Preparation of Two-Dimensional Fe₃O₄ Nanodisks and Nanosheets Transformed from α-Fe₂O₃ Analogues and Their Applications in Magnetic Field-Assisted Microalgal Harvesting Biorefinery Processes

Abstract

Two-dimensional Fe 3 O 4 nanodisks (NDs) and nanosheets (NSs) were prepared from the thermal transformation of α-Fe 2 O 3 NDs and NSs, respectively. As a shape-controlling additive, the Al 3+ concentration controlled their sizes and disk-or sheet-like morphologies. Analyses of α-Fe 2 O 3 and Fe 3 O 4 NDs and NSs reveal anisotropic nanoarchitectures of highly crystalline hematite and magnetite domains, respectively. Magnetic hysteresis showed that α-Fe 2 O 3 NDs and NSs were weak superparamagnetic, while Fe 3 O 4 NDs and NSs were soft ferrimagnetic. The magnetic harvesting of microalga Haematococcus pluvialis (H. pluvialis) using Fe 3 O 4 NDs and NSs was demonstrated, resulting in a harvesting efficiency (HE) of >∼95% at less than 1 min of incubation. The efficient extraction of astaxanthin (ATX) from H. pluvialis was demonstrated through the laceration of the cell wall by Fe 3 O 4 NDs and NSs under mild ultrasonications. ATX extraction efficiencies (EEs) of up to ∼83% with minimal cellular damage under <10 min of mild ultrasonication were obtained using Fe 3 O 4 NSs. The potential recycling feasibility of Fe 3 O 4 NDs and NSs was assessed by measuring their HE and EE after five magnetic recovery and regeneration cycles using scrubbing procedures. This study revealed a synergistic enhancement of the HE and EE by integrating magnetic nanomaterial-based harvesting and extraction, offering novel process solutions for highly efficient microalgal biorefineries.