Uranium Removal from Aqueous Solution by Banyan Leaves: Equilibrium, Thermodynamic, Kinetic, and Mechanism Studies

Abstract

Biosorption is an effective and economic method to treat wastewater containing low concentrations of uranium pollutants. In this study, banyan leaves (BLs) were used as a biosorbent to adsorb uranium ions. The biosorption behavior, thermodynamics, and kinetics of this adsorption process, as well as its mechanism, were investigated. The results indicated that the optimal adsorption effect was achieved when the initial pH was 3.0, the initial uranium concentration was 100 mg L−1, the absorbent dosage was 5 g L−1, and the temperature was chosen to be 293 K. The biosorption rate could be described better by a pseudo-second-order model than a pseudo-first-order model, an Elovich kinetic model, and a double-constant speed kinetic model. The adsorption of uranium(VI) ions proceeded very rapidly in the first 30 min, which subsequently slowed down, and finally reached equilibrium after 50 min. The biosorption isotherm of uranium adsorbed onto BLs could be described better by Freundlich isotherm than Langmuir and Temkin isotherms. This pattern of uranium adsorption onto BL was not achieved by single-molecule form, but rather by multiple forms of adsorption. The thermodynamics parameters, namely, ΔH0, ΔS0, and ΔG0, of adsorption of uranium(VI) ions onto BLs were calculated according to the results of the experiment, which revealed that the biosorption processes were both endothermic and spontaneous. The authors investigated the active sites of BLs for biosorption, and the results proved that −OH, C=O, P−O, and Si=O played an important role in biosorption.