G O El-Sayed, H A Dessouki, S S Ibrahim
The adsorption of Ni(II) and Cd(II) ions onto rice straw was investigated. The effects of pH, initial adsorbent concentration, initial metal ion concentration, contact time and temperature were studied in batch experiments. The Langmuir and Freundlich models were applied and the adsorption equilibrium followed the two adsorption isotherms. The maximum adsorption capacities for Ni(II) and Cd(II) were 35.08 and 144.19 mg/g, respectively. Thermodynamic parameters such as free energy change (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°) have been calculated on the basis of Langmuir constants. The negative values of ΔG° were indicative of the spontaneity of the adsorption process. The negative values of ΔH° revealed the exothermic nature of the adsorption process whereas the positive ΔS° reflected an irregular increase of the randomness at the adsorbent/solution interface during the sorption of metal ions.
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