The removal of heavy metal cations by sulfidated nanoscale zero-valent iron (S-nZVI): The reaction mechanisms and the role of sulfur

In this study, the reaction between sulfidated nanoscale zero-valent iron (S-nZVI) and heavy metal cations as well as the role of sulfur were investigated. The results showed the corrosion products of S-nZVI were lepidocrocite ( γ -FeOOH) or/and magnetite (Fe 3 O 4 ), depending on heavy metal species. While the removal of Hg(II), Ag(I), Cu(II), and Pb(II) by S-nZVI was rapid and could achieve over 99% within 5 mins, the removal of Ni(II) and Zn(II) was low in efficiency and unstable. Sulfur was existed as iron sulfides at fresh S-nZVI, but was displaced by the heavy metals and formed the related sulfide compound, or oxidized to S 0 and SO 4 2- . The removal mechanisms are strongly dependent on the solubility product constant (K sp ) of metal sulfides. For Hg(II) and Ag(I), with K sp of corresponding metal sulfides much lower than that of iron sulfide, the removal mechanism is the displacement reaction. For Cu(II) and Pb(II), with K sp of corresponding metal sulfides moderately lower than that of iron sulfide, the removal mechanisms are the displacement reaction and complexation with surface groups of S-nZVI. For Zn(II) and Ni(II), whose K sp of corresponding metal sulfides slightly lower than that of iron sulfide, are mainly removed by complexation with surface groups of S-nZVI.