|摘要: ||The use of five chemical modification processes, including phosphorylation, sulfonation, sodium thiosulfate, citric acid and formaldehyde modification, to modify the lignocellulosic substrate of Leucaena lucocephala into adsorbent which could actually enhance the removal efficiency of metal ions (Cd2+ and Pb2+) from aqueous solution.|
Parameters that may affect the metal ions adsorption efficiency including solution pH, contact time, kinds of metal ions, initial metal concentration, competition metal ions and reaction temperature were conducted. The best adsorption efficiencies of Cd2+ and Pb2+ with adsorbents at optimal operation condition were sequenced as phosphorylation (91.8 mg-Pb2+/g, 61.2 mg-Cd2+/g ), sulfonation (46.2 mg-Pb2+/g, 18.4 mg-Cd2+/g), citric acid (43.8 mg-Pb2+/g, 16.4 mg-Cd2+/g ), formaldehyde modification (41.0 mg-Pb2+/g, 19.2 mg-Cd2+/g), sodium thiosulfate (30.8 mg-Pb2+/g, 12.4 mg-Cd2+/g), and nonmodified adsorbents (16.8 mg-Pb2+/g, 10.4 mg-Cd2+/g). The adsorption capacities of Cd2+ and Pb2+ depended on solution pH. The maximum adsorbed capacity of both heavy metals happened at pH 6.5. The adsorption equilibrium reached after 60 minutes of contact time with batch kinetic studies of each metal ion and different adsorbents.
The experimental isothermal equilibrium data were evaluated with Freundlich, Langmuir, and Dubinin–Radushkevich equation isotherm models to calculate the isothermal adsorption constants and to decide the adsorption behaviour being chemical or physical reaction. Three adsorption kinetic models, Pseudo–first order rate equation, Pseudo–second order rate equation, and Intraparticle diffusion equation, were used to evaluate the experimental data. Under the effects of reaction temperature, initial metal concentration and competition metal ions, pseudo–second order rate equation model best fitted to experimental data. The thermodynamic parameters (enthalpy change, free energy change, and entropy change) for both metal ions adsorption into six adsorbents with increasing temperature from 15 to 60 ℃ indicated that the Pb2+ adsorption was exothermic and feasible, and the Cd2+ adsorption was endothermic and feasible. The competition of binary heavy metal ions into six kinds of adsorbents also studied. The binding strength of Pb2+ was stronger than Cd2+ and increased with the initial heavy metal ions concentration.