The sorption of Cu(II) ions onto natural (clinoptilolite) and synthetic (Na-X and Na-P1) zeolites was investigated. The synthetic zeolite samples were obtained from coal fly ash under hydrothermal crystallization conditions. The following issues were studied under batch conditions: the influence of the initial concentration of the Cu(II) ions, dose of zeolites, anion kind, and pH of solution. The experimental data were modelled with the Freundlich, Langmuir, and Dubinin-Radushkevich adsorption isotherms and their goodness-of-fit levels were compared. The affinity of the Cu(II) ions for synthetic zeolites was much higher than for the natural ones. The maximum sorption capacity of the synthetic zeolites (Na-P1 and Na-X) was 256 mg·g^-1 and 141 mg·g^-1, respectively. It was 10.6 and 6 times higher, respectively, than the clinoptilolite sorption capacity (24 mg·g^-1). The uptake of the Cu(II) ions occurred in two processes, i.e. the ion exchange and adsorption/precipitation in accordance with the initial concentration of the Cu(II) ions in the solution. Study of the pH and anion kind influence on the zeolite sorption capacity shows that the synthetic zeolites were effective sorbents of the Cu(II) ions, also in the acidic solutions.
The Freundlich and Dubinin-Radushkevich equations provided the best description of the sorption for the Cu(II) ions onto the synthetic zeolite systems when the initial Cu(II) ion concentration was 500-1,000 mg·L^-1. On the other hand, the Langmuir and Dubinin-Radushkevich isotherms described the sorption most properly when the concentrations were above 1,000 mg·L^-1.