As a physicochemical process, adsorption involves the mass transfer of a solute (adsorbate) from a fluid phase to an adsorbent surface through weak atomic and molecular forces (physi­cal adsorption) or through weak chemical bonds (chemical adsorption). For a chemical adsorption, the adsorbate forms a monolayer on the surface of the adsorbent. Chemical adsorption methods for detoxification include the use of zeolite (Eken-Saragoglu and Arslan 2000), eartomaceous earth (Ribeiro et al. 2001), activated charcoal (Silva and Roberto. 2001), wood charcoal (Miyafuji et al. 2003), diatomacenous earth (Ribeiro et al. 2001), polymeric adsorbents (Weil et al. 2002), mixed bed resin (Tran and Chambers 1986), and ion-exchange resins (Sarvari Horvath et al. 2004; Chandel et al. 2007).

Zeolites are widely used as ion-exchange beds in domestic and commercial water purifica­tion, softening, and other applications. Zeolites have a porous structure that can accommodate a wide variety of cations, such as Na+, K+, Ca2+, Mg2+, and others, which are loosely held and can readily be exchanged in a contact solution. Eken-Saragoglu and Arslan (2000) conducted detoxification tests with CaO and combinations with zeolite during ethanol production from corn cob hemicellulose hydrolysate by Pichia stipitis and Candida shehatae. They found that the single neutralization method did not support high ethanol production (2.8 g/L) during fermentation of hydrolysates by C. shehatae with only 2.8 g/L ethanol obtained. However, neutralization + zeolite treatments significantly increased final ethanol concentration to approximately 6.0 g/L.

In the study of Maddox and Murray (1983) , fermentation of crude hydrolysate of Pinus radiata by Clostridium acetobutylicum was not successful. After a decolorization or a steam­stripping detoxification treatment, the n-butanol fermentation was still not successful. However, a combination of the two gave a butanol concentration of up to 1.6 g/L. When activated carbon (150 g/L) was added to the hydrolysate, about 30% sugar was removed from the hydrolysates. Maddox and Murray also used anion and cation exchange resins to remove inhibitory compounds. They observed a butanol concentration of 5.7 g/L after the fermenta­tion, which represented a yield of 17% based on sugar utilized. Wood charcoals were also tested for removal of inhibitors such as furan and phenolic compounds in wood hydrolysates (Miyafuji et al. 2003). Wood charcoals prepared at various temperatures were found to selectively remove only the inhibitors without reducing the levels of fermentable sugars. A wood charcoal treatment with a wood charcoal weight to hydrolysates ratio of 0.07 could enhance the fermentation of wood hydrolysates (Miyafuji et al. 2003). The wood charcoals prepared at higher temperatures had enhanced ability to remove inhibitors, compared with wood charcoals prepared at lower temperatures (Miyafuji et al. 2003).

Polymeric adsorbents can also be used to remove aldehydes, such as furfural, that inhibit fermentation. Weil et al. (2002) investigated the removal of furfural from a biomass hydro­lysate using XAD-4 and XAD-7. The XAD-4 showed higher specificity for furfural removal than XAD — 7, and it also had little interaction with glucose. The fermentation of XAD-4- treated hydrolysate with E. coli K011 was nearly as rapid as the control medium that was formulated with sugars of the same concentration. Liquid chromatographic analysis showed that furfural concentrations were less than 0.01 g/L compared with the initial concentrations that were in the range of 1-5 g/L. The fermentation of the resulting biomass hydrolysate also confirmed that the concentration of furfural in the hydrolysate caused negligible inhibition (Weil et al. 2002).

Tran and Chambers (1986) compared a molecular sieve detoxification treatment with a mixed bed ion resin treatment. The molecular sieve method decreased the concentration of xylose, acetic acid, and furfural by 10%, 40%, and 82%, respectively. The mixed bed ion resin treatment, to a lesser extent, removed 7% of xylose, 20% of acetic acid, and 20% of furfural. Molecular sieve treatment produced higher ethanol (9.9 g/L) upon fermentation than mixed bed resin treatment of lignocellulosic hydrolysates (8.0g/L).

Ion exchange resin treatment is one of most efficient methods for lignocellulosic hydro­lysates detoxification (Larsson et al. 1999) . Wooley et al. ( 1999) used weak base resins (Amberlyst A20, Rohm & Haas, Paris, France) to treat dilute — acid- pretreated poplar and reported 88% removal of acetic acid and a 100% sugar recovery. Nilvebrant et al. (2001) tested the effects of an anion exchanger (AG1-X8, BioRad Laboratories, Richmond, CA), a cation exchanger (AG50 W-X8, BioRad Laboratories), and plain resins (XAD-X8, BioRad Laboratories) on detoxification of dilute-acid-pretreated spruce and fermentation by baker’s yeast. For ethanol productivity, the performance of the resins was in the sequence of AG1-X8 > XAD-X8 > AG50-X8. Sarvari Horvath et al. (2004) compared six different anion-exchange resins with different properties for detoxification of a hydrolysate from dilute-acid-pretreated spruce. The resins tested were featured by styrene-, phenol-, and acrylic-based matrices and strong as well as weak functional groups. Fractions of the hydrolysate were collected and analyzed for fermentable sugars and inhibitors, and the effect on the fermentability was evaluated using S. cerevisiae. An initial adsorption of glucose was found to occur in the strong alkaline environment, leading to glucose accu­mulation in the resin at a later stage. The fractions collected from strong anion-exchange resins with styrene-based matrices displayed the best fermentability: a sevenfold increase in ethanol productivity compared with untreated hydrolysate. The fractions from a strong anion exchanger with acrylic-based matrix and a weak exchange with phenol-based resin displayed an intermediate improvement in fermentability, a four — to fivefold increase in ethanol productivity. The fractions from two weak exchangers with styrene — and acrylic — based matrices displayed a twofold increase in the productivity. Phenolic compounds were more efficiently removed by resins with styrene — and phenol-based matrices than by resins with acrylic-based matrices. Volumetric productivity and ethanol yield increased after treatment with all six resins. The volumetric productivity in the reference fermentation, 2.2g/L/h, was eight times higher than in the untreated hydrolysates. The volumetric pro­ductivity in the anion-exchange-treated samples was two to seven times higher than in the untreated hydrolysate. For all adsorption-based detoxification methods, the reuse or recovery of the adsorbate will determine the economics and viability of the process.