Zeolites as Adsorbents and Photocatalysts for Removal of Dyes from the Aqueous Environment

This study investigated the potential of zeolites (NH4BETA, NH4ZSM-5, and NaY) to remove two frequently used dyes, methylene blue (MB) and rhodamine B (RB), from an aqueous environment. The removal of dyes with zeolites was performed via two mechanisms: adsorption and photocatalysis. Removal of dyes through adsorption was achieved by studying the Freundlich adsorption isotherms, while photocatalytic removal of dyes was performed under UV irradiation. In both cases, the removal experiments were conducted for 180 min at two temperatures (283 K and 293 K), and dye concentrations were determined spectrophotometrically. Additionally, after photodegradation, mineralization was analyzed as chemical oxygen demand. A computational analysis of the structures of MB and RB was performed to gain a deeper understanding of the obtained results. The computational analysis encompassed density functional theory (DFT) calculations and analysis of two quantum-molecular descriptors addressing the local reactivity of molecules. Experimental results have indicated that the considered zeolites effectively remove both dyes through both mechanisms, especially NH4BETA and NH4ZSM-5, due to the presence of active acidic centers on the outer and inner surfaces of the zeolite. The lowest efficiency of dye removal was achieved in the presence of NaY zeolite, which has a lower SiO2/Al2O3 ratio. A more efficient reduction was completed for RB dye, which agrees with the computationally obtained information about reactivity.


Introduction
Clean and unpolluted water is an irreplaceable resource that enables life on Earth. Water quality has deteriorated yearly due to rapid economic and industrial progress [1][2][3]. The primary source of pollution is industrial, agricultural, and communal wastewater, which may adversely affect the environment and human health when inadequately treated before being discharged into the recipient [4]. Textile, graphic, food, pharmaceutical, photographic, and cosmetic industries use dyes in the production process, producing many colored wastewaters characterized by high oxygen consumption [5][6][7][8]. Even in low concentrations in the water, the presence of dyes harms aquatic organisms' life due to their toxic, mutagenic, and carcinogenic properties [7][8][9]. In long-term contact with humans, dyes can cause severe damage to the kidneys, liver, and central nervous system. In addition, dyes may cause respiratory problems or the appearance of allergic reactions and dermatitis [7,[10][11][12][13].
tion under the influence of UV radiation in the presence of these zeolites. After photodegradation, the degree of mineralization was analyzed as chemical oxygen demand (COD) to gain a better insight into the photocatalytic process efficiency. Quantum-mechanical calculations within the density functional theory (DFT) approach gave us important insights into the reactivity of MB and RB, which helped us to interpret the obtained results, and explain the difference in degradation efficiency of these two dyes.

Results of Adsorption Observation
The Freundlich isotherm and specific adsorption parameters describe the adsorption suspension of RB-zeolites and MB-zeolites. To establish the most appropriate adsorption equilibrium correlation and the accuracy in parameter prediction of non-linear isotherm models were compared and discussed (Figures 1-6).

Results of Adsorption Observation
The Freundlich isotherm and specific adsorption parameters describe the adsorption suspension of RB-zeolites and MB-zeolites. To establish the most appropriate adsorption equilibrium correlation and the accuracy in parameter prediction of non-linear isotherm models were compared and discussed (Figures 1-6).
The Freundlich adsorption isotherm is mainly used for expressing the adsorption of non-idealized systems where the adsorbent surface is energetically heterogeneous, indicating the formation of multilayers. During adsorption from the solution, the expression applies: where q e is the equilibrium amount of adsorbed substance per unit mass of adsorbent (x m ⁄ ), c is the equilibrium concentration, and KF and n are specific empirical constants [14,50,57]. The experimental results of adsorption observation of RB from the aqueous environment on NH4BETA, NH4ZSM-5, and NaY zeolite are presented in Figures 1-3 and  7 (where x/m is the amount of adsorbed substance in equilibrium, γe is the adsorbate concentration at equilibrium, and γo is the initial concentration of adsorbate). Adsorption parameters (k, n, ∆adsHm presented in Tables S1 and S2) were determined from the graphs of functional dependence ln / vs. ln (Figures S1-S12).     Based on the obtained results regarding the adsorption of RB (Figures 1-3) and MB (Figures 4-6) on NH4BETA, NH4ZSM-5, and NaY zeolites, one may determine that the adsorption isotherms pass through several plateaus (type of Freundlich adsorption isotherm), and according to Giles belong to S4 group, indicating multi-layered physical adsorption. The only exception occurs for the RB-NH4ZSM-5 suspension, at 293 K, where one plateau is registered, which suggests that the adsorption is a monolayer and that, in addition to physical adsorption, chemisorption also occurs.
Physical adsorption results from weak intermolecular interactions, while chemisorption is based on the exchange of valence electrons between the adsorbent and adsorbate. One may not always set the boundary between physisorption and chemisorption. Observing RB and MB's adsorption on zeolites can provide interesting information about possible interactions with the zeolite surface. The results show that NaY zeolite was the worst adsorbent for RB and MB dye. Obtained results can be explained by the fact that the acidity of the surface was decisive for adsorption, i.e., the presence of Lewis and Brønsted active centers located on the outer and inner zeolite surfaces.  The higher ratio of SiO2/Al2O3 in NH4BETA and NH4ZSM-5 zeolites indicates the presence of active sites of the acidic character of different strengths, which favor a better interaction with these dyes. The surface of high-silicate zeolites is more homogeneous with an organophilic nature, in contrast to the zeolite surface with a lower content of SiO2/Al2O3, which is more selective for water and other polar molecules [51,54,58].   The higher ratio of SiO2/Al2O3 in NH4BETA and NH4ZSM-5 zeolites indicates the presence of active sites of the acidic character of different strengths, which favor a better interaction with these dyes. The surface of high-silicate zeolites is more homogeneous with an organophilic nature, in contrast to the zeolite surface with a lower content of SiO2/Al2O3, which is more selective for water and other polar molecules [51,54,58].   The greater degree of removal of RB ( Figure 7) and MB ( Figure 8) dyes by adsorption on NH4BETA zeolite compared to NH4ZSM-5 zeolite can be explained by the larger specific surface area of NH4BETA zeolite, and this is supported by the value of the constant k (Tables S1 and S2), which is the highest for the suspension MB-NH4BETA (k = 54.87) and RB-NH4BETA (k = 15.35); this shows a very high affinity between this adsorbent and adsorbates. Based on the value of the constant n (Tables S1 and S2), it can be concluded that there is a relatively strong interaction between the active centers of adsorbents and adsorbates and that it is the strongest at the temperature of 293 K for the RB-NH4ZSM-5 suspension.  The Freundlich adsorption isotherm is mainly used for expressing the adsorption of non-idealized systems where the adsorbent surface is energetically heterogeneous, indicating the formation of multilayers. During adsorption from the solution, the expression applies: where q e is the equilibrium amount of adsorbed substance per unit mass of adsorbent (x/m), c is the equilibrium concentration, and K F and n are specific empirical constants [14,50,57]. The experimental results of adsorption observation of RB from the aqueous environment on NH 4 BETA, NH 4 ZSM-5, and NaY zeolite are presented in Figures 1-3 and 7 (where x/m is the amount of adsorbed substance in equilibrium, γ e is the adsorbate concentration at equilibrium, and γ o is the initial concentration of adsorbate). Adsorption parameters (k, n, ∆ ads H m presented in Tables S1 and S2) were determined from the graphs of functional dependence ln x/m vs. lnγ e (Figures S1-S12). Based on the obtained results regarding the adsorption of RB (Figures 1-3) and MB (Figures 4-6) on NH 4 BETA, NH 4 ZSM-5, and NaY zeolites, one may determine that the adsorption isotherms pass through several plateaus (type of Freundlich adsorption isotherm), and according to Giles belong to S4 group, indicating multi-layered physical adsorption. The only exception occurs for the RB-NH 4 ZSM-5 suspension, at 293 K, where one plateau is registered, which suggests that the adsorption is a monolayer and that, in addition to physical adsorption, chemisorption also occurs.
Physical adsorption results from weak intermolecular interactions, while chemisorption is based on the exchange of valence electrons between the adsorbent and adsorbate. One may not always set the boundary between physisorption and chemisorption. Observing RB and MB's adsorption on zeolites can provide interesting information about possible interactions with the zeolite surface. The results show that NaY zeolite was the worst adsorbent for RB and MB dye. Obtained results can be explained by the fact that the acidity of the surface was decisive for adsorption, i.e., the presence of Lewis and Brønsted active centers located on the outer and inner zeolite surfaces.
The higher ratio of SiO 2 /Al 2 O 3 in NH 4 BETA and NH 4 ZSM-5 zeolites indicates the presence of active sites of the acidic character of different strengths, which favor a better interaction with these dyes. The surface of high-silicate zeolites is more homogeneous with an organophilic nature, in contrast to the zeolite surface with a lower content of SiO 2 /Al 2 O 3 , which is more selective for water and other polar molecules [51,54,58].
The greater degree of removal of RB ( Figure 7) and MB ( Figure 8) dyes by adsorption on NH 4 BETA zeolite compared to NH 4 ZSM-5 zeolite can be explained by the larger specific surface area of NH 4 BETA zeolite, and this is supported by the value of the constant k (Tables S1 and S2), which is the highest for the suspension MB-NH 4 BETA (k = 54.87) and  (Tables S1 and S2), it can be concluded that there is a relatively strong interaction between the active centers of adsorbents and adsorbates and that it is the strongest at the temperature of 293 K for the RB-NH 4 ZSM-5 suspension. The greater degree of removal of RB ( Figure 7) and MB ( Figure 8) dyes by adsorption on NH4BETA zeolite compared to NH4ZSM-5 zeolite can be explained by the larger specific surface area of NH4BETA zeolite, and this is supported by the value of the constant k (Tables S1 and S2), which is the highest for the suspension MB-NH4BETA (k = 54.87) and RB-NH4BETA (k = 15.35); this shows a very high affinity between this adsorbent and adsorbates. Based on the value of the constant n (Tables S1 and S2), it can be concluded that there is a relatively strong interaction between the active centers of adsorbents and adsorbates and that it is the strongest at the temperature of 293 K for the RB-NH4ZSM-5 suspension.  With the increase in temperature, the adsorption capacity of NH4BETA and NH4ZSM-5 zeolites for RB dye also increases, as indicated by the total number of adsorbed molecules (by 8% on NH4ZSM-5; for I plateau by 63%, and for II plateau by 45% on NH4BETA). This could mean that the dye molecules, in addition to weak van der Waals interactions, partially bonded to the adsorbent surface even by chemical bonding. The value of the adsorption heat also confirms this for NH4ZSM-5 zeolite, which, although it remained within the limits of physisorption, increased with the temperature increase. RB molecules were probably attached to the active centers of the zeolite by hydrogen bonds via hydrogen atoms from hydroxide groups located on the surface of the zeolite and electronegative nitrogen or oxygen atoms in the dye structure.
The calculated value Q (Table S1) indicates that most molecules of RB dye were adsorbed to the surface of the NH4ZSM-5 zeolite and that the number of adsorbed molecules increases with the increase in temperature (from 53 to 58%). In endothermic reactions, with an increasing temperature, the number of adsorbed species increases because their mobility increases. The total number of adsorbed molecules of RB dye on the NH4BETA zeolite at 293 K amounted to 29%, which indicates that the dye molecules on the NH4BETA zeolite became bonded to the active centers on the outer surface and that the large molecules of RB dye could not reach the inner surface of the zeolite, which is composed of pores and cavities.
With the increase in reaction temperature, the quantity of adsorbed RB molecules on NaY zeolite decreases, which indicates physical adsorption, and the value of adsorption heat also confirms this. Based on the data presented in Table S2, it can be noticed that the adsorption capacity of NH4BETA for MB dye increases with an increasing temperature (I plateau by 23%). In contrast, the adsorption capacity of other zeolites decreases with the With the increase in temperature, the adsorption capacity of NH 4 BETA and NH 4 ZSM-5 zeolites for RB dye also increases, as indicated by the total number of adsorbed molecules (by 8% on NH 4 ZSM-5; for I plateau by 63%, and for II plateau by 45% on NH 4 BETA). This could mean that the dye molecules, in addition to weak van der Waals interactions, partially bonded to the adsorbent surface even by chemical bonding. The value of the adsorption heat also confirms this for NH 4 ZSM-5 zeolite, which, although it remained within the limits of physisorption, increased with the temperature increase. RB molecules were probably attached to the active centers of the zeolite by hydrogen bonds via hydrogen atoms from hydroxide groups located on the surface of the zeolite and electronegative nitrogen or oxygen atoms in the dye structure.
The calculated value Q (Table S1) indicates that most molecules of RB dye were adsorbed to the surface of the NH 4 ZSM-5 zeolite and that the number of adsorbed molecules increases with the increase in temperature (from 53 to 58%). In endothermic reactions, with an increasing temperature, the number of adsorbed species increases because their mobility increases. The total number of adsorbed molecules of RB dye on the NH 4 BETA zeolite at 293 K amounted to 29%, which indicates that the dye molecules on the NH 4 BETA zeolite became bonded to the active centers on the outer surface and that the large molecules of RB dye could not reach the inner surface of the zeolite, which is composed of pores and cavities.
With the increase in reaction temperature, the quantity of adsorbed RB molecules on NaY zeolite decreases, which indicates physical adsorption, and the value of adsorption heat also confirms this. Based on the data presented in Table S2, it can be noticed that the adsorption capacity of NH 4 BETA for MB dye increases with an increasing temperature (I plateau by 23%). In contrast, the adsorption capacity of other zeolites decreases with the increase in temperature, which can also be seen based on the total number of adsorbed MB molecules on the zeolite surface. The highest value of Q is for the MB-NH 4 ZSM-5 suspension at 283 K, but this value decreased with the temperature increase. Additionally, the number of total adsorbed MB molecules on the surface of the NaY zeolite decreased by 37% with the increased adsorption temperature, indicating that the adsorbate molecules became tied to the active centers by van der Waals forces or dipole bonds. This is also confirmed by the calculated values of adsorption heat (∆ ads H m ), which decreased with the increase in adsorption temperature.
When we compare RB and MB dyes, the higher degree of RB removal in the presence of NH 4 ZSM-5 zeolite can be explained by the greater possibilities of binding to the outer zeolite surface because at pH > 3, RB dye behaves both as a weak base and as a weak acid. The high degree of MB dye removal in the presence of NH 4 BETA and NaY zeolites can be explained by the smaller dimensions of the MB molecules, which, in addition to binding to the outer surface, are most probably also bound to the active centers of the inner zeolite surface [17,49,59].

Photodegradation and Mineralization
When considering the photocatalytic activity of zeolites, many studies focus on zeolites containing framework heteroatoms or a modification of zeolites with oxides or metal ions precisely because of the improvement in photocatalytic capabilities [60]. This is evident from research in which zeolites were used as supports for TiO 2 photocatalysts [61,62] or zeolite-based composites were doped with metal ions [62], whereby a higher efficiency of dye degradation was achieved than in the presence of zeolites and TiO 2 individually. However, one must not ignore the fact that zeolites are crystalline aluminosilicates built from tetrahedra of silicon and aluminum interconnected through oxygen atoms, and it is generally known that porous SiO 2 exhibits photocatalytic activity via siloxane bridges, which generate siloxy radicals under the influence of UV radiation (below 390 nm) [63,64] accordingly, in this manuscript, the photocatalytic activity of the original samples of synthetic zeolites in the removal of RB and MB dyes was tested.
First, the photolysis of RB and MB was examined under UV radiation to see the contribution of adding zeolites to these systems ( Figure S13). Slightly higher efficiency of direct photolysis was observed at a higher temperature in the case of both dyes. However, direct photolysis is significantly less efficient in the case of both dyes compared to the system with zeolites (Figures 9 and 10). The results of photodegradation of RB and MB dyes in the presence of NH 4 BETA, NH 4 ZSM-5, and NaY zeolites, under the influence of UV radiation for 180 min, are given in Figures 9 and 10. to the outer surface, are most probably also bound to the active centers of the inner zeolite surface [17,49,59].

Photodegradation and Mineralization
When considering the photocatalytic activity of zeolites, many studies focus on zeolites containing framework heteroatoms or a modification of zeolites with oxides or metal ions precisely because of the improvement in photocatalytic capabilities [60]. This is evident from research in which zeolites were used as supports for TiO2 photocatalysts [61,62] or zeolite-based composites were doped with metal ions [62], whereby a higher efficiency of dye degradation was achieved than in the presence of zeolites and TiO2 individually. However, one must not ignore the fact that zeolites are crystalline aluminosilicates built from tetrahedra of silicon and aluminum interconnected through oxygen atoms, and it is generally known that porous SiO2 exhibits photocatalytic activity via siloxane bridges, which generate siloxy radicals under the influence of UV radiation (below 390 nm) [63,64] accordingly, in this manuscript, the photocatalytic activity of the original samples of synthetic zeolites in the removal of RB and MB dyes was tested.
First, the photolysis of RB and MB was examined under UV radiation to see the contribution of adding zeolites to these systems ( Figure S13). Slightly higher efficiency of direct photolysis was observed at a higher temperature in the case of both dyes. However, direct photolysis is significantly less efficient in the case of both dyes compared to the system with zeolites (Figures 9 and 10). The results of photodegradation of RB and MB dyes in the presence of NH4BETA, NH4ZSM-5, and NaY zeolites, under the influence of UV radiation for 180 min, are given in Figures 9 and 10.   One of the critical roles in the process of photocatalytic degradation is played by adsorption, which mainly depends on the affinity of the catalyst for the substrate, the specific surface area of the catalyst, and the nature of the solvent. After adsorption, the photodegradation of adsorbed molecules commences under the influence of radiation. As already mentioned, zeolites with high SiO 2 /Al 2 O 3 content, such as NH 4 BETA and NH 4 ZSM-5, show a high affinity for removing dyes.
The obtained results of photodegradation of MB and RB dyes under UV radiation (Figures 9 and 10) indicate that the highest efficiency of photodegradation at 283 K was achieved in the presence of NH 4 ZSM-5 zeolite, then NH 4 BETA, and finally NaY zeolite. It can be seen in Table S3 that NH 4 ZSM-5 zeolite has the highest ratio of SiO 2 and NaY has the lowest.
The presence of acid-base centers in the zeolite structure, which have electron donor and electron acceptor properties, prevents the possible recombination of electrons and holes, leading to a higher photodegradation efficiency. The results of the photodegradation of MB dye in the presence of zeolite at 293 K follow the same trend as at 283 K. The increase in temperature did not significantly affect the degree of degradation of MB dye in the presence of NH 4 BETA zeolite. In contrast, in the presence of NH 4 ZSM-5 and NaY zeolite, there is an insignificant decrease in the degree of degradation, which is probably the effect of the physisorption of the dye molecules on the catalyst surface. In the case of RB dye, the highest efficiency of photodegradation at 293 K was achieved in the presence of NH 4 BETA zeolite (83.3%). It can be observed that the degree of removal of RB is increased by the increase in temperature reaction, which is most likely the effect of the increased frequency of molecular collisions in the solution.
The photodegradation of MB and RB molecules in the presence of zeolite under UV radiation includes the generation of electron and hole pairs. Electrons in the conduction zone are unstable and pass to adsorbed oxygen molecules, where superoxide radicals are formed, while holes in the valence zone can be captured by the molecules of dye or water, creating • OH. The adsorbed dye molecules can be degraded by radical species and mineralized into less toxic products.
Based on the structures of RB and MB dyes, it can be expected that depending on the degree of mineralization, in addition to CO 2 and H 2 O, inorganic ions NH + 4 , NO + 3 , and SO 2− 4 are also formed. According to the results shown in Figures 11 and 12, it can be concluded that the highest degree of mineralization of MB dye (at 283 K) and RB dye (at 283 K and 293 K) was achieved in the presence of NH 4 ZSM-5, then NH 4 BETA, and the lowest degree of mineralization was achieved in the presence of the NaY zeolite. The obtained results of mineralization agree with the results of photodegradation. The mineralization degree is lower than the degree of photodegradation, which indicates the presence of various intermediates, whose mineralization is often slower than the degradation of the initial compound. In the case of MB dye, the degree of mineralization at 293 K is the highest in NH 4 ZSM-5 zeolite and the lowest in NH 4 BETA zeolite.  The presence of multiple intermediates is also indicated by the change in the pH value of the solution during the photocatalytic process. As seen in Table 1, during the photodegradation of RB dye in the presence of all zeolites, the pH value decreases (due to the formation of acidic intermediates), except in the presence of NaY zeolite at 293 K, where the pH value increases. During the decomposition of MB dye, the pH value decreases in the presence of NH4ZSM-5 zeolite, as well as NH4BETA (283 K), while in the presence of the NaY zeolite, the pH value increases due to the formation of base intermediates.   The presence of multiple intermediates is also indicated by the change in the pH value of the solution during the photocatalytic process. As seen in Table 1, during the photodegradation of RB dye in the presence of all zeolites, the pH value decreases (due to the formation of acidic intermediates), except in the presence of NaY zeolite at 293 K, where the pH value increases. During the decomposition of MB dye, the pH value decreases in the presence of NH4ZSM-5 zeolite, as well as NH4BETA (283 K), while in the presence of the NaY zeolite, the pH value increases due to the formation of base intermediates.  The presence of multiple intermediates is also indicated by the change in the pH value of the solution during the photocatalytic process. As seen in Table 1, during the photodegradation of RB dye in the presence of all zeolites, the pH value decreases (due to the formation of acidic intermediates), except in the presence of NaY zeolite at 293 K, where the pH value increases. During the decomposition of MB dye, the pH value decreases in the presence of NH 4 ZSM-5 zeolite, as well as NH 4 BETA (283 K), while in the presence of the NaY zeolite, the pH value increases due to the formation of base intermediates. During the decomposition of RB dye, radical species probably first attack nitrogen atoms (N-deethylation process), whereby various intermediates with an aromatic ring are formed. After that, reactive radicals attack carbon atoms, resulting in the cleavage of chromophores and oxidation of intermediates to carboxylic acids, preventing the increase of pH value, as per the results presented in Table 1. Decomposition of a -C-S=S-functional group starts with the electrophilic attack of radical species on the -C-S=S-functional group in the structure of the MB dye molecule, where sulfate ions are formed through sulfoxide, sulfone, and sulfonic acid. In addition, the attack of radicals on nitrogen atoms can lead to the formation of substituted anilines, phenols, aldehydes, or carboxylic acids, which can further be mineralized to CO 2 and H 2 O. The degree of mineralization of RB dye is lower than that of MB dye (except for RB-NH 4 BETA at 293 K), which is probably a consequence of its forming a more significant number of intermediates. The appearance of a more significant number of intermediates in the case of RB dye is expected, given that the RB molecule is larger than the MB dye.

Computational Analysis
To better understand the experimental results, the influence of the structure of the tested dyes was analyzed. MB and RB are dyes that consist of cation and Cl anion. The starting geometries of MB and RB have been taken from the ChemSpider library and subjected to geometrical optimizations. Since there is a noncovalent interaction between the cation and anion in these cases, we had to apply the dispersion-corrected variant of the B3LYP functional. The geometrically optimized structures of MB and RB are presented in Figure 13, with the indicated distances between the cation and Cl anion. During the decomposition of RB dye, radical species probably first attack nitrogen atoms (N-deethylation process), whereby various intermediates with an aromatic ring are formed. After that, reactive radicals attack carbon atoms, resulting in the cleavage of chromophores and oxidation of intermediates to carboxylic acids, preventing the increase of pH value, as per the results presented in Table 1. Decomposition of a -C-S=S-functional group starts with the electrophilic attack of radical species on the -C-S=S-functional group in the structure of the MB dye molecule, where sulfate ions are formed through sulfoxide, sulfone, and sulfonic acid. In addition, the attack of radicals on nitrogen atoms can lead to the formation of substituted anilines, phenols, aldehydes, or carboxylic acids, which can further be mineralized to CO2 and H2O. The degree of mineralization of RB dye is lower than that of MB dye (except for RB-NH4BETA at 293 K), which is probably a consequence of its forming a more significant number of intermediates. The appearance of a more significant number of intermediates in the case of RB dye is expected, given that the RB molecule is larger than the MB dye.

Computational Analysis
To better understand the experimental results, the influence of the structure of the tested dyes was analyzed. MB and RB are dyes that consist of cation and Cl anion. The starting geometries of MB and RB have been taken from the ChemSpider library and subjected to geometrical optimizations. Since there is a noncovalent interaction between the cation and anion in these cases, we had to apply the dispersion-corrected variant of the B3LYP functional. The geometrically optimized structures of MB and RB are presented in Figure 13, with the indicated distances between the cation and Cl anion. In both cases, the Cl anion was placed near the cations' positively charged atoms. This means that in the case of the MB, the Cl anion was placed above the sulfur atom, while in the case of the RB, the Cl anion was placed above the nitrogen atom. In the case of the MB, the Cl anion moved significantly to the plane corresponding to the central ring and shifted to interact with the nearby hydrogen atom noncovalently. In the case of the RB, the Cl anion practically remained in the vicinity of the positively charged nitrogen, which disagrees with the finding reported by Delgado and Selsby [65]. Namely, in their computational study, the Cl anion shifted towards the central part of the RB. However, the disagreement between these results is expected since they applied the Hartree-Fock method for geometrical optimizations, which is the level of theory that neglects the electron correlation. There is a significant difference in terms of the shortest distance In both cases, the Cl anion was placed near the cations' positively charged atoms. This means that in the case of the MB, the Cl anion was placed above the sulfur atom, while in the case of the RB, the Cl anion was placed above the nitrogen atom. In the case of the MB, the Cl anion moved significantly to the plane corresponding to the central ring and shifted to interact with the nearby hydrogen atom noncovalently. In the case of the RB, the Cl anion practically remained in the vicinity of the positively charged nitrogen, which disagrees with the finding reported by Delgado and Selsby [65]. Namely, in their computational study, the Cl anion shifted towards the central part of the RB. However, the disagreement between these results is expected since they applied the Hartree-Fock method for geometrical optimizations, which is the level of theory that neglects the electron correlation. There is a significant difference in terms of the shortest distance between the Cl anion and the cationic fragment since the Cl anion is much closer to the cationic fragment in the case of the MB dye.
Computational analysis in this work encompassed calculations of the MEP and ALIE quantities, two well-known quantum molecular descriptors describing the local reactivity of the molecules. The MEP descriptor is one of the most frequently calculated quantities to identify molecules' sites with electron abundance or deficiency. Another popular descriptor for addressing the local reactivity of molecules is the ALIE quantity. MEP is a substantial quantity that reveals which molecular sites are prone to interact with other molecules based on electrostatic interactions. One molecule's electron-abundant site would react with the electron-deficient site of the other molecule. An equally important but somewhat less frequently applied descriptor is ALIE, which reveals the molecular sites where the lowest amount of energy is required to remove an electron. In other words, this descriptor indicates the molecular sites sensitive to electrophilic attacks. Essentially, both MEP and ALIE descriptors were introduced into practice by Professor Politzer and his coworkers [66][67][68][69][70]. The most practical way to analyze the values of MEP and ALIE descriptors is by their mapping to the electron density surface, which was performed in this work. MEP and ALIE surfaces of the MB and RB dyes are presented in Figure 14.
of the molecules. The MEP descriptor is one of the most frequently calculated quantities to identify molecules' sites with electron abundance or deficiency. Another popular descriptor for addressing the local reactivity of molecules is the ALIE quantity. MEP is a substantial quantity that reveals which molecular sites are prone to interact with other molecules based on electrostatic interactions. One molecule's electron-abundant site would react with the electron-deficient site of the other molecule. An equally important but somewhat less frequently applied descriptor is ALIE, which reveals the molecular sites where the lowest amount of energy is required to remove an electron. In other words, this descriptor indicates the molecular sites sensitive to electrophilic attacks. Essentially, both MEP and ALIE descriptors were introduced into practice by Professor Politzer and his coworkers [66][67][68][69][70]. The most practical way to analyze the values of MEP and ALIE descriptors is by their mapping to the electron density surface, which was performed in this work. MEP and ALIE surfaces of the MB and RB dyes are presented in Figure 14. The analysis of the minimal and maximal values of the MEP descriptor provides a further understanding of the results related to dye removal via adsorption. Namely, the lowest MEP values for both MB and RB dyes are practically the same and are equal to −75.73 kcal/mol and −72.91 kcal/mol, respectively. The similar lowest MEP values are expected since the Cl anion bears the negative charge in both cases. However, there is a huge difference in the maximal MEP values in favor of the RB dye. Namely, the MB dye's highest MEP value is 34.35 kcal/mol, while the RB dye's highest MEP value is 56.16 kcal/mol. This discrepancy in the highest MEP values between the MB and RB dyes shows that the RB dye is far more prone to interact with other structures based on electrostatic interactions. Indeed, the adsorption study in this work indicates that the RB dye is removed to a greater extent than the MB, which agrees with the computational results. The analysis of the minimal and maximal values of the MEP descriptor provides a further understanding of the results related to dye removal via adsorption. Namely, the lowest MEP values for both MB and RB dyes are practically the same and are equal to −75.73 kcal/mol and −72.91 kcal/mol, respectively. The similar lowest MEP values are expected since the Cl anion bears the negative charge in both cases. However, there is a huge difference in the maximal MEP values in favor of the RB dye. Namely, the MB dye's highest MEP value is 34.35 kcal/mol, while the RB dye's highest MEP value is 56.16 kcal/mol. This discrepancy in the highest MEP values between the MB and RB dyes shows that the RB dye is far more prone to interact with other structures based on electrostatic interactions. Indeed, the adsorption study in this work indicates that the RB dye is removed to a greater extent than the MB, which agrees with the computational results. Conversely, the minimal and maximal ALIE values of MB and RB dyes are very similar, so they are expected to have comparable sensitivity toward electrophilic attacks.

Chemicals and Solutions
The standard dye solutions (0.5-5 mg/dm 3 ) of MB and RB (products of the Merck company, Germany) were prepared by diluting the basic solutions. Thehe absorption maxima were then determined, and calibration curves were constructed (Figures S14 and S15). Synthetic zeolites were used during the work (products of Zeolyst International company, Kansas City, KS, USA): NH 4 BETA (from the BEA group, mark CP 814E), NH 4 ZSM-5 (from the MFI group, mark CBV 3024E), and NaY (from the FAU group, mark CBV 100).

Structural Analysis of Zeolites
All zeolites' specific surfaces (Sp) were determined by the BET method on the Flowsorb II-2300 instrument (Table S3).
Additionally, the FT-IR spectra of all zeolites were recorded on the Thermo Scientific Nicolet iS10 FT-IR Spectrometer with a resolution of 4 cm −1 and 32 scans to identify the surface acidity of the zeolite (Figures S16-S18).

Adsorption Experiments
Adsorption experiments of the dyes MB and RB from the aqueous environment on NH 4 BETA, NH 4 ZSM-5, and the NaY zeolite were performed at 283 K and 293 K for 180 min. iThe reaction suspension was always thermostated (Thermostat: WiseCircu WCR, model WCR-P22, Witeg, Wertheim, Germany). The adsorbent mass was 0.25 g (exact weight ±10 −4 g) in contact with 50cm 3 of adsorbate solutions of different concentrations (0.5-5 mg/dm 3 ). The results are presented as curves of functional dependence x/m on the concentration at equilibrium γ e., from which the quantity of adsorbed dye on one adsorption monolayer was read, i.e., on the plateau. The amount of adsorbed dye (x) was calculated from the difference in concentrations before and after adsorption, while x/m represents the quantity of adsorbed dye per adsorbent mass unit. Other adsorption parameters, the number of adsorbed molecules on individual plateaus (N) and their total surface (S), as well as the parameter Q, which represents the ratio of the surface of adsorbed molecules and the specific surface of the zeolite, were calculated. The parameters k and n, as well as the adsorption heat (∆H ads ), were determined by calculations and graphic presentation (from the diagram of dependence ln x/m vs. ln γ e ).

Photodegradation
Photodegradation of the dyes MB (1 mg/dm 3 ) and RB (1 mg/dm 3 ) under UV radiation was performed in a photochemical cell made of double-walled Pyrex glass in the presence of NH 4 BETA, NH 4 ZSM-5, and the NaY zeolite. Into the cell, 20 cm 3 of the aqueous solution of dye and 0.1 g of zeolite were weighed (exact weight ±10 −4 g), followed by sonification in an ultrasonic bath for 15 min, to achieve a uniform size of the catalyst particles. The photochemical cell was placed on a magnetic stirrer, and the suspension was continuously mixed during the irradiation with a constant oxygen flow. Photodegradation was performed at 283 K and 293 K for 180 min and a high-pressure mercury lamp with a suitable concave mirror was used as a source of UV radiation (Philips, HPL-N, 125 W, with emission bands in the area of UVA radiation at 304, 314, 335 and 366 nm, with an emission maximum at 366 nm).

Analytical Procedures
Dye concentrations before and after adsorption were determined spectrophotometrically on the Perkin Elmer UV/VIS Spectrometer Lambda 25 instrument.
The concentration of dyes in specific time intervals after photodegradation was determined on the double-beam T80 + UV-vis Spectrometer (UK), at a fixed slit width (2 nm), using a 1 cm quartz cell and computer-loaded UV Win 5 data software.
The pH value of standard dye solutions was determined using a glass electrode (AmpHel pH electrode, Hanna Instruments, Cluj Napoca, Romania) connected to the pH meter (Bench pH meters, Hanna Instruments, Cluj Napoca, Romania). For monitoring the pH during the degradation, a combined glass electrode (pH-Electrode SenTix 20, Xylem Analytics Germany Sales GmbH & Co. KG, WTW, Weilheim, Germany) connected to the pH meter was used (pH/Cond 340i, Xylem Analytics Germany Sales GmbH & Co. KG, WTW, Weilheim, Germany).
The COD was determined according to Standard Method 410.4 declared by EPA, United States Environmental Protection Agency. The calibration curve was obtained using HOOCC 6 H 4 COOK as a standard solution, and R 2 was 0.995 ( Figure S19). Aliquots of 2.5 cm 3 were taken from the reaction mixture after 90 min of photodegradation experi-ments. The COD concentration was determined spectrophotometrically by measuring the absorbance of the formed Cr 3+ at a fixed slit width (2 nm) using a quartz cell (1 cm optical length) and computer-loaded UV Win 5 data software. Absorbance was recorded on a double-beam T80 + UV-vis Spectrometer. The evolution of absorbance of the digested solution was recorded at 600 nm. The samples were digested at 150 • C for 2 h.

Computational Methods
Molecular DFT calculations on the RB and MB were performed by applying the dispersion corrected version of a B3LYP functional [71], namely the B3LYP-D3 variant [71,72], in combination with the 6-31G(d,p) basis set [73,74]. At the mentioned level of theory, the structures of RB and MB were first geometrically optimized to reach the ground states. Frequency calculations were further performed to ensure that the actual equilibrium states were identified, confirmed by the inexistence of the imaginary frequencies. After obtaining the ground states of the MB and RB, molecular electrostatic potential (MEP) and average local ionization energy (ALIE) were calculated to analyze the local reactivity properties of dyes, using the M06-2X functional [75] and the same basis set.

Conclusions
Zeolites have a significant place among the most commonly used adsorbents thanks to their specific crystal structure consisting of pores, channels, and cavities of different dimensions, which results in a large internal surface area available for the removal of various organic pollutants, including dyes. The properties of zeolite are determined, among other things, by active centers of different acidity (Lewis and Brønsted type) located on the outer and inner surfaces of the zeolite. Recently, zeolites have increasingly been used as catalysts in photodegradation processes. This study observed the adsorption and photodegradation of the selected organic dyes, MB and RB, in the presence of zeolite material from BEA, MFI, and FAU groups. The characterization of all adsorption suspension s is given through the Freundlich adsorption isotherm, and the corresponding parameters were determined. The results showed that adsorption is still one of the leading methods in wastewater treatment because a high degree of dye removal was achieved under these experimental conditions. NH 4 BETA and NH 4 ZSM-5 zeolites are very effective adsorbents for these dyes due to acid centers of different strengths responsible for the adsorption, while the NaY zeolite proved to be the least effective. The higher degree of adsorption on the NH 4 BETA zeolite compared to NH 4 ZSM-5 can be attributed to a higher specific surface area (about 40% higher). Adsorption of dyes on all zeolites occurs according to the principle of multi-layer physical adsorption, except for the RB dye on NH 4 ZSM-5, where chemisorption partially occurs. In addition, the high efficiency of removing these organic pollutants was achieved by heterogeneous photocatalysis, where the highest degree of photodegradation of dyes was performed in the presence of a NH 4 ZSM-5 zeolite (highest SiO 2 /Al 2 O 3 ) primarily and secondarily in the presence of a NH 4 BETA zeolite, probably due to the formation of siloxy radicals. Siloxy radicals with hydroxyl and superoxide radicals contributed to a high degree of dye degradation. Compared to the photodegradation efficiency of dyes, the lower degree of mineralization could be explained by the formation of various decomposition intermediates, which requires a longer irradiation time.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/molecules27196582/s1, Table S1: Summary overview of parameters for adsorption of RB from the aqueous environment on selected zeolites at two temperature; Table S2: Summary overview of parameters for adsorption of MB from the aqueous environment on selected zeolites at two temperature; Table S3: Characterization of zeolites; Figure S1: Functional dependence ln x/m of ln γ e for the suspension RB-NH 4 BETA at 283 K; Figure S2: Functional dependence ln x/m of ln γ e for the suspension RB-NH 4 BETA at 293 K; Figure S3: Functional dependence ln x/m of ln γ e for the suspension RB-NH 4 ZSM-5 at 283 K; Figure S4: Functional dependence ln x/m of ln γ e for the suspension RB-NH 4 ZSM-5 at 293 K; Figure S5: Functional dependence ln x/m of ln γ e for the suspension RB-NaY at 283 K; Figure S6: Functional dependence ln x/m of ln γ e for the suspension RB-NaY at 293 K; Figure S7: Functional dependence ln x/m of ln γ e for the suspension MB-NH 4 BETA at 283 K; Figure S8: Functional dependence ln x/m of ln γ e for the suspension MB-NH 4 BETA at 293 K; Figure S9: Functional dependence ln x/m of ln γ e for the suspension MB-NH 4 ZSM-5 at 283 K; Figure S10: Functional dependence ln x/m of ln γ e for the suspension MB-NH 4 ZSM-5 at 293 K; Figure S11: Functional dependence ln x/m of ln γ e for the suspension MB-NaY at 283 K; Figure S12: Functional dependence ln x/m of ln γ e for the suspension MB-NaY at 293 K; Figure S13: Degree of photolytic degradation of RB and MB dye under the UV radiation at two temperatures after 180 min radiation; Figure S14: Calibration curve of MB dye; Figure S15: Calibration curve of RB dye; Figure S16: FT-IR spectra NH 4 BETA zeolite; Figure S17: FT-IR spectra NH 4 ZSM-5 zeolite; Figure S18: FT-IR spectra NaY zeolite; Figure  Funding: This paper is funded through the EIT's HEI Initiative SMART-2M project, supported by EIT Raw Materials, financed by the European Union.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Written informed consent was obtained from the patients to publish this paper. Data Availability Statement: Not applicable.

Conflicts of Interest:
The authors declare no potential conflict concerning this article.