Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China.
The School of Materials Science and Engineering, Tongji University, Shanghai 201804, China.
J Hazard Mater. 2022 Feb 15;424(Pt C):127486. doi: 10.1016/j.jhazmat.2021.127486. Epub 2021 Oct 10.
The naturally-formed iron (hydr)oxides on the surface of zero valent iron (ZVI) have long been considered as passivation layer and inert phases which significantly reduce the reaction activities when they are employed in environmental remediation. Although it seems there are no direct benefits to keep these passivation layers, here, we show that such phases are necessary intermediates for the transformation to iron sulfides through an anion exchange pathway during sulfidation of ZVI. The pre-formed (hydr)oxides undergo a phase evolution upon aging and specific phases can be effectively trapped, which can be confirmed by a combination of different characterization techniques including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRPD), and X-ray absorption near edge structure (XANES) spectroscopy. Interestingly, after sulfidation, the resultant samples originated from different (hydr)oxides demonstrate different activities in the Cr(VI) sequestration. The XANES investigation of Fe K edge and Fe L edge indicates Fe remains the same after sulfidation, suggesting a non-redox, anion exchange reaction pathway for the production of iron sulfides, where O anions are directly replaced with S. Consequently, the structural characteristics of the parent (hydr)oxides are inherited by the as-formed iron sulfides, which make them behave differently because of their different structural natures.
天然形成的铁（氢）氧化物在零价铁（ZVI）表面长期以来被认为是钝化层和惰性相，当它们被用于环境修复时，会显著降低反应活性。尽管保持这些钝化层似乎没有直接的好处，但在这里，我们表明，在 ZVI 的硫化过程中，通过阴离子交换途径，这些相是向铁硫化物转变的必要中间相。在老化过程中，预先形成的（氢）氧化物会经历相演变，并且可以有效地捕获特定的相，这可以通过包括扫描电子显微镜（SEM）、X 射线光电子能谱（XPS）、X 射线粉末衍射（XRPD）和 X 射线吸收近边结构（XANES）光谱在内的不同表征技术的组合来证实。有趣的是，硫化后，来自不同（氢）氧化物的所得样品在 Cr(VI)螯合方面表现出不同的活性。Fe K 边和 Fe L 边的 XANES 研究表明，硫化后 Fe 保持不变，这表明铁硫化物的生成是一种非氧化还原、阴离子交换反应途径，其中 O 阴离子被直接取代为 S。因此，母体（氢）氧化物的结构特征被新形成的铁硫化物继承，由于其不同的结构性质，它们的表现也不同。
