| Water is one of the most basic requirements for human daily life. Flocculation is widely applied in the primary treatment of drinking water. However, traditional inorganic flocculants, such as aluminum and iron salts, are suffered to increase in metal ions concentration in water, which may have human health implications after long-term drinking. It is strategically important to develop environment-friendly and high-efficient flocculants. Recently, more attentions have been paid to natural polymer based flocculants in water treatment, since they are believed to be wide-resource, nontoxic, environment-friendly and high-efficient materials. Furthermore, they have been even acclaimed as "Green Flocculants of21st Century". However, disadvantages such as poor reactivity, low molecular weight and poor solubility have limited their practical applications. Chemical modifications are employed to improve their performances.In this work, a strong cationic chitosan based flocculant (Chitosan-CTA) has been prepared, and3-chloro-2-hydroxypropyl trimethyl ammonium chloride (CTA) was used as etherification agent. A series of Chitosan-CTA samples with various substitution degrees have been obtained. The flocculation properties of this kind of cationic chitosan (Chitosan-CTA) were systematically studied at both the laboratory and pilot scales. At the laboratory scale, the effects of flocculant dosage, temperature, original turbidity of untreated water, pH and various ions present in test water were investigated by beaker experiment using kaolin suspension as synthetic water. Moreover, raw water from the Zhenjiang part of the Yangtze River in China was used as test water for the further investigation of the flocculation properties at the pilot scale. The influences of dosage, sedimentation time, and mechanical mixing rate on the flocculation properties were studied by orthogonal test, and the optimal experimental conditions were obtained. The effect of the substitution degree of CTA on the flocculation properties was also investigated at both the laboratory and pilot scales. The flocculation processes are well described by the flocculation kinetics model of particle collisions in combination with fractal theory, providing the necessary theoretical basis for the design and application of novel flocculants. Several interesting results have been drawn:(1) At the laboratory scale, the dosage of the flocculants, neither too high nor too low, could result in better flocculation effects, and the zeta potential was near zero at the optimal dosage around0.05mg/L, which further proved that the flocculation mechanism of charge neutralization was dominant. And more flocculants are required as the original turbidity of synthetic water increases, while the dosage requirements would decrease with temperature increase. Chitosan-CTA has better flocculation properties in acid conditions. In addition, flocculation performances of Chitosan-CTA are always promoted by salt additives for compression of the electric double layer of the kaolin particles by metal ions. Furthermore, in comparison with traditional flocculants, Chitosan-CTA has lower dosage requirement but higher flocculation performances. In addition, the floc sizes and its fractal dimensions have been also measured by light scattering during the flocculation process. The floc sizes and its fractal dimension both increase until the dosages reach the optimal one, after which flocs aggregation no longer takes place and the floc sizes as well as its fractal dimension even decrease because of steric stabilization and electrostatic repulsion. Moreover, the floc sizes and fractal dimension always increase with increasing time and reach the plateau after flocculation equilibrium in each dosage.(2) At the pilot scale, it is found that the flocculant dosage is the most important factor to the flocculation properties among the three test external factors in this current work. Furthermore, the optimal flocculation conditions are obtained as follows:dosage of flocculants is1.1mg/L, sedimentation time is20min, and mechanical mixing rate of Nos.1,2, and3tanks in the triple coagulative sedimentation tank is200,100, and50rpm, respectively. In addition, the results of the conventional water quality test indicates that the water treated by strong cationic chitosan-based flocculants as mentioned above is fully measured up the Chinese standard for drinking water quality of GB5749-2006. Moreover, in comparison with polyferric sulfate, current used flocculant in Zhenjiang Water Supply Company, the optimal dosage of Chitosan-CTA are much lower only fortieth of that. Although this chitosan based flocculant is quite expensive, it still shows higher cost-effective in water treatment.(3) Results drawn from the flocculation test at both the laboratory and pilot scales indicate that Chitosan-CTA with higher content of CTA shows better flocculation performance in each dosage because Chitosan-CTA with more cationic ions on the chain backbone has more efficient charge neutralization and bridging flocculation effects. Furthermore, flocculation kinetics model of particles collisions are employed and the theoretic simulated curves fits the experimental data quite well, supporting the aforementioned discussion further.Above all, the flocculating experiments at laboratory and pilot scale both indicate that this strong cationic chitosan-based flocculant shows good flocculating performances in water treatment, and has wide prospects of practical applications. |
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