Influence of anode material on electrochemical oxidation for the treatment of tannery wastewater

Effective degradation of bentazone by two-dimensional and three-phase, three-dimensional electro-oxidation system: kinetic studies and optimization using ANN

Bentazone is a broad-leaved weed-specific herbicide in the pesticide industry. This study focused on removing bentazone from water using three different methods: a two and three-dimensional electro-oxidation process (2D/EOP and 3D/EOP) with a fluid-type reactor arrangement using tetraethylenepentamine-loaded particle electrodes and an adsorption method. Additionally, we analysed the effects of two types of supporting electrolytes  (Na2SO4 and NaCl) on the degradation process. The energy consumption amounts were calculated to evaluate the obtained results. The degradation reaction occurs 3.5 times faster in 3D/EOP than in 2D/EOP at 6 V in Na2SO4. Similarly, the degradation reaction of bentazone in NaCl occurs 2.5 times faster in 3D/EOP than in 2D/EOP at a value of 7.2 mA/cm2. Removal of bentazone is significantly better in 3D/EOPs than in 2D/EOPs. The use of particle electrodes can significantly enhance the degradation efficiency. The study further assessed the prediction abilities of the machine learning model (ANN). The ANN presented reasonable accuracy in bentazone degradation with high R2 values of 0.97953, 0.98561, 0.98563, and 0.99649 for 2D with Na2SO4, 2D with NaCl, 3D with Na2SO4, and 3D with NaCl, respectively.

Dissolution of mixed oxide(MOX) fuel in nitric acid:A review

The fast neutron reactor is an internationally promising fourth-generation reactor. The main fuel for this reactor is a mixed oxide fuel, and its reprocessing is currently one of the technical challenges being tackled by various countries. One of the difficulties in the reprocessing of mixed oxide (MOX) fuel lies in the nitric acid dissolution process. The high Pu content in MOX fuel can lead to issues such as solvent radiolysis, nuclear criticality, increased insoluble residues, and slow dissolution rates during the nitric acid dissolution process. These challenges have yet to be effectively addressed. This article discusses the chemical aspects of nitric acid dissolution of MOX fuel and investigates the impact of fuel manufacturing processes, the addition of metal catalyst ions, hydrofluoric acid addition, fuel plutonium content, dissolution temperature, and ultrasonic assistance on the nitric acid dissolution of MOX fuel. A review of various countries' engineering practices related to MOX fuel dissolution is presented. Based on the research findings and experiences, a potentially feasible future industrial processing route for MOX fuel is proposed, and future research priorities are outlined.

The dual built-in electric fields across CoS/MoS2 heterojunctions for energy-saving hydrogen production coupled with sulfion degradation

Substituting the sluggish oxygen evolution reaction with the sulfur oxidation reaction can significantly reduce energy consumption and eliminate environmental pollutants during hydrogen generation. However, the progress of this technology has been hindered due to the lack of cost-effective, efficient, and durable electrocatalysts. In this study, we present the design and construction of a hierarchical metal sulfide catalyst with a gradient structure comprising nanoparticles, nanosheets, and microparticles. This was achieved through a structure-breaking sulfuration strategy, resulting in a "ball of yarn"-like core/shell CoS/MoS2 microflower with CoS/MoS2/CoS dual-heterojunctions. The difference in work functions between CoS and MoS2 induces an electron polarization effect, creating dual built-in electric fields at the hierarchical interfaces. This effectively modulates the adsorption behavior of catalytic intermediates, thereby reducing the energy barrier for catalytic reactions. The optimized catalyst exhibits outstanding electrocatalytic performance for both the hydrogen evolution reaction and the sulfur oxidation reaction. Remarkably, in the assembled electrocatalytic coupling system, it only requires a cell voltage of 0.528 V at 10 mA cm-2 and maintains long-term durability for over 168 h. This work presents new opportunities for low-cost hydrogen production and environmentally friendly sulfion recycling.

Electrochemical oxidation and mechanism of sulfanilamide from groundwater in a flow-through system using carbon fiber (CF) anode

Metal-based anodes have been used for a long time in the electrochemical oxidation processes to remediate groundwater. However, the high cost of this technique as well as the release of potentially toxic metals (ex, lead), are major barriers being fully implemented. As an alternative of metal-based anodes, in recent years, carbon-based anodes have been paid attention due to their eco-friendliness and cost-effectiveness. This study evaluated the oxidation performance of carbon fiber (CF) anode in a flow-through system. The CF anode degraded 45-87% of the target pollutant (sulfanilamide), depending on the current intensity applied. However, no further degradation of sulfanilamide was observed after the cathode, indicating that sulfanilamide degradation occurred mainly at the anode. This study also determined the effect of electrolytes on electrochemical oxidation using chloride (Cl-), sulfate (SO42-), bicarbonate (CO3-), and synthetic groundwater. Cl- and SO42- electrolytes were converted electrochemically into active species, thereby enhancing sulfanilamide degradation, while the bicarbonate and groundwater electrolytes inhibited oxidation performance by scavenging hydroxyl radicals. A series of scavenger tests and characterization showed that the direct oxidation and hydroxyl radicals involved the sulfanilamide degradation. Especially, the production of hydroxyl radicals is more favorable in high currents than in low currents. That is, CF anode contributed to the degradation by direct oxidation of carbon-based electrodes and generation of hydroxyl radicals. In summary, this study highlights how a CF anode is capable of effectively degrading organic pollutants via anodic oxidation.

Electrochemical sulfate production from sulfide-containing wastewaters and integration with electrochemical nitrogen recovery

Electrochemical methods can help manage sulfide in wastewater, which poses environmental and health concerns due to its toxicity, malodor, and corrosiveness. In addition, sulfur could be recovered as fertilizer and commodity chemicals from sulfide-containing wastewaters. Wastewater characteristics vary widely among wastewaters; however, it remains unclear how these characteristics affect electrochemical sulfate production. In this study, we evaluated how four characteristics of influent wastewaters (electrolyte pH, composition, sulfide concentration, and buffer strength) affect sulfide removal (sulfide removal rate, sulfide removal efficiency) and sulfate production metrics (sulfate production rate, sulfate production selectivity). We identified that electrolyte pH (3 × difference, i.e., 25.1 to 84.9 μM h-1 in average removal rate within the studied pH range) and sulfide concentration (16 × difference, i.e., 82.1 to 1347.2 μM h-1 in average removal rate) were the most influential factors for electrochemical sulfide removal. Sulfate production was most sensitive to buffer strength (6 × difference, i.e., 4.4 to 27.4 μM h-1 in average production rate) and insensitive to electrolyte composition. Together, these results provide recommendations for the design of wastewater treatment trains and the feasibility of applying electrochemical methods to varying sulfide-containing wastewaters. In addition, we investigated a simultaneous multi-nutrient (sulfur and nitrogen) process that leverages electrochemical stripping to further enhance the versatility and compatibility of electrochemical nutrient recovery.

Performance of the Dual-Chamber Fungal Fuel Cell in Treating Tannery Wastewater

Fungi are typically expressed as excellent microorganisms that produce extracellular enzymes used in the bioaccumulation phenomenon. In this study, laboratory-scale dual-chamber fungal fuel cells (FFCs) were applied as an alternate approach for the available degradation of complex organic pollutants represented in chemical oxygen demand (COD) and total nitrogen (TN), as well as inorganic pollutants represented as total chromium (Cr), and the generation of bioenergy represented in output voltages (V), power density (PD) and current density (CD), as applied to tannery effluent. Aspergillus niger strain, (A. niger), which makes up 40% of the fungal population in tannery effluent was examined in a training study for efficient hexavalent chromium bioaccumulation, especially in high concentrations. The trained A. niger showed a faster growth rate than the untrained one in broth media containing different loaded chromium concentrations. For an external resistance of 1000 Ω, two FFCs were utilized, one with electrolytic matrices including phosphate buffer solution (PBS) and bicarbonate buffer solution (BBS), and the other without electrolytic matrices, where the energy generation and treatment efficacy of the two dual-chamber FFCs were evaluated for a period of 165 h. At 15 h, the electrolytic FFCs showed a high voltage output of 0.814 V, a power density of 0.097 mW·m−2, and a current density of 0.119 mAm−2 compared to the non-electrolytic FFC. At 165 h, the electrolytic FFCs showed high removal efficiency percentages for COD, TN, and total Cr of up to 77.9%, 94.2%, and 73%, respectively, compared to the non-electrolytic FFC.

Synthesis and characterization of cylindrical electrode with sucrose binder as advanced electrode materials for copper 3D-electro-oxidation

This study produced a biomass-based cylindrical electrode containing sucrose (an organic binder). The Cu2+ removal performance of the synthesized sucrose-bonded cylindrical electrode was evaluated in a 3-phase 3-dimensional electro-oxidation reactor (3D-EO) and the classical electro-oxidation method (2D-EO). Sodium Dodecyl Sulfate (SDs) was grafted onto activated carbon and used as microelectrode in 3D-EO reactors. SDs grafting resulted in a 57% reduction in the micropores of activated carbon. Therefore, the surface area of carbon after grafting decreased from 1328 m2/g to 580 m2/g. The sucrose-bonded cylindrical electrode has a rich carbon structure and consists of 84.04 wt% C, 12.10 wt% O and 3.20 wt%Si. According to CV measurement, the sucrose-bonded cylindrical electrode gives a surface reaction against Cu2+ at voltages lower than -0.62 V. Increasing the potential difference from 1V to 3V in 2D-EO and 3D-EO processes led to the removal of Cu2+ from the solution. The 3D-EO reactor achieved a removal rate of 87.12% at 3V. The 100 ppm solution was treated with a 3D-EO reactor containing 6 g/L of PC/SDs400Ws for 60 min, successfully removing 91.22% of Cu2+.

Hydroxyl radicals in anodic oxidation systems: generation, identification and quantification

Anodic oxidation has emerged as a promising treatment technology for the removal of a broad range of organic pollutants from wastewaters. Hydroxyl radicals are the primary species generated in anodic oxidation systems to oxidize organics. In this review, the methods of identifying hydroxyl radicals and the existing debates and misunderstandings regarding the validity of experimental results are discussed. Consideration is given to the methods of quantification of hydroxyl radicals in anodic oxidation systems with particular attention to approaches used to compare the electrochemical performance of different anodes. In addition, we describe recent progress in understanding the mechanisms of hydroxyl radical generation at the surface of most commonly used anodes and the utilization of hydroxyl radical in typical electrochemical reactors. This review shows that the key challenges facing anodic oxidation technology are related to i) the elimination of mistakes in identifying hydroxyl radicals, ii) the establishment of an effective hydroxyl radical quantification method, iii) the development of cost effective anode materials with high corrosion resistance and high electrochemical activity and iv) the optimization of electrochemical reactor design to maximise the utilization efficiency of hydroxyl radicals.

Tannery wastewater treatment: conventional and promising processes, an updated 20-year review

Mismanagement of various wastes especially waste water produced by tanning processes has caused serious environmental problems and ultimately impaired human health. Constant efforts have been making to alleviate the pollution of tannery wastewater (TWW), yet terminal treatment still takes dominance. In this review, research on TWW treatment from 2000 to 2021 was summarized, and main methods such as coagulation and flocculation, adsorption, biological treatment, membrane filtration, advanced oxidation process were briefly discussed. More detailed introduction was given to the method of electrochemical treatment since it has excellent performance such as environmental friendliness and high efficiency, hence attracting more and more research attention in recent years. In view of the harsh physi-chemical conditions of TWW, integrated or combined treatment methods are accordingly recommended with better performance and multi-function, however comprehensive studies on optimization of methods combination and cost-effectiveness are needed. The certain issues that the residue Cr in treatment sludge and high salinity in effluent still remain were put forward in this work and potential solutions were provided. Moreover, this review proposed the perspective that realizing multi-function, recycling, and intensification should be the developing direction for future TWW treatment. This review is expected to provide a general guide for researchers who aspire to ameliorate TWW pollution problems and understand various methods utilized in this field.

Graphical abstract

Application of lead oxide electrodes in wastewater treatment: A review

Electrochemical oxidation (EO) based on hydroxyl radicals (·OH) generated on lead dioxide has become a typical advanced oxidation process (AOP). Titanium-based lead dioxide electrodes (PbO₂/Ti) play an increasingly important role in EO. To further improve the efficiency, the structure and properties of the lead dioxide active surface layer can be modified by doping transition metals, rare earth metals, nonmetals, etc. Here, we compare the common preparation methods of lead dioxide. The EO performance of lead dioxide in wastewater containing dyes, pesticides, drugs, landfill leachate, coal, petrochemicals, etc., is discussed along with their suitable operating conditions. Finally, the factors influencing the contaminant removal kinetics on lead dioxide are systematically analysed.

Decolorization of textile wastewater by electrooxidation process using different anode materials: Statistical optimization

The presence of reactive dyes in textile wastewater is a serious environmental concern due to their associated mutagenic and carcinogenic effects. The present study aims to analyze the effect of different anodic materials on the decolorization of a real textile wastewater effluent. For this purpose, four different anodic materials-TiO2-coated platine, TiO2-coated ruthenium dioxide (RuO2) (viz., RuO2), titanium dioxide (TiO2), and graphite-were connected, respectively, to titanium dioxide (TiO2) used as a cathode electrode. Color and cost optimization studies were performed using the response surface methodology and the Box-Behnken experimental design (BBD). According to ANOVA results, the R2 values for Pt/TiO2, RuO2/TiO2, TiO2/TiO2, and graphite/TiO2 electrode pairs were found to be 97.4%, 93.8%, 92.44%, and 92.2%, respectively, indicating a good compatibility as it is close to one. The results show that color removal efficiencies at the optimal conditions were 86.3%, 90.8%, 91.5%, and 93.6% for Pt/TiO2, graphite/TiO2, TiO2/TiO2, and RuO2/TiO2, respectively. Furthermore, energy consumption cost at the optimum conditions was also evaluated, and the results were as follows: Pt/TiO2 (0.95 €/m3), graphite/TiO2 (0.74 €/m3), TiO2/TiO2 (0.31 €/m3), and RuO2/TiO2 (0.26 €/m3). Consequently, this research paper shows that all of the tested anodic materials give satisfactory color removal efficiencies higher than 86%. When energy consumption and color removal are considered together, the use of TiO2/TiO2 and RuO2/TiO2 pairs would be preferred. PRACTITIONER POINTS: Anodic contribution was investigated for decolorization of textile wastewater by electrooxidation process. Graphite, TiO2-coated Pt, TiO2-coated RuO2, and TiO2 were used as anode materials. Highest color removal with lowest energy consumption was achieved with TiO2-coated RuO2 anode material (93.6%).

Establishment of electrochemical treatment method to dye wastewater and its application to real samples

It is of great significance to study the treatment of organic dye pollution. In this work, a method of electrochemical treatment for reactive blue 19 dye (RB19) wastewater system was established, and it was applied to the actual dye wastewater treatment. The effects of applied voltage, electrolyte concentration, electrode spacing, and initial concentration on the removal effect of RB19 have been studied in detail. The results show that the removal rate of RB19 can reach 82.6% and the chemical oxygen demand (CODcr) removal rate is 54.3% under optimal conditions. The removal of RB19 in the system is mainly the oxidation of hydroxyl free radicals. The possible degradation pathway is inferred by ion chromatography: hydroxyl free radicals attack the chromophoric group of RB19 to make it fall off, and then decompose it into ring-opening. The product is finally oxidized to CO2 and water. The kinetic fitting is in accordance with the zero-order reaction kinetics. At the same time, using the established electrochemical system to treat the actual dye wastewater has also achieved good results. After 3 hours of treatment, the CODcr removal rate of the raw water is 44.8%, and the CODcr removal of the effluent can reach 89.5%. The degradation process conforms to the zero-order reaction kinetics. The result is consistent with the electrochemical treatment of RB19.

Efficient removal of ammoniacal nitrogen from textile printing wastewater by electro-oxidation considering the effects of NaCl and NaOCl addition

In this study, an electro-oxidation (EO) process using graphite electrodes as electrode pairs was used for the removal of chemical oxygen demand (COD), ammoniacal nitrogen (NH₄⁺-N), and color from real textile printing wastewater. The effects of solution pH, sodium chloride (NaCl) dosage, sodium hypochlorite (NaOCl), which is the oldest and still most important chlorine-based bleach, dosage, and oxidation time were investigated on the removal efficiencies. Operating conditions for the EO reactor were applied to current density 1 mA/cm², distance between the electrodes: 2 cm, 150 min operation time, and stirring speed of 500 rpm. At optimum conditions: pH 9.5, applied current density 1 mA/cm², NaCl dosage of 8 g/L, NaOCl dosage of 44.4 mg/L and 150 min electro-oxidation time, the obtained removal efficiencies were 86.5% and 91.1% for chemical oxygen demand (COD) and ammoniacal nitrogen, respectively. Efficiency was increased to 91.1% for ammoniacal nitrogen from 21.7% after applying EO combined with NaOCl addition compared to individual NaOCl addition.

Electro-enhanced chlorine-mediated ammonium nitrogen removal triggered by an optimized catalytic anode for sustainable saline wastewater treatment

Electrochemical technology has unique superiorities in chlorine-mediated pollutant oxidation, but has limited application in saline wastewater treatment due to inadequate efficiency and high energy consumption. To promote electrochemical oxidation capacity, a novel but low-cost electrode containing TiO₂/Co-WO₃/SiC was prepared and optimized, achieving highly efficient chlorine-mediated ammonium nitrogen oxidation (98.3 ± 2.2% in 120 min, with initial NH₄⁺-N of 10.2 ± 0.5 mg L^-1) in a simple electrochemical system with supplied current density only at 1.00 mA cm^-2. Comparing with unmodified carbon fiber cloth, the catalytic anode achieved 96.0% nitrogen selectivity, enhanced the system current efficiency by 20.6% and reduced the energy consumption by 54.4%, making the treatment of simulated mariculture wastewater both energy-saving (36.5 ± 2.8 kWh kg^-1 NH₄⁺-N) and cost-effective (1.45 US$ m^-3), comparing with previously reported electrochemical processes (54-622 kWh kg^-1 NH₄⁺-N). The nitrogen content (<1 mg L^-1) in the treated wastewater, containing only 0.18 mg L^-1 NH₄⁺-N, meets the discharge standard of mariculture wastewater. The promoted electrochemical oxidation should be attributed to the chloride derived species (HOCl and ClO^-) and related active species (Cl, ClO, OH, etc.). This easily prepared and reusable catalytic electrode is a promising alternative to conventional anode materials in sustainable electrochemical treatment of saline wastewater.

Fuzzy Optimization for the Remediation of Ammonia: A Case Study Based on Electrochemical Oxidation

This case study covers the application of the fuzzy optimization in simultaneously satisfying various constraints that include the compliance of ammonia and nitrate concentrations with stringent environmental standards. Essential components in the multi-criteria decision-making analysis is in the utilization of the Box-Behnken design (BBD) response equations, cost equations and the cumulative uncertainty of response towards the sodium chloride dosage, current density and electrolysis time parameters. The energy consumption in the electrochemical oxidation of ammonia plays an essential role in influencing the total operating cost analysis. The determination of boundary limits based on the global optimum resulted in the complete ammonia removal and USD 64.0 operating cost as its maximum boundary limits and the 40.6% ammonia removal and USD 17.1 as its minimum boundary limits. Based on the fuzzy optimal results, the overall satisfaction level incurred a decrease in adhering with a lower ammonia standard concentration (10 mg/L at 80.3% vs. 1.9 mg/L at 76.1%) due to a higher energy consumption requirement. Global optimal fuzzy results showed to be highly cost efficient (232.5% lower) as compared to using BBD alone. This demonstrates the practicality of fuzzy optimization applications in the electrochemical reactions.

Stormwater herbicides removal with a solar-driven advanced oxidation process: A feasibility investigation

The solar driven advanced oxidation process (AOP) has the potential to be developed as a passive stormwater post-treatment method. Despite its widespread studies in wastewater treatment, the applicability of the process for micropollutant removal in stormwater (which has very different chemical properties from wastewater) is still unknown. This paper investigated the feasibility of three different AOP processes for the degradation of two herbicides (diuron and atrazine) in pre-treated stormwater: (i) photoelectrochemical oxidation (PECO), (ii) electrochemical oxidation (ECO), and (iii) photocatalytic oxidation (PCO). The durability of different anode materials, the effects of catalyst loading, and solar photo- and thermal impacts under different applied voltages were studied. Boron-doped diamond (BDD) was found to be the most durable anode material compared to carbon fiber and titanium foil for long-term operation. Due to the very low electroconductivity of stormwater, a high voltage was required, causing severe oxidation of the carbon fiber material. PECO achieved the best degradation results compared to ECO and PCO, with over 90% degradation of both herbicides in 2 h under 5 V, following a first-order decay process (with a half-life value of 0.40 h for diuron and 0.58 h for atrazine). The voltage increase had a positive impact on the oxidation processes, with 5 V found to be the optimal applied voltage, while catalyst loading had a negligible effect. Interestingly, the solar thermal effect plays a dominant role in enhancing the performance of the PECO process, which indicates the potential of integrating a photovoltaic chamber with a PECO system to harness both the light and heat of solar energy for stormwater treatment.

Phosphate recovery and simultaneous nitrogen removal from urine by electrochemically induced struvite precipitation

The direct discharge of urine into water bodies leads to environmental pollution, and an increase in the water treatment cost, whereas recycling of the nutrients in urine is of significant economic value. A single-compartment reactor was investigated for the recycling of phosphate and simultaneous removal of nitrogen from urine wastewater by electrochemical magnesium induction, and electrochemical oxidation for the removal of residual nitrogen from the supernatant. The results demonstrated that phosphate recovery capacity was greater than 11 mg P cm^-2 h^-1 at a current density of 15 m A cm^-2 and anodizing time of 20 min; the removal rates of ammonium and total nitrogen in the synchronous electrochemical oxidation were 80% and 75%, respectively, at a current density of 45 m A cm^-2 and anodizing time of 60 min. The anodizing time and initial pH were determined to be critical control factors in the electrochemical struvite induction and nitrogen electrochemical oxidation. The on-site electrochemical nitrogen oxidation could rapidly utilize the alkaline supernatant following phosphate recovery. Thus, the integration of the single-compartment reactor, electrochemical magnesium dosage, and simultaneous nitrogen electrochemical oxidation demonstrates potential for application to decentralized reactors to treat source-separated urine.

Effect of polarity shift on the performance of electrocoagulation process for the treatment of produced water

A novel electrocoagulation (EC) system for the treatment of produced water (PW), with a provision to shift polarity is designed with the aim of reducing the problems resulting from electrode passivation. The enhancement of performance in terms of the rate and completion of the pollutant removal, energy and electrode consumption, sludge formation and ultimately the operating cost has been investigated by performing statistically designed experiments using RSM. The switching mode operation of EC had brought about significant improvement in the oil removal than the conventional mode. The polarity changeover in short span of batch recirculation time (BRT) was found to be more effective. Greater current density (CD) and supporting electrolyte concentration (SC) were found to enable a still lower switchover time (SOT) so that changeover frequency can further be increased. It was possible to remove a maximum of 99% COD and 98% Oil & Grease from a PW sample having initial concentration of 360 mgL^-1of oil (1280 mgL^{- 1}COD) at BRT of 15 min, CD of 1.6 Adm^-2, SC of 3 gL^-1 and SOT of 1 min. The combination of operating variables (BRT- 3 min, CD- 2.14 Adm^-2SC- 3 gL^-1and SOT- 1.9 min) giving minimum operating cost (0.65 US$/kg COD removed) achieving oil removal (88%) meeting discharge standards was found out by optimizing the RSM models for cost and oil removal. The method can be considered as an effective alternative for treating PW especially in offshore oil basins where time and space are the major constraints.

Enhancement of S(IV)-Cr(VI) reaction in p-nitrophenol degradation using rice husk biochar at neutral conditions

In this study, biochar R550, obtained from rice husk charred at 550 °C, was used to detoxify Cr(VI) and organic pollutant p-nitrophenol (PNP) with the cooperation of sulfite, simultaneously. Cr(VI) was mainly reduced by sulfite, and the reduction was accelerated by biochar. Also, the reactive oxygen species formed in-situ as a result of enhanced oxidation of sulfite with Cr(VI)/R550 system and the activation of O₂ by R550, led to the degradation of PNP. Production of the radicals viz., SO₃^-, SO₄^- and OH was followed by Electron Paramagnetic Resonance (EPR) study, and the predominant role of SO₄^- towards PNP degradation was confirmed by radical quenching tests. The reaction completed biochar sample was undergone the X-ray photoelectron spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR) spectral analysis, which suggested that the carboxyl group of the biochar triggered and enhanced the reactivity of Cr(VI) via coordination linkage, which in turn activated the sulfite and converted the SO₃^2- into SO₄^- to a higher extent by overcoming the undesirable transformation of SO₃^2- to SO₄^2-. Such results could also be verified by the other three biochar (Oxi-R550, H-R550, and R800). The results of the present study shed light on the mechanism of biochar mediated sulfite activation process by Cr(VI) and also assured the viability and green approach of the technique in detoxifying the industrial effluents containing Cr(VI) and organic pollutants.

A low-voltage pulse electrolysis method for the degradation of anthraquinone and azo dyes in chloride medium by anodic oxidation on Ti/IrO2 -RuO2 -SnO2 electrodes

Wastewater produced by the textile industry containing azo dyes and anthraquinone dyes is significant source of pollution to the environment and is toxic for aquatic life. To overcome the high-energy cost of traditional electrochemical oxidation, a custom-built power supply device for the degradation of anthraquinone and azo dyes by low voltage of 15.0-20.0 V pulsed discharge was investigated. Titanium coated with mixed oxide (Ti/IrO₂ -RuO₂ -SnO₂ ) plates and pure titanium plates were used as the anode and cathode, respectively, for the generation of chlorine in the dye solution. For the anthraquinone dye Reactive Blue 19, 60.0% of the chemical oxygen demand (COD) and 22.0% of the total organic carbon (TOC) were removed using this system. A comparison of the direct current electrolysis and pulsed discharge revealed that using the pulsed discharge method reduced the energy cost by 68.6%. UV-visible, LC-MS, and GC-MS were used to identify the intermediate compounds formed during the degradation of Reactive Blue 19. The results indicate that in the process of oxidation by chlorine/hypochlorite, the chromophore group was first oxidized to -NH₂ , followed by decolorization via chlorination of the aromatic rings. The results confirm that low-voltage pulse electrolysis can be used for the degradation of industrial dyes in waste effluents. PRACTITIONER POINTS: Low-voltage pulse electrolysis can be used for the degradation of industrial dyes and/or dyes in waste effluents. For anionic dye Reactive Blue 19, 60.0% of COD and 22.0% of TOC were removed using low-voltage (20.0 V) pulse electrolysis. The pulsed discharge method reduced the energy cost of this degradation process by 68.6% compared with direct current electrolysis. The intermediate compounds formed during the degradation of Reactive Blue 19 were confirmed by UV-visible spectroscopy, LC-MS, and GC-MS.

Electrochemical treatment of industrial sulfidic spent caustic streams for sulfide removal and caustic recovery

Alkaline spent caustic streams (SCS) produced in the petrochemical and chemical manufacturing industry, contain high concentrations of reactive sulfide (HS^-) and caustic soda (NaOH). Common treatment methods entail high operational costs while not recovering the possible resources that SCS contain. Here we studied the electrochemical treatment of SCS from a chemical manufacturing industry in an electrolysis cell, aiming at anodic HS^- removal and cathodic NaOH, devoid of sulfide, recovery. Using a synthetic SCS we first evaluated the HS^- oxidation product distribution over time, as well as the HS^- removal and the NaOH recovery, as a function of current density. In a second step, we investigated the operational aspects of such treatment for the industrial SCS, under 300 A m^-2 fixed current density. In an electrolysis cell receiving 205 ± 60 g S L^-1 d^-1 HS^- over 20 days of continuous operation, HS^- was removed with a 38.0 ± 7.7 % removal and ∼80 % coulombic efficiency, with a concomitant recovery of a ∼12 wt.% NaOH solution. The low cell voltage obtained (1.75 ± 0.12 V), resulted in low energy requirements of 3.7 ± 0.6 kW h kg^-1 S and 6.3 ± 0.4 kW h kg^-1 NaOH and suggests techno-economic viability of this process.

Electrochemical degradation of dye on TiO2 nanotube array constructed anode

A high oxygen evolution potential (2.6V) and conductivity of Ti/TiO₂ NTs/Ta₂O₅-PbO₂ anode was fabricated by mixed metal oxide. A well-aligned TiO₂ nanotubes was successfully prepared by using 1-butyl-3-methylimidazolium tetrafluoroborate as the electrolyte. The surface structure of anodes were characterized by scanning electron microscope, X-ray diffraction and energy dispersive X-ray spectroscopy. During the electrochemical degradation experiments, the effects of different anodes, current density, initial pH value and concentration were discussed. The results showed that co-doped Ta₂O₅ coating is an effective method to improve the surface morphology and the electrochemical characterization of Ti/TiO₂ NTs/PbO₂. At the initial pH value of 3 and current density of 12 mA cm^-2, the removal rates of Acid Orange 7 and total organic carbon with Ti/TiO₂ NTs/Ta₂O₅-PbO₂ anode were almost 100% and 98.3%. Comparing with Ti/PbO₂ anode at the same charge consumption (3 A h L^-1), the instantaneous current efficiency of the Ti/TiO₂ NTs/Ta₂O₅-PbO₂ anode and Ti/TiO₂ NTs/PbO₂ anode increased by 40.0% and 27.1%, respectively. The highest rate of k_.OH on Ti/TiO₂ NTs/Ta₂O₅-PbO₂ anode was 12.4 μmol (L min)^-1. The organic dyes are oxidized into CO₂ and H₂O by ^.OH radical. The reaction process and mechanism during the electrochemical degradation were discussed.

Simultaneous removal of hydrogen sulfide and ammonia using a combined system with absorption and electrochemical oxidation

Hydrogen sulfide (H₂S) and ammonia (NH₃), common impurities in biogas, need to be removed before utilizing it. In this study, a combined system, which consisted of an absorption column and an electrochemical oxidation reactor, was tested to simultaneously remove these impurities. The effects of the current density and the chemical loading rate on the system performance were investigated. Firstly, the mass transfer coefficients for the absorption column were determined at various gas flow rates. More mass of NH₃ was transferred, compared with that of H₂S, because of its higher solubility. In the electro-oxidation reactor, reactive chlorine species (RCSs) were generated and oxidized both H₂S and NH₃; however, NH₃ started to degrade only after H₂S was completely eliminated. At a current density of 400 A/m², the current efficiencies of H₂S and NH₃ were 23.1% and 5.9%, respectively. In the combined system, the removal efficiency of H₂S was closely related to the mass ratio of the H₂S transferred and the RCSs generated. The removal efficiency of H₂S was greater than 99% when the ratio was less than 1. The mass transfer potential and the oxidation kinetics should be balanced to improve the system performance for the simultaneous removal of H₂S and NH₃.

Electrochemical oxidation with the aerobic pretreatment process for sulfate-rich tannery effluent

The anaerobic feed of tannery effluent was treated using a new invention of an integrated approach: electrochemical oxidation with aerobic pretreatment, which reduces the chemical oxygen demand (COD) and sulfur/sulfide gas formation. Bacterial consortium was used in the present study isolated from a common effluent treatment plant (CETP). Microbial community analysis of anaerobic feed of tannery effluent (AFTE) was done by next generation sequencing. Under aerobic treatment, 79% and 85% of COD reduction were achieved during 3rd and 5th days of the aerobic process. The electrochemical oxidation process was applied for 60 min to reduce the remaining COD using the current density of 20 mA/cm². Ti-TiO₂/IrO₂/RuO₂-coated mesh and titanium sheet were used as anode and cathode respectively in an electrochemical reactor. A separate electrooxidation experiment was also carried out with galvanostatic mode of constant current density (20 mA/cm²) which enhanced the duration of electrochemical oxidation up to 13 h for complete reduction of COD concentration. UV-Vis and NMR spectroscopy were used to confirm the degradation of organic matter in the tannery effluent during aerobic and electrooxidation processes, where aerobic bacterial degradation is significant. The presence of mixed salt chloride and sulfate was recovered and the elemental composition was confirmed by EDAX analysis.

Anode materials for sulfide oxidation in alkaline wastewater: An activity and stability performance comparison

Electrochemical sulfide removal can be attractive as a zero-chemical-input approach for treatment of waste streams such as spent caustics coupled to caustic recovery. A key concern is possible decline in catalytic activity, due to passivation from deposited elemental sulfur (S⁰) on the anode surface and stability limitation, due to sulfide oxidation under highly alkaline conditions. In this study, six commercially available electrode materials (Ir Mixed Metal Oxide (MMO), Ru MMO, Pt/IrOx, Pt, PbOx and TiO₂/IrTaO₂ coated titanium-based electrodes) were tested to investigate the impact of the electrocatalyst on the process efficiency in terms of sulfide removal and final product of sulfide oxidation, as well as to determine the stability of the electrocatalyst under high sulfide concentrations (50 mM Na₂S) and high alkalinity (pH > 12). Short-term experiments showed that the catalyst type impacts the anode potential and the sulfide oxidation reaction products. Longer-term experiments under current densities up to 200 A m^-2 showed a high differentiation in stability performance among the catalysts. Ru MMO was the most active towards sulfide oxidation with a coulombic efficiency of 63.2 ± 0.5% at an average anode potential of 0.92 ± 0.17 V vs SHE. Ir MMO was the most stable, preserving 100% of its original catalyst loading during the tests. The results demonstrated that Ru MMO and Ir MMO were the most suitable electrode materials for sulfide oxidation under highly alkaline conditions, while the need for establishing a good trade-off between activity, stability and cost still persists.

Remediation of ammonia-contaminated groundwater in landfill sites with electrochemical reactive barriers: A bench scale study

Leachate plumes originating from leaking landfills often cause the contamination of groundwater in subsurface. Ammonia nitrogen in the contaminated groundwater is usually hard to be attenuated due to the hypoxic condition in subsurface environment. In this study, an active chlorine mediated electrochemical reactive barrier (ACM-ERB) consisting of inert electrodes is proposed for the remediation of ammonia-contaminated groundwater because an elevated level of chloride ions was often found in the groundwater polluted by leachate plumes. Bench-scale experiments were conducted to evaluate the prototype of this remedial technology and to study the variables affecting the performance of ACM-ERB. The results showed that ammonia in the simulated groundwater can be effectively converted into nitrogen rather than undesirable nitrite/nitrate. RuO₂/Ti anode was better than PbO₂/Ti anode for the sake of ammonia removal. In the presence of naturally occurring level of bicarbonate, the electrode arrangement with an upstream cathode offered weak alkaline pH and therefore favored the removal of ammonia in the initial stage of experiment. Higher current densities and bicarbonate concentrations were favorable to the removal of ammonia. An ammonia removal efficiency up to 70% was achieved for 20 mg/L NH₄⁺-N influent, when the operating conditions were 250 mg/L chloride ions, 500 mA current, -80 mm water level and 6 mL/min flow rate. Polarity reversal could prevent the formation of scale on electrodes, thereby allowing the long-term operation of the ACM-ERB system in groundwater. Moreover, in the experiment using diluted leachate as influent solution, ammonia was preferentially removed relative to the organic contaminants. The present study demonstrates that ACM-ERB is a promising method to cope with the ammonia-contaminated groundwater in landfill sites.

Membrane electro-oxidizer: A new hybrid membrane system with electrochemical oxidation for enhanced organics and fouling control

The synergistic combination of membrane filtration with advanced oxidation is of particular interest for next-generation wastewater treatment technologies. A membrane electro-oxidizer (MEO) hybridizing a submerged microfilter and an electrochemical cell was developed and investigated for tertiary treatment of secondary industrial (textile) wastewater effluent. Laboratory- and pilot-scale MEO systems were designed and evaluated for treatment efficiency and membrane fouling control. The MEO achieved substantial removal of color (50-90%), turbidity (>90%), and bacteria (>4 log) as well as chemical oxygen demand (13-31%) and 1,4-dioxane (∼25-53%). Fluorescence-based parallel factor analysis disclosed the degradation of humic-like organics with fluorophores. Size exclusion chromatograms with organic carbon detection confirmed the removal of specific organic molecules with ∼100 Da. While investigating the effects of oxidant quenching agents, reactive chlorine species and hydrogen peroxide were found to be most responsible for the anodic oxidation of secondary effluent organics. The efficacy of membrane fouling mitigation by the MEO was greater when higher electric current densities were applied, but was not dependent on the number of electrochemical cells installed. The MEO is a promising technology for enhanced organics removal with simultaneous fouling control due to its multifunctional active oxidants.

Electrochemical Oxidation of Resorcinol in Aqueous Medium Using Boron-Doped Diamond Anode: Reaction Kinetics and Process Optimization with Response Surface Methodology

Electrochemical oxidation of resorcinol in aqueous medium using boron-doped diamond anode (BDD) was investigated in a batch electrochemical reactor in the presence of Na₂SO₄ supporting electrolyte. The effect of process parameters such as resorcinol concentration (100–500 g/L), current density (2–10 mA/cm²), Na₂SO₄ concentration (0–20 g/L), and reaction temperature (25–45°C) was analyzed on electrochemical oxidation using response surface methodology (RSM). The optimum operating conditions were determined as 300 mg/L resorcinol concentration, 8 mA/cm² current density, 12 g/L Na₂SO₄ concentration, and 34°C reaction temperature. One hundred percent of resorcinol removal and 89% COD removal were obtained in 120 min reaction time at response surface optimized conditions. These results confirmed that the electrochemical mineralization of resorcinol was successfully accomplished using BDD anode depending on the process conditions, however the formation of intermediates and by-products were further oxidized at much lower rate. The reaction kinetics were evaluated at optimum conditions and the reaction order of electrochemical oxidation of resorcinol in aqueous medium using BDD anode was determined as 1 based on COD concentration with the activation energy of 5.32 kJ/mol that was supported a diffusion-controlled reaction.

Electrochemical treatment of tannery wastewater-Raw, coagulated, and pretreated by AOPs

This study on tannery wastewater treatment showed that indirect electrooxidation by chlorine generated at a Ti/SnO₂/PdO₂/RuO₂ (SPR) anode led to full ammonia removal, and a decrease in chemical oxygen demand (COD) of up to 77.0%. A combined process of coagulation + H₂O₂/UV + electrooxidation allowed us to achieve a decrease in COD of up to 97.5%. Equations describing the kinetics of the decrease in COD, the relationship between the decrease in COD and current, and the relationship between the current efficiency of COD reduction and the initial concentrations of the reagents were established. The changes in the Adsorbable Organically Bound Halogen (AOX) value were determined, and the individual compounds, including chloroorganics, present in raw and treated wastewater were identified by gas chromatography mass spectrometry (GC-MS). Values of AOX increased in the initial phase of electrooxidation, while afterwards they decreased.

Nitrate and carbon matter removals from real effluents using Si/BDD electrode

This work investigated the electrochemical treatment of four real effluents which were municipal wastewater (MWW), human urine (HU), river water (Wadi), and slaughterhouse wastewater (SHWW). The treatment was performed on a Boron-Doped Diamond (BDD) as anode/cathode material with an applied current density of 35.7 mA cm^-2 and without any reagent addition. Effluent characterization before treatment indicated that nitrogen pollution existed essentially as ammonium/ammonia ions, low level of nitrate, and in some cases as nitrite form. Organic pollution was also determined by COD values which were 920, 7300, 320, and 2280 mg O₂ L^-1 for MWW, HU, Wadi, and SHWW effluents, respectively. The effectiveness of the electrochemical oxidation/reduction was assessed by nitrogenous compounds and COD removals. Obtained removals underlined the simultaneous oxidation and reduction at the BDD interfaces of the main species present in the real effluents as well as the electro-generated ones. Results confirmed the high performance of BDD electrode for removal of coexistent pollutants from the studied matrix. The oxidation of organic matter and ammonium/ammonia as well as the kinetic of COD degradation was rapid in acidic medium (HU case) than that in neutral and alkaline medium (MWW, Wadi, and SHWW). Moreover, the phytotoxicity test showed less toxic behavior only in the cases of MWW and SHWW with a Germination Index equal to 58.8 and 72.2 %, respectively. The EC and ACE were also evaluated for all studied wastewaters, and the lowest EC value (0.03 kWh (g COD)^-1) was obtained for the more charged effluent (HU).

Phenolic wastewaters depuration by electrochemical oxidation process using Ti/IrO2 anodes

The electrochemical oxidation (EO) of phenolic wastewaters mimicking olive oil mill effluents was carried out in a batch stirring reactor using Ti/IrO₂ anodes, varying the nature (NaCl and Na₂SO₄) and electrolyte concentration (1.8-20 g L^-1), current density (57-119 mA cm^-2) and initial pH (3.4-9). Phenolic content (TPh) and chemical oxygen demand (COD) removals were monitored as a function of applied charge and over time. The nature of the electrolyte greatly affected the efficiency of the system, followed by the influence of the current density. The NaCl concentration and the initial pH influenced the process in a lesser extent. The best operating conditions achieved were 10 g L^-1 of NaCl, current density of 119 mA cm^-2 and initial pH of 3.4. These parameters led to 100 and 84.8% of TPh and COD removal, respectively. Under these conditions, some morphological differences were observed by SEM on the surface of the anode after treatment. To study the potential toxicity of the synthetic effluent in neuronal activity, this mixture was applied to rat brain slices prior to and after EO. The results indicate that although the treated effluent causes a smaller depression of the neuronal reactive oxygen species (ROS) signal than the untreated one, it leads to a potentiation instead of recovery, upon washout. Furthermore, the purification of a real olive mill wastewater (OMW), with the organic load of the synthetic effluent, using the same optimised operating conditions, achieved total phenolic compounds abatement and 62.8% of COD removal.This study demonstrates the applicability of this EO as a pre-treatment process of a real effluent, in order to achieve the legal limit values to be discharged into natural streams regarding its organic load.

Environmental Pollution, Toxicity Profile and Treatment Approaches for Tannery Wastewater and Its Chemical Pollutants

Leather industries are key contributors in the economy of many developing countries, but unfortunately they are facing serious challenges from the public and governments due to the associated environmental pollution. There is a public outcry against the industry due to the discharge of potentially toxic wastewater having alkaline pH, dark brown colour, unpleasant odour, high biological and chemical oxygen demand, total dissolved solids and a mixture of organic and inorganic pollutants. Various environment protection agencies have prioritized several chemicals as hazardous and restricted their use in leather processing however; many of these chemicals are used and discharged in wastewater. Therefore, it is imperative to adequately treat/detoxify the tannery wastewater for environmental safety. This paper provides a detail review on the environmental pollution and toxicity profile of tannery wastewater and chemicals. Furthermore, the status and advances in the existing treatment approaches used for the treatment and/or detoxification of tannery wastewater at both laboratory and pilot/industrial scale have been reviewed. In addition, the emerging treatment approaches alone or in combination with biological treatment approaches have also been considered. Moreover, the limitations of existing and emerging treatment approaches have been summarized and potential areas for further investigations have been discussed. In addition, the clean technologies for waste minimization, control and management are also discussed. Finally, the international legislation scenario on discharge limits for tannery wastewater and chemicals has also been discussed country wise with discharge standards for pollution prevention due to tannery wastewater.

Evaluation of the Treatment Process of Landfill Leachate Using the Toxicity Assessment Method

Landfill leachate is composed of a complex composition with strong biological toxicity. The combined treatment process of coagulation and sedimentation, anaerobics, electrolysis, and aerobics was set up to treat landfill leachate. This paper explores the effect of different operational parameters of coagulation and sedimentation tanks and electrolytic cells, while investigating the combined process for the removal efficiency of physicochemical indices after processing the landfill leachate. Meanwhile, a battery of toxicity tests with Vibrio fischeri, zebrafish larvae, and embryos were conducted to evaluate acute toxicity and calculated the toxicity reduction efficiency after each treatment process. The combined treatment process resulted in a 100% removal efficiency of Cu, Cd and Zn, and a 93.50% and an 87.44% removal efficiency of Ni and Cr, respectively. The overall removal efficiency of chemical oxygen demand (COD), ammonium nitrogen (NH₄⁺-N), and total nitrogen (TN) were 93.57%, 97.46% and 73.60%, respectively. In addition, toxicity test results showed that the acute toxicity of landfill leachate had also been reduced significantly: toxicity units (TU) decreased from 84.75 to 12.00 for zebrafish larvae, from 82.64 to 10.55 for zebrafish embryos, and from 3.41 to 0.63 for Vibrio fischeri. The combined treatment process was proved to be an efficient treatment method to remove heavy metals, COD, NH₄⁺-N, and acute bio-toxicity of landfill leachate.