Electro-Peroxone and Photoelectro-Peroxone Hybrid Approaches: An Emerging Paradigm for Wastewater Treatment

Abstract

Electrochemical advanced oxidation practices (EAOPs), remarkably, electro-peroxone (EP), photoelectro-peroxone (PEP), and complementary hybrid EP approaches, are emerging technologies on accountability of complete disintegration and elimination of wide spectrum of model pollutants predominantly biodegradable, recalcitrant, and persistent organic pollutants by engendering powerful oxidants in wastewater. A concise mechanism of EP and PEP approaches along with their contribution to free radical formation are scrutinized. Furthermore, this chapter provides a brief review of EP, PEP, and complementary hybrid EP-based EAOPs that have pragmatically treated laboratory-scale low- and high-concentrated distillery biodigester effluent, refractory pharmaceutical, textile, herbicides, micropollutant, organic pollutant, acidic solution, landfill leachates, municipal secondary effluents, hospital, and industries-based wastewater. Afterward, discussion has further extended to quantitatively evaluate energy expenditures in terms of either specific or electrical energy consumptions for EP and PEP practices through their corresponding equations.

1. Introduction

In current scenario, diverse industrial setups have been expanded very rapidly. Consequently, numerous industrial effluents particularly textile, oil, and gas, pharmaceutical, paint, fertilizer, petrochemical, metal, and mining industries have made major contribution to wastewater. These industrial effluents contain toxic dyes, nitrates, 2,4-dichlorophenoxyacetic acid herbicide (2,4-D herbicide), toxic heavy metals, pharmaceutical waste, organic waste, total ammonia nitrogen (TAN), micropollutants, and so on [1]. Wastewater comprising these noxious chemicals is lethal to humans as well as aquatic life. In this frame of reference, several techniques have been exploited for wastewater treatment in the literature explicitly, biological techniques, chemical procedures, and physical methods. Since biological techniques were constrained due to toxic contaminants, long processing time, as well as insufficient degradation of pollutants, that is, perfluorinated compounds is devoid of biological disintegration owing to 533 kJ mol⁻¹ energy content required to fracture C▬F bond [2, 3], and physical adsorption is a nondestructive method, which could not oxidize pollutants entirely, solely accountable for shifting pollutants from one phase to another as well as pricey method for a powerful adsorbent, which cannot regenerate [4], and chemical methods, which increase cost as well as generate toxic sludge [5].

Over the last few decades, diverse advanced oxidation processes, namely peroxone, ozonation, and electro-oxidation, have been carried out for wastewater treatment through hydroxyl-free radical (HȮ) production [6, 7]. Peroxone technique is a blend of hydrogen peroxides (H₂O₂) and ozone (O₃), and on this account several free radicals are produced, which oxidize waste organic compound present in water, but requirement of hydrogen peroxide enhances its cost as well as its storage and transport problem [8]. Furthermore, peroxone has shortcomings of low oxygen ozone conversion rate and been suppressed under neutral and acidic environments [9]. Likewise, ozonation is highly resourceful practice for treatment of large bio-recalcitrant-based wastewater; such type of waste usually required high quantity of energy to decay and has the ability to resist microbes; being an oxidizing agent, a large number of intermediates are generated by ozone, which initiated chain reaction and hence degraded waste. On the contrary, ozone reacts with naturally occurring bromide ions in water to form carcinogenic bromates as side products [10] and has less oxidation potential of 2.07 as well as inadequacy of degrading ozone refractory compounds [11]. Although electro-oxidation process has been provoked in treatment of refractory compounds [12] and micropollutants-based wastewater, nevertheless it has a drawback of more energy consumption (3–5 V) during electrolysis [13]. Additionally, electrochemical-based electrocoagulation techniques are where current is passed across wastewater solution containing electrodes, and metallic ions released from dissolution of anode result in coagulation via counter ions in corresponding solution and suspended waste particle made cluster at bottom, it has drawback of electrode encapsulation via oxide layer, and hence, it was not a continuous technique [14].

To overcome these dilemmas of traditional advanced oxidation techniques, researchers have been devising various electrochemical advanced oxidation practices notably, electro-Fenton, photoelectro-Fenton, electro-peroxone, and photoelectro-peroxone for wastewater treatment. Nonetheless, homogeneous electro-Fenton and photoelectro-Fenton techniques catalyzed degradation of persistent organic pollutants only under acidic media, and its alternative heterogeneous techniques could conduct full mineralization of same pollutants under neutral pH [15]. In this circumstance, hybrid electro-peroxone (EP) and photoelectro-peroxone (PEP) have been accredited for wastewater treatment under alkaline, neutral, acidic pH, posed good disintegration, and mineralization rates [16, 17, 18]. As a matter of fact, EAOPs are hybrid approaches, which have been constructed by integrating two or more practices for enhanced ȮH formation to accelerate abatement of pollutants in wastewater [19]. As a matter of fact, ȮH species is the second strongest oxidant with 2.8 V oxidation potential usually prompting nonselective attacks on C▬H bond to oxidize and mineralize pollutants very swiftly as demonstrated through Eq. (6) [20]. Additionally, ȮH could randomly demolish refractory pollutants when existing satisfactorily in water and exploited admirable degradation rate of 10⁸ to 10¹⁰ M⁻¹ s⁻¹ [21].

Similarly, electro-peroxone is basically hybrid of two elementary approaches, which includes ozonation and electrolysis. In this context, all these techniques were taken into an account to mitigate their drawbacks and develop a novel method named electro-peroxone by putting all together [22]. Solely, oxygen was injected into ozone generator, which interleaved its inlet sparged effluent within cathode at electrolytic cell, where oxygen reduction via two electrons at cathode was main culprit of in situ hydrogen peroxide generation founded on Eq. (1). Electrochemically formed H₂O₂ subsequently catalyzed transformation of ozone into ȮH by means of peroxone reaction as discussed viaEq. (2). Henceforth, electrochemical formation of H₂O₂ and peroxone reactions are the two key reactions of hybrid electro-peroxone approach [23]. Other reactions could have taken place via EP process as elaborated with Eqs. (3)–(5) [24, 25]. Major gratification of EP technique is to produce low sludge, comparatively cost-effective, manageable, and continuous production of H₂O₂, alleviate energy intake owing to good rate flow within the system, which promotes mass transfer and convection [26].

$${\mathrm{O}}_2+{\text{2H}}^{+}+{\text{2e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$E1

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_3\to \mathrm{ȮH}+{{\mathrm{Ȯ}}_2}^{-}+{\mathrm{H}}^{+}+{\mathrm{O}}_2$$E2

$${\text{2H}}_2{\mathrm{O}}_2+{\text{2O}}_3\to \mathrm{ȮH}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{HȮ}}_{2^{-}}+{\text{3O}}_2$$E3

$${\text{2O}}_3+{\mathrm{OH}}^{-}\to \mathrm{ȮH}+{\mathrm{Ȯ}}_{2^{-}}+{\text{2O}}_2$$E4

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_3+{\mathrm{e}}^{-}\to \mathrm{ȮH}+{\mathrm{O}}_{2^{-}}+{\mathrm{OH}}^{-}$$E5

$$\mathrm{R}+\dot{\mathrm{O}}\mathrm{H}\to {\mathrm{CO}}_2+{\mathrm{H}}_2{\mathrm{O}}_2$$E6

Even though EP is an expedient approach, its rate of degradation of pollutants usually diminishes with acidity of solution; these acids further make complex with ions, thereby preventing their oxidation. Furthermore, much quantity of O₃ is consumed during EP process [27]. Therefore, existing techniques were modified by incorporating UV light as energy source into electro-peroxone to devise hybrid PEP approach. Photo-electro-peroxone is fundamentally hybrid of three elementary approaches, which include ozonation, electrolysis, and photolysis; these methodologies were coupled to endorse full abatement of pollutants by ȮH formation, which could be proceeded either through Eq. (7) or through (8)via PEP approach [28]. This process is expedience with elegant performance even at acidic media where photo-synthesized electron within conduction band of a semiconductor bismuth oxychloride (BiOCl) interacts with ozone to yield ozone-free radicals (Ȯ₃⁻) based on Eqs. (10) and (11); afterward, Ȯ₃⁻ subsequently will take H⁺ and then finally convert into ȮH as discussed in Eqs. (12) and (13) [2, 29]. Moreover, activation of ozone and H₂O₂ is being abetted by PEP. Likewise, PEP technique has demonstrated 98% efficiency for decontamination of total organic carbon (TOC) from wastewater with specific energy consumption of 0.66 kWh (gTOC_removed)⁻¹, while same amount of pollutants could be refined via UV/O₃ and electro-peroxone with specific energy consumption of 3.56 and 1.07 kWh (gTOC_removed)⁻¹ sequentially, via low reaction rate. That is way photoelectro-peroxone and electro-peroxone are privileged over conventional hybrid advanced oxidation techniques such as UV-integrated electrolysis (UV/electrolysis) and ozone (UV/O₃) for wastewater treatment [30].

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_3+\mathrm{ⱨⱱ}\to 2\dot{\mathrm{O}}\mathrm{H}+{\mathrm{O}}_2$$E7

$${\mathrm{H}}_2{\mathrm{O}}_2+\mathrm{ⱨⱱ}\to 2\dot{\mathrm{O}}\mathrm{H}$$E8

$${\mathrm{O}}_3+\mathrm{ⱨⱱ}\to \mathrm{O}+{\mathrm{O}}_2$$E9

$$\text{BiOCl}+\mathrm{ⱨⱱ}\to \text{BiOCl}-{\mathrm{ⱨ}}^{+}+\text{BiOCl}-{\mathrm{e}}^{-}$$E10

$${\mathrm{O}}_3+\text{BiOCl}-{\mathrm{e}}^{-}\to {\mathrm{Ȯ}}_{3^{-}}$$E11

$${\mathrm{H}}^{+}+{\mathrm{Ȯ}}_{3^{-}}\to {\mathrm{HȮ}}_3$$E12

$${\mathrm{HȮ}}_3\to \mathrm{HȮ}+{\mathrm{O}}_2$$E13

This chapter study aimed to theoretically probe environmentally friendly, cost-effective, comparatively less energy consuming, no secondary toxic side product instigating, and highly versatile novel techniques for wastewater treatment. In this context, recently EAOPs-based hybrid EP and PEP approaches have been discussed for wastewater treatment. Photo-electroperoxone and EP have vividly treated distillery biodigester effluent [31], refractory pharmaceutical [32], hospital [33], ballast water [34], herbicides [18], micropollutants [35], organic pollutant [30], acidic [2], landfill leachates [36], industrial [37], and municipal secondary effluents [26]-based wastewater. Degradation rate of pollutants could be written in terms of rate law to demonstrate chemical kinetic of pollutants during wastewater treatment by electro-peroxone approach. When uniform current is provided to reactor, HȮ formation rate also turns out to be constant and $k\left[H\dot{O}\right]$ in Eq. (14) becomes equal to k_app based on Eq. (15) pseudo-first-order rate constant. Here, $\frac{-d\left[P\right]}{\mathit{dt}}$ is rate of disintegration of pollutants in solution; while [P], [HȮ], k, and k_app denote concentration of pollutants and hydroxyl-free radicals in wastewater, absolute rate constant, and apparent rate constant, respectively [38, 39].

$$\mathrm{\text{Rate}}=\frac{-\mathrm{d}\left[\mathrm{P}\right]}{\mathrm{dt}}=\mathrm{k}\left[\mathrm{P}\right]\left[\mathrm{H}\dot{\mathrm{O}}\right]$$E14

$$\mathrm{\text{Rate}}=\frac{-\mathrm{d}\left[\mathrm{P}\right]}{\mathrm{dt}}={\mathrm{k}}_{\mathrm{app}}\left[\mathrm{P}\right]$$E15

2. Setup devised for electro-peroxone (EP) and photoelectro-peroxone (PEP)

Experimental setup has been devised for degradation of pollutants in wastewater through EP and PEP approaches, which is illustrated in Figure 1. Wastewater treatment was processed in air-proof semi-batch reactor [40]. One-liter wastewater was incorporated inside the reactor, and pairs of electrodes, that is, cathode and anode 1 cm apart, were interleaved in the middle of the reactor. Quartz jacket-enclosed UV lamp was perpendicularly immersed in reactor for UV photolysis during PEP process. Bubble diffuser and magnet stirrer bar were placed to diffuse mixture of ozone and O₂ gases in aqueous solution and to mix content inside the reactor. Electrolytic operations such as EP and PEP were performed under galvanostatic conditions via direct current power supply in the presence of supporting electrolyte [36]. Additionally, constant temperature was maintained via water flow around reactor chamber. Ozone generator was operated to attain ozone and oxygen mixture from inlet oxygen supplied through oxygen cylinder. It was connected to ozone meter to estimate ozone concentration within reactor’s inlet and outlet channels, which subsequently attached with gas flow meter [40]. Desired quantity of ozone was incorporated inside reactor by modifying flow rate of inlet ozone gas.

Figure 1.

Schematic illustration of reactor devised for electro-peroxone and photoelectro-peroxone for wastewater treatment image reproduced from ref. [18]. 1-oxygen cylinder, 2-rotameter, 3-ozone generator, 4-reactor, 5-power supply, 6-stirrer, 7-anode, 8-cathode, 9-fine bubble diffuser, 10-UV-lamp, 11-UV source.

3. Electro-peroxone approaches for diverse wastewater treatment

In the literature, a wide spectrum of wastewater applications such as textile, pharmaceutical, biodigester effluents, refractory compounds, and real wastewater treatments have been successfully conducted by researchers as demonstrated by Table 1.

Category of wastewater	Anode	Cathode	Standard reaction conditions	References
Secondary effluent from coal industry	Dimension-ally stable anode (DSA)	Natural air diffusion electrode	pH: 4, current: 200 mA, electrolyte: 0.3 M Na2SO4, treatment time: 3 h, inlet ozone dose: 6 mg min^−1, flow rate: 100 mL min^−1	[37]
BDE of rice grains	Al	Polytetrafluoroethylene (PTFE)	pH: 6, current: 0.032 mA m^−2, electrolyte: 0.15 M Na2SO4, treatment time: 50 min, inlet ozone dose: 135 mg L^−1, flow rate: 70 L min^−1	[31]
Hospital wastewater	Pt sheet	Activated carbon fiber	pH: 9, current: 400 mA, electrolyte: 0.05 M Na2SO4, inlet ozone dose: 5 g h^−1, flow rate: 1 L min^−1	[33]
Ballast water	Perforated DSA	PTFE and carbon black-modified graphite felt (GF)	pH: 7, current: 50 mA, aeration rate: 50 mL min^−1, temperature: 25°C, E. coli: 10^6–10^7 CFU mL^−1, flow rate: 7 mL min^−1	[34]
Refractory OMPs	Mixed oxides of ruthenium and iridium (RuO2/IrO2) coated Ti plate	Stainless steel plate	Current: 60 mA, treatment time: 15 min, inlet ozone dose: 7 mg L^−1, concentration of model pollutant: 150 μg L^−1, flow rate: 0.15 L min^−1	[35]
LEV	Pt mesh	Carbon fiber composite	pH: 6.8, current: 140 mA, electrolyte: 0.05 mol L^−1 Na2SO4, treatment time: 15 min, inlet ozone dose: 47 mg L^−1, temperature: 25°C, flow rate: 0.08 mg L^−1	[41]
TC and disinfections	Perforated dimension stable anode	Carbon black and PTFE-modified GF	pH: 7, current: 50 mA, electrolyte: 0.05 M Na2SO4, aeration rate: 50 mL min^−1, E coli: 1000 CFU mL^−1, concentration of model pollutant: 700 μg L^−1, flow rate: 35 mL min^−1	[26]
Multiple FQs	Pt plate	Fe-modified carbonized MOF	pH: 4.2, current: 210 mA, treatment time: 10 min, inlet ozone dose: 40.2 mg L^−1, temperature: 25°C, concentration of model pollutant: 20 mg L^−1, flow rate: 50 mL min^−1	[42]
Carbamazepine	Carbon rod	CeOx/GF	pH:5, current: 0.05 mA, electrolyte: 0.05 Na2SO4 mol L^−1, treatment time: 60 min, ozone output: 50 mg h^−1, temperature: 25°C, concentration of model pollutant: 10 mg L^−1, flow rate: 0.5 L min^−1	[43]
Antibiotics and biocides	Pt	Carbon-PTFE	Phosphate buffer: 50 mM, current: 35 mA, electrolyte: Na2SO4 50 mM, inlet ozone dose: 4.5 mg L^−1, temperature: 15°C, concentration of model pollutant: 10 μg L^−1, flow rate: 0.35 L min^−1 for each	[44]
Acid orange 7	Graphite (4 cm^2)	Graphite (4 cm^2)	pH:7.7, current: 0.5 A, anode to cathode ratio: 6: 6, electrolyte: 0.1 M Na2SO4, treatment time: 10 min, temperature: 25°C, concentration of model pollutant: 500 mg L^−1, ozone flow rate: 8.5 L min^−1	[45]
AR14	Pt sheet	Carbon-PTFE (XC-72 carbon powder)	pH:10, current: 0.7 A, electrolyte: 0.1 M Na2SO4, treatment time: 30 min, temperature: 25°C, concentration of model pollutant: 400 mg L^−1, flow rate: 0.25 L min^−1	[46]
AV19	Ti|IrSnSb-oxide plate	3D GDE	pH: 3, current density: 20 mA cm^−2, electrolyte: 0.05 M Na2SO4, inlet ozone dose: 14.5 mg L^−1, temperature: 25°C, concentration of model pollutant: 40 mg TOC L^−1, electrolyte flow rate: 2 L min^−1, pressure exerted at GDE: 3 psi	[47]
CV	Pt rod	Stainless steel wool	pH: 9, current: 0.1 A, electrolyte: 100 mg L^−1 NaCl, inlet ozone dose: 2 mg L^−1, temperature: 22°C, concentration of model pollutant: 50 mg L^−1, peroxide: 15 mmol L^−1	[48]

Table 1.

Diverse EP approaches are exemplified for diverse wastewater treatment under standard reaction conditions.

Textile industries are producing huge volume of wastewater nearly 30–50 cm³ water volume ton⁻¹_dyes. Subsequently, dyestuff effluents (10–20 mg L⁻¹) have been discharged into sewages and rivers [49, 50]. Treatment of textile wastewater is inappropriate through biological treatment meanwhile it culprits toxic secondary by products at the end, alternatively several oxidants have been reported which were being restricted on accountability of structural intricacy of dyestuffs. Consequently, textile wastewater treatment is prompted by EAOPs. [51, 52, 53]. Anionic dye Acid Orange 7(C₆H₁₁N₂NaO₄S) containing wastewater (500 mg L⁻¹) has been fully decontaminated with 90% and 99% exclusion of TOC and chemical oxygen demand (COD), respectively, within 90 minutes through EP approach carried out in cylindrical reactor [54]. Likewise, Acid Orange 7 was pulverized in a cylindrical baffled reactor to boost exchange among reactants and well-organized electrode arrangement by EP approach. Acid Orange 7 (500 mg L⁻¹) mineralization cleared out 92% TOC and 99% COD were declined within 90 minutes at pH 7.7 with large electrode surface area ratio (6:6) by which degradation was considerably enhanced [45]. Similarly, Acid red 14 (AR 14) wastewater (400 mg L⁻¹) has been disintegrated in Ep-based Box-Behnken experimental setup. Full disintegration of AR 14 was accomplished, and 69% COD exclusion was achieved within 60 minutes at 10 pH [46]. Another attempt has been made on decomposition of crystal violet (CV) with K_app of 2.69 × 10⁻² and 2.87 × 10² min⁻¹, for 100 and 200 mg L⁻¹ CV in wastewater, respectively. About 98% CV was eliminated at pH range of 7–9 within 5 minutes through combination of electrolysis/peroxone/H₂O₂, and corresponding treated wastewater was manifested no toxicity for microbes. Electro-peroxone approaches were provoked decolorization at alkaline media [48]. A novel approach has been made to smash Acid Violet 19 (AV19) in a lab-scale filter-press-based plant employing 3-D gas diffusion electrode as a cathode to attain better oxygen reduction reaction via EP. This led to 60% mineralization and 100% decolorization, at 3 psi pressure that was employed to gas diffusion electrode with acidic pH medium (3). It was demonstrated that AV19 disintegration was consequence of in situ generated peroxide coupled with ozonation as well as an anodic oxidation [47].

Recurring detection of personal care and pharmaceutical compounds in water has increased health heedfulness and environmental considerations [55, 56]. Wastewater treatment plant could not completely eliminate antibiotics through traditional activated sludge and sedimentation techniques; as a result, these have been monitored in secondary wastewater effluents in certain quantity [56, 57, 58]. In contrast to traditional ozonation practices, ozone recalcitrant micropollutants notably chloramphenicol, ibuprofen, and clofibric acid have been effectively pulverized and accelerated degradation kinetics through triggering HȮ production by means of EP approach [59, 60]. Numerous advanced oxidation techniques have been launched to smash bio-recalcitrant paracetamol (PCT). EP approach has exhibited good efficiency in full disintegration of PCT with rate constant of 0.1662 min⁻¹ [32]. Levetiracetam (LEV) removal was a bit challenging owing to polar structures, which was not susceptible to ozone degradation [61]. Extremely water-soluble antiepileptic drug LEV has been manifested pseudo-first-order degradation kinetics via EP approach by means of promoted synergy between O₃ and H₂O₂ for actively electrochemical generation of HȮ and led to withdrawn of 53.4% LEV at 15 minutes from wastewater [41]. Numerous antibiotics notably ciprofloxacin, norfloxacin, ofloxacin, and trimethoprim have been degraded with EP and tracked pseudo-first-order kinetics. Outcomes revealed that EP technique effectively eradicates antibiotics and ozone inert biocides within short time and lessen energy consumption than that of ozonation process [44]. Similarly, tetracycline (TC) and microbes were smashed into organic acids and after mineralization totally excluded from wastewater by EP approach [44]. Highly stable polyacrylonitrile-based carbon fiber cathode was designed for mineralization of phenol, where oxidation promoted transformation of pyridinic-N of polyacrylamide into pyridonic-N through EP, which endorsed cathode for oxygen reduction reaction. Major gratification of this procedure was 30-fold recyclability of fabricated cathode as well as cathodic potentials declined energy expenditure from 91.5% (simple cathode) to 48.2% (current cathode) for H₂O₂ formation [62]. Electro-peroxone approach was conducted by fabricating iron-modified carbonized metal organic framework (MOF)-based cathode for treatment of biostatic drugs such as fluoroquinolones (FQs); these were selectively 99% decomposed by O₃ and the rest of its ozone-reluctant transformed intermediates and side products were removed by HȮ. Moreover, HȮ was also liable to overall TOC removal, and its formation was promoted by surface functionalities of MOF-based cathode through synergic effect of adsorption and activation [42]. Aforementioned hybrid EP approaches have been successfully applied for the treatment of synthetic wastewater.

Real wastewater in contrast to synthetic wastewater is more complex, having abundant organic micropollutants with varieties of molecular structures accompanying physicochemical properties [63, 64]. Biochemical oxygen demands (BODs)/COD ratio has been commenced for probation of biodegradability prospects and wastewater encompassing 0.4 or its onward ratio has manifested good biodegradability as well as decline in bio-toxicity [65]. Electro-peroxone approach was carried out for processing of reverse osmosis concentrate obtained from industrial coal wastewater. As a result, 92% decolorization, 89% UV₂₅₄, and 71.2% TOC have been eradicated within 6 hours [37]. After treatment, 91.3% color reduction and 99.9% COD elimination were detected in distillery biodigester effluents (BDE) through EP approach, and low cost of 1 m³ BDE/2$ and less sludge formation are major gratification of this approach [31]. Chloramphenicol and clofibric acid such as ozone obstinate micropollutants in surface water have been oxidized by EP system and subsequently resulting in hypochlorous and hypobromous acids, which are the main culprit of engendering chlorinated and brominated derived byproducts were efficiently quenched by electrochemically produced H₂O₂ in surface water [35]. Several antibiotics conspicuously ciprofloxacin, norfloxacin, ofloxacin, and trimethoprim have been eliminated from secondary wastewater effluents through EP [44]. Electro-peroxone practice was also employed to process municipal secondary effluents with negligible disinfection side products as well as 65% COD and 44% BOD were declined [26]. Diverse 89 pharmaceutical compounds were examined in terms of organic micropollutants (OMPs) exclusively existing in a real wastewater to evaluate their ozone reactivities and physiochemical properties by means of quantitative structure activity relationship (QSAR) for the sake of kinetic assessment. Pharmaceutical compounds having partial charge moieties, and branched, electrophiles, and lowest unoccupied molecular orbital energies were categorized as ozone-resistant compounds with ozone rate constant (k_O3) < 10² M⁻¹ s⁻¹ and were degraded by sole EP. Conversely, ozone reactive pharmaceuticals accompanying with nucleophilic species, highly occupied molecular orbital energies, and conformation contingent charge descriptors were deemed to be ozone reactive with rate constant greater than 10² M⁻¹ s⁻¹ would be rapidly eradicated by EP and ozonation [66]. Additionally, ultrasound coupled EP system and virgin EP has been applied for textile industry effluent at 5.8 pH. 93% and 99% decolorization have been accomplished through virgin EP and the integrated ultrasound EP process within 60 minutes after treatment [67]. Similarly, real pesticide wastewater has been treated via 3D/EP system with elimination of 97.5% TOC and 95.1% COD up to 500 and 300 minutes sequentially; consequently, long reaction time contributes to more cost although 3-D/EP is more cost effective to that of 2-D EP. Moreover, 30 minutes later BODs/COD ratio has been incremented from 0.049 to 0.571 [68].

In context of acidic wastewater treatment, few attempts have been made to mend EP process. Tannic acid has been oxidatively smashed by EP approach in two phases firstly tannic acid pulverized via O₃ and HȮ, which were accountable for carboxylic acid like intermediates. Consequently, more intermediate formation lowered pH of wastewater, and thereby HȮ was also declined. Somehow it has been overcome via increasing inlet ozone as well as adjusting current and unlike ozonation more than 10% efficacy has been achieved via EP [69]. Electrode-separated compartmental-based EP approach was carried out to eliminate para-aminobenzoic acid (PABA) from wastewater solution. As a result, 63.6%–89.5% PABA has been abolished by contribution of cathodic and anodic side reactions, respectively, at 10 minutes as well as incremented pseudo-first-order reaction kinetics and HȮ formation [70].

3.1 Miscellaneous electrode texture-based electro-peroxone approaches for wastewater treatment

Carbon nanotubes (CNTs) have exhibited brilliant adsorption to pollutants, and this tendency along with adsorption kinetic was further enhanced in terms of electro-sorption by employing them as electrodes [71]. Furthermore, CNTs have been demonstrated good electrochemical oxidation of pollutants, good chemical stability, electrical conductivity, and noteworthy mechanical strength during electrolysis [72] and photolysis [73]. Advanced oxidation approach has been integrated with adsorption to construct hybrid system for actively pulverization of pollutant in wastewater [74, 75]. Pharmaceutical compounds, particularly diclofenac sodium (DS), were completely fragmented by carbon nanotubes-polytetrafluoroethylene (CNTs-PTFE) electrode over five consecutives cycles exploiting pseudo-second-order kinetics. Where negatively charge diclofenac sodium was exhibited electro-sorption to CNTs-PTFE anode afterward, adsorption phenomena switched this anode into cathode and corresponding adsorbed pollutants were subsequently disintegrated by EP approach within 10 minutes and 99% TOC were eliminated after 1 hour [76]. Likewise, copper ferrite-modified carbon nanotubes (CuFe₂O₄/CNTs) were used as catalysts having brilliant recyclability to decomposed fluconazole (FLC) wastewater through EP. Catalyst has adsorbed FLC on its sphere and enhanced FLC mass transfer to electrode surface and thereby eliminated 89% FLC and integrated adsorption-EP technique has contributed 10% efficiency to virgin EP approach [77].

Similarly, carbon nitride-multiwall carbon nanotubes-based nanocomposite (n-C₃N₃/MWCNT) catalyst has actively smashed sodium oxalate in wastewater by endorsing adsorption of pollutant and accelerating electron transfer, which trigger O₃ and O₂ electro reduction [78]; consequently, H₂O₂ and Ȯ₃⁻ were generated, which has further enhanced HȮ formation [79]. On account of large surface area, activated carbons are good in elimination of micropollutants (MPs); on the contrary, MPs saturated activated carbons having high affinity for adsorbates pose a major challenge in regeneration of electrode, which was overwhelmed by oxidation of MPs through ozonation process [80] but some sorts of MPs were inert toward ozonation reaction [64]. In this frame, EP coupled with ozonation to exclude diverse MPs, namely trimethoprim, ciprofloxacin, perfluorooctanoic acid, carbamazepine, diclofenac, and benzotriazole from wastewater and efficiently pulverized MPs from ozone. Afterward, ozone-resistant MPs were disintegrated via EP with simultaneous regeneration of powdered activated carbons (PAC). In contrast to virgin PAC, all MPs have been exploited more than 100% efficacy for PAC regeneration except diclofenac and perfluorooctanoic acid (PFOA) [81].

Electro-peroxone approaches have been well organized at alkaline and neutral pH; on the contrary, its progress was constrained at acidic pH, which bounds rate constant of H₂O₂ as of 9.6 × 10⁶ to 0.01 M⁻¹ s⁻¹ for 11 to 3 pH, respectively. Hence, reaction between ozone and deprotonated peroxide has no more yielded reactive oxygen species [22]. Manganese carbon nitride-carbon nanotubes (C₃N₄-Mn/CNT) composite catalyst overcomes drawbacks of disintegrating pollutants in strongly acidic solution via EP reaction. Moreover, C₃N₄-Mn/CNT heterogeneous catalyst has been accelerated peroxone reaction between H₂O₂ and O₃, and decomposed oxalic acid within 30 minutes at pH 3 for up to 5 cycles [82]. Additionally, C₃N₄-Mn/CNT-integrated EP system has been evaluated for disintegration efficiency of oxalic acids at a wide range of pH, Outcomes reveal that 57.1- and 2.6-fold increments have been achieved in integrated system, at 3 and 9 pH as compared with virgin EP [82].

Traditional EP approaches were mostly carried out by commencing 2-D electrode system, which have been demonstrated low mass transfer; therefore, to boost electrode performances for additional optimization of conducted treatment were suggested for forthcoming generation [78, 83]. Reticulated enamel carbon, graphite felt, polytetrafluoroethylene, and carbon felt-based cathodic materials were manifested O₂ reduction for H₂O₂ formation [84]. Unlike conventional 2D-electrode in EP approach, 3D-electrode system could considerably promote the electrochemical efficiency of reactor owing to large surface area, which boosted H₂O₂ formation [85]. TiO₂-loaded granular-activated carbon (TiO₂-GAC) as a 3D electrode in EP system was applied for decomposition of diuron, which is a phenyl urea herbicide wastewater, hybrid 3-D/EP system was demonstrated two times more pseudo-first-order disintegration rate (effectiveness) than those of sole EP system. Diurons were adsorbed by TiO₂-GAC and later polarized to synthesize microelectrodes, which yields ȮH. Moreover, TiO₂-GAC has considerably enhanced H₂O₂ formation in a corresponding solution [68]. Being a 3-D activated carbon system, carbon felt (CF) has been shown elegant electrolytic proficiency, good mechanical stability, and cost effective [86]. N-doped-reduced graphene oxides (N-rGOs) supported carbon as well-designed cathode was demonstrated to improve oxygen reduction feedback for H₂O₂ generation, better conductivity, boosted electrocatalysis, and electron transfer rate [87, 88]. Diuron was completely smashed at 9 pH within 15 minutes through EP approach using versatile N-rGO/CF-based cathode electrode. Furthermore, N-rGO/CF exploited good efficiency in H₂O₂ formation and lessen energy expenditures for 10 cycles continuously to that of sole CF cathode. This system has led to processed real pesticide wastewater having COD of 3680 mg L⁻¹ after processing till 360 minutes, and COD was declined to 47.7 mg L⁻¹. Moreover, BOD/COD ratio of 0.4 and 0.04 has been obtained for processed and unprocessed real pesticide wastewater, respectively [88]. Another attempt was made in which a filter-press flow cell integrated with three-dimensional air diffusion electrode-based lab scale plant was devised to disintegrate levofloxacin and 63% mineralization accomplished at 3 pH [89]. Likewise, GF was modified with cerium oxides (CeOx) to well-designed cathode, and H₂O₂ exhibited chemisorption with CeOx; consequently, it will prompt reaction with O₃ as compared with bulk H₂O₂. Consequently, CeOx/GF-EP system has been manifested 69.4% TOC exclusion in disintegration of carbamazepine within 60 minutes at pH range of 5–9 with upright fivefold recyclability. In contrast to traditional EP, this strategy is perquisite for degradation of refractory organic pollutants under acidic media by proficiently activating ozone, upgraded surface hydrophilicity, and lessen energy expenditure for electro-generation of H₂O₂ [43].

A novel hybrid approach comprising three electrodes in EP system for oxalate-containing wastewater has been developed. After elaboration of reaction mechanism, it was suggested that all reactions in combination subsidizes HȮ formation in EP. In contrast to two electrode systems, three electrodes system could be comparatively privilege in providing precise control and purifying salt-rich wastewater [79].

3.2 Complementary hybrid electro-peroxone approaches for wastewater treatment

To proficiently mineralize and eliminate a wide range of biodegradable contaminants along with refractory pollutants from wastewater by low electrical energy requirement in cost effective and easy ways, some conventional approaches conspicuously biological treatment, ultrasound, electrocoagulation, and low-pressure filtration were coupled with electro-peroxone to devise novel complementary hybrid electro-peroxone system [67, 90, 91]. In this circumstance, synergy overcome constrained individual approaches that have low efficiency independently and accelerated attenuation of wastewater in terms of complementary hybrid electro-peroxone system.

Bio-electroperoxone (Bio-EP) approach has been devised for a two-way treatment of pharmaceutical wastewater, where microbes biodegrade some compounds at electrically bound biofilm reactor (EBBR), and the rest of all compounds that did not undergo biological oxidation was pulverized via EP approach. Integrated Bio-EP has been eliminated 89% TOC, 84% suspended solids, 99.99% deactivated microbes, and 92.20% decolorized wastewater [39]. Another attempt was made to pulverize recalcitrant contaminants particularly methylene blue. In this system, self-sustained energy achieved from microbial fuel cell-based cathode by Bio-EP process was supplied and 83% methylene blue has been eliminated within 30 minutes by exploiting pseudo-first-order kinetics with 2.05 h⁻¹ rate constant during pulverization [90].

Hydroxyl-free radicals could be synthesized by fracturing bubbles cavitation in aqueous medium through ultrasound (US) based on Eq. (16) [92]. Moreover, US also splits up ozone and peroxide based on Eqs. (17) and (18) [93, 94]. Integrated US/EP approach was applied to fragmentize acid orange 7 at pH 7, which has manifested 88% mineralization, 99% decolorization, and 85% COD elimination with pseudo-first-order kinetics [67].

$${\mathrm{H}}_2\mathrm{O}+⦆⦆⦆\to \mathrm{HȮ}+\stackrel{·}{\mathrm{H}}$$E16

$${\mathrm{H}}_2{\mathrm{O}}_2+⦆⦆⦆\to \mathrm{HȮ}+\mathrm{HȮ}$$E17

$${\mathrm{O}}_3+⦆⦆⦆\to {\mathrm{O}}_{\text{2}}+\mathrm{O}$$E18

Shale gas fracturing flowback water (SGFFW) was processed with electroperoxone-integrated electrocoagulation (EC/EP) or ECP approach and led to 82.5% COD exclusion up to 90 minutes, with 29.9% average current effectiveness. In ECP technique, coagulant hydroxyl-aluminum at anode eliminates colloids and suspended items [95] as well as catalyzed HȮ formation by reaction with O₃ to breakdown pollutants via EP at cathode [96]. Likewise, peroxi-coagulation was integrated with EP; thereby, high efficiency with less reaction time was obtained than that of virgin EP with full decolorization, and 92.2% UV₂₅₄ and 72.2% TOC were eliminated [37].

Low-pressure filtration was coupled with EP approach to design hybrid electro-peroxone filtration (EPF) system, which was continuously eliminated 64.87% ibuprofen (IBU) at less filtration pressure (0.8 kPa) within 8 seconds in its very low concentration of 1 mg L⁻¹. IBU elimination efficacy was comparatively three times than that of individual efficiencies achieved from electrochemical filtration and ozone filtration. In contrast to sole EP, EPF was promoted mass transfer of HȮ and O₃ owing to membrane permeation drift [91].

Downflow bubble column electrochemical reactor (DBCER) has led to incremented mass transfer and contact area owing to energetically liquid inflow, and small bubble formation takes place to cause commotion as well as did not let out electrochemically synthesized in situ oxygen viaEq. (19) rather it was dispersed within cell [97, 98]. As a matter of fact, under large current density and low pH, HȮ production would favor based on Eq. (20), which subsequently subsidizes oxidation at electrode surface viaEq. (21) [99]. DBCER with boron-doped diamond (BDD) electrode system has also been prerequisites for in situ formation of O₂, O₃, and H₂O₂ to accomplish EP process with 75% TOC exclusion at pH 3 phenol smashed by ozone and 65% TOC with drawn at pH 7 phenol pulverized by H₂O₂ within 6 hours [98].

$${\text{2H}}_2\mathrm{O}\to {\mathrm{O}}_2+{\text{4H}}^{+}+{\text{4e}}^{-}$$E19

$$\mathrm{BDD}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{BDD}\left(\mathrm{HȮ}\right)+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$E20

$$\mathrm{BDD}\left(\mathrm{HȮ}\right)+\mathrm{R}\to \mathrm{BDD}+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$E21

4. Photoelectro-peroxone practices for wastewater treatment and their comparison with EP

Simple ozonation, photolysis, and electrolysis mechanisms were integrated to fashion novel hybrid PEP approach for wastewater treatment to overcome shortcoming of low mineralization rate during EP at acidic pH and dwindle corresponding electrical energy consumption. Henceforth, PEP and EP approaches have been contributed to synergistic effect, which has been quantitatively determined through enhancement factor calculated by Eqs. (22) and (23) [55].

$$\text{Enhancement}=\frac{\left({\mathrm{k}}_{\mathrm{PEP}}\right)}{\left({\mathrm{k}}_{\mathrm{O}}\right)+\left({\mathrm{k}}_{\mathrm{UV}}\right)+\left({\mathrm{k}}_{\mathrm{E}}\right)}$$E22

$$\text{Enhancement}=\frac{\left({\mathrm{k}}_{\mathrm{EP}}\right)}{\left({\mathrm{k}}_{\mathrm{O}}\right)+\left({\mathrm{k}}_{\mathrm{E}}\right)}$$E23

Where k_EP, k_PEP, k_O, k_UV, and k_E denote rate constants during pollutant disintegration for EP, PEP, ozonation, photolysis, and electrolysis, respectively. Enhancement factors along with degradation rate constants have been comparatively incremented during PEP approaches than those of EP for same wastewater treatment (e.g., Table 2).

Category of wastewater	Conducted approach	Anode	Cathode	Apparent degradation rate constant Kapp	Enhancement factor	Ref.
Nitrobenzene	PEP	12 cm^2 platinum sheet	40 cm^2 carbon-PTFE	93.0^a	5.8	[30]
—	EP	—	—	86.7^a	5.5	[30]
Chlorobenzene	PEP	—	—	212^a	8.4	[30]
—	EP	—	—	164^a	6.7	[30]
Benzaldehyde	PEP	—	—	112^a	4.7	[30]
—	EP	—	—	98.0^a	4.1	[30]
1,4-Dioxane	PEP	Platinum/titanium (Pt/Ti)	Carbon-PTFE	2.237^b	33.4	[100]
—	EP	—	—	1.749^b	28.2	[100]
Nitrophenol	PEP	50 cm^2 carbon belt	50 cm^2 BDD	0.145^c	13.2	[40]
—	EP	—	—	0.074^c	6.7	[40]
2,4-D herbicide	PEP	Titanium	Graphene	0.0009^d	ND	[18]
MY	ZVI/PEP	Platinum	Graphite felt	0.1926^e	1.54	[101]
Reactive Yellow F3R	PEP	Ti/TiO3	Graphite	1.176^e	1.38	[102]
—	EP	—	—	0.834^e	1.28	[102]
PFOA	PEP	rGO/BiOClfilm-based photoanode	Graphite	17.5^f	4.73	[2]
CBZ	EP	Carbon rod	CeOx/GF	2 × 10^−2g	3.12	[43]
Organic contaminant	Bio-EP	Nematic liquid crystal display electrode	Pt coated titanium	0.0177^g	ND	[39]
Methylene blue	Bio-EP	Activated carbon granules	XC-72 carbon black	0.237^g	ND	[90]

Table 2.

Comparison between PEP and EP techniques has been demonstrated on basis of enhancement factors along with degradation rate constants for wastewater treatment.

rGO/BiOCl = reduced graphene oxide/bismuth oxy-chloride, ND = not determined, b = K_app × 10³ s⁻¹, a = K_app × 10² min⁻¹, c = K_app^NP min⁻¹, d = (min)⁻¹ reaction rate constant (K_obs), e = h⁻¹, f = K_app × 10⁴ min⁻¹, g = min⁻¹.

Organic pollutants containing plentiful wastewater have been magnificently treated by PEP. In this framework, derivatives of benzene particularly nitrobenzene, chlorobenzene, and benzaldehyde containing wastewater were processed through electro-peroxone and photoelectro-peroxone approaches. Although both approaches have been drawn out 98% TOC, PEP exhibited good degradation kinetics, and consumed less energy than that of EP and other advanced oxidation processes, which have been exploited slow degradation kinetics and used up high energy [30]. Similarly, 1,4-dioxane, a major contributor to refractory organic pollutant, is exclusively found in industrial wastewater and landfill leachates and was disintegrated with 33 times proficient pseudo-first-order rate constant via PEP as compared with UV photolysis, ozone, and electrolysis. Photoelectro-peroxone approach has discharged 98% TOC after 1,4-dioxane mineralization in wastewater solution. Solely 37% TOC was drawn out via EP owing to reliant on pH, which gradually lower consequently interfere with ȮH^⧿ due to resulting intermediates of 1,4-dioxane decomposition notably, carboxylic acid [100]. Furthermore, 4-nitrophenol comprising wastewater has been processed by PEP technique along inserting BDD electrode as an anode that accelerates numerous free radical creations on its sphere and other conventional electrochemical-based advanced oxidation processes. As a result, all mineralized TOC was excluded from wastewater solution in 45 minutes via PEP [40]. Likewise, TOC elimination was taken into an account for mineralization degree of 4-chlorophenol, benzotriazole, metanil yellow (MY), TC, and carmoisine with 85, 84.2, 65.6, 62.4, and 60.2% TOC removal, respectively. It could be considered that pollutants or compounds with high carbon content were deemed to have low TOC removal owing to compacted structure [101]. Additionally, PFOA was hardly smashed by advanced oxidation techniques and HȮ is also somewhat inactive for PFOA [103, 104]. Therefore, PFOA has been 56.1% decomposed by PEP within 3 hours manifesting pseudo-first order kinetics [2].

Similarly, PEP approaches have been eliminated herbicides at both alkaline and neutral pH. In this context, 2,4-D herbicide was entirely degraded within 25 minutes and its degradation kinetics has exploited first-order reaction rate by PEP approach having rate constant of about 2.5-folds higher than the rate constant of EP. Furthermore, 58.9% TOC has been wiped out during 2,4 D mineralization at pH of 7 from wastewater solution. On contrary to stainless steel and graphite felt, cathodic-activated carbon has promoted reaction rate by engendering H₂O₂ [28]. Likewise, another attempt has been made to boost 2,4-D herbicide disintegration through UV-assisted PEP. Complete fragmentation of 2,4-D herbicide in solution (58 mg L⁻¹) was obtained at 5.6 pH in 112 minutes along with its 91% elimination; moreover, 76% TOC has been withdrawn during 2,4-D herbicide mineralization after 2 hours. Low pH and 85% COD along with trapping assessment revealed that both species ȮH and Ȯ₂⁻ have contributed to wastewater treatment [18]; hence, approximately at slightly acidic pH reaction could be proceeded between H⁺ ions and Ȯ₃⁻ to produce reaction active species (HȮ⁻) viaEq. (24) [105].

$${\text{2Ȯ}}_{3^{-}}+{\mathrm{H}}^{+}\to \mathrm{H}{\dot{\mathrm{O}}}_2\to {\mathrm{HȮ}}^{-}+{\mathrm{O}}_2$$E24

$${\mathrm{O}}_2+{\text{2H}}^{+}+{\text{2e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$E25

Furthermore, PEP-based some attempts have been made in textile wastewater treatment. In this circumstance, MY dye containing wastewater has been processed by incorporating zero-valent iron (ZVI) as a nano-catalyst in the solution, which was further followed by PEP process and accelerated wastewater treatment. This hybrid PEP/ZVI approach has successfully decolorized wastewater solution (50 mg L⁻¹) at acidic pH 3 within 25 minutes, as acidic media promotes H₂O₂ electrolytically based on Eq. (25) [101]. Moreover, reactive yellow F3R (RY F3R) wastewater was pulverized by PEP manifesting first-order kinetics and 97.66% decolorization and 84.64% TOC has been excluded with 14 and 1.4 times more degradation rate constant as compared with photolysis and EP sequentially.

Moreover, real textile wastewater also has been treated by PEP effectively by withdrawing TOC [102] and decolorization rate could be promoted by incorporating transition metals that in turn produce Fenton reagent. Fe⁺² triggers ozone activation and hydroxyl-free radical formation as discussed in Eqs. (26) and (27) [28, 106].

$${\mathrm{Fe}}^{+2}+{\mathrm{O}}_3\to {\mathrm{FeO}}^{+2}+{\mathrm{O}}_2$$E26

$${\mathrm{FeO}}^{+2}+{\mathrm{H}}_2\mathrm{O}\to \text{2HȮ}+{\mathrm{Fe}}^{+3}+{\mathrm{OH}}^{-}$$E27

$${\mathrm{COD}}_{\text{exclusion}\%}=\frac{{\mathrm{C}}_{\mathrm{f}}–{\mathrm{C}}_0}{{\mathrm{C}}_0}\times 100$$E28

In addition, COD parameter was applied to analyze pollutant concentration in landfill leachate and lower the pollutant concentration, and lesser oxidant would be acquired; hence, lower COD exclusion would be attained. Percentage of COD exclusion could be calculated by Eq. (28) where C₀ and C_f denote quantity of COD that has been consumed by leachate before and after its treatment [107]. In this frame, 83% COD exclusion has been achieved at 5.6 pH through PEP [36].

5. Energy expenditures for electro-peroxone, complementary hybrid EP, and photoelectro-peroxone approaches

Some amount of energy has required to perform electrochemical oxidation of wastewater. In this framework, specific energy is mandatory to disintegrate pollutants in innumerable wastewater treatment. Therefore, Eqs. (29) and (30) have been proposed to estimate energy supplied during EP, PEP, and complementary hybrid EP approaches [94].

$${\mathrm{SEC}}_{\mathrm{EP}}=\frac{\mathrm{U}\times \mathrm{I}\times \mathrm{t}+\mathrm{r}\times {\mathrm{CO}}_3}{\left({\mathrm{TOC}}_0–{\mathrm{TOC}}_{\mathrm{t}}\right)\times \mathrm{V}}$$E29

$${\mathrm{SEC}}_{\mathrm{PEP}}=\frac{{\mathrm{P}}_{\mathrm{UV}}\times \mathrm{t}+\mathrm{U}\times \mathrm{I}\times \mathrm{t}+\mathrm{r}\times {\mathrm{CO}}_3}{\left({\mathrm{TOC}}_0–{\mathrm{TOC}}_{\mathrm{t}}\right)\times \mathrm{V}}$$E30

$${\mathrm{SER}}_{\mathrm{PEP}}=\frac{\mathrm{UIT}\times {\mathrm{U}}_{\mathrm{\text{photolysis}}}\times {\mathrm{rCO}}_3}{\left({\mathrm{COD}}_0–{\mathrm{COD}}_{\mathrm{t}}\right)\times \mathrm{V}}$$E31

$${\mathrm{SEC}}_{\mathrm{EP}}=\frac{\mathrm{U}\times \mathrm{I}\times \mathrm{t}+\mathrm{r}\times {\mathrm{CO}}_{\mathrm{3}}}{\left(\left[\mathrm{PCT}\right]{}_0-\left[\mathrm{PCT}\right]{}_{\mathrm{t}}\right)\times \mathrm{V}}$$E32

$$\text{Energy consumption}=\frac{\mathrm{U}\times \mathrm{I}\times \mathrm{t}}{\mathrm{V}}$$E33

$$\mathrm{EC}=\frac{\mathrm{U}\times \mathrm{I}\times \mathrm{t}}{\mathrm{V}\times ∆\left({\mathrm{TOC}}_{\exp }\right)}$$E34

$$\mathrm{EC}=\frac{\left(\mathrm{V}\times \mathrm{I}+\mathrm{\text{ozone generator energy}}\right)\times \mathrm{t}\times 1000}{{\mathrm{C}}_{\text{dye removal}}\times \mathrm{\text{cell volume}}}$$E35

$${\mathrm{SEC}}_{\mathrm{EP}}=\frac{\mathrm{U}\times \mathrm{I}\times \mathrm{t}+\mathrm{C}\times \mathrm{Q}\times \mathrm{t}\times \mathrm{R}}{\left({\mathrm{C}}_0–{\mathrm{C}}_{\mathrm{t}}\right)\mathrm{V}}$$E36

$$\mathrm{EEC}=\frac{\mathrm{U}\times \mathrm{I}\times \mathrm{t}+{\mathrm{Q}}_{\text{gas}}\times \mathrm{a}\times {\mathrm{CO}}_3}{\mathrm{V}}\times 10{-}^3$$E37

$$\mathrm{EEC}=\frac{1000\times \mathrm{U}\times \mathrm{I}\times \mathrm{t}+{\mathrm{Q}}_{\text{gas}}\times \mathrm{a}\times {\mathrm{CO}}_3}{\left({\mathrm{TOC}}_0-{\mathrm{TOC}}_{\mathrm{t}}\right)\mathrm{V}}$$E38

Where SEC_EP and SEC_PEP are the specific energy consumptions for EP and PEP sequentially measured in kWh (gTOC_removed)⁻¹, and SER_PEP is the specific energy consumption or electrical energy requirement measured in kWh (gCOD_removed)⁻¹. U is an average cell voltage (V), I denotes current (A), t represents reaction time (h), r symbolizes energy requirements for ozone formation (kWh (kgO₃)⁻¹), C_O3 designates ozone quantity consumed during EP and PEP approaches, TOC₀ and TOC_t indicate total organic carbon in the solution at 0 time and any time t (mg L⁻¹), [PCT]₀ and [PCT]_t symbolize concentrations of unprocessed and processed paracetamol (PCT), respectively, V shows solution volume (L), P_UV denotes power of UV lamp (W) [108], Δ(TOC_exp) represents change in the concentration of TOC [89], (C₀ − C_t) is the concentration of LEV in untreated and treated wastewater sequentially, R denotes energy expenditure for ozone formation, C symbolizes concentration of inlet ozone, Q indicates flow rate of gaseous ozone [41], a represents energy attained for ozone formation, Q_gas designates feed gas volume, and C_O3 reveals feed gas comprising ozone concentration [26].

Total organic carbon was effectively discarded from wastewater during mineralization of benzene derivatives through SEC_EP and SEC_PEP, of 1.07 and 0.66 kWh (gTOC_removed)⁻¹ respectively [30]. Similarly, SEC_EP and SEC_PEP of 0.22 and 0.30 kWh (gTOC_removed)⁻¹ have been achieved by removing TOC from 1,4-dioxane containing wastewater sequentially [100]. Nitrophenol decomposition has been expended SEC_EP and SEC_PEP, of 7.5 and 4.1 kWh (gTOC_removed)⁻¹, respectively, for entire elimination of TOC. In addition to PEP, BDD electrode dramatically enhanced reaction kinetic; therefore, deducted energy requirement [40] SER_PEP of 1.5 kWh (gCOD_removed)⁻¹ has been consumed in landfill leachate treatment using Eq. (31) [36]. Entire PCT breakdown via EP has expended SEC_EP of 0.1164 kWh (gPCT_removed)⁻¹ based on Eq. (32) [32]. 1.676 and 22.86 kWh m⁻³ energy have been expended during hybrid bio-EP and solely EP, respectively, calculated through Eq. (33) [39]. Electrolytic energy consumption (EC) of 0.27 kWh (gTOC_removed)⁻¹ was obtained viaEq. (34) during levofloxacin mineralization employing 3-D perforated electrode by EP [89]. 37.7% and 41.1% COD have been excluded via EP and 3-D TiO₂/GAC system within 90 minutes by exhausting electrical energy of 0.1 and 0.08 kWh (gCOD_removed)⁻¹, respectively [68]. Additionally, 39.2% and 43.6% COD have been eliminated from real pesticide wastewater with energy expenditures of 0.088 and 0.079 kWh (gCOD_removed)⁻¹via CF-EP and N-rGOs/CF-EP system sequentially [88]. Eq. (35) was taken into an account; afterward, 53 kWh kg⁻¹ (kg denote weight of removed dye) energy has been estimated during Acid Orange 7 disintegration with 99% COD exclusion during EP in cylindrical reactor [54]. To diminish SEC, another attempt was made in which Acid Orange 7 was entirely pulverized (99% COD_removal) through EP with EC of 8 kWh kg⁻¹ founded on Eq. (35) [45]. AV19 dye has been 60% mineralized with energy expenditure of 0.085 kWh (gTOC_removed)⁻¹ by laboratory-scale EP plant equipped with 3-D electrode based on Eq. (34) [47]. Similarly, LEV drug was smashed through EP approach with SEC_EP of 0.326 kWh (gLEV_removed)⁻¹ on the basis of Eq. (36) [41]. Electrical energy consumption (EEC) of 0.47 kWh m⁻³ has been calculated through Eq. (37) in decontamination of municipal wastewater and TC disintegration via EP approach [26]. In the same way, IBU elimination through EPF system used up 0.16 kWh m⁻³ energy [91]. Sequential EEC of 1136.8 and 828.4 kWh (kg_TOC)⁻¹ have been achieved for virgin EP and hybrid peroxi-coagulation/EP system founded on Eq. (38) [37]. Likewise, 99.9% COD exclusion has been achieved during treatment of BDE of rice grains through EP approach via EEC of 3.8 kWh m⁻³ [31].

Overall, PEP dwindled almost 45% specific energy consumption than that of EP approaches for a same category of wastewater under unchanged reaction conditions; nevertheless, some exceptions may be commenced conspicuously in degradation of 1,4-dioxone. Complementary hybrid EP approaches have foremost expedience of comparatively reducing energy expenditures to that of a virgin EP as well as enhanced abatement of pollutants in wastewater treatment. In this milieu, bio-electroperoxone system offered much indulgence by in taking very low energy. In contrast to conventional 2-D electrodes, latest 3-D electrodes-based EP approaches have been manifested less energy consumption.

6. Conclusions

High-operating cost-advanced oxidation processes on accountability of derisory performance to wastewater treatment have been exploited sundry shortcomings, which urge necessity for EAOPs-based alternative techniques. In this framework, to exaggerate traditional 2-D EP system has been transformed into 3-D EP by modification in electrode texture as a result, more peroxide formation was catalyzed by large electrode surface area as well as considerably SEC were also dwindled. Notwithstanding PEP approaches were established to overcome dilemma of existing EP techniques under harsh conditions, where UV accelerated further prevailing hydroxyl-free radicals and synergistic effect of individual mechanisms involved in PEP have been substantially boosted enhancement factor along with degradation kinetics of pollutants in wastewater thereby diminishing energy expenditures in the form of SEC_PEP. Additionally, to improve some conventional methods more conspicuously filtration, electrocoagulation and biological treatments were coupled with EP to devise novel complementary hybrid EP-based EAOPs, which have demonstrated pragmatic mineralization effectiveness and declined required electrical energy consumption.

Over the last decade, EAOPs in wake of nonselective oxidation and prohibition of secondary products have been acquainted for wastewater treatment. Henceforth, EAOPs more conspicuously novel complementary hybrid EP and PEP approaches could be more economical option for wide spectrum of synthetic and real wastewater treatment along with reducing energy expenditures, which could be fruitful from laboratory to large scale.

Conflict of interest

The authors declare no conflict of interest.