Patagonian salt marsh soils and oxidizable pedogenic pyrite: solid phases controlling aluminum and iron contents in acidic soil solutions

The sulfidic materials present in salt marshes could be oxidized forming sulfuric acid, increasing the toxic levels of both Al and Fe available to plants. The objectives of this study were: (a) to evaluate the mechanism of acid generation from the oxidation of sulfidic materials, and (b) to predict solid phases governing the dissolved Fe and Al concentrations in soils at low pH. The study was conducted in 14 soil profiles associated to eight salt marshes situated along the Atlantic coast of Patagonia. The potential acidity was estimated by the peroxide-oxidizable sulfuric acidity method (POSA). To predict the availability of Fe and Al at low pH, the solid phase equilibrium that governs the solubility of these elements through ion activity of the products was determined. The scanning electron microscopy analysis in sulfidic materials reveals the occurrence of framboidal pyrite. The relative variability of POSA at low pH values may indicate retention of sulfates by Al and Fe hydroxides, producing the formation of basic sulfates of iron and aluminum. At pH>5.5, Fe2+ and Al3+ activities show an equilibrium with amorphous oxy-hydroxides of Fe(OH)(3) and gibbsite, respectively. As the pH begins to decline below 5.5, Fe2+ and Al3+ activities show an equilibrium with respect to soil-Fe(OH)(3) and Al(OH)(3) amorphous, respectively. While for more acidic conditions, the solid phase in predicting both Fe2+ and Al3+ activities was basic iron sulfate and jurbanite. The acidic soil solutions with pH<3, Fe2+ and SO42- activities show an equilibrium with goethite and melanterite, respectively. As a consequence of acid generation, phosphorus adsorption by aluminum and iron oxide minerals was detected.