Hydroxyl and sulfate radicals-based electrochemical advanced oxidation process for treating dye-bearing wastewater – A review

The presence of DWW can potentially diminish the entry of sunlight into aquatic environments, thereby disrupting the process of photosynthesis. Furthermore, the delicate film of dyes that develops at water surfaces has the potential to lower the targeted oxygen (O₂) concentration (Pereira & Alves 2012). Thus, it can affect both aquatic plants and animals. Furthermore, the chemical oxygen demand (COD) and biochemical oxygen demand (BOD) in real DWW were reported to be 150–30,000 and 80–6,000 mg L⁻¹, respectively, and resulted in harmful environmental and human health effects (Samsami et al. 2020). Therefore, DWW must be adequately treated to minimize its negative environmental impact.

Several techniques have been applied to treat the DWW. These include coagulation/flocculation (Dalvand et al. 2016; Kristianto et al. 2019), adsorption (Chahinez et al. 2020; Kara et al. 2021), ion exchange (Bayramoglu et al. 2020; Shao et al. 2021), membrane processes (Derouich et al. 2020; Yang et al. 2020), biodegradation (El Bouraie & El Din 2016; Przystaś et al. 2018), and advanced oxidation processes (AOPs) (Siwińska-Stefańska et al. 2018; Mansour et al. 2023). All these dye removal methods are well capable of treating the DWW. But, each of these methods has its own set of benefits and drawbacks. Coagulation is an economically viable option for treating DWW; however, high sludge production and disposal are the major problems (Mcyotto et al. 2021). The adsorption using activated carbon (Kannan & Sundaram 2001), biomass (Vedula & Yadav 2022), waste material (Hameed & Ahmad 2009), etc. is an efficient procedure in terms of cost, efficiency, flexibility, simplicity, etc. Activated carbon as an adsorbent has been proven costly, but many low-cost adsorbents have recently been reported. However, the regeneration of spent adsorbents is considered a significant issue (Messaoudi et al. 2022). Membrane filtration has been highly efficient for low-concentrated DWW because high dye concentration may lead to membrane blockage, but membranes are susceptible to clogging and fouling (Hakami et al. 2020). So, membranes need to be cleaned or changed regularly, necessitating high capital and energy costs. These passive treatment methods separate the dyes from wastewater, which again needs further treatment through advanced processes. The biological treatment process (active treatment method) using microorganisms such as bacteria (Ansari et al. 2022), fungi (Saroj et al. 2015), and algae (Mahalakshmi et al. 2015) is found to be an environmentally friendly and easy approach in dealing with DWW. However, most of the biological studies concentrate on bio-sorption rather than catabolic activities. The bio-kinetic parameters are yet to be studied in depth. Furthermore, the biological processes are very slow and may not be effective at high concentrations of dyes.

The use of the electrochemical advanced oxidation process (EAOP) has solved many of the drawbacks mentioned in the other methods up to some extent. They are based on in-situ production of hydroxyl radicals (OH^•) and sulfate radicals (SO4^−•), which work as excellent oxidizing agents that can degrade persistent organic pollutants present in the wastewater. Hydroxyl radical-based EAOP (HR-EAOP) consists of two main methods: anodic oxidation (AO), which is a direct method of producing OH^• from water (Petrucci et al. 2015) while electro-Fenton (EF) is an indirect method in which a mixture of hydrogen peroxide (H₂O₂) and ferrous salt (Fe²⁺) is used to electrochemically produce OH^• (Adachi et al. 2022).

Numerous studies have been reported on the treatment of DWW by HR-EAOP (Hamad et al. 2018; Abdelhay et al. 2021; Latha et al. 2024). But recently, sulfate radicals have been given much attention because of their better lifetime and redox potential (Flanagan et al. 1984; Wang et al. 2022). Persulfate (PS) (Chanikya et al. 2021) and peroxymonosulfate (PMS) (Yuan et al. 2021) are two primary sources of SO4^−•, which can be activated by different methods such as metal or non-metal catalysts, heat, microwave, ultrasound, and electrochemistry. Among these methods, electrochemical activation, i.e., sulfate radicals-based EAOP (SR-EAOP), has been neglected despite its significant potential for degrading organic pollutants.

Over the past five years since 2019, there has been a significant escalation in the volume of literature addressing the treatment of DWW across various academic journals and conference proceedings. Figure S1 shows that the allocation of efforts in treating DWW highlights a noticeable emphasis, with approximately 40–50% of the work concentrated on AO and EF methodologies. In contrast, a mere 10% has been dedicated to exploring SR-EAOP.

This review paper aims to thoroughly analyze and review the mechanisms and applications of HR-EAOP and SR-EAOP in treating synthetic and real DWW. AO and EF processes have been discussed specifically in HR-EAOP. Additionally, the review aims to analyze the performance of sole HR-EAOP with combined HR-EAOP and SR-EAOP in a single electrochemical cell in degrading dye pollutants, elucidating their synergistic effects and potential for practical implementation in wastewater treatment systems.

The textile sector represents one of the most substantial consumers of water, and the effluent produced from these industrial activities is markedly contaminated. Untreated textile effluent encompasses a diverse array of pollutants, including synthetic dyes, hazardous chemicals, suspended particulates, toxic heavy metals, and organic contaminants. The origins of these pollutants are predominantly associated with the dyeing, printing, and finishing stages of production. The effluent is commonly distinguished by color, fluctuations in pH, total suspended solids (TSS), total dissolved solids (TDS), hardness, chlorides, COD, BOD, oil, and grease, all of which present considerable environmental hazards when released untreated into aquatic ecosystems. The characteristics of DWW are given in Table S1 (Chockalingam et al. 2019).

There are two broader categories in which dyes can be classified. The first category is based on the chemical structure of dyes, while the second one is based on their application on the substrate. Table S2 represents different dyes such as azo, nitro, xanthene, acridine, and anthraquinone, which are classified based on their molecular structure (Ali 2010). The chemical properties of dyes depend on two components. i.e., chromophore and auxochrome. A chromophore is a functional group (azo (–N = N–), carbonyl (>C = O), and nitro (–NO₂)) that absorbs the light in the visible range, and it will decide the color of a particular dye. An auxochrome is also a functional group [hydroxyl (–OH), amino (–NH₃), and sulfonate (–SO₃–)] groups that will intensify the color of a dye. Based on substrate application, dyes have been classified as acid, basic, direct, direct, disperse, reactive, sulfur, and vat (Table S3) (Kuehni 2005). Classification by usage or application is the principal system adopted by the Colour Index (Shore 1998). Azo dyes are the largest class, with a broad range of colors and structures. In the fiscal year 2020, the annual production volume of azo dyes across India amounted to 8,500 metric tons (Bayomie et al. 2020).

In the realm of dyeing operations, two essential procedures unfold: exhaustion and fixation. The assessment of dyeing efficiency hinges upon these procedures. Typically, the complete exhaustion of dye to 100% saturation is unattainable. Instead, a range of 80–90% exhaustion is often feasible, with the residual dye subsequently discharged, contributing to DWW. Table 1 lists residual dyes percentages, which depend on the type of dyes (Periyasamy et al. 2019). These residual dyes further cause turbidity, impacting light penetration and ultimately harming the photosynthetic capacity of underwater flora and fauna. Additionally, the dyeing process elevates TDS due to salt usage. The manifestation of these issues underscores the need for the remediation of DWW.

The first contaminant to be recognized in DWW is the color because dyes are visible even in very low concentrations (<1 ppm), affecting the aesthetic appearance of water bodies (Malik 2003). A wide variety of aquatic organisms, including algae, macrophytes, and fish, can be significantly impacted by the presence of DWW. Research on the effects of optilan red dye on microalgae, specifically the Chlorella genus, revealed a marked decline in critical phytopigments such as chlorophyll and carotenoids (Gita et al. 2021). Similarly, methylene blue (MB) dye has been shown to inhibit the photosynthesis process in microalgae by directly disrupting the synthesis of key chlorophyll molecules (Moorthy et al. 2021). Aquatic macrophytes are also vulnerable to dye contamination. Studies on the impact of vat blue 20 and direct red (DR) 89 dyes on Lemna gibba, a commonly studied duckweed, showed adverse effects on both morphological and physiological characteristics, leading to chlorosis, necrosis, reduced growth, frond detachment, and decreased photosynthesis (Olaru et al. 2016). DWW has been found to impair several macrophyte parameters, including chlorophyll content, frond number, total frond area, and dry weight (Mazur et al. 2018). Fishes have also been badly affected by the DWW. A significant reduction in red blood cell count was observed in Gambusia affinis, a widely studied freshwater species (Song et al. 2017). Additionally, reactive blue 203 and maxilon blue 5G dyes have been tested on zebra fish (Danio rerio), revealing deoxyribonucleic acid (DNA) damage in brain tissues and abnormalities such as tail malformation, pericardial edema, and microphthalmia (Köktürk et al. 2021).

DWW presents significant health hazards to humans. These pollutants infiltrate the human system via the ingestion of fish and seafood, which accumulate toxins from polluted aquatic environments. Synthetic dye basic red 9 breaks down into carcinogenic aromatic amines under anaerobic conditions, and their disposal in water bodies has the potential for allergic dermatitis, skin irritation, mutations, and cancer (Sivarajasekar & Baskar 2014). Disperse red 1 dye, in particular, has demonstrated mutagenic effects on human hepatoma cells and lymphocytes, increasing the frequency of micronuclei in these cells (Fernandes et al. 2015). Furthermore, synthetic dyes, such as disperse azo dyes, have been linked to increased malondialdehyde production and oxidative stress, which result in histopathological damage to both renal and hepatic tissues (Methneni et al. 2021). Various harmful components of textile dyes are known to induce oxidative stress and cause DNA damage. Exposure to azo dyes such as sudan I and sudan II, along with their metabolites, triggers both mutagenic and epigenetic changes, leading to alterations in DNA structure and conformation, which can interfere with replication and repair processes, ultimately resulting in premature cell death (Bienstock et al. 2022).

AOPs embody a set of methodologies for degrading DWW. Rooted in the generation of exceedingly reactive oxidizing agents, namely OH^• (Oturan & Aaron 2014) and SO4^−• (Fedorov et al. 2020), these processes proficiently catalyze the oxidation and mineralization of dyes, leading to the formation of innocuous end products such as water and carbon dioxide (CO₂).

EAOP harnesses the synergistic effects of electrochemistry and advanced oxidation, offering a promising solution for the degradation of recalcitrant dye molecules. In EAOPs, electrochemical reactions occur at the electrode surface, generating highly reactive species OH^• and SO4^−• (Sirés et al. 2014). These reactive species possess strong oxidative capabilities, facilitating the breakdown of complex dye molecules into smaller, more biodegradable fragments. The treatment of DWW by HR-EAOP and SR-EAOP is discussed in the subsections below.

HR-EAOPs are broadly classified as direct and indirect oxidation based on the production of OH^•. AO and EF are the direct and indirect methods of EAOP, respectively. Both these oxidation methods are explained separately in the sections below.

There exist two principal mechanisms through which dyes undergo degradation, namely electrochemical conversion: the transformation of persistent organic pollutants into biodegradable byproducts, including short-chain carboxylic acids (Geneste 2018), and electrochemical combustion or incineration (the complete mineralization of organic pollutants into CO₂, water, and inorganic ions) (Comninellis & De Battisti 1996).

In AO, the anode materials are of paramount importance as they markedly influence the overall efficiency of the process. The classification of anodes can be divided into two principal categories: active anodes and non-active anodes. This distinction primarily hinges on the interaction of the anode material with adsorbed OH^•. In the scenario of active anodes, OH^• is chemically adsorbed onto the surface of the anode, resulting in a partial degradation of organic dyes. This occurrence of chemical adsorption leads to the formation of higher oxides or superoxides (Migliorini et al. 2011). Anodes such as platinum (Pt), titanium (Ti), iridium dioxide (IrO₂), tin oxide (SnO₂), and ruthenium oxide (RuO₂) serve as examples of active anodes (Nidheesh et al. 2018). Conversely, OH^• are physically adsorbed onto non-active anodes, including lead dioxide (PbO₂) and boron-doped diamond (BDD) (Panizza & Cerisola 2008).

Dye removal efficiency for two different anodes, BDD and PbO₂, was evaluated for methyl red (MR) degradation (Panizza & Cerisola 2008). BDD anode gave better dye removal efficiency because of the high O₂ overvoltage, which enhances the production of OH^•. Similarly, methyl orange (MO) dye degradation efficiency was compared using three different anodes, Ti/Ru/SnO₂, BDD, and PbO₂ (Labiadh et al. 2016). Partial oxidation was permitted using Ti/Ru/SnO₂ due to the accumulation of oxidation intermediates. On the other hand, complete oxidation was obtained using both BDD and PbO₂ anodes due to the formation of hydroxyl radicals at the same rate. Electrochemical degradation of methyl violet dye was compared between BDD and Pt anodes, while stainless steel (SS) was the cathode in both cases (Hamza et al. 2009). The Pt/SS combination resulted in a lower mineralization rate because the hydroxyl radical partially oxidized the dye compared to complete oxidation in the case of BDD/SS.

The impact of three distinct supporting electrolytes, namely sodium chloride (NaCl), sodium sulfate (Na₂SO₄), and sodium nitrate (NaNO₃), was investigated concerning the degradation of atrazine dye (Zhu et al. 2019). Dye removal efficiencies of 98, 84, and 77%, were achieved utilizing NaCl, Na₂SO₄, and NaNO₃, respectively, in 60 min. Similar patterns were observed by Li et al. (2020), where NaCl yielded a higher total organic carbon (TOC) removal of 57%, as compared to 24% yielded by Na₂SO₄ at 15 mA in 120 min. Chlorides exhibited superior electrolytic characteristics compared to sulfates due to their propensity for indirect oxidation mediated by electrogenerated active Cl₂ (Baddouh et al. 2019). This additional mechanism, alongside direct oxidation facilitated by hydroxyl radicals, contributes to the heightened reactivity. The resultant blend of chlorine, hypochlorite, and hypochlorous acid, generated through chloride oxidation, demonstrates heightened decolorization efficiency (Panizza & Cerisola 2003).

The initial concentration of dyes determines the kinetics of the dye degradation mechanisms. Two distinct oxidation mechanisms for acid yellow (AY) were identified in the presence of BDD anodes (Rodriguez et al. 2009). At lower concentrations of the dye, the oxidation kinetics adhered to pseudo-first-order behavior and were governed by mass transport phenomena. Conversely, at elevated dye concentrations, the degradation of AY conformed to zero-order kinetics, with the reaction kinetics being primarily regulated by charge transfer processes. Elevating the initial dye concentration causes a reduction in the dye degradation rate. This phenomenon is attributable to the tendency of dye molecules to aggregate as the concentration rises, forming clusters characterized by diminished diffusivity (Baddouh et al. 2019). Consequently, the diffusion rate of dye molecules to the anode surface decreases. Additionally, the generation rate of hydroxyl radicals diminishes, thereby leading to a commensurate decrease in the rate of dye oxidation. Consequently, this inhibition facilitated an enhanced dye removal efficiency.

The degradation kinetics exhibited greater favourability within an acidic pH (Petrucci et al. 2015). At pH 3, a significant decolorization of over 85% was observed within 2 min. In contrast, at pH 7 and 11, dye removal was merely around 30% during the same time. However, following the initial rapid decolorization under acidic conditions, the reaction rate decreased, leading to total decolorization times similar to those at elevated pH levels. Likewise, complete dye removals were accomplished at an acidic pH of 3 after durations of 30 min (Baddouh et al. 2019) and 45 min (Tang et al. 2020b) of reaction. The presence of H⁺ ions within the acidic solution acted as an inhibitor, impeding the competing reactions of O₂ evolution and OH^• generation (Dai et al. 2016). Another reason that may cause elevated degradation within acidic pH may be the fact that under acidic conditions, the equilibrium among the active Cl₂ species shifts toward HOCl, which is known to possess a higher oxidation potential than OCl⁻ (Petrucci et al. 2015).

Temperature variation affects the AO process efficiency (Araújo et al. 2014). The COD removal rate for a mixture of dyes, novacron yellow and remozal red, increased from 30 to 45% with temperature rise from 25 to 40 °C due to enhanced molecular kinetic energy promoting reactions between OH^• and dye molecules. However, increasing temperature from 40 to 60 °C led to decreased COD removal. This decline is attributed to the predominance of side reactions, such as O₂ evolution, at elevated temperatures, which detracts from dye oxidation. Similar behavior was observed by Kazm & Najim (2022), when the COD removal rate enhanced from 20 to 45% when the temperature increased from 20 to 40 °C.

Functional groups and side chains play a key role in the decolorization kinetics, affecting the efficiency of the processes. The effects of diverse functional groups in four azo dyes, namely reactive orange (RO) 16, reactive violet (RV) 4, reactive red (RR) 228, and reactive black (RB) 5, were analyzed (Soares et al. 2017). The presence of an additional sulfonic group in RV 4, as compared to RO 16, enhances electrophilic attack by OH^• radicals, which induces hydroxylation of the pollutant and cleavage of the azo bond, enhancing degradation. The slower kinetics in RB 5 treatment is due to an additional azo bond. More azo bonds increase the conjugated π system, raising the activation energy for electrophilic attack by OH^•. Higher activation energy contributes to the recalcitrance of the pollutant. Thus, this effect leads to a marked reduction in the decolorization rate. The least decolorization rate of RR 228 is linked to the ciclopropil side chain. The structure of RR 228 contains electronegative heteroatoms that increase carbon nucleophilicity, affecting oxidant interaction. Thus, OH^• preferentially attacks side chain atoms, which slows azo bond cleavage.

Other studies on the degradation of synthetic DWW using AO, such as acid green (AG) 50 (El-Ashtoukhy & Amin 2010), reactive green (RG) 19 (Petrucci et al. 2015), RV 2 (Hamad et al. 2018), MB (Baddouh et al. 2019; Sharan et al. 2023), reactive brilliant yellow (RBY) (Tang et al. 2020b), and direct blue (DB) 86 (Kumar & Gupta 2022), are present in Table 2.

The AO process has also been used to treat the real DWW. Ti/RuO₂ electrode was used as an anode for the treatment of real DWW AO (Kaur et al. 2017). At the optimized condition of current 1.66 A, time 80 min, and pH 5.49, COD and dye removal efficiencies were obtained as 78.8 and 99.1%, respectively. A similar study was done by using BDD as the anode material in the treatment of real DWW (Zou et al. 2017). At optimum conditions of CD 60 mA cm⁻², pH 2, and 3,000 mg L⁻¹ NaCl, complete COD removal was obtained within 3 h. A comparative analysis was done between anodes, Ti/RuO₂-IrO₂, and BDD in the treatment of a real DWW (Okur et al. 2022). At optimized conditions of pH 4, CD 80 mA cm⁻², time 5 h, BDD showed dye, TOC, and COD removal of 93, 75, and 48%, respectively, whereas Ti/RuO₂-IrO₂ had a dye, TOC, and COD removal of 88, 62, and 41%, respectively. Similar studies that have been done by other researchers (Radha et al. 2009; Martínez-Huitle et al. 2012; de Moura et al. 2015) on the treatment of real DWW are summarized in Table 3.

The electrochemical regeneration of Fe²⁺ ion in the EF process offers notable benefits compared to the traditional Fenton process, particularly regarding efficiency and operational simplicity (Nidheesh et al. 2024). This regeneration facilitates sustained OH^• generation, which is essential for the effective degradation of pollutants.

The concentration of H₂O₂ greatly influences the dye removal efficiency. H₂O₂ production depends on certain factors, such as operating conditions (O₂ solubility, temperature, pH) and cathode properties. So, acidic pH, ambient temperature, and an appropriate cathode material are essential for better production rates (Sirés et al. 2014). If it is available in excess amounts, hydroperoxyl radical (HO₂^•) may be formed, which has a lower oxidation potential than OH^• (Haber & Weiss 1934). Similarly, excess of Fe²⁺ leads to the accumulation of iron sludge. Thus, the [H₂O₂]/[Fe²⁺] ratio has a significant role in the degradation efficiency of dyes.

The efficiency of the EF process is mainly dependent on the choice of cathode materials being used. The effectiveness of cathode materials in the EF process hinges on several critical requirements: low catalytic activity for H₂O₂ decomposition (Zhao et al. 2024), high overvoltage for hydrogen evolution reaction (Liu et al. 2024), and high stability and conductivity. Carbonaceous materials have been mostly used as the cathode in the EF process. Several carbonaceous materials such as carbon cloth (CC) (García-Rodríguez et al. 2016), activated carbon fiber (ACF) (Ergan & Gengec 2020), reticular vitreous carbon (Rivera et al. 2022), carbon nanotubes (CNT) (Tang et al. 2020a), graphite (Adachi et al. 2022), carbon-felt (CF) (Khan et al. 2023), and carbon aerogels (Zhang et al. 2023).

The degradation of RR 195 in dyeing wastewater was studied using an EF cell consisting of a fixed bed cathode (carbon cylinders) and steel anode (screens horizontally packed) (Elbatea et al. 2021). The dye and COD removals of 100 and 96%, respectively, were attained for 50 mg L⁻¹ of dye at a CD of 2 mA cm⁻² within 60 min of contact time. The performance of three different cathode materials, ACF and CC, was evaluated in the treatment of MO dye using the EF process (García-Rodríguez et al. 2016). CF exhibited the highest degradation of 98% after 30 min, while CC yielded 62.5% degradation. CF electrodes generated more H₂O₂ than CC in the EF process due to reduced O₂ reduction overpotential, enhancing reactivity for organic pollutant degradation.

A comparative study was done on the electrochemical degradation of MR by the EF process using two different electrolytes, Na₂SO₄ and NaCl (Ma & Zhou 2009). NaCl proved to be a better electrolyte because of the indirect oxidation by active chlorine and less sensitivity towards parameter variation.

Incorporating heteroatoms like nitrogen into carbon materials markedly improves the EF process's ability to degrade DWW. Nitrogen-doped ordered mesoporous carbon exhibited enhanced electrocatalytic properties compared to its nitrogen-free counterpart, thereby facilitating the breakdown of organic contaminants such as brilliant red X3B through a reduction in the overpotential for O₂ reduction and an increase in OH^• generation vital for dye degradation (Peng et al. 2014). A highly stable nitrogen-doped carbon nano fibers led to complete decolorization of acid orange 7 with TOC removal of 92.4% at pH 3 and 93.3% at pH 6 (Barhoum et al. 2021). Nitrogen's higher electronegativity compared to carbon facilitates charge redistribution and enhances p-electron activity, thereby destabilizing O–O bonds and optimizing OOH intermediate adsorption on carbon substrates, which promotes H₂O₂ synthesis, leading to increased scholarly interest in nitrogen-doped carbon materials for their notable selectivity and efficacy in H₂O₂ production (Chen et al. 2018).

The application of catalysts has significantly intensified to improve dye degradation. Decolorization of MB dye was studied using the Ir-ZSM 5 catalyst. With a pH level of 2, a catalyst concentration of 150 mg L⁻¹ and a current of 150 mA, complete degradation was obtained because of the large surface area, high crystallinity, and uniform active phase dispersion of the catalyst (El Jery et al. 2023).

A notable trend in the electrochemical field is the implementation of three-dimensional (3D) electrochemical cells within the EF process. The 3D EF process employs polarized electrode particles between anode and cathode to enhance pollutant treatment by reducing reactant-electrode distance, improving mass transfer, and thus improving pollutant degradation. A 3D EF process, featuring a Pt anode, SS cathode, and particle electrode, synthesized from steel slag and manganese, was investigated to eliminate rhodamine (Rh) B dye in an aqueous medium. The system exhibited a degradation efficiency of 93.22% in 80 min without an air supply, while the efficiency reached 100% in 50 min with an air supply (Wang et al. 2014). A 3D EF process consisting of a magnetic multi-walled CNT (MWCNTs)-magnetite (Fe₃O₄) nano-composite (MWCNTs/Fe₃O₄) as a particle electrode with Ti/TiO₂-RuO₂-IrO₂ as an anode was utilized for the degradation of RB 5 dye (Iranpour et al. 2018). MWCNTs were made up of multiple concentric layers of graphene sheets rolled into tubes. At optimal conditions of pH 5.13, a MWCNTs/Fe₃O₄ concentration of 55.27 mg L⁻¹, a CD of 15.86 mA cm⁻², and an electrolysis duration of 57.91 min, dye and COD removals of 98.20 and 91.96%, respectively, were achieved. This notable efficacy can be ascribed to the influence of (MWCNTs/Fe₃O₄) nano-composite, which enhances H₂O₂ production and facilitates improved mass transfer, thereby promoting better interaction between dye and OH^•.

A comparative analysis of the degradation efficacy of EF and PEF processes was conducted for the removal of acid blue (AB) 29 dye (Salazar et al. 2019). The EF process achieved a total TOC removal of 35%. In contrast, the PEF process resulted in approximately 50% TOC removal following 5 h of electrolysis at a minimal irradiance of 8 W m⁻². Upon increasing the irradiance to a peak value of 25 W m⁻², nearly 90% TOC removal was accomplished after 5 h, while complete TOC elimination was observed at an irradiance of 30 W m⁻² in under 5 h. Titchou et al. (2022) investigated the degradation of DR 23 dye through EF and PEF processes, revealing that PEF achieved complete TOC removal in 6 h, surpassing the 97% removal attained by EF (Titchou et al. 2022). Gomathi et al. (2023) studied the degradation of a group of azo and thiazine dyes and obtained 96 and 92% dye and COD removal efficiencies over a period of 40 min (Gomathi et al. 2023). Similar results were obtained by other researchers who reported 97% COD reduction for eosin yellow dye after a period of 60 min (Mansour et al. 2023).

In a study, 96.6% COD and 93.5% dye removals were obtained for RB 5 at 800 mg L⁻¹ H₂O₂ dosage using the SEF process, while the EF process yielded a slightly less 95.1% COD and 91.4% dye removals (Şahinkaya 2013). A comparative analysis of EF and SEF processes in the degradation of acid black 172 dye revealed that the EF process achieved 82 and 70.5% removals of dye and COD, respectively, whereas the SEF process demonstrated superior efficiency with 95.5 and 94.5% removals of dye and COD, respectively (Mahmoudi et al. 2022).

Bio-electro-Fenton (BEF) technology combines the bio-electrochemical system with the well-known EF process, offering promising applications in environmental remediation (Olvera-Vargas et al. 2016). A typical BEF system consists of two separate chambers, the anode and cathode chambers, which are divided by a separator that enables essential chemical reactions while keeping each chamber isolated. The anode chamber requires a strictly anaerobic environment for optimal biochemical activity, while the cathode chamber operates under aerated conditions, essential for producing H₂O₂, a key element for the system's overall effectiveness (Dios et al. 2014). The process offers a unique and cost-effective advantage over the conventional EF process by harnessing the oxidation of organic matter to generate electricity, rather than depending on traditional power sources. In the BEF reactor's anode, electroactive bacteria break down organic material, releasing electrons and protons. These electrons flow through external circuits, while protons pass through proton exchange membranes to the cathode. At the cathode, a two-electron reduction of O₂ produces H₂O₂, which then reacts with Fe²⁺ to form hydroxyl radicals.

Treatment of RB 5 dye was performed using a BEF process (Dios et al. 2014). Marine sediment was used as a substrate in the anodic compartment, while graphite was used as cathodic material, with a U-shaped salt glass tube between them. A complete decolorization was obtained in 15 min of electrolysis. A high voltage of around 1,000 mV was determined too. A four-electrode BEF system was designed to enhance the oxidation of MO dye using a (ACF/Fe₃O₄) nano-composite functional cathode (Berhe et al. 2023). The BEF system achieved a power of 4,128 mW, with an open-circuit voltage of around 280 mV over a 25-day period. Under optimal conditions of MO concentration of 20 mg L⁻¹, external resistance of 100 Ω, pH 3.5, and aeration rate of 300 mL min⁻¹, the system demonstrated a degradation efficiency of 82.5 ± 4.27% after 40 h. A BEF system characterized by low energy consumption was developed for the remediation of MB dye (Wang et al. 2022). At a pH of 3, with an external resistance of 100 Ω and an airflow rate of 400 mL min⁻¹, the system demonstrated Coulombic efficiency, Faraday efficiency, H₂O₂ production, COD, and dye removal of 1.26%, 74.25%, 20.18 mg L⁻¹, 31.58%, and 93.50%, respectively. Open-circuit voltage and power of 960 mV and 1,648 mW were obtained.

Other studies on the degradation of synthetic DWW through EF processes, such as AY 36 (Cruz-González et al. 2010), novacron blue (NB) (Rêgo et al. 2014), MB (Loloei & Rezaee 2016), malachite green (MG) (Teymori et al. 2020), MO (Adachi et al. 2022), and AB 25 (TaheriAshtiani & Ayati 2022), are given in Table 4.

EF has also been employed to treat real DWW. A comparative assessment was done involving the utilization of the EF process to evaluate the decolorization of acid red 18, a synthetic dye, in contrast to real DWW (Malakootian & Moridi 2017). The EF process employed iron plates as dual-functioning anode and cathode electrodes. Under the optimized operating conditions characterized by a pH of 3, an applied voltage of 30 V, an H₂O₂ concentration of 1.45 mg L⁻¹, and an electrolyte concentration (NaCl) of 100 mg L⁻¹, a remarkably high decolorization efficiency of 99.9% was achieved. Conversely, when applied to real DWW, a slightly attenuated decolorization efficiency of 90.5% was attained. This attenuation was due to the scavenging of OH^• by the wastewater matrix. Another comparison study of the degradation efficiency of RhB with real DWW was reported using the EF process (Anil et al. 2022). MnFe₂O₄-GO was used as the catalyst, while the electrodes were Pt-coated over titanium (Ti) and graphite felt; 97.51% of dye degradation efficiency for RhB was obtained at the optimized condition of pH 3, voltage 3 V, and catalyst concentration of 20 mg L⁻¹. But, when the same optimized condition was applied to the real TWW, the degradation efficiency was reduced to 61.24%. The decrease in degradation efficiency may be attributed to the presence of varying combinations of dyes. Some other similar works which have been done by different researchers (Wang et al. 2010; Ghanbari & Moradi 2015; Kaur et al. 2017; GilPavas & Correa-Sánchez 2019; Kuleyin et al. 2021) in the treatment of real DWW are summarized in Table 5.

For numerous decades, efforts have been dedicated to treating DWW using HR-EAOPs. However, SR-EAOPs have recently gained popularity in treating DWW. Sulfate radicals have performed better than hydroxyl radicals because of certain advantages such as superior lifetime and redox potential (Nidheesh & Rajan 2016). PS and PMS are the two main sources to produce sulfate radicals. However, the direct interaction of these sources with the wastewater will result in very low degradation efficiency (Liang & Bruell 2008). So, these sources first need to be activated into sulfate radicals. There are many activation methods that can be used to activate these sources, such as metal or non-metal catalysts (Ren et al. 2015), heat (Zrinyi & Pham 2017), microwave (Yang et al. 2009), ultrasound (Gayathri et al. 2010), and electrochemistry (Chanikya et al. 2021).

Significant research on the utilization of SR-AOPs for treating DWW remains limited. However, some notable studies conducted in this domain have been summarized below. The treatment of basic violet 16 dye was investigated by SR-EAOP, using iron as both anode and cathode (Hasani et al. 2021). At optimal conditions of pH 5, voltage of 11.43 V, PS dose of 90 mg L⁻¹, initial dye concentration of 45 mg L⁻¹, and electrolysis time of 48.5 min, the dye and COD removal efficiencies were 95 and 57.14%, respectively.

The generation of sulfate radicals strongly depends on the type of anode materials being used. Different anode materials, such as BDD (Miao et al. 2020; Santos et al. 2020), graphite (Ayati & Ghorbani 2021), platinum (Li et al. 2019), and niobium pentoxide (Nb₂O₅) (Tao & Luu 2023), have been used and have given desired results. Comparative analysis of the two anodes, Ti/BDD and Ti/SnO₂-Nb₂O₅, was studied in the treatment of DWW using SR-EAOP (Tao & Luu 2023). Ti/SnO₂-Nb₂O₅ yielded 73.04% dye, 41.32% COD, and 39.22% TOC removals in 150 min, while Ti/BDD yielded higher 94.78%, 64.57% COD, and 41.57% TOC removals even in less time of 120 min. The higher removals in Ti/BDD may be attributed to the higher production of sulfate radicals. A similar comparative analysis was performed for the two anodes, BDD and Pt, in the treatment of tartrazine dyes (Yao et al. 2022). At a CD of 6 mA cm⁻², with a PS concentration of 25%, Pt yielded 58% dye removals, while BDD yielded 68% dye removals after a period of 20 min.

Introducing metallic catalysts into the PS system could improve the overall degradation efficiency of DWW. Iron and cobalt were coated on mesoporous silica (SBA-15), called (Fe–Co/SBA-15) catalyst, was fabricated for the activation of PS to degrade orange II dye (Cai et al. 2014). At an initial dye concentration of 100 mg L⁻¹, PS = 2,000 mg L⁻¹, CD of 8.40 mA cm⁻², and pH 6, a very low decolorization efficiency of around 10% were obtained after 60 min using SR-EAOP system because of the very low oxidation ability of PS. When 1,000 mg L⁻¹ of the catalyst was added into the PS system, decolorization efficiency was slightly enhanced to 18.2%.

The effectiveness of removing reactive brilliant blue was assessed by a PS-based process at various electrolytes: nitrates, chlorides, and sulfates, each at a concentration of 15 mMol L⁻¹ (Li et al. 2019). By applying a CD of 10 mA cm⁻² and performing electrolysis for 60 min, with a PS concentration of 5 mM, the results revealed that chloride-based electrolytes achieved the highest decolorization efficiency of 97.95%. In contrast, nitrate and sulfate electrolytes yielded 92.08 and 59.66% efficiency, respectively.

A comparative analysis of 3D-(SR-EAOPs) and 2D-(SR-EAOPs) was studied in the treatment of AB 113 dye (Rahmani et al. 2020). The 3D model consisted of graphite anode, SS cathode, and activated carbon as the third electrode. At optimized conditions of pH 5.5, persulfate concentration of 22.59 mM, electrode potential of 15 V, retention time of 24.99 min, and granular activated carbon (GAC) of 12.5 g, the 3D model obtained a higher 90.51% dye removal efficiency as compared to 70.25% obtained in the 2D model. The enhanced reaction kinetics and improved dye removal efficiency in 3D models, as opposed to 2D models, can be attributed to the increased surface area facilitating greater interaction between persulfate ions and dye molecules.

Graphene oxide (GO) nanoparticles can improve the decolorization efficiency of DWW by enhancing the current efficiency. The effect of GO nanoparticles was studied in the treatment of AB 25 dye (Ayati & Ghorbani 2021). Graphite rods were taken as both anode and cathode. It was found that GO nanoparticles reduced the reaction time by achieving around 95% decolorization efficiency, from 60 min (without GO) to 40 min under the optimized condition.

A real DWW was treated using an anode made of Pt/Ti and a cathode made of iron plate and PS as a source of surface radicals (Chanikya et al. 2021). A substantial 76.6% reduction in COD was reported after 60 min of electrolysis, with the optimal PS dose of 500 mg L⁻¹. Interestingly, when the PS concentration was pushed to its maximum at 1,000 mg L⁻¹, the initial stages of electrolysis exhibited the highest level of COD removal. However, this efficiency gradually diminished over the course of 60 min. This decline was attributed to elevated side reactions resulting from the exceptionally high PS concentration.

Some authors have studied the treatment of DWW by combining hydroxyl and sulfate radicals in a single electrochemical cell. Cai et al. (2014) compared the degradation efficiencies of the alone AO system and the AO/PS systems in the treatment of orange II dye (Cai et al. 2014). In the AO system, an 89.4% decolorization rate was observed, while in the AO/PS system, a slightly higher decolorization rate of 91.2% was obtained. Degradation of anthraquinone dye was studied with platinum and graphite taken as anode and cathode, respectively (Li et al. 2019). Under a separate AO system, only 37% dye degradation was obtained at 60 min. When PS was added into the AO system (AO/PS), efficiency was highly enhanced to 92.5%. Degradation of MG dye was performed, and decolorization efficiency between AO and AO/PS systems was compared (Miao et al. 2020). BDD was taken as the anode material. A very lower decolorization efficiency (28.46%) was obtained in the AO alone system, while it was greatly enhanced to 95.92% with the persulfate activated (AO/PS) system. A similar comparison was made between AO and (AO/PS) systems in treating tartrazine azo dye using BDD and SS, as anode and cathode, respectively (Santos et al. 2020). About 54% of dye degradation was obtained in AO at a CD of 16.7 mA cm⁻² and at 360 min. This limited degradation can be attributed to the minimal generation of hydroxyl radicals from the Na₂SO₄ electrolyte because of its low conductivity. Upon the introduction of 36 mM of PS (AO/PS), the decolorization rate experienced a notable enhancement, reaching 77%. This improvement can be attributed to the efficient activation of both PS and sulfate, leading to the generation of the optimum amount of sulfate radicals. A comparative analysis was studied between AO/PS and EF/PS systems in the treatment of tartrazine dye solutions, using 25% PS in both systems (Yao et al. 2022). Platinum was used as an anode in both systems. In the AO/PS system, a dye removal efficiency of 76% was obtained, while a higher 93% was obtained in the EF/PS system at a CD of 9 mA cm⁻² after a period of 20 min, which may be attributed to the better activation efficiency of PS in the presence of electrogenerated H₂O₂.

Therefore, based on the above discussions, it has been ascertained that sulfate radicals, characterized by their elevated oxidation potential (2.5–3.1 V), exhibit superior dye removal efficiency as compared to hydroxyl radicals (1.8–2.7 V) (Xia et al. 2020). SR-EAOP exhibits efficient performance under moderate pH conditions (pH 4–9), a conveniently applicable range for wastewater treatment. Conversely, EAOPs relying on hydroxyl radicals demonstrate their primary efficacy under acidic conditions (Manos et al. 2020). One significant benefit of the sulfate radical compared to the hydroxyl radical is its extended half-life, lasting between 30 and 40 μs, which is far above the half-life of hydroxyl radicals, which is merely 20 μs. This results in enhanced mass transfer and increased interactions between the organic compounds and sulfate radicals. Consequently, this highlights the efficient harnessing of radical species for degradation purposes (Ahmed et al. 2012). When non-active anodes such as BDD are employed as the anode in SR-EAOP, hydroxyl radicals are generated concurrently with sulfate radicals. This cooperative interaction enhances the system's dye degradation efficiency (Santos et al. 2020). Sulfate radicals exhibit a preference for organics, enabling the breakdown of targeted compounds by selectively degrading specific functional groups.

On the other hand, hydroxyl radicals lack selectivity, engaging in a range of reactions such as electron transfer, hydrogen atom abstraction, and more, as they indiscriminately react with organic substances (Wacławek et al. 2015). Thus, in order to arrive at a definitive deduction, it is imperative to undertake further comprehensive research endeavors. This entails the investigation of diverse electrode materials and a variety of electrolytes. Additionally, the EF process can be synergistically amalgamated with the SR-EAOP to facilitate a comprehensive comparative analysis between the HR-EAOP and SR-EAOP methodologies.

EAOPs, based on the electrochemical generation of the hydroxyl and sulfate radicals, have shown great potential to treat DWW. HR-EAOP has been found to be treating all kinds of dyes. In AO, the anodic material is the main component affecting the dye degradation efficiency. Pt, IrO₂, SnO₂, RuO₂, PbO₂, and BDD are the different anodes that have been analyzed. BDD has shown the best degradation efficiency out of all materials because of the efficient mass transfer and production of highly efficient hydroxyl radicals. NaCl has shown better performance than Na₂SO₄ and NaNO₃ because of the additional advantages of mediated oxidation by chlorides along with hydroxyl radicals. Degradation at lower concentrations of the dye has been governed by mass transport phenomena, while charge transfer processes have been governed at elevated dye concentrations. An increase in CD has also increased the degradation of DWW. A rise in temperature has also led to increased efficiencies, but very high temperatures have led to decreased efficiency because of the O₂ evolution.

In EF, carbonaceous materials, such as CC, ACF, reticular vitreous carbon, CNT, graphite, and CF, has been mainly used as the electrode material. The introduction of nitrogen into carbonaceous material has markedly improved the EF process's ability to degrade DWW. The pH, Fe²⁺ concentration, and H₂O₂ concentration are the major factors affecting the EF process's degradation efficiency. Optimum degradation has been attained at an acidic pH of 3, while it decreases at a higher pH due to the formation of ferric hydroxide precipitate. The ratio of [H₂O₂] and [Fe²⁺] has a significant role in the degradation efficiency of dyes. The application of PEF, SEF, and BEF has greatly improved the degradation of DWW when compared with the conventional EF process because of the higher generation of hydroxyl radicals and less generation of the sludge.

SR-EAOP has demonstrated significant efficacy in treating DWW. Persulfate and peroxymonosulfate serve as primary sulfate radical precursors. Various activation techniques include metal or non-metal catalysts, thermal methods, microwave application, ultrasound, and electrochemical processes. The incorporation of metallic catalysts has enhanced the degradation efficiency within the PS system for DWW. The limitations of HR-EAOP are mitigated by SR-EAOP, which exhibits superior degradation capabilities in DWW. Sulfate radicals have displayed marginally superior degradation efficiencies relative to hydroxyl radicals, owing to their elevated oxidation potential (2.5–3.1 V), broader pH applicability (2–9), and extended half-life (30–40 μs). An additional benefit of SR-EAOP is the synergistic effect of sulfate and hydroxyl radicals, which further enhances degradation efficiency.

Research on dye degradation has predominantly concentrated on azo dyes. Future studies should also investigate other dye categories, including anthraquinone, xanthene, and phthalocyanine derivatives. It is essential to note that these methods have mainly been validated in laboratory settings. Implementing these techniques in practical scenarios is vital for their real-world applicability. Most SR-EAOP research has concentrated on BDD anodes, while the potential of other anode materials remains largely unexplored. Thus, it is crucial to evaluate the performance of various anode materials to determine their effect on DWW degradation efficiency. A hybrid HR-EAOP and SR-EAOP system has demonstrated superior performance compared to the HR-EAOP system alone; however, comparative research between SR-EAOP and HR-EAOP is limited. Addressing this gap in research requires comprehensive studies to elucidate the comparative advantages and disadvantages of both methodologies. Such comparative studies will promote a deeper understanding of each approach's relative strengths and weaknesses.

M.M.A has contributed to the conceptualization of the article and written the first draft of the article. A.D. helped to draft and review the article. A.A. has reviewed the article to its final shape.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.