Co-Precipitation
Often the solubility of an element is lower in the soil solution than predicted by the solubility product because the solubility of an ion is lower in mixed ionic solution than in pure ionic solutions. One possible reason for the reduced solubility is due to co-precipitation. Co-precipitation is the incorporation of trace element into mineral structure during solid solution formation and recrystallization of minerals. This process will reduce the mobility and toxicity of the toxic trace elements that are incorporated into the mineral.
Minerals will only incorporate elements into their structure that have similar ionic radii as the elements composing the mineral. For example, during the formation of calcite, Mn2+, Cd2+, and Fe2+ can possibly be incorporated into the mineral structure. In the formation of Fe and Al-oxides, Cr3+, Mn3+, and V3+ may be incorporated into the structure. Mn and Fe-oxides have more possibility for co-precipitation than Al-oxides and aluminosilicate minerals. Co-precipitation reactions are also controlled by the rate of soil mineral dissolution.
Precipitation reactions in soils have the ability to reduce the toxicity and mobility of harmful trace elements that may be present in the soil system. These reactions occur naturally in the soil. Surface precipitation has been used as a method of natural attenuation in marshland near San Francisco. Buried paint chips containing zinc were found in the soil at the site. It was determined that the zinc that was leaching was precipitating out of soil solution into an insoluble and immobile form. The company was permitted to put a soil cap over the contaminated area, saving them the cost of excavation and incineration. Currently, precipitation is not commonly used as a method of attenuating polluted soil. This is because there are many factors that may cause the precipitates to dissolve and again become mobile. Also, other chemical reactions are competing with precipitation in the soil and may cause the pollutant to remain soluble.