Removal of Persistent Organic Contaminants by Electrochemically Activated Sulfate

Design, Validation, and Fabrication of a Tailored Electrochemical Reactor Using 3D Printing for Studies of Commercial Boron-Doped Diamond Electrodes

Boron-doped diamond (BDD) electrodes are the most effective and resistant electrodic materials to perform advanced oxidation processes. Having a reactor that can provide adequate hydrodynamic conditions is mandatory to use these electrodes effectively. In this work, the diamond anode electrochemical reactor (E3L-DAER) is designed to fulfill this necessity. Several features are included to improve its efficiency, like conic inlet/outlet, flow enhancers, and a reduced interelectrode gap. The fluid dynamic validation has been performed using computer fluid dynamics (CFD) calculations, residence time distribution (RDT) curves, and mass transfer analysis. The reactor has been made using a three-dimensional (3D) printing stereolithography (SLA) technique, which allows us to build chemical-resistant reactors with nonstandard and tailored features in a cheap and fast way. The obtained results demonstrate that the designed reactor has the required fluid dynamics properties to perform reliable BDD electrode studies and applications. Finally, a BDD electrode was used to test the production of different oxidants such as persulfate, peroxophosphate, and chlorine-derived species.

Structural dependence of PFAS oxidation in a boron doped diamond-electrochemical system

Driven by long-term persistence and adverse health impacts of legacy perfluorooctanoic acid (PFOA), production has shifted towards shorter chain analogs (C4, perfluorobutanoic acid (PFBA)) or fluorinated alternatives such as hexafluoropropylene oxide dimer acid (HFPO-DA, known as GenX) and 6:2 fluorotelomer carboxylic acid (6:2 FTCA). Yet, a thorough understanding of treatment processes for these alternatives is limited. Herein, we conducted a comprehensive study using an electrochemical approach with a boron doped diamond anode in Na2SO4 electrolyte for the remediation of PFOA common alternatives, i.e., PFBA, GenX, and 6:2 FTCA. The degradability, fluorine recovery, transformation pathway, and contributions from electro-synthesized radicals were investigated. The results indicated the significance of chain length and structure, with shorter chains being harder to break down (PFBA (65.6 ± 5.0%) < GenX (84.9 ± 3.3%) < PFOA (97.9 ± 0.1%) < 6:2 FTCA (99.4 ± 0.0%) within 120 min of electrolysis). The same by-products were observed during the oxidation of both low and high concentrations of parent PFAS (2 and 20 mg L-1), indicating that the fundamental mechanism of PFAS degradation remained consistent. Nevertheless, the ratio of these by-products to the parent PFAS concentration varied which primarily arises from the more rapid PFAS decomposition at lower dosages. For all experiments, the main mechanism of PFAS oxidation was initiated by direct electron transfer at the anode surface. Sulfate radical (SO4•-) also contributed to the oxidation of all PFAS, while hydroxyl radical (•OH) only played a role in the decomposition of 6:2 FTCA. Total fluorine recovery of PFBA, GenX, and 6:2 FTCA were 96.5%, 94.0%, and 76.4% within 240 min. The more complex transformation pathway of 6:2 FTCA could explain its lower fluorine recovery. Detailed decomposition pathways for each PFAS were also proposed through identifying the generated intermediates and fluorine recovery. The proposed pathways were also assessed using 19F Nuclear Magnetic Resonance (NMR) spectroscopy.

Sustainable Electrochemical Activation of Self-Generated Persulfate for the Degradation of Endocrine Disruptors: Kinetics, Performances, and Mechanisms

This study presents an electrolysis system utilizing a novel self-circulation process of sulfate (SO42-) and persulfate (S2O82-) ions based on a boron-doped diamond (BDD) anode and an activated carbon fiber (ACF) cathode, which is designed to enable electrochemical remediation of environmental contaminants with reduced use of chemical reagents and minimized residues. The production of S2O82- and hydrogen peroxide (H2O2) on the BDD anode and ACF cathode, respectively, is identified as the source of active radicals for the contaminant degradation. The initiator, sulfate, is identified by comparing the degradation efficiency in NaSO4 and NaNO3 electrolytes. Quenching experiments and electron paramagnetic resonance (EPR) spectroscopy confirmed that the SO4-· and ·OH generated on the ACF cathode are the main reactive radicals. A comparison of the degradation efficiency and the generated S2O82-/H2O2 of the divided/undivided electrolysis system is used to demonstrate the superiority of the synergistic effect between the BDD anode and ACF cathode. This work provides evidence of the effectiveness of the philosophy of "catalysis in lieu of supplementary chemical agents" and sheds light on the mechanism of the generation and transmission of reactive species in the BDD and ACF electrolysis system, thereby offering new perspectives for the design and optimization of electrolysis systems.

Electrochemical degradation of key drugs to treat COVID-19: Experimental analysis of the toxic by-products formation (PCDD/Fs)

Drug consumption has grown exponentially in recent decades, particularly during the COVID-19 pandemic, leading to their presence in various water sources. In this way, degradation technologies for pollutants, such as electrochemical oxidation (ELOX), have become crucial to safeguard the quality of natural resources. This study has as its starting point a previous research, which demonstrated the efficacy of ELOX in the removal of COVID-19 related-drugs, such as dexamethasone (DEX), paracetamol (PAR), amoxicillin (AMX), and sertraline (STR), using the electrolytes NaCl and Na2SO4. The present research aims to study the potential risks associated with the generation of toxic by-products, during the ELOX of cited drugs, specifically focusing on the highly chlorinated persistent organic pollutants (POPs), such as polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Dioxins and furans can be formed potentially in electrochemical systems from precursor molecules or non-precursor molecules in chloride medium. First, the degradation of the parent compounds was found to be complete. At this point, a comprehensive investigation was conducted to identify and analyse the by-products formed during the degradation process; precursors of PCDD/Fs, such as chlorophenols or hydroquinones were identified. Additionally, in continuation of the previous study, PCDD/Fs congeners were investigated, revealing elevated concentrations; the highest concentration obtained was for the congener 1,2,3,4,6,7,8-HpCDF (234.6 pg L-1 in NaCl) during degradation of the AMX. Finally, an assessment of the toxicity based on TEQ values was conducted, with DEX exhibiting the highest concentration among all compounds: 30.1 pg L-1 for NaCl medium. Therefore, the formation of minor by-products should not be underestimated, as they can significantly enhance the toxicity of the final sample, so the selection of the appropriate remediation technology, as well as the optimization of experimental operating variables, is determining in the treatment of pharmaceutical-contaminated waters.

Peroxymonosulfate-Based Electrochemical Advanced Oxidation: Complication by Oxygen Reduction Reaction

Peroxymonosulfate (PMS)-based electrochemical advanced oxidation processes (EAOPs) have received widespread attention in recent years, but the precise nature of PMS activation and its impact on the overall process performance remain poorly understood. This study presents the first demonstration of the critical role played by the oxygen reduction reaction in the effective utilization of PMS and the subsequent enhancement of overall pollutant remediation. We observed the concurrent generation of H2O2 via oxygen reduction during the cathodic PMS activation by a model nitrogen-doped carbon nanotube catalyst. A complex interplay between H2O2 generation and PMS activation, as well as a locally increased pH near the electrode due to the oxygen reduction reaction, resulted in a SO4•-/•OH-mixed oxidation environment that facilitated pollutant degradation. The findings of this study highlight a unique dependency between PMS-driven and H2O2-driven EAOPs and a new perspective on a previously unexplored route for further enhancing PMS-based treatment processes.

Novel heterogeneous Fenton catalysts for promoting carbon iron electron transfer by one-step hydrothermal synthesization

Carbon materials play a crucial role in promoting the Fe(III)/Fe(II) redox cycle in heterogeneous Fenton reactions. However, the electron transfer efficiency between carbon and iron is typically low. In this study, we prepared a novel heterogeneous Fenton catalyst, humboldtine/hydrothermal carbon (Hum/HTC), using a one-step hydrothermal method and achieved about 100 % reduction in Fe(III) during synthesis. Moreover, the HTC continuously provided electrons to promote Fe(II) regeneration during the Fenton reaction. Electron paramagnetic resonance (EPR) and quenching experiments showed that Hum/HTC completely oxidized As(III) to As(V) via free radical and non-free radical pathways. Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) and two-dimensional correlation spectroscopy (2D-COS) analyses revealed that monodentate mononuclear (MM) and bidentate binuclear (BB) structures were the dominant bonding methods for As(V) immobilization. 40 %Hum/HTC exhibited a maximum As(III) adsorption capacity of 167 mg/g, which was higher than that of most reported adsorbents. This study provides a novel strategy for the efficient reduction of Fe(III) during catalyst synthesis and demonstrates that HTC can continuously accelerate Fe(II) regeneration in heterogeneous Fenton reactions.

Manganese-nitrogen co-doped biochar (MnN@BC) as particle electrode for three-dimensional (3D) electro-activation of peroxydisulfate: Active sites enhanced radical/non-radical oxidation

A novel manganese-nitrogen co-doped biochar (MnN@BC) was synthesized and used as particle electrodes in three-dimensional (3D) electro-activation of peroxydisulfate (PDS) for the degradation of refractory organic pollutants. All the spectroscopy (EDS, XRD, XPS, FTIR, and Raman) results indicated that Mn-N nanoclusters were successfully deposited and embedded in BC. The material appeared graphitized structure with more defects after Mn-N doping. MnN@BC in 3D electro-activation of PDS (E/MnN@BC/PDS) exhibited excellent performance in carbamazepine (CBZ) removal, with removal efficiency and degradation rates of 96.84% and 0.0582 min-1, respectively. Besides, MnN@BC was favorable for adsorption, electron transfer, and reactive oxidizing species (ROS) formation. MnN@BC had good recyclability in the E/MnN@BC/PDS system by the recycled experiments and characterization. Furthermore, quenching experiments, probe experiments, and electron paramagnetic resonance (EPR) analyses suggested that •OH and 1O2 were the main ROS in the E/MnN@BC/PDS system, and the non-radical oxidation take a key part. In addition, this system achieved excellent CBZ degradation under wide pH range of 3-11, had good tolerance to natural organic matter and inorganic ions, and was efficient to various water matrices and other refractory organic pollutants. These findings provided new insights into particle electrode design and mechanisms enhancement in electro-activated PDS systems.

Electrochemical degradation of ciprofloxacin through a DoE-driven optimization in a filter-press type reactor under batch recirculation mode

In this work, the electrochemical degradation of ciprofloxacin (CIP) was studied in a filter-press-type reactor without division in a batch recirculation manner. For this purpose, two boron-doped diamond (BDD) electrodes (as cathode and anode) were employed. Also, the optimal operating conditions were found by response surface methodology (RSM) following a central composite face-centered design with three factors, namely current intensity (i), initial pH (pH0), and initial concentration ([C]0) with two responses, namely remotion efficiency (η) and operating cost. Optimal operating conditions were i = 3 A, pH0 = 8.49, and [C]0 = 33.26 mg L-1 within an electrolysis time of 5 h, leading to a maximum removal efficiency of 93.49% with a minimum operating cost of $0.013 USD L-1. Also, a TOC analysis shows an 80% of mineralization extent with an energy consumption of 5.11 kWh g-1 TOC. Furthermore, the CIP degradation progress was followed by mass spectrometry (LC/MS) and a degradation pathway is proposed.

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

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

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

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

Green and Sustainable Ultrasound-Assisted Anodic Electrochemical Preparation of Graphene Oxide Dispersions and Their Antioxidant Properties

A fast method for preparing aqueous graphene oxide (GO) dispersions by electrochemical oxidation of a graphite anode without preliminary intercalation with oxidizing agents is proposed. Ultrasonic probing was used in the modulation mode of ultrasonic waves (work/rest) for more efficient graphite oxidation-exfoliation. It is shown that the 4/2 s mode of ultrasonic modulation is the most effective due to the probe material's low corrosion while maintaining the optimum synthesis temperature not exceeding 30-35 °C and achieving the best characteristics of the resulting product. Three cases of anodic oxidation of graphite to obtain graphene oxide were considered: (1) a combined cathode-anode compartment, (2) a split cathode-anode salt-bridged compartment, and (3) separated anode compartment with a 3.5 kDa dialysis membrane. It was determined that the approach to synthesis with a divided cathode-anode compartment makes it possible to obtain GO sheets with fewer defects compared to chemical methods or methods with a combined cathode-anode compartment and makes it possible to control the oxidation degree of the material (C:O ratio) by varying the current density. The prepared samples showed good stability for more than six months. The spectral and morphological characteristics were studied. Using chemiluminometry in the luminol/Co(II)/H₂O₂ system, the antioxidant properties concerning three key reactive oxygen species (H₂O₂, superoxide anion radical, and hydroxyl radical) were demonstrated. It was also shown that the prepared GO dispersions do not induce lipid and phospholipid peroxidation.

Formation of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) in the electrochemical oxidation of polluted waters with pharmaceuticals used against COVID-19

The COVID-19 pandemic has produced a huge impact on our lives, increasing the consumption of certain pharmaceuticals, and with this, contributing to the intensification of their presence in wastewater and in the environment. This situation demands the implementation of efficient remediation technologies, among them, electrochemical oxidation (ELOX) is one the most applied. This work studies the application of ELOX with the aim of eliminate pharmaceuticals used in the fight against COVID-19, assessing its degradation rate, as well as the risk of formation of toxic trace by-products, such as unintentional POPs like polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). To this end, model solutions containing 10 mg L^-1 of dexamethasone (DEX), paracetamol (PAR), amoxicillin (AMX), and sertraline (STR) with two different electrolytes (NaCl and Na₂SO₄) have been evaluated. However, electrochemical systems that contain chloride ions in solution together with PCDD/Fs precursor molecules may lead to the formation of these highly toxic by-products. So, PCDD/Fs were quantified under conditions of complete degradation of the drugs. Furthermore, the presence of PCDD/Fs precursors such as chlorophenols was determined, as well as the role of Cl^-, Cl^• and SO 4 • - radicals in the formation of the by-products and PCDD/Fs. The maximum measured concentration of PCDD/Fs was around 2700 pg L^-1 for the amoxicillin case in NaCl medium. The obtained results emphasise the importance of not underestimating the potential formation of these highly toxic trace by-products, in addition to the correct selection of oxidation processes and operation variables, in order to avoid final higher toxicity in the medium.

Electrochemical activation of persulfate by Al-doped blue TiO2 nanotubes for the multipath degradation of atrazine

The combination of electrolysis and persulfate activation (E/PDS) is a cost-effective method for the treatment of refractory organics. However, persulfate is difficult to be activated into radicals at the anode, resulting in insufficient electro-activation efficiency. Herein, Al doped blue TiO₂ nanotube electrodes (Al-bTNT) were first employed as cost-effective anode materials to fully activate PDS to radicals. In E/PDS, the kinetic constant of atrazine removal by Al-bTNT (0.048 min^-1) substantially outperformed the other anodes, including the blue TiO₂ nanotube electrodes (bTNT) (0.024 min^-1), Ti₄O₇ (0.02 min^-1), and B doped diamond (BDD) anodes (0.023 min^-1). The Al-bTNT-E/PDS exhibited a low energy consumption (E_EO = 0.72 kWh m^-3) and a high mineralization rate. Based on the results of electron paramagnetic resonance, quenching experiments, and probe experiments, we propose that atrazine degrades in the Al-bTNT-E/PDS system mainly via a novel radical pathway that involves both·OH and SO₄·^- and the generated SO₄·^- is responsible for the enhanced removal rate. The oxygen vacancies (V_O) generated from interstitial Al may serve as the active sites to adsorb and dissociate the persulfate molecules based on extensive characterizations. The attempt at soil-washing wastewater disposal indicated the synergistic system possessed good potential for future practical application.

Integrating anodic sulfate activation with cathodic H2O2 production/activation to generate the sulfate and hydroxyl radicals for the degradation of emerging organic contaminants

Conventional electrocatalytic degradation of pollutants involves either cathodic reduction or anodic oxidation process, which caused the low energy utilization efficiency. In this study, we successfully couple the anodic activation of sulfates with the cathodic H₂O₂ production/activation to boost the generation of sulfate radical (SO₄^·-) and hydroxyl radical (·OH) for the efficient degradation of emerging contaminants. The electrocatalysis reactor is composed of a modified-graphite-felt (GF) cathode, in-situ prepared by the carbonization of polyaniline (PANI) electrodeposited on a GF substrate, and a boron-doped diamond (BDD) anode. In the presence of sulfates, the electrocatalysis system shows superior activities towards the degradation of pharmaceutical and personal care products (PPCPs), with the optimal performance of completely degrading the representative pollutant carbamazepine (CBZ, 0.2 mg L^-1) within 150 s. Radicals quenching experiments indicated that ·OH and SO₄^·- act as the main reactive oxygen species for CBZ decomposition. Results from the electron paramagnetic resonance (EPR) and chronoamperometry studies verified that the sulfate ions were oxidized to SO₄^·-radicals at the anode, while the dissolve oxygen molecules were reduced to H₂O₂ molecules which were further activated to produce ·OH radicals at the cathode. It was also found that during the catalytic reactions SO₄^·-radicals could spontaneously convert into peroxydisulfate (PDS) which were subsequently reduced back to SO₄^·-at the cathodes. The quasi-steady-state concentrations of ·OH and SO₄^·-were estimated to be 0.51×10^-12 M and 0.56×10^-12 M, respectively. This study provides insight into the synergistic generation of ·OH/SO₄^·- from the integrated electrochemical anode oxidation of sulfate and cathode reduction of dissolved oxygen, which indicates a potential practical approach to efficiently degrade the emerging organic water contaminants.

Electrochemical degradation of PFOA and its common alternatives: Assessment of key parameters, roles of active species, and transformation pathway

This study investigates an electrochemical approach for the treatment of water polluted with per- and poly-fluoroalkyl substances (PFAS), looking at the impact of different variables, contributions from generated radicals, and degradability of different structures of PFAS. Results obtained from a central composite design (CCD) showed the importance of mass transfer, related to the stirring speed, and the amount of charge passed through the electrodes, related to the current density on decomposition rate of PFOA. The CCD informed optimized operating conditions which we then used to study the impact of solution conditions. Acidic condition, high temperature, and low initial concentration of PFOA accelerated the degradation kinetic, while DO had a negligible effect. The impact of electrolyte concentration depended on the initial concentration of PFOA. At low initial PFOA dosage (0.2 mg L^-1), the rate constant increased considerably from 0.079 ± 0.001 to 0.259 ± 0.019 min^-1 when sulfate increased from 0.1% to 10%, likely due to the production of SO₄^•-. However, at higher initial PFOA dosage (20 mg L^-1), the rate constant decreased slightly from 0.019 ± 0.001 to 0.015 ± 0.000 min^-1, possibly due to the occupation of active anode sites by excess amount of sulfate. SO₄^•- and ^•OH played important roles in decomposition and defluorination of PFOA, respectively. PFOA oxidation was initiated by one electron transfer to the anode or SO₄^•-, undergoing Kolbe decarboxylation where yielded perfluoroalkyl radical followed three reaction pathways with ^•OH, O₂ and/or H₂O. PFAS electrooxidation depended on the chemical structures where the decomposition rate constants (min^-1) were in the order of 6:2 FTCA (0.031) > PFOA (0.019) > GenX (0.013) > PFBA (0.008). PFBA with a shorter chain length and GenX with -CF₃ branching had slower decomposition than PFOA. While presence of C-H bonds makes 6:2 FTCA susceptible to the attack of ^•OH accelerating its decomposition kinetic. Conducting experiments in mixed solution of all studied PFAS and in natural water showed that the co-presence of PFAS and other water constituents (organic and inorganic matters) had adverse effects on PFAS decomposition efficiency.

Degradation of rhodamine B by persulfate activated with green tea iron nanoparticles

Green iron tea nanoparticles (GT-Fe NPs) were used as persulfate(PS) activators to oxidize rhodamine B (RhB) in this study. Optimized oxidative degradation condition was 0.033 mM Fe, 5 mM PS at pH 3.0 and 298 K with an initial RhB content of 50 mg/L. After 120 min of RhB degradation utilizing GT-Fe NPs activated PS, 99% of RhB reduction was achieved, while 98% RhB reduction with PS activated by citric acid-Fe²⁺(CA-Fe) with the same amount of Fe²⁺. This RhB reduction was due to the delayed release of Fe(II) in the GT-Fe NPs. The addition of GT-Fe NPs enhanced the synthesis of OH· and SO₄^-· while inhibiting the formation of O₂^-·. A possible RhB degradation pathway was the chromophore destruction and ring-opening processes using GT-Fe NPs/PS, which produced a range of low molecular weight carboxylic acids (oxalic acid, lactic acid, acetic acid, and formic acid). GT-Fe NPs seem to be a promising persulfate activator in comparison to common activators such as CA-Fe.

Sulfate radicals-based advanced oxidation processes for the degradation of pharmaceuticals and personal care products: A review on relevant activation mechanisms, performance, and perspectives

Owing to the rapid development of modern industry, a greater number of organic pollutants are discharged into the water matrices. In recent decades, research efforts have focused on developing more effective technologies for the remediation of water containing pharmaceuticals and personal care products (PPCPs). Recently, sulfate radicals-based advanced oxidation processes (SR-AOPs) have been extensively used due to their high oxidizing potential, and effectiveness compared with other AOPs in PPCPs remediation. The present review provides a comprehensive assessment of the different methods such as heat, ultraviolet (UV) light, photo-generated electrons, ultrasound (US), electrochemical, carbon nanomaterials, homogeneous, and heterogeneous catalysts for activating peroxymonosulfate (PMS) and peroxydisulfate (PDS). In addition, possible activation mechanisms from the point of radical and non-radical pathways are discussed. Then, biodegradability enhancement and toxicity reduction are highlighted. Comparison with other AOPs and treatment of PPCPs by the integrated process are evaluated as well. Lastly, conclusions and future perspectives on this research topic are elaborated.

Electro-activating of peroxymonosulfate via boron and sulfur co-doped macroporous carbon nanofibers cathode for high-efficient degradation of levofloxacin

To address the difficulty of precisely regulating the two-electron oxygen reduction reaction (2e-ORR) and investigate the synergistic effect of hydrogen peroxide (H₂O₂) and peroxymonosulfate (PMS), a heterogeneous electro-catalyst was synthesized via carbonation of boron (B) and sulfur (S) co-doping electrospun nanofibers containing iron and cobalt (B, S-Fe/Co@C-NCNFs-900), and used to degrade levofloxacin (Levo) in the electro-activating PMS with self-made cathode material (E-cathode-PMS) system. The morphological, structural, and electrochemical characteristics have been investigated. The results showed that B and S co-doping could remarkably enhance electron transfer and manage two-electron oxygen reduction, which was more favorable for H₂O₂ generation. Levo degradation efficiency could reach 99.63% with a reaction rate of 0.3056 min^-1 in 20 min under the appropriate conditions (pH = 4, current = 20 mA, and [PMS] = 8.0 mM). The steady-state concentration of singlet oxygen (¹O₂) was calculated to be 669.17 × 10^-14 M, which was 15.42, 29.74, and 45.00 times respectively than that of HO₂·/O₂·^- (43.40 × 10^-14 M), ·OH (22.25 × 10^-14 M) and SO₄^-·(14.87 × 10^-14 M), signifying that ¹O₂ was the predominant reactive oxygen species (ROS) involved in Levo removal. The high TOC removal (74.19%), low energy consumption (0.14 kWh m^-3 order^-1), few intermediates toxicity, and excellent Levo degradation efficiency for complex wastewater with various anions and matrixes showed the prospective practical applications of the E-cathode-PMS system. Overall, this study provides a useful strategy to regulate and control the 2e-ORR pathway.

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

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

Electro-Induced Carbon Nanotube Discrete Electrodes for Sustainable Persulfate Activation

In electrochemical advanced oxidation processes (EAOPs), the rate-limiting step is the mass transfer of pollutants to the electrodes due to the limited active surface areas. To this end, we established a three-dimensional (3D) EAOP system by coupling conventional graphite electrodes with dispersed carbon nanotubes (CNTs). The electrodes (particularly the anode) induced electric field spontaneously polarized CNTs into dispersed reactive particle electrodes (CNT-PEs) in the solution, which remarkably promoted electrochemical activation of peroxydisulfate (PDS) to generate surface CNT-PDS* complexes and surface-bound radicals (SBRs). Based on the excited potential (E_CNT-PEs) at different positions in the 3D electric field, CNT-PEs were activated into three states. (i) E_CNT-PEs < E_organic, CNT-PEs are chemically inert toward DCP oxidation; (ii) E_organic < E_CNT-PEs < E_water, CNT-PEs will oxidize DCP via an electron-transfer process (ETP); (iii) E_CNT-PEs > E_water, both CNT-PDS* complexes and the anode will oxidize water to produce SBRs. Thus, DCP could be oxidized by CNT-PDS* complexes via ETP to form polychlorophenols on the CNT surface, causing rapid deactivation of the micro-electrodes. In contrast, SBRs attack DCP directly into chloride ions and hydroxylated products, maintaining the surface cleanliness and activity of CNT-PEs for long-term operations.

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

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

Formation of chlorate and perchlorate during electrochemical oxidation by Magnéli phase Ti4O7 anode: inhibitory effects of coexisting constituents

Formation of chlorate (ClO₃⁻) and perchlorate (ClO₄⁻) as by-products in electrooxidation process has raised concern. In the present study, the formation of ClO₃⁻ and ClO₄⁻ in the presence of 1.0 mM Cl⁻ on boron doped diamond (BDD) and Magneli phase titanium suboxide (Ti₄O₇) anodes were evaluated. The Cl⁻ was transformed to ClO₃⁻ (temporal maximum 276.2 μM) in the first 0.5 h on BDD anodes with a constant current density of 10 mA cm², while approximately 1000 μM ClO₄⁻ was formed after 4.0 h. The formation of ClO₃⁻ on the Ti₄O₇ anode was slower, reaching a temporary maximum of approximately 350.6 μM in 4.0 h, and the formation of ClO₄⁻ was also slower on the Ti₄O₇ anode, taking 8.0 h to reach 780.0 μM. Compared with the BDD anode, the rate of ClO₃⁻ and ClO₄⁻ formation on the Ti₄O₇ anode were always slower, regardless of the supporting electrolytes used in the experiments, including Na₂SO₄, NaNO₃, Na₂B₄O₇, and Na₂HPO₄. It is interesting that the formation of ClO₄⁻ during electrooxidation was largely mitigated or even eliminated, when methanol, KI, and H₂O₂ were included in the reaction solutions. The mechanism of the inhibition on Cl⁻ transformation by electrooxidation was explored.

Ferrocene-modified Uio-66-NH2 hybrids with g-C3N4 as enhanced photocatalysts for degradation of bisphenol A under visible light

Designing graphitic carbon nitride (CN) based heterostructured photocatalysts with high catalytic activity is highly desired for peroxymonosulfate (PMS) activation to degrade organic pollutants from water. Herein, a novel heterostructured composite (U-F@CN) consisting of ferrocene-modified Uio-66-NH₂ (U-F) and CN was synthesized. The U-F@CN exhibited superior photocatalytic performance to degrade bisphenol A (BPA) in the presence of PMS under visible light. The experimental results indicated that BPA could be removed entirely by U-F@CN within 60 min under visible light irradiation. In addition, the outstanding photocatalytic activity could be maintained at high level in a wide pH range, appropriate temperature region and natural water condition. Benefiting from the good chemical stability, outstanding optical property and in-situ generation of interfacial heterojunction of U-F@CN, the interfacial transport of photogenerated charges could follow the Z-scheme mechanism, which can accelerate the charge separation and transport to yield abundant reactive active species (ROS) to efficiently active PMS and under visible light. This work provides a novel approach to design CN-based heterostructured photocatalysts with high stability and superior photocatalytic activity for environmental remediation.

Effect of electrolyte composition on electrochemical oxidation: Active sulfate formation, benzotriazole degradation, and chlorinated by-products distribution

Electrochemical oxidation is an effective technique for treating persistent organic pollutants, which are hardly removed in conventional wastewater treatment plants. Sulfate and chloride salts commonly used and present in natural wastewater influence the electrochemical degradation process. In this study, the effect of electrolyte composition on the active sulfate species (SO₄^●⁻ and S₂O₈²⁻) formation, benzotriazole degradation-a model organic compound, and chlorinated by-products distribution have been investigated while using a boron-doped diamond (BDD) anode. Different Na₂SO₄:NaNO₃ and Na₂SO₄:NaCl ratios with constant conductivity of 10 mS/cm were used in the experiments and applied anode potential was kept constant at 4.3 V vs. Ag/AgCl. The electrogenerated SO₄^●⁻ and S₂O₈²⁻ formation were faster in 10:1 and 2:1 Na₂SO₄:NaNO₃ ratios than in the 1:0 ratio. The ^●OH-mediated SO₄^●⁻ production has prevailed in 10:1 and 2:1 ratios. However, ^●OH-mediated SO₄^●⁻ production has hindered the 1:0 ratio due to excess chemisorption of SO₄²⁻ on the BDD anode. Similarly, the faster benzotriazole degradation, mineralization, and lowest energy consumption were achieved in the 10:1 Na₂SO₄:NaNO₃ and Na₂SO₄:NaCl ratio. Besides, chlorinated organic by-product concentration (AOX) was lower in the 10:1 Na₂SO₄:NaCl ratio but increased with the increasing chloride ratio in the electrolyte. LC-MS analysis shows that several chlorinated organic transformation products were produced in 0:1 to 2:1 ratio, which was not found in the 10:1 Na₂SO₄:NaCl ratio. A comparatively higher amount of ClO₄⁻ was formed in the 10:1 ratio than in 2:1 to 0:1 ratio. This ClO₄⁻ formation train evidence the effective ^●OH generation in a sulfate-enriched condition because the ClO₄⁻ formation is positively correlated to ^●OH concentration. Overall results show that sulfate-enriched electrolyte compositions are beneficial for electrochemical oxidation of biorecalcitrant organic pollutants.

Electrochemical oxidation processes for PFAS removal from contaminated water and wastewater: fundamentals, gaps and opportunities towards practical implementation

Electrochemical oxidation (EO) is emerging as one of the most promising methods for the degradation of recalcitrant per- and poly-fluoroalkyl substances (PFASs) in water and wastewater, as these compounds cannot be effectively treated with conventional bio- or chemical approaches. This review examines the state of the art of EO for PFASs destruction, and comprehensively compares operating parameters and treatment performance indicators for both synthetic and real contaminated water and wastewater media. The evaluation shows the need to use environmentally-relevant media to properly quantify the effectiveness/efficiency of EO for PFASs treatment. Additionally, there is currently a lack of quantification of sorption losses, resulting in a likely over-estimation of process' efficiencies. Furthermore, the majority of experimental results to date indicate that short-chain PFASs are the most challenging and need to be prioritized as environmental regulations become more stringent. Finally, and with a perspective towards practical implementation, several operational strategies are proposed, including processes combining up-concentration followed by EO destruction.

A state of the art review on electrochemical technique for the remediation of pharmaceuticals containing wastewater

Pharmaceutical wastewater is a frequent kind of wastewater with high quantities of organic pollutants, although little research has been done in the area. Pharmaceutical wastewaters containing antibiotics and high salinity may impair traditional biological treatment, resulting in the propagation of antibiotic resistance genes. The potential for advanced oxidation processes (AOPs) to break down hazardous substances instead of present techniques that essentially transfer contaminants from wastewater to sludge, a membrane filter, or an adsorbent has attracted interest. Among a variety of AOPs, electrochemical systems are a feasible choice for treating pharmaceutical wastewater. Many electrochemical approaches exist now to remediate rivers polluted by refractory organic contaminants, like pharmaceutical micro-pollutants, which have become a severe environmental problem. The first part of this investigation provides the bibliometric analysis of the title search from 1970 to 2021 for keywords such as wastewater and electrochemical. We have provided information on relations between keywords, countries, and journals based on three fields plot, inter-country co-authorship network analysis, and co-occurrence network visualization. The second part introduces electrochemical water treatment approaches customized to these very distinct discarded flows, containing how processes, electrode materials, and operating conditions influence the results (with selective highlighting cathode reduction and anodic oxidation). This section looks at how electrochemistry may be utilized with typical treatment approaches to improve the integrated system's overall efficiency. We discuss how electrochemical cells might be beneficial and what compromises to consider when putting them into practice. We wrap up our analysis with a discussion of known technical obstacles and suggestions for further research.

Electrochemical removal and recovery of uranium: Effects of operation conditions, mechanisms, and implications

Removing and recovering uranium (U) from U-mining wastewater would be appealing, which simultaneously reduces the adverse environmental impact of U mining activities and mitigates the depletion of conventional U resources. In this study, we demonstrate the application of a constant-voltage electrochemical (CVE) method for the removal and recovery of U from U-mining wastewater, in an ambient atmosphere. The effects of operation conditions were elucidated in synthetic U-bearing water experiments, and the cell voltage and the ionic strength were found to play important roles in both the U extraction kinetics and the operation cost. The mechanistic studies show that, in synthetic U-bearing water, the CVE U extraction proceeds exclusively via a single-step one-electron reduction mechanism, where pentavalent U is the end product. In real U-mining wastewater, the interference of water matrices led to the disproportionation of the pentavalent U, resulting in the formation of tetravalent and hexavalent U in the extraction products. The U extraction efficacy of the CVE method was evaluated in real U-mining wastewater, and results show that the CVE U extraction method can be efficient with operation costs ranging from $0.55/kgU ~ $64.65/kgU, with varying cell voltages from 1.0 V to 4.0 V, implying its feasibility from the economic perspective.

Electrochemical elimination of Microcystis aeruginosa with boron-doped diamond anode in different electrolyte systems: chemical and biological mechanisms

The chemical and biological mechanisms of electrochemical elimination of Microcystis aeruginosa (M. aeruginosa) using boron-doped diamond (BDD) anode were comparatively explored in three different electrolytes (chloride, sulfate, and phosphate solutions). The most efficient elimination of M. aeruginosa was observed in chloride solution, which was attributed to the greatest total long-lived oxidants from the favorable formation of active chlorine. Moreover, the high permeability of active chlorine resulted in profound intracellular damages to chlorophyll-a, microcystin-LR (MC-LR), superoxide dismutase (SOD) enzyme, and DNA in the chloride system. The change of membrane permeability and degradation of the released MC-LR induced by active chlorine were further confirmed by the increase of extracellular MC-LR in the initial 5 min and a complete decay in the subsequent 15 min, while the change in morphology of algae cells was insignificant from SEM images. In sulfate and phosphate electrolytes, membrane damages were much more pronounced based on lipid peroxidation observation, although changes in cell morphology was found more significant in phosphate system. The higher concentrations of oxidants (·OH, O₃, H₂O₂, S₂O₈^2-) generated in sulfate than in phosphate solution explained the greater efficiency of electrochemical elimination of M. aeruginosa in the sulfate electrolyte in terms of changes of cell density, OD₆₈₀, chlorophyll-a, MC-LR, lipids, SOD enzyme, and DNA.

Experimental insights into anodic oxidation of hexafluoropropylene oxide dimer acid (GenX) on boron-doped diamond anodes

GenX is the trade name of the ammonium salt of hexafluoropropylene oxide dimer acid (HFPO-DA) and is used as a replacement for the banned perfluorooctanoic acid (PFOA). However, recent studies have found GenX to be more toxic than PFOA. This work deals with the electrochemical degradation of HFPO-DA using boron-doped diamond anodes. For the first time, an experimental study was conducted to investigate the influence of sulfate concentration and other operating parameters on HFPO-DA degradation. Results demonstrated that sulfate radicals were ineffective in HFPO-DA degradation due to steric hindrance by -CF₃ branch. Direct electron transfer was found as the rate-determining step. By comparing degradation of HFPO-DA with that of PFOA, it was observed that the steric hindrance by -CF₃ branch in HFPO-DA decreased the rate of electron transfer from the carboxyl head group even though its defluorination rate was faster. Conclusively, a degradation pathway is proposed in which HFPO-DA mineralizes to CO₂ and F- via formation of three intermediates.

Electrochemical activation of peroxymonosulfate (PMS) by carbon cloth anode for sulfamethoxazole degradation

Electrochemical activation of peroxymonosulfate (PMS) at carbon cloth anode (E (Carbon cloth Anode)/PMS system) was investigated for sulfamethoxazole (SMX) degradation. The results indicated that PMS could be activated at carbon cloth anode during electrolysis, resulting in the improvement of SMX degradation. The degradation efficiency of SMX was facilitated with the higher PMS concentration and current density, respectively. The degradation rate constant of SMX increased with the rising pH from 3.6 to 6.0, and reached the highest value at pH 6.0, and then decreased with further increasing pH to 8.0. The presence of chloride ion (Cl^-, 5-100 mM) significantly enhanced SMX degradation, while addition of humic acid (HA, 1-5 mgC L^-1) inhibited SMX degradation. Addition of carbonate (HCO₃^-, 5-20 mM) had a negligible impact on SMX degradation. Small amounts of phosphate (PO₄^3-, 0-5 mM) could promote degradation, while a large amount of PO₄^3- (10-20 mM) inhibited the degradation. Moreover, the quenching experiments demonstrated that sulfate radical (SO₄·^-), hydroxyl radical (·OH) and singlet oxygen (¹O₂) contributed to SMX degradation in E (Carbon cloth Anode)/PMS system. The degradation intermediates of SMX were identified by LC-MS/MS and the degradation pathways were deduced to be hydroxylation, the cleavage of S-N bond, and oxidation of aniline group. Moreover, the micronucleus test of Vicia faba root tips indicated that the E (Carbon Cloth Anode)/PMS system could reduce the genetic toxicity of SMX contaminated water to some extent.

Recent advances in photocatalytic remediation of emerging organic pollutants using semiconducting metal oxides: an overview

Many untreated and partly treated wastewater from the home and commercial resources is being discharged into the aquatic environment these days, which contains numerous unknown and complex natural and inorganic compounds. These compounds tend to persist, initiating severe environmental problems, which affect human health. Conventionally, physicochemical treatment methods were adopted to remove such complex organic chemicals, but they suffer from critical limitations. Over time, photocatalysis, an advanced oxidation process, has gained its position for its efficient and fair performance against emerging organic pollutant decontamination. Typically, photocatalysis is a green technology to decompose organics under UV/visible light at ambient conditions. Semiconducting nanometal oxides have emerged as pioneering photocatalysts because of large active surface sites, flexible oxidation states, various morphologies, and easy preparation. The current review presents an overview of emerging organic pollutants and their effects, advanced oxidation processes, photocatalytic mechanism, types of photocatalysts, photocatalyst support materials, and methods for improving photodegradation efficiency on the degradation of complex emerging organic pollutants. In addition, the recent reports of metal-oxide-driven photocatalytic remediation of emerging organic pollutants are presented in brief. This review is anticipated to reach a broader scientific community to understand the first principles of photocatalysis and review the recent advancements in this field.

ZIF-8-derived carbon-modified g-C3N4 heterostructure with enhanced photocatalytic activity for dye degradation and hydrogen production

In this study, a metal-organic framework (ZIF-8)-derived nitrogen-doped carbon (C-ZIF)-modified g-C₃N₄ composite was directly prepared by pyrolysis. C-ZIF@g-C₃N₄ composites with different loadings were prepared by changing the precursor content. The charge transfer process that occurs at the interface of the ZIF-8-derived nitrogen-doped carbon/g-C₃N₄ heterostructures strongly hinders the recombination of photoinduced electron-hole pairs, thereby effectively enhancing the photocatalytic activity of C-ZIF@g-C₃N₄. Meanwhile, the unique hierarchical inorganic/organic heterostructure provides an abundance of active sites for photocatalytic reactions. Upon visible-light irradiation, C-ZIF@g-C₃N₄ with an optimal ratio of g-C₃N₄ to C-ZIF components exhibits both enhanced photocatalytic activity and excellent separability during the degradation of dye and hydrogen evolution compared to unmodified g-C₃N₄.

Interpretation of high perchlorate generated during electrochemical disinfection in presence of chloride at BDD anodes

Perchlorate is a disinfection by-product (DBP) of serious health concern. Herein, the long sought mechanism of high perchlorate production during electrochemical disinfection at boron-doped diamond (BDD) anode in the presence of chloride was elucidated. The generated perchlorate at BDD during electrochemical disinfection (in 10 mM NaCl) in 60 min reached 0.125 mM, which was 830 times higher than the EPA standard. In contrast, perchlorate at PbO₂ and SnO₂ anodes was below the detection limit. Further experiments employing NaClO₃ revealed that the conversion ratio from ClO₃^- to ClO₄^- in 10 h at BDD (98%) was considerably higher than PbO₂ (13%) and SnO₂ (12%). Such significant difference among anodes was fully interpreted with a two-step mechanism. The first step is essential to produce ·ClO₃ by oxidizing ClO₃^- at electrodes. Otherwise, the conversion to perchlorate would be impossible even with excessive ·OH, which was verified with the photocatalysis process. The second step is the perchlorate generation with radical reaction between ·ClO₃ and ·OH, where the primary role of ·OH was substantiated by scavenging test. Interestingly, the capability of perchlorate production was correlated with free ·OH instead of the total amount of ·OH. Despite the similar abilities of electron transfer between anodes and ClO₃^-, much higher free ·OH exists at BDD anode than at PbO₂ and SnO₂ anodes through chronoamperometry experiments and work function characterization, which reasonably provides interpretation of high perchlorate production at BDD anode.