The adsorption isotherms of M2+ ions (M = Mg, Ca, Sr or Ba) were determined at pH 7.0 and at different temperatures. The adsorbent, δ‐MnO2, was converted to the K+ form prior to adsorption and about 1.5 mol K+ ions were released per mol of M2+ ions adsorbed. The adsorption capacity at a given temperature increased in the series: Mg2+ < Ca2+ ≦ Sr2+ < Ba2+. This was explained by an ion exchange mechanism between hydrated ions: K+ ions in the outer Helmholtz layer and M2+ ions in the bulk of the solution. The radii of the hydrated ions decreased in the series: Mg2+ > Ca2+ > Sr2+ > Ba2+. The adsorption of M2+ ions at pH values below the point of zero charge (pH 3.3) was significant for Mg2+ ions only. Although adsorption was not strictly reversible, the results fitted the Langmuir isotherm and ‘apparent heats of adsorption’, Q, were calculated. The endothermic heats (Q = 20,18, 11 and 5 kJ mol−1 for Mg2+, Ca2+, Sr2+ and Ba2+ adsorption respectively) indicated positive entropy contributions which are expected for the adsorption mechanism suggested. The decrease in Q down the alkaline‐earth group was correlated to the entropy effects and to the hydration numbers of the cations.
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