High-solid NaOH-urea pretreatment of maize stover and enzymatic hydrolysis kinetic modeling

To address the engineering challenges in lignocellulosic biorefining of maize stover—specifically the high energy consumption of single alkali pretreatment and the substantial water usage caused by low solid payload—this study draws on the NaOH-urea method used in the paper industry for processing non-wood fibers. By pretreating maize stover, optimizing reaction conditions, and further integrating cellulase hydrolysis, we achieved efficient production of monosaccharides required for biorefining. Through comprehensive consideration of technical feasibility, mass transfer efficiency, and economic costs, the optimal pretreatment conditions for the high-solid system were determined as follows: solid payload, 25%; NaOH, 9%; urea, 5%; pretreatment temperature, 50 ℃; pretreatment time, 24 h; overcoming the technical limitations of traditional alkali methods that rely on high temperatures and extremely low solid payload. In parallel, we combined the Langmuir isotherm model, substrate surface hydrophobicity distribution to systematically reveal how the changes in maize stover structure induced by the combined NaOH-urea pretreatment affect the reaction kinetics of cellulose and hemicellulose hydrolysis. The research progresses sequentially from macroscopic process optimization to reaction kinetic responses and then to microstructuralchanges, elucidating the "synergistic wall-disruption" mechanism of the combined pretreatment from both temporal and spatial dimensions, thereby achieving strong theoretical depth and completeness. At the theoretical level, the conventional Michaelis–Menten model fails to adequately describe reaction processes in heterogeneous systems. We introduce a fractal kinetic model to characterize the high-solid heterogeneous enzymatic hydrolysis system of maize stalk. The fractal kinetic modeling revealed that the NaOH-urea combined pretreatment significantly increased the rate coefficient and the theoretical limit concentration of the enzymatic hydrolysis. Specifically, the fractal exponent (h) of glucose enzymatic hydrolysis decreased significantly from 0.847 8 in the untreated raw material to 0.796 5, indicating improved homogeneity, reduced mass transfer resistance, and enhanced accessibility of the cellulose-enzyme reaction system. Conversely, the fractal exponent of xylose enzymatic hydrolysis anomalously increased from 0.721 3 to 0.835 4, revealing that the residual hemicellulose network exhibits a more complex spatial heterogeneity and severe steric hindrance during the middle and late stages of degradation. Our research confirm the distinct roles of NaOH and urea in dismantling the lignocellulosic recalcitrance barrier. NaOH primarily cleaves ether linkages and substantially reduces substrate hydrophobicity from 1.88 to 0.44 L/g. Urea, as an effective hydrogen-bond disruptor and physical protective layer, promotes extreme swelling and disintegration of the cellulose substrate while minimizing excessive damage to the cellulose backbone caused by the strong alkali. This synergistic effect increased the E_max of cellulose to 421.84 mg/g, thereby improving heterogeneous mass transfer and the enzymatic catalytic microenvironment under high-solid conditions. In summary, the NaOH-urea combined pretreatment not only significantly reduces the heating energy consumption and wastewater discharge in industrial fermentation, but also suppresses the formation of fermentation inhibitors such as furfural. The residual urea in the system serves as a high-quality nitrogen source for microorganisms, eliminating the need for additional nitrogen supplementation in subsequent efficient bioconversion processes, such as integrated ethanol-methane coproduction from straw and high-solid substrate fermentation. This pretreatment strategy and the associated kinetic analysis models are not limited to maize stover; they also offer a universally applicable methodology with strong industrial potential for the low-carbon and cost-effective conversion of other high-lignin agricultural residues, including rice husks and bamboo.