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 absorption spectroscopy (XAS) to compare the sorption of Mn­(II), Co­(II), Ni­(II), Zn­(II), and Cd­(II) with sulfated green rust (GR) in anoxic pre-equilibrated suspensions at pH 8 over a timespan of 1 h to 1 week. The XAS data suggest that all 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 extensive sorption of Co­(II), Ni­(II), and Zn­(II) that continues throughout the entire experimental timeframe. We attribute these observations to differences in the affinity and extent of divalent metal substitution in Fe­(II) sites of the GR lattice as controlled by ionic size. Divalent metals smaller than Fe­(II) [i.e., Co­(II), Ni­(II), and Zn­(II)] are readily accommodated and undergo coprecipitation during GR dissolution–reprecipitation. In contrast, divalent metals larger than Fe­(II) [i.e., Mn­(II) and Cd­(II)] have a low affinity for substitution and remain coordinated at the surface following limited exchange with Fe­(II)(s) at GR particle edges. These results imply that GR may strongly affect the solubility of Co­(II), Ni­(II), and Zn­(II) in reducing geochemical systems but will have little impact on the retention of Cd­(II) and Mn­(II).