Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, The Chinese Academy of Sciences, Nanjing 210008, China; University of Chinese Academy of Sciences, Beijing 100049, China.
Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, The Chinese Academy of Sciences, Nanjing 210008, China.
Environ Int. 2019 May;126:234-241. doi: 10.1016/j.envint.2019.02.036. Epub 2019 Feb 26.
Apart from surface complexation, precipitation of minerals also plays an important role in reducing the mobility and transport of heavy metals in the environment. In this study, Cd(II) sorption species on surfaces of γ-AlO at pH 7.5 were characterized using multiple techniques. Results show that in addition to adsorption complexes, Cd hydroxide phases (Cd(OH) precipitates and Cd(OH) polynuclear complexes) were formed at the initial stages of Cd(II) sorption and gradually transformed to CdCO with time. In addition, Cd(II) formed CdAl layered double hydroxide (LDH) on γ-AlO under various conditions, independent of temperature and Cd loadings. The formation of Cd hydroxide phases and CdAl LDH could be ascribed to surface-induced precipitation because the bulk solution was undersaturated with respect to hydroxides. CdAl LDH formation on the Al-bearing mineral here is rather surprising because typically this occurs with elements of ionic radii similar to that of Al; this formation is unknown for metals such as Cd(II) with a much larger ionic radius. The thermodynamic feasibility of CdAl LDH formation was further confirmed by laboratory synthesis of CdAl LDH and density function theory (DFT) calculations. These results suggest that Cd precipitation on Al-bearing minerals can be an important mechanism for Cd immobilization in the natural environment. Additionally, the finding of CdAl LDH formation on Al-bearing minerals and the thermodynamic stability of CdAl LDH provides new insights into the remediation of Cd-polluted soils and aquatic systems.
除了表面络合作用外，矿物沉淀在降低环境中重金属的迁移和运输方面也起着重要作用。在本研究中，使用多种技术对 pH 值为 7.5 时γ-AlO 表面上 Cd(II)的吸附物种进行了表征。结果表明，除了吸附络合物外，在 Cd(II)吸附的初始阶段还形成了 Cd 氢氧化物相（Cd(OH)沉淀和 Cd(OH)多核络合物），并随着时间的推移逐渐转化为 CdCO。此外，在不同条件下，Cd(II)在γ-AlO 上形成了 CdAl 层状双氢氧化物（LDH），这与温度和 Cd 负载无关。由于与氢氧化物相比，体相溶液处于过饱和状态，因此可以将 Cd 氢氧化物相和 CdAl LDH 的形成归因于表面诱导沉淀。在含 Al 的矿物上形成 CdAl LDH 是相当令人惊讶的，因为通常只有与 Al 的离子半径相似的元素才能形成 LDH；而对于 Cd(II)等具有更大离子半径的金属，这种形成是未知的。通过实验室合成 CdAl LDH 和密度泛函理论（DFT）计算进一步证实了 CdAl LDH 形成的热力学可行性。这些结果表明，含 Al 矿物上的 Cd 沉淀可以是自然环境中 Cd 固定化的重要机制。此外，在含 Al 的矿物上形成 CdAl LDH 以及 CdAl LDH 的热力学稳定性为修复 Cd 污染的土壤和水生系统提供了新的思路。
