Adsorption of tannic acid on chitosan-montmorillonite as a function of pH and surface charge properties

Chitosan, a natural biopolymeric cation, is a candidate to modify montmorillonite for the adsorption of anions. As an anionic organic pollutant the adsorption of tannic acid was studied. Because of protonation/deprotonation reactions of both chitosan-montmorillonite and tannic acid, the adsorption process is strongly pH-dependent. The objective of this work is to characterize the pH dependency of adsorption in combination with surface charge determinations. Montmorillonite was modified with different amounts of chitosan, corresponding to 20–1000% of the cation exchange capacity (CEC). The deacetylation degree of chitosan was determined by polyelectrolyte titration and was found to be 74%. The uptake of chitosan was determined by the C-content. The interlayer expansion was investigated by X-ray powder diffraction. The adsorption capacity for tannic acid was investigated with the batch technique at pH 3, 4, 5 and 8. As a measure for the adsorption properties, the electrokinetic surface charge was determined with a particle charge detector. The uptake of chitosan by montmorillonite is up to 152% (1.69 molc kg− 1) of the CEC. The resulting anion exchange capacity of chitosan-montmorillonite calculated from C-content is 0.43 molc kg− 1. At low loadings with chitosan (24.7 and 49.5% uptake), a monolayer is formed in montmorillonite. At an uptake of 96.8%, a bilayer structure is observed, which becomes more dominant at higher loadings. On the external surface, a monolayer of chitosan was formed. From pH 4 to 8, the surface charge of all modified montmorillonites is with − 9 to 8 mmolc kg− 1 close to the point of zero charge. The maximal adsorption capacity for tannic acid is found with 240 g kg− 1 (0.14 molc kg− 1) at pH 4. The adsorption process fits in well with the Freundlich isotherm. At lower as well as higher pH values the adsorption capacity decreases up to about 25%. Most probably the exchange sites in the interlayer do not contribute to the adsorption of tannic acid. The observed surface charge is lower than the adsorbed amount of tannin. It is thought that tannin is adsorbed also by van der Waals forces besides ionic forces.