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

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 30 min. The cadmium removal efficiency increases rapidly with rising pH in the range of 3.0-8.6, maximum removal capacity of 66.9 mg 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 20 min 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). (C) 2014 Elsevier B.V. All rights reserved.