The effect of pH and anion composition on the aging of freshly precipitated iron(III) hydroxide sludges has been examined. The rate of transformation of the kinetically favored iron(III) oxyhydroxide, ferrihydrite, to its crystalline analogues, hematite (α-Fe2O3) and goethite [α-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 υT   nitrate > υT   chloride > υT   sulfate (υT = 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.

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The use of the solubility domain approach for providing representative effluent treatment quality assurance standards for the hydroxide precipitation of Zn 2+ , Pb 2+ , and Fe 3+ from various wastewater composition types has been modeled and experimentally validated. Solubility domain calculations were based on the precipitated phases isolated from systems representing the upper and lower limits of potential effluent chemical compositions. All such phases resembled their mineralized counterparts with a lower degree of structural order. For all systems modeled, the experimentally observed solubilities were generally encompassed within the predicted solubility domains. Validation of the approach using real treatment data demonstrated the effectiveness of the solubility domain method in predicting optimum operating condition ranges for wastewater treatment. Prediction accuracy was largely dependent on the nature of the solubility-limiting phase. With the exception of carbonate-rich systems, changes to the aqueousphase complexation had a minimal impact on solubility in comparison to selection of the solubility-limiting phases.

Lime and magnesia are commonly used to treat metal (e.g., iron)-laden waters and wastewaters, which can generate large volumes of sludge with their attendent handling and landfill problems. In order to address possible reductions in generated sludge and subsequent landfill volumes, the effects of pH and concentration of Ca2+ and Mg2+ on the initial precipitation and subsequent phase transformations of iron(III) hydroxide sludges have been investigated. In both Cl- and SO4 2- media, ferrihydrite (5Fe2O3·9H2O) was found to be the kinetically preferred phase independent of pH and Ca2+ concentration, with residual Ca2+ solubility being controlled by adsorptive processes at pH values more alkaline than the point of zero charge of ferrihydrite. In contrast, ferrihydrite was the only phase present at pH values <9 when the mole fraction x of Mg2+, where x = Mg2+/(Mg2+ + Fe3+), was less than 0.1, whereas above this value a multicomponent phase related to the mineral pyroaurite [Mg6Fe2(OH)16CO3·4H2O] (termed a-pyroaurite in this study) was also formed. Transformation of ferrihydrite (in the absence of a-pyroaurite) occurred with time to produce hematite (α-Fe2O3) and/or goethite [α-FeO(OH)]. Increases in both Ca2+ and Mg2+ concentration promoted the formation of hematite at the expense of goethite, but reduced the observed rate of transformation. The pyroaurite-related material did not act as a precursor to the formation of either hematite or goethite. The comparative sludge volumes (cm3/g) of the solids investigated in this study decreased in the following order:  ferrihydrite (29.3) > goethite (20.6) > a-pyroaurite (∼15.0) > hematite (7.8).

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