THE EFFECT OF CADMIUM ON THE TRANSFORMATION OF FERRIHYDRITE INTO CRYSTALLINE PRODUCTS AT PH 8

Ferrihydrite, prepared in the presence of 0 to 20 mole % Cd in the sol ution, was used to study the transformation of ferrihydrite into cryst alline products. The result showed that the presence of Cd strongly re tards the transformation of ferrihydrite into crystalline products, su ppressing the formation of goethite and leading to a product which eve ntually consists entirely of hematite at pH 8 and at 70 degrees C. The fraction of hematite in the transformation products increased with in creasing level of Cd in the system. When 9 mole % Cd was present, the transformation product consisted entirely of hematite. The chemical an alysis and XRD data showed that Cd was incorporated into the lattice o f iron oxides, Cd-hematite and Cd-goethite being formed. The mole % Cd which replaced iron in the iron oxides increased with increasing leve l of Cd in the system below 9 mole % Cd. Above this value, but below 2 0 mole % the mole % of Cd incorporated in the lattice of iron oxides w as constant at about 2.9 mole %. The volume of the unit cell of Cd-goe thite increased with increasing level of Cd in the system until the go ethite production was entirely suppressed. The volume of the unit cell of Cd-hematite also increased with increasing level of Cd, below 9 mo le % of Cd in the system. Above this value, it was constant. Scanning electron microscopic examination showed that the presence of Cd affect ed the morphology of hematite more than that of goethite. The goethite grew from ferrihydrite as acicular crystals independent of the amount of Cd in the system. The shape of hematite particles varied from irre gular platelets with lower Cd level, to ellipsoids, with higher Cd lev el in the system, and it also suggested that Cd prevented the formatio n of goethite by hindering the dissolution of ferrihydrite rather than by interfering with nucleation and growth of goethite from solution. The rate of transformation was studied at pH 8, 50 degrees C and 70 de grees C. The transformations were first order reactions at both temper atures.