| Water,one of the most widely distributed and largest in nature,is the essential material basis for all biological organism and life activities,and plays a connecting role for the whole ecosystem.Therefore,water has a decisive meaning for the survival of biological and human beings.However,the rapid economic development has brought a variety of water pollution problems,resulting the water resources faced with the double dilemma--inadequate reserves and increasing pollution,and the problem has been noted by different countries in different areas.In the conventional process of water treatment,flocculation as a pre-treatment process,not only need efficient pollutant removal capacity,but its treatment effect and secondary hazards in the process will have an impact on the follow-up process.Therefore,the flocculation has a decisive effect on the final treatment effect.It is the core problem to improve the flocculation effect by developing new flocculants which are high efficiency,economy and green.Meanwhile,in-depth studying of there treatment effect and mechanism have a guiding significance for the management of actual polluted water.Due to the traditional flocculants have the disadvantages of large amount of dosage,high content of residual monomer and little source of raw materials,more researcher focus on the natural flocculants gradually.Chitosan,which has a certain antibacterial property,good biodegradability and low toxicity,has many advantages such as chitosan that can be chemically modified on the molecular chain,which is favored by many researchers.Among them,chitosan is favored by many researchers because of its positive charge,antibacterial property,good biodegradability and low toxicity.In this paper,acrylamide(AM)with high molecular weight and methacrylamide propyltrimethylammonium chloride(MAPTAC)with positively charged quaternary ammonium groups were selected as the grafting monomer to modified the chitosan(CTS)under the ultraviolet initiation system to improve the solubility and positive charge density of chitosan,and synthesized the new flloculant with high efficiency,low consumption,dual-effect of flocculation and sterilization finally.In addition,the synthesis conditions were optimized,and the structure were characterized.Finally,it was applied to the kaolin and E.coli suspensions to verify flocculation performance,and confirmed flocculation and sterilization mechanism in depth.The main research work and conclusions are as follows:(1)Preparation of CTS-g-PAMA: In order to improve the solubility of chitosan,it was pre-treated with carboxymethylation firstly.Secondly,adding the appropriate amount of chitosan and acetic acid solution to the reaction system,after being completely dissolved,add the quantitative monomer—AM and MAPTAC.Thirdly,the reaction solution was filled with nitrogen for 30 min.Then,placing the reaction bottle under the UV lamp device for a cetain time after adding the initiator—azobisisobutyl imidazoline hydrochloride(VA-044).Finally,taking the sanmple to extracted with absolute ethanol and acetone,and drying to constant weight to obtain the new flocculant—CTS-g-PAMA(2)In the preparation of CTS-g-PAMA,the intrinsic viscosity and grafting ratio of the product under different conditions were determined by changing the initiator concentration,total monomer concentration,mass ratio of monomer to chitosan,molar ratio of monomer and illumination time.And the optimal range of synthesis was selected.Then the response surface method was used to determine the optimum preparation conditions according to the results of single factor experiments.(3)The solubility and Zeta potential of CTS and CTS-g-PAMA were compared,the results showed that the CTS-g-PAMA has better solubility and positive charge density,and the isoelectric point at pH 8,which enhanced flocculation process of dispersion and electrical neutralization effect.(4)The strcture of CTS-g-PAMA and CTS,monomer(AM andMAPTAC)were compared by spectroscopic apparatus: Nuclear Magnetic Resonance Hydrogen Spectroscopy(1H-NMR),Fourier transforms infrared spectroscopy(FTIR),UV spectrum(UV)and X-ray diffraction(XRD).The results showed that the structures of CTS-g-PAMA and CTS are similar,but the vibrational peak or absorption peak of quaternary ammonium groups appeared in the spectrum,meanwhile the crystallinity decreased and the crystal structure collapsed after grafting.Then the surface morphology and thermal stability of CTS-g-PAMA and CTS were compared by scanning electron microscopy(SEM)and differential thermal-thermogravimetric analysis(DSC-TG).The results show that the surface of CTS-g-PAMA is fluffy,distributed with many pores and folds,which is helpful to enhance the ability of sweeping.And the thermal stability of CTS-g-PAMA is better than CTS,the difference of weight loss process also indicates that there is some difference between the two structures.(5)Taking the residual turbidity percentage(RT%)of supernatant as the target value,the flocculation performance of CTS-g-PAMA for kaolin suspension was evaluated under different pH,dosagec and grafting ratio.Meanwhile,the flocculation effects of PAMA,PAM and PAC at the same conditions were compared,and the flocculation mechanism was analysised based on the results of Zeta potentials.The results showed that the CTS-g-PAMA exhibited excellent flocculation effect at very low dosage,which was superior to conventional flocculant.In the flocculation process,the bridging effect due to the high molecular weight of AM and long linear chain structure of CTS and the electrical neutralization due to the quaternary ammonium groups of MAPTAC,as well as sweeping ability due to the large specific surface area was combined well to remove the contaminants.(6)The CTS-g-PAMA was applied to the treatment of Escherichia coli suspension.Taken the residual turbidity percentage(RT%)and the optical density(OD600)of supernatant as the target values,the effect of pH,flocculant dosage and grafting ratio on flocculation efficiency were investigated.Meanwhile,the flocculation effects of PAMA,PAM and 1277 at the same conditions were compared,and the flocculation sterilization mechanism of CTS-g-PAMA was studied accoding to the results of Zeta potential,β-galactosidase activity and three-dimensional fluorescence spectroscopy.The results showed that the flocculation and bactericidal effect of CTS-g-PAMA was more significant than that of other products.With the removal mechanism of kaolin,combined the electricity neutralization,bridging and sweeping effect very well.At the same time,CTS-g-PAMA can change the permeability of E.coli cell membrane,and impede the normal metabolism of cells,which play a role in killing bacteria.Thus there are no "re-stabilization" phenomenan under the excess dosage.All in all,the CTS-g-PAMA exhibited well performance on the flocculation and sterilization with good prospects for development. |
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