Thermodynamics of Peat-, Plant-,\nand Soil-Derived Humic Acid\nSorption on Kaolinite

Abstract

Humic acids (HAs) form coatings on clays and minerals\nthat can play an important role in nutrient and contaminant\nmigration in soil and water. Humic acid−clay mineral\ninteractions are known to be affected by pH and ionic\nstrength, but little attention has been paid to the effects\nof temperature. In this paper we report the stoichiometry and\nthermodynamics of interactions of aqueous HAs (isolated\nfrom two peats, two soils and a marine alga with a\nmethod that removes lipids) with kaolinite clay, Al₂Si₂O₅(OH)₄, at seven temperatures from 5.0 to 35.0 °C in 0.05 M\nNaCl at pH 3.5. All the sorption isotherms exhibit\nconsecutive steps ascribed to HA monolayer and bilayer\nformation, respectively. Site capacity comparisons suggest\ndifferent HA molecular conformations on kaolinite.\nLinearly correlated enthalpy and entropy changes for HA\nsorption point to the importance of hydration and dehydration\nin the sorption mechanism.