مجله آب و خاک (Jul 2024)
Investigating Lead Adsorption Behavior in the Presence of Carbon Dot (Kinetic, Isotherm and Thermodynamics)
Abstract
Introduction The rapid growth of technology, industry, and development of cities has led to an increase in heavy metal pollution in freshwater sources and greywater across the world. The use of different adsorbents in order to remove some heavy metals from aquatic environments is a topic that has been addressed many times in different studies. However, the use of inexpensive absorbents with high adsorption capacity and high efficiency is the priority of many researchers especially when they are discussing the removal of heavy metals from the aquatic environment. Nanomaterials by having exceptional properties such as high efficiency of adsorption, high specific surface area, and fast adsorption can be used to remove metal pollutants from aquatic environments. Carbon dot (CD), among various nanomaterials (carbon-based nanomaterials (CNM), including carbon nanotubes (CNTs), graphene) are suitable adsorbents for heavy metals removal due to their specific surface area and many binding sites. Carbon dots are nanoparticles that lack a specific dimension and fall under the category of carbon nanomaterials, measuring over 10 nm in size. They possess various qualities, including being environmentally friendly, simple to create, highly compatible with living organisms, stable, and capable of switching emission on and off based on the excitation wavelength. Additionally, they can be customized for specific uses due to their high carbon content, which can reach up to 99.9%. These characteristics have generated significant interest among researchers in various fields. In this study, the influence of the fungal carbon dots on the adsorption capacity and kinetics, isotherms, and thermodynamics of lead was investigated. Materials and Methods Alternaria alternata provided by the Department of Plant Protection at Ferdowsi university of Mashhad. It was recultured and fungal exopolysaccharide was extracted and then was converted into carbon dot using the hydrothermal method. Fungal exopolysaccharide autoclaved in a Teflon container at a temperature of 200 °C. Lead adsorption of synthesized fungal carbon dots was investigated. Lead adsorption tests by fungal carbon dots were performed in laboratory conditions. Lead concentrations (100, 200, 300, 400, 500, 750 and 1000 mg L-1), contact time (5, 10, 15, 20, 25, 30 and 60 minutes), pH (2, 4, 6, 7, 8, 9, 10 and 11), amount of carbon dots (nanosorbent) (50, 100, 200, 300, 400, 500, 750 and 1000 mg), ionic strength of the solution (0.1, 0.01 and 0.001 M potassium chloride) and solution temperature (25, 30, 35, 40 and 45 °C) was considered for kinetic tests. The data obtained from the kinetic tests were fitted using non-linear regression analysis using Statistica 7.0 software with the kinetic models of intraparticle diffusion, Lagergren (pseudo-first order) and pseudo-second-order. Thermodynamic results were calculated from the data of lead adsorption isotherms at temperatures of 25, 35 and 45 °C. Thermodynamic parameters to analyze the effect of temperature on metal adsorption, such as free energy change, enthalpy change and entropy change, were estimated using thermodynamic equations. Results and Discussion The initial lead concentration had a great effect on the adsorption rate it by carbon dot, and the highest and lowest percentage of lead adsorption with values of 90.65 and 44.2% were observed in two concentrations of 300 and 1000 mg L-1 of lead, respectively. With the increase of pH up to 8, the amount of lead adsorption by fungal carbon dot increased significantly. However, with further increase in pH, this trend was reversed and the amount of adsorption decreased. The results showed that lead adsorption by carbon dot increased with the decrease of potassium chloride molarity. By increasing the amount of carbon dot in the solution, the amount of lead adsorption increased, and the highest adsorption was observed at the concentration of 300 mg L-1 of carbon dot. The results of the experiment also showed that with increase in temperature, the adsorption rate increased at first and then decreased. Based on these results, as the contact time between the absorbent and lead increased, the amount of adsorption by the carbon dots also increased. The maximum adsorption was observed at 25 minutes, which was considered the equilibrium time. As shown in the results, the pseudo-second-order model shows the kinetics of Pb adsorption better than the two pseudo-first-order models and intraparticle diffusion. In this model, R2 values are between 0.9989 and 0.9994, and Qe is almost equal to the equilibrium value. According to these results, the decrease of values DG° with the increase in temperature means that the adsorption of lead increases with the increase in temperature, which shows that the adsorption process is more favorable with the increase in temperature, or in other words, it is a spontaneous reaction. Also, the positivity of the reaction enthalpy value (DH°) shows the endothermic nature of the adsorption process. The positivity of the entropy value (DS°) indicates the increase of disorder of the system between the adsorbent material and the solution during the process of lead adsorption by the carbon dot. Conclusion In total, the results showed that the carbon dot is a very good absorbent for removing lead from the water environment. In the experimental condition when the initial concentration of lead was 300 mg L-1, temperature was 25 °C, adsorbent concentration was 0.3 g L-1, reaction time was 25 minutes, and pH 8, the amount of lead adsorption increased significantly. It seems that fungal carbon dot is a safe and relatively cheap adsorbent and suitable for removing lead metal from the solution environment.
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