Reduction and removal of aqueous mercury(II) by micro-sized zero-valent iron/indium

Abstract

The micron-scale zero-valent iron (ZVI) is commonly used for removal of heavy metals in water. In order to maximize the removal efficiency of aqueous Hg(II) by ZVI, the surface of ZVI was doped with diverse metals (i.e., Ni, Cu, In, and Sn). Then the synthesized bimetal particles were used in the batch experiment in a slurry form, and the Hg(II) removal and reduction efficiency were confirmed in 60 and 120 minutes. Among them, In-modified ZVI (ZVI/In) showed the highest Hg(II) removal efficiency of 99 % or more, and also produced the largest concentration of Hg(0), 104 μmol/m3, in the headspace. Kinetic experiments were carried out with varying ratio of ZVI/In, and it was found that larger amount of Hg(II) can be removed with increased In content. As the In concentration in solution increased from 0.5 mM to 2 mM, more Hg(0) was detected in the headspace, in general, however, when excessive amount of In, 18 mM, was added in solution, Hg(0) in the headspace was remarkably decreased. It suggests that excessive In can decrease the Hg(II) reduction rate or increase the amalgamation rate with Hg(0). Surface characterization of ZVI/In using SEM, TEM, EDS, XPS, and XRD analysis was conducted to explain removal mechanism of Hg(II) by ZVI/In. Relatively even distribution of In on the thin ferric oxides shell of ZVI was confirmed by the TEM-EDS image. The same analysis revealed that there was no significant difference in the distribution of In before and after the Hg(II) reaction. Furthermore, ZVI/In removed more than 99 % Hg(II) during continuous recycle test of seven times, suggesting water molecules reduced to H2 by ZVI might be adsorbed by In as atomic hydrogen, which mainly generates catalytic activity of Hg(II) reduction with a long-term stability. Overall, catalytic property of In, providing atomic hydrogen, on the ZVI surface and amalgamation of In-Hg are considered to be the key factors controlling the reduction and removal Hg(II).