration and ultraviolet irradiance promote the micropollutant degradation. However, the presence of bicarbonate and humic acid inhibit micropollutant degradation. The mechanism of micropollutant abatement was elaborated based on reactive species contributions, density functional theory calculation and degradation routes. Free radicals (HO^•, Cl^•, ClO^• and Cl₂^•-) could be generated by chlorine photolysis and subsequent propagation reactions. The concentrations of HO^• and Cl^• are 1.14 × 10^-13 M and 2.0 × 10^-14 M in optimal condition, respectively, and the total contributions of HO^• and Cl^• for the degradation of atrazine, primidone, ibuprofen and carbamazepine are 24 %, 48 %, 70 % and 43 %, respectively. The degradation routes of four micropollutants are elucidated based on intermediate identification, Fukui function and frontier orbital theory. Micropollutants can be effectively degraded in actual wastewater effluent, and the small molecule compound proportion increases during effluent organic matter evolution. Compared with photolysis and electrolysis, the coupling of the two processes has potential for energy saving in micropollutant degradation, which shed light on the prospects of ultraviolet light-emitting diode coupling with electrochemical process for effluent treatment.

Selective Removal of Emerging Organic Contaminants from Water Using Electrogenerated Fe(IV) and Fe(V) under Near-Neutral Conditions

Fe(IV) and Fe(V) are promising oxidants for the selective removal of emerging organic contaminants (EOCs) from water under near-neutral conditions. The Fe(III)-assisted electrochemical oxidation system with a BDD anode (Fe(III)-EOS-BDD system) has been employed to generate Fe(VI), while the generation and contributions of Fe(IV) and Fe(V) have been largely ignored. Thus, we examined the feasibility and involved mechanisms of the selective degradation of EOCs in the Fe(III)-EOS-BDD system under near-neutral conditions. It was found that Fe(III) application selectively accelerated the electro-oxidation of phenolic and sulfonamide organics and made the oxidation system be resistant to interference from Cl^-, HCO₃^-, and humic acid. Several lines of evidence indicated that EOCs were decomposed via direct electron-transfer process on the BDD anode and by Fe(IV) and Fe(V) but not Fe(VI), besides HO^•. Fe(VI) was not generated until the exhaustion of EOCs. Furthermore, the overall contributions of Fe(IV) and Fe(V) to the oxidation of phenolic and sulfonamide organics were over 45%. Our results also revealed that Fe(III) was oxidized primarily by HO^• to Fe(IV) and Fe(V) in the Fe(III)-EOS-BDD system. This study advances the understanding of the roles of Fe(IV) and Fe(V) in the Fe(III)-EOS-BDD system and provides an alternative for utilizing Fe(IV) and Fe(V) under near-neutral conditions.

In situ generation of highly localized chlorine by laser-induced graphene electrodes during electrochemical disinfection

Laser-induced graphene (LIG) has gained popularity for electrochemical water disinfection due to its efficient antimicrobial activity when activated with low voltages. However, the antimicrobial mechanism of LIG electrodes is not yet fully understood. This study demonstrated an array of mechanisms working synergistically to inactivate bacteria during electrochemical treatment using LIG electrodes, including the generation of oxidants, changes in pH-specifically high alkalinity associated with the cathode, and electro-adsorption on the electrodes. All these mechanisms may contribute to the disinfection process when bacteria are close to the surface of the electrodes where inactivation was independent of the reactive chlorine species (RCS); however, RCS was likely responsible for the predominant cause of antibacterial effects in the bulk solution (i.e., ≥100 mL in our study). Furthermore, the concentration and diffusion kinetics of RCS in solution was voltage-dependent. At 6 V, RCS achieved a high concentration in water, while at 3 V, RCS was highly localized on the LIG surface but not measurable in water. Despite this, the LIG electrodes activated by 3 V achieved a 5.5-log reduction in Escherichia coli (E.coli) after 120-min electrolysis without detectable chlorine, chlorate, or perchlorate in the water, suggesting a promising system for efficient, energy-saving, and safe electro-disinfection.

Electrochemical degradation of acrylic acid using Ti/Ta2O5-IrO2 electrode

Acrylic acid (AA) is widely used as a raw material in the industrial production of various chemicals. Its extensive use has produced environmental problems that need to be solved. The Ti/Ta₂O₅-IrO₂ electrode, a type of dimensionally stable anode, was used to investigate the electrochemical deterioration of AA. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis showed that IrO₂ existed as an active rutile crystal and as a TiO₂-IrO₂ solid solution in Ti/Ta₂O₅-IrO₂ electrode with a corrosion potential of 0.212 V and chlorine evolution potential of 1.30 V. The effects of current density, plate spacing, electrolyte concentration, and initial concentration on the electrochemical degradation of AA were investigated. Response surface methodology (RSM) was used to determine the ideal degradation conditions: current density 22.58 mA cm^-2, plate spacing 2.11 cm, and electrolyte concentration 0.07 mol L^-1, and the highest degradation rate reached was 95.6%. Free radical trapping experiment verified that reactive chlorine played a dominant role in the degradation of AA. The degradation intermediates were analyzed by GC-MS.

Effects of electrochemical intervention on the remediation of black-odorous water: insights into microbial community dynamics and functional shifts in sediments

Black-odorous water is a severe environmental issue that has received continuous attention. The major purpose of the present study was to propose an economical, practical, and pollution-free treatment technology. In this study, the in situ remediation of black-odorous water was conducted by applying different voltages (2.5, 5, and 10 V) to improve oxidation conditions of the surface sediments. The study investigated the effects of voltage intervention on water quality, gas emissions, and microbial community dynamics in surface sediments during the remediation process. The results indicated that the voltage intervention can effectively increase the oxidation-reduction potential (ORP) of the surface sediments and inhibit the emissions of H₂S, NH₃, and CH₄. Moreover, the relative abundances of typical methanogens (Methanosarcina and Methanolobus) and sulfate-reducing bacteria (Desulfovirga) decreased because of the increase in ORP after the voltage treatment. The microbial functions predicted by FAPROTAX also demonstrated the inhibition of methanogenesis and sulfate reduction functions. On the contrary, the total relative abundances of chemoheterotrophic microorganisms (e.g., Dechloromonas, Azospira, Azospirillum, and Pannonibacter) in the surface sediments increased significantly, which led to enhanced biochemical degradability of the black-odorous sediments as well as CO₂ emissions.

Multivariate optimization of characteristic parameters of continuous-flow system with a front buffer tank for industrial reverse osmosis concentrate treatment

Industrial reverse osmosis concentrate (ROC) was electrochemically oxidized using a continuous-flow system (CFS) with a front buffer tank. Multivariate optimization including Plackett-Burman (PBD) and central composite design based on response surface method (CCD-RSM) was implemented to investigate the effects of characteristic (e.g., recirculation ratio (R value), ratio of buffer tank and electrolytic zone (R_V value)) and routine (e.g., current density (i), inflow linear velocity (v) and electrode spacing (d)) parameters. R, v values and current density significantly influenced chemical oxygen demand (COD) and NH₄⁺-N removal and effluent active chlorine species (ACS) level, while electrode spacing and R_V value had negligible effects. High chloride content of industrial ROC facilitated the generation of ACS and subsequent mass transfer, low hydraulic retention time (HRT) of electrolytic cell improved the mass transfer efficiency, and high HRT of buffer tank prolonged the reaction between the pollutants and oxidants. The significance levels of COD removal, energy efficiency, effluent ACS level and toxic byproduct level CCD-RSM models were validated by statistical test results, including higher F value than critical effect value, lower P value than 0.05, low deviation between predicted and observed values, and normal distribution of calculated residuals. The highest pollutant removal was achieved at a high R value, a high current density and a low v value; the highest energy efficiency was achieved at a high R, a low current density and a high v value; the lowest effluent ACS and toxic byproduct levels were achieved at a low R value, a low current density and a high v value. Following the multivariate optimization, the optimum parameters were decided to be v = 1.2 cm h^-1, i ≥ 8 mA cm^-2, d ≥ 4, R_V = 10-20 and R = 1 to achieve better effluent quality (i.e., lower effluent pollutant, ACS and toxic byproduct levels).

A review on three-dimensional electrochemical technology for the antibiotic wastewater treatment

The potential genotoxicity and non-biodegradability of antibiotics in the natural water bodies threaten the survival of various living things and cause serious environmental pollution and destruction. Three-dimensional (3D) electrochemical technology is considered a powerful means for antibiotic wastewater treatment as it can degrade non-biodegradable organic substances into non-toxic or harmless substances and even completely mineralize them under the action of electric current. Therefore, antibiotic wastewater treatment using 3D electrochemical technology has now become a hot research topic. Thus, in this review, a detailed and comprehensive investigation was conducted on the antibiotic wastewater treatment using 3D electrochemical technology, including the structure of the reactor, electrode materials, the influence of operating parameters, reaction mechanism, and combination with other technologies. Many studies have shown that the materials of electrode, especially particle electrode, have a great effect on the antibiotic wastewater treatment efficiency. The influence of operating parameters such as cell voltage, solution pH, and electrolyte concentration was very significant. Combination with other technologies such as membrane and biological technologies has effectively increased antibiotic removal and mineralization efficiency. In conclusion, the 3D electrochemical technology is considered as a promising technology for the antibiotic wastewater treatment. Finally, the possible research directions of the 3D electrochemical technology for antibiotic wastewater treatment were proposed.

Application of Electrochemical Oxidation for Water and Wastewater Treatment: An Overview

In recent years, the discharge of various emerging pollutants, chemicals, and dyes in water and wastewater has represented one of the prominent human problems. Since water pollution is directly related to human health, highly resistant and emerging compounds in aquatic environments will pose many potential risks to the health of all living beings. Therefore, water pollution is a very acute problem that has constantly increased in recent years with the expansion of various industries. Consequently, choosing efficient and innovative wastewater treatment methods to remove contaminants is crucial. Among advanced oxidation processes, electrochemical oxidation (EO) is the most common and effective method for removing persistent pollutants from municipal and industrial wastewater. However, despite the great progress in using EO to treat real wastewater, there are still many gaps. This is due to the lack of comprehensive information on the operating parameters which affect the process and its operating costs. In this paper, among various scientific articles, the impact of operational parameters on the EO performances, a comparison between different electrochemical reactor configurations, and a report on general mechanisms of electrochemical oxidation of organic pollutants have been reported. Moreover, an evaluation of cost analysis and energy consumption requirements have also been discussed. Finally, the combination process between EO and photocatalysis (PC), called photoelectrocatalysis (PEC), has been discussed and reviewed briefly. This article shows that there is a direct relationship between important operating parameters with the amount of costs and the final removal efficiency of emerging pollutants. Optimal operating conditions can be achieved by paying special attention to reactor design, which can lead to higher efficiency and more efficient treatment. The rapid development of EO for removing emerging pollutants from impacted water and its combination with other green methods can result in more efficient approaches to face the pressing water pollution challenge. PEC proved to be a promising pollutants degradation technology, in which renewable energy sources can be adopted as a primer to perform an environmentally friendly water treatment.

Electrochemical Oxidation of Organic Pollutants in Aqueous Solution Using a Ti4O7 Particle Anode

Anodes based on substoichiometric titanium oxide (Ti₄O₇) are among the most effective for the anodic oxidation of organic pollutants in aqueous solutions. Such electrodes can be made in the form of semipermeable porous structures called reactive electrochemical membranes (REMs). Recent work has shown that REMs with large pore sizes (0.5-2 mm) are highly efficient (comparable or superior to boron-doped diamond (BDD) anodes) and can be used to oxidize a wide range of contaminants. In this work, for the first time, a Ti₄O₇ particle anode (with a granule size of 1-3 mm and forming pores of 0.2-1 mm) was used for the oxidation of benzoic, maleic and oxalic acids and hydroquinone in aqueous solutions with an initial COD of 600 mg/L. The results demonstrated that a high instantaneous current efficiency (ICE) of about 40% and a high removal degree of more than 99% can be achieved. The Ti₄O₇ anode showed good stability after 108 operating hours at 36 mA/cm².

Pulsed Electrolysis of Boron-Doped Carbon Dramatically Improves Impurity Tolerance and Longevity of H2O2 Production

Electrocatalytic water treatment has emerged in the limelight of scientific interest, yet its long-term viability remains largely in the dark. Herein, we present for the first time a comprehensive framework on how to optimize pulsed electrolysis to bolster catalyst impurity tolerance and overall longevity. By examining real wastewater constituents and assessing different catalyst designs, we deconvolute the complexities associated with key pulsing parameters to formulate optimal sequences that maximize operational lifetime. We showcase our approach for cathodic H2O2 electrosynthesis, selected for its widespread importance to wastewater treatment. Our results unveil superior performance for a boron-doped carbon catalyst over state-of-the-art oxidized carbon, with high selectivity (>75%) and near complete recoveries in overpotentials even in the presence of highly detrimental Ni2+ and Zn2+ impurities. We then adapt these fine-tuned settings, obtained under a three-electrode arrangement, for practical two-electrode operation using a novel strategy that conserves the desired electrochemical potentials at the catalytic interface. Even under various impurity concentrations, our pulses substantially improve long-term H2O2 production to 287 h and 35 times that attainable via conventional electrolysis. Our findings underscore the versatility of pulsed electrolysis necessary for developing more practical water treatment technologies.

Sensing of chemical oxygen demand (COD) by amperometric detection-dependence of current signal on concentration and type of organic species

The standard method to determine chemical oxygen demand (COD) with K₂Cr₂O₆ uses harmful chemicals, has a long analysis time, and cannot be used for on-site online monitoring. It is therefore necessary to find a fast, cheap, and harmless alternative. The amperometric determination of COD on boron-doped diamond (BDD) electrodes is a promising approach. However, to be a suitable alternative, the electrochemical method must at least be able to determine the COD of water samples independently of the contained substances. Therefore, the current signal as a function of various organic materials was investigated for the first time. It was shown that the height of the signal current depended on the type of organic matter in single-substance solutions and that this substance dependency increases with the amount of COD. Those findings could be explained by the mechanism proposed for this reaction, showing that the selectivity of the reaction depends on the ratio of the concentration of hydroxyl radicals and organic species. We give an outlook on how to improve the method in order to increase the linear working range and avoid signal variance and how to further explain the signal variance.

Electrochemical removal of gaseous benzene using a flow-through reactor with efficient and ultra-stable titanium suboxide/titanium-foam anode at ambient temperature

Catalytic oxidation technology is currently considered as a feasible approach to degrade and mineralize volatile organic compounds (VOCs). However, it is still challenging to realize efficient removal of VOCs through catalytic oxidation at room temperature. In our study, a novel flow-through electrocatalytic reactor was designed, composed of porous solid-electrolyte, gas-permeable titanium sub-oxides/titanium-foam (TiSO/Ti-foam) as anode and platinum coated titanium foam (Pt/Ti-foam) as cathode. This device could oxidize nearly 100% of benzene (10 ppm) to carbon dioxide at a current density of 1.2 mA/cm² under room temperature. More importantly, the device maintained excellent stability over 1000 h. Mechanism of benzene mineralization was discussed. Hydroxyl radicals generated on the TiSO/Ti-foam anode played a crucial role in the oxidation of benzene. This study provides a promising prototype of the electrochemical air purifier, and may find its application in domestic and industrial air pollution control.

Efficient detoxification of textile wastewater by applying Chenopodium album nanoparticles and its application in simulated metal-bearing effluents removal

Large-scale pollution of water and soils bodies is associated with the discharge of the untreated textile industry effluents. Halophytes grows on saline lands and accumulate secondary metabolites and other stress protective compounds. Utilization of Chenopodium album (halophytes) to synthesize zinc oxide (ZnO) and their efficiency to treat different concentrations of textile industry waste water is proposed in this study. Potential of nanoparticles textile industry waste water effluents was also analyzed by exposing different concentrations of nanoparticles (0 (control), 0.2, 0.5, 1 mg) and time intervals of 5, 10, and 15 days. The absorption peaks by UV region, FTIR and SEM analysis were used characterized on ZnO NPs for the first time. FTIR analysis showed the preens of various functional groups and vital phytochemicals that can play its role in the formation of nanoparticles that can be used for trace elements removal and bioremediation. SEM analysis indicated that the pure ZnO NPs synthesis ranged from 30 to 57 nm. Results shows that the green synthesis of halophytic nanoparticles presents maximum removal capacity after 15 days exposure to 1 mg of ZnO NPs. Hence, the prepared ZnO Nps from halophytes can be a viable solution for treating the textile industry effluents before they are discharged into water bodies for sustainable environmental growth and environmental safety.

Electrochemical oxidation of carbamazepine in water using enhanced blue TiO2 nanotube arrays anode on porous titanium substrate

In this study, a blue TiO₂ nanotube arrays anode on porous titanium substrate (Ti-porous/blue TiO₂ NTA) was successfully fabricated by facile anodization and in situ reduction, and was used to investigate the electrochemical oxidation of carbamazepine (CBZ) in aqueous solution. The surface morphology and crystalline phase of the fabricated anode were characterized by SEM, XRD, Raman spectroscopy and XPS, and the electrochemical analysis confirmed that blue TiO₂ NTA on Ti-porous substrate had larger electroactive surface area, better electrochemical performance and higher ⋅OH generation ability than that on Ti-plate substrate. The removal efficiency of 20 mg L^-1 CBZ in 0.05 M Na₂SO₄ solution reached 99.75% at 8 mA cm^-2 after 60 min electrochemical oxidation, and the rate constant was 0.101 min^-1 with low energy consumption. EPR analysis and free radical sacrificing experiments showed that ⋅OH played a key role in the electrochemical oxidation. The possible oxidation pathways of CBZ were proposed through the identification of degradation products, and the main reactions may involve deamidization, oxidization, hydroxylation and ring-opening. Compared with Ti-plate/blue TiO₂ NTA anode, Ti-porous/blue TiO₂ NTA anode displayed excellent stability and reusability, and is promising to be used in the electrochemical oxidation of CBZ in wastewater.

A Novel Strategy of Combined Pulsed Electro-Oxidation and Electrolysis for Degradation of Sulfadiazine

A combination of the peroxymonosulfate (PMS) electro-activation process and the electro-oxidation process driven by a pulsed electric field (PEF) was used to degrade sulfadiazine (SND) wastewater. Mass transfer is the limiting step of electrochemical processes. The PEF could enhance mass transfer efficiency by reducing the polarization effect and increasing the instantaneous limiting current compared with the constant electric field (CEF), which could benefit the electro-generation of active radicals. The degradation rate of SND after 2 h was 73.08%. The experiments investigated the effects of operating parameters of pulsed power supply, PMS dosage, pH value and electrode inter distance on the degradation rate of SND. The predicted response value of single-factor performance experiments was obtained as 72.26% after 2 h, which was basically consistent with the experimental value. According to the quenching experiments and EPR tests, both SO₄^•- and •OH were present in the electrochemical processes. The generation of active species were significantly greater in the PEF system than that in the CEF system. Moreover, four kinds of intermediate products were detected during the degradation by LC-MS. This paper presents a new aspect for electrochemical degradation of sulfonamide antibiotics.

Changes of dissolved organic matter fractions and formation of oxidation byproducts during electrochemical treatment of landfill leachates: Development of spectroscopic indicators for process optimization

Electrochemical oxidation (EO) is an attractive option for treatment of dissolved organic matter (DOM) in landfill leachate but concerns remain over the energy efficiency and formation of oxidation byproducts ClO₃^- and ClO₄^-. In this study, EO treatment of landfill leachates was carried out using representative active and nonactive anode materials, cell configurations and current densities. Size exclusion chromatograms coupled with 2D synchronous and asynchronous correlation analysis showed that the sensitivity of DOM fractions to EO degradation was dependent on the anode material. The nonactive boron-doped diamond (BDD) anode demonstrated the best performance for DOM oxidation. The humic acid-like fraction (HA, 2.5-20 kDa) predominated the visible absorbance of landfill leachates at λ ≥400 nm, and it generally had the highest reaction rates except the occurrence of the pH-induced denaturation and precipitation of the proteinaceous biopolymer fraction (BP, >20 kDa). During the EO treatment of landfill leachate with BDD anode, the UV absorbance spectra of landfill leachates at wavelengths <400 nm were affected by the formation of free chlorine. Instead, the decrease of Abs420 was found to be a good indicator of the shift of the oxidation from predominantly HA fraction to the proteinaceous BP fraction. The behavior of the Abs420 parameter was also indicative of the transition from the energy-efficient oxidation of DOM to the dominance of side reactions of chlorine evolution and the subsequent formation of ClO₃^- and ClO₄^-. These findings suggest that the EO treatment of landfill leachate can be optimized by adjusting the current density with feedback signals from the online monitoring of Abs420, to achieve a trade-off between degradation of DOM and control of ClO₃^- and ClO₄^-.

A review of advanced oxidation process towards organic pollutants and its potential application in fracturing flowback fluid

Fracturing flowback fluid (FFF) including various kinds of organic pollutants that do harms to people and new treatments are urgently needed. Advanced oxidation processes (AOPs) are suitable methods in consideration with molecular weight, removal cost and efficiency. Here, we summarize the recent studies about AOP treatments towards organic pollutants and discuss the application prospects in treatment of FFF. Immobilization and loading methods of catalysts, evaluation method of degradation of FFF, and continuous treatment process flow are discussed in this review. In conclusion, further studies are urgently needed in aspects of catalyst loading methods, macromolecule organic evaluation methods, industrial process, and pathways of macromolecule organics' decomposition.

Ozone- and Hydroxyl Radical-Mediated Oxidation of Pharmaceutical Compounds Using Ni-Doped Sb-SnO2 Anodes: Degradation Kinetics and Transformation Products

Electrochemical oxidation provides a versatile technique for treating wastewater streams onsite. We previously reported that a two-layer heterojunction Ni-Sb-SnO₂ anode (NAT/AT) can produce both ozone (O₃) and hydroxyl radical (^•OH). In this study, we explore further the applicability of NAT/AT anodes for oxidizing pharmaceutical compounds using carbamazepine (CBZ) and fluconazole (FCZ) as model probe compounds. Details of the oxidation reaction kinetics and subsequent reaction products are investigated in the absence and presence of chloride (Cl^-) and sulfate (SO₄ ^2-). In all cases, faster or comparable degradation kinetics of CBZ and FCZ are achieved using the double-layered NAT/AT anode coupled with a stainless steel (SS) cathode in direct comparison to an identical setup using a boron-doped diamond anode. Production of O₃ on NAT/AT enhances the elimination of both parent compounds and their transformation products (TPs). Very fast CBZ degradation is observed during NAT/AT-SS electrolysis in both NaClO₄ and NaCl electrolytes. However, more reaction products are identified in the presence of Cl^- than ClO₄ ^- (23 TPs vs 6). Rapid removal of FCZ is observed in NaClO₄, while the degradation rate is retarded in NaCl depending on the [Cl^-]. In SO₄ ^2--containing electrolytes, altered reaction pathways and transformation product distributions are observed due to sulfate radical generation. SO₄ ^·- oxidation produces fewer hydroxylated products and promotes the oxidation of aldehydes to carboxylic acids. Similar trend in treatment performance is observed in mixtures of CBZ and FCZ with other pharmaceutical compounds in latrine wastewater and secondary WWTP effluent.

Three-dimensional structured electrode for electrocatalytic organic wastewater purification: Design, mechanism and role

Considering the growing need in decentralized water treatment, the application of electrocatalytic processes (EP) to achieve organic wastewater purification will be dominant in the near future due to high efficiency, small reactor assembly as well as the flexibility of operation and management. The catalytic performance of electrode materials determines the development of this technology. Among them, the unique three-dimensional (3D) structure electrode shows better performance than two-dimensional (2D) electrode in increasing mass transfer, enhancing adsorption and exposing more active sites. Hence, this review starts with the introduction of definition, classification, advantages and disadvantages of 3D electrode materials. Then a critical discussion on the design and construction of 3D electrode materials for organic wastewater purification application is provided. Next, the removal mechanism of organic pollutants on the surface of 3D electrode, the role of 3D structure, the design of reactor with 3D electrode, the conversion and toxicity of degradation products, electrode energy efficiency, stability and cost, are comprehensively reviewed. At last, current challenges and future perspectives for the development of 3D electrode materials are addressed. We deem that this review will provide a valuable insight into the design and application of 3D electrodes in environmental water purification.

Energetic evaluation of phenol wastewater treatment by reverse electrodialysis reactor using different anodes

Efficient electrode materials are essential to convert salinity gradient energy into oxidative degradation energy and electrical energy by reverse electrodialysis reactor (REDR). In this context, comparative experiments of REDR using different anodes (Ti/IrO₂-RuO₂, Ti/PbO₂ and Ti/Ti₄O₇) were conducted. The effects of output current and electrode rinse solution (ERS) flowrate on mineralization efficiency and energy output were discussed. Results demonstrated that the COD removal rate(η_COD) rose almost linearly with output current and ERS flowrate when using Ti/Ti₄O₇ anode, but excessive operating conditions caused a slow increase or even decrease of η_COD when using Ti/IrO₂-RuO₂ or Ti/PbO₂ anodes. The order of electrode system potential loss (E_ele) for the three anodes was Ti/Ti₄O₇> Ti/PbO₂> Ti/IrO₂-RuO₂. High E_ele was beneficial to η_COD but had a negative effect on the net output power (P_net) of REDR. Regardless of the applied anodes, increasing the current and decreasing the ERS flowrate was detrimental to P_net due to higher E_ele. Based on these findings, four energy efficiency parameters were defined to evaluate energy recovery from multiple perspectives by linking energy output with mineralization capacity. They were electrode efficiency (η_ele), energy efficiency (EE), general current efficiency (GCE) and energy consumption (EC), respectively. Results showed that REDR with Ti/Ti₄O₇ anodes and suitable operating conditions achieved the optimal energy indicators and mineralization efficiency, which provided an efficient and economical option for wastewater treatment and energy recovery.

Treatment of wastewater from the washing process of a municipal solid waste collection container by electrochemical treatment using different anode materials: a statistical optimization

An underground municipal solid waste (MSW) container should be washed periodically to prevent/reduce odor and leachate production. In this study, the treatment process of wastewater derived from the washing process of an MSW container was investigated using the electrochemical (EC) treatment process with different anode materials (Fe, TiO₂, and graphite). Response surface methodology (RSM) was used to evaluate the effect of process parameters such as initial pH, applied current, and reaction time on chemical oxygen demand (COD), Tannin/Lignin, and color removals. According to the results obtained from the RSM models, all process parameters were significant. The optimum process parameters in terms of COD removal were derived from the models for each anode material. Under the optimized conditions, the COD removals were determined to be 93.25%, 75.95%, and 98.46% for Fe-Fe, TiO₂-Fe, and graphite-Fe electrode pairs, respectively. The color and Tannin/Lignin removals were determined as 98.12% and 77.78% for the Fe-Fe, 92.76% and 98.45% for the TiO₂-Fe and 94.50% and 79.56% for the graphite-Fe electrode pair, respectively. The specific energy consumption (SEC) values were found as 46.95, 300.02, and 32.95 kWh/m³ for each electrode combination given above, respectively. In terms of both removal efficiencies and SEC, the most effective anode material was determined as graphite, followed by iron.

Emerging Phosphate-Functionalized Co3O4/Kaolinite Composites for Enhanced Activation of Peroxymonosulfate

The Fenton-like reaction, as one of the most efficient strategies to generate radical species for the degradation of environmental pollutants, has attracted considerable attention. However, engineering low-cost catalysts with excellent activity by phosphate surface functionalization has seldom been used for the activation of peroxymonosulfate (PMS). Herein, emerging phosphate-functionalized Co₃O₄/kaolinite (P-Co₃O₄/Kaol) catalysts have been prepared by hydrothermal and phosphorization. Kaolinite nanoclay with rich hydroxyl groups plays a vital role in realizing phosphate functionalization. The results indicate that P-Co₃O₄/Kaol shows superior catalytic performance and excellent stability to the degradation of Orange II, which could be attributed to the existence of phosphate that promotes the adsorption of PMS and the electron transfer of Co²⁺/Co³⁺ cycles. Furthermore, the ^•OH radical was identified as the dominating reactive species for the degradation of Orange II compared to the SO₄^•- radical. This work could offer a novel preparation strategy for emerging functionalized nanoclay-based catalysts for effective pollutant degradation.

Electrochemical-based approaches for the treatment of forever chemicals: Removal of perfluoroalkyl and polyfluoroalkyl substances (PFAS) from wastewater

Electrochemical based approaches for the treatment of recalcitrant water borne pollutants are known to exhibit superior function in terms of efficiency and rate of treatment. Considering the stability of Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are designated as forever chemicals, which generating from various industrial activities. PFAS are contaminating the environment in small concentrations, yet exhibit severe environmental and health impacts. Electro-oxidation (EO) is a recent development that treats PFAS, in which different reactive species generates at anode due to oxidative reaction and reductive reactions at the cathode. Compared to water and wastewater treatment methods those being implemented, electrochemical approaches demonstrate superior function against PFAS. EO completely mineralizes (almost 100 %) non-biodegradable organic matter and eliminate some of the inorganic species, which proven as a robust and versatile technology. Electrode materials, electrolyte concentration pH and the current density applying for electrochemical processes determine the treatment efficiency. EO along with electrocoagulation (EC) treats PFAS along with other pollutants from variety of industries showed highest degradation of 7.69 mmol/g of PFAS. Integrated approach with other processes was found to exhibit improved efficiency in treating PFAS using several electrodes boron-doped diamond (BDD), zinc, titanium and lead based with efficiency the range of 64 to 97 %.

Challenges and Future Roadmaps in Heterogeneous Electro-Fenton Process for Wastewater Treatment

The efficiency of heterogeneous electro-Fenton technology on the degradation of recalcitrant organic pollutants in wastewater is glaringly obvious. This green technology can be effectively harnessed for addressing ever-increasing water-related challenges. Due to its outstanding performance, eco-friendliness, easy automation, and operability over a wide range of pH, it has garnered significant attention from different wastewater treatment research communities. This review paper briefly discusses the principal mechanism of the electro-Fenton process, the crucial properties of a highly efficient heterogeneous catalyst, the heterogeneous electro-Fenton system enabled with Fe-functionalized cathodic materials, and its essential operating parameters. Moreover, the authors comprehensively explored the major challenges that prevent the commercialization of the electro-Fenton process and propose future research pathways to countervail those disconcerting challenges. Synthesizing heterogeneous catalysts by application of advanced materials for maximizing their reusability and stability, the full realization of H₂O₂ activation mechanism, conduction of life-cycle assessment to explore environmental footprints and potential adverse effects of side-products, scale-up from lab-scale to industrial scale, and better reactor design, fabrication of electrodes with state-of-the-art technologies, using the electro-Fenton process for treatment of biological contaminants, application of different effective cells in the electro-Fenton process, hybridization of the electro-Fenton with other wastewater treatments technologies and full-scale analysis of economic costs are key recommendations which deserve considerable scholarly attention. Finally, it concludes that by implementing all the abovementioned gaps, the commercialization of electro-Fenton technology would be a realistic goal.

In-situ Electrochemical Synthesis of H2O2 for p-nitrophenol Degradation Utilizing a Flow-through Three-dimensional Activated Carbon Cathode with Regeneration Capabilities

The growing ubiquity of recalcitrant organic contaminants in the aqueous environment poses risks to effective and efficient water treatment and reuse. A novel three-dimensional (3D) electrochemical flow-through reactor employing activated carbon (AC) encased in a stainless-steel (SS) mesh as a cathode is proposed for the removal and degradation of a model recalcitrant contaminant p-nitrophenol (PNP), a toxic compound that is not easily biodegradable or naturally photolyzed, can accumulate and lead to adverse environmental health outcomes, and is one of the more frequently detected pollutants in the environment. As a stable 3D electrode, granular AC supported by a SS mesh frame as a cathode is hypothesized to 1) electrogenerate H2O2 via a 2-electron oxygen reduction reaction on the AC surface, 2) initiate decomposition of this electrogenerated H2O2 to form hydroxyl radicals on catalytic sites of the AC surface 3) remove PNP molecules from the waste stream via adsorption, and 4) co-locate the PNP contaminant on the carbon surface to allow for oxidation by formed hydroxyl radicals. Additionally, this design is utilized to electrochemically regenerate the AC within the cathode that is significantly saturated with PNP to allow for environmentally friendly and economic reuse of this material. Under flow conditions with optimized parameters, the 3D AC electrode is nearly 20% more effective than traditional adsorption in removing PNP. 30 grams of AC within the 3D electrode can remove 100% of the PNP compound and 92% of TOC under flow. The carbon within the 3D cathode can be electrochemically regenerated in the proposed flow system and design thereby increasing the adsorptive capacity by 60%. Moreover, in combination with continuous electrochemical treatment, the total PNP removal is enhanced by 115% over adsorption. It is anticipated this platform holds great promises to eliminate analogous contaminants as well as mixtures.

Electrochemical Removal of Nitrogen Compounds from a Simulated Saline Wastewater

In the last few years, many industrial sectors have generated and discharged large volumes of saline wastewater into the environment. In the present work, the electrochemical removal of nitrogen compounds from synthetic saline wastewater was investigated through a lab-scale experimental reactor. Experiments were carried out to examine the impacts of the operational parameters, such as electrolyte composition and concentration, applied current intensity, and initial ammoniacal nitrogen concentration, on the total nitrogen removal efficiency. Using NaCl as an electrolyte, the N_TOT removal was higher than Na₂SO₄ and NaClO₄; however, increasing the initial NaCl concentration over 250 mg·L^-1 resulted in no benefits for the N_TOT removal efficiency. A rise in the current intensity from 0.05 A to 0.15 A resulted in an improvement in N_TOT removal. Nevertheless, a further increase to 0.25 A led to basically no enhancement of the efficiency. A lower initial ammoniacal nitrogen concentration resulted in higher removal efficiency. The highest N_TOT removal (about 75%) was achieved after 90 min of treatment operating with a NaCl concentration of 250 mg·L^-1 at an applied current intensity of 0.15 A and with an initial ammoniacal nitrogen concentration of 13 mg·L^-1. The nitrogen degradation mechanism proposed assumes a series-parallel reaction system, with a first step in which NH₄⁺ is in equilibrium with NH₃. Moreover, the nitrogen molar balance showed that the main product of nitrogen oxidation was N₂, but NO₃^- was also detected. Collectively, electrochemical treatment is a promising approach for the removal of nitrogen compounds from impacted saline wastewater.

Insight into advanced oxidation processes for the degradation of fluoroquinolone antibiotics: Removal, mechanism, and influencing factors

The enrichment and transport of antibiotics in the environments pose many potential hazards to aquatic animals and humans, which has become one of the public health challenges worldwide. As a widely used class of antibiotics, fluoroquinolones (FQs) generally accumulated in the environments as traditional sewage treatment plants cannot completely remove them. Advanced oxidation processes (AOPs) have been shown to be a promising method for the abatement of antibiotic contamination. In this review, influencing factors and relevant mechanisms of FQs removal by various AOPs were summarized. Compared with other AOPs, photocatalytic ozone may be considered as a cost-effective method for degrading FQs. Finally, the benefits and application restrictions of AOPs were discussed, along with proposed research directions to provide new insights into the control of FQs pollutant via AOPs in practical applications.

PbO2 materials for electrochemical environmental engineering: A review on synthesis and applications

Lead dioxide (PbO₂) materials have been widely employed in various fields such as batteries, electrochemical engineering, and more recently environmental engineering as anode materials, due to their unique physicochemical properties. Key performances of PbO₂ electrodes, such as energy efficiency and space-time yield, are influenced by morphological as well as compositional factors. Micro-nano structure regulation and decoration of metal/non-metal on PbO₂ is an outstanding technique to revamp its electrocatalytic activities and enhance environmental engineering efficiency. The aim of this review is to comprehensively summarize the recent research progress in the morphology control, the structure constructions, and the element doping of PbO₂ materials, further with many environmental application cases evaluated. Concerning electrochemical environmental engineering, the lead dioxide employed in chemical oxygen demand detection, ozone generators, and wastewater treatment has been comprehensively reviewed. In addition, the future research perspectives, challenges and the opportunities on PbO₂ materials for environmental applications are proposed.

Removal of Iodine-Containing X-ray Contrast Media from Environment: The Challenge of a Total Mineralization

Iodinated X-ray contrast media (ICM) as emerging micropollutants have attracted considerable attention in recent years due to their high detected concentration in water systems. It results in environmental issues partly due to the formation of toxic by-products during the disinfection process in water treatment. Consequently, various approaches have been investigated by researchers in order to achieve ICM total mineralization. This review discusses the different methods that have been used to degrade them, with special attention to the mineralization yield and to the nature of formed by-products. The problem of pollution by ICM is discussed in the first part dedicated to the presence of ICM in the environment and its consequences. In the second part, the processes for ICM treatment including biological treatment, advanced oxidation/reductive processes, and coupled processes are reviewed in detail. The main results and mechanisms involved in each approach are described, and by-products identified during the different treatments are listed. Moreover, based on their efficiency and their cost-effectiveness, the prospects and process developments of ICM treatment are discussed.

Wastewater treatment by anodic oxidation in electrochemical advanced oxidation process: Advance in mechanism, direct and indirect oxidation detection methods

Electrochemical Advanced Oxidation Process (EAOP) has been applied to the degradation of refractory pollutants in wastewater due to its strong oxidation capacity, high degradation efficiency, simple operation, and mild reaction. Among electrochemical processes, anodic oxidation (AO) is the most widely used and its mechanism is mainly divided into direct oxidation and indirect oxidation. Direct oxidation means that pollutants are oxidized at the anode by direct electron transfer. Indirect oxidation refers to the generation of active species during the electrolytic reaction, which acts on pollutants. The mechanism of AO process is controlled by many factors, including electrode type, electrocatalyst material, wastewater composition, pH, applied current and voltage levels. It is very important to explore the reaction mechanism of electrochemical treatment, which determines the efficiency of the reaction, the products of the reaction, and the extent of reaction. This paper firstly reviews the current research progress on the mechanism of AO process, and summarizes in detail the different mechanisms caused by influencing factors under common AO process. Then, strategies and methods to distinguish direct oxidation and indirect oxidation mechanisms are reviewed, such as intermediate product analysis, electrochemical test analysis, active species detection, theoretical calculation, and the limitations of these methods are analyzed. Finally some suggestions are put forward for the study of the mechanism of electrochemical advanced oxidation.

New Magnetically Assembled Electrode Consisting of Magnetic Activated Carbon Particles and Ti/Sb-SnO2 for a More Flexible and Cost-Effective Electrochemical Oxidation Wastewater Treatment

Magnetic activated carbon particles (Fe3O4/active carbon composites) as auxiliary electrodes (AEs) were fixed on the surface of Ti/Sb-SnO2 foil by a NdFeB magnet to form a new magnetically assembled electrode (MAE). Characterizations including cyclic voltammetry, Tafel analysis, and electrochemical impedance spectroscopy were carried out. The electrochemical oxidation performances of the new MAE towards different simulated wastewaters (azo dye acid red G, phenol, and lignosulfonate) were also studied. Series of the electrochemical properties of MAE were found to be varied with the loading amounts of AEs. The electrochemical area as well as the number of active sites increased significantly with the AEs loading, and the charge transfer was also facilitated by these AEs. Target pollutants’ removal of all simulated wastewaters were found to be enhanced when loading appropriate amounts of AEs. The accumulation of intermediate products was also determined by the AEs loading amount. This new MAE may provide a landscape of a more cost-effective and flexible electrochemical oxidation wastewater treatment (EOWT).

A novel conductive carbon-based forward osmosis membrane for dye wastewater treatment

Forward osmosis (FO) membrane fouling is one of the main reasons that hinder the further application of FO technology in the treatment of dye wastewater. To alleviate membrane fouling, a conductive coal carbon-based substrate and polydopamine nanoparticles (PDA NPs) interlayer composite FO membrane (CPFO) was prepared by interfacial polymerization (IP). CPFO-10 membrane prepared by depositing 10 mL of PDA NPs solution exhibited an optimum performance with water flux of 7.56 L/(m²h) for FO mode and 10.75 L/(m²h) for pressure retarded osmosis (PRO) mode, respectively. For rhodamine B and chrome black T dye wastewater treatment, the water flux losses were reduced by 21.6%, and 14.5% under the voltages of +1.5 V, and -1.5 V, respectively, compared with no voltage applied after the device was operated for 8 h. The applied voltage had little effect on the fouling mitigation performance of the CPFO membrane for neutral charged cresol red. After the device was operated for 4 cycles, the rejection rates of dyes wastewater treated by the CPFO membranes with applied voltage were close to 100%. The flux decline rate and flux recovery rate of CPFO membrane for rhodamine B and chrome black T wastewater treatment under application of +1.5 V and -1.5 V voltage after 4 cycles were 11.6%, 99.2%, and 16.7%, 98.9%, respectively. Therefore, the voltage-applied CPFO membrane still maintained good rejection and antifouling performance in long-term operation. This study provides a new insight into the preparation of conductive FO membranes for dye wastewater treatment and membrane fouling control.

A comprehensive review on green perspectives of electrocoagulation integrated with advanced processes for effective pollutants removal from water environment

The rapidly expanding global energy demand is forcing a release of regulated pollutants into water that is threatening human health. Among various wastewater remediating processes, electrocoagulation (EC) has scored a monumental success over conventional processes because it combines coagulation, sedimentation, floatation and electrochemical oxidation processes that can effectively decimate numerous stubborn pollutants. The EC processes have gained some attention through various academic and industrial publications, however critical evaluation of EC processes, choices of EC processes for various pollutants, process parameters, mechanisms, commercial EC technologies and performance enhancement via other degradation processes (DPs) integration have not been comprehensively covered to date. Therefore, the major objective of this paper is to provide a comprehensive review of 20 years of literature covering EC fundamentals, key process factors for a reactor design, process implementation, current challenges and performance enhancement by coupling EC with pivotal pollutant DPs including, electro/photo-Fenton (E/P-F), photocatalysis, sono-chemical treatment, ozonation, indirect electrochemical/advanced oxidation (AO), and biosorption that have substantially reduced metals, pathogens, toxic compound BOD, COD, colors in wastewater. The results suggest that the optimum treatment time, current density, pulse frequency, shaking speed and spaced electrode improve the pollutants removal efficiency. An elegant process design can prevent electrode passivation which is a critical limitation of EC technology. EC coupling (up or downstream) with other DPs has resulted in the removal of organic pollutants and heavy metals with a 20% improved efficiency by EC-EF, removal of 85.5% suspended solid, 76.2% turbidity, 88.9% BOD, 79.7% COD and 93% color by EC-electroflotation, 100% decolorization by EC-electrochemical-AO, reduction of 78% COD, 81% BOD, 97% color by EC-ozonation and removal of 94% ammonia, 94% BOD, 95% turbidity, >98% phosphorus by aerated EC and peroxicoagulation. The major wastewater purification achievements, future potential and challenges are described to model the future EC integrated systems.

Efficient Removal of Ammonia Nitrogen by an Electrochemical Process for Spent Caustic Wastewater Treatment

Spent caustic wastewater produced in a soda plant has a high concentration of ammonia nitrogen (NH4+-N). As excessive NH4+-N discharging into water bodies would cause eutrophication as well as destruction to the ecology balance, developing an efficient technology for NH4+-N removal from the spent caustic wastewater is imperative in the current society. In this study, an electrochemical process with graphene electrodes was designed for the NH4+-N removal in the spent caustic wastewater. The removal efficiency of the NH4+-N during the electrochemical process could reach 98.7% at 4 A in a short treatment time (within 120 s) with an acceptable energy consumption (6.1 kWh/m3-order). NO3− and NO2− were not detected during the electrochemical process. An insignificant amount of NH2Cl, NHCl2, and NCl3 produced in the treatment suggested that little of the NH4+-N reacted with chlorine, that is, chlorination played a negligible role in the NH4+-N removal. By electron equilibrium and nitrogen conversion analysis, we think that NH4+-N was primarily converted to NH2(ads) on the surface of a graphene electrode by one-electron transfer during the direct oxidation of the electrochemical process. Due to the high calcium ion (Ca2+) in the spent caustic wastewater, the electrode scale significantly increased to 1.4 g after treatment of 240 s at 4 A. By X-ray diffraction (XRD) analysis, the composition of the electrode scale is portlandite Ca(OH)2. Although the electrode scale was obvious during the electrochemical treatment, it could be alleviated by alternating the electrode polarity. As a result, the life and efficiency of the graphene electrode for NH4+-N removal could remain stable for a long time. These results suggest that the electrochemical process with a graphene electrode may provide a competitive technology for NH4+-N removal in spent caustic wastewater treatment.

Enhanced dewaterability of lake dredged sediments by electrochemical oxidation of peroxydisulfate on BDD anode

Dredged sediments, as a product of mitigating endogenous pollution of rivers and lakes, cause severe environmental pollution without suitable disposal. To reduce dredged sediments, the electrochemical oxidation (EO) of peroxydisulfate (PS) on a boron-doped diamond (BDD) anode (EO/BDD-PS) was utilized to enhance the dewaterability of the dredged sediments. The soluble chemical oxygen demand increased in the EO/BDD-PS system, and more than 70.0% of the specific resistance to filtration was reduced by EO/BDD-PS within 20 min. The optimal conditions were determined to be as follows: current density, 30 mA cm^-2; PS dosage 4 g L^-1; and initial pH, 6.96. After treatment with EO/BDD-PS, the electronegativity of the sludge flocs was alleviated and the particle size increased from 7.61 to 10.64 μm. Furthermore, proteins and polysaccharides were degraded, and tightly bound extracellular polymeric substances (TB-EPS) and loosely bound EPS (LB-EPS) were effectively transported to soluble EPS (S-EPS). Furthermore, humification of organic matter occurred in S-EPS and LB-EPS when the dredged sediment was treated with EO/BDD-PS. Dominant hydroxyl radicals (•OH) and sulfate radicals (SO₄•^-) were generated in the EO/BDD-PS system. Moreover, the efficiency of the filtrate as an electrolyte decreased slightly after recycling five times. Therefore, this method may be economical for enhancing the dewaterability of dredged sediments.