Skip to main content
NCBI home page
RSC Advances logoLink to RSC Advances
. 2026 Oct 12. Online ahead of print. doi: 10.1039/d6ra06639a

Sustainable synthesis of a Zr-based UiO-66 MOF from PET waste for UV-assisted degradation of organic contaminants

Samar H Tawakey a,✉, A K Abou-Seif a, Ali H Gemeay a, Mohamad M Ayad a,b,✉
PMCID: PMC13656015  PMID: 42865877

Abstract

The coexistence of polyethylene terephthalate (PET) waste and persistent organic pollutants in aquatic environments requires comprehensive circular-economy strategies to valorize PET waste into functional materials for water treatment. In this study, post-consumer PET was converted to terephthalic acid (TPA) via a neutral hydrothermal depolymerization process and was used as an organic linker for the synthesis of a Zr-based UiO-66. The recovered linker and synthesized UiO-66 were characterized by FTIR, PXRD, TEM/SAED, nitrogen adsorption–desorption, UV-vis spectroscopy, electrochemical impedance spectroscopy, and Mott–Schottky analysis. PXRD confirmed UiO-66 crystallinity, while TEM revealed nano-sized polyhedral particles with an average size of approximately 120 nm. The material exhibited a BET surface area of 822.2 m2 g−1, total pore volume of 0.379 cm3 g−1, and average pore diameter of 2.93 nm. Optical and electrochemical analyses showed a band gap of about 3.51 eV, n-type semiconducting behavior, and a flat-band potential of −0.68 V vs. Ag/AgCl. The photocatalytic activity of PET-derived UiO-66 was evaluated using methylene blue (MB) under UV light irradiation. Under optimized conditions, the catalyst achieved about 97.3% degradation with an apparent pseudo-first-order rate constant of 0.024 min−1, which was about 12 times higher than that of direct photolysis. Radical scavengers' experiments showed that hydroxyl radicals (˙OH) acted as major reactive species, followed by superoxide radicals (˙O2−) and photogenerated holes (h+). The catalyst showed good recyclability with about 86% activity retained after five cycles. These results offer an environmentally sustainable approach to recycle PET waste into a useful photocatalyst for water treatment.

Terephthalic acid recovered from post-consumer PET waste was utilized to synthesize UiO-66, demonstrating a sustainable waste-to-resource strategy for the photocatalytic degradation of methylene blue in water.graphic file with name d6ra06639a-ga.webp

1. Introduction

The industrial revolution in the late 18th century marked a significant turning point in the increase of water pollution induced by humans. Rapid industrialization and urbanization have led to the deterioration of aquatic ecosystems by discharging untreated effluents into the natural water bodies and an increase in waterborne illnesses in densely populated areas.1,2 Various persistent organic and inorganic pollutants such as synthetic dyes, surfactants, insecticides, medicines, flame retardants, heavy metals and personal care items have been introduced into aquatic habitats through industrial and home activities. These contaminants are resistant to natural breakdown and build in the environment and seep into ground water. Their long-term existence is a great challenge to environmental sustainability and constitute substantial concerns to public health including possible harm for future generations.3,4

The accumulation of plastic waste, particularly polyethylene terephthalate (PET), has emerged as a major environmental concern owing to its widespread use, durability, and resistance to degradation. PET based items such as beverage bottles and food packaging materials can be physically degraded to microplastics that can enter aquatic habitats. The contamination of wastewater is much more hazardous because microplastics have been detected in freshwater and marine habitats.5 Chemical depolymerization is a viable pathway to recover 1,4-benzenedicarboxylic acid (terephthalic acid) from PET, breaking down the polymer chains to monomeric units.6,7 Conventional depolymerization processes happen under extreme acidic or basic conditions, which are related to corrosive by-products and significant energy requirements.8 In contrast, neutral depolymerization by hydrolysis or alcoholysis under mild circumstances is a greener and more sustainable method, which allows the efficient recovery of linker precursors without the generation of hazardous residues.9,10

Metal organic frameworks (MOFs) have been considered in recent years as very promising materials due to their modular architecture, significantly large surface area, tunable porosity, and structural variety. Metal organic frameworks have been successfully employed in a variety of applications, including gas storage, catalysis, drug delivery, sensing, and, most recently, wastewater remediation.11,12 UiO-66 is one of the many MOF families. It is a zirconium-based framework made from Zr6 nodes and 1,4-benzenedicarboxylic acid. UiO-66 displayed outstanding thermal and chemical stability, high surface area and excellent photocatalytic activity. These qualities make UiO-66 a very good candidate for the degradation of organic pollutants in aquatic environments.13 It is worth mentioning that the monomer unit of PET polymers and the organic linker in UiO-66 have structural similarities.14 This commonality gives a rare opportunity to upcycle PET trash into MOF precursors, treating two environmental challenges simultaneously: plastic pollution and water contamination.

Advanced treatment techniques have been developed with an extensive amount of effort to effectively eliminate contaminants from wastewater. Conventional treatment approaches for the removal of organic pollutants include adsorption, precipitation, membrane separation, and biological treatments eliminate the risk to some extent.15,16 Among emerging alternatives, photocatalytic degradation of organic pollutants has been proven to be a simple, cost-effective, and environmentally friendly method.17 While photocatalysis minimizes secondary pollution and enables the degradation of a broad spectrum of organic compounds under mild operational conditions, unlike conventional techniques.18,19 Effective multifunctional photocatalyst development is still a major research challenge. In response, various nanomaterials have been studied, such as metal oxides, inorganic and organic hybrids, and carbon-based nanostructures.20

This work shows the sustainable synthesis of UiO-66 via neutral depolymerization of post-consumer PET for recovery of terephthalic acid. The work provides an integrated circular strategy for the simultaneous valorization of PET waste and photocatalytic wastewater treatment. The photocatalytic performance of the synthesized UiO-66 was evaluated for the degradation of MB as a model organic dye pollutant under UV irradiation. The substance showed strong activity which confirms its environmental significance and possible practical application.

2. Experimental

2.1. Materials

Zirconium(iv) chloride (ZrCl4, anhydrous) was obtained from Sigma-Aldrich/Merck, France. Commercial 1,4-benzenedicarboxylic acid (terephthalic acid; H2BDC), 98%) was purchased from Merck. N,N-dimethylformamide (DMF, 99%) and methanol (99.8%, HPLC grade) were acquired from Fisher Chemical, UK. Hydrochloric acid (HCl, 37%) was obtained from Scharlau, Spain. Sodium hydroxide pellets (analytical grade reagent) were obtained from Fisher Chemical, UK. Methylene blue (MB) was obtained from ICN Pharmaceuticals, USA. p-benzoquinone (p-BQ for synthesis) was obtained from Merck-Schuchardt, Germany. Isopropanol (IPA, 99.9%) was purchased from Loba Chemie, India. Ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) and sodium chloride (NaCl) were gained from Eastrin Fine Chemicals S.LTD, Italy. Sodium sulfate (Na2SO4) was acquired from Merck, Germany. A commercially available 6 W UV lamp was purchased from Amazon.eg (Egypt) was used for the photocatalytic experiments. The lamp operates at a wavelength of 254 nm, according to the technical specification provided for the corresponding lamp model. All materials employed in this study were of analytical grade and used as received without additional purification. Deionized water (DI) was used for all solution preparations and glassware washing.

2.2. Characterization

Fourier Transform Infrared (FTIR) spectra were obtained using a Perkin–Elmer Spectrum BX spectrometer throughout the range of 400–5000 cm−1. To verify functional groups and confirm structural consistency of both the PET-derived and commercial terephthalic acid. The powder X-ray diffraction (XRD) spectrum was obtained to identify the crystalline structure of the produced material utilizing a Shimadzu XRD-6100 with Cu Kα radiation (λ = 1.5406 Å).The diffraction patterns were acquired across a 2θ range of 5–80° at a scan rate of 2° min−1 to assess the crystalline structure of the synthesized UiO-66.The shape and particle size of the photocatalyst were examined using a transmission electron microscope (TEM, JEOL JEM-2100F) operated at 200 kV with carbon grids (200 mesh). The specific surface area and pore size distribution were evaluated via N2 adsorption–desorption isotherms at 77 K utilizing a BELSORP mini II (BEL Japan). The specific surface area was computed using the Brunauer–Emmett–Teller (BET) method, while the pore volume and size distribution were assessed by t-plot and Barrett–Joyner–Halenda (BJH) analyses, respectively. The UV-visible spectra of the photocatalyst were obtained using a Lambda spectrophotometer UVD-2960 within the spectral range of 250–800 nm in order to evaluate optical absorption capabilities. TOC analysis utilizing a QP1680 TOC/TN analyzer (Hach, USA) based on high temperature catalytic combustion and non-dispersive infrared (NDIR) detection of the evolved CO2.

2.3. Neutral hydrolysis of PET and recovery of terephthalic acid

The experimental scheme for terephthalic acid (TPA) recovery from PET wastes is summarized in Fig. 1a. The depolymerization of PET was carried out under neutral conditions, using water as the reaction medium, to minimize environmental impact and avoid the use of corrosive reagents. This approach aligns with recently reviewed green depolymerization methods that emphasize milder conditions, reduced energy input, and minimal secondary waste generation.10 Post-consumer PET plastic bottles were collected from recycling bins located on our university campus, cleaned thoroughly with distilled water, and dried at 60 °C to remove moisture. The bottles were then cut into small flakes (approximately 5–10 mm) to increase the surface area and facilitate depolymerization. 10.00 g of PET chips and two grams of NaCl were placed into a Teflon-lined stainless-steel autoclave with a volume of 250 mL, to which 100 mL of deionized water was added. The autoclave was adequately sealed, positioned in an oven, heated to 200 °C, and sustained at this temperature for 8 hours.8 After the required reaction time, the autoclave was removed from the oven and allowed to cool to room temperature. The resulting solid product was collected by filtration, thoroughly washed with ethanol and deionized water, and subsequently dried in an oven at 100 °C to obtain a white TPA powder. Following filtration, washing, and drying, approximately 7.00 g of TPA was recovered from 10.00 g of post-consumer PET waste. The TPA yield was calculated according to: TPA yield (%) = (mass of recovered TPA/theoretical TPA mass) × 100.

Fig. 1. Preparation of PET-derived UiO-66 and its photocatalytic application. (a) Recovery of TPA from post-consumer PET bottles. (b) Synthesis of UiO-66 using the recovered TPA linker and ZrCl4 precursor under solvothermal conditions. (c) Photograph of the homemade UV photoreactor. (d) Schematic representation of the reactor setup used for (MB) photocatalytic degradation.

Fig. 1

Open in a new tab

Based on the theoretical TPA mass of approximately 8.65 g obtainable from 10.00 g of PET, the calculated TPA yield was approximately 80.9%.

2.4. Synthesis of UiO-66 from PET-derived terephthalic acid

The terephthalic acid (H2BDC linker) recovered from the neutral depolymerization of PET was used as the organic ligand for the synthesis of UiO-66. The synthesis was adapted from previously reported procedures with minor modifications.21 In a typical solvothermal procedure, 122 mg of terephthalic acid (TPA) and 125 mg of zirconium tetrachloride (ZrCl4) were separately dissolved in 5 mL of N,N-dimethylformamide (DMF), corresponding to a ZrCl4 molar ratio of (1 : 1.37). The two solutions were mixed, stirred, and sonicated for 30 minutes to ensure complete mixing. The sealed bottle was then placed in an oven and heated at 80 °C for 24 hours under static conditions to promote solvothermal crystallization. The white precipitate was recovered by centrifugation after it cooled down to room temperature. It was then extensively cleaned with DMF and methanol to get rid of any remaining solvent and unreacted precursors. After that, the product was dried for 12 hours at 120 °C to produce pure UiO-66. The experimental workflow is summarized in Fig. 1b. The synthesized MOF was stored in a desiccator prior to characterization and photocatalytic testing.

2.5. Electrochemical measurements

Electrochemical characterizations, comprising electrochemical impedance spectroscopy (EIS) and Mott–Schottky analysis, were conducted using BioLogic SP-150 potentiostat controlled by EC-Lab software (version 11.62) all measurements were performed in a 0.2 M Na2SO4 solution with a three-electrode configuration comprising a glassy carbon electrode (2 mm diameter) as the working electrode, a platinum wire as the counter electrode, and an Ag/AgCl reference electrode.

Mott–Schottky measurements were performed over a potential range of −1.0 to +1.0 V vs. Ag/AgCl using a staircase potentiostatic impedance method with an AC amplitude of 10 mV. Impedance data were collected over a frequency range of 1 kHz to 1 MHz, and the Mott–Schottky plot presented in this work was evaluated at 1000 Hz (1 kHz).

For electrode preparation, UiO-66 suspension was prepared by dispersing 5 mg of the synthesized UiO-66 powder in 1 mL of N,N-dimethylformamide (DMF) containing 20 µL of Nafion. The mixture was sonicated for 30 min to obtain a homogeneous suspension. Subsequently, 50 µL of the suspension was drop-cast onto the glassy carbon electrode (GCE) surface and dried at 50 °C to obtain the UiO-66-modified GCE used as the working electrode.

2.6. Photocatalytic activity test

The adsorption behavior of MB on UiO-66 was first evaluated separately under dark conditions for up to 120 min. Based on the adsorption profile, a dark equilibration period of 60 min was applied before UV irradiation in the subsequent photocatalytic experiments.

The photocatalytic performance of the synthesized catalyst was assessed via the photocatalytic degradation of MB dye utilizing a handmade photocatalytic reactor, as seen in Fig. 1c. In a standard experiment, 100 mL of MB aqueous solution (10 mg L−1) was combined with 20 mg of catalyst prior to UV irradiation, and the suspension was mixed using a magnetic stirrer for 60 minutes in the absence of light to achieve adsorption–desorption equilibrium. Subsequently, the suspensions were subjected to UV irradiation while continuous stirring was sustained during the photocatalytic reaction. At 15 minutes intervals, 2 mL aliquots of the suspension were withdrawn and centrifuged to remove the solid catalyst. The residual MB content in the supernatant was quantified using UV-visible spectrophotometry at λmax = 664 nm. The photocatalytic degradation efficiency was determined using eqn (1):

Degradation efficiency (%) = [(C0 − Ct)/C0)] × 100 1

where C0 is the initial MB concentration after dark adsorption equilibrium and Ct is the MB concentration at irradiation time t. A series of photocatalytic studies were performed to assess the influence of solution pH, starting dye concentration, and catalyst dosage on degradation efficiency. Following each photocatalytic experiment, the utilized UiO-66 was collected, ultrasonically washed with methanol to remove residual MB, then dried prior to reuse. The photocatalyst's reusability was evaluated during five successive cycles.

2.7. Point of zero charge (pHpzc) analysis

The point of zero charge (pHpzc) of UiO-66 was measured utilizing the pH-drift method. In a typical procedure, 50 mg of UiO-66 was suspended in 50 mL of 0.1 M NaCl solution, and the initial pH of each suspension was adjusted to 2, 4, 6, 8, 10, and 12 using 0.1 M HCl or 0.1 M NaOH. The suspensions were stirred for 60 min to guarantee homogeneous dispersion, after which the initial pH (pHi) was documented. The mixtures were stirred for 48 h at ambient temperature to attain equilibrium. Subsequently, the final pH (pHf) was measured, and ΔpH (pHf − pHi) was plotted as a function of pHi. The pHpzc was taken at the point where ΔpH equals zero.22,23

3. Result and discussion

3.1. Structural analysis of TPA derived from PET waste

The recovery of TPA was achieved through neutral depolymerization of PET waste under hydrothermal conditions. The mechanism of neutral hydrolysis is proposed to go through cleavage of the ester linkages in the PET backbone in the presence of NaCl and water. NaCl attacks the C Created by potrace 1.16, written by Peter Selinger 2001-2019 O bond resulting in electrolytic instability of adjacent bonds, subsequently leading to the cleavage of the polymer chain.24 The water molecules react with the PET polymer chains under heating and pressure to produce TPA and ethylene glycol (EG).25 The depolymerization process can be explained as following. PET + H2O → TPA + EG.3.1.

To verify the successful hydrolysis of PET waste and to investigate the functional groups in TPA produced by neutral depolymerization process. FTIR analysis was performed for both TPA derived from PET waste and compared against commercial standard TPA. Fig. 2a illustrates the FT-IR spectra, confirming the successful transformation of polymer into monomeric organic linker. The presence of a broad absorption band at around 3452 cm−1 in both TPA samples corresponds to the O–H stretching vibration and arises from strong intermolecular hydrogen bonding between carboxylic acid groups.26,27 The stretching vibrational bands at 1634 cm−1 and 1560 cm−1 of C Created by potrace 1.16, written by Peter Selinger 2001-2019 C in benzene ring confirm the terephthalic aromatic backbone.28–30 In contrast to PET the absence of characteristic ester bands at around 1730 cm−1 and 1250 cm−1 indicates the effective cleavage of ester linkages during depolymerization.29,31

Fig. 2. Structural, morphological, and textural characteristics of the synthesized UiO-66 photocatalyst. (a) The FTIR spectra of PET-derived and commercial terephthalic acid. (b) A comparison between the simulated and synthetic UiO-66 PXRD patterns. (c) Illustration of UiO-66 crystal structure. (d) TEM image of the polyhedral morphology of UiO-66 particles with an average size of about 120 nm. (e) SAED pattern showing polycrystalline nature of synthesized UiO-66. (f) BET isotherm measured at 77 K with inset illustrating the BJH pore size distribution.

Fig. 2

Open in a new tab

3.2. Structural, morphological, and surface area analysis

The crystallographic structure of the synthesized UiO-66 was investigated by powder X-ray diffraction. As shown in Fig. 2b, the PXRD pattern of the synthesized UiO-66 matched the simulated pattern of the fcu UiO-66 crystal structure obtained from the crystallographic information file (CIF no. 733458). The characteristic diffraction peaks observed at 2θ values of approximately 7.6° and 8.5° were assigned to the typical reflections of (111) and (002) crystallographic planes of UiO-66. Additional diffraction peaks at 12.3°, 17.5°, 22.6°, 25.9°, 31.1°, and 43.7° were consistent with that reported in the literature and indicating the successful preparation of UiO-66.32,33

The morphology and particle size of the synthetized UiO-66 were examined by TEM. As shown in Fig. 2d, the UiO-66 crystals synthesized from PET-derived terephthalic acid exhibited a well-defined polyhedral morphology, which is characteristic of zirconium-based MOFs prepared via solvothermal routes. The particles are compact faceted crystals with relatively smooth edges, indicating high crystallinity and homogeneous nucleation throughout growth. The average particle size was estimated to be around 120 nm indicating the creation of nanosized UiO-66 particles providing accessible surface sites for better interaction with pollutant molecules during the photocatalytic processes.34 The selected area electron diffraction (SAED) pattern (Fig. 2e) displayed diffraction rings, confirming the polycrystalline nature of the synthetized material and the presence of many small crystalline domains randomly orientated in the sample.35

Surface area and pore size distribution of UiO-66 were measured by N2 adsorption–desorption analysis. Prior to the nitrogen adsorption–desorption analysis, the UiO-66 sample was subjected to a high-vacuum degassing procedure (recorded pressure of 7.391 × 10−5 Pa) at 120 °C for 12 h. This pretreatment step was conducted to ensure the complete evacuation of adsorbed moisture, residual solvents, and volatile gases from the internal porous network. Following degassing, the adsorption measurements were performed utilizing N2 gas at 77.36 K. As shown in Fig. 2f, UiO-66 exhibited a type I isotherm, which indicates the microporous property of UiO-66 framework and the strong adsorption uptake at low relative pressure.36 The BET specific surface area was 822.2 m2 g−1, and the total pore volume was 0.379 cm3 g−1.37 The BJH pore size plot (inset of Fig. 2f) revealed an average pore diameter of 2.93 nm, indicating the existence of additional mesoporosity besides the intrinsic micropores of UiO-66. This larger pore size compared to idealized microporous UiO-66 indicates the presence of structural defects commonly observed when using waste-derived linkers. These mesopores are greatly useful for the photocatalysis, where the methylene blue molecules can rapidly diffuse into the interior framework and improve the overall degradation rate.

3.3. Optical and electronic properties

The optical properties of the synthesized UiO-66 material were investigated by UV-vis absorption spectroscopy to determine the light absorption behavior and band gap energy. As shown in Fig. 3a, the UV-vis spectrum presents a prominent absorption band in the ultraviolet region, which can be attributed to the ligand-to-metal charge transfer transitions from the organic terephthalate linker to the zirconium oxide clusters within the UiO-66 framework. This absorption behavior is typical for Zr-based MOF and has been reported extensively in the literature.38

Fig. 3. Optical and electrochemical band-structure analysis of the synthesized UiO-66 photocatalyst. (a) UV-visible absorption Spectrum of UiO-66. (b) Tauc plot obtained from UV-visible absorption data for calculating the optical band gap energy. (c) Mott–Schottky plot used to estimate the flat-band potential and semiconductor behavior of UiO-66. (d) Proposed band position diagram of UiO-66.

Fig. 3

Open in a new tab

The optical band gap was determined from the Tauc plot shown in Fig. 3b, plot of (αhv)nversus photon energy (hν) where α represents absorption coefficient and n represents the nature of the electronic transition. The band gap energy was obtained from the linear portion of the (αhv)1/2vs. hν plot by extrapolating to the energy axis assuming an indirect permitted transition.39,40

The estimated band gap value for UiO-66 was about 3.51 eV, which means that the material absorbs mainly in the UV region. This value is in agreement with published band gaps for UiO-66 and similar Zr-based MOFs, suggesting that the material is a broad band gap semiconductor.41 The semiconductor characteristics of UiO-66 modified glassy carbon electrode (UiO-66/GCE) were investigated by Mott–Schottky (MS) analysis in Na2SO4 electrolyte at a frequency of 1000 Hz. The positive slope of the Mott–Schottky plot of 1/C2vs. applied potential confirms the n-type semiconductor behavior of the UiO-66 film as seen in Fig. 3c.42

The flat-band potential (Efb) was obtained by extrapolating the linear fit to the x-axis, Efb = −a/b, where a and b are the intercept and slope of the fitted line, respectively. The flat-band potential was calculated to be about −0.68 V vs. Ag/AgCl by using the fitting parameters derived from the MS plot. For n-type semiconductors, the flat-band potential is usually near to the conduction band edge, therefore the obtained value is an estimation of the conduction band position of UiO-66.43 The MS plot exhibits a good linear behavior (R2 = 0.996) which further verifies the dependability of the fitting and suggests the establishment of a stable semiconductor/electrolyte interface. The results indicate that the UiO-66 deposit on the GCE is semiconducting with suitable electronic characteristics for electrochemical and photoelectrochemical applications.

3.4. Photocatalytic performance of UiO-66

The photocatalytic performance of UiO-66 was evaluated by investigating the photodegradation of MB, which served as a model pollutant under UV irradiation, with the removal efficiency using eqn (1). Because adsorption can contribute to the overall removal and strongly affect the photocatalytic mechanism. Therefore, the adsorption and photodegradation of MB experiments were carried out under the same conditions to illustrate photocatalytic performance.

The adsorption–desorption phenomena of MB on UiO-66 were evaluated by stirring the suspension in dark for 120 min. As shown in Fig. 4a, the MB absorbance gradually decreased during the dark period and reached equilibrium after 120 min with a maximum adsorption of 14.65% under these conditions. After adsorption equilibrium was reached, the photocatalytic degradation of MB over UiO-66 was tested under UV irradiation (Fig. 4b); the MB absorbance decreased with increasing irradiation time up to 120 min. To distinguish photocatalytic degradation from direct photolysis and adsorption, another experiment was done without using the catalyst in which no change in the solution absorbance was observed over time. The corresponding Ct/C0 profiles under different conditions are presented in Fig. 4c, confirming that MB removal in the presence of UiO-66 under UV irradiation is significantly higher than that due to photolysis alone. To further verify the photocatalytic activity of UiO-66, the degradation kinetics were analyzed using pseudo-first-order model (eqn (2)):

ln(C0/Ct) = kt 2

where k is the apparent rate constant and t is the irradiation time.44 The rate constant was obtained by the linear plot of ln(C0/Ct) vs. the irradiation time, which showed an excellent linear fit (R2 = 0.994; Fig. 4d). The degradation reaction rate constant for UV + UiO-66 was k = 0.024 min−1, which is approximately 12 times higher than that of photolysis (k = 0.002 min−1), demonstrating the significant photocatalytic activity of UiO-66.

Fig. 4. Photocatalytic degradation performance and kinetic analysis of (MB) over the synthesized UiO-66 photocatalyst under UV irradiation. (a) UV-visible absorption spectra of MB during the dark adsorption stage. (b) Time-dependent UV-visible absorption spectra of MB during photocatalytic degradation. (c) Variation of (Ct/C0) as a function of time, showing MB removal during dark adsorption and subsequent UV irradiation in the presence and absence of UiO-66. (d) Pseudo-first-order kinetic plots for MB degradation under UV irradiation.

Fig. 4

Open in a new tab

The mineralization of MB was investigated by TOC analysis utilizing a QP1680 TOC/TN analyzer (Hach, USA) based on high temperature catalytic combustion and non-dispersive infrared (NDIR) detection of the evolved CO2. The TOC removal efficiency was estimated as follows:TOC removal (%) = [(TOCi − TOCt)/TOCi] × 100where TOCi and TOCt are the TOC concentration at the beginning and after irradiation time, respectively. The TOC concentration was decreased from 6.6 mg C L−1 before irradiation to 0.7 mg C L−1 after 120 min of photocatalytic treatment which indicated a TOC removal effectiveness of about 89.4%. This considerable reduction of TOC also shows the thorough mineralization of MB-originated organic carbon during photocatalysis, and not solely decolorization of the dye solution.

3.4.1. Effect of pH

One of the most important factors influencing the performance of a photocatalyst in aqueous environments is the pH of the solution. pH significantly changes position of the energy bands, particle surface charge, oxidation and reduction potential reactive species generation, and catalyst stability.45,46 As shown in Fig. 5a, the point of zero charge (pHpzc) of UiO-66 is approximately 3.56.47 At pH < pHpzc, the UiO-66 surface is positively charged (Zr–OH2+ groups), leading to partial electrostatic repulsion with the cationic MB+ molecules and slightly reduced adsorption. However, as the pH increases to near-neutral values (pH 4–6), the surface becomes less protonated, allowing optimal MB adsorption and enhanced interfacial charge transfer. Although MB stays positively charged at higher pH, photocatalytic efficacy diminishes dramatically. This reduction is mostly due to the hydrolytic disintegration of the UiO-66 framework in alkaline conditions, where deprotonation of Zr–O–C connections causes partial collapse of the porous structure and loss of active sites.48,49 Moreover, the generation of hydroxyl radicals (˙OH) becomes less favorable because excess OH− ions act as radical scavengers, reducing the overall availability of reactive oxygen species (ROS).50 Together, these effects suppress photocatalytic degradation in basic media. The Ct/C0 plots at different pH values revealed that the photodegradation efficiency decreases with the increase in pH (Fig. 5b); this pH dependent behavior is governed by ionic nature of MB and the surface charge of UiO-66. To further understand how pH affects photocatalysis kinetics, pseudo-first-order rate constants were calculated by plotting ln(C0/Ct) versus time at different pH levels (Fig. 5c). The highest rate constant was recorded at pH 4, whereas the reaction rate exhibited a gradual decline with the elevation of pH, which is consistent with the degradation efficiencies that were recorded. Fig. 5d, summarizes the effect of pH on the photodegradation of MB over UiO-66 catalyst a cross a pH range of 2–9 using 20 mg of catalyst and 100 mL of 10 mg L−1 dye solution under UV irradiation (6 W UV lamp) at 25 °C for 120 min.

Fig. 5. 5Solution pH's effect on the photocatalytic degradation of (MB) over UiO-66 under UV irradiation. (a) Point of zero charge analysis of UiO-66, showing a pHpzc of approximately 3.56. (b) Time-dependent (Ct/C0) profiles for MB degradation at different pH values. (c) Pseudo-first-order kinetic plots of MB degradation at different pH values. (d) Summary for MB degradation efficiency at different initial pH values.

Fig. 5

Open in a new tab

3.4.2. Effect of initial dye concentration

The effect of initial dye concentration was investigated by varying the initial concentration from 5 to 50 mg L−1 while keeping the catalyst dose constant (20 mg of UiO-66 in 100 mL solution) under UV irradiation at 25 °C for 120 min. Fig. 6a and b present the MB degradation profiles (Ct/C0versus time) and the corresponding pseudo-first-order kinetic plots at different initial dye concentrations (5–50 mg L−1). A clear decrease in degradation rate was observed with increasing MB concentration. At lower dye concentrations, MB was rapidly degraded, whereas higher concentrations resulted in significantly slower degradation. The pseudo-first-order kinetic model provided a good linear fit for all concentrations (Fig. 6b), confirming that the degradation process followed apparent first-order kinetics under the studied conditions. The calculated rate constants decreased progressively with increasing dye concentration, indicating that pollutant loading plays a critical role in determining reaction kinetics.

Fig. 6. Effects of initial MB concentration and UiO-66 dosage on photocatalytic degradation under UV irradiation. (a) Time-dependent (Ct/C0) profiles and (b) pseudo-first-order kinetic plots for MB degradation at different initial dye concentrations. (c) MB degradation efficiency as a function of initial dye concentration. (d) Time-dependent (Ct/C0) profiles and (e) pseudo-first-order kinetic plots for MB degradation at different UiO-66 dosages. (f) MB degradation efficiency as a function of photocatalyst dosage.

Fig. 6

Open in a new tab

The corresponding pseudo-first-order rate constants were 0.0397, 0.0241, 0.0215, 0.0167, 0.0101, and 0.00655 min−1 for initial MB concentrations of 5, 10, 20, 30, 40, and 50 mg L−1, respectively. The decrease in photocatalytic efficiency with increasing MB concentration is further summarized in Fig. 6c. The degradation efficiency dropped from approximately 99.4% at 5 mg L−1 to 44% at 50 mg L−1, highlighting the adverse effect of high pollutant concentrations. This behavior can be attributed to two main factors. First, at higher MB concentrations, a larger fraction of the UiO-66 surface becomes occupied by dye molecules, which can reduce the availability of surface sites involved in photocatalytic reactions and promote charge-carrier recombination; consequently, the formation and utilization of reactive oxygen species, particularly hydroxyl radicals (˙OH), becomes less effective. Second, the higher dye concentration increases the color intensity of the solution and causes a stronger inner-filter (light screening) effect, which limits photon penetration into the bulk solution.51 As a result, fewer photons reach the catalyst surface, decreasing the generation of photogenerated electron–hole pairs and lowering the overall production of oxidizing species such as ˙OH. Therefore, the degradation rate and overall photodegradation efficiency decrease as the initial MB concentration increases.

3.4.3. Effect of photocatalyst dosage

The influence of UiO-66 dosage on the photodegradation was evaluated by varying the catalyst amount 5–40 mg under pH 4, while the MB concentration, light intensity, and other experimental parameters stayed constant. As shown in Fig. 6d, increasing the catalyst dose initially enhanced the degradation rate, reflecting the increased availability of active sites and improved light absorption. This increase in photocatalytic efficiency could be due to the increase of the photo catalyst active centers, thus increasing the concentration of generated degradation radicals.52 However, an additional increase in photocatalyst dose to 40 mg resulted in a minor decrease in the efficiency of the degradation. The decline in photocatalytic efficiency at excessive catalyst loadings is attributed to particle agglomeration and increased light scattering, which reduce photon penetration and active-site accessibility.53 Additionally, high catalyst loadings may hinder mass transfer and promote recombination of photogenerated electron–hole pairs, thereby counteracting the benefits of increased surface area. Kinetic analysis (Fig. 6e) demonstrated that the apparent rate constant increased with catalyst dose up to an optimal value (20 mg), beyond which it decreased. The corresponding pseudo-first-order rate constants were 0.0149, 0.0198, 0.0241, and 0.0116 min−1. The highest photocatalytic performance was obtained at a catalyst dosage of 20 mg, whereas a further increase to 40 mg resulted in a decrease in both degradation efficiency and the apparent kinetic rate constant. This trend is summarized quantitatively in Fig. 6f, where the maximum degradation efficiency (98%) was achieved at 20 mg of UiO-66.

3.4.4. Effect of reaction temperature

The effect of reaction temperature on the photocatalytic degradation of methylene blue (MB) over UiO-66 was studied at actual solution temperatures of 15, 25, 35 and 45 °C, with all other experimental conditions maintained constant. As shown in Fig. 7a, the rate of photocatalytic degradation increased slowly with rising temperature. The possible reason for this promotion is the acceleration of molecular mobility and mass transfer, facilitating the interactions between MB molecules and reactive species generated on the surface of photocatalyst. It is known that the kinetics of heterogeneous photocatalysis is generally influenced by temperature by altering the molecular mobility, mass transfer, surface interactions, and the rates of elementary reaction steps.54 The temperature-dependent degradation kinetics were evaluated using the pseudo-first-order kinetic model. The obtained kinetic parameters are summarized in Table 1.

Fig. 7. Effect of reaction temperature on the photocatalytic degradation of MB over UiO-66: (a) variation of (Ct/C0) with irradiation time at 15, 25, 35, and 45 °C; (b) Arrhenius plot of ln k versus (1/T); and Eyring plot of ln(k/T) versus (1/T).

Fig. 7

Open in a new tab
Table 1. Temperature-dependent pseudo-first-order kinetic parameters for the photocatalytic degradation of MB over UiO-66.
Temperature (K) k (min−1) (R2)
288.15 0.02028 0.9784
298.15 0.02411 0.9942
308.15 0.03273 0.9786
318.15 0.04515 0.9694
Open in a new tab

The temperature dependence of the reaction rate was further analyzed using the Arrhenius and Eyring equations. The Arrhenius analysis is one of the common methods used to describe the effect of temperature on the photocatalytic degradation kinetics and to determine the apparent activation energy.54,55 The Arrhenius equation can be written as:

ln k = ln A − Ea/RT 3

where A is the pre-exponential factor, Ea is the apparent activation energy, R is the universal gas constant, and T is the absolute temperature. As shown in Fig. 7b, the Arrhenius plot of ln k versus (1/T) showed good linearity R2 = 0.9764. The apparent activation energy was calculated to be 20.6 kJ mol−1 from the slope of the fitted line. The relatively low apparent activation energy is consistent with the observed increase of the photocatalytic degradation rate with increasing temperature.

The activation parameters were further determined using the Eyring equation:

3.4.4. 4

where kB is the Boltzmann constant, h is Planck's constant, ΔH* is the activation enthalpy, and ΔS* is the activation entropy. The Eyring plot of ln(k/T) versus (1/T) shown in Fig. 7b, showed satisfactory linearity (R2 = 0.9688). From the slope and intercept of the fitted line, the apparent activation enthalpy and activation entropy were calculated to be 18.0 kJ mol−1 and −249 J mol−1, respectively.

The corresponding activation Gibbs free energy was calculated using the following equation:

ΔG* = ΔH* − TΔS* 5

The calculated ΔG* at 298.15 K, was approximately 93.48 kJ mol−1. This positive value represents the free-energy barrier for the formation of the activated state. In general, the kinetic and activation-parameter analyzes indicate that the increase in reaction temperature increases the apparent photocatalytic degradation rate of MB over UiO-66.

3.5. Photocatalytic mechanism

To identify the main reactive species responsible for the photodegradation of MB, radical trapping experiments were performed using selective scavengers. The three main active species involved in heterogenous photocatalysis process are hydroxyl radicals (˙OH), photogenerated holes (h+) and superoxide radicals (˙O2−).56 Accordingly, EDTA-2Na, p-benzoquinone (PBQ), and isopropanol (IPA) were employed as scavengers for photogenerated holes (h+), superoxide radicals (˙O2−), and hydroxyl radicals (˙OH), respectively, to clarify the photocatalytic degradation pathway.57,58 As shown in Fig. 7b, the addition of IPA caused a significant decrease in MB degradation efficiency from about 97.3% to about 47%, suggesting that ˙OH radicals are the dominant oxidative species. PBQ and EDTA-2Na caused moderate reductions in degradation efficiency, to 80% and 90%, respectively, indicating that ˙O2− and h+ also contribute to the degradation, but play a secondary role.

Based on these results and photocatalytic behavior of UiO-66 under UV irradiation, a mechanism is proposed (Fig. 7a). Upon UV irradiation, UiO-66 is excited and generates electron–hole pairs through excitation of electrons from the valence band (VB) to conduction band (CB).59 In aqueous solution the photogenerated holes react with OH− ions to create ˙OH radicals, while the photogenerated electrons reduce dissolved oxygen (O2) to superoxide radical anions (˙O2−). The generated (˙O2−) can subsequently undergo further reduction and protonation to produce additional ˙OH radicals. The reactive radicals then attack the MB molecules, degrading it into CO2 and H2O as well as low toxicity products.60–62 The photocatalytic reaction steps can be summarized as it is shown below:

UiO-66 + hν → UiO-66(e− + h+) 6
h+ + H2O → ˙OH + H+ 7

h+ + OH− → ˙OH

e− + O2 → ˙O2− 8
˙OH/˙O2− + MB → CO2 + H2O + other products 9

3.6. Reusability and structure stability

Photocatalyst reusability is an important consideration when assessing commercial viability and environmental sustainability. Reproducibility experiments were carried out to evaluate the stability of the synthesized UiO-66 over five consecutive cycles of MB photodegradation. As described in Fig. 7c, the photocatalytic efficiency for MB degradation reached approximately 97.3% in the first cycle and remained high, at approximately 86% by the fifth cycle. This represents a relatively minor decrease in performance (12.2% total loss), which is comparable to some of previously reported results.63 The ability to maintain over 86% efficiency after five uses confirms that the zirconium-based framework (UiO-66) synthesized via neutral depolymerization maintains its structural integrity in aqueous environments.

The structural stability of UiO-66 was also confirmed by XRD measurement after five consecutive cycles. The characteristic diffraction peaks of UiO-66, especially the peaks at 2θ 7.6° and 8.5°, shown in Fig. 8d, are still detected after five photocatalytic cycles. The presence of these reflections confirms the crystalline framework of UiO-66 was well kept during the consecutive photocatalytic treatments. Finally, the similarity of the principal diffraction peaks before and after photocatalysis reveals the remarkable structural stability of UiO-66 for five successive applications.

Fig. 8. (a) Proposed photocatalytic degradation mechanism of methylene blue over UiO-66s. (b) Effect of different scavengers on MB degradation efficiency. (c) Reusability performance of the UiO-66 photocatalyst for methylene blue degradation under UV irradiation over five consecutive cycles. (d) XRD patterns of UiO-66 before photocatalysis and after five successive photocatalytic degradation cycles.

Fig. 8

Open in a new tab

4. Conclusion

This study reports a circular strategy for the conversion of post-consumer PET waste to a functional UiO-66 photocatalyst by neutral hydrothermal depolymerization and solvothermal MOF synthesis. The recovery of terephthalic acid from PET was confirmed by FTIR analysis. The formation of crystalline, nanoscale, high surface area UiO-66 was verified by PXRD, TEM/SAED and N2 adsorption–desorption study. The BET surface area of the prepared material was 822.2 m2 g−1, pore volume was 0.379 cm3 g−1 and average pore diameter was 2.93 nm. From optical and Mott–Schottky experiments, the band gap was found to be 3.51 eV, the material acts as an n-type semiconductor with flat band potential of approximately −0.68 V vs. Ag/AgCl. The produced UiO-66 from PET showed excellent photocatalytic activity for degradation of MB under UV irradiation, up to 97.3% removal under optimized experimental conditions (initial MB concentration = 10 mg L−1, UiO-66 dosage = 20 mg, and pH = 4), with the apparent rate constant of 0.024 min−1. The catalyst also retained 86% of its activity after five cycles of reuse. Scavenger experiments indicated that the predominant oxidative species were ˙OH radicals, with minor contributions from ˙O2− and h+. In conclusion, the work shows the potential of PET waste as a sustainable source of linkers for the synthesis of value-added MOF photocatalysts. Further improvements should include a wider range of pollutant testing, mineralization analysis, post-cycle structural characterization and benchmarking with state-of-the-art photocatalysts to further evidence practical applicability.

Author contributions

Samar H. Tawakey: conceptualization, methodology, investigation, formal analysis, data curation, visualization, and writing – original draft. Ali K. Abouseif: supervision. Ali H. Gemeay: supervision, formal analysis, visualization, and writing – review and editing. Mohamad M. Ayad: supervision, formal analysis, visualization, and writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgments

The authors gratefully acknowledge support from the Academy of Scientific Research & Technology (ASRT), Egypt, and the Czech Academy of Sciences, Czech Republic, through the Mobility Plus Project (ASTR-25-03). During the preparation of this manuscript, the authors used ChatGPT and QuillBot solely for language editing, paraphrasing, grammatical correction, and improvement of clarity. The authors critically reviewed and edited all AI-assisted content and take full responsibility for the accuracy, integrity, and final content of the manuscript.

Data availability

All data supporting the findings of this study are included within the article.

References

  1. UNESCO, Water for People, Water for Life: The United Nations World Water Development Report 1, UNESCO Publishing, Paris, 2003. https://unesdoc.unesco.org/ark:/48223/pf0000129556, accessed 20 March 2026 [Google Scholar]
  2. UNESCO, Water, A Shared Responsibility: The United Nations World Water Development Report 2, UNESCO Publishing, Paris, 2006. https://unesdoc.unesco.org/ark:/48223/pf0000143120, accessed 20 March 2026 [Google Scholar]
  3. Su J. Yuan M. Han L. Deng H. Chang J. Zhuang Y. Wang J. Zhang Y. Ultrathin metal organic framework nanosheets with rich defects for enhanced fluoride removal. Chem. Eng. J. 2023;451:138989. doi: 10.1016/j.cej.2022.138989. [DOI] [Google Scholar]
  4. Wang J.-S. Yi X.-H. Xu X. Ji H. Alanazi A. M. Wang C.-C. Zhao C. Kaneti Y. V. Wang P. Liu W. Yamauchi Y. Eliminating tetracycline antibiotics matrix via photoactivated sulfate radical-based advanced oxidation process over the immobilized MIL-88A: Batch and continuous experiments. Chem. Eng. J. 2022;431:133213. doi: 10.1016/j.cej.2021.133213. [DOI] [Google Scholar]
  5. Yip A. McArthur O. D. Ho K. C. Aucoin M. G. Ingalls B. P. Degradation of polyethylene terephthalate (PET) plastics by wastewater bacteria engineered via conjugation. Microb. Biotechnol. 2024;17:e70015. doi: 10.1111/1751-7915.70015. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Ohmura S. D. Muroi C. Sakata H. Wada M. Miyoshi N. Depolymerization of waste PET with phosphoric acid-modified silica gel under microwave irradiation. J. Polym. Environ. 2017;25:250–257. doi: 10.1007/s10924-016-0802-7. [DOI] [Google Scholar]
  7. Teke S. Saud S. Bhattarai R. M. Ali A. Nguyen L. Denra A. Nguyen D. B. Mok Y. S. Optimization of PET depolymerization for enhanced terephthalic acid recovery from commercial PET and post-consumer PET bottles via low-temperature alkaline hydrolysis. Chemosphere. 2024;365:143391. doi: 10.1016/j.chemosphere.2024.143391. [DOI] [PubMed] [Google Scholar]
  8. Stanica-Ezeanu D. Matei D. Natural depolymerization of waste poly(ethylene terephthalate) by neutral hydrolysis in marine water. Sci. Rep. 2021;11:4431. doi: 10.1038/s41598-021-83659-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Ma M. Wang S. Liu Y. Yu H. Yu S. Ji C. Li H. Nie G. Liu S. Insights into the depolymerization of polyethylene terephthalate in methanol. J. Appl. Polym. Sci. 2022;139:e52814. doi: 10.1002/app.52814. [DOI] [Google Scholar]
  10. Onwucha C. N. Ehi-Eromosele C. O. Ajayi S. O. Schaefer M. Indris S. Ehrenberg H. Uncatalyzed neutral hydrolysis of waste PET bottles into pure terephthalic acid. Ind. Eng. Chem. Res. 2023;62:6378–6385. doi: 10.1021/acs.iecr.2c04117. [DOI] [Google Scholar]
  11. Singh S. Sivaram N. Nath B. Khan N. A. Singh J. Ramamurthy P. C. Metal organic frameworks for wastewater treatment, renewable energy and circular economy contributions. npj Clean Water. 2024;7:124. doi: 10.1038/s41545-024-00408-4. [DOI] [Google Scholar]
  12. Maponya T. C., Makgopa K., Modibane K. D. and Hato M. J., Metal–organic frameworks for heavy metal removal from water, in Metal–Organic Framework Nanocomposites: from Design to Application, ed. A. Khan, M. Jawaid, A. M. A. Asiri, W. Ni and M. M. Rahman, CRC Press, Boca Raton, 2020, pp. 13–52, 10.1201/9780429346262-2 [DOI] [Google Scholar]
  13. Ahmadijokani F. Molavi H. Rezakazemi M. Tajahmadi S. Bahi A. Ko F. Aminabhavi T. M. Li J.-R. Arjmand M. UiO-66 metal-organic frameworks in water treatment: A critical review. Prog. Mater. Sci. 2022;125:100904. doi: 10.1016/j.pmatsci.2021.100904. [DOI] [Google Scholar]
  14. Dyosiba X. Ren J. Musyoka N. M. Langmi H. W. Mathe M. Onyango M. S. Feasibility of varied polyethylene terephthalate wastes as a linker source in metal-organic framework UiO-66(Zr) synthesis. Ind. Eng. Chem. Res. 2019;58:17010–17016. doi: 10.1021/acs.iecr.9b02205. [DOI] [Google Scholar]
  15. Zhu M.-X. Lee L. Wang H.-H. Wang Z. Removal of an anionic dye by adsorption/precipitation processes using alkaline white mud. J. Hazard. Mater. 2007;149:735–741. doi: 10.1016/j.jhazmat.2007.04.037. [DOI] [PubMed] [Google Scholar]
  16. Li W. Mu B. Yang Y. Feasibility of industrialscale treatment of dye wastewater via bio-adsorption technology. Bioresour. Technol. 2019;277:157–170. doi: 10.1016/j.biortech.2019.01.002. [DOI] [PubMed] [Google Scholar]
  17. Wang L. Li Z. Chen J. Huang Y. Zhang H. Qiu H. Enhanced photocatalytic degradation of methyl orange by porous graphene/ZnO nanocomposite. Environ. Pollut. 2019;249:801–811. doi: 10.1016/j.envpol.2019.03.071. [DOI] [PubMed] [Google Scholar]
  18. Wang C.-C. Li J.-R. Lv X.-L. Zhang Y.-Q. Guo G. Photocatalytic organic pollutants degradation in metal-organic frameworks. Energy Environ. Sci. 2014;7:2831–2867. doi: 10.1039/C4EE01299B. [DOI] [Google Scholar]
  19. Yang F. He X. Xin T. Yang H. Bai L. Gao L. Wang Y. A series of polymer-supported polyoxometalates as heterogeneous photocatalysts for degradation of organic dye. Molecules. 2023;28:3968. doi: 10.3390/molecules28093968. [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Lad V. N. and Murthy Z. V. P., Advanced materials for photocatalytic applications: The challenge ahead, in Handbook of Smart Photocatalytic Materials: Fundamentals, Fabrications and Water Resource Applications, ed. C. M. Hussain and A. K. Mishra, Elsevier, 2020, pp. 3–8 [Google Scholar]
  21. Mansouri M. Sadeghian S. Mansouri G. Setareshenas N. Enhanced photocatalytic performance of UiO-66-NH2/TiO2 composite for dye degradation. Environ. Sci. Pollut. Res. 2021;28:25552–25565. doi: 10.1007/s11356-020-12098-9. [DOI] [PubMed] [Google Scholar]
  22. Meskelu K. Taddesse A. M. Teju E. Bezu Z. Enhanced photocatalytic performance of ternary g-C3N4/UiO-66/Ag3VO4 composite for methylene blue degradation under visible light irradiation. Results Chem. 2024;11:101786. doi: 10.1016/j.rechem.2024.101786. [DOI] [Google Scholar]
  23. Bakatula E. N. Richard D. Neculita C. M. Zagury G. J. Determination of point of zero charge of natural organic materials. Environ. Sci. Pollut. Res. 2018;25:7823–7833. doi: 10.1007/s11356-017-1115-7. [DOI] [PubMed] [Google Scholar]
  24. Campanelli J. R. Cooper D. G. Kamal M. R. Catalyzed hydrolysis of polyethylene terephthalate melts. J. Appl. Polym. Sci. 1994;53:985–991. doi: 10.1002/app.1994.070530801. [DOI] [Google Scholar]
  25. Chen J., Li Z., Liu G. and Ni P., Hydrothermal depolymerization of waste PET, in Proceedings of the 2010 4th International Conference on Bioinformatics and Biomedical Engineering, IEEE, 2010, 10.1109/ICBBE.2010.5517887 [DOI] [Google Scholar]
  26. Nason A. K. Jerozal R. T. Milner P. J. Suntivich J. Reactive crystallization via metal-organic-framework formation enables separation of terephthalic acid from textile impurities. ACS Sustain. Chem. Eng. 2023;11:18–22. doi: 10.1021/acssuschemeng.2c05496. [DOI] [Google Scholar]
  27. McMurry J., Organic Chemistry, Cengage Learning, Boston, 9th edn, 2016 [Google Scholar]
  28. Chinglenthoiba C. Mahadevan G. Zuo J. Prathyumnan T. Valiyaveettil S. Conversion of PET bottle waste into a terephthalic acid-based metal-organic framework for removing plastic nanoparticles from water. Nanomaterials. 2024;14:257. doi: 10.3390/nano14030257. [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Reusch W., Infrared spectroscopy, Michigan State University, https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/spectrpy/infrared/irspec1.htm, accessed 8 May 2026 [Google Scholar]
  30. Miller L. M. and Coates J. P., Interpretation of infrared spectra: A practical and systematic approach, in Encyclopedia of Analytical Chemistry, ed. R. A. Meyers, John Wiley & Sons, 2025, pp. 1–24, 10.1002/9780470027318.a5606.pub2 [DOI] [Google Scholar]
  31. Awaja F. Pavel D. Recycling of PET. Eur. Polym. J. 2005;41:1453–1477. doi: 10.1016/j.eurpolymj.2005.02.005. [DOI] [Google Scholar]
  32. Xu J. Wang B. Wang Y. Yang J. Li C. Electron interaction-enhanced photocatalytic degradation of tetracycline over Fe/UiO-66. Chemistry. 2025;7:197. doi: 10.3390/chemistry7060197. [DOI] [Google Scholar]
  33. Xie W. Yang X. Hu P. Cs2.5H0.5PW12O40 encapsulated in metal-organic framework UiO-66 as heterogeneous catalysts for acidolysis of soybean oil. Catal. Lett. 2017;147:2772–2782. doi: 10.1007/s10562-017-2189-z. [DOI] [Google Scholar]
  34. Semivrazhskaya O. O. Salionov D. Clark A. H. Casati N. P. M. Nachtegaal M. Ranocchiari M. Bjelić S. Verel R. van Bokhoven J. A. Sushkevich V. L. Deciphering the mechanism of crystallization of UiO-66 metal-organic framework. Small. 2023;19:2305771. doi: 10.1002/smll.202305771. [DOI] [PubMed] [Google Scholar]
  35. Zhu L. Zhang D. Xue M. Li H. Qiu S. Direct observations of the MOF (UiO-66) structure by transmission electron microscopy. CrystEngComm. 2013;15:9356–9359. doi: 10.1039/C3CE41122B. [DOI] [Google Scholar]
  36. Singh S. Sharma S. Umar A. Jha M. Mehta S. K. Kansal S. K. Nanocuboidal-shaped zirconium based metal organic framework for the enhanced adsorptive removal of nonsteroidal anti-inflammatory drug, ketorolac tromethamine, from aqueous phase. New J. Chem. 2018;42:1921–1930. doi: 10.1039/C7NJ03851H. [DOI] [Google Scholar]
  37. Liao X. Wang X. Wang F. Yao Y. Lu S. Ligand modified metal organic framework UiO-66: A highly efficient and stable catalyst for oxidative desulfurization. J. Inorg. Organomet. Polym. Mater. 2021;31:756–762. doi: 10.1007/s10904-020-01808-y. [DOI] [Google Scholar]
  38. Hendrickx K. Vanpoucke D. E. P. Leus K. Lejaeghere K. Van Yperen-De Deyne A. Van Speybroeck V. Van Der Voort P. Hemelsoet K. Understanding intrinsic light absorption properties of UiO-66 frameworks: A combined theoretical and experimental study. Inorg. Chem. 2015;54:10701–10710. doi: 10.1021/acs.inorgchem.5b01593. [DOI] [PubMed] [Google Scholar]
  39. Ghobadi N. Band gap determination using absorption spectrum fitting procedure. Int. Nano Lett. 2013;3:2. doi: 10.1186/2228-5326-3-2. [DOI] [Google Scholar]
  40. Andrade P. H. M. Volkringer C. Loiseau T. Tejeda A. Hureau M. Moissette A. Band gap analysis in MOF materials: Distinguishing direct and indirect transitions using UV-vis spectroscopy. Appl. Mater. Today. 2024;37:102094. doi: 10.1016/j.apmt.2024.102094. [DOI] [Google Scholar]
  41. Gayathri K. Vinothkumar K. Teja Y. N. Al-Shehri B. M. Selvaraj M. Sakar M. Balakrishna R. G. Ligand-mediated band structure engineering and physiochemical properties of UiO-66 (Zr) metal-organic frameworks (MOFs) for solar-driven degradation of dye molecules. Colloids Surf., A. 2022;653:129992. doi: 10.1016/j.colsurfa.2022.129992. [DOI] [Google Scholar]
  42. Gelderman K. Lee L. Donne S. W. Flat-band potential of a semiconductor: Using the Mott-Schottky equation. J. Chem. Educ. 2007;84:685. doi: 10.1021/ed084p685. [DOI] [Google Scholar]
  43. Sakai N. Ebina Y. Takada K. Sasaki T. Electronic band structure of titania semiconductor nanosheets revealed by electrochemical and photoelectrochemical studies. J. Am. Chem. Soc. 2004;126:5851–5858. doi: 10.1021/ja0394582. [DOI] [PubMed] [Google Scholar]
  44. Rescigno R. Sacco O. Venditto V. Fusco A. Donnarumma G. Lettieri M. Fittipaldi R. Vaiano V. Photocatalytic activity of P-doped TiO2 photocatalyst. Photochem. Photobiol. Sci. 2023;22:1223–1231. doi: 10.1007/s43630-023-00363-y. [DOI] [PubMed] [Google Scholar]
  45. Nasikhudin M. D. Kusumaatmaja A. Triyana K. Study on photocatalytic properties of TiO2 nanoparticle in various pH condition. J. Phys.: Conf. Ser. 2018;1011:012069. doi: 10.1088/1742-6596/1011/1/012069. [DOI] [Google Scholar]
  46. Salvador P. Influence of pH on the potential dependence of the efficiency of water photo-oxidation at n-TiO2 electrodes. J. Electrochem. Soc. 1981;128:1895. doi: 10.1149/1.2127760. [DOI] [Google Scholar]
  47. Kim H.-G. Choi K. Lee K. Lee S. Jung K.-W. Choi J.-W. Controlling the structural robustness of zirconium-based metal organic frameworks for efficient adsorption on tetracycline antibiotics. Water. 2021;13:1869. doi: 10.3390/w13131869. [DOI] [Google Scholar]
  48. Kim J. Y. Kang J. Cha S. Kim H. Kim D. Kang H. Choi I. Kim M. Stability of Zr-based UiO-66 metal-organic frameworks in basic solutions. Nanomaterials. 2024;14:110. doi: 10.3390/nano14010110. [DOI] [PMC free article] [PubMed] [Google Scholar]
  49. Bůžek D. Demel J. Lang K. Zirconium metal-organic framework UiO-66: Stability in an aqueous environment and its relevance for organophosphate degradation. Inorg. Chem. 2018;57:14290–14297. doi: 10.1021/acs.inorgchem.8b02360. [DOI] [PubMed] [Google Scholar]
  50. Nakabayashi Y. Nosaka Y. The pH dependence of OH radical formation in photo-electrochemical water oxidation with rutile TiO2 single crystals. Phys. Chem. Chem. Phys. 2015;17:30570–30576. doi: 10.1039/C5CP04531B. [DOI] [PubMed] [Google Scholar]
  51. Zhang D. Lv S. Luo Z. A study on the photocatalytic degradation performance of a [KNbO3]0.9-[BaNi0.5Nb0.5O3−δ]0.1 perovskite. RSC Adv. 2020;10:1275–1280. doi: 10.1039/C9RA07310H. [DOI] [PMC free article] [PubMed] [Google Scholar]
  52. Li Y. Jin Z. Zhao T. Performance of ZIF-67-derived fold polyhedrons for enhanced photocatalytic hydrogen evolution. Chem. Eng. J. 2020;382:123051. doi: 10.1016/j.cej.2019.123051. [DOI] [Google Scholar]
  53. Davis R. J. Gainer J. L. O'Neal G. Wu I.-W. Photocatalytic decolorization of wastewater dyes. Water Environ. Res. 1994;66:50–53. doi: 10.2175/WER.66.1.8. [DOI] [Google Scholar]
  54. Tran H. D. Nguyen D. Q. Do P. T. Tran U. N. P. Kinetics of photocatalytic degradation of organic compounds: a mini-review and new approach. RSC Adv. 2023;13:16915–16925. doi: 10.1039/D3RA01970E. [DOI] [PMC free article] [PubMed] [Google Scholar]
  55. Nisar A. Saeed M. Muneer M. Usman M. Khan I. Synthesis and characterization of ZnO decorated reduced graphene oxide (ZnO-rGO) and evaluation of its photocatalytic activity toward photodegradation of methylene blue. Environ. Sci. Pollut. Res. 2022;29:418–430. doi: 10.1007/s11356-021-13520-6. [DOI] [PubMed] [Google Scholar]
  56. Sha Z. Chan H. S. O. Wu J. Ag2CO3/UiO-66(Zr) composite with enhanced visible-light promoted photocatalytic activity for dye degradation. J. Hazard. Mater. 2015;299:132–140. doi: 10.1016/j.jhazmat.2015.06.016. [DOI] [PubMed] [Google Scholar]
  57. Wang Y. L. Zhang S. Zhao Y. F. Bedia J. Rodriguez J. J. Belver C. UiO-66-based metal organic frameworks for the photodegradation of acetaminophen under simulated solar irradiation. J. Environ. Chem. Eng. 2021;9:106087. doi: 10.1016/j.jece.2021.106087. [DOI] [Google Scholar]
  58. Elmorsy E. S. Amer W. A. Mahrous A. Ayad M. M. Insight into the novel ZIF-8@N-CQDs/ZIF-67 nanocomposite for photocatalytic degradation of methylene blue under visible light irradiation. Mater. Sci. Eng. B. 2023;298:116900. doi: 10.1016/j.mseb.2023.116900. [DOI] [Google Scholar]
  59. Mimouni R. Askri B. Larbi T. Amlouk M. Meftah A. Photocatalytic degradation and photo-generated hydrophilicity of methylene blue over ZnO/ZnCr2O4 nanocomposite under stimulated UV light irradiation. Inorg. Chem. Commun. 2020;115:107889. doi: 10.1016/j.inoche.2020.107889. [DOI] [Google Scholar]
  60. Shibata H. Ogura Y. Sawa Y. Kono Y. Hydroxyl radical generation depending on O2 or H2O by a photocatalyzed reaction in an aqueous suspension of titanium dioxide. Biosci., Biotechnol., Biochem. 1998;62:2306–2311. doi: 10.1271/bbb.62.2306. [DOI] [PubMed] [Google Scholar]
  61. Li S. Yang S. Liang G. Yan M. Wei C. Lu Y. Regulation and photocatalytic degradation mechanism of a hydroxyl modified UiO-66 type metal organic framework. RSC Adv. 2023;13:5273–5282. doi: 10.1039/D3RA00004D. [DOI] [PMC free article] [PubMed] [Google Scholar]
  62. Khan I. Saeed K. Zekker I. Zhang B. Hendi A. H. Ahmad A. Ahmad S. Zada N. Ahmad H. Shah L. A. Shah T. Khan I. Review on methylene blue: Its properties, uses, toxicity and photodegradation. Water. 2022;14:242. doi: 10.3390/w14020242. [DOI] [Google Scholar]
  63. Aboraia A. M. Al-omoush M. Solayman M. Saad H. M. H. Khabiri G. Saad M. Alsulaim G. M. Soldatov A. V. Ismail Y. A. M. Gomaa H. A heterostructural MoS2QDs@UiO-66 nanocomposite for the highly efficient photocatalytic degradation of methylene blue under visible light and simulated sunlight. RSC Adv. 2023;13:34598–34609. doi: 10.1039/D3RA06299F. [DOI] [PMC free article] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

All data supporting the findings of this study are included within the article.

Articles from RSC Advances are provided here courtesy of Royal Society of Chemistry

RESOURCES