Perovskites as Catalysts in Advanced Oxidation Processes for Wastewater Treatment

Abstract

Advanced oxidation processes (AOPs), based on the formation of highly reactive radicals are able to degrade many organic contaminants present in effluent water. In the heterogeneous AOPS the presence of a solid which acts as catalyst in combination with other systems (O₃, H₂O₂, light) is required. Among the different materials that can catalyse these processes, perovskites are found to be very promising, because they are highly stable and exhibit a high mobility of network oxygen with the possibility of forming vacancies and to stabilize unusual oxidation states of metals. In this review, we show the fundaments of different kinds of AOPs and the application of perovskite type oxides in them, classified attending to the oxidant used, ozone, H₂O₂ or peroxymonosulfate, alone or in combination with other systems. The photocatalytic oxidation, consisting in the activation of the perovskite by irradiation with ultraviolet or visible light is also revised.

1. Introduction

Advanced oxidation processes (AOPs) are based on the generation of radical intermediates, mainly hydroxyl radicals (HO•), in amount enough to be able to attack and oxidize either partially or fully most of the recalcitrant chemicals present in the effluent water, such as pesticides, dyes, pharmaceuticals and so on [1]. The processes based on the free radicals occur at higher rates of degradation than those based on other chemical oxidation technologies and are not highly selective [2,3]. The high oxidation potential of hydroxyl radicals (2.80 v) make them capable of attacking organic compounds by abstracting a hydrogen atom or by adding to the double bonds, carrying out their mineralization by transformation into more oxidized intermediates, carbon dioxide, water and inorganic salts. These reactions of hydroxyl radicals with organic compounds can be written as follows:

HO• + R-H → H₂O + R•

(1)

HO• + C=C → HO-C-C•

(2)

HO• + Ph-H → Ph-H(OH)•

(3)

AOPs can be classified in several categories, depending on the different reagent systems used for the generation of hydroxyl radicals. Attending to the reaction medium, these advanced oxidation processes can be classified either as homogeneous or heterogeneous [4]. The first ones can be subdivided in turn in those using energy (ultraviolet or visible radiation, ultrasound energy, electrical energy) and those not involving energy (ozone (O₃) in alkaline medium, O₃/H₂O₂ and H₂O₂/homogeneous catalyst, generally Fe²⁺, known as Fenton process). The heterogeneous processes can be classified in four main groups: (i) catalytic ozonation, which uses the combination of O₃ and a solid catalyst; (ii) photocatalytic ozonation, under the action of O₃/light (UV or visible)/solid catalyst; (iii) Fenton-like processes, which are produced by the action of H₂O₂/solid catalyst, containing mainly the Fe²⁺/Fe³⁺ couple but also other transition metal ions with multiple oxidation states; when they are combined with the action of light they are called Photo-Fenton processes; and iv) photocatalytic oxidation, by combination of light (UV or visible) and a solid catalyst.

It must be remarked that the single ozonation or the use of only H₂O₂ belong to the class of chemical oxidation technologies, as they work on the direct attack of the oxidants, not being considered as AOPS, because they do no generate hydroxyl radicals by themselves [5]. Only when O₃ and H₂O₂ are combined between them or its individual action is supplemented by other dissipating energy components, such as UV/visible light or ultrasound or by activation with a catalyst, the formation of free radicals occurs and they can be considered AOPs.

In the last fifteen years some reviews in literature have devoted to show the state of art of AOPS for wastewater treatment [4,5,6,7] and in particular Fenton and photo-Fenton processes [8,9]. Most of these oxidation technologies are usually expensive and in addition, they are unable to completely degrade the organic compounds present in real wastewater and they cannot process the large volumes of waste generated. However, AOPs can degrade the residue up to a certain level of toxicity and then the intermediate can be furtherly degraded by the conventional methods. Furthermore, the combination of AOPs (as a pre-treatment or post-treatment stage) with a biological treatment contributes to reduce operating costs of the global process [10].

As mentioned above, the heterogeneous advanced oxidation processes generally use solid catalysts in combination with other systems (O₃, H₂O₂, light) to carry out the degradation of organics. The main advantage of heterogeneous catalysts with respect to the homogeneous ones is the facility of separation of the product and of the recovery of the catalyst. However, to be applied in the industry, heterogeneous catalysts must satisfy some specifications, such as high activity, thermal, mechanical, physical and chemical stability and resistance to the deactivation.

Perovskite-type oxides of the general formula ABO₃, where A is a rare earth metal and B a transition metal, have attracted the attention of many scientists because of their unique structural features. They have a well-defined structure, which allows the introduction of a wide variety of metal ions in both A and B positions [11,12]. Its structure is represented in Figure 1. The partial substitution of these cations by other foreign leads to changes in the oxidation states of metal ions and to the formation of oxygen vacancies. The thermal and hydrothermal stability of perovskites is quite high and as a result, they can be applied to gas or solid reactions carried out at high temperatures or liquid-phase reactions occurring at low temperatures [13,14]. The high mobility of network oxygen and the stabilization of unusual oxidation states confer them a diversity or properties, which allows their application as solid oxide fuel cells [15], magnetic and electrode materials [16], chemical sensors [17], adsorbents [18] and heterogeneous catalysts in industrial reactions [11,14,19]. One of the potential applications of perovskites is as catalysts in carbon-based electrodes, which has incited many scientists to study the mechanism of the catalytic decomposition of H₂O₂ by perovskites [20,21,22].

Catalysts used in oxidation technologies can be classified as follows: (i) metal catalysts, usually supported on a metal oxide surface (TiO₂, Al₂O₃, ZrO₂ and CeO₂) or on active carbon; (ii) metal oxide catalysts and (iii) organometallic catalysts. Different heterogeneous catalysts have been applied in some AOPs. As an example, the use of several heterogeneous systems containing iron species stabilized in a host matrix, such as oxides [23,24,25,26], clays [23,27,28,29,30], zeolites [29,31,32] or carbon materials [33,34,35,36,37] in the Fenton and the Fenton-like processes has been reported. In the last years perovskites have been applied as Fenton catalysts because of their versatile composition and high stability [19]. Furthermore, the existence of redox active sites in B cations and oxygen vacancies may facilitate the transformation of H₂O₂ into HO• [38,39,40].

There are some reviews in literature describing the employment of clays and mineral oxides, mainly of iron, in Fenton-like processes [23,25,26]. However, until our knowledge, there is no any review reporting the use of perovskites in AOPs. In the present paper we describe the utilization of perovskites and like-perovskite oxides in this kind of processes, classified as a function of the oxidant reactant responsible for generating the free radicals, by alone or combined with other systems, giving place to the hybrid methods. Thus, in first place, in Section 2, we report the processes based on ozone, O₃, including catalytic ozonation (O₃/catalyst) and photocatalytic ozonation (O₃/catalyst/light). In the following section we revise the use of H₂O₂ as oxidant in three kinds or systems: (i) Fenton-like reactions (H₂O₂/catalyst); (ii) photo Fenton-like reactions (H₂O₂/catalyst/light); and (iii) catalytic wet peroxide oxidation, CWPO, (H₂O₂/catalyst/air). In the last fifteen years a new chemical oxidant, peroxymonosulfate (PMS), has aroused a great interest as alternative to others as H₂O₂ or O₃. For that reason, the fourth section is devoted to describing the use of PMS, activated by a catalyst (perovskite) or by the combination of both a catalyst and light irradiation. The photocatalytic oxidation, consisting in the activation of perovskite by irradiation with UV or visible light, is revised in Section 5. Finally, a short section devoted to the degradation under dark ambient conditions, show some examples in which perovskites catalyse the oxidation of organics in the absence of light and without additional chemical oxidant. Although these processes should not be considered strictly AOPS, because radicals are not formed, some authors consider them as novel advanced oxidation technologies for low cost treatment of wastewaters.

2. Processes Based on Ozone

Heterogeneous catalytic ozonation is getting an enormous attention in the treatment of drinking and waste water, because of its capacity to improve the mineralization and degradation of organic pollutants, its manufacturing simplicity and economic nature. There are many advantages to the use of ozone compared to other conventional technologies due to its high oxidizing power (2.07 V). Ozone can degrade organic pollutants by direct electrophilic attack with molecular ozone or by indirect attack with hydroxyl radicals HO•, which are generated through its decomposition process. Single ozonation has been widely used in water and wastewater because is an effective oxidation process. However, several disadvantages can limit its application such as slow and incomplete oxidation or mineralization (the intermediates are not totally oxidised to CO₂ and water) and the low solubility and stability of ozone in water. At first sight, heterogeneous catalytic ozonation does not present these drawbacks although much research is still needed. Catalytic ozonation utilizes solid catalysts in order to improve the decomposition of ozone and to enhance the production of hydroxyl radicals, HO•. In general, the heterogeneous catalysts decompose the ozone into caged or free radicals or simply they adsorb reactants facilitating their reaction. To date, several types of heterogeneous ozonation catalysts such as metal oxides, supported metal oxides and carbon materials have been tested with promising results. Perovskites-type metal oxides constitute undoubtedly an interesting alternative due to the high stability under aggressive conditions, high degree of stabilization of transition metals in their oxidation states and high oxygen mobility. However, the catalytic ozonation mechanism with perovskites is still a challenge for chemists and not many detailed studies are available. Most of the studies have been carried out by the same research group [41,42,43,44].

The first studies concerning catalytic ozonation with active perovskites appeared in 2006 [41]. The authors studied the ozonation decomposition of pyruvic acid, a refractory substance typically produced after oxidation of phenol-like compounds, in the presence of LaTi_0.15Cu_0.85O₃ perovskite. This perovskite was active and stable in the ozonation process being oxalic and acetic acids the only intermediates formed. Experimental results clearly indicated that typical operating parameters like ozone concentration, mass of catalyst or temperature, performed a key role on the pyruvic acid ozonation. The catalyst exhibited high stability and its catalytic activity improved after the first use. For instance, after 150 min of reaction the pyruvic acid had practically been eliminated, in contrast to the 67% of conversion achieved with fresh catalyst for the same period. Regarding kinetic considerations, authors proposed a Langmuir–Hinshelwood mechanism derived from bi-adsorption of pyruvic acid and ozone on different active sites and successive reaction of pyruvic acid with the O• radicals on the surface through reactions 1 and 2, before occurring the desorption of formed products (see Scheme 1).

Carbajo et al. [42] extended the study of the removal of pyruvic acid from water through catalytic ozonation to other perovskites, LaTi_1-xCu_xO₃ and LaTi_1-xCo_xO₃, which were compared with other catalysts, such as Ru-Al₂O₃, Ru-CeO₂, FeO(OH) and MCM-41, this last impregnated with copper or cobalt. The results showed that only perovskites and Ru-CeO₂ catalysts increased significantly the pyruvic acid depletion with respect to the produced in the absence of catalyst. Conversion values of 80% were reached after 2 h of catalytic ozonation. Due to the low adsorption capacity of the catalysts to adsorb the pyruvic molecules, authors concluded that the catalytic ozonation mechanism was governed by surface reactions involving adsorbed ozone and dissolved pyruvic acid.

Another pioneering work with LaTi_0.15Cu_0.85O₃, the same catalyst used in Reference [41], was carried out to eliminate gallic acid, a primary intermediate of benzoic acid oxidation [43]. The role of different operating variables was studied. Whereas the catalyst and ozone doses exerted a positive influence in the ozonation rate, the increment in the initial acid gallic concentration diminished the conversion. The activity of the catalyst in terms of acid elimination was kept for consecutive cycles. However, the catalyst displayed a partial deactivation in terms of total organic carbon (TOC) elimination after the second reuse. In any case, the TOC degree was still higher than the one achieved in the non-catalytic system.

Finally, Carbajo et al. [44] went a step further in analysing the activity of the same catalyst, LaTi_0.15Cu_0.85O₃, in the ozonation of four real phenolic wastewaters coming from agro-industrial field, a wine distillery industry, olive debittering and from olive oil production. The main goal was to study the activity and stability of the catalyst together with the influence of the different operating variables. The results suggested that if enough time was allowed the catalytic ozonation of the phenolic mixture achieved 100% of mineralization. Moreover, the increment of temperature promoted the mineralization level.

Some of the compounds that cannot be easily removed from drinking water or wastewater by classical treatments are pharmaceutical compounds. Catalytic ozonation allows high removal of organic carbon of these compounds being the most appropriate process. As a first approach Beltran et al. [45] tested two copper and cobalt perovskites, LaTi_0.15Cu_0.85O₃ and LaTi_0.15Co_0.85O₃, as catalysts to remove in the presence of ozone sulfamethoxazole, a synthetic antibiotic usually found in municipal wastewaters. Some experiments were also carried out in the presence of activated carbon, as promoter of the activation of ozone. The results showed that catalytic or promoted ozonation were not necessary to eliminate sulfamethoxazole from water, because it can be removed only by the action of ozone. However, from a practical point of view, the combined ozone processes are clearly recommended in order to remove the resulting total organic carbon (TOC). The catalytic ozonation of two pharmaceutical compounds, the drug diclofenac and the synthetic hormone 17-ethynylstradiol, was also conducted on the same perovskites by the same authors [46], obtaining similar results to those observed in their previous work. Both compounds can be eliminated by direct ozonation; however, when copper perovskite was used, the TOC removal reached the 90% after 2 h of reaction.

To date, the main advantage of catalytic ozonation is the ability to improve the mineralization degree achieved at the end of the process. In this sense, non-substituted perovskites type LaBO₃ (B=Fe, Ni, Co and Mn) and substituted perovskites type LaB_xCu_1-xO₃ (B=Fe and Al), have been proposed as effective catalysts in ozonation processes of oxalic acid and dye C.I. Reactive Blue 5 [47]. Most of perovskites tested showed better performances in the catalytic ozonation of oxalic acid than single ozonation. On the contrary, in the case of the removal of dye, conversion values reached through single ozonation were slightly higher than in catalysed systems. However, the results in terms of TOC removal were better in the presence of catalyst and LaCoO₃ allowed almost complete mineralization of dye after 3 h of reaction under the conditions tested. A key factor in the removal of contaminants seems to be the presence of lattice oxygen vacancies, which are able to activate adsorbed species.

In general, the catalytic activity seems to be enhanced by high surface area and easy access of reactants to the active sites. Then high surface area could be of interest to improve the activity of catalysts. Concerning to that, Afzal et al. [48] studied the behaviour of high surface area perovskites for catalytic ozonation of 2-chlorophenol. A nanocasting technique (NC), using SBA-15 as a template, was employed for the synthesis of NC-LaMnO₃ and NC-LaFeO₃ catalysts, with high surface area. Authors compared these catalysts with the same perovskites synthesized by conventional citric acid (CA) assisted route, CA-LaMnO₃ and CA-LaFeO₃, as well as with Mn₃O₄ and Fe₂O₃. They found that NC-perovskites, containing easily accessible active sites, showed higher catalytic activity (80% of TOC removal) than their counterpart CA-perovskites (35% of TOC removal). Mn₃O₄ and Fe₂O₃ were the worst catalysts.

Bromide, Br−, is usually present as micro-emerging pollutant in some water matrixes to be degraded. The ozonation treatment leads to the formation of bromate, BrO₃⁻, which is considered a potential carcinogen to humans by the World Health Organization, therefore being necessary its removal from the water. Zhang et al. [49] tested two perovskites, LaCoO₃ and LaFeO₃, as catalysts for the simultaneous removal of BrO₃⁻ and benzotriazole (BZT) in the presence of ozone. 71% of BrO₃⁻ was eliminated and BZT was completely degraded in only 15 min when LaCoO₃ was used. LaFeO₃ resulted in being inactive for BZT degradation; however, it reduced BrO₃⁻ to HOBr/OBr⁻ efficiently. The surface hydroxyl groups present in both perovskites were key in the involved reactions.

When the catalytic ozonation does not lead to a high degree of mineralization, it is necessary to combine it with other oxidation systems. The integration of catalytic ozonation and photocatalysis seems to be the most appropriate solution [50]. Photocatalytic ozonation is an advanced ozonation route allowing high removal of organic carbon by combining the beneficial effects of ozonation with the generation of hydroxyl radicals via electron hole formation (free radical oxidation). Considering that perovskites have been successfully used in catalytic ozonation, some authors studied the O₃/UVradiation/perovskites system. Thus, Rivas et al. [51] carried out the advanced oxidation of pyruvic acid in presence of LaTi_0.15Cu_0.85O₃ with several oxidation systems: O₃, UV radiation, O₃/UV radiation, O₃/perovskite, UV radiation/perovskite, O₃/UV radiation/perovskite, H₂O₂/UV radiation, H₂O₂/UV radiation/perovskite, being O₃/UV radiation/perovskite the system investigated in more detail. The efficiency of the oxidation systems was examined in terms of the economic cost as a function of removal percentage of pyruvic acid and TOC. As expected, ozone was not able of eliminating pyruvic acid, displaying conversion values around 20% after 3 h of reaction. In contrast, application of UV radiation led to 40% of pyruvic acid elimination. Finally, for the combined O₃/UV radiation/perovskite system, the pyruvic acid removal reached 100%, while the mineralization degree obtained was 80%. Therefore, under the operating conditions investigated, the photocatalytic ozonation seems to be the best option.

3. Processes Based on Hydrogen Peroxide

3.1. Fenton-like Reactions (H₂O₂/catalyst)

The homogeneous Fenton system implies the reaction of Fe²⁺ with H₂O₂ to generate hydroxyl radicals (Equation (4)), with a high reactivity and high oxidant power, capable of oxidize organics according to Equations (1)–(3).

Fe²⁺ + H₂O₂ → Fe³⁺ + HO• + HO⁻

(4)

The generated HO• radicals can re-combine with Fe²⁺:

Fe²⁺ + HO• → Fe³⁺ + HO⁻

(5)

The ferric ions formed may decompose the hydrogen peroxide into water and oxygen, following the Equations (6)–(10), in which ferrous ions and radicals are also generated. The reaction of H₂O₂ with Fe³⁺ is referred in literature as Fenton-like reaction [52].

Fe³⁺ + H₂O₂ → Fe-OOH²⁺ + H⁺

(6)

Fe-OOH²⁺→ HO₂• + Fe²⁺

(7)

Fe²⁺ + HO₂• → Fe³⁺ + HO₂⁻

(8)

Fe³⁺ + HO₂• → Fe²⁺ + O₂ + H⁺

(9)

HO• + H₂O₂ → H₂O + HO₂•

(10)

Other reactions involving radicals in the Fenton process are:

H₂O₂ + HO• → HO₂• + H₂O

(11)

HO₂• + HO₂• → H₂O₂ + O₂

(12)

Notice that by reaction (11) H₂O₂ acts as sink for HO•, diminishing the oxidizing power of the Fenton reactants.

The homogeneous Fenton system, which implies the reaction of Fe²⁺/Fe³⁺ in solution with H₂O₂, has several drawbacks. By one hand, the chemical reactivity of iron is strictly dependent on the pH and only at pH ≈ 3, all three Fenton-active species of Fe²⁺, Fe³⁺ and Fe(OH)²⁺ coexist together. On the other hand, the final effluent contains high metal concentrations, which have to be recovered by additional treatment. In the heterogeneous systems Fe²⁺ and Fe³⁺ are part of a solid, which results in different advantages, especially related to recovery of the catalyst and the low leaching of ions.

The Fenton-like reactions involved in a heterogeneous system are the following:

S-Fe²⁺ + H₂O₂ → S-Fe³⁺ + HO• + HO⁻

(13)

S-Fe³⁺ + H₂O₂ → S-Fe²⁺ + HO₂• + H⁺

(14)

S-Fe²⁺ + HO• → S-Fe³⁺ + HO⁻

(15)

S-Fe³⁺ + HO₂• → S-Fe²⁺ + O₂ + H⁺

(16)

Organic + HO• → R• + H₂O →…→ CO₂ + H₂O

(17)

where S represents the surface of the catalyst. The reaction (14) is rate-limiting since its rate constant is ca. four orders of magnitude lower than that of reaction (13).

Although the classical Fenton system is based on the use of Fe²⁺/H₂O₂, other elements with multiple redox sates (like chromium, cerium, copper, cobalt, manganese and ruthenium) can directly decompose H₂O₂ into HO• through conventional Fenton-like pathways [53]. Therefore, perovskites containing these elements, mainly in B position, can be used in this kind of reaction.

Rhodamine B (RhB) is one of the most studied organic pollutants in water in the Fenton-like reactions [54,55,56,57,58]. The first study reporting the use of perovskites as catalysts for the removal of RhB in Fenton-like reactions was carried out by Luo et al. [54]. In this work, BiFeO₃ magnetic nanoparticles (BFO MNPS) prepared by sol-gel method were tested in the degradation of RhB in the presence of H₂O₂ at 25 °C and pH = 5. According to the isoelectric point of BFO MNPs (I.P. = 6.7), under these conditions, the anionic form of the dye (pKa = 3.7) interacts easily via electrostatic forces with the positively charged catalyst particles. By selecting initial H₂O₂ and catalyst concentrations as 10 mM and 0.5 g/L, respectively, 95.2% of RhB was degraded in 90 min and a TOC removal of 90% was achieved within 2 h, in contrast to the removal of only 10% of RhB in the presence of Fe₃O₄ nanoparticles. By Montecarlo (MC) simulations authors concluded that after the adsorption of H₂O₂ molecules on the surface hollow sites of BFO MNPs facets, they are activated to generate HO• radicals, which then decompose RhB into other smaller organic compounds and CO₂. BFO MNPs showed excellent chemical stability during reaction (as checked by XPS), being reusable for at least five cycles, without a significant loss of activity. BFO MNPs were also tested in the degradation of methylene blue and phenol, leading to 79.5% and 82.1% of removal, respectively.

Zhang et al. [55] synthesized a series of Cu-doped LaTiO₃ perovskite (LaTi_1−xCu_xO₃, x = 0.0–1.0) by a sol-gel method, which resulted be very efficient for the degradation of RhB with H₂O₂ in a pH range of 4–9. In contrast to the absence of activity of sample containing only titanium, the coexistence of Ti³⁺/Ti⁴⁺ and Cu⁺/Cu²⁺ in the perovskite structure of partially substituted samples allowed the degradation of 8 mg/L of RhB through redox cycles involving the transformation of H₂O₂ into HO• and HO₂•/O₂•⁻. For a H₂O₂ concentration of 10 mM, about 84% of RhB was decolorized within 2 h in the presence of 1.4 g/L of LaTi_0.4Cu_0.6O₃. Notice that the amount of RhB degraded was slightly lower than the observed by Luo et al. [54]; although the catalyst amount used in Reference [55] was almost three times higher, the initial concentration of RhB was approximately the double. The reduction of H₂O₂ to O₂, which is carried out by oxygen vacancy [22], was not observed in this reaction.

The surface area of perovskites is low and as a consequence, the interaction between the contaminants and the active sites is limited. In order to improve the catalytic efficiency of perovskite-like oxides by increasing their surface area, some strategies have been developed, such as their supporting on mesoporous silica supports [56,57,58] or in honeycombs [59] and the formation of nanocomposites [60,61].

In this sense, La-FeO₃/SBA-15 [56] was more efficient than non-supported LaFeO₃ for catalysing RhB oxidation in the presence of H₂O₂ under ambient conditions due to a synergic effect between the large capacity of mesoporous SBA-15 for RhB adsorption and the high number of active sites exposed in LaFeO₃ nanoparticles for reacting with H₂O₂. The best catalyst was the sample containing many oxygen vacancies (as deduced from XPS results), which are a key factor influencing the performance of these catalysts in oxidation reactions. The catalyst was efficient in a wide pH range (2–10). Under the optimum conditions, a degradation of RhB of 87% was achieved after 3 h. No leaching of Fe³⁺ was observed in the solution after reaction, the contribution of homogeneous Fenton reaction being discarded. The stability of La-FeO₃/SBA-15 was also confirmed by carrying out four cycles of reutilization, which showed no deactivation of the sample. The catalyst was also applied for the degradation of other organic dyes, achieving a decomposition of 66% for methylene blue and 42% for brilliant red X-3B and direct scarlet 4BS.

The good synergy between the support and the LaFeO₃ perovskite was explored by the same authors [58], who tested different supports based on mesoporous silica, such as SBA-15, SBA-16 and MCF and on nanosized silica powders (NSP). Different factors influence on the catalytic behaviour for degradation of RhB. By one hand, the RhB adsorption on the support is a crucial step of the reaction and as a result, the combination of LaFeO₃ with a non-porous support showing a low capacity of adsorption decomposed the RhB in a little extent. On the other hand, a network of pores with short length is necessary to allow the transportation of RhB to the active sites of LaFeO₃. In this sense, the shorter the pore length, the faster the RhB molecules reached the catalytic centres and were oxidized (see the transport process in Figure 2). Authors concluded that LaFeO₃ supported on MCF containing randomly distributed pores with short length was the best catalyst for oxidative degradation of RhB in aqueous solution, achieving a removal of the contaminant of 97% in 2 h.

SBA-15 was also used by the same authors as support of a perovskite-type oxide La₂CuO₄ containing a few amounts of CuO [57]. The solid was tested in the degradation of RhB and organic dyes, including reactive brilliant red X-3B, direct scarlet 4BS and methylene blue under ambient conditions. The catalyst was active in a wide pH range (2–10) and depletion of RhB between 85% and 95% was produced after 3 h, depending on the amount of catalyst. The mineralization of RhB into CO₂ was completed and the catalyst could be recycled. Although the activity decreased in ca. 14% in the fifth cycle, it could be recovered after a treatment of the used catalyst in air at 500 °C for 2 h.

Another approach for modifying the surface properties and reactivity of perovskites is the formation of nanocomposites [60,61]. In this regard, a novel 3D perovskite-based composite BiFeO₃/carbon aerogel (BFO/CA) prepared by sol-gel method led to a 95% of degradation of ketoprofen in 150 min and a TOC removal of 60% after 5 h [60]. These activities values were significantly higher than those obtained for bulk BFO and nano BFO, due to the higher reducibility of Fe³⁺ and Co³⁺ species in the composite, as deduced from TPR studies and to the dispersion of active sites not only on the surface of CA support but along the 3D structure of CA. Furthermore, the catalyst was active in a wide pH range of 3–7 and the leaching of iron was low.

A La_1+xFeO₃ (L_1+xFO, 0 ≤ x ≤ 0.2) nanocomposite formed between LaFeO₃ and an inert La₂O₃, resulted to be twice more active for degradation of methyl orange that the pristine LaFeO₃ [61]. The modification of surface properties, such as surface Fe²⁺ concentration, surface defects, H₂O₂ adsorption capacity and charge-transfer rate led to an enhanced Fenton-like activity in the composite. The most notorious aspect of this work was that the major reactive species were not hydroxyl radicals but singled oxygen (¹O₂), as deduced from in situ electron paramagnetic resonance analysis and radical scavenging experiments. Authors proposed the corresponding mechanism of ¹O₂-based composite/H₂O₂ system. 100% of contaminant was degraded in 90 min at pH = 3 and a total organic carbon (TOC) removal of 96% was achieved after 4 h.

Other contaminants degraded by LaFeO₃ perovskite, in this case auto supported, were different pharmaceutical and herbicides [62]. Among them, sulfamethoxazole (SMX) was completely removed in LaFeO₃–H₂O₂ system after 2 h at neutral pH. By formation of a surface complex between LaFeO₃ and H₂O₂, the O-O bond in H₂O₂ is weakened and chemical environment of iron changes, the Fe^3+/Fe²⁺ redox potential decreasing significantly, which accelerates the cycle of Fe^3+/Fe²⁺ and produces more HO• and O₂•⁻/ HO₂• radicals, enhancing the Fenton-like removal of organic compounds. The TOC removal was 22% in 2 h and SMX was transformed into simpler aliphatic acids, mostly biodegradable.

Due to its abundance in most of wastewater effluents and its toxicity, phenol is a usual organic compound model in developing methods for water remediation, including AOPs. The removal of phenol and phenolic compounds has been tested in Fenton-like reactions on different perovskites, mainly containing iron or copper in B position [63,64,65]. LaFeO₃ and BiFeO₃ were tested by Rusevova et al. [63] in the degradation of phenol. The influence of reaction temperature, catalyst and H₂O₂ concentrations and pH, on the catalytic behaviour was studied. The rate constant for phenol degradation, which increased with temperature, was 3-fold higher when initial reaction pH diminished from 7 to 5. Conversion values of phenol of 90–95% were achieved after 6 h and leaching of metals was negligible. The most new-fangled aspect of the study was that in order to settle the nature of the active oxidizing species authors used compound specific stable isotopic analysis (CSIA) as alternative to other conventional techniques. Based on their results, authors concluded that the major species involved in phenol degradation were hydroxyl radicals. They extended the application of both LaFeO₃ and BiFeO₃ to the removal of methyl terc-butyl ether (MTBE), for which a depletion of 80% was obtained after 6 h.

Different perovskite-like oxides LaBO₃ (B = Cu, Fe, Mn, Co, Ni) synthesized using the Peccini method were tested in Fenton-like degradation of phenol but only LaCuO₃ and LaFeO₃ were active [64]. Authors studied the recyclability of the catalysts during 3 cycles for LaCuO₃ and 40 cycles for LaFeO₃.The induction period observed in the first cycle for LaFeO₃ was significantly shortened for the second and successive cycles. In this sense, a degradation of phenol of 75% was produced in 5 h in the second cycle, in contrast to the 22% observed in the first one. The reasons for this improvement in the activity were an increase in the surface concentration of oxygen containing species (water and carbonate) involved in the transformation and the formation of dispersed particles of iron oxides on the surface. The TOC conversion of 21–22% after 10 h did not change for the different cycles.

Hammouda et al. [65] prepared ceria perovskite composites CeO₂-LaCuO₃ and CeO₂-LaFeO₃, which were more active for the degradation of bisphenol than non-doped perovskites, especially at short reaction times. Furthermore, CeO₂ improved the stability of perovskites towards leaching of metals. Authors attributed the enhancement in the activity to the fact that, as observed by XPS, more Ce³⁺ ions were formed in the ceria-perovskite catalysts, due to an electron transfer from the transition metal of perovskites to the CeO₂. As a result, more oxygen radicals were formed by interaction of H₂O₂ with Ce³⁺, favouring the Fenton-like degradation of the contaminant. By following the evolution of the intermediates formed, authors proposed a mechanism of reaction and a degradation pathway.

In order to improve the catalytic ability of BiFeO₃ nanoparticles to degrade recalcitrant pollutants, some authors have proposed an in-situ surface modification by using chelating agents [66,67]. In this regard, the bisphenol A (BPA) degradation in a wide pH range (4–9) was accelerated when the nano-BiFeO₃ were modified by adding different ligands to the Fenton solution, such as tartaric acid, formic acid, glycine, nitrilotriacetic acid and ethylenediaminetetraacetic acid (EDTA) [66]. EDTA was the most efficient chelating agent, mainly because of a higher HO• formation from the H₂O₂ decomposition. Under the optimum conditions 91.2% of BPA was removed within 2 h, in contrast to the 20% of BPA degraded with unmodified BFO. Although the use of chelating agents increased the contribution of Fenton homogeneous reaction by formation of soluble iron complexes, the trend observed in the BPA degradation for reactions carried out with different ligands did not follow the order of leached ions, indicating the irrelevant contribution of homogenous reaction to BPA degradation. As EDTA was the most efficient chelating agent, it was also used by same authors [67] as ligand for BiFeO₃ in the degradation of triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol), a broad-spectrum antibacterial agent widely used in personal and health care products. When pristine BFO were used, triclosan was mainly transformed into 2,4-dichlorophenol, a carcinogenic compound. The addition of EDTA modified significantly the dechlorination ratio of triclosan, which increased from 26.4% for H₂O₂-BFO sample up to 97.5% in the chelated system. Triclosan was degraded almost completely in 3 h under the optimal conditions.

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 into HO• [39,40]. In this sense, some perovskites containing partially substituted manganese in B position, have been tested in Fenton-like reactions for the degradation of methylene blue (MB) [68], different dyes [69] and paracetamol [70].

Maghalaes et al. [68] tested LaMn_1-xFe_xO₃ and LaMn_0.1-xFe_0.9Mo_xO₃ perovskites in the decomposition of H₂O₂ to O₂ and in the oxidation of MB. The presence of manganese in the perovskites seemed to play an important role on the H₂O₂ decomposition rate, which decreased with the amount of Mn substituted by Fe and/or Mo. However, LaMnO₃ was not active for the MB discoloration, which suggested that it was able to transform the H₂O₂ into O₂ but it was unable to form the HO• radicals, necessary to degrade the dye molecules. On the contrary, samples substituted by Mo degraded MB up to 20% in 1 h.

Jahuar et al. [69] synthesized a series of manganese-substituted lanthanum ferrites having compositions LaMn_xFe_1-xO₃ (x = 0.1–0.5) by a sol–gel auto-combustion method, which were used as catalysts in the removal of anionic dyes (Remazol Turquoise Blue, Remazol Brilliant Yellow) and cationic dyes (MB, Safranine-O) by the action of H₂O₂, in the absence and presence of visible-light. The initial pH of solution was fixed in all cases to the value of 2. Unsubstituted LaFeO₃ produced a low dye degradation for long time periods, exhibiting a poor catalytic activity under dark conditions. However, the partial substitution of iron by manganese led to catalysts able to degrade over 90% of dye in time periods of 150–300 min, due to the Fenton-like activity of manganese ions, capable of existing in various oxidation states. In the presence of light, an enhancement in the catalytic activity was produced and degradation times were reduced to 25–70 min.

The contribution of manganese ions to Fenton-like reaction was, on the contrary, discarded by Carrasco-Díaz et al. [70] in the decomposition of paracetamol by H₂O₂ under mild reaction conditions (25 °C and pH ≈ 6) in the presence of LaCu_xM_1-xO₃ (0.0 ≤ x ≤ 0.8, M = Mn, Ti) perovskite-like oxides prepared by amorphous citrate decomposition. Degradation values of paracetamol between 80% and 97% were achieved after 5 h. XPS studies of the catalysts allowed authors to conclude that Cu^2+/Cu⁺ were the catalytically active species, the catalysts containing a higher amount of copper at the surface, mainly as Cu²⁺, being the most active. The titanium and manganese species seemed not to be responsible of the enhanced activity observed in some of the substituted samples with respect to that of LaCuO₃. The catalysts were recyclable for at least three cycles and a negligible leaching of metals was produced. TOC values of 47–54% were achieved.

Finally, some mathematical analysis of the heterogeneous oxidations of contaminants by perovskites have been carried out. More concretely, mathematical modelling of photo-Fenton-like oxidation of acetic acid by LaFeO₃ has been reported [71,72]. From the experimental results authors concluded that the main reactions occurring in the system were the complete mineralization of acetic acid by H₂O₂ due to the presence of the catalyst and the decomposition of H₂O₂ into water and O₂ in the homogeneous phase. Therefore, this kind of reaction should not be considered as an AOP, because no hydroxyl radicals were formed.

Table 1. Oxides type perovskite used in Fenton-like and photo Fenton-like reactions of organic pollutants in aqueous solution.

			Concentration			Treatment Efficiency
Reference	Catalyst	Target Pollutant	Catalyst	Pollutant	H2O2	Degradation of Pollutant	TOC Removal
[54]	BiFeO3	RhB, phenol, MB	0.5 g/L	4.79 g/L	10 mM	95.2% (RhB) in 90 min	90% in 2 h
[55]	LaTi1-xCuxO3 (x = 0.0–1)	RhB	1.4 g/L	8 g/L	10 mM	84% in 2 h	-
[56]	LaFeO3/SBA-15	RhB, MB, brilliant red X-3B, direct scarlet 4BS	2 g/L	9.1 mg/L	0.34 mM	87% in 3 h (RhB) 66% MB 42% for the others	-
[57]	La-Cu-O/SBA-15	RhB and other dyes	Not indicated, [RhB]/[catalyst]= 0.0045, 0.077	-	-	85-95% in 3 h	100% in 3 h
[58]	LaFeO3/SBA-15, SBA-16, MCF and non-porous silica	RhB	2 g/L	9.6 mg/L	0.34 mM	97% in 2 h for LaFeO3/MCF	-
[60]	BiFeO3/carbon aerogel (BFO/CA)	ketoprofen	0.3 g/L	40 mg/L	12 mM	95% in 150 min	60% in 5 h
[61]	La1+xFeO3 (L1+xFO, 0 ≤ x ≤ 0.2) nanocomposite	Methyl orange	0.5 g/L	5 mg/L	0.198 M	100% in 90 min (pH 3)	96% in 4 h
[62]	LaFeO3	Different herbicides and pharmaceutical	1.4 g/L	3 mg/L	23 mM	100% of SMX in 2 h (pH 6.5)	22% in 2 h
[63]	LaFeO3, BiFeO3	phenol, MTBE	0.01–1 g/L	25 mg/L phenol 50 mg/L MTBE	3 g/L = 88 mM	90–95% phenol in 6 h 80% MTBE in 6 h	-
[64]	LaBO3 (B= Cu, Fe, Mn, Co, Ni)	phenol	5 g/L	0.01 M	0.7 M	85% in 10 h	21% in 10 h
[65]	Ceria-LaCuO3, ceria-LaFeO3	Bis-phenol	0.2 g/L	20 mg/L	10 mM	98% in 45 min (ceria-LaCuO3) 92% in 42 min (ceria-LaFeO3)	-
[66]	BiFeO3 modified by chelating agents	BPA	0.5 g/L	0.1 mM	10 mM	91.2% in 2 h	-
[67]	BiFeO3 (BFO); BFO modified by EDTA	Triclosan	0.5 g/L	34.5 mM	10 mM	82.7% in 3 h (BFO) 100% in 30 min (BFO-EDTA)	-
[68]	LaMn1-xFexO3, LaMn0.1-x Fe0.9MoxO3	MB	30 mg (volume not indicated)	0.1 g/L	2.9 mM	20% in 1 h (LaMn0.01Fe0.9Mo0.09O3)	-
[69]	LaMnxFe1-xO3 (x = 0.1–0.5), absence and presence of visible light	Anionic and cationic dyes	0.2 g/L	15 mg/L cationic dyes 60 mg/L anionic dyes	17 mM	Anionic dyes: 66–98% in 4 h (no light); 90–95.8 in 70 min (light) Cationic dyes: 90-99.4% in 5 h (no light); 98–99.7% in 30 min (light)	-
[70]	LaCuxM1-xO3 (0.0 ≤ x ≤ 0.8, M= Mn, Ti)	paracetamol	0.2 g/L	50 mg/L	13.8 mM	80–97% in 5 h	47–54% in 5 h
[73]	LaMnO3 +UV light	phenol	0.6 g/L	0.1 g/L	14.8 mM	99.92% in 4 h	-
[74]	Catalyst (Bi0.97Ba0.03FeO3, BiFe0.9Cu0.1O3, Bi0.97Ba0.03 Fe0.9Cu0.1O3) + visible light	2-chlorophenol	0.4 g/L	50 mg/L	10 mM	100% in 70 min (BiFe0.9Cu0.1O3, Bi0.97Ba0.03 Fe0.9Cu0.1O3) 63% in 70 min (Bi0.97Ba0.03FeO3)	68% in 1 h (BiFe0.9Cu0.1O3) 73% in 1 h (Bi0.97Ba0.03 Fe0.9Cu0.1O3)
[75]	EuFeO3 (EFO) + visible light	RhB	1 g/L	5 mg/L	0.2 mM	37% EFO+vis. light 50% EFO+ H2O2 71% EFO+ H2O2+ vis. light	-
[76]	LaFe1-xCuxO3 + visible light	Methyl orange	0.8 g/L	10 mg/L	8.8 mM	92.9% in 1 h	-
[59]	LaFeO3 + UV light Pt/LaFeO3 + UV light	tartrazine	81.25 g/L	40, 60, 80 mg/L	0.0019–0.0076 mol/h	43% in 3 h (LaFeO3) 63% in 3 h (Pt/LaFeO3) (40 mg/L and 0.0038 mol/h) 100% in 30 min (catalyst + UV light)	45% in 3 h (LaFeO3); 65% in 3 h (Pt/LaFeO3) (40 mg/L and 0.0038 mol/h) 100% in 40 min (catalyst + UV light)
[77]	LaMeO3 (Me= Mn, Co, Fe, Ni, Cu)(cordierite, and Pt/LaMnO3/cordierite	acetic acid	9.4–40 g/L	1.26 g/L	83 mM	-	54% in 5 h (Pt/LaMnO3) 60% in 5 h (LaFeO3)
[78]	LaFeO3/corundum with different loads of LaFeO3	acetic acid, ethanol, acetaldehyde, oxalic acid	93.75 g/L	0.5 g/L	83 mM	-	97% (acetic acid); 53% (ethanol); 62% (acetaldehyde) and 95% (oxalic acid) in 5 h
[79]	LaFeO3/corundum	MTBE	9.4-4 g/L	0.5 g/L	0–42 mM	-	100% in 2 h
[80]	LaFeO3, BiFeO3, LaTi0.15 Fe0.85 O3 and BiTi0.15 Fe0.85 O3	methylparaben	0.1 g/L	5 mg/L	0.5 mM	82.8% in 90 min	-
[81]	graphene-BiFeO3+ + EDTA + visible light	TTBBA	0.5 g/L	0.011 g/L	20 mM	80% in 15 min	62.8% in 3 h

RhB: Rhodamine B; MB: Methylene Blue; MTBE: methyl tert-butyl ether; BPA: bis-phenol A; TTBBA: tetrabromobisphenol A.

summarizes the conversion values and reaction conditions for the use of perovskites in the degradation of different organics by Fenton-like reactions and photo Fenton-like reactions, these last being revised in the following section.

3.2. Photo Fenton-Like Reactions (H₂O₂/Catalyst/Light)

In the photocatalytic oxidation processes, the electron–hole pairs in the catalyst are produced via the irradiation of the UV light and the oxidative radicals are formed between the catalyst and water interface [82]. The formation rate of HO• radicals in photo-Fenton processes is higher than in Fenton processes. While the Fenton reaction is governed principally by Equation (4) leading to the formation of HO• radicals, in the Photo-Fenton process occurs, in addition, the photolysis of H₂O₂:

H₂O₂ + hν → 2 HO•

(18)

and the photo reduction of Fe³⁺:

Fe³⁺ + H₂O + hν → HO• + Fe²⁺ + H⁺

(19)

Different perovskite oxides, non-supported [69,73,74,75,76], supported on monoliths [59,77,78,79,80] or in form of composites [81], have been used as catalysts for the degradation of several organics in the presence of H₂O₂ under light irradiation conditions.

A LaMnO₃ perovskite prepared by co-precipitation method resulted to be an excellent photo-Fenton catalyst of oxidation of phenol [73]. The phenol conversion (99.92% in 4 h) obtained when LaMnO₃ was activated by UV radiation in the presence of stoichiometric amount of H₂O₂ necessary to degrade phenol, was even higher than the achieved when using TiO₂ as catalyst (98% in 4 h). Furthermore, LaMnO₃ could be regenerated by calcination after reaction, yielding to similar catalytic performance to that of the first cycle.

In Reference [75] a series of iron perovskites containing europium in B position, EuFeO₃ (EFO), calcined at different temperatures, was used for the photodegradation of RhB by combination of visible light and H₂O₂. By the action of visible light, electron-hole pairs were formed in EFO nanoparticles and electrons were easily trapped by H₂O₂, leading to the formation of HO• radicals. In addition, a complex between Fe³⁺ at the surface (≡Fe³⁺) and H₂O₂ was formed and ≡Fe³⁺ was transformed into ≡Fe²⁺, generating HO• and HO₂•, which decomposed RhB. Authors studied the effect of the calcination temperature of the catalysts on the band gap, microstructure and photocatalytic activity. The perovskite calcined at 750 °C showed the best catalytic behaviour, due to the combination of good crystallinity and appropriate BET surface area and band gap. The photodegradation of 37% of RhB after 3 h increased up to the 71% when H₂O₂ was added, which proves the Fenton-like activity of EFO nanoparticles.

One strategy to improve photocatalytic activity of ABO₃ perovskites is 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. In this regard, 2-chlorophenol (2-CP), was degraded in the presence of three metal doped BiFeO₃ (BFO) nanoparticles, H₂O₂ and visible light [74]. BiFeO₃ perovskite was substituted either in A position (Bi_0.97Ba_0.03FeO₃) or B position (BiFe_0.9Cu_0.1O₃) and in both (Bi_0.97Ba_0.03 Fe_0.9Cu_0.1O₃). After only 70 min of visible light irradiation, Cu-doped BFO and Ba-Cu co-doped BFO almost completely removed 2-CP. The mineralization degree reached was of 68% and 73%, respectively. Authors concluded that in addition to the participation of Fe²⁺/Cu⁺ couple active for the formation of HO•, the oxygen vacancies on the surface can also participate by activating H₂O₂ molecules to form a lattice oxygen, which is furtherly desorbed as O₂.

More recently Phan et al. [76] studied the efficiency of LaFeO₃ (LFO) perovskites, doped with Cu in B position (LaFe_1-xCu_xO₃), in the photo-Fenton decolorization of methyl orange (MO). Interestingly, the substitution of Cu into Fe-site in LFO modified the light absorption property of perovskite, as noted in the UV–vis absorption spectra and the corresponding band gap energy of LFO and LaFe_1-xCu_xO₃ shown in Figure 3a,b, respectively. Notice that all the samples had suitable band gap energy for organic pollutant degradation under visible light irradiation. When Fe was substituted by 15 mol% of Cu (LFO-15Cu), the MO degradation rate was improved in ca. 60% and 92.9% of MO was removed in only 1 h at an initial solution pH of 6. Under the optimum conditions, the photocatalytic performance of LFO-15Cu was also evaluated for two cationic dyes, rhodamine B (RhB) and methylene Blue (MB), obtaining even better results: 99.4% of degradation for RhB and 98.8% for MB in 60 min.

LaFeO₃ or Pt/LaFeO₃ perovskites supported on honeycomb monoliths have been tested in the degradation of tartrazine, a not-biodegradable dye used in food industries [59] by continuous flow of H₂O₂ in the presence of UV light at different pH values. The natural pH of solution (near 6) was the best operating condition, under which 100% of tartrazine was discoloured after 30 min of irradiation and mineralization was complete after 40 min. On the contrary, the discoloration was 50% under acidic condition (pH 3) and 55% under basic condition (pH 9), after 30 min of irradiation. In the first case, the excess of H⁺ could react with HO• and produce water subtracting hydroxyl radicals necessary for the decomposition of tartrazine [83]. In the second one, H₂O₂ was accumulated in liquid medium because under alkaline conditions H₂O₂ has a very high stability [84] and as a consequence, the production of hydroxyls radicals was limited.

Supported LaMeO₃ (Me= Mn, Co, Fe, Ni, Cu) perovskites were prepared by impregnation of thin wall of monolithic honeycomb cordierite support with different active phase loadings and tested in the photo-Fenton oxidation of acetic acid [77]. In the case of LaMnO₃ sample, the honeycomb was also impregnated with 0.1% of Pt. Photo-Fenton activity was closely related to the amount of active phase supported on monolithic carrier and LaFeO₃ and Pt/LaMnO₃ perovskites were the best catalysts in terms of reaction rate. The addition of Pt enhanced the initial rate of acetic acid degradation, achieving the highest TOC removal, 18% in 1 h; however it did not enhance the catalytic performance after 5 h.

Different loads of LaFeO₃ perovskites supported over corundum monoliths were studied by the same authors in the photo-Fenton degradation of several organics [78] under UV irradiation. 97% of TOC removal was attained in the degradation of acetic acid after 4 h with the catalyst containing 10.64 wt% of LaFeO₃ and values of 53, 62 and 95% were achieved when ethanol, acetaldehyde and oxalic acid, respectively, were used as model pollutant.

The excellent catalytic behaviour of this catalyst was extended to other contaminants, as methyl terc-butyl ether (MTBE) [79]. About 100% of TOC removal and complete mineralization of MTBE into CO₂ and water was achieved if H₂O₂ was continuously dosed during irradiation time of 2 h at solution pH of 6.7. Although the TOC removal obtained by the combination of H₂O₂ and UV light in the absence of catalyst was quite high (about 97%), a very significant formation of CO was observed, indicating the importance of LaFeO₃ for the improvement in the mineralization of MTBE.

The degradation of methylparaben was studied in the presence of four ABO₃ perovskite catalysts (A: La, Bi and B: Fe, Ti-Fe) supported on a monolithic structure [80]. BiFeO₃ was the best catalyst and under the optimum conditions 82.8% of pollutant was degraded in only 90 min. In the absence of UV light, only 10% of methylparaben was removed. An interesting aspect of this work was the study of toxicity carried out by cress seed, showing an inhibition of only 1.09% in the growing of roots, which demonstrated the low toxicity of the products of degradation.

The combination of photocatalytic activity and oxidizing power of H₂O₂ was also applied for the degradation of tetrabromobisphenol A (TBBPA) with a graphene-BiFeO₃ composite as catalyst [81]. The catalytic activity was influenced by calcination temperature, pH, presence or not of EDTA, dosage of H₂O₂ and load of catalyst. The degradation of TBBPA approximately followed a kinetics of pseudo first order. Under the most favourable conditions and in the presence of EDTA the rate constant of TBBPA degradation with the graphene-BiFeO₃ was 5.43 times higher than that of BiFeO₃ and 80% of TBBPA was removed in 15 min. This enhancement in the catalytic activity was attributed to the increasing of the adsorption capacity (due to a large surface area) and to the high electron transfer ability of graphene in the composite, which favoured the generation of reactive species. The composite was stable and could be reused for five cycles without loss of catalytic activity.

3.3. Catalytic Wet Peroxide Oxidation (H₂O₂/Catalyst/Air)

Catalytic wet peroxide oxidation (CWPO) processes are based on the degradation of contaminants by the combined action of a solid catalyst, hydrogen peroxide and air in aqueous solution. In a certain extent, they are similar to Fenton-like processes, because they use H₂O₂ as oxidant; however CWPO processes are carried out in the presence of a flow of air or under pressurized air. They can work with high oxidation efficiency in a wide range of pH without leaching or production of sludge.

Apart from Fenton-type catalysts, including zeolitic materials or composite metal oxides, a reduced number of perovskites has also been tested for CWPO applications. It is important to note that very little theoretical and experimental information is available and there are only a few examples of CWPO using perovskites as heterogeneous catalysts.

The first application of a perovskite in CWPO was carried out by Ovejero et al. [85], who compared the activity of LaTi_0.45Cu_0.55O₃ with that of other catalysts containing Fe or Cu in the degradation of phenol. The reactions were carried out at 100 °C in a system pressurized with air at 1 MPa. The perovskite led to a complete elimination of phenol and a TOC removal of 92% in 45 min. The leaching of copper was 22%; however it was significantly lower than the leaching measured for other catalysts (between 64 and 74%). Considering the fact of the high stability of copper ions in perovskite structures, it is probable that most of the leached copper proceeded from La₂CuO₄ oxide, phase detected by XRD together with the perovskite phase. LaTi_0.45Cu_0.55O₃ was reused in a second cycle, the TOC removal decreasing only to 90%.

Three years later, the same authors extended the study of the CWPO degradation of phenol to other perovskites, in order to elucidate the influence of different reaction conditions (temperature, H₂O₂ peroxide concentration, catalyst concentration and air pressure) on the performance [86]. Three perovskites of LaTi_1–xCu_xO₃ composition, with different substitution degree, were tested in the reaction. TOC removal values comprised between 88 and 94% were achieved at 100 °C under air pressure (1 MPa) and stoichiometric amount of H₂O₂ after 2 h. The temperature exerted a significant effect on the activity and only a 15% of TOC removal was reached when reaction was developed at 40 °C. The catalysts could be easily regenerated by calcination in air, leading to similar activity in the second run.

Less drastic operation conditions that the reported above were applied by Faye et al. [87] for the degradation of the same contaminant, phenol, by the action of several LaFeO₃ perovskites, synthesized by self-combustion method by varying the glycine/NO₃⁻ molar ratio. Thus, authors used a flow of air at atmospheric pressure and mild temperatures, 25 or 40 °C. Depending on synthesis conditions, strong differences in the structural, textural and reducibility characteristics were observed. The perovskite having the highest surface area exhibited the highest TOC abatement (76%) and very low iron leaching (0.27 wt%) after 4 h of reaction at 40 °C. The perovskites were better catalysts than Fe₂O₃, for which a TOC removal of only 10% was reached after 4 h.

4. Processes Based on Peroxymonosulfate

4.1. Peroxymonosulfate Activated by Perovskites (PMS/catalyst)

The peroxymonosulfate ion (PMS) can be considered as a derivate of hydrogen peroxide by replacing one H-atom by a SO₃⁻ and is stable as triple potassium salt (2KHSO₅·KHSO₄·K₂SO₄), known by the commercial names of Oxone (from DuPont) and Caroat (from Evonik). PMS is quite stable in water solution and over a wide pH range and shows great potential for generating both sulphate and hydroxyl radicals. Sulphate radicals have a higher half-life than hydroxyl radicals (30–40 ls vs. 20 ns) and they are more selective to react with organics containing unsaturated bonds or aromatic rings during electron transfer, having some operation advantages. Recently, a review of AOPs based on sulphate radicals generated from PMS and persulfate has been published [88]. PMS in an unsymmetrical oxidant that, in the absence of any activator, can partially oxidize some organic compounds according to the redox potential of 1.82 V of reaction (Equation (20)).

HSO₅⁻ + 2H⁺ + 2 e⁻ → HSO₄⁻ + H₂O

(20)

In order to generate sulphate radical, the decomposition of PMS must be carried out in presence of an activator, such as transition metals, ultraviolet irradiation (UV), microwave (MW), ultrasound (US), electron conduction and homogeneous and heterogeneous catalysts. The different methods for activation of PMS as well as their application for the removal or persistent organics have been recently revised by Ghanbari et al. [89]. Several nanostructured oxides and carbon materials have been tested as heterogeneous catalysts for PMS activation [90,91]. In spite of the high reactivity of nanostructured oxides, their low recyclability is a problem to be solved. Carbon materials show in general low activities and stabilities. Among heterogeneous catalysts used for PMS activation, materials based on cobalt play an important role. Thus, cobalt oxides, Co-metal oxide and Co-carbon-based supports have been applied for the degradation of different pollutants in presence of PMS. The mechanism for oxidation of organics by Co-assisted decomposition of PMS has been proposed as follows [92,93,94]:

Co²⁺ + HSO₅⁻ → Co³⁺ + SO₄•⁻ + OH⁻

(21)

Co²⁺ + H₂O → CoOH⁺ + H⁺

(22)

CoOH⁺ + HSO₅⁻→ SO₄•⁻ + CoO⁺ + H₂O

(23)

CoO⁺ + 2H⁺ → Co³⁺ + H₂O

(24)

Co³⁺ + HSO₅⁻ → Co²⁺ + SO₅•⁻ + H⁺

(25)

Co²⁺ + SO₄•⁻ → Co³⁺ + SO₄^2-

(26)

SO₄•⁻ + OH⁻→ SO₄^2- + HO•

(27)

SO₄•⁻ (SO₅•⁻, HO•) + organics →…..→ CO₂ + H₂O

(28)

Therefore, three types of reactive radicals including sulphate (SO₄•⁻), peroxy-sulphate (SO₅•⁻) and hydroxyl radicals HO• can be generated during PMS activation by cobalt, although peroxy-sulphate radical is less efficient to attack the organic compounds due to its weak oxidizing ability (E(SO₅•⁻/ SO₄²⁻) = 1.1 V).

Transition mixed metal spinels, iron-based heterogeneous catalysts and other transition metal oxides have also been studied in this system [89]. However, the use of perovskites for activation of PMS has not been revised. In the present review we show some examples of the application of oxide-like perovskites as activators of PMS for the degradation of organics in waters (

Table 2. Degradation of pollutants by the combination of peroxymonosulfate (PMS) and oxides type perovskite.

			Concentration			Treatment Efficiency
Reference	Catalyst	Target Pollutant	Catalyst	Pollutant	PMS	Degradation	TOC Removal
[95]	ACoO3 (A = La, Ba, Sr and Ce)	phenol	0.2 g/L	20 mg/L	0.1 mM	95% in 180 min (LaCoO3 and SrCoO3) 80% in 180 min (BaCoO3 and CeCoO3)	81% in 6 h (LaCoO3 and SrCoO3) 35% in 6 h (BaCoO3 and CeCoO3)
[96]	PrBaCo2O5+δ	phenol, MB	0.1 g/L (for phenol) 0.05 g/L (for MB)	20 mg/L phenol10 mg/L MB	21.2 mM (for phenol) 2.3 mM (for MB)	100% phenol in 30 min 100% MB in 15 min	39.3% at pH 2, 82% at pH 9
[97]	LaCoO3/ZrO2	RhB	0.1 g/L	10 mg/L	0.1 g/L	100% in 60 min	-
[98]	SrCo1−xTixO3−δ (SCTx, x = 0.1, 0.2, 0.4, 0.6)	phenol	0.1 g/L	20 mg/L	11.9 mM	85% in 15 min for SCT0.4	76.2% in 2 h
[99]	Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF)	phenol	0.1 g/L	20 mg/L	6.5 mM	100% in 30 min	-
[100]	LaCo0.4Cu0.6O3	phenol	0.1 g/L	20 mg/L	0.65 mM	100% in 12 min	-
[101]	LaCo1−xMnxO3+δ (LCM, x = 0, 0.3, 0.5, 0.7 and 1.0)	phenol	0.1 g/L	20 mg/L	3.25 mM	100% in 20 min for LaCoO3.002 100% in 40 min for LaCo0.5Mn0.5O3.053	67% in 40 min
[102]	LaCoO3 (LCO) LCO-SiO2, CTAB-LCO	PBSA	0.5 g/L	5 mg/L	5 mM	100% in 30 min (LCO-SiO2) 100% in 5 min (LCO and CTAB-LCO)	-
[103]	LaCoO3	four herbicides (metazachlor, tembotrione, tritosufuron and ethofumesate)	0.5 g/L	1 mg/L (each)	0.1 mM	95% metazachlor, 85% tembotrione, 5% tritosufuron; 45% ethofumesate t = 60 min; pH = 7	-
[104]	LaMO3 (M= Fe, Ni, Cu, Co)	RhB	0.1 g/L	10 mg/L	0.6 mM	42% in 60 min (LaFeO3) 60% in 60 min (LaNiO3) 45% in 60 min (CuFeO3) 98% in 60 min (LaFeO3)	-
[105]	LaFeO3	diclofenac	0.6 g/L	0.15 mM	0.3 mM	100% in 1 h	50% in 2 h
[106]	La0.8Ca0.2Fe0.94O3-δ and Ag-La0.8Ca0.2Fe0.94O3-δ	MB, phenol and Rh6G	1.0 g/L	10 mg/L MB	0.24 g/L (for MB and Rh6G) 0.48 g/L (for phenol)	84–90% MB in 45 min 100% phenol in 15 min 100% Rh6G in 10 min	-
[107]	BiFeO3 + visible light	RhB	1 g/L	5 mg/L	5 mM	63% in 40 min (25 °C) 93% in 40 min (45 °C)	-
[108]	LaCoO3-TiO2 (Co:Ti = 0:1–1:0) + UVA light	four herbicides: metazachlor, tembotrione, tritosufuron and ethofumesate	0.5 g/L	1 mg/L (each)	0.1 mM	90% metazachlor, 97% tembotrione, 20% tritosufuron; 70% ethofumesate t = 60 min; pH = 7	-
[109]	Sr2CoFeO6 + UV light	bisphenol F	0.3 g/L	20 mg/L	0.1 mM	75% in 2 h	90% in 6 h

MB: Methylene Blue; PBSA: 2-phenyl-5-sulfobenzimidazole acid; RhB: Rhodamine B; Rh6G: Rhodamine 6G.

).

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 [95,96,97] or partially substituted by other cations [98,99,100,101]. And as occurred in the Fenton-like reactions, phenol is again one of the most studied pollutant [95,96,98,99,100,101].

A series of cobalt-perovskite catalysts, ACoO₃ (A = La, Ba, Sr and Ce) was tested by Hammouda et al. [95] in the degradation of phenol by action of PMS. LaCoO₃ and SrCoO₃ showed the best catalytic performance, leading to a depletion of 95% of phenol in 3 h and a TOC removal of 65% in 6 h, in contrast to the 80% of removed phenol and 35% of mineralization degree reached with BaCoO₃ and CeCoO₃. Phenol degradation followed the pseudo first order kinetics and the intermediate formed were identified as catechol, hydroquinone and benzoquinone. Only between 7 and 12% of phenol was removed by physical adsorption. The activity was not related to the textural properties but to the content of cobalt of samples, the removal of phenol increasing with cobalt amount. The degradation of phenol by PMS in absence of perovskite was only of 10% after 3 h, which indicated the low oxidation power of PMS as compared to sulphate radicals formed in the presence of catalyst.

Su et al. [96] found that both hydroxyl and sulphate radicals were responsible for the degradation of phenol and methylene blue (MB) in the presence of PMS and a mixed ionic−electronic conducting (MIEC) double perovskite, PrBaCo₂O_5+δ (PBC) over a wide pH range, although the sulphate were the major radicals for promoting the degradation of organics. In addition, the oxygen vacancies in perovskite structure played a key role in the activation of PMS and in facilitating easier valence-state changes of the cobalt ions. The PBC catalysed the phenol oxidation with a TOF that was ∼196-fold higher than that of the classical Co₃O₄ spinel and 100% phenol was removed in 30 min. In the case of MB only 15 min were necessary to produce the complete degradation.

Zirconia-supported LaCoO₃ perovskite, LaCoO₃/ZrO₂ and its corresponding LaCoO₃ powder, were used to degrade RhB in the presence of PMS [97]. The nanocomposite showed a much higher catalytic activity than LaCoO₃ to activate PMS, in spite of the fact that it contained only 12.5 wt% of LaCoO₃. RhB was completely degraded in only 60 min and the nanocomposite could be reused for several cycles without activity loss.

Different perovskites of cobalt in B position partially substituted by Ti, Fe, Cu or Mn have been tested in the degradation of phenol [98,99,100,101]. In this sense, SrCo_1−xTi_xO_3−δ (SCTx, x = 0.1, 0.2, 0.4, 0.6) perovskites exhibited an excellent activity for phenol degradation under a wide pH range, leading to a faster oxidation than Co₃O₄ and TiO₂ [98]. The order of activity was SCT0.2 ≈ SCT0.1 > SCT0.4 > SCT0.6, therefore the rate of phenol oxidation decreasing with the content of cobalt. The effects of operating conditions and initial pH on the catalytic activity were studied for the SCT0.4/PMS system. At pH ≥ 7 the catalyst led to an optimized performance in terms of higher TOC removal, minimum Co leaching and good catalytic stability, which can overcome the common problems of Fenton reaction and provide a promising application for real wastewater treatments under neutral or alkaline conditions. Less than 5% of phenol was removed by adsorption during the 90 min period and the same amount was degraded by PMS in 90 min in the absence of catalyst.

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) perovskite was very effective for PMS activation to produce free radicals and the subsequent degradation of phenol [99]. On the contrary, it was not active in the production of radicals from activation of other peroxides, such as H₂O₂ or peroxydisulfate (PDS). Authors found that the oxygen vacancies and the metal ions in A position with a less electronegativity than cobalt in the perovskite structure play a key role by conferring cobalt sites a high charge density for interacting with PMS via a rapid charge transfer process and to produce free radicals, resulting in a higher activity when compared to a Co₃O₄ spinel. Thus, 100% of phenol was removed in 30 min with BSCF, in contrast to the 45% of degradation reached with Co₃O₄. Authors concluded that the PMS activation by BSCF gave rise to the generation of both hydroxyl and sulphate radicals.

LaCo_1-xCu_xO₃ (x=0-1) perovskites prepared via sol-gel method with citric acid as organic complexing agent were also tested in the PMS-phenol system [100]. LaCo_0.4Cu_0.6O₃ was the best catalyst, showing a removal efficiency of 100% in only 12 min and a TOF value of 1 h⁻¹, which was 2.5 times higher than that obtained by Duan et al. (0.4 h⁻¹) [99] for the same concentrations of catalyst and pollutant, although the PMS dosage used in the first case was ten times lower. No significant change on surface of the catalyst was observed after the oxidation reaction, proving the high stability of LaCo_0.4Cu_0.6O₃, although an activity loss of 20% was produced after fourth cycle, due to the poisoning of active sites by adsorption of degradation intermediates. The redox species involved in the mechanism were not only the Co²⁺/Co³⁺ pair but also the Cu⁺/Cu²⁺ couple, which reacted with both SO₄•⁻ and HO•.

Miao 2018 [101] synthesized a series of LaCo_1−xMn_xO_3+δ (LCM, x = 0, 0.3, 0.5, 0.7 and 1.0) perovskites, calcined at different temperatures, showing over stoichiometric oxygen. Authors found that the interstitial oxygen plays a key role in the catalytic activity for degradation of phenol, in such way that a proper amount of interstitial oxygen promotes the electron transfer rate of the perovskite but an excess hinders this process. The most active catalyst was that containing only cobalt in B position, that is, LaCoO₃, which led to a complete depletion of phenol in only 20 min. Among all the substituted catalysts, LaCo_0.5Mn_0.5O_3.053, calcined at 900 °C, exhibited the best performance, due to its high interstitial oxygen ion diffusion rate. Furthermore, its stronger relative acidity contributed to an enhanced stability. Phenol was completely degraded with this catalyst after 40 min. Considering that manganese ions are much cheaper and less toxic than cobalt ions, these substituted perovskites are an appropriate alternative for the activation of PMS.

LaCoO₃ perovskite has proved to be very efficient for the activation of PMS in the degradation of different organic pollutants [102,103,104]. The degradations of aqueous solutions of 2-phenyl-5-sulfobenzimidazole acid (PBSA) using PMS activated with LaCoO₃ perovskites prepared by three different methods was investigated by Pang et al. [102]. LaCoO₃ was prepared by a normal precipitate method (sample named as LCO), by introduction of cetyltrimethylammonium bromide (CTAB-LCO) and by a hydrothermal method with the adding of silicon (LCO-SiO₂). LCO-SiO₂ was active in a wider pH range (4–8), leading to a complete removal of PBSA in 30 min and showing a very low leaching of metal ions. On the contrary, LCO and CTAB-LCO presented a contribution of the homogeneous reaction to the total activity, due to the leached metals, which resulted in the PBSA depletion of 100% in only 5 min. From studies with radical quenchers and from identified intermediates authors concluded that for LCO-SiO₂ the activation of PMS resulted from the combination of SO₄•⁻ and electronic transfer reaction. However, in the case of LCO and CTAB-LCO, both SO₄•⁻ and HO• radicals were involved.

More recently, Solís et al. [103] have reported the combination of LaCoO₃ and PMS for the removal of various aqueous herbicides (metazachlor, tembotrione, tritosufuron and ethofumesate). The catalyst amount exerted a positive influence on herbicides conversion, which increased when the load of catalyst did from 0.5 to 1.5 g/L. An increment in the pH values reduced cobalt leaching and decreased PMS depletion. As the point of zero charge (PZC) value of LaCoO₃ was 9.08, at acidic or neutral pH the catalyst surface is positively charged and as a result it interacts more easily with the anions from PMS. With respect to the influence of PMS concentrations, those equal or above 0.5 mM produced the instantaneous removal of metazachlor, tembotrione and ethofumesate while tritosulfuron required almost one hour to be completely degraded when 0.5 mM of PMS was used.

The high activity of cobalt ions for PMS activation was confirmed by Lin et al. [104], who tested a series of LaMO₃ perovskites (M=Co, Cu, Fe and Ni) in the removal of RhB. Once more, LaCoO₃ was the most active, followed by LaNiO₃, LaCuO₃ and LaFeO₃. The mechanism of reaction was studied by addition of different scavengers or radical inhibitors. Authors found that both Co³⁺/Co²⁺ and La³⁺/La⁴⁺ ions decomposed PMS yielding mainly sulphate radicals and hydroxyl radicals in lesser extent. By comparison of the obtained rate constants with other from literature, authors concluded that LaNiO₃, LaCuO₃ and LaFeO₃ were no competitive with other existing catalysts, because of the low activity of Ni, Cu and Fe for PMS activation, in contrast to LaCoO₃, which exhibited a rate constant comparable or even higher than those reported for other catalysts.

Iron in B position of perovskites has resulted to be also an efficient cation for the PMS activation [105,106]. The oxidative degradation of diclofenac (DCF), a non-steroidal anti-inflammatory drug, was carried out in the presence of LaFeO₃ and PMS [105]. DFT studies allowed authors to conclude that a strong interaction occurs between the Fe (III) sites on LaFeO₃ surface and PMS, with the formation of an inner-sphere complex and the transfer of electrons from PMS to Fe (III). Sulphate radicals were identified as the major responsible for DCF degradation by the LaFeO₃ /PMS system. Although 100% of DCF was removed in only 1 h, the mineralization was only of 50% and fifteen different intermediates were formed.

La_0.8Ca_0.2Fe_0.94O_3-δ and Ag-La_0.8Ca_0.2Fe_0.94O_3-δ were tested by Chu et al. [106] in the removal of phenol, MB and rhodamine 6G. From electrochemical impedance spectra, authors concluded that Ag nanoparticles and lattice oxygen vacancies improve the p-type conductivity of the perovskite. Furthermore, the O₂ of solution is adsorbed on the oxygen vacancies and as a consequence, in order to replace the lost oxygen, more SO₅•⁻ react generating more sulphate radicals. Under the optimum conditions, around 84–90% of MB was degraded in only 45 min. When using Ag-La_0.8Ca_0.2Fe_0.94O_3-δ rhodamine 6G and phenol were completely removed in 15 and 10 min, respectively. This perovskite was also very efficient for the removal of Escherichia coli.

4.2. Peroxymonosulfate Activated by the Combination of Perovskites and Light Irradiation (PMS/Catalyst/Light)

The combination of PMS activation by a perovskite and light irradiation has been applied for the degradation of different pollutants [107,108,109]. Rhodamine B (RhB) was used as model of organic pollutant for studying the PMS activation by a BiFeO₃ microsphere in presence of visible light [107]. To confirm the contribution of the oxidizing radical species, ethanol (EtOH), t-butanol (t-BuOH) and 1,4-benzoquinone (BQ) were employed as radical scavengers. The results indicate that the main generated radical species during the activation of PMS by BiFeO₃ were HO• and SO₄•⁻ radicals. However, the O₂•⁻ radicals, which are formed at longer reaction times, also play an important role in the degradation of RhB. Authors explained the reaction mechanism as follows. BiFeO_3, with narrow band gap energy (1.92 eV), can be easily excited by visible light and the electrons and holes generated by action of light can react with Fe³⁺ and RhB, respectively. Simultaneously, Fe³⁺ and Fe²⁺ can also activate PMS to yield SO₄•⁻ and SO₅•⁻ radicals, which degrades RhB. About 63% of RhB was degraded in 40 min by BiFeO₃/PMS/vis light system, in contrast to 43% of removal in the absence of perovskite. The catalyst was used in three consecutive cycle runs without significant loss of photocatalytic activity.

The combination of a cobalt perovskite (LaCoO₃) with a photocatalyst (TiO₂) in different molar ratios (Co:Ti= 0:1–1:0) was used to activate PMS for the oxidation of a mixture of four herbicides (metazachlor, tembotrione, tritosulfuron and ethofumesate) [108]. Same authors had previously tested LaCoO₃/PMS system for the degradation of the same herbicides [103]. In general, LaCoO₃-TiO₂ with Co:Ti ratios in the range 0.1:1 to 0.5:1 showed a higher activity than the rest of solids tested, although a Co/Ti ratio of 0.1:1 was enough to reach enhanced degradation rates when compared to pristine titania or pure perovskite. The number of degraded herbicides by LaCoO₃-TiO₂/PMS system was 3.5–5 times higher in the presence of UVA-light and the reaction followed a second order kinetic, depending of concentration of both PMS and herbicides. As complete mineralization was not achieved, authors carried out studies to assess the potential phytotoxicity of the accumulated intermediates, concluding that all samples did show no phytotoxicity after 180 min of treatment for PMS concentrations ≥ 0.15 mM.

A double cobalt perovskite, Sr₂CoFeO₆, was tested in the mineralization of bisphenol F (BPF) in neutral medium by activation of PMS under UV irradiation [109]. Neither direct UV photolysis nor PMS alone degraded the BPF and Sr₂CoFeO₆ exhibited higher activity (75% of BPF degradation in 2 h) than the corresponding single perovskites, SrCoO₃ (60%) and SrFeO₃ (35%), caused probably by an accelerated reduction of Fe³⁺ in the presence of cobalt ions. UV irradiation also improved the mineralization degree of BPF and values of TOC removal in 6 h increased from 65% to 90% when Sr₂CoFeO₆/PMS system was irradiated. As a novel aspect of this work, authors studied the influence of chemicals co-existing in the natural water matrix, such as humic acid and inorganic anions (Cl⁻, HCO₃⁻ and CO₃^2-), on the inhibition of degradation of bisphenol.

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].

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 [111,112,113,114,115,116,117,118,119,120,121,122], 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 [111,112,114,119,120,121,122].

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.

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, La_1-xLi_xFeO₃ (with x = 0, 3, 5 and 7%) were active under light irradiation for the degradation of MB and arcylon effluents, La_0.97Li_0.03FeO₃ 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 La_0.9Ca_0.1FeO₃ 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 [126,127,128,129]. 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. Oxides type perovskite applied as photocatalysts for the degradation of different organic contaminants in aqueous solution.

Reference	Catalyst	Preparation Procedure	Contaminant	Light (Irradiation Source)
[111]	LaFeO3	Sol–gel	RhB	Visible
[112]	LaFeO3	Sol–gel, SBA-16 as template	RhB	Visible
[114]	LnFeO3 (Ln = La, Sm)	Sol–gel	RhB	Visible
[119]	LaFeO3	Hydrothermal	RhB	Visible
[120]	ReFeO3 microspheres (Re: La, Sm, Eu, Gd)	Microwave	RhB	Visible
[121]	LaFeO3, g-C3N4	Solvothermal	RhB	Visible
[122]	LaFeO3-rGO	Sol-gel	MB and RhB	Visible
[116]	LaFeO3	Sol–gel, Microwave	Methyl orange and MB	Visible
[117]	LaFeO3	Microwave	MB	Visible
[115]	Li-doped LaFeO3	Sol-gel	MB and arcylon	UV-visible
[118]	LaFeO3, Ca-doped LaFeO3	Reverse microemulsion	MB	Visible
[113]	LaFeO3, La2FeTiO6	Sol–gel	p-Chlorophenol	Visible
[123]	LaCoO3	Surface-ion adsorption	MB, methyl orange and neutral red	UV
[124]	SrTiO3/CeO2	Dry (SrCO3, CeO2)-wet (sol-gel Ti(OC4H9)4) composition	Reactive black 5	UV
[125]	SrTiO3/CeO2	Dry (SrCO3, CeO2)-wet (sol-gel Ti(OC4H9)4) composition	Direct Red 23	UV
[126]	BaBiO3	Solid state, hydrothermal	RhB, water splitting	Visible
[127]	KBiO3	Solid phase heating	RhB, crystal violet, MB, phenol	Visible
[128]	Bi2Fe4O9	Hydrothermal method	phenol, aqueous ammonia	Visible
[130]	LaMnO3–diamond	Sol-gel	Acid red C	Visible
[131]	Au NP/ KNbO3	Deposition/precipitation + thermal reduction	Sec-phenethyl alcohol	Visible
[132]	S, C-co-doped SrTiO3	Calcination	2-Propanol	Visible
[129]	BiFeO3 nanoparticles	Sol-gel	Methyl orange	UV-Visible
[133]	LaCoO3 nanofibres	Electrospinning	RhB	UV
[134]	PrFeO3 (nanotubes, nanofibers, nanoparticles)	Electrospinning, annealing	RhB	Visible
[136]	Ag3PO4/LaCoO3	Liquid deposition	Bisphenol	Visible
[137]	LaFeO3/TiO2	Two-step	MB	Fluorescent
[138]	LaFeO3/TiO2	Hydrothermal	Methyl orange	Visible
[139]	CsPbBr3/TiO2	Wet-impregnation	Benzyl Alcohol	Visible

MB: Methylene Blue; RhB: Rhodamine B.

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⁺ to form HO• radicals, which can oxidize 4-CP• radicals into CO₂. LaCoO_3-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 LaCoO_3-x, achieving degradation values of 40%. Sr₀.₈₅Ce_0.15Fe_O3-δ [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 LaNiO_3-δ 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 LaNiO_3-δ and the formation of lanthanum carbonate.

SrFeO_3-δ 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 Ca_xSr_1-xCuO_3-δ (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 (Fe^2+/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.