Research Of Alkaline Oxidative Pretreatment For Rice Straw

Development of biomass energy which is prepared by lignocellulose is the main way to solve the problems of energy and environment. The methods of pretreatment of rice straw were studied in this paper.The composition, enzyme hydrolysis saccharification and the surface morphology of rice straw were studied after the pretreatment of rice straw by NaOH,H2O2/NaOH and03/NaOH. The mechanism of pretreatment was discussed, and the kinetics of ozone oxidization on lignin was investigated as well.Pretreatment was evaluated by the conversion of rice straw hemicellulose and cellulose to reducing sugars, the results showed that all the pretreatment dropped the content of lignin, increased the content of cellulose and accelerated the enzymatic saccharification of rice straw. The enzymatic scarification ratio of the pretreated straw was improved obviously. The best pretreatment effect was obtained by03/NaOH. Under the same enzymatic hydrolysis conditions(using31.2g enzyme preparation per gram of substrate,45℃, pH5.0,120h), by NaOH pretreatment(sodium hydroxide2.0%, solid-to-liquid ratio1:12, temperature30℃, reaction time24h), H2O2/NaOH pretreatment (sodium hydroxide2.0%, hydrogen peroxide2.0%, solid-to-liquid ratio1:12, temperature40℃, reaction time24h, the optimal pretreatment condition),03/NaOH pretreatment (after the milled rice straw was mixed with2.0%NaOH solution and shaken by an incubator at140rpm at30℃for20h, ozone pretreatment was performed by continuously sparging ozone through the suspension for4h) and untreatment, the enzymatic scarification ratios of rice straw were74.90%、83.23%、92.57%、53.53%, respectively.The results of SEM images of the rice straw show that the outer surface of the untreated rice straw is coated tidily with silica cells, suberized cells and silica swelling. However the silica cells, suberized cells and silica swelling which coated on the surface of rice straw have been removed partly by NaOH pretreatment, removed entirely by H2O2/NaOH and03/NaOH pretreatment. The inner substances of thin-walled cells were removed partly by NaOH pretreatment, removed entirely by and03/NaOH pretreatment. The cell walls became loosened and the porosity was increased since the inner substances of the thin-walled cells were completely lost, thus03/NaOH, H2O2/NaOH, NaOH pretreatment is much better than untreated on the increasing of the effect surface of enzymatic hydrolysis and accelerating of enzymatic hydrolysis velocity. The method of03/NaOH pretreatment shows the best effect among those pretreatments.There is no big difference on the mechanism of NaOH and H2O2/NaOH pretreatment, either bond of the lignin was broken down by NaOH, high molecular weight lignin is decomposed into small molecular weight lignin and dissolved in solution. At the same time, ester bond which contacted lignin and hemicellulose was broken due to saponification by NaOH, thus thehydrogen bond strength decreased between hemicellulose and cellulose, and so that the rice straw expensed, the difference between those two methods was that H2O2promoted the solve of materials instead of opening epoxidation degradation of lignin. While O3make lignin open epoxidation degradation, meanwhile, other materials were also oxidation, due to the oxidation of O3is much stronger than that of H2O2.The component contents change and enzyme hydrolysis sacchrification rate of rice straw before and after the pretreatment show that the contents of lignin is not the decisive factor to the enzyme hydrolysis saccharification, the change of effective specific surface area of the rice straw have very important influence after pretreatment.The decomposition kinetics results show that the ozone oxidation degradation of lignin was first order reaction under the condition of alkaline, and its reaction activation energy was about45kJ/mol, the mechanism of lignin oxidative degradation was hydroxyl free radical oxidation, during the oxidative degradation, the lignin was take place from high molecular to small molecular weight, from benzene propane to benzoic acid by oxidation to carboxylic acids and aldehydes, and finally to acetic acid, carbon dioxide and water by oxidation.