Study On Performance Enhancement And Mechanism Of Buoy Bead Flotation Of Microalgae Biomass

With the rapid development of agriculturalization and industrialization,a large number of wastewater and waste gas are discharged,which has caused irreversible harm to environment.Microalgae,as a kind of microorganism widely existing in nature,has short generation cycle and high photosynthetic efficiency.It can use inorganic nutrients in wastewater and waste gas for biomass accumulation.Meanwhile,microalgae biomass can be used for the production of high-value products such as biodiesel,food supplement and soil conditioner.Therefore,the development of microalgae industry can reduce environmental pollution and promote economic production.However,the high production cost restricts the development of microalgae industrialization.In order to develop microalgae industry,the problems existing in each production process such as cultivation,harvesting and refining needed to be optimized.Based on this,the buoy bead flotation was used to harvest microalgae.The pretreatment,disruption and the selection of low density microspheres（LDMs）were studied to improve the harvesting efficiency.Meanwhile,the mechanism of buoy bead flotation process was investigated.Extended Derjaguin-Landau-Verwey-Overbeek（XDLVO）theory was used to analyze the interaction between microalgae cells and LDMs in the flotation process.The aim of this study was to enrich the flotation theory and explore efficient,convenient and environment-friendly microalgae harvesting technology,so as to provide a scientific basis for promoting the large-scale industrialization of microalgae.Main conclusions of this research were listed as follows:（1）Based on heat-added pretreatment,an improved bead flotation method was proposed.Compared with traditional method,the harvesting efficiency of microalgae had been raised in this improved method.The harvesting efficiency of Chlorella vulgaris,Scenedesmus sp.,Spirulina platensis,Microcystis aeruginosa and Anabaena variabilis after heat-added pretreatment was higher than 90%.In addition,the zeta potential of LDMs changed from negative to positive after surface modification,resulting in the formation of microalgal aggregation.When the p H of microalgal suspension was 6-8,the harvesting efficiency of Chlorella vulgaris,Scenedesmus sp.and Microcystis aeruginosa was 91.63%,98.50%and99.37%,respectively.This method was convenient and did not need to adjust the p H value of water.Besides,low dosage of floculants were needed during floation process.Thus,this method had strong industrial application potential.（2）In oder to avoid the separate step,a novel integrated method of microalgae harvesting and disruption was developed.The cavitation of ultrasonic can break the cell membrane and improve the disruption efficiency of microalgal cells.After ultrasonic pretreatment,different proportions of n-hexane:isopropanol mixture（HIP）were used to extract lipid from microalgal biomass.The highest lipid extraction efficiency of Chlorella vulgaris was 18.91%at ultrasonic time of 13 min,HIP ratio of 4,solid concentration of 13.6 g/L and the ultrasonic power of 254W.The content of polyunsaturated fatty acids biodiesel by ultrasound-assisted extraction methods was lower and degree of saturated fatty acids was higher than that of B&D method.（3）Walnut protein extract（WPE）was used as pre-floculants to harvest microalgae,and walnut lipid droplets emulsion（WLDE）was applied in buoy bead flotation process.The harvesting efficiency of Chlorella vulgaris and Scenedesmus sp.exceeded 95%.Walnut protein can enhance the bridging and sweeping ability of chitosan.In flotation process,the harvesting efficiency of Chlorella vulgaris and Scenedesmus sp.was higher than 95%at p H of 7.The proportion of walnut lipid droplets/chitosan and the dosage of mixture floculants depended on the sepcies of microalgae.However,because Spirulina platensis suspeneded in high ion concentration medium,the bridging and sweeping ability of chitosan were weak,resulting in the harvesting efficiency of Spirulina platensis at a low level,up to 70%.（4）XDLVO theory was used to calculate the interaction between micoalgal cells and LDMs in the process of harvesting,and revealed the mechanism of the interaction between particles in buoy bead flotation process.It was found that pre flocculant and chitosan coated LDMs can compress the electric double layer on the surface of microalgae cells and reduced U_ES between microalgae cells and LDMs.U_XDLVO changed from repulsive to attractive,so the harvesting efficiency was improved.However,heat-added pretreatment can improve the harvesting efficiency by changing U_AB.When WLDE was used as the LDMs,the energy barrier of U_XDLVO between microalgae and WLDE-5 was eliminated,or the energy barrier remained stable,but the secondary minimum value was significantly reduced.Therefore,choosing WLDE-5 as LDMs can significantly improve the harvesting efficiency of microalgae.However,for Spirulina platensis,the energy barrier increased with the decreas of secondary minimum value,so WLDE-5 did not significantly improve the recovery efficiency of Spirulina platensis.After desalting,the energy barrier was eliminated,and Spirulina platensis can achieve a high harvesting efficiency.