**5. Photocatalytic Oxidation (Light/catalyst)**

Within AOPs, photocatalyst-based degradation methods represent an interesting research field where there has been continuous development. Heterogeneous photocatalysis is widely recognized as an effective technology for treating waters containing some refractory organic compounds through the photogeneration of oxidizing radicals such as HO• and O2−•. It is a green technology with broader application prospect and compared with the traditional chemical oxidation, photocatalysis is usually non-toxic, non-corrosive and harmless to the environment. The photocatalytic oxidation is based on the use of a semiconductor and ultraviolet-visible (UV-vis) radiation (see Figure 4). The fundamental step of the process is generation of electron-hole pairs, which requires absorption of photons with adequate energy and promotion of electrons from the valence band to the conduction band. The photogenerated charge carriers participate in a series of reactions producing highly reactive radicals. One of the most relevant applications of this technique is the degradation of environmental pollutants in aqueous wastewater into less harmful products. These treatments are very appropriate because of their on-place use and because they do not have extra energy consumption. The degradation of organics is normally accomplished of semiconductors such as zinc oxide (ZnO) or titanium dioxide (TiO2). The latter is currently the most popular photocatalyst due mainly to its specific photocatalytic properties like for example strong oxidizing power, high chemical stability and relative inexpensiveness. Recently, different photocatalytic materials capable of efficiently working with sunlight, based mostly on TiO2 and their combination with solar collectors, have been revised [110].

**Figure 4.** Schematic illustration of a model photocatalytic system showing the contribution of hole-electron couples to the formation of radicals.

Unfortunately, the use of TiO2 catalysts is limited by the large band gap of TiO2 (Eg = 3.2 eV), being active only under UV light. Considering that the sunlight is composed mainly of visible light (43%) while only 4% of the spectrum is UV light, visible-light photocatalytic performance is desirable in order to effectively utilize the sunlight. In this sense, considerable effort is being devoted to developing

alternative heterogeneous photocatalysts, which are active under visible light. Among the various materials, some perovskites have been considered as a promising photocatalysts, since they present high activity in the long band of visible-light. They can be used alone or combined with TiO2 in form of composites, with the aim of narrowing the bandgap of this oxide.

One of the perovskites more widely studied as photocatalyst is LaFeO3 [111–122], due to its narrow band gap (often less than 3.0 eV), which can be excited easily under visible light or UV light irradiation. It can be used auto supported or in form of composites and rhodamine B (RhB) has been tested by different authors as model molecule in the evaluation of its photocatalytic activity [111,112,114,119–122].

In Reference [111] LaFeO3 particles prepared by sol-gel method were able to degrade RhB in 24% under visible irradiation, showing a higher activity than that of international P-25TiO2. Authors found a reverse correlation between crystallite size and photocatalytic activity, the most active sample being that exhibiting the smallest size. Values of degradation comprised between 15 and 50% were reached when LaFeO3 nanoparticles were prepared by using silica SBA-16 as template [112]. The high surface area and crystallinity of samples were responsible for the adsorption and photocatalytic degradation of RhB, respectively.

Li et al. [114] prepared different samples containing LnFeO3 (Ln = La, Sm) nanoparticles by sol–gel method at different calcination temperatures. 80% of RhB was degraded in 2 h by LaFeO3 under visible light, in contrast to the 20% obtained with SmFeO3. When H2O2 was added to the reaction media, the photocatalytic activity improved due to the synergistic effect between the semiconductor photocatalysis and Fenton-like reaction. Complete degradation of RhB was achieved after 3 h of reaction when microspheres composed of perovskite LaFeO3 nanoparticles, prepared by hydrothermal method, were used as photocatalysts [119]. In this case, the hydrothermal reaction conditions and the concentration of citric acid played an essential role in the development of LaFeO3 microspheres. The efficiency of LaFeO3 microspheres was higher than that of LaFeO3 prepared by microwave assisted method [120] (95% of degradation of RhB in 3 h).

As an alternative way to obtain perovskite-type nanoparticles, graphitic carbon nitride (g-C3N4) has been combined with LaFeO3 by using a solvothermal method [121] according to the scheme of preparation shown in Figure 5. The synergistic interaction between LaFeO3 and g-C3N4 improved the separation efficiency of photogenerated electron-hole pairs. Then the photocatalytic activity for the degradation of RhB was 19 times higher than that of LaFeO3 under visible light irradiation. In addition, the catalysts kept excellent stability after four cycles.

**Figure 5.** Schematic illustrating the synthesis of the LaFeO3/g-C3N4 heterojunction. With permission from [121].

To achieve more homogeneous LaFeO3 nanoparticles, Ren et al. [122] used reduced graphene oxide as template. They obtained nanoparticles anchored on graphene oxide by combining the sol-gel method and high-temperature annealing. The LaFeO3–rGO can work under visible-light irradiation as an efficient catalyst for the degradation of RhB and MB, the bandgap of LaFeO3 nanoparticles on reduced graphene oxide being of 1.86 eV. The oxidation process was dominated by the electron transfer since the presence of rGO facilitated the electron-hole separation.

Methylene blue (MB) has been degraded under the photocatalytic reactions with LaFeO3 perovskites [116,117] and LaFeO3 doped in A position [115,118], whose activity is strongly influenced by the process of synthesis. There are many methods to prepare LaFeO3 such as sol-gel, co-precipitation, electro-spinning, citric acid complex, stearic acid solution combustion or glycine combustion at high temperature. Among these methods, the sol-gel process has been proven to be one of the most effective [116] and MB and methyl orange were completely degraded after visible light irradiation for 4 h of LaFeO3 synthesized by sol-gel method and calcined under vacuum microwave. The photodegradation process of MB on LaFeO3 followed a pseudo-first-order kinetic process.

However, sol-gel method and solid-state reactions need annealing at a high temperature which results in short homogeneity and high porosity of the samples without control on the particle size. As an alternative way, the microwave assisted synthesis allows preparing nanoparticles with small size, narrow size distribution and high reactive ability in a time-saving process. In this regard, LaFeO3 with high crystallinity and sphere-like shape was able to decolorize a MB aqueous solution in only 90 min of exposure to visible light [117].

A strategy to improve the photocatalytic efficiency of perovskites is by doping in A position [115,118]. Thus, a series of LaFeO3 perovskites doped with Li, La1-xLixFeO3 (with x = 0, 3, 5 and 7%) were active under light irradiation for the degradation of MB and arcylon effluents, La0.97Li0.03FeO3 showing the highest activity [115]. LaFeO3 and Ca-doped LaFeO3, synthesized via reverse microemulsion without additional high temperature calcination process [118] were active in the photocatalytic degradation of MB under the action of visible light. When La0.9Ca0.1FeO3 was used instead of LaFeO3, the degradation rate of MB improved 30%.

LaFeO3 and its corresponding double perovskite, La2FeTiO6, have also been tested in the degradation of p-chlorophenol [113] under visible light. Authors correlated the photocatalytic activity with differences in structure or surfaces properties. Thus, the optical property in the visible light region and the inferior symmetry at the Fe nucleus of La2FeTiO6 resulted in a better performance with respect to that of LaFeO3.

Apart from LaFeO3 other perovskites have been used as photocatalysts. In this regard, the fact that LaCoO3 hollow nanospheres exhibit a band gap of 2.07 eV makes this compound a promising candidate material for photocatalytic applications. Fu et al. [123] studied the photocatalytic degradation of MB, methyl orange and neutral red under UV irradiation. UV–vis analysis showed that LaCoO3 hollow nanospheres exhibited excellent photocatalytic activity, reaching degradation values near 90% in 100 min for the three contaminants. Moreover, the authors discussed the influence of temperature and time of calcination on the structures of LaCoO3 and its formation mechanism.

Alkali earth titanates, such as SrTiO3, are perovskite-type oxides based on a Ti-O polyhedron, showing a similar energy band structure to that of TiO2. Furthermore, SrTiO3 shows a wide absorption band in the ultraviolet region in the 300–400 nm range. As a result it can be used as photocatalyst, whose activity can be improved by incorporation of CeO2 to the structure, which shows strong UV absorption in the 300–450 nm range. In this sense, a SrTiO3/CeO2 composite was used in the photodegradation of two azo dyes, C.I. Reactive Black 5 [124] and C.I. Direct Red 23 [125]. The influence of pH on the photoactivity was studied in the first case. Authors extended the study to the influence of other parameters, such as catalyst dose, concentration of dye, pH value, irradiation intensity and use of KI as scavengers, in the second one. Under the optimum conditions, a complete degradation of C.I. reactive black 5 and C.I. direct red 23 was achieved in 120 min and 60 min, respectively. Authors proposed a tentative degradation pathway based on the sensitization mechanism of photocatalysis.

Recently, bismuth-based perovskites have also attracted interest as photocatalysts due to their particular electronic structure that reduces the charge mobility and the band gap to ∼2 eV [126–129]. BaBiO3 powders with a base centred monoclinic structure exhibited good activity for the water-splitting reaction and the degradation of rhodamine B dye under visible light [126]. Authors demonstrated

that the catalytic activity strongly depended on the crystallinity of the materials, BaBiO3 prepared by solid state being the catalyst with the highest crystallinity, the lowest resistance to the charge transfer and the greatest photocatalytic performance. Another bismuthate, KBiO3, was investigated as a visible-light-driven photocatalyst in the degradation of organic pollutants, such as RhB, crystal violet, MB and phenol [127]. The difference between the degradation mechanisms of these organic pollutants under the action of KBiO3 depended on competition of the photocatalysis, redox reaction and adsorption mechanisms. In the case of RhB and crystal violet, the redox potentials are higher than that of KBiO3 (1.59 eV) but lower than its band gap energy (2.04 eV), thus only the adsorption and photooxidation controlled the reactions. MB presents a redox potential lower than the band gap of KBiO3 and in this case the reaction was controlled by both the photooxidation and chemical oxidation. In the case of phenol, photooxidation was observed at the end steps of the process whereas quick adsorption and chemical oxidation were determined at the initial stage. A double Bi-perovskite, Bi2Fe4O9, composed of nanoplates, which behaves as a multiband semiconductor [128] was tested in the photocatalytic oxidation to aqueous ammonia oxidation and phenol under visible light irradiation. The catalyst displayed a higher activity when compared to its bulk material. The improvement of photocatalytic performance of Bi2Fe4O9 could be due to the efficient electron-hole separation that may act as electron-hole recombination centres. The photocatalytic performance for phenol oxidation enhanced when an appropriate amount of, H2O2, was added, which can act as a strong electron scavenger and also as a promoter of the Fenton-like reaction.

LaMnO3 perovskite is another promising photocatalyst owing to its catalytic and electrical properties, price, nontoxicity and high stability. However, a high combination rate of electron/hole pairs and the agglomeration of particles are some of the perovskite limitations. Semiconductor coupling with carbon materials is believed to induce cooperative o synergistic interactions retarding the fast recombination of the charge carriers and getting better the photocatalytic activity. In this sense, Huang et al. [130] observed a higher efficiency for photodegradation of acid red C-3GN over a series of LaMnO3-diamond composites than for LaMnO3. In the composites, the perovskite particles are uniformly distributed on the diamond surface creating a network structure, which increases the active sites and the absorption of dye molecules. The composite showed the best photocatalytic activity when the mass ratio was 1LaMnO3/2diamond.

AuNP/KNbO3 have shown photocatalytic activity in the photooxidation of sec-phenethyl alcohol to acetophenone under the action of visible light in the presence of H2O2 [131]. Photophysical properties of KNbO3 and TiO2 are fundamentally similar, with band gaps near 3.2 eV; however, the particle size of KNbO3 presents advantages over TiO2 since small particle size of TiO2 make difficult its separation from reaction solutions. The activity of this AuNP-decorated KNbO3 was superior to that of undecorated KNbO3.

In recent years most researchers have concentrated their attention on modifying perovskite by doping with a transition metal or non-metal to improve the catalytic activity. The doping allows the recombination of centres of electron-hole pairs in the semiconductor particles. For instance, the doping with C and S atoms improved the photocatalytic activity of SrTiO3 for oxidation of 2-propanol [132], because a new absorption edge in the visible light region was produced.

Several articles report the benefit of using nanostructures to improve the photocatalytic activity with respect to the bulk samples [129,133,134]. Thus, perovskite-type BiFeO3 nanoparticles showed increased degradation ability of methyl orange under visible light irradiation with respect to bulk BiFeO3, probably due to the higher surface area of the nanoparticles [129]. As a result, more than 90% of MO was decolorized after 8 h under UV-vis irradiation and after 16 h under visible light when BiFeO3 nanoparticles were used. In contrast, only 70% of dye was degraded after 16 h by the action of UV-vis light in the presence of bulk BiFeO3. Dong et al. [133] found that LaCoO3 nanofibers prepared at different temperatures presented better photocatalytic activity for the degradation of RhB than LaCoO3 particles. Nanofibers exhibited an increased surface, providing more photocatalytic active sites on the inner/outer surfaces that led to a complete degradation of RhB in only 50 min for the

best catalyst, which resulted to be that with a high degree of crystallinity of LaCoO3 nanofibers and containing some residual carbon. PrFeO3 porous nanotubes showed high optical absorption in the UV-visible region and an energy band gap of 1.97 eV [134], displaying a higher photocatalytic activity in the degradation of RhB than PrFeO3 nanofibers or PrFeO3 nanoparticles. Thus, 46.5% RhB was degraded by the nanotube sample in 6 h, whereas decolorization efficiency was 29.5% and 15.7% for nanofibers and nanoparticle samples, respectively. PrFeO3 nanotubes were used for three cycles of photodegradation, showing a slight deactivation.

Silver orthophosphate (Ag3PO4) has been extensively studied due to its good activity as photocatalyst for organic pollutants degradation under the action of visible light [135]. However, the photocorrosion together with the formation of metallic Ag in the surface of catalyst limit the stability and cause a decrease in the activity and reusability. Several strategies have been proposed to improve the photostability of Ag3PO4, among them, the synthesis of composites perovskite-type (ABO3) has attracted considerable attention due to its high catalytic activity. Guo et al. [136] synthesized and characterized several Ag3PO4/LaCoO3 composites with different ratios, which were evaluated in the photocatalytic degradation of bisphenol A. For the composite containing 10% of LaCoO3, the contaminant was completely degrader after 40 min, achieving a TOC removal of 77.3%. Furthermore, the authors investigated in detail the degradation intermediates and the photocatalytic mechanism.

It has been also found that by introducing perovskites in a TiO2 matrix, a beneficial effect in the photocatalytic activity of TiO2 can be obtained. Thus, Gao et al. [137] synthesized multi-modal TiO2-LaFeO3 composite films by a two-step method, which exhibited high photocatalytic activity in the degradation of MB aqueous solution (60% of degradation in 1 h). In comparison with TiO2 and LaFeO3 materials, the composite obtained exhibited good microstructural properties and high specific surface area. The introduction of LaFeO3 not only improved the photocatalytic activity and the hydrophilicity but also influenced the interfacial charge transfer process. The same composite, TiO2-LaFeO3, was prepared by Dhinesh et al. [138] by a hydrothermal method and tested in the degradation of methyl orange in aqueous solution under visible light irradiations. TiO2-LaFeO3 composite exhibited enhanced visible light photocatalytic properties in comparison with LaFeO3 nanoparticles due to a synergetic effect.TiO2 causes the inhibition of the recombination between photoinduced electron and hole pairs and LaFeO3 perovskite played an important role in extending light absorption into the visible region. Halide perovskite CsPbBr3/TiO2 composites [139] also showed an enhanced activity in the selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation. Action spectra and electron spin resonance studies showed that photo-excited electrons generated within CsPbBr3 were transferred to the conduction band of TiO2, forming, via the reduction of oxygen, superoxide radicals. 50% of benzyl alcohol was oxidized after 20 h of irradiation.

Table 3 summarizes the applications of perovskite-like oxides synthesized by different methods as photocatalysts for the degradation of different organics.



## **6. Processes under Dark Ambient Conditions**

As shown above, to date, great success has been achieved for producing visible-light photocatalysts by doping perovskites or designing composites. Other perovskites have been proven active in AOPS involving an additional chemical oxidant, such as ozone, hydrogen peroxide or peroxymonosulfate. Nevertheless, both kinds of processes using or light or chemical oxidant are expensive, thus limiting their practical applications. Therefore an effort has to be made to find new catalysts capable of working under dark ambient conditions, as potential low-cost alternative for the remediation of waters. A few examples of the application of perovskites for the degradation of organics in waters in dark ambient conditions are shown next.

A layered perovskite La2NiO4 crystal [140] can act as a round-the-clock photocatalyst and efficiently degrade phenolic pollutants in the dark. This photocatalyst can produce photoelectrons not only by visible light irradiation but also from some reactant molecules in the dark leading to the degradation of 4-chlorophenol (4-CP). 4-CP− anions can donate electrons to La2NiO4, what is followed by the reaction with dissolved oxygen to generate O2-• and reaction with H<sup>+</sup> to form HO• radicals, which can oxidize 4-CP• radicals into CO2. LaCoO3-x (x= 0-0.075) [141] calcined at different temperatures also displayed activity for the degradation of methyl orange in the dark. 17% of dye was removed with the best catalyst in 45 h in the absence of light. The degradation rate improved under the visible light due to the optical property of LaCoO3-x, achieving degradation values of 40%. Sr0.85Ce0.15FeO3-<sup>δ</sup> [142] can also work in the dark after thermal activation. SrFeO3 is known as photocatalyst since the bandgap energy values are comprised between 1.80 and 3.75 eV; however, the doping with Ce improves its redox properties and exerts a positive role in oxidation reactions. The catalyst was applied to remove RhB and Orange II from aqueous solution. In the first case the degradation after 7 h only increased from 40 to 60% in the presence of visible light. On the contrary, for the degradation of Orange II it was necessary to irradiate the catalyst, because only 5% was removed in the dark.

Other examples of the use of perovskites as catalysts for degradation of contaminants in aqueous solution in the absence of an oxidant or light have been described. In this regard, methyl orange (MO) was degraded by a layered perovskite, La4Ni3O10, without additional reagents or external energy [143]. The dye degradation occurred via electrons transfer from the dye molecules to the perovskite and then to the dissolved oxygen, which acted as electrons acceptor. The same dye, MO, was degraded by LaNiO3-<sup>δ</sup> under dark ambient conditions (room temperature and atmospheric pressure) [144]. Under the optimum conditions 94.3% of MO was degraded after 4 h. Authors concluded that MO was decomposed by two synergic effects derived from nickel present at the surface of LaNiO3-<sup>δ</sup> and the formation of lanthanum carbonate.

SrFeO3-<sup>δ</sup> perovskite synthesized by a combined high temperature and high-energy ball milling process was active in the degradation or bisphenol A (BPA) and Acid Orange 8 under dark ambient conditions [145]. The complete degradation of BPA was produced after 24 h, with a TOC removal of 83%. In the case of dye, the full decolorization was attained in only 1 h. By last, more recently, Chen et al. [146] tested a series of CaxSr1-xCuO3-<sup>δ</sup> (x= 0-1) perovskites in the removal of Orange II dye, widely used in the textile industry. Samples containing a higher amount of Ca were more active for the degradation of Orange II in dark conditions. A depletion in concentration of dye of 80% was reached in only 10 min, which increased to 95% after 1 h. However the mineralization was partial only and some by-products were formed, reaching a TOC removal of 60%. The catalysts were stable after 9 cycles of reusing.

## **7. Summary and Perspectives**

In this paper we have summarized the applications of perovskites as catalysts in heterogeneous advanced oxidation processes for the degradation of pollutants present in waters. Processes have been classified and revised according to the oxidant employed in the process, that is, ozone, hydrogen peroxide and peroxymonosulfate, which can be used alone or combined with light irradiation. The photocatalytic oxidation, consisting in the activation of a catalyst, in this case, a perovskite, by irradiation with UV or visible light, has also been revised.

The various systems described here using perovskites were shown to effectively degrade and remove specific pollutants from waters. Phenol has been the most studied but other pollutants, such as dyes (especially rhodamine B and methylene blue), phenolic compounds, herbicides and some drugs have also been reported. Although single ozonation (in the absence of catalyst) has been widely used in water and wastewater because is an effective oxidation process, the use of a catalyst improves the decomposition of ozone and the production of hydroxyl radicals and overall increases the degree of mineralization of the contaminants. However, the use of perovskites in this type of processes is very limited and only a few studies have been carried out. Significantly higher is the number of papers related to the application of perovskites in Fenton-like processes, using H2O2 as producer of radicals. The stabilization of cations with unusual oxidation states and redox properties in the perovskite network make these oxides good candidates for this type of reaction. Thus, most of reported perovskites contain iron (Fe2+/Fe3+), the Fenton reactant by excellence, in B position but other transition metals, such as copper, manganese or titanium have also been resulted active for the Fenton-like degradation of contaminants. When the H2O2/perovskite system is combined with the irradiation of light (photo-Fenton process) the degradation of contaminant and TOC removal generally increase because the rate of production of HO• radicals is higher.

Several examples of perovskites as activators of peroxymonosulfate (PMS) for the production of radicals able to degrade organics present in waters have been presented. It should be remarked that the treatment with PMS is more expensive than other AOPs, due to the price of the reagents. As cobalt is catalogued as one of the best transition metals in the homogeneous activation of PMS, most of the described examples for the heterogeneous activation by perovskites are based on those containing cobalt in B position, alone or substituted by other cations.

It should be pointed that one of the AOPS in which perovskites have been more extensively applied is the heterogeneous photocatalysis, because they present high activity in the long band of visible-light. These processes generally lead to higher TOC removal than processes based on a chemical oxidant. Perovskites have been used in photocatalytic degradation or organics alone or combined with TiO2 in form of composites, in this way narrowing the bandgap of this oxide. Additionally, the main strategy to improve the photocatalytic activity has been the substitution of the element in A or B position, which leads to the introduction of defects into the narrow band gap and to the formation of oxygen vacancies, which inhibit the recombination between the photogenerated electrons and the holes. Again, as in Fenton-like processes, perovskites containing iron in B position have been the most studied.

Perovskites have resulted been active by alone in the revised AOPs. However, majority of studies were carried out in semi-batch and batch reactors, while continuous fixed bed reactors, which are promising from the practical point of view, have not extensively studied for treatment of real wastewaters. In this sense it can be expected that in the future more studies will be devoted to them.

The low surface area of perovskites implies a limited interaction with the contaminants. In order to increase the surface area, new synthesis methods have been applied and some strategies have been developed, such as their supporting on mesoporous silica supports, honeycombs or the formation of composites. Another strategy to improve catalytic activity of perovskites in AOPs is the hetero-doping in order to produce more active sites of the low-valence B-site transition metals (i.e., Fe2+, Cu+ and Ti3+) or to introduce oxygen vacancies, which can facilitate the transformation of H2O2 (or PMS) into HO•. Furthermore, the use of nanostructures improves the catalytic behaviour with respect to the bulk samples. Considering all of this, we think that research of forthcoming years will be addressed to design new synthesis methods which allow the obtaining of perovskites in form of nanostructures, nanoparticles or nanofibers and also to search new materials based on perovskites containing other different active cations and exhibiting higher surface areas, which can be extended to the removal of other persistent contaminants present in waters.

By last, it should be remarked the high cost of AOPs, involving light or chemical oxidants, which usually have to been constantly fed to keep the process operative. Recent studies reporting promising results of the application of perovskites for degradation of some organics under dark ambient conditions, should encourage researchers in a short-term future to the search of similar systems capable of degrading other contaminants without necessity of using energy or reagents, which would considerably reduce the cost of the process.

**Author Contributions:** M.L.R.C. chose the topic and designed the organization of paper; M.L.R.C. and E.C. performed the literature search and wrote the article. M.L.R.C. corrected and revised the manuscript according to comments of referees.

**Acknowledgments:** M.L.R.C. thanks the supporting by the Spanish Ministry of Science and Innovation (CTM2014-56668-R).

**Conflicts of Interest:** The authors declare no conflict of interest.
