Harvesting Microalgae Biomass Using Magnetic Nanoparticles from Iron-Rich Particulate Material

Using magnetic nanoparticles for efficient microalgae harvesting has shown promising results with enhanced harvesting efficiency within a reduced time frame. However, the cost of nanoparticle synthesis presents a potential constraint the widespread implementation of this technique. In this way, this work investigated, for the first time, the efficacy of magnetic nanoparticles (MNPs) derived from particulate matter for harvesting microalgae (Chlorella sp.) both naked and functionalized with commercial tannin. The nanoparticles were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Fourier transform infrared (FTIR), and zeta potential. The MNPs present in the particulate material were magnetite with an estimated size (based on 1000 particles) of around 30.5 ± 9.56 nm. Harvesting efficiency (HE %) was optimized using full factorial experiments, in which the most influential variables (pH and MNPs concentration) were combined at different levels, achieving optimal harvesting efficiency. The MNPs (naked) exhibited higher harvesting efficiency (HE % = 86%; MNPs concentration = 1250 mg. L^-1; pH = 3) than functionalized nanoparticles (HE % = 77%; MNP-TANs concentration = 1100 mg/L; pH = 3.5); however, a higher MNPs concentration was necessary in a lower pH. The functionalization contributed to particle stabilization, increasing its reuse cycles from 3 (MNSs) to 7 cycles (MNP-TANs). At pH 10 (the final pH of the microalgae cultivation), both exhibited a similar HE % of 60%.