MODELING PH-DEPENDENT SULFATE ADSORPTION IN THE BS HORIZONS OF PODZOLIZED SOILS

A model was developed that predicted the pool of adsorbed SO4 in the Bs horizons of northern Scandinavia as a function of pH and the solution SO4 concentration, assuming that 2 H+ are consumed for every SO42- adsorbed. To accomplish this, a sequential leach procedure with acid MgSO4 was used which facilitated the determination of SO4 adsorption capacity and the characterization of the adsorption isotherm. The Temkin equation showed the best fit within the range of SO4 concentrations used in this study. The variability between different soils in terms of adsorption capacities and isotherm slopes was modeled using linear and multiple regressions with other soil characteristics. Model results showed good agreement with other experimental observations in northern Scandinavia. The model, however, underestimated SO4 adsorption for soils from southern Scandinavia, which may be because of regional differences in, for example, adsorbent crystallinity. Furthermore, the results show that most pristine soils in northern Scandinavia retain 20 to 40% as much SO4 as they would retain if they were located in a severely acidified area. Therefore SO4 desorption following a decreasing atmospheric S deposition would only involve part of the adsorbed SO4. Only a small hysteresis effect was observed when the adsorption isotherm was compared to the desorption isotherm for one soil.