Soil Restoration: Cleaning Contaminated Earth

Soil pollution is a common problem, especially in urban areas and former industrial sites, where human activities such as manufacturing, industrial dumping, and waste disposal can lead to the presence of harmful substances in the soil. Soil contamination can have serious implications for human health and the environment, and it is crucial to address and treat polluted soil to mitigate these risks. The good news is that polluted soil can indeed be cleaned and restored through various remediation techniques. The specific approach depends on factors such as the type and extent of the contamination, the location, and the intended land use. Common methods include soil containment, landfill capping, bioremediation, chemical oxidation, soil washing, and thermal treatment. Each technique has its advantages and considerations, and selecting the appropriate method ensures effective and sustainable soil remediation.

Soil containment

To achieve this, heavy-duty liners are placed under and around the contaminated soil to prevent the migration of contaminants into the surrounding soil or groundwater. These liners must be periodically checked for damage.

If soil containment is chosen as a strategy, additional measures must be taken, including periodic inspections for settlement, the monitoring of groundwater and surface water, and the collection of additional samples over time. If these precautions are taken, containment can often meet any long-term and short-term exposure prevention goals.

A landfill cap can then be added to serve as a barrier layer that prevents toxic effects on nearby areas. This cap can be made of varying materials depending on the type of contamination. For example, the cap could be constructed using a layering system of vegetated soil and the use of plants or asphalt concrete.

Landfill caps

The design of landfill caps varies depending on the site and its intended functions. They can range from a simple one-layer system of vegetated soil to a complex multi-layer system of soils and geosynthetics. The complexity of the system depends on the climate, with less complex systems being used in dry climates and more complex systems in wet climates. The cap's critical components are the barrier layer and the drainage layer.

The barrier layer is constructed using low-permeability materials like clay or geosynthetic clay liners (GCLs) to prevent water from passing into the waste. The most effective single-layer caps are made of concrete or asphalt, which form a strong barrier. On the other hand, multi-layered caps consist of a vegetative layer, a drainage layer, and a low-permeability layer of compacted clay and a synthetic liner.

The drainage layer is made of high-permeability materials that carry away water that percolates into the cap. The slope of the cap is essential for optimal performance, as flat areas can limit drainage. The thickness of the layers also matters, with capillary layers typically ranging from 0.30 to 0.60 metres and capillary blocks from 0.15 to 0.30 metres.

In addition to these layers, other materials may be used to increase slope stability and control gas emissions. Landfill caps can be temporary or permanent, and they are most effective when the underlying waste is above the water table. While a cap alone cannot prevent groundwater from flowing horizontally through the waste, it can prevent the vertical entry of water.

Bioremediation

The process involves stimulating the growth of microbes that use contaminants as their energy and food source. These microbes convert contaminants into harmless substances such as water and gases like carbon dioxide. Bioremediation requires the right temperature, nutrients and foods to be successful. Amendments such as molasses, vegetable oil or air can be added to the environment to create optimal conditions for microbes to flourish and accelerate the process.

The process can take several months to several years, depending on the size of the contaminated area, the concentration of contaminants, temperature, soil density and whether bioremediation is done in situ or ex situ.

Chemical oxidation

During chemical oxidation, a reactive chemical oxidant is injected into the contaminated soil to destroy the pollutants. This method is particularly useful for treating organic contaminants like TPH (total petroleum hydrocarbons), BTEX (benzene, toluene, ethylbenzene, and xylene), and PCBs (persistent organic pollutants). It is also effective in treating groundwater contamination, as the oxidant can be added to groundwater, allowing it to permeate through the soil.

The chemical oxidation process involves the use of oxidizing agents, such as hydrogen peroxide, hypochlorite, chlorine dioxide, potassium permanganate, and ozone. These agents are commonly used for wastewater purification. The choice of oxidizing agent depends on the specific contamination being treated.

One example of chemical oxidation is the use of Peroxone, a combination of hydrogen peroxide and ozone, to treat soils contaminated with Total Petroleum Hydrocarbons (TPH). This process involves the simultaneous oxidation of the pollutants by the exchange of electrons, which activates the decomposition of hydrogen peroxide into OH radicals. These radicals then react with the organic compounds, breaking them down into harmless products like water (H2O) and carbon dioxide (CO2).

Another variation is the Fenton process, which involves the use of Fenton's reagent, along with ultraviolet rays, to activate the decomposition of hydrogen peroxide into OH radicals.

The benefits of chemical oxidation include its ability to treat deep-level soil contamination and its relatively short application time. However, a drawback is the potential impact on indigenous soil microbial diversity. While some studies show that chemical oxidation can inhibit microbial diversity and delay population recovery, others indicate that microbial populations can recover and regenerate contaminant degradation abilities over time.

Overall, chemical oxidation is a viable method for treating polluted soils, particularly those contaminated with hydrocarbons and organic compounds.

Soil washing

The soil washing process can be broken down into six steps:

• Pretreatment: The soil is pretreated to remove debris and large objects such as rocks. Larger materials may be crushed if contaminants are known to be present in them.
• Separation: The highly contaminated fine-grained material is separated from the coarse material using physical separation methods such as vibratory screens, hydrocyclones, or magnetic separators.
• Coarse grain treatment: The coarse material is treated to remove any remaining contaminants. Common treatments include attrition scrubbing or high-pressure wash sprays.
• Fine grain treatment: The fine-grained material is treated with surfactants, solvents, acids, or bases to facilitate the separation of contaminants.
• Process water treatment: The wash water is treated to remove fine materials and contaminants before disposal.
• Residuals management: The fine grain sediment, which contains high concentrations of contaminants, is treated and/or disposed of. Treatment methods include thermal desorption, incineration, composting, land farming, solidification, and stabilisation.

Frequently asked questions