Properties of synthetic goethites with Co for Fe substitution

Isomorphic substitution in goethites is common in nature and the properties of goethites generally change as a function of the degree of substitution (e.g. Al-goethites). In synthetic goethites, substitution by other elements such as Co is also known. Recent literature indicates that the influence of Al and Ca on the unit-cell dimensions of goethite is similar. In contrast to Al-goethites, however, little is known about other properties of Co-goethites and in this study some properties of synthetic Co-goethites were investigated by XRD, IR, TEM, TGA and reductive dissolution techniques. Eight goethite samples (S1 to S8) with varying Co concentrations were synthesized from mixed alkaline solutions of Fe(III) nitrate and Co(II) nitrate, aged at 63 degrees C and ambient pressure. The goethites contained up to 9.5 mol.% Co. Their redness increased with Co concentration, e.g. 0.5 Y 6.0/6.4 for S1 and 6.4 YR 3.3/3.2 for S8. Surface area ranged from 46 to 88 m(2)/g. Unit-cell parameters a, b, c and nu all showed a negative linear dependency on the Co concentration of the goethites. Transmission and diffuse reflectance IR spectrometry showed the presence of strong bands which were interpreted as nu-OH, delta-OH and gamma-OH vibrations. The delta-OH and gamma-OH band positions showed a positive linear dependency on the Co concentration of the samples. Dehydroxylation occurred between 280 and 315 degrees C and dehydroxylation peak positions tended to decrease with increasing Co concentrations. As with Al-goethites, Co-goethite reductive dissolution rates decreased parabolically with increasing substitution. X-ray diffraction and IR analyses, TGA and congruent reductive dissolution suggest the existence of single phases, i.e. Co-goethites of varying degrees of isomorphic substitution.