Effect of pH and anion type on the aging of freshly precipitated iron(III) hydroxide sludges

The effect of pH and anion composition on the aging of freshly precipitated iron(lll) hydroxide sludges has been examined. The rate of transformation of the kinetically favored iron(lll) oxyhydroxide, ferrihydrite, to its crystalline analogues, hematite (alpha-Fe2O3) and goethite [alpha-FeO(OH)], was promoted with increasing pH. The influence of anion type on the transformation rate was related to the affinity of the anions for the surface of the ferrihydrite particles, with rates decreasing in the order v(T nitrate) > v(T chloride) > v(T sulfate) (v(T) = rate of transformation). The relative composition and crystal morphology of the product species was found to be dependent on both the anion type and the pH of the system. Hematite formation was favored at pH values near the point of zero charge of ferrihydrite (pH 7-9), whereas goethite formation was favored outside of this region. A correlation between enhanced hematite formation and both the relative affinities of the anions for the ferrihydrite particles and their relative aqueous phase complex stability constants was evident. This is thought to be a manifestation of the competing formation mechanisms of hematite and goethite.