The effect of background anion on cation exchange reactions,such as Na-Ca and Na-Cu exchange reactions,on montmorillonites has been studied,but the results are not always clear and discrepancies exist in the literature.In this study,the exchange of zine(Zn^2+)for sodium(Na^+)on Wyoming montmorillonite was investigated at 298°K using Cl^-,ClO4^-,NO3^-,OAc^-,and SO4^2- solution media at a constant total metal charge concentration of 0,0200 molc L^-1.Results indicated that the clay CEC values were essential similar for Cl^-,ClO4^-,NO3^- and SO4^2- solution media with an average CEC of 0.856±0.008 molc kg^-1;in an OAc^- solution the clay CEC was much higher than that in other anion media.The specific adsorption of Zn(SAZn),as defined by the extraction of Zn using 0.05 mol L^-1 Na2-EDTA,was different in the various background solutions.The highest value for SAZn was 0.359±0.0350 molc kg^-1,which occurred in OAc solution.There was essentially no difference in the total apparent adsorbed metals(the sum of adsorbed equivalents of Na and Zn per kilogram of clay,Q)among the various background solutions.The average Q for all anion media was 0.807±0.011 molc kg^-1 and was independent of exchanger composition.Experimental results indicated that there were no significant monovalent cation complexes such as ZnCl^+ or ZnNO3^+ that were adsorbed by montmorillonite.The Na-Zn exchange isotherms indicated that there was an adsorption preference for Zn over Na on Wyoming montmoorillonite.
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