Optimization of microalgal biomass (Scenedesmus dimorphus) for maximized bioethanol production through response surface methodology (RSM)

Abstract

Microalgae show great potential as a bioethanol feedstock, primarily attributed to their rapid growth rates and higher carbohydrate content. The pretreatment of biomass is recognized as a crucial step in bioethanol production, as it is essential for providing fermentable reducing sugars (monosaccharides) necessary for yeast (Saccharomyces cerevisiae) growth. It is essential to optimize this step individually for each influencing parameter to enhance overall efficiency. The present study focuses on the optimization of the growth medium with the starvation conditions and microwave-assisted acid hydrolysis (with H₂SO₄) of Scenedesmus dimorphus through RSM (response surface methodology), investigating the impact of temperature, time of hydrolysis, acid concentration, and acid volume on the yield of carbohydrate, reducing sugar and bioethanol. Preliminary studies facilitated the identification of an appropriate strategy for cultivation in a growth medium and determining the extent to which the variables must be optimized. The medium optimization resulted in a higher carbohydrate yield (30%) in the Zarrouk medium under starved conditions. The generated models forecasted the maximum reducing sugar content (0.59 g/L; R² = 0.977) after 11.87 min of acid hydrolysis under the temperature (145.16 °C) with an acid concentration of 1.25 N and acid volume (2.86 mL) whereas the maximum carbohydrate conversion yield (0.45 g/g) and bioethanol yield (20.5 wt%). When comparing the efficiency of carbohydrate conversion, the optimized process demonstrated an impressive 95.2% conversion rate, whereas the conventional process exhibited a significantly lower conversion rate of 62.18%. In summary, this research successfully identified the optimal growth medium and refined process conditions, ultimately maximizing the production of bioethanol.

Graphical abstract