Wet Oxidation Pretreatment
Wet oxidation is a chemical pretreatment technique that involves exposure of biomass to water and air or oxygen at a high temperature. This pretreatment process involves the subcritical oxidation of organics or oxidizable inorganic components at elevated temperatures in the range of 125-320°C and pressures in the 0.5-2 MPa range, using oxygen or air as the oxidant [212, 213]. Energy requirements for wet oxidation pretreatments are relatively low since the only thermal energy required for these pretreatments is the difference in enthalpy between the incoming and outgoing streams. However, the capital cost for wet oxidation pretreatments is higher than other pretreatment techniques, and the operating costs are mainly the power to produce compress air [213]. Wet oxidation can be used to fractionate lignocellulosic material by solubilizing hemi — cellulose and removing lignin [214]. During wet oxidation, lignin is decomposed to carbon dioxide, water, and carboxylic acids [214, 215]. The amount of lignin removed in the wet oxidation process ranges from 50% to 70% depending on conditions used and type of biomass pretreated. Martin et al. have reported that for sugarcane bagasse, 50% of lignin can be removed after 15 minutes of wet oxidation pretreatments, which resulted in 57.4% conversion of cellulose, compared to only 35% lignin removal and 48.9% cellulose conversion for steam explosion [216]. However, Martin and coworkers also found that the amount of byproducts formed was almost always higher for pretreatment by wet oxidation than by steam explosion. Byproducts obtained included succinic acid, glycolic acid, formic acid, acetic acid, phenolic compounds, and furfural, which would have negative effects on further downstream processing due to inhibition [216].
The wet oxidation technique is very effective in removing dense wax coating containing silica and protein in biomass forms such as straw, reed, and other cereal crop residues [217]. In a wet oxidation pretreatment experiment on wheat straw with pretreatment time of 10 minutes, Pederson and coworkers reported 400 and 200 g/ kg dry matter for glucose and xylose, respectively, after 24 hours at 50°C using an enzyme mixture of 36 FPU/g Celluclast-1.5 L and 37 CBU/g of Novozyme-188 [218]. Difficult biomass forms such as grape stalk, which contains tannins, a chemical that complicates delignification, have also been shown to benefit from the wet oxidation process. Ping and coworkers have shown that up to 50% cellulose conversion can be obtained from wet oxidation pretreated grape stalk compared to 25% conversion with sulfuric acid pretreatment [219]. The wet oxidation technique has been reported for the pretreatment of wheat straw [215, 218, 212], rice husk [214], sugarcane bagasse [220], corn stover [221], rape straw [222], and tobacco stalks [223].
A number of researchers have seen the benefits of combining wet oxidation with other pretreatment methods to further increase the yield of sugars after enzymatic hydrolysis [224-227]. Combining wet oxidation with alkaline pretreatment, as well as adding a base such as sodium carbonate during the wet oxidation, has been shown to reduce the formation of byproducts, thereby decreasing inhibition [228]. A combination of wet oxidation and steam explosion is also known as wet explosion. In this technique the biomass not only undergoes the chemical reaction described above, but also undergoes physical rupture due to steam explosion [225]. The main advantages in combining wet oxidation with steam explosion is the ability of the combined process to handle larger pieces of biomass and to operate at higher biomass loadings per given volume of pretreatment solvent medium [226, 227].
Georgieva and coworkers studied the wet explosion pretreatment of wheat straw using three different oxidizing agents (H2O2,
O2, and air) [226]. The effect of the pretreatment was evaluated based on glucose and xylose liberated during enzymatic hydrolysis. Their results showed that pretreatment with the use of O2 as oxidizing agent was the most efficient in enhancing overall convertibility of the raw material to sugars and minimizing generation of furfural as a byproduct. They found that the wet explosion pretreatment method enabled relatively high yields from both enzymatic hydrolysis and simultaneous saccharification and fermentation (SSF) to be obtained when performed on unwashed slurry with 14% dry matter and a low enzyme loading of 10 FPU/g cellulose in an industrial acceptable time frame of 96 h. Cellulose and hemicellulose conversion from enzymatic hydrolysis were 70 and 68%, respectively, and an overall ethanol yield from SSF was 68% [226]. The biomass saccharification step has shown to benefit from the addition of an acid soaking step prior to wet explosion as well. This may be because acid pretreatment helps to hydrolyze the hemicelluloses, while wet explosion will expose more enzyme binding sites for the saccharification step [225].