Photodegradation of Methylene Blue and Crystal Violet by Zr-Modified Engelhard Titanium Silicate 10

Abstract

The present work focuses on the removal of dyes from polluted water, and, more precisely, the targets are crystal violet (CV) and methylene blue (MB). For this purpose, a series of Zr-modified catalysts based on microporous Engelhard Titanium Silicate 10 (ETS-10) were developed and synthesized. Aiming at improvement in the photodegradation efficiency and stability of ETS-10, Zr centers replacing part of Ti ones were introduced during the synthesis procedure. The obtained Na-K-ETS-10/xZr catalysts were characterized by X-ray powder diffraction (XRD), wavelength dispersive X-ray fluorescence (WDXRF), N₂ physisorption and Fourier transform infrared spectroscopy (FTIR). The photocatalytic properties of Na-K-ETS-10/xZr- (x = 5, 10, 15 and 20 wt% Zr) catalysts were studied in terms of water purification from crystal violet and methylene blue. The Na-K-ETS-10/xZr wt% x = 6 catalyst appeared to be the most efficient in the photodegradation of CV and MB, removing nearly 100% of the dyes. Kinetic studies showed that the removal of CV and MB is a rapid process and one, which obeys the non-linear pseudo-second-order model.

1. Introduction

Our society faces a critical challenge tied with the increasing demand for water resources, as only 3.5% of earth’s water is freshwater. This includes basins, lakes, rivers, groundwater and ice, all of which require careful monitoring. Various factors, such as environmental contaminants, technological advances, evolving water treatment methods and a deepened understanding of the link between water quality and human well-being, have led global health bodies, such as the World Health Organization (WHO) [1] and the European Union (EU), to establish and refine drinking water quality standards (Water Framework Directive 2000/60/EU (WFD) and Drinking Water Directive (2020/2184)) [2]. Numerous industries contribute to water pollution, with textile, pharmaceutical and mining industries among them. The textile industry, in particular, is a major polluter due to the release of dyes, chemicals and untreated wastewater [3,4,5]. Noteworthy pollutants in the textile industry include reactive dyes (prone to leaching), azo dyes (potentially carcinogenic breakdown products), indigo dye (from denim production) and direct dyes with poor fixation on fabrics. Crystal violet (CV) and methylene blue (MB), categorized as basic dyes, are used in textiles and biological staining [6]. However, CV and MB are stable in harsh conditions, pose environmental and health risks, and resist conventional degradation methods [7]. Considering the need for effective removal, various methods have been implemented, including chemical, physical and biological approaches [8,9]. Photocatalysis stands out due to its simplicity, absence of secondary pollution and recyclability. Zeolite-type porous materials, combining adsorption and photocatalysis, offer a promising solution [10]. These materials, characterized by a developed surface and good thermal stability, can undergo ion exchange, allowing for changes in their chemical composition and physicochemical properties [10,11]. Titanium Silicate 10 (ETS-10) has shown promising photocatalytic applications in various fields. ETS-10 is a zeolite-type material with a microporous framework characterized by pores ranging from 4 to 10 Å. The unique feature of ETS-10 is the replacement of aluminum (Al) with titanium (Ti), adopting an octahedral configuration [TiO₆]²⁻. In photocatalysis, ETS-10 has demonstrated significant potential due to several advantageous properties. First, the Al/Ti substitution results in a more acid-resistant compound, enhancing stability under varying conditions. This stability is crucial for maintaining catalytic activity over extended periods [12]. Additionally, the material exhibits high ionic conductivity, good acidity and pH stability, contributing to its effectiveness as a photocatalyst. The photocatalytic activity of ETS-10 is particularly relevant in the degradation of organic dyes. In the presence of UV-Vis radiation, ETS-10 can initiate photoreactions, leading to the breakdown of dye molecules. Furthermore, ETS-10 offers simplicity in its application as a photocatalyst, often involving a straightforward reactor setup. Its ability to avoid secondary pollution caused by degraded organic substances and its recyclability make it an environmentally friendly choice for water purification [13]. The main drawbacks associated with ETS-10 photocatalytic application are related to its strict synthesis and modification conditions, its limited visible light absorption, surface area constraints—e.g., it requires microporosity—its deactivation due to factors such as agglomeration, reduction in surface area or changes related to structural and physicochemical properties.

In this context, ETS-10 zeolite modification with Zr (Na-K-ETS-10/xZr) is explored in this work. The study includes a repeatable and scalable hydrothermal synthesis, determining the limit of Zr/Ti substitution. The characterization of the synthesized materials was preformed using XRD, WDXRF, FTIR and N₂ physisorption. The catalytic degradation of organic dyes CV and MB by the obtained Na-K-ETS-10/xZr samples is repeatable and achieved using white light irradiation. The data disclose 95–98% photodegradation, catalyst reutilization in several (four) cycles, regeneration of the catalytic activity up to the starting efficiency using ethanol washing. The PSO kinetics and photodegradation experiments conducted involving scavengers (IPA or EDTA) suggest a classical mechanism of photodegradation involving ^•OH and ^•O₂⁻ free radicals.

2. Materials and Methods

2.1. Materials

All starting reagents for the synthesis of Na-K-ETS-10, Na-K-ETS-10/xZr forms and those used for photocatalytic activity experiments were purchased from Sigma Aldrich (Schnelldorf, Germany) or Alfa Aesar (Heysham, UK) and used without additional purification. The reagents used included nanosized silicon dioxide (Fumed SiO₂, 0.2–0.3 µm aggregates, Sigma Aldrich), nanosized titanium dioxide (TiO₂, 99.6%, particle size 21 nm, Sigma Aldrich), sodium hydroxide (NaOH, 97%, Sigma Aldrich), potassium fluoride (KF, purity Alfa Aesar), Zirconium(IV) chloride (ZrCl₄, 99.9%, Sigma Aldrich), methylene blue hydrate (>97%, Sigma Aldrich), crystal violet (97%, Sigma Aldrich). The water used was ultrapure water (electrical conductivity—0.055 µS/cm, Q-FRONT EDI/BIO, Adrona, Rīga, Latvia).

2.1.1. Synthesis of Na-K-Engelhard Titanium Silicate 10 (Na-K-ETS-10)

The titanosilicate Na-K-ETS-10 was synthesized from a viscous gel with a molar composition 7.65 Na₂O:1.36 K₂O:5.44 H₂SO₄:1.82 TiO₂:10 SiO₂:101 H₂O [14,15,16]. The initial viscous gel was obtained by a slow addition of an acidic suspension of nanosized TiO₂ (Solution A) to an alkaline aqueous solution of nanosized SiO₂ (Solution B) with vigorous stirring at room temperature. In a typical procedure, Solution A was prepared by adding 0.8 g of nanosized TiO₂ to 2 mL of distilled water and 1.6 ml of conc. H₂SO₄, followed by homogenization for 1 h with stirring. The resulting TiO₂-H₂SO₄-H₂O mixture appeared as a milky-white suspension. Solution B was obtained by adding 3.36 g of NaOH to 8 mL of distilled water with stirring. After dissolution of NaOH (~20 min), 3.30 g of nanosized SiO₂ was slowly added with vigorous stirring until complete dissolution. The resulting light-yellow solution was homogenized for additional 1 h. Next, Solution A was added dropwise to Solution B, under constant stirring (for approximately 45 min). The resulting white viscous gel was homogenized for 25 min; then, 0.87 g of KF was added with additional 5 min of stirring. The pH of the viscous gel should be around 10.5. Finally, the gel was transferred to a 20 mL Teflon or PPL-lined autoclave, and crystallization of Na-K-ETS-10 was achieved under autogenous pressure at 230 °C for 72 h. The obtained product was vacuum-filtered and washed several times with distilled water until the pH reached ~7–8 and dried at 60 °C for 2 h. The synthesis steps are visualized in Scheme 1.

2.1.2. Functionalization of Na-K-ETS-10 with Zr

The partial substitution of Ti with Zr in the ETS-10 framework was performed in situ by adding 5, 10, 15 or 20 mol% ZrCl₄ to the initial viscous gel at the expense of the same amount of TiO₂. Thus, the molar composition of the initial gel can be given as 7.65 Na₂O:1.36 K₂O:5.44 H₂SO₄:xZrCl₄:(1.82 − x) TiO₂:10 SiO₂:101 H₂O, where x = 0.09, 0.18, 0.27 and 0.36. The procedure for preparing the initial viscous gel was similar to that for obtaining Na-K-ETS-10, with the exception that the required amount of ZrCl₄ was added to the acidic suspension containing TiO₂. Crystallization was performed in stainless-steel Teflon or PPL-lined autoclave under autogenous static conditions for 72 h at 230 °C. The obtained products were vacuum-filtered and washed several times with distilled water until the pH reached ~7–8 and dried at 60 °C for 2 h. The prepared Zr-modified samples (before performing WDXRD analyses) were labeled Na-K-ETS-10/xZr, where x = 5, 10, 15 and 20, corresponding to the added amount of ZrCl₄.

2.2. Methods of Characterization

2.2.1. Powder X-ray Diffraction (PXRD) Analysis

Powder X-ray diffraction analysis was performed on Empyrean (Malvern Panalytical, Almelo, The Netherlands) diffractometer equipped with a PIXcel3D detector and copper X-ray source (CuKα = 1.5406 Å). The diffracting patterns were collected in the 3–70° 2Theta range, under operating conditions of 40 kV and 30 mA and a step size of 0.013°.

2.2.2. Wavelength Dispersive X-ray Fluorescence (WDXRF) Spectroscopy

The chemical composition of the synthesized samples was determined on a Supermini200 WDXRF spectrometer (Rigaku, Tokyo, Japan). Data collection was performed at 50 kV and 4.00 mA. Each sample was crushed and then pressed to obtain a tablet. The sample/tablet was placed in a holder with an irradiated area of 30 mm in diameter. The weight ratio of the amount of sample to the amount of glue (Acrawax C powder) was 5:1. A semi-quantitative method (SQX) was used to determine the elemental composition.

2.2.3. Specific Surface Area Analysis—N₂ Physisorption

The specific surface area and porosity of Na-K-ETS-10 and Zr-modified Na-K-ETS-10 were analyzed using 3Flex surface analyzer (Micromeritics, Norcross, GA, USA). Before analysis, the samples were degassed in situ at 90 °C for 5 h under vacuum (>1.10⁻⁶ mmHg). The physisorption experiments were carried out under liquid nitrogen (77 K) using N₂ probe molecule. Quantitative information for the specific surface area (S, m²·g⁻¹), micropore volume (V_m, cm³·g⁻¹), pore size distribution, etc., was obtained by analyzing the resulting N₂ adsorption/desorption isotherms. Brunauer, Emmett and Teller (BET) specific surface areas were calculated from adsorption data in the relative pressure range from 0.05 to 0.31 [17]. The total pore volume was estimated based on the amount adsorbed at a relative pressure of 0.96 [18]. Micropore volumes were determined using the t-plot method [19] and the Horvath–Kawazoe micropore algorithm [20]. Pore size distributions (PSDs) were calculated from nitrogen adsorption data using an algorithm based on the ideas of Barrett, Joyner and Halenda (BJH) [21]. The mesopore diameters were determined as the maxima on the PSD for the given samples.

2.2.4. Fourier Transform Infrared (FTIR) Spectroscopy

The samples were examined using a Tensor 37 (Bruker, Berlin, Germany) spectrometer using KBr pellets. For each sample, 128 scans were collected at a resolution of 2 cm⁻¹ over the wavenumber region 4000–400 cm⁻¹.

2.3. Photocatalytic Degradation Experiments

The effective photocatalytic degradation of the organic dyes—crystal violet and methylene blue—in the presence of Zr-substituted Na-K-ETS-10 was evaluated using UV-Vis spectroscopy. The details of the experiments were as follows: 50 mg of zeolite Na-K-ETS-10/xZr was added to 10 mL of a 10 µg/mL CV or MB dye solution and stirred for 60 min at ambient conditions. The absorption resulting from the photodegradation of the dye was monitored at 585 nm for CV and 660 nm for MB using Biowave S2100 spectrophotometer (Biochrom WPA, Cambridge, UK) or Cary 4000 (Agilent, Santa Clara, CA, USA). The kinetics of the photodegradation reactions were examined based on the change in dye concentration by measuring the characteristic absorbance peak at different irradiation times. The efficiency of photodegradation was determined using the following equation [4,22]:

$$Degradation Efficiency\left(\%\right)=\frac{{(C}_0-C)}{C_0}\times 100$$

(1)

where C₀ and C are the solution concentration at t = 0 and after some irradiation time.

3. Results and Discussion

One of the drawbacks associated with ETS-10 is related to its synthesis conditions. This is indeed valid, as we attempted several recipes for obtaining a monophasic Na-K-ETS-10 (without the use of organic structure-directing agent), but in most cases, the synthesis yielded Na-K-ETS-10 and a second phase, e.g., quartz, ETS-4, AM-1 or a significant amorphous halo. For example, the use of TiCl₄ solution with SiO₂ (not nanosized) resulted in a mixture of crystalline and amorphous phases. The combination of TiCl₄ solution and nanosized SiO₂ produced ETS-10 and quartz or ETS-10 and ETS-4 depending on the time and temperature. When titanium isopropoxide was employed as a Ti source, the results were similar. Noh et al. [14] claimed that for reproducible synthesis of ETS-10, the Ti source must be nanosized. This was confirmed in our experiments when P25 Degussa (anatase to rutile at an 80:20 ratio) was employed in the synthesis, yielding only small amounts of impurities. In their synthesis, Noh et al. [14] chose commercial Ti(SO₄)₂ or TiOSO₄ as the Ti sources and sodium silicate or Ludox-AS40 solutions as the Si sources. In our case, Ti was introduced as a TiO₂ nanopowder dissolved in H₂O/H₂SO₄ solution. The silica source was introduced as amorphous fumed silica (nano silica) dissolved in NaOH/H₂O solution. In order to minimize the quartz “formation”, the SiO₂-NaOH/H₂O dissolution was carried out in a plastic beaker. The obtained molar composition was 7.65 Na₂O:2.72 KF:5.44 H₂SO₄:1.82 TiO₂:10 SiO₂:101 H₂O. This molar composition fell within the crystallization region for ETS-10 proposed by Noh et al. [14], although the total H₂O content was significantly reduced, resulting in formation of a viscous gel. Potassium fluoride was added to the gel just before its transfer into the Teflon/PPL vessel, and the pH was adjusted to around 10.5 (preventing the formation of ETS-4 at higher pH). Tentatively, KF helps the stabilization of the small 7-membered cages in the ETS-10 framework. Finally, the optimal temperature for the synthesis of Na-K-ETS-10 was found to be 230–240 °C.

The above considerations resulted in the synthesis of XRD pure Na-K-ETS-10 and also the partially substituted Zr forms. All samples obtained were characterized by WDXRF, PXRD, FTIR and N₂ adsorption/desorption for micro-/mesoporosity, specific surface and PSDs.

3.1. Physicochemical Characterization of Na-K-ETS-10/xZr Form

The chemical composition of the obtained samples was investigated by WDXRF (

Table 1. WDXRF—Elemental analysis of the synthesized Na-K-ETS-10/xZr samples.

Synthesis Batch Composition	Na, wt%	Si, wt%	K, wt%	Ti, wt%	Zr, wt%
Na-K-ETS-10/5Zr	9.90	58.85	7.15	20.84	3.26
Na-K-ETS-10/10Zr	9.63	45.23	8.56	30.37	6.21
Na-K-ETS-10/15Zr	12.07	42.10	13.93	23.78	8.13
Na-K-ETS-10/20Zr	7.97	34.97	9.53	35.44	12.10

). Based on the WDXRF results, it is visible that although the starting amounts of Zr are 5, 10, 15 and 20 wt%, the final amounts are ~3, 6, 8 and 12 wt%. Thus, not all of the supplied Zr managed to incorporate and substitute Ti in the ETS-10 framework. In addition, the amounts of Si exhibit a gradual decrease as the amounts of Zr increase (Table 1). This divergence in the expected chemistry suggests that the introduction of Zr atoms, in addition to replacing Ti atoms, produces Si vacancies in the ETS-10 framework [23] (

Table 2. Chemical composition of the synthesized samples.

Synthesized Sample Composition	Chemical Composition
Na-K-ETS-10	Na 3.28 K 1.37 (Ti 3.48 Si 15.52 O 41.48)
Na-K-ETS-10/3 wt% Zr	Na 3.23 K 1.37 (Ti 3.27 Zr 0.27 Si 15.73 O 41.27)
Na-K-ETS-10/6 wt% Zr	Na 3.55 K 1.85 (Ti 5.37 Zr 0.58 Si 13.63 O 43.37)
Na-K-ETS-10/8 wt% Zr	Na 5.01 K 3.36 (Ti 4.71 Zr 0.85 Si 14.29 O 42.71)
Na-K-ETS-10/12 wt% Zr	Na 3.32 K 2.33 (Ti 7.08 Zr 1.27 Si 11.92 O 45.08)

). The explanation is related to the greater ionic radius of Zr than Ti and the possible partial dissolution of silica on the outer surfaces of the particles.

The data from WDXRF suggesting an imbalance in the chemical composition of the synthesized Na-K-ETS-10/xZr (x = 3, 6, 8 and 12) samples with the increase in Zr content are further supported by the powder X-ray diffraction results employed to monitor the syntheses and for phase identification. The PXRD patterns of the Na-K-ETS-10 and Na-K-ETS-10/xZr zeolites of the catalysts studied are shown in Figure 1. The PXRD data revealed that the starting Na-K-ETS-10 matched closely the ICDD pattern 01-073-9935 [24], and thus, the conclusion was that the synthesized samples are monophasic.

In the case of samples with x = 3 and x = 6 Zr, the PXRD shows that the structure is preserved, although some of the diffraction maxima at higher angles have lower intensity or are undetected. On the other hand, the PXRD of samples containing higher amounts of Zr—8 wt% and 12 wt%—reveals the destruction of the ETS-10 framework. This is also visualized in the SEM micrographs (Figure 2).

The nitrogen adsorption/desorption isotherms for Na-K-ETS-10 and Na-K-ETS-10/xZr samples are shown in Figure 3, and the textural parameters determined from analyzing the isotherms are listed in

Table 3. Surface of the Na-K-ETS-10 and Na-K-ETS-10/xZr samples.

Sample	SBET m2·g−1	Vt cm3·g−1	Average Pore Diameter nm
Na-K-ETS-10	19.53	0.052	10
Na-K-ETS-10/3Zr	118.00	0.054	2.1
Na-K-ETS-10/6Zr	144.56	0.064	2.3
Na-K-ETS-10/8Zr	7.35	0.003	7.8
Na-K-ETS-10/12Zr	9.15	0.003	7.7

. In Figure 3, one can clearly see that the N₂ adsorption/desorption isotherms for Na-K-ETS-10/xZr with x = 3 and x = 6 Zr possess a microporous part, while those for x = 8 and x = 12 lack microporous adsorption. This is in accordance with PXRD data and is related to the collapse of the ETS-10 structure at higher Zr content. The N₂ adsorption/desorption isotherms of Na-K-ETS-10/x = 3 and 6 Zr wt% can be classified as a combination of type Ia and IV, according to the IUPAC classification [18], with H3 type hysteresis loop indicating the micro-/mesoporous texture of the materials. The specific surface area (SSA) of the samples is in the interval between 7.35 and 144.56 m²/g. The mesoporous diameters due to textural microporosity were found in the range of 2.1–10 nm. The highest surface area (144.56 m²/g) was registered for the Na-K-ETS-10/6 wt% Zr sample.

The FTIR spectra of Na-K-ETS-10 and Na-K-ETS-10/xZr (x = 3, 6, 8 and 12) are shown in Figure 4. The most intensive bands are observed in the 1000–400 cm⁻¹ region of the FTIR spectra and correspond to the framework vibrations. The band around 1645 cm⁻¹ is the result of deformation vibrations of bound water in the structure. The broad intensive bands observed in all spectra in the 3500–3200 cm⁻¹ region were associated with the asymmetric stretching vibrations of O-H (water) groups. For all samples, a broad band at 1037 cm^–1 is present. This band is attributed to the Si–O–Si stretching vibrations. The bands at 750 and 548 cm⁻¹ with varied intensities are related to Ti–O, Si–O and O–Ti–O vibrations, respectively. The lowest band (443 cm⁻¹) is mainly attributed to O–Si–O, O–Ti–O and Ti–O vibrations. These bands correlate well with the characteristic peaks reported previously for Na-K-ETS-10 [25,26,27,28]. Although there is a destruction of the crystalline structure (PRXD and N₂ physisorption) in samples Na-K-ETS-10/8 wt% Zr and Na-K-ETS-10/12 wt% Zr, the FTIR spectra remain quite steady, and only the intensity drop and disappearance of the skeletal related band at 740 cm⁻¹ are registered. This suggests that the nuclei or embryonic structural precursors are present, but the long-range order is not pronounced.

3.2. Photocatalytic Activity of Patterns of the Studied Samples

3.2.1. Photodegradation of Crystal Violet and Methylene Blue

The photodegradation of crystal violet and methylene blue promoted by white light irradiation and the presence of Na-K-ETS-10/xZr (where x = 3 or 6 wt% Zr) was investigated. The specifically Zr-modified samples were chosen based on the results from PXRD and N₂ adsorption/desorption analyses disclosing the preservation of the ETS-10 structure, the presence of microporosity and higher specific surface area (SSA) compared to the parent Na-K-ETS-10 sample. The results for crystal violet and methylene blue photodegradation are provided as a decrease in absorbance and as relative concentration (C/C₀) versus time (min) plots in Figure 5, Figure 6 and Figure 7, respectively. For both dyes, the photodegradation is close to 100% (96.3 and 90.1% removal of CV, and 96.2 and 98.2% removal of MB by Na-K-ETS-10/3Zr and Na-K-ETS-10/6Zr, respectively). However, the speed of the degradation process is not the same, and it depends on the catalyst employed. When Na-K-ETS-10/6Zr is used, the photodegradation of both dyes proceeds significantly faster, reaching a plateau around the 10th min. On the other hand, Na-K-ETS-10/3Zr promotes a time-prolonged photodegradation, reaching a plateau around the 55th min for CV and 70th min for MB.

The reusability and recovery (regeneration) performances of the catalyst employed are among the key selection parameters for subsequent scale-up. The reusability of Na-K- Na-K-ETS-10/6Zr was investigated for four cycles of 15 min photodegradation of CV or MB (Figure 8). During the first three cycles, the photodegradation reaction is performed for 15 min; then, the solution is exchanged with a fresh crystal violet or methylene blue. For the fourth cycle, in addition to the exchange of solution, the catalyst is washed with ethanol (1 g catalyst in 50 mL) and dried at 60 °C. In the case of CV, for the first three cycles, the photodegradation efficiency of Na-K-ETS-10/6Zr drops slowly from 96.9 to 92.9% (Figure 8a). For MB, the drop in efficiency is more pronounced, e.g., from 98.9 to 79.2% (Figure 8b). In both cases, the regeneration performed by washing the catalyst with ethanol restores its starting efficiency (Figure 8, fourth cycle). Thus, the synthesized catalysts are suitable for repeated use in the photodegradation of crystal violet and methylene blue dyes from polluted waters.

3.2.2. Kinetic Studies

The most common practice for determining photodegradation kinetic parameters involves the use of a linearized form of pseudo-first-order (PFO) and pseudo-second-order (PSO) models [29,30]. This is accomplished by plotting for pseudo-first-order (PFO) and pseudo-second-order (PSO) parameters, as dictated by Equations (2) and (3), respectively. The parameters are then estimated from the slope and intercept of the best fit line. However, other linear forms of PFO and PSO have been utilized for kinetic parameter estimation; these include the works by Uddin et al. [31] and Robati [32]. All linear forms of PSO can be applied at any t > 0 and are also suitable for a prolonged period of time. The model, which fits a photodegradation process, should be applicable to the process at any time t. Linear PFO modeling also requires prior knowledge of q_e for a straightforward parameter estimation using either Equations (2) or (3). It is applicable only during the initial stage of the photodegradation process. This could be due to the discontinuity in the linear forms of PFO as soon as q_e is attained.

The kinetic equation for the linear pseudo-first-order kinetic model [30,33,34] is represented as

$$\mathit{log}(q_e-q_t)=\mathit{log}\left(q_e\right)-\left(\frac{K_1}{2.303}\right)t$$

(2)

where q_t is the photodegradation capacity at a particular time, and K₁ is a rate constant for the first-order reaction. The values of the rate constant, maximum photodegradation capacity and correlation coefficient are shown in

Table 4. Linear PFO and PSO parameters from Na-K-ETS-10/3Zr and Na-K-ETS-10/6Zr.

Sample	Pseudo-First-Order Crystal Violet	Pseudo-Second-Order Crystal Violet	Pseudo-First-Order Methylene Blue	Pseudo-Second-Order Methylene Blue
Na-K-ETS-10/3Zr	qt = 3.5146 k1 = 0.0032 R2 = 0.9964	qe = 1.6985 k2 = 0.1876 R2 = 0.9828	qt = 0.8414 k1 = 0.0055 R2 = 0.8650	qe = 6.8121 k2 = 0.0099 R2 = 0.9851
Na-K-ETS-10/6Zr	qt = 198.83 k1 = 0.0076 R2 = 0.6237	qe = 0.0579 k2 = 5.2132 R2 = 0.9999	qt = 3.0918 k1 = 0.2562 R2 = 0.8038	qe = 2.0298 k2 = 1.0655 R2 = 0.9964

. These were obtained by plotting C₀/C versus contact period, as shown in Figure 9a,b.

The kinetic equation for the linear pseudo-second-order kinetic model [29,30,34,35] is represented as

$$\frac{t}{q_t}=\frac{1}{k_2{q_e}^2}+\left(\frac{1}{q_e}\right)t$$

(3)

The values of t/q_t versus t were plotted as shown in Table 4 and Figure 9c,d.

Non-linear modeling uses a predefined objective (OF) [29,30], which facilitates the estimation of model parameters. The OF used is the sum of squares of the distances or differences between the experimental and calculated or predicted values of the response variable (Equation (4)) [29].

$$OF=\sum _{i=1}^n{\left(y_i-{\hat{y}}_i\right)}^2$$

(4)

where y_i is the experimental response for ith observation; ${\hat{y}}_i$ is the calculated or predicted value of y_i; and n is the total number of observations or data points. A model perfectly fitted with a dataset would have the OF equal to zero. The kinetic parameters are estimated by minimizing the above OF (i.e., least-squares minimization or least-squares regression). Non-linear modeling using PFO and PSO, which utilize fractional uptake, F(t), is defined in Equation (5):

$$F\left(t\right)=\frac{q\left(t\right)}{q_e}$$

(5)

The PFO and PSO models can be transformed or reduced to Equations (6) and (7), respectively.

$$F\left(t\right)=q_e(1-e^{-K_{1}t})$$

(6)

$$F\left(t\right)=\frac{k_2}{1+k_{2}t}$$

(7)

where k₂ = k₂q_e, F(t) is the amount adsorbed at time t; q_e is the amount remaining after equilibrium of the photodegradation reaction; K₁ is the pseudo-first-order rate constant; k₂ is the pseudo-second-order rate constant.

The advantage of using non-linear modeling is having a single unknown parameter. In 2017, Tan and Hameed [30] suggested non-linear modeling as a better technique compared to linear regression, as it provides more realistic kinetic parameters. The results for non-linear PFO and PSO are shown in

Table 5. Non-linear PFO and PSO parameters for pseudo first order and pseudo second order.

Sample	Pseudo-First-Order Crystal Violet	Pseudo-Second-Order Crystal Violet	Pseudo-First-Order Methylene Blue	Pseudo-Second-Order Methylene Blue
Na-K-ETS-10/3Zr	qt = 5.2791 k1 = 0.0030 R2 = 0.9649	qt = 1.8305 k2 = 0.1010 R2 = 0.9958	qt = 7.54·10−16 k1 = 4.3·10−5 R2 = 0.9460	qt = 160.0229 k2 = 2.12·10−6 R2 = 0.9458
Na-K-ETS-10/6Zr	qt = 5.9701 k1 = 0.0743 R2 = 0.9971	qt = 1.8150 k2 = 4.1875 R2 = 0.9998	qt = 7.1800 k1 = 0.0497 R2 = 0.9986	qt = 2.1794 k2 = 0.5020 R2 = 0.9822

and Figure 10a,b for PFO, and Figure 10c,d for PSO.

In wastewater treatment, e.g., dye removal, an effective adsorbent should possess both high photodegradation capacity and rapid photodegradation rate. Kinetic models were developed to comprehend the mechanism and enhance the efficiency of the photodegradation process on a larger scale. To explore the potential rate-determining step, such as mass transfer processes, the linear and non-linear pseudo-first-order and pseudo-second-order models were employed. The determined equilibrium sorption capacity (q_t) values obtained from the non-linear pseudo-second-order model are in agreement with the experimental data for MB. On the other hand, the results obtained for the equilibrium sorption capacity (q_t) for CV correspond to a non-linear pseudo-first-order model.

Furthermore, the correlation coefficient values for all the chosen dyes are also close to unity. As a result, the photodegradation process can be effectively described by the non-linear pseudo-second-order kinetic model for MB and non-linear pseudo-first-order model for CV. On the other hand, the linear pseudo-first-order kinetics and linear pseudo-second-order kinetics exhibit low R² values and fail to accurately predict the adsorbed dye amounts. Consequently, the linear pseudo-first-order and linear pseudo-second-order models are unsuitable for describing this particular system.

The sample of Na-K-ETS-10/6Zr leads to fast photodegradation of CV and MB in wastewater, probably because of the larger surface (144.56 m²/g⁻¹). Due to its large surface area, the adsorbent interacts very well with the adsorbate, and the interaction leads to the rapid kinetics and degradation of the dyes.

3.2.3. Plausible Photodegradation Mechanism of MB and CV by Na-K-ETS-10/6Zr

The photodegradations of MB and CV are well-studied processes [36,37,38,39,40,41]. According to the available literature, the degradation of MB and CV can be described in two major steps: (1) the formation of ^•OH and ^•O₂⁻ free radicals and (2) the progressive oxidation breakage of MB and CV molecules promoted by those free radicals into CO₂, H₂O, SO₄^2– and NH₄⁺ [42,43,44]. The role of ^•OH and ^•O₂⁻ free radicals in our photodegradation study was checked indirectly by adding isopropyl alcohol (IPA) as ^•OH scavenger and EDTA-Na₂ as ^•O₂⁻ scavenger (Figure 11). Moreover, the experiments were conducted with Na-K-ETS-10/3Zr over Na-K-ETS-10/6Zr because of the slower photodegradation kinetics and with the assumption that the photodegradation mechanism is similar. The occurrence of IPA clearly inhibits the photodegradation of CV and MB in the presence of Na-K-ETS-10/3Zr. This inhibition is very well pronounced for CV (photodegradation does not start) and partial for MB (the photodegradation efficiency drops to 50%). On the other hand, the addition of EDTA-Na₂ to the reaction mixture exhibits some photodegradation inhibition for CV and basically no incidence in the MB degradation process. The oxidative breakage of the MB and CV molecules was indirectly confirmed by the absence of signals from ¹H NMR spectra related to organic molecules or fragments. The NMR data were collected after the reaction was performed in a 20 mg/mL D₂O solution of CV or MB. Due to the very high concentration of the dyes, the reaction time was extended to three days. The NMR results of the solution following CV photodegradation (Figure S1) do not disclose the presence of low molecular intermediates; the only plausible remains may be some C–OH, C–NH₂ fragments (though with extremely low concentration). In the case of MB, the concentration was too high, and the reaction could not be initiated (sonication was also attempted).

The insights of the mechanism require knowledge of both physicochemical characterization and photodegradation results. As mentioned before, Na-K-ETS-10/6Zr has higher SSA and microporosity than Na-K-ETS-10/3Zr. This is particularly important, as the non-linear PSO model was chosen as the best model describing the photodegradation process. In other words, the non-linear PSO model suggests that the surface interactions of the dyes and the catalysts are the driving force behind the photodegradation process, e.g., higher surface area, faster degradation process. In terms of chemical composition, it is clear from the WDXRF analysis that Zr displaces Ti in the framework. However, the Si amounts also diminish. In small amounts, the targeted Ti→Zr is easily accommodated by the Na-K-ETS-10 framework (Figure 12a,b). The bulkier Zr⁴⁺ requires a lengthier Zr–O bond; in addition, the Zr–O bond energy is higher than Ti–O, and as a result, the expanding ETS-10 framework produces Ti vacancies. The breakage of the Ti–O–Ti chains produces TiOH centers. With the increase in Zr amounts, the number of TiOH centers increases, and in addition, Si vacancies are produced (Figure 12c). The Ti→Zr substitution for Zr > 8 wt% cannot be accommodated by the ETS-10 framework, and the PXRD reveals its collapse. The TiOH centers in the presence of defects in the Na-K-ETS-10/3Zr and 6Zr framework suggest that the photocatalytic mechanism must be similar to TiO₂ nanoparticles (Figure 12d).

The photodegradation efficiencies of other catalysts toward MB and CV taken from the literature are provided in

Table 6. Comparative degradation efficiencies of various catalysts toward CV and MB.

Catalyst	Degradation Efficiency (%)		Ref.
	Crystal Violet	Methylene Blue
FeSnO2 (Lawsonia inermis leaves)	84.0	76.0	[45]
MgO NPs	99.2		[46]
MgO-Bentonite	83.2		[46]
GA-cl-poly(acrylamide)@ C3N4/BiOI nanocomposite hydrogel	88.0		[47]
Ag3PO4/TiO2@Ti3C2	99.2	94.4	[48]
Closite 30B/ZnO/Ag2O	99.2	98.4	[49]
CoFe2O4/TiO2	83.0	95.0	[50]
Fe3O4@inulin	91.1		[51]
Zn–Mg–Al/LDH	87.4		[52]
Peroxymonosulfate	42.0		[53]
HAP–MnFe2O4	88.0		[54]
ZnO nanosheet arrays		98.0	[55]
CdSe nanoparticles		92.8	[56]
mussel shell		99.6	[57]
TN450 coated		90.6	[58]
ZnO/20%Cu-DPA		87.0	[59]
TiO2-Ag		97.3	[60]
Fe-Ni-Cd TMNP’s		79.4	[61]
ZnO:Co5%		62.6	[62]
R-WO3/R-TiO2/CC		68.0	[11]
calcined keratin char-TiO2 composite film		87.4	[63]
cellulose/TiO2 microbeads		95.0	[64]
Ag/TiO2		90.9	[65]
ETS-10/6Zr	96.1	97.8	This work

for comparison. The values of the reported efficiencies of Na-K-ETS-10/6Zr compete with the most promising candidates [40].

4. Conclusions

The purpose of this study was an investigation of the use of microporous Na-K-ETS-10/xZr for wastewater treatment—more specifically, the photodegradation of CV and MB dyes. Na-K-ETS-10/xZr, x = 3 and x = 6 forms were successfully synthesized using the conventional hydrothermal technique without the use of structure-directing agents or templates. When the amounts of Zr were increased (x > 10 wt%), the ETS-10 structure was not conserved. The N₂ adsorption/desorption isotherms exhibited micro-/mesoporous features for both 3Zr and 6Zr catalysts; however, the SSA of the samples containing 6 wt% Zr was higher (144 versus 118 m².g⁻¹). The two synthesized Zr forms of Na-K-ETS-10/xZr (x = 3 and x = 6) were investigated for the treatment of water samples contaminated by crystal violet and methylene blue dyes. The results revealed that the efficiency of the photodegradation process for CV and MB was nearly complete (96–98%). The catalysts showed good reusability, with a slight drop in efficiency after three cycles—92.8% for CV and 79.2 for MB—and a complete regeneration of efficiency after a simple ethanol wash. The kinetic studies showed that the removal of CV and MB was a rapid process, which obeyed the non-linear pseudo-second-order model, with a strong affinity for the zeolite surface. The kinetics of the photodegradation process for the sample containing 6 wt% Zr (Na-K-ETS-10/6Zr) appeared to be faster than those for the sample containing 3 wt% Zr. The excellent photodegradation efficiency and kinetics of the obtained zeolites, along with their conserved reusability and easy regeneration, make them promising materials for industrial application.