Comparison between Radio Frequency (RF) and Traditional Heating Assisted Alkaline Pretreatment on Lignocellulosic Biomass

Abstract

Pretreatment plays an important role in making the cellulose accessible for enzyme hydrolysis and subsequent conversion because it more or less destroys resistance and recalcitrance of biomass. In this study, radio frequency (RF) assisted dielectric heating was utilized in the alkaline (NaOH) pretreatment. The substrates included agricultural residues (corn stover), herbaceous crops (switchgrass), and hardwood (sweetgum) to softwood (loblolly pine). Pretreatment was performed at 90℃ for both RF and traditional water bath (WB) heating for one hour after overnight soaking in NaOH solution (0.2g NaOH/g Biomass). Pretreated materials were characterized by chemical compositional analysis; enzyme hydrolysis, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The glucan yield from hydrolysis yield using RF heating method for these four feedstocks were 89.6%, 72.6%, 21.7% and 9.9%, while with conventional heating technique the yields were 89.4%, 51.8%, 19.6% and 9.8%. Interestingly, RF heating raised glucan yield on switchgrass and sweetgum but not much on corn stover or loblolly pine. The SEM images and FTIR spectra agreed with composition analysis and results of enzyme hydrolysis. Moreover, the acetic acid of switchgrass hydrolysate after radio frequency heating was 2.19g/L compared to that of traditional water bath heating at 1.58g/L. After pretreatment, the practicality of using switchgrass hydrolysate medium to grow recombinant E-coli utilizing pBHR68 plasmid for production of polyhydroxybutyrate (PHB), a biodegradable plastic, was explored in this study. Switchgrass hydrolysates after alkaline pretreatment assisted by radio frequency heating and traditional water bath heating (original and added carbon source), as well as M9 medium (control group), were used as culture media. The RF media was shown to be optimal for PHB concentration produced, with final dry cell weight (DCW) 6.30 ± 0.11 g/L, PHB concentration 2.25 ± 0.13 g/L. Moreover, Switchgrass hydrolysates after alkaline pretreatment assisted by radio frequency heating and traditional water bath heating (original and added carbon source) were used as culture media for acetone-butanol-ethanol (ABE) fermentation by Clostridium beijerinckii. The hydrolysate was used after pH adjustment without sediment removal for ABE fermentation. ABE got to the maximum in the first medium with butanol concentration 3.9 g/L and total ABE concentration 5.91 g/L, corresponding to the maximum ABE yield (0.45) in the first medium, indicating that the enzymatic hydrolysates after alkaline pretreatment assisted by radio frequency was best for Clostridium beijerinckii growth. However, the difference of the switchgrass hydrolysate between radio frequency and traditional water bath heating was smaller with the addition of yeast extract, which verified that the radio frequency probably broke down into some nutrients in favor of ABE fermentation.