Preparation Of Cationic And Amphoteric Starch-based Coagulants And Study On The Effect Of Molecular Structure On Coagulation Properties

Coagulation technology,as one of the most widely used water treatment technologies,can effectively remove particles and natural organic matter in water.Compared to inorganic coagulants with the problems of metal ions and monomer residues,natural polymers,with their environmental-friendly,safe and non-toxic properties,have attracted growing research attention.However,previous research on the interaction of molecular structure and coagulation performance of polymeric coagulants was not systematic and comprehensive.In this project,starch polymer possessing the advantages of abundant sources and low cost is selected as raw material.After modification,two series of graft cationic and amphoteric starch-based coagulant were synthesised with different graft chain length and charge density.After characterization of their molecular structure and chemical composition,they were utilized to treat simulated surface water containing high organic matter respectively.In this process,the composition and physical and chemical properties of the coagulation system were adjusted for comprehensively investigating the effects of the molecular structure of the two types of polymers on their coagulation properties.Some specific experimental results are illustrated as below:Through the coagulation experiment,it was found that the coagulation properties of the two series of organic polymer both showed an upward tendency,followed by a decreasing trend with the increase of the dosage.Through comparison within the same group of starch-based coagulant,the results showed that charge density and graft-chain length all played a significant role during coagulation.When the dosage was insufficient,the coagulation performance of these polymers would improve with the increase of their charge density,resulting in the reduce of the optimal dosage.Based on this,the charge neutralization was considered as the domain mechanism in this condition.On the other hand,when the dosage was excessive,polymers with various average graft-chain lengths would demonstrate different degrees of floc restabilization,indicating that the correlation between the coagulation performance and their graft-chain length.By changing the composition and the physical and chemical properties of thecoagulation system,it was found that the basic characteristics of the pollutants and the molecular structure of the two types of polymers were all affected,and ultimately manifested as the changes in coagulation performance.To be more specific,when the p H or ionic strength of the solutions became lower or higher,the charge density of the coagulant would get higher,its molecular chains would stretch,and the negative charge on the surface of the pollutants would be partially neutralized.Therefore,this condition was beneficial for the coagulant to collide with the pollutants.And when the p H and ionic strength of the solution changed opposite,the dissociation of the cationic group would be suppressed and the molecular chain would tend to shrink,the condition of which was quiet disadvantageous for coagulation.A mathematical model was established to further explore the relationship between the molecular structure and coagulation characteristics of high molecular polymers.The results showed that charge density and graft chain length were important factors affecting the coagulation properties of starch-modified polymers: when the dosage was insufficient,the polymer coagulation effect was positively correlated with the total charge and graft chain length;when the dosage was excessive,the coagulation effect was inversely related to the two structural factors.