Wet oxidation pretreatment of lignocellulosic residues of sugarcane, rice, cassava and peanuts for ethanol production

Wet oxidation (WO) pretreatment of sugarcane bagasse, rice hulls, cassava stalks and peanut shells was investigated. WO was performed at 195 °C for 10 min, with 2 g kg−1 of Na2CO3 and under either 3 or 12 bar of oxygen. Oxygen pressure and the type of raw material used had a major effect on the fractionation of the materials, formation of sugars and by-products, and cellulose enzymatic convertibility.

Cellulose content in the solid fraction increased after pretreatment of all materials, except rice hulls. The greatest increase, from 361 g kg−1 to almost 600 g kg−1, occurred for bagasse. The solubilisation of individual components was different for each material. Bagasse xylan was solubilised to a large extent, and 45.2% of it was recovered as xylose and xylo-oligosaccharides in the liquid fraction.

In the prehydrolysates of rice hulls around 40% of the original glucan was recovered as gluco-oligosaccharides, due to hydrolysis of starch contained in grain remains. The formation of by-products was modest for all the materials, but increased with increasing oxygen pressure. The highest yield of acetic acid (34–36 g kg−1 of raw material) and furfural (0.7–1.8 g kg−1) occurred for bagasse.

The pretreatment enhanced the enzymatic convertibility of cellulose giving the best result (670.2 g kg−1) for bagasse pretreated at the highest oxygen pressure. However, for the other materials the pretreatment conditions were not effective in achieving cellulose conversions above 450 g kg−1. Some enzymatic conversion of xylan was observed.

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