Electrochemical treatment of organic pollutants in landfill leachate using a three-dimensional electrode system

Refractory degradable dissolved organic matter (R-DOM) driving nitrogen removal by the electric field coupled iron‑carbon biofilter (E-ICBF): Performance and microbial mechanisms

Researches on the advanced nitrogen (N) removal of municipal tailwater always overlooked the value of refractory degradable dissolved organic matter (R-DOM). In this study, a novel electric field coupled iron‑carbon biofilter (E-ICBF) was utilized to explore the performance and microbial changes with polyethylene glycol (PEG) as the representative R-DOM. Results demonstrated that the removal efficiencies of E-ICBF for nitrate nitrogen (NO3--N), ammonia nitrogen (NH4+-N), and total nitrogen (TN) improved by 28.76 %, 12.96 %, and 28.45 %, compared to quartz sand biofilter (SBF). Moreover, removal efficiencies of NO3--N and TN in E-ICBF with R-DOM went up by 12.11 % and 14.02 % compared to methanol. Additionally, both PEG and the electric field reduced the microbial richness and diversity. However, PEG promoted the increase of denitrifying bacteria abundance including unclassified_f_Comamonadaceae, Thauera, and unclassified_f_Gallionellaceae. The electric field improved the abundances of genes related to N removal (hao, nasC, nasA, nifH, nifD, nifK) and PEG further enhanced the effect. The abundances of key enzymes [EC:1.7.5.1], [EC:1.7.2.1], [EC:1.7.2.4], and [EC:1.7.2.5] decreased due to the addition of PEG and the electric field mitigated the negative influence. Additionally, the electric field changed relationships between microorganisms and pollutant removal, and improved interspecific relationships between denitrifying bacterial genera and other genera in E-ICBF.

A chemical coating strategy for assembling a boron-doped diamond anode towards electrocatalytic degradation of late landfill leachate

The high efficiency electrocatalytic degradation of late landfill leachate is still not an easy task due to the complexity and variability of organic pollutants. A chemical coating strategy for assembling a boron-doped diamond anode (BDD) towards electrocatalytic degradation of late landfill leachate was adopted and studied. The results shows the high removal rates of organic carbon (TOC) and ammonia nitrogen (NH3-N) after electrochemical oxidation for 5 h can reach 99% and 100%. Further, the organic migration and transformation depends on current density, A/V value, initial pH, electrochemical degradation time, and composition of the stock solution. Specifically, alkaline conditions can increase both TOC and NH3-N removal rates, which is reflected in the NH3-N removal rate of 100% when the pH is 8.5 after only 5 h. The types of organic matter decreased from 63 species to 24 species in 5 h, in which the removal of fulvic acids is superior to that of soluble biometabolites. Amides/olefins and phenolic alcohols are all degraded and converted into other substances or decomposed into CO2 and H2O by BDD, accompanied by the continuous decomposition of alcohol-phenols into alkanes. In all, this study provides a core reference on electrocatalytic degradation of late landfill leachate.

Intensive Treatment of Organic Wastewater by Three-Dimensional Electrode System within Mn-Loaded Steel Slag as Catalytic Particle Electrodes

Developing a green, low-carbon, and circular economic system is the key to achieving carbon neutrality. This study investigated the organics removal efficiency in a three-dimensional electrode reactor (3DER) constructed from repurposed industrial solid waste, i.e., Mn-loaded steel slag, as the catalytic particle electrodes (CPE). The CPE, a micron-grade material consisting primarily of transition metals, including Fe and Mn, exhibited excellent electric conductivity, catalytic ability, and recyclability. High rhodamine B (RhB) removal efficiency in the 3DER was observed through a physical modelling experiment. The optimal operating condition was determined through a single-factor experiment in which 5.0 g·L-1 CPE and 3 mM peroxymonosulfate (PMS) were added to a 200 mL solution of 10 mM RhB under a current intensity of 0.5 A and a 1.5 to 2.0 cm distance between the 2D electrodes. When the initial pH value of the simulated solution was 3 to 9, the RhB removal rate exceeded 96% after 20 min reaction. In addition, the main reactive oxidation species in the 3DER were determined. The results illustrated that HO• and SO4•- both existed, but that the contribution of SO4•- to RhB removal was much lower than that of HO• in the 3DER. In summary, this research provides information on the potential of the 3DER for removing refractory organics from water.

Pretreatment of hypersaline and high-organic wastewater with a three-dimensional electrocatalytic system: a pilot-scale study

The three-dimensional electrocatalytic oxidation (3DEO) is a promising electrochemical system in the treatment of refractory wastewater, but still far from large-scale applications. In this work, we prepared 146.5 Kg Ti-Sn-Sb@γ-Al2O3 particle electrodes to construct a 3DEO system for the pretreatment of hypersaline and high-organic wastewater in an industrial park sewage plant, with activated carbon particle electrodes as a comparison. The average COD removal rates of Ti-Sn-Sb@γ-Al2O3 and activated carbon-based 3DEO systems were 24.43 and 48.73%, respectively, and the energy consumption of the two 3DEO systems were 102.8 and 31.4 kWh/Kg COD, respectively. However, compared to the negligible enhancement of wastewater biodegradability in the activated carbon 3DEO system, the Ti-Sn-Sb@γ-Al2O3 3DEO system greatly improved the biochemical index (B/C) from 0.021 to 0.166 (by 690.5%). Due to its superior catalytic capacity, Ti-Sn-Sb@γ-Al2O3 outperforms activated carbon in improving biodegradability as the latter relies mainly on adsorption. The results of this work provide a 3DEO engineering practice experience on the pretreatment of hypersaline and high-organic wastewater.

Preparation of SnO2-Sb/attapulgite (AP) clay particulate electrode for efficient phenol electrochemical oxidation

A novel SnO2-Sb/AP (attapulgite) particle electrode was prepared for three-dimensional electrocatalytic oxidation (3D/EO) of organic pollutants using a co-sintering method. The electrochemical properties and micromorphology were determined using polarization, cyclic voltammetry (CV), and field emission scanning electron microscope (FE-SEM), and compared with activated carbon (AC), AP, and TiO2/AP particle electrodes. Besides, their potential application in the electrochemical degradation of phenol was investigated. The SnO2-Sb/AP particle electrode exhibited higher electrochemical activity than other particle electrodes due to its large number of active sites, low transfer coefficient (α, 0.12), and high-volt ampere charge (q*, 1.18 C·cm-2). The electrochemical CODCr degradation efficiency (100%) of phenol on SnO2-Sb/AP particle electrodes is much higher than for other particle electrodes. Moreover, an excellent stability of the SnO2-Sb/AP particle electrode is also verified by repeated experiments. These results indicate that the SnO2-Sb/AP particle electrodes broaden the application area of clays and are expected to be a promising method for 3D/EO.

Generation and engineering applications of sulfate radicals in environmental remediation

Sulfate radical (SO4•-)-based advanced oxidation processes (AOPs) have become promising alternatives in environmental remediation due to the higher redox potential (2.6-3.1 V) and longer half-life period (30-40 μs) of sulfate radicals compared with many other radicals such as hydroxyl radicals (•OH). The generation and mechanisms of SO4•- and the applications of SO4•--AOPs have been examined extensively, while those using sulfite as activation precursor and their comparisons among various activation precursors have rarely reviewed comprehensively. In this article, the latest progresses in SO4•--AOPs were comprehensively reviewed and commented on. First of all, the generation of SO4•- was summarized via the two activation methods using various oxidant precursors, and the generation mechanisms were also presented, which provides a reference for guiding researchers to better select two precursors. Secondly, the reaction mechanisms of SO4•- were reviewed for organic pollutant degradation, and the reactivity was systematically compared between SO4•- and •OH. Thirdly, methods for SO4•- detection were reviewed which include quantitative and qualitative ones, over which current controversies were discussed. Fourthly, the applications of SO4•--AOPs in various environmental remediation were summarized, and the advantages, challenges, and prospects were also commented. At last, future research needs for SO4•--AOPs were also proposed consequently. This review could lead to better understanding and applications of SO4•--AOPs in environmental remediations.

Research status of volatile organic compound (VOC) removal technology and prospect of new strategies: a review

As an important component of air pollution, the efficient removal of volatile organic compounds (VOCs) is one of the most important challenges in the world. VOCs are harmful to the environment and human health. This review systematically introduced the main VOC control technologies and research hotspots in recent years, and expanded the description of electrocatalytic oxidation technology and bimetallic catalytic removal technology. Based on a three-dimensional electrode reactor, the theoretical design of a VOC removal control technology using bimetallic three-dimensional particle electrode electrocatalytic oxidation was proposed for the first time. The future research focus of this method was analyzed, and the importance of in-depth exploration of the catalytic performance of particle electrodes and the system reaction mechanism was emphasized. This review provides a new idea for using clean and efficient methods to remove VOCs.

Enhancement of excess sludge dewatering by three-dimensional electro-Fenton process based on sludge biochar

Deep dewatering of waste activated sludge (WAS) is still a challenge due to high content of bound water and non-Newton fluid properties of sludge flocs. Electro-Fenton (EF) can enhance sludge dewaterability, however, low pH needed in homogeneous EF and fine flocs after EF conditioning influenced deep dewatering of sludge and the subsequent resource recovery disposal. In this study, a three dimension electro-Fenton (3D-EF) using Fe modified sludge biochar (Fe@SBC) as particle electrode, heterogeneous Fenton catalyst and skeleton builder for deep dewatering of sludge under neutral pH was proposed. Fe@SBC obtained at 800 °C exhibited high capacity of H₂O₂ electrogeneration and activation due to high conductivity and content of 2e-ORR selectivity functional groups. With promoted generation of H₂O₂ and hydroxyl radical (•OH), 3D-EF with Fe@SBC showed higher decomposition of bound extracellular polymeric substances (EPS) and disintegration of cells in sludge flocs, resulting in releasing bound and intracellular water into free water. Compared with EF, 3D-EF with Fe@SBC800 had higher ability in breaking macromolecules of protein and polysaccharide, as well as removing -COOH and -NH₂ groups in EPS, which could facilitate release of bound water trapped in EPS and self-coagulation of fine flocs. During subsequent filtering process, Fe@SBC could enhance sludge filterability as skeleton builder. A synergetic effect of strong oxidation and physical conditioning were proposed in 3D-EF sludge dewaterability with Fe@SBC, and the improved oxidation by Fe@SBC was supposed to play the major role.

Persulfate-enhanced continuous flow three-dimensional electrode dynamic reactor for treatment of landfill leachate

Compared with sequencing batch reactor, continuous flow dynamic reactors are more conducive to promotion and application. In this study, the ability of a three-dimensional (3D) electrode dynamic reactor to remove pollutants in the landfill leachate was investigated, in which landfill leachate entered through continuous flow. Either increased of current density or the decreased of flow rate was conducive to the removal of pollutants. The optimal process parameters for current density and flow rate were 16 mA cm^-2 and 0.75 L h^-1, respectively. When the current density was constant at 16 mA cm^-2 and the flow rate was kept at 0.75 L h^-1, 60.02% of total organic carbon (TOC), 96.50% of chroma, 64.98% of chemical oxygen demand (COD) and 99.46% of ammonia nitrogen (NH₃-N) were removed. The characteristic peaks of refractory organic pollutants were reduced by 97.95%. After the reaction, the biological oxygen demand (BOD)/COD was increased from 0.24 to 0.32. As one of the emerging trace organics in landfill leachate, 85.90% of ibuprofen (IBU) was removed. The results showed that the 3D electrode dynamic reactor constructed in this study could reduce the TOC, refractory trace organic pollutant, NH₃-N and chroma in the landfill leachate. The 3D electrode dynamic reactor constructed in this research has application potential in the field of landfill leachate treatment.

Application of electro-Fenton and photoelectro-Fenton processes for the degradation of contaminants in landfill leachate

Worldwide, most solid waste ends its life in landfill sites, which have a significant environmental impact in several respects. In particular, rainfall over landfill sites results in the production of an aqueous leachate containing compounds having low biodegradability, high toxicity, and a high organic load. For this reason, this study aims to investigate the applicability of electro-Fenton (EF) and photoelectro-Fenton (PEF) processes as alternative for treating a local landfill effluent with high organic content (chemical oxygen demand (COD) = 2684.7 mg-O₂ L ^-1) in a continuous-flow reactor (using, for first time, this kind of system with higher electrodes area of 35 cm²) using boron-doped diamond anode (Nb/BDD) and a carbon felt cathode (FC) electrodes. The effects of current density j (30, 60 and 90 mA cm^-2) and UV radiation wavelength (UVA and UVC) were studied to evaluate the treatment efficiency as well as the energy consumption. Results clearly showed that, the best efficiencies removing organic matter, in terms of COD, were about 66%, 68% and 89% with an energy consumption of only 19.41, 17.61 and 17.59 kWh kg COD^-1 for EF, PEF-UVA and PEF-UVC respectively, at 90 mA cm^-2 after 4 h of operation. The treatment of this kind of effluent produced organic and inorganic by-products, the acetic and formic acids as well as NO₂^-, NO₃^-, and NH₄⁺, being assessed their concentrations.

Electrochemical treatment for leachate membrane retentate: Performance comparison of electrochemical oxidation and electro-coagulation technology

With the widespread use of membrane in advanced treatment of leachate, China produces a large amount of leachate membrane retentate (LMR) (≈23.4 million tons) annually, which is usually treated by incineration or recirculation in engineering, but these technologies have many drawbacks. LMR is suitable for electrochemical treatment due to its high electrical conductivity. This study compared the performances of electrochemical oxidation (EO) and electro-coagulation (EC) technology on LMR treatment under different experimental conditions, including anode material, current density, initial pH and reaction time. We found that EO optimal conditions achieved 70.1%, 83.1%, 78.7%, 98.7%, and 69.7% removal of total organic carbon (TOC), UV absorption (at 254 nm), chromaticity, ammonia nitrogen (NH₃-N), and total nitrogen (TN), respectively. Compared with EO, EC exhibited a similar removal ability for orgainics and better removals of chroma, but much less performance for removing nitrogen pollutants in the same reaction time, that is, removals of NH₃-N and TN were only 31.5% and 36.2%, respectively. Meanwhile, EC showed much higher instantaneous current efficiency of COD than EO under its optimal reaction time (120 min). In addition, the UV-Vis spectra and 3D fluorescence spectra indicated that EO exhibited relatively outstanding performance in decomposing dissolved organic matter (DOM) with rather complicated structures than EC. Also, the flow field-flow fractionation technique demonstrated that EO preferentially destroy humic-like, large molecular weight DOM, and converting them to smaller molecules, which resulted in more volatile organic compounds in EO samples than EC samples. While EC had little selectivity in the removal of organics, except humic-like DOM with relative small molecular. These findings can provide a theoretical basis for the electrochemical treatment of LMR.

Advanced oxidation processes and selection of industrial water source: A new sight from natural organic matter

Natural organic matter (NOM) refers to the dissolved organic matter in natural water that can pass through 0.45 μm filter membrane. As a pivotal role in the surface water body, it has a significant effect on the efficiency of AOPs. In this study, Excitation emission matrix - parallel factor (EEM-PARAFAC) analysis is used to elucidate the changes of NOM fluorescence peaks after electrochemical oxidation process, two-dimensional correlation spectroscopy (2D-FTIR-COS) and Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) are utilized to clarify the molecular characteristics of NOM in surface water and the effects of electrochemical oxidation on NOM molecules. The results indicate that parts of NOM molecules are mineralized into simple compounds and precursors of refractory organic matters produced by some NOM molecules after AOPs. It is concluded that the precursors of these refractory organic matters may belong to terrestrial humus (C2). Therefore, for the purpose of avoiding more refractory organic pollutants produced by NOM which can reduce the performance of AOPs in the water treatment process, factories should choose water sources with less humus as industrial water supply, or degrade humus by physical or chemical methods before industrial water supply.

Application of perovskite oxides and their composites for degrading organic pollutants from wastewater using advanced oxidation processes: Review of the recent progress

In the recent years, perovskite oxides are gaining an increasing amount of attention owing to their unique traits such as tunable electronic structures, flexible composition, and eco-friendly properties. In contrast, their catalytic performance is not satisfactory, which hinders real wastewater remediation. To overcome this shortcoming, various strategies are developed to design new perovskite oxide-based materials to enhance their catalytic activities in advanced oxidation process (AOPs). This review article is to provide overview of basic principle and different methods of AOPs, while the strategies to design novel perovskite oxide-based composites for enhancing the catalytic activities in AOPs have been highlighted. Moreover, the recent progress of their synthesis and applications in wastewater remediation (pertaining to the period 2016-2022) was described, and the related mechanisms were thoroughly discussed. This review article helps scientists to have a clear outlook on the selection and design of new effective perovskite oxide-based materials for the application of AOPs. At the end of the review, perspective on the challenges and future research directions are discussed.

Electrochemical Oxidation of Landfill Leachate after Biological Treatment by Electro-Fenton System with Corroding Electrode of Iron

Electrochemical oxidation of landfill leachate after biological treatment by a novel electrochemical system, which was constructed by introducing a corroding electrode of iron (Fe_c) between a boron-doped diamond (BDD) anode and carbon felt (CF) cathode (named as BDD–Fe_c–CF), was investigated in the present study. Response surface methodology (RSM) with Box–Behnken (BBD) statistical experiment design was applied to optimize the experimental conditions. Effects of variables including current density, electrolytic time and pH on chemical oxygen demand (COD) and ammonia nitrogen (NH₃-N) removal efficiency were analyzed. Results showed that electrolytic time was more important than current density and pH for both COD and NH₃-N degradation. Based on analysis of variance (ANOVA) under the optimum conditions (current density of 25 mA·cm⁻², electrolytic time of 9 h and pH of 11), the removal efficiencies for COD and NH₃-N were 81.3% and 99.8%, respectively. In the BDD–Fe_c–CF system, organic pollutants were oxidized by electrochemical and Fenton oxidation under acidic conditions. Under alkaline conditions, coagulation by Fe(OH)₃ and oxidation by Fe(VI) have great contribution on organic compounds degradation. What is more, species of organic compounds before and after electrochemical treatment were analyzed by GC–MS, with 56 kinds components detected before treatment and only 16 kinds left after treatment. These results demonstrated that electrochemical oxidation by the BDD–Fe_c–CF system has great potential for the advanced treatment of landfill leachate.

A review on the landfill leachate treatment technologies and application prospects of three-dimensional electrode technology

With the expansion of urbanisation, the total amount of solid waste produced by urban residents has been increasing, and the problem of municipal solid waste disposal has also been aggravated. Landfill leachate treatment technologies could be divided into three categories: biological, physical and advanced oxidation treatment technology. Among them, advanced oxidation treatment technology has a good effect on the treatment of landfill leachate with little secondary pollution and has excellent application potential. Three-dimensional (3D) electrode technology, as a new type of advanced oxidation technology, could remove refractory pollutants in water and has attracted considerable attention. This article aims to (1) compare existing landfill leachate treatment technologies, (2) summarise 3D electrode technology application scenarios, (3) discuss the advantages of 3D electrode technology in landfill leachate treatment and (4) look ahead the future directions of 3D electrode technology in landfill leachate treatment. We hope that this article will be helpful to researchers who are interested in the field of landfill leachate treatment.

Fabrication of novel three-dimensional Fe3O4-based particles electrodes with enhanced electrocatalytic activity for Berberine removal

Reasonable design of three-dimensional (3D) catalytic particle electrodes (CPEs) is crucial for achieving efficient electrocatalytic oxidation of organic pollutants. Herein, the novel Fe₃O₄/SnO₂/GO (FO/SO/GO) particle electrode has been developed and serviced to the 3D electrocatalytic berberine hydrochloride oxidation system with DSA (RuO₂-IrO₂-SnO₂/Ti) electrode as anode and GDE (gas diffusion electrode) electrode as the cathode. Compared with 2D systems and other CPEs, FO/SO/GO electrode shows excellent electrocatalytic activity and remarkable stability for BH removal, that is, the removal rate of BH is 94.8% within 90 min, and the rate constant is 0.03095 min^-1. More importantly, after five cycles, the ternary composite still maintains a strong ability to oxidize pollutants. The structural characterization and electrochemical measurement further uncover that the electron transfer ability and electrocatalytic oxidation efficiency are highly dependent on the surface structure regulation of CPEs. Furthermore, the quenching experiments show that hydroxyl radicals are the main active species in the 3D electro-Fenton (EF) system, which can oxidize BH molecules adsorbed on the surface of GO to CO₂, H₂O, or other products. The results could potentially provide new insights for designing and fabricating more stable and efficient 3D CPEs electrocatalytic removal of organic pollutants in the future.

Effect of chloride ions on the simultaneous electrodialysis and electrochemical oxidation of mature landfill leachate

An attempt has been made to improve the treatment efficiency of mature landfill leachate prior to the existing biological treatment. In this study, electrochemical oxidation (EO) was applied as a pre-treatment to remove organic contaminants and was simultaneously combined with electrodialysis (ED) to remove ionic constituents, such as ammonium and phosphate. A laboratory-scale electrochemical reactor was designed by utilizing a carbon graphite anode and a stainless steel cathode and separated by an anion exchange membrane (AEM) and cation exchange membrane (CEM), creating a three-compartment reactor. The oxidation of the organic pollutant would occur in the anodic compartment, while the targeted ammonium and phosphate ions would be migrated and accumulated in the central compartment. The treatment process was performed in a batch recirculation time of 12 h at a constant supplied current of 0.25 A and evaluated by means of the initial leachate pH (i.e., original pH value of 7.85; adjusted pH value of 5.50 and 8.50) and three different initial chloride concentrations. The higher the chloride concentration in the leachate, the higher the removal efficiency, except for total phosphate. The highest chemical oxidation demand (COD) removal was 86.2% (0.88 g W^-1 h^-1), at an initial leachate pH value of 7.85 with the addition of 2 g L^-1 of NaCl. Furthermore, under the same conditions, the ammonium, total phosphate, and chloride removals were 85% (0.44 g W^-1 h^-1), 89% (0.08 g W^-1 h^-1), and 83% (0.69 g W^-1 h^-1), respectively. Also, the concentrated ionic compounds in the central compartment can lower the energy consumption and can possibly be further treated or managed.

Simultaneous Removal of Nitrogen and Refractory Organics from a Biologically Treated Leachate by Pulse Electrochemical Oxidation in a Multi-channel Flow Reactor

Electrochemical oxidation (EO) is often used in the advanced treatment of refractory wastewater. However, in a conventional EO process of direct-current (DC) power supply, oxide layers often form on the anodes, which not only hinder the oxidation reaction on them but also cause higher energy consumption. In this paper, a biologically treated leachate (BTL) of municipal solid waste (MSW) was comparably treated by EO with DC (DC-EO), monopulse (MP-EO), and double pulse (DP-EO) power source models in a home-made multi-channel flow reactor. The effects of process parameters of current density (I _A), superficial liquid velocity (U _L), pulse frequency (f _P), duty ratio (R _D), and so forth on the removal efficiency of chemical oxygen demand (COD) (RE_COD), total organic carbon (TOC) (RE_TOC), and total nitrogen (TN) (RE_TN) were investigated simultaneously. Average energy consumption () and organic composition of the treated effluent of DC-EO and MP-EO were also compared comprehensively, and a new mechanism of MP-EO has been proposed accordingly. Under optimal conditions, 2 L of BTL was treated by MP-EO for 180 min, and the RE_COD, RE_TOC, and RE_TN could reach as high as 80, 30, and 80%, respectively. Compared with DC-EO, the of MP-EO is reduced by 69.27%. Besides, the kinds of organic matter in the treated effluent of MP-EO are reduced from 53 in the BTL to 11, which is much less than in the DC-EO process of 29 kinds. Therefore, the MP-EO process exhibits excellent removal performance of organics and TN and economic prospects in the treatment of refractory organic wastewater.

Enhanced nitrogen removal in an electrochemically coupled biochar-amended constructed wetland microcosms: The interactive effects of biochar and electrochemistry

The interactive effects of both biochar (BC) and electrochemistry (EC) can affect nitrogen (N) removal process. However, little is known about how this function in constructed wetland (CW) systems. In this study, an electrochemically (EC) coupled BC-amended saturated subsurface vertical flow constructed wetland (BECW) systems were established to enhance nitrogen (N) removal. Other three CW systems: without BC and EC (CW); with EC only (ECW); and with BC only (BCW) were performed as controls. Results indicated that the total nitrogen (59.88%-93.03%) and nitrate‑nitrogen (83.14%-100%) of the BECW system were significantly enhanced (p < 0.05) compared with the control systems. Treated WWTP tail-water could meet Class-IV of the Surface Water Quality Standard (GB3838-2002) in China by the BECW system. The enhanced N removal in the BECW system could be attributed to (1) the autotrophic denitrification process in which H₂ and Fe²⁺ provided by the cathode and anode acted as electron donors; and (2) BC addition acting as substrate could improve the activity, diversity and richness of microorganisms. Microbial community analysis further indicated that high N removal in the BECW system was significantly dependent on the synergy between the heterotrophic and autotrophic denitrifiers, facilitated by BC and EC interaction. Results illustrate that the BECW system is a feasible and eco-sustainable technology for treating low C/N tail-water from WWTPs. This work provides a novel and fundamental understanding of the electrochemically coupled biochar-amended CW system. These results could serve as a theoretical basis for the engineered applications in the deep purification of WWTPs' tail-water.

Landfill leachate treatment by persulphate related advanced oxidation technologies

Landfill leachate is produced from garbage decomposition with highly toxic and bio-refractory compounds, which poses serious harm to environmental security and human health. Thus, it is urgent to treat landfill leachate properly. Persulfate (PS) oxidation has attracted extensive attentions in terms of fast reaction speed, non-selectivity to target pollutants and thorough oxidation. In recent years, PS oxidation has been widely adopted for landfill leachate purification. However, the related results have been rarely summarized. In this review, the treatment of landfill leachate by PS oxidation system is discussed systematically including oxidants, activation modes and oxidation mechanisms. In addition, the current situation of PS oxidation system and other coupled systems for landfill leachate treatment is also summarized. Finally, the challenges and future research directions of landfill leachate treatment based on PS oxidation process are proposed. Meaningfully, this review will provide valuable references for the development of landfill leachate treatment process, promoting the application of advanced oxidation technology in landfill leachate treatment.

Progress in research and development of particle electrodes for three-dimensional electrochemical treatment of wastewater: a review

A three-dimensional (3D) electrochemical technology is regarded as a very effective industrial wastewater treatment method as it has high treatment efficiency, high current efficiency, and low energy consumption, and especially can completely mineralize nonbiodegradable organic pollutants. The core of the 3D electrochemical technology is a particle electrode, and the particle electrode plays several important roles for removing pollutants during the electrochemical reaction process. Many types of particle electrodes have been developed and used for different types of wastewater treatment. In this paper, a comprehensive review on the research and development of particle electrodes of the 3D electrochemical reactors for wastewater treatment is conducted. Specifically, the role that the particle electrode plays during the 3D electrochemical treatment of wastewater is thoroughly investigated and systematized. In addition, the different types of particle electrodes used in the 3D electrochemical wastewater treatment are classified into several types according to the presence or absence of a catalyst and the main components of the particle electrode or carrier. Also, focusing on the recent research results, the structural characteristics, performance, advantages and defects, and the role of catalyst components of each particle electrodes are evaluated. Finally, the direction and prospect of future research on the particle electrode is presented.

Electro-Persulfate Processes for the Treatment of Complex Wastewater Matrices: Present and Future

Complex wastewater matrices present a major environmental concern. Besides the biodegradable organics, they may contain a great variety of toxic chemicals, heavy metals, and other xenobiotics. The electrochemically activated persulfate process, an efficient way to generate sulfate radicals, has been widely applied to the degradation of such complex effluents with very good results. This review presents the fundamentals of the electro-persulfate processes, highlighting the advantages and limitations, followed by an exhaustive evaluation on the application of this process for the treatment of complex industrial effluents. An overview of the main relevant experimental parameters/details and their influence on the organic load removal is presented and discussed, having in mind the application of these technologies at an industrial scale. Finally, the future perspectives for the application of the electro-persulfate processes in the treatment of complex wastewater matrices is outlined.

The preparation of black titanium oxide nanoarray via coking fluorinated wastewater and application on coking wastewater treatment

Coking wastewater is extremely toxic with poor biodegradability owing to the presence of refractory organics. Black titanium oxide nanotube array (BTN), not only photocatalyst but also electrocatalyst, is with definite potentiality in organic wastewater treatment. Here, we firstly developed an electrochemical method, using fluorinated coking wastewater as electrolyte rather than traditional fluorinated ethylene glycol, to prepare titanium oxide nanoarray economically. Unexpectedly, suspended pollutants and ammonia nitrogen in coking wastewater were removed in BTN preparation. Moreover, the as-prepared BTN could be further employed as photocatalyst or electrocatalyst to degrade dissolved organic matter in coking wastewater. As an electrocatalyst, BTN possessed the comparable •OH production activity about 9.9 × 10^-15 M S^-1 to boron-doped diamond, high oxygen evolution potential around 2.75 V, and high selectivity of chlorine production. Moreover, the biodegradability of treated coking wastewater could be effectively improved by using BTN as electrocatalyst in electrochemical oxidation, and the BOD/COD was from 0.19 to above 0.3 in 4 h at current density of 2 mA cm^-2. The energy consumption was about 63-68 kWh kgCOD^-1, lower than that of various reported electrodes. This study provided an economical and environmentally friendly method to prepare BTN, which was with positive application prospect in the field of coking wastewater treatment.

Magnetite nanoparticles: Synthesis methods - A comparative review

Iron oxide-based nanoparticles have gathered tremendous scientific interest towards their application in a variety of fields. Magnetite has been particularly investigated due to its readily availability, versatility, biocompatibility, biodegradability, and special magnetic properties. As the behavior of nano-scale magnetite is in direct relation to its shape, size, and surface chemistry, accurate control over the nanoparticle synthesis process is essential in obtaining quality products for the intended end uses. Several chemical, physical, and biological methods are found in the literature and implemented in the laboratory or industrial practice. However, non-conventional methods emerged in recent years to bring unprecedented synthesis performances in terms of better-controlled morphologies, sizes, and size distribution. Particularly, microfluidic methods represent a promising technology towards smaller reagent volume use, waste reduction, precise control of fluid mixing, and ease of automation, overcoming some of the major drawbacks of conventional bulk methods. This review aims to present the main properties, applications, and synthesis methods of magnetite, together with the newest advancements in this field.

Electrocatalytic ozonation process supplemented by EDTA-Fe complex for improving the mature landfill leachate treatment

The present study was to enhance catalytic ozonation process (COP) using ferric (Fe)- ethylenediaminetetraacetic acid (Fe-EDTA) integrated with an electrocoagulation (EC) process for landfill leachate pretreatment. For this purpose, the effect of operating parameters such as ozone and Fe-EDTA concentrations, current, initial pH, and reaction time were investigated. The findings revealed that the EC process and single ozonation process (SOP) could alone reduce chemical oxygen demand (COD) in landfill leachate by 23% and 39%; respectively. Moreover, integration of both processes at 100 mA current, 400 mg h^-1 ozone concentration, and 3 h reaction time could significantly improve COD reduction to 70%. As well, current efficiency and ozone consumption in the proposed system could considerably develop compared with EC process and SOP. The integrated electro-catalytic SOP using Fe-EDTA could be operated at neutral pH value, which the COD removal efficiency was obtained 79.7%. Subsequently, biochemical oxygen demand (BOD₅)/COD ratio of effluent increased to 0.64. Examining pseudo-first-order and pseudo-second-order kinetics, it was realized that constant rate in the system had augmented. These results also indicated that the modified process using Fe-EDTA was a promising landfill leachate pretreatment technique that could significantly enhance COD removal efficiency and BOD₅/COD ratio, and ultimately decrease time and sludge production.

Review on electrochemical system for landfill leachate treatment: Performance, mechanism, application, shortcoming, and improvement scheme

Landfill leachate is a type of complex organic wastewater, which can easily cause serious negative impacts on the human health and ecological environment if disposed improperly. Electrochemical technology provides an efficient approach to effectively reduce the pollutants in landfill leachate. In this review, the electrochemical standalone processes (electrochemical oxidation, electrochemical reduction, electro-coagulation, electro-Fenton process, three-dimensional electrode process, and ion exchange membrane electrochemical process) and the electrochemical integrated processes (electrochemical-advanced oxidation process (AOP) and biological electrochemical process) for landfill leachate treatment are summarized, which include the performance, mechanism, application, existing problems, and improvement schemes such as cost-effectiveness. The main objective of this review is to help researchers understand the characteristics of electrochemical treatment of landfill leachate and to provide a useful reference for the design of the process and reactor for the harmless treatment of landfill leachate.

Removal of organic compounds from cooling tower blowdown by electrochemical oxidation: Role of electrodes and operational parameters

The reuse of cooling tower blowdown (CTBD) in the cooling tower itself requires CTBD deionization and a pre-treatment before deionization to remove organic compounds (OCs) that induce membrane fouling. This study assesses the potential of electrochemical oxidation (EO) with a boron-doped diamond (BDD) and a Ti/RuO₂ mixed-metal oxide (MMO) anode for CTBD pre-treatment. Also, the influence of the applied current density (j), initial pH, hydrodynamic conditions, and supporting electrolyte on the process performance was evaluated. Results show that COD and TOC removal were 85 and 51%, respectively, with the BDD-anode; however, they were 50 and 12% with MMO-anode at a j-value of 8.7 mA cm^-2 and neutral pH. An increased j-value increased the COD and TOC removal; however, different pHs, hydrodynamic conditions, and the addition of supporting electrolytes had a minor impact on the removal with both anodes. Liquid chromatography-organic carbon detection analysis showed that the OC in CTBD mainly consisted of humic substances (HS). EO with the BDD-anode resulted in 35% HS mineralization, while the rest of the HS were partially oxidized into low molecular weight compounds and building blocks. However, HS mineralization was limited with the MMO-anode. The mineralization and oxidation were accompanied by the formation of organic and inorganic chlorinated species. These species increased the toxicity to Vibrio fischeri 20-fold compared to the initially low-toxic CTBD. Thus, EO with a BDD-anode is a promising pre-treatment technology for the removal of OCs before CTBD deionization, but measures to minimize the chlorinated species formation are required before its application.

Electrochemical degradation of ibuprofen using an activated-carbon-based continuous-flow three-dimensional electrode reactor (3DER)

We developed a continuous-flow three-dimensional electrode reactor (3DER) to remove ibuprofen (IBP) from water. The effects of the operating parameters on the 3DER performance were investigated. The 3DER was constructed by filling a conventional two-dimensional electrode reactor with granular activated carbon, which acted as particle electrodes. The IBP removal efficiency of the 3DER was 98% in 4 h, which was 2.5 times higher than the removal efficiency for the two-dimensional electrode reactor. IBP removal kinetics tests indicated that the current density (1-20 mA/cm²) correlated better than the other operating parameters with the first-order rate constant (k). The flow rate affected the IBP removal kinetics to a small degree. Chloride and sulfate supporting electrolyte concentrations between 17 and 100 mM affected the IBP removal kinetics in opposite ways. Increasing the chloride concentration increased k, but increasing the sulfate concentration decreased k. Radical quenching experiments indicated that much more IBP degradation occurred through both indirect and direct oxidation mechanisms in the 3DER than in the two-dimensional electrode reactor. The particle electrodes caused hydroxyl radicals to form when the 3DER treatment was started, but the particle electrodes later acted as third electrodes and favored direct oxidation of IBP.