| With the deteriorating human living environment and the shortage of natural resources,microalgae,as a microorganism that can degrade pollutants and produce energy,has attracted widespread attention.Utilizing municipal sewage to cultivate microalgae can realize the recycling of sewage,reduce the cost of municipal sewage treatment,and reduce the cost of microalgae biodiesel process.Microalgae is a bottleneck that restricts the development of the entire process due to its small individual cells,relativ ely dispersed in solution,and low dry weight of cells.Based on the background that existing microalgae harvesting methods cannot meet the requirements of high efficiency and economy,a new magnetic flocculant was developed for harvesting chlorella and Scenedesmus obliquus cultivated in urban sewage.In this project,magnetic nano-iron ferric oxide was prepared by co-precipitation method,and the synthesis conditions were optimized.Then,a sol-gel method was used to composite polyaluminum chloride(PAC)on the surface of nanometer-sized ferric oxide,and the optimal mixing ratio of the two raw materials was determined to be 1: 3.Scanning electronic microscopy(SEM),Xray diffraction(XRD),Vibrating sample magnetometer(VSM)and Zeta potential analysis methods were used to characterize the magnetic materials before and after wrapping.The materials before and after wrapping are spheroidal,with a relatively uniform size,about 60 ～ 80 nm;the materials before and after wrapping have superparamagnetism,and the saturation magnetic strengths are 92.32 emu / g and 73.53 emu / g,respectively;-13.9 m V and 46.5 m V,the synthetic magnetic flocculant yielded more than 95% of microalgae within 3 minutes.The prepared magnetic flocculant was used to harvest Chlorella and Scenedesmus obliquus,and the harvest conditions were optimized by single factor and response surface experiments.In a single factor experiment,the effects of the dosage,p H,and stirring speed on the harvest efficiency were explored.The optimal harvest conditions for chlorella were 40 mg/L with a magnetic flocculant,p H 8,and a stirring rate of 300 rpm.The best harvesting conditions for S.obliquus are that the magnetic flocculant is added at 60 mg/L,the p H is 7,and the mixing rate is 300 rpm.The optimal parameters for harvesting chlorella with magnetic flocculant based on response surface optimization are: the dosage of magnetic flocculant is 31.92 mg/L,the p H is 6.6,the stirring rate is 443 rpm,and the harvest efficiency under this condition is 98.45%;Response surface optimization The optimal parameter conditions for magnetic flocculant harvesting of S.obliquus are: the dosage of magnetic flocculant is 55.36 mg/L,p H is 6.3,stirring rate is 396 rpm,and the harvest efficiency under this co ndition is 98.56%..Based on the experimental optimization results,the mechanism of the magnetic flocculation method was investigated from the Zeta potential and the microscopic morphology of the floc.It was concluded that the main mechanism of the harvest process was electrical neutralization,and that magnetic particles played a key role in the flocculation process.After the microalgae was harvested,nano-sized ferric tetroxide was recovered by the chloroform methanol method,and recombined for reuse to reduce costs.And this method does not reduce the oil content of microalgae and the oil quality of biodiesel. |
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