Application of Pillared Clays for Water Recovery

Abstract

In recent years, efforts have been made in developing new and more efficient water purification methods and the synthesis of catalysts with greater catalytic activity that are more stable and can be used in wide pH ranges. Pillared clays represent a viable alternative for removing organic contaminants. The clays, usually smectites, are modified by inserting inorganic pillars (Al, Zr, Cr, Fe, Ti, Ga, and Mn) between the layers of the clay, increasing its surface area, porosity, catalytic activity, and thermal stability. This review describes the importance of using pillared clays with different polyoxycations in Fenton, photo-Fenton, ozonation, wet catalytic oxidation of hydrogen peroxide, and photocatalysis processes. Pillared iron clays (Fe-PILCs) are promising catalysts capable of generating hydroxyl radicals that can oxidize organic contaminants, thus facilitating their removal. The current challenges of the PILC application at industrial scale are also discussed.

1. Introduction

Nowadays, addressing water pollution problems is of paramount importance to mitigate the environmental impact that different anthropogenic activities have had on bodies of water.

In 2010, the United Nations General Assembly recognized the human right to drinking water and sanitation as essential for human life, establishing in 2015 the new Sustainable Development Goal (SDG 6) “Ensure the availability and sustainable management of water and sanitation for all” [1]. Throughout these years, efforts have been made to involve politicians, nations, and interested groups in various water and sanitation projects to develop efficient technologies for water management.

Although many developed countries have achieved universal access to safe drinking water (73% of the global population in 2022), this situation is very different for many less developed regions [2]. Figure 1 shows the distribution of drinking water worldwide in 2022.

Drinking water from rivers, lakes, and ponds is noticeably different from wastewater in terms of contamination extent [3]. Wastewater recovery and purification are critical challenges in environmental management and human health preservation, particularly as global water demand continues to increase due to urbanization, industrialization, and agriculture, among other activities generating large quantities of untreated or insufficiently treated wastewater [4,5], often containing highly water-soluble carcinogens, heavy metals [4], and other contaminants. This is worrying due to its high negative impact on ecosystems [6], since in high concentrations, it can cause severe diseases in humans and in both aquatic and terrestrial animals.

For the above, efficient treatment processes are necessary to eliminate the various pollutants in water, which result from discharges of different anthropogenic activities (industries, agriculture, hospitals, and households) [7,8].

The treatment of wastewater is crucial for sustainable development and requires effective treatment systems. Currently, there are a handful of methods and technologies for wastewater treatment, which are usually divided into primary, secondary, and tertiary treatment techniques (Figure 2), which frequently incorporate biological, physical, and chemical procedures.

The election of treatment typically relies on factors such as wastewater characteristics, cost, feasibility, efficiency, environmental impact, and operational challenges [3].

Advanced oxidation processes (AOPs), used as tertiary treatments, represent a viable option for water recovery since they are based on the generation of radicals with high oxidation–reduction potential such as hydroxyl radical (2.8 V), for the oxidative decomposition of contaminants, using low chemical consumption, and in most of the cases, eliminating sludge formation [3,9,10].

Due to their stability, catalytic activity and relatively easy recovery, heterogeneous catalysts are generally preferred in advanced oxidation processes. They are easily separated from the reaction mixture, can be used in successive runs, do not generate sludge, and are effective in a wide pH range.

Different supports such as synthetic and natural zeolites, carbons, mesoporous silica materials (SBA-15, MCM-41), metal–organic frameworks (MOFs), and clays have been used as main constituents of catalytic systems for advanced oxidation reactions (Fenton-heterogeneous, photo-Fenton, ozonation, catalytic wet oxidation of hydrogen peroxide (CWPO), and photocatalysis) for wastewater treatment. Figure 3 depicts the distribution of scientific articles in the literature regarding the use of heterogeneous catalysts in advanced oxidation processes to remove contaminants in water from 2020 to date, according to SCOPUS (search date: January 2025).

Figure 3 shows that the highest percentage of manuscripts dealing with catalyzed advanced oxidation processes focuses on clays as supports of catalytically active metals in the oxidation processes of organic compounds; for this reason, clays have become potential materials for the degradation of contaminants.

Clays are hydrated phyllosilicates (Al₂Si₂O₅(OH)₄) containing Fe³⁺, Mg²⁺, and other cations [11,12], forming a layered or sheet structure. These sheets are either tetrahedral or octahedral, the former being constituted by one Si atom bonded to four oxygen atoms (SiO₄), while octahedral sheets (Al(Mg)O₈) pose a high content of Al or Mg, bonded to oxygen atoms shared in different layers [12]. Due to the incomplete neutralization of oxygen, the clay layers have a negative charge, which is balanced by hydrated cations (Na⁺, K⁺, or Ca⁺) located in the interlayer space [13].

Clay minerals are classified into a 1:1 type consisting of one tetrahedral and one octahedral sheet (kaolinite, halloysite, rectorite, chrysotile) and a 2:1 type consisting of two tetrahedral sheets and one octahedral sheet between them (smectites: laponite, bentonite, montmorillonite, sepiolite, laponite) [12,14]; the structure of the smectite is shown in Figure 4.

Clay minerals have been used in various processes to remove recalcitrant contaminants in solutions [16] and in a wide variety of industrial processes [13] due to their abundance, low cost, thermal stability, swelling properties, and environmental friendliness [17,18]. In addition, the interlayer space can be easily modified, increasing their porosity [19], which makes them excellent catalytic supports. The traditional process for increasing the porosity of clays is the pillaring method, where metallic polyoxocations are introduced in solution into the interlayer space. When dehydrated, the corresponding metallic oxide is obtained. Another alternative process is clay exfoliation, which consists of separating its layers using surfactants such as urea, hydrazine hydrate, formamide, potassium acetate, and N-methylformamide hydrate, causing a significant increase in the surface area, in mesopores and macropores [20,21,22]. In addition to presenting high thermal stability above 700 °C [23], this spacing that exfoliated clays presents allows them to be used mainly in adsorption processes [17,24]. In the case of pillared clays, metallic species can be inserted into the interlayer space that provides catalytic activity to the material to be used in advanced oxidation processes.

This review aims to summarize and analyze the use of pillared clays to remove organic contaminants in advanced oxidation processes, considering their effectiveness in Fenton, photo-Fenton, ozonation, catalytic wet hydrogen peroxide oxidation processes (CWPO), and photocatalytic reactions. It also describes the activity of pillared clays in water recovery and treatment in each of these processes.

2. Pillared Clays and Advanced Oxidation Processes

Advanced oxidation processes (AOPs) are a potential alternative for efficient wastewater treatment. These are based on the production and use of hydroxyl radicals (•OH) for decomposing organic or recalcitrant contaminants. These hydroxyl radicals are strongly reactive and not very selective [25], as well as being non-toxic. They are also potent oxidants (E⁰ = 2.8 V) only below fluorine (E⁰ = 3.03 V) and react between 10⁶ and 10¹² times faster than other oxidants such as ozone [26], achieving in most cases complete conversion of pollutants towards CO₂ and H₂O or partial mineralization and obtaining less-toxic intermediates [27]. Some of the most common AOPs include Fenton, photo-Fenton, ozonation, photocatalysis, electrochemical oxidation, and sonolysis [28,29].

2.1. Fenton Process

Fenton is an advanced oxidation process that employs iron salts (Fe²⁺) to catalyze hydrogen peroxide (H₂O₂) dissociation in aqueous solution [27,30] to generate hydroxyl radicals (•OH) according to Equations (1) and (2),

$${\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2 \to {\mathrm{F}\mathrm{e}}^{3+}+ •\mathrm{O}\mathrm{H}+\mathrm{O}{\mathrm{H}}^{-}$$

(1)

$${\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2 \to {\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}}^{+}+\mathrm{H}{\mathrm{O}}_2^{•}$$

(2)

Equation (1) is considered the central reaction in the Fenton process where ferrous ions (Fe²⁺) (k₁ ≈ 70 M⁻¹ s⁻¹) [31] rapidly oxidize towards ferric ions (Fe³⁺). Fe²⁺ ions are regenerated in Equation (2) with a slower rate constant (k₂ = 0.0001–0.01 M⁻¹ s⁻¹) [32]. Also, the hydroperoxyl radical facilitates the regeneration of the ferrous ion, as shown in Equation (3) (k₃ = 1.2 × 10⁶ M⁻¹ s⁻¹) [33].

$${\mathrm{F}\mathrm{e}}^{3+}+\mathrm{H}{\mathrm{O}}_2^{•} \to {\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{O}}_2+{\mathrm{H}}^{+}$$

(3)

In the Fenton process, the reaction time to mineralize organic compounds is shorter than other advanced oxidation methods [34]. Generally, this process occurs at pH 3, which promotes the generation of hydroxyl radicals (•OH) [35]; however, when pH increases, the iron oxyhydroxides and ferric hydroxide are produced, causing the precipitation of iron, i.e., sludge formation. Furthermore, the fact that the reaction rate of Equation (2) is slower than that of Equation (1) causes an excessive production of iron hydroxides, facilitating the formation of sludge [36].

When working at lower pH (<2), there is the presence of iron complex species [Fe(H₂O)₆]²⁺ and stable oxonium ions (H₃O₂)⁺, reducing the activity of Fe²⁺ and H₂O₂ [37]. The reaction rate in a Fenton process relies on the amount of iron, and the concentration of the oxidant directly influences the extent of contaminants mineralization, so the ratio of [Fe²⁺]₀/[H₂O₂]₀ and the organic compounds affect the reaction pathways in the Fenton process [38]. Furthermore, other drawbacks of using homogeneous systems in Fenton processes are the high consumption of H₂O₂ [39], the addition of chemicals to maintain the acidic pH to ensure iron ions remain in solution [40], and the complicated recovery of iron from sludge, requiring additional treatments. This can be overcome by using heterogeneous iron catalysts, where the leaching of metal ions is low, they are more stable during the process, less dependent on pH, non-corrosive, easily recovered, environmentally friendly, and have higher activity [41]. In addition, these catalysts can easily be recovered from the reaction and reused in subsequent reactions [42].

Several studies suggest that Fenton reactions using heterogeneous catalysts occur on the surface of the catalysts or in layers very close to the surface. However, only some active sites are available to carry out Fenton reactions. Additionally, steric hindrance makes access to them difficult; one of the disadvantages of heterogeneous iron catalysts is the lower reaction rate compared to homogeneous catalysis [42,43].

The heterogeneous Fenton process involves three steps: (1) adsorption of organic compounds on the surface, (2) in situ generation and attack of •OH radicals on said compounds [44], and (3) desorption of the products from the catalyst surface [45]. Furthermore, the Fenton process can be enhanced with UV radiation or sunlight, a process known as photo-Fenton [46].

Application of Pillared Clays in Heterogeneous Fenton Processes

An innovative approach to improving water recovery processes is using pillared clays. These are modified clay materials with inorganic pillars inserted between the layers of natural clay. This alteration significantly increases the clay’s structural stability, porosity and surface area, making it a highly effective medium for water treatment applications.

Pillared clays are porous materials obtained by exchanging cations of the clay (smectite) interlayers with inorganic polyoxocations (Al, Zr, Cr, Fe, Ti, Ga, and Mn) [25,47,48], which combine with oxygen to form stable metal oxide pillars [49]. Depending on the type of polyoxocation used, pillared clays can have different applications as adsorbents, photocatalysts, filler materials, etc. [16,50,51]. Inserting pillars between the clay layers alters the physiochemical characteristics and catalytic performance [52].

The basal spacing of clays is achieved when the polyoxocations are introduced. Through a calcination step, they are converted into pillars of metal oxides by dehydration and dihydroxylation, causing the interlayers to remain separated and preventing their collapse [53], as shown in Figure 5. Clays modified with pillars exhibit surfaces and microporosity of up to four times more than the clay without pillars [49], and have strong acidity and high thermal stability [54]. In general, it is known that Lewis acid sites in a pillared clay are located on the metal oxides that compose it. In contrast, Brönsted acid sites are in the structure of the clay layers associated with the OH groups, and the pillars also contribute to Brönsted acid sites [55,56]. In addition, due to the steric hindrance caused by the micropores formed, shape selectivity in these materials is facilitated [13].

Pillared iron clays (Fe-PILCs) typically exhibit basal spacings of 21–25 Å (corresponding to d_{0 0 1}); this spacing includes the thickness of the laminae, which is 9.6 Å [57,58]. The formation of polymeric species of Fe₂O₃ permanently joins the sheets; these species are obtained when the Fe cations are dehydrated and dehydroxylated [57]. Previous works confirmed the presence of ferrous oxide (Fe²⁺) and ferrous ferric oxide (Fe₃O₄) in Fe-PILCs. Fe₃O₄ (magnetite) is an oxide that contains a mixture of iron with two oxidation states (Fe²⁺/Fe³⁺) [58,59].

The catalytic action of the iron species of Fe-PILCs allows the generation of •OH radicals and thus achieves the elimination of organic compounds through the Fenton process. These iron-pillared clays in a Fenton process have efficiently degraded nitroaromatic compounds such as nitrobenzene [60].

Table 1. Summary of iron-pillared clays for the Fenton process.

Organic Pollutants	Catalyst	Reaction Conditions	Remotion	Ref
Levofloxacin (LVX)	Iron-silica pillared clay	Catalyst loading = 0.5 g/L Concentration LVX = 50 mg/L pH = 3.0 Temperature = 40 °C 500 mg/L H2O2 Reaction time = 180 min in 5 consecutive runs	XLVX = 90%	[61]
Sulfanilamide (SA)	Fe, Al-Montmorillonite -pillared clay	Catalyst loading = 3 g/L Concentration SA= 0.29 mmol/L pH = 3.1 and 4.1 Temperature = 30–60 °C H2O2/SA = 1–36 mol/mol Reaction time = 360 min	XSA = 95–99%	[62]
Cinnamic acid (CA)	Fe-bentonite	Concentration CA = 120 mmol/L pH = 2.9 Temperature = 80 °C Fe-cinnamic acid molar ratio = 10 H2O2/ molar ratio = 83 Reaction time = 60 min	XCA = 90%	[63]
Pesticides (NPT)	Fe-pillared bentonite	Catalyst loading = 1 g/L Concentration NPT = 5 mg/L pH = 3.0 Temperature = 40 °C H2O2/NPT = 23.2 mg/L for 5 mg/L NPT	XNPT = 100%	[64]
Nitrobenzene (NB)	Fe-pillared kaolinite	Catalyst loading = 1.0 g/L Concentration NB = 75 mmol/L pH = 3.0 ± 0.05 Temperature = 35 °C 10 mmol/L H2O2 Reaction time = 180 min 10 mmol/L H2O2	XNB > 85%	[60]
Acid Green 25 (AG25)	Fe-Ipoh clay	Catalyst loading = 1.25 g/L Concentration AG25 = 50 mg/L pH = 3.0 Temperature = 30 °C 6.7 mM H2O2 Reaction time = 120 min	X = 95%	[65]
Orange II	Fe/pillared saponite	Catalyst loading = 90 mg/L Concentration Orange II = 0.1 mM pH = 3.0 Temperature = 70 °C 6 mM H2O2 Reaction time = 240 min	XOrange II = 91%	[66]
Orange II	Fe-Pillared bentonite clay (nanocatalysts)	Catalyst loading = 1 g Concentration Orange II = 0.2 mM pH = 3.0 Temperature = 30 °C 10 mM H2O2 Reaction time = 120 min	XOrange II = 100%	[67]
Phenol	Al/Fe-pillared clay (bentonite)	Catalyst loading = 5 g/L Concentration Phenol = 5 × 10−4 mol/L pH = 3.7 Temperature = 25 °C 0.1 mol/L H2O2 Reaction time = 180 min	Xphenol = 100%	[68]
4-chlorophenol (4-CP)	Al/Fe-pillared clay (montmorillonite)	Catalyst loading = 0.5 g/L Concentration 4-CP = 155.6 μmol/L pH = 3.5 Temperature = 30 °C H2O2:4-CP = 13.5:1 (molar ratio) Reaction time = 240 min	X4-CP = 100%	[69]

shows some of the studies using PILCs in Fenton processes for removal of organic compounds.

As shown in Table 1, the use of iron-pillar clays employed in wastewater treatment allows for reaching high percentages of degradation of organic compounds in times of up to 6 h; in addition, using a pH of 3.0 allows the formation of hydroxyl radicals (•OH), but with the advantage of an easy recovery of the catalyst from the reaction medium [70]. It can also be observed that iron-pillared clays can remove a wide variety of organic compounds such as drugs, dyes, pesticides, and phenols. Furthermore, reaction temperatures are usually around 30 °C, since some authors [66] suggest that high reaction temperatures can decompose hydrogen peroxide into oxygen and water, affecting the mineralization time of organic compounds and favoring the leaching of iron, affecting the stability of the catalyst.

2.2. Photo-Fenton Processes

The photo-Fenton process is a variant of the typical Fenton process, where UV radiation (λ < 380 nm) or visible light (λ = 380–780 nm) is used to increase the efficiency of •OH generation, enhancing the process [71]. Additionally, UV radiation can help regenerate Fe²⁺ from Fe³⁺, making the Fenton process more efficient. The regenerated Fe²⁺ reacts with H₂O₂ to form •OH radicals and Fe³⁺.

The chemical reactions occurring in a photo-Fenton process are described by Equations (4)–(7) [72,73],

$${\mathrm{F}\mathrm{e}}^{3+}+\mathrm{h}\mathrm{v}\to {\mathrm{F}\mathrm{e}}^{2+}+•\mathrm{O}\mathrm{H}+{\mathrm{H}}^{+}$$

(4)

$${\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{F}\mathrm{e}}^{3+}+ •\mathrm{O}\mathrm{H}+{\mathrm{O}\mathrm{H}}^{-}$$

(5)

$${\mathrm{H}}_2{\mathrm{O}}_2+\mathrm{h}\mathrm{v}\to 2(•\mathrm{O}\mathrm{H})$$

(6)

$$•\mathrm{O}\mathrm{H}+\mathrm{o}\mathrm{r}\mathrm{g}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{c} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{a}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{n}\mathrm{t}\mathrm{s}\to \mathrm{o}\mathrm{x}\mathrm{i}\mathrm{d}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{d} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s}$$

(7)

2.2.1. Application of Pillared Clays in Heterogeneous Photo-Fenton Processes

Clays, as catalyst supports in the photo-Fenton process, offer a viable and reassuringly straightforward synthesis alternative due to iron’s easy incorporation into their structure. Additionally, iron-pillared clays (Fe-PILCs) present an interesting option, thanks to their high stability and catalytic activity across a range of pH values from 3 to 7 [58,72,74]. This straightforward process and their high stability make them effective in treating various pollutants in wastewater, offering a promising future for wastewater treatment. As previously stated, Fe-PILCs have within their structure FeO (Fe²⁺) and magnetite (Fe²⁺/Fe³⁺). Figure 6 schematically shows the photo-Fenton process employing iron-pillared clays.

Chen and Zhu [75] have proposed a simultaneous (heterogeneous–homogeneous) mechanism for the mineralization of Orange II using pillared iron clays (bentonite) under UV radiation. Firstly, for the heterogeneous mechanism, Fe³⁺ ions present on the clay surface are converted to Fe²⁺ by the effect of UV light (Equation (4)). Subsequently, the Fe²⁺ ions are oxidized by H₂O₂, forming OH radicals (Equation (5)). Likewise, the UV light favors the formation of more OH radicals (Equation (6)), which oxidize organic molecules (Orange II) (Equation (7)). They have also postulated that the formation of intermediate compounds (oxalic acid, phthalic acid, and 2-carboxybenzeneacetic acid) formed by the degradation of the dye in solution cause some iron ions to be leached, forming organic iron complexes that could be mineralized by oxidation of the hydroxyl radical and by a ligand-to-metal charge transfer (LMCT) process decarboxylating the organic intermediaries driven by photons. At the end of the cycle, these ions could return to the pillared clay’s surface and continue the cycle, as seen in Scheme 1. These results also coincide with those proposed by Soon and Hameed [76] pillared iron clays based on laponite clay.

According to Nidheesh et al. [77], the photo-Fenton process has advantages such as (a) reduction in the formation of iron sludge in comparison to the traditional Fenton process; (b) extra generation of hydroxyl radicals through the breakdown of H₂O₂; (c) swift production of ferrous ions (Fe²⁺) when exposed to UV light.

Fe-PILCs have been used in photo-Fenton to remove organic compounds such as dyes (azo-dye Acid Black [78], azo-dye Orange II [79], Methylene Blue [80], Malachite Green and Red Congo [81], phenol [82], tyrosol [83], toluene [84], antibiotics [85], and other recalcitrant compounds. Some advantages of Fe-PILCs in photo-Fenton processes are their stability, their capacity to work with pH values higher than 3.0, and where leaching of Fe ions is minimal, which allows this type of catalyst to be used in various reaction cycles [25], extra generation of hydroxyl radicals through the breakdown of H₂O₂.

The Fe²⁺ and Fe³⁺ ions in the pillared iron clay structure allow oxidation reactions to occur at pH > 3.0, with negligible iron leaching [86,87]. Furthermore, iron as pillar is very active in forming $•$OH radicals.

Guo et al. [88] suggested using rectorite (clay material) and iron to simultaneously remove tetracycline hydrochloride and Cr(VI) in a pH range of 3.8–8.1 and sunlight. A degradation of 97.5% of tetracycline hydrochloride and 98.1% of Cr(VI) was obtained. The efficiency of these catalysts continued up to 5 reaction cycles, and their mesoporous structure facilitated the molecular diffusion of H₂O₂ and contaminants. Likewise, species such as goethite (Fe(OH)O) facilitated electron transfer, improving the removal of contaminants [89].

2.2.2. Influence of Copper in Fe-PILCs

As previously mentioned, Fe-PILCs have the advantage of working at pH > 3.0, and iron is very active in generating hydroxyl radicals. However, some authors [90] suggest that incomplete mineralization of organic products may occur when working at pH circumneutral, due to the formation of stable complexes. It is one of the reasons why some authors [58,91,92] propose the introduction of additional metals into iron-pillared clays, thus enhancing the efficiency of the photo-Fenton process and allowing the oxidation reactions to occur at pH values greater than 3.0.

Copper, aluminum, and titanium are some of the metals that have been incorporated into iron-pillared clays (Fe-PILCs) to carry out the removal of organic contaminants using photo-Fenton [58,93,94].

Copper as oxide is a Lewis acid that has very advantageous photocatalytic properties. It is a non-toxic material, is very stable over long periods, is environmentally friendly, and has strong light absorption [95].

Previous studies [76,96] have shown that, under acidic conditions, copper’s catalytic activity is lower than iron’s. However, copper shows higher catalytic activity under neutral or alkaline pH conditions. Furthermore, the interaction of Cu and Fe ions increases the generation of radicals [97], because the electron transfer in the reaction medium can be accelerated [95].

These effects have been reported by Khankhasaeva et al. [62] who employed pillared clays (Fe/Cu/Al-Montmorillonite) for total mineralization of sulfanilamide in 160 min and a pH value of 3.5–4.0, observing that the three-metal catalyst (Fe/Cu/Al-Montmorillonite) had higher activity than the two-metal systems (Cu/Al-Mt and Fe/Al-Mt); this is due to the interaction between copper and iron ions. Furthermore, the excellent stability of these catalysts allowed their reuse up to four times without losing notable catalytic activity (99–94%).

Hurtado et al. [58] reported the use of iron-pillared clays with copper (Cu/Fe-PILC) in the mineralization of paracetamol under neutral pH conditions by photo-Fenton process, obtaining about 80% mineralization at almost neutral pH conditions; this result was only 2% less than the degree of mineralization achieved at acidic pH after 180 min of reaction. Moreover, Hadjltaief et al. [94] reported the effectiveness of pillared iron clays doped with copper (Cu/Fe–PILC Tunisian clay) in mineralizing phenol. Their research, conducted in the presence of UV–C irradiation and a range of pH from 3 to 7, showed that total mineralization was achieved after 40 min of reaction. The stability of the catalyst was maintained even after several reaction cycles (three times), and the leaching of the metals was insignificant (below 0.5 ppm), further confirming the effectiveness of the Cu/Fe-PILC catalyst.

Meijide et al. [89] mentioned that the stability of copper-/iron-pillared clays depends on the molar ratio of both metals (Fe-Cu), where copper is about 15 times less than iron.

As mentioned earlier, pillared clays have within their structure Brönsted and Lewis acid sites. This surface acidity favors the catalytic activity in the removal of organic pollutants. Dorado et al. [98] found that Fe-PILC has an acidity of 0.317 mmol NH₃/g, while a pillared iron clay with copper (Cu/Fe-PILC) (twice exchanged) shows a total acidity of 0.663 mmol NH₃/g. They proposed that the Na⁺ ions present in Fe-PILC are replaced by Cu²⁺ (a Lewis acid cation). The ability of Cu²+ to be reduced to Cu⁺ facilitates the reduction of Fe³⁺ to Fe²⁺, thereby accelerating the production of $•$OH.

The above agrees with the work of Sun et al. [99], who proposed that in a Fe–Cu bimetallic catalyst, Fe²⁺ ions are primarily regenerated through the reaction between Fe³⁺ and Cu⁺, as shown in Equation (8), instead of the reduction of Fe³⁺ and H₂O₂, as shown by Equation (2).

$${\mathrm{C}\mathrm{u}}^{+}+{\mathrm{F}\mathrm{e}}^{3+}\to {\mathrm{C}\mathrm{u}}^{2+}+{\mathrm{F}\mathrm{e}}^{2+}$$

(8)

Wei et al. [100] studied synthesizing magnetite (ferrous ferric oxide) for removing pyridine by the photo-Fenton process and circumneutral pH. Their findings suggest that the reaction of Cu²⁺ with H₂O₂ is a crucial factor in the production of more $•$OH radicals through their reduction to Cu⁺ (Equation (9)). Likewise, this Cu⁺ ion can also react with H₂O₂ and obtain Cu²⁺ and more hydroxyl radicals (•OH) (Equation (10)). Another way to obtain more hydroxyl radicals is when Cu²⁺ reacts with H₂O under UV radiation (Equation (11)).

$${\mathrm{C}\mathrm{u}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{C}\mathrm{u}}^{+}+ •{\mathrm{O}}_2^{-}+{\mathrm{H}}^{+}$$

(9)

$${\mathrm{C}\mathrm{u}}^{+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{C}\mathrm{u}}^{2+}+ •\mathrm{O}\mathrm{H}+{\mathrm{O}\mathrm{H}}^{-}$$

(10)

$$\equiv {\mathrm{C}\mathrm{u}}^{2+}+{\mathrm{H}}_2\mathrm{O}+\mathrm{h}\mathrm{v}\to \equiv {\mathrm{C}\mathrm{u}}^{+}+ •\mathrm{O}\mathrm{H}+{\mathrm{H}}^{+}$$

(11)

Finally, another benefit of copper in iron catalysts is its role as a “protector” of iron, as reported by Zhang et al. [101]. They used copper in an acidic medium, where it reacted with oxygen to produce hydrogen peroxide. This hydrogen peroxide, a result of the copper’s reaction, effectively prevents the iron from being oxidized, thereby ensuring the catalyst’s effectiveness.

$${\mathrm{C}\mathrm{u}}^{+}+{\mathrm{O}}_2+2{\mathrm{H}}^{+}\to {\mathrm{C}\mathrm{u}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2$$

(12)

2.3. Ozonation Processes

Ozonation is an advanced oxidation process, and a promising option for wastewater treatment. It efficiently eliminates refractory organic compounds in effluents from various industrial processes. Ozonation, unlike other AOPs, does not generate sludge, ensuring a clean operation. It produces oxygen and water as final products through the decomposition of residual ozone [102]. Ozone solubilizes sludge, decreasing biomass, resulting in a null of sludge generation [103,104].

Ozone (O₃) has a strong oxidation capacity (E = 2.07 V). It is composed of a weak single bond and a strong double bond that reacts effectively with various organic and inorganic compounds [105]. The primary partial reactions of ozone in water are Equations (13) and (14) [106],

$${\mathrm{O}}_3+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$

(13)

$${\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\to {\mathrm{O}}_2+2{\mathrm{O}\mathrm{H}}^{-}$$

(14)

Ozone generation occurs through different processes, and the corona discharge method is the most used. This method uses an electrical discharge to separate oxygen molecules and form ozone. It involves electrodes where air is collected and passed through. While it can be expensive, its clear advantages make it an important study area such as water treatment [107].

Ozone can be used to oxidize organic pollutants in wastewater in two ways: (i) direct oxidation with molecular ozone [107] and (ii) by indirect reaction generating free radicals •OH [108,109]; these two reaction routes are shown in Figure 7. The selectivity and reaction rate of ozone are both significantly influenced by pH.

The direct reaction, which typically occurs beneath acidic conditions (pH < 4), shows a high selectivity in attacking unsaturated bonds in organic compounds, but proceeds at a slow rate. Conversely, the indirect reaction under alkaline conditions (pH > 7) leads to rapid ozone decomposition and the generation of •OH radicals. This indirect reaction, while faster, is less selective with some compounds dissolved in water. Hydroxyl radicals (•OH), in the indirect ozonation mechanism, initiate a chain reaction that includes initiation, propagation, and termination. For an acidic-to-neutral pH, the indirect reactions start with the reaction in Equation (15) [103,110], while for an alkaline pH, ozone decomposes rapidly because the amount of hydroxyl ions (OH⁻) in the solution increases, initiating chain reactions; the reactions involved are shown in Equations (16) and (17) [103,105]. From these reactions, hydroperoxyl ion (${\mathrm{H}\mathrm{O}}_2^{-}$), superoxide anion ${(\mathrm{O}}_2^{-})$, and ozone anion (O₃⁻) are produced [103,110]. The hydroperoxide ion plays a crucial role in this sequence, reacting rapidly to generate hydroperoxyl radical ($\mathrm{H}{\mathrm{O}}_2^{•}$) [106], which can react again to obtain oxygen and hydroxyl radical (Equation (18)), or hydrogen peroxide (Equation (19)) [111],

$${\mathrm{O}}_3+{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{O}}_2^{-}+\mathrm{H}{\mathrm{O}}_2^{•}$$

(15)

$${\mathrm{O}}_3+{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{H}\mathrm{O}}_2^{-}+{\mathrm{O}}_2$$

(16)

$${\mathrm{O}}_3+\mathrm{H}{\mathrm{O}}_2^{-}\to \mathrm{H}{\mathrm{O}}_2^{•}+ \left({•\mathrm{O}}_3^{-},•{\mathrm{O}}_2^{-},•\mathrm{O}\mathrm{H}\right)$$

(17)

$${\mathrm{O}}_3+\mathrm{H}{\mathrm{O}}_2^{•}\to 2{\mathrm{O}}_2+•\mathrm{O}\mathrm{H}$$

(18)

$$2\left(\mathrm{H}{\mathrm{O}}_2^{•}\right)\to {\mathrm{O}}_2+{\mathrm{H}}_2{\mathrm{O}}_2$$

(19)

On the other hand, it has been observed that ozone alone does not achieve effective mineralization of recalcitrant organic pollutants in wastewater treatment. In addition, some works have reported that the chemical structure of compounds such as amines, ketones, binary, and ternary salts are not mineralized by ozone alone [112,113], so combining it with advanced oxidation processes, such as hydrogen peroxide and UV light are necessary.

Ozone-based AOPs can oxidize recalcitrant compounds in biodegradable products [114,115]. For this reason, more efficient ozonation processes integrate H₂O₂, UV radiation, and metal catalysts [116,117]. Catalytic ozonation can be classified as homogeneous or heterogeneous depending on the type of catalyst employed.

2.3.1. Homogeneous Catalytic Ozonation

This catalytic ozonation process improves the elimination of several organic contaminants from wastewater [118,119]. Various metal ions (Cr³⁺, Mn²⁺, Fe²⁺, Ni²⁺, Co²⁺, Cu²⁺, and Zn²⁺) have been employed to remove recalcitrant organic pollutants by homogeneous catalytic ozonation [120,121]. However, water pollution from the use of metal ions is becoming an important problem [122,123]. To avoid these drawbacks and to increase the surface area, efficiency, and stability of the catalyst [124], these active species and metallic oxides have been immobilized on supports such as bentonite, sepiolite, montmorillonite, silica, and activated carbon [123,124].

2.3.2. Heterogeneous Catalytic Ozonation

This catalytic ozonation process promotes more efficient and faster oxidation reactions [125]. The advantages of heterogeneous catalytic ozonation include high stability and efficiency, complete mineralization of contaminants, no additional or secondary contamination, no need for a continuous supply of reagents, and the ability to reuse and regenerate the catalyst [126,127].

Various heterogeneous catalysts for ozonation have their own physicochemical and catalytic characteristics. Their greater surface area and more active sites allow for better catalytic performance [128], generally metal oxides are used as heterogeneous catalytic ozonation (

Table 2. Summary of heterogeneous ozonation catalysts for the removal of organic pollutants in wastewater.

Organic Pollutants	Catalyst	Reaction Conditions	Remotion	Ref
Wastewater from yeast production	MnO2	CODo = 880 mg/L Catalyst loading = 6 g/L O3 350 m/L pH = 12.0 Reaction time = 20 min	56.02%	[129]
Petroleum refinery wastewater	Mn-Fe-Cu/Al2O3	CODo = 2825 mg/L Catalyst loading = 7 g/L O3 2.9 g/h pH = 8.2 Reaction time = 60 min	67.1%	[42]
Wastewater from paper production	Fe-NZ	CODo = 400 mg/L Catalyst loading = 10 g/L O3 300 m/L pH = 9.0 Reaction time = 120 min	71.01%	[130]
Organic wastewater (high salt content)	Ca-C/Al2O3	CODo = 100–126 mg/L Catalyst loading = 400 g/L O3 12 m/L pH = 8.36 Reaction time = 40 min	64.40%	[131]
Hypersaline wastewater	SnOx-MnOx/Al2O3	CODo = 500 mg/L Catalyst loading = 40 g/L O3 6 m/L pH = 7.0 Reaction time = 240 min	93.80%	[132]
Phenacetin (PNT)	CuFe2O4	[PNT]o = 0.2 mM Catalyst loading = 2 g/L O3 0.36 m/L pH = 7.72 Reaction time = 180 min	70%	[133]
Sulfamethoxazole (SMX)	Fe2O3/CeO2/Actived Carbon	[SMX]o = 40 mg/L Catalyst loading = 2 g/L O3 48 mg/L pH = 7.5 Reaction time = 15 min	86%	[134]
Ibuprofen (IBU)	Fe2O3/Al2O3@SBA-15	[IBU]o = 10 mg/L Catalyst loading = 1.5 g/L O3 30 m/L pH = 7.0 Reaction time = 60 min	90%	[135]
Reactive Black 5 dye (RB5)	Ag-Ce-O	[RB5]o = 100 mg/L Catalyst loading = 0.7 g/L O3 60 L/h pH = 10.0 Reaction time = 80 min	88%	[136]
4-Chlorophenol (4-CP)	MnFe2O4	[4-CP]o = 100 mg/L Catalyst loading = 1 g/L O3 5 mg/L pH = 6.21 Reaction time = 30 min	95.7%	[137]
Indigo carmine (IC)	Fe-pillared clay	[IC]o = 1000 mg/L Catalyst loading = 100 mg/L O3 0.045 L/min pH = 3.0 Reaction time = 60 min	100%	[138]

). In addition, chemisorption reactions involving ozone, the catalytic surface, and organic compounds occur simultaneously [9].

According to the above results, heterogeneous catalysts can effectively remove various organic pollutants in wastewater by combining the oxidizing characteristics of ozone with the activity of catalysts [125]. Table 2 further illustrates the diversity in the pH range used, which underlines the importance of this parameter in the catalytic ozonation process and its impact on the degradation pathway. In the direct reaction, molecular ozone is transferred from the gaseous to the liquid phase, reacting with organic compounds, and in the indirect reaction, the ozone generates various radicals [139].

Ozonation with Fe-Pillared Clay

Pillared iron clays (Fe-PILC) have also been shown to be effective in degrading indigo carmine (IC) by ozonation [138].

Bernal et al. [138] reported that ozone combined with Fe-PILCs allowed the degradation of indigo carmine to isatine, which decreased by up to 72% in 60 min.

The interaction of ozone with iron-pillared clays can improve the generation of •OH radicals, according to Equations (20) and (21) [140],

$${\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{O}}_3\to \mathrm{F}\mathrm{e}{\mathrm{O}}^{2+}+{\mathrm{O}}_2$$

(20)

$$\mathrm{F}\mathrm{e}{\mathrm{O}}^{2+}\stackrel{{\mathrm{H}}_2{\mathrm{O}}_2}{\to }{\mathrm{F}\mathrm{e}}^{3+}+•\mathrm{O}\mathrm{H}+\mathrm{O}{\mathrm{H}}^{-}$$

(21)

The iron species in the pillared clay may generate the ferrous species shown in Equation (22),

$$6{\mathrm{H}}^{+}+{\mathrm{O}}_3+\mathrm{F}\mathrm{e}\mathrm{O}\to {\mathrm{F}\mathrm{e}\mathrm{O}}^{2+}+3{\mathrm{H}}_2\mathrm{O}$$

(22)

Through the following reaction (Equation (23)), ferrous ions (Fe²⁺) can be obtained from ferric ions (Fe³⁺),

$${\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{F}\mathrm{e}}^{2+}+\mathrm{H}{\mathrm{O}}_2^{•}+{\mathrm{H}}^{+}$$

(23)

Likewise, Bernal et al. [138] reported that no significant effect was observed on the ozonation rate by pure bentonite, but there was when Fe-PILCs were used. According to the above results, iron-pillared clays represent a viable option. When combined with ozone, they are capable of degrading and mineralizing recalcitrant contaminants such as indigo carmine present in wastewater.

2.4. Catalytic Wet Hydrogen Peroxide Oxidation Process

Catalytic wet oxidation of hydrogen peroxide (CWPO) is a promising advanced oxidation process that utilizes hydrogen peroxide as an oxidizing agent and a heterogeneous catalyst capable of degrading organic and inorganic compounds from wastewater under moderate pressure and temperature conditions [141,142]. This process is based on the decomposition of H₂O₂ on the surface of a catalyst to generate hydroxyl radicals (•OH) and hydroperoxide (${\mathrm{H}\mathrm{O}}_2^{•}$). In fact, the incorporation of H₂O₂ in heterogeneous catalysis favors a greater participation of the hydroxyl radicals present in the reaction environment [143].

Pillared aluminum or iron clays represent a viable option for CWPO because these materials combine their porosity with the activity of the active sites, as well as for its abundance, stability, and low cost [144].

Table 3. Summary of pillared clays in CWPO for the degradation of organic pollutants in wastewater.

Organic Pollutants	Catalyst	Reaction Conditions	Remotion	Ref
Phenol compounds in coffee wastewater	Al/Fe-pillared bentonite	CODo = 38,416 mg/L Catalyst loading = 4.10 g H2O2 = 0.17 mol/L Flow rate = 2 mL/h pH = 4.2 Reaction time = 96 h Temperature = 25 °C Eight reuse cycles	Xphenol = 62.4% Selectivity to CO2 = 67.5%	[145]
Phenol	Fe-Silica pillared clay (SPC)	[Phenol]o = 500 mg/L Catalyst loading = 2 g/L H2O2 = 0.1 mol/L Flow rate = 2 mL/h pH = 3.7 Reaction time = 120 min Temperature = 30 °C Three reuse cycles	Xphenol = 99.5%	[146]
Tyrosol	Cu/Al-pillared clay	[Tyrosol]o = 3.6 mmol/L Catalyst loading = 1.0 g/L H2O2 = 0.068 M Flow rate = 30 mL/h pH = 5.6 Reaction time = 60 min Temperature = 25 °C	Xtyrosol = 100%	[147]
Phenol	Al-Ce-Fe/bentonite	[Phenol]o = 47 ppm (5 × 10−4 M) Catalyst loading = 0.5 g H2O2 = stoichiometric amount pH = 3.7 Reaction time = 4 h Temperature = 25 °C	Xphenol = 100%	[148]
Organic matter in surface water	Al/Fe-pillared clay	[DOC]o = 7.0 mg/L Catalyst loading = 3.81 g/L H2O2 = 0.037 mg/mg C. mg Fe Flow rate = 0.56 cm3/min pH = dependent on sampling location Reaction time = 4 h Temperature = dependent on sampling location	90%	[149]

summarizes works using pillared clays with various metals, such as copper, iron, manganese, and platinum.

The catalytic activity of pillared clays is due to the active sites (metals) located in the pillars that form it and in the structure of the clay sheets [54,55]. Notably, adding H₂O₂ significantly enhances the participation and formation of the hydroxyl radicals in the reaction medium. These radicals depend on the concentration of the catalyst and the reaction temperature and are produced by a redox process when hydrogen peroxide meets the catalyst [150], as shown in the following reactions (Equations (24)–(30)), where R-H is the organic pollutant:

$${\mathrm{F}\mathrm{e}}_{\left(\mathrm{P}\mathrm{I}\mathrm{L}\mathrm{C}\right)}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{F}\mathrm{e}}_{\left(\mathrm{P}\mathrm{I}\mathrm{L}\mathrm{C}\right)}^{2+}+\mathrm{H}{\mathrm{O}}_2^{•}+{\mathrm{H}}^{+}$$

(24)

$${\mathrm{F}\mathrm{e}}_{\left(\mathrm{P}\mathrm{I}\mathrm{L}\mathrm{C}\right)}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{F}\mathrm{e}}_{\left(\mathrm{P}\mathrm{I}\mathrm{L}\mathrm{C}\right)}^{3+}+•\mathrm{O}\mathrm{H}+{\mathrm{O}\mathrm{H}}^{-}$$

(25)

$$\mathrm{R}-\mathrm{H}+•\mathrm{O}\mathrm{H}\to \mathrm{R}•+{\mathrm{H}}_2\mathrm{O}$$

(26)

$$\mathrm{R}• + {\mathrm{O}}_2 \to \mathrm{ROO}•$$

(27)

$$\mathrm{R}\mathrm{O}\mathrm{O}•+\mathrm{R}-\mathrm{H}\to \mathrm{R}\mathrm{O}\mathrm{O}\mathrm{H}+\mathrm{R}•$$

(28)

$$\mathrm{R}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{F}\mathrm{e}}_{\left(\mathrm{P}\mathrm{I}\mathrm{L}\mathrm{C}\right)}^{2+}\to \mathrm{R}\mathrm{O}•+{\mathrm{F}\mathrm{e}}_{\left(\mathrm{P}\mathrm{I}\mathrm{L}\mathrm{C}\right)}^{3+}+{\mathrm{O}\mathrm{H}}^{-}$$

(29)

$$\mathrm{R}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{F}\mathrm{e}}_{\left(\mathrm{P}\mathrm{I}\mathrm{L}\mathrm{C}\right)}^{3+}\to \mathrm{R}\mathrm{O}\mathrm{O}•+{\mathrm{F}\mathrm{e}}_{\left(\mathrm{P}\mathrm{I}\mathrm{L}\mathrm{C}\right)}^{2+}+{\mathrm{H}}^{+}$$

(30)

As Table 3 shows, Fe-PILCs with various metals (Cu, Ce, Al) are highly efficient in the catalytic wet oxidation hydrogen peroxide of aqueous effluents with various recalcitrant pollutants. In the case of copper–aluminium pillared clays, the oxidation state of aluminium is Al³⁺. Therefore, the oxidation–reduction reaction between Al³⁺ and H₂O₂ cannot generate hydroxyl radicals, so other metals such as copper with different oxidation states (Cu⁺ and Cu²⁺) must be incorporated, which decompose directly with H₂O₂ and produce •OH radicals.

When pillared clays are used in CWPO, the reactions can be carried out under mild pressure and temperature conditions, which makes the process more feasible. In addition to presenting high stability and catalytic activity, their leaching is insignificant, probably due to their structure, making them potential catalysts for the degradation of organic compounds.

2.5. Photocatalysis

Photocatalysis is an advanced oxidation process employed to degrade organic pollutants at low concentrations in wastewater [151]. It is generally carried out at room pressure and temperature, and oxidizing compounds are not necessary to add to the reaction medium [152,153].

In the process of photocatalysis, photonic energy, such as ultraviolet radiation, visible light, or sunlight, is converted into chemical energy by photocatalysts, typically semiconductors like TiO₂, ZnO, ZnS, Fe₂O₃, GaP, MoS₂, CdS, CeO₂, AgCl, WO₃, ZrO₂, SnO₂, BiVO₄, BiOBr, CdS, SiO₂, and WS₂ [151,154,155]. When photons, with energy equal to or greater than the band gap of the material (BG), hit the photocatalyst, an electron (e⁻) is excited from the valence band (VB) to the conduction band (CB), creating a hole (h⁺) in the valence band. This process generates the electron–hole pair (e⁻h⁺), which will participate in the reduction and oxidation of organic pollutants until complete mineralization (H₂O and CO₂) or until obtaining low-toxicity species [156,157] (Figure 8).

Chakravorty and Roy [154] reported that oxidation-reduction reactions take place on the surface of photocatalysts according to the following reactions (Equations (31)–(35)),

Photo excitation,

$$\mathrm{P}\mathrm{h}\mathrm{o}\mathrm{t}\mathrm{o}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}+\mathrm{h}\mathrm{v}\to {\mathrm{e}}^{-}+{\mathrm{h}}^{+}$$

(31)

Charge carrier trapping of e⁻:

$${\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}\to {\mathrm{e}}_{\mathrm{T}\mathrm{R}}^{-}$$

(32)

Electron hole recombination:

$${\mathrm{e}}_{\mathrm{T}\mathrm{R}}^{-}+{\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+}\left({\mathrm{h}}_{\mathrm{T}\mathrm{R}}^{+}\right)\to {\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}+\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{t}$$

(33)

Oxidation of hydroxyls:

$${\mathrm{O}\mathrm{H}}^{-}+{\mathrm{h}}^{+}\to •\mathrm{O}\mathrm{H}$$

(34)

Photo-degradation of R-H (organic pollutant):

$$\mathrm{R}-\mathrm{H}+\mathrm{O}\mathrm{H}\to {•\mathrm{R}}^{\prime }+{\mathrm{H}}_2\mathrm{O}$$

(35)

TiO₂ is among the most used photocatalysts since it presents an important difference between the valence (BV) and conduction (BC) bands, and its reaction generates a considerable oxidation power of the hydroxyl radical [154]. However, recent efforts have been made to develop photocatalysts with greater surface area and improve their catalytic activity in the visible region [159,160].

For this reason, various materials have been used as supports for TiO₂ or different photocatalysts to achieve their activity in visible light. Some of these materials are activated carbon [161], zeolites [162,163], silica [164,165], and clays [155,166]. These supports provide a high surface area, porosity, and easy recovery; in the case of titanium oxide, they prevent its aggregation [159].

Clays have been of the most important supports mentioned above because they are easy to obtain, relatively low cost, porous, chemically stable, non-toxic, and have high mechanical resistance [167,168].

Table 4. Photocatalytic degradation of contaminants in wastewater using pillared clays.

Organic Pollutants	Photocatalyst	Reaction Conditions	Degradation	Ref
Metoprolol tartrate (MET)	TiO2-FeAB (acidified bentonite) TiO2/NB (natural bentonite)	[MET] = 3 × 10−5 M Catalyst loading = 1 g/L UV visible pH = 6.2 Temperature = 20 °C Reaction time = 300 min	80% 70%	[169]
Textile wastewater	TiO2-anthill clay	[Textile wastewater] = 3.01 mg/L CODo = 2.89 mg/L Catalyst loading = 2.05 wt% Sunlight pH = 2.0 Reaction time = 1.07 h Four cycles of reuse	70.92%	[170]
Blue 19 dye	Nb2O5-bentonite	[Blue 19] = 30 mg/L Catalyst loading = 0.5 g 125 W mercury vapor lamp pH = 3.0 Reaction time = 2 h	98%	[171]
Acetaminophen	TiO2-ZnO-cloisite clay	[ACE] = 10 mg/L Catalyst loading = 250 mg/L Solar light Reaction time = 10 h	100%	[172]
Phenol	Fe-TiO2-Bentonite	[Phenol] = 5 × 10−4 M Catalyst loading = 0.5 g UV lamp (λ = 330–390 nm) pH = 3.7 Reaction time = 240 min	≅90%	[173]
Herbicides (bromacil, chlorotoluran and sulfosulfuron)	TiOSO4-laponite	[Herbicides] = 5 ppm Catalyst loading = 50 mg UV radiation Reaction time = 60 min	80%	[174]
Solophenyl red 3BL (SR3BL)	TiO2-pillared montmorillonite	[SR3BL] = 100 mg/L Catalyst loading = 2.5 g/L Solar radiation pH = 5.8 Reaction time = 220 min	95%	[175]
Rhodamine B (RHB) and tetracycline (TC)	g-C3N4-montmorillonite	[RHB] = 30 ppm [TC] = 100 ppm Catalyst loading = 2 g/L Visible light Reaction time = 360 min	87% (RhB) 76% (TC)	[176]
Tetracycline (TC)	NiO-montmorillonite	[TC] = 10 mg/L Catalyst loading = 0.8 g/L UV radiation (λ = 296 nm) Visible light pH = 6.8–7.1 Reaction time = 180 min Five cycles of use	94.2% (UV) 75.5% (Visible light)	[177]
2,4-dichlorophenoxyacetic acid (2,4-D)	Ti-pillared clay	[2,4-D] = 20 mg/L Catalyst loading = 120 mg (10.5 mmol of Ti/clay) UV radiation Reaction time = 90 min	≅70%	[178]
Acetaminophen (ACT)	Montmorillonite-Fe2O3/starch	[ACT] = 10 mg/L Catalyst loading = 0.75 g/L UVA radiation (λ = 365 nm) pH = 7.1 Reaction time = 80 min Four cycles of use	91%	[179]
Trimethoprim (TMP)	Cr3+/Ti-pillared clay montmorillonite	[TMP] = 25 mg/L Catalyst loading = 1 g/L UV radiation Reaction time = 180 min	76%	[180]
Triclosan (TCS)	TiO2/Al-pillared clay	[TCS] = 90 μmol/L Catalyst loading = 1 g/L UV radiation pH = 4.0 Reaction time = 120 min	85.15 ± 0.49%	[181]
Methyl orange (MO)	TiO2 pillared sericite	[MO] = 20 mg/L Catalyst loading = 50 mg UV radiation Reaction time = 80 min	98.5%	[182]

shows some works for the photocatalytic degradation of organic contaminants using pillared clays.

Table 4 shows that using various clays as supports for different semiconductors results in high percentages of degradation of organic contaminants using UV light, visible light, or sunlight. Therefore, it is demonstrated that pillared clays are viable for photocatalytic reactions to remove contaminants from water.

3. Future Perspectives

Pillared clays have been proven to play a crucial role in the generation of hydroxyl radicals, key species in advanced oxidation processes used to remove organic compounds in wastewater, so it is expected that these types of clays will continue to be useful for water recovery. Nevertheless, the application of pillared clays on advanced oxidation processes at industrial scale still faces a long road and some challenges to overcome. In this context, it is essential to implement more efficient synthesis methods and leave behind traditional approaches that demand large amounts of water for PILC production. This can significantly influence properties such as acidity, thermal stability, and porous structure, so it is important to optimize the process to make it more economically viable and decrease the environmental impacts without sacrificing efficiency.

It is also important to further extend the research on pillared clays and conduct lifecycle assessments to establish the environmental impacts of both, the catalyst synthesis and the wastewater treatment. In this sense, global warming potential, GWP (carbon footprint), is an environmental impact indicator that must be included in such studies. Within the advanced oxidation processes catalyzed by pillared clays, some of the stages affecting GWP are the activation of the catalyst through calcination and mixing during reaction, also energizing lamps used in photochemical processes by nonrenewable resources. Thus, it is of paramount importance to identify the active metal species required for the different advanced oxidation processes to optimize calcination temperatures and time, thus reducing the overall process’s carbon footprint. In addition, and also in the context of processes activated by light, pillared clays with copper are promising because of their activity in the visible region of the electromagnetic spectrum.

Despite the great diversity of applications that clays have, there is practically no work that refers to immobilized clays, which would be very advantageous to avoid high-pressure drops when handling high volumes in the treatment of contaminated water. Some works have focused on the extrusion of honeycomb monoliths, but none on the immobilization of pillared clays on monoliths. The importance of these supports lies in their use in bubble columns which, together with pillared clays, could intensify the advanced oxidation processes.

Finally, it is worth mentioning that by focusing on the application of pillared clays on oxidation processes, the chemical reduction implied in many cases has been generally overlooked, and this may become an important research line to convert pollutants like CO₂ into value-added compounds.

4. Conclusions

According to the report in this work, pillared clays demonstrate their potential as catalysts in advanced oxidation processes (Fenton, photo-Fenton, ozonation, catalytic peroxidation, and photocatalysis) intending to assist in the recovery and purification of wastewater from industrial, agricultural, hospital, and household sectors, with their treatment being crucial for achieving sustainable development.

The main lines of research focus on the use of pillared clays with different polyoxocations (Al, Fe, Zr, Mn, or Ti) or on the modification of clays with metals to optimize their catalytic activity or achieve the selective degradation of some contaminants, as well as allowing their use with UV light, visible light, or solar radiation.

Likewise, the use of pillared iron clays (Fe-PILCs) to remove a wide variety of organic contaminants, such as drugs, pesticides, dyes, and phenols from wastewater is shown.

In addition, pillared clays have several advantages that make them interesting, such as their porosity, stability, mechanical resistance, non-toxicity, easy recovery, and ability to work with pH values greater than 3.0, with minimal leaching of iron ions, maintaining their use in various reaction cycles. In most cases, complete mineralization of organic contaminants is achieved, or fewer contaminating intermediates are obtained.

Despite the above, pillared clays still have a long way to go to be used on an industrial scale. To achieve this, it is necessary to intensify the synthesis processes using, for example, ultrasound, the introduction of very specific polyoxocations to increase catalytic activity under visible light, and the identification of active species to optimize calcination temperatures and purification methods to achieve more sustainable and environmentally friendly processes.