Heavy Metals in Wastewater: The Effect of Electrolyte Composition on the Precipitation of Cadmium(II) Using Lime and Magnesia

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^?, SO₄ ^2? and CO₃ ^2? 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 (CaSO₄.2H₂O) and calcite (CaCO₃) at higher SO₄ ^2? and CO₃ ^2? concentrations as secondary precipitates using lime as the precipitant, and the hydroxy-sulfate Mg₂(OH)₂SO₄.xH₂O, nesquehonite (MgCO₃.4H₂O), hydromagnesite Mg₅(OH)₂(CO₃)₄.4H₂O] 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 β-Cd(OH)₂ formation in the Mg²⁺-containing Cd(II)/Cl^? and Cd(II)/SO₄ ^2? systems.