Characterization of the binding site affinity distribution of a surfactant-modified clinoptilolite

Abstract The kinetics, adsorption isotherms, equilibration time and pH effect of a surfactant-modified activated clinoptilolite (SMZ) on the removal of toxic Cr(VI) species from aqueous solutions were studied. Simultaneously, the Cr(III) sorption characteristics of the activated clinoptilolite (NaZ) were also studied. The cation and anion exchange capacities of each zeolite were calculated and in the case of the Cr(VI)-SMZ system several models were studied for adsorption isotherm description. Experimental binding isotherms were fitted by non-linear regression to Langmuir, Freundlich and Jovanovic – as well as to hybrid models – isotherms and the affinity distribution corresponding to the Freundlich isotherm was used to extract binding parameters. It has been demonstrated for the first time that the affinity distribution analysis combined with the Freundlich binding model allows the characterization of the SMZ binding properties for Cr(VI). In comparison to other surfactant-modified clinoptilolites, this activated zeolite proved to have improved properties for Cr(VI) adsorption. The results obtained demonstrate the possibilities of the surfactant-modified activated zeolite for separation of Cr(VI) from Cr(III) and hence, its speciation. Its potentiality for chromium speciation using a flow injection system with a minicolumn packed with SMZ is outlined.