Multifunctional Role of Magnetic Nanoparticles in Efficient Microalgae Separation and Catalytic Hydrothermal Liquefaction

In this work, the efficiency of extracting algae from culture medium using magnetic nanoparticles (MNPs), converting the algal/particle slurry to biocrude using hydrothermal liquefaction (HTL), and successfully recycling the MNPs from the char phase was fully demonstrated for the first time. MNPs were synthesized by coprecipitation and used to extract algae from aqueous phase at a separation efficiency (SE) of 99%. The SE was optimized at pH 4. Liquefaction of algal/MNPs slurry gave a biocrude yield of 37.1% while algae only yielded 23.2%. The percentage areas in the GC-MS chromatogram corresponding to hydrocarbons (HCs) in Zn-ferrite catalyzed and uncatalyzed biocrude were 46.5% and 19.9%, respectively, while the percentage areas of heptadecane from Zn-ferrite catalyzed and uncatalyzed biocrude were 37.8% and 10%, respectively. Furthermore, the percentage area of heteroatom compounds in biocrude reduced substantially when liquefaction was done in the presence of Zn/Mg ferrites. The nanoparticles were recovered from biochar by sonication and recycled at a SE of 96.1%. Recycling of MNPs for magnetic separation of algae and catalytic HTL could lower the cost of microalgae harvesting and improve the yield and quality of biocrude. This could potentially reduce the cost of advanced biofuel processing from microalgae, making them more affordable in comparison to petroleum-derived fuels.