Microwave Assisted Synthesis Of Amphoteric Chitosan And Its Application As Flocculant For Water Treatment

Chitosan is the product of deacetylated chitin, and it is widely distributed in thearthropods, crustaceans, fungi and yeast. Moreover it is the only cationic naturalpolymer flocculant in nature. However, it is necessary to be modified when chitosan isused as flocculant, as it has smaller molecular weight and is only soluble in dilute acidrather than in water or organic solvents. The chitosan molecules contains manychemically active groups such as amino, hydroxyl group. In addition, the amino on theC₂-position is called the primary amino group, which has a strong activity andprovides a great potential for the modification of chitosan. Currently the chitosanderivatives were prepared mainly through a variety of chemical modification methodssuch as chitosan-acyl, carboxy, alkylated, crosslinked, hydroxy, and graft copolymeri-zation. In this paper, graft copolymerizationthe which was one of the most commonmodification method was used to improve physical and chemical properties ofchitosan for better flocculation effect.Amphoteric chitosan flocculant （chitosan-acrylamide-acrylic acid, CTS-g-AM-AA） was prepared by the method of microwave irradiation. When ceric ammoniumnitrate was used as a free radical initiator, acrylamide （AM） and acrylic acid （AA）were grafted onto the framework of chitosan under the nitrogen atmosphere. Throughthe analysis of SEM （scanning electron microscopy）, FTIR （fourier transform infraredspectroscopy） and TG-DSC, the morphology and thermal properties of graftcopolymer were further characterized. The study showed that the method ofmicrowave irradiation could prepare the CTS-g-AM-AA flocculant successfully, andgreatly shorten the reaction time, and improve the graft ratio and graft efficiency.Under the optimal reaction conditions（reaction temperature50℃、reaction time12min、initiator concentration4mmol/L、mass ratio of acrylamide and chitosan6：1）,the raft rate of the graft copolymers were76%, and the grafting efficiency could reach12%. Characterization analysis results showed that, CTS-g-AM-AA graft copolymerwas successfully synthesized by microwave method, and compared with formerproduct, its surface morphology changed significantly and its flocculation performa-nce improved. But its thermal stability weakened, and it would depolymerized moreeasily under high temperature. The amphoteric chitosan （chitosan-acrylamide-acrylic acid, CTS-g-AM-AA）prepared by microwave irradiation was used as a flocculant for the treatment ofsimulated kaolin suspension and printing and dyeing wastewater, and the effects of thepH of wastewater and the flocculant concentration on flocculation performance wasstudied. The results showed that, compared with chitosan and polyacrylamide, graftcopolymer of CTS-g-AM-AA had better flocculation properties in wastewatertreatment, and the modified chitosan had a wider range of applicable pH. Inwastewater treatment, when the dosage of CTS-g-AM-AA graft copolymer was100mg/L, the turbidity removal rate could reach99.28%, and the COD removal rate couldreach87.64%.Amphoteric chitosan flocculant CTS-g-AM-AA, chitosan and polyacrylamidewere used as flocculant to treat kaolin suspension, and by calculating the fractaldimension of the precipitation floc after flocculation experiments, the density of thefloc produced by the three flocculants were investigated. The results showed that, thefractal dimension of floc precipitation was the largest and the floc density was thebest when the amphoteric chitosan flocculant of chitosan-acrylamide-acrylic acid wasused to treatment analog kaolin suspension.