The chemical methods of detoxification are based on the addition of certain chemical compounds that vary the conditions of the aqueous medium provok­ing changes in the pH, formation of precipitates, or the direct transformation of the toxic compounds. Among these methods, the ionic interaction of the ionic exchange resins can be included in this group of detoxification methods. The most employed chemical methods of detoxification are shown in Table 4.7. By neutral­ization, the solubility of many inhibitory substances is changed. This allows their removal by a later filtration or adsorption. However, the addition of alkali up to very high pH values (alkaline detoxification) leads to the formation of a significant amount of precipitate composed by calcium salts (if lime is used), which entrains the inhibitory compounds or causes them to settle. In addition, many inhibitors are unstable at pH higher than 9. Alkaline treatment is considered one of the best detoxification methods since a high percentage of substances such as furalde- hydes and phenolic compounds can be removed by this method, improving the fermentability of the resulting liquid medium especially when biomass hydrolyz- ates pretreated with dilute acid are employed (Persson et al., 2002a). The addition of calcium hydroxide (overliming) or ammonium has shown better results than the use of sodium or potassium hydroxide. Some methods to determine the optimal

Physical Methods for Detoxification of Pretreated Biomass

Remarks Reduction of acetic acid and phenolic compounds in nonvolatile fraction; roto — evaporation 93% yield of ref. fermn.; removal: 54% acetic acid, 100% furfural, 29% vanillin; roto-evaporation Solv: diethyl ether (solv.); yield comparable to ref. fermn.; removal of acetic, formic, and levulinic acids, furfural, HMF Solv.: ethyl acetate; 93% yield of ref. fermn.; removal: 56% acetic acid, 100% furfural, 100% vanillin, 100% hydroxybenzoic acid Solv.: ethyl acetate; removal of low molecular phenolic compounds Solv.: ethyl acetate; EtOH yield (SSF): detoxified hz. 0.51 g/g, undetox, hz. 0 g/g; high degree of phenolic removal	References Palmqvist and Hahn-Hagerdal (2000a)
Palmqvist and Hahn-Hagerdal (2000a) Palmqvist and Hahn-Hagerdal (2000a)
Palmqvist and Hahn-Hagerdal (2000a)
Palmqvist and Hahn-Hagerdal (2000a) Cantarella et al. (2004)
Continued

Chemical Methods for Detoxification of Pretreated Biomass

References

Precipitation or removal of toxic compounds; 10% lower yield for Pichia sp.

EtOH yield (SSF): detoxified hz. 0.86 g/g, undetox, hz. 0 g/g Yield comparable to ref. fermn.; 20% removal of furfural and HMF

Removal of acid acetic, furfural and part of phenolic compounds

7.5% lower yield for Pichia sp.

39% reduction in fermentation time Saha et al. (2005a)

Reduction in fermn. time: SSF Saha et al. (2005b)

-18%, SHF-67%

Removal: 51% furfural, 51% HMF, Martinez et al. (2000, 2001) 41% phenolic compounds, 0% acetic acid; overliming at 60°C or 25°C, at high temperature, the required amounts of lime and acid are reduced

Continued

Chemical Methods for Detoxification of Pretreated Biomass

Source: Adapted from Sanchez, O. J., and C. A. Cardona. 2008. Bioresource Technology 99:5270-5295. Elsevier Ltd.

Note: Reference fermentation (ref. fermn.) refers to fermentation carried out in a glucose-based medium without inhibitors; hz = hydrolyzate.

amount of lime to be added in dependence on the acid content of the hydrolyzate have been developed (Martinez et al., 2001). The positive effects of alkaline treat­ment on the hydrolyzate fermentability cannot be explained only by the removal of inhibitors. It has been postulated that this detoxification method may have possible stimulating effects on ethanol-producing microorganisms (Persson et al., 2002a).

Ionic exchange also has been studied as a detoxification method showing a high efficiency for removing inhibitors. This method can be considered as a special case of adsorption because ionized groups of the ionic exchange resin (the adsor­bent) interact electrostatically with the charged molecules of inhibitors. In par­ticular, some anionic exchange resins are used to eliminate phenolic compounds as a consequence of the strong bonds formed between quaternary ammonium groups of the resin (positively charged) and phenols (negatively charged). The rest of substances that do not interact with the resin pass through the adsorbent leading to a detoxified hydrolyzate. Besides the high resin cost, one drawback of this method lies in the fact that the content of fermentable sugars in the hydrolyz — ate can be reduced (Oliva, 2003). In the model process developed for NREL, the ionic exchange was proposed as a detoxification method for ethanol production process using poplar wood as the feedstock (Wooley et al., 1999). The biomass pretreated with dilute sulfuric acid at 190°C and high pressure is cooled by flash­ing, which removes 61% furfural and HMF as well as 6.5% of the acetic acid released from hemicellulose. The liquid fraction of the pretreated biomass is sent to an ionic exchange column whose effluent undergoes overliming to enhance the detoxification efficiency. Then, sulfuric acid is added to remove the calcium and suspended solids by forming a precipitate of calcium sulfate (gypsum). This design was based on data obtained at pilot scale level using a column with a diam­eter of 20 mm and a length of 1 m containing Amberlyst A20, a weak base resin. The regeneration of the resin is accomplished by passing the eluent (ammonium) through the column. Xie et al. (2005), in turn, demonstrated the successful detox­ification of corn stover hydrolyzate using a polymeric adsorbent without the need of a subsequent alkalinization. In contrast, the newer model process for ethanol production for corn stover designed for NREL only suggested the overliming as the detoxification method (Aden et al., 2002).

Different methods of detoxification that combine physical and chemical prin­ciples have been proposed, such as the neutralization with CaO or Ca(OH)2 fol­lowed by the addition of activated carbon and filtration to remove the acetic acid (Olsson and Hahn-Hagerdal, 1996). For lignocellulosic materials pretreated by pyrolysis and hydrolyzed with dilute acid, the utilization of several adsorbents, such as activated carbon, diatomite, bentonite, and zeolites, after the treatment by neutralization has been also studied (Yu and Zhang, 2003).