**1. Introduction**

Among the environmental questions of the present, water pollution by emergent contaminants, such as pharmaceuticals and pesticides, is a serious problem, making their removal a challenging task [1]. In particular, the use of pesticides has increased over the years to improve the production of agricultural goods and to satisfy the contextual growth of world population. Pesticides are a wide group of chemical compounds classified as persistent hazardous pollutants owing to a very high time of retention in water and giving rise to accumulation in sediment and in water effluents. They are also easily transferred over a long distance [2]. Their presence in the environment, especially in water, even at low concentrations, is a serious problem for both living organisms and human health.

Advanced Oxidation Processes (AOPs) are among the new, green and performing solutions for the removal of pesticides from water [3,4]. In these processes, the oxidation of the hazardous contaminants is obtained through the production of highly reactive radical species, such as •O<sup>2</sup> −, •O<sup>3</sup> <sup>−</sup>, or OH• . Different AOPs can be simultaneously utilized to avoid the generation of by-products

in treated water [5]. In this context, the photocatalysis and the Fenton process are two of the most promising AOPs [6]. The degradation of pesticides in water by means of photocatalysis allows to efficiently remove these pollutants with a moderate formation of secondary products and the selectivity of the process can be enhanced if peculiar materials (such as molecularly imprinted photocatalysts) are employed [7–10]. The hydroxyl radicals are formed in this process after the irradiation of a semiconductor photocatalyst with UV or a solar/visible light source with the consequent formation of photoelectrons in the conduction band and photoholes in the valence band of the photocatalyst [11]. The Fenton process involves the reaction between Fe2<sup>+</sup> and hydrogen peroxide to give the hydroxyl radicals (reaction 1):

$$\mathrm{Fe}^{2+} + \mathrm{H}\_{2}\mathrm{O}\_{2} \rightarrow \mathrm{Fe}^{3+} + \mathrm{OH}^{\bullet} + \mathrm{OH}^{-} \tag{1}$$

The further reaction of the ferric ions with the excess of H2O<sup>2</sup> re-generates the ferrous ions with the formation of the hydroperoxyl radicals (HOO• ) (reaction 2):

$$\text{Fe}^{3+} + \text{H}\_2\text{O}\_2 \rightarrow \text{Fe}^{2+} + \text{HOO}^\* + \text{H}^+ \tag{2}$$

The regeneration of ferrous ions can be accelerated, enhancing also the efficiency of the overall degradation process, in the presence of visible or near ultraviolet irradiation (i.e., the photo-Fenton process, reactions 3-5), with the consequent formation of further hydroxyl radicals [12,13]. Furthermore, some Fe(III)–carboxylate complexes originated from the coordination of Fe3+, and organic intermediates can adsorb in the UV–vis region, and other Fe2<sup>+</sup> species can be formed through the ligand-to-metal charge transfer (LMCT) (reaction 4). Finally, also the zero-valent iron species can be considered a source of Fe2<sup>+</sup> (reaction 5) [14].

$$\text{Fe}^{3+} + \text{hv} + \text{H}\_2\text{O} \rightarrow \text{Fe}^{2+} + \text{OH}^\bullet + \text{H}^+ \tag{3}$$

$$[\text{Fe}^{3+}\text{ (RCO}\_2\text{)}]^{2+} + \text{hv} \rightarrow \text{Fe}^{2+} + \text{CO}\_2 + \text{R}^{\bullet} \tag{4}$$

$$\text{Fe}^{0} + \text{hv} \rightarrow \text{Fe}^{2+} + 2\text{e}^{-} \text{ (}\lambda < 400 \text{ nm)}\tag{5}$$

The photo-Fenton process was successfully applied in the degradation of various pesticides and pharmaceuticals under solar light irradiation [15,16].

Among the various semiconductors used for the photocatalytic applications, recently, cerium oxide (CeO2, commonly called *ceria*), a largely used catalyst in many thermo-catalytic reactions [17,18], was examined as an alternative to the most used metal oxide photocatalysts (such as TiO<sup>2</sup> and ZnO [19–22]). The most attractive properties of CeO<sup>2</sup> are: the lower band-gap (around 2.7–2.8 eV) compared to TiO<sup>2</sup> and ZnO, making the material sensitive to visible light; the presence of empty 4f energy levels that facilitate the electron transfers; the high stability in the reaction medium; the high oxygen mobility related to the reversible Ce4+/Ce3<sup>+</sup> transformation, and the ability to form nonstoichiometric oxygen-deficient CeO2-x oxide [23]. The presence of defect centres in the CeO2, together with the high oxygen mobility and the consequential redox properties can be exploited in the Fenton-like reactions, that in this case, are different to the radical-attacking mechanism of conventional iron-based Fenton process, are driven by the interaction between the hydrogen peroxide and the surface defective Ce3<sup>+</sup> centres (reactions 6–8, [24,25]):

$$\rm{Ca^{3+}} + \rm{H\_2O\_2} \rightarrow \rm{Ce^{4+}} + \rm{OH^{\bullet}} + \rm{OH^{-}} \tag{6}$$

$$\mathrm{H\_2O\_2 + OH^\* \to H\_2O + HOO^\*}\tag{7}$$

$$\rm{Ce^{4+}} + \rm{HOO^{\bullet}} \rightarrow \rm{Ce^{3+}} + \rm{H^{+}} + \rm{O\_{2}} \tag{8}$$

The conventional iron species catalysts in the Fenton process require a strict operating pH range (between 3 and 4), thus increasing the overall process cost, whereas with CeO<sup>2</sup> it is possible to work at neutral pH [26].

On the basis of the above considerations, in this work we have studied the degradation of a largely used insecticide, i.e., the imidacloprid (C9H10ClN5O2), by the comparison between three different AOPs: solar photocatalysis, Fenton and solar photo-Fenton, taking advantage of the wide versatility of CeO<sup>2</sup> that can be used both as a photocatalyst and as a Fenton-like reagent. *Catalysts* **2020**, *10*, x FOR PEER REVIEW 3 of 16

The imidacloprid (hereafter called "IMI") is a neonicotinoid pesticide, which acts similarly to the natural insecticide nicotine [27]. Although IMI is not directly used in water, it is commonly transferred to water channels, and it presents a high leachability [28,29]. For its high toxicity, solubility, and stability, the presence of IMI in water even at low concentrations is a serious environmental concern. The imidacloprid (hereafter called "IMI") is a neonicotinoid pesticide, which acts similarly to the natural insecticide nicotine [27]. Although IMI is not directly used in water, it is commonly transferred to water channels, and it presents a high leachability [28,29]. For its high toxicity, solubility, and stability, the presence of IMI in water even at low concentrations is a serious

The Fenton-like process through ceria is activated by the presence of non-stoichiometric Ce3<sup>+</sup> centres on the surface of CeO<sup>2</sup> (reaction 6, [30,31]). One of the simpler methods to induce these defects is the exposure of CeO<sup>2</sup> to sunlight [32]. Indeed, the interaction of CeO<sup>2</sup> with the efficient UV solar photons (i.e., the photons with an energy higher than the band-gap of CeO2) led to a release of the labile ceria surface oxygens with the formation of CeO2-x defects. After the loss of oxygen, the cerium atoms adopted the most stable configuration, i.e., the Ce3<sup>+</sup> oxidation state. environmental concern. The Fenton-like process through ceria is activated by the presence of non-stoichiometric Ce3+ centres on the surface of CeO<sup>2</sup> (reaction 6, [30,31]). One of the simpler methods to induce these defects is the exposure of CeO2 to sunlight [32]. Indeed, the interaction of CeO<sup>2</sup> with the efficient UV solar photons (i.e., the photons with an energy higher than the band-gap of CeO2) led to a release of the labile ceria surface oxygens with the formation of CeO2-x defects. After the loss of oxygen, the cerium

In this context, we have synthetized two different types of CeO2: the first through one of the most employed preparation procedures for this oxide, as the precipitation with KOH from a cerium nitrate solution [23,33]; the second type using the same synthesis but irradiating the samples just after calcination with a solar lamp and in the presence of a H<sup>2</sup> flow, with the aim to further increase the surface defects on CeO2. Interestingly, with this original modified strategy we have obtained a "grey CeO2" instead of the typical yellow coloured ceria. The possibility to generate further defects on CeO<sup>2</sup> with solar exposure in a reducing atmosphere was especially exploited, due to a synergistic mechanism able to in situ provide the Ce3<sup>+</sup> species, mainly in the solar photo-Fenton tests. atoms adopted the most stable configuration, i.e., the Ce3+ oxidation state. In this context, we have synthetized two different types of CeO2: the first through one of the most employed preparation procedures for this oxide, as the precipitation with KOH from a cerium nitrate solution [23,33]; the second type using the same synthesis but irradiating the samples just after calcination with a solar lamp and in the presence of a H<sup>2</sup> flow, with the aim to further increase the surface defects on CeO2. Interestingly, with this original modified strategy we have obtained a "grey CeO2" instead of the typical yellow coloured ceria. The possibility to generate further defects on CeO<sup>2</sup> with solar exposure in a reducing atmosphere was especially exploited, due to a synergistic mechanism able to in situ provide the Ce3+ species, mainly in the solar photo-Fenton tests.
