Biofuel production from wet microalgae biomass: Comparison of physicochemical properties and extraction performance

A one-step process that integrates cell disruption with lipid extraction from wet microalgae was proposed by using n-hexane/formic acid. The scanning electron microscopy showed that raw microalgae cells were completely disrupted. The derivative thermogravimetric analysis indicated that formic acid could lead to a fast-thermal decomposition of microalgae residues at the temperatures ranged from 175 to 280 °C. The Fourier transform infrared spectroscopy revealed that the intensities of CO vibrations increased with an increase of the carbonyl compounds. When microalgae were treated with this mixed solvent at an n-hexane/formic acid ratio of 9:2 (v/v) and an addition dosage of 80 mL/g wet microalgae at 80 °C for 2 h, the optimal crude lipid yield was 256.3 mg/g dry weight. A high fatty acid methyl ester (FAME) content of 83.6% was achieved, dominated by palmitic acid, palmitoleic acid, oleic acid, and linoleic acid. Compared with the conventional methods including n-hexane, ethanol, n-hexane/methanol, and chloroform/methanol without pre-treatment, the crude lipid yields increased by 15%–402%. Compared with the chloroform/methanol extraction supplemented by ultrasound, microwave, hydrothermal, and dilute nitric acid pre-treatments, the achieved FAME contents increased by 79%–99%. Additionally, the energy yield from n-hexane/formic acid extraction reached up to 8.55 kJ/g volatile solid.