The exchange of Na + ‐montmorillonite with Cu 2+ was studied using X‐ray diffraction, electron spin resonance (esr) and a specific Cu 2+ electrode in order to investigate the hydration and exchange properties of mixed Cu 2+ ‐Na + clay systems. With < 30% of the exchange sites occupied by Cu 2+ , the clays behave much as pure Na + ‐montmorillonite and swell freely in water. Larger exchange levels of Cu 2+ produce more ordered interlayers at low relative humidities. At high relative humidities, this ordered structure is reduced and a solution‐like environment is produced as the aluminosilicate sheets further separate. Exchange isotherms demonstrate a large preference for Cu 2+ over Na + at low Cu 2+ exchange levels but preference for Na + at high Cu 2+ levels. In contrast, Cu 2+ ‐Mg 2+ exchange does not show this effect, indicating that certain exchange sites on the silicate structure preferentially adsorb divalent cations. Thus, Cu 2+ ions are effectively removed from solution by Na + ‐montmorillonite, but are not adsorbed in preference to other divalent cations.
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Murray B. McBride (1976) studied this question.