New insights into the different adsorption kinetics of gallic acid and tannic acid on minerals via 1H NMR relaxation of bound water

The slower adsorption of lower molecular weight organic molecules remains poorly understood. This study investigated the adsorption kinetics of gallic acid (GA) and tannic acid (TA) on kaolinite (Kao), montmorillonite (Mon) and hematite (Hem), with an emphasis on the role of the bound water on the minerals. The lower adsorption of TA and GA on Kao than on Mon attributed to the lower specific surface area of Kao. Because of the electrostatic attraction, the adsorption of TA and GA on Hem was higher than that on Mon, even the specific surface area of the former was much lower than that of the later. The adsorption rates of TA on the three minerals were generally two orders of magnitude higher than those of GA. The adsorption kinetics of GA was strongly diffusion dependent; however, the diffusion process had limited influence on TA adsorption kinetics. The decreased c values of the intraparticle diffusion model of GA with increasing ionic strength provided additional direct evidence for the diffusion-dependent adsorption and the reduced hindrance by bound water via hydration layer compression. However, hydration layer compression had no effect on TA adsorption kinetics. The reduced 1H NMR relaxation rate of bound water indicated that the bound water quantity on minerals decreased with increasing ionic strength, which proved the occurrence of hydration layer compression. This study highlighted the importance of bound water and the relative sizes of organic molecules in the adsorption kinetics of organic compounds on minerals, which should be carefully considered for their environmental fate studies.