SULFATE ADSORPTION ON A VARIABLE CHARGE SOIL AND ON REFERENCE MINERALS

The adsorption of sulfate on deltaAl2O3, on aluminum-coated TiO2, and on an acid forest soil from the Harz Mountains, Germany, was investigated, as well as the potential effect of this adsorption on the rate of weathering of deltaAl2O3. The experimental data indicate that: (1) sulfate ions are adsorbed only on positively charged surfaces; (2) the adsorption of sulfate is dependent on the ionic strength of the solution; (3) sulfate ions have no effect on the weathering rate of aluminum oxide surfaces. The data could be described within the framework of the Triple Layer Model by the equilibrium: > MOH2+ + SO42- + pH+ = (> MOH2+) (SO4)Hpp-1 The intrinsic constant K of this equilibrium is, for the different systems investigated, found to be in the range: for p = 0: 4.8 < logK(SO4) < 5.5 for p = 1: 9.3 < logK(HSO4) < 12.7 Although these results indicate an initial electrostatic bonding of sulfate ions to mineral surfaces, longer-term studies are required to establish whether covalent bonding may eventually develop with aging or with temporary high concentrations of sulfate during dry periods.