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Copper, lead and zinc sorption on Wyoming montmorillonite Cooper, Roger Brian

Abstract

Exchange adsorption of Cu²⁺ , Pb²⁺ and Zn²⁺ for Na on Wyoming mont-morillonite, in chloride and nitrate electrolyte solutions less than 10⁻³ M total heavy metal concentration, can be modelled with a simple equilibrium ion-exchange reaction: Me²⁺ + 2 Na{Mont} = Me{Mont}₂ + 2 Na⁺ where Me. and Mont represent a given heavy metal and montmorillonite. Equilibrium constants for this reaction are equal for Cu²⁺ and Zn²⁺ at 3.0 ± 1, slightly less than that for Pb²⁺ at 5.0 ± 1, and comparable to constants calculated for monopositive exchange of K⁺ for Na⁺ (3.0 ± 1) and H⁺ for Na⁺ (2.5 ± .5). Above 10⁻³ M total metal concentration, heavy metal exchange for Na is complicated by variable total Na exchange capacity and by apparent anionic interferences. CuCl₂ and ZnCl₂ electrolyte solutions are more effective Na exchangers at 0.1 M than Cu(NO₃)₂, Pb(NO₃)₂, KC1 or HC1 solutions at the same concentration. Retardation factors of Cu, Pb and Zn spots eluted across thin layers of Na-montmorillonite (supported by silica gel) by aqueous NaCl and NaNO₃ solutions of 0.05 to 3.0 M concentration suggest, when interpreted with a rudimentary ion-exchange mass transfer model, that metals migrate mainly as monopositive species—perhaps as monohydroxo-complexes within the clay micelle. Precipitation of Pb chloride or hydroxy-chloride is indicated by multiple Pb spots on NaCl-eluted chromatograms.

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