Ozone Pretreatment
Ozone is a powerful oxidant that is well known in the water treatment industry and in the paper industry for pulp bleaching. It is highly reactive towards compounds with conjugated double bonds and functional groups with high electron densities. Therefore, the moiety most likely to be oxidized in ozonization of lignocellulosic biomass materials is lignin due to its high content of carbon-carbon double bounds. The ozone degradation is essentially limited to lignin and hemicellulose is slightly attacked, but cellulose is hardly affected. During this chemical pretreatment, ozone attacks lignin releasing soluble compounds of smaller molecular weight, mainly organic acids such as formic and acetic acid, which can result in a drop in pH from 6.5 to about 2.
Ozone can be used to degrade lignin and hemicellulose in many lignocellulosic materials such as loblolly pine [229], sweetgum [229], cotton stalks [205], wheat straw [230-232], Miscanthus [229], and poplar sawdust [233]. Vidal and Molinier reported that the rate of enzymatic hydrolysis of wheat straw increased by a factor of 5 following 60% removal of the lignin from wheat straw by ozone pretreatment [233]. Enzymatic hydrolysis yield increased from 0% to 57% as the percentage of lignin decreased from 29% to 8% after ozonolysis pretreatment of poplar sawdust. The main advantages linked to the ozone pretreatment process is that lignin can be effectively removed and the soluble byproducts which might interfere with subsequent hydrolysis or fermentation can be washed off after the pretreatment. Another advantage is that pretreatment can be carried out at ambient temperature [233]. However, a large amount of ozone is required, making the process expensive.