State Key Laboratory of Biogeology and Environmental Geology , China University of Geosciences , Wuhan , Hubei China.
Department of Environmental Science and Engineering , Fudan University , Shanghai , China.
Environ Sci Technol. 2019 May 21;53(10):5768-5777. doi: 10.1021/acs.est.9b00498. Epub 2019 Apr 22.
Dissolved Mn(III) species have recently been recognized as a significant form of Mn in redox transition zones, but their speciation, stability, and reactivity are poorly understood. Besides acting as the intermediate for Mn redox chemistry, Mn(III) can undergo disproportionation producing insoluble Mn oxides and aqueous Mn(II). Using pyrophosphate(PP) as a model ligand, we evaluated the thermodynamic and kinetic stability of Mn(III) complexes. They were stable at circumneutral pH and were prone to (partial) disproportionation at acidic or basic pH. With an initial lag phase, the kinetics of Mn(III)-PP disproportionation was autocatalytic with the produced Mn oxides promoting the disproportionation. X-ray diffraction and the average Mn oxidation state indicated that the solid products were not pure Mn(IV) oxides but a mixture of triclinic birnessite and δ-MnO. Addition of synthetic analogs of the precipitates eliminated the lag phase, confirming their catalytic roles. Thermodynamic calculations adequately predicted the stability regime of Mn(III)-PP. The present results refined the constant for Mn(PP) formation, which allows a consistent and quantitative prediction of equilibrium speciation of Mn(III)-Mn(II)-birnessite with PP. A simple and robust model, which incorporated the thermodynamic constraints, autocatalytic rate law, and verified reaction stoichiometry, successfully simulated all kinetic data.
溶解态的 Mn(III) 最近被认为是氧化还原过渡带中 Mn 的一种重要形态，但人们对其形态、稳定性和反应性知之甚少。Mn(III) 除了作为 Mn 氧化还原化学的中间产物外，还可以发生歧化反应，生成不溶性 Mn 氧化物和水合 Mn(II)。本研究使用焦磷酸盐(PP)作为模型配体，评估了 Mn(III)配合物的热力学和动力学稳定性。它们在近中性 pH 值下稳定，在酸性或碱性 pH 值下容易发生（部分）歧化反应。Mn(III)-PP 歧化反应动力学具有初始滞后阶段，呈自催化特征，生成的 Mn 氧化物促进了歧化反应。X 射线衍射和平均 Mn 氧化态表明，固体产物不是纯 Mn(IV)氧化物，而是三方钠锰矿和 δ-MnO 的混合物。添加沉淀的合成类似物消除了滞后阶段，证实了它们的催化作用。热力学计算充分预测了 Mn(III)-PP 的稳定范围。本研究结果改进了 Mn(PP)形成常数，使得能够对含有 PP 的 Mn(III)-Mn(II)-钠锰矿的平衡形态进行一致和定量预测。一个简单而稳健的模型，结合热力学限制、自催化速率定律和验证的反应计量学，成功地模拟了所有动力学数据。
