*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 <sup>−</sup> 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)).

$$\text{HSO}\_5^- + 2\text{H}^+ + 2\text{ e}^- \rightarrow \text{HSO}\_4^- + \text{H}\_2\text{O} \tag{20}$$

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 [92–94]:

$$\rm{Co}^{2+} + \rm{HSO}\_5^- \rightarrow \rm{Co}^{3+} + \rm{SO}\_4\bullet^- + \rm{OH}^- \tag{21}$$

$$\text{Co}^{2+} + \text{H}\_2\text{O} \rightarrow \text{CoOH}^+ + \text{H}^+ \tag{22}$$

$$\text{CoOH}^+ + \text{HSO}\_5^- \rightarrow \text{SO}\_4\bullet^- + \text{CoO}^+ + \text{H}\_2\text{O} \tag{23}$$

$$\rm{CoO^{+}} + 2H^{+} \rightarrow \rm{Co^{3+}} + H\_{2}O \tag{24}$$

$$\rm{Co}^{3+} + \rm{HSO}\_5^- \rightarrow \rm{Co}^{2+} + \rm{SO}\_5\bullet^- + \rm{H}^+ \tag{25}$$

$$\text{Co}^{2+} + \text{SO}\_4\text{\bullet}^- \rightarrow \text{Co}^{3+} + \text{SO}\_4\text{\bullet}^{2-} \tag{26}$$

$$\mathrm{SO\_4\bullet}^- + \mathrm{OH}^- \rightarrow \mathrm{SO\_4}^{2\circ} + \mathrm{HO\bullet} \tag{27}$$

$$\text{CO}\_4\bullet^-\text{(SO}\_5\bullet^-, \text{HO}\bullet) + \text{organic} \rightarrow \dots \dots \rightarrow \text{CO}\_2 + \text{H}\_2\text{O} \tag{28}$$

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•−/ SO4 <sup>2</sup>−) = 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).


#### *Catalysts* **2019** , *9*, 230


**Table 2.** *Cont*.

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

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, PrBaCo2O5+<sup>δ</sup> (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 [98–101]. In this sense, SrCo1−xTixO3−<sup>δ</sup> (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.

Ba0.5Sr0.5Co0.8Fe0.2O3-<sup>δ</sup> (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.

LaCo1-xCuxO3 (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]. LaCo0.4Cu0.6O3 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 LaCo0.4Cu0.6O3, 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•<sup>−</sup> and HO•.

Miao 2018 [101] synthesized a series of LaCo1−xMnxO3+<sup>δ</sup> (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, LaCo0.5Mn0.5O3.053, 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 [102–104]. 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•<sup>−</sup> and electronic transfer reaction. However, in the case of LCO and CTAB-LCO, both SO4•<sup>−</sup> 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.

La0.8Ca0.2Fe0.94O3-<sup>δ</sup> and Ag-La0.8Ca0.2Fe0.94O3-<sup>δ</sup> 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•<sup>−</sup> react generating more sulphate radicals. Under the optimum conditions, around 84–90% of MB was degraded in only 45 min. When using Ag-La0.8Ca0.2Fe0.94O3-<sup>δ</sup> 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*.
