Removal of Fermentation Inhibitors Formed during Pretreatment of Biomass by Polymeric Adsorbents

Abstract

The production of aldehydes that are microbial inhibitors may occur when hexoses and pentoses in an aqueous solution are exposed to temperatures above 150 °C under acidic conditions common to acid-catalyzed lignocellulose biomass pretreatment. Concentrations greater than 0.1% of the degradation product, furfural, strongly inhibit fermentation, as was confirmed for hydrolysate that contained 0.5% (w/o) furfural. Methods of furfural removal that have been reported include sulfite or alkali addition to achieve chemical reduction, ion exchange, hydrophobic adsorption, and irreversible adsorption on activated carbon. This paper reports the removal of furfural from biomass hydrolysate by a polymeric adsorbent, XAD-4, and desorption of the furfural to regenerate the adsorbent using ethanol. Liquid chromatographic analysis showed that furfural concentrations were less than 0.01 g/L compared to the initial concentrations that were in the range of 1−5 g/L. Fermentation of the resulting biomass hydrolysate with recombinant Escherichia coli ethanologenic strain K011 confirmed that the concentration of furfural in the hydrolysate caused negligible inhibition. Fermentation of XAD-4-treated hydrolysate with E. coli K011 was nearly as rapid as the control medium that was formulated with reagent-grade sugars of the same concentration. Ethanol yields for both fermentations were 90% of theoretical. Modeling of the adsorptive properties of this styrene-based adsorbent indicates that it is suitable for on−off chromatography and could be useful in a continuous processing system for removing small amounts of aldehydes that might otherwise inhibit fermentation.

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“…For example, polymeric adsorbent XAD-4 was used to adsorb furfural from acid-catalyzed biomass hydrolysate before ethanol fermentation for removal of inhibitory contaminant [147]. This study indicated that such polymeric adsorbents are a very good source of detoxification of the biomass hydrolysate before anaerobic fermentation.…”

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“…all produce cocktails of inhibitory chemicals that act to suppress the activities of yeast (and bacteria) in converting hydrolysate sugars to ethanol. Various approaches have therefore been adopted to alleviate the deleterious effects of these inhibitors 98,[126][127][128]136 For example, these can be reduced using steam stripping, nanofiltration membranes, supercritical fluid extraction, or polymeric adsorbent materials (e.g., amberlite resins).…”

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“…The conversion of glucose to ethanol during fermentation of the enzymatic hydrolyzate is not difficult provided there is an absence of inhibitory substances such as furfural, hydroxyl methyl furfural, or natural wood-derived inhibitors such as resin acids (Weil et al 2002). While there are no known natural organisms able to ferment five-carbon (C5) sugars, new yeast strains are being developed that may be able to process both C6 and C5 sugars at high yields (Ragauskas et al 2006).…”