Interactions of aromatic acids with montmorillonite: Ca2+- and Fe3+-saturated clays versus Fe3+–Ca2+-clay system

Aromatic acids are involved in important soil processes, such as mobilization of microelements by plants and formation of humic components. Interactions of aromatic acids (benzoic, vanillic, p-coumaric, syringic and ferulic) with Ca2+- and Fe3+-saturated montmorillonite were studied, and the role of aqueous Ca2+ in aromatic acid adsorption to Fe3+-montmorillonite was investigated. Removal of aromatic acids at pH 3 and changes in partial molar free energy of adsorption of benzoic acid were higher for Fe3+-montmorillonite than for Ca2+-montmorillonite. Adsorption of aromatic acids on Ca2+-montmorillonite decreased similarly with increasing pH. The aromatic acids could be divided into two groups according to their behavior on the surface of Fe3+-montmotillonite with increasing pH: i) ferulic and syringic and ii) benzoic, vanillic, and p-coumaric. Adsorption of the studied acids by Fe3+-montmorillonite was affected by their molecular structures. Adsorption of benzoic acid by Fe3+-montmorillonite increased in the presence of 5 mM CaCl2. Adsorption of aqueous Ca2+ on Fe3+-montmorillonite was high (0.59 mmoc/g), and was not influenced by the concentration of benzoic acid. Desorption of benzoic acid from Fe3+–Ca2+-montmorillonite was greater than that from Fe3+-montmorillonite. For the former system, adsorption hysteresis of benzoic acid increased with increasing acid concentration. The current study demonstrates a role of aqueous Ca2+: when adsorbed from the soil solution to iron-containing surfaces, it can change their adsorptive properties towards low-molecular-weight acids.