When contaminating substances are not used, stored or disposed of safely, they can infiltrate the site in which they are produced or handled, seeping into and contaminating the soil. Contaminated soil is most often found in urban areas and industrial sites due to activities such as manufacturing, industrial dumping, land development, waste disposal or excessive fertilizer or pesticide use.
Depending on the type of contaminant and method in which it enters, the surface layer of soil can be contaminated, or the contaminated can be released further down in the soil. Leaks from buried tanks, sewage pipes or landfills usually cause the latter.
When removing contaminated soil, it’s important to consider also removing contaminated groundwater, and how the contaminants could have spread from your site to another via waterways or dust.
Contaminated soil can be harmful to people and animals through ingestion (of plants grown in the soil), breathing or absorption through the skin. It can have harmful health effects, and effects on the land use capabilities of the site.
Biological Treatment/ Remediation
Biological treatment or biological remediation is removing contaminated soil by using biological processes to degrade, transform, or otherwise remove contaminants from soil – the “biological processes” being bacteria or fungi. They use some contaminants as food sources, making them effective at breaking down:
- Halogenated organic solvents
- Plastic polymers
- Non-chlorinated pesticides and herbicides
- Nitrogen compounds
One of the ways to biologically remediate soil is by planting various plants that release substances through their roots which support the existing microorganisms in the soil in removing the contaminants. This is a process of phytoremediation called rhizosphere biodegradation.
Other biological treatments including phytoremediation include phyto-accumulation/ phyto-extraction, phyto-volatilization, and phyto-degradation.
Phyto-accumulation or phyto-extraction is planting plants that absorb the contaminants (usually water-soluble metals like lead), and then removing the plants from the site.
Phyto-volatilization is planting plants that absorb organic contaminants and release them into the air.
Phyto-degradation is planting plants that metabolise and destroy contaminants in their plant tissues.
Phytoremediation methods are especially effective against oil-contaminated soil.
Chemical methods for decontaminating soil include chemical oxidation, and soil stabilisation/ soil solidification/ chemical fixation
Chemical oxidation converts contaminated soils into non-hazardous soils by injecting chemical oxidants into the soil or groundwater. This can be done where the soil already is, rather than removing the contaminated soil before decontaminating. Chemical oxidants destroy TPH, BTEX and PCBs, as these contaminants react to oxidants.
During soil stabilisation/ soil solidification/ chemical fixation, immobilising agents are added to the soil, effectively locking it in. This reduces the soil’s leachability or bioavailability so contaminants can’t reach any receptors, converting it to a less dangerous form. Soil stabilisation also gives the ground higher resistance and lower permeability, making it more suitable for construction work.
One method of soil stabilisation is phyto-stabilisation, which is planting plants that produce immobilising compounds, keeping the contaminants harmless and in place. Phytoremediation methods are especially effective against oil-contaminated soil.
Physical soil decontaminating methods include soil washing and solvent extraction. Both of these methods require removing the contaminated soil before decontaminating.
When soil is washed with a liquid wash solution, fine-grained soils like silts and clays are washed away along with the contaminants (as many contaminants bind to fine soils), while the coarse-grained soils such as sands and gravels are kept and safely reused. Soil washing doesn’t destroy or remove the contaminants from the fine-grained soils, however, so these must still be disposed of in a licenced facility.
Solvent extraction essentially separates organic and inorganic compounds by adding them to liquids that don’t mix together, such as water and organic solvents, and shaking the solution. The organic compounds will be more soluble in the organic solvents, and the inorganic compounds will be more soluble in the water. Once the water is removed and disposed of, the organic solvent can be distilled down to organic compounds and be reused. The aim is to get rid of the inorganic compounds (which are inorganic contaminants) and keep the organic ones. However, this method also doesn’t destroy or remove the contaminants from the water, so it must be disposed of in a licenced facility.
Thermal methods of soil remediation include pyrolysis/ thermal desorption, and thermal destruction. The contaminates soil needs to be removed before these methods are performed.
Thermal desorption (AKA pyrolysis) uses heat to increase the volatility of contaminants to a degree that they can be separated from the soil. After separation, the contaminants can be destroyed using direct thermal destruction – which is akin to incineration.
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