Nanoencapsulation of hexavalent chromium with nanoscale zero-valent iron:High resolution chemical mapping of the passivation layer

作     者：Xiao-yue Huang Lan Ling Wei-xian Zhang 

作者机构：State Key Laboratory for Pollution ControlCollege of Environmental Science and EngineeringTongji UniversityShanghai 200092China Shanghai Institute of Pollution Control and Ecological SecurityShanghai 200092China 

出 版 物：《Journal of Environmental Sciences》 (环境科学学报（英文版）)

年 卷 期：2018年第30卷第5期

页      面：4-13页

核心收录：

学科分类：083002[工学-环境工程] 0830[工学-环境科学与工程（可授工学、理学、农学学位）] 08[工学] 

基　　金：supported by the National Natural Science Foundation of China(Nos.21677107,51578398) the Fundamental Research Funds for the Central Universities(No.0400219363) 

主　　题：Nanoscale zero-valent iron (nZVI) Hexavalent chromium Solid phase reaction Passivation Spherical-aberration-correctedscanning transmission electronmicroscopy (Cs-STEM) 

摘      要：Solid phase reactions of Cr（Ⅵ） with Fe（0） were investigated with spherical-aberration-corrected scanning transmission electron microscopy（Cs-STEM） integrated with X-ray energy-dispersive spectroscopy（XEDS）. Near-atomic resolution elemental mappings of Cr（Ⅵ）–Fe（0） reactions were acquired. Experimental results show that rate and extent of Cr（Ⅵ） encapsulation are strongly dependent on the initial concentration of Cr（Ⅵ） in solution. Low Cr loading in nZⅥ（〈1.0 wt%） promotes the electrochemical oxidation and continuous corrosion of n ZⅥ while high Cr loading（〉1.0 wt%） can quickly shut down the Cr uptake. With the progress of iron oxidation and dissolution, elements of Cr and O counter-diffuse into the nanoparticles and accumulate in the core region at low levels of Cr（Ⅵ）（e.g., 〈 10 mg/L）. Whereas the reacted n ZⅥ is quickly coated with a newly-formed layer of 2–4 nm in the presence of concentrated Cr（Ⅵ）（e.g., 〉 100 mg/L）. The passivation structure is stable over a wide range of pH unless pH is low enough to dissolve the passivation layer. X-ray photoelectron spectroscopy（XPS） depth profiling reconfirms that the composition of the newly-formed surface layer consists of Fe（Ⅲ）–Cr（Ⅲ）（oxy）hydroxides with Cr（Ⅵ） adsorbed on the outside surface. The insoluble and insulating Fe（Ⅲ）–Cr（Ⅲ）（oxy）hydroxide layer can completely cover the n ZⅥ surface above the critical Cr loading and shield the electron transfer. Thus, the fast passivation of nZⅥ in high Cr（Ⅵ） solution is detrimental to the performance of nZⅥ for Cr（Ⅵ） treatment and remediation.