Department of Soil Science, College of Agriculture and Bioresources, University of Saskatchewan, Saskatoon, SK S7N 5A8, Canada.
Department of Soil Science, College of Agriculture and Bioresources, University of Saskatchewan, Saskatoon, SK S7N 5A8, Canada.
Chemosphere. 2021 Aug;276:130150. doi: 10.1016/j.chemosphere.2021.130150. Epub 2021 Mar 4.
Boron is an essential plant micronutrient responsible for several important functions. Boron availability in soils may be influenced by binding with soil organic matter (SOM), particularly with aromatic diol and polyphenol groups on SOM. The mechanism by which aromatic diols bind boron, however, remains unclear. The objective of this work is to further investigate interaction between boric acid and varying concentrations of an aromatic, polyphenolic SOM analogue (tannic acid at 5, 10 and 20 g L) from pH = 5-9. UV/Visible spectroscopy showed boric acid enhanced tannic acid deprotonation at pH = 7.0 and 9.0, resulting in singly deprotonated tannic acid subunits. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) showed boric acid/tannic acid binding for all concentrations at pH = 7 and 9, whereas binding at pH = 5.0 was observed only at 20 g L tannic acid. Uncomplexed boron species were not evident at pH = 9.0, but were detectable at pH = 7.0 at lower tannic acid concentrations and prevalent at pH = 5.0, qualitatively indicating binding affinity increases from pH = 5.0 to 9.0. ATR-FTIR results indicated tetrahedral coordination of boron upon complexation to tannic acid with a monodentate mechanism. These results collectively highlight a transition of solution planar boric acid to a tetrahedral, monodentate coordination with a single phenol group in tannic acid polyphenols. This contrasts with previous spectroscopic studies, which indicated bidentate tetrahedral or monodentate trigonal planar orientations prevail at aromatic diol sites. This work presents a previously unobserved boric acid coordination mechanism to an SOM analogue and, therefore, may better inform prediction and modeling of boron behavior in soils.
硼是一种必需的植物微量元素，负责多种重要功能。土壤中硼的可用性可能受到与土壤有机质（SOM）结合的影响，特别是与 SOM 上的芳香二醇和多酚基团结合。然而，芳香二醇结合硼的机制仍不清楚。本工作的目的是进一步研究硼酸与不同浓度芳香、多酚 SOM 类似物（单宁酸在 5、10 和 20 g L）之间的相互作用，pH 值为 5-9。紫外/可见光谱表明，在 pH 值为 7.0 和 9.0 时，硼酸增强了单宁酸的去质子化，导致单去质子化的单宁酸亚基。衰减全反射傅里叶变换红外光谱（ATR-FTIR）表明，在 pH 值为 7 和 9 时，所有浓度下硼酸/单宁酸都发生了结合，而在 pH 值为 5.0 时仅在 20 g L 单宁酸时观察到结合。在 pH 值为 9.0 时未检测到未络合的硼物种，但在 pH 值为 7.0 时在较低的单宁酸浓度下可检测到，在 pH 值为 5.0 时更为常见，定性表明结合亲和力从 pH 值 5.0 增加到 9.0。ATR-FTIR 结果表明，硼酸与单宁酸络合后呈四面体形，以单齿配位机制配位。这些结果共同强调了溶液中平面硼酸向单宁酸中四面体、单齿配位的转变，其中单齿配位与单宁酸多酚中的一个酚基团有关。这与先前的光谱研究形成对比，先前的光谱研究表明，双齿四面体或单齿三角平面取向在芳香二醇位点上占主导地位。这项工作提出了一种以前未观察到的 SOM 类似物的硼酸配位机制，因此可能更好地为土壤中硼行为的预测和建模提供信息。
