Efficacy And Mechanism Of Amphiphilic Chitosan-modified Microbubbles To Enhance Air Flotation For Algae Removal

Eutrophication of lake and reservoir water resources leads to a large number of algae reproduction,resulting in water quality problems such as odor,algal toxins and disinfection by-products,which seriously endangers the quality of drinking water and the safety of water supply.Dissolved air flotation（DAF）process is widely used in drinking water treatment industry.However,in the aspect of algae removal,traditional DAF still has some problems,such as electrostatic repulsion between algae cells and microbubbles,limited sweeping volume of microbubbles in water,low probability of collision and adhesion with algae cells.In recent years,positive modification DAF（Posi-DAF）can overcome the shortcomings of traditional DAF and effectively improve the algae removal rate by adding cationic modifier to the dissolved air system to produce microbubbles with positive charge on the surface.Therefore,it has been widely concerned at home and abroad.It is expected to become a research hotspot to seek safe and efficient bubble modifier.Chitosan（CTS）is a kind of natural nontoxic cationic polymer with excellent flocculation.In this study,CTS was modified to enhance its solubility and associate hydrophobic groups to make it have amphiphilic structure.The influencing factors of the preparation process of amphiphilic CTS were explored,the best preparation conditions were determined,and a simple,efficient,green and environmental friendly new type of CTS was developed microbubble modifier.In addition,Posi-DAF algae removal experiment was carried out to investigate the adhesion effect of amphiphilic CTS and bubbles,determine the optimal type and dosage of amphiphilic CTS,optimize the working parameters of the air dissolving system,and explore the dominant adhesion mechanism and collaborative removal mechanism of microbubble-amphiphilic CTS-AOM-algae cell system.Through the preparation of amphiphilic CTS and the research on the influencing factors by response surface methodology（RSM）,the significant degree of the influence of carbon chain length,raw material dosage ratio,reaction time and reaction temperature on the degree of substitution of response value was determined,and the preparation conditions of amphiphilic CTS were optimized.In addition,the functional groups of the products were characterized by fourier transform infrared spectrometer（FTIR）,which proved that hydrophilic quaternary ammonium group and hydrophobic alkyl group were successfully introduced into CTS molecular chain.The water solubility of CTS was enhanced in the whole p H range,and C4-HTCC was the best.In addition,the crystallinity was weakened and the thermal stability was decreased,which was more obvious with the increase of the length of alkyl carbon chain.At the same time,the surface morphology of amphiphilic CTS became rough and loose due to the decrease of crystallinity.According to Posi-DAF experiment,when the dissolved gas pressure was 0.5MPa,the reflux ratio was 20%,C4-HTCC was selected as the bubble modifier,and the dosage was1.0mg/l,the decontamination effect was the best.Both C4-HTCC and C8-HTCC had good decontamination effect.C4-HTCC had good surface charge modification effect on microbubbles because of its good water solubility and easy to ionize in water to produce protonated amino(-NH³⁺),which could promote the adhesion of foam and improve the removal rate of algae cells under electrostatic attraction;the relative strength of hydrogen bond between C8-HTCC and algae cells and organic matter was higher,which indicated that C8-HTCC had better adsorption and bridging ability.However,the overall decontamination situation showed that the intermolecular hydrogen bond between amphiphilic CTS modified microbubbles and algae cells and organic matter was not the main mechanism to affect the algae removal ability,and electrostatic attraction might be the main mechanism of enhancing the adhesion of foam and flocs.The results of algal organic matter（AOM）and Posi-DAF experiments showed that AOM was mainly composed of high molecular biopolymers,such as protein and polysaccharide.The increase of AOM content played a positive role in the removal of algae cells and other organic pollutants,and reached the best value when AOM=0.8mg/L.C4-HTCC had the best removal effect in the whole range of AOM dosage,but due to the short butyl carbon chain,electrostatic attraction was the main adhesion mechanism,the intermolecular hydrogen bonding was weak,and the adsorption bridging ability between C4-HTCC and AOM was not as good as that of C8-HTCC,so it was less affected by the change of AOM content.Compared with AOM=0.2mg/L,the removal efficiency of C8-HTCC on algae cells,UV₂₅₄ and turbidity increased by 18.28%,14.86%and 20.18%respectively.Its removal efficiency of humic acid and other macromolecular substances was even better than that of C4-HTCC,which was due to the formation of stable intermolecular hydrogen bonds with microbubbles and AOM,good adsorption and bridging ability,and the network structure formed had a good net trapping and sweeping effect on macromolecular substances.But on the whole,the best removal rate of C8-HTCC was still lower than that of C4-HTCC.In conclusion,electrostatic attraction and hydrophobicity were the main mechanisms to promote the adhesion of microbubble-amphiphilic CTS-AOM-algal cells,followed by adsorption bridging and net sweeping.