Exploration of Zinc Oxide Nanoparticles for Efficient Photocatalytic Removal of Methylene Blue Dye: Synthesis, Characterization and Optimization ^†

Abstract

Water pollution, particularly through industrial effluents, is a significant environmental challenge. The present study explores the synthesis, characterization, and photocatalytic application of zinc oxide nanoparticles (ZnO NPs) for the degradation of Methylene Blue (MB) dye. ZnO NPs were synthesized via the hydrothermal method, and their structural and morphological features were examined using X-ray diffraction, Transmittance Electron Microscopy, and FESEM techniques. A systematic study was carried out to investigate the effects of catalyst mass dosage, initial dye concentration, and light intensity on photocatalytic degradation efficiency. Results show that the synthesized ZnO NPs are effective in MB dye degradation, and the process adheres to first-order kinetics. This work not only demonstrates the potential of ZnO NPs in addressing industrial dye pollution but also contributes valuable insights toward the development of cost-effective and environmentally sustainable water treatment solutions.

1. Introduction

Water pollution encompasses any chemical or physical changes in water, either indirectly or directly, that negatively impact human life or render water unsuitable for desired applications. Water pollution profoundly affects the lives of individuals, families, and entire communities. As an essential requirement for all living organisms, water pollution poses a threat that could potentially end life on Earth. Industrial wastewater discharges from commercial and manufacturing processes, including accidental spills or intentional releases into surface water, untreated sewage discharges, and chemical contaminants, such as chlorine from treated wastewater, result in the release of wastes and contaminants into runoff flowing into surface water. There is an increasing trend in water contamination affecting human life across industrialized countries [1,2]. Among the most perilous compounds found in water are heavy metals, oils, and dyes. Particularly, organic dyes are produced by various local industries, including textiles, paper, plastics, leather, food, cosmetics, and more. Organic dyes are considered one of the largest and most crucial chemical groups used in the industrial world. Approximately 4.5 million tons of dyes were produced in 1996, with the majority intended for use in the textile industry [3,4]. While most dyes are inert or nontoxic, some possess significant toxicity to humans [5]. The presence of dyes in water impedes light penetration and adversely affects photosynthesis. Many dyes are complex in structure and synthesis, making them challenging to remove from contaminated water solutions [6].

The dyestuff industry has seen rapid growth, with about 60,000 tons of dyes discharged into the environment as pollutants worldwide, and 80% of this waste is azo dyes [7,8,9,10,11]. zinc oxide (ZnO NPs) is a promising nontoxic and low-cost catalyst with notable photocatalytic potential for degrading water pollutants. Its application remains limited due to challenges such as low efficiency, photocorrosion, and high recovery costs [12]. ZnO’s high biocompatibility and visible spectrum optical emission have led to recent applications in the healthcare industry for biotherapeutic purposes, including cancer treatment [13,14]. The photocatalytic behavior of ZnO exhibits promising results, with unique reactions and benefits, including its white, odorless, crystalline nature, water insolubility, nontoxicity, 3.3 eV energy gap, high melting point, and distinct optical, acoustic, and electronic properties. Its potential applications extend to device development and photoelectronics, such as sensors, laser diodes, and photocells. ZnO’s ability to degrade fabric dye in just 1.5 h further illustrates its potential as an environmentally friendly solution to dye pollution.

2. Materials and Methods

2.1. Materials

The materials used in the experiment include oxalic acid and zinc acetate obtained from Germany (Sigma-Aldrich, Taufkirchen, Germany), and Methylene Blue (MB) dye. A standard solution of MB dye was prepared by dissolving 0.1 g in 100 mL of double-distilled water to achieve a concentration of 100 mg/L.

2.2. Preparation of Zinc Oxide Nanoparticles by Hydrothermal Method

Zinc oxide nanoparticles (ZnO NPs) were synthesized using a hydrothermal system. An amount of 5 g of oxalic acid was mixed with 5 g of zinc acetate in 100 mL of water, and the mixture was stirred magnetically for 30 min at room temperature. The blend was then placed in a Teflon cup and transferred to a thermal system inside an autoclave at 160 °C for 24 h. After completion, the resulting solution was washed with distilled water several times and dried at 80 °C for 24 h. The dried product was then ground and calcined in an electric furnace to yield the ZnO NPs nanocomposite.

2.3. Photocatalytic Experiments

The photocatalytic activity of the prepared ZnO NPs was examined through the degradation process of MB dye under UV light. A mass of 0.6 g of catalyst was suspended in 50 mg/L of MB dye, serving as the pollutant solution. Initially, a stable equilibrium of dye adsorption with ZnO NPs was established by stirring the solution in the dark for 90 min. Every 10 min, a 3 mL aliquot of the solution was taken and centrifuged to remove the supernatant particles. The MB dye solution’s absorbance was analyzed at 665 nm. The photocatalytic degradation efficiency of MB dye was calculated using Equation (1):

$$\mathrm{PDF} (\%)=\frac{C_0-C_t}{C_0}\times 100$$

(1)

where $C_0$ is the initial concentration of the dye at the start of the test, and $C_t$ is the dye concentration after a given period of testing (t) [15].

3. Result and Discussion

3.1. Characterization of ZnO NPs

The X-ray diffraction (XRD) pattern of zinc oxide nanoparticles (ZnO NPs) was analyzed to determine their phase purity and crystalline structure.

3.1.1. X-ray Diffraction Analysis

The XRD pattern did not show any additional peaks that would indicate the presence of impurities, suggesting that the synthesized nanoparticles of zinc oxide are of a single phase. Any impurity patterns that were observed could be attributed to the effects of calcination.

Nine distinct peaks were detected at diffraction angles of 31.7°, 34.4°, 36.2°, 47.5°, 56.5°, 62.82°, 66.4°, 67.9°, and 69.10°. These peaks correspond to the $\left(100\right)$, $\left(002\right)$, $\left(101\right)$, $\left(102\right)$, $\left(110\right)$, $\left(103\right)$, $\left(200\right)$, $\left(112\right)$, and $\left(201\right)$ reflection planes, in that order [16]. All the detected peaks could be attributed to the hexagonal Wurtzite structure of ZnO, as confirmed by the JCPDS card (no. 36-1451). The highly crystalline nature of the synthesized ZnO NPs is evidenced by the very sharp and robust peaks, as shown in Figure 1.

The absence of extraneous peaks and the confirmation of the hexagonal Wurtzite structure contribute to the overall understanding of the high quality and purity of the synthesized ZnO NPs. These characteristics are essential for their application in photocatalytic processes and other potential uses. Further sections discuss additional experimental results and their implications for the current study.

3.1.2. Transmittance Electron Microscopy (TEM) Analysis

Transmittance Electron Microscopy (TEM) was utilized to investigate the morphology, crystal structure, and particle size of ZnO NPs. As depicted in Figure 2, the TEM images reveal that the surface particle structure of the ZnO NPs exhibits a globular stacked configuration. This observation provides insights into the physical appearance and arrangement of the nanoparticles.

3.1.3. FESEM Analysis

The morphology of the nano zinc oxide prepared by the hydrothermal method was further investigated using Field Emission Scanning Electron Microscopy (FESEM). The images shown in Figure 3 reveal zinc oxide in small white spherical shapes that are closely packed with one another. This structural evidence supports the formation of zinc oxide nanoparticles with different nanoscale dimensions, specifically at 1 $\mathsf{\mu }$m and 200 nm scales [17].

The analyses from both the TEM and FESEM techniques present a comprehensive view of the morphology and structure of the synthesized ZnO NPs. The observations underline their suitability for applications where specific structural properties are essential, such as in photocatalytic degradation processes.

3.2. Effect of Mass Dosage

Determining the optimal quantity of catalyst is a crucial stage in scaling up the photocatalytic process, as it influences both the economics of the method and the downstream processing required to separate the photocatalyst from the reaction medium. The effect of ZnO NPs on the degradation of dye was studied within the range of 0.2–0.6 g under various optimal conditions. Figure 4 illustrates the influence of the catalyst’s mass on decolorization efficiency. As the weight of ZnO NPs increased from 0.2 to 0.6 g, the PhotoDegradation Efficiency (PDE%) improved from 39.56% to 89.71% after 1 h [18,19,20].

3.3. Effect of Concentration of MB Dye

Figure 5 shows the influence of the initial concentration of MB dye on degradation efficiency. A significant decrease in removal percentage was observed with an increase in the initial concentration of MB dye. This can be attributed to the saturation of the catalyst’s active sites due to the excess adsorption of dye molecules, thereby hindering the activity of radicals (OH•) responsible for the degradation reaction. Another plausible reason is the screening effect of UV light, which increases with a higher dye concentration. When the mass dosage of adsorbent was 0.4 g and the concentration of MB dye increased from 25 to 150 mg/L, the removal percentage improved from 88% to 69.16% after 1 h of photodegradation [21,22].

These findings reflect the complex interplay between the mass of the catalyst and the concentration of the dye, highlighting the need to carefully optimize these parameters to achieve efficient photocatalytic degradation.

3.4. Effect of Light Intensity (L.I.)

The influence of light intensity, ranging from 1.2 to 2.3 mw/cm², was studied by altering the distance between the light source and the exposed surface of the material. The photodegradation of MB dye under the effect of varying light intensities was investigated using 0.4 g of ZnO NPs and a concentration of MB dye at 50 mg/L. As shown in Figure 6, it was determined that the reactions consistently follow first-order kinetics. The rate of photocatalytic degradation and PhotoDegradation Efficiency (PDE%) increased with rising UV light intensity. More radiation is available to excite the catalyst in this scenario, and thus, additional charge carriers are generated, leading to an elevated rate of photocatalytic removal [18].

This observation emphasizes the significance of light intensity in enhancing photocatalytic reactions. It suggests the need for carefully calibrated exposure levels to achieve the most effective degradation rate, without causing unnecessary energy consumption.

4. Conclusions

In this work, ZnO nanoparticles were prepared using the sono-hydrothermal technique, presenting an accessible method for production. The research data demonstrate that the degradation of MB dye can be efficiently achieved using ZnO NPs in aqueous solutions and dispersions under UV irradiation. The photocatalytic degradation of MB dye proved to be most effective in solutions at low concentrations and under higher-intensity light. An increase in the mass of the ZnO NPs catalyst, ranging from 0.2 to 0.6 g, led to an improvement in the PDE% from 39.56% to 89.71% after 60 min. This study highlights the potential of ZnO nanoparticles as an effective catalyst for the degradation of dyes, contributing valuable insights to the field of environmental remediation.