Trace Metal Adsorption onto an Acid Mine Drainage Iron(III) Oxy Hydroxy Sulfate

The adsorption properties of Fe-rich precipitates in acid mine drainage (AMD) systems differ from those of pure hydrous iron(III) oxides, and this can lead to inaccurate predictions of trace metal adsorption and attenuation. Adsorption edges for Cu, Pb, Zn and Cd adsorption onto a poorly ordered, goethite-bearing iron(III) oxy hydroxy sulfate, precipitated in an AMD system in New Zealand, have been compared to those for adsorption onto synthetic schwertmannite and two-line ferrihydrite. Adsorp tion of Cu and Zn onto the AMD iron(III) oxy hydroxy sulfate was greater than onto synthetic schwertmannite, which was in turn greater than onto two-line ferrihydrite. The two factors considered most likely to enhance Cu and Zn adsorption on the AMD iron(III) oxy hydroxy sulfate were (i) the formation of ternary complexes between the oxide surface, adsorbed SO4 and the metal ion and (ii) bacterially mediated formation of the AMD precipitate. Cd adsorption was similarly enhanced on AMD iron(III) oxy hydroxy sulfate but unaffected by SO4, which did not adsorb at the relatively high pH conditions required for Cd adsorption. Although Pb did appear to form ternary complexes with SO4, Pb adsorption onto both AMD iron(III) oxy hydroxy sulfate and synthetic schwertmannite was less than adsorption onto two-line ferrihydrite.