Sulphate sorption in relation to properties of podzolic and brunisolic soils in northeastern Ontario

Abstract

An insight into sulphate (SO42−) retention by podzolic and brunisolic soils was obtained by examining SO42− sorption by B horizons from forest soils in northeastern Ontario. The amount of water soluble and adsorbed SO42− present in the soils and the ability to adsorb additional SO42− was evaluated. Relationships between soil pH, organic carbon and different iron (Fe) and aluminum (Al) fractions with water soluble and adsorbed SO42− were examined. Using a surface complexation approach, modelling was carried out to predict the soil SO42− sorption capacity at any given pH. The potential SO42– adsorption capacity was also measured by applying adsorption equations. Both the Freundlich and Langmuir sorption isotherms gave comparative fits to the SO42− adsorption data. The soils contained small amounts of water extractable SO42− (0.1 to 0.8 mmole kg–1). There was a significant positive correlation between water extractable SO42− and organic carbon in the podzolic soils but not for the brunisolic soils. Approximately 90% of the sulphate extracted was contained in the insoluble SO42− fraction. Sulphate sorption by both soil types was most closely related to amorphous inorganic Al. In brunisolic soils, total adsorbed SO42– also had a strong positive correlation with the oxalate extractable silica fraction and a negative correlation with pH. Sulphate adsorption simulations were close to the measured values. Key words: Soluble SO42−, adsorbed SO42−, Podzol, Brunisol, amorphous Al oxide, SO42− adsorption model