Flocculation properties of biodegradable amphoteric chitosan-based flocculants

► Preparation of amphoteric chitosan-based flocculants. ► Notable improved solubility and salt-resistance of the amphoteric flocculants. ► Investigation of the flocculation performances at the laboratory and pilot scales. ► Study on the flocculation mechanism from the conformations of polymeric flocculants and flocculation kinetics. In this study, a kind of amphoteric chitosan-based flocculants (3-chloro-2-hydroxypropyl trimethyl ammonium chloride (CTA) modified carboxymethyl chitosan, denoted as CMC-CTA) have been prepared. The amphoteric flocculants showed notable improvement in solubility and salt-resistance in the whole pH range compared with its precursors, i.e., chitosan and cationic chitosan. The flocculation properties of CMC-CTA were systematically studied at both the laboratory and pilot scales. At laboratory scale, kaolin suspension was employed as synthetic water in the beaker experiment. The effects of pH, temperature, and original turbidity of untreated water on the dosage and flocculation performances were investigated, respectively. At pH below and above the isoelectric point of CMC-CTA, the optimal flocculation performances were obtained with a dosage of 0.03–0.20 mg/L and 0.8–1.0 mg/L, respectively. The flocculation mechanism was discussed in detail from the morphologies of polymeric flocculants and flocculation kinetics: The shape and charge of polymer chains in the solution greatly influenced the final flocculation performances; The flocculation kinetics agreed well with the particle collision model. Moreover, raw water from the Zhenjiang part of the Yangtze River in China was used for the further investigation of the flocculation properties at the pilot scale. The influences of dosage, sedimentation time, and mechanical mixing rate to the flocculation properties were studied by orthogonal testing. The optimal experimental condition was then obtained. Compared with the currently used flocculants in the Zhenjiang Water Factory, i.e., polyferric sulfate, which has an optimal dosage around 40 mg/L, the value of CMC-CTA (1.1 mg/L) decreased greatly. Above all, the flocculation experiments at the laboratory and pilot scales both indicate that the amphoteric chitosan-based flocculants have good flocculation performances in a wide pH range.