Interfacial properties of some hydrous manganese dioxides in 1-1 electrolyte solution
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AbstractInterfacial properties of four synthetic hydrous manganese dioxide samples, two belonging to birnessite (7 Å manganite) and two to amorphous δ-MnO2 varieties, in aqueous 1:1 electrolyte solutions have been studied. Chemical analysis shows that the samples are only partially oxidized and have high water contents. Electron micrographs indicate that the samples occur in aggregates, consisting of finely divided spherical or plate-shaped particles, though the porosities of some of them are low and these, therefore, exhibit low surface area. Both salt and potentiometric titrations reveal that the pHpzc of the samples varies from 2.25 to 3.0 depending upon the preparation method. However, the IEP values of the samples as determined from the electrophoretic mobility measurements tend to shift to higher pH values with respect to pHpzc. All the samples have very high surface charge, due to which specific adsorption of some counterions also takes place. Calculation of the concentrations of surface species shows that at pH near pzc surface complexation is negligibly small and the point of zero charge is possibly characterized by the presence of equal amounts of positive and negative charges.

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