弱磁场对已钝化零价铁除Se(IV)过程的加速作用和反应机制

Passivation of zero valent iron (ZVI) has become an obstacle of applying ZVI for contaminants removal either in permeable reactive barrier or in industrial wastewater treatment. Thus, improving the reactivity of the passivated ZVI is of significance. Weak magnetic field (WMF) is proposed to accelerate Se(IV) removal by passivated ZVI and to rejuvenate the reactivity of passivated ZVI in this study. Various passivated ZVI samples with passive layers of different constituents and contents will be synthesized and the synthesized passivated ZVI samples will be characterized carefully with various techniques. A reactor with applied weak magnetic field (WMF) will be constructed and the batch experiments will be conducted in this reactor. The relationship among the WMF intensity, the properties of passive layer, and the WMF-induced enhancement in Se(IV) removal by passivated ZVI will be elucidated. A continuous reactor with permanent magnets to provide WMF will be designed and the influence of WMF on Se(IV) removal by passivated ZVI in a continuous process will be determined. The mechanisms of Se(IV) removal by passivated ZVI in the presence or absence of WMF will be determined. Electrochemical experiments will be performed to determine the rate-limiting step of Se(IV) removal by aged ZVI and the influence of WMF on the rat-limiting step. Finite element software will be employed to calculate the distribution and gradient of induced magnetic field by passivated ZVI in an applied WMF. Experiments with iron wire will be designed to support the results of theoretical calculation and thus the role of WMF in Se(IV) removal by passivated ZVI will be clarified. This project will not only offer a brand-new and environmental-friendly method of activating passivated ZVI but also set up the foundation of water treatment magneto-chemistry.

零价铁广泛应用于受污染地下水修复和工业废水处理中，但钝化问题已成为推动零价铁技术应用的重要障碍，如何提高已钝化零价铁的反应活性已成为研究的焦点，本课题提出利用弱磁场来实现已钝化零价铁的活化。模拟零价铁在地下水修复和工业废水处理中的实际反应过程，制备含有不同性质、不同钝化膜含量的已钝化零价铁并进行系统地表征，以Se(IV)为目标污染物，建立弱磁场对已钝化零价铁除Se(IV)过程的加速程度与磁场强度、钝化膜性质和含量之间的关系；设计连续流反应器，利用永磁铁提供的磁场实现对已钝化零价铁连续除Se(IV)过程的加速；解析弱磁场对已钝化零价铁除Se(IV)过程中Se(IV)的价态转化、去除机理的影响；根据电化学实验、磁学测量、磁学软件的计算结果和较宏观尺度实验的验证，明确弱磁场在加速已钝化零价铁除Se(IV)过程中的作用机制。课题有望为已钝化零价铁提供一种有效的活化技术，破解零价铁应用的技术难题。

本课题主要完成了两部分研究。.第一，证实了弱磁场是一种具有广阔应用前景的活化已钝化零价铁活性的方法。无外置弱磁场条件下，不同条件制备的老化铁除Se(IV)的反应活性呈现不同程度的降低或升高。外置弱磁场能够提升对应体系反应速率，提升倍数达到4.6-39.6倍。此外，进一步明确了钝化膜性质与解钝化所需磁场强度大小间的关系。当老化铁表面的钝化膜主要为Fe3O4或较薄的α-Fe2O3、γ-FeOOH时，磁场强度仅为1 mT即可恢复老化铁去除Se(IV)的反应活性；如果钝化膜的厚度较大且主要成分为α-Fe2O3、γ-FeOOH等，弱磁场的强度则需要2-5 mT才能有效缓解老化铁的反应活性。.第二，探明了弱磁场强化零价铁反应活性的作用机制。尽管前期研究已经初步明确了弱磁场强化零价铁除污染性能的作用机制主要与洛伦兹力和磁场梯度力有关，但是这二者力在弱磁场对零价铁活性的强化作用中到底扮演了一个什么样的角色仍然未知。研究发现，通过将弱磁场对零价铁活性的强化作用分别与洛伦兹力和磁场梯度力的固有系数作相关性分析发现，弱磁场强化作用与前者无明显关系，而与磁场梯度力的系数呈现非常明显的线性相关。为进一步阐明弱磁场对零价铁腐蚀活性的作用机制，利用上海光源BL08U1A和BL14W1的两组线站对零价铁的腐蚀行为进行了系统地表征，可以明晰弱磁场能够使得零价铁的腐蚀产物在零价铁颗粒表面呈现非常明显的非均匀的分布，而这就是源于磁场梯度力作用了顺磁性离子（如Fe2+、Fe3+等），使其由磁感应强度较低的位置向磁感应强度较高的位置方向迁移。这进一步证实，弱磁场的对零价铁反应活性的强化作用机制主要与磁场梯度力有关，而与洛伦兹力关系不大。

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