Abstract
In this work, the use of Fe3O4 magnetic nanoparticles in the adsorption of rhodamine 6G solution was evaluated via absorbance and fluorescence spectroscopy using the UV-Vis spectrometer. The adsorption mechanism of rhodamine 6G on Fe3O4 was determined with respect to the adsorbent dosage (2.5–10 mg/L) and treatment time (0–150 min). The experimental data revealed that the fluorescence quantum yield of rhodamine 6G was inversely proportional to the percentage of dye removal. The highest efficiency of dye removal was obtained at 10 mg/L Fe3O4, and the adsorption capacity was about 150 mg/g, together with a reduced treatment time of 30 min, owing to active adsorption of Fe3O4. We believe that our study makes a significant contribution to the literature because Fe3O4 magnetic nanoparticles were found to be able to quench rhodamine 6G dye molecules, which will aid in eliminating toxic and hazardous pollutants from dye wastewater, which are otherwise detrimental to the environment and human health.
1. Introduction
Nowadays, the textile industry, being the highest user of dyes for fiber coloration, is faced with the challenge of disposal of dye wastewater worldwide. Effluents derived from many manufacturing units are sometimes discharged into water resources without any treatment, partly due to economic and technical restrictions. Because most dyes are stable against oxidizing agents and sunlight, nonremoval of color from dye wastewater is becoming one of the main water pollutants. They cause severe environmental hazards, not only affecting the aesthetic merit but also decreasing the penetration of light, and it can even prove to be carcinogenic for humans. As a result, various physical, chemical, and biological techniques have been developed to eliminate dissolved inorganic/organic compounds by converting them into harmless end products. Among these treatment approaches, physical adsorption on activated carbon is a very practical one to produce effluents composing low level of these compounds [1–3]. However, the use of alternative substitutes to eliminate such dyes from other materials like nanomaterials has emerged as they are able to treat a large amount of dye wastewater and are not time-consuming and produce a low amount of contaminants. In recent years, magnetic nanoparticles have gained considerable interest with regard to environmental concerns. Among them, Fe3O4 is considered as a good candidate capable of removal of original dyes [4]; however, there is no empirical study on the relationship between the adsorption process of dyes and fluorescence quantum yield. Fe3O4 nanoparticles have been proved to be biocompatible with low-toxicity and are applicable for a variety of biomedical researches. For instance, they are commonly used as a contract agent, therapeutic agent, and sensing probe for magnetic resonance imaging, hyperthermia therapy, and targeted-drug delivery, respectively [5]. However, although there are many common cationic rhodamine family dyes used in the textile industry, only a few studies have been focused on adsorption behaviors of rhodamine 6G. Therefore, in this study, we select rhodamine 6G as a dye model to elucidate the interaction mechanism between Fe3O4 magnetic nanoparticles and rhodamine 6G dye. Based on dosages of Fe3O4 magnetic nanoparticles, it is possible to observe both amplification and quenching of fluorescence of dye molecules.
2. Materials and Methods
Iron-(II,III) oxide or Fe3O4 with particle sizes in the range of 50–100 nm and cationic dye rhodamine 6G were purchased from Sigma-Aldrich, USA. The dye powder was first dissolved in deionized water for use as a stock solution. The stock solution was, then, diluted in two proportions. The first portion was used to acquire a calibration curve as y = 79289x−0.0032, where y and x represent the absorbance peak and rhodamine 6G concentration, respectively, as shown in Figure 1.
The second portion was used further as adsorbate. Next, the adsorption experiments were performed by mixing 1 mL dye solution of fixed concentration (3 µM rhodamine 6G) and 1 mL adsorbent of known quantity (2.5–10 mg/L Fe3O4) under a sonicator at the room temperature of 26°C and pH 7.0. The treatment time was varied from 0 to 150 min. At every 30 min time interval, the mix solution was studied under the UV-Vis spectrometer by measuring the absorbance at wavelength of 525 nm and fluorescence at wavelength 555 nm. More details of the optical setup were explained in [6]. Fluorescence quantum yield, efficient removal of dye, and amount of dye adsorbed on adsorbent were calculated using the following equations:
Fluorescence quantum yield (Q) was calculated by [6]where I and A are the integrated fluorescence intensity and absorbance peak, respectively; n is the refractive index of the solvent; and subscript REF is the reference fluorophore. The dye concentration was calculated from the calibration curve.
Efficiency of dye concentration removal was calculated as follows:
Amount of dye adsorbed on the adsorbent at equilibrium state was calculated as follows:
3. Results and Discussion
Adsorbent dosage is an imperative factor in the determination of its adsorption capacity on an adsorbate. To verify the effect of the adsorbent dosage on rhodamine 6G adsorption, adsorption experiments were performed by mixing the Fe3O4 dosage (2.5, 5.0, 7.5, and 10.0 mg/L) in a solution form, for certain amount of rhodamine 6G of 3.0 µM at a room temperature of 26°C and pH 7.0 and for time intervals from 0 to 150 min. The absorption and fluorescence intensities of the mixed solution are displayed in Figures 2 and 3, respectively, and the absorbance peak and integrated fluorescence intensity of the solution are shown in Figures 4 and 5, respectively. The absorption and fluorescence intensities of Fe3O4 mixed with rhodamine 6G are found to be remarkably lower than those of rhodamine 6G alone. They all tend to decrease as dosages of Fe3O4 magnetic nanoparticles increase, owing to the following reasons. Since a surface-to-volume ratio of Fe3O4 is high, it intensely adsorbs rhodamine 6G. A reduction in the fluorescence intensity will thus occur due to the inner filter effect [7]. The fluorescence intensity reduces not only due to the strong adsorption capacity of Fe3O4 but also due to the fluorescence energy resonance transfer of rhodamine 6G by the Fe3O4 nanoparticles. It is a nonradiative energy transfer from an excited donor fluorophore to an acceptor though nonradiative dipole-dipole coupling [8–10]. Hence, it is possible to quench the rhodamine 6G dye molecule by Fe3O4.
A reduction of both absorption and fluorescence intensities further results in a significant decrease in the fluorescence quantum yield, calculated using equation (1) and as shown in Figure 6. The fluorescence quantum yield is in the range of 0.81–0.88. In Figure 7, the fluorescence quantum yield is plotted with respect to the dye removal calculated using equation (2). The fluorescence quantum yield is inversely proportional to dye removal. That is, with the increase in dye removal, the fluorescence quantum yield gradually decreases. The relationship is linear, which further confirms that Fe3O4 magnetic nanoparticles are an efficient fluorescence quencher for rhodamine 6G. The observed fluorescence mechanism of rhodamine 6G under the influence of Fe3O4 magnetic nanoparticles possibly involves fluorescence resonance energy transfer between the dye molecule and Fe3O4 magnetic nanoparticles.
The effect of Fe3O4 dosage on rhodamine 6G concentration removal shows a small change, as displayed in Figure 8; however, the adsorption capacity of Fe3O4 calculated using equation (3) shows a considerable change, as exhibited in Figure 9. The adsorption capacity is in the range of 150–600 mg/g. It is seen that the treatment time profile of rhodamine 6G uptake is a smooth and continuous curve leading to a saturation stage, indicating the available monolayer coverage of rhodamine 6G on the surface of Fe3O4.
In all cases, it is clearly seen that the adsorption of rhodamine 6G is rapid at the first treatment time of 30 min because the mass transfer driving force is large. Adsorption of rhodamine 6G remains constant after reaching an equilibrium state because rhodamine 6G reaches a boundary layer and then possibly diffuses into the tiny pores of Fe3O4. Furthermore, the equilibrium time is shorter at a higher Fe3O4 dosage. This is because, as the Fe3O4 dosage increases, the surface area of Fe3O4 will increase, leading to higher availability of active adsorbent sites to adsorb rhodamine 6G from aqueous solution. Furthermore, at each equilibrium time, it is found that the percentage of rhodamine 6G concentration removal tends to increase from 52 to 57% when the Fe3O4 dosage is increased from 2.5 to 10.0 mg/L, respectively. It is believed that the removal percentage would not go beyond 60% even when more than 10 mg/L of Fe3O4 is added. These results are in good agreement with those in [4]. Thus, the highest efficiency of dye removal is achieved at 10 mg/L Fe3O4, whereas the corresponding adsorption capacity is about 150 mg/g in this study. This value is still higher in comparison with the results reported over the last ten years [11, 12] in other studies on the adsorption of different dyes on different adsorbents, except for [13]. The best treatment time in our adsorption process is 30 min, involving low operation cost that is very important for industry-scale application of an adsorbent. Since rhodamine 6G concentration removal cannot reach 100% similar to [4], partly due to the aggregation of Fe3O4 surface area available to rhodamine 6G and consequently a reduction in the diffusion path length, application of UV light irradiation is suggested to be added in the present process in order to further improve the adsorption capacity. It is expected that the dye removal might go beyond 60% in that case and a comparative study will be presented in the future.
4. Conclusions
The effectiveness of Fe3O4 magnetic nanoparticles as an adsorbent for the elimination of rhodamine 6G dye solution was investigated in this study, by using a UV-Vis spectrometer. Further, amplification and quenching of fluorescence of dye molecules were observed with respect to the dosage of Fe3O4 nanoparticles. Upon the addition of Fe3O4 in rhodamine 6G solution, the absorption and fluorescence intensities were significantly reduced due to the adsorption of rhodamine 6G molecules on the surface of Fe3O4 and the quenching of rhodamine 6G as a fluorophore. The fluorescence quantum yield of rhodamine 6G was therefore decreased, conforming to an increase in dye removal. The highest efficiency of dye removal was achieved at 10 mg/L Fe3O4, whereas the corresponding adsorption capacity was approximately 150 mg/g at a shortened treatment time of 30 min. It is expected that Fe3O4 magnetic nanoparticles will be applied as an adsorbent for elimination of a variety of toxic and hazardous pollutants from dye wastewater because the magnetic nanoparticles have a high surface-to-volume ratio and appropriate pore size for adsorption of dye molecules.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding this study.
Acknowledgments
This research was funded by the King Mongkut’s Institute of Technology, Ladkrabang, Bangkok 10520, Thailand.