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 Ca2+ and Mg2+ on the initial precipitation and subsequent phase transformations of iron(III) hydroxide sludges have been investigated. In both Cl- and SO42- 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-pyroa...