Production of Bioethanol by Electrochemical Redox Coupled with Microbial Cells Using Lignocellulosic Biomass

Abstract

Bioethanol, blended with gasoline (petrol), is used as liquid transportation fuel worldwide. Local production and use of bioethanol supports local economies, decreases a country’s carbon footprint and promotes self-sufficiency. The latter is especially important for bio-resource rich, land-locked countries such as Nepal that are seeking alternative transportation fuels and technologies to produce them. Bioethanol is a renewable resource that is dominantly produced from either sucrose or starchy biomass. As Nepal is rich in agricultural sector, use of residual lignocellulosic biomass from plants can be a better alternative for bioethanol production. The lignocellulosic biomass composition of plants differ depending on the locality and seasonal changes. We have evaluated the suitability of four different sources of lignocellulosic biomass, viz., Ipomoea carnea, Phragmites karka, Saccharum spontaneum and Zea mays cobs for obtaining reducing sugar which can be used for bioethanol production in Nepal. S. spontaneum was found to be the best among the four as an economic source of lignocellulosic biomass since it has better degradation capabilities, high caorific value and relatively high total reducing sugar (TRS) content (612.2±11.5 mg·g vii

-1 biomass). Hot water pretreatment of S. spontaneum biomass at 100 o C for 2 h followed by hydrochloric acid hydrolysis (TRS = 330.4±20.5 mg·g -1 biomass) were found to be the best for release of fermentable sugars. The variations in characteristics of lignocellulosic biomass before and after pretreatment with hot water at 100 o C for 2 h were investigated by using differential thermo gravimetric curve (DTG), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy methods. The XRD and FTIR analysis showed that pretreatment reduced the amorphous nature of cellulose and increased crystalline characteristics. The content of glucose (untreated: 246.7±4.0 and pretreated: 235.1±5.0 mg·g -1 biomass) and xylose (untreated: 86.6±3.9 and pretreated: 62.5±3.0 mg·g -1 biomass) determined in untreated and pretreated (hot water at 100 o C for 2 h) biomass suggested that while the cellulose loss during pretreatment was minimal, hemicellulose content was lost significantly. The conventional method to produce ethanol is via microbial fermentation and it comes with limitations. We have demonstrated the feasibility of using a bio-electrochemical system to improve ethanol production by yeast. In this system, externally supplied 4V was used to drive the chemical reactions to generate higher levels of ethanol in the yeast cultures. In the present study, we have identified two highly efficient ethanol producing yeast strains, viz., Saccharomyces cerevisiae (CDBT2) and Wickerhamomyces anomalous (CDBT7) out of twelve isolates and used them in a bioelectrochemical cell to enhance ethanol production. CDBT2 and CDBT7 were cultured in anodic and cathodic compartments with fine platinum coated platinum anode and neutral red coated graphite-felt cathode, ethanol production was drastically enhanced by 52.8±0.44% in average. The above experiments were repeated using lignocellulosic biomass hydrolysates (Saccharum spontaneum by pretreating with hot water for 2 h at 100 o C followed by hydrolysis with 0.5M HCl) as substrate resulted better enhancement in ethanol production (61.8±0.12%). Use of cellulose acetate in place of nafion membrane as anodic and cathodic partition better enhanced ethanol production by 6.30±0.22%. The enhancement in expression of alcohol dehydrogenase and pyruvate decarboxylase was seen when voltage was supplied. The results concluded that CDBT2 and CDBT7 yeast strains produced ethanol efficiently from both glucose and lignocellulosic biomass hydrolysate. The ethanol production was enhanced in electrochemical cell in the presence of low level of external voltage. Ethanol production was further enhanced with the better involvement of electron transport systems, when neutral red was deposited on cathode and platinum nanoparticles were coated on the platinum anode and cellulose acetate as partition membrane. This can be an optimal method for commercial ethanol production from biomass hydrolysate after up scaling.