Agriculture, Ecosystems and Environment, 47 ( 1993 ) 87-102 Elsevier Science Publishers B.V., Amsterdam 87 Sulfate adsorption on a variable charge soil and on reference minerals Laurent Charier*, Nancy Dise 1, Werner Stumm Institute of Aquatic Sciences (E.A. W.A.G.), CH 8600 Dubendorf, Switzerland Abstract The adsorption of sulfate on JAl203, 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 $AI203. 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: > MOH~ + SO ~- +pH + = ( > MOH+ ) (SO4) H~,- t The intrinsic constant K of this equilibrium is, for the different systems investigated, found to be in the range: for p=0: 4.8<1og Kso, < 5.5 for p= 1: 9.3<1og KHso, < 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. Introduction The capacity of soils to adsorb sulfate is important in assessing the impact of acid deposition on terrestrial ecosystems. Since basic cations such as cal- cium and magnesium are leached from soils with sulfate as a counter-ion (Johnson and Cole, 1980), sulfate retention can act as a buffer on cation depletion and thereby on soil acidification. Moreover, deciphering sorption mechanisms is important for predicting both the adsorption capacity of a soil and the rate of sulfate desorption, should the input of sulfate decrease. *Corresponding author: Environmental Geochemistry Group, LGIT, University of Grenoble-I (VJF), BP53, F-38041 Grenoble, Cedex 9, France. ~Present address: Norwegian Institute of Water Research (NIVA), P.O. Box 69, Korsvoll N- 0808, Oslo 8, Norway. © 1993 Elsevier Science Publishers B.V. All rights reserved 0167-8809/93/$06.00