**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].

$$\text{Enhanement} = \frac{(\text{k}\_{\text{PEP}})}{(\text{k}\_{\text{O}}) + (\text{k}\_{\text{UV}}) + (\text{k}\_{\text{E}})} \tag{22}$$

$$\text{Enhanement} = \frac{(\text{k}\_{\text{EP}})}{(\text{k}\_{\text{O}}) + (\text{k}\_{\text{E}})} \tag{23}$$

Where kEP, kPEP, kO, kUV, and kE 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**).

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 electroperoxone 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<sup>⧿</sup> 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


*rGO/BiOCl = reduced graphene oxide/bismuth oxy-chloride, ND = not determined, <sup>b</sup> = Kapp <sup>10</sup><sup>3</sup> <sup>s</sup> 1 , <sup>a</sup> = Kapp 102 min<sup>1</sup> , c = KappNP min<sup>1</sup> , d = (min)<sup>1</sup> reaction rate constant (Kobs), e = h<sup>1</sup> , <sup>f</sup> = Kapp <sup>10</sup><sup>4</sup> min<sup>1</sup> , g = min<sup>1</sup> .*

### **Table 2.**

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

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

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, cathodicactivated 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�<sup>1</sup> ) 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 Ȯ<sup>2</sup> � have contributed to wastewater treatment [18]; hence, approximately at slightly acidic pH reaction could be proceeded between H+ ions and Ȯ<sup>3</sup> � to produce reaction active species (HȮ�) *via* Eq. (24) [105].

$$2\dot{\mathbf{O}}\_{3^{-}} + \text{H}^{+} \rightarrow \text{H}\dot{\mathbf{O}}\_{2} \rightarrow \text{H}\dot{\mathbf{O}}^{-} + \text{O}\_{2} \tag{24}$$

$$\text{O}\_2 + 2\text{H}^+ + 2\text{e}^- \rightarrow \text{H}\_2\text{O}\_2 \tag{25}$$

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�<sup>1</sup> ) 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].

$$\text{Fe}^{\cdot+2} + \text{O}\_3 \rightarrow \text{FeO}^{\cdot+2} + \text{O}\_2 \tag{26}$$

$$\rm{FeO}^{+2} + \rm{H}\_2\rm{O} \rightarrow 2\rm{H}\dot{\rm{O}} + \rm{Fe}^{+3} + \rm{OH}^- \tag{27}$$

$$\text{COD}\_{\text{exclusion}\%} = \frac{\text{C}\_{\text{f}} - \text{C}\_{0}}{\text{C}\_{0}} \times 100 \tag{28}$$

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 Cf 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].
