Precipitation is becoming the most widely selected means for removing heavy metals from groundwater in pump-and-treat operations. It is used as a pretreatment process with other technologies (such as chemical oxidation or air stripping), where the presence of metals would interfere with treatment. After water is pumped to the surface, precipitation converts soluble heavy metals to insoluble metals that settle and/or are filtered out of the water.
The precipitation process usually adjusts pH, adds chemicals that stimulate precipitation, adds coagulants, and mixes the fluid in a device called a flocculator. (Flocculation refers to coalescence of particles.) The chemical precipitants, coagulants, and flocculator are all used to increase particle size through aggregation. Commonly used precipitants include carbonates, sulfates, sulfides, lime, and other hydroxides. The precipitants generate very fine particles that are held in suspension. Coagulants are often added to aggregate the suspended particles. Mixing in a flocculator following the addition of coagulants promotes contact among the particles, which in turn promotes particle growth and settling. In anaerobic environments, bacteria react with soluble metals to form insoluble metal precipitants.
Limitations and Concerns
Precipitation may be difficult for contaminated water with multiple metals. As the precipitation process for one metal is enhanced, the additive may prevent precipitation of another.
The precipitant that is removed must be tested for leaching prior to land disposal. There is concern that, after disposal, metals will leach from precipitants, due to changing environmental conditions such as moisture and pH.
If the process is aimed at removing radionuclides, the amounts of radionuclides that can be removed depend on the precipitant, the concentration of radionuclides present in the aqueous waste, and the pH of the solution. In all cases, the radioactive precipitant must be disposed of carefully.
Hexavalent chromium requires extra treatment prior to coagulation and flocculation.
The addition of chemicals must be carefully controlled so that unacceptable concentrations in treatment effluent will not occur. Discharged treated water often requires pH adjustment.
If the process is targeting radioactive substances, treatability studies should be conducted to select the best ion exchange materials and to determine the best operating parameters for chemical precipitation.
The technology can be used to remediate groundwater containing heavy metals, including their radioactive isotopes. Precipitation is used mainly to convert dissolved metals into particulates that can be removed by coagulation and filtration.
Technology Development Status
Because of its success in meeting requirements for the discharge of treated wastewater, metal precipitation is recognized as a proven process for use in remedial activities such as groundwater treatment. It is commercially available.
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