Statistical physics-based analysis of the adsorption of Cu2+ and Zn2+ onto synthetic cancrinite in single-compound and binary systems

Adsorption isotherms of Cu2+ and Zn2+ onto cancrinite were investigated in single-compound and binary systems at pH 6.0 and room temperature. The maximum adsorption capacity of Cu2+ was higher than Zn2+ in both the systems. This result could be related to Pauling electronegativity and hydrated ionic radius of these metals, making the cancrinite more selective for Cu2+ than Zn2. The separation factor (Cu,Zn) was greater than unity confirming the higher selectivity for Cu2+ adsorption. Langmuir and Freundlich models fitted well the single-compound adsorption isotherms. In order to obtain a better interpretation of the adsorption of Cu2+ and Zn2+ in single and binary systems, a monolayer and an exclusive extended monolayer models were adopted. According to these statistical physics models results, the receptor sites of cancrinite showed to be more selective for Cu2+ (n = 0.94) than Zn2+ (n= 0.77) in single-compound systems. Due to the competitive effect arising between Cu2+ and Zn2+ on the same receptor site, the n values were reduced in binary system to 0.68 and 0.39 for Cu2+ and Zn2+, respectively. Furthermore, the adsorption energies in singlecompound systems were equal to 23.78 and 19.23 kJ/mol for Cu2+ and Zn2+, respectively. In binary system, the adsorption energies of the studied ions decreased and their values in all the systems indicated the existence of a physisorption process. The reduction of the adsorbed capacity ratio between the binary and single-compound systems (i.e. Qb/Qs) highlighted the antagonistic effect between copper and zinc ions. Finally, thermodynamic parameters were investigated in detail.