纳米零价铁团聚体结构及复杂水环境演化机制研究

Goal of this proposed research is to investigate structure and properties of the agglomerates of nanoscale zero-valent iron (nZVI) and their evolution in aqueous solution. Owing to its high reactivity and multifunctional property, studies and applications of nZVI attract many interests in recent decades. nZVI tend to agglomerate into micro-sized flocs in aqueous solution, which are considered to be an undesirable form in its application. In fact, these agglomerates are ubiquitous in any aqueous solutions containing nZVI, and play an important role in nZVI reactions. So far, few have been studied and little is known about these agglomerates. Agglomerates to be studied in the proposed research include those of nZVIs from difference sources, fresh and the oxidized nZVI, bare and surface-modified nZVI, reference nanoparticles such as magnetic nanoscale iron oxides and copper nanoparticles. The agglomerates will be characterized from nanometer level to millimeter scale using a number of technologies, such as aberration corrected scanning transmission electron microscope (Cs-STEM), scanning electron microscope (SEM), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), zeta potential analyzer, particle-size analyzer and Turbiscan dispersion stability analyzer. Parameters of the agglomerate structure, such as stacking density, size distribution of packing holes and agglomerate strength, will be studied and compared. Effects of particle component on the structure, such as those of Fe(0) content and iron oxide shell on the strength, will be evaluated. Influence of particle oxidation and wastewater impurities (e.g., surfactants) on the agglomerate structure will also be assessed, with the results from the analyses of inter-particle forces and particle components. We will also evaluate the impact of mechanical mixing and sonication on agglomerate disaggregation. Hydrodynamic properties of the aggregates will be assessed via settling and mixing experiments on bench and pilot scale respectively. Suspension stability of the agglomerates will be evaluated using Turbiscan stability index (TSI). Results from the proposed research will provide a thorough examination of nZVI agglomerates and offers quantitative data on their structure and physical properties. The results will be helpful to the study of the colloidal behavior and the environmental chemistry of nZVI in aqueous solution. The methodology used is also applicable to the studies of other superfine particles for water treatment use.

纳米零价铁（nZVI）在水处理的研究应用近年得到关注，而对其在水中主要赋存状态之一—nZVI团聚体尚不了解。本研究拟在“单体成分-团聚体结构-宏观水动力学”三个层次，考察nZVI团聚体在水环境中微结构、行为属性及在复杂环境与水力条件下的演化。研究首先考察典型nZVI团聚体基本结构（如堆积孔分布、团聚强度等），获得量化参数并分析其随nZVI被氧化变化规律；通过多种颗粒单体及团聚微结构表征、团聚动力学分析及参照体系比较，阐明结构及环境因素（零价铁含量、表面活性剂等）作用下团聚属性演变规律，揭示微观作用机制并摸清关键控制条件；研究并考察量化不同尺度下nZVI团聚体典型水力条件宏观水动力学行为。研究摸清nZVI团聚体结构及相关行为参数，揭示纳米颗粒团聚体属性与其单体结构及微观作用力之间关系；结果为nZVI胶体行为调控提供微观解释及数据支持，也为涉及超细颗粒的环境化学过程研究提供数据参考和方法借鉴。

本项目均按计划实施，已经完成研究目标。研究从团聚体角度切入分析纳米零价铁（nZVI）水动力学行为，研究了常见水动力学条件下（搅拌、超声分散、沉降、压缩）的其团聚体的结构、属性及变化，并进行扩展探索与水处理中常用的各种絮体的性质进行对比。结果表明，nZVI团聚体在涉及高颗粒浓度的实际应用极为普遍，不同来源nZVI颗粒其团聚体结构存在显著差异。研究拓展了nZVI研究视角，揭示了其在水中真实状态，结果为其工程应用提供参考；研究并拓展了领域研究方法，如引入非入侵原位拍摄表征、图像测速、光学微流变等表征手段，给出纳米颗粒团聚体结构性质的统计量化结果，反映传统沉降观测中难以观察到的体系内部的细节信息，拓展了研究融合。研究考察了团聚体结构特征如尺寸、形貌（纵横比、投影面积）、三维分形维数等，并通过原位测速技术推算其团聚体密度，并量化了其典型水动力学速度，并表征了典型水动力学状态下团聚体堆积体及压缩体的结构（流变及动态压缩）。纳米颗粒团聚体在高剪切力条件下会出现破碎，但即使在极端湍流剪切条件，其仍然维持微米级别，其破碎至纳米级别需依赖其他措施（如球磨破碎）。研究验证了球磨对纳米颗粒团聚体的强化破碎效果，进一步研究了在典型水化学组分（pH、离子强度、有机质）存在下，球磨破碎纳米颗粒团聚体的效果及变化。研究开创性地使用稳定性分析仪（Turbiscan）模拟实际应用中的重力分离环境，原位表征小尺度范围纳米零价铁团聚体的重力沉降过程，研究其重力沉降模式转化，获取部分量化参数描述悬浮体系稳定性、上清液与沉淀层基本属性，提出一种表征颗粒在水环境中重力沉降过程的新思路。基于微米尺度团聚体在高剪切强度下结构尺寸稳定性及演变规律，研究发明了无机絮体包覆的超细粉体复合体系以强化其在反应器中流态化，并研究了其在典型水动力学条件下物理化学性能、流化性能、重力分离效率、稳定性以及反应活性；团聚体结构稳定性等研究结果也支撑解释了纳米铁在资源回收中的富集效应及应用。