People use numerous methods to decontaminate polluted soils, including on-site treatment and off-site removal of the contaminated soil. These methods are designed to lessen the chance of further environmental harm at the site or from run-off during heavy rains. But none of these is considered ideal for the total cleanup of contaminated land and remediators usually use more than one technique.
Containing Polluted Soils
The physical removal of contaminated soil is a cleanup method where people either dump the soil in a landfill or incinerate in. These methods can cause new problems by either confining the polluted soil without decontamination, or burning the toxic material and causing air pollution. Contaminants in the soil can be either solid or liquid and are physically or chemically mixed with the soil particles. The treatment method depends on the type of contamination. Toxic organic chemicals such as solvents and pesticides are usually incinerated, while heavy metals are broken down into compounds that can’t react with other materials. In certain situations, such as landfills, the soil is isolated on-site to prevent spreading the contamination from rain water run-off. This containment method at landfills typically involves using multiple impermeable layers of plastic liners and dense clay caps.
Soil washing is the process of using chemical solvents to remove toxic materials from the soil. This allows the clean-up of polluted soil in place (in situ) without having to excavate. By pumping chemicals into the ground, this pump-and-treat method flushes the harmful toxins from the ground by moving them toward wells that pump it out for processing. In-situ flushing works best where the soil beneath the polluted area is not permeable, like clay. Removing heavy metals involves chemical treatments using organic and inorganic acids and chelating agents. This method can also generate secondary hazardous wastes that can require even more treatment.
Contaminants like solvents and heating oil are present in the soil as liquids and can slowly dissolve into the groundwater in which case cleaners typically use chemicals like detergent-related surfactants, and cosolvents like ethanol or methanol, to dissolve these pollutants.
Bioremediation involves inciting the growth of naturally occurring waste-eating microorganisms to help remove soil contaminants, a method considered to be the most environmentally sound. The use of microorganisms to degrade organic contaminants in the soil can happen either after excavation or at the site. The microorganisms break down contaminants by using them as a food source during the aerobic processes, which requires oxygen. If remediators use this method after removing the soil, they use a slurry bioremediation method in which they mix the soil in water to keep the soil suspended and the microbes in contact with the contaminants. The solid-phase bioremediation method places the excavated soil in an isolated site where it’s tilled with water and nutrients to spark the growth of the microorganisms.