Sequestration of Cd(II) with nanoscale zero-valent iron (nZVI): Characterization and test in a two-stage system

•Introducing nZVI to sequestrate Cd(II) in batch experiments and flow reactors.•Oxidized nZVI has lower Cd(II) adsorption capacity.•Investigating the possibility of Cd(II) being reduced to Cd(0) by nZVI.•Proving potential of nZVI for large scale application. Nanoscale zero-valent iron (nZVI) for removal of cadmium from polluted water was investigated. Batch experiments were conducted at 1:100 mass ratio of Cd to nZVI to investigate low, medium and high concentration levels of species. Effect of solution pH was importantly evaluated on the removal efficiency of Cd(II) by nZVI. The removal process is fast and can reach equilibrium in less than 30min. The cadmium removal efficiency increases rapidly with rising pH in the range of 3.0–8.6, maximum removal capacity of 66.9mg Cd(II)/g nZVI was observed. Benchmark tests were also conducted with oxidized nZVI (nZVI oxidized with oxygen bubbling), which had much lower removal efficiency of Cd(II) under identical conditions. Flow experiments with a two-stage reactor were performed to examine the effects of hydraulic retention time, influent cadmium concentration, nZVI recycle ratio and nZVI dose. Removal efficiency over 91% was achieved within a residence time of 20min in a two-stage flow reactor. Higher nZVI recycle ratio and longer hydraulic retention time enhanced removal efficiency. Transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray powder diffraction were employed to characterize nZVI before and after the reactions. Data suggest that Cd(II) is sequestrated within nZVI by adsorption or surface complex formation with no apparent reduction of Cd(II).