EFFECT OF CADMIUM(Ⅱ) AND ANION TYPE ON THE AGEING OF FERRIHYDRITE AND ITS SUBSEQUENT LEACHING UNDER NEUTRAL AND ALKALINE CONDITIONS

摘要

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 (NO_3~-, Cl~-, and SO_4~(2-) ), was investigated at 20℃ 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 amount of Cd(Ⅱ). At a Cd(Ⅱ):Fe(Ⅲ) mole ratio of 1%, the transformation rate is NO_3~- > Cl~- > SO_4~(2-), which correlates with the relative affinities of the anions for the ferrihydrite surface. The presence of Cd(II) promotes hematite formation at pH 9 and 10, whereas at pH 11 goethite is almost the sole product. With increasing initial Cd(?) concentrations, increasing incorporation of Cd(Ⅱ) into the products is observed. For 5 mole % Cd(Ⅱ), ~ 2.5 mole % of Cd(Ⅱ) is included in the transformation products, principally hematite, while at pH 11, with 1 mole % Cd(Ⅱ), all of the Cd(Ⅱ) incorporates into the goethite lattice. Transmission electron micrographs show that the presence of Cd(Ⅱ) leads to a reduction in size and promotes the twinning of goethite crystals, and can result in ellipsoidal-shaped hematite crystals. Leachability of Cd(Ⅱ) from fresh and aged coprecipitated Cd(Ⅱ)-ferrihyrdite is dependent on the extent of transformation of the ferrihydrite, with 70-90% of the Cd(Ⅱ) leachable from ferrihydrite, while goethite is able to incorporate and remove more Cd(Ⅱ) than hematite.