Ozone treatment is one way of reducing the lignin content of lignocellulosic wastes. This results in an increase of the in vitro digestibility of the treated material, and unlike other chemical treatments, it does not produce toxic residues. Ozone can be used to degrade lignin and hemicellulose in many lignocellulosic materials such as wheat straw [149], bagasse, green hay, peanut, pine [150], cotton straw [151], and poplar sawdust [152]. Research indicated [153] ozone is highly reactive toward compounds incorporating conjugated double bonds and functional groups with high electron densities. Therefore, the moiety, most likely to be oxidized in ozonization of lignocellulosic materials, is lignin due to its high content of C=C bonds. Thus, during the ozonolysis, the degradation is mainly limited to lignin. Ozone attacks lignin releasing soluble compounds of less molecular weight, mainly organic acids such as formic and acetic acid [153]. The main advantages linked to this process are the lack of any degradation products that might interfere with subsequent hydrolysis or fermentation and the reactions occurring at ambient temperature and normal pres­sure. Furthermore, the fact that ozone can be easily decomposed by using a catalytic bed or increasing the temperature means that processes can be designed to minimize environmental pollution. A drawback of ozonolysis is that a large amount of ozone is required, which can make the process expensive and less applicable [154]. However, recently Hu et al. [155] demonstrated that a lower charge of ozone could be used to enhance the enzymatic digestibility of cellulose, if the ozone-treated biomass was not washed and the in-situ generated acids were employed in a subsequent DAP.

8.2 Summary

The effects of different chemical pretreatment technologies on the structure of lig- nocellulose are summarized in this section. In addition, the environment impacts of these pretreatments are also briefly discussed. Some directions and perspectives are also proposed for the future chemical pretreatment technologies.