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

文献类型： 外文期刊

第一作者： Liu Jin

作者： Liu Jin;Yang Jianjun;Zeng Xibai;Wang Jian;Yang Jianjun;Sparks, Donald

作者机构：

关键词： soil carbon sequestration;multivalent cation;STXM;NEXAFS;ferrihydrite

期刊名称：ACTA CHIMICA SINICA （ 影响因子：2.668； 五年影响因子：1.839 ）

ISSN： 0567-7351

年卷期： 2017 年 75 卷 6 期

页码：

收录情况： SCI

摘要： 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 L-3-edge, respectively, to reveal the molecular structures of C, Si and Fe. Our results indicated the similar correlation of C-Fe (R-2=0.965) to C-Si (R-2=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 L-3-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.

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