Soil Contamination: Cleaning Up Polluted Earth

Soil contamination, also known as soil pollution or land pollution, is caused by the presence of xenobiotic (human-made) chemicals or other alterations in the natural soil environment. It is typically caused by industrial activity, agricultural chemicals, or improper disposal of waste. Soil contamination can have serious health implications and significantly limit potential land use. The cleanup or remediation of polluted soil involves various methods such as excavation and disposal, bioremediation, chemical oxidation, soil washing, thermal treatment, and soil stabilisation. The chosen method depends on factors including the type and extent of contamination, location, and intended land use.

Bioremediation: Using bacteria and fungi to remove contaminants

Bioremediation is a process that uses biological processes to degrade, transform, or remove contaminants from soil and water. This natural and sustainable remediation technique uses organic materials to initiate biological processes in the soil that eventually remove contaminants.

Bioremediation is often used to mitigate pollutants like halogenated organic solvents and compounds, hydrocarbons, nitrogen compounds, some metals, and non-chlorinated pesticides and herbicides. This process is preferred over chemical methods as it avoids the use of additional chemicals and is environmentally friendly. However, it can take months or even years to complete.

During bioremediation, microorganisms such as bacteria and fungi are used because they use the contaminants as a food source. These microorganisms break down the contaminants through biological activity, removing or immobilizing soil pollutants. For example, bacteria can break down substances like pesticides in the soil, while fungi can use contaminants as a source of nutrients.

Bioremediation is particularly useful for treating contamination that occurs at deep levels, such as total petroleum hydrocarbons (TPH), benzene, toluene, ethylbenzene, xylene (BTEX), and persistent organic pollutants (POPs). This process is also effective for treating organic contaminants in difficult-to-access areas, such as soils beneath buildings.

While bioremediation is a preferred method for its environmental benefits and cost-effectiveness, it is important to note that it is a slow process. In cases where rapid remediation is required, such as during a leak, chemical processes like chemical oxidation may be more suitable.

Chemical oxidation: Injecting reactive chemical oxidants to destroy contaminants

Chemical oxidation is a process that involves injecting reactive chemical oxidants into contaminated soil to destroy contaminants. This method is particularly useful when contamination occurs at deep levels below a structure, and it can be used to treat a variety of organic contaminants, including total petroleum hydrocarbons (TPH), benzene, toluene, ethylbenzene, xylene (BTEX), and persistent organic pollutants (PCBs).

This process is often chosen for its versatility and ability to treat contaminants in difficult-to-access areas. It is also effective at remediating chlorinated solvents and petroleum products. When selecting an oxidant, it is important to consider factors such as the compatibility of the injected form with the targeted lithology and the safety of its use on the specific project site. The physical form of the oxidant should be chosen based on factors like lithology, contaminant mass, and injection approach.

Before beginning an in-situ chemical oxidation (ISCO) project, it is crucial to understand how the contaminant mass is distributed based on soil type, geology, lithology, and hydrogeology. This knowledge helps in efficiently targeting the treatment zone and avoiding unnecessary costs. Additionally, the success of ISCO relies on the efficient delivery of reagents to the contaminated area. The residence time of reagents in the contaminated area should also be considered to optimize their delivery efficiency.

Safety is a critical aspect of the chemical oxidation process. Workers handling oxidizing chemicals must avoid skin and eye contact, prevent ingestion, and refrain from breathing dust or mist. Stringent personal protective equipment (PPE) requirements, tooling compatibility, and site-specific job hazard analysis (JHAs) and health and safety programs (HASPs) are essential components of the safety planning process.

Chemical oxidation is a widely used remediation technology, especially in China, where it is applied to large-scale polluted sites. It is considered a mature, economical, and effective method for treating contaminated areas. However, it is important to monitor the impact of ISCO on environmental quality, as the decomposition products of some oxidants may exceed environmental standards.

Soil washing: Using a liquid wash solution to remove fine-grained soils and contaminants

Soil washing is an effective method for removing hazardous contaminants from soil. The process involves washing the soil with a liquid wash solution, scrubbing the soil, and then separating the clean soil from the contaminated washwater. The wash solution is typically comprised of cleaning fluid and water, and sometimes chemical additives.

The theory behind soil washing is that contaminants are more likely to bind to fine-grained soils, such as silts and clays, which, in turn, are more likely to bind to coarse-grained soils. Therefore, the goal of soil washing is to separate these contaminated fine-grained soils from the cleaned coarse-grained soils, and to remove the contaminants from the fine soils into the washing solution.

The process of soil washing can be divided into several steps:

• Pretreatment: The soil is pretreated to remove debris and large objects, such as rocks, through physical separation methods.
• Washing: The soil is then washed with the liquid wash solution, which may include chemical additives, to remove the contaminants from the fine-grained soils.
• Scrubbing: The soil is vigorously mixed with the wash solution, and the contaminants are desorbed from the soil particle surface.
• Separation: The clean coarse-grained soils are separated from the contaminated fine-grained soils and washwater.
• Treatment: The contaminated washwater is treated with appropriate technologies based on the types and concentrations of contaminants present. Treatment options may include sand filters, coagulation, flocculation, dissolved air flotation, and granular activated carbon, among others.
• Disposal: The contaminated fine-grained soils and treated washwater are disposed of as necessary, following local regulations and requirements.

Soil washing is a cost-effective technique as it reduces the amount of material that requires further treatment. It is particularly effective for sandy and gravelly sediments, while fine-grained sediments can be more challenging to decontaminate. In such cases, mechanical separation methods may be used to remove the water from the soil. Additionally, surfactants can be added to the wash solution to facilitate separation and enhance the performance of washing.

Soil stabilisation: Adding immobilising agents to reduce contaminants' leachability and bioavailability

Soil contamination can have serious health implications for humans, animals, and plants, and can significantly limit land use. The most appropriate remediation technique depends on the type and level of contaminants present, the physical attributes of the site, and the remediation objectives.

Soil stabilisation is a remediation technique that involves adding immobilising agents to reduce contaminants' leachability and bioavailability. This technique can be used to treat a wide range of inorganic and organic contaminants, including metals, asbestos, radionuclides, and pesticides. Stabilisation can be achieved in two main ways:

• Modifying the contaminant in the ground to a less dangerous form: This process involves stabilising the contaminant in the ground to make it less dangerous. For example, common cementing agents such as Portland cement and lime can be added to the soil to reduce the solubility, bioavailability, and leachability of contaminants.
• Solidification: This process involves reducing the mobility of the contaminant by binding it in place so it can't reach any receptors. For example, soil washing uses a liquid wash solution to separate and remove fine-grained soils, such as silts and clays, along with the contaminants that are bound to them.

Soil stabilisation can be performed in situ (in the ground) or ex situ (out of the ground), depending on the aim of the process and the regulatory program under which the work is performed. It can also improve the geotechnical competency of the ground, making it more suitable for construction work due to higher resistance and lower permeability.

Before remediation work begins, it is critical to develop a customised soil transportation plan, complete any necessary regulatory paperwork, and ensure proper disposal of contaminated soil according to local regulations and environmental protection standards.

Thermal treatment: Using heat to destroy or move contaminants

Thermal treatment is a method of remediating contaminated soil that involves the use of heat to destroy or move contaminants. This process can be applied in situ, meaning directly on-site, without the need for excavation or transportation of the polluted soil, which results in significant cost savings.

There are several techniques used in thermal treatment to introduce heat into the contaminated soil. One method is electrical resistance heating, which employs arrays of electrodes to generate a concentrated flow of current, producing temperatures greater than 100°C. This rapid form of remediation can effectively treat soil and groundwater in less than 40 days. Another technique is thermal conduction, also known as electrical conductive heating or in situ thermal desorption, where heat is supplied to the soil through steel wells or by covering the ground surface with a blanket. The use of a blanket is more suitable for shallowly polluted soil, while steel wells are utilised for deeper contamination.

During the thermal treatment process, contaminants may be destroyed or moved. Some contaminants are destroyed by the high temperatures, while others are transformed into gases (volatilised) and become more mobile. These volatilised contaminants can then be extracted through collection wells and transported to an ex situ treatment unit for further cleanup. This makes thermal treatment particularly effective for dense or light nonaqueous phase liquids (DNAPLs or LNAPLs).

While thermal treatment offers quick and reliable remediation, it also has some drawbacks. The high temperatures required can be energy-intensive and may damage soil properties. Additionally, the environmental compatibility and sustainability of these technologies have not been extensively explored. As a result, there is no universally appropriate thermal treatment technology, and the choice of method depends on the specific contamination scenario and site characteristics.

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