Metals as Catalysts for Ozonation

*Jacqueline A. Malvestiti, Rodrigo P. Cavalcante, Valdemar Luiz Tornisielo and Renato Falcão Dantas*

### **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.

**Keywords:** transition metals, pH, catalytic mechanism, HO• , drawback

### **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 [4–8]. Ozone has a twopolar 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 oxidationreduction 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].

$$\rm O\_3 + HO\_2^- \rightarrow O\_3^- \\ \bullet + HO\_2\bullet \tag{1}$$

$$\bullet \bullet \bullet \bullet ^{-}\_{2}\bullet \bullet \bullet ^{-}\_{3}\bullet + \bullet \_{2} \tag{2}$$

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].

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 (�10<sup>8</sup> –10<sup>9</sup> M�<sup>1</sup> 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 ( moles of OH produced moles ofO3 consumed) varies between 10% and 40% during conventional ozonation of water and municipal wastewater [16–19].

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<sup>+</sup> 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

**Figure 1.** *Ozone reaction by the Criegge mechanism.*

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 –, CO3 2�, HPO4 <sup>2</sup>� and H2PO4 �) (Eqs. (3)–(6)) [20, 21] the radical chain reaction [21].

$$\text{HO}^{\bullet} + \text{HCO}\_{3}^{-} \rightarrow \text{OH}^{-} + \text{HCO}\_{3}^{\bullet} \tag{3}$$

$$\rm{HO}^{\bullet} + \rm{CO}\_{3}^{2-} \rightarrow \rm{OH}^{-} + \rm{CO}\_{3}^{-} \tag{4}$$

$$\text{HO}^\* + \text{H}\_2\text{PO}\_4^- \rightarrow \text{OH}^- + \text{H}\_2\text{PO}\_4^\* \tag{5}$$

$$\text{HO}^{\bullet} + \text{HPO}\_{4}^{2-} \rightarrow \text{OH}^{-} + \text{H}\_{2}\text{PO}\_{4}^{-} \tag{6}$$

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 (<sup>1</sup> O2) [18–22], 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, 18–23].

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• [19–24].

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) [29–40].

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• [41–43]. 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**.



### **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].
