Phytomediated synthesis of WO₃ nanoparticles using Solanum lycopersicum fruit extract for enhanced photocatalytic activity of 2,4-dichlorophenol

It is increasingly recognized that the environmental implications of nanotechnologies should be understood better before further technological development takes place. The toxic organic chemicals discharged from various industrial processes and human activities contaminate the global water supply, which is a serious problem for living beings. Heterogeneous photocatalysis (one kind of advanced oxidation process (AOP)) has been considered a cost-effective alternative for the purification of dye-containing wastewater (Grzechulska & Morawski 2002; So et al. 2002). It has been proposed as a promising process to remove pollutants from air and water streams due to the possibility of organic pollutants’ oxidation to CO₂ and H₂O in the presence of semiconductors (Su et al. 2012; Affam & Chaudhuri 2013; Cybula et al. 2014; Zhao et al. 2014; Saxena et al. 2016; Handojo et al. 2020; Hamza et al. 2023; Fatima et al. 2024; Shakoor et al. 2024; Tayyab et al. 2024). This process is very useful for removing highly toxic and hazardous organic pollutants that are miscible in water and difficult to remove through conventional methods.

Aromatic compounds are common pollutants in the waste effluent discharged by many factories and industry processes, including chemical plants, petroleum refineries, phenolic resins, caprolactam textile facilities, and some pharmaceutical processes (Li et al. 2009; Wang et al. 2009). Among the aromatic compounds, phenolic products are toxic to humans and aquatic organisms and are listed as one of the most common and serious environmental contaminants in water (Li et al. 2009). Due to the toxicity and consequent health hazards of phenolic compounds, these products have been placed on the list of priority pollutants by the US Environmental Protection Agency (Zhou et al. 2011). Chlorophenols are widely generated from a number of industrial manufacturing processes, such as pesticide, paint, solvent, pharmaceutical, wood-preserving chemicals, coke-oven, and pulp industries (Kilic & Cinar 2008). These compounds pose severe problems to the environment because they are carcinogens and mutagens. For example, 2,4-dichlorophenol (2,4-DCP) may cause some pathological symptoms and changes in human endocrine systems. Their mode of exposure is through the skin and the gastrointestinal system (Ranjit et al. 2008; Jia et al. 2012). They have been used extensively in many industrial products such as petrochemicals, pharmaceuticals, dyes, pulps, pesticides, and paints (Hameed et al. 2008; Zhang et al. 2010; Zhou et al. 2010; Ren et al. 2011; Chung et al. 2013); therefore, chlorophenols are common chloroaromatic pollutants (Olaniran & Igbinosa 2011). Removing these contaminants from water is a significant challenge because of ever-increasing pollution and the shortage of high-quality fresh water (Liu et al. 2012). The removal of these hazardous organic pollutants has become necessary and important for environmental safety.

Most of the methods used for the transformation of wastes and pollutants and treating wastewater are physical, chemical, or biological. Chemical transformations involve the application of reagents and reaction conditions to transform and treat target species. Moreover, recent studies have demonstrated that photocatalysis can be used to mineralize organic compounds or degrade dyes under UV irradiation. Tungsten oxide (WO₃), an important n-type semiconductor, has received wide attention owing to its promising application for electrochromic and photochromic devices, secondary batteries, and photocatalysts (Hong et al. 2009; Ghazal et al. 2022; Matalkeh et al. 2022). In addition, WO₃ possesses a small band gap energy falling within the solar spectrum (about 2.7–2.8 eV for WO₃), which implies that it has potential applications under visible light in the environmental field. However, the photocatalytic activity of WO₃ under visible light is limited due to the drawback that WO₃ has a conduction band (CB) edge lying in a position unfavourable for single-electron reduction of O₂, leading a poor photocatalyst for organic degradation under O₂ conditions. The photocatalytic and sensing applications of several types of WO₃ nanomaterials have been reported, which show great potential for human health, environmental protection and remediation, and energy conversion (Inoue et al. 1995; Bamwenda & Arakawa 2001). However, several fundamental issues must be addressed before they are economically viable for large-scale industrial applications. For example, pure WO₃ nanomaterials are usually not efficient photocatalysts because of the high electron–hole recombination rate and the difficulty in the reduction of oxygen. Owing to the high activation energy of reaction with gas molecules, the high work temperature and slow response time also restrict their gas sensing applications (Ozkan Zayim et al. 2003). Due to the wide range of applications, nanoparticles (NPs) of WO₃ have been synthesized by various experimental techniques, such as pyrolysis (Pokhrel et al. 2009), thermal decomposition (Zhao & Miyauchi 2008), sol–gel (Zheng et al. 2011), colloidal process (Sun et al. 2000), and ion-exchange methods (Lu et al. 2002).

Biogenic synthesis is useful not only because of its reduced environmental impact (Shankar et al. 2004; Anastas & Zimmerman 2007; Dahl et al. 2007) compared with some of the physicochemical production methods but also because it can be used to produce large quantities of nanoparticles that are free of contamination and have a well-defined size and morphology. Biosynthetic routes can actually provide nanoparticles of a better-defined size and morphology than some of the physicochemical methods of production. Compared with the use of whole plant extracts and plant tissue, the use of plant extracts for making nanoparticles is simpler. Plant extract–mediated synthesis is an increasing focus of attention. Processes for making nanoparticles using plant extracts are readily scalable and may be less expensive compared with the relatively expensive methods based on microbial processes. Plant extracts may act both as reducing agents and stabilizing agents in the synthesis of nanoparticles. The source of the plant extract is known to influence the characteristics of the nanoparticles. This is because different extracts contain different concentrations and combinations of organic reducing agents.

Tomatoes, or Solanum lycopersicum, are a healthy, nutrient-rich vegetable that have a broad range of anti-disease properties. The main ingredient in many meals is the tomato. The plant is native to South West America and is a member of the nightshade family, Solanaceae. Tomatoes include lycopene as well as a number of phenolic chemicals, flavonoids, and vitamins that have strong antioxidant properties. Plants are essential to the development of novel medications. As reducing agents, pro-oxidant metal complexes, free radical scavengers, and singlet oxygen production quenchers, antioxidants are crucial in the prevention of human diseases (Matei et al. 2021; Trombino et al. 2021). Therefore, S. lycopersicum has numerous highly reactive chemical functions that use reducing agents and stabilizing agents in the synthesis of nanoparticles.

However, to date there has been no report on the synthesis and photocatalytic behaviour of biosynthesis of citric acid (CA)–modified WO₃ nanoparticles using S. lycopersicum fruit extract for the degradation of 2,4-dichlorophenol. In this study, we explore the use of a conventionally produced composite material made of biosynthesis of WO₃ nanoparticles modified with CA using S. lycopersicum for degradation of a 2,4-DCP solution. We aim to determine the optimal amount of composite material and the most suitable method for realizing the most efficient and cost-effective water treatment process. Distinguished visible-light-driven photocatalytic properties for organic pollutant decomposition are observed for the doped WO₃ nanoparticles. The results will shed light on the approach to exploit the superior performance of the materials.

S. lycopersicum fruit juice was extracted from fresh tomatoes and centrifuged for 10 min to separate the solid matter. The S. lycopersicum fruit extract was added into 50 mL of 0.3 M sodium tungstate dehydrate (Na₂WO₄.2H₂O) solution and continuously stirred. CA was added to the abovementioned solution dropwise. The stirring condition was maintained for 2 h until the colour of the solution changed from white to yellow. The solid precipitate obtained was purified by repeated centrifugation at 10,000 rpm for 10 min. It was then dried in an air hot oven at 80 °C overnight and thereafter calcined at 400 °C for 4 h in a muffle furnace. The final bio-CA/WO₃ product was coded as BCAWO and bare WO₃ was coded as WO.

Here C₀ is the concentration of dye before irradiation and C is the concentration of dye after a certain irradiation time.

The synthesized nanoparticles were characterized by the following methods. UV-vis diffuse reflectance spectrum (UV-vis DRS) measurements were carried out in a JASCO V-550 double beam spectrophotometer with a photomultiplier tube (PMT) detector equipped with an integrating sphere assembly, using BaSO₄ as a reference sample. The powder X-ray diffraction patterns were measured in an X-ray diffractometer (XPERT PRO X-ray) using Cu Kα irradiation at 25 °C. Structural assignments were made with reference to the JCPDS powder diffraction files. The surface morphology was examined using scanning electron microscopy (SEM-JSM 6701F- 6701) in both secondary and backscattered electron modes and the elemental analysis was also detected by an energy dispersive X-ray spectroscopy (EDS) attached to the SEM. The absorption spectra were obtained using a UV-Vis spectrophotometer (JASCO-V-530). The surface morphology was also analysed by transmission electron microscopy (TECNAI G2 model). The functional group was predicted by a JASCO-4200 Fourier transform infrared spectrometer (FT-IR).

The HRTEM image of BCAWO suggested that the sample has a well-defined crystalline structure, which is crucial for excellent photocatalytic activity. The images clearly show the presence of secondary material capping with a thickness of nm, which may be assigned to bioorganic compounds present in the fruit extract. Figure 4(c) shows the SAED pattern of BCAWO, which reveals the polycrystallinity of the prepared sample.

Five different adsorbent dosages (0.5, 1, 1.5, 2, and 2.5 g) were studied for the removal of 2,4-DCP at a fixed initial concentration of 100 mg/L. Adsorbent dosage plays a significant role in the percentage of 2,4-DCP removal. It was clearly evident from the results that a gradual increase in the adsorbent dose leads to high percentages of 2,4-DCP removal. For each type of adsorbent, an adsorbent dose of 2.5 g showed the highest 2,4-DCP removal capacity, whereas 2,4-DCP removal efficiency was negligible for 0.5 g adsorbent dose. This may be due to increased adsorbent surface area and the availability of more adsorption sites resulting from the increased adsorbent dosage. This factor contributes to greater efficiency of 2,4-DCP removal. So, removal efficiency increases up to the optimum dosage beyond which removal efficiency decreases. This indicates that 2,4-DCP removal was effective in the dose ranges studied. The observed effect of catalyst loading on phenol degradation by solar photocatalysis is that although the number of active sites in solution increases with catalyst loading, a point appears to be reached where light penetration is compromised because of excessive particle concentration, resulting in optimum catalyst loading at which the degradation is the maximum.

Successful application of the photocatalytic oxidation system requires the investigation of the dependence of the photocatalytic degradation rate on the substrate concentration. The effect of initial 2,4-DCP concentration on photocatalytic degradation of phenol was studied with different initial 2,4-DCP concentrations of 10, 30, 50, 70, and 100 mg/L with initial pH of 4 and with BCAWO loading of 2.5 g/L.  Figure 7(c) shows that the percentage degradation of 2,4-DCP decreased with increase in initial concentration of 2,4-DCP. With the fixed catalyst loading, when the initial phenol concentration increases, since the generation of OH⁻ does not increase, the probability of 2,4-DCP molecules to react with ⁻OH decreases, therefore, the rate of 2,4-DCP degradation decreases. With 100 mg/L initial 2,4-DCP concentration, the degradation did not initiate for the first few min, showing the delay in photocatalysis. The delay may be owing to the generation of reactive hydroxyl radicals. At a higher initial 2,4-DCP concentration, the 2,4-DCP molecules present in the solution are higher leading to a higher number of collisions between the 2,4-DCP molecules themselves and thus the molecules compete for active sites. This competitive effect may lead to the displacement of 2,4-DCP molecules nearer to the catalyst surface by other colliding 2,4-DCP molecules, thus giving less time for them to adsorb on the surface active sites. Such a phenomenon would have resulted in delayed initiation of photocatalysis at high initial 2,4-DCP concentrations.

An attractive feature of photocatalytic materials is that the inherent structure of the catalyst is stable towards oxidative decomposition and that the photocatalyst can be facilely separated from the dispersion by simple centrifugation after reaction, which facilitates the reuse of the catalyst. In addition, as mentioned previously, the release of degraded intermediates from the catalyst surface can also prevent the catalyst surface from blocking and poisoning by the intermediates. In our experiments, the stability and reusability of the BCAWO were examined by repetitive use of the catalyst. The solution resulting from the photocatalytic degradation of 2,4-DCP was filtered and washed, and the photocatalyst was dried. The dried catalyst samples were used for the degradation of 2,4-DCP, employing similar experimental conditions. Under the present investigation, it was observed that the dissolution of the catalyst was found to be negligible. Catalyst samples showed considerably reproducible photocatalysis activity up to five cycles for the degradation of 2,4-DCP.

The literature (Sherlya et al. 2014; Nezamzadeh-Ejhieh & Ghanbari-Mobarakeh 2015; Hoseini et al. 2017; Ali et al. 2020; Gnanasekaran et al. 2021; Moja et al. 2024) presents photodegradation studies utilizing various photocatalysts for the removal of 2,4-DCP in aqueous solutions. Table 2 summarizes the findings of this comparison with previously published reports. When compared to earlier studies (Table 2), it can be concluded that the time needed for maximum removal of 2,4-DCP was shorter, at 180 min.

Improvement of these simple techniques to utilize solar radiation more efficiently could provide more economic solutions for the degradation of organic compounds such as 2,4-DCP from water. However, due to the periodic availability of this source of energy, it cannot be used for continuous treatment of wastewater. Under such conditions, solar energy may be used during daytime and non-monsoon periods. Another alternative is to store wastewater generated during the night and treat it during daytime using a fluidized bed reactor operated in batch with recycle mode, so as to handle large quantities of wastewater. Thus, the BCAWO nanoparticles can be effectively used for the treatment of 2,4-DCP-contaminated water by solar photocatalysis. It is also evident that the reaction takes place via the formation of singlet oxygen, superoxide, and hydroxyl radicals. This methodology has an additional advantage of using the visible light of solar energy for the degradation of organic pollutants in water. Further studies to apply this synergistic technique using solar energy are underway.

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

The authors declare there is no conflict.