Title : Influence of microwave-assisted hydrotropic pretreatment on the level of delignification of various types of biomass
Abstract:
Structural polysaccharides (cellulose and hemicellulose), which are the main components of lignocellulosic biomass, can be an inexpensive and easily available source of carbon used in biosynthesis processes; however, they must be susceptible to enzymatic degradation. An increase in the susceptibility of cellulose and hemicellulose to enzymatic hydrolysis is achieved during pretreatment aimed at reducing crystalline areas in cellulose, increasing the porosity of cellulose and hemicellulose and, primarily, removing lignins. The study was aimed at evaluating the effectiveness of the delignification of pine chips, beech chips and wheat straw, taking into account changeable process conditions of microwave-assisted pretreatment with sodium cumene sulfonate (NaCS) as the hydrotrope. Tests were conducted with the microwave generator at a constant power of 600 W, but with a variable concentration of NaCS (10, 20, 40%, w/v), time (10, 30, 60 min) and pressure (39, 78, 117 PSI). The highest level of biomass loss as a result of pretreatment was determined for wheat straw, and the lowest for pine chips. The maximum level of extraction of components achieved as a result of microwave-assisted hydrotropic pretreatment was ca. 55% for wheat straw, ca. 45% for beech chips, and 35% for pine chips. At constant NaCS concentration, regardless of the type of lignocellulosic biomass, increased loss of biomass was recorded as a result of pretreatment at increasing pressure and exposure. The highest loss of mass after microwave-assisted hydrotropic pretreatment was obtained at 117 PSI for 60 minutes, regardless of the biomass type. Microwave-assisted hydrotropic pretreatment also caused extraction of hemicellulose from biomass, lowering the content of this polysaccharide in the analysed samples. The effectiveness of the extraction of hemicellulose from biomass is correlated with biomass extractives. The pretreatment method used enables an almost complete extraction of hemicellulose from wheat straw from the level of 29-30% DW and from pine chips (original content 12-13% DW). The extraction of lignins from biomass was less intense in comparison to hemicellulose. In the case of the use of 10% w/v NaCS, increased content of lignins in the biomass was recorded, while when 20% w/v NaCS was used, the content of lignins was constant regardless of the conditions of microwave-assisted pretreatment and biomass type. The observed increase in lignin content in biomass results from the use of constant weights for determination of lignocellulose components. The lowering of lignin content in biomass was determined at as much as 40% w/v NaCS, although the level of extraction depended on the type of biomass. Conclusions facilitating the highest delignification of raw materials used include NaCS concentration at 40% w/v, pressure of 117 PSI and duration of 60 minutes. The obtained biomass after microwave-assisted hydrotropic pretreatment is a potential source of carbon that can be used in microbiological biosynthesis processes.
