Adsorption from aqueous solution by δ-manganese dioxide I. Adsorption of the alkaline-earth cations

The adsorption isotherms of M 2+ ions (M = Mg, Ca, Sr or Ba) were determined at pH 7.0 and at different temperatures. The adsorbent, δ‐MnO 2 , was converted to the K + form prior to adsorption and about 1.5 mol K + ions were released per mol of M 2+ ions adsorbed. The adsorption capacity at a given temperature increased in the series: Mg 2+ < Ca 2+ ≦ Sr 2+ < Ba 2+ . This was explained by an ion exchange mechanism between hydrated ions: K + ions in the outer Helmholtz layer and M 2+ ions in the bulk of the solution. The radii of the hydrated ions decreased in the series: Mg 2+ > Ca 2+ > Sr 2+ > Ba 2+ . The adsorption of M 2+ ions at pH values below the point of zero charge (pH 3.3) was significant for Mg 2+ 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 Mg 2+ , Ca 2+ , Sr 2+ and Ba 2+ 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.