Adsorption from aqueous solution by δ‐manganese dioxide II. Adsorption of some heavy metal cations

Abstract

AbstractThe adsorption isotherms of M2+ ions (M2+ = Ni2+, Co2+, Cd2+, Zn2+ and Mn2+) on the K+‐form of δ‐MnO2, at pH 6 and at different temperatures, fitted the Langmuir equation and an apparent heat of adsorption, Q, was found to be ‐ 78, ‐ 33, ‐ 34, ‐ 19 and ‐ 15 kJ mol−1 respectively. The adsorption capacity of δ‐MnO2 increased in the series: Ni2+ < Co2+ < Cd2+ ± Zn2+ < Mn2+. This was nearly the order of decrease in the radii, r′, of the hydrated ions, estimated from hydration enthalpies. An ionexchange mechanism between hydrated K+ ions in the outer Helmholtz layer and hydrated M2+ ions in the solution, suggests positive entropy contributions which offset the endothermic Q. The proposed mechanism is in agreement with the observed sequence of adsorption capacity and with the decrease in Q in the above series, except for Co2+ adsorption (possibly complicated by the oxidation of Co2+ by δ‐MnO2). The adsorption of the cations is probably accompanied by the exchange with Mn ions from the solid. There was evidence of specific adsorption below the point of zero charge (pH 3.3). The adsorption isotherms of Mn2+ ions at pH 7 were higher than those at pH 6 and Q was found to be ‐ 19 kJ mol−1. As the ionic strength increased, the adsorption isotherm of Mn2+ ions at pH 7 and 298 K shifted to lower values. Adsorption isotherms of Cu(II)ions at pH 3.5 and of Fe(III) at pH 2 represent specific adsorption and Q was found to be ‐ 74 and ‐ 13 kJ mol−1 respectively.