Supported TiO₂ Photocatalysis of Spiked Contaminants in Water and Municipal Wastewater

Abstract

An aqueous mixture of three compounds (atrazine, carbamazepine, and p-chlorobenzoic acid) has been treated by photochemical processes including photolysis and photocatalysis with 10.7% TiO₂ supported on ceramic foams of mullite. Experiments were conducted in both ultrapure water and in a secondary effluent from a municipal wastewater treatment plant. Radiation at 365 nm was totally inefficient in the photolytic process carried out in ultrapure water; however, some sensitization phenomena were observed when municipal wastewater was used as a bulk matrix. In the latter case, conversion values in the range of 20–30% were obtained after 2 h. The photocatalytic process was much more effective experiencing conversions above 80% after just 80 min of reaction. The nature of the matrix used exerted a significant influence. Use of municipal wastewater slowed down the process due to the scavenging character of the natural organic matter content. Test runs in the presence of carbonates and t-butyl alcohol suggested that radical carbonates play some role in contaminant abatement, and secondary radicals generated after the t-BuOH attack by HO^● radicals should also be considered in the reaction mechanism. A pseudo-empirical mechanism of reactions sustains the experimental result obtained, acceptably modeling the effects of a water matrix, scavenger addition, and radiation volumetric photon flux.

1. Introduction

Water is essential for all living beings [1]. Although a significant portion of the Earth’s surface is covered by water, only about 2.5% of it is freshwater that can be directly used for domestic, agricultural, or industrial purposes [2]. As a result, less than 1% of the world’s water is readily accessible for human use, making it a precious resource. This is because 70% of freshwater is either frozen in glaciers or trapped in deep aquifers. Many contaminants are released into aquatic environments through various pathways, including domestic wastewater, hospital discharges, improper manufacturer disposal, and wastewater treatment plants [3,4]. In some cases, these contaminants are not effectively removed by conventional water and wastewater treatment processes, such as coagulation, sedimentation, filtration, and biological processes, and they can remain in surface waters at low concentrations, typically in the range of ng-μg L⁻¹ [5,6]. The water treatment process has become increasingly challenging from technical, economic, and environmental perspectives due to the growing diversity and complexity of wastewater matrices. Therefore, it is essential to develop more robust and versatile advanced treatment technologies to effectively remove contaminants [7]. Advanced oxidation processes (AOPs) eliminate these contaminants by generating reactive radicals, such as the hydroxyl radical, sulfate radical, and chloride radical [8,9,10]. Light-driven advanced oxidation processes are a promising solution for remediation contaminants in drinking water treatment. Recently, light-emitting diodes (LEDs) have emerged as a more cost-effective, environmentally friendly, and sustainable alternative to mercury lamps for UV generation, commonly referred to as UV-LEDs. Photocatalysis in the presence of titanium dioxide (TiO₂) is widely employed for photo-oxidation processes due to its high chemical stability, availability, non-toxicity, and low cost [6]. In water treatment and disinfection technologies using UV light, germicidal UVC (260 nm) has been predominantly favored over other UV wavelengths, such as UVA (365 nm) and UVB (310 nm). UVA-LEDs have been recently employed in TiO₂ photocatalysis. The use of immobilized catalysts presents a promising technology to enhance the sustainability of photocatalytic processes by enabling the reuse of spent catalysts [11]. While catalysts in suspension are generally more efficient, their difficult recovery limits their practical application [12]. In this sense, a variety of supports such as glass, silica, polymers, alumina, or hollow fiber membranes using techniques like dip-coating, electrophoretic deposition, or photo-etching have emerged as viable alternatives [13,14]. Although the photocatalytic activity of immobilized systems is often lower than that of suspended catalysts, they offer notable advantages under real-world conditions, particularly for the treatment of secondary effluents, where matrix effects are less significant and the trade-offs between efficiency and practicality can be balanced [13,14].

The aim of the present study is to simultaneously evaluate the performance of supported TiO₂ in removing contaminants from both ultrapure water and urban wastewater. Carbamazepine, atrazine, and p-chlorobenzoic acid were selected as model contaminants due to their frequent occurrence in aquatic environments and use as probe compounds in radical rates’ determination [9,15,16]. A novel aspect of this study includes the first evaluation of micropollutant removal from wastewater using UVA-LEDs in combination with TiO₂ photocatalysts supported on commercial Al₂O₃/SiO₂ foam materials. In this sense, ceramic materials have shown promising properties as a photocatalyst support. Hence, Figueredo and coworkers [13] reported the superior performance and stability of ceramic-supported TiO₂ when a secondary effluent was treated. Danfa et al. [17] studied the use of clay aggregates (Leca^®). These authors pointed out the loss of TiO₂ by erosion processes. Martin Gonzalez and collaborators [18] highlighted the potential of scaling up the Imazalil degradation and mineralization at a semi-pilot level using open-cell ceramic foams. Pellegrino [19] showed the performance of a cordierite-honeycomb-supported TiO₂ film both in pure and wastewater.

The scavenging effects of carbonates and bicarbonates in wastewater, as well as tert-butanol on the photocatalytic kinetic parameters, were also investigated.

2. Results and Discussion

2.1. Photocatalytic Degradation Influence of a Water Matrix

First, some experiments were carried out to examine the role of a water matrix (ultrapure and wastewater) to eliminate the selected target compounds. To assess the potential direct photolysis of probe compounds, uncontrolled pH photolytic runs were carried out in the presence of a mixture of the three contaminants, 1 µM of each. Control runs revealed that the direct photolysis in ultrapure water by UVA-LEDs did not lead to any significant elimination of the selected compounds after 1 h of treatment. However, the photo-degradation by UVA-LEDs using the urban wastewater matrix (see

Table 1. Parameters of the municipal wastewater treatment plant (MWWTP).

Parameter (Units)	Value
pH	8.2–8.4
Conductivity (µS/Cm) (25 °C)	670
Turbidity (NTU)	5
COD (mg O2 L−1)	36
BOD5 (mg O2 L−1)	11
Total Organic Carbon, TOC (mg L−1)	12
Inorganic Carbon, IC (mg L−1)	25
Absorbance 254 nm	0.24
Chloride, Cl− (mg L−1)	90
Nitrate, NO3− (mg L−1)	30
Sulfate SO42− (mg L−1)	63

) revealed a partial conversion of contaminants. Conversion values of roughly 30, 20, and 18% after 120 min were achieved for CAR, ATZ, and pCBA, respectively, when UVA-LEDs were used. Since none of the three compounds absorb light at 365 nm, these results cannot be explained based on the radiation spectrum of contaminants. The most plausible reason relies on the sensitizing character of natural organic matter (NOM) present in wastewater. NOM contains various chromophoric groups (like aromatic rings, phenolic groups, and conjugated double bonds) that can absorb sunlight, particularly UV and visible light. Upon absorption, these chromophores can be photoexcited, leading to generation of reactive species such as the triplet excited state that can participate in energy or electron transfer reactions. The triplet state can react with dissolved oxygen to form reactive oxygen species (ROS) such as singlet oxygen (¹O₂), the superoxide radical (O₂•⁻), or even hydroxyl radicals (HO•) (indirectly, via secondary reactions). These ROS can then oxidize organic micropollutants, including pharmaceuticals and pesticides like in this study, even if those contaminants do not absorb light themselves [20,21].

Next, a series of reactions in ultrapure water/municipal wastewater were carried out to assess the performance of the photocatalytic process in the oxidation of the mixture of micropollutants by UVA-LEDs/CF₂. As indicated in the experimental section, adsorption of micropollutants was allowed for 30 min in the dark. This time was proven long enough to achieve adsorption equilibrium. It was established that hardly 1–5% of the initial amount of compounds was adsorbed onto the photocatalysts.

Figure 1 depicts the evolution of the model compounds with time during the course of photocatalytic experiments under the UVA-LEDs/CF₂ system.

As shown in Figure 1, the presence of CF₂ in the photolytic tubular reactor notably enhances the removal efficiency of the model pollutants when the process is carried out in Milli-Q water. After one hour of reaction with six LEDs, conversion rates of approximately 80% for carbamazepine (CAR), 90% for para-chlorobenzoic acid (pCBA), and 70% for atrazine (ATZ) were achieved.

In contrast, when the same experiment was conducted with the pollutants dissolved in urban wastewater, the degradation rates significantly declined. After 60 min, the conversion dropped to 70% for CAR, 65% for pCBA, and 55% for ATZ. This reduction can be attributed to the presence of total organic carbon (TOC), inorganic carbon (IC), and other radical scavengers, which compete with the target pollutants for the photogenerated reactive species. Additionally, the adsorption of organic and inorganic constituents onto the catalyst surface may hinder the photocatalytic activity, further limiting the degradation efficiency.

2.2. Influence of Volumetric Photon Flux

The influence of the volumetric photon flux was also checked by repeating the same experiment in wastewater when four and two LEDs were switched on. Figure 2 shows this influence, revealing the importance of this parameter in the performance of the process, especially regarding the parent compound depletion. Differences were also noticed when other global parameters such as chemical oxygen demand (COD) or total organic carbon (TOC) were monitored. No effect was observed, however, when absorbance at 254 nm was considered (see Figure 2, right).

In order to evaluate the extent to which radiation flux influences the reaction kinetics, a simplified pseudo-empirical model was employed. The process was modeled under the assumption that the target pollutants are degraded by photogenerated species (such as holes or surface hydroxyl radicals), regardless of whether adsorption occurs. In other words, the generation of reactive species is considered the rate-limiting step, rather than the adsorption of the pollutants onto the catalyst surface.

The formation of these reactive species (holes, hydroxyl radicals, singlet oxygen, or others) is assumed to depend on both the TiO₂ concentration and the incident radiation flux (e.g., the number of LEDs employed), given that both the TiO₂ concentration and radiation intensity are constant over time during the experiments. Reaction (1) was simplified and treated as a constant-flux process for modeling purposes.

$$\mathrm{h}\nu {+\mathrm{TiO}}_2\stackrel{{\mathrm{k}}_{\mathrm{Photocat}}}{\to }\mathrm{hole}/\mathrm{HO}•$$

(1)

$${\mathrm{C}}_{\mathrm{i}}+\mathrm{hole}/\mathrm{HO}•\stackrel{{\mathrm{k}}_{\mathrm{Ri}}}{\to }\mathrm{Intermediate}$$

(2)

$$\mathrm{Intermediate}+\mathrm{hole}/\mathrm{HO}•\stackrel{{\mathrm{k}}_{\mathrm{Ri}}}{\to }\mathrm{Final} \mathrm{Product}$$

(3)

The proposed reaction mechanism does not consider the probable interference of the matrix composition and is only intended to ascertain the dependency of incident radiation on the process. A more detailed mechanism is proposed later. To make the approach more realistic, the reaction of target compounds with ROS leads to the generation of intermediates capable of sequestering and competing for the reactive species through Equation (3). Rate constants k_Ri are assumed to be of the same order than the reported values for hydroxyl radicals, i.e., 1.4–8.8, 5.0–9.0, and 2.4–18.0 × 10⁹ M⁻¹s⁻¹ for CAR, pCBA, and ATZ, respectively, [22,23,24,25,26,27,28,29,30,31,32,33,34].

A generic value of 2.0 × 10⁹ M⁻¹s⁻¹ was given to the rate constant in reaction (3). As observed, the interval of values in the literature for k_Ri is high and the value used determines the reactivity of the compounds. Since the purpose is to assess the influence of radiation and no water matrix effects are considered, k_Ri were adjusted within the interval of validity to model the process in wastewater. Once k_Ri were fixed, the rate constant k_Photocat was fitted to minimize the differences between modeled and experimental concentration data. Equations (1)–(3) were applied to the design equation of a perfectly mixed batch reactor leading to the corresponding differential equations which were solved by the fourth-order Runge–Kutta method. As inferred from Figure 3, a clear linear relationship is obtained between the rate of ROS generation and the volumetric photon flux.

To assess the nature of the inhibitory effect of the wastewater matrix content, the next step was to complete some experiments in ultrapure water doping this matrix with some well-known radical quenchers or typical scavengers detected in urban wastewaters.

2.3. Effect of Radical Scavengers Addition

2.3.1. Phosphates Addition

Phosphates can influence photocatalytic processes in various ways, depending on their concentration, the type of photocatalytic surface, and the experimental conditions. Phosphates can strongly adsorb onto the photocatalyst surface (TiO₂), blocking active sites and reducing the adsorption of pollutants [35]. This adsorption is particularly relevant in acidic and neutral media, as phosphate groups can form complexes with surface sites on TiO₂. Phosphates can modify the surface charge of the photocatalyst, affecting the attraction or repulsion of pollutants and reactive species such as hydroxyl radicals (HO•). Additionally, depending on the pH, phosphates can shift the point of zero charge (PZC) of TiO₂, influencing its photocatalytic efficiency [36].

Phosphates can also act as electron scavengers, trapping photogenerated electrons and thus inhibiting the overall photocatalytic process. This reduces the generation of reactive species like hydroxyl radicals (HO•), which are essential for pollutant degradation. Additionally, under certain conditions, phosphate ions can enhance photocatalytic reactions by facilitating the formation of surface complexes with metal ions, which can improve charge separation and enhance the photocatalytic performance.

Some photocatalytic experiments were conducted in the presence of CF₂, applying UVA light after the addition of phosphates. Figure 4 shows the evolution of the normalized concentration of each compound in the aforementioned experiments.

As observed, the addition of phosphates to the reaction media has no appreciable effect on the kinetics, suggesting, therefore, that adsorption of target compounds onto the catalyst surface is not a crucial stage in the process. Accordingly, either the adsorption of contaminants is not the limiting stage or, alternatively, oxidation takes place in the liquid bulk by free hydroxyl radicals or any other ROS in the solution.

Since the experiments were conducted in ultrapure water (ensuring no interference from other compounds), an attempt was made to model the process based on reactions (1)–(3). The fitting process resulted in a photodegradation rate constant (k_Photocat) of 9 × 10⁻⁹ M s⁻¹ and hydroxyl radical rate constants of 7.0 × 10⁹, 5.0 × 10⁹, and 4.0 × 10⁹ M⁻¹s⁻¹ for CAR, pCBA, and ATZ, respectively. These values fall within the range of those reported in the literature.

2.3.2. Carbonates Addition

Carbonates and bicarbonates can act as scavengers of hydroxyl radicals, decreasing their availability for pollutant degradation. These reactions produce carbonate radicals which have lower oxidation potential than hydroxyl radicals:

$$\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}+\mathrm{H}{\mathrm{O}}^{•}\to \mathrm{C}{\mathrm{O}}_3^{-•}+{\mathrm{H}}_2\mathrm{O} \mathrm{k}=8.5·{10}^6 {\mathrm{M}}^{-1}{\mathrm{s}}^{-1}$$

(4)

$$\mathrm{C}{\mathrm{O}}_3^{2-}+\mathrm{H}{\mathrm{O}}^{•}\to \mathrm{C}{\mathrm{O}}_3^{-•}+\mathrm{O}{\mathrm{H}}^{-} \mathrm{k}=3.9·{10}^8 {\mathrm{M}}^{-1}{\mathrm{s}}^{-1}$$

(5)

As a result, the efficiency of photocatalytic degradation is reduced, especially when the process relies primarily on hydroxyl radical-mediated oxidation. Studies have shown that increasing carbonate concentrations can significantly suppress photocatalytic performance by depleting HO●, leading to a decreased rate of pollutant removal [37]. Carbonate radicals have a redox potential of approximately 1.78 V, significantly lower than HO● (2.8 V), making them less effective in degrading many organic pollutants [38,39,40]. However, in some cases, carbonate radicals can participate in selective oxidation reactions, particularly with electron-rich organic compounds. The presence of carbonate radicals may shift the photocatalytic degradation pathway, favoring different intermediate formation and slower reaction kinetics [41].

Figure 5 illustrates the partial inhibition of the process when 50 mM of carbonates at pH 8 were introduced into the reaction medium. The process was then modeled in ultrapure water, and, in addition to the mechanisms described in reactions (1)–(3), reactions (4) and (5) were incorporated. Theoretical calculations suggested that hydroxyl radicals were nearly entirely sequestered by the bicarbonate ion, resulting in a minimal conversion of target compounds, which did not align with the experimental data. Previous findings imply that either other non-scavenged reactive oxygen species (ROS) contribute to the process or, alternatively, carbonate radicals play a significant role in the removal of target pollutants. As a result, the proposed pseudo-empirical mechanism was extended to include the following reactions:

$${\mathrm{C}}_{\mathrm{i}}+{\mathrm{CO}}_3^{-•}\stackrel{{\mathrm{k}}_{\mathrm{Ci}}}{\to }\mathrm{Intermediate}$$

(6)

$$\mathrm{Intermediate}+{\mathrm{CO}}_3^{-•}\stackrel{{\mathrm{k}}_{\mathrm{Ci}}}{\to }\mathrm{Final} \mathrm{Products}$$

(7)

The following k_Ci_values were obtained from the literature: 1.25, 1.3, and 0.94 × 10⁷ M⁻¹s⁻¹ for CAR, pCBA, and ATZ, respectively [42]. Given the concentration of bicarbonate in the reaction medium, hydroxyl radicals (HO●) were effectively prevented from reacting with the target pollutants. As a result, the ratio of reaction rates between HO● radicals and bicarbonate to HO● radicals and the target pollutants was calculated as (0.05 × 5 × 10⁸)/(10⁻⁶ × 5 × 10⁹) = 5 × 10³, a value high enough to significantly impede the process. Nevertheless, the reactivity of carbonate radicals with the contaminants allowed for their partial conversion, as experimentally confirmed.

2.3.3. Tert-Butyl Alcohol (t-BuOH) Addition

Finally, the well-known hydroxyl radical scavenger tert-butyl alcohol (t-BuOH) was used to confirm the crucial role played by this species. tert-butyl alcohol shows a relatively high reactivity towards hydroxyl radicals in the order of 3.8–7.6·10⁸ M⁻¹ s⁻¹.

A photocatalytic experiment carried out in the presence of 20 mM in t-BuOH led to a conversion of roughly 20% of the target compounds after 60 min, regardless of the pollutant considered (Figure 6). Once more, given the ratios of reactions between the hydroxyl radical and t-BuOH or contaminants (0.02 × 5 × 10⁸)/(10⁻⁶ × 5 × 10⁹) = 2 × 10³, the theoretical conversion of target compounds should be minimal, and the 20% conversion experimentally obtained after 60 min is not justified. A plausible explanation would involve the generation of organic radicals capable of propagating the radical chain reaction [43,44,45]. When t-BuOH reacts with HO•, hydrogen abstraction at the α-position (tertiary carbon) is the predominant pathway, leading to the formation of a tert-butoxy radical (t-BuO•):

(CH₃)₃COH + •OH→(CH₃)₃CO• + H₂O

(8)

The tert-butoxy radical (t-BuO•) is highly reactive and can undergo different transformations such as fragmentation into a methyl radical (•CH₃):

(CH₃)₃CO•→(CH₃)₂CO + •CH₃

(9)

The methyl radical (•CH₃) is reactive and can further react with molecular oxygen to form methyl peroxyl radicals (CH₃O₂•). Additionally, the t-BuO• radical can react with O₂, leading to the formation of tert-butyl peroxy radical (t-BuOO•), which can propagate oxidation reactions:

(CH₃)₃CO• + 1/2O₂→(CH₃)₃COO•

(10)

These peroxy radicals are key intermediates in atmospheric and aqueous-phase oxidation processes [46]. Further oxidation can lead to the formation of peroxides and other carbonyl-containing compounds, influencing advanced oxidation processes such as photocatalysis and Fenton reactions [47].

Given the uncertainty in the organic radicals formed from t-BuOH oxidation and the reactivity of these radicals towards the model compounds here used, for simplicity purposes, it was decided to just consider the formation of the methyl radical and further reaction of this species with CAR, pCBA, ATZ, and the first intermediates:

•CH₃ + C_i → Intermediate

(11)

•CH₃ + Intermediate → Final products

(12)

The rate constants for the reactions of methyl radicals with organic compounds are not extensively documented and exhibit significant variability, depending on the nature of the organic substrates and the experimental conditions [48]. In this study, the rate constants for reactions (11) and (12) were adjusted to match both the experimental and calculated data. The resulting value for the rate constant is treated as a modeling parameter, rather than a precise reflection of the actual process, since not all intermediates and reactions are considered, and some radicals are omitted from the analysis. A value of 50 M⁻¹s⁻¹ was found to be suitable to satisfactorily explain the partial conversion of pollutants when the hydroxyl radical pathway was suppressed.

Additionally, an experiment conducted in the presence of both bicarbonate (0.05 M) and t-BuOH (0.02 M) confirmed that, under these particular operating conditions, the scavenging power of t-BuOH predominates and, again, the conversion observed for CAR, pCBA, and ATZ was due to the radicals generated after the attack of the tertiary alcohol by hydroxyl radicals (see Figure 7).

2.3.4. Removal of Alkalinity from Wastewater

Finally, to elucidate why the photocatalytic process is slowed down when applied in urban wastewater if compared to results obtained in ultrapure water, an experiment was conducted by previously removing carbonates from the wastewater (acidification + air stripping).

As observed in Figure 8, no appreciable effect of carbonate removal was experienced when the photocatalytic experiment was carried out in municipal wastewater. Consequently, it is suggested that the inhibitory effect of the urban wastewater must be attributed to the ability of natural organic matter (NOM), capable of scavenging radicals, to absorb light or to compete by TiO₂ active sites.

The proposed model was finally completed by introducing the composition of the wastewater shown in Table 1, that is, incorporating the concentration of TOC (NOM), carbonates, and associated reactions, including reaction (13)

$$\mathrm{TOC}+\mathrm{hole}/\mathrm{HO}•\stackrel{{\mathrm{k}}_{\mathrm{Ri}}}{\to }\mathrm{Intermediate}$$

(13)

As stated previously, the initiation stage (reaction (1)) was given the value obtained in ultrapure water (9 × 10⁻¹⁰ M s⁻¹), while the constant in (11) was given a value typically found in the literature (10⁷−10⁸ M⁻¹ s⁻¹) [49].

Moreover, the slight sensitizing nature of the wastewater towards the photolysis of CAR, pCBA, and ATZ was taken into consideration by adding simple first-order reactions such as:

$${\mathrm{C}}_{\mathrm{i}} \underset{Sensitizer}{\overset{h\nu }{\to }}\mathrm{Intermediate}$$

(14)

Rate constants in (14) were fitted, leading to values of 5.5, 2.9, and 4 × 10⁻⁵ s⁻¹ for CAR, pCBA, and ATZ, respectively.

As observed in Figure 8, experimental data were acceptably simulated by considering the set of reactions (1)–(14), including the effect of promoted photolysis, ROS scavenging by NOM, and the role of carbonates (R² = CARB: 0.981, pCBA: 0.985, ATZ: 0.990).

3. Materials and Methods

3.1. Reagents

The three micropollutants (Atrazine, ATZ, CAS: 1912-24-9; Carbamazepine, CAR, CAS: 298-46-4; and p-Chlorobenzoic acid, p-CBA, CAS: 74-11-3) were analytical standard-grade (>99%). Commercial compounds were purchased from Sigma-Aldrich, Louis, MO, USA. Two different water media were tested in this study. Ultrapure water was purified by a Millipore Milli-Q water academic system (18.2 MΩ cm⁻¹). Wastewater was collected from the secondary sedimentation unit of the municipal wastewater treatment plant (MWWTP) Rincon de Caya in the city of Badajoz, Spain. Water was collected after the activated sludge biological oxidation. Water samples were then filtered through cellulose filters (Whatman, Maidstone, UK, grade 1) in order to remove particles in suspension and stored in a freezer at 4 °C. Table 1 summarizes the main characteristics of MWWTP.

3.2. Photocatalyst Synthesis and Characterization

The synthesis of the supported catalyst referenced the work of Figueredo and co-authors [13]. Ceramic foams (CF) of mullite (Al₂O₃/SiO₂ 20 ppi, Vukopor^®, Lanik, Boskovice, Czech Republic) were used as TiO₂ support; 30 mm diameter × 22 mm width and 3.1 ± 0.3 g average weight pieces were obtained from commercial pieces (nominal size 50 × 50 × 22 mm) after several washing steps with boiling ultrapure water and drying in oven. TiO₂ immobilization was carried out by dip-coating (dip-coater ND-DC, Nadetech Innovations, Navarra, Spain), following the procedure already reported in the bibliography [50]. Briefly, the pieces were submerged for 60 s in a 150 g L⁻¹ TiO₂ P25 suspension in ultrapure water at pH 1.5 (with HNO₃) at an immersion/emersion speed of 0.65 mm s⁻¹. Then, they were dried in an oven at 110 °C for 24 h and calcined at 500 °C for 2 h (heating rate: 5 °C min⁻¹). Finally, the pieces were rinsed with water, dried, and stored until use. In some cases, a new coating cycle (impregnation + calcination) was applied. The synthesized materials have been called CF2.

The composition of CF2 (wt%) according to WDXRF analysis was the following: (Al₂O₃: 62.1; SiO₂: 23.9; MgO: 1.39; TiO₂: 10.7; K₂O: 0.53; Na₂O: 0.53; Fe₂O₃: 0.41; CaO: 0.36). The nitrogen adsorption–desorption isotherm revealed that the BET surface area, S_BET, was 3.032 m².g⁻¹. The morphology of the supported catalyst shows the irregular multilayer deposition of TiO₂ on the foam surface [13].

3.3. Experimental Setup and Procedure

A tubular photoreactor equipped with a borosilicate glass tube (28 mm internal diameter, 500 mm length, total volume 0.36 L) was used in all the experiments. The reactor was situated above the air-cooled radiation system equipped with 6 LEDs (LZ4-04UV00, LED ENGIN; 3 W radiant power each, λ_max 365 nm). A volumetric photon flux of I_{UVA, 365nm} = 3.73 × 10⁻⁵ Einstein L⁻¹s⁻¹ was determined using nitrite as actinometer 1.5 × 10⁻⁴ M [1]. The wall of the tube was covered by a stainless-steel reflector. The photoreactor was filled with 23 CF2 pieces, and the total estimated amount of TiO₂ was 7.6 g. The reactor was connected to an agitated tank (Pyrex, 1.3 L capacity) with inlets/outlets for liquid and sampling. The tank was initially loaded with 1 L of water to be treated and then pumped using a peristaltic pump. The solution of the liquid was kept recirculating at a rate of 7.7 Lh⁻¹, which provided enough turbulence to consider perfect mixing conditions.

Degradation experiments were carried out using aqueous solutions of the mixture of ATZ, CAR, and pCBA as probe compounds in both MilliQ water and urban wastewater under identical experimental conditions. The initial concentration for each compound was 1 µM (i.e., in mg L⁻¹, 0.216 ATZ; 0.236 CAR; and 0.157 pCBA). Prior to irradiation, the solution was circulated in the tube in the dark for 30 min in order to establish the adsorption equilibrium of compounds onto the TiO₂ supporting catalyst (a preliminary study indicated a negligible adsorption after 30 min; this time was assumed to be sufficient to reach adsorption equilibrium, if any). After that, the LEDs were switched on, and the reaction was started. Reaction samples were withdrawn at intervals through a sampling port for analyses.

3.4. Analytical Methods

The analysis of the three aqueous micropollutants at initial concentrations of 1 µM was carried out by means of an HPLC-DAD apparatus (Hitachi, Elite LaChrom, San Jose, CA, USA). The stationary phase used was a Phenomenex C-18 (5 μm, 3 × 150 mm) column. A50:50 (v/v) mixture of 0.1% H₃PO₄ acidified water and acetonitrile was pumped at a rate of 1 mL min⁻¹, obtaining retention times of 11.2, 12.4, and 13.8 min for CAR, pCBA, and ATZ, respectively. UV detection was conducted at 210 nm.

Total organic carbon (TOC) and inorganic carbon (IC) were analyzed in a Shimadzu TOC-VCSH analyzer equipped with automatic sample injection. Dissolved organic carbon (DOC) mineralization was measured using Hach Lange LCK414 commercial tests. pH solution was measured in a GLP 21 + Crison^® pH meter. Conductivity and turbidity were determined in a Crison^® 524 conductimeter device and 2100 IS Hach^® turbidimeter, respectively.

4. Conclusions

The following conclusions can be drawn from this study:

Direct photolysis of atrazine, carbamazepine, and p-chlorobenzoic acid using UVA-LEDs (365 nm) was ineffective in ultrapure water. However, partial conversion in municipal wastewater suggests the occurrence of sensitization phenomena, likely induced by natural organic matter (NOM).

The photocatalyst supported on mullite foams (CF₂) exhibited high activity under UVA irradiation in ultrapure water, with pollutant conversion values exceeding 80% after 60 min. In contrast, the performance was notably reduced in municipal wastewater, due to the scavenging effects of TOC, IC, and other competitive species.

The photocatalytic process was dependent on photon flux, with higher LED intensity resulting in increased degradation rates. This confirms that under the tested conditions, the process is limited by the generation of reactive species rather than by pollutant adsorption.

The addition of phosphate ions had negligible effects on the degradation kinetics, while carbonate and tert-butyl alcohol significantly inhibited the process, confirming their scavenging role of hydroxyl radicals. Nevertheless, partial pollutant removal was still observed, indicating the possible involvement of other reactive oxygen species such as carbonate radicals or organic intermediates.

A pseudo-empirical kinetic model, including hydroxyl and carbonate radical pathways as well as the sensitizing effect of NOM, was developed and successfully fitted to the experimental data. The model adequately predicted pollutant conversion under various experimental conditions.

Carbonate removal from the municipal wastewater did not lead to an improvement in photocatalytic efficiency, reinforcing the dominant role of NOM in suppressing photocatalytic activity in real matrices.

Overall, the use of TiO₂ supported by mullite foams combined with UVA-LEDs represents a promising strategy for the degradation of micropollutants in water. However, the presence of radical scavengers and natural matrix components must be considered for the practical application of this technology in real water treatment scenarios.