Microwave assisted pretreatment of sweet sorghum bagasse for bioethanol production

Abstract

The growing demand for energy in the world, the implications of climate change, the increasing damages to our environment and the diminishing fossil fuel reserves have created the appropriate conditions for renewable energy development. Biofuels such as bioethanol can be produced by breaking down the lignocellulosic structure of plant materials to release fermentable sugars. Sweet sorghum bagasse has been shown to be an important lignocellulosic crop residue and is potentially a significant feedstock for bioethanol production. The aim of this study was to investigate suitable microwave assisted pretreatment conditions of sweet sorghum bagasse for bioethanol production. A chemical pretreatment process of sweet sorghum bagasse, using different concentrations (1 to 7 wt%) of sulphuric acid (H2SO4) and calcium hydroxide (Ca (OH)2) was applied to break up the lignocellulosic matrix of sweet sorghum bagasse. The pretreated broth, which contained pentose and hexose sugars, was fermented using a combination of Zymomonas mobilis ATCC31821 and Saccharomyces cerevisiae to produce bioethanol at pH 4.8 and 32oC for 24 hours. The highest reducing sugar yield of 0.82 g/g substrate was obtained with microwave irradiation at 180 W for 20 minutes in a 5 wt% sulphuric acid solution. The highest ethanol yield obtained was 0.5 g/g from 5 wt% H2SO4 pretreated bagasse at 180 W using a 10:5% v/v of Saccharomyces cerevisiae to Zymomonas mobilis ratio, whereas for 3 wt% Ca (OH)2 microwave pretreatment, a sugar yield of 0.27 g/g substrate was obtained at 300 W for 10 minutes. Thereafter, an ethanol yield of 0.13 g/g substrate was obtained after 24 hours of fermentation when using a 10:5% v/v of Saccharomyces cerevisiae to Zymomonas mobilis ratio. The effect of microwave pretreatment on the bagasse was evaluated using Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR) analysis. The reducing sugars formed were quantified using High Performance Liquid Chromatography (HPLC). The results showed that microwave pretreatment using 5 wt% H2SO4 is a very effective pretreatment that can be used to obtain sugars from sweet sorghum bagasse. The analytic results also showed physical and functional group changes after microwave pretreatment. This confirms that microwave irradiation is very effective in terms of breaking up the lignocellulose structure and improving fermentable sugar yield for fermentation. Bioethanol yields obtained from microwave pretreatment using different solvents also show that Saccharomyces cerevisiae and Zymomonas mobilis ATCC31821 is a good combination for producing ethanol from sweet sorghum bagasse. Sweet sorghum bagasse is clearly a very effective and cheap biomass that can be used to produce bioethanol, since very high yields of fermentable sugars were obtained from the feedstock.