Alkaline Pre-treatment of Gmelina arborea Leaves and Bark for Enhanced Bioethanol Production

Abstract

Lignocellulosic biomass from Gmelina arborea leaves and bark represents a promising non-edible feedstock for second-generation bioethanol, addressing food-fuel conflicts and environmental concerns associated with fossil fuels. The structural complexity of lignocellulose limits the accessibility of fermentable sugars during hydrolysis. Pre-treatment methods such as alkaline treatment are widely employed to disrupt lignin structures and enhance cellulose availability. This study investigated the effectiveness of alkaline pre-treatment using sodium hydroxide (NaOH) on the leaves and bark of Gmelina arborea for bioethanol production. Biomass samples were treated with 2.0 M, 2.5 M, and 3.0 M NaOH solutions for 24 hours at room temperature, washed, neutralized to pH 4–6 with 1 M H₂SO₄, and fermented with Saccharomyces cerevisiae for 5 days under anaerobic conditions. Ethanol was recovered by distillation (78 °C) and purified by rotary evaporation. The highest ethanol yield (25 ml) was obtained from leaf biomass pre-treated with 2.5 M NaOH, while bark biomass yielded the highest ethanol volume (18 ml) at 3.0 M NaOH. Lower concentrations (2.0 M) produced lower yields but higher purity (densities as low as 0.724 ± 0.013 g/cm³ for bark). Ethanol density ranged from 0.724 ± 0.013 to 0.952 ± 0.01 g/cm³. Residue fermentation yielded no ethanol, confirming the necessity of hydrolysate processing. Alkaline pre-treatment improved ethanol recovery from both tissues, with biomass-specific optima (2.5 M for leaves; 3.0 M for bark). Gmelina arborea residues therefore warrant further investigation as a second-generation bioethanol feedstock, provided yields are confirmed by sugar quantification and mass-balance calculations in future work.Gmelina arborea