**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.

**Keywords:** electro-peroxone, photoelectro-peroxone, wastewater, complementary hybrid EP approaches, energy consumption

## **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<sup>1</sup> energy content required to fracture CdF 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 biorecalcitrant-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 photoelectroperoxone 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 [16–18]. 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 CdH 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 10<sup>8</sup> to 1010 M<sup>1</sup> s <sup>1</sup> [21].

*Electro-Peroxone and Photoelectro-Peroxone Hybrid Approaches: An Emerging Paradigm… DOI: http://dx.doi.org/10.5772/intechopen.102921*

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 *via* Eq. (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].

$$\mathrm{O}\_{2} + 2\mathrm{H}^{+} + 2\mathrm{e}^{-} \xrightarrow{\quad} \mathrm{H}\_{2}\mathrm{O}\_{2} \tag{1}$$

$$\text{H}\_2\text{O}\_2 + \text{O}\_3 \xrightarrow{\text{C}^\cdot \text{C}^\cdot \text{H}^+} \text{\text{\textdegree{}OH}^\cdot + \text{\textdegree{}O}\_2\text{\textdegree}^- + \text{H}^+ + \text{O}\_2 \tag{2}$$

$$\text{2H}\_2\text{O}\_2 + 2\text{O}\_3 \xrightarrow{\text{H}\_2\text{O}\_2} \text{\text{\textdegree OH}} + \text{H}\_2\text{O} + \text{H}\dot{\text{O}}\_2 + \text{3O}\_2\tag{3}$$

$$\text{2O}\_3 + \text{OH}^- \xrightarrow{-} \xrightarrow{-} \text{\cdot\text{OH} + \text{\cdot}\_2\text{-} + 2\text{O}\_2} \tag{4}$$

$$\text{CH}\_2\text{O} + \text{O}\_3 + \text{e}^- \xrightarrow{\text{---}} \text{\text{\textdegree OH}} + \text{O}\_{2^-} + \text{OH}^- \tag{5}$$

$$\text{R} + \dot{\text{OH}} \xrightarrow{\text{---}} \text{CO}\_2 + \text{H}\_2\text{O}\_2 \tag{6}$$

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 photosynthesized electron within conduction band of a semiconductor bismuth oxychloride (BiOCl) interacts with ozone to yield ozone-free radicals (Ȯ<sup>3</sup> �) based on Eqs. (10) and (11); afterward, Ȯ<sup>3</sup> � 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 (gTOCremoved) �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 (gTOCremoved) �<sup>1</sup> 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].

$$\text{H}\_2\text{O} + \text{O}\_3 + \text{hv} \xrightarrow[\text{K}]{} 2\dot{\text{O}}\text{H} + \text{O}\_2 \tag{7}$$

$$\text{H}\_2\text{O}\_2 + \text{hv} \xrightarrow[]{} \text{I} \text{\textasciicircum} \text{2\text{\textquotedblleft}OH} \tag{8}$$

$$\text{O}\_3 + \text{hv} \xrightarrow[-]{} \text{O} + \text{O}\_2 \tag{9}$$

$$\text{BiOCl} + \text{hv} \xrightarrow{\text{-} \longrightarrow \text{-} \atop \text{BiOCl}} \text{BiCl} - \text{h}^+ + \text{BiOCl} - \text{e}^- \tag{10}$$

$$\text{O}\_3 + \text{BiOCl} - \text{e}^- \xrightarrow{\text{---} \atop \text{O}\_3-} \text{O}\_3\text{-} \tag{11}$$

$$\rm H^{+} + \dot{O}\_{3^{-}} \xrightarrow{\cdot \longrightarrow \longrightarrow} \rm HO\_{3} \tag{12}$$

$$\text{H}\dot{\text{O}}\_{3} \xrightarrow[]{} \text{H}\dot{\text{O}} + \text{O}\_{2} \tag{13}$$

This chapter study aimed to theoretically probe environmentally friendly, costeffective, 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 electroperoxone approach. When uniform current is provided to reactor, HȮ formation rate also turns out to be constant and *k HO*\_ in Eq. (14) becomes equal to kapp based on Eq. (15) pseudo-first-order rate constant. Here, �*d P*½ � *dt* is rate of disintegration of pollutants in solution; while [P], [HȮ], k, and kapp denote concentration of pollutants and hydroxyl-free radicals in wastewater, absolute rate constant, and apparent rate constant, respectively [38, 39].

$$\text{Rate} = \frac{-\text{d}[\text{P}]}{\text{dt}} = \text{k}[\text{P}]\left[\text{H}\dot{\text{O}}\right] \tag{14}$$

$$\text{Rate} = \frac{-\text{d}[\text{P}]}{\text{dt}} = \text{k}\_{\text{app}}[\text{P}] \tag{15}$$
