THE ROLE OF BASIC ALUMINUM SULFATE MINERALS IN CONTROLLING SULFATE RETENTION IN THE MINERAL HORIZONS OF 2 SPODOSOLS

We collected soil solutions during the spring snowmelt at two depths (25 and 75 cm) using tension-free lysimeters in two Humic Cryorthods (Hermine and Coniferous) located in forested catchments. We also extracted soil solutions by centrifugation from the B and the C horizons of these soils. Although most data points cluster around the solubility line of natural gibbsite (pKs = 33.2), the relationship between Al3+ activity and solution pH showed a marked departure from Al(OH)3 solubility with a slope of −1.91. Moreover, at pH values below 4.6, solutions were slightly undersaturated with respect to natural gibbsite. Results from thermodynamic calculations suggested that the Hermine B, and probably the Coniferous B, horizons have attained chemical conditions sufficient to enter the stability field of a basic aluminum sulfate mineral formed in the presence of Al(OH)3. Mineral stability relationships indicated that the solid phase of concern could be alunite (pKs = 85.4) and that the apparent saturation status of the soil solutions with respect to alunite increased with increasing reaction time. However, physical evidence for the presence of alunite in these horizons is still lacking. Solutions from the C horizons were all clearly undersaturated with respect to any Alx(OH)y(SO4)z mineral. While the possible formation of alunite could be affecting sulfate retention in these acid soils, the results do not exclude the pH-dependent sulfate adsorption mechanism proposed by a number of investigators. This study emphasizes the need for caution when interpreting the results of sorption experiments carried out in conditions of saturation with respect to a mineral phase involving the adsorbate. © 1990 Williams and Wilkins.