UV-initiated Preparation Of Chitosan-based Grafted Flocculant With Amphiphilic Structure And Its Water Treatment Application

The development and application of water treatment agents is an important means to improve water pollution control technology,which can effectively enhance the removal effect of various pollutions and improve the quality of effluent.As an important part in the field of water treatment agents,the development of new,efficient,safe and environmentally friendly flocculants has become the research hotspot.The traditional natural flocculants have the disadvantages of few active functional groups and poor water solubility.In order to solve the problem,this paper selected the natural polysaccharide chitosan(CS)as the grafting backbone to improve the biodegradability of the flocculant.Acrylamide(AM)and amphiphilic monomer acryloyloxyethyl dimethylbenzyl ammonium chloride(AO)were grafted on the backbone of CS,which increased the number of active sites of flocculant and improved the water solubility of CS.Based on the idea,the flocculation-enhancing flocculant CS-g-P(AM-AO)was successfully synthesized through UV-light irradiation way.AO monomer has the hydrophilic cationic quaternary ammonium group and the hydrophobic benzyl group in molecular structure.The unique structural feature of AO can enhance the surface activity and hydrophobic association effect of the graft copolymer.During the graft copolymerization process,the amphiphilic properties help to form the microblock distribution of the functional monomer AO in the branch of CS-g-P(AM-AO),thereby enhancing the flocculation performance of CS-g-P(AM-AO).Furthermore,combined with the novel magnetic separation technology,the core/shell magnetic chitosan-based graft copolymer Mag@CS-g-PAO was synthesized.This paper systematically investigated the structural characteristics of the two graft copolymers through a series of characterization methods.In addition,the two graft copolymers were applied a variety of polluted water.Moreover,this paper systematically investigated their removal mechanism.The specific research contents and conclusions are as follows:(1)The characterization results of FTIR,¹H NMR spectra and TG/DSC results confirmed the successful synthesis of CS-g-P(AM-AO)by UV-light irradiation.SEM images indicated that CS-g-P(AM-AO)had the irregular,porous surface structure.The special surface morphology helps to improve the water solubility of CS-g-P(AM-AO).In the synthesis process of CS-g-P(AM-AO),the single-factor experiment was used to explore the effects of total monomer molar concentration,the molar concentration of the initiator,the molar ratio of the monomer AO to CS,the irradiation time and the solution p H on the intrinsic viscosity and grafting efficiency of CS-g-P(AM-AO).Based on the above results,Box-Behnken response surface methodology was used to investigate the significant effects and interaction effects of the three factors.Under the optimum conditions,the intrinsic viscosity and grafting efficiency of CS-g-P(AM-AO)were551.2ml/g and 78.1%,respectively.In addition,CS-g-P(AM-AO)showed the superior water solubility in aqueous solutions with the p H range of 2.0?10.0,which would improved its application perfomence.(2)By calculating the monomer reactivity ratio,the sequence distribution of the grafted branch during the graft copolymerization process was investigated.Results showed the reactivity ratios of the two monomers AM and AO were 0.497 and 1.450,respectively.The results suggested that monomer AM was more prone to copolymerization and the monomer AO is more prone to homopolymerization during the graft copolymerization.The results of sequence length distribution showed that the distribution of AO sequences and the proportion of long segments became wider and larger,as the increase of the dose of AO.Due to the hydrophobic association effect,hydrophobic microdomains are easily formed between AO molecules during the graft polymerization reaction,which helps monomer AO to form homopolymerization chain segments on the CS backbone.Furthermore,the formation mechanism of microblock structure during CS-g-P(AM-AO)graft copolymerization process was revealed.(3)CS-g-P(AM-AO),CS-g-PAM and commercially cationic flocculant CPAD were applied to the dewatering experiments of municipal sludge.During sludge dewatering process,the dewatering performance of CS-g-P(AM-AO)was better than CPAD and CS-g-PAM.When the dosage of CS-g-P(AM-AO)was 6 mg/g TSS,the values of FCMC,SRF and CST were 77.98%,3.83×10¹¹ cm/g and 4.9 s,respectively.The charge neutralization and the hydrophobic association of the microblock structure enhance the sludge dewatering effect of CS-g-P(AM-AO),and weaken the re-stability when CS-g-P(AM-AO)dosage was excessive.In addition,the hydrophobic association of CS-g-P(AM-AO)improved the removal rate of PN in the S-EPS and TB-EPS components,and reduced the viscosity and compressibility of sludge.According to the results of sludge floc morphology,it was found that there were more voids on the surface of the sludge floc after CS-g-P(AM-AO)conditioning.The results of floc sedimentation rate and size distribution showed that the floc after CS-g-P(AM-AO)conditioning had a faster sedimentation rate,a larger average particle size and stronger shear resistance.(4)CS-g-P(AM-AO)was used to the treatment of orange II dyeing wastewater.The effects of flocculant dosage,initial solution p H value,and initial dye concentration on the flocculation performance were investigated.Compared with CS,CS-g-PAM and cationic flocculant CPAD,CS-g-P(AM-AO)exhibited the best turbidity and decolorization effect,and had the good acid and alkali resistance.Zeta potential and floc FTIR spectra results further demonstrated that the microblock structure distribution of functional monomer AO greatly improved the interaction possibility between the functional chain of CS-g-P(AM-AO)and dye pollutants.The enhanced charge neutralization and hydrophobic association effects improved the dye removal efficiency of CS-g-P(AM-AO).(5)The results of FTIR,TG,XRD and SEM showed that the copolymer PAO was successfully grafted onto the surface of the core-shell magnetic chitosan.Magnetic graft copolymer Mag@CS-g-PAO was prepared.Magnetization curves results indicated that the saturation magnetization of Mag@CS-g-PAO is 12.03 meu/g,which showed Mag@CS-g-PAO has a good magnetic response in aqueous solution.Mag@CS-g-PAOs and Mag@CS were applied to the treatment of three azo dye wastewaters including orange II,acid red and amaranth red.Results showed that the removal rates of Mag@CS-g-PAOs for the three azo dye were far better than those of Mag@CS under the same dosage.The order of removal effect of Mag@CS-g-PAO for three dye was acid red>orange II>amaranth red.The optical microscopy,FTIR and XPS spectra analysis results showed that the charge neutralization of the Mag@CS-g-PAO and the?-?stacking of the benzyl group and the aromatic group of the dye molecule enhanced the Mag@CS-g-PAO specific adsorption effect of dye molecule.Due to the weak bridging ability of Mag@CS-g-PAO,obvious flocs could not be formed during the decolorization process.In addition,Mag@CS-g-PAO exhibited the good regeneration and reuse properties for the treatment of orange II,acid red and amaranth red.