The performance of modified clinoptilolites (zeolites) from two different sources (South Africa and the USA) for the adsorption of Ni, Cd and Pb from synthetic industrial effluent contaminated with metal concentration levels at 50, 150 and 500 ppm was evaluated. The selectivity of the clinoptilolite for the adsorption of Ni, Cd and Pb was investigated with mixed feed solutions containing all three ions in equal concentrations and single-component concentrations containing only one of the ions. The homoionic forms of the clinoptilolite were made of Na, K and Ca. Batch experiments were then conducted to measure the uptake of metals by the zeolites. The zeolites were characterised using SEM, XRD and BET. The South African clinoptilolite showed a higher surface area and pore volume (17.52m/g and 0.047cm/g respectively) than the USA zeolite (12.26m/g and 0.028cm/g respectively) for the Na homoionic form. According to the equilibrium studies, the selectivity sequence was found to be Pb > Cd > Ni, with good fits being obtained using Langmuir and Freundlich adsorption isotherms for low metal concentrations. Examples of equilibrium adsorption capacities for RSA and USA clinoptilolite modified with Na for Pb were 26.94 mg/g and 27.06 mg/g when RSA-Na and USA-Na were used respectively. The adsorption was found to depend on the homoionic form of the zeolite and to a lesser extent the source of the zeolite. The selectivity of a particular zeolite for a particular heavy metal can be altered by the homoionic form of the zeolite. Overall, the adsorption capacity of the USA clinoptilolite was higher than the adsorption capacity of the SA clinoptilolite, revealing the potential of clinoptilolite in metal-polluted industrial effluent treatment.
Research ArticleHomoionic forms of clinoptilolite forms, respectively. The positive changes in enthalpy provide an indication that the sorption reaction is endothermic for Ni(II). The Gibbs free energy values were all negative except for Ca 2+ -exchanged clinoptilolite at 298.15 K and 308.15 K, for which the values were positive 3.10 kJ/mol and 0.53 kJ/mol, respectively. The entropy values for Ni 2+ sorption were also positive; values of 0.12 kJ/mol.K, 0.08 kJ/mol.K and 0.14 kJ/mol.K were obtained for the Na + , K + and Ca 2+ forms, respectively. As expected, the enthalpy obtained from the Van't Hoff plot is dependent not only on the metal ion being adsorbed, but also on the ion being displaced. Pretreatment of the zeolite enhances the removal efficiency provided that monovalent ions are used for the pretreatment. Thus clinoptilolite is an effective low-cost absorbent for the removal of Ni 2+ from aqueous solutions.
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