Study On Synthesis Mechanism Of Organic Flocculant Based On Efficient Initiation System And Its Application In Water Treatment

Flocculation is one of the most widely used water treatment technologies to remove suspended colloid particles and some organic pollutants from water.The efficient initiation system is beneficial to the low-cost synthesis of organic polymer flocculants.Its effect on the molecular structures,characteristics and performances of flocculants should be further studied.In this paper,two chitosan-based flocculants(CS-g-DAC_（T）and CS-g-DAC_（V）)were successfully prepared by the traditional UV initiation method using the traditional photoinitiator,2,2’-Azobis[2-（2-imidazolin-2-yl）propane]dihydrochloride(V_A-044),or titanium dioxide（TiO₂）.The differences of initiation efficiency of two initiation method and composition,structure and properties of two self-prepared flocculants were compared through a variety of characterization methods.The initiation mechanisms of free radicals and the synthesis mechanisms of flocculants were emphasized.CS-g-DAC_（T）and CS-g-DAC_（V）were used to treat wastewater containing kaolin,sodium humate and fulvic acid,respectively.The flocculation performance and flocculation mechanism of two flocculants were studied.In addition,a cationic polyacrylamide flocculant（P（AM-DAC））was synthesized through visible light initiation using TiO₂and potassium persulfate（K₂S₂O₈）as the initiator system.Electron spin resonance（ESR）was used to analyze the free radical initiation process originating from TiO₂and K₂S₂O₈and synthesis mechanism of P（AM-DAC）.The self-prepared P（AM-DAC）was used to treat simulated antibiotic wastewater,and its flocculation performance and flocculation mechanism were investigated.The main conclusions of this paper are as follows:（1）The composition,structure and properties of CS-g-DAC_（T）and CS-g-DAC_（V）were studied by Fourier transform infrared spectroscopy（FTIR）,thermogravimetric analysis/differential thermal gravity（TGA/DTG）,X-ray photoelectron spectroscopy（XPS）and nuclear magnetic resonance(~1HNMR and ¹³CNMR).It was found that CS-g-DAC_（T）and CS-g-DAC_（V）had the same functional groups and thermal stability.The ratio of C=N to C-N was 1.33:1 in CS-g-DAC_（T）molecular while the value was0.192:1 in CS-g-DAC_（V）molecular after calculating the integral area of their characteristic peak in XPS spectrum.In the study of synthesis mechanism,photogenic holes（h⁺）,superoxide radical（O₂·^-）and hydroxyl radical（·OH）were generated by TiO₂and V_A-044under UV irradiation.Compared with TiO₂,V_A-044produced more O₂·^-and less·OH,which contributed to the generation of monomer reaction free radicals.The initiation time of V_A-044was shorter,and the CS-g-DAC_（V）synthesized by V_A-044had a higher molecular weight.V_A-044had higher initiation efficiency,which improved the synthesis efficiency of natural organic polymer flocculants.（2）CS-g-DAC_（T）had the better removal efficiencies and formed larger flocs compared with CS-g-DAC_（V）in water treatment containing some pollutants.CS-g-DAC_（T）and CS-g-DAC_（V）had the similar charge neutralization.The bridging of CS-g-DAC_（T）played an essential role in the removal colloidal particles,which weakened the negative effects of charge reversal.In the treatment of colloidal particles,the S_fof CS-g-DAC_（T）,CS-g-DAC_（V）and PAC were all higher than that of commercial polyacrylamide.It indicated that the binding force between flocculant and pollutants was strong,and the generated flocs were dense.The high R_fvalue of CS-g-DAC_（T）and CS-g-DAC_（V）indicated that they owned the strong bridging as well.（3）P（AM-DAC）was synthesized through copolymerization of Acrylamide（AM）with DAC using the initiation system of TiO₂and persulfate under visible light irradiation.In the initiation process,the electron generated by TiO₂activated persulfate that yielded SO₄·^-.The oxidation potential of SO4·^-was higher than·OH.The initiation system of TiO₂and persulfate had high initiation efficiency.The molecular weight of P（AM-DAC）reached 284.136 k Da under visible light irradiation of 4 h.Its energy consumption of initiation and synthesis was low.P（AM-DAC）had good thermal stability.The synthesized P（AM-DAC）possessed-OH,-N-C=O and-C=O-functional groups and stable intermolecular ionic bonds.（4）The removal rate of tetracycline hydrochloride by P（AM-DAC）was 76.84%that was higher than that treated by commercial polyacrylamide in the treatment of simulated antibiotic wastewater.P（AM-DAC）had the wider optimal dosage range.P（AM-DAC）increased the particle size of flocs and the sedimentation rate of flocs.According to the analysis of zeta potential,charge neutralization was not the only flocculation mechanism in the treatment,and bridging was indispensable.Al atoms accounted for a relatively high proportion in the flocs generated by P（AM-DAC）flocculation according to the analysis of chemical atomic composition of flocs in SEM-EDS.It indicated that P（AM-DAC）was easily interact with PAC and tetracycline hydrochloride,promoting the removal of tetracycline in water.