Thermodynamics of Organic Acid Sorption to Goethite

ABSTRACT Adsorption to minerals is a key mechanism in stabilizing organic carbon in soils. We used isothermal titration calorimetry (ITC) to quantify the thermodynamics of binding of citric acid, oxalic acid, and salicylic acid to four goethites with different specific surface areas (SSA, 14–120 m 2 g −1 ). Thermodynamic parameters could be determined for sorption of citric and salicylic acids, while flocculation of particles prevented their quantification for sorption of oxalic acid. For citric acid adsorption, ∆H shifted from −23.5 ± 0.57 to −27.0 ± 0.47 kJ mol −1 and ∆S from −8.8 ± 1.54 to −29.9 ± 0.13 J mol −1 K −1 with increasing SSA and broader (110) diffraction peaks of goethite, thus reducing ∆G from −20.7 ± 0.02 to −18.0 ± 0.03 kJ mol −1 . Salicylic acid adsorption was more exothermic ( ∆H −40.53 ± 1.93 kJ mol −1 ) and accompanied by a larger loss of entropy ( ∆S −65.1 ± 1.91 J mol −1 K −1 ), possibly due to chelation of its ortho hydroxyl and carboxyl groups to single iron atoms on the mineral surface. These results demonstrate that ITC can decipher adsorption thermodynamics of organic ligands to mineral surfaces, but ligand‐induced flocculation can render the interpretation of results difficult. Crystallite size and lattice defects of adsorbent minerals influence the thermodynamics of sorption by determining the conformation of organic molecules sorbed to goethite surfaces.