Soil contamination has dangerous effects for the human health and agricultural activities. Largest portion of pollution sources introduced to soil is due to anthropogenic activities and electronic industries. If the soil pollution is found to be at a risk level after the sampling and monitoring of the soil area, remediation technologies are determined to be used. The goal of remediation is removing or reducing the pollutants from soil. There are various remediation technologies with different working principles, their effectiveness and cost in separate categorizations. To choose the most beneficial and available technology that can be applied at high efficiency among types of remediation technologies, characterization of site, contamination type, the goal of removal percentage and extend cost effectiveness should be considered.

Remediation of soil through various technologies is the result of sampling and monitoring when the pollution is found to be at a risk level that requires intervention to maintain the favorable environmental conditions for all living creatures. [1]

The main objective of remediation technologies is to bring the polluted soil to sustainable environmental conditions and to reduce the pollutants to minimum as possible. If the pollutants cannot be removed, then their uptake must be prevented by some other prevention focused remediation technologies. [1]

Regards to the scale of pollution, the risk level and financial availability as well as the time constraints on a contaminant removal project or an overall treatment of soil may take place right on the polluted place which is an in-situ method or the polluted soil may be completely excavated and transported to another area to be remediated in special facilities where has designed tanks and prepared beds for any coherent remediation technique. [1]

In the scope of remediation technologies

· Biological,

 · Thermal,

 · Physical,

· Chemical Remediation Techniques can be pointed out.

Remediation techniques can be conducted as two types as following: in situ techniques and ex-situ techniques. [2]

In-situ techniques are applied to the contamination source in the area of the source. They generally do not require excavation and transportation of contaminated soil. These kind of techniques disturbs soil less and their treatment cost is also less in contrast to ex-situ systems. However, for the installing and monitoring of these remediation devices may cost higher than expected depending on the pollution case. In the scope of in-situ remediation techniques, weather, soil permeability, contamination depth, and potential deep leaching of chemicals should be taken into account carefully. [2]

Ex-situ techniques includes the excavation of soil from the contaminated site and transportation of the soil to an off-site facility for treatment or disposal. Ex-situ techniques requires additional costs for soil excavation, transport disposal and site-refilling. This type of techniques are able to achieve better results in a shorter tie period than in-situ techniques. [2]

Either in-situ or ex-situ, the main goal of any remediation technique should be contaminant removal/extraction rather than immobilization and containment. [2]

Biological Remediation

1)      Bioventing

Bioventing is one of the in-situ technologies for vadose zone (has low water content relative to the saturated zone below the water table and is therefore commonly referred to as the unsaturated zone) remediation whereby uses indigenous microorganisms present in the soil to degrade contaminants and treat the soil. This degradation occurs when sufficient oxygen, adequate amount of nutrients and appropriate level of moisture are supplied to stimulate those microorganism. After degradation of hydrocarbons, they are converted into less harmful compounds. This conversion encourages the minimization of volatilization while optimizing the biodegradation. [3]

Bioventing has become a reliable technique since it has simplicity, operates at high efficiency, has lower cost than other technologies and causes minimal site disturbance yet the most important disadvantage of bioventing is extended time required for remediation. In addition to that, bioventing needs providence of nutrient and oxygen to maintain microbial activity. [3]

In the USA, bioventing has been approved in 38 states and in all 10 Environmental Protection Agency (EPA) regions. [4]

1)      Bio-stimulation

Bio-stimulation is one of the natural remediation techniques where nutrients such as phosphorus, nitrogen, oxygen, or carbon is supplied to the soil to improve the biodegradation of the pollutants. There are factors that might limit the bio-stimulation in soil like, microorganism type, nutrients, pH, temperature, moisture, oxygen, soil properties and contaminant presence. Lack of these factors lead to remediation process to get inefficient. To determine the optimum ranges of these factors, properties of the contaminated site and contaminants must be known. This is a pretreatment investigation to remediate the soil in the most efficient way. Identifying the nutrients and standardization of them also helps to improve the biodegradation of the contaminants by indigenous microorganism at the contaminated area.

2)      Vermiremediation

Vermiremediation term is used to define the use of earthworm species in the removal of contaminated soils or in the degradation of non-recyclable compounds. Earthworms are highly able to survive from contaminants in soil such as pesticides, herbicides and PAHs. At least when the concentration is maintained at tolerable level. Besides, they are also able to accelerate the removal process of pollutants by changing the properties of soil both physically and chemically. They increase the available carbon and nitrogen in the soil by their excreted body fluids such as urine and mucus, ingesting soil with organic materials during transition of the nutrient towards their gut and by their burrowing ability. [7]

However, earthworms need well-defined environmental conditions to provide this remediation. Firstly, they need enough nutritional substance to be active enough to remediate. To assure their food sources organic wastes, manure or wastewater sludge can be applied on contamination site. Secondly, the earthworms react unfavorably to extreme soil conditions such as extreme salt, pH and heavy metal content. Therefore, extreme conditioned soils may not be remediated by this technique or the contamination site may need a preliminary condition adjustment for earthworms. Thirdly, soil climatic conditions should support a proper environment for earthworms while also water content (moisture) should also be adequate for their activity. [7]

Eisenia fetida are the most commercialized earthworm species since their biology and ecology are well described. In addition to that they are also commercially available and easy to keep in laboratory environments which is the main reason why they are the most studied type of earthworms in remediation techniques. [9]

3)      Phytoremediation

Phytoremediation is a technique which utilizes plants to remove, transfer stabilize or degrade contaminants in soil, sediment or even water bodies. It is considered to be cost-effective, promising and environmentally friendly technology. Phytoremediation term includes two words which comes from Latin and can be explained as “phyto”- meaning- plant and “remedium” which is a suffix meaning to clean or restore. This combination can refers collection of plants that are either naturally occurring, or genetically engineered so as to provide treatment for contaminated environments. [10]

Some plants which are exposed to metalliferous soil as they are growing up acquires an ability to tolerate and even accumulate high concentrations of metallic compounds without demonstrating any sign of toxicity caused by metals. These plants are the basis of phytoremediation technique. [10]

Depending on the contaminant type and site conditions, the phytoremediation technique may change as for containment (phyto-immobilization and phytostabilization) or removal (phytoextraction and phytovolatilization) purposes. [10]

Thermal Remediation

1)      Surface Capping

Surface capping is a simple remediation method that covers up the contaminated zone with a water-proof material to form a stable protection surface so that the uptake of contaminants through dermal contact, ingestion, inhalation etc. can be prevented. This technique is actually not a remediation method as no efforts for removal of heavy metals are made. Regardless to the fact that it is not a remediation method, it eliminates the risk of exposure enough. The surface cap also serves as an impermeable filter to prevent further contamination of groundwater. However, the capped soil loses its quality to support soil ecosystem especially in the scope of supporting plant growth. This capped area can only be used for civil purposes if it is going to be utilized for any purpose. [2]

 

Surface capping is a remediation method for highly polluted soils which is decided to be not used again for agricultural purposes. This method is only applicable to small areas since the construction of proof material can become harder to cover up as the area becomes larger. The depth and seasonal fluctuation of soil and the slope of soil should be taken into account as designing capping material and constructing it so that the sliding risk can be reduced to safe level. [2]

1)      Electro-Kinetic Extraction

Electro-kinetic extraction is the technique of removing heavy metals from pollution site by electrical adsorption. As the low level of direct current is applied to soil via electrodes inserted into the ground cation in the soil begins to migrate towards cathode while the anions in the soil starts to migrate towards anode at the attractive force of acquired electrical field. After the metal contaminants are concentrated on their polarized electrodes, they are either precipitated, electroplated or pumped towards to above of soil via extraction wells as to complete removal procedure. [2]

Electrokinetic extraction of heavy metals involves desorption/ dissolution followed by transport. When the concentration of heavy metals in the soil solution becomes below the soil sorption capacity, chemical additives are typically needed to help mobilize sorbed metals. Higher energy consumption is additionally required to facilitate the transport of low-concentration metal ions. [2] Currently, the electro-kinetic remediation technique is still at the developing phase. There are number of demonstrations and pilot-scale projects employing the technique, but full-scale applications are rarely witnessed. [2]

Chemical Remediation

1)      Soil Washing

The purpose of this technique is to remove hazardous contaminants from soils by mechanical agitation or reduce pollutant volume by adding water or some chemicals depending on the type of pollutant. Pollutants are aimed to attach to fine soil materials (i.e. sand and gravel) rather than coarse soil materials (i.e. silt and clay). This methods is an ex-situ technique. Ex-situ means the cleaning of the contaminated soil in a place other than the environment. Most of the pollutants are usually found in fines and clean sand and gravel after washing leave materials that are easy to reuse or destroy. [2]

1)      Soil Flushing

Soil flushing method is an in-situ removal technique to detract from soil by propagating an extraction fluid inside through soil. After that, extraction fluid is recovered to be reused and eventually should be treated. Groundwater and extraction fluids which recovered from adsorbed pollutants are subjected to public wastewater treatment processes or reworked to meet appropriate discharge standards before being recycled to streams. [12]

Generally, the technique is based on injecting or infiltrating extraction fluid into soil. The formula of the soil can be altered depending on the target contaminant. Especially, increasing the efficiency in heavy metal removal is tried to be reached by changing the formula with various chelating compounds and acidic solutions. The tests conducted to observe most efficient agent have concluded as EDTA. [2]

So overall, to reclaim contaminated soils or to remove the contamination from the soil, remediation technologies are used. These technologies differ in their place of application which are in situ remediation and ex situ remediation. In situ remediation is less costly than ex situ remediation. Other than solidifying contaminants inside the polluted soil, extracting the pollutants from soil is more favored thought it takes longer time. The capacity of application of the remediation methods can be started with characterization of site geography, contamination type in site, the goal of removal percentage and extend cost effectiveness in correlation with available budget feasibility requirements and public acceptability. To choose the most beneficial and efficient technology these factors should be taken into account.

Thank you for reading!

References

1] P. D. Marsal, Soil Pollution, Origin, Monitoring, Remediation, Verlag Berlin Heidelberg: Springer, 2008.

[ 2] W. L. W. S. M. G. Lianwen Liu, «Remediation techniques for heavy metalcontaminated soils: Principles and applicability,» Science of the Total Environment, no. 633, pp. 206-219, 2018.

[ 3] R. G. Z. Stella M. C. Bezerra, «Bioventing of Gasoline-Contaminated Soil:Some Aspects for Optimization,» ResearchGate, 2014.

[ 4] M. S. H. F. M. M. K. (. M.-M. Mohammad Mehdi Amin1, «Soil remediation via bioventing, vapor extraction and transition regime between vapor extraction and bioventing,» International Journal of Environmental Health Engineering, cilt 2, no. 6, 2013.

 [ 5] L. Molina-Barahona, R. Rodrigues-Vazquez and M. Hernandez-Velasco, "Diesel removal from contaminated soils by biostimulation and supplementation with crop residues," Applied Soil Ecology, pp. 165-175, 2004.

 [ 6] M. Tyagi, M. M. R. d. Fonseca and C. C. C. R. d. Carvalho, "Bioaugmentation and biostimulation strategies to improve the effectiveness of bioremediation processes," Biodegradation, pp. 231-241, 2010.

[ 7] L. D. D. A.-B. M. C.-R. Jacobo Rodriguez-Camposa, «Potential of earthworms to accelerate removal of organic contaminants from soil: A review,» Applied Soil Ecology, no. 79, pp. 10-25, 2014.

[ 8] *. G. C. H. b. D. B. G. a. Armin Kavehei a, «Effects of red earthworms (Eisenia fetida) on leachability of lead minerals in soil,» Environmental Pollution, no. 237, pp. 851-857, 2018. 26 [ 9] I. F. A. Ogheneruemu Abraham Ekperusi, «Bioremediation of heavy metals and petroleum hydrocarbons in diesel contaminated soil with the earthworm: Eudrilus eugeniae,» Springer Plus, cilt 4, no. 540, 2015.

 [ 1 0] P. K. P. &. L. Y. Li, «Phytoremediation Technology: Hyper-accumulation: Metals in Plants,» Water Air Soil Pollution, no. 184, pp. 105-126, 2007.

[ 1 1] C. M. R. Lee A. Newman, «Phytodegradation of organic compounds,» Current Opinion in Biotechnology, cilt 15, no. 3, pp. 225-230, 2004.

[12] R. H. E. Lombi, «REMEDIATION OF POLLUTED SOILS: Soil-Flushing,» Encyclopedia of Soils in the Environment, 2005.