| Water is one of the most essential substances for survival, daily life and production activity of humans. However, increasing worldwide contamination of water has become one of the crucial environmental problems nowadays. Therefore, it is urgently needed to develop green and efficient water treatment technologies. During the past decades, a multitude of processes for the purification of water have been studied for a wide variety of pollutants. Among them, flocculation is widely applied because of its high efficiency, facile operation and low cost. During the flocculation process, the choice of flocculants greatly affects the final effluent quality. Since traditional metal-based and synthetic polymeric flocculants have been found to cause potential secondary pollution, extensive attention has been given to natural polymeric ones which offer inherent advantages such as wide availability, renewability and environment-friendliness. Among various types of natural polymers, chitosan is one of the most outstanding candidates due to the presence of large numbers of primary amino groups on its molecular backbone. Furthermore, amphoteric chitosan-based flocculants containing both cations and anions, obtained through proper chemical modification by introducing new functional groups, not only can be used for the removal of different charged contaminants, but also enjoy merits of wider applicable pH range, enhanced coordination ability and better salt-tolerance.In the current work, three novel kinds of amphoteric chitosan-based flocculants were synthesized using two-step methods, respectively. They were modified by introducing functional groups of small molecular weight (3-chloro-2-hydroxypropyl trimethyl ammonium chloride modified carboxymethyl chitosan, denoted as CMC-CTA), grafting neutral polymeric branches (carboxymethyl chitosan-graft-polyacrylamide copolymer, denoted as CMC-g-PAM) and grafting positively charged polymeric branches (carboxymethyl chitosan-grqft-poly[(2-methacryloyloxyethyl) trimethyl ammonium chloride], denoted as CMC-g-PDMC), respectively. Then, flocculation performances of the three flocculants for the removal of several different contaminants in water were systematically studied, and the flocculation mechanisms in various flocculation systems were simultaneously discussed in detail. At last, a new modified model of flocculation kinetics in combination with fractal geometry of flocs was developed. On the basis of the modified model, an in-situ monitoring technique of flocculation kinetics was established, and validated in different flocculation systems using amphoteric chitosan-based flocculants aforementioned. Several interesting results have been drawn:(1) All the three kinds of flocculants prepared under suitable conditions exhibited satisfied flocculation performances for the removal of kaolin suspension in a wide pH range. Charge neutralization effect or patching effect was the dominant flocculation mechanism when the surface charges of flocculants and kaolin suspended particles were opposite; yet bridging effect predominated when both flocculants and kaolin suspended particles had negative surface charges. Among the three kinds of flocculants, CMC-CTA strengthened charge attraction between flocculants and kaolin particles under acid condition; CMC-g-PAM improved bridging effect of flocculants; CMC-g-PDMC combined the advantages of both above, displaying obviously enhanced properties in aspects of optimal dosage, pH sensitivity and floc properties.(2) All the three kinds of flocculants prepared under suitable conditions were proven effective in flocculating both anionic dye methyl orange (MO) and cationic dye light yellow7GL at corresponding favored pH values. The optimal pH values were4and11for the removal of MO and7GL, respectively. For all the three kinds of flocculants, charge neutralization effect was the predominant flocculation mechanism in flocculating dye molecules. Besides, the grafted chains in CMC-g-PAM and CMC-g-PDMC also contributed much to bridging and sweeping effects, resulting in better floc properties.(3) In addition to the removal of inorganic kaolin particles and organic dye molecules, CMC-g-PDMC also demonstrated high flocculation performance for the removal of microorganism Escherichia coli (E. coli), due to large amounts of quaternary ammonium groups on the grafted chains. Besides traditional charge attraction and bridging effects of the flocculant, CMC-g-PDMC also exerted a "breaking-cell-walls" effect resulting from the positively charged branches of PDMC: The flocculant could break the cell walls of E. coli, engender leakage of intracellular substances, and finally kill the bacteria.(4) The modified model of flocculation kinetics in combination with fractal theory and the in-situ monitoring technique based on it represented high data accuracy and experimental feasibility for various flocculation systems, demonstrating high application potential. The theoretic simulated curves were in good agreement with the experimental data with high determination coefficients (R2) larger than0.90, and were especially accurate for description of the initial stage in flocculation (R2>0.99). Based on the kinetics constants fitted through the modified model, the kinetic constants for particles’ aggregation of CMC-g-PDMC were significantly larger than those of CMC-CTA and CMC-g-PAM.In light of the results above, the amphoteric chitosan-based flocculants after proper chemical modification in this work exhibited satisfied flocculation performance for the removal of several different contaminants in water;a reliable and simple-operated in-situ monitoring technique of flocculation kinetics based on the model modified by fractal theory was established. All these flocculants and the monitoring method of flocculation kinetics were promising in real water treatment. |
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