Effect of Cadmium(Ii) and Anion Type on the Ageing of Ferrihydrite and its Subsequent Leaching under Neutral and Alkaline Conditions

Abstract

The effect of cadmium(II) on the transformation of ferrihydrite[with Cd(II):Fe(III) ratios ranging from 0 to 5 mole %] in neutral and alkaline media (pH 7-11), combined with the effects of electrolyte type (NO3-, Cl-, and SO4-2), was investigated at 20 °C over a period of 1 yr. The presence of Cd(II) strongly retards the conversion of ferrihydrite into hematite and/or goethite at pH 7–10, with decreases in the rate of transformation dependent on the amountof Cd(II). At a Cd(II):Fe(III) mole ratio of 1%, the transformation rate is NO3- > Cl- > SO4-2, which correlates with the relative affinitiesof the anions for the ferrihydrite surface. The presence of Cd(II) promotes hematite formation at pH 9 and 10, whereas atpH 11 goethite is almost the sole product. With increasinginitial Cd(II) concentrations, increasing incorporationof Cd(II) into the products is observed. For 5 mole %Cd(II), ∼ 2.5 mole % of Cd(II) is included in thetransformation products, principally hematite, while at pH 11, with 1 mole % Cd(II), all of the Cd(II) incorporates into thegoethite lattice. Transmission electron micrographs show that the presence of Cd(II) leads to a reduction in size and promotesthe twinning of goethite crystals, and can result in ellipsoidal-shaped hematite crystals. Leachability of Cd(II) fromfresh and aged coprecipitated Cd(II)-ferrihyrdite is dependent onthe extent of transformation of the ferrihydrite, with 70–90% of the Cd(II) leachable from ferrihydrite, while goethite is ableto incorporate and remove more Cd(II) than hematite.