Removal of furan and phenolic compounds from simulated biomass hydrolysates by batch adsorption and continuous fixed-bed column adsorption methods
Introduction
Dilute-acid pretreatment with sulfuric acid has been the most widely used method for biomass pretreatment, which can generate hydrolyzed xylose-rich stream and digestible cellulose substrate for enzymatic hydrolysis (Davis et al., 2015). Most hemicellulose in biomass can be effectively degraded into sugar monomers at a sulfuric acid concentration of 0.5–1.0 wt% and a hydrolysis temperature of 140–190 °C (Lim and Lee, 2014). However, inhibiting compounds are inevitably generated during the dilute-acid pretreatment, which can produce negative effects in downstream fermentation. These inhibitors include aliphatic carboxylic acids, furans, and phenolic compounds. The removal of such inhibitors has been reported using various separation methods including alkali treatment, vacuum evaporation, activated charcoal adsorption, ion exchange resin adsorption, nanofiltration, and their combinations (Bras et al., 2014, Geddes et al., 2015, Persson et al., 2002, Ra et al., 2015a, Ra et al., 2015b, Santos et al., 2014, Soleimani et al., 2015, Zhang et al., 2015b). However, because these researchers’ endeavors have been mostly concerned with completely removing inhibitors in an attempt to improve ethanol yield, the loss of sugar has not been thoroughly considered.
Activated charcoal has been widely used as an adsorbent to remove inhibitors such as furans and phenolic compounds whose hydrophobicity is much greater than sugars and aliphatic carboxylic acids in lignocellulosic hydrolysates (Mussatto et al., 2004, Ra et al., 2015a, Ra et al., 2015b, Rodrigues et al., 2001, Sahu et al., 2008, Santos et al., 2014, Soleimani et al., 2015, Sulaymon and Ahmed, 2008). This is because activated charcoal is able to adsorb these compounds due to its large adsorption capacity and strong hydrophobicity. The detoxification of biomass hydrolysates by activated charcoal adsorption has been limited to the application to batch adsorption systems. Many batch adsorption works (Kamal et al., 2011, Kudahettige-Nilsson et al., 2015, Lee et al., 2011, Mussatto et al., 2004, Schirmer-Michel et al., 2008, Soleimani and Tabil, 2013) have documented sugar loss during the detoxification treatment. Despite being linked to activated charcoal dosage and hydrolysate composition (Mateo et al., 2013), sugar loss has been disregarded in most works (Soleimani et al., 2015, Ra et al., 2015a, Ra et al., 2015b) and many researchers have not yet begun to develop a systematic approach to reduce sugar loss. Therefore, it should be noted that use of an optimal dosage of activated charcoal can minimize consumption of the adsorbent as well as loss of fermentable sugars during the detoxification.
Fig. 1 displays a diagram which depicts a new overall sugar purification process where all inhibitors have been removed from biomass hydrolysates. In the first step of the overall process, a batch adsorption equilibrium method, a continuous fixed-bed column adsorption method, and a combination of the two can be introduced to remove furans and phenolic compounds. As its subsequent steps, the first and the second emulsion liquid membranes (ELMs) can be used to remove aliphatic carboxylic acids and sulfuric acid, respectively. Highly selective removal of the acids from the simulated hydrolysates without the organic cyclic compounds were realized through the two-step ELM process by Lee, 2013, Lee, 2014, Lee, 2015. In each ELM process, it was possible to achieve a very high degree of acid extraction while still preserving a majority of the xylose. Accordingly, the second and the third steps of the overall sugar purification process displayed in Fig. 1 will be no longer discussed. The objective of this work was therefore to develop the first step of the overall process using several activated charcoal adsorption technologies with which furan and phenolic compounds could be selectively separated from simulated hydrolysates, while keeping sugar loss at a minimum.
Section snippets
Reagent preparations
Simulated biomass pretreatment hydrolysates were prepared by dissolving d-xylose (EP grade, Tokyo Chemical Industry) as a sugar, sulfuric acid (GR grade, Matsunoen Chemicals), acetic acid (GR grade, Junsei Chemical) as an aliphatic carboxylic acid, furfural (Tokyo Chemical Industry) as a furan, and 4-hydroxybenzoic acid (99%, Aldrich) and vanillic acid (97%, Aldrich) as phenolic compounds into deionized water (18.2 MΩ, Simplicity, Millipore). Each concentration was determined based on the
Adsorption isotherm of single component
Adsorption equilibrium experiments were carried out to investigate how strongly each component in simulated hydrolysates was adsorbed onto activated charcoal, and to estimate whether selective removal of a furan (furfural) and two phenolic compounds (4-hydroxybenzoic acid and vanillic acid) from the hydrolysates in activated charcoal adsorption systems was feasible. Fig. 2 shows the adsorption isotherm of each single component in the batch activated charcoal adsorption systems, which is
Conclusions
The removal efficiency of furan and phenolic compounds using activated charcoal was much higher than that of xylose, sulfuric acid, and acetic acid throughout batch adsorption equilibrium experiments. It was closely related to hydrophobicity of each component in the simulated hydrolysates, due to a strongly hydrophobic nature of activated charcoal. The four treatment strategies were tested to remove furan and phenolic compounds, which are composed of batch adsorption equilibrium and continuous
Acknowledgements
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2010-0023390) and by the Ministry of Education (NRF-2015R1D1A1A09058205).
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