## 3 Purification Routes Applied to Lignocellulosic Acid Hydrolysates
### Adsorption
#### 3.4.1 Activated Charcoal  
The most important parameter for an efficient adsorption with AC is the hydrolysate:AC ratio [76]. Optimum ratios to ensure good removal rates of impurities with no adsorption of sugars appeared to be about 200:1 to 50:1 (_w/w_) depending on the concentration of the hydrolysate components [50; 72; 77]. pH also strongly influences the adsorption process, weak organic acids (phenols, acetic acid) being most readily adsorbed in the non-ionized state and consequently low pHs favor their adsorption,whereas the ionized form of the weak acids are poorly adsorbed at high pHs [50; 73; 78]. Contact time between activated charcoal and hydrolysate for an optimal adsorption was at least 20 min, and equilibrium was reached in about 60 min [76]. The temperature also influences the adsorption of the inhibitors, such as aromatic compounds. For instance, temperatures close to room temperature led to better adsorption of phenolic compounds by about 20% than an adsorption experiment run at 60 \({}^{\circ}\)C on an acid _Eucalyptus_ wood hydrolysate [77].  
The addition of AC to neutralized _Eucalyptus_ wood acid hydrolysate with a hydrolysate:AC ratio of 200:1 (_w/w_) at 40 \({}^{\circ}\)C led to about 80% lignin adsorption, whereas xylose was almost totally recovered in the hydrolysate (less than 2% adsorption) [77]. The adsorption of lignin improved by 28% the consumption of xylose during the downstream fermentation step [77]. When the production of ethanol is the target of the biorefinery, the lignocellulosic acid hydrolysate are often pH adjusted to 5.5 before AC adsorption as the fermentation of the sugars occurred at this pH [68; 72]. This rise in pH of the hydrolysate can also be beneficial for the adsorption of the inhibitory compounds, as low pH can lead to the adsorption of sulfuric acid, which makes the surface of the AC less hydrophobic, thus reducing the adsorption of the inhibitory compounds [79].  
The comparison of alkalinization, overlining, evaporation, liquid/liquid extraction, and adsorption with AC showed that adsorption with AC was the most efficient method for the removal of fermentation inhibitors while minimizing sugar losses [73]. The use of adsorption also decreases the cost of the process as low temperatures (20 to 40 \({}^{\circ}\)C) are used, as well as low ratios of hydrolysate:charcoal (200:1 to 50:1 (_w/w_)).  
On a synthetic solution of 20% (_w/w_) sulfuric acid, containing glucose, furfural, HMF, and acetic acid, AC was found to adsorb furans and acetic acid more effectively than a cation exchange resin [79]. However, in order to give value to the adsorbed fermentation inhibitory compounds, desorption of these compounds is important as well. Usually, in the adsorption/desorption process, regeneration or desorption time is comparable to the duration of the loading or adsorption step. Unlike cation exchange resin, the regeneration of granulated activated charcoal (GAC) with water was not feasible; ethanol 50% (_v/v_) was required to desorb furans and acetic acid in a limited duration [79]. Overall, AC led to the best performance compared to resins when high purity is required, and when ethanol can be used to regenerate the adsorbent.