Compositional changes of rice straw fibers after pretreatment with diluted acetic acid: Towards enhanced biomethane production

Abstract

In the present study, diluted acetic acid was used for the pretreatment of rice straw in order to improve its bioconversion into biomethane. After pretreatment with 2% acetic acid for 24 h, cellulose and hemicellulose significantly reduced by 8.9% and 18.1%, respectively. The degradation of cellulose and hemicellulose resulted in significant increase of relative lignin ratio by 14.9%. Cumulative biomethane production after 28 days of anaerobic digestion of straw pretreated with 2%, 4% and 6% for 24 h was 24.3%, 15.2%, and 18.1%, respectively, higher than the untreated sample. Longer pretreatment for 36 and 48 h showed the same trend with relatively low biomethane yield. Thus, 24 h pretreatment with 2% acetic acid improved biomethanation over other pretreatments. Scanning electron microscopy and Fourier transform infrared spectroscopy confirmed the partial degradation of rice straw and modification of lignin structure by un-bonding the ether chain within lignin molecules as a result of pretreatment. Pretreatment with 2% acetic acid for 24 h resulted in the maximum biomethane energy yield of 6.29 MJ kg−1 dry straw, 47.3% energy conversion efficiency and the maximum net profit of US$ 106.4 ton−1 dry straw. In conclusion, application of 2% acetic acid for 24 h is a promising approach for high lignocellulose degradation efficiency at mild conditions due to the relatively short pretreatment time and enhanced biomethanation.