Enhanced methane production from corn stover by anaerobic digestion: Positive effects of deep eutectic solvents...

The conversion of lignocellulosic biomass to renewable energy by anaerobic digestion (AD) has received great concern, and the efficiency is unfortunately limited by the highly stubborn structure of lignocellulose. Herein, two deep eutectic solvents (DES), ChCl-LA and K₂CO₃-EG, were employed to selectively fractionate corn stover (CS) to improve AD performance and methane production. The ChCl-LA and K₂CO₃-EG achieved delignification rates of 52.91 % and 55.94 %, and cellulose recovery rates of 84.96 % and 82.37 %. Additionally, the methane yields increased from 17.51 mL/g-VSS for the untreated CS to 41.00 and 51.99 mL/g-VSS upon ChCl-LA and K₂CO₃-EG pretreatment. Remarkably, both ChCl-LA lignin (M_n = 863, M_w = 1291) and K₂CO₃-EG lignin (M_n = 881, M_w = 1258) exhibited low molecular weights along with high purities of 78.57 % and 84.05 %, respectively. The supplementation of ChCl-LA lignin and K₂CO₃-EG lignin during AD of microcrystalline cellulose increased methane yields by 51.8 % and 36.8 %, respectively. Lignin structure analysis revealed that ChCl-LA lignin was enriched with syringyl units of β-O-4 linkages and unconjugated carbonyl groups, forming a linear redox-active structure. This effect potentially facilitates the direct interspecies electron transfer (DIET), and the enrichment of DIET-active microbial taxa such as Flexilinea and Odoribacter was observed. Moreover, ChCl-LA lignin addition upregulated the key metabolic genes of pyruvate kinase (K01624, +5 %), methylmalonyl-CoA mutase (K05606, +18 %), and acetyl-CoA synthetase (K12780, +60 %), which supported the enhanced activity in glycolysis, volatile fatty acid metabolism, and acetoclastic methanogenesis. This study proposes a novel DES pretreatment before AD to valorize lignocellulose and enhance methane production.