化学学报 ›› 2017, Vol. 75 ›› Issue (6): 617-620.DOI: 10.6023/A17030082 上一篇    下一篇

所属专题: 铁环境化学

研究论文

STXM-NEXAFS研究铁(III)离子诱发柠檬酸在高岭石表面的固定机制

刘瑾a, 杨建军b,d, 曾希柏b, Wang Jianc, Donald Sparksd   

  1. a 中国农业科学院农业资源与农业区划研究所 北京 100081;
    b 中国农业科学院农业环境与可持续发展研究所 北京 100081;
    c 萨斯卡切温大学加拿大光源 加拿大萨斯卡切温省萨斯卡通市创新大道44号 S7N 2V3;
    d 特拉华大学 特拉华环境研究所 美国特拉华州纽瓦克市科学街221号 19711
  • 收稿日期:2017-03-01 出版日期:2017-06-15 发布日期:2017-04-12
  • 通讯作者: 杨建军, Wang Jian E-mail:yangjianjun@caas.cn;jian.wang@lightsource.ca
  • 基金资助:

    项目受中国农业科学院海外“青年英才”创新工程和国家自然科学基金(Nos.41601313,U1632134)资助.

Fe(III)-induced Sequestration of Citric Acid on Kaolinite Surface Probed by STXM-NEXAFS Spectroscopy

Liu Jinaa, Yang Jianjunb,d, Zeng Xibaib, Wang Jianc, Donald Sparksd   

  1. a Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081;
    b Institute of Environmental and Sustainable Development in Agriculture, Chinese Academy of Agricultural Science, Beijing 100081;
    c Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Canada, S7N 2V3;
    d Delaware Environmental Institute, University of Delaware, Newark 19711, USA
  • Received:2017-03-01 Online:2017-06-15 Published:2017-04-12
  • Contact: 10.6023/A17030082 E-mail:yangjianjun@caas.cn;jian.wang@lightsource.ca
  • Supported by:

    Project supported by the Innovation Funds of the Chinese Academy of Agricultural Science for Overseas Elite Youth Program, and National Natural Science Foundation of China (Nos. 41601313, U1632134).

热带地区土壤有机碳的稳定性受有机碳与高岭石、三价铁离子[Fe(Ⅲ)]交互作用的强烈影响.低分子量有机酸(LMWOAs)是土壤有机碳中最易被微生物降解的组分,研究LMWOAs在高岭石-Fe(Ⅲ)-LMWOAs三相系统中的固定机制对于理解热带地区土壤有机碳循环具有重要意义;然而LMWOAs在该三相系统中固定机制仍不清楚,并缺乏分子水平上的直接证据.本研究以柠檬酸(CA)为LMWOAs的典型代表,通过采集Fe/CA初始物质的量比2.0条件下获取的吸附样品,利用同步辐射X射线扫描透射显微术(STXM)在亚微米尺度上表征C,Fe和Si的分布特征,发现C-Fe分布的相关性与C-Si分布相当;并在高C,Fe微区中利用C的K边和Fe的L3边X射线近边吸收精细结构谱(NEXAFS)在分子水平上揭示了Fe在不同富集微区分别以水铁矿和高岭石-Fe(Ⅲ)-CA三元配合体形式存在,从而表明水铁矿诱发的吸附/共沉淀以及以Fe(Ⅲ)为架桥形成高岭石-Fe(Ⅲ)-CA三元配合作用是CA固定的重要机制.本研究成果对于理解热带地区土壤中CA稳定性和低分子量有机酸的碳固定和循环机制提供了重要科学依据.

关键词: 土壤碳固定, 多价阳离子, X射线扫描透射显微术, X射线近边吸收精细结构谱, 水铁矿

Organic carbon (OC) stability in tropical soils is strongly affected by the mutual interactions of OC, kaolinite and the Fe(Ⅲ) cation. Low molecular weight organic acids (LMWOAs) represent the most biodegradable constituent of OC in soils. Therefore, investigating retention mechanisms of LMWOAs in kaolinite-Fe(Ⅲ)-LMWOAs systems are of significant importance in understanding the sequestration and cycling of OC in tropical soils. However, retention mechanisms of LMWOAs in the kaolinite-Fe(Ⅲ)-LMWOAs system remains unclear, and there is a lack of direct evidence at the molecular level. In this study, citric acid (CA) was chosen as a model compound for LMWOAs and a sorption sample was collected after batch experiment using kaolinite, Fe(Ⅲ), and CA as reagents at pH 3.5 and at an initial Fe/CA molar ratio of 2.0. Synchrotron-based scanning transmission X-ray microspectroscopy (STXM) was applied to characterize the distribution of carbon (C), iron (Fe) and silicon (Si), representing CA, Fe(Ⅲ) cation/Fe hydroxides and kaolinite respectively, at the submicron scale in the sorption sample. After that, the hot spots of C and Fe were selected and further probed by STXM coupled with Near-edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy at the C and Si K-edge and the Fe L3-edge, respectively, to reveal the molecular structures of C, Si and Fe. Our results indicated the similar correlation of C-Fe (R2=0.965) to C-Si (R2=0.960) in the sorption sample, which implied a close relationship between CA and Fe hydroxides as well as kaolinite. We also found two Fe species as ferrihydrite and kaolinite-Fe(Ⅲ)-citrate complex coexisted in the Fe-enriched hot spots probed by STXM coupled with Fe L3-edge NEXAFS spectroscopy. These results provide direct evidence of the contribution of CA retention through ferrihydrite-induced adsorption/coprecipitation and ternary complexation of CA and kaolinite via an Fe bridge in the investigated ternary system. This study will enhance our understanding of the stability of CA and the sequestration and cycling of LMWOAs in tropical soils.

Key words: soil carbon sequestration, multivalent cation, STXM, NEXAFS, ferrihydrite