Interfacial behavior of some synthetic MnO 2 samples during their adsorption of Cu 2+ and Ba 2+ from aqueous solution at 300°K

Abstract

Interfacial characteristics of a number of synthetic MnO 2 samples of different crystalline modifications during their adsorption of Cu 2+ and Ba 2+ from aqueous solutions have been investigated. The pH values of the samples both in water and 20% NH 4Cl solutions have been determined in order to assess qualitatively their surface acidity. The results indicate that many of the samples contain some free surface acidity and therefore could not be correlated with the adsorption capacity of Cu 2+. On the other hand, surface OH groups determined by the exchange of K + from KI in anhydrous dimethyl formamide medium gives better correlation with the adsorption capacity of Cu 2+ despite a few deviations. However, the ratio of the moles of charge released to the moles of charge adsorbed is much less than the theoretical value of 2, indicating that in addition to normal coulombic force considerable adsorption takes place through chemical interaction. This is further supported by the adsorption of both Cu 2+ and Ba 2+ on MnO 2 samples at their respective pH (PZC) values or below. Total surface potential and free energy of specific adsorption of Cu 2+ and Ba 2+ on MnO 2 have been calculated and correlated with the adsorption characteristics of the ions.