The Study On Heat-aided Flotation For Harvesting Microalgae

As a promising raw material,microalgae has attracted more and more attention.However,the high cost and low recovery limit the development of microalgae applied to industrialization.In order to improve the recovery and save the cost in harvesting process,the traditional foam flotation method was optimized by heating the microalgae as pretreatment.Besides,the influence was explored of heating pretreatment on the recovery of Chlorella vulgaris(C.vulgaris)and Scenedesmus dimorphus(S.dimorphus).The mechanism of heat-aided improving efficiency was analyzed by scanning electron microscope,the surface properties of microalgae cells and the extended Derjaguin-Laudau-Verwey-Overbeek were used to calculate the interfacial energy between microalgae and water,microalgae cells and bubbles.Furthermore,the response surface design was employed to optimize processes of heat-aided flotation,screen the notable factors,obtain the prediction model and determine the optimal harvesting parameters.The main conclusions of this study were as follows:(1)Before the optimization,the highest recovery of S.dimorphus was 91.84% under the heating temperature of 90?.The recovery of S.dimorphus was higher than C.vulgaris.Compared with the traditional flotation methods,this harvesting method was not suitable for C.vulgaris.According to the results of microscope,the heating pretreatment made the microalgae cells aggregate,which was beneficial to the subsequent processes of foam flotation.(2)The surface properties of microalgae can be affected by heating pretreatment,which was mainly by improving the hydrophobicity of two species of microalgae,decreasing the absolute value of Zeta potential and the effect on the integrity of the algae cells.According to the results of linear fitting,it could be found that the surface hydrophobicity of the microalgae had a strong negative correlation with the absolute value of Zeta potential.Heating pretreatment can improve the hydrophobicity of microalgae by changing the properties of the microalgae surface groups.Therefore,the harvesting efficiency of microalgae can be increased.(3)The hydrophobic attraction generated by Lewis acid-base interactions was the essential factor in the interaction between algal cells and bubbles in water.The higher the temperature was,the greater hydrophobic attraction and higher recovery were.Under the same conditions,the hydrophobic attraction of S.dimorphus was larger than that of C.vulgaris.Therefore,the recovery rate of S.dimorphus was higher than C.vulgaris.The lower the Hydrophile Index was,the better the floatability of microalgae cells and higher recovery rate was obtained.And the hydrophilic index could be used to screen out microalgae species suitable for heat-aided flotation.(4)According to the analysis of Plackett-Burman screening experiments,it was found that temperature(?),rotational speed(rpm)and flotation time(min)had significant influence on the recovery of microalgae.The three factors selected were optimized by Box Behnken design.Moreover,it was found that they all had significant effects on the recovery of microalgae,especially for temperature.The optimum parameters of two species microalgae were obtained by response surface analysis.The optimum parameters of C.vulgaris were: heating temperature 90?,rotational speed 1300 rpm,flotation time 15 min.Under these conditions,the recovery rate of C.vulgaris was 49.950 ± 2.375%;the optimum parameters of S.dimorphus was: heating temperature 90?,rotational speed 1200 rpm,flotation time 15 min,and the recovery rate was 94.372±2.565%.