Sonocatalytic removal of naphthalene from an aqueous solution using ZnO nanoparticles

Water covers almost 70% of the earth's surface. Still, only a tiny fraction of this can be used for domestic purposes. Recycling and reusing water is a better option for accomplishing the water demand, so wastewater treatment has become indispensable to fulfil the water demand worldwide (Suresh et al. 2021) for agricultural, industrial, and domestic usage (Kumari et al. 2018; Suresh et al. 2018; Gupta et al. 2021).

Different forms of polycyclic aromatic hydrocarbons (PAHs) are found in the environment due to the incomplete combustion of fossil fuel, wood, coal, tar, and grasses. Some PAH compounds are manufactured in the industry for various applications which might enter into the groundwater and surface water (Gupta et al. 2021). The degradation of PAHs is complex because they have low solubility and their concentration in drinking water standards is in the range of 20 μg/l (U.S. Environmental Protection Agency 1986).

According to the US Environment Protection Agency (USEPA) (U.S. Environmental Protection Agency 1986), there are 16 PAH priority pollutants based on their toxicities, and one of them is naphthalene. In fossil fuels, naphthalene exists naturally, and when they are burnt, naphthalene is released into the atmosphere (Jia & Batterman 2010). Naphthalene is also used as a carbamate insecticide, surface-active agent, resin, dye intermediate, synthetic tanning agent, a moth repellent, and also as a raw material for various organic chemicals (Agency for Toxic Substances and Disease Registry (ATSDR) 1995; U.S. Environmental Protection Agency 1998). Naphthalene exposure can produce adverse effects on human health.

There are different techniques available for the degradation of PAHs, of which two basic approaches are the biological and chemical methods. These methods mainly contain degradation steps along with phase-separation steps (Gupta et al. 2021). Biodegradation is a conventional method used to degrade environmental pollutants (Ukiwe et al. 2013). Degradation of PAHs by chemical processes also depends on many factors, such as molecular structure, molecular weight, and temperature. Sometimes, surfactants are used to increase the PAH's solubility to improve the degradation efficiency (Pandey et al. 2017).

Conventional methods are not sufficient for the complete mineralization of environmental pollutants. Advanced oxidation processes (AOPs) are a valuable option for degradation purposes because they can oxidize a wide range of contaminants. AOPs are mainly based on the generation of oxidants for the oxidation of organic compounds. This water purification technique is environmental-friendly, and in this method, there is no transfer of pollutants from one phase to another (Sharma et al. 2011; Miklos et al. 2018).

There is reactive oxygen generation in AOPs such as sonolysis, Fenton, UV-based processes, and ozonation. These AOPs have some drawbacks (Schrank et al. 2004; Ramesh Raja & Suresh 2011; Vincenzo Naddeo 2013; Brienza & Katsoyiannis 2017; Mishra et al. 2017; Gupta et al. 2021). Ozonation involves high operating costs, high energy consumption, and requires process safety measures. The Fenton-based process is limited to acidic conditions, which boost operating costs through chemical use in adjusting the pH and iron sludge formation.

Recent research is focused on sonochemical techniques because of their ease of operation and OH^• radical generation in the absence of any added chemicals. Ultrasound technology is a versatile and environment-friendly method (Khadhraouia et al. 2021). The sonochemical method uses ultrasound for degradation purposes (Pang et al. 2011; Sathishkumar et al. 2016; Gupta et al. 2021). This process can degrade different types of organic pollutants. Gupta et al. (2021) reviewed extensively the sonochemical degradation of PAHs. Hybrid techniques involving sonochemistry and other AOPs are increasingly used. Sponza & Oztekin (2010) studied PAH degradation in a petrochemical industry wastewater using TiO₂-based sonochemical treatment. They found that the maximum PAH removal was 97% at an optimum condition of 20 mg/l of Fe²⁺, 60 °C and 150 min of sonication time.

Different photocatalysts have been used for degrading organic compounds in an ultrasonic reactor such as TiO₂, zinc oxide (ZnO), Sn nanoparticles, Er³⁺:Y₃Al₅O₁₂/TiO_2. Composite, and GaN:ZnO (Wang et al. 2009; Kou et al. 2010; Sponza & Oztekin 2010, 2011; Vela et al. 2012; Wang et al. 2014). ZnO nanoparticles have been widely used for degradation of pollutants from waste stream because of their luminescent materials, good photocatalytic property, and large free exciton binding energy so that excitonic emission processes can persist at or even above room temperature (Hisatomi et al. 2014; Roy et al. 2016; Bharati & Suresh 2017; Chouchene et al. 2017; Kusiak-Nejman et al. 2021). ZnO particles also have environmental advantages such as low corrosion, waste minimization, and recycling of the catalyst. Karavasilis & Tsakiroglou (2021) investigated methylene blue dye removal onto ZnO-coated glass beads under a UV irradiation light source. They found maximum removal of methylene blue dye. Mustapha et al. (2020) reviewed synthesis methods for TiO₂ and ZnO particles, and their nanocomposites for wastewater treatment. They found ZnO anchored onto clay are good promising sequesters and have been explored for wastewater remediation. Roy & Chakraborty (2021) used ZnO nanorods for photodegradation of methylene blue and found 65% degraded of the initial concentration within 50 min.

The main sources of several PAHs are petroleum spills, fires, and unconverted fossil fuels. Therefore, it is necessary to evaluate the competitive degradation of PAHs under both equal and non-equal proportion mixtures and also to quantify the interference and interaction of one component with the degradation of the other. No information is available in the literature for the naphthalene degradation and simultaneous with other PAHs from an aqueous solution in the presence of ZnO particles.

In this work, sonocatalytic degradation of naphthalene was carried out in the presence of ZnO particles. The effect of process parameters such as the mass of the ZnO catalyst, voltage, temperature, and time on the naphthalene degradation rate was studied. The present paper also aims to apply the response surface methodology (RSM) design to carry out experiments on the degradation rate using ZnO particles. An analysis of variance (ANOVA) has been performed for the raw and noise data in order to identify the significant parameters affecting the degradation rate, and their effects on the response characteristics have been quantified.

All the chemicals and reagents used were of analytical reagent grade from SRL Pvt. Ltd (Indore, India). All solutions were prepared using double distilled water (DDW).

The catalyst was prepared using the thermal decomposition method as described by Razmara et al. (2019). 0.136 g of zinc chloride was mixed with 20 ml of water and stirred at room temperature for 1 h. An ethanolic solution combined with 10 ml of potassium thiocyanate (extra pure, ACS, 99%) was mixed with an aqueous solution of quinoline, which was prepared by mixing 0.258 g of quinoline in 10 ml of water. A mixture of the above solutions was sonicated for 30 min, and then the solvent was evaporated to obtain colourless ZnO powder. ZnO was kept in a muffle furnace at 650 °C for 5 h. ZnO in the form of white powder was obtained and kept in a desiccator until further use.

All experiments were carried out in an ultrasonic horn type reactor with a tip diameter of 5 mm–1.5 cm (Leelasonic, Mumbai, India). The tip of the horn was submerged in the liquid. The reactor was operated at a frequency of 20 kHz and a maximum power dissipation of 260 W. The aqueous solutions of naphthalene were prepared by adding the required quantity of naphthalene in water. About 50 ml of the naphthalene solution was taken into a beaker and subjected to sonication. The experiments were carried out in the absence and presence of the ZnO catalyst. The effect of temperature on the naphthalene degradation was studied using a hot plate assisted with the ultrasonic reactor. The samples were analysed by high-performance liquid chromatography (HPLC, Water, Bangalore, India) to quantify the amount of naphthalene before and after sonication.

Table 1 shows the chromatographic conditions of the HPLC analysis used in this study. The retention time of naphthalene was 7.405 min under the experimental conditions described above. The experiments were repeated thrice, and the error bar of three consecutive replicates was ∼5%. Scanning electron microscopy (SEM) with energy-dispersive atomic X-ray (EDAX) spectroscopy (JEOL Model, Japan) was used to determine the morphological patterns along with the surface chemical composition of ZnO. Furthermore, an Brunauer, Emmett, and Teller (BET) surface area analyzer was used for determining the surface area and pore volume. Fourier Transform Infra-Red (FTIR) Spectroscopy (Shimadzu, Model: IR Affinity-IS, Japan) analysis was performed between 500 and 4,000 cm⁻¹ to determine different functional groups in the ZnO catalyst.

The elemental composition of the ZnO catalyst was as follows: Zn 25.18%, O 37.03%, Mg 3.20%, K 0.83%, Ca 4.05%, N 5.07%, Na 4.13%, Cl 3.43%, and Fe 0.32%. From the SEM analysis, the particle size of the ZnO catalyst ranged from 28 (±0.5) to 30 (±0.9) nm (data not shown). The surface area was found to be 82.43 m²/g. Razmara et al. (2019) reported the surface area of ZnO nanoparticles to be 76.34 m²/g.

This is mainly due to the catalyst surface with the bubble–liquid interfacial region, oxidation of OH• radicals with bubble-enhanced ZnO surface (Adewuyi 2001), which progressively degrades the catalyst surface filled with naphthalene molecules interlocked with the bubble-detached mechanism. From the results of Psillakis et al. (2004), degradation of PAHs was not successfully completed by using sonolysis. This was mainly due to the detrimental effect on the initial degradation rates and required some matrix components or catalyst-related materials to enhance the degradation of PAHs. Furthermore, with a lower initial concentration of naphthalene, degradation was less and slow due to the hydrophobic and semi-volatile characteristics of PAHs (Manoli & Samara 1999). Synthetic ZnO was more effective in removing reactive red dye than commercial ZnO (Varasanyi 1974) after 60 min. Psillakis et al. (2004) reported a higher degradation of naphthalene at a higher concentration (450 μg/l) as compared to the lower initial concentration (150 μg/l) in 10 min while using a sonicator. In a naphthalene degradation study, the presence of Fe²⁺ enhanced degradation of 4-n-nonylphenol and 3-chlorophenol (Nagata et al. 2000; Yim et al. 2003; Velmurugan & Swaminathan 2011).

Figure 3 shows the degradation of naphthalene in the presence and absence of the ZnO catalyst. The highest degradation of naphthalene was observed on the ZnO catalyst surface as compared to without catalyst in the sonolysis. The values are 80.13, 85.38, 88.23, 88.31, and 26.3%; 34.12, 34.48, and 34.56%, respectively, for with and without the ZnO catalyst with a reaction time of 20, 40, 50, and 60 min.

Figure 5(b) shows the degradation of naphthalene was 12.55, 23.9, 32.63, 55.34, 70.73, 75.8, 83.13, 83.63%, respectively, for 50, 100, 200, 220, 240, 260, 280, and 300 V. The optimum voltage was found to be 280 V for maximum degradation of naphthalene. The degradation efficiency is affected by the power of the sonochemical reactor (Figure 5(b)). The degradation efficiency calculation started from 200 V onwards because in most of the literature data, experiments are mainly performed at 200 V. The degradation efficiency is the highest at 280 V, which is the maximum power of the sonochemical reactor in which experiments were performed.

Supplementary Material, Figure S1 shows the relationship plot of the pseudo-first-order rate, and Table 2 shows the kinetic parameters and R² value of the pseudo-first-order rate. It was observed that R² values were more close to 1 in all concentrations in the case of the pseudo-first-order kinetic model. Theoretically, when the R² value is nearer to 1.0, the data fit is considered excellent. Psillakis et al. (2004) reported the first-order model was fitted for degradation of PAH mixtures using the sonochemical method. As shown in Table 2, the rate constant of the apparent pseudo-first-order equation increased from 0.00596, 0.00668, 0.00786, 0.01198, 0.01457, 0.02109, 0.02491, 0.03858, and 0.02810 min⁻¹ as the initial concentration increased from 0.5 to 100 mg/l. This shows a positive effect on the rate of the naphthalene degradation process.

The rate constant increases with the number of active sites on the ZnO catalyst surface with increasing initial concentration (Kee et al. 2022). However, Supplementary Material, Figure S1 shows that the rate constant slightly decreased (0.02810 min⁻¹) with initial concentration of 100 mg/l as compared to the initial concentration of 80 mg/l (0.03858 min⁻¹). This is due to the saturation of solute or completely filled pores of the ZnO particle by solute via mass transfer resistance with chemical rate control (Suresh & Sundaramoorthy 2015). Therefore, 80 mg/l may be the maximum concentration of naphthalene that the ZnO catalyst particles can easily degrade. A similar observation is reported for diazinon degradation at 300 mg/l initial concentration and phenol degradation using photocatalytic technology (Osarumwense et al. 2015). Table 2 also shows the second-order kinetic rate constant, which varied from 0.0006 to 0.01432 l mg/min. However, no trends in the values as compared to the first-order kinetic rate constant were found.

The provided design consisted of four independent variables at three levels and 29 samples with three repetitions, in which the range of independent variables was determined by pre-tests and according to the present study. The range and levels of independent variables and related codes are listed in Table 3. Design-Expert software (version 6) was used to design the test and analyze the results.

The numerical values of the correlation coefficient (R²), the adjusted correlation coefficient (Adj. R²), and the predicted correlation coefficient (Pred. R²) (Table 3) in the fitted model are not significantly different from each other, which indicates a very good correlation between the results obtained by the experimental method and the predicted values by the statistical method. The results showed that the quadratic model is suitable for the obtained data (P-value = 0.0001) and in optimal process, performance conditions were set equal to 100%. Regression analysis with a high correlation coefficient (R² = 0.989) and the model F-value of 6.94 indicated the adequacy of the model.

ANOVA was tested on the output raw data and the noise associated with every test runs to identify the distinct characteristic between the control or signal factors. The contribution of individual parameters is weighted to enforce control on the degradation rate. The ANOVA results for raw and noise data with the degradation rate were used to identify the factors, which would significantly affect the quality characteristic of the output responses, or the factors can be relaxed. Considering the ANOVA and quadratic model, time, temperature, and voltage were the most important parameters affecting the efficiency of the degradation rate process. From the results, the model F-value below 0.05 indicates the model terms are significant, whereas model F-value greater than 0.1 indicates the model terms are not significant.

The effect of the ZnO catalyst mass in the range 0.5–1.5 g/l on the naphthalene removal is shown in Figure 6. According to Figure 6, as the mass of the ZnO catalyst increases, the efficiency of the process increases and reaches its maximum at a ZnO catalyst mass of 1.5 g/l, and an increase beyond that reduces the efficiency with a steady state value. The effect of time in the range of 10–60 min on the naphthene removal is shown in Figure 6. According to Figure 6, as the time increases, the efficiency of the process increases until an equilibrium condition was reached. This is because initially a maximum number of vacant surface sites are available, which, however, decreases as the degradation proceeds.

Similarly, the effect of the voltage and temperature followed the same trend. The increasing effect of temperature indicates the endothermic nature of the process in the presence of ZnO particles which can be ascribed to various reasons (Ramesh Raja & Suresh 2011; Suresh et al. 2014). An increase in the chemical potential of the organic molecules at higher temperature helps in the penetration of these molecules to the inner surface of ZnO particles. This helps in increasing the possibility of bonding between the solute–surface and the functional groups present on the adsorption sites of ZnO particles (Ibrahim & Asal 2017).

The initial chromatograms of the naphthalene solution (standard solution) and the treated solutions showed a peak of naphthalene at 7.41 min. After 132 min, the observed peak area decreased, and new peaks were identified at 2.15 (±0.07), 3.5 (±0.04), 3.6 (± 0.03)–3.7 (± 0.04), 3.9 (±0.05), and 4.1 (±0.08) min (not shown here). These peaks indicated the intermediate species produced by the degradation of naphthalene. After 252 min, naphthalene had been completely degraded as its corresponding peak had disappeared.

Several authors reported the amino and substituted naphthalene compounds through HPLC analysis, which mainly includes 2-naphthyl amine, 1-naphthyl amine, 1.5-diaminonaphthalene, 2-methylnaphthalene, other alkyl naphthalenes and nitronaphthalenes (Haritash & Kaushik 2009; Jing et al. 2014) in a similar retention time as intermediate products. Different derivatized peaks in the range between 3.6 (± 0.03) and 3.7 (± 0.04) min of retention time were found due to other PAH compounds.

In this work, degradation of naphthalene was done using a sonochemical reactor and ZnO nano-catalyst. The RSM design of experiments was performed to evaluate the effect of different parameters on the degradation rate. The ZnO catalyst was prepared for enhancing the degradation efficiency, which was synthesized using the thermal decomposition method. The FTIR analysis shows the characteristic peak of ZnO stretching. This analysis also shows the absorption of water on the surface of the ZnO nanoparticles. The FTIR spectra analysis was affected by the morphology of the catalyst and the size of nanoparticles. The degradation efficiency of naphthalene becomes almost double in the presence of the catalyst when compared with the degradation efficiency without the catalyst. The optimum parameters were 50 min of reaction time, 2.2 g/l of ZnO catalyst mass, 30 °C, 280 V, and 80 mg/l of initial concentration under constant conditions such as 20 kHz and 260 W of power intensity. The kinetic studies showed that the degradation reaction order is of first order.

Considering the ANOVA and quadratic model, time, temperature, and voltage were the most important parameters affecting the efficiency of the degradation process. From the results, model F-value of less than 0.05 indicates model terms are significant, whereas model F-value greater than 0.1 indicates model terms are not significant. To extend this study to real PAH samples, simultaneous degradation behaviour of naphthalene was carried out with other PAH compounds such as acenaphthylene, fluorene, and phenanthrene from an aqueous solution in the presence of ZnO. The results found that the percent degradation values of naphthalene, acenaphthylene, fluorene, and phenanthrene were 64, 48, 42, 37, respectively, from an equal mixture solution. However, the degradation efficiency was lower for different proportion mixtures as compared to the equal mixture solution.

The reusability test revealed that the synthesized ZnO particles were highly stable within at least five repeated cycles. Therefore, synthesized ZnO nanoparticles can be a promising catalyst for PAH degradation with a great reusability potential.

The authors are thankful to MANIT Bhopal (India) for providing necessary institutional facilities and encouragement. P.K. is thankful to MANIT Bhopal for providing a Ministry of Education, Govt. of India fellowship to carry out this research work.

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

The authors declare there is no conflict.