Standard Enthalpies of Adsorption of Di- and Trivalent Cations on Alumina (original) (raw)

1997, Journal of Colloid and Interface Science

in view of practical applications. For example, the tempera-Standard enthalpies of Ni(II), Gd(III), and Y(III) adsorption ture of water in a natural environment and in technological on alumina can be estimated from the temperature effect on apparprocesses is often significantly different from room temperaent equilibrium constants of surface reactions. The results are relature. tively insensitive to the choice of the adsorption model, although Adsorption of cations increases when the temperature inthe calculated apparent equilibrium constants of surface reactions creases and the other parameters (pH, initial concentration at a given temperature are model dependent and they often differ of heavy metal ions, ionic strength, solid-to-liquid ratio) are by several orders of magnitude from one model to another. The kept constant (12). Simultaneously, the point of zero charge conditions of the experiment-solid-to-liquid ratio, initial concen-(PZC) is shifted to lower pH values when the temperature tration of multivalent cations in solution, presence of various 1:1 and 2:1 electrolytes at various concentrations-do not signifi-increases (13). Figure 1 shows that at higher temperatures cantly affect the calculated values of standard enthalpies of adthe surface charge density s 0 (and probably also the surface sorption. Assuming that the total number of surface sites and the potential) assumes more negative values at a given pH; thus, number of protons released per one adsorbed heavy metal cation the electrostatic conditions are more favorable for adsorption do not depend on temperature, the standard enthalpies of adsorpof cations. Adsorption of cations at oxides at constant s 0 is tion on alumina at constant pH are 65 kJ/mol for Ni, 90 kJ/mol relatively insensitive to temperature (14). Adsorption data for Gd, and 90 kJ/mol for Y. The presence of alkaline earth metal are often presented in the form of percentage uptake (pH) cations (up to 0.05 mol dm 03) does not significantly influence the curves, at a given initial concentration of heavy metal ions, adsorption of nickel and gadolinium on alumina. Thus, barium ionic strength, and solid-to-liquid ratio. When the temperaand magnesium do not compete with heavy metal cations for the ture increases, these S-shaped curves are shifted to lower surface sites. ᭧ 1997 Academic Press pH values. The shift of pH 50 (the pH value at which 50%