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

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.