A biorefinery approach for high value-added bioproduct (astaxanthin) from alga Haematococcus sp. and residue pyrolysis for biochar synthesis and metallic iron production from hematite (Fe2O3)

Abstract The present study aimed to investigate a biorefinery approach on natural astaxanthin production from the microalgae Haematococcus lacustris and biomass residue for biochar production through pyrolysis. Further, the biochar obtained from the H. lacustris residue was explored for iron oxide reduction using hematite iron ore through a thermogravimetric analyzer (TGA). The alga H. lacustris cultivated in a low-cost polythene made photobioreactor (PBR) for a period of 32 days. During cultivation, several parameters were optimized for the enhanced biomass and astaxanthin accumulation. The results revealed that the highest biomass productivity of 3 g L−1 was found in acetate amended culture medium. Similarly, the addition of an extra carbon source (acetate) supported the highest astaxanthin productivity of 203 mg L−1, and it was observed during the red phase on day 32. The PBR volumetric biomass productivity of H. lacustris was estimated at up to 79.8 tonnes/acre/year. In this study, the alga H. lacustris grown in polythene made PBR produced the highest natural astaxanthin yield of 6.76%/cell dry weight (CDW). Further, the residual biomass was pyrolyzed through a non-catalytic process to obtain the maximum biochar yield. A mini pellet was synthesized using H. lacustris biochar and hematite (Fe2O3), and it was taken for metallic iron production under a nitrogen atmosphere. The results revealed that the biochar was supported for maximum metallic iron (Fe) production at ≥975 °C. This is the first study demonstrating the utilization of microalgae H. lacustris for natural astaxanthin production and biomass residue for metallic iron (Fe) production at low-cost methods.