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

• Published in: Biology and fertility of soils Vol. 26; no. 3; pp. 157 - 163
• Main Authors: Martinez, C.E, Kleinschmidt, A.W, Tabatabai, M.A
• Format: Journal Article
• Language: English
• Published: Berlin Springer 01-02-1998
• Subjects: adsorption; Agronomy. Soil science and plant productions; Biological and medical sciences; Chemical, physicochemical, biochemical and biological properties; concentration; Earth sciences; Earth, ocean, space; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; Geochemistry; ionization; Mineral components. Ionic and exchange properties; organic acids and salts; Physics, chemistry, biochemistry and biology of agricultural and forest soils; Soil and rock geochemistry; soil pH; Soil science; soil types; sorption; sorption isotherms; sulfates; variable charge soils; Chile; Costa Rica; South America; Central America; America; Aliphatic compound; Property of soil; Composition effect; Arenoic acid; Experimental study; Sulfates; SOIL CHEMISTRY; Organic acids; Molecular weight; Tropical soil; Edaphic factor; Adsorption; Low; Variable charge; Concentration effect; Phenols; pH; SOIL TYPES; Physicochemical properties
• ISSN: 0178-2762; 1432-0789
• DOI: 10.1007/s003740050361

Sulfate (SO4(-2)) movement and transport in soils has received considerable attention in recent years. In most soils, SO4(-2) 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 SO4(-2) 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. In 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 SO4(-2) 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 SO4(-2) 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 SO4(-2) adsorbed by the two soils, with oxalic, tartaric, and citric acid showing the greatest effect. The 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 SO4(-2) adsorbed, suggesting chemisorption of SO4(-2) and the release of hydroxide ions. The ionization fraction values of the organic acids at the equilibrium pH were correlated with the amounts of SO4(-2) 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.