Dynamics and Mechanisms of Mn(II), Co(II), Ni(II), Zn(II), and Cd(II) Sorption onto Green Rust Sulfate

Batch kinetic experiments are combined with X-ray absorptionspectroscopy(XAS) to compare the sorption of Mn-(II), Co-(II), Ni-(II), Zn-(II), andCd-(II) with sulfated green rust (GR) in anoxic pre-equilibrated suspensionsat pH 8 over a timespan of 1 h to 1 week. The XAS data suggest thatall five divalent metals coordinate at Fe-(II) sites of the GR sorbent,whereas the batch results show that GR exhibits bimodal sorption behavior,with fast but limited uptake of Mn-(II) and Cd-(II) and much more extensivesorption of Co-(II), Ni-(II), and Zn-(II) that continues throughout theentire experimental timeframe. We attribute these observations todifferences in the affinity and extent of divalent metal substitutionin Fe-(II) sites of the GR lattice as controlled by ionic size. Divalentmetals smaller than Fe-(II) [i.e., Co-(II), Ni-(II),and Zn-(II)] are readily accommodated and undergo coprecipitation duringGR dissolution-reprecipitation. In contrast, divalent metalslarger than Fe-(II) [i.e., Mn-(II) and Cd-(II)] havea low affinity for substitution and remain coordinated at the surfacefollowing limited exchange with Fe-(II)((s)) at GR particleedges. These results imply that GR may strongly affect the solubilityof Co-(II), Ni-(II), and Zn-(II) in reducing geochemical systems butwill have little impact on the retention of Cd-(II) and Mn-(II). Ionic size controls the affinity andmechanism of sorptionof non-redox-active divalent metals by green rust: divalent metalssmaller than Fe-(II) undergo extensive coprecipitation, while divalentmetals larger than Fe-(II) remain coordinated at the surface.