Design of functional magnetic particles and their application to microalgae separation

Abstract

Microalgae feedstocks are leading candidates for large-scale production of sustainable biochemicals and biofuels due to the huge potentials of microalgae including high biomass and lipid productivity, carbon neutrality, wide range of end-products, growth adaptability, and lesser land requirement. However, the overall process, starting from microalgae cultivation and ending in conversion to biofuels, involves complicated pro-cess units and faces technological and economic challenges for commercialization as this time. Recently, the application of multifunctional nanoparticles has been suggested as a powerful tool to open a possibility for commercialization of microalgae-based biofuels. Chapter 2 is aimed to review extensive research to improve process efficiency of microalgal biorefin-ery in the literature. Attention will be focused mainly on the nanoparticle-aided microalgae harvesting, extrac-tion and conversion. With respect to microalgae harvesting, various functionalized magnetic nanoparticles for enhancing harvesting efficiency will be introduced. Furthermore, nanoparticles with multiple functions or recyclability to reduce process costs are covered. In the literature, aminoclay-conjugated nanoparticles used to increase lipid extraction yield have been reported. The various nanocatalysts for enhancing conversion yield or upgrading biodiesel are presented. In chapter 3, magnetophoretic harvesting of microalgae is conducted through a three-step process, which includes functionalization of magnetic particles by (3-aminopropyl)triethoxysilane (APTES), magnetic separation, and detachment of magnetic particles by increasing pH to higher than the isoelectric point. De-tachment process is specifically focused and found that the use of larger magnetic particles is more efficient for detachment of magnetic particles from algae-particle conglomerates. The detaching efficiency improves from 12.5% to 85% when the particle size is increased from 108 nm to 1.17 $\mu$m. In chapter 4, efficient microalgal harvesting and integrated technology are suggested as a way to im-prove economic feasibility of microalgae-based biodiesel. Tri-functional (cationic, magnetic, and lipophilic) carbon microparticles filled with magnetite ($Fe_3O_4$) are synthesized through one-step aerosol spray pyrolysis and applied in microalgal harvesting and serial microalgal lipid entrapment. Carbon microparticles are tri-functional in the following respects: (i) the cationic carbon microparticles facilitate flocculation with anionic microalgae due to electrostatic attractions

(ii) the magnetic properties of the carbon microparticles, owing to embedded magnetites, enable the separation of microalgal flocs from very dilute cultures (~2 g $L^{-1}$) with a separation efficiency of 99%

and (iii) the lipophilicity enables the recovery of lipid droplets extracted from oleaginous microalgae. Microalgal lipids are directly separated through adsorption onto magnetic carbon mi-croparticles from concentrated microalgal slurries after harvesting. The tri-functionality may facilitate the integrated use of magnetic carbon microparticles in microalgal biorefineries and the tri-functional microparti-cles could potentially be applied in various areas such as biomedicine, catalysis, magnetism, energy materials, and environmental remediation. In chapter 5, it is suggested to integrate downstream processes (harvesting and cell disruption) by using cationic surfactant-decorated $Fe_3O_4$ nanoparticles to mitigate complexity and cost in the biorefinery, The quaternary ammonium heads of cationic surfactants play an important role in not only flocculating nega-tively charged microalgae but also weakening thick cell wall. Harvesting efficiency and cell damaging effect are evaluated in accordance with the studied three cationic surfactants, including cetrimonium bromide (CTAB), cetylpyridinium chloride (CPC), and cetylpyridinium bromide (CPB). CTAB decorated $Fe_3O_4$ nano-particles, which are found to be the most effective, achieve microalgae harvesting with 96.6 % efficiency at a dosage of 0.462 g particle/g cell and 99.8% at a 0.92 g particle/g cell. To recycle magnetic nanoparticles and obtain high purity microalgal biomass, the detachment of microalgae from microalgae- $Fe_3O_4$ flocs is con-ducted by adding the anionic surfactant, sodium dodecyl sulfate (SDS). The detached CTAB decorated $Fe_3O_4$ nanoparticles shows a steady reuse efficiency of about 80%. Furthermore, microalgae harvesting by CTAB decorated Fe3O4 nanoparticles contributes to the great improvement in the total extracted lipid content and the greener wet extraction, though energy-intensive cell disruption step is omitted, demonstrating cell disrup-tion ability of CTAB decorated $Fe_3O_4$ nanoparticles. In conclusion, this dissertation covers the state of art in nanoparticle-based microalgal biorefinery and research progress in the design of functional magnetic particles and provides a guide for future improvements in microalgal biorefinery.