Valorization of microalgal biomass to value-added products using integrated supercritical CO2 extraction and sub-critical hydrothermal liquefaction

Abstract

The study depicts the use of different extraction techniques to utilize each fraction of microalgal consortium biomass (Chlorella sp. and Phormidium sp.) for its conversion to value-added products (VAPs). The algal biomass was rich in proteins (30.8%), lipids (28.1%), carbohydrates and essential amino acids (36.29% of total protein). The lipid yield obtained after supercritical CO2 extraction (SFE) was found to be 3.0 ± 0.4% compared to the conventional soxhlet extraction (28.2 ± 2.3%) with chloroform: methanol (1:1 v/v). However, SFE extracted lipid showed higher Polyunsaturated fatty acids (PUFAs) content (20.68%) with the presence of eicosapentaenoic acid, docosahexaenoic acid, α-linolenic acid, linoleic acid and arachidonic acid. The PUFA content in lipid obtained after soxhlet extraction was only 3.48%. The residual biomass after SFE showed recovery of 17.1 ± 0.7% proteins and 27.1 ± 0.9 mg g−1 of total carotenoids. Subcritical-hydrothermal liquefaction (HTL) of the microalgae at 230 °C and 2.7 MPa produced 30.7% biocrude (dried biomass basis) containing aldehyde/ketone (37.14%), aromatics (27.63%) and alkanes/alkenes/alkynes (15.3%) along with other compounds. This fraction consisted of VAPs targeting multiple industries-food (11%), fuel (22%), pharmaceutical (56%) and cosmeceutical (12%). A novel approach including sequential SFE-HTL process showed a lower biocrude yield (8.6%) but showed upgradation in biocrude quality having 50% aromatic compound of which >48% were pyrazines derivatives (flavoring agents for the food industry) and 20.8% pharmaceutically valuable compounds, besides extracting PUFAs from SFE. Hence, a new microalgal valorization approach has been developed by integrating two extraction techniques to produce multiple VAPs to target a minimum waste generation approach.