Williamson Research Centre for Molecular Environmental Science, Department of Earth and Environmental Sciences, The University of Manchester, Manchester M13 9PL, UK.
Diamond Light Source Ltd., Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, UK.
J Hazard Mater. 2022 Jun 5;431:128564. doi: 10.1016/j.jhazmat.2022.128564. Epub 2022 Feb 24.
Molybdenum (Mo) is a key trace element and a contaminant in many environments including mine tailings and acid mine drainage systems. Under oxic conditions Mo exists in a number of forms, including mono-molybdate (Mo(VI)O) and various poly-molybdate species (e.g. Mo(VI)O) depending on the geochemical conditions (e.g. pH). The mobility and bioavailability of Mo is often controlled by sorption to mineral surfaces, including iron (oxyhydr)oxides e.g. hematite (FeO). This study uses adsorption isotherms, PHREEQC geochemical modeling, Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR), and X-ray Absorption Spectroscopy (XAS) to holistically characterise the molecular scale adsorption of molybdate to hematite as a function of pH (3-12) and Mo(VI) concentration (0.01 × 10 - 2 × 10 M). PHREEQC and ATR-FTIR indicated both pH and Mo concentration are important variables when forming mono- vs. poly- molybdate and suggest low pH (≤ 4) and high Mo(VI) concentration (≥ 5 × 10 M) contribute to the formation of a poly-molybdate surface species on the hematite surface. XAS found Mo adsorbed to hematite via an inner-sphere corner-sharing bidentate binuclear complex with an octahedral mono-molybdate structure at a Mo concentration of 0.6 × 10 M across the pH range, and at a Mo(VI) concentration of 5 × 10 M and pH over 5. This is the first direct observation of octahedrally coordinated Mo(VI) adsorption species on hematite, and this information has broad implications for the mobility and transport of Mo as a contaminant in the environment.
钼（Mo）是一种关键的微量元素，也是许多环境（包括尾矿和酸性矿山排水系统）中的污染物。在氧化条件下，钼以多种形式存在，包括一钼酸盐（Mo（VI）O）和各种多钼酸盐物种（例如 Mo（VI）O），具体取决于地球化学条件（例如 pH 值）。钼的迁移性和生物可利用性通常受矿物表面吸附的控制，包括铁（氧）氢氧化物，例如赤铁矿（FeO）。本研究使用吸附等温线、PHREEQC 地球化学模型、衰减全反射-傅里叶变换红外光谱（ATR-FTIR）和 X 射线吸收光谱（XAS）全面描述钼酸盐在赤铁矿表面的分子尺度吸附随 pH 值（3-12）和 Mo（VI）浓度（0.01×10 - 2×10 M）的变化。PHREEQC 和 ATR-FTIR 表明，形成单-和多-钼酸盐时，pH 值和 Mo（VI）浓度都是重要的变量，并且低 pH 值（≤4）和高 Mo（VI）浓度（≥5×10 M）有助于在赤铁矿表面形成多钼酸盐表面物种。XAS 发现，Mo 通过内球角共享双核配合物吸附到赤铁矿上，在 Mo 浓度为 0.6×10 M 时，在整个 pH 范围内形成八面体单钼酸盐结构，在 Mo（VI）浓度为 5×10 M 和 pH 值高于 5 时也是如此。这是首次直接观察到八面体配位的 Mo（VI）吸附物种在赤铁矿上的存在，这一信息对 Mo 作为环境污染物的迁移和输运具有广泛的意义。
