Manganese ferrite nanoparticle-algal cell interaction mechanisms for potential application in microalgae harvesting

Algal biofuel is a promising green energy for the future, but harvesting algae remains a major challenge. To overcome this obstacle, magnetic separation using magnetic nanoparticles is proposed as a simple, highly efficient yet cost-effective method to collect microalgae. Magnetite (Fe₃O₄) nanoparticles are the typical materials of choice, but their performance varies depending on the pH of the water, furthermore, they can be toxic to algal cells. Hence, a more stable and non-toxic alternative replacement for Fe₃O₄ is needed. In this work, we explore the use of biocompatible manganese-containing magnetic ferrite nanoparticles (NPs) to harvest Chlorella sorokiniana and Scenedesmus obliquus microalgae. Using this novel NP, we achieved a harvesting efficiency of roughly 90% for Chlorella sorokiniana and 80% for Scenedesmus obliquus up to three cycles consistently throughout a wide pH range of 2–12. This was due to the high stability and reversible attachment of the NPs to the algal cells. Surface analysis of the NPs-Algae by Fourier transformed infrared (FTIR), the microbial adhesion to hydrocarbons (MATH), zeta potential, and the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory indicated acid-base interactions and hydrophobicity effects are the driven forces for NPs-algae interaction instead of simple electrostatic attraction. Overall, our study provided a more efficient magnetic harvesting approach for algae and a more in-depth understanding of the separation mechanisms to improve and advance the algae biofuel industry.