Modeling the distribution of aluminum speciation in soil water equilibria with the mineral phase jurbanite

Abstract

This paper presents the use of an equilibrium-based computer model to investigate the speciation of Al in soil solutions that are assumed to be in equilibrium with mineral phase jurbanite.The model predicts the distribution of various inorganic Al species,Al-organic matter complexes,and polymeric-Al species in solution as a function of pH.Using input data from several published sources, the model was used to demonstrate how change in soil solution composition will impact Al solution chemistry in equilbrium with several possible Al solid phases: jurbanite,basaluminite,alunite and gibbsite.Emphasis was placed on jurbanite due to its reported presence in soils impacted by acidic deposition.In the presence of jurbanite,the model predicts that SO4 2- will have a substantial influence on the distribution of Al species and the concentration of total soluble Al,while the concentrations of organically complexed and fluoride complexed Al are minimal in the pH range studied.The model was also used to predict Al speciation for published soil solution data, assuming the soil solutions were in equilibrium with jurbanite.Predicted concentrations of total dissolved Al,inorganic Al and Al-or-ganic complexes agreed within an order of total dissolved Al,inorganic Al and Al-organic complexes agreed within an order of magnitude,however,the model consistently over predicts dissolved concentrations using the current set of equilibrium constants.Nevertheless,the model results imply that the presence and subsequent dissolution of jurbanite in soils impacted by acidic deposition will markedly influence soil solution Al chemistry.