Delignification characteristics of rice straw pretreatment combining biogas slurry immersion with freeze–thaw

The challenge associated with lignocellulosic biomass pretreatment is efficiently overcoming biomass recalcitrance at low cost and non-pollution. This study aims to make full use of the by-product biogas slurry rich in ammonia nitrogen and microorganisms from anaerobic digestion and the cold resources of winter in cold regions and propose a combined pretreatment method of biogas slurry immersion and then freeze–thaw. The physiochemical characteristics of pretreated rice straw were studied in terms of liquid–solid ratio, immersion temperature, immersion duration, and number of freeze–thaw cycles. Lignin removal first increased then decreased with increasing temperature and liquid–solid ratio during biogas slurry immersion and increased with prolonged immersion duration. Hydrolytic acidification bacteria played a more crucial pretreatment role during biogas slurry immersion than free ammonia. The freezing–thawing treatment could further break the chemical bonds between lignin and carbohydrates in the rice straw after soaking in biogas slurry and reduce the crystallinity of cellulose. The lignin removal gradually increased with the increase in the number of freezing–thawing repeated, and the maximal lignin removal from the rice straw undergoing biogas slurry immersion with a liquid–solid ratio of 15:1 at 30 °C for 3 days and then five freeze–thaw repetitions reached 59.90%, and its crystallinity index reduced by 17.23% compared to raw rice straw. The carbon to nitrogen ratio of pretreated rice straw decreased to below 25, and a certain amount of volatile fatty acids could be accumulated, respectively, after biogas slurry immersion and freeze–thaw. The combined pretreatment method of biogas slurry immersion and freeze–thaw would be a promising approach to promote the low-cost operation and large-scale production of the straw pretreatment and biogas industry, especially in regions with natural cold resources.