Effect of single and combined pretreatment strategies applied to Chlorella vulgaris biomass on bioethanol production

Abstract

Microalgae serve as an attractive feedstock for the production of third-generation bioethanol because of their rich carbohydrates. The conversion of biomass to ethanol necessitates a saccharification step in which complex carbohydrates are hydrolyzed into sugars that can be fermented to bioethanol. The recalcitrance of microalgal cell envelopes, which are difficult to depolymerize and render internal carbohydrate reserves inaccessible to hydrolysis, impedes carbohydrate mobilization. Saccharification occurs through acidic or alkaline hydrolysis, enzyme hydrolysis, or a combination of the three methods. Mobilization of carbohydrates in large amounts requires expensive pretreatment techniques. The goal of this study was to produce bioethanol via chemo-enzymatic pretreatment of Chlorella vulgaris biomass using single and combined pretreatment strategies. The single pretreatments were acid, alkaline, and enzymatic hydrolysis, and combined pretreatment strategies were acid and enzyme hydrolysis; alkaline and enzyme hydrolysis; acid, alkaline and enzyme hydrolysis. For single pretreatment strategies, the highest reducing sugar yields were; 1.70 g mL⁻¹ at 1.2 mol L⁻¹ of NaOH carried out at 120 °C for 30 min, 1.42 g mL⁻¹ at 0.2 mol L⁻¹ of H₂SO₄ carried out at 121 °C for 30 min and 1.31 g mL⁻¹ with 1.0 × 10⁷ spores/mL of A. niger incubated at room temperature (28°C ± 2) for 7 days. For the combined pretreatment strategies, NaOH and enzyme hydrolysis produced the highest reducing sugar yield of 1.90 g mL⁻¹. The highest bioethanol yield of 5% w/v was obtained with the NaOH and enzyme hydrolysis pretreatment strategy. Further studies to understand the effect of various factors during the pretreatment and fermentation stages will help optimize and improve the bioethanol yield from microalgal feedstock.

Keywords:

• Bioethanol
• microalgae
• pretreatment
• hydrolysis
• fermentation
• sugar

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.