An Overview of Environmental Catalysis Mediated by Hydrogen Peroxide

Abstract

The use of hydrogen peroxide (produced in situ or ex situ) as the main agent in oxidative processes of environmental pollutant removal is widely studied. The degradation of water pollutants, such as dyes, pharmaceuticals, cosmetics, petroleum derivatives, and even pathogens, has been successfully obtained by different techniques. This review gives an overview of the more recent methods developed to apply oxidative processes mediated by H₂O₂ and other reactive oxygen species (ROS) in environmental catalysis, with particular attention to the strategies (Fenton-like and Bio-Fenton, photo- and electro-catalysis) and the materials employed. A wide discussion about the characteristics of the materials specifically studied for hydrogen peroxide activation, as well as about their chemical composition and morphology, was carried out. Moreover, recent interesting methods for the generation and use of hydrogen peroxide by enzymes were also presented and their efficiency and applicability compared with the Fenton and electro-Fenton methods discussed above. The use of Bio-Fenton and bi-enzymatic methods for the in situ generation of ROS seems to be attractive and scalable, although not yet applied in full-scale plants. A critical discussion about the feasibility, criticalities, and perspectives of all the methods considered completes this review.

1. Introduction

The depletion of water resources is a theme of global concern and it is more and more exacerbated by increasing water consumption (especially connected to certain agricultural and industrial activities [1,2]), contamination sources and climate changes that bring about aridity/desertification issues [3,4]. As a consequence, access to clean water is progressively more limited and the need for the reuse of wastewater is mandatory worldwide [3,4]. Wastewater has to be treated to fulfill the quality requirements before the reintroduction in specific systems (e.g., agricultural, industrial, potable, etc.) [5].

Wastewater depuration is typically a multi-step procedure in which most of the chemical and pathogenic contaminants are successfully removed [4]. Nevertheless, recalcitrant and new kinds of not commonly monitored pollutants (Contaminants of Emerging Concern, such as pharmaceuticals and personal care products), detected at trace/sub-trace levels in water bodies and often not removed by traditional depuration processes, represent a further hurdle to the achievement of high-quality water standards [4,6,7]. For this reason, advanced methods should support these processes, among which the most studied ones since the 1980s are the so-called Advanced Oxidation Processes (AOPs) [8]. The methods involving the presence of Reactive Oxygen Species (ROS) with strong oxidizing properties underlie AOPs’ definition and are aimed at bringing about the partial/total mineralization of chemical pollutants to carbon dioxide, water and inorganic ions up to disinfection from pathogens [7,9,10]. The main ROS are non-radical species such as H₂O₂ and singlet oxygen (¹O₂), and highly reactive free radicals, such as hydroxyl (^●OH), hydroperoxide (HOO^•), superoxide (O₂^●−), carbonate anion (CO₃^●−), and sulfate (SO₄^●−) radical, deriving from the activation of precursors (hydrogen peroxide, persulfate/peroxydisulfate, peroxymonosulfate and sodium percarbonate) via oxidation/reduction reactions [7,11,12,13,14].

The oxidation processes exploiting H₂O₂ and the derived ROS are the focus of the present review. Many different treatments have been proposed [11,15,16,17,18], also in a variety of combinations and with different triggering sources (ultrasounds, UV/vis light, heat and electrochemical energy) [7,15,19,20,21,22,23,24], as summarized in Figure 1.

In Figure 1, the methods that are mainly recognized to be capable of inducing an in situ generation of ROS [17,25,26,27] have been underlined since the use of such methods can overcome the problems related to high costs and excessive consumption of hydrogen peroxide, as well as avoid the hazards associated with its transport, handling, and storage in significant quantities [25,27,28]. A further distinction can be made considering homogeneous and heterogeneous processes. In this latter category, solid catalytic materials are applied for the generation of active species in the aqueous medium through catalytic ozonation, photocatalytic and electrochemical systems, heterogeneous Fenton-like processes, etc. [29,30,31]. Heterogeneous catalyst-based methods are considered greener to permit both easier recycling of active materials and a decrease in costs and pollution [4]. For this reason, these will be more explore more in depth in this excursus.

From what has been seen so far, the two most common methods to produce hydrogen peroxide in heterogeneous systems are photocatalysis and electrocatalysis (Figure 2). In order to briefly introduce the basilar mechanism of a photocatalytic process (Figure 2A), it is possible to use the well-known semiconductor TiO₂ as an example. The mechanism of the UV/TiO₂ implies that when titania is irradiated with light energy equal to or higher than its band gap, an electron (e⁻) can be excited from the valence band to the conduction band, leaving a hole (h⁺) in the valence band. If charge separation is maintained, the paired e⁻-h⁺ may migrate to the surface of the photocatalyst. In the aqueous phase, the photoinduced h⁺ can oxidize surface hydroxyl groups or surface-bond water molecules to produce hydroxyl radicals and other ROS [32,33]. Regarding electrocatalysts, they have to initiate an Oxygen Reduction Reaction (ORR) to generate hydrogen peroxide via two e⁻ or four e⁻ processes (Figure 2B). Therefore, the involved materials have to possess suitable electronic configuration and redox properties, conductivity, low overpotential, fast kinetics and good Faraday efficiency to allow for oxygen reduction [34,35,36,37].

The methods based on the use/production of hydrogen peroxide and related ROS are mainly founded on the following reactions (1–19) [8,17,21,22,40]. Equation (1) is related to ozone-related systems, Equations (2)–(6) to the peroxone (O₃/H₂O₂) system, Equations (5)–(11) to photolysis and photocatalytic (hν)/thermocatalytic (ΔT) reactions in the presence of a heterogeneous catalyst (“Cat.” refers to a generic semiconductor), and Equations (6) and (12)–(18) to Fenton and Fenton-like processes. Equation (18), in particular, is an example of radical recombination that can restore H₂O₂ [15]. Equation (19) is related to ultrasound (US)-assisted reactions, in which sound waves can lead to cavitation phenomena, involving vapor- and gas-filled microbubbles, and to the generation of high temperature and high pressure. Finally, the formation of organic radicals (R^●) during these processes introduces new variables into this chain of reactions [41].

3O₃ + H₂O → 2 ^●OH + 4O₂

(1)

H₂O₂ → HO₂⁻ + H⁺

(2)

HO₂⁻ + O₃ → ^●OH + O₂⁻ + O₂

(3)

O₃ + H₂O + hν → H₂O₂ + O₂

(4)

H₂O₂ + hν → 2 ^●OH

(5)

H₂O₂ + ^●OH → H₂O + HOO^●

(6)

Cat. + hν/ΔT → e⁻ _{(conduction band)} + hole⁺ _{(valence band)}

(7)

hole⁺ _{(valence band)} + OH⁻ _(surface) → ^●OH

(8)

hole⁺ _{(valence band)} + H₂O _(adsorbed) → ^●OH + H⁺

(9)

e⁻ _{(conduction band)} + O_{2 (adsorbed)} → O₂^●−

(10)

H⁺ + O₂^●− → HOO^●

(11)

H₂O + hν → ^●OH + H^● when λ(hν) < 242 nm

(12)

Fe²⁺ + H₂O₂ → Fe³⁺ + ^●OH + OH⁻

(13)

Fe³⁺ + H₂O₂ → Fe²⁺ + HOO^● + H⁺

(14)

^●OH + Fe²⁺ →Fe³⁺ + OH⁻

(15)

Fe³⁺ + HOO^● → Fe²⁺ + O₂H⁺

(16)

Fe²⁺ + HOO^● + H⁺ → Fe³⁺ + H₂O₂

(17)

2HOO^● → H₂O₂ + O₂

(18)

H₂O + US → ^●OH + H^●

(19)

It is worth briefly discussing the Fenton processes and the distinction between Fenton and Fenton-like ones. In the classical homogeneous Fenton process, low pH values (around 3.0) are necessary to avoid the precipitation of iron oxyhydroxides from Fe(II), resulting in higher costs [42,43,44]. Furthermore, when complex real matrices have to be depolluted, there is the risk of precipitation of the iron catalyst, leading to the formation of sludge and undesired by-products [7]. To overcome these limitations, Fenton-like processes are designed to use oxidants other than hydrogen peroxide and/or transition metals other than Fe²⁺ (e.g., Fe³⁺, Cu²⁺/Cu⁺, iron-based minerals, nano zero-valent iron, etc.), and/or employ heterogeneous catalysts and/or external energy sources to create similar reactions [29,44,45,46,47]. All the aspects concerning the chemical development of AOP methods involving H₂O₂ will be addressed in Section 2.1.

As anticipated, another important point that must be considered concerns the production of H₂O₂ to be used in these reactions. In recent years, biological systems for the in situ generation of H₂O₂ have been carefully taken into account since they allow for the production of hydrogen peroxide under mild conditions, avoiding the use of critical solvents and reducing or avoiding the formation of toxic by-products.

One of the most exploited enzymes for this purpose is Glucose Oxidase (GOx), which catalyzes the oxidation of β-d-glucose to d-glucono-δ-lactone (further hydrolyzed into gluconic acid) and hydrogen peroxide in the presence of molecular dioxygen, as summarized in Equation (20).

β-d-glucose + O₂ + H₂O → gluconic acid + H₂O₂

(20)

This catalytic mechanism is also widely exploited in Bio-Fenton processes that actually use H₂O₂ enzymatically produced as a reagent for the Fenton reaction [48]. Unlike the classic Fenton reaction, the Bio-Fenton process also occurs at pH values near neutrality owing to the synergy with all the GOx reaction products [49]. A wide discussion about the environmental application of the Bio-Fenton technique will be made in Section 2.2 of this review, together with a description of innovative bi-enzymatic systems coupling GOx and other enzymes.

Finally, a section about H₂O₂ detection techniques and another on the critical assessment of the whole H₂O₂-mediated systems will be presented to enrich the overview of the practical implications of these depuration treatments. The global structure of this review will cover the topics indicated in Figure 3.

2. Recent Developments in AOP Systems Involving H₂O₂

This section will examine different material- and enzyme-based systems involving hydrogen peroxide and its derived ROS for decontaminating wastewater from several types of contaminating agents, ranging from synthetic dyes (often found downstream textile industries) to pharmaceuticals and other harmful chemicals derived from different production activities to biological pathogens.

2.1. Advanced Materials

By searching on the Scopus database for studies with the query “H₂O₂ AND generation AND wastewater” and further limiting the search to the keywords “Hydrogen peroxide”, 407 documents were found. Of these research studies, 96 possess “Iron”, 77 “Iron compounds”, 32 “Ferric Ion”, 17 “Ferric compounds”, 38 “Ferrous Ion”, 109 “Fenton”, 23 “Photo-Fenton”, and 42 “Electro-Fenton” as further keywords. This circumstance makes clear the strong contribution of iron-based chemistry and Fenton-type reactions when environmental decontamination mediated by hydrogen peroxide is attempted.

Table 1. Iron-based systems for pollutant removal mediated by H₂O₂ (except for two cases, all the works reported were published within the last 5 years) *.

System	Pollutants	Conditions	Performance	Mechanism and Notes	Ref.
1. Fe(III)-functionalized polyacrylonitrile/polypropylene fiber mesh	Bisphenol A (BPA)	Best setup: 75 ppm of BPA, 300 ppm of H2O2, pH = 3, 60 °C.	100% BPA abatement in <30 min; effects of temperature and pH. Fast degradation in Rotating Catalytic Reactor.	Fenton process; reusable catalyst but poisoning by the intermediates.	[54]
2. Zn-Carbon Nanotubes (CNTs) in the presence of Fe2+	4-chlorophenol (4-CP)	Conc. 4-CP = 50 mg L−1; Conc. Fe2+ 20 mg L−1, pH = 2.0. O2 was fed in the reaction mixture (400 mL min−1).	Abatement of 4-CP and TOC: 98.8% and 87.4%, respectively (20 min). When 4-CP was spiked in real wastewater, the abatements of 4-CP and TOC were 47.0% and 45.6%. Effects of pH, Zn-CNTs dosage and Fe2+ amount.	Fenton: in situ generation of a high concentration of H2O2, rapid regeneration of Fe2+ from the reduction of Fe3+ by Zn and high adsorption ability of Zn-CNT towards pollutants.	[55]
3. Graphitic-C3N4 QDs with FeOOH	Tetracycline (TC), p-nitrophenol (PNP), 2,4-dinitrophenol (2,4-DNP)	500 W Xe lamp with a 420 nm cut-off; addition of H2O2; optimal pH = 7.	Abatement: TC ca. 90% (2 min), PNP 90% (10 min), 2,4-DNP ca. 90% (5 min). Dependence on catalyst/pollutant ratio, pH, conc. H2O2.	Photo-Fenton: main action of •OH with the aid of O2•−and h+; photogenerated e− in CB favored Fe3+/Fe2+ cycling.	[56]
4. Ultrathin porous Graphitic-C3N4 nanosheets with amorphous FeOOH QDs	Oxytetracycline (OTC)	300 W Xe lamp with a 420 nm cut-off; pH = 7.	Highest efficiency for 20%FeOOH-composite degrading 86.23% of OTC (120 min) and TOC removal of 48.6%.	Photo-Fenton; Graphitic-C3N4 in situ produced H2O2, improving transport of photogenerated e−-h+ pairs; FeOOH generated •OH.	[57]
5. α-Fe2O3/Graphitic-C3N4	Rhodamine B (RhB), tetracycline hydrochloride (TC-H)	Simulated solar light with 300 W Xe lamp; neutral pH.	Degradation of RhB 96% (90 min) and TC-H 95% (150 min).	Photo-Fenton: effective separation and transfer of photogenerated charge carriers; H2O2 photoproduction on g-C3N4; •OH generation from H2O2 decomposition on α-Fe2O3; O2•− and h+ played supporting role.	[58]
6. Magnetite/maghemite NPs coated with waste-sourced bio-based substances (BBS)	Phenol (PH)	For Fenton: addition of H2O2 5 × 10−4 M; photoactivation by a lamp with max emission at 365 nm; pH = 3.5.	100% PH degradation (5 min). Reusability allowed by the constant Fe release from NPs.	Fenton and photo-Fenton processes, but higher efficiency in Fenton mode. In photo-Fenton •OH are generated. BBS acted preventing catalyst oxidation and Fe precipitation.	[59]
7. Glucose-mediated Fe3O4 magnetic NPs	Methylene blue (MB), Cr(VI)	For Fenton and photo-Fenton: addition of H2O2; photocatalytic reduction of Cr(VI); photoactivation with vis. light (250 W).	MB degradation at pH 9: 93% in 75 min by photo-Fenton and 92% in 120 min by Fenton process. 100% photoreduction of Cr(VI) in 25 min. Influence of H2O2 concentration, pH, catalyst/pollutant dosage.	Higher efficiency of both Fenton and photo-Fenton ascribed to the novel synthesis method. •OH was detected as primary ROS. The catalyst was stable and reusable.	[60]
8. CuO-Fe2O3 heterojunction	Quinoline Yellow (QY)	Best setup: H2O2 and QY concentrations 27.6 mM and 100 mg L−1, respectively; pH = 3, 40 °C.	100% QY removal in ca. 60 min. Dependence on pH, conc. H2O2 and dye, catalyst dose, temperature.	Photo-Fenton and Fenton (no strong effect of irradiation); recyclability.	[61]
9. Fe3O4/Cu magnetic NPs prepared using Rosmarinus officinalis leaves aqueous extract	Methyl Orange (MO), imipenem (IMI), imatinib mesylate (IMA)	300 W Xe lamp with a 420 nm cut-off.	MO, IMI, IMA degradation of 96.6%, 81.8% and 84%, respectively, after 5400 s; TOC decrement.	Photocatalytic process with production of H2O2 and O2●− as main ROS; reusability of the catalyst; beneficial effect of R. officinalis leaves extract.	[62]
10. Mexican Natural Zeolite-based Membrane	Reactive Black 5 (RB5)	Best setup: pH= 3, conc. H2O2 = 3 g L−1, conc. RB5 = 100 mg L−1, added FeCl3 = 0.013 g L−1, LED lamp emission at 405 nm (2.2 W), permeation flux ≅ 467 cm3 m−2 h−1.	92.3% discoloration in 30 min; progressive TOC decrement.	Photo-Fenton; reusable membranes, but possible Fe leaching.	[63]
11. Triphase MIL-101(Fe)/Graphitic-C3N4/hydrophobic carbon cloth	Methyl Orange (MO), methylene blue (MB), rhodamine B (RhB), rhodamine 6G (Rh6G)	300 W Xe lamp; dye solution = 10 ppm, pH = 3. Catalyst (size: 2.5 cm2) hydrophilic surface immersed in solution, while the hydrophobic part was exposed to air for O2 supply.	Abatement: MO 99%, MB 99%, RhB 98%, Rh6G 97% (130 min); high reaction rate constant.	Photo-Fenton with photoactivated in situ production of H2O2, promoted by triphase design of the Z-scheme heterojunction with a favored pathway for O2 transfer. Photoinduced e− and h+ separation efficiency; •OH and O2•− were the main ROS.	[64]
12. MoS2-Fex composite	Sulfadiazine (SD)	Best catalyst: MoS2-Fe75; Optimal conditions: H2O2 addition, SD = 10 mg L−1, pH = 6.5.	SD degradation 91.1% (90 min); effect of catalyst dosage, pH, H2O2 feeding way.	Fenton: MoS2-Fex selectivity for 1O2. “Small amount for multiple times” feeding way of H2O2 increased MoS2-Fex stability and SD degradation rate, reducing H2O2 decomposition. Formation of Fe sludge was much reduced than nano-iron powder. Long-term effectiveness of the MoS2-Fex/H2O2 system.	[65]
13. MoS2 in the presence of Fe3+	Bisphenol A (BPA), benzoic acid (BA), sulfadiazine (SDZ), rhodamine B (RhB), carbamazepine (CBZ), 4-acetamidophenol (APAP), ciprofloxacin (CIP), tetracycline hydrochloride (TC-H)	Optimal system: 1.0 mM H2O2, 0.3 g L−1 MoS2 and 0.15 mM Fe3+, pH = 3.0.	Abatement (60 min): CBZ 65.9%, APAP 79.1%, SDZ 84.1%, BA 86.0%, RhB 90.8%, CIP 92.5%, BPA 93.0%, TC 100%. Effects of pH and concentrations of pollutant, H2O2, Fe3+, MoS2.	Fenton: strong oxidative intermediate Mo6+ peroxo-complexes besides •OH radicals. Stable and reusable catalyst.	[66]
14. TiO2-supported Fe (FeTi-ox)	Acetaminophen (AAP), benzoic acid (BA), carbamazepine (CBZ), phenol (PH)	Conc. H2O2 = 10 mM and conc. pollutant = 5−10 μM, pH = 7.	Degradations (2 h): AAP 100%, PH 55%, while BA and CBZ were not removed. Pollutants selectively reacted with •OH (BA and CBZ) or with both •OH and Fe(IV) (AAP and PH).	Fenton: relevant interaction H2O2-TiO2 forms a peroxo−titania complex Fe(III)-Ti-OOH, which reacted further with H2O to give surface oxidant Fe[IV]-O2+ even in the presence of Cl−, HCO3− ions and organic matter. Reusability of FeTi-ox.	[67]
15. P25 (TiO2) in the presence of US and Fe2+/Fe3+	Bisphenol A (BPA), sulfadiazine (SDZ)	US at 400 kHz alone or in the presence of P25 under vis. light (LED lamp emitting at 400–630 nm).	With US and Fe2+: >90% degradation of BPA and >80% for SDZ in 30 min. In the presence of Fe2+/Fe3+, US, P25 and vis. light, 100% SDZ abatement in 60 min.	Homogeneous sono-Fenton: in situ generation of H2O2 and •OH. Sono-photo-Fenton: P25 promoted Fe2+/Fe3+ cycling by the photoproduced e−. P25 favored reaction at circumneutral pH and pollutant mineralization.	[68]
16. Iron-cobalt oxide nanosheets (CoFe-ONSs)	Tetracycline (TC)	Conc. TC 50 mg L−1, conc. H2O2 20mM, neutral pH.	TC removal 83.5% (50 min). Effects of catalyst dosage, conc. H2O2, pH, temperature, conc. TC, anions and water sources.	Fenton: •OH were the main ROS. Redox cycles of FeII/FeIII and CoII/CoIII enhanced •OH generation. Negligible Fe ions leaching from catalyst (reusability).	[69]
17. Magnetic ZnO@Fe3O4 composite	p-nitrophenol (p-NP)	Conc. p-NP 35 mg L−1, pH = 3 (optimal); 100 W incandescent lamp (400–1700 nm).	100% p-NP removal in 60 min. Effect of temperature.	Photo-Fenton: self-generation of H2O2, with primary role of •OH; catalyst reusability.	[70]
18. ZnFe2O4/BiVO4 heterojunction	Methylene blue (MB).	Best composition: 0.15ZnFe2O4/BiVO4.MB conc. 40 mg L−1, eventual addition of H2O2; photoactivation with 300 W Xe lamp.	MB degradation with photocatalytic method (2 h): 83.7%; with Photo-Fenton (1 h): 98.8%.	Photocatalysis: h+ had a main role in degrading MB; photo-Fenton: activation of H2O2 (•OH production) by photogenerated e−; photogenerated carriers separation efficiency.	[71]
19. LaFeO3 prepared from citric acid and LaFeO3 synthesized from waste-sourced bio-based substances	4-methylphenol (4-MP) and crystal violet (CV)	Conc. pollutant 10 mg L−1; pH = 8–10; 1500 W Xe lamp with a 340 nm cut-off.	LaFeO3 prepared from waste-sourced bio-based substances removed 100% CV and ca. 40% 4-MP. The citric acid-derived LaFeO3 photodegraded 90% of 4-MP and 30% of CV.	Photocatalysis: different LaFeO3 efficiencies were ascribed to different ζ-potentials. A homogeneous photo-Fenton process can occur when LaFeO3 synthesized from waste-derived substances releases Fe and carboxylate ions.	[72,73]
20. Cerium, Cobalt, Copper-doped Strontium Ferrate (SCFCC)	Escherichia Coli	Best material with 20% Cu doping. Bacteria conc. 1.0–3.0 × 105 CFU/mL. SCFCC/inoculum ratio = 1 g/50 mL. Tests in the dark, and after thermo- or UV-activation (max 70 °C heating and UV-A 300 W lamp, respectively).	Max. bacterial removal 55% in the dark, 98% after UV, 40% after thermal activation.	Photo/Thermo-catalysis: formation of H2O2 and •OH. Activity was also influenced by SCFCC metal ions’ redox couples and oxygen vacancies.	[74]
21. Peanut shell-derived biochar (PBC) in the presence of Fe2+	Bisphenol A (BPA), dimethyl phthalate (DMP), sulfamethoxazole (SMX)	Degradation tests conducted in a three-electrode cell aerated with O2 (0.4 L min−1); voltage applied 0.5–1.1 V; pH = 3.0, conc. Fe2+ 0.2 mM and conc. pollutants 20 mg L−1.	Removal efficiencies of 98–100% within 15 min; mineralization efficiencies of 83–100%.	Electro-Fenton: PBC hierarchical porous structure and defects caused a high surface area, electrical and ionic conductivity. The presence of OOH/C–O–C and N on the surface ensured a high two-electron ORR selectivity for H2O2 production, accelerated Fe2+ regeneration, also enabling •OH accumulation.	[75]
22. Ferromagnetic activated carbon from rubber seed hull	Bezaktiv Brilliant Blue (BBB)	Best setup: pH = 3, conc. H2O2= 17 mol L−1, conc. BBB = 100 mg L−1.	BBB removal > 75% in all conditions (4 h). Effects of pH, conc. H2O2 and pollutant, catalyst dosage.	Fenton; reusable catalyst.	[76]
23. Natural pyrite (FeS2)	Carbamazepine (CBZ)	Addition of tartaric acid (TA), citric acid (CA), ascorbic acid (AA); simulated sunlight (300 W Xe lamp).	No CBZ degradation with pyrite; CBZ abatement: 70%, 60%, 53% in pyrite/TA, pyrite/CA, pyrite/AA systems, respectively under irradiation (30 min). Effect of catalyst dosage, pH, conc. CBZ.	Photo-Fenton; in situ generated H2O2 without extra pH adjustment; organic acids can form complex with Fe in pyrite, promoting Fe(II) dissolution. Upon irradiation, pyrite is excited to generate photo-e−, able to reduce oxygen to produce H2O2 and •OH.	[77]
24. Goethite (α-FeOOH)	Bisphenol A (BPA)	H2O2 addition (1.0 mM), conc. BPA 0.1 mM; reaction allowed under acidic, neutral and weakly alkaline conditions.	BPA degradation 75.9% after 240 min; pH dependency.	Fenton; •OH production. Good structural stability of catalyst; higher efficiency with H2O2 in comparison to persulfate oxidation system due to the limited radical scavenging.	[78]
25. Heterogeneous Nanoscale Zero Valent Ion (n-ZVI)	Glyphosate (GLY)	Eventual addition of H2O2. Optimal pH 3–4.	Up to 87% of GLY removal (30 min) in Fenton mode. Effects of pH, conc. H2O2, n-ZVI dosage. Efficiency also in tap water (100% degradation in 40 min), despite potentially interfering ions.	Adsorption and Fenton (after H2O2 addition).	[79]

* Legend: QDs = Quantum Dots; CB = conduction band; e⁻ = electrons; h⁺ = holes; λ cut-off < 420 nm indicates visible light; TOC = Total Organic Carbon; NPs = nanoparticles; ORR = Oxygen Reduction Reaction. For the sake of clarity, when mechanisms are indicated as “Fenton” for brevity, they have to be considered Fenton-like processes.

shows a selection of recent works reporting iron as the main component or coadjuvant in remediation routes. Indeed, the opportunities of obtaining Fe-containing materials from natural sources (entries 10, 23 and 24 in Table 1) or preparing different kinds of iron-based compounds are uncountable [41,47,50,51,52,53]. Moreover, even in Fenton and photo-Fenton processes defined as “heterogeneous”, iron can be added as a homogeneous reactant (entries 13, 15 and 21 in Table 1).

In the next subsections, the discussion, starting from the data of Table 1, will bring up several subjects, citing material types from 0D to 3D, from inorganic catalysts to organic substances, and from mono-element to binary and mixed systems, referring to the possibility of tuning the compositions with dopants or with the formation of composites/hybrids. The surface physical properties (area, porosity, surface charge), the chemical defects, such as oxygen vacancies, and a hint regarding the role of synthesis have also been taken into account. In Figure 4, the main points of this study are depicted.

2.1.1. Considerations on Structures and Morphologies

In heterogeneous systems, the role of materials is determined not only by their chemical activity in a strict sense, but also by other physical–chemical characteristics, such as the materials’ structures and morphologies. As the catalyst surface in the H₂O₂-mediated processes is involved in the adsorption and dissociation of oxygen/water molecules and oxygenated intermediates, a favorable surface charge, and the increase in the specific surface area and, thus, the number of active sites has been pursued as strategies to accelerate the reactions [80,81]. In this regard, Farhadian et al. [82] highlighted the importance of the support within composites/functionalized materials in a review focused on mesoporous silica with high surface area and high pore volume, capable of loading and dispersing catalytically active iron species for heterogeneous Fenton oxidation reactions. Indeed, the large cavities of the silica support can facilitate the mass transfer processes, and the high surface area of these materials increases the number of active sites, allowing for a more intimate contact with contaminants. Strategies for enhancing the catalytic performance of iron/silica composites were indicate, such as the simultaneous loading of different metals and the design of multimodal pore supports, surface functionalization to favor the distribution of active sites, and the use of radiation, ultrasound, and electrolysis.

In the same direction, entries 1–5 and 11 in Table 1 report the presence of 0D (quantum dots), 1D (carbon nanotubes) and 2D materials (polyacrylonitrile fiber mesh, graphitic carbon nitride, carbon cloth), also in combination. Salunkhe et al. [83] dedicated a review to 0D–2D synergistic nanocomposites for photocatalytic applications: the size-dependent bandgap tunability and effective charge carrier production of quantum dots can be exploited together with graphitic-C₃N₄ features, such as low toxicity, chemical and thermal stability, visible light absorption, n-type semiconducting property, and a configurable bandgap. Graphitic-C₃N₄ has been widely used for the photocatalytic degradation of pollutants alone or in co-presence with other compounds, such as MgO, Bi₂O₃, MoO₃/Ag, Au/Ni₂P, etc. [84,85,86,87,88,89,90,91]. Among the attempts to exploit graphitic-C₃N₄, Torres-Pinto et al. prepared these materials as metal-free photocatalysts able to generate hydrogen peroxide and, in turn, ^•OH in the presence of dissolved oxygen when irradiated by visible light [92]. Adding low dosages of dissolved iron promoted a Fenton process and the C₃N₄ photocatalytic system improved the mineralization of resorcinol, phenol, gallic acid, and benzoic acid at a natural pH by approximately 1.2 times.

Graphene and reduced graphene oxide (rGO), 2D versatile carbon materials, have demonstrated excellent mechanical properties, high charge-carrier mobility, high specific surface area, a large number of functional groups and good electrocatalytic activity, attracting increasing attention for environmental applications as adsorbents, photocatalysts and electrocatalysts [93]. Many composites have been designed, as well, with doping elements, nano zero-valent iron, oxides, mixed oxides, and sulfides in binary and ternary systems [94,95,96,97,98,99,100]. An interesting application of these materials is also the possibility of transforming them from 2D to 3D systems, such as aerogels, sponges and foams, which possess enhanced porosity and lightness as inner characteristics [101,102]. For instance, a novel cathode of macroporous graphene aerogel (GA) with a high specific surface area was proposed for the electro-Fenton reaction, efficiently and continuously producing in situ H₂O₂ to degrade antibiotic ciprofloxacin [97]. The good mineralization efficiency was attributed to the relevant number of macropores of GA that acted as reaction traps to accelerate the electro-generation of H₂O₂, subsequently decomposed by Fe²⁺ to form ^•OH. Simultaneously, the strong charge transfer ability of GA was beneficial to the conversion of Fe³⁺/Fe²⁺, making this material a promising candidate material for electro-Fenton cathode due to its low cost, high efficiency and corrosion resistance.

For similar reasons connected to morphology, biochars and active carbons have been widely employed in H₂O₂-mediated processes (entries 21 and 22 in Table 1). Indeed, active carbons are produced from carbon-based compounds—often non-renewable (i.e., coal, petroleum residues, peat, lignite and polymers)—by the combination of a pyrolytic process and chemical or physical treatments through activators such as chemicals (acids, alkalis, salts) or gases (i.e., steam, CO₂, N₂) [103,104]. Biochars, instead, were initially defined as carbonized biomass (formed under a low O₂ environment from agricultural residue, animal waste, or refuse of woody plants) and they can also be subjected to further physical–chemical transformations to improve surface features [103,104]. For both active carbons and biochars, the raw material, together with the production technique and operating temperature, have an important influence on product yield and composition [101]. In their review, for instance, Ribeiro et al. [105] discussed the various activities of carbon materials as catalysts on their own and hybrid magnetic carbon nanocomposites in catalytic wet peroxide oxidation reactions for the degradation of organic pollutants. Apart from purely iron-based compounds that are currently being developed [106,107], several other doping/functionalizing moieties have been used on biochar and active carbons, such as Ag₃PO₄ [108], MnO₂, nitrogen [109], boron [110], B/N/graphene [111], N/S/Fe [112], FeAl-layered double hydroxide [113], copper oxides [114,115,116], CuNi [117], and Sr/Ce [118].

Other efforts to improve the morphological and textural features have been made by acting directly on inorganic active phases, as in the examples of entries 10 and 11 in Table 1, where crystalline porous solids [119] (zeolites and a metal–organic framework, MOF) have been used, taking advantage of to their large surface area, tunable porosity and chemistry [120,121,122]. In particular, in entry 11, MIL (Materials of Institute Lavoisier) indicates one of the most popular MOFs and MIL-101 (Fe) is the one assembled by 1,4-benzene dicarboxylate ligand and iron(III) cation [123].

2.1.2. Considerations on Chemical Compositions and Stability

Among the inorganic species used as active phases in hydrogen-peroxide-mediated processes, the simplest compositions are zero-valent metal nanoparticles (e.g., Fe⁰, Al⁰, Zn⁰, Cu⁰) [124,125,126,127,128,129,130], which bear strong chemical reducibility, high efficiency, and large specific surface although the main drawbacks are the tendency towards agglomeration and oxidization [131]. Nanoscale zero-valent iron (n-ZVI) is certainly the most utilized in environmental applications, see entry 25 in Table 1. The typical reaction initiating the whole process is the slow-releasing source of dissolved Fe²⁺ in acidic solutions, starting a Fenton-like pathway [41,79,127]. Al⁰ was reported to be able to produce hydrogen peroxide in situ in water at neutral conditions and further decompose H₂O₂ into ^•OH in an acidic solution [125]. Other reaction types that zero-valent metal nanoparticles can support are, for example, simple photocatalysis or ozone-based processes [124,128].

Other important categories of materials exploited in H₂O₂-mediated processes are metal sulfides, metal oxides and mixed oxides. Metal sulfides have been presented in Table 1 (entries 12 and 13) in combination with iron since they can accelerate the Fe³⁺/Fe²⁺ cycle through the exposed reducing metal active sites or directly participate as a co-catalyst in the reaction, activating the oxidant to generate active radicals [41]. However, these compounds can be employed as principal active phases. According to the previous findings, there are two general mechanisms commonly proposed for metal sulfide-based AOPs. One is the improvement of the electron transfer efficiency resulting from the reductive S²⁻ on the catalyst surface; the other one is the protons’ capture by unsaturated S atoms on the surface of the metal sulfide, resulting in the formation of H₂S and exposition of metal active sites with reducing properties [132]. In general, the behavior of these compounds can be designed by choosing the most adequate metal (Fe, W, Co, Zn, Mo, Pb, Cr, Cu) in the sulfide since this factor has been reported to influence the performances [41,132]. An example of a further fine-tuning effort was conducted by Bai et al. [133], who synthesized MoS₂ photocatalysts with sulfur vacancies that in situ produced H₂O₂ and hydroxyl radicals. The reaction was favored by the transformation of variable metal molybdenum atom, Mo(IV)/Mo(VI), that effectively enhanced the degradation efficiency towards tetracycline and diclofenac.

Among binary oxides, one of the first and most studied is TiO₂, which is very well known as a semiconductor photocatalyst, as explained in the Introduction. The limitations of this system are mainly attributable to the high band gap of TiO₂, which requires UV light to trigger the titania activity, and the rapid recombination of e⁻-h⁺ pairs, with consequent photocatalytic efficiency reduction and radiation energy loss [134]. For this reason, many efforts have been made to overcome these issues through metal and non-metal doping, surface modification, and fabrication of composites with other materials. Iron and/or H₂O₂ assistance is a popular strategy in the literature [135,136,137]; here, two significant examples of the combination of titania with iron, H₂O₂ and ultrasound to start different Fenton-like processes are reported in Table 1, entries 14 and 15. More detailed information on the modulation titania band gap, surface reactivity, charge transfer, and stability, among other properties, has been extensively reported in ad hoc reviews [138,139,140,141,142].

To break free from titanium oxide in photocatalysis and iron oxides in Fenton-like systems (entries 5–9, 16 and 17 in Table 1), a plethora of other binary oxides and their various combinations have been developed, such as cobalt, copper, zinc, cerium, tin, and tungsten oxides [143,144,145,146,147,148,149,150,151,152]. Different but interesting studies also concern metal peroxides able to release H₂O₂, as recently reported for MgO₂ in dye degradation and bacterial decontamination [153], and for CaO₂ in trichloroethylene abatement [154].

Mixed oxides in different crystalline structures, such as perovskite (ABO₃), scheelite (ABO₄) and spinel types (AB₂O₄) [155,156,157], have been widely employed in environmental depollution (entries 18–20 in Table 1). They are exploited for their ability to degrade pollutants both in the presence and in the absence of hydrogen peroxide through their inherent semiconducting and redox properties, or by forming heterojunction structures. Often, they are employed as photocatalysts and electrocatalysts [158,159,160,161,162,163,164,165]. These materials with at least two metal elements in the basic structure have the advantage of being regulable in many characteristics, starting from the composition, which can bear multiple doping elements, creating several combinations. Moreover, in many studies, the possibility of inducing non-stoichiometry and charge compensation mechanisms is considered another important tool to create defects such as oxygen vacancies inducing high oxygen mobility, unusual element valence states, such as Fe(IV), and active redox couples [18]. For perovskite-type oxides, for instance, activity descriptors based on the occupancy by electrons in the orbitals of the active metal were proposed to explain material catalytic properties [166].

In general, when oxide materials are involved, catalytic activities have also been related to the degree and symmetry of the crystalline structure [80,167].

Regarding the use of magnetic iron-based oxides, they have been presented as a convenient tool not only to perform heterogeneous Fenton-like reactions, but also to allow for a better and easier separation from the water medium [50]. Many works have described these systems; for instance, in Table 1, they are exemplified in entries 6, 7, 9, and 17. Actually, the possibility of recovery, regeneration and reuse of the active phase in decontamination procedures is a fundamental parameter to define their sustainability. This aspect has been stressed in many studies and is highlighted in Table 1 (entries 1, 6–10, 13, 14, 16, 17, 22).

As partially anticipated in Section 2.1.1 discussing 2D carbon materials, such as graphitic carbon nitride, a novel frontier of H₂O₂ generation through photocatalysis is the replacement of traditional inorganic photocatalysts with organic ones, addressing remarkable advantages, such as narrow bandgap, adjustable band edge potentials, ability to control surface configurations, and tunability of structural units to promote efficient charge separation and transfer [168]. Various strategies to improve the activity and stability of organic photocatalysts have been explored, such as the construction of donor–acceptor structures, the design of conjugated structures, the incorporation of heteroatoms, the enhancement of the internal electric field, and the substitution of functional groups. Some examples of these compounds considered by Zhang et al. [168] are resorcinol-formaldehyde resins, poly(3-(4-ethylnylphenyl)ethynyl)pyridine, and Covalent Heptazine Frameworks, among others. Hydrogen peroxide generation was reported to occur by the anthraquinone-mediated oxygen reduction reaction (ORR), the radical-related ORR, the water oxidation reaction (WOR), and the dual ORR and WOR pathways.

Moreover, the role of natural-inspired organic substances as coadjutants in catalytic and Fenton-like reactions for water depollution has been explored both in homogeneous and heterogeneous systems. In particular, humic-like substances (HLSs), given the intrinsic influence of humic acids in natural ecosystems, have been investigated [4]. In a recent review, García-Ballesteros et al. [169] pointed out that HLSs, which can even be isolated from different wastes, are photosensitive and are able to generate ROS such as hydroxyl radicals and singlet oxygen or triplet excited states upon irradiation (Equations (21)–(24)) [4,170]. This circumstance is particularly convenient when HLSs are complexed with iron to promote homogeneous (photo)-Fenton-like processes at mild pH, preventing Fe deactivation [171].

HLS + hν → ³HLS*

(21)

HLS + hν → HLS^+● + e⁻_aq

(22)

³HLS* + H-Substrate → HLS^● + Substrate^●

(23)

HLSH^● + O₂ → HLS + HOO^●

(24)

Novel hybrid materials for heterogeneous catalysis have also been synthesized using HLSs to induce photocatalytic properties [172,173]. In some cases, these materials were produced by combining HLSs with iron-based oxides to drive heterogeneous (photo)-Fenton (or mixed heterogeneous/homogeneous) processes (as in entries 6 and 19 in Table 1) [170].

The effects of ligands in heterogeneous Fenton-like systems have also been explored with different chemical substances, like organic acids (entry 23 in Table 1). For instance, EDTA (ethylenediaminetetraacetic acid) was found to be able to enhance the activation rate of H₂O₂ via the decrease in the redox potential of the Fe(III)/Fe(II) pair. Additionally, it improved the utilization efficiency of hydrogen peroxide by preserving the electron-rich Fe(II) under the attack of O₂^●− and HOO^● [174]. Also, in the case of semiconductor oxides, the influence of ligands and complexes has been studied to enhance the photocatalytic performances [175,176,177], and this further underlines the importance of even more careful engineering of materials to make decontamination processes more efficient.

Lastly, the modulation of the physical–chemical properties of catalysts has also been carried out by fine-regulating, during the synthesis, both the operating parameters and the precursors’ types, which act to stabilize the final materials as well as impart specific properties, for instance, specific surface charge and exposed functional groups [178,179,180] (see entries 7, 9 and 19 in Table 1). A simplified roadmap for the development of functional materials for H₂O₂-mediated processes is summarized in Figure 5.

2.2. Enzyme-Driven Processes

2.2.1. Glucose-Oxidase-Based Bio-Fenton

The applications of the Bio-Fenton approach in water remediation are increasing since the range of treatable pollutants is quite broad. In Bio-Fenton remediation processes, the hydrogen peroxide necessary to activate the Fenton reaction is mainly provided by an enzymatic reaction catalyzed by Glucose Oxidase (GOx) that occurs in mild pH conditions. GOx is a stable oxidoreductase able that use oxygen as an electron acceptor to catalyze β-d-glucose to d-glucono-δ-lactone by using the coenzyme FAD (Flavin Adenine Dinucleotide) as an electron carrier [181]. The oxidation of β-d-glucose leads to the reduction of one FAD molecule to the hydrogenated form FADH₂; successively, d-glucono-δ-lactone is non-enzymatically hydrolyzed to gluconic acid and the reduced coenzyme is re-oxidized to FAD by a molecule of dioxygen, producing a molecule of H₂O₂ for each reaction cycle, as in Figure 6.

GOx in vivo is usually coupled with Catalase, which rapidly causes the dismutation of H₂O₂; on the contrary, in environmental applications, GOx is mainly exploited when coupled to a system able to activate Fenton or Fenton-like reactions. Table 2 provides a summary of recent studies concerning several classes of environmental contaminants removed by the Bio-Fenton reaction. To optimize the hydrogen peroxide production, different aspects have to be taken into consideration. The amount of H₂O₂ produced depends on both the concentration of the biocatalyst and glucose. Indeed, according to the Michaelis–Menten model, at low concentrations of glucose, the rate of H₂O₂ production is linearly proportional to its initial amount, but the reaction rate decreases when the glucose concentration increases until reaching a maximum value, which, in turn, depends on the concentration of GOx [48,183].

Huang and co-workers [184] highlighted the complex role of glucose in the Bio-Fenton reaction, reporting that, at a too-high concentration, it could also act as a hydroxyl radical scavenger, inhibiting the oxidation reaction. The optimization of hydrogen peroxide generation could be obtained, therefore, by using specific techniques, which allow for the evaluation of the influence of different variables on the reaction outcomes [185].

Gluconic acid formed from the oxidation of glucose makes the process a chelate-Bio-Fenton, since it is a well-known chelating agent for ferrous and ferrate ions and is also used in non-enzymatic AOP processes to avoid precipitation of iron hydroxides at neutral pH [186]. Ahuja et al. [49] observed a 100% Fe(III) chelation with a ligand–ion ratio of 1:1 at pH 4 and an increasing concentration of chelate complexes with pH, demonstrating the advantage of using Bio-Fenton at circumneutral pH. The same authors [49] also demonstrated that the gluconate-complex could directly react with H₂O₂ or be the source of iron ions.

In order to ensure high efficiency in milder conditions, the Bio-Fenton reaction can be performed with iron citrate instead of iron sulfate [187,188,189,190]. However, in general, the introduction of iron as a homogeneous reactant can lead to the formation of iron-containing sludge [191,192]. Therefore, heterogeneous Fe sources such as iron oxides and oxyhydroxides, like Fe₃O₄ (magnetite), γ-Fe₂O₃ (maghemite), α-Fe₂O₃ (hematite), α-FeOOH (goethite) [193], or green rust [194] have also been employed. These solids prevent sludge formation, improve Fenton efficiency in a wider range of pH, and allow for their reuse.

A change in concentration and availability of iron ions with pH is a fundamental factor that influences the efficiency of the Fenton reaction [195]; furthermore, it is well known that pH affects the enzymatic activity. Wang et al. [196] found that the removal of trichloroethene by a Bio-Fenton reaction carried out with GOx immobilized on magnetic particles at the equilibrium state did not depend on the pH, but this parameter affected the reaction rate that decreased by approximately 60% when the pH increased from 3.6 to 9. Liu and co-workers [197] demonstrated that strongly acidic and alkaline environments significantly inhibited the 4-chlorophenol degradation, decreasing from 87.6% at pH 7 to only 55.2% at pH 3 and 35.8% at pH 11. This degradation trend follows the hydrogen peroxide production, which is minimal at pH 3 and 11, while it has a maximum at pH 7. The authors underlined that during the Bio-Fenton reaction, whatever the initial pH value, there was an increase in the acidity of the solution due to the production of gluconic acid. Therefore, the efficiency of the reaction was only maintained if the variation in pH was limited within the activity range (4.5–7).

Another crucial factor is the temperature. An optimal range between 25 and 60 °C for GOx catalytic capability was reported. As for the pH, the temperature mainly influences the kinetics of the reaction more than its final outcome. Huang et al. [184] found a 2.8-fold increase in k_obs for trichloroethene degradation when the temperature increased from 15 to 40 °C. A similar trend was also reported by Karimi et al. [198] for the decolorization of malachite green; in this case, a slight reduction in the decolorization kinetic was already observed at 35 °C, probably due to a partial denaturation of the enzyme. On the contrary, Wang et al. [196] showed that enzyme immobilization could improve enzyme thermal stability, reporting for GOx immobilized on magnetic particles an enhancement in activity with increasing the temperature from 15 to 40 °C, with consequent reduction in the time needed to reach the equilibrium from 96 h to 24 h.

The coupled effect of thermal stability and improvement in the enzymatic activity was also reported by Zhao et al., who found that 3,4-dimethylaniline was removed with a higher rate by GOx immobilized on Kaoline (Kaoline@GOx) than a free GOx system [183]. The authors suggested that this effect was due to an enhancement in GOx spatial conformation when the enzyme was fixed on the support.

Recently, some studies have reported an improvement in Bio-Fenton efficiency when appropriate UV-light irradiation was applied. Liu and co-workers [197] observed the evolution of hydrogen peroxide produced by GOx immobilized on Kaoline in the presence of organic green rust as a source of iron. Changing the UV-light intensity, the equilibrium concentration of H₂O₂ first increased with the irradiation (from 0 to 150 µW cm⁻²) and then decreased with further raising the irradiance up to 400 µW cm⁻². Consequently, the degradation percentage of 4-chlorophenol followed the same trend. The authors proposed that the mechanism at the basis of the variation in the enzymatic activity could be related to the photo-excitation of the cofactor FAD into FAD*, which induced a glucose oxidase stronger reactivity. Indeed, the FAD cofactor shows high absorption capacity at 250–300 nm and strong UV light tolerance [199]. Obviously, excessive light intensity causes enzyme inhibition and denaturation.

Very few works investigate the Bio-photo-Fenton strategy for environmental remediation. Ghatge and co-workers [200] studied the degradation of sulfonated polyethylene employing glucose oxidase immobilized on TiO₂. They demonstrate the synergy between Bio-Fenton and light irradiation in the production of reactive species following the concentration of the degradation products. Indeed, compared to the Bio-Fenton degradation using free GOx, 21-fold and 17-fold higher amounts of acetic acid and butanoic acid were, respectively, released by the Bio-photo-Fenton reaction after incubation for 6 h.

Table 2. GOx Bio-Fenton systems for pollutant removal mediated by H₂O₂ (acronym not explicated elsewhere: COD = Chemical Oxygen Demand, U = unit).

Pollutant	Performances	Conditions	Notes	Ref
1.Trichloroethene (TCE)	Removal after 192 h - TCE 5 mg L−1: 76.2% - TCE 50 mg L−1: 94.1%.	200 mg magnetic nanoparticles (MIG); 2.5 mM Glucose.	- Gox immobilized on MIG - Recycled for 4 cycles - Effectiveness in the ranges of T 15–45 °C and of pH 3.6–9.0 - Influence of inorganic ions: Ca2+ >Mg2 + >Cu2+ and H2PO4−> Cl−> SO42−.	[196]
2. 4-chlorophenol (4-CP)	Removal after 250 min - GOx@Kaolin/OGR: 77.5% - GOx@Kaolin/OGR/UV: 96.1%.	4-CP: 5 mg L−1 UV: 150 μW cm−2 T: 25 ± 1 °C OGR: 250 mg L−1 GOx@Kaolin: 2.5 U mL−1 Glucose: 5 mmol L−1.	- GOx immobilized on Kaolin (GOx@Kaolin) coupled with organic green rust (OGR, source of iron) and enhanced by UV light - Reusability 6 cycles.	[197]
3.Trichloroethylene (TCE)	- After 24 h at pH 7 Ground water: 30% or 48% if doped with additional H2O2 (after 3 h).	TCE: 60 mg L−1 Glucose: 60 mM GOx: 1 mg mL−1 Fe: 25 mg L−1.	- GOx in solution - Organic matter of ground water acts as a radical scavenger.	[201]
4. Polycyclic aromatic hydrocarbons (PAHs): Naphthalene (NAP), Anthracene (ANT), Pyrene (PYR)	- After 48 h, neutral pH, removal%: 95.1%, 75.4%, and 85.2% for NAP, ANT, and PYR - COD reduction%: 28.6%, 13.8%, and 30.8% for NAP, ANT, and PYR.	PAH: 50 mg L−1 each GOx: 10 U Fe(III)citrate: 0.1 mM Glucose: 2 mM.	- GOx in solution - Evaluation of PAHs concentration and COD to evaluate the mineralization - Bio-Fenton as pre-treatment to enhance PAHs removal by activated sludge: COD removal from 33% to 72%.	[189]
5. Bisphenol A (BPA)	After 10 days of incubation Removal of 80%.	BPA: 0.1 mM GOx: 10 U Glucose: 32 mM Fe(III)citrate: 0.5 mM pH 5.3.	- GOx in solution.	[188]
6. Chloro-acetanilide herbicides: acetochlor, alachlor, metolachlor, propachlor, butachlor	After 5 days, degradation %: acetochlor: 72.8% alachlor: 73.4% metolachlor: 74.0% propachlor: 47.4% butachlor: 43.8%.	GOx: 10 U pH 5.5 Fe(III)citrate: 0.5 mM Glucose: 32 mM Each herbicide 0.1 mM.	- GOx in solution - Influence of chemical structure of herbicides and particularly of R-O-R’ groups.	[187]
7. Trace organic contaminant (TrOCs) (mefenamic acid, ketoprofen, caffeine, carbamazepine, trimethoprim, fenofibrate, diuron, carbendazim, thiabendazole)	After 360 min pH 7 mefenamic acid: 68.54% ketoprofene: 44.7% caffeine: 36.1% carbamazepine: 44.1% trimethoprim: 46.4% fenofibrate: 20.3% diuron: 89.4% carbendazim: 73.1% thiabendazole: 88.9%.	Glucose: 1 M GOx: 100 U mL−1 pH 7 50 µg L−1 mix of TrOCs T: 30 °C H2O2:FeSO4 = 50:1.	- GOx produced from Aspergillus niger using Casuarina equisetifolia biomass in a pilot-scale - Municipal wastewater as a matrix - Influence of the rate H2O2: FeSO4.	[185]
8. Trichloroethylene (TCE)	After 8 h Removal of 78%.	Glucose: 2.5 mM Fe(II): 0.5 mM GOx: 10 U mL−1.	- GOx in solution - Efficiency maintained in the pH range 3–6 and in a T range 15–30 °C.	[184]
9. Sulfonated polyethylene (SPE)	After 6 h with free GOx, concentration degradation products: Acetic acid: 0.22 mM Butanoic acid: 0.01 mM; After 6 h with TiO2-GOx: degradation product conc.: Acetic acid 4.78 mM Butanoic acid 0.17 mM.	GOx free: Glucose: 32 mM GOx: 1 U mL−1 pH 5.5, T: 30 °C SPE: 1 mg mL−1; TiO2-GOx: As above, except for 10 U of TiO2-GOx Xe lamp 150 W, 400 nm cut off.	- GOx both free and immobilized on TiO2 particles - Degradation follows studying the product formation (acetic acid, butanoic acid, isovaleric acid, 1,2-ethanediol monoacetate).	[200]
10. Atrazine (ATZ)	After 360 min 72.8% removal.	Phosphate buffer: 5 mM GOx: 10 μmol min−1 Glucose: 3 mM Ferric citrate: 0.5 mM ATZ: 0.1 mM pH 5.8.	- GOx in solution - Toxicological assay and by-products study.	[190]
11. 3, 4-Dimethylaniline (3, 4 DMA)	After 180 min, removal of 86.55%.	3, 4-DMA: 30 mg L−1 Green Rush: 1 mM in Fe(II) and 1 mM in Fe(III) Glucose: 5 mM pH 7 Kaolin@GOx: 2.5 U mL−1.	- GOx immobilized on Kaolin (Kaolin@GOx) - Organic green rust as a source of iron.	[183]

Table 2. GOx Bio-Fenton systems for pollutant removal mediated by H₂O₂ (acronym not explicated elsewhere: COD = Chemical Oxygen Demand, U = unit).

Pollutant	Performances	Conditions	Notes	Ref
1.Trichloroethene (TCE)	Removal after 192 h - TCE 5 mg L−1: 76.2% - TCE 50 mg L−1: 94.1%.	200 mg magnetic nanoparticles (MIG); 2.5 mM Glucose.	- Gox immobilized on MIG - Recycled for 4 cycles - Effectiveness in the ranges of T 15–45 °C and of pH 3.6–9.0 - Influence of inorganic ions: Ca2+ >Mg2 + >Cu2+ and H2PO4−> Cl−> SO42−.	[196]
2. 4-chlorophenol (4-CP)	Removal after 250 min - GOx@Kaolin/OGR: 77.5% - GOx@Kaolin/OGR/UV: 96.1%.	4-CP: 5 mg L−1 UV: 150 μW cm−2 T: 25 ± 1 °C OGR: 250 mg L−1 GOx@Kaolin: 2.5 U mL−1 Glucose: 5 mmol L−1.	- GOx immobilized on Kaolin (GOx@Kaolin) coupled with organic green rust (OGR, source of iron) and enhanced by UV light - Reusability 6 cycles.	[197]
3.Trichloroethylene (TCE)	- After 24 h at pH 7 Ground water: 30% or 48% if doped with additional H2O2 (after 3 h).	TCE: 60 mg L−1 Glucose: 60 mM GOx: 1 mg mL−1 Fe: 25 mg L−1.	- GOx in solution - Organic matter of ground water acts as a radical scavenger.	[201]
4. Polycyclic aromatic hydrocarbons (PAHs): Naphthalene (NAP), Anthracene (ANT), Pyrene (PYR)	- After 48 h, neutral pH, removal%: 95.1%, 75.4%, and 85.2% for NAP, ANT, and PYR - COD reduction%: 28.6%, 13.8%, and 30.8% for NAP, ANT, and PYR.	PAH: 50 mg L−1 each GOx: 10 U Fe(III)citrate: 0.1 mM Glucose: 2 mM.	- GOx in solution - Evaluation of PAHs concentration and COD to evaluate the mineralization - Bio-Fenton as pre-treatment to enhance PAHs removal by activated sludge: COD removal from 33% to 72%.	[189]
5. Bisphenol A (BPA)	After 10 days of incubation Removal of 80%.	BPA: 0.1 mM GOx: 10 U Glucose: 32 mM Fe(III)citrate: 0.5 mM pH 5.3.	- GOx in solution.	[188]
6. Chloro-acetanilide herbicides: acetochlor, alachlor, metolachlor, propachlor, butachlor	After 5 days, degradation %: acetochlor: 72.8% alachlor: 73.4% metolachlor: 74.0% propachlor: 47.4% butachlor: 43.8%.	GOx: 10 U pH 5.5 Fe(III)citrate: 0.5 mM Glucose: 32 mM Each herbicide 0.1 mM.	- GOx in solution - Influence of chemical structure of herbicides and particularly of R-O-R’ groups.	[187]
7. Trace organic contaminant (TrOCs) (mefenamic acid, ketoprofen, caffeine, carbamazepine, trimethoprim, fenofibrate, diuron, carbendazim, thiabendazole)	After 360 min pH 7 mefenamic acid: 68.54% ketoprofene: 44.7% caffeine: 36.1% carbamazepine: 44.1% trimethoprim: 46.4% fenofibrate: 20.3% diuron: 89.4% carbendazim: 73.1% thiabendazole: 88.9%.	Glucose: 1 M GOx: 100 U mL−1 pH 7 50 µg L−1 mix of TrOCs T: 30 °C H2O2:FeSO4 = 50:1.	- GOx produced from Aspergillus niger using Casuarina equisetifolia biomass in a pilot-scale - Municipal wastewater as a matrix - Influence of the rate H2O2: FeSO4.	[185]
8. Trichloroethylene (TCE)	After 8 h Removal of 78%.	Glucose: 2.5 mM Fe(II): 0.5 mM GOx: 10 U mL−1.	- GOx in solution - Efficiency maintained in the pH range 3–6 and in a T range 15–30 °C.	[184]
9. Sulfonated polyethylene (SPE)	After 6 h with free GOx, concentration degradation products: Acetic acid: 0.22 mM Butanoic acid: 0.01 mM; After 6 h with TiO2-GOx: degradation product conc.: Acetic acid 4.78 mM Butanoic acid 0.17 mM.	GOx free: Glucose: 32 mM GOx: 1 U mL−1 pH 5.5, T: 30 °C SPE: 1 mg mL−1; TiO2-GOx: As above, except for 10 U of TiO2-GOx Xe lamp 150 W, 400 nm cut off.	- GOx both free and immobilized on TiO2 particles - Degradation follows studying the product formation (acetic acid, butanoic acid, isovaleric acid, 1,2-ethanediol monoacetate).	[200]
10. Atrazine (ATZ)	After 360 min 72.8% removal.	Phosphate buffer: 5 mM GOx: 10 μmol min−1 Glucose: 3 mM Ferric citrate: 0.5 mM ATZ: 0.1 mM pH 5.8.	- GOx in solution - Toxicological assay and by-products study.	[190]
11. 3, 4-Dimethylaniline (3, 4 DMA)	After 180 min, removal of 86.55%.	3, 4-DMA: 30 mg L−1 Green Rush: 1 mM in Fe(II) and 1 mM in Fe(III) Glucose: 5 mM pH 7 Kaolin@GOx: 2.5 U mL−1.	- GOx immobilized on Kaolin (Kaolin@GOx) - Organic green rust as a source of iron.	[183]

2.2.2. Bi-Enzymatic Processes for Water Treatment

Enzymatic Cycles Activated by H₂O₂

Many enzymes require H₂O₂ to carry out their catalytic activity. Among them, peroxidases are the most studied for environmental applications.

Peroxidases are able to bind H₂O₂ to their Fe-heme prosthetic group. In the resting state, the Fe(III) is coordinated by four nitrogen atoms belonging to heme pyrrolic groups and one N atom of the side chain of an amino acid. As shown in Figure 7, the fifth ligand can be the nitrogen of the distal histidine or the sulfur of a cysteine residue. The sixth coordination position is usually free or occupied by a water molecule [202], which is easily substituted by an H₂O₂ molecule. In a typical peroxidase active site, the distal side of the heme is characterized by the presence of one histidine and one arginine residues, which stabilize the coordination with the H₂O₂ molecule and favor the subsequent steps of the catalytic cycle [203,204].

In the presence of organic or inorganic substrates, H₂O₂ oxidizes the Fe(III) to a ferryl (Fe(IV)=O) radical, which can be reduced by inducing two steps of one-electron oxidation of a wide variety of compounds with the production of two molecules of organic or inorganic radicals and water (Figure 8A) [205].

A quite different mechanism was reported for the heme-thiolate peroxidases, such as chloroperoxidase (CPOs) and aromatic-peroxygenase (APOs).

CPO from Caldariomyces fumago fungi was the first discovered halogenating enzyme [206,207]. In addition to substrate halogenation, it also exhibits peroxidase, catalase, and cytochrome P450-like activities [208]. The catalytic cycle leads to halide (chloride, bromide, and iodide, but not fluoride) oxidation, but this is not fully understood.

Wagenknecht and Woggon [209] proposed the mechanism reported in Figure 8B, in which the ferryl cation radical reacts with halides, reducing to Fe(III) and forming a complex capable of catalyzing the chlorination of the substrate. The Fe(V)=O reduction is favored by the three amino acid residues (GLU183, HIS105 and ASP106) that directly take part in its protonation. In the absence of halides, this enzyme could act as a normal peroxidase and follow the scheme in Figure 8A.

APOs are peroxygenases, enzymes able to transfer oxygen from peroxides to organic substrates (aliphatic, aromatic and heterocyclic) [210]. For these reasons, they are very useful for organic functionalization and synthesis reactions.

Dye-decolorizing peroxidases (DyPs) are probably the most recent heme-containing enzyme family discovered. Indeed, they were isolated in 1999 from the basidiomycetous fungus Bjerkandera adusta Dec 1 strain [211]. DyPs are so called due to their capability to oxidize synthetic high-redox potential anthraquinone-type dyes [212], but they can also react with other substrates like β-carotene and aromatic sulfides [213]. This different catalytic activity is probably related to some differences in the active site, which contains an aspartate residue in place of the histidine on the distal side [213], and a different arrangement of the secondary structure with the presence of β-sheets. Recent studies on the catalytic cycle suggest an electron transfer from the active site to the enzyme’s surface occurs, allowing for the oxidation of large molecules that cannot physically enter the active site and favoring the relatively low specificity of these enzymes. This long electron transfer pathway seems to involve tryptophan and tyrosine residues that can form stable radicals and directly participate in the transfer [214].

Glucose Oxidase Coupling with Peroxidases

There is extensive research about the employment of free enzymes, such as peroxidases, for environmental applications [215], but in the last few decades, articles relating to immobilized enzymes have become increasingly widespread. As reported by Rigoletto et al. [216], different inorganic supports have been developed not only to improve enzymes’ stability but also to produce in situ H₂O₂ to initiate the catalytic cycle [217,218,219].

It is well known that peroxidases could be inactivated by high concentrations of hydrogen peroxide [220,221,222]; therefore, a controlled in situ H₂O₂ production could be useful to avoid these drawbacks and maintain a stable enzymatic activity.

The possibility of exploiting the enzymatic generation of hydrogen peroxide coupling a peroxidase with GOx in a self-sustaining bi-enzymatic system has become widely studied, mainly for bio-sensing [223,224], but also more recently for environmental application. Although, in some studies, GOx is added to the solution as a homogeneous reagent together with glucose [225], the most widespread trend is to immobilize GOx on a solid support or to co-immobilize it with peroxidase in order to guarantee its reuse.

Co-immobilization of enzymes could enhance the system performances due to a reduction in mass transfer between peroxidase and H₂O₂. Furthermore, the formation of gluconic acid induced by GOx in the peroxidase microenvironment helps to reach a favorable pH value for peroxidase, thereby accelerating the reaction and enhancing the catalytic cycle. The resulting consumption of H₂O₂ prevents its accumulation on GOx, avoiding its inactivation [226,227]. This synergy ensures improved system performances, as reported by Gao and co-workers, who observed a higher Orange G decolorization with GOx and CPO co-immobilized on magnetic graphene oxide (MGO) with respect to those obtained employing GOx and CPO immobilized individually [228]. The co-immobilization on magnetic material also permitted an easy recovery of the support and the reusability of the system. Indeed, the authors reported that GOx-CPO-MGO could be employed for six cycles of water treatment with more than 38% residual activity retained after the last cycle.

Enhanced performances of co-immobilized enzymes have also been reported by Gu and co-workers [229]. The authors employed dopamine and modified cellulose–chitosan composite beads for a covalent loading of Horseradish peroxidase (HRP) and GOx and tested them to degrade acridine in wastewater, obtaining an almost complete removal. However, the removal efficiency was reduced from 99.0% to 61.2% after six reaction cycles; the decrease was tentatively ascribed to the mediator attack on the amino acid groups on the surface of the bi-enzyme system or to the loss of enzymes during application and washing.

Studies on reusability and stability for long storage periods are fundamental to estimating the real applicability of the bi-enzymatic systems in industrial applications. Actually, the literature confirms the possibility of adopting this strategy for the treatment of a large number of contaminants, from dyes [230,231] to other organic contaminants such as ferulic and caffeic acids [232], Bisphenol A [226], and endocrine disruptors [233].

3. Detection Methods for H₂O₂

In this work, different strategies for in situ hydrogen peroxide production have been identified. Important tools for the optimization of these reactions are represented by H₂O₂ detection and quantification methods.

According to most of the studies cited in this review, the commonest strategies employed in the environmental field are colorimetric assays. They are widely used because of their simple setup and easy spectrophotometric detection. They are usually based on chemical reactions involving hydrogen peroxide as a reagent with the formation of colored products. A summary of the literature concerning these methods is shown in Table 3.

Some colorimetric assays date back to the 1980s, but they are still employed, such as the I³⁻ assay, in which the H₂O₂ concentration is determined following the changes in absorption at 352 nm wavelength [57,133,184,234,235], or the reaction between 4-aminoantipyrine and phenol in the presence of peroxidase where the maximum absorption at 505 nm of the red product is monitored [234,236,237]. Another method is the vanadate one, based on the measure of 450 nm [238,239,240]. Moreover, some colorimetric assays are commercialized as a kit, like the Amplex^TM red hydrogen peroxide/peroxidase assay kit [241] (Thermofisher, Waltham, MA, USA) and the PAKTEST [242] (Kiouritsu Chemical Check Lab., Yokohama, Japan). However, not all colorimetric methods are exploitable for monitoring hydrogen peroxide production during AOP treatments. For example, permanganate titration cannot be used since Fe²⁺ ions react with permanganate, affecting H₂O₂ detection [240].

Recently, enzyme-mimetic materials have been developed for luminescent sensing of H₂O₂ in bioassays. They show a very low limit of detection (LOD) and are based on fluorescence [243,244].

Another important class of hydrogen peroxide detection methods is represented by electrochemical sensors in which H₂O₂ can be oxidized or reduced on the electrodes’ surface. They show higher sensitivity and selectivity, cost-effectiveness, relatively shorter response time, and miniaturization capabilities. The research in this field is focused on overcoming drawbacks such as overpotentials and low kinetics by modifying the electrode surfaces with different materials, including dyes, redox proteins, metal oxides, and redox polymers, among others [245]. Recently, the interest in nanomaterials has grown and there is no shortage of combinations with the materials listed above that can improve H₂O₂ electrocatalytic sensing.

As seen in the previous paragraphs, a wide range of heme-containing proteins exists in nature that can use H₂O₂ in their catalytic cycles (peroxidases, cytochrome-c, hemoglobin, myoglobin, hemin, etc.), and, for this reason, they are also exploitable in H₂O₂ detection. Different studies suggest that HRP-based electrodes show good affinity with H₂O₂ and noticeable performances [246,247]. In order to ensure an optimized electron transfer, these proteins are often immobilized on highly conductive materials, both organic and inorganic, with a large surface area [248]. Interesting examples of these materials that also find application in the bio-medical sector are gold nanoparticles [249,250,251], nanometric metal oxides [252,253], metal nanoparticle–MOF [254,255,256,257], carbon nanotube–MOF [258] and metal nanoparticle–polymers [259,260].

Despite their efficiency, redox proteins could degrade over time and in harsh environmental conditions. To overcome these drawbacks, the development of inorganic materials with enzyme-mimetic activity is becoming more and more attractive. Most of these inorganic materials are mono- and bi-metals nanoparticles (AuNPs, Ag-NPs, Pt-NPs, Ag-AuNPs, Ni-CoNPs, etc.) or transition metals oxides and, often, to avoid aggregation on the electrode surface, they have been dispersed in polymeric matrices or introduced in MOFs. A broad description of these electrodes can be found in the comprehensive review carried out by Duanghathaipornsuk and co-workers [248].

Among electrochemical sensors with peroxidase-like activity, those based on Prussian Blue (PB) and its Analogues (PBAs) are largely used. They are metals hexacyanoferrate and can catalyze the reduction of H₂O₂ at low potentials. There is a wide variety of PBAs containing one or two transition metals that occupy the outer coordination sphere and that define the selectivity for the detection of different analytes, both organic and inorganic [261]. PBAs employed for H₂O₂ detection usually contain iron coupled with another transition metal like Ni [262,263], Co [264,265] or Mn [264,266].

With the enhancement of nanotechnologies, researchers developed PB nanoparticles [267] to improve pH stability and sensitivity. Zang and co-workers studied the influence of nanoparticle size on electrocatalytic performance, finding that the sensitivity drops exponentially with increasing particle dimensions [268]. These nanozyme-based electrodes could also be affected by PB degradation during the electrochemical H₂O₂ reduction due to the solubilization of ferric hexacyanoferrate induced by the formation of reactive species [269]. Since this could limit the real use of the sensor, stabilization strategies, such as nanoparticles’ coverage or dispersion in different polymers, have been designed. For example, Unzuncar et al. [270] developed a two-layer interface based on carboxymethyl cellulose and poly(3,4-ethylene dioxythiophene) with high stability and sensing behavior, not affected by interfering molecules present in tap water. More detailed syntheses of PB and PBA for electrochemical sensing can be found in some recent reviews [261,271].

Finally, many other enzyme-free electrochemical-based sensors for H₂O₂ are based on a wide range of materials, i.e., metal–organic frameworks [272], carbon nanocomposites [273], graphene oxide [274], and MXenes [275].

It must also be mentioned that some methods employing liquid-chromatographic analysis for the detection and quantification of hydrogen peroxide concentration in solution were developed. Some of them are based on the deprotonation of H₂O₂ to HO₂⁻, which occurs when the pH of the eluent is higher than the acid-dissociation coefficient of H₂O₂ (pKa = 11.6). The revelation via a UV-detector can be carried out after separation in an ionic chromatography column with a LOD of 0.027 mg L⁻¹ [276]. Other studies proposed the use of acidified potassium iodide solution as a mobile phase and the replacement of the reverse phase column with a series of capillary columns. Tantawi et al. [277] found a LOD of 8.29 × 10⁻⁴ mM employing this detection strategy. The authors reported that the method also shows high robustness by maintaining a high regression coefficient and excellent sensitivity in real matrices.

Since H₂O₂-mediated processes lead to the formation of ROS, their quantification is also extremely important for monitoring the depuration stages. Although it is not within the scope of this review, we can cite Electron Paramagnetic Resonance (EPR) and scavenging techniques as the main strategies for ROS identification. Specific papers have been devoted to summarizing the state-of-the-art on these topics [278,279].

Table 3. Spectrophotometric methods for H₂O₂ detection and quantification (acronyms not explicated elsewhere: NADPH = reduced Nicotinamide Adenine Dinucleotide Phosphate; TMB = 3,3′,5,5′-tetramethylbenzidine; ABTS = 2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid).

System	Wavelength	Conditions	Ref
1. Phenolphthalein method	552 nm	- 0.24 mL of phenolphthalein stock solution: 0.02 g mL−1 Phenolphthalein solution containing 10 g of NaOH, 5 g of zinc - CuSO4 solution: 0.48 mL, 0.01 M - 100 mL of water.	[234]
2. Iodide method	352 nm	-2 mL of solution A (66 g L−1 KI; 0.2 g L−1 of (NH4)6Mo7O24; 2 g L−1 of NaOH) - 2 mL of solution B (20 g L−1 of potassium hydrogen phthalate) - 6 mL of Peroxide samples.	[57,133,234,235]
3. Oxidation of NADPH	340 nm	- Phosphate buffer: 0.04 M, pH 7.75 - EDTA: 4 × 10−4 M - Sodium azide: 4.2 × 10−3 M - Glutathione peroxidase: 8 × 10−8 M - Reduced glutathione: 3 × 10−3 M - NADPH: 5.6 × 10−5 M - Glutathione reductase: 1 U -Peroxide sample.	[234]
4. DMAB/MBTH/HRP	590 nm	- 3-(dimethylamino)benzoic acid (DMAB): 5 × 10−4 M - 3-methyl-2-benzothiazolinonehydrazone (MBTH): 2 × 10−5 M - Acetate buffer: 0.1 M, pH 5.5 -HRP.	[280]
5. 4-aminoantypirine/phenol/HRP	505 nm	- 4 mL of 4-aminoantipyrine/phenol reagent (2.34 g L−1 of phenol, 1g L−1 of 4-aminoantipyrine, 0.001 M phosphate buffer pH 6.9, 2.5 µM HRP) - 6 mL of peroxide sample.	[234,236,237]
6. Nanoparticles decorated Ce2(WO4)3 nanosheets (CWNSs)	652 nm	- TMB: 100 µL 8 mM - CWNSs: 70 µL, 1000 µg mL−1 - Phosphate buffer: 400 µL, 50 mM pH 4 - MilliQ water: 330 µL - H2O2: 100 µL - LOD: 0.15 µM.	[281]
7. Ammonium metavanadate	450 nm	- Metavanadate 6.2 mM - Sulfuric acid 0.058 M - LOD: 143 µM.	[238,239,240]
8. Fe3O4 magnetic nanoparticles (MNPs) with peroxidase mimetics	545 nm	- ABTS: 24 µL, 60 mM - Fe3O4 MNPs: 10 µL, 3.74 mg mL−1 - Acetate buffer: pH 4185 µL - H2O2: 24 µL - Incubation 45 °C for 10 min and then diluted with 900 µL of water (after MNPs removal) and analyzed.	[282]
9. Peroxidase-mimicking metal−organic framework containing catalytic Cu2+ and luminescent Tb3+: PA-Tb-Cu MOF (PA = m-phthalic acid)	545 nm Fluorescence (310 nm exciting wavelength)	- Acetate buffer: 960 μL, 10 mM, pH 5.05 - 20 μL of PA-Tb-Cu - MOF suspension: 11.73 mg mL−1 - Ascorbic acid: 10 μL, 200 mM - H2O2 sample + water up to a final volume of 1 mL - Measure after 20 min incubation - LOD: 0.2 µM.	[244]
10. CeO2 nanoparticles doped with Eu3+	590 nm Fluorescence (330 nm exciting wavelength)	- Samples prepared in potassium phosphate buffer (KPi) or Phosphate Buffered Saline (PBS) or 10% Farmigene Stain Buffer (FBS) - 125 g L−1 of nanoparticles - Measure after 30 min of incubation - LOD: 150 nM.	[243]
11. Cobalt/bicarbonate system	260 nm	- Sodium oxalate: 25 μL,16.34 mM - Cobalt chloride: 25 μL, 67.8 mM - 2 mL of sample Finally, the reaction volume is made up to 2.5 mL with 270 μL of saturated sodium bicarbonate solution.	[185,201,283,284]
12. Titanium oxalate	385 nm	- 10 mL of peroxide sample - 1 mL of 1 M sulfuric acid - 1 mL of 50 g L−1 potassium titanium oxalate solution - 13 mL of water - Measure after 5 min of incubation - LOD: 29 µM.	[77,125,285]
13. N,N-diethyl-p-phenylenediamine (DPD)	551 nm	- DPD reagent (27 mL water, 3 mL phosphate buffer, 6 µL methanol, 50 µL of 10 g L−1 of DPD solution prepared in sulfuric acid 0.5 M, 50 µL HRP 1g L−1) - LOD: 0.77 µM.	[64,285,286]
14. p-hydroxyphenyl acetic acid (POHPAA)	406–410 nm Fluorescence (315 nm exciting wavelength)	- POHPAA reagent (POHPAA 270 mg L−1, HRP 30 mg L−1, NaOH 1 M, potassium hydrogen phthalate 8.2 g L−1 pH 5.8) - LOD: 0.16 µM.	[285,287,288]

Table 3. Spectrophotometric methods for H₂O₂ detection and quantification (acronyms not explicated elsewhere: NADPH = reduced Nicotinamide Adenine Dinucleotide Phosphate; TMB = 3,3′,5,5′-tetramethylbenzidine; ABTS = 2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid).

System	Wavelength	Conditions	Ref
1. Phenolphthalein method	552 nm	- 0.24 mL of phenolphthalein stock solution: 0.02 g mL−1 Phenolphthalein solution containing 10 g of NaOH, 5 g of zinc - CuSO4 solution: 0.48 mL, 0.01 M - 100 mL of water.	[234]
2. Iodide method	352 nm	-2 mL of solution A (66 g L−1 KI; 0.2 g L−1 of (NH4)6Mo7O24; 2 g L−1 of NaOH) - 2 mL of solution B (20 g L−1 of potassium hydrogen phthalate) - 6 mL of Peroxide samples.	[57,133,234,235]
3. Oxidation of NADPH	340 nm	- Phosphate buffer: 0.04 M, pH 7.75 - EDTA: 4 × 10−4 M - Sodium azide: 4.2 × 10−3 M - Glutathione peroxidase: 8 × 10−8 M - Reduced glutathione: 3 × 10−3 M - NADPH: 5.6 × 10−5 M - Glutathione reductase: 1 U -Peroxide sample.	[234]
4. DMAB/MBTH/HRP	590 nm	- 3-(dimethylamino)benzoic acid (DMAB): 5 × 10−4 M - 3-methyl-2-benzothiazolinonehydrazone (MBTH): 2 × 10−5 M - Acetate buffer: 0.1 M, pH 5.5 -HRP.	[280]
5. 4-aminoantypirine/phenol/HRP	505 nm	- 4 mL of 4-aminoantipyrine/phenol reagent (2.34 g L−1 of phenol, 1g L−1 of 4-aminoantipyrine, 0.001 M phosphate buffer pH 6.9, 2.5 µM HRP) - 6 mL of peroxide sample.	[234,236,237]
6. Nanoparticles decorated Ce2(WO4)3 nanosheets (CWNSs)	652 nm	- TMB: 100 µL 8 mM - CWNSs: 70 µL, 1000 µg mL−1 - Phosphate buffer: 400 µL, 50 mM pH 4 - MilliQ water: 330 µL - H2O2: 100 µL - LOD: 0.15 µM.	[281]
7. Ammonium metavanadate	450 nm	- Metavanadate 6.2 mM - Sulfuric acid 0.058 M - LOD: 143 µM.	[238,239,240]
8. Fe3O4 magnetic nanoparticles (MNPs) with peroxidase mimetics	545 nm	- ABTS: 24 µL, 60 mM - Fe3O4 MNPs: 10 µL, 3.74 mg mL−1 - Acetate buffer: pH 4185 µL - H2O2: 24 µL - Incubation 45 °C for 10 min and then diluted with 900 µL of water (after MNPs removal) and analyzed.	[282]
9. Peroxidase-mimicking metal−organic framework containing catalytic Cu2+ and luminescent Tb3+: PA-Tb-Cu MOF (PA = m-phthalic acid)	545 nm Fluorescence (310 nm exciting wavelength)	- Acetate buffer: 960 μL, 10 mM, pH 5.05 - 20 μL of PA-Tb-Cu - MOF suspension: 11.73 mg mL−1 - Ascorbic acid: 10 μL, 200 mM - H2O2 sample + water up to a final volume of 1 mL - Measure after 20 min incubation - LOD: 0.2 µM.	[244]
10. CeO2 nanoparticles doped with Eu3+	590 nm Fluorescence (330 nm exciting wavelength)	- Samples prepared in potassium phosphate buffer (KPi) or Phosphate Buffered Saline (PBS) or 10% Farmigene Stain Buffer (FBS) - 125 g L−1 of nanoparticles - Measure after 30 min of incubation - LOD: 150 nM.	[243]
11. Cobalt/bicarbonate system	260 nm	- Sodium oxalate: 25 μL,16.34 mM - Cobalt chloride: 25 μL, 67.8 mM - 2 mL of sample Finally, the reaction volume is made up to 2.5 mL with 270 μL of saturated sodium bicarbonate solution.	[185,201,283,284]
12. Titanium oxalate	385 nm	- 10 mL of peroxide sample - 1 mL of 1 M sulfuric acid - 1 mL of 50 g L−1 potassium titanium oxalate solution - 13 mL of water - Measure after 5 min of incubation - LOD: 29 µM.	[77,125,285]
13. N,N-diethyl-p-phenylenediamine (DPD)	551 nm	- DPD reagent (27 mL water, 3 mL phosphate buffer, 6 µL methanol, 50 µL of 10 g L−1 of DPD solution prepared in sulfuric acid 0.5 M, 50 µL HRP 1g L−1) - LOD: 0.77 µM.	[64,285,286]
14. p-hydroxyphenyl acetic acid (POHPAA)	406–410 nm Fluorescence (315 nm exciting wavelength)	- POHPAA reagent (POHPAA 270 mg L−1, HRP 30 mg L−1, NaOH 1 M, potassium hydrogen phthalate 8.2 g L−1 pH 5.8) - LOD: 0.16 µM.	[285,287,288]

4. Criticalities and Perspectives

The processes based on H₂O₂ reactivity for environmental remediation have been investigated in depth in terms of general lab-scale degrading performances, as revealed from the significant number of research documents reported in this review. However, for applicability in real-scale conditions, other aspects have to be taken into account, whose schematic representation is depicted in Figure 9.

4.1. Product Characterization, Toxicity and Influence of Environmental Factors

The first point concerns the properties’ study of the products formed during the depuration procedure and their impact on the environment. The optimum should be to reach the complete mineralization of the pollutants into water and carbon dioxide, but this is not straightforward, not only because the degradation efficiency can be lower than 100%, but also because some treatments, such as those based on enzymes, occur through radical reactions, which do not lead to the mineralization of the contaminants.

Therefore, toxic intermediates and by-products can be produced [289,290]. In this sense, Hofman-Caris et al. [291] found that UV/hydrogen peroxide processes in drinking water treatment may generate mutagenic by-products in particular conditions, mainly influenced by parameters such as nitrate concentration (that can be subjected to photolysis), the presence of natural organic matter, the UV spectrum of the lamps, and the UV dose applied. The role of UV light has also been described in [292]. The comparison among the toxicity levels of H₂O₂/UV, photo-Fenton and heterogeneous photocatalysis (TiO₂/H₂O₂/UV) processes for the treatment of strongly polluted colored wastewater was analyzed as well [293,294]. The use of the system with titania resulted in the most effective method to remove toxins, organic material, and color.

It is, therefore, necessary not only to identify the formed by-products [187,188], but also to evaluate the toxicity of the resulting solution [190] and estimate COD values as an indirect measurement of pollutant mineralization [189,295]. In this regard, Liu et al. compared the COD values after the classic Fenton treatment and Bio-Fenton pre-treatment, followed by active sludge treatment of real wastewater spiked with chlorophenol [197]. The authors found that the resulting COD after the combined treatment was two times lower than that obtained with classic Fenton only, suggesting that the combination of different approaches improves the biodegradability of the selected contaminant.

Regarding biological contaminants, urban wastewater treatment plants are among the main hotspots of antibiotic resistance spread into the environment. It was found that UV/H₂O₂ processes may not be effective in minimizing the antibiotic resistance spread potential into the environment since the death of bacterial cells, which results in DNA release into treated water, may pose a risk for antibiotic resistance transfer to other bacteria present in the receiving water body [296].

Moreover, the control of the hydrogen peroxide quantity used as the oxidant is important since it was found that toxicity increases by increasing H₂O₂ addition, causing not only its undesirable excess in the effluent, but, in the absence of effective separation from the heterogeneous active phase, also catalyst poisoning [38,293].

In general, applying remediation strategies to real conditions means considering the influence of all the environmental factors that are intrinsic to the complexity of real systems, such as the co-presence of ions and organic matter in water matrices that can compete with the target chemical pollutants [201,297,298].

Indeed, wastewater or natural water contains inorganic ions (Ca²⁺, Mg²⁺, Cl⁻, H₂PO₄⁻, etc.) and dissolved organic matter that can influence the efficiency of the treatments. For instance, the reduction of trichloroethene removal by Bio-Fenton treatment in the presence of Cu²⁺ was demonstrated [196], as well as the inhibition of the GOx activity due to the reaction with H₂O₂, which resulted in a reduced production of hydroxyl radical [299]. A similar effect was also observed for Mg²⁺, which is known to hinder the Fenton reaction [300]. Anions, such as H₂PO₄⁻, Cl⁻ and SO₄²⁻, can form complexes with iron ions, reducing their availability for the Fenton reaction [196]. Dissolved organic matter (DOM) in natural water, similarly, can act as a radical scavenger, limiting the number of reactive species available for substrate oxidation and leading to a decrement in pollutant degradation [301]. This kind of competition was also observed by Ravi et al., who reported a reduction in trichloroethylene degradation by the Bio-Fenton reaction carried out with free GOx in groundwater [201].

Also, in the case of pathogen contamination, this control is fundamental. Malvestiti et al. [302] verified the efficiency of O₃, O₃/H₂O₂ and UV/H₂O₂ treatments to disinfect municipal effluents and the influence of carbonate, nitrate and industrial contaminants. The results showed that all AOP treatments were affected by the presence of nitrate and, particularly, carbonate. These ions reduced the inactivation of total coliforms and Escherichia coli. Ozone disinfection was the most affected method by scavenging compounds.

Additionally, the selectivity of the decontamination process towards specific pollutants is, in general, another rising theme in environmental research to ensure a higher low-cost/effectiveness ratio and lower secondary pollution [303].

4.2. Processes’ Scalability, Cost Analysis and Environmental Impact

Real applications presuppose the scalability of the processes moving from laboratory scale to pilot plants and full-scale plants. At the basis of the scalability process, there are several critical points that have to be considered, such as oxidant conversion efficiency, location of radical production and delivery (i.e., in situ generation or need for a transport step) [304], and the study of kinetic parameters and degradation pathways [305]. All of these have to be initially optimized through the development of proper catalysts with high stability and activity; for instance, in the case of electrodes, they have to be characterized by high activity and corrosion resistance, long working life span and limited synthesis costs [305].

At the same time, the geometry and configuration of the reactors (e.g., batch mode vs. continuous-flow systems), integration with different depuration systems (like biological treatments), and operational costs have to be considered [304,306,307,308].

Obviously, each strategy shows intrinsic strengths and critical aspects that can limit its development on a large scale. For instance, the high operational costs of lamps, together with the difficult design of photoreactors and lower performances at neutral pH, are challenges for photo-Fenton full-scale applications [45], whereas the difficulty in distributing the formation of cavities in the reactor could be the limiting factor of sono-Fenton scale-up [309]. More complete considerations on AOP scalability have been discussed in [289,304], which also reported lists of pilot plants and full-scale plants of these wastewater remediation approaches.

It is clear that the overall evaluation of the feasibility of depuration procedures must involve the techno-economic aspects together with the environmental impact [101,310].

One widespread factor in evaluating the AOP processes is the consideration of the demand for energy. An example of compared systems was reported by Maniakova et al. [298]; the authors claimed that a solar-driven process catalyzed by N-doped TiO₂ was not yet competitive with the system of Fe²⁺/H₂O₂/ethylenediamine-N,N’-disuccinic acid (EDDS, chelating agent) under sunlight in the removal of CECs from secondary treated urban wastewater because higher irradiation times were necessary, which would result in a larger surface area for solar reactors.

From an economic point of view, the criterion of Electric Energy per Order (EEO), which takes into account the electric power, the removal efficiency and the flow rate [311], could be used to compare the cost of different AOP strategies [311,312], but it does not consider other operating costs.

To have a more comprehensive overview of total process expense, several authors proposed to evaluate the sum of the single source of cost: (i) power, (ii) chemicals, and (iii) sludge/waste management (including transportation and disposal) [313,314].

A new criterion to compare AOPs has been proposed, namely the Accumulated Oxygen-equivalent Chemical-oxidation Dose (AOCD), which comprises the accumulation of the oxidant dose normalized to the treatment time. AOCD also takes into account other important parameters such as current density, irradiance, wavelength, active surface area and lamp, distance of the lamp, faradaic and quantum yields, number of electrons exchanged, residence time, etc., depending on the AOP technique considered. The development of tools for AOP comparison is fundamental to help legislators determine and choose the best available techniques for water reuse as defined by the Industrial Emissions European Union Directive 2010/75/EU [315].

Regarding the evaluation of the processes’ environmental impacts, the life cycle assessment (LCA) methodology is a valuable and widespread support tool [316]. What emerges from most of the LCA outputs is that the main environmental hotspots of AOP strategies are, in order of relevance, the energy supply, the H₂O₂ employment, and, finally, the auxiliary chemicals.

Since energy is recognized as the main impacting factor, an appropriate choice of energy source is fundamental. For instance, García-Montaño et al. [317] compared different photo-Fenton strategies and noticed that the use of solar irradiation decreased the environmental impacts between 8.3 and 57.3%. Similar results have been reported by Ttofa et al. [318], who found that using solar energy to also power auxiliary equipment, such as pumps, lessened the calculated environmental impacts by up to 83%.

It can, therefore, be stated that investment in renewable energy could be effective in decreasing the environmental repercussions of AOPs.

In studies that report a lower energy imprint, the contribution of chemicals to the environmental impact grows, with a particular influence brought about by the use of hydrogen peroxide and auxiliary chemicals (i.e., NaOH necessary to reach a proper water pH after the treatment), or because of the substances released during the process [316].

From the materials’ and chemicals’ point of view, Costamagna et al. [319] investigated the photo-Fenton processes through LCA methodology. The photo-Fenton reaction carried out at circumneutral conditions, using the already mentioned soluble bio-organic substances (BBS) as auxiliary agents, was compared with the traditional photo-Fenton run at pH 2.8. The evaluation was carried out both at the laboratory level and at the pilot plant scale. Working in mild conditions reduced the environmental burden associated with the use of chemicals, but the decrement in effectiveness significantly increased the overall impact of the system.

In the case of heterogeneous catalysis, the focus is primarily on the sustainability of the materials, from their synthesis, possibly using greener strategies, to performances and recyclability [101,320,321]. Some studies highlight that the choice of heterogeneous Fenton can significantly reduce the environmental impact of the water treatments owing to the reuse of the catalyst. On the other hand, a higher amount of H₂O₂ seems to be required to degrade the same amount of pollutant, making the research of the least impactful compromise necessary [322].

4.3. Final Considerations

As emerges from the discussion of the recent literature, the panorama of AOP techniques is very diversified and effective for the remediation of different contaminated aqueous matrices. An example of the evolution of depuration systems over time is represented by Fenton-like processes, which can overcome the main issues of classical Fenton reactions, such as the addition of hydrogen peroxide, careful pH regulation in a narrow acid pH range and formation of sludge, although other drawbacks are still required to be solved. The regulation of the amount and type of energy involved, the low on-site production efficiency of hydrogen peroxide, the design of reactors (not only in terms of operation modes, but also chemical stability) and operational costs have to be optimized [16,27,45,323,324,325,326]. Issues related to the best compromise among efficiency, energy and chemical demand, operating and monitoring aspects together with the economic convenience must similarly be adjusted in processes based on photo-, electro-catalysis, etc.

It is, therefore, difficult to establish an absolute ranking of all the different techniques since there are a multitude of facets that should be taken into consideration. However, it is possible to summarize the main aspects emerging from the comparative reading of the literature as follows:

• The use of AOPs in mild pH conditions can reduce additional process costs related to the salinity increase, as it is induced in classic Fenton treatment by acidification and further neutralization [315];
• In situ hydrogen peroxide production avoids high costs and hazards associated with its transport, handling, and storage;
• Homogeneous processes such as ozonization have been investigated and already applied in the full-scale treatment of urban wastewater effluents [327];
• Heterogeneous catalysis shows several advantages with respect to homogeneous processes, such as the recyclability of the catalyst, which can contribute both to cost reduction and limitation of the environmental impact. However, this strategy does not seem completely ready for full-scale application due to catalyst preparation costs, effectiveness, stability, and reactor configurations [328];
• The main hotspot of the environmental impact is energy and, consequently, an appropriate choice of energy source can significantly reduce the total impact of the process; thus, investment in renewable energies should be preferred;
• Since wastewaters are characterized by a complex composition, their contamination cannot be solved by employing a single remediation technique. Effective hybrid systems combining AOP techniques and/or biological treatments are reported in the literature [315];
• AOP strategies have been reported among the best available technologies in a recent review about wastewater reuse in European countries [315];
• Limited scale-up and techno-economic analysis are available for Bio-Fenton treatments. Enzyme cost and stability could represent critical points for its real applications; however, some studies suggest that GOx production could be cost-effective and scalable [185]. Moreover, as widely described above, the immobilization of glucose oxidase improves its stability towards oxidation and deactivation due to environmental factors and ensures its reusability, making Bio-Fenton suitable for actual wastewater treatment;
• Although, to the best of the authors’ knowledge, there are no reported pilot or full-scale plants that integrate Bio-Fenton in wastewater treatments, its effectiveness as a pretreatment to be coupled with conventional active sludge was highlighted by Wang et al. [189].