Fan Yuke, Wang Lijun, Putnis Christine V, Zhang Wenjun
College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China.
Institut für Mineralogie, University of Münster, 48149 Münster, Germany.
Inorg Chem. 2024 Apr 15;63(15):6909-6921. doi: 10.1021/acs.inorgchem.4c00353. Epub 2024 Apr 2.
The coprecipitation of iron (Fe) and phosphorus (P) in natural environments limits their bioavailability. Plant root-secreted organic acids can dissolve Fe-P precipitates, but the molecular mechanism underlying mobilizing biogenic elements from highly insoluble inorganic minerals remains poorly understood. Here, we investigated vivianite (Fe(PO)·8HO) dissolution by organic acids (oxalic acid (OA), citric acid (CA), and 2'-dehydroxymugineic acid (DMA)) at three different pH values (4.0, 6.0, and 8.0). With increasing pH, the vivianite dissolution efficiency by OA and CA was decreased while that by DMA was increased, indicating various dissolution mechanisms of different organic acids. Under acidic conditions, weak ligand OA (HCO > CO at pH 4.0 and CO at pH 6.0) dissolved vivianite through the H effect to form irregular pits, but under alkaline condition (pH 8.0), the completely deprotonated OA was insufficient to dissolve vivianite. At pH 4.0, CA (HCit > HCit > HCit) dissolved vivianite to form irregular pits through a proton-promoted mechanism, while at pH 6.0 (HCit > Cit) and pH 8.0 (Cit), CA dissolved vivianite to form near-rhombohedral pits through a ligand-promoted mechanism. At three pH values ((H)DMA > (H)DMA at pH 4.0, (H)DMA at pH 6.0, and (H)DMA and one deprotonated imino at pH 8.0), strong ligand DMA dissolved vivianite to form near-rhombohedral pits via ligand-promoted mechanisms. Raman spectroscopy showed that the deprotonated carboxyl groups (COO) and imino groups were bound to Fe on the vivianite (010) face. The surface free energy of vivianite coated with OA decreased from 29.32 mJ m to 24.23 mJ m and then to 13.47 mJ m with increasing pH, and that coated with CA resulted in a similar pH-dependent vivianite surface free-energy decrease while that coated with DMA increased the vivianite surface free energy from 31.92 mJ m to 39.26 mJ m and then to 49.93 mJ m. Density functional theory (DFT)-based calculations confirmed these findings. Our findings provide insight into the mechanism by which organic acids dissolved vivianite through proton and ligand effects.
自然环境中铁（Fe）和磷（P）的共沉淀限制了它们的生物有效性。植物根系分泌的有机酸可以溶解铁 - 磷沉淀物，但从高度不溶性无机矿物质中 mobilizing 生物成因元素的分子机制仍知之甚少。在这里，我们研究了在三种不同pH值（4.0、6.0和8.0）下有机酸（草酸（OA）、柠檬酸（CA）和2'-脱氧麦根酸（DMA））对蓝铁矿（Fe₃(PO₄)₂·8H₂O）的溶解情况。随着pH值的升高，OA和CA对蓝铁矿的溶解效率降低，而DMA的溶解效率升高，这表明不同有机酸的溶解机制各不相同。在酸性条件下，弱配体OA（在pH 4.0时HCOO⁻ > COO²⁻ 且在pH 6.0时为COO²⁻）通过H⁺效应溶解蓝铁矿形成不规则凹坑，但在碱性条件（pH 8.0）下，完全去质子化的OA不足以溶解蓝铁矿。在pH 4.0时，CA（HCit²⁻ > HCit⁻ > Cit³⁻）通过质子促进机制溶解蓝铁矿形成不规则凹坑，而在pH 6.0（HCit⁻ > Cit²⁻）和pH 8.0（Cit³⁻）时，CA通过配体促进机制溶解蓝铁矿形成近菱形凹坑。在三个pH值下（在pH 4.0时（H）DMA⁺ > （H）DMA，在pH 6.0时为（H）DMA，在pH 8.0时为（H）DMA和一个去质子化的亚氨基），强配体DMA通过配体促进机制溶解蓝铁矿形成近菱形凹坑。拉曼光谱表明，去质子化的羧基（COO⁻）和亚氨基与蓝铁矿（010）面上的Fe结合。随着pH值升高，涂覆OA的蓝铁矿表面自由能从29.32 mJ/m²降至24.23 mJ/m²，然后降至13.47 mJ/m²，涂覆CA的蓝铁矿表面自由能也有类似的pH依赖性降低，而涂覆DMA的蓝铁矿表面自由能从31.92 mJ/m²增加到39.26 mJ/m²，然后增加到49.93 mJ/m²。基于密度泛函理论（DFT）的计算证实了这些发现。我们的研究结果深入了解了有机酸通过质子和配体效应溶解蓝铁矿的机制。 （注：原文中“mobilizing”可能有误，暂按原样翻译，未作修改）
