Assessing the photodegradation efficiency of benzene, toluene, and xylene (BTX): A comparative investigation using activated charcoal (AC), zeolitic imidazolate framework-8 (ZIF-8), and zirconium metal–organic framework (Zr-MOF)

Voltaic organic compounds (VOCs), i.e., benzene, toluene, and xylene (BTX), are the common pollutants found everywhere including air, water, etc. Importantly, BTX is the principal aromatic element in petroleum products that are used for plastic production. Environmental contamination due to high BTX migration capacities, water solubility, toxicity, and volatility is a major public concern worldwide (Paixão et al. 2007; Farhadian et al. 2009; Mukherjee & Bordoloi 2012). The United States Environmental Protection Agency (US EPA) has set the maximum allowed values in water for benzene, toluene, and xylene at 0.5, 0.1, and 10 ppm, respectively (Agency for Toxic Substances and Disease Registry) (Wongbunmak et al. 2020). To manage and remediate BTX contamination, it is necessary to conduct thorough research to identify the most effective catalytic and sensing technologies for detecting, removing, and separating BTX from natural and household sources. While traditional approaches like flocculation, sedimentation, and filtration have been used, they have limitations in completely removing BTX (Chi et al. 2024). Adsorption systems, particularly those using activated carbon, have gained popularity due to their high adsorption capacity and efficiency in removing VOCs (Yang et al. 2019). However, adsorption alone may not sufficiently degrade BTX, highlighting the need for advancements in treatment technologies.

In recent years, heterogeneous photocatalysis has emerged as a promising alternative for BTX degradation (Bratovcic 2019). Some limitations, including fouling, catalyst separation, and recovery or regeneration of photocatalysts during heterogeneous photocatalysis, are major concerns for possible real-world applications. Nevertheless, the destruction of dangerous pollutants through advanced oxidation processes (AOPs) via catalytic degradation, in which strong oxidizing catalysts, including advanced porous materials such as nanoporous metal–organic frameworks (MOFs) and covalent organic frameworks (COFs), coupled with UV light (wavelength of light 200–300 nm) to form extremely reactive hydroxyl radical species (i.e., OH•), have shown better alternatives to overcome the above limitations (Ollis 1988; Sun et al. 2022). This technique utilizes photoactive materials that, when exposed to light, produce reactive oxygen species (ROS) such as hydroxyl radicals (•OH), which can degrade BTX into less harmful byproducts like CO₂ and H₂O (Ollis 1988; Sun et al. 2022). Various materials have been studied for their photocatalytic abilities, including titanium dioxide (TiO₂), zinc oxide (ZnO), and MOFs (Ahmad et al. 2024). Binas et al. (2019) demonstrated that Mn-doped TiO₂ nanoparticles effectively degrade gaseous BTX under UV and visible light, showcasing the enhanced efficiency of doped photocatalysts (Binas et al. 2019). Similarly, zeolitic imidazolate frameworks (ZIF-8), a subclass of MOFs, have shown considerable promise due to their high surface area, tunable pore structure, and stability under UV exposure.

MOFs, particularly zirconium-based MOFs (Zr-MOFs), have drawn significant interest in recent studies. Zr-MOFs offer excellent thermal and chemical stability, alongside their high surface area and unique structural features that facilitate electron transfer, which is crucial for effective photocatalytic degradation. These properties make Zr-MOFs a competitive candidate for BTX degradation. In parallel, AC continues to be widely used due to its high adsorption capacity and cost-effectiveness. However, its amorphous nature can limit photocatalytic performance when compared to crystalline frameworks like MOFs. MOFs such as ZIF-8, TiO₂-based photocatalysts, and UiO-66 have demonstrated notable efficiency in the photocatalytic degradation of VOCs due to their high surface area and tunable pore structures. For instance, TiO₂-based materials have shown efficacy in VOC degradation but often suffer from rapid electron–hole recombination, limiting their catalytic efficiency under prolonged irradiation (Jamaludin et al. 2022). UiO-66 MOFs, another Zr-based variant, exhibit excellent chemical stability but are less effective for BTX degradation due to their higher band gap, reducing electron transfer efficiency (Tahir et al. 2023). Comparatively, Zr-MOF used in our study shows a reduced band gap (4.75 eV), which enhances charge separation, allowing for improved photocatalytic performance and electron transfer capability in BTX degradation. Studies on ZIF-8 highlight its utility in VOC degradation under UV–visible light but underscore limitations in the degradation of complex compounds like xylene due to its restricted pore size (Li et al. 2024).

Different studies have been performed on BTX degradation (%) using porous materials, as listed in Table 1. In the literature, we have found that solvents play an important role in the photodegradation process. However, to the best of our knowledge, the role of solvents in advanced porous materials, including MOFs and COFs, for photodegradation applications has never been studied.

The selection of an appropriate solvent is crucial in determining the efficiency of photocatalytic processes. Acetonitrile (ACN) has emerged as a preferred solvent for photocatalytic applications, particularly in conjunction with substrates such as activated charcoal (AC), zeolitic imidazolate framework (ZIF-8), and zirconium metal–organic framework (Zr-MOF). Its selection is justified by its distinctive physicochemical properties, including a high dielectric constant (37.5), large dipole moment (3.92 D), low viscosity, and low boiling point, which collectively create optimal experimental conditions for photodegradation processes (Garnayak & Patel 2015; Myneni et al. 2022). Studies have demonstrated that ACN's properties not only enhance the solubility of organic compounds but also facilitate strong interactions between the photocatalyst and BTX molecules (Nikhar et al. 2023). Furthermore, ACN aids in the stabilization of reactive intermediates, thereby enhancing degradation kinetics, while simultaneously promoting better dispersion and adsorption of BTX on photocatalyst surfaces. These characteristics collectively contribute to improved overall photocatalytic efficiency, making ACN an ideal solvent choice for such applications (Garnayak & Patel 2015; Myneni et al. 2022).

In this study, initially, AC, ZIF-8, and Zr-MOF were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area analysis, and spectroscopic analysis techniques. Based on these excellent features, comparative photodegradation analyses of BTX have been demonstrated in ACN, respectively. UV irradiation was used to determine the primary volatile oxidation products of BTX during photocatalysis. The degradation efficiency of the photocatalyst and its intermediate products on the catalyst surface was also determined.

Zirconyl chloride octahydrate (ZrOCl₂·8H₂O, 99%, Sigma-Aldrich), terephthalic acid (C₈H₆O₄, 99%, Sigma-Aldrich), benzene (99%, SDFCL), toluene (99.5%, SDFCL), xylene (99%, SDFCL), acetonitrile (ACN, HPLC grade, 99.9%, TCI), and N,N′-dimethylformamide (DMF, 99%, SDFCL) are utilized, as well as distilled water. Commercially available AC and ZIF-8 were obtained from Sigma-Aldrich. All chemicals used were of analytical grade.

AC and ZIF-8 were used as-received from Sigma-Aldrich without further modification or synthesis procedures. These materials were characterized prior to use in degradation studies.

The Fourier transform infrared (FTIR) spectra of AC revealed characteristic peaks at specific wavenumbers. These included peaks at 1,100–1,180 cm⁻¹ indicating C–O stretching, peaks at 3,300–3,500 cm⁻¹ representing O–H stretching, peaks at 1,670 cm⁻¹ indicating aromatic C = C ring stretching, and a peak at 780 cm⁻¹ corresponding to C–H out-of-plane bending. In contrast to Zr-MOF and ZIF-8, the FTIR spectra of the commercially available AC showed broader diffraction peaks, suggesting its amorphous nature. The AC contained several functional groups, including C–OH groups that were linked to aliphatic and aromatic sites.

The FTIR spectra of ZIF-8 exhibited distinct bands in certain spectral ranges. These bands corresponded to vibrations of the imidazole unit and were observed between 990 and 1,150 cm⁻¹. These vibrations represented complete ring stretching and in-plane ring bending. Also, C–H and C–N bonds were observed during the stretching vibration at 760 and 990 cm⁻¹. Additionally, besides the framework bands, peaks were observed in the range of 2,000–2,400 cm⁻¹. These vibrations overlapping with the 2,200–2,400 cm⁻¹ region made it difficult to determine the distinctive (C–N) bands, which in turn made it challenging to determine the acid site strength and concentration (Tsai 2014; Abdelhamid et al. 2017). Stretching around 3,400 cm⁻¹ shows the aromatic ring structure due to imidazolate.

SEM was used to clearly observe the morphology of the as-prepared Zr-MOF, and the image of the crystals showed that they were disordered and agglomerative (shown in Figure 4(c)). These data provide important insights into the structural properties of the materials, which are necessary for understanding their performance in the degrading application.

UV–visible spectroscopy of AC, ZIF-8, and Zr-MOF was conducted by preparing a sample of each material, which was then suspended in a suitable solvent. These samples were subsequently analyzed using a spectrophotometer to measure the absorbance intensity of light passing through them. The absorbance wavelengths for AC, ZIF-8, and Zr-MOF were observed at 200, 265, and 268 nm, respectively. The absorbance spectrum of AC at 200 nm indicates the presence of aromatic rings or conjugated double bonds within the material. In the case of ZIF-8, the absorbance response at 265 nm suggests the ability of zinc ions to absorb UV radiation, leading to the formation of a chemical linkage with the linker. Similarly, Zr-MOF exhibits an absorbance wavelength at 268 nm, indicating the transmission of absorbed energy through the Zr-linked BDC linker structure. Supplementary Figure S1(a)–S1(c) represents the absorbance spectra of AC, ZIF-8, and Zr-MOF, respectively, providing a comprehensive visual representation of their UV–visible absorption characteristics.

The energy differential between the lowest unoccupied energy level (conduction band (CB)) and the highest occupied energy level (valence band (VB)) of a material is referred to as its band gap. The band gap is a critical factor in photocatalysis because it affects a material's ability to absorb light and produce charge carriers. It is unclear whether differences in photocatalytic degradation of MOFs and AC samples are related to these properties alone, as factors such as UV absorbance, band gap, oxygen defect, and photogenerated hole–electron recombination also affect the catalyst's performance (Zhang et al. 2019). Using a Tauc plot to analyze the UV–visible absorption of AC, ZIF-8, and Zr-MOF reveals that their respective band gaps (E_g) are 5.54, 5.45, and 4.75 eV (shown in Supplementary Figure S1(a)–S1(c)). The findings suggest that Zr-MOF has an advantage in charge separation due to the reduction in E_g, which improves photocatalytic efficiency and encourages the recombination of photogenerated electron–hole pairs. This is advantageous for the photocatalytic degradation application.

Zr-MOF has shown remarkable potential as a catalyst for BTX degradation, surpassing AC and ZIF-8 in effectiveness. The efficacy of these catalysts can be elucidated through the lowest unoccupied molecular orbital (LUMO)–highest occupied molecular orbital (HOMO) mechanism, which is associated with the band gap. In this context, the LUMO represents the catalyst's ability to accept electrons, while the HOMO signifies the pollutant's capacity to donate electrons (E_g = 4.75 eV).

These characteristics stem from its unique structural features. Therefore, Zr-MOF emerges as an ideal candidate for catalytic degradation of BTX pollutants, outperforming AC and ZIF-8, making it a promising and effective catalyst in addressing BTX contamination challenges. Analysis of the LUMO–HOMO interactions reveals that the most favorable electron transfer occurs when there is a minimal band gap between the LUMO of the catalyst and the HOMO of the pollutant. The Zr-MOF catalyst stands out in this regard, exhibiting a superior BTX degradation efficiency due to its high electron affinity and excellent electron transfer capability, which result in the smallest band gap.

The specific surface areas of AC, ZIF-8, and Zr-MOF were determined through physical adsorption using a BET study. ZIF-8 exhibited the highest specific surface area at 1,122.1 m²/g, followed by AC at 711.5 m²/g and Zr-MOF at 535.4 m²/g. The nitrogen adsorption–desorption isotherm of Zr-MOF displayed a Type IV behavior, indicative of heteropores comprising interconnected micropores and mesopores (Anderson & Stylianou 2017; Liang & Zhao 2018; Zhang et al. 2021). The surface area of all photocatalysts is shown in Supplementary Figure S2, with Zr-MOF having the lowest surface area yet still exhibiting rapid catalytic degradation efficiency.

Photocatalytic efficiency can be influenced by pore volume and diameter. Zr-MOF, ZIF-8, and AC had pore volumes of 0.628, 0.369, and 0.291 cm³/g, respectively. Higher pore volume and surface area may boost catalyst catalytic activity. The photocatalytic degradation efficiency of BTX compounds is typically predicated on the adsorption of reactant molecules onto the catalyst surface, followed by chemical processes that break down the compound into smaller, less harmful products.

In conclusion, the photocatalytic degradation efficacy of AC, ZIF-8, and Zr-MOF is influenced by their specific surface area and pore volume. Surprisingly, our findings revealed that Zr-MOF, despite having the lowest surface area, exhibited rapid catalytic degradation, positioning it as a strong contender in catalytic degradation processes. The experimental results demonstrated that the specific surface areas of AC and MOFs, along with their degradation efficiencies toward BTX, did not perfectly align, indicating that the catalyst with the highest degradation efficiency did not necessarily possess the largest specific surface area. Therefore, it can be inferred that other factors such as pore volume and pore size diameter may also play a crucial role in influencing the photocatalytic efficiency of AC, ZIF-8, and Zr-MOF catalysts, in addition to the specific surface area.

Photocatalytic degradation serves as a widely employed technique for pollutant removal from both water and air. However, the accurate assessment of degradation efficiency is hindered by background noise and interference within the analytical system. To overcome this challenge, blank testing is commonly utilized to analyze photocatalytic degradation in the absence of target analytes. This approach facilitates the identification of alternative degradation pathways and enables adjustments for subsequent tests (Giannakoudakis & Bandosz 2014).

Photocatalytic degradation of BTX compounds involves complex processes facilitated by ROS generated upon light irradiation. Each catalyst in this study – AC, ZIF-8, and Zr-MOF – contributes uniquely to the degradation mechanisms due to their structural and chemical properties.

• 1. Activated carbon (AC): AC primarily contributes to BTX degradation through its high surface area and adsorption capacity, allowing BTX molecules to concentrate on its surface. However, as an amorphous material, AC has limited intrinsic photocatalytic properties. In the presence of UV light, AC can produce minor amounts of ROS, primarily through surface-bound oxygen functionalities. These ROS, such as hydroxyl radicals (•OH), initiate oxidation reactions on adsorbed BTX molecules, leading to gradual breakdown into intermediate products and eventually CO₂ and H₂O. The efficiency of AC in BTX degradation is enhanced in the presence of ACN, which improves the adsorption of BTX on the AC surface and stabilizes intermediate radicals during photodegradation.

• 2. ZIF-8: ZIF-8, a zeolitic imidazolate framework, has a high surface area and porous structure, which enhances the adsorption of BTX molecules. Upon UV irradiation, the Zn ions within the ZIF-8 structure act as active sites for the generation of ROS, such as superoxide anions (⁠⁠) and hydroxyl radicals (•OH). These ROS attack the aromatic rings of BTX, breaking down these compounds into smaller intermediates. The structure of ZIF-8, specifically its rhombic dodecahedron geometry, facilitates electron transfer and reduces electron–hole recombination, which improves photocatalytic efficiency. In the ACN medium, ZIF-8 displays increased dispersion of BTX molecules, enhancing their interaction with ROS and leading to more effective degradation.

• 3. Zirconium-based MOF (Zr-MOF): Zr-MOF, characterized by its strong Zr–O clusters, exhibits robust photocatalytic activity in BTX degradation. Zr-MOFs are known for their high chemical stability and ability to facilitate electron transfer, which is essential for efficient ROS generation. Upon UV exposure, Zr-MOFs produce a high concentration of ROS, primarily hydroxyl radicals (•OH) and superoxide anions (⁠⁠), through the activation of Zr–O bonds. The generated ROS rapidly degrade BTX compounds, following a pathway of aromatic ring cleavage, oxidation, and subsequent mineralization to CO₂ and H₂O. The presence of Lewis acidic sites in Zr-MOF enhances its interaction with BTX molecules, and the ACN solvent further aids by optimizing the solubility and dispersion of BTX, ensuring efficient degradation pathways.

In this study, catalytic degradation was determined and studied for BTX by AC, ZIF-8, and Zr-MOF. The degradation of BTX was monitored and calculated after every 10 min. Then, the calibration curve was plotted as per the rate of benzene degraded with effect of buffer time. The regression and degradation % (time taken to degrade >85%) was plotted. This regression contributes to an overall relationship with the solute and analyte during the irradiation period and letting them entrap and engulf the BTX residues via photocatalyst and letting them degrade simultaneously.

This indicated that Zr-MOF is more susceptible to take part in degradation reaction. Moreover, its higher pore volume enhances its ability to provide sufficient free surface area for the efficient movement of free radicals, thus initiating the degradation of BTX, more effectively than AC and ZIF-8. The Zr–O cluster present in Zr-MOF plays a crucial role in the catalytic degradation of BTX. The degradation rate of BTX in the presence of these photocatalysts depends on their size, chemical moieties, and surface adsorption characteristics. Benzene, being the smallest molecule among the BTX group, exhibits the highest affinity for AC surface adsorption. Its breakdown leads to the formation of phenol and catechol, which are further oxidized to produce CO₂ and H₂O. Toluene, being larger than benzene, degrades to benzaldehyde, benzoic acid, and benzyl alcohol, which are also oxidized to CO₂ and H₂O. The larger size of xylene contributes to its slower breakdown rate. Xylene, being the largest in BTX, has the lowest surface adsorption. During its breakdown, toluene and benzene are produced, and ultimately lead to CO₂ and H₂O production.

The regression and degradation percentages of BTX for all three photocatalysts are presented in Table 2, offering comprehensive insights into their catalytic performance and efficiency in BTX degradation.

The investigation revealed that all three materials exhibited favorable degradation responses despite their distinct structural variations. Notably, larger pore sizes were found to enhance the degradation activity, and a corresponding increase in pore volume substantially improved the efficiency of catalytic degradation. This enhancement can be attributed to the larger surface area available for adsorption, resulting in accelerated kinetic rates. Thus, a larger pore volume plays a critical role in significantly enhancing the efficiency of catalytic degradation. The degradation percentages achieved in this study surpassed those reported in earlier research, as presented in Table 1. The surface characteristics (pore size and pore volume) with absorbance and band gap reading observed during this study have been mounted in Table 3.

Furthermore, photooxidation assumes a pivotal role in the catalytic degradation of BTX using AC, ZIF-8, and Zr-MOF. The rate of photooxidation is contingent upon various factors, including the specific type of photocatalyst employed, the intensity and wavelength of the light source, the concentration of BTX compounds, and the presence of other chemicals or contaminants (Kočí et al. 2009). The process involves illuminating a photocatalyst with light, triggering its activation, and subsequently engaging with oxygen to generate ROS like hydroxyl radicals. These ROS then interact with the BTX compounds, facilitating their degradation (Binas et al. 2019). By quantifying photooxidation rates, it becomes possible to assess the efficacy of the photocatalytic degradation process and optimize conditions to achieve maximum degradation efficiency.

In this study, AC and ZIF-8 both exhibit photooxidation capabilities for BTX contaminants due to their better catalytic activity and adsorption surface. However, Zr-MOF has shown the best catalytic activity among the three and achieved the best degradation efficiency. The rate of degradation of BTX varies due to the structure and rate of oxidation rate susceptibility. Notably, a simpler chemical structure of benzene makes it more susceptible to oxidation than toluene and xylene, resulting in varying photooxidation rates and further facilitates the degradation mechanism. Zr-MOF exhibits the highest photooxidation rates for benzene, toluene, and xylene at 0.0118, 0.0111, and 0.0107 mg/L/min, respectively, as shown in Table 4.

Moreover, Zr-MOF promotes BTX photooxidation by creating reactive species like hydroxyl radicals with its Lewis acid sites. The photooxidation rates for BTX are notably higher than those for other pollutants with structurally comparable chemical structures. This may be because more complex compounds have more reactive sites to react with the Zr-MOF catalyst's hydroxyl radicals. However, AC and ZIF-8 also promote photooxidation, but with slightly lower rates. The catalytic activity of AC is mainly attributed to its adsorption surface for pollutants, while ZIF-8's metal ions act as catalysts for reactive species generation and provide an excellent adsorption surface. The simple chemical structure of benzene makes it more reactive to photooxidation than toluene and xylene, which have more complex structures.

In the case of AC, while it lacks intrinsic photocatalytic properties, its high surface area and porosity enable it to act as an efficient adsorbent, concentrating BTX molecules at active sites and potentially producing limited ROS under specific conditions. When exposed to UV light, AC may act as a photosensitizer, generating small amounts of hydroxyl (•OH) and superoxide anions (⁠⁠), which partially degrade BTX compounds through successive hydroxylation reactions (Kočí et al. 2009). However, AC's primary role in degradation is through adsorption, and any mineralization of BTX is slow and incomplete compared with MOFs.

ZIF-8 and Zr-MOF, both MOFs with photocatalytic properties, demonstrate a more robust degradation mechanism. When irradiated with UV–visible light, both materials generate electron–hole pairs, where electrons in the CB reduce O₂ to and holes in the VB oxidize water or hydroxide ions to •OH (Binas et al. 2019). These ROS are central to the degradation process, as they actively attack BTX molecules adsorbed on the catalyst surface, resulting in their oxidation and mineralization into CO₂ and H₂O. In ZIF-8, BTX degradation primarily involves successive hydroxylation, forming hydroxylated intermediates that ultimately degrade. The smaller pore size of ZIF-8 limits the interaction with larger molecules like xylene, resulting in a lower degradation rate for xylene than benzene and toluene.

Zr-MOF, with its highly stable structure, large pore volume, and low band gap, exhibits enhanced photocatalytic activity due to efficient electron–hole separation and abundant Lewis acid sites. These features contribute to increased ROS generation, allowing for rapid degradation of BTX. The degradation pathways of BTX involve oxidation to form intermediates like phenols, benzoic acid, and benzyl alcohol, which are further mineralized. Zr-MOF's structure supports sustained ROS production, enabling superior degradation efficiency relative to both AC and ZIF-8.

The superior photocatalytic performance of Zr-MOF over AC and ZIF-8 can be attributed to its unique structural characteristics, as revealed through BET surface area analysis, SEM, XRD, and FTIR studies. Although Zr-MOF has a lower BET surface area (535.4 m²/g) than ZIF-8 and AC, its higher pore volume (0.628 cm³/g) and distinctive pore structure enable effective pollutant adsorption and enhanced interaction with BTX compounds. This structure provides Zr-MOF with ample space for adsorbing and immobilizing BTX molecules, thus facilitating greater contact with active sites during UV irradiation.

The SEM analysis reveals Zr-MOF's disordered, agglomerative surface, creating accessible sites for pollutant adsorption, which supports rapid degradation kinetics. XRD results confirm the crystalline nature of Zr-MOF, indicating a robust framework that enhances stability under UV light, while FTIR analysis reveals Zr–O–Zr and Zr–OH groups that serve as reactive sites, contributing to hydroxyl radical formation crucial for photocatalysis. Together, these structural features promote efficient charge separation and active radical production, ultimately improving the degradation efficiency of BTX compounds.

In combination, Zr-MOF's lower band gap, substantial pore volume, stable crystalline structure, accessible surface morphology, and abundant reactive sites provide a highly efficient photocatalytic platform. These characteristics collectively support Zr-MOF's superior performance in BTX degradation by enabling efficient pollutant adsorption, sustained charge separation, and high radical generation under UV light. This synergy underscores Zr-MOF's potential as a robust photocatalyst, with promising applications in broader environmental remediation efforts.

Zr-MOF demonstrates enhanced photocatalytic performance for the degradation of BTX; however, its practical application is hindered by several challenges, including stability under operational conditions and elevated synthesis costs relative to traditional materials such as AC. The structural integrity of Zr-MOF may deteriorate with prolonged UV exposure and fluctuating environmental factors, which could diminish its degradation efficiency over time. Additionally, the synthesis of Zr-MOF requires expensive precursors and intricate processing methods, leading to increased overall costs. In contrast, activated carbon is readily accessible, economically viable, and exhibits considerable stability, rendering it a more cost-effective option despite its comparatively lower degradation efficiency. Therefore, the decision between Zr-MOF and activated carbon involves a balance between performance and financial considerations, indicating that Zr-MOF may be more suitable for applications where high efficiency is prioritized over cost limitations. Future research should aim to enhance the economic feasibility and stability of Zr-MOF to expand its practical applications.

In our study, the degradation of BTX compounds is primarily facilitated by the generation of ROS under UV–visible light irradiation, with Zr-MOF showing the highest efficiency among the tested materials. To enhance our understanding of these processes, we have drawn parallels with recent studies on the role of co-oxidants in boosting photocatalytic efficiency, such as those conducted on parabens degradation (Sheikhmohammadi et al. 2020). These studies demonstrated that co-oxidants, when combined with photocatalysts, significantly improve degradation rates by promoting ROS generation and accelerating pollutant breakdown.

Similarly, recent research on Co-ZIF/WO₃ heterostructures for selective NOx reduction and cefixime removal (Alamgholiloo et al. 2024) has shown that combining materials with distinct catalytic properties can significantly improve catalytic efficiency through synergistic effects. The study on Co-ZIF/WO₃ for NOx reduction demonstrates that the introduction of WO₃ not only enhances electron–hole separation but also provides additional active sites that improve the interaction between pollutants and catalysts. This mechanism is like the enhanced ROS production observed in our Zr-MOF system, which also benefits from improved charge separation and stability, thus supporting higher degradation efficiency for BTX compounds.

In parallel, the use of WO₃/Co-ZIF nanocomposites in cefixime removal through machine learning optimization highlights the potential of combining materials with high adsorption and photocatalytic capacities (Sheikhmohammadi et al. 2024). The WO₃/Co-ZIF composite facilitated pollutant breakdown via efficient ROS generation, a mechanism closely related to that of Zr-MOF in our study. The application of machine learning in optimizing parameters for photocatalytic degradation underscores the importance of fine-tuning material properties to maximize ROS production and pollutant interaction, which could be a promising approach for future studies on BTX degradation in MOF-based systems.

In conclusion, ACN was identified as an excellent solvent for significantly enhancing the effectiveness of BTX photodegradation processes using AC, ZIF-8, and Zr-MOF. The adsorbed BTX molecules react with generated hydroxyl radicals (•OH) to form intermediate compounds such as phenol, cresols, and benzoic acid, which can be further oxidized to CO₂ and H₂O. The photodegradation (%) followed the sequence B > T > X under specific environmental conditions. Notably, within ACN, benzene exhibited the highest degradation rate, followed by toluene and xylene. This difference in degradation rates is attributed to their respective diffusion rates with the selected photocatalysts, as follows: Zr-MOF > ZIF-8 > AC.

The findings of this study demonstrate Zr-MOF's high photocatalytic efficiency in degrading BTX pollutants, suggesting its potential applicability in wastewater treatment systems. However, the practical implementation of Zr-MOF in industrial settings requires careful consideration of scalability, cost-effectiveness, and environmental impact.

Currently, Zr-MOF synthesis involves steps such as solvothermal processing, which can be both time-intensive and require precise control of reaction conditions. Scaling this process for industrial-scale applications may demand adjustments to reduce synthesis time and simplify operational requirements. Recent advancements in scalable synthesis approaches for MOFs, such as microwave-assisted or continuous flow synthesis, offer promising alternatives to streamline Zr-MOF production, potentially reducing manufacturing complexity.

While Zr-MOF demonstrates higher degradation efficiencies than traditional materials like AC and other MOFs, its synthesis involves higher costs due to zirconium precursors and energy requirements. However, the material's extended stability and reusability could offset initial investment costs by reducing the frequency of replacement in treatment systems. Additionally, ongoing research into alternative, more cost-effective synthesis routes, such as metal–organic gels or alternative zirconium sources, could make Zr-MOF more economically viable for broader wastewater treatment applications.

Using Zr-MOF in wastewater treatment presents both opportunities and challenges regarding environmental sustainability. On the one hand, Zr-MOF's high efficiency in pollutant degradation can significantly reduce the concentration of hazardous organic compounds, lessening the ecological burden on aquatic systems. However, the environmental impact of Zr-MOF synthesis and potential leaching of zirconium during use need to be evaluated in long-term applications. Future studies could focus on developing more sustainable production methods, as well as assessing the material's biodegradability and lifecycle impacts to ensure safe and environmentally friendly deployment.

Beyond BTX compounds, Zr-MOF's structural stability and catalytic properties make it a viable candidate for degrading other persistent organic pollutants, such as phenols, pesticides, and endocrine-disrupting compounds (EDCs). This versatility could position Zr-MOF as a multifunctional catalyst in comprehensive wastewater treatment systems, where the degradation of diverse contaminants is essential. In summary, while Zr-MOF shows promising catalytic potential, addressing factors such as synthesis scalability, cost reduction, and environmental safety will be key to realizing its real-world application potential in wastewater treatment. Further research into optimizing Zr-MOF synthesis and lifecycle assessment will strengthen its viability as a practical solution in environmental remediation.

Zr-MOFs demonstrate significant catalytic potential for degradation processes; however, it is crucial to evaluate their long-term stability and possible environmental effects. A primary concern is the leaching of metal ions, especially Zr ions, which may lead to environmental contamination and health hazards. To address this challenge, various strategies can be investigated, such as surface functionalization, encapsulation, and post-treatment methods. Nonetheless, additional research is required to comprehensively understand the environmental consequences of employing Zr-MOFs in real-world applications.

The application of AC, ZIF-8, and Zr-MOF as photocatalysts for environmental remediation introduces a range of opportunities and challenges regarding sustainability and economic viability. AC is advantageous due to its affordability and widespread availability, making it suitable for large-scale implementations. However, its photocatalytic efficiency is limited, and the necessity for frequent regeneration due to surface saturation can elevate operational expenses and energy demands, thereby affecting overall sustainability. Although AC can be regenerated through high-temperature methods or chemical treatments, these processes may generate environmental burdens through emissions and waste, thereby limiting its lifecycle benefits.

In contrast, ZIF-8 offers moderate enhancement in photocatalytic efficiency compared to AC by producing ROS when exposed to UV–visible light. Nonetheless, its stability diminishes with extended exposure, resulting in structural degradation and the potential leaching of zinc ions. The synthesis of ZIF-8 often involves organic solvents, which raises environmental concerns; however, advancements in green synthesis techniques, such as water-based or solvent-free methods, may mitigate these issues. Economically, the moderate performance of ZIF-8 in relation to its production costs poses scalability challenges, although improvements in stability could prolong its lifecycle and lower long-term expenses.

Zr-MOF exhibits exceptional photocatalytic performance, stability, and efficient ROS generation, positioning it as the most effective catalyst among the three candidates for the degradation of BTX. Nonetheless, its synthesis is hindered by the use of costly zirconium precursors and energy-demanding conditions, which contribute to elevated production costs and environmental concerns. Despite Zr-MOF's strong stability allowing for reuse across multiple cycles with minimal loss in efficiency, it is imperative to tackle these initial production challenges to enhance its broader applicability. Investigating more economical synthesis methods, such as utilizing alternative zirconium sources or implementing milder reaction conditions, could render Zr-MOF a viable option for large-scale applications.

Overall, this comparative study demonstrates that AC, ZIF-8, and Zr-MOF are excellent photocatalytic materials in ACN, effectively promoting the photooxidation of BTX contaminants in industrial and environmental settings. Importantly, Zr-MOF exhibited the highest photooxidation rate for BTX degradation within 60 min. This study provides valuable insights into the potential application of photocatalysis as a treatment method for BTX contamination, underscoring the importance of carefully selecting the catalyst, solvent, and adsorbent additive combination to achieve efficient catalytic degradation. Further research is required to comprehensively understand the mechanisms governing the different degradation rates and to optimize the conditions for maximum degradation efficiency.

The authors acknowledge the assistance and support provided by the Department of Biotechnology Engineering and Food Technology at Chandigarh University (Gharuan) and the Department of Biotechnology, Parul Institute of Technology, Parul University, Vadodara (Gujarat) to complete this research work.

S.N.: investigation, visualization, data curation, data validation, data analysis, manuscript preparation, and editing. M.C.: conceptualization, supervision, manuscript preparation, manuscript reviewing, and editing. All authors have read and agreed to the published version of the manuscript.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.