**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 electrosorption 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 Ȯ<sup>3</sup> 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 <sup>10</sup><sup>6</sup> to 0.01 M<sup>1</sup> <sup>s</sup> <sup>1</sup> 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].

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

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<sup>1</sup> after processing till 360 minutes, and COD was declined to 47.7 mg L<sup>1</sup> . 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 oxalatecontaining 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 pseudofirst-order kinetics with 2.05 h�<sup>1</sup> 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-firstorder kinetics [67].

$$\text{(H}\_2\text{O} + \text{)}\text{)} \rightarrow \text{H}\dot{\text{O}} + \dot{\text{H}} \tag{16}$$

$$\rm{H\_2O\_2 + ()() \rightarrow H\dot{O} + H\dot{O}} \tag{17}$$

$$(\bullet \bullet + \bullet)) \to \bullet 2 + \bullet \tag{18}$$

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 electroperoxone 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�<sup>1</sup> . 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 *via* Eq. (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 *via* Eq. (21) [99]. DBCER with boron-doped diamond (BDD)

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

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].

$$\text{2H}\_2\text{O} \rightarrow \text{O}\_2 + 4\text{H}^+ + 4\text{e}^- \tag{19}$$

$$\text{BDD} + \text{H}\_2\text{O} \rightarrow \text{BDD} \text{(H\dot{O})} + \text{H}^+ + \text{e}^- \tag{20}$$

$$\text{BDD} \left( \text{H} \dot{\text{O}} \right) + \text{R} \rightarrow \text{BDD} + \text{CO}\_2 + \text{H}\_2\text{O} + \text{H}^+ + \text{e}^- \tag{21}$$
