Study On Preparation And Flocculating Performance Of The PAC-Modified Starch Composite Coagulant

With the development of economy,the situation of environmental pollution is becoming more and more severe,especially in water pollution.“Coagulation,Precipitation,Filtration,Disinfection”is a conventional water treatment process,in which coagulant is the core,and the performance of coagulant directly affects the quality of treated sewage.The purpose of this paper is to prepare an efficient composite coagulant to improve the treated sewage quality and reduce the concentration of Al³⁺of conventional water treatment process.The main contents of this research were as follows:?1?In this paper,poly-aluminum chloride?PAC?was prepared with calcium aluminate by acid-leaching technology.The experimental results showed that when the calcium aluminate,hydrochloric acid dosage?volume fraction was 62.5%?,temperature and reaction time were 20.0g,80.0ml,95?and 2.5h respectively,the aluminum oxide content and basicity of PAC was achieved 10.25%,36.55%.?2?Modified starch?MSt?was prepared by Semi-dry Method with corn starch?St?as raw material and Glycidyl trimethyl ammonium chloride?GTA?as etherifying agent.The reaction conditions were optimized by response surface methodology.The experimental results showed that when the St:H₂O:NaOH:GTA=20.0:7.0:0.29:4.73and temperature and reaction time were 65?and 2.5h respectively,the performance of MSt was the best.?3?Calculating the mass of PAC and MSt required for the preparation of PAC-MSt composite flocculant with a certain composite ratio.A certain mass of self-made PAC was dissolved in water and placed on a magnetic agitator to stir slowly.Then,adding certain mass of MSt to the PAC solution and quickly stirring until MSt dissolve completely.The stirred solution was transferred to the volumetric bottle and the volume was fixed.The PAC-MSt composite coagulants solution with a certain proportion can be prepared,and coagulants'topography structure was measured by FT-IR?fourier transform infrared spectrometer?,which preliminarily proved the success of PAC-MSt composite coagulant.?4?PAC-MSt composite coagulant was applied to the treatment of reactive brilliant red X-3B simulated printing and dyeing wastewater.The experimental results showed that PAC-MSt composite coagulant had better coagulation performance than PAC.When the dosage of PAC?4%?was 100 mg/L,the removal rate of X-3B reached 70.02%.Under the same conditions,the removal rate of X-3B was only 63.27%.When the dosage of PAC was 160 mg/L,the removal rate of X-3B reached 68.78%.Under the same conditions,the composite coagulant's performance of PAC?4%?is less affected by the change of pH value and salt concentration than PAC.The coagulation and precipitation kinetics model of X-3B in the range of 250-450 mg/L conforms to the first-order reaction model.?5?PAC-MSt composite coagulant was applied to the treatment of micro-polluted raw water treatment.The experimental results showed that PAC-MSt composite coagulant had better coagulation performance than PAC,and the optimal composite proportion of different wastewater is different.When the dosage of PAC?6%?is 40mg/L,the residual turbidity of raw water is 1.4 NTU,and when the dosage of PAC is 50mg/L,the residual turbidity of raw water is 2.0 NTU.When the dosage of PAC?6%?is35 mg/L,the removal rate of total organic carbon?TOC?of raw water is the highest,which is 53.86%,and when the dosage of PAC is 50 mg/L,the removal rate of TOC is the highest,which is 52.49%.When the settling time is more than 40 minutes,the TOC of the adsorbed part by precipitation flocs will dissolve and the removal rate of TOC will decrease.Excessive dosage of PAC?6%?will make the removal rate of TOC decrease rapidly,because residual MSt will increase TOC,so the optimum dosage of PAC-MSt composite coagulant should be confirmed by considering comprehensively according to the pollutant index which need to be controlled.