Optimization of culture conditions of Scenedesmus sp. algae and catalytic performance of a NiFe2O4@SiO2/MgO magnetic nano-catalyst for biodiesel production

Abstract

This study aims to optimize the lipid content of Scenedesmus sp. for high-yield biodiesel production. Three factors affecting the culture conditions, namely salinity, nitrogen concentration, and light intensity, were selected and their effects on the maximum lipid content were investigated using the Box Behnken design. The results showed that the maximum lipid content (32.7% of algal dry weight) was obtained in the algal samples cultured under the optimized conditions. A core-shell magnetic nano-catalyst, NiFe₂O₄@SiO₂/MgO, was synthesized and used to produce biodiesel via the transesterification reaction. The nano-catalyst was characterized by field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray powder diffraction analysis (XRD), vibrating sample magnetometry (VSM), elemental mapping techniques, transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FT-IR). Using the Box Behnken design and keeping the temperature constant, the molar ratio of methanol to oil, the amount of catalyst, and the time were optimized to achieve the maximum yield of biodiesel. The maximum yield of biodiesel was 95.3% under the optimal conditions.

Fig. 1. Stepwise preparation of NiFe₂O₄@SiO₂/MgO core–shell magnetic nano-catalyst.

Fig. 2. The FT-IR spectra of NiFe₂O₄ (a), NiFe₂O₄@SiO₂ (b), and NiFe₂O₄@SiO₂/MgO (c).

Fig. 3. The XRD pattern of NiFe₂O₄ (a), MgO (b), NiFe₂O₄@SiO₂ (c), and NiFe₂O₄@SiO₂/MgO (d).

Fig. 4. The VSM analysis of NiFe₂O₄ (a) and NiFe₂O₄@SiO₂/MgO (b).

Fig. 5. FE-SEM images of the NiFe₂O₄ (a), NiFe₂O₄@SiO₂ (b) and NiFe₂O₄@SiO₂/MgO (c).

Fig. 6. TEM images of the NiFe₂O₄ (a), NiFe₂O₄@SiO₂ (b) and NiFe₂O₄@SiO₂/MgO (c).

Fig. 7. EDS spectrum of the NiFe₂O₄@SiO₂/MgO.

Fig. 8. Picture of the elemental mapping of the NiFe₂O₄@SiO₂/MgO.

Fig. 9. The growth curve of the control medium (a) and 15 treatments (b) for lipid content optimization. (L, N, and S are related to light intensity, nitrate concentration, and salinity, respectively, and −1, 0, 1 is related to levels of experiments).

Fig. 10. The response surface plots: (a) interactive effect of light intensity and NO₃ concentration, (b) interactive effect of light intensity and salinity concentration, (c) interactive effect of NO₃ concentration and salinity concentration.

Fig. 11. Response surface plots: (a) interactive effect of methanol to oil molar ratio and amount of catalyst, (b) interactive effect of amount of catalyst and reaction time, (c) interactive effect of methanol to oil molar ratio and reaction time.

Fig. 12. GC-MS spectrum of Scenedesmus sp. biodiesel synthesized under optimum conditions.

Fig. 13. The reusability of the NiFe₂O₄@SiO₂/MgO under optimized reaction condition.

Fig. 14. FE-SEM image (a) and XRD pattern (b) of the NiFe₂O₄@SiO₂/MgO after recycling.