Sulfate adsorption by variable charge soils: Effect of low-molecular-weight organic acids

Sulfate (SO42-) movement and transport in soils has received considerable attention in recent years. In most soils, SO42- coexists with a variety of natural organic compounds, especially organic acids. Studies were conducted to assess the effect of low-molecular-weight organic acids (eight aliphatic and five aromatic acids) on SO42- adsorption by variable charge soils from Chile and Costa Rica. The effects of type of organic acid, pH, type of soil, and organic acid concentration were investigated. Tn one experiment, a 1.0 g soil sample was equilibrated with 25 ml 0, 0.5, 1.0, 2.0, 4.0, or 6.0 mM K2SO4 in 1 mM NaCl in the presence or absence of 5 mM citric acid. In the second set of experiments, the adsorption of 2 mM SO42- in soils at pH 4 or pH 5 in the presence or absence of one of 13 organic acids at a concentration of 2 mM or 5 mM was studied. Results showed that citric acid significantly decreased SO42- adsorption by the two soils. Sulfate adsorption decreased with increasing pH of the equilibrium solution. Aliphatic acids, with the exception of cis-aconitic acid, decreased the amount of SO; adsorbed by the two soils, with oxalic, tartaric, and citric acid showing the greatest effect. Th-differences in pH values of the equilibrium solutions in the presence and absence of organic acids were significantly, but: negatively, correlated with the amount of SO42- adsorbed. suggesting chemisorption of SO42- and the release of hydroxide ions. The ionization fraction values of the organic acids at the equilibrium pH were correlated with the amounts of SO42- adsorbed, suggesting that the protonation of surface hydroxyl groups of the mineral phase increased as the strength of the ionization of the acid increased, thus creating more positively charged surfaces.