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Article

Eco-Friendly Synthesis of TiO2/ZIF-8 Composites: Characterization and Application for the Removal of Imidacloprid from Wastewater

by
Lucija Bogdan
1,*,
Ana Palčić
2,*,
Marina Duplančić
1,
Mirela Leskovac
1 and
Vesna Tomašić
1
1
Faculty of Chemical Engineering and Technology, University of Zagreb, Marulićev Trg 19, 10000 Zagreb, Croatia
2
Institute Ruđer Bošković, Bijenička Cesta 54, 10000 Zagreb, Croatia
*
Authors to whom correspondence should be addressed.
Processes 2023, 11(3), 963; https://doi.org/10.3390/pr11030963
Submission received: 7 February 2023 / Revised: 13 March 2023 / Accepted: 15 March 2023 / Published: 21 March 2023
(This article belongs to the Special Issue Catalysis in Advanced Oxidation Processes for Environmental Remediation)
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Abstract

:
The aim of this work was to develop hybrid TiO2/ZIF-8 photocatalysts and test their activity for the removal of agricultural pollutants in water. The hybrid photocatalysts were prepared by an innovative method involving hydrothermal synthesis at 150 °C using a mechanochemically synthesized zeolitic imidazolate framework (ZIF-8) and titanium tetraisopropoxide as a titanium dioxide (TiO2) precursor. Three composite photocatalysts with different mass fractions of titanium dioxide (5, 50, and 95 wt%) were synthesized and characterized, and their adsorption and photocatalytic properties investigated for the removal of imidacloprid. The equilibrium adsorption test showed that ZIF-8 is a good adsorbent and can adsorb 65% of the model component under the working conditions used in this work, while the hybrid photocatalysts can adsorb 1–3% of the model component. It is assumed that the adsorption is hindered by the TiO2 layer on the surface of ZIF-8, which blocks the interactions of ZIF-8 and imidacloprid. A significant decrease in band gap energies (3.1–3.6 eV) was observed for the hybrid TiO2/ZIF-8 photocatalysts compared to the values obtained with ZIF-8 (5 eV), depending on the mass fractions of TiO2. The highest removal efficiency of imidacloprid was achieved with the hybrid photocatalysts containing 5 wt% TiO2.

1. Introduction

Due to the continuous increase in the concentration of organic pollutants, especially pesticides, in water ecosystems, there is a great need to find technology for efficient treatment of industrial and municipal wastewater. Concerns are particularly growing due to the presence of the widely used insecticide, imidacloprid, and neonicotinoids in general, in unintentionally treated areas in the form of dust generated by the operation of seed sowing machines, thereby affecting non-target beneficial insects [1,2]. Also, as it is soluble in water, imidacloprid has been observed in lakes and streams which presents a danger for various types of aquatic organisms [2,3,4,5]. Numerous studies have been conducted with the aim of studying the consequences of the use of imidacloprid on various species of bees and ladybugs. The studies showed an increase in mortality, damage to nervous system tissues, vision, and smell, as well as slowed growth and development of these insects. Moreover, drastic changes in their reproductive behavior were reported [6,7,8]. In addition, the systemic nature of imidacloprid enables the distribution of its metabolites within the plant, and its presence was detected in the nectar and pollen of some plants [2,9,10,11,12,13,14].
Over the past ten years metal-organic frameworks (MOFs) have received considerable attention as a research hotspot in both adsorption and photocatalysis [15,16,17,18,19]. It is worth highlighting the great chemical and functional adaptability of MOFs, which results from the use of different organic ligands, metals, and metal clusters, as well as different preparation methods and crystallization conditions. Various procedures for the synthesis of MOFs are described in the literature (e.g., hydrothermal, solvothermal, mechanical, microwave, electrochemical, diffusion, ion thermal syntheses, etc.) [20,21]. The application of MOFs in the photocatalytic decomposition of organic pollutants and NOx, antibacterial agents, solar fuels and for obtaining photoelectrochemical (PEC) energy further emphasizes the impressive potential of such materials [19,22,23,24,25,26,27,28,29]. Significant advances have also been made in the field of zeolites and advanced materials with a large specific surface area and uniform structure, which opens new possibilities for the preparation of composite photocatalysts and further improvement of the photocatalytic activity of existing photocatalysts whose constituent part is titanium dioxide [29,30,31,32]. The zeolitic imidazolate framework-8 (ZIF-8 or Zn (II)-imidazolate) consists of zinc (Zn) as a metal center and 2-methylimidazole (2-MIM) as organic linker. With Zn–N bond, ZIF-8 is one of the best known examples of stable framework structures in aqueous solutions [33,34]; it possesses higher chemical and thermal stability compared to other MOFs [35,36] and, in addition, its unique property is its superior adsorption capacity [37]. According to the opinion of some authors, the combination of ZIF-8 with appropriate semiconductor material reduces the recombination rate of charge carriers and enhances photocatalytic activity [38,39], thus making it a favorable candidate for utilization in photocatalytic processes.
Photocatalysis using titanium dioxide is a well-established method of decomposition of organic pollutants, yet the required activation by UV radiation presents a severe disadvantage that limits its use [40,41,42]. The attention of researchers in the field of photocatalysis is particularly focused on the innovative design of photocatalysts with the aim of solving problems related to band gap energy, charge transfer, and inhibition of photocatalytic reaction by charge recombination. The photocatalytic activity of existing photocatalysts, especially TiO2, can be improved with various strategies, such as using organic and inorganic compounds as photosensitizers (dye sensitization), combining semiconductor materials of different energy levels, and doping/decoration with metals or nonmetals to inhibit the recombination rate and thus increase the quantum yield [43,44,45,46]. In the scope of extensive research in this area, various methods of doping titanium dioxide with nonmetals [47,48,49] or other photoactive metals [50,51,52,53,54] have been developed, as well as other TiO2 modification procedures aimed at reducing the band gap as well as the rate of charged carrier recombination either by irradiating the surface with UV light [55] or by influencing the charge of the surface [56,57,58,59,60,61,62]. An expanding strategy of enhancing photoactivity is synthesizing composite materials by combining existing photocatalytic materials, such as TiO2, with zeolites and mesoporous materials, as well as MOFs, thus taking advantage of the complementary benefits of different materials [63].
This study provides a simple and effective strategy for the rational design and fabrication of MOF-derived samples using an unconventional synthesis route as well as an environmentally friendly method of mechanochemical synthesis for the preparation of composite or hybrid materials that can be used for the low-cost and effective removal of a neonicotinoid insecticide (imidacloprid) from wastewater, combining the advantages of adsorption and photocatalysis utilizing TiO2–the best known semiconductor photocatalyst so far–and ZIF-8, a representative of crystalline metal-organic framework materials, i.e., a relatively new and still insufficiently studied group of porous materials with extremely large application possibilities in various fields [64,65,66,67,68,69]. Photocatalysts based on MOFs can be considered third-generation photocatalysts [70]. However, the majority of photocatalytic degradation studies using MOFs are focused on the photodegradation of organic dyes [71]. To the best of our knowledge, this is the first report utilizing mechanochemically synthesized ZIF-8 and the hybrid TiO2/ZIF-8 materials as possible photocatalytically active substances for pesticide removal.

2. Materials and Methods

2.1. Photocatalyst Synthesis

The synthesis of the photocatalyst was carried out in two stages. In the first stage, the metal-organic framework material ZIF-8 was synthesized mechanochemically, and in the second stage, hydrothermal synthesis was used to prepare the hybrid TiO2/ZIF-8.
The ZIF-8 material was synthesized using the procedure proposed by Martinez et al. [64]. The mechanochemical synthesis lasted 55 min and was carried out in two steps. In the first part, 2-methylimidazole (Sigma Aldrich, St. Louis, MO, USA, 99%) and zinc oxide (Ventron, Mumbai, India, 99.9%) were added in two polymethylmethacrylate grinding jars in a mass ratio of 2:1. Two grinding balls made of zirconium dioxide with a diameter of 7 mm were added to the containers. The jars were closed, secured against the powder leakage, and placed in the Retsch MM 400 ball mill, Germany. The first phase of grinding was carried out for 10 min at a milling frequency of 30 Hz. After 10 min, the mill was stopped to add a few drops of ethanol (Gram Mol, Zagreb, Croatia, abs.) and several crystals of ammonium nitrate (TTT, Novaki, Croatia, p.a.) to the homogenized mixture of reactants. After this, the grinding vessels were again secured against powder leakage, the second stage of the mechanochemical synthesis lasted for 45 min at a grinding frequency of 30 Hz. At the end of the synthesis, the resultant powder was recovered from the milling jar ready for the characterization of the product.
Hydrothermal synthesis was used to prepare a TiO2/ZIF-8 photocatalyst by employing autoclave-in-autoclave synthesis approach [69]. The ZIF-8 prepared by mechanochemical treatment was mixed in ethanol with titanium tetraisopropoxide (Acros, Geel, Belgium, 98%) at 300 rpm overnight. The prepared mixture was transferred to a 50 mL autoclave Teflon liner. Distilled water (6 mL) was placed in a 250 mL Teflon-lined steel autoclave and a 50 mL liner and the reaction mixture placed inside. Hydrothermal synthesis of the composite TiO2/ZIF-8 photocatalyst was carried out overnight at 150 °C. Three composite photocatalysts with different mass fractions of titanium dioxide were synthesized, e.g., CAT-1 (5% TiO2), CAT-2 (95% TiO2) and CAT-3 (50% TiO2). The system was cooled to room temperature and the product was washed with distilled water to neutral pH, which was followed by indicator paper. The resulting powdery product was dried in an oven at 65 °C.

2.2. Characterization of Photocatalysts

The crystallinity of ZIF-8 and synthesized dry powder catalysts was determined by X-ray diffraction (XRD) on Malvern PANalytical X’Pert Pro Multipurpose Powder Diffractometer, UK, under CuKα radiation (λ = 1.54 Å) at 40 kV and 8 mA. The infrared (IR) spectra was determined using the FrontierTM FT-IR Spectrometer within the wavenumber 400 cm−1 and 4000 cm−1. The band gap for mechanochemically synthesized ZIF-8 and hybrid catalysts was determined by using Tauc’s method from the Kubelka-Munk function obtained by the EvolutionTM 350 Uv-Vis Spectrometer, Thermo Fisher, with a scan speed of 600 nm min−1. The temperature stability of the samples was determined by thermogravimetric and calorimetric analysis (TGA-DSC). The TGA curves were determined by heating ~10 mg of the sample from 25 to 900 °C with a heating speed of 10 °C min−1 using TA Instruments Q500, USA, while the DSC curves were obtained on the Mettler Toledo DSC 823e by using two heating cycles, first from 25 to 105 °C and second from 25 to 250 °C (10 °C min−1). The morphology and particle size of the powders were examined by a Tescan Vega 3 Easyprobe, Czech Republic, scanning electron microscope (SEM). The samples were steamed with gold and palladium. The particle size was also determined using laser diffraction on Mastersizer 2000, Malvern, UK, with particle refractive index (RI) 2.741 and dispersant RI 1.330.

2.3. Adsorption and Photocatalytic Experiments

The photocatalytic removal of imidacloprid (Sharda Cropchem Limited, Mumbai, India) was carried out in a suspension reactor using an aqueous solution of imidacloprid with an initial concentration of 10 mg L−1, a total reaction volume of 80 mL, and a photocatalyst mass of 80 mg at a constant mixing speed of 250 rpm. The irradiation source of the UV-A radiation was the Pen-Ray lamp, Upland, CA model 90-0019-01 with λmax = 365 nm.
At the beginning and the end of the reaction, the following parameters were determined: pH value using a pH meter, and dissolved oxygen concentration measured with an oximeter. The initial pH value of the imidacloprid solution was ~5.3, and the desired pH value of 6.5 was achieved using a 0.01 M sodium hydroxide (VWR International, Leuven, Belgium) solution. The solution was mixed with a magnetic stirrer at room temperature at a speed of 250 rpm.
The photocatalytic removal of imidacloprid was carried out in a total period of 240 min. Before the photocatalytic process began, the reaction mixture was stirred for 30 min in the absence of light radiation to allow the adsorption equilibrium of imidacloprid molecules on the surface of the catalyst particles. After that, the lamp was inserted into the middle of the suspension reactor using a quartz tube. The photocatalytic reaction was monitored by taking aliquots of the reaction mixture (~0.5 mL) at certain time intervals (15, 30 and 60 min) and measuring the change in imidacloprid concentration using high performance liquid chromatography (HPLC) instrument (Shimadzu, Japan) equipped with a UV-Vis detector (254 nm). Separation and quantification were performed on an Agilent Zorbax C18 column (250 × 4.6 mm; 5 m, Santa Clara, CA, USA in gradient elution mode using two mobile phases of water and acetonitrile containing 0.3 v% formic acid at a flow rate of 1.0 cm3 min−1. In the initial period of the reaction, when the largest concentration changes were expected, measurements were performed every 15 min, then every 30 min and, after the 120th min of the reaction, every 60 min. Aliquots of the reaction mixture were filtered on a polyvinyl difluoride (PVDF) syringe microfilter (0.22 μm) and stored in ampoules for HPLC analysis. The change in the imidacloprid conversion and the evolution of the peak area of the photocatalytic transformation products detected by the LC-QToF-MS technique as a function of irradiation time for degradation over modified TiO2 has been reported in previous work [55]. Since the photoactivity of pure ZIF-8 was not observed in the current study, we assume that a similar reaction mechanism can be attributed to the composites, as for TiO2.
As mentioned, preliminary measurements included testing the equilibrium adsorption of the model component on the surface of the photocatalyst particles in an identical experimental system. The adsorption test was conducted in the absence of light i.e., with the photoreactor covered with aluminum foil.

3. Results and Discussion

The XRD patterns of the prepared hybrid samples were compared with the patterns of the mechanochemically synthesized ZIF-8 and commercial TiO2 P25 obtained on the same device (Figure 1). It was found that the pattern of ZIF-8 is consistent with the literature data, with characteristic peaks at 2θ = 7.44°; 10.46°; 12.78°; 14.9°; 16.5° and 18° [64,65,66,67,68,69,72,73,74,75]. The newly formed hybrid composite catalysts CAT-1 and CAT-3 show a dominant ZIF-8 structure with characteristic peaks at 7.44°, 10.46° and 12.78°. The intense peak at 7.44° indicates that it is a highly crystalline form of ZIF-8 and the resulting composite. Indications of the peaks corresponding to the structure of TiO2, specifically anatase in the structure of CAT-1 and CAT-2 composites, are also visible.
The synthesized CAT-2 catalyst largely coincides with the structure of TiO2, more precisely with the structure of anatase with characteristic peaks at 25.28°, 37.93° and 48.34°. Such results were expected considering that the said hybrid predominantly contained TiO2 (95%).
In Figure 2 the IR spectra of the synthesized composite photocatalysts with mechanochemically synthesized ZIF-8 is presented. In the “fingerprint” area (inset), the bands of CAT-1, CAT-3 and CAT-2 represent an overlap of ZIF-8 and TiO2 bands depending on the corresponding proportion of each material in the samples. The small peaks at 3135 and 2929 cm−1 are associated with C-H stretching vibrations [76]. The absorption peaks between 1300 and 1500 cm−1 can be assigned to the entire ring stretching [77,78]. The peaks at 1147 and 995 cm−1 can be attributed to a C-N stretch mode [77,79], whereas the characteristic peaks at 760 cm−1 and 700 cm−1 could be assigned to a C-H bending mode and the ring out-of-plane bending vibration of the 2-methylimidazole (2-MIM), respectively [77]. Linear vibrations of the TiO2 molecule exhibit bands in the region between 700 cm−1 and 500 cm−1, yet are difficult for detection by FT-IR spectroscopic analysis as can be seen in the respective spectra of CAT-2 (95% TiO2) and TiO2. In addition, as the content of TiO2 in CAT-1 and CAT-3 rises, the transmittance value of the ZIF-8 bands in that region decreases and the bands become less resolved. The FT-IR spectrum for pure TiO2, shown in Figure 2, is in good agreement with earlier reports [80,81,82,83].
Infrared spectroscopy was also used to gain insight into the adsorption of imidacloprid from a model solution on the surface of the composite catalysts. In the Figures S1 and S2, in Supplementary Materials, the IR spectra of ZIF-8, CAT-1, CAT-2 and CAT-3 are shown after application in the systems for testing adsorption and photocatalytic properties. Similarity of the spectra of used materials and the as-prepared materials indicates that during adsorption and during photocatalysis there is no change in the structure of the composite catalysts. The only difference is the presence of the band at 1310 cm−1 in the used samples so it can be assigned to imidacloprid. The highest intensity of this band for pure ZIF-8 sample suggests the largest amount of insecticide is adsorbed on the ZIF-8 sample.
The span of the band gap energy, Eg, is a very important parameter that is considered when examining the applicability of a photocatalyst in photocatalytic processes regarding wavelength range. Herein, the value of the energy gap was estimated using Tauc’s method. It should be stressed that ZIF-8 is not a photocatalytically active substance. Mechanochemically synthesized ZIF-8 shows an energy gap value of 5.0 eV (Figure 3, inset). During the hydrothermal synthesis, a layer of titanium dioxide was applied to the surface of ZIF-8, which led to a reduction of the band gap energy of the resulting composites. It can be noted that the size of the band gap depends on the mass fraction of the photocatalytically active component, i.e., TiO2. The addition of 5% titanium dioxide to the surface of ZIF-8 results in a reduction of the band gap from 5.0 eV to 3.6 eV, with 50% TiO2 to 3.5 eV, and with 95% TiO2 to 3.1 eV.
Thermogravimetric analysis (TG/TGA) and scanning calorimetric analysis (DSC) were performed to determine the thermal stability of the prepared samples. Thermogravimetric analysis (Figure S3A–H, Supplementary Materials) of all samples was carried out using TA Q500 in a stream of air and a stream of nitrogen, and the samples were heated from room temperature (~25 °C) to 900 °C.
Thermogravimetric analysis showed that both ZIF-8 and the hybrid catalysts are thermally stable and applicable in a wide range of temperatures up to 400 °C. Due to the high thermal stability of the inorganic components of the analyzed samples, it is assumed that water and smaller organic molecules remaining after the synthesis were removed in the early stages of the analysis, followed by the decomposition of ZIF-8 and 2-methylimidazole.
Next, a differential scanning calorimetry (DSC) analysis of the samples was carried out with two heating cycles (10 °C /min), the first from room temperature to 105 °C to remove excess water and organic matter remaining from the synthesis, and the second from room temperature to 250 °C. As can be seen in Figure S4 in Supplementary Materials, the DSC curves of all samples contain peaks that indicate an endothermic process, i.e., heat absorption. As it is an endothermic process, the area under the peak corresponds to the change in the latent heat of melting (ΔHm) and is proportional to the amount of crystalline matter in the sample. Given that the expected dependence of the increase in crystallinity of the sample and the melting temperature depending on the mass fraction of TiO2 was not observed, it is assumed that the crystallinity of the sample depends on the quality of the mechanochemically synthesized ZIF-8. Some researchers suggest that ZIF-8 recrystallizes in water which can cause the results to differ [74,75].
The morphology and size of the particles were examined by SEM and laser light diffraction. In Figure 4 the morphology of the samples obtained by SEM is shown. Particles of ZIF-8 and composite ZIF-8/TiO2 photocatalysts form agglomerates of different sizes, and it can be observed that larger agglomerates are formed in systems with more TiO2 (CAT-2 and CAT-3). Meanwhile, CAT-1 and ZIF-8 form agglomerates of smaller sizes with characteristic dimensions up to approximately 20 μm; CAT-3 particles form agglomerates of 20–50 μm, while the dimensions of agglomerates in CAT-2 are greater than 50 μm.
The number-based particle size distribution is given in Figure 5. As the proportion of titanium dioxide in the photocatalyst gets higher, the particle size is shifted towards lower values and the width of the particle size distribution decreases. The largest proportion of particles in the ZIF-8 sample was observed between 0.357 μm and 0.485 μm, in CAT-1: 0.416–0.564 μm, in CAT-2: 0.278–0.377 μm, and in CAT-3: 0.285–0.377 μm. It can be observed that larger agglomerates are found in the samples containing particles of smaller sizes.
After the characterization of the samples, their adsorption and photocatalytic properties were tested in the system for the removal of the neonicotinoid insecticide, imidacloprid. Adsorption properties were tested in a suspension reactor in the absence of light, while photocatalytic properties were tested in an identically prepared suspension reactor, with a centrally located Pen-Ray lamp as a source of UVA radiation. The ZIF-8 sample proved to be an extremely good adsorbent. After 120 min of conducting the experiment, it adsorbed 65% of the model component, and equilibrium adsorption of imidacloprid was achieved after approximately 30 min (Figure 6). On the other hand, the synthesized hybrid photocatalysts showed poor adsorption properties. In 120 min of conducting the experiment, only 1–3% of the model component was adsorbed. A possible reason for the reduced adsorption efficiency is the surface layer of titanium dioxide that blocks the active centers of ZIF-8 as well as the particle agglomeration.
Figure 6 illustrates the imidacloprid adsorption over mechanochemically synthesized ZIF-8 and hybrid TiO2/ZIF-8 samples. The results obtained show that the adsorption capacity of ZIF-8 at an initial imidacloprid concentration of 1 g L−1 is 0.3842 mg g−1, while the adsorption capacities of the composite samples are compatible with the TiO2 content in the samples, i.e., the sample containing 5% TiO2 (CAT-1) showed the highest adsorption capacity of 0.030 mg g−1, followed by CAT-3 (50% TiO2) with 0.0016 mg g−1, while CAT-2, which contained 95% TiO2, showed very low adsorption capacity. The results obtained are consistent with our expectations (e.g., the pure ZIF-8 exhibits the highest adsorption capacity) and as the TiO2 weight fraction increased, the adsorption capacity of the hybrid samples decreased, which can be attributed to the low specific surface area of TiO2.
Table S1 in Supplementary Materials gives an overview of the adsorption capacities of some MOF composites found in the literature and a comparison with the results obtained in this study [76,84,85,86,87,88].
After establishing the adsorption equilibrium, the reaction mixture was irradiated with a Pen-Ray lamp working at 365 nm. Figure 7 illustrates the changes of the normalized imidacloprid concentrations (CA/CA0) with irradiation time and testing the obtained results on the assumed model. The kinetic analysis was performed for a conventional batch reactor (closed system), assuming a pseudo-first-order kinetic model of reaction and root mean square deviations (RSMD) used as a correlation criterion between the experimental and theoretical data predicted by the proposed kinetic model. The estimated values of the pseudo-first-order rate constant (kA) based on the experimental results using the method of non-linear optimization and the RSMD are presented in Table 1. From the values of RMSD, it can be concluded that a good agreement of the experimental results with the values obtained to the assumed model has been achieved. The reaction rate constants, derived from the first-order kinetic curves (Figure 7) for the photodegradation of imidacloprid from the highest to the lowest, are given in the order of CAT-1 (9.129 × 10−4 min−1), CAT-2 (7.790 × 10−4 min−1) and CAT-3 (3.371 × 10−4 min−1) catalysts.
It is difficult to compare the kinetic results obtained with those in the literature due to either the lack of reported information or due to the different reaction conditions and photocatalysts applied. Examples dealing with photocatalytic degradation of imidacloprid or some other neonicotinoids via MOF-based composites are scarce and mostly refer to a different type of potential photocatalysts and various reaction conditions [89,90,91].
Figure 8 shows changes in pH and dissolved oxygen concentration during the photocatalytic process. The decrease in pH can be attributed to the formation of mineral salts during photocatalysis. During the photocatalytic removal of imidacloprid there is a decrease in the concentration of dissolved oxygen, which is in line with expectations. It is well known that the presence of oxygen reduces the recombination of charge carriers, i.e., oxygen as an electron acceptor with electrons of the conduction band (CB) can lead to the formation of superoxide anion radicals (O2−), which play an extremely important role in the photodegradation mechanism of neonicotinoids.
Surprisingly, during the photocatalytic process there was an increase in the value of total organic carbon (not shown here). A possible explanation is that some of the organic molecules remained confined in the ZIF-8 samples during synthesis, which blocks the access of imidacloprid molecules to the catalytically active centers. A potential solution is the drying of hybrid photocatalysts during their preparation at slightly higher temperatures compared to the conditions used in this work (65 °C), especially since the thermal analysis showed a high thermal stability of the prepared samples. Nevertheless, it is observed that when ZIF-8 and the hybrid ZIF-8/TiO2 are applied, the imidacloprid concentration decreases significantly, which is reflected in the imidacloprid removal efficiencies ranging from 8.4% to 55.1% (Figure 9).
For the reusability experiments (Figure 10), the most active photocatalyst (CAT-1) was recovered by centrifugation at 600 rpm for 5 min, and the supernatant discarded. The photocatalyst was then dried at 65 °C for 6 h. The recovered powder was then poured into a fresh solution and used under the same conditions as in the previous photocatalytic experiments. After four cycles of measurement, the removal efficiency of imidacloprid slightly decreased with each cycle after 240 min of irradiation, which can be attributed to a slight loss of material during recycling, i.e., transferring from the centrifuge tube to the reaction vessel. The obtained results indicate the high stability of the photocatalyst under the specified operating conditions, which is particularly important for its practical application.

4. Conclusions

This work deals with the preparation, detailed characterization, and application of hybrid photocatalysts for the removal of imidacloprid as a representative of agricultural pollutants. Hybrid photocatalysts were prepared by an innovative method that combines the advantages of mechanochemical and hydrothermal synthesis. The aim of the syntheses was to combine TiO2 as a proven photocatalytically active component and ZIF-8 as representative of zeolitic imidazolate frameworks that possess unique structural, textural and physico-chemical properties and, therefore, are promising materials for the preparation of the hybrid photocatalysts. Our intention was to improve the adsorption capacity and to reduce the band gap energies which, as expected, would result in a shift of the working area of the hybrid TiO2/ZIF-8 photocatalyst towards the wavelengths of the visible region of solar radiation. A detailed characterization was carried out using different methods, such as XRD, infrared spectroscopy, UV/Vis spectroscopy, thermogravimetry, DSC analysis, SEM and the laser diffraction method.
The obtained results show that hybrid TiO2/ZIF-8 materials with favorable adsorption properties and reduced band gap energies compared to ZIF-8 can be successfully prepared by using the method described in this study. Although the prepared hybrid photocatalysts showed only moderate and still insufficient activity, we believe that this work provides useful guidelines for the design and development of new hybrid photocatalysts with great potential for visible light application for environmental remediation.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr11030963/s1, Figure S1: IR spectra of the samples after imidacloprid adsorption, Figure S2: IR spectra of the samples after photocatalytic testing, Figure S3: TGA curves of the samples in nitrogen (A, C, E, G) and air (B, D, F, H) for mechanochemically synthesized ZIF-8 (A, B), CAT-1 (C, D), CAT-2 (E, F) and CAT-3 (G, H), Figure S4: DSC curves of the studied series of samples; Table S1: An overview of the adsorption capacities of MOF composites found in literature.

Author Contributions

Conceptualization, A.P., M.D. and V.T.; methodology, A.P. and M.D.; investigation, L.B. and M.D.; resources, A.P. and V.T.; data curation, L.B., A.P., M.D. and M.L.; writing-original draft preparation, L.B.; writing-review and editing, A.P. and V.T.; visualization, L.B. and M.D.; supervision, A.P. and V.T.; funding acquisition, A.P. and V.T. All authors have read and agreed to the published version of the manuscript.

Funding

The research was funded by the Croatian Science Foundation under the projects IN-PhotoCat (IP-2018-01-8669) and SMALLPORE (UIP-2019-04-4977).

Data Availability Statement

The data presented in this study are available upon reasonable request from the corresponding authors.

Conflicts of Interest

The authors declare no conflict of interest.

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Processes 11 00963 g001 550
Figure 1. XRD patterns of the studied series of samples.
Figure 1. XRD patterns of the studied series of samples.
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Figure 2. IR spectra of the studied series of samples.
Figure 2. IR spectra of the studied series of samples.
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Figure 3. UV-Vis diffuse reflectance spectra and the Kubelka–Munk function of the studied samples (inset).
Figure 3. UV-Vis diffuse reflectance spectra and the Kubelka–Munk function of the studied samples (inset).
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Figure 4. SEM images of the samples in 1kx (A,C,E,G) and 5kx (B,D,F,H) magnification for ZIF-8 (A,B), CAT-1 (C,D), CAT-2 (E,F) and CAT-3 (G,H).
Figure 4. SEM images of the samples in 1kx (A,C,E,G) and 5kx (B,D,F,H) magnification for ZIF-8 (A,B), CAT-1 (C,D), CAT-2 (E,F) and CAT-3 (G,H).
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Figure 5. Particle size distribution of the prepared samples.
Figure 5. Particle size distribution of the prepared samples.
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Figure 6. Adsorption of imidacloprid on mechanochemically synthesized ZIF-8 and on hybrid TiO2/ZIF-8 samples.
Figure 6. Adsorption of imidacloprid on mechanochemically synthesized ZIF-8 and on hybrid TiO2/ZIF-8 samples.
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Figure 7. Pseudo-first-order kinetic curves of the imidacloprid photodegradation.
Figure 7. Pseudo-first-order kinetic curves of the imidacloprid photodegradation.
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Figure 8. Changes in pH and dissolved oxygen concentration after photocatalytic tests (240 min).
Figure 8. Changes in pH and dissolved oxygen concentration after photocatalytic tests (240 min).
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Figure 9. Comparison of the imidacloprid removal efficiency on different samples.
Figure 9. Comparison of the imidacloprid removal efficiency on different samples.
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Figure 10. Reusability of CAT-1 for the photocatalytic removal of imidacloprid.
Figure 10. Reusability of CAT-1 for the photocatalytic removal of imidacloprid.
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Table
Table 1. Estimated values of the reaction rate constant (kA) and the corresponding root mean square deviations (RMSD) for the kinetic model of pseudo-first-order.
Table 1. Estimated values of the reaction rate constant (kA) and the corresponding root mean square deviations (RMSD) for the kinetic model of pseudo-first-order.
SamplekA (×104), min−1RMSD × 103
CAT-19.1293.144
CAT-27.7906.092
CAT-33.3711.309
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Bogdan, L.; Palčić, A.; Duplančić, M.; Leskovac, M.; Tomašić, V. Eco-Friendly Synthesis of TiO2/ZIF-8 Composites: Characterization and Application for the Removal of Imidacloprid from Wastewater. Processes 2023, 11, 963. https://doi.org/10.3390/pr11030963

AMA Style

Bogdan L, Palčić A, Duplančić M, Leskovac M, Tomašić V. Eco-Friendly Synthesis of TiO2/ZIF-8 Composites: Characterization and Application for the Removal of Imidacloprid from Wastewater. Processes. 2023; 11(3):963. https://doi.org/10.3390/pr11030963

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Bogdan, Lucija, Ana Palčić, Marina Duplančić, Mirela Leskovac, and Vesna Tomašić. 2023. "Eco-Friendly Synthesis of TiO2/ZIF-8 Composites: Characterization and Application for the Removal of Imidacloprid from Wastewater" Processes 11, no. 3: 963. https://doi.org/10.3390/pr11030963

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