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 5mM CaCl₂. Adsorption of aqueous Ca²⁺ on Fe³⁺-montmorillonite was high (0.59mmo_c/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.