Soil Remediation Technologies

1.     Introduction

The industrial and social accidents are considered to be the major reasons of pollutants to enter into the environment; these accidents may include waste disposal leakages, transportation spills and storage leakages in industrial facilities. The petroleum contaminated sites are reported to have clean up problems significantly and needed to develop the soil and water remediation technologies on the basis of new technologies. The quick, feasible and deployable remediation technologies are required to be developed which are applicable with the wide range of the physical settings of numerous ongoing processes. The potential dangers have been recognized by public and government industry that the chemical ingredients included PCBs (polychloro biphenyls), TPH (total petroleum hydrocarbons), heavy metals, pesticides and PAHs (polycyclic aromatic hydrocarbons) which are subjected directly to the environment and the human health.

Many remediation technologies have been developed in order to treat the environmental contaminations including soil, wastewater, leachate and ground water which have been contaminated by many pollutants (Riser-Roberts, 1998). In particular contaminated soil conditions, a combination of remediation technologies can be used to achieve the optimum condition for the prevailing circumstances. The contamination can be reduced to an acceptable and safe level by deploying chemical, physical and biological remediation technologies in conjunction with one another (Reddy et al., 1999; RAAG, 2000). This report describes the numerous soil remediation technologies and human impacts on the environment. The sources of ground pollutions have been assessed with the respect of the human activities.

The technology has been discussed with its applicability, concerns, processes, limitations, advantages and parameters specific to the site. The selection of the particular remediation technologies is greatly dependent upon the characteristics of the site under consideration, costs, and regulatory requirements with time constraints of projects (Riser-Roberts, 1998; Reddy et al., 1999).

The selection of the appropriate technology is important step for the successful remediation of the contaminated sites. The treatment of the contaminated soils is only dependent upon the proper design selection and its adjustment based on the varying contaminants and the properties of the soil with the performance of the applied system (Chareneau et al, 1995; API 1993; Alexander, 2003; Demars et al. 1995; Anderson 1995; USEPA, 1995–1998;  Reddy et al. 1999; RAAG, 2000; Feng et al., 2001; Hejazi, 2002; Kao et al. 2001; Li et al. 2002; Abbott et al. 2002; Guerin et al. 2002; Barnes; 2003; Chu 2003; Halmemies et al. 2003; Khan and Husain 2003; Juhasz et al. 2003; Liang et al. 2003; Wilk 2003; Wait and Thomas 2003).

2.     Human impacts on the environment

Much technological developments have been made in recent decade to benefit the human communities. The human engineered materials and inventions have improved the quality of life with the improved human survival rate. The improved technologies have helped humans to survive in the harsh habitats comfortably. The human communities are consuming water, food and oxygen and producing waste. Energy is being used for cooling, transportation, heating and for running industries and businesses. For survival humans interact with biotic and abiotic aspects of the environment like other living things ion earth.

During the development of the new technologies humans are less concerned about their impacts on the biotic and abiotic elements of the ecosystem or environment. Large quantities of waste materials have been reported to be produced by the human activities. Some of the waste is organic and can be returned to the soil by the waste recycling procedures. These recycling activities can remove the major quantities of contaminations from soil and thus the water underneath. Many communities prefer to send their wastes to the landfills, the landfill sites are found to be leaked under certain circumstances and produce toxic chemicals into the surrounding soil and water. Such toxic substances are found to cause the death of the living things. Human activities have impacted the following environmental elements

2.1.          Air

Human activities including transporting, heating and treating fossil fuels and industrial elements have polluted the air significantly. Such human activities have emitted large quantities of the greenhouse gases and thus the cause of the ozone layer depletion.

2.2.          Water

Human has affected the water level by its high consumption. The flow of water through the natural watersheds has been affected by the development of the human communities. The quality of water has also been affected by the casual and improper disposal of the industrial chemicals. Chemical spills have greatly poisoned the oceans, lakes, rivers and underground waters.

2.3.          Habitat

The human activities have destructed the natural habitat as fewer producers can be found within the food chains. These activities have lead to the extinction of numerous species that has disturbed the ecosystems greatly.

Figure 1: soil contamination and effects on water quality

3.     Sources of the ground pollution

The main sources of the land pollution include industrial factories, human sewage, nuclear and chemical plants, oil refineries, littering, landfills, debris construction, mining and oil spills etc.

The ground pollution has been resulted mainly due to the urbanization which has consumed forestland. The raw material is in great demand that has lead to the destruction and exploitation of the forests. Water is in ore demand that requires the construction of reservoirs thus loosing the land. Greater demand of food has also increased the agricultural land that also has destroyed the forests. Tons and tons of the domestic waste has been dumped every day, the improper dispose off of this waste can result in the devastating impacts on the ground water and soil. Incinerators are commonly used to treat the waste from homes, offices and industries; however a large amount of waste cannot be burnt and is termed as dumping grounds.  Such activities have increased the solid wastes and the quality of ground water and soil.

New dumping grounds are required to be found for the accommodation of the increasing wastes. Pesticides and herbicides are being used to improve the yield of the crop that has also polluted the ground and has washed out into the soil and water bodies. Land pollution has also been increased by the industrial activities. Huge holes are found to be drilled out during the open cast mining activities that have developed deep pools which have dangerous depths. The soil has been considered to be polluted by the mining wastes that may contain numerous poisonous substances.

4.     Soil remediation technologies

The researchers and practitioners have developed many soil remediation technologies; few of them are discussed in this section

4.1.          Soil washing

Water combined with other solvents is used to wash the soils that proceed to the scrubbing processes by various mechanical equipments. For the dissolution of the specific contaminants solvents are selected which are also required to be assessed for their health and environmental impacts (Asante-Duah, 1996; Feng et al., 2001; Chu and Chan, 2003; Urum et al., 2003). The coarse soil is typically separated from the fine soil by the soil washing process. The smaller soil particles sorbed and bind by the hydrocarbon contaminants which may separate the larger particles from the smaller ones and reduces the contaminated soil volume (Riser-Roberts, 1998).

The majority of the clay and particles of silt can be found in the smaller soil volume which can be subjected to the other treatment methods for further remediation of soil i.e. bioremediation and incineration. The larger volume of the soil will be considered as the clean volume and can be used to serve the purpose of backfill (USEPA, 1996a; RAAG, 2000; Chu and Chan, 2003). Other technologies can be combined with the soil washing for efficient decontamination of the soil.

4.2.          Soil vapor extraction

Vacuum extraction or soil venting which is also known as soil vapor extraction technique is another accepted, cost effective and recognized soil remediation technology. The technology is especially effective for the soil which is contaminated greatly by the SVOCs and VOCs (Suthersan, 1997; Zhan and Park, 2002; Halmemies et al., 2003). Horizontal or vertical wells are installed in this technique in the contaminated soil area. The evaporation process is aided by the use of the air blowers. The volatile components resulted from the contaminated mass can be evaporated by the vacuums that can be applied near the contamination source through wells.

Extraction wells after then used to remove them permanently. Carbon adsorption methods are then employed to treat the extracted vapors before their complete removal into the atmosphere (USEPA, 1995a). The biodegradation of the less volatile contaminants can be stimulated by the sub surfaces that are provided by the soil vapor extraction technique (USEPA, 1996b, 1998a; Halmemies et al., 2003; Harper et al., 2003). The procedure can also be used for the treatment of the contaminated ground water and stripping of air. The figure 2 showed a simplified form of the soil remediation technique.

Figure 2: soil vapor extraction process

4.3.          Chemical extraction

The process of separating hazardous components from the soil is called the chemical extraction process. The wastes present in sledges, soils and sediments are not to be destroyed when subjected to this technique. The remediation procedure reduces the soil volume. An extracting chemical is used in this technique and is different from the simple soil washing process which use only water or water additives. The process is varied according to the type of the equipment used, chemical employed and type of the operation considered (Barnes; 2003).

Figure 3: chemical extraction

The soil is usually graded in the fine and coarse fraction by the physical steps of separation before subjected to the remediation technique of the chemical extraction. An assumption is generally made to consider the fine part to contain more contaminants. The kinetics of extraction can be enhanced by the physical separation processes that separate the heavy metals and related particulates from the soil.

4.4.          Acid Extraction

Another effective extracting element is acid for decontamination oft eh soil. The contaminants of the heavy metals are usually removed by using the hydrochloric acid. The coarse solids are removed first from the soils through the physical screening procedures. In the extraction unit the acid can be introduced to the contaminated soil. Depending upon the type of the soil the residence time of the process is varied for particular unit, contaminants, soil type and the concentration of the contaminants. The mixture of the extracting element and the soil is pumped out of the mixing tank continuously. Hydro-cyclones are used to separate the extracting element and the soil. The soils are then transported to the rinse system after the extraction process. The metals and acids are then removed by rinsing the soil with rinsed water. The precipitants which are available commercially are used to generate the rinse waters and extraction solutions (Abbott et al. 2002). These participants include sodium hydroxide, lime, and other formulations with flocculants for the removal of metals and reformation of the acid. The concentrated form of the metals is highly suitable for the potential recovery. The water is then removed from the soils and any residual acid is neutralized by mixing it with fertilizers or lime.

4.5.          Solvent Extraction

A common form of the chemical extraction is a solvent extraction which uses organic solvents as the extracting elements. Depending upon the technique is increasingly use with other technologies i.e. stabilization, solidification, soil washing, and incineration. In some situations the technology has also been used as a standalone remediation process. The target contaminants can be removed with the extraction of the metals which are organically bounded with the soil. The treated soil matrix may contain the traces of solvent which have made the solvent toxicity a major consideration. After treating with the best demonstrated available technology the mixture is then returned to the site. The chemical extraction technology of the soil remediation has the medium term duration of maintenance and operation.

4.6.          Land disposal

Another waste treatment process is called land filling. Double liners are required to be built around the landfills that permit the introduction of the hazardous contaminants from the offsite, release response programs, collection of leachate, systems for the detection of any leakage and ground water monitoring (Barnes; 2003). The criteria of accepting variety of wastes have restricted the types to wastes to be disposed off in the landfills. The onsite remedial actions and generators are subjected to the landfill remedial units. The standards and permitted limits are required to be met by reducing the size or the requirements depending upon the conditions of the specific waste or the site (Halmemies et al. 2003). The disposal units are preferred to be left in the places which are prior to the promulgation of the predefined standards and rules of the waste disposal policies.

Figure 4: A typical landfill

4.7.          Incineration

The organic contaminants in the soil can be burned at high temperatures i.e. 870-1,200 °C in the presence of oxygen. The refractory and halogenated contaminants are burned and volatilized in the presence of oxygen at as much high temperature as 870 to 1,200 °C. The combustion is usually sustained and initiated by the auxiliary fuels.  The efficiency for the destruction and removal of the contaminants by the incinerators is usually 99.99%. The efficiency has been found to be exceeded up to 99.9999% for PCBs and dioxins (Halmemies et al. 2003). However further treatments are required by the combustion residuals and off gases.

Figure 5: incineration

5.     Discussions

For the remediation technologies the development is required in the methods for acceleration of the aquifer cleanups through soil treatments. The multi phase plumes may be found in the soil and ground water which have different directions to move. The contaminants in most of the circumstances are difficult to be located and thus small amounts of contaminants can become the reasons of contaminating the larger volume sites. The efficiency of the treatments processes depends upon the contaminants source and the time required remediating the soil and water bodies. From the source zone of contaminants the discharge of small and long term impacts for the contaminant discharge or the flux that is being emanating from the contaminant zone source. The ecological indicators are required for the comprehensive assessment of the strategies of the risk management because of the persistent presence of the sediment underscores and PCBs in soil.

The conditions of the ecosystems and the adverse effects or the impacts of the contaminants have been assessed through biological indictors. The contaminated sediments are required to be remediated through the effective indicators and risk management of the sediments which are contaminated through the strategies of the risk management (Wait and Thomas 2003). For the control and identification of the contaminants the useful indication and monitoring techniques have been developed in recent decade. The performance of the extractive elements can be accessed through evaluating procedures or the natural recovery techniques. The in situ remediation techniques are required to be developed continuously in the geological formations of difficult habitat. The evaluating programs have been developed in chemical oxidation and in situ thermal procedures according to the broad array conditions of the typical geological fields.

The effective technologies have been developed for the remediation of the soil contaminated with metals which has made the in situ procedures more in demand. Numerous standards are required to be set out for the level of industrial and house emissions to the ground water and soil. The contaminants of the soil are required to be processed through the remediation procedures which can also keep the waterways and other ground water bodies protected from the contamination. The natural background levels can be compared with the standards used in the recent remediation techniques and new standards can be adjusted.

The appropriate residential standards are required to be adjusted along with the industrial area zoned emissions emanating in the nearby colonies. The compliance are required to be monitored against all standards which is quite a critical practice and emissions are required to be detected and reported to the local community or the other authorities. The continued and significant breaches are important to be ensured for enforcement or a jail sentence for the persons responsible for the pollution. Continuous breaches are however not mush preferred terms than the compliance. The remediation technologies should be considered as a normal expense of running an industry or a business. The significant penalties can be developed for the polluters to stay aware of the environmental safety standards.

6.     Conclusion

After scrutinizing the evidence it has been concluded that human activities are responsible for the degradation of the land surfaces. The misuse of the land elements has leaded the practitioners to develop procedures for the assessment of the ground pollution. The improper disposal of the waste is the major cause of the land pollution. Exploitation of the minerals, improper disposal of the industrial and urban wastes, inadequate agricultural practices to use the soil properly are considered to be the major factors ground pollution. The natural habitats have been destroyed and environments are polluted by the industrial revelation which has set out the series of events into motion and developed hazardous impacts for animals and human beings.

There is large number of soil remediation technologies available. However a variety of conditions specific to the particular sites require different types of the remediation technologies. The choice of the technology and the strategy of the soil remediation are dependent upon the site conditions, contaminant types, contaminant source and source control measures. The technology selection process is also been effected by the potential impacts of the numerous remedial measures. In some circumstances more than one remediation technologies are required to address the contamination issues effectively (Anderson, 1995).

The process trains are usually made to remove the hazardous materials and contaminants present in the soil that might affect the ground water quality. Such situation may include the use of the biological treatments without or with the enhancement remediation technologies. For petroleum contaminated sites the option of train processes is considered to be the most suitable option, where the hot spots are handled greatly by the thermal treatments and physical separations, biological processes are not recommended significantly. Thus consequently during the selection of the redial process, cost effective and efficient technologies must be preferred. In decision making process it has been to be crucial to select one or more remediation technologies particularly. Many conflicting parameters are considered while the decision making process of soil remediation technologies.

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