Buoy-bead Flotation Harvesting Technology Applied On The Microalgae Chlorella Vulgaris And The Mechanisms Of Physicochemical Interactions Among Microalgae-medium-bead Multiphase Interfaces

With the continuous improvement of people's living standard,the demands for energy,green food,medicine and many other products are increasing with each passing day,resulting in the short supply of their raw materials?such as traditional fossil energies and crops?.Microalgae products have attracted widespread attention,mainly by virtue of their low cost and easy cultivation conditions.However,the difficulties of microalgae harvesting have limited the development of microalgae industry,as conventional approaches to flotation and harvesting of microalgae are expensive and energy-consuming.Therefore,developing a novel harvesting method is of vital significance for promoting the development of algae industry.This paper proposes a new buoy-bead flotation method using the common species Chlorella vulgaris,and introduces the surface response method to investigate the influence of different factors on harvesting efficiency;in combination with the extended Derjaguin-Laudau-Verwey-Overbeek?XDLVO?theory,it probes into the action mechanism between buoy-bead and algae,and obtains the following results:1)Replacing the bubbles by sodium silicoborate buoy-beads as the upload carrier in the traditional air flotation method,a new flotation technology,buoy-bead flotation technology is developed.By comparing the four buoy-bead materials of sodium silicoborate,fly ash,hollow glass beads and latex particles,the floating beads were compared.The efficiency of buoy-bead flotation is closely related to the material properties,and sodium borosilicate was found to be the most effective way to harvest algae compared with other common 3 types of hollow buoy-bead.Among sodium borosilicate,hollow glass,latex and fly ash materials,the sodium borosilicate acheved highest harvesting efficiency,which in concentration of 1 g/L63.24%harvesting effiency was acheved while the rest of the three materials ranging from10-25%.At the same time,the harvesting efficiency is better than air flotation without adding flotation reagents.The hydrophobic and hydrophilic of buoy-bead is an important parameter to harvesting efficiency.2)According to response surface methodology,stirring speed,microsphere concentration and microsphere diameter are the three significant factors influencing harvesting efficiency.By optimizing the three influencing factors,the optimal harvesting conditions were obtained as follows:stirring speed 133 rpm,microsphere diameter 56?m,microsphere concentration 0.546 g/L.The optimal harvesting efficiency was predicted to be83.7%while the experimental verification value was 85.4±3.2%,so the model was in good agreement with the experimental results.3)In order to further improve the harvesting efficiency buoy-bead flotation,a new technology coupled with microalgae pre-flocculation floatation is proposed.The experimental results show that the highest harvesting efficiencies were 88.52% and 92.04%,respectively.The adsorption experiments show that the two preflocculants on the surface of Chlorella vulgaris are electrostatic adsorption and patch adsorption.By detecting the surface characteristics of microalgae,FeCl₃ is found to flocculate by changing the surface electric property of microalgae and reducing the electrostatic repulsion between microalgae.the buoy-bead is insert in after flocs formed.However,chitosan is adsorbed on the surface of Chlorella vulgaris by patch adsorption.Meanwhile,a large number of chitosan aggregates are also attached to the surface of the buoy-beads after pre-flocculation of chitosan,which can form a bridge between the microalgae and the surface of the buoy-beads,promoting the formation of"Chlorella vulgaris-buoy bead"aggregate.4)Based on the coating technology of chemical precipitation method,moringa seed extract and polydimethyldiallyl ammonium chloride?PDDA?are coated on the surface of sodium silicoborate microsphere.Two new types of floating beads?SLPMs and moringa floating beads?are developed.The optimal particle concentrations for the two new materials were both 0.7 g/L;SLPMs had the highest harvesting efficiency?98.43%?at pH 9,while moringa beads had the highest harvesting efficiency?89.13%?at pH 7.The influence of pH was greater on the harvesting efficiency of moringa beads.Acording to Zeta potential,FTIR and other characterization experiments,the mechanism of the new float material to improve chlorella recovery is revealed as follows:?1?PDDA is coated on the surface of sodium silicateborate,which leading to the outermost layer of the float is positive charged,and the electrostatic repulsion between the microalgae becomes attraction.Under the strong electrostatic attraction,a"microalgae jacket"is formed around SLPMs,and a higher harvesting efficiency can be achieved by compressing the double electric layer.?2?moringa extract coated on the surface of sodium silicateborate changes the surface hydrophobicity,so that the hydrophilic repulsion between the Chlorella vulgaris and buoy-bead becomes hydrophobic attraction.At the same time,moringa seed contains the active substance that causes microalgae flocculation,which will be released into the algal solution during the harvesting process.Then the microalgae attach to the surface of buoy-bead in the form of flocculation,thus improving the recovery rate of microalgae.The research can provide theoretical basis and technical support for the development of high efficiency,low energy consumption and environmentally friendly microalgae harvesting technology and the industrialization of microalgae biological manufacturing.