The synergistic effect of nickel-iron-foam and tripolyphosphate for enhancing the electro-Fenton process at circum-neutral pH

Mechanistic and quantitative profiling of electro-Fenton process for wastewater treatment

Electro-Fenton (EF) process represents an energy-efficient and scalable advanced oxidation technology (AOT) for micropollutants removal in wastewaters. However, mechanistic profiling and quantitation of contribution of each subprocess (i.e., adsorption at electrode, coagulation, radical oxidation, electrode oxidation/reduction, and H₂O₂ oxidation) to the overall degradation are substantially unclear, resulting in difficulty in tunability and optimization for different treatment scenarios. In this study, we investigated degradation kinetics of a target micropollutant in an EF system. The contribution of all possible subprocesses was elucidated by comparing the observed degradation rate in the EF system with the sum of the kinetics in each subprocess. The results indicated that the overall degradation can be attributed to the synergistic action of the above-mentioned subprocesses. The radical oxidation accounts for 87% elimination, followed by electrode reoxidation/reduction of 7.7%. These results not only advance the fundamental understanding of synergistic effect in EF system, but also open new possibilities to optimize these techniques for better scalability. In addition, the methodology in this study could potentially boost the in-depth exploration of subprocess contribution in other Fenton-like systems.

Critical Review on the Mechanisms of Fe2+ Regeneration in the Electro-Fenton Process: Fundamentals and Boosting Strategies

This review presents an exhaustive overview on the mechanisms of Fe³⁺ cathodic reduction within the context of the electro-Fenton (EF) process. Different strategies developed to improve the reduction rate are discussed, dividing them into two categories that regard the mechanistic feature that is promoted: electron transfer control and mass transport control. Boosting the Fe³⁺ conversion to Fe²⁺ via electron transfer control includes: (i) the formation of a series of active sites in both carbon- and metal-based materials and (ii) the use of other emerging strategies such as single-atom catalysis or confinement effects. Concerning the enhancement of Fe²⁺ regeneration by mass transport control, the main routes involve the application of magnetic fields, pulse electrolysis, interfacial Joule heating effects, and photoirradiation. Finally, challenges are singled out, and future prospects are described. This review aims to clarify the Fe³⁺/Fe²⁺ cycling process in the EF process, eventually providing essential ideas for smart design of highly effective systems for wastewater treatment and valorization at an industrial scale.

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

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

Recent advances in application of heterogeneous electro-Fenton catalysts for degrading organic contaminants in water

Over the last decades, advanced oxidation processes (AOPs) have been widely used in surface and ground water pollution control. The heterogeneous electro-Fenton (EF) process has gained much attention due to its properties of high catalytic performance, no generation of iron sludge, and good recyclability of catalyst. As of October 2022, the cited papers and publications of EF are around 1.3 × 10^-5 and 3.4 × 10^-3 in web of science. Among the AOP techniques, the contaminant removal efficiencies by EF process are above 90% in most studies. Current reviews mainly focused on the mechanism of EF and few reviews comprehensively summarized heterogeneous catalysts and their applications in wastewater treatment. Thus, this review focuses on the current studies covering the period 2012-2022, and applications of heterogeneous catalysts in EF process. Two kinds of typical heterogeneous EF systems (the addition of solid catalysts and the functionalized cathode catalysts) and their applications for organic contaminants degradation in water are reviewed. In detail, solid catalysts, including iron minerals, iron oxide-based composites, and iron-free catalysts, are systematically described. Different functionalized cathode materials, containing Fe-based cathodes, carbonaceous-based cathodes, and heteroatom-doped cathodes, are also reviewed. Finally, emphasis and outlook are made on the future prospects and challenges of heterogeneous EF catalyst for wastewater treatments.

Boosting generation of reactive oxygen and chlorine species on TNT photoanode and Ni/graphite fiber cathode towards efficient oxidation of ammonia wastewater

Photoelectrocatalytic (PEC) process combining the merits of photocatalysis and electrocatalysis is considered as a promising ammonia oxidation technology for water treatment. However, some key issues, such as the limited in situ generation of oxidants on photoanode, slow mass transfer problem and generation of nitrate/nitrite by-products hinder the further application of PEC process in the treatment of ammonia pollutant. In this study, the graphite felt (GF) cathodes modified by different transition metals (Ni, Fe, Mn, Co, Cu) were screened by physicochemical and photoelectrochemical characterizations. The results show that the Ni-GF cathode with more Ni⁰ uniformly distributed on the GF surface had the best electrocatalytic activity to generate H₂O₂. The PEC system composed of 10.0 wt% Ni-GF cathode and optimized titania nanotubes (TNTs) photoanode selectively converted about 96.1% ammonia to N₂ within 90 min. Compared with the single TNTs photoanode system, the ammonia oxidation reaction rate constant of the synergistic PEC oxidation system was increased by about two times, which demonstrated the role of the oxidants simultaneously generated on both anode and cathode. The in situ generated reactive oxygen-based oxidants and chlorine-based oxidants interacted together, and ClO• acted a leading role in the ammonia oxidation which were confirmed by quenching and probe experiments. In addition, the contributions of •OH and ClO• were significantly improved in the synergistic PEC oxidation system, compared with the single TNTs photoanode system. Furthermore, the nitrate by-products generated by the ammonia oxidation were further reduced on the Ni-GF cathode. The large amount of active chlorine and active oxygen generated on the electrode diffused into the bulk, effectively overcoming the mass transfer limitation of direct oxidation. Therefore, the developed TNTs photoanode/Ni-GF cathode system can continuously and efficiently convert ammonia to N₂ without the formation of nitrate/nitrite by-products.

A microbubble-assisted rotary tubular titanium cathode for boosting Fenton's reagents in the electro-Fenton process

To improve cathodic H₂O₂ accumulation and Fe³⁺ reduction synchronously in the electro-Fenton (EF) process, a microbubble-assisted rotary tubular titanium cathode (MRTTC) was designed for the first time. By utilizing this MRTTC, H₂O₂ accumulation improved by 4.05-fold, along with a 200% enhancement in iron reduction compared to the conventional EF process. This promotion is mainly attributed to a considerably higher oxygen mass transfer, which reduces the thickness of the adhered diffusion layer. The oxygen mass transfer coefficient (K_La) also improved from 0.0073 s^-1 to 0.012 s^-1 at a rotational speed of 300 rpm. In addition, the microbubble-assisted cathode further improved the K_La to 0.047 s^-1. The synergistic effect between the rotating and microbubble-assisted cathodes further intensified H₂O₂ accumulation in MRTTC. Apart from H₂O₂ promotion, the iron reduction rate was elevated because the newly formed O₂^-• provided an additional reduction pathway for Fe³⁺ reduction in addition to the cathodic path. The effectiveness of MRTTC was confirmed by treating a benchmark organic pollutant, sulfamerazine (SMR), where approximately 100% SMR decay was obtained in 3 h. The results show that MRTTC is a novel and promising design in EF for antibiotic wastewater treatment.

High-precision and on-line measurement of dissolved organic matter in Electro-Fenton process based on dual wavelength analysis with combination of fluorescence emission and ultraviolet absorption spectroscopy

Electro-Fenton (EF) process is a significant water treatment method for the degradation of dissolved organic matter (DOM) in water and has been widely studied and applied in the past decade to degrade various dissolved organics. During water treatment, in order to monitor the degradation efficiency, it is in dire need to develop rapid and accurate methods that are favorable to assay on-line and on-site to provide feedback in a timely manner. UV-Visible absorption spectroscopy and Excitation-Emission Matrix (EEM) fluorescence spectroscopy techniques are most potential to realize on-line DOM measurement, but the measurement accuracy is unsatisfactory because of the obligatory involvement of iron-containing interferents. This study aims to simplify the measurement system complexity while overcoming the effect of iron-containing interferents during the measurement. An intrinsic relationship between the measured DOM concentration and the ultraviolet absorption at λ₁ and the light intensities of the fluorescence emission at λ₂ is derived theoretically and proved, based on which the influence of iron ions and their complexes on the spectrum can be eliminated, thus the content of DOM in the Electro-Fenton process is accurately determined. The proposed dual wavelength analysis with combination of fluorescence emission and ultraviolet absorption spectroscopy can achieve high precision (R²=0.9882,RMSE=0.0131mg/L). Furthermore, the on-line measurement design, called ultraviolet absorption-fluorescence emission dual wavelength analyzer, only includes one ultraviolet LED and two photodetectors. Its structure is simple and suitable for on-line monitoring DOM in EF process.

The electro-Fenton regeneration of Granular Activated Carbons: Degradation of organic contaminants and the relationship to the carbon surface

Electrochemical regeneration of Granular Activated Carbon is an emerging treatment option to restore adsorption capacity in systems designed to remove organic contaminants from aqueous solutions. The electro-Fenton process is one such electrochemical process and it is reviewed along with other members of its family including Photoelectro-Fenton and Heterogeneous electro-Fenton and electro-Fenton like reactions, for its ability to regenerate Granular Activated Carbons contaminated with organics. The behaviour of critical operating parameter such as pH, current, catalyst concentration and initial contaminant concentration are reviewed to find optimal operating conditions. The relationship between electro-Fenton regeneration and the chemical and physical surface of the carbon is also explored. Understanding regeneration mechanisms and the optimal operating conditions enables these technologies to be used commercially and to be scaled-up and treat contaminated waters more efficiently.

Cost comparison of advanced oxidation processes for wastewater treatment using accumulated oxygen-equivalent criteria

Advanced oxidation processes (AOPs) have received a lot of attention over the years as advanced physico-chemical polishing wastewater treatments to remove biorefractory pollutants. Additionally, many studies report their excellent degradation and mineralization performance as stand-alone technologies too, demonstrating the versatility of these processes; however, there is a lack of suitable methods to compare the performance (in terms of removal efficiency and operating costs) of different AOPs in the same conditions. In this context, the goal of this paper is to propose a systematic investigation by introducing a novel criterion, namely the accumulated oxygen-equivalent chemical-oxidation dose (AOCD), to systematically compare the diverse AOPs available: ozonation, H₂O₂ photolysis, Fenton, photo-Fenton, electro-Fenton and photoelectro-Fenton (paired with anodic oxidation, for the latter two). For each of these, the cost efficiency was determined by optimizing the operating conditions for the removal of phenol, selected as a model pollutant (1.4 mM, equivalent to 100 mg-C L^-1). The operating costs considered sludge management, chemical use and electricity consumption. Among all AOPs, electro-Fenton was the most cost-effective (108 - 125 € m^-3), notwithstanding the mineralization target (50%, 75% and 99%), owing to its electrocatalytic behavior. Chemical Fenton proved competitive too up to 50% of mineralization, meaning that it could also be considered as a cost-effective pre-treatment solution. AOCD was the lowest for electro-Fenton, which could be attributed to its excellent faradaic yield, while UV-based processes generally required the highest dose. The AOCD criterion could serve as a baseline for AOP comparison and prove useful for the legislator to determine the "best available techniques" as defined by the Industrial Emissions European Union Directive 2010/75/EU.

Accelerated oxidation of microcystin-LR by Fe(II)-tetrapolyphosphate/oxygen in the presence of magnesium and calcium ions

Fe(II)-tetrapolyphosphate complexes are known to activate molecular oxygen (Fe(II)-TPP/O₂) to produce reactive oxidants (most likely, Fe(IV)-TPP complexes) that are capable of degrading refractory organic contaminants in water. This study found that magnesium and calcium ions (Mg²⁺ and Ca²⁺) accelerate the degradation of micfrocystin-LR (MC-LR), the most toxic and abundant cyanotoxin, by the Fe(II)-TPP/O₂ system. The addition of Mg²⁺ and Ca²⁺ increased the observed rate constant of MC-LR degradation by up to 4.3 and 14.8 folds, respectively. Mg²⁺ and Ca²⁺ accelerated the MC-LR degradation in the entire pH range, except for the alkaline region with pH > ca. 10. The addition of Mg²⁺ and Ca²⁺ also reshaped the pH-dependency of the MC-LR degradation, greatly increasing the rate of MC-LR degradation at neutral pH. It was found that Mg²⁺ and Ca²⁺ accelerate the reaction of Fe(II)-TPP complexes with oxygen, resulting in faster production of reactive oxidants. The findings from cyclic voltammetry and potentiometric titration suggest that Mg²⁺ and Ca²⁺ form ternary complexes with Fe(II)-TPP, which exhibit higher reactivity with oxygen. Due to the effects of Mg²⁺ and Ca²⁺, the rate of MC-LR degradation by the Fe(II)-TPP/O₂ system was even higher in natural water than in deionized water.

Unconventional electro-Fenton process operating at a wide pH range with Ni foam cathode and tripolyphosphate electrolyte

We propose an unconventional electro-Fenton (EF) system with a nickel-foam (Ni-F) cathode and tripolyphosphate (3-PP) electrolyte at near-neutral pH (EF/Ni-F-3-PP) to overcome pH restrictions in EF while preventing Ni-F corrosion. Response surface modelling was used to optimize the main operating parameters with a model prediction analysis (R² = 0.99): pH = 5.8, Fe²⁺ = 3.0 mM and applied current = 349.6 mA. Among the three variables, the pH exerted the highest influence on the process. Under optimal conditions, 100 % of phenol removal was achieved in 25 min with a pseudo-first-order apparent rate constant (k_app) of 0.2 min^-1, 3.2-fold higher than the k_app of EF/Ni-F with SO₄^2- electrolyte at pH 3. A mineralization yield of 81.5 % was attained after 2 h; furthermore, it was found that 3-PP enhanced H₂O₂ accumulation by preventing bulk H₂O₂ decomposition. Finally, toxicity evaluation revealed the formation of toxic by-products at the early stages of treatment, which were totally depleted after 2 h, demonstrating the detoxifying capacity of the system. In conclusion, this study shows for the first time the potential of Ni-F as a cathode for EF under near-neutral conditions, rendered possible by the 3PP electrolyte. Under these conditions, the Ni-F corrosion issue could be alleviated.

Effect of Fe2+, Mn2+ catalysts on the performance of electro-Fenton degradation of antibiotic ciprofloxacin, and expanding the utilizing of acid mine drainage

In this work, the removal of ciprofloxacin (CIP) was studied by electro-Fenton (EF) technique using different molar ratio of Mn²⁺/Fe²⁺ based on a chemically modified graphite felt (MGF) cathode. The CIP removal efficiency reached 95.62% in 30 min and the removal efficiency of total organic carbon (TOC) reached 94.00% in 8 h under optimal conditions (50 mg/L initial CIP concentration, 400 mA applied current, 2:1 M ratio of Mn²⁺/Fe²⁺, and 3 initial pH value). A possible pathway of CIP degradation was supposed according to the analysis of the by-products detected during the EF process. An expanding experiment for CIP removal was also conducted by using acid mine drainage (AMD) rich in iron and manganese to replace the homogeneous solution in EF, and the CIP removal efficiency of 89.00% in 60 min under the optimal conditions may assign new perspectives for organic pollutants removals by utilizing AMD.

A highly efficient flow-through electro-Fenton system enhanced with nitrilotriacetic acid for phenol removal at neutral pH

Low pH requirement is one of the biggest limitations of the application of traditional Fenton and electro-Fenton (EF) process because Fe^II/Fe^III would precipitate at high pH. In this study, a flow-through EF system operated in batch recirculation mode was constructed. Nitrilotriacetic acid (NTA) was used as a chelating agent in the EF system (NTA/EF) to keep iron soluble at high pH values, producing OH by reaction of H₂O₂ generated in situ with Fe^IINTA that obtained by the reduction of Fe^IIINTA at the cathode. This flow-through NTA/EF system accelerated the mass transfer of target molecules to the electrode surface and showed high efficiency for phenol removal at pH 5-8 with rate constants (k) at around 0.26 min^-1, higher than that of the batch test (k = 0.15 min^-1) and EF process without NTA (k = 0.16 min^-1). The influences of aeration rate, current, flow rate, Fe dose, the ratio of NTA to Fe, pH, and initial phenol concentration on the phenol removal were investigated. The system could be used for at least 3 times for phenol removal without obvious efficiency decline. The flow-through NTA/EF system is promising for the removal of organic contaminants in a wide pH range.

Waste-wood-derived biochar cathode and its application in electro-Fenton for sulfathiazole treatment at alkaline pH with pyrophosphate electrolyte

For the first time, a biomass-derived porous carbon cathode (WDC) was fabricated via a facile one-step pyrolysis of recovered wood-waste without any post-treatment. The WDC along with pyrophosphate (PP) as electrolyte were used in electro-Fenton (EF) at pH 8 for sulfathiazole (STZ) treatment. The H₂O₂ accumulation capacity of WDC was optimized via the following parameters: pyrolysis temperature, applied current and electrolyte. Results showed that the WDC cathode prepared at 900 °C achieved the highest H₂O₂ accumulation (13.80 mg L^-1 in 3 h) due to its larger electroactive surface area (28.81 cm²). Interestingly, it was found that PP decreased the decomposition rate of H₂O₂ in solution as compared to conventional electrolyte, which resulted in higher H₂O₂ accumulation. PP allowed operating EF at pH of 8 due to the formation of Fe²⁺-PP complexes in solution. Moreover, Fe²⁺-PP was able to activate oxygen to produce OH. In this way, the degradation of STZ took place through four main pathways: 1) via OH from the Fe²⁺-PP complex, 2) via OH from EF reactions, 3) via surface OH at the boron doped diamond electrode (BDD) and 4) via SO₄- from BDD activation. Finally, microtox tests revealed that some toxic intermediates were generated during WDC/BDD/PP EF treatment, but they were removed at the end of the process.

Tripolyphosphate-assisted electro-Fenton process for coking wastewater treatment at neutral pH

The first application of a novel electro-Fenton (EF) for coking wastewater (CW) treatment at the original pH (6.80) by using tripolyphosphate (TPP) ligand was proposed. Total organic carbon (TOC) decay of CW followed a pseudo-first kinetic rate constant with an apparent rate constant (k_app) of 1.07 × 10^-2 min^-1 for the EF in the presence of TPP (EF/TPP), which was 2.10 times higher than that of conventional EF (k_app = 5.10 × 10^-3 min^-1) working at pH 3. The high efficiency of EF/TPP at neutral pH was mainly attributed to the newly formed Fe-O-P coordination in the iron-ligand compound (Fe²⁺-TPP) supported by UV-absorption spectra results, activating oxygen to produce ^•OH and hence enhancing the oxidation capacity. Key operating parameters of CW mineralization by EF/TPP including Fe²⁺ concentration and pH value were systematically investigated. Excitation-emission matrix (EEM) spectra technique was used to assess the variance of dissolved organic matters during the EF/TPP process. Results showed an 81% mineralization of CW after 3 h electrolysis coupled with a low energy consumption (0.129 kWh g^-1 TOC) which were obtained by the EF/TPP process. Microtox toxicity demonstrated that TPP could reduce the toxicity of raw CW and importantly, it showed that EF/TPP was effective for detoxification. Mechanism study via simulated matrix with similar components as CW revealed that ^•OH produced both from Fenton and Fe²⁺-TPP activation together with the generated active chlorine was responsible for CW mineralization. In summary, the TPP-assisted EF process was presented as a promising technique for extending coking wastewater treatment at near-neutral pH with a high mineralization.