基于近红外光谱的玉米秸秆吸湿过程水分子结构演变规律研究

Evolution of Water Molecular Structure during Moisture Absorption in Corn Stover Based on Near-infrared Spectroscopy

摘要:

秸秆作为重要的农业生物质资源,其含水率直接影响热值、机械性能、生物转化效率及储存安全性。然而,现有研究多关注水分变化宏观结果,缺乏对秸秆吸湿过程中微观机制的深入探究。本研究基于近红外光谱技术,系统探究了玉米秸秆的吸湿动力学及水分微观赋存状态演变规律,揭示了玉米秸秆吸湿过程中含水率变化与水分子结构演变之间的内在联系。研究结果表明,玉米秸秆在吸湿过程中,随着含水率的增加,其内部逐渐建立起水分动态平衡。吸湿本质上是水分子氢键网络的动态重组过程:水分由初始阶段以含1~3个氢键的弱结合态为主,逐步演变为含4个氢键的强结合有序水团簇。在加湿初期,溶剂化层水尚未显现;至2 h,由于无机盐离子的水合作用,溶剂化层水分子开始出现;后期因自由水增多导致离子浓度稀释,其含量逐渐下降。在25℃、相对湿度90%条件下,粒径与吸湿能力之间呈非线性关系,过度粉碎会导致孔隙塌陷,抑制水分迁移;而中等粒径(0.25~0.85mm)的样品在水分响应方面表现最优。

Abstract:

The microscopic mechanisms and molecular-level changes governing moisture absorption in crop straw were not fully understood. To clarify the relationship between macroscopic moisture variation and microscopic water structure, near-infrared (NIR) spectroscopy was employed to investigate moisture absorption kinetics and the evolution of water retention forms in corn stover with different particle sizes. As the particle size was reduced, the specific surface area was increased, the internal pore structure was reorganized, and capillary condensation was consequently enhanced. As a result, the initial water uptake rate was accelerated, the time required to reach moisture equilibrium was shortened, and the equilibrium moisture content was increased. During moisture absorption, the hydrogen-bond network of water molecules was continuously reorganized. Weakly bound water containing 1~3 hydrogen bonds was gradually transformed into larger and more ordered strongly bound water structures characterized by four hydrogen bonds. Solvation-layer water was not detected during the initial wetting stage but was observed after 2 h of moisture absorption, mainly because of the hydration of inorganic salt ions. Its proportion was subsequently reduced as the local ion concentration was diluted by increasing free water. A nonlinear relationship between particle size and water uptake capacity was also identified. At 25℃ and a relative humidity of 90% , the microporous structure was found to be damaged by excessive grinding, and water migration pathways were obstructed. In contrast, the most favorable moisture response and greater hydrogen-bond network stability were observed for corn stover with particle size of 0.25~0.85mm.The molecular-level mechanism of water structure evolution during moisture absorption was clarified, and a theoretical basis was provided for safe straw storage, biomass pretreatment optimization, and rapid NIR moisture detection.