Inorganic contaminants, such as heavy metals , which exhibit toxicity effects when present at low levels, can be difficult to treat. The level of toxicity can be well below the metal's solubility concentration, and for this reason precipitation, a common treatment method for the removal of heavy metal, does not provide the needed effluent quality. Cadmium , for instance, has a reported maximum contaminant level (MCL) of 10 μg/l, with a 5 μg/l proposed level. Based upon theoretical solubility calculations, alkaline precipitation may only reduce the level of cadmium to 140 μg/l. Hence, additional methods of wastewater treatment are often required to meet environmental regulations. One such treatment technique is chemical coprecipitation.
Coprecipitation is a process in which a solid is precipitated from a solution containing other ions. These ions are incorporated into the solid by adsorption on the surface of the growing particles, physical entrapment in the pore spaces, or substitution in the crystal lattice. Adsorption is one of the principle mechanisms of coprecipitation. It is a process in which the solid species , or adsorbent, is added to a solution containing other ions, called adsorbates. In this case, the adsorbates are bound to the solid's surface by physical or chemical interactions between one adsorbate and the adsorbent.
In solution, coprecipitation and adsorption are thus related by the time during which an adsorbate is present. The type of adsorbent present also affects the extent of ion uptake from solution. One solid used to perform this treatment is ferric oxide. For example, iron coprecipitation of cadmium has been reported to yield a residual cadmium concentration of about 3 μg/l. Several solid and solution variables must be considered when designing and optimizing coprecipitation as a treatment option. These include the equilibrium concentration of the species in solution, the suspension pH , and the presence of other interacting ions. Also, the properties of the solid adsorbent are important. These include type of solid formed, surface area available, age, and surface charge. In short, coprecipitation is controlled by a number of important variables which are specific to a given system. Thus treatability studies must be performed to fully optimize this wastewater treatment process and produced the necessary effluent quality.
[James W. Patterson ]
Leckie, J., et al. Adsorption/Coprecipitation of Trace Elements from Water with Iron Oxyhydroxide. EPR5 Final Report, 1980.
A Review of Solid-Solution Interactions and Implications for the Control of Trace Inorganic Materials in Water Treatment. AWWA Committee Report. Vol. 80, 1988.