The Effect of Acetic Acid and Acetate Ions on Sorption–Desorption of a Mixture of Phenolic Acids by Modified Kaolinite

Abstract

The sorption and desorption of a mixture of phenolic acids by the kaolinite modified with aluminum hydroxide was studied in the presence of acetate buffer. The experiments were performed in a 1-mL flow-through microcolumn in 5 and 50 mM sodium acetate buffer (pH 4.5) at a flow rate of 0.5 mL/min. The total concentration of phenolic acids was 0.01 mg/mL (0.06 mM); the concentration of each acid was 0.01 mM. Sodium acetate buffer (5 and 50 mM, pH 4.5) and 50 mM sodium acetate buffer supplemented with 0.1 mM oxalic acid were used for desorption. The concentration of phenolic acids was determined by reverse phase high-pressure liquid chromatography. The following order of sorption was observed: gallic > protocatechuic $$ \gg $$ p-hydroxybenzoic ~ vanillic ~ ferulic ~ syringic acid. The sorption of phenolic acids in 50 mM buffer comprised 18–35% of their sorption in 5 mM buffer, suggesting the competition of acetate ions and phenolic acids for binding sites on the mineral. The order of desorption of phenolic acids was opposite to the order of sorption and generally correlated with the stability constants of the phenolic acid complexes with aluminum hydroxide. All acids, except for gallic and protocatechuic acids, were weakly bound to the mineral and were almost completely (88–98%) desorbed with 5 mM acetate buffer. The total desorption of gallic and protocatechuic acids by all eluents comprised 25 and 45% of their sorbed amount, respectively. Desorption was significant only in 50 mM acetate buffer (12 and 23%) and in the same buffer with 0.1 mM oxalic acid (10 and 15%). Thus, it has been shown that the distribution of phenolic acids between the solid phase and solution is largely determined by the presence of competing aliphatic compounds. Based on desorption experiments, we propose possible types of complexes of phenolic acids and aluminum hydroxide on the surface of the mineral.