Metals as Catalysts for Ozonation

Abstract

Ozonation is an efficient process for water and wastewater treatment, widely used for the disinfection and oxidation of organic pollutants. This process is effective, however, some pollutants are ozone-resistant. For better oxidation, enhanced production of hydroxyl radicals (HO•) can be obtained through the transition metals insertion in solution, known as homogeneous catalytic ozonation. These metals may react directly with O3 to produce HO• or interact with organics such as humic substances in the water matrix to promote O3 transformation to HO•. In this chapter, a short review of the homogeneous catalytic ozonation, including key aspects, such as pH effect, metals concentration, catalytic mechanisms, drawbacks of the homogeneous catalytic ozonation application, and the possible solution for it was provided.

1. Introduction

Access to clean and safe drinking water has become an emergency concern and requires immediate action. The population growth with consequent city development, especially in developing and emerging countries, has increased the volume of municipal wastewater produced every year and, this is the major contributor to a variety of water pollution problems [1].

Innovations in water and wastewater technologies are needed to solve challenges of climate change, resource shortages, emerging contaminants, urbanization, sustainable development, and demographic changes [2]. About 47% of the world’s population has no access to clean and reliable drinking water supply and, according to the WWDR [3], this ratio is expected to increase 57% by 2050.

The removal of the new and wide range of pollutants, especially those of emerging concern in secondary effluents started to be included in several legislations around the world. However, most of the current wastewater treatment plants (WWTP) were not designed to remove these types of pollutants, thus an additional advanced tertiary treatment is necessary to achieve this goal.

Ozonation is considered one of the most effective methods for disinfection and removal of organic pollutants, even in low concentrations [4, 5, 6, 7, 8]. Ozone has a two-polar resonance structure, which makes ozone behave as both electrophilic and nucleophilic dipoles [4]. The organic pollutants’ reactivity is selective, occurring mainly by specific reaction pathways, such as electrophilic, nucleophilic, or dipolar addition reactions [9] and the reactions predominate at low pH levels [10]. These reactions are known as direct reactions.

Direct reactions can be divided into four categories. The first one is the oxidation-reduction reaction, which occurs mostly due to the electron transfer process, such as the reactions between O₃ and HO₂⁻ (or O₂^–•) (Eqs. 1 and 2) [11, 12].

$${\mathrm{O}}_3+\mathrm{H}{\mathrm{O}}_2^{-}\to {\mathrm{O}}_3^{-}\bullet +{\mathrm{HO}}_2\bullet$$E1

$${\mathrm{O}}_3+{\mathrm{O}}_2^{-}\bullet \to {\mathrm{O}}_3^{-}\bullet +{\mathrm{O}}_2$$E2

The second one is the cycloaddition reaction, which generally occurs between an unsaturated compound (with a carbon double bond or π electrons) and an electrophilic compound, forming a new compound. The cycloaddition reaction mechanism between O₃ and olefinic substance was proposed by Criegee (Figure 1): (1) formation of primary ozonide (or five-member ring); (2) generation of the zwitterion; (3) different reaction pathways of zwitterion and formation the final products, such as ketones, aldehydes or acids (in aqueous solution) [13].

Figure 1.

Ozone reaction by the Criegge mechanism.

The third one is the electrophilic substitution reaction, in which the ozone, as an electrophilic agent, attacks the nucleophilic position of the organic substances and substitute one part of the organic molecule. The last one is the nucleophilic reaction, in which the ozone molecule can react with molecules at their electrophilic positions, especially, when the compound contains carbonyl or double and triple nitrogen carbon bonds [14].

The indirect reaction occurs when the hydroxyl radical (HO^•) and other reactive oxygen species (ROS), are formed by O₃ decomposition, it’s a nonselective oxidant and highly reactive with almost all types of organic moieties at diffusion-controlled rates (∼10⁸–10⁹ M⁻¹ s⁻¹), which may promote the complete degradation of organic pollutants [10, 15], prevailing at high pH levels.

Therefore, ozone-resistant pollutants are abated almost exclusively by ROS, mainly HO^• oxidation during ozonation [15, 16], and are usually less effectively abated due to the low HO^• yield from O₃ decomposition in real water matrices. The HO yield ($\frac{\text{moles of}\mathrm{OH}\text{produced}}{\text{moles of}{\mathrm{O}}_3\text{consumed}}$) varies between 10% and 40% during conventional ozonation of water and municipal wastewater [16, 17, 18, 19].

Ozone decomposition is the result of chain reactions with initiation, propagation, and ending phases [4]. The reaction between ozone and OH⁻ ions form hyperoxides radicals HO₂^• (initiation phase). HO₂^• is in equilibrium with the superoxide radical (O₂^•−), and the reaction between ozone and superoxide radical produces ozonide (O₃^•−) which reacts with H⁺ to form HO₃^•. Then HO₃^• is dissociated into HO^• and O₂^•, and the reaction between O₃ and HO^• forms HO₄^• (propagation phase). The ending phase occurs with the dissociation of HO₄^• into HO₂^• and O₂. However, the presence of inorganic and organic matter could initiate promote and prohibit the radical chain reaction [20]. In fact, a wide variety of compounds are able to initiate (i.e. hydrogen peroxide, humics, reduced metals, formate), to promote (i.e. primary and secondary alcohols, humics, ozone itself) or to inhibit (i.e. tertiary alcohols, HCO₃^–, CO₃²⁻, HPO₄²⁻ and H₂PO₄⁻) (Eqs. (3)–(6)) [20, 21] the radical chain reaction [21].

$${\mathrm{HO}}^{•}+{{\mathrm{HCO}}_3}^{-}\to {\mathrm{OH}}^{-}+{{\mathrm{HCO}}_3}^{•}$$E3

$${\mathrm{HO}}^{•}+{{\mathrm{CO}}_3}^{2-}\to {\mathrm{OH}}^{-}+{{\mathrm{CO}}_3}^{-•}$$E4

$${\mathrm{HO}}^{•}+{\mathrm{H}}_2{{\mathrm{PO}}_4}^{-}\to {\mathrm{OH}}^{-}+{\mathrm{H}}_2{{\mathrm{PO}}_4}^{•}$$E5

$${\mathrm{HO}}^{•}+{{\mathrm{HPO}}_4}^{2-}\to {\mathrm{OH}}^{-}+{\mathrm{H}}_2{{\mathrm{PO}}_4}^{-}$$E6

Ozone decomposition in water is strongly pH-dependent and occurs faster with an increase in pH [4].

In order to increase the production of hydroxyls radicals (HO^•), and at the same time increase the oxidation capacity, ozonation can be performed in the presence of catalysts, namely catalytic ozonation.

The catalytic decomposition of O₃ in the presence of catalysts can lead to various ROS, such as ozonide radical (O₃^•–), hydroxyl radical (HO^•), superoxide radical (O₂^•–), hydrogen peroxide (H₂O₂), and singlet oxygen (¹O₂) [18, 19, 20, 21, 22], these ROS and O₃ can react with pollutants simultaneously, thus bringing about their abatement. Due to the enhanced transformation of O₃ to ROS, higher abatement efficiencies can often be obtained for ozone-resistant pollutants during catalytic ozonation compared to conventional ozonation [9, 18, 19, 20, 21, 22, 23].

The catalytic ozonation through the transition metals insertion in solution is known as homogeneous catalytic ozonation. They may react directly with O₃ to produce HO^• or interact with organics such as humic substances in the water matrix to promote O₃ transformation to HO^• [19, 20, 21, 22, 23, 24].

The reaction mechanism follows two main pathways. The first one is based on the acceleration of ozone decomposition by the generation of the ^•O₂⁻ and ^•O₃⁻ radicals and subsequently HO^• formation [24, 25]. The other one is based on the formation of complexes between the catalyst and the organic compound, followed by a final oxidation reaction [25, 26]. Therefore, metal ions are able to enhance the efficiency of single ozonation for the removal of different organic compounds in aqueous solution, particularly those recalcitrant to direct ozone oxidation [27, 28]. In this catalytic process, the pH of the solution and the concentration of the transition ion can influence both the efficiency of the process and its mechanism [25]. The most widely metal ions used as catalysts of the ozonation process are Mn(II), Fe(III), Fe(II), Co(II), Cu(II), Zn(II), and Cr(III) [29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40].

The heterogeneous catalytic ozonation uses solid catalysts (e.g., metal oxides and metals on supports). There are two ways for O₃ to be decomposed in this system: at the catalyst surface and/or react with organics adsorbed on the catalyst surface to produce HO^• [41, 42, 43]. Many noble metals and metal oxides, immobilized or not on supports, have been used for heterogeneous catalytic ozonation [44, 45], being the order of catalytic activity for the decomposition of ozone following one [46]: Pt > Pd > Ag > Ru, Rh, Ir > Ni > Cd > Mn > Fe > Cu > Zn, Zr ≫ Co, Y, Mo, Ti, Au.

In this chapter, we only discuss homogeneous catalytic ozonation. Nevertheless, there are many homogeneous catalytic ozonation systems described in the literature. Some of them are collected in Table 1.

Catalyst	Pollutant	Operational conditions	Ozonation efficiency (%)	Reference
Co(II) and Fe(II)	p-chlorobenzoic acid (p-CBA) and benzotriazole	p-CBA and benzotriazole: 500 μg/L; Ozone concentration: 2 mg/L; Catalyst dose: 1 mg/L; pH 7.8, T: 23 ± 2°C.	Degradation efficiency O3/Fe(II): p-CBA: 57%; benzotriazole: 98%	[29]
Fe(II)	Acid black Azo Dye	Ozone concentration: 70 mg/L,; Dye concentration: 200 mg/L; pH 6, Reaction time: 20 min	Degradation efficiency: 95.5%	[30]
Mn(II),Fe(II), Fe(III), Zn(II), Co(II), Ni(II)	C.I. Reactive Red 2 (RR2)	Catalyst dose: 0.6 mM; Ozone concentration::200 ml/min; Dye concentration: 100 mg/L; pH: 2 and 5	Degradation efficiency: pH 2: Mn(II) > Fe(II) ≥ Fe(III) > Zn(II) > Ni(II) > Co(II); pH 5: Fe(III) > Fe(II) > Ni(II) ≥ Zn(II) ≥ Co(II) ≥ Mn(II)	[31]
Mn(II), Pb(I), Cu(II), Zn(II), Fe(II), Ti(II)	2-chlorophenol	Catalyst dose: 0–2 ppm; 2-chlorophenol: 100 ppm; Ozone concentration: 18 mg/min; Reaction time: 20 min, pH = 3.	Degradation efficiency: Pb^+, Cu^2+, Zn^2+: 65%; Ti^2+ or Fe^2+: 80%; Mn^2+: 90.5%	[32]
Fe(III) and Fe2O3/Al2O3	Oxalic acid	Ozone concentration: 30 mg/L, Gas flow rate: 24 L/h, Catalyst concentration: 1 mg/L, Al2O3 or Fe2O3/Al2O3: 1.25 g/L (part. Size: 1.6–2 mm), Reaction time: 3 h. T: 20°C. pH 2.5.	Degradation efficiency: 7% homogeneous (Fe(III)), 30% heterogeneous (Fe2O3/Al2O3)	[33]
Fe(II), Co(II), Cr(III), and Mn(II)	Di-(2-ethyl hexyl) phthalate (DEHP)	Ozone concentration: 96 mg/min. Flow rate of 2 L/min, DEHP concentration: 300 μg/L, Reaction time: 120 min. T:20 ± 0.5°C.	Degradation efficiency: Cr(III): the most active catalyst: 75% removal. Co(II): the least active catalyst: 45%	[34]
Co(II)	p-chlorobenzoic acid (pCBA)	Ozone concentration: 0.1 mM, Catalyst concentration:10-5 M Co(II), pCBA concentration: 3 Μm, Reaction time: 60 min, near-neutral pH, T24 °C.	Degradation efficiency: 75%	[35]
Mn(II)	2,4-dichlorophenol (DCP)	Ozone concentration: 8.2 mg/L, Ozone gas-flow rate: 200 mL/min, Catalyst concentration: 200 mg/L, DCP concentration:10 mg/L. Reaction time: 30 min	Degradation efficiency: 100%, TOC removal: > 80%	[36]
Ni(II), Fe(II), Mn(II), Zn(II), Sr.(II), Cr(III), Cd(II), Hg(II), and Cu(II)	1,3,6-naphthalenetrisulfonic acid (NTS)	Ozone concentration: 1.04 × 10–4 mol/dm3	Degradation efficiency: Fe(II): 79% and Mn(II): 72%	[37]
		NTS concentration:1.23 × 10–4 mol/dm^3.
		Catalyst concentration: 0.1 millieq/dm^3
		Reaction time: 30 min, pH: 2 and 7
Electro-permanganate	Nitrobenzene (NB), Atrazine (ATZ), Sulfamethoxazole (SMX), Diclofenac (DCF), Phenol, and Carbamazepine (CBZ)	Ozone concentration: 1 mg/L, 60 mL/ min	Degradation efficiency: CBZ and phenol: 100%. DCF: 87.08%, SMX: 84.23%, ATZ: 38.43%, NB: 21.56%.	[38]
		initial organic concentration = 0.060 mM
		pH = 5, T:293 K, Reaction time: 20 min
		Catalyst concentration = 0.100 mM
Ce(III)	Phenol	Ozone concentration: 0.13 g/L, Flow rate: 100 ml/min, Reaction time: 120 min, Catalyst concentration: 50 mM, Phenol concentration: 2.7 mM	TOC removal: 71%, Degradation efficiency: 99%	[39]
Fe(II)	Reactive Red 2 (RR2)	Catalyst concentration: 0.9 Mm	k2: 2248 M/s	[40]
		Ozone concentration: 2 mg/L/min
		RR2 dyeconcentration: 0.45 mM, pH 7
		Reaction time: 70 min

Table 1.

Catalytic ozonation processes of pollutants in water and wastewater.

1.1 Mechanism of homogeneous catalysis

The metal ions (Fe²⁺, Cu²⁺, Cr²⁺, Mn²⁺, Ni²⁺, Co²⁺, Cd²⁺, Ag⁺, Zn²⁺, etc) influence the rate of reaction, the selectivity of ozone oxidation, and the efficiency of ozone utilization. A variety of different mechanisms has been proposed to explain the metal ions effects on ozonation, but there are two major mechanisms of homogeneous catalytic ozonation [22, 31, 35].

1.1.1 Mechanism 1: Decomposition of ozone by metal ions leading to the generation of free radicals

The oxidation mechanism of organic compounds via ozonation is dependent on the pH of the reaction medium, (i) at basic pH ozone decomposes producing HO^• radicals and other radical species in solution (Eqs. (7)–(9) and, (ii) at acidic pH, ozone is stable and reacts directly with organic substrates [31]. As it is well described in the literature, the generation of free radicals can subsequently oxidize the organic compounds more efficiently [23].

$${\mathrm{O}}_3+{\mathrm{OH}}^{-}\to {{\mathrm{O}}_2}^{•-}+{{\mathrm{HO}}_2}^{•}$$E7

$${\mathrm{O}}_3+{{\mathrm{O}}_2}^{•-}\to {\mathrm{O}}_2+{{\mathrm{O}}_3}^{•-}$$E8

$${{\mathrm{O}}_3}^{•-}+{\mathrm{H}}^{+}\to {{\mathrm{HO}}_3}^{•}\to {\mathrm{O}}_2+{\mathrm{HO}}^{•}$$E9

The homogeneous catalytic ozonation occurs mostly at acidic pH values because at the real pH range for waters/wastewaters (6–8), the effect of metal ions is almost diminished [32].

In general, the mechanism of metal-catalyzed ozone decomposition with the generation of HO^• radicals can be briefly expressed according to Eqs. (10)–(12) [31], being very similar to the Fenton process. The metal ions react with ozone or enhance its decomposition to generate HO^• radicals and their regeneration occurs via the oxidation by HO₂^−• radicals [9, 10].

$${\mathrm{M}}^{\mathrm{n}+}+{\mathrm{O}}_3+{\mathrm{H}}^{+}\to {\mathrm{M}}^{\left(\mathrm{n}+1\right)+}+{\mathrm{HO}}^{•}+{\mathrm{O}}_2$$E10

$${\mathrm{O}}_3+{\mathrm{HO}}^{•}\to {\mathrm{O}}_2+{{\mathrm{HO}}_2}^{–•}$$E11

$${\mathrm{M}}^{\left(\mathrm{n}+1\right)+}+{{\mathrm{HO}}_2}^{–•}+{\mathrm{OH}}^{-}\to {\mathrm{M}}^{\mathrm{n}+}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2$$E12

The formation of HO^• would be scavenged in the presence of excess metals (Eq. 13) [20, 21], so the optimization of catalyst dosage is also vital for catalytic ozonation process [23, 40].

$${\mathrm{M}}^{\mathrm{n}+}+{\mathrm{HO}}^{•}\to {\mathrm{M}}^{\left(\mathrm{n}+1\right)+}+{\mathrm{HO}}^{-}$$E13

One of the biggest challenges of this review was to find publications that represented the conditions found in the real waters (aquatic environments and wastewaters), for both the contaminants concentrations and pH. Since that, in the real waters, the contaminants concentrations range from ng/L and at the pH near neutral and most works present high values for them.

We gathered some works using metal ions for ozone catalysis and most of them report that Mn(II) and Fe(II) were the metals that showed the best results to increase ROS production. Sánchez-Polo and Rivera-Utrilla [37] tested the Mn(II) and Fe(II) ions as catalysts for the removal of 1,3,6-naphthalenetrisulfonic acid at acidic pH values; Xiao et al. [36] used the Mn(II) for the removal of 2,4-dichlorophenol; Ni et al. [32] used various metals for the removal of 2-chlorophenol at the acidic pH value and they found that Mn(II) was the most efficient catalyst tested. Okawa et al., [47] found that the presence of Fe(III) and Mn(II) enhanced the degradation of 2,4-dichlorophenol by ozone in acetic acid. However, Trapido et al. [48] observed no catalytic activity of Mn(II) for ozonation of dinitrobenzene. Wu et al. [31], Li et al. [49] and Ma & Graham [50] identified the optimal concentrations for metals ions to act as a catalyst for the decomposition of ozone into HO^• radicals, in order to remove emerging contaminants, with an emphasis here on C.I. reactive red 2, alachlor and atrazine.

1.1.2 Mechanism 2: Complexes formation between organic molecule and the catalyst

In this mechanism, the metal ions combine with organic molecules to form complexes, which are then oxidized by O₃ and other oxidizing species [23].

Pines and Reckhow [35] reported that high mineralization of oxalic acid takes place via ozonation in the presence of Co(II) ions. This process was determined to have a high reaction rate, which increase with a decrease in pH. To prove that the oxidation reaction did not depend on the formation of HO^• radicals, the researchers tested the reaction rate in the presence of tert-butanol, which is known as a HO^• radical scavenger. Based on the results obtained, the authors confirmed that it is not changed in mineralization rates, proving that the HO^• radicals was not responsible for mineralization of oxalate in the Co(II)/O₃ system (Figure 2).

Figure 2.

Oxalic acid catalytic ozonation mechanism by means of the Co(II)/O₃ system.

Soon after the work of Pines and Reckhow [35], Beltrán et al. [51] tested the same oxalic acid mineralization in the Co(II)/O₃ system with the presence of tert-butanol and also confirmed that the HO^• radicals were not responsible for mineralization of oxalic acid. Continuing their work with the mineralization of oxalic acid, Beltrán et al. [33] also found that Fe(III) ions act as a catalyst for the mineralization of oxalic acid in the same way as Co(II). In both cases (Co and Fe) ozone reacts with both negatively and positively charged complex moieties and HO^• radicals are formed as secondary by-products. They proposed a sequence of reactions to explain the process (Eqs. (14)–(18)):

$${\mathrm{Fe}}^{3+}+{\mathrm{C}}_2{{\mathrm{O}}_4}^{-}\to {\mathrm{FeC}}_2{{\mathrm{O}}_4}^{+}$$E14

$${\mathrm{FeC}}_2{{\mathrm{O}}_4}^{+}+{\mathrm{C}}_2{{\mathrm{O}}_4}^{-}\to \mathrm{Fe}{{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2}^{-}$$E15

$$\mathrm{Fe}{{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2}^{-}+{\mathrm{C}}_2{{\mathrm{O}}_4}^{-}\to \mathrm{Fe}{{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3}^{3-}$$E16

$${\mathrm{FeC}}_2{{\mathrm{O}}_4}^{+}+{\mathrm{O}}_3\to {\text{2CO}}_2+{\mathrm{Fe}}^{3+}+{{\text{2O}}_3}^{\bullet -}$$E17

$$\mathrm{Fe}{{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2}^{-}+{\text{2O}}_3\to {\text{2CO}}_2+\mathrm{Fe}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}^{+}+{{\text{2O}}_3}^{\bullet -}$$E18

$$\mathrm{Fe}{{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3}^{3-}+{\text{2O}}_3\to {\text{2CO}}_2+\mathrm{Fe}{{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2}^{-}+{{\text{2O}}_3}^{\bullet -}$$E19

Andreozzi et al. [52] explained the mechanism of the catalytic effect observed in Mn²⁺/O₃ systems, as related to the formation of Mn(III) complex with oxalate ions (Ox). The molecular ozone attacked the oxalate ion radical and that leads to the formation of HO^• radicals [53] (Eqs. (20)–(25)):

$${\mathrm{Mn}}^{2+}+{\mathrm{O}}_3+{\text{2H}}^{+}\to {\mathrm{Mn}}^{4+}+{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$E20

$${\mathrm{Mn}}^{2+}+{\mathrm{Mn}}^{4+}\to {\mathrm{Mn}}^{3+}$$E21

$${\mathrm{Mn}}^{4+}+\mathit{n}{\mathrm{Ox}}^{2-}\to {\mathrm{Mn}}^{3+}{\left({\mathrm{Ox}}^{2-}\right)}_{\mathit{n}}$$E22

$${\mathrm{Mn}}^{3+}{\left({\mathrm{Ox}}^{2-}\right)}_{\mathit{n}}\to {\mathrm{Mn}}^{2+}+{\mathrm{Ox}}^{-}+\left(\mathit{n}-1\right){\mathrm{Ox}}^{2-}$$E23

$$\mathrm{O}{\mathrm{x}}^{\bullet -}+{\mathrm{O}}_3+{\mathrm{H}}^{+}\to {\text{2CO}}_2+{\mathrm{O}}_2+{\mathrm{OH}}^{•}$$E24

$${\mathrm{OH}}^{•}+{\mathrm{Ox}}^{2-}\to \dots$$E25

The transition metals are very important catalysts due to their characteristics [54]:

1. Bonding ability: Transition-metal catalysis is founded on the principle that electron donation from a metal to a ligand is accepted by an antibonding orbital of the ligand, thereby weakening one of the bonds in the ligand. Without this, the initial step of bond activation in many catalytic processes would simply not occur [55].

2. Wide choice of ligands: Transition elements readily form alloys and lose electrons to form stable cations, forming a wide variety of stable coordination compounds, in which the central metal atom or ion acts as a Lewis acid and accepts one or more pairs of electrons [54].

3. Ligand effects: the variation of the steric or electronic environment at the active site of a ligand can influence the behavior of a transition metal catalyst;

4. Oxidation state variability and the co-ordination number;

5. Ability to readily interchanges between oxidation states during a catalytic reaction: transition metals can be readily involved in redox processes.

In summary, in mechanism 2, the ozone may equally efficiently attack neutral, positively and negatively charged metal-complex species, which could be a major reaction pathway for catalytic ozonation, especially for some low-molecular-weight acids, such as oxalic acid [35, 50].

2. Concluding remarks

It is well described in the literature that catalytic ozonation in homogeneous phase is effective in removing a wide range of industrial effluents, products from the pharmaceutical industry, pesticides, and recalcitrant organics. However, homogeneous catalytic ozonation has the disadvantage of producing secondary contaminants from the addition of metallic ions [56]. In addition to the possibility of the residual concentration of metals exceeding regulatory limits for drinking water. Therefore, one more step in this system must be considered, to remove the metal ions from the treated matrices. This is the major drawback of applying homogeneous catalytic ozonation, especially when this process is applied for drinking water treatment. As a promising alternative to this inconvenience, heterogeneous catalysis appears which uses metals in the solid state (metallic oxides and metals on supports).

Acknowledgments

The authors thank the financial support of the São Paulo Research Foundation (FAPESP) [grant numbers #2022/00454-0; 2022/04015-1; #2019/26210-8], project CAPES/COFECUB [grant number 88881.191742/2018-2100], and National Council for Scientific and Technological Development (CNPq) [grant number 311674/2021-6].

Conflict of interest

The authors declare no conflict of interest.