Since soil is a nonrenewable natural resource, it is crucial for plant development, biomass decomposition and recycling. Provided that humans eat food that is cultivated in soil, it is crucial for Earth's survival. Additionally, it gives us clothing, papermaking fiber, and fuel wood. Although it recycles nutrients and serves as a habitat for bacteria but is ignored. The accumulation of harmful substances (radioactive materials, toxic compounds, chemicals, and pharmaceutical residues) in the soil that have negative effects on plants and animals is referred to as soil contamination. Industrial processes, agricultural practices, mining operations, home wastes, hospital wastes and unrestricted nuclear facilities are the main contributors of these contaminants. These contaminants are hazardous and pose a serious risk to the environment, food safety and human life.

Techniques for soil pollution remediation are offered by emerging research and technology. These methods, which include physical, biological, thermal, combined, and chemical cleanup, can be used in-situ or ex-situ.

Chemical Treatment

Around the world, several approaches, and techniques (physical, chemical, and biological) have been used to remediate contaminated soil. In chemical remediation, certain chemicals that can either chemically transform or eliminate organic contaminants are introduced to the polluted soil. These treatment procedures primarily use hydrogen peroxide and CO2 oxidation as chemical oxidants to eliminate contaminants. These techniques' efficacy depends on the type of pollutant, the organic matter present in the soil, and the texture of the soil. They have drawn a lot of interest from scientists due to their quick restoration process and economical nature. There are two types of chemical treatment: in situ and ex situ.

In Situ Chemical Remediation Technologies

The in situ chemical technologies for soil remediation are as follows:

·         Chemical Oxidation

·         Lasagna Process

·         Dechlorination

·         Soil flushing

·         Chemical fixation

·         Stabilization/ Solidification (S/S) method

Chemical Oxidation

In situ chemical oxidation is a remediation technique that is frequently used to destroy a variety of contaminants (heavy metals, hydrocarbons) found in soil. The most often employed oxidants in this method are potassium manganite and hydrogen peroxide-based Fenton's reagent, which are injected or mixed directly into the soil and groundwater to oxidize contaminants. Although it is utilized less commonly, ozone can also be employed as an oxidant. When a material accepts an electron to oxidize a target species, its oxidation state is reduced. The toxicity of the initial pollutants is subsequently removed by oxidizing the target species. Hot water under extreme pressure (between 250 and 300 °C) has also been employed as an oxidant (extracting agent). The process is known as supercritical water oxidation (SWO). In situ chemical oxidation uses a strong catalyst to ensure efficient oxidation and the use of safe chemicals in the fields while extending the life of the treatment.

Lasagna Process

Due to its layered structure and capacity to combine in-situ treatment methods with electro-kinetics contamination transfer, the Lasagna in-situ technology has been named and developed to treat polluted soils with low permeability.

During the Lasagna soil remediation process, electrodes are placed on the outer edges of a contaminated area. Electro-osmotic flow is created as direct current is applied to the electrodes, which causes the water to move through the soil. Contaminants that are water soluble are transported from the cathode to the anode. The pollutants are either caught or eliminated as they pass through the treatment zone situated along the flow path.

The clayey soil that has been contaminated with soluble organic chemicals can be effectively treated by combining electro osmosis within situ bioremediation. The method depends on a number of factors, including the electric field strength, chemical loading, type of soil, type of microorganisms, and either pre-growth on the support or inoculation into the treatment zone, as well as PH, permeability, adsorption and buffering capacity, and geochemical process (like acid-base reaction, dissolution or precipitation, redox reactions, complexation and speciation).

In Situ Chemical Flushing

In situ chemical flushing is a cutting-edge method for treating contaminated soil and ground water. It involves injecting chemicals through vertical wells into the polluted zone, which is typically the vadose zone (soil above the water table) or saturated zone, or occasionally both. Before being discharged or reinjected, the injected solution passes through the contaminated area, where it is treated and removed.

By making the pollutants more solubilized and mobile, the in situ flushing process aims to enhance the traditional pump and treat remediation strategy by accelerating the procedure. This technology is the cutting edge-method for treating low-solubility substances like Dense Non-Aqueous Phase Liquid (DNAPL), which can persist in soil for decades.

The efficiency of the chemical flushing remediation is directly related to the type and quantity of the flushing chemical introduced to the soil. An excellent flushing agent for getting petroleum hydrocarbons out of the soil and groundwater is surfactant.

The process is less expensive. However, this remediation method is not suitable for porous soils and is not advised for clay soils.

Anaerobic Dechlorination

Most of the global contamination may be caused by organic molecules that contain chlorines. By blocking the enzymatic attack site, chlorine frequently prevents aerobic bacteria from degrading these substances, allowing the pollutants to persist in the environment. Over the past ten years, awareness of anaerobic bacteria' ability to reductively dechlorinate most of these chemicals has increased.

Numerous commercial Aroclors include highly chlorinated PCBs that are resistant to aerobic breakdown until they undergo anaerobic microbial dechlorination, which dechlorinates them to a substantial extent.

The dechlorination of PCBs is affected by: PCB concentration, Bioavailability, Inhibitors, Temperature and Nutrients.

Dechlorination is therefore a feasible, effective, and economical process to remove chlorinated substances from the environment.

Chemical Fixation

Chemical fixation is the process of introducing chemicals or other materials to contaminated soil in order to stable the contaminants in a less hazardous form and lower their risk to the environment, as well as their bioavailability and mobility. The treatment procedure is a proven way to change a waste's chemical, physical, or compositional characteristics. By transforming the targeted pollutants into their least soluble, mobile, or poisonous formulation, it lowers the risk potential of certain wastes.

Stabilization/ Solidification (S/S) Technology

Stabilization/solidification (S/S) technologies treat hazardous, radioactive, and mixed wastes using chemical, physical, or a combination of both. While stabilization techniques lower the hazardous potential of waste by making pollutants less mobile, soluble, or toxic, solidification techniques compress the waste into a compact solid state.

In conclusion, this review discusses different in situ chemical remediation technologies for soil contamination. The effort is made to highlight efficient and cost-effective in situ chemical technologies to remediate heavy metals, organic and inorganic contaminants present in the soil. Furthermore, it also focuses on different chemicals and reagents (treatment mixtures and combinations) used in studies to reduce and remove pollutants present in soil.

Keywords: Insitu Techniques, Chemical Remediation, Soil Treatment

About the Author: Muhammad Tahir is a postgraduate scholar in environmental sciences and deeply passionate about environmental studies and eager to pursue opportunities to deepen his knowledge and engagement in this area to make positive changes in this world.