Polycation adsorption on montmorillonite: pH and T as decisive factors for the kinetics and mode of chitosan adsorption

Abstract

AbstractAdsorption of the polycation chitosan on montmorillonite was studied at different pH values from 4.5 to 6.5 and at temperatures of 25, 50 and 75ºC. The amount of chitosan adsorbed increased with temperature, indicating that temperature is a decisive factor. At pH 5.0 and 25ºC the amount adsorbed was 1.18 molc kg–1, whereas it was 1.5 times higher (1.79 molc kg–1) at 75ºC. The uptake of chitosan increased significantly at higher pH. This can be attributed to the decrease in the degree of protonation. Surface charge and X-ray diffraction measurements indicate that most of the chitosan is adsorbed in the interlayer, where mono- and bilayer structures are formed.The kinetics of chitosan adsorption also depend on temperature and pH. At ⩾ 50ºC, the adsorption mechanism of chitosan on montmorillonite is closest to the intraparticle diffusion model, whereas at lower temperature (25ºC) the adsorption process is closer to the pseudo-second order model. The pH of the solution affects the protonation degree of chitosan and the mode of adsorption on montmorillonite, but not the adsorption rate.For chitosan-montmorillonite prepared at pH 5.0 and 75ºC, the effective anion exchange capacity (AEC) was found to be 80% (0.36 molc kg–1) of the calculated value. The relatively large AEC and the location of most of the anion exchange sites in the interlayer make chitosan-montmorillonite an interesting prospect as an adsorbent for water-purification procedures.