Effect of Ca2+, Mg2+, and Anion Type on the Aging of Iron(III) Hydroxide Precipitates

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 Ca 2+ and Mg 2+ on the initial precipitation and subsequent phase transformations of iron(III) hydroxide sludges have been investigated. In both Cl - and SO 4 2 - media, ferrihydrite (5Fe 2 O 3 ·9H 2 O) was found to be the kinetically preferred phase independent of pH and Ca 2+ concentration, with residual Ca 2+ 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 Mg 2+, where x = Mg 2+ /(Mg 2+ + Fe 3+ ), was less than 0.1, whereas above this value a multicomponent phase related to the mineral pyroaurite [Mg 6 Fe 2 (OH) 16 CO 3 ·4H 2 O] (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-Fe 2 O 3 ) and/or goethite [α-FeO(OH)]. Increases in both Ca 2+ and Mg 2+ 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 (cm 3 /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).