Bioenergy production and metallic iron (Fe) conversion from Botryococcus sp. cultivated in domestic wastewater: Algal biorefinery concept

Highlights: • Botryococcus sp. grown in domestic sewage produced 3.2 g/L of dry biomass. • Botryococcus produced 94.1 wt% of biodiesel using tungstated zirconia catalyst. • Lipid extracted residues yield 41 wt% of biochar after pyrolysis at 500 °C. • Thermogravimetric curve show that the metallic iron synthesis was found at 990 °C. • Botryococcus is a potential material for conversion of hematite to metallic iron (Fe). -- Abstract: This study focused on a novel approach for biodiesel production and metallic iron synthesis using biochar obtained from the biomass residue of green microalgae Botryococcus sp. Hematite (Fe₂O₃) is one of the most important iron ore used in steelmaking industries. Thus, we proposed this work for the development of algal biorefinery concept at commercial scale. This work contains two phases; in the first phase, the alga was successfully cultivated on the domestic wastewater at large scale using a low-cost photobioreactor, which provided significant biomass and lipid yield. To reduce the cost involved in biomass harvesting, an auto-flocculation technique was implemented and harvested 94.8% of biomass without adding any flocculants. The biodiesel extraction was performed in an ultrasonic bath with a frequency of 25 kHz using a tungstated zirconia as a heterogeneous acid catalyst, which produced 94.1 wt% of biodiesel yield. The kinetic studies were investigated at various reaction temperature and confirmed that the reaction followed a pseudo-first-order kinetic model. The activation energy and pre-exponential factor for the transesterification reaction were found to be 45.3861 kJ mol⁻¹ and 2.6956 min⁻¹, respectively. In the second phase, the lipid extracted residue was converted to biochar through pyrolysis process, and the yield obtained was 41 wt%. The obtained biochar was utilized for metallic iron synthesis, and this reaction was carried out in a thermogravimetric analyzer equipped with Fourier-transform infrared spectroscopy. The results showed that the reduction behaviors was occurred in a stepwise manner rendering to the temperature and the metallic iron synthesis was found at 990 °C.