Interactions between chitosans and bacterial suspensions: adsorption and flocculation

Chitosans with different chemical composition were applied to flocculate Escherichia coli suspensions. The adsorption of chitosans was followed both by applying fluorescence labeled chitosans and monitoring changes in zeta potential values of the bacterial cells. The effects of chitosan molecular weight and composition, identified by the fraction of acetylated units (F-A), were examined, as well as environmental conditions such as pH and ionic strength. The adsorption of chitosans to E. coli cells increased strongly with pH, the adsorbed amounts increased approximately 40% if pH was increased from pH 5 to 6.5. Despite their low charge density, the chitosans with higher F-A adsorbed in higher amounts and reversed the cell surface charge most effectively. The chitosans with low molecular weights adsorbed most. Ionic strength did not affect the adsorption of highly acetylated chitosan with F-A 0.49, whereas for chitosan with F-A 0.01, adsorption increased with ionic strength. The combination of flocculation and adsorption data clearly showed that charge neutralization was not the main flocculation mechanism. Several results pointed to bridging as one dominating mechanism for flocculation. (C) 2002 Elsevier Science B.V. All rights reserved.