Adsorption of copper (II) ions from aqueous solution by five silica samples

Abstract

Abstract Samples of BDH precipitated silica, Degussa TK900 (non‐porous) and Crosfield xerogels [A (mesoporous), B (mesoporous) and C (microporous)] were characterised by chemical analysis, by Coulter particle‐size analysis, by electron microscopy and by their BET surface area. The point of zero charge, determined by potentiometric acid/alkali titration, was at pH 2.8–2.9 for all samples. The surface charge density for the non‐porous sample was lower than that for the porous samples. The results of the initial rate of exchange of the silicas with tritiated water suggested an exchange with the physically held water, which is lost at 393 K. The adsorption isotherms of Cu (II), determined by atomic absorption spectrophotometry at pH 6 and 323 K, followed the Langmuir equation. The limiting adsorption capacity decreased in the order: precipitated silica > B > A > C > TK900. This order could not be correlated with the BET nitrogen surface area or the pore volumes of the samples. However, the order was the same as the Na + content of the silicas. Desorption runs established that adsorption was not reversible, in fact desorption was significant at equilibrium concentrations approaching zero. An apparent endothermic heat of adsorption of Cu (II) on precipitated silica, calculated from the data of the Langmuir isotherms at different temperatures, was 23 kJ mol −1 . The increase in the adsorption of Cu (II) as the pH increased may be ascribed to the adsorption of hydrolysed species. From the Grahame equation, the estimated free energy of specific adsorption of Cu (II) on precipitated silica near the point of zero charge was‐13 kJ mol −1 .