Polyvinylpyrrolidone mediated electrospun ZnO-ZnFe2O4 composite nanofibers for removing malachite green from water
In this issue, we will introduce polyvinylpyrrolidone mediated electrospun ZnO-ZnFe2O4 composite nanofibers!
Sradhanjali Raut* , Shraban Kumar Sahoo.(https://www.sciencedirect.com/science/article/pii/S2666845924001065)
*School of Applied Sciences, Centurion University of Technology and Management, Odisha, India
Water pollution caused by synthetic dyes has emerged as a significant environmental issue due to its persistence and impact on ecosystems and human health. Malachite Green (MG), one of these synthetic dyes, is widely used across various fields, including textiles, leather, and pharmaceuticals. However, it is resistant to conventional water treatment methods, is difficult to decompose in aquatic environments, and is toxic. Therefore, there is a need for technologies capable of efficiently removing MG from water. Among the various treatment methods, adsorption removal is highly effective in eliminating trace pollutants such as organic compounds and some metals, thanks to its ability to selectively remove specific substances using adsorbents.
Nanofibers have garnered attention for their application as adsorbents due to their large specific surface area, high porosity, and ease of functionalization. In particular, electrospun nanofibers created using the electrospinning method can enhance the adsorption effect of pollutants and efficiently facilitate their removal by leveraging their high specific surface area and adjustable properties.
Zinc oxide (ZnO) and zinc ferrite (ZnFe₂O₄) are well-known nanomaterials with properties suitable for water treatment. ZnO exhibits photocatalytic activity, enabling the decomposition of organic pollutants, while zinc ferrite promotes the removal of pollutants through adsorption and separation processes utilizing its magnetic properties. By combining these two materials, it is possible to enhance each of their characteristics, potentially resulting in a material with superior water purification performance. This study focuses on fabricating ZnO-ZnFe₂O₄ composite nanofibers using the electrospinning method and evaluating their function and properties as an adsorbent for the effective removal of MG dye from water.
〈Material〉
Polyvinylpyrrolidone (PVP), zinc acetate anhydrous, iron(III) acetylacetonate, and malachite green (MG) were dissolved in ethanol to prepare a solution. Zinc acetate and iron(III) acetylacetonate were then added to this solution in ratios of Zn= 1:2, 1:1, and 2:1, followed by stirring. Nanofibers were fabricated from this solution using the electrospinning method. The fibers were then calcined at 500°C to remove the PVP, resulting in ZnO-ZnFe₂O₄ nanofibers.
The fabricated nanofibers were characterized using powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). Subsequently, a series of batch adsorption experiments were conducted to investigate the adsorption behavior of MG dye on the surface of the adsorbent. The concentration of non-adsorbed dye in the treated solution was determined using UV-visible spectroscopy.
＜Result＞
Characteristics Evaluation
The combination of peaks corresponding to both ZnO and ZnFe₂O₄ was observed, indicating that both phases coexist in the nanocomposite. The peak intensity varied depending on the ratio of zinc to iron.
When comparing the three types of nanofibers with Zn-Fe ratios of Zn-Fe (1-1), Zn-Fe (2-1), and Zn-Fe (1-2), it was confirmed that the Zn-Fe (1-1) nanofiber had a larger surface area, as evidenced by its higher N₂ adsorption.
Based on these results, the subsequent experiments were conducted using Zn-Fe (1-1) nanofibers.
Next, FE-SEM images of the Zn-Fe (1-1) nanofiber at different resolutions were observed, revealing the formation of one-dimensional continuous nanofibers. FE-SEM-EDX analysis confirmed the presence of zinc, oxygen, and iron peaks, indicating the existence of both ZnO and ZnFe₂O₄. However, there is a possibility of detecting impurities or surface contaminants. The diameter of the nanofibers was found to be between 40 and 60 nm.
These results confirm the successful fabrication of ZnO-ZnFe₂O₄ nanofibers, which are a binary oxide material.
(Sradhanjali Raut, Shraban Kumar Sahoo.: Polyvinylpyrrolidone mediated electrospun ZnO-ZnFe2O4 composite nanofibers for removing malachite green from water, Results in Surfaces and Interfaces , Issue 17,1st October 2024. より引用)
MG Adsorption Experiment
The adsorption of MG dye onto the nanofiber surface was tested across a pH range of 3 to 9. The results showed that the adsorption capacity increased with higher pH levels (Figure 3b).
(Sradhanjali Raut, Shraban Kumar Sahoo.: Polyvinylpyrrolidone mediated electrospun ZnO-ZnFe2O4 composite nanofibers for removing malachite green from water, Results in Surfaces and Interfaces , Issue 17,1st October 2024. より引用)
Effect of pH on MG Adsorption and Kinetic Studies
From the data measuring the surface charge of the nanofibers at varying pH levels, the point of zero charge (pHₚZₚC) was determined to be 6.4.
When pH < pHₚZₚC, the surface of the material is positively charged, whereas when pH > pHₚZₚC, it becomes negatively charged. Under basic conditions, electrostatic attraction occurs between the negatively charged adsorbent surface and the cationic form of MG present in the aqueous solution, resulting in maximum adsorption efficiency. In acidic conditions, electrostatic repulsion between the positively charged surface and the cationic MG reduces the adsorption efficiency.
Time and kinetic studies of MG dye adsorption revealed that the adsorption efficiency initially increases with longer contact time, attributed to the availability of active sites on the nanomaterial surface. This phase is rapid, indicating favorable initial adsorption kinetics. As equilibrium approaches around 60 minutes, the adsorption rate slows, suggesting a diffusion-controlled process or saturation of available binding sites. Kinetic models, such as pseudo-first-order (PFO) and pseudo-second-order (PSO), were employed to elucidate these behaviors, aiding in the understanding of the adsorption mechanism and optimal operating conditions. The nonlinear equations of both models were used to calculate various related kinetic parameters, revealing that MG adsorption onto the Zn-Fe (1-1) nanofiber surface follows the PFO kinetics.
The concentration experiment for MG dye adsorption (Figure 3d) shows that a higher initial concentration of MG increases the adsorption capacity until saturation is reached. Isotherm studies, such as the Langmuir model, analyze the equilibrium adsorption behavior. The Langmuir model assumes monolayer coverage and uniform energy, describing adsorption onto a finite number of identical sites. In contrast, the Freundlich model represents adsorption on heterogeneous surfaces where affinity varies, indicating multilayer coverage. The Langmuir model is applied for monolayer adsorption, while the Freundlich model is suitable for more complex and non-uniform surfaces.
Nonlinear equations were used to calculate various parameters related to these isotherm models, which are summarized in Table 2.
(Sradhanjali Raut, Shraban Kumar Sahoo.: Polyvinylpyrrolidone mediated electrospun ZnO-ZnFe2O4 composite nanofibers for removing malachite green from water, Results in Surfaces and Interfaces , Issue 17,1st October 2024. より引用)
NaCl, NaOH, and HCl were used to evaluate regeneration; 93% of the total regeneration was obtained with HCl. After regeneration, the material was dried and washed with water until pH balance was achieved, and efficiencies were found to be maintained up to 80% for each of the five cycles.
〈Summary〉
ZnO-ZnFe₂O₄ composite nanofibers were fabricated using the electrospinning method. The surface area of the nanofibers was found to be the largest when the Zn-Fe ratio was 1:1. Furthermore, adsorption experiments showed that MG adsorption depended on the pH of the solution, with maximum adsorption capacity observed at high pH due to electrostatic interactions between the cationic dye and the negatively charged nanofiber surface.
Kinetic and isotherm studies revealed that MG adsorption followed the pseudo-first-order kinetic model and the Langmuir isotherm model. The regeneration experiments demonstrated that the adsorbent possesses sufficient chemical stability and can remove the adsorbed MG species for reuse.
Based on these results, it was confirmed that the fabricated ZnO-ZnFe₂O₄ nanofibers are effective adsorbents for removing MG dye from water.
The large surface area of nanofibers is certainly effective in adsorption. It became clear that the adsorption rate is influenced by the ratio of adsorbents in the material and the morphological changes of the nanofiber surface due to pH variations in the aqueous solution, making it crucial to identify the optimal ratio and pH. I hope that the application of nanofibers in adsorbents and filtration systems will continue to expand, addressing environmental issues such as water pollution!