Adsorption of cadmium, nickel and lead ions: equilibrium, kinetic and selectivity studies on modified clinoptilolites from the USA and RSA

Abstract

The performance of modified clinoptilolites (zeolites) from two different sources (South Africa and the USA) for the adsorption of Ni²⁺, Cd²⁺ and Pb²⁺ 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 Ni²⁺, Cd²⁺ and Pb²⁺ 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 Ca²⁺. 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 (17.52m²/g and 0.047cm³/g respectively) than the USA zeolite (12.26m²/g and 0.028cm³/g respectively) for the Na⁺ homoionic form. According to the equilibrium studies, the selectivity sequence was found to be Pb²⁺ > Cd²⁺ > Ni²⁺, 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.