Generation,Properties And Templating Functionality Of Bulk Micro And Nanobubbles

Recently,microbubbles and nanobubbles have attracted much attention due to their unique physicochemical properties.Compared with conventional bubbles,bulk micro and nanobubbles have unique characteristics,such as a large specific surface aera,long stagnation,high mass transfer efficiency and free radical generation,etc.,so it has broad application prospects in the wastewater treatment,biological medicine,agriculture production and so on.Among them,it has attracted great attention in the preparation of functional porous materials as a green template.And it is needed to advance the fundamental research of micro and nanobubbles with the rapid development of applications.The study on the physicochemical properties and potential applications of micro and nanobubbles are denpendence of their controllable formation.At present,a variety of the formation methods of micro and nanobubbles have been developed,including the electrolysis of water,ultrasound,gas-liquid mixture,etc.These methods have their own advantages and disadvantages,and it is difficult to understand the physicochemical properties of micro and nanobubbles in depth owing to the difference in the various factors and formation condition.Although the unique properties of micro and nanobubbles have been gradually revealed with the deepening of research,their generation and stability mechanism still are controversial,and there is the lack of the reasonable and convincing explanation for some puzzling phenomena.Therefore,it is of great theoretical significance and far-reaching application value to explore the feasible method for generating micro and nanobubbles accompanied by the study on the physicochemical properties and the stability mechanism,and to further guide their application in preparing new functional materials.Therefore,in this thesis,several methods for generating different kinds of micro and nanobubbles were explored based on the principles of pressure change,hydrodynamic cavitation,ultrasonic cavitation.The bulk micro and nanobubbles with different gases were generated including oxygen,nitrogen,carbon dioxide and air.And it was discussed on the formation,stability mechanism and factors of generated uncoated nanobubbles and protein-coated microbubbles.On this basis,the gelatin/chitosan-coated microbubbles were used to prepare bio-aerogels with interconnected pores as templates.In the gelatin system,microbubbles were stabilized by the combination of gelatin and SDS,while the microbubbles were fixed in the gel network via controlling pre-crosslinking process in the chitosan system,which ensures the good combination of microbubbles and gelation.The prepared porous bio-aerogels display the excellent performance in the selective oil-water separation and the adsorption for multiple pollutants,which reveals the template advantages of micro and nanobubbles in preparing porous bio-aerogels,and provides new materials for the highly efficient treatment of water pollution.The main contents and innovations about this work are divided into the following parts:(1)Based on the periodic pressure change,the generation of uncoated nanobubbles with different gases was investigated with the help of a self-designed device at normal temperature and pressure.The formation of nanobubbles was confirmed,and the influence of gas and operating parameters on the size and stability of nanobubbles was investigated.The oxygen,nitrogen and carbon dioxide-filled nanobubbles formed by this method have excellent stability,among which CO2 nanobubbles are slightly less stable,but remain stable for more than 24 h.The stability mechanism of nanobubbles was further discussed,and it was found that the charges at the interface of nanobubbles play an important role in its stability.In this part,the research on the generation of uncoated nanobubbles at normal temperature and pressure,enriches the generation method of bulk nanobubbles,providing a theoretical foundation for the further study on the physicochemical properties and the potential applications of bulk nanobubbles.(2)Based on hydrodynamic cavitation,the shell-coated microbubbles containing oxygen,nitrogen and carbon dioxide were generated by utilizing the baffled high intensity agitation(BHIA)cell at the low concentration of bovine serum albumin(BSA).The effect of operating parameters and gas properties on the microbubbles was investigated systematically,and the stability mechanism was further discussed.The prepared microbubbles show good stability because of the formation of thick film derived from the adsorption of BSA on the gas-liquid interface,which supports for the applications of shell-coated microbubbles with different gas in various fields.At the same time,the influence of microbubbles on the behaviors of protein molecules was investigated,and it was found the abundant gas-liquid interface would induce the conformation changes and arrangement of BSA,resulting in the change of aggregation behaviors of protein molecules.Besides,the effect of metal ions on the stability of protein-coated microbubbles was studied.There was the obvious difference in the interaction between microbubbles and different metal ions.Based on the above understandings,the application prospect of microbubbles in protein extraction and purification,and the selective flotation for metal ions may be revealed.(3)The route of preparing gelatin-based aerogels with the high porosity and high oil absorption was explored by introducing microbubbles into the gelation based on the generation of microbubbles stabilized by gelatin/SDS complexes.The addition of SDS not only improves the surface activity of the composite system,but also increases the bulk viscosity,providing a basis for the generation and stability of the considerable microbubbles,and guaranteeing the role of the microbubbles-template during the rapid gelation.The prepared gelatin-based aerogels were characterized.It was found that the introduction of extensive microbubbles not only endows aerogels with abundant and interconnected pores to ensure the low density,high connectivity and high adsorption capability,but also enhances the mechanical properties of the aerogels.Compared with the template-free gelatin-based aerogels,the adsorption capacity of the prepared aerogels was greatly improved,and it has good reusability.Furthermore,it can selectively adsorb oil/organic solvents from water after the hydrophobic modification This study would provide a useful reference for preparing the porous bio-materials with superior properties,and reveal the potential prospect of gelatin-based aerogels in the treatment of oil/organic solvents leakage.(4)Preparing the porous chitosan aerogels using the microbubble-template was explored based on the generation of chitosan-coated microbubbles via ultrasound without the addition of additional additives.It is found that although there is the low surface activity of chitosan system,a large number of chitosan-coated microbubbles could be produced by ultrasound in the high viscosity solution.Therefore,in order to ensure the effectiveness of microbubbles-template,the pre-crosslinked process properly was adopted by using genipin derived from the natural materials as a crosslinking agent,and the microbubbles were introduced by ultrasound,which leads to the fact that the microbubbles were firmly locked in the gel.Finally,the porous chitosan aerogels were obtained and characterized.The aerogels prepared have the interconnected pores and oleophobicity underwater,which can be used for the separation of insoluble oils in water.Owing to the interconnected pores,the water flux in oil-water separation was significantly improved,and it has good reusability.Moreover,the formation of porous structure significantly accelerates the adsorption rate of heavy metal ions and organic dyes in water,and improves the adsorption capacity deriving from the exposure of more active sites in aerogels.And the insoluble oil and soluble contaminants can simultaneously be removed by the prepared aerogels from water.This study provides a universal and green way to prepare functional porous biomaterials with high porosity via the microbubble-template without the addition of additional additives,and provides a good candidate material for the highly efficient treatment of water pollutionTo sum up,the generation methods of the uncoated nanobubbles and biomaterials-coated microbubbles were systematically explored in this paper,and the stability mechanisms of formed microbubble and nanobubbles were discussed.On this basis,the template roles of micro and nanobubbles in the preparation of functional porous biomaterials were further investigated.This research not only enriches the generation methods and basic theoretical understanding of micro and nanobubbles,but also expands its applications,which has both important theoretical significance and far-reaching application value.