Ultrasound–acid modified Merremia vitifolia biomass for the biosorption of herbicide 2,4-D from aqueous solution

Abstract In this work, a biosorbent was prepared by the ultrasound–acid treatment of Merremia vitifolia plant and tested for the removal of 2,4-dichlorophenoxyacetic acid (2,4-D), a phenoxy herbicide. Optimal values of five batch biosorption parameters namely stirring speed, contact time, biosorbent dosage, initial pH and initial adsorbate concentration were experimentally obtained in sequential manner for an enhanced biosorption capacity. The kinetics of the biosorption of 2,4-D were best described by the pseudo first order kinetic model (R2 = 0.99) and the biosorption equilibrium data was successfully fitted to the Langmuir adsorption isotherm (R2 = 0.99) with a maximum biosorption capacity of 66.93 mg g−1. The mechanism of biosorption was investigated using two intraparticle diffusion models (Weber and Boyd), Dubinin–Radushkevich isotherm model and electrostatic interactions. The presence of intraparticle and film diffusion limitations for the biosorption was confirmed along with the physical and chemical nature of the biosorption. The thermodynamic parameters of the biosorption were calculated using the equilibrium data obtained at four different temperatures. The entropy change for biosorption was found to be negative indicating the decreased randomness at the interface. Desorption studies were carried out using different solvents and the percentages of desorption were compared.