2.3.2. Mechanism of Photocatalytic Removal of Organic Pollutants

Most authors agreed that the photocatalytic degradation of organic substrates (in this case, the phenol) follows Langmuir–Hinshelwood kinetics (Equation (38)) [70]:

$$r = -\frac{d\mathbf{C}}{dt} = \frac{k\mathbf{K}\mathbf{C}}{1 + \mathbf{K}\mathbf{C}}\tag{38}$$

where *r* represents the initial rate of reaction, *C* is the concentration of the reactant, *t* is the irradiation time, *k* is the rate constant, and *K* is the adsorption coefficient of the reactant. When the concentration of the pollutant is in the millimolar range, Equation (38) can be simplified to the apparent rate order equation (Equation (39)) [71]:

$$
\ln \frac{\mathcal{C}\_0}{\mathcal{C}} = k \mathcal{K} t = \mathcal{K}\_{app} t \tag{39}
$$

where *C* and *C0* are the concentrations of pollutants at time *t* and *t0*, respectively, and *Kapp* is the apparent-first order rate constant (in min−1). It is assumed that the reaction occurs on the surface of the photocatalyst. The mechanism of the photocatalytic reaction implies two steps: (i) the degradation of phenol to intermediates (e.g., hydroxylated and short-chain compounds) and (ii) the mineralization of the by-products to carbon dioxide and water [72].

Sobczynski et al. [73] admitted that during the UV irradiation of TiO2, the phenol reacted with photogenerated holes (h+) or photoinduced hydroxyl radicals, resulting in a variety of hydroxylated reaction by-products. The photocatalytic reaction proceeded via multiple steps, in which the principal intermediates were hydroquinone, catechol, and p-benzoquinone. Also, another variety of aliphatic intermediates formed in the reaction mixture, which finally converted to non-toxic end products (Figure 9).

**Figure 9.** Possible mechanism of the phenol mineralization in the presence of TiO2 under UV-light irradiation. Reproduced with permission from ref. [73]. Copyright 2004 Elsevier.

Guo and co-workers [74] focused on the identification of the intermediate produced by phenol photodegradation on TiO2 using GC-MS and HPLC tools. They agreed that the main reaction intermediates were hydroquinone, resorcinol, catechol, 1,2,3-benzenetriol, (E)-2-butenedioic acid, 2-hydroxy-propaldehyde hydroxy-acetic acid, 3-hydroxy-propyl acid, and glycerol. The •OH radicals are highly reactive species that attack the phenyl ring of the phenol yielding to catechol, resorcinol, and hydroquinone. Further, the phenyl rings will break up to give malonic acid, followed by short-chain organic acids (e.g., maleic, oxalic, acetic, formic acids, glycolic acid), and finally, CO2. Based on the detected by-products, the authors established that besides hydroxyl radicals, the •H was also a significant active free radical in the degradation pathways. During the photocatalytic process, H<sup>+</sup> or •<sup>H</sup> is scavenged by oxygen to form HO2• radicals, which finally convert to hydroxyl radicals. However, these authors concluded that the principal reactive species responsible for organics degradation was •OH radical.

Wysocka et al. [75] carried out the photocatalytic degradation of phenol over Memodified TiO2/SiO2@Fe3O4 nanocomposites (Me = Pd, Au, Pt, Cu) obtained by ultrasonicassisted sol-gel method. In their study, the Pd- and Cu-TiO2/SiO2@Fe3O4 photocatalysts displayed the highest photo-oxidation rate of phenol and mineralization. •O2 <sup>−</sup> and •OH were the active species involved in the photodegradation process. These radicals attack the phenyl ring yielding catechol, hydroquinone, and benzoquinone generation, followed by oxalic acid and CO2 formation (Figure 10). Conversely, different pathways occurred for the hydroquinone and catechol oxidation. Catechol was directly oxidized to oxalic acid and further mineralized to unharmful products. The hydroxylated by-products were oxidized to aliphatic carboxylic acids and finally to CO2. The authors found that for Pt-TiO2/SiO2@Fe3O4 nanocomposite, a lack of catechol after 60 min of irradiation resulted in a low mineralization rate. It has been postulated that the enhanced photocatalytic activity of Pd- and Cu-modified photocatalysts were due to increases in the number of adsorption sites and efficient charge carrier separation. In the case of the Au-TiO2/SiO2@Fe3O4 sample, the keto-enol tautomeric equilibrium retarded the rate of the phenol mineralization.

**Figure 10.** Schematic illustration of the phenol photocatalytic degradation mechanism for TiO2/SiO2@Fe3O4 magnetic catalysts modified with: (**a**) copper; (**b**) gold; (**c**) palladium, and (**d**) platinum nanoparticles. Reproduced with permission from ref. [75]. Copyright 2018 MDPI.

The photomineralization reaction depends on several factors, such as (i) the synthesis conditions (e.g., synthesis route, thermal treatment), (ii) the physicochemical properties of the catalyst (e.g., phase composition, morphology, particle size, surface area, porosity, bandgap energy), and (iii) the operational parameters (e.g., solution pH, initial concentration of the organic substrate, the mass of catalyst, wavelength, reaction temperature, radiant flux, and design of the reactor) [76].
