RENEWABLE CHEMICALS PRODUCTION FROM AQUATIC WEED: PISTIA STRATIOTES

Abstract

A renewable chemical that can replace fossil fuels is bioethanol. Pistia stratiotes, an aquatic weed, was used as the substrate for the synthesis of ethanol in the current investigation. Initially, chemical composition was analyzed and shows this weed contains ash content (18.36%), total solid content (92.85 %), moisture content (7.15%), cellulose content (25.90%), hemicellulose content (18.44%), lignin content (25.25%), starch content (0.6 %), protein content (21.38 %). This composition shows that it contains a high amount of cellulose and hemicellulose which can be utilized as a carbon source for ethanol production after pretreatment. Reducing structural recalcitrance and enhancing hydrolysis efficiency are crucial factors for increasing fermentable sugars and the production of valuable products. Response surface methodology was employed to optimize acidic pretreatment and alkaline pretreatment. In the alkaline pretreatment, cellulose content was exposed and this was further followed by enzymatic hydrolysis, aiming to enhance the saccharification of Pistia stratiotes. On the other hand in the acidic pretreatment direct hydrolysis was done and reduced sugar was directly followed by the fermentation process. Alkaline concentration, time and temperature (0.5 - 3 % NaOH, 30 - 60 minutes, 60 - 120 °C) were taken as independent variables in the optimization of alkaline pretreatment. The substrate comprises lignin, hemicellulose, and cellulose. The NaOH concentration during pretreatment significantly influenced the delignification process, resulting in an increased cellulose content. The highest cellulose content was achieved with 2.47 % NaOH at 120 ºC for 60 minutes, leading to enhanced cell porosity and facilitating greater enzyme saccharification accessibility. Under these optimized conditions, the sample exhibited 51.66 % cellulose content. Enzymatic hydrolysis of this cellulose was performed with a commercially available cellulase enzyme. It resulted in 31.06 g/L of reduced sugar liberation after 72 hours. In the Optimization of acidic pretreatment conditions, three independent variables including acid concentration (0.15 - 3.1 % H2SO4 concentration), time (12.96 - 97.04 minutes) and temperature (93.18 - 126.82 °C) with responses like sugar and cellulose concentration. Subsequently, the optimized condition was subjected to fermentation for the production of ethanol using Saccharomyces cerevisiae and Pichia stipites strains. The physiochemical characteristics of the cellulose obtained after pretreatment were analysed using SEM, FTIR, and TGA. The maximum amount of sugar is produced under optimal conditions (2.5% acid concentration) at 120°C for 15 minutes. For the v fermentation stain two microbial strains were procured by NCIM, Pune, these are Saccharomyces cerevisiae and Pichia stipitis. Initial sugar was taken as 10g/l in the hydrolysate, which is consumed by the strains with maximum sugar conversion rate (0.24 g/l/h). The quantification of the ethanol was done by gas chromatography. Maximum sugar consumption by S. Cerevisiae and P. stipites was 85.9 % and 87.9 %, respectively. Thus, we present the optimized acidic pretreatment conditions for maximum production of sugar, and thereby, the maximum yield of ethanol from Pistia stratiotes using S. Cerevisiae and P. stipitis. This study demonstrates that acidic and alkaline pretreatment of Pistia stratiotes significantly increased its reduced sugar content and cellulose content, leading to a higher sugar yield during enzymatic hydrolysis and maximum ethanol was produced by the P. stipitis strains because it can convert both pentose and hexose sugar into the ethanol under anaerobic conditions.