Sono-assisted sulfuric acid process for the recovery of fermentable sugars, valuable byproducts from rice straw

Abstract

Bioethanol, derived from lignocellulosic biomass, has emerged as a sustainable and green alternative to non-renewable gasoline fuel. Bioethanol production is facing the challenge of economical and efficient recovery of sugars from biomass, which is mainly governed by the pretreatment and hydrolysis steps. The biorefinery concept (recovery of multi-product from biomass) may make bioethanol production cost competitive with gasoline. In this study, it was focused to develop a single step sono-assisted sulfuric acid process (SSP) for rice straw biorefinery to recover: sugars for bioethanol, demineralized residue for pure silica and porous residue for bioachar. In the first step, critical factors of sono-assisted sulfuric acid pretreatment (sonication time, temperature and acid concentration) were optimized using response surface methodology. SSP produced 31.78 g of sugar/100g of dry biomass at 80˚C and 10% sulfuric acid concentration after 50 min of sonication. SSP led to > 50% total sugar recovery from RS in one-step. Total sugar recovery reached up to 85.9% upon dilute acid hydrolysis of SSP treated rice straw compared to 85.5% sugar recovery employing enzymatic hydrolysis. Process economics (energy requirement and reagent cost) showed that SSP-dilute acid hydrolysis proved viable technology for commercialization compared to SSP-enzymatic hydrolysis process. In the second part of this study, SSP treated rice straw was used as a precursor to produce pure mesoporous silica. SSP treatment removed more than 96% of mineral oxides from residual rice straw. This residual waste was combusted at 750˚C for 15 min, which produced ≥ 96% pure mesoporous silica. The hydrolyzed rice straw (without any further processing) was used as biosorbent to remove Methylene blue dye from aqueous solution. The maximum monolayer adsorption capacity obtained was not that high (8.30 mg/g) compared to other biosorbents. Thus, SSP treated rice straw was combusted at 450˚C for one hou...