Abstract
BackgroundPretreatment is a crucial step for valorization of lignocellulosic biomass into valuable products such as H2, ethanol, acids, and methane. As pretreatment can change several decisive factors concurrently, it is difficult to predict its effectiveness. Furthermore, the effectiveness of pretreatments is usually assessed by enzymatic digestibility or merely according to the yield of the target fermentation products. The present study proposed the concept of “precise pretreatment,” distinguished the major decisive factors of lignocellulosic materials by precise pretreatment, and evaluated the complete profile of all fermentation products and by-products. In brief, hemicellulose and lignin were selectively removed from dewaxed rice straw, and the cellulose was further modified to alter the crystalline allomorphs. The subsequent fermentation performance of the selectively pretreated lignocellulose was assessed using the cellulolytic, ethanologenic, and hydrogenetic Clostridium thermocellum through a holistic characterization of the liquid, solid, and gaseous products and residues.ResultsThe transformation of crystalline cellulose forms from I to II and from Iα to Iβ improved the production of H2 and ethanol by 65 and 29%, respectively. At the same time, the hydrolysis efficiency was merely improved by 10%, revealing that the crystalline forms not only influenced the accessibility of cellulose but also affected the metabolic preferences and flux of the system. The fermentation efficiency was independent of the specific surface area and degree of polymerization. Furthermore, the pretreatments resulted in 43–45% of the carbon in the liquid hydrolysates unexplainable by forming ethanol and acetate products. A tandem pretreatment with peracetic acid and alkali improved ethanol production by 45.5%, but also increased the production of non-ethanolic low-value by-products by 136%, resulting in a huge burden on wastewater treatment requirements.ConclusionCellulose allomorphs significantly affected fermentation metabolic pathway, except for hydrolysis efficiency. Furthermore, with the increasing effectiveness of the pretreatment for ethanol production, more non-ethanolic low-value by-products or contaminants were produced, intensifying environmental burden. Therefore, the effectiveness of the pretreatment should not only be determined on the basis of energy auditing and inhibitors generated, but should also be assessed in terms of the environmental benefits of the whole integrated system from a holistic view.
Highlights
Pretreatment is a crucial step for valorization of lignocellulosic biomass into valuable products such as H2, ethanol, acids, and methane
After centrifugation at 16,000×g for 10 min, the contents of alcohols and acids in the supernatants of the liquid samples were determined by gas chromatography (6890N-FID, Agilent, USA) using a system equipped with a DB-WAXETR1 capillary column (30 m × 0.53 mm i.d. × 1 μm) according to the protocol described previously [24]
It is noted that the values of dissolved organic carbon (DOC), ethanol, acetic acid, and H2 of the control batch were subtracted from those of the experimental samples
Summary
Pretreatment is a crucial step for valorization of lignocellulosic biomass into valuable products such as H2, ethanol, acids, and methane. The effectiveness of pretreatments is usually assessed by enzymatic digestibility or merely according to the yield of the target fermentation products. The effectiveness of pretreatments is invariably assessed by enzymatic hydrolysis or digestibility [11, 16,17,18,19,20,21], or according to the yield of the target fermentation products (mainly ethanol) [22]. While the fate of hydrolysates or non-target fermentation products from the pretreated biomass is seldom considered. Factors such as whether or not and how pretreatments affect a subsequent microbial fermentation are seldom discussed or included with the several potential mechanisms. If hydrolysis is increased and fermentation is decreased by pretreatment, the reduced fermentation efficiency is often attributed to the formation of inhibitory by-products during pretreatment [6]
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