Utilizing Zn(Cu/Cr)Al-Layered Double Hydroxide-Based Photocatalysts for Effective Photodegradation of Environmental Pollutants

Abstract

Layered double hydroxides (LDHs) and their derived mixed oxides are emerging as a promising class of biocompatible inorganic lamellar nanomaterials. The detailed structure and textural characteristics of the synthesized LDH-based materials were examined using X-ray diffraction, Fourier transform infrared spectroscopy, and N₂ adsorption/desorption isotherm. This study explored the removal efficiency of pharmaceutical tolperisone hydrochloride (TLP), as well as the herbicides quinmerac (QUI) and clomazone (CLO) from water, using dried and calcined LDH-based photocatalysts under simulated solar irradiation and UV irradiation. A higher removal efficiency was observed using UV irradiation, for all substrates. The most effective removal was achieved using ZnAl photocatalysts thermally treated at 100 °C (ZnAl 100) and 500 °C (ZnAl 500). The highest removal rates were observed in the TLP/ZnAl 100 and QUI/ZnAl 100 systems, achieving ~79% and ~86% removal after 75 min of treatment under UV. In contrast, the CLO/ZnAl 100 and CLO/ZnAl 500 systems achieved ~47% removal of CLO. Furthermore, this study investigated the role of reactive species to elucidate the mechanisms of photodegradation under UV. It was found that in the degradation of TLP and QUI in the presence of ZnAl 100 and ZnAl 500, the superoxide anion radical played the most important role.

1. Introduction

The availability of clean water is becoming an increasingly critical issue affecting the global economy and growing populations. The unsustainable exploitation of natural water resources has resulted in widespread pollution, which contributes to the global drinking water crisis. This overuse has also disrupted aquatic ecosystems and changed geological cycles. The key concern is the contamination of water by micropollutants, such as pharmaceuticals, personal care products, industrial chemicals, and pesticides [1,2]. Each year, approximately 400 million tons of pollutants are discharged into the aquatic environment. These substances are often persistent, non-biodegradable, and capable to bioaccumulate, leading to significant environmental hazards [3].

Pharmaceuticals, even in low concentrations, exist in wastewater and can ultimately enter surface water, groundwater, or agricultural lands. This contamination poses various health risks to both aquatic ecosystems and human populations [2,4]. Tolperisone hydrochloride (TLP), a piperidine derivative, is commonly used to treat conditions such as multiple sclerosis, myelopathy, and painful muscle spasms in orthopedic and rheumatologic diseases. As a centrally acting muscle relaxant, TLP effectively reduces elevated muscle tone and tension without side effects [5]. To prevent potential harmful effects on non-target organisms, unnecessary contact with TLP should be avoided [6]. According to the European Directive 67/548/EEC [7], TLP is highly toxic to aquatic ecosystems and must not be allowed to enter groundwater, watercourses, or sewage systems, even in minimal amounts.

Pesticides have been extensively used in agriculture to manage weeds, insects, and fungi to protect crops. However, pesticide residues have been spotted in the air, water, soil, vegetables, and animals, posing significant health risks to humans. Numerous diseases linked to pesticide pollution have been documented, including cancer, hematological disorders, pulmonary dysfunction, immune system deficiencies, and congenital deformities [8]. For instance, quinmerac (QUI), an auxinic herbicide, is particularly important for the post-emergence control of cleavers, speedwells, and other broad-leaved weeds in cereals, oilseed rape, and sugar beet crops [9]. QUI is approved as an active substance by EC Directive 91/414 and authorized for use in the EU, while it exhibits high to very high mobility in soil and is stable in the pH range of 5–9. According to the European Food Safety Authority [10], it degrades slowly under simulated summer sunlight. Even though QUI offers agricultural benefits, its environmental impact is concerning due to the contamination of water bodies and its undesirable effects on non-target organisms, particularly on aquatic living organisms. Besides the above-mentioned post-emergence herbicide, rice cultivation also requires extensive use of various agrochemicals. Among these, clomazone (CLO) is a soil-applied herbicide that selectively controls various grasses and broadleaf weeds. CLO reaching sensitive plants and inhibits the synthesis of chlorophyll and carotenoids, resulting in pigment-deficient foliage [11]. This herbicide is typically a refractory toxic organic pollutant, which leads to serious environmental issues due to the large, remaining amount of residue in wastewater and in the environment [12,13].

Aquatic ecosystems and human health are at serious risk from the mentioned organic effluents, which are highly resistant to natural degradation or elimination. Fortunately, researchers around the world are actively developing techniques to remove pollutants from wastewater [14]. Various techniques, such as electrolysis, oxidation, adsorption, and photocatalysis, are employed to eliminate pollutants. Photodegradation is favored for its high efficiency and environmental friendliness. Current research focuses on the use of photocatalysts, which degrade pollutants in wastewater using environmentally friendly materials [15].

Pursuing effective semiconductor photocatalysts for wastewater remediation is one of the greatest challenges in environmental science. Layered double hydroxides (LDHs) have emerged as promising photocatalysts, gaining increasing attention in fields like water treatment, CO₂ reduction, and water splitting, due to their unique supramolecular structures and specific properties. LDH-based materials offer an innovative approach to the design, optimization, and mechanistic study of photocatalytic pollutant degradation. Their excellent physicochemical properties, such as large surface area, tunable optical properties, high anion exchange capacity, broad light absorption range, ease of synthesis, low cost, and remarkable recyclability, make them ideal candidates for environmental applications [16,17,18,19]. LDHs are notable for their structural flexibility, which allows for the incorporation of various metal cations and interlayer anions, making them highly tunable. The LDH structure can be described by the stacking of charged brucite-like layers consisting of a divalent metal ion M²⁺ (e.g., Ca²⁺, Cu²⁺, Zn²⁺, Mg²⁺, Ni²⁺), octahedrally coordinated to six hydroxyl groups, in which part of the divalent cations (M²⁺) are substituted by the trivalent ions M³⁺ (e.g., Al³⁺ Fe³⁺, Cr³⁺, In³⁺) [20]. These nanocomposites are produced using various synthetic techniques, such as reconstitution of the coprecipitation method and the ion exchange method. Calcination of LDHs triggers many structural and textural changes that usually lead to enhanced catalytic performance. LDHs have received much attention due to their easy preparation, usually by co-precipitation; their low cost; and their wide applicability because of their flexible structures. Innovations in LDH-based materials aim to enhance their performance across a range of applications [21,22].

In recent years, there have been numerous reports on the development and application of LDH-based nanocomposites. Specifically, LDHs are promising nanomaterials for removing various pollutants, as their tunable structure offers substantial potential for tailored environmental remediation solutions. For that reason, research efforts have focused on exploring novel catalysts with excellent ion-exchange and adsorption capacities, large surface areas, and suitability for photocatalytic processes [22].

In this study, firstly, various Zn(Cu/Cr)Al LDH-based materials were synthesized and prepared employing the coprecipitation method under different experimental conditions. Considering that the synthesis route should be simple, cost-effective, and environmentally friendly, the coprecipitation method was chosen to meet the mentioned requirements. Furthermore, due to their favorable properties (optical, textural, and structural), as well as stability and non-toxicity, the selected metal constituents and their oxides have been known to be promising materials for photocatalytic application in various fields of research. The newly synthesized photocatalysts were characterized by applying different techniques, X-ray powder diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). In addition, Brunauer–Emmer–Teller (BET), and Brunauer–Joyner–Hallenda (BJH) were employed for the determination of surface properties. Moreover, the photocatalytic activity of the LDH-based photocatalysts was tested in the removal of TLP, QUI, and CLO from water, employing simulated solar irradiation (SSI) and UV irradiation. Furthermore, the photocatalytic degradation pathway of selected pollutants was also determined using various scavengers.

2. Materials and Methods

2.1. Synthesis and Preparation of LDH-Based Photocatalysts

Photocatalysts were synthesized using the low supersaturation co-precipitation method. Metal precursors that were used for the synthesis of ZnAl-layered double hydroxides (LDHs) were Zn(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O, dissolved in 1 mol dm⁻³ solution containing 30 mol % of Al and 70 mol % of Zn. For the synthesis of ZnCr LDHs, precursors were Zn(NO₃)₂·6H₂O and Cr(NO₃)₃·9H₂O, dissolved in 1 mol dm⁻³ solution containing 30 mol % of Cr and 70 mol % of Zn were. ZnCrAl LDHs were synthesized using the same metal precursors dissolved in 1 mol dm⁻³ solution, containing 25 mol % of Al, 5% mol of Cr, and 70 mol % of Zn, whereas 1 mol dm⁻³ solution containing 30 mol % of Al, 5 mol % of Cu, and 65 mol % of Zn, was used for the synthesis of ZnCuAl LDHs. Metal precursor solutions and base solutions (0.67 mol dm⁻³ Na₂CO₃; 2.25 mol dm⁻³ NaOH) were continuously added (4 cm³ min⁻¹) at a constant temperature (40 °C) maintaining the constant pH of 9.4 in the reaction mixture. The mixture was vigorously stirred (300 rpm). Precipitation products were aged (12 h, 40 °C, 200 rpm), washed with distilled water until pH 7, dried (24 h; 100 °C), and calcined at 500 °C for 5 h in air. Dried samples (LDHs) were denoted as, ZnAl 100, ZnCr 100, ZnCrAl 100, ZnCuAl 100, and calcined samples (mixed oxides) as, ZnAl 500, ZnCr 500, ZnCrAl 500 and ZnCuAl 500.

2.2. LDH-Based Photocatalyst Characterization Techniques

XRD was used for identification of crystalline phases. XRD analysis was conducted with Rigaku MiniFlex 600 with Cu-Kα radiation (λ = 0.15406 nm; 2θ range from 5 to 70°; scan rate = 0.02 s⁻¹). The crystallite size was calculated using the Scherrer equation:

$$D=k\cdot {\displaystyle \frac{\lambda }{\beta \cdot \cos \theta }},$$

(1)

where D (nm) represents the crystallite size, k the shape function  (0.9), λ (nm) the X-ray wavelength, θ the angle of diffraction, and β  the full width at half maximum of the most intense peak.

The FTIR spectra of the powdered samples were recorded using an Alpha FTIR spectrometer (BRUKER Optics, Leipzig, Germany). The FTIR spectra were recorded with a spectral resolution of 4 cm⁻¹ in the range of 4000–400 cm⁻¹, with 24 averaged scans per measurement.

In order to determine the band gap energy of the developed photocatalytic material, the diffuse reflectance R∞ of the powdered samples were measured by UV-VIS spectrophotometer Evolution 600 (Thermo Scientific, Lenexa, KS, USA), using the DRA-EV-600 diffuse reflectance integrating sphere accessory in the range between 240 nm and 840 nm, with a step of 1 nm and speed of 30 nm min⁻¹.

Textural analysis was performed by low temperature nitrogen adsorption at −196 °C (Microtrac Belsorp Max II—MicrotracBEL, Osaka, Japan). The surface area was calculated using the BET method. The pore size distribution and the cumulative pore volume were determined by the BJH method, applied to the desorption branch of the isotherm. The external surface area, which represented the mesopore surface area without micropores, was calculated using the t-plot method.

2.3. Photocatalytic Experiments

The key properties of the investigated pollutants, pharmaceutical TLS, herbicides QUI and CLO, are summarized in Table S1 (Supplementary Material). Photocatalytic experiments were conducted as described in previous studies by our group [23], with a detailed procedure available in the Supplementary Material. All the measurements were carried out in triplicates and the error bars are shown in the figures. The following chemicals: NaF (Merck, Zug, Switzerland), ethanol (Sigma-Aldrich, Burlington, MA, USA), p-benzoquinone (≥98%, Sigma-Aldrich, Burlington, MA, USA), and H₂O₂ (30% (v/v), Sigma-Aldrich, Burlington, MA, USA) were utilized to investigate the radical mechanism involved in the photodegradation process.

2.4. Analytical Methods

Details on the experimental conditions for UFLC−DAD, UV energy flux measurements, and pH determinations are provided in the Supplementary Materials.

3. Results and Discussion

3.1. Characterization of LDH-Based Photocatalysts

XRD patterns of all dried photocatalysts are presented in Figure 1. Sample ZnAl 100 exhibited intense, sharp, symmetric reflections, which indicate high crystallinity. Reflection peaks at 2θ values of 11.96°, 23.78°, 34.21°, 39.3°, 49.7°, 59.79°, and 62° were assigned to the (003), (006), (012), (015), (018), (110), and (113) reflection planes, characteristic of layered double hydroxide (JCPDS card no. 51-1525) [24,25,26]. Additional reflections that could be assigned to the zincite ZnO phase were also detected in this sample. Similar diffractograms of ZnAl 100 and ZnCuAl 100 samples were observed. Nevertheless, the intensity of the ZnCuAl 100 sample reflections was lower, probably due to the incorporation of copper in the LDH structure.

The layered structure was also confirmed for sample ZnCr 100; however, its broad, low intensity, asymmetric peaks indicate very low crystallinity. Moreover, XRD reflections of sample ZnCrAl 100 also indicate much lower crystallinity when compared to the ZnAl 100 sample, from which can be concluded that even a small amount of chrome has a negative effect on LDH crystallinity. XRD analysis also confirmed that synthesized LDHs exhibit R3m rhombohedral symmetry [27,28].

The XRD analysis of the calcined samples (Figure 2) showed the collapse of the original layered structure and the formation of mixed oxides. After thermal treatment, the ZnO zincite phase (JCPDS card no. 36-1451) was the dominant phase in all samples, and it could be correlated with the series of Bragg reflections corresponding to the 2θ values of 32.08, 34.5, 36.2, 47.4, 56.55, and 67.8°. An additional mixed oxide phase, overlapping with the ZnO phase, that corresponds to the Zn(Al)O non-stoichiometric mixed oxide phase (JCPDS card no. 51-0037), was detected in samples ZnAl 500, ZnCuAl 500, and ZnCrAl 500 [29,30]. Broad peaks of the ZnAl 500, ZnCuAl 500, and ZnCrAl 500 samples suggest low crystallinity, probably due to the overlapping of reflections of the large amount of non-stoichiometric mixed oxides with reflections of the crystalline ZnO phase. Furthermore, sharp, high intensity reflections of sample ZnCr 500 originate from the presence of a highly crystalline ZnCr₂O₄ spinel phase (JCPDS card no. 73-1962). The small amount of Cr ions in sample ZnCrAl 500 did not induce the formation of spinel phase ZnCr₂O₄.

Crystallite size D, given in

Table 1. Structural, textural, and optical parameters of dried and calcined samples.

Sample	ZnAl 100	ZnCuAl 100	ZnCr 100	ZnCrA l100	ZnAl 500	ZnCuAl 500	ZnCr 500	ZnCrAl 500
D 1 (nm)	13.04	25.36	4.21	11.28	2.6	32.6	17.7	3.4
SBET 2 (m2 g−1)	21.2	20.3	116.8	69.7	86	32	24.4	86
Vp 3 (cm3 g−1)	0.46	0.31	0.19	0.14	0.53	0.39	0.23	0.10
Stplot 4 (m2g−1)	21.1	19.7	117.4	68.9	84.8	31.4	23.6	85.8
Eg 5 (eV)	3.66	-	3.66	3.55	3.76	-	3.64	3.55

¹ D—crystallite size; ² S_BET—BET surface area; ³ Vp—pore volume; ⁴ S_tplot—t-plot surface area; ⁵ E_g—energy band gap.

, decreased after calcination for the ZnAl and ZnCrAl samples, due to significant changes in their structure [31], which is in accordance with the lower crystallinity of the newly formed mixed oxides, noted in the XRD analysis. However, the sample ZnCuAl 500 exhibited a slight growth in crystalline size after calcination, probably because of a larger amount of crystalline ZnO phase present in this sample. The most notable change in crystallite size was detected for sample ZnCr 500, increased after calcination, concluding that the crystallinity of these samples is improved with the increase in temperature treatment, probably due to the formation of a highly crystalline spinel phase [32].

FTIR spectroscopy was conducted to provide better insight into the composition and functional groups of the synthesized and calcined samples. The FTIR spectra of LDH samples are presented in Figure 3. The broad and intense band around 3500 cm⁻¹ present in all samples was attributed to the stretching vibration of the hydroxyl OH groups and the water molecules in the layered structure, whereas a weak shoulder peak recorded at 3200 cm⁻¹ for samples ZnAl 100, ZnCuAl 100, and ZnCrAl 100 was assigned to the O–H–O vibration of the interlayer water molecules [33,34,35]. The presence of a bending mode band of the water molecules is also noted as a weak shoulder at 1700 cm⁻¹. The intercalated ${\mathrm{C}\mathrm{O}}_3^{2-}$ anions in the interlayer of double hydroxide were observed at 1355 cm⁻¹, 835 cm⁻¹, and 667 cm⁻¹ [34,35]. Bands at 553 cm⁻¹ and 430 cm⁻¹ could be attributed to the metal oxide M–O and M–O–H stretching [35]. After calcination (Figure 4), all peaks characterizing the C–O stretching vibrations were decreased considerably due to the collapse of the layered structure [34,35]. The stretching band originating from the metal oxide phases intensified, which is in good accordance with the XRD results. Very intense peaks at lower wavenumbers for sample ZnCr 500 could be explained with the presence of the spinel phase detected on XRD.

The UV-Vis diffuse reflectance spectra were applied for the energy band gap determination (E_g) using the Kubelka–Munk transformation [36,37]. The calculated data for E_g, for direct transition, are presented in Table 1. E_g values correspond to wavelengths from UVA spectrum, which is part of the solar irradiation spectrum. Nevertheless, these values indicate a rather wide band gap.

Textural analysis results are presented in Table 1, and Figure 5 and Figure 6. The adsorption/desorption curve for sample ZnAl 100 (Figure 5a) corresponds to the type II isotherm from the IUPAC classification, characteristic of non-porous or macroporous adsorbents with unrestricted monolayer/multilayer adsorption. The hysteresis loop of the H3 type appears at high relative pressures (p/p₀ > 0.9), indicating a mesoporous structure with wedge-shaped pores in sample ZnAl 100 [38,39]. After calcination (sample ZnAl 500), the shape of the isotherm and hysteresis loop at high relative pressures (p/p₀ > 0.9) remained the same (Figure 5b) with the addition of a H4 hysteresis loop type shape at relative pressures between 0.4 and 0.8, probably due to the evolution of the gasses during the thermal decomposition and formation of smaller slit shape mesopores. Wide monomodal pore size distribution for sample ZnAl 100 (Figure 6a) revealed the presence of larger mesopores with diameters around 100 nm, resulting in the relatively small surface area (21.2 m² g⁻¹). After thermal treatment (sample ZnAl 500), pore size distribution changes (Figure 6b), showing the presence of a larger amount of mesopores with diameters around 100 nm with the addition of a sharp peak at ~4 nm, corresponding to the smaller mesopores (formed during LDH thermal decomposition and evolution of H₂O and CO₂ gasses), causing the formation of larger surface area (86 m² g⁻¹).

The adsorption–desorption isotherm of the ZnCuAl 100 sample (Figure 5a) is very similar to the isotherm of the ZnAl 100 sample and corresponds to the type II isotherm from the IUPAC classification with the H3 hysteresis loop type. However, the amount of adsorbed gas was slightly lower, which could be correlated to the low presence of smaller mesopores (<10 nm) detected in this sample (Figure 6a) and lower pore volume (Table 1) in comparison to the ZnAl 100 sample. After calcination (Figure 5b), the isotherm type does not change, but the hysteresis is slightly wider and its formation starts at relative pressure of ~0.5, probably because of the formation of smaller slit-shaped mesopores. The presence of smaller mesopores was confirmed with the BJH pore size distribution, which revealed a bimodal size distribution with a distinctive peak at ~4 nm and a wide peak at larger values of pore diameter.

The sample ZnCr 100 (Figure 5a) exhibited a type IV IUPAC adsorption isotherm and a H2 hysteresis loop type, characteristic for mesoporous materials with ink-bottle-shaped pores [28,38]. After thermal decomposition, in sample ZnCr 500 (Figure 5b), the steep region of the isotherm type IV shifted to high relative pressures (p/p₀ > 0.9), and the shape of the hysteresis loop changed to the H1 type, corresponding to the networks of ink-bottle pores, where the width of the neck size distribution is similar to the width of the pore/cavity size distribution [40]. Sample ZnCr 100 displayed a monomodal distribution with smaller mesopores, indicated by the presence of the intense peak at around ~4 nm, instigating the formation of a larger surface area (116.8 m² g⁻¹). The presence of larger mesopores in the calcined sample ZnCr 500 (Figure 6b) and the complete disappearance of smaller mesopores induced a significant surface area decrease (24.4 m² g⁻¹).

The presence of the 5 mol% of Cr in sample ZnCrAl 100 resulted in the isotherm and hysteresis loop like in the ZnAl 100 sample (type II with H3 hysteresis loop), with a wider hysteresis loop shape forming at lower relative pressures, around 0.5 (Figure 5a). After thermal decomposition, in sample ZnCrAl 500 (Figure 5b), isotherm and hysteresis loop shape remained the same with the difference in hysteresis loop forming at higher relative pressures (around 0.7). Sample ZnCrAl 100 showed a wide pore size distribution (from 4 to 200 nm), with a peak at 60 nm and a shoulder at 20 nm. An indication of a small peak at around 4 nm, explains the higher surface area (69.7 m² g⁻¹) when compared to the ZnAl 100 sample (21.2 m² g⁻¹). It can be concluded that even a small amount of Cr (5 mol %) induces an increase in surface area. After calcination (Figure 6b), an indication of a small peak at around 4 nm disappeared, the distribution became slightly narrower (from 10 to 100 nm), with a peak at 50 nm and a shoulder at ~10 nm, the amount of mesopores increased, resulting in the slight increase in specific surface area (86 m² g⁻¹).

The mesoporous structure for all samples was also confirmed by the t-plot, given in Table 1, since the calculated specific area with this method revealed values very close to the BET surface area, from which can be concluded that no micropores were present.

3.2. Photocatalytic Activity of the Newly Synthesized LDHs Photocatalysts in the Removal of Selected Organic Pollutants Under Different Types of Irradiation

Efficiencies of the newly synthesized LDH-based photocatalysts were investigated in the removal of selected emerging pollutants under SSI and UV irradiation.

To begin with, the photocatalytic efficiency of different LDHs and their derived mixed oxide photocatalysts were evaluated in the TLP removal under SSI. As shown in Figure 7a, the calcined samples exhibited a higher photocatalytic activity compared to the dried samples. The highest efficiency was observed with ZnCuAl 500 and ZnCrAl 500, achieving 57.5% and 57.0% TLP removal, respectively. On the other hand, significant adsorption occurred in both systems under SSI, which is an undesirable effect of photocatalytic degradation. Specifically, after 15 min of sonification in the dark, 26.0% of TLP was absorbed in the system with ZnCuAl 500, while in the case of ZnCrAl 500, 32.0% of the TLP was adsorbed. Contrary to expectation, in the first period of photocatalytic degradation in the presence ZnAl 500 and ZnAl 100, the concentration of TLP was higher than its initial equilibrium concentration, which may be ascribed to the desorption of TLP induced by irradiation [41]. Considering the examined herbicides, the highest QUI removal was achieved in the presence of ZnAl 500 (Figure 7b). Namely, 88.7% was removed after 75 min of treatment, while the adsorption was 60.0%. Observing the results in the case of CLO, 52.1% was removed in the presence of ZnAl 500, while in the system with ZnCr 500, 51.6% was removed, as is illustrated in Figure 7c. Taking the adsorption of CLO into account, the highest level (44.0%) was achieved with ZnCr 500.

Considering the results of TLP degradation in the reaction suspension under UV irradiation (Figure 8a), the highest removal efficiency (79.0%) was observed with ZnAl 100, followed by ZnAl 500 with 74.0% of TLP removal. In all studied systems, adsorption occurred during the 15 min sonification in the dark, prior to the photodegradation experiments. Furthermore, in the case of ZnCrAl 500, the TLP concentration was increased due to the possible desorption during the irradiation. In the case of QUI removal under UV light (Figure 8b), similarly to TLP, ZnAl 100 was the most efficient photocatalyst, removing 86.0% of QUI from the suspension after 75 min of treatment. Beside the high photocatalytic activity, significant adsorption was noted with ZnAl 500, namely, 58.0% of QUI was adsorbed. Regarding the CLO removal (Figure 8c) under UV irradiation, ZnCr 500 demonstrated the highest efficiency, removing 64.0% after 60 min of photocatalytic process. On the other hand, ZnAl 100, ZnAl 500, and ZnCr 100 materials showed lower efficiencies, with around 47.0% of CLO removal. The highest level of adsorption, prior to photocatalytic treatment, was observed in the system with ZnCr 500, reaching 52.0% of CLO adsorption.

In general, the highest adsorption of all tested pollutants was observed in the case of the calcined ZnAl samples, which could be explained by the higher surface and higher amount of adsorption sites. Furthermore, the best overall performance of the ZnAl 100 and ZnAl 500 photocatalysts could be related to the presence of the highest amount of photocatalytically active ZnO phase (zincite) and the Zn(Al)O non-stoichiometric mixed oxide phase. Favorable phase composition and possible heterojunction effects between active phases promote photon absorption, confirming the crucial role of the ZnO phase in the photocatalytic removal of pollutants [28,29,31].

3.3. Effect of the Addition Various Scavengers on Photocatalytic Process

Since ZnAl 100 and ZnAl 500 were proved to be the most efficient LDH photocatalysts in the removal of selected organic pollutants, the role of different reactive species (free and adsorbed hydroxyl radicals (${\mathrm{HO}}^{•}$), photogenerated holes (h⁺), and superoxide anion radicals (${\mathrm{O}}_2^{•-}$)) in the photodegradation was investigated by the addition of various scavengers, under UV irradiation.

In order to examine the possible degradation mechanism pathway in case of TLP, experiments were carried out in the presence of NaF, p-benzoquione, ethanol, and H₂O₂ (Figure 9a,b). The lowest photocatalytic activity was achieved in the presence of p-benzoquinone, which indicates that in the TLP degradation process, ${\mathrm{O}}_2^{•-}$ plays an important role. In the presence of both of the investigated catalysts, p-benzoquione and ethanol had similar effects on the degradation process. Namely, the decreased activity can be explained by the competition between TLP and p-benzoquinone for ${\mathrm{O}}_2^{•-}$. In the case of ethanol in the system UV/ZnAl 500, 34.0% of TLP (Figure 9b) was degraded, which indicates that the free ${\mathrm{HO}}^{•}$ also take place in the degradation process. In addition, the adsorbed ${\mathrm{HO}}^{•}$ were also responsible for the TLP degradation, since in the presence of NaF, the degradation efficiency was decreased to 56.0% (Figure 9b), compared to the UV/ZnAl 500 system without scavengers. Finally, the slightest change was observed using H₂O₂. These findings showed that H₂O₂ also decreased the removal efficiency, but in a much weaker way compared to the other scavengers. Similar findings were found in the study by Šojić Merkulov et al. [42], where it was determined that in the presence of H₂O₂ with concentrations above 2.0 mmol dm⁻³, the ${\mathrm{HO}}^{•}$ can recombine to ${\mathrm{HO}}_2^{•}$, which can further react with the remaining ${\mathrm{HO}}^{•}$, resulting in ineffective oxygen and water. In addition, in a study by Bognár et al. [4], the degradation mechanism of TLP was investigated in detail and similar findings were obtained. Namely, based on the mentioned study, it can be highlighted that besides ${\mathrm{O}}_2^{•-}$, the adsorbed ${\mathrm{HO}}^{•}$ are also necessary in the degradation of TLP, which is in accordance with the current results. Furthermore, 12 various degradation intermediates of TLP were identified and determined.

On the other hand, the same scavengers were added to the suspension of QUI/ZnAl 100 and QUI/ZnAl 500, under UV irradiation. As can been seen in Figure 10a, the addition of H₂O₂ to the reaction suspension slightly decreased the removal of QUI during 60 min of UV irradiation in the presence of ZnAl 100. Within the same time frame, the addition of p-benzoquinone and ethanol had a more significant influence on the final QUI removal efficiency in system ZnAl 100. Under the same conditions (during 75 min treatment), using p-benzoquinone, 52.0% of QUI was removed, while in the presence of ethanol it was 54.0% (Figure 10a). The influence of scavenger on the system UV/ZnAl 500 is presented in Figure 10b. The lowest activity was found in the case of p-benzoquinone (55.0%). A slightly increased process efficiency was observed with NaF, where 88.0% of QUI was removed. Interestingly, an increased process efficiency was also revealed in the presence of ethanol (91.0%) and H₂O₂ (93.0%). The enhanced photocatalytic efficiency of the ZnAl 500 photocatalyst in the QUI degradation with NaF can be explained by adsorption of fluoride ions on the photocatalyst. Thus, the increased photodegradation efficiency can be ascribed either to the h⁺ direct oxidation or to the enhanced generation of free ${\mathrm{HO}}^{•}$, since the formation of adsorbed ${\mathrm{HO}}^{•}$ was significantly inhibited [43,44,45]. Moreover, increased process efficiency was also shown in the presence of ethanol (91.0%) and H₂O₂ (93.0%) is probably due to the increase in the ${\mathrm{HO}}^{•}$ radical concentration [46]. In the study conducted by Despotović et al. [41], it was also found that the heterogeneous catalysis of QUI primarily occurs via ${\mathrm{HO}}^{•}$. The results also showed that the disappearance of herbicide led to the formation of several organic intermediates.

4. Conclusions

In this paper, we report on the photoactivity of the newly synthesized LDH-based photocatalysts. These photocatalysts were prepared using a low supersaturation coprecipitation method and thermal treatment in order to obtain derived mixed oxides. Moreover, the photocatalytic activity of the newly synthesized materials was examined in the removal of the TLP pharmaceutical and the selected herbicides, QUI and CLO, from water under the SSI and UV irradiation. The successful synthesis of the LDH-based materials was proven by the applied characterization techniques (XRD, FTIR, BET, and BJH). Regarding the photocatalytic studies, based on the obtained results, each photocatalyst showed a different adsorption affinity for tested compounds. In general, a higher removal efficiency was observed using UV irradiation in comparison to SSI, for all substrates. The most effective removal was achieved using ZnAl 100 and ZnAl 500 after 75 min of treatment. Moreover, the degradation mechanism of selected pollutants (TLP and QUI) was also examined by adding various scavengers, in the presence of ZnAl 100 and ZnAl 500, under UV irradiation. Based on the obtained results, both in the case of TLP and QUI degradation, ${\mathrm{O}}_2^{•-}$ played the most important role.