几种典型地带性土壤中胡敏素结构特征及对金属离子吸附特性的研究

The chemical structure of soil humic substances and their interaction with metal ions are one of the key and frontier issues in the research of international soil science. Humic substances can be divided into three main components, namely, humic acid, fulvic acid, and humin. Humin typically comprises more than 50% of soil organic matter, and plays an important role in soil fertility improvement and ecological environment protection. However, because of its insolubility, heterogeneity, and structural complexity, humin is the least understood among the three components of soil humic substances. To date, the chemical structure of soil humin and its interaction with metal ions remain unclear. In a recent study, we found that the adsorption of metal ion (Cu2+) on humin from forset soil is more mobilizable than its adsorption on the corresponding humic acid. Thus, further research is needed to clarify the interaction of humin and metal ions in soils. In this project, we plan to collect seven typical zonal soil samples from different climate areas of China. Soil humin will be isolated using NaOH-Na4P2O7 extraction, and will be further fractionated into iron-linked humin, clay-combined humin, and insoluble humin. The chemical structure of humin and its fractions will be characterized using elemental analysis, FTIR, 13C CPMAS NMR, Py-GC/MS, and synchrotron-based C-1s NEXAFS spectroscopy. The adsorption behaviors of metal ions (Pb2+, Cd2+ and Cr3+) on humin and its fractions will be studied as a function of pH, ionic strength, temperature, metal ion concentration, and contact time in the absence or presence of humic acid or inorganic minerals by batch equilibrium method. Conventional FTIR and synchrotron-based XAS (NEXAFS, XANES and EXAFS) spectroscopies will be used to examine the mechanisms of metal ion adsorption on humin. The differences of structural characteristics and metal ion adsorption properties between humin and humic acid will be compared. The main objectives of the research are to clarify the nature of soil colloid and the mechanism of solid-liquid interface reaction at the molecular level, to identify the contribution of humin in the adsorption of metal ions by humic substances of soil, to understand comprehensively the accumulation, migration, and transformation processes of metal ions in soil, and to provide theoretical basis for the forecasting of bioavailability and toxicity of metal ions, the regulation of soil fertility, and the remediation of contaminated soils.

土壤腐殖物质化学结构以及与金属离子的相互作用关系，是国际土壤学研究的重点和前沿课题之一。土壤腐殖物质三组分中，尽管胡敏素占有机碳、有机氮的绝大部分，并在土壤肥力和生态环境等方面发挥重要作用，但由于其研究复杂性远高于其它两种腐殖物质组分，致使目前对其化学结构以及与金属离子相互作用关系都尚不清楚。本项目拟针对我国不同气候带的几种典型地带性土壤，在对胡敏素及其组分化学结构进行表征的基础上，研究胡敏素、胡敏素分组组分、胡敏素与胡敏酸共存、以及胡敏素与无机矿物共存时对金属离子的吸附作用；利用常规和同步辐射谱学技术揭示吸附的微观机理；同时比较胡敏素与胡敏酸结构特征及吸附特性的差异，为从分子水平上揭示土壤胶体本性及固液相界面反应机制，明确胡敏素在土壤腐殖物质组分吸附金属离子中的贡献及其机理，全面认识金属离子进入土壤后的累积、迁移和转化规律，进而为预测金属离子生物有效性和毒性以及污染土壤修复提供理论依据。

土壤腐殖物质化学结构及与金属离子相互作用关系，是国际土壤学研究的重点和前沿课题之一。土壤腐殖物质组分中，尽管胡敏素（HU）占有机碳、有机氮的绝大部分，但其化学结构及与金属离子相互作用关系仍不清楚。本项目以我国几种地带性土壤为供试土壤，在对HU进行结构表征基础上，研究了它们对金属离子的吸附作用及其机理，同时比较了HU与胡敏酸（HA）在上述方面的差异。主要结果如下：1）土壤腐殖物质组分以HU含碳量最高，而HU组分中又以不溶性HU含碳量最高。2）去除无机矿物后，HU的烷基碳和羰基碳增加，而烷氧碳和芳香碳减少；与HA相比，HU呈不规则的颗粒状结构，具有蓬松、粗糙和多孔的表面，H/C和C/N比值较高且含有较多脂族类化合物、多糖类化合物、肽键氮和硅酸盐杂质，而O/C比值、芳基类化合物、木质素类化合物、酚类化合物、含氮化合物和羧基碳含量较低；HU组分中，碱溶性HU脂族性较低，而非碱溶性HU烷基性较低，粘粒结合HU含较多的直链烷/烯烃类化合物，而非溶解性HU含较多的多糖类化合物。3）无论单一还是多种金属离子共存，它们在HU上的吸附量随pH值、接触时间、金属离子浓度和反应温度增加而增加，但随离子强度增加而下降；吸附动力学符合准二级动力学方程，吸附等温线符合Langmuir和Freundlich方程；吸附过程需要能量，是自发、吸热和自由度增加的缔合反应；与无机矿物、EDTA、有机酸以及多种金属离子共存时，均降低了HU对某种金属离子的吸附量；不同腐殖物质组分对金属离子吸附贡献率为富里酸>HA>HU及铁结合HU>粘粒结合HU>不溶性HU。4）吸附金属离子后，HU的表面形态发生了改变，羧基、羟基、芳香氮和伯胺氮参与了金属离子的吸附作用，金属离子是以内层配位的扭曲八面体构型存在，吸附条件不影响其局域配位结构。上述研究有助于从分子水平上揭示土壤胶体本性及固液相界面反应机制，明确HU在土壤腐殖物质吸附金属离子中的贡献及其机理，为预测金属离子生物有效性和毒性及污染土壤修复提供理论依据。