Recovery of Chlorella biomass using Fe3O4 and Fe3O4@SiO2 magnetic nanoparticles: isotherm and thermodynamic characteristics of adsorption

Harvesting microalgae by centrifugation and filtration is very costly, affecting technical and economical feasibility of downstream processing of algal biomas to bioproducts. Flocculation is a potential technique for recovery of algae biomass. Particularly, flocculation using magnetic nanoparticles offers high separation efficiency due to their high surface area and magnetic properties. In this work, the synthesized magnetic nanoparticles including Fe3O4 and Fe3O4@SiO2 were used to harvest C. sorokiniana TH01 from aqueous suspension. The effects of pH, adsorbent dosage, algal biomass concentration, temperature and reaction time on harvesting performance were investigated. It was revealed that the optimal C. sorokiniana TH01 harvesting conditions for Fe3O4 were pH of 5, adsorbent dosage of 0.5 g/L, biomass concentration of 2-2.5 g/L, temperature 25(o)C and reaction time 20-30 min. For Fe3O4@SiO2, the optimal conditions were pH of 7, adsorbent dosage of 0.6 g/L, with the same biomass concentration, temperature and reaction time as Fe3O4. Under the optimal conditions, Fe3O4 and Fe3O4@SiO2 can be regenerated up to three cycles with harvesting efficiency till remained over 55%. Adsorption of C. sorokiniana TH01 on Fe3O4 and Fe3O4@SiO2 exhibited the best fitness with Langmuir isortherm with R-2 and maximal adsorption capacity estimated of 0.995 and 6.55 g/g and 0.9971 and 9.53 g/g, respectively. Thermodynamics study revealed that an adsorption of C. sorokiniana TH01 on Fe3O4 and Fe3O4@SiO2 was exothermic process. Furthermore, XRD, EDS, SEM, TGA and FT-IR data confirmed the successful binding of microalgae cells on surfaces of Fe3O4 and Fe3O4@SiO2. The super-magnetization intensities (> 22 emu/g) of the synthesized materials demonstrated an excellent separation capability with external magnetic devices.