Surface modification of rubber (Hevea brasiliensis) leaves for the adsorption of copper ions: kinetic, thermodynamic and binding mechanisms

BACKGROUND: This research describes the adsorption of copper ions from aqueous solutions following the modification of rubber (Hevea brasiliensis) leaves with formaldehyde solution. The main objectives of this research were to identify the binding mechanisms of copper ions on the chemically modified rubber leaves by spectroscopic techniques and to investigate the effects of several important physicochemical parameters such as pH, copper concentration, contact time, adsorbent dose and temperature on copper removal. RESULTS: Based on a kinetic study, the pseudo-second-order model was found to fit the experimental results well, while the Boyd kinetic model indicated that the rate-determining step was due to film diffusion. Adsorption isotherms were modelled by the Langmuir and Freundlich isotherm equations, with the former providing a better fit for the data. Based on the Langmuir model, the maximum adsorption capacities of Cu(II) ions at 300, 310 and 320 K were 8.36, 8.61 and 8.71 mg g(-1), respectively. Thermodynamic parameters such as the Gibbs free energy (Delta G degrees), enthalpy (Delta H degrees) and entropy changes (Delta S degrees) were calculated. The adsorption process was spontaneous as the values of Delta G degrees were negative, and endothermic as higher adsorption capacities were recorded at higher temperatures. More than 80% of copper ions bound on the adsorbent were able to be desorbed using 0.02 mol L-1 HCl, HNO3 and EDTA solutions. Besides ion exchange, surface complexation could also play a major role in copper binding. CONCLUSION: Due to its relative abundance and satisfactory adsorption capacity, the modified rubber leaves can be considered as a good low-cost adsorbent for removing copper ions from dilute aqueous solutions. (C) 2008 Society of Chemical Industry