The effect of effluent composition on the efficiency of hydroxide precipitation of Cd(II), using both lime and magnesia as precipitants, has been modelled by the solubility domain approach in order to provide wastewater effluent treatment limits, and has been experimentally validated. Common anionic species such as Cl-, SO42- and CO32- have been treated. Solubility domain calculations were based on those phases that were found to determine metal solubility for systems representing the upper and lower limits of potential effluent chemical compositions. The isolated phases were found to resemble their mineralized counterparts, although in several cases exhibited a lower degree of structural order. Experimentally determined Cd(II) solubilities were generally encompassed within the calculated solubility domains, indicating that solubility domain predictions provide effluent treatment quality assurance ranges for the hydroxide precipitation process. The formation of gypsum (CaSO4 center dot 2H(2)O) and calcite (CaCO3) at higher SO42- and CO32- concentrations as secondary precipitates using lime as the precipitant, and the hydroxy-sulfate Mg-2(OH)(2)SO4 center dot xH(2)O, nesquehonite (MgCO3 center dot 4H(2)O), hydromagnesite [Mg-5(OH)(2)(CO3)(4) center dot 4H(2)O] and brucite [Mg(OH)2] when employing magnesia was shown to have little effect on the observed residual Cd(II) solubility, although Mg(OH)(2) did promote beta-Cd(OH)(2) formation in the Mg2+-containing Cd(II)/Cl- and Cd(II)/SO42- systems.
Water Air and Soil Pollution Vol. 165, no. 1-4, p. 131-152