耦合浮珠-超声辅助溶剂萃取法用于微藻采收及油脂提取

为了优化微藻生物柴油生产工艺，开发高效低耗的微藻采收与油脂提取技术，该研究使用优化浮珠浮选工艺对小球藻进行采收，随后选取小球藻-表面层状聚合物浮珠聚集体进行破壁提油处理，并通过响应面优化破壁工艺，建立一种新型耦合浮珠-超声辅助溶剂萃取工艺。结果表明，在超声时间为13 min，正己烷：异丙醇体积比例为4，微藻质量浓度为13.6 g/L，超声功率为254 W时，油脂提取效率较高，为18.91%。相比传统气浮法与超声辅助溶剂萃取法，该法采收效率、细胞破壁效率和饱和脂肪酸含量都达到了较高水平，分别为98.36%、90.19%和37.03%。因此，耦合浮珠-超声辅助溶剂萃取工艺是一种有效提取小球藻细胞中油脂的工艺。研究结果为微藻生物柴油制备工艺的发展提供科学依据。

Microalgal harvesting and lipid extraction by coupling buoyant-bead and ultrasound-assisted solvent extraction method

Microalgae can widely be considered as one of the most promising bioenergy feedstocks. There is no competition with crops, where microalgae do not require arable land for cultivation. There is also no influence on the supply or price of food crops, compared with conventional oil crops. However, the harvesting and lipid extraction of microalgae have been the major challenges in the microalgae industry. Traditional harvesting is time-consuming, energy-intensive, and/or not eco-friendly, particularly to separate microalgae cells, including centrifugation, gravity sedimentation, flocculation, and flotation. A buoy-bead flotation is emerging for harvesting the microalgae in recent years. The dried biomass powder or wet concentrate can also be used for lipid extraction after microalgae harvesting and concentration. The cost of lipid extraction accounts for around 30%-40% of the total biodiesel production. Bead milling, homogenizer, microwave, and ultrasound are commonly-used mechanical disruptions. Among them, ultrasound-assisted extraction has widely been used to extract intracellular components, due to its high energy efficiency easy to be commercialized on a large scale. Specifically, the extraction time can be shortened to 1/10, while the extraction efficiency can increase by 50-500 times, compared with the control. In this study, surface-layered polymeric microspheres (SLPMs) were used in the buoy-bead flotation for harvesting microalgae. After that, the ultrasound-assisted extraction was utilized to break the cell wall, and then to extract lipid from microalgae. In harvesting, the zeta potential of flocs was analyzed to compare the harvesting efficiency of microspheres with flocculants and surface-modified microspheres by a single factor. In lipid extracting, a novel approach was developed to couple the buoyant beads and ultrasound-assisted solvent extraction for higher efficiency. Mathematical modeling and central composite design (CCD) were used to statistically optimize the effect of ultrasonic time, the ratio of hexane and isopropanol, microalgal concentration, and transducer power on lipid yield. The optimum operation condition was determined to compare with different lipid extraction. The compositions of extracted lipids were then characterized using gas chromatography/mass spectrometry analysis (GC-MS). It was found that the SLPMs achieved a higher harvesting efficiency of 98.36%, compared with the surfactant/flocculant and sodium silicate microspheres. Consequently, the maximum lipid yield was 18.91 % under an optimal combination: the ultrasonic time of 13 min, the hexane: isopropanol ratio of 4, microalgal concentration of 13.6 g/L, and transducer power of 254 W. Fourier transform infrared demonstrated that the content of lipid, polysaccharide and proteins increased significantly on the surface of microalgal cells, with the increase of ultrasonic time. More importantly, ultrasound can also damage the cell structure of microalgae cells. A higher cell disruption efficiency and small particle size were achieved in the coupled approach, compared with ultrasonic-assisted solvent extraction. Additionally, compared with the modified Bligh & Dyer method, the buoyant beads and ultrasound assisted solvent extraction (BBUASE) method has lower polyunsaturated fatty acid content and higher saturated fatty acid content. Thus, the BBUASE can be expected to serve as a highly efficient way to produce fatty acid methyl ester and raw biodiesel in the modern microalgae industry.