Study on Adsorption of Ag+ by Waste Tea: Adsorption Kinetics, Thermodynamics, Isotherm Properties

Abstract

Aims: In order to explore the adsorption effect of tea on heavy metal ions in industrial wastewater, Ag+ is used as the research object in this paper. Background: In recent years, heavy metal pollution in water has seriously affected human health and the stability of the ecological environment. In order to reduce the harmfulness of heavy metals, various countries have issued a variety of control standards for heavy metals in water, but there are still great restrictions in the prevention and control technology and level of heavy metal pollution. Therefore, how to effectively treat heavy metal pollution in water has become a hot topic in the field of water pollution management. Objective: The optimized conditions of the adsorption are obtained. Properties of the thermodynamics, adsorption kinetics, and adsorption isotherm are obtained. Methods: In order to determine the best adsorption conditions for Ag+, the influence of factors such as pH value, initial concentration of Ag+, tea dosage, contact time, and adsorption temperature on the adsorption effect of tea is studied. The thermodynamics, adsorption kinetics, and adsorption isotherm are studied. Results: The results showed that when the temperature is 25°C, the pH of the solution was 3.5, the amount of adsorbent was 2.5 g/L, the initial concentration of Ag+ was 125 μg/L and the contact time was 30min, the adsorption rate was highest, reaching 98.11%. The thermodynamic study of adsorption showed that at room temperature and above (298.15-318.15 K), ΔG° < 0, indicating that the adsorption process can be spontaneous. The value of ΔGo in this study is between -20 and -80 kJ/mol, indicating that this is a physicochemical adsorption process. ΔHo = -80.111 kJ/mol < 0, indicating that the adsorption process of Ag+ is exothermic. ΔSo = -188.977 J/(mol·K) < 0, indicating that the adsorption is a process of entropy reduction. The adsorption kinetics study showed that the adsorption equilibrium capacity of different concentrations had a large gap with the experimental results, and the correlation coefficient was small by fitting the quasi-first-order kinetic equation and combining it with the experimental measurements. When the quasi-second-order kinetic equation was used, the calculated values of the equilibrium adsorption capacity of each concentration were basically close to the experimentally measured values, and the correlation coefficient was large, so the kinetics of the adsorption system of Ag+ by tea conformed to the quasi-second-order kinetic equation. The adsorption isotherm of this adsorption process is accorded with the Freundlich model and belonged to heterogeneous adsorption. Conclusion: Tea is a good adsorbent and has the potential for adsorption of Ag+.