methanol oxidation [36]. The limitations like high-cost approach, non-availability, and restrictions in poisoning of intermediates [37, 38] while using pure Pt, narrows Pt’s attraction, and utility as EC are now masked by other suitable alternatives [39]. Conductivity of the EC is improved by surface modification, such that cathode attains a high electrocatalytic property. This can be best achieved by functionalization modules of NMs [40]. A schematic depiction of electrocatalytic degradative action in the presence of FNMs that easily removes the contaminants, to protect the water system is shown in Figure 1.2.
An effective catalytic reducing ability was noticed by Qiu, L. et al., while using Au/PPy NTs as EC in gauging fuel cell’s capability for waste effluent water management. 4-NP was the targeted compound in this experiment [41]. FNM nano-TiO2/C membrane, an EC got by sol-gel technique, had an efficient removal of diesel oil (100%) from wastewater. Authors Yang, Y. et al., observed that ECMR was a key enhancer in this situation [42]. The choice of material, a primary factor in wastewater management, was focused in the reports of Bankole, M.Y. et al., where PHB/CNTs and P-CNTs (P-Purified) had a worthy removal of heavy metals like [Fe (15.92% and 15.11%), Cr (98.19% and 99.80%), As (99.95% and 99.99%), Cd (99.34% and 98.68%), Pb (98.85% and 99.44%), Cu (83.08% and 82.91%), Zn (18.34% and 21.80%), and Ni (77.95% and 78.06%)] in redox fashion [43]. Nitrobenzene, a carcinogenic contaminant associated with dyes, pesticides, explosives, and pharmaceuticals, was successfully resolved and degraded to 99.8% in 5 h by FNM TiO2-NTs/SnO2-Sb/PbO2 electrocatalytically with an increased stability at a potential of 2.00 V [44]. In a similar protocol, the working trials were conducted for electrocatalytic action for degrading the organic contaminant benzoic acid. The mesoporous structured material TiO2-NTs/m-SnO2-Sb that was produced by electro-deposition process had a significant removal efficacy [45]. Pd/TiO2 NTs got by electro-chemical deposition method excellently removed 2,4,5-PCB (90%) by electrocatalytic dehydro-halogenation process [46].
Figure 1.2 Electrocatalytic degradative action to protect the water system.
1.3 Electro-Fenton/Hetero Electro-Fenton as FNMs
Fenton’s redox chemistry employs ·OH released between the reacting species (H2O2 + Fe2+) for the decomposition of target pollutants (TPs), where electro-Fenton (EF) or photoelectro Fenton (P-EF) have prominent roles. Hetero-EF (H-EF) utilizes solid nanocatalyst as a supporter for reducing H2O2 → ·OH. The disadvantage of small pH range (acidic) is overcome by solid supporters when used. The effluents released into the water system have a wide range of pH [47]. Micro-porous/meso-porous FNMs offer best solutions for degrading OPs in the water bodies. Research communities are focusing on this segment for protection of environmental crises using Fe/other transition metal/metal oxides as cathodic FNMs in H-EF methods.
Cathodic FNMs are got by (i) uni/multi step synthesis of low-density porous-solids (C aerogels), (ii) modified conducting FNMs with Fe, and (iii) carbonaceous solids supported with Fe or other components as FNMs [48–50]. Formation of sludge as Fe-hydroxides, as in normal Fentons, is retarded or inhibited, thus improving the efficiency and availability of catalyst for its activity. Hence, less energy utilization and a cost-effective approach is favored. Similarly, reusability and recyclability for many trials were observed while using cathodic FNMs of Fe2O3/N-C by [51] and Fe-Cu-C aerogel [52]. However, Fe when strengthened with other metals (transition) embedded in it, results in a redox reaction with catalytic decomposition, and is favored with the increase in efficacy of the electrocatalytic system to bring about degradation of TPs [53–55]. A figurative description of the functionalized catalytic activity is shown in Figure 1.3.
In a typical report of Cui, L. et al., MO decomposition by H-EF was proved to be accelerated by FNM - Fe3O4/MWCNTs, when prepared by solvothermal process. Degradability of the TP was noted to be 90.3% (3 h) with reusability to 5 runs, at pH (3). This system with two compartments of FNM membrane required no external additives, but had a potency in green wastewater treatment techniques [56]. Zhao, H. et al. reported that Fe3O4@Fe2O3/ACA (activated C aerogel) as cathodic in this EF routine degraded (90%) of OP-pesticide imidacloprid (30 min) and TOC (60 min) in pH range of (3–9) [57]. Haber-Weiss model inferred that Fe2+ aided the decomposition of peroxide to form ·OH. ·OH and ·O2− contribute for the degradation of OP. Mesoporous FNMs MnCo2O4-CF (C felt) as cathodic EF with excellent porosity and large modified surface area prepared showed a powerful degrading capacity for CIP (100%) an antibiotic in 5 h and TOC (75%) in 6 h [58]. Mn2+/Mn3+, Co3+/Co2+ with e− transfers enhanced peroxide decomposition to form ·OH and ·OOH required for five cycles degradation.
Figure 1.3 Electro-Fenton functionalized catalytic degradative activity for water bodies.
Table 1.1 Electro-Fenton (EF)/Hetero-Electro-Fenton (H-EF) catalyst as FNMs.
| FNMs as catalyst | Type | Year | Process | Current/Voltage | Parametric expressions | Solution evolved (% degradation) | Reusable cycles | Remarks | Ref. |
| BGA-GDE | EF | 2019 | Hydrothermal | 4.5 mA cm−2 | pH (3–9) | 60 min | BPA (~89.65%) | 5 TOC (~90%) | 5 | · OH | pseudo-1st-order kinetics | [62] |
| RGO-Ce/WO3 NS/CF | EF | 2018 | Hydrothermal | 300–400 mA | pH (3) | 1h | CIP (100%) | 5 | · O2−, H2O2, ·OH | Ce-WO3 improved adsorption | [63] |
| ACF-HPC | EF | 2019 | Hydrothermal, carbonization | (16, 20, 24) mA cm−2 | pH (3, 7, 9) | 40, 180 min | Phenol (93.8%) | 5TOC (85.7%) | 5 | Enhanced formation of H2O2, ·OH | Low-cost | [64] |
| Fe-C/PTFE | H-EF | 2015 | Ultra-sonification | 100 mA | pH (6.7) |120 min | 2,4-DCP (95%) | | pseudo-1st-order kinetics | promoters: H2O2, ·OH | Cheap | [65] |
