Ultrasonic-assisted lignocellulose pretreatment of amaranth stem for bioethanol production

Abstract

The depletion of fossil fuels, increasing energy demands in the world, and the effects associated with global warming has led to a search for alternative energy forms that are renewable such as bioethanol. Bioethanol produced from biomass such as a plant or organic waste could help reduce the production of carbon dioxide (CO2). First generation ethanol which is derived from food crops can offer some CO2 reduction benefits, but has the disadvantage of competing with food crops, which limits the production of first generation ethanol. Second generation ethanol offers the potential of providing novel biofuels. Second generation bioethanol is produced by breaking down the lignocellulose plant structure into fermentable sugars, which can further be fermented into ethanol. However, one major drawback in bioethanol production is lower ethanol yields due to ineffective pretreatment. The aim of this study was to assess the effectiveness of ultrasonic pretreatment in combination with acid and alkali treatment to liberate fermentable sugars from amaranth lignocellulose for ethanol production. High Performance Liquid Chromatography (HPLC) was used to analyze and quantify the sugars and ethanol, and the solid residues were characterized by Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). The effect of energy input (54 -423kJ.g-1), sonication time (15-60minutes), calcium hydroxide concentration (10 - 50 g.kg-1 in Ca(OH)2 in water) and sulphuric acid concentration (10 – 50g.kg-1 H2SO4 in water) and biomass loading (10 - 100g.kg-1 biomass) on sugar and ethanol yields were studied. The highest sugar yield using ultrasonic-assisted dilute acid pretreatment (340g.kg-1 substrate) was obtained at 270 kJ.g-1 energy input for 30min in the presence of 30 g.kg-1 H2SO4 in water. The highest sugar yield using ultrasonic-assisted dilute alkali pretreatment (240 g.kg-1 substrate) was obtained at 270 kJ.g-1 energy input for 30 min in a solution of 30 g.kg-1 Ca(OH)2 in water. The highest ethanol yield of 110 g.kg-1 of biomass was obtained after 24 hours of fermenting an alkaline pretreated hydrolysate, and 90 g.kg-1 ethanol yield was obtained from an acid pretreated hydrolysate. The combined pretreatment was shown to be effective in liberating fermentable sugars from amaranth stem with ultrasonic-assisted dilute acid pretreatment resulting in 92% conversion of total sugars. With reduced pretreatment time, and relatively low catalyst concentrations, assisted ultrasonic pretreatment is an economical attractive lignocellulose pretreatment step.