Contamination of soil in oil refinery has been an environmental issue of modern industrialization in many countries. The main reason for this contamination is the contaminated products generated during the activities associated with purifying and refining petroleum in oil refinery. Those activities including distillation, chemical treatment, product transfer and storage and so on are the contributors of the contaminants. These consist of the petroleum hydrocarbons, asbestos, metals, some inorganic compounds and etc[1]. There are a lot of effective remediation technologies such as pump-and-treat (PAT), soil washing, thermal desorption, bioremediation and etc. However, some of them are causing inhibition of soil fertility or even destruction to ecosystem. Therefore, this encourages my evaluation on phytoremediation- a variation of bioremediation which has been an emerging technology for remediation of petroleum hydrocarbons since late 1990s[2].
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Phytoremediation is a cost-effective in-situ treatment which uses up vegetation to clean up the petroleum-contaminated soils and groundwater. In general, it is to utilize the ability of the “special” plants’ roots associated bacteria to take up, accumulate, and breakdown the contaminants (e.g. TPHs) located in soils and ground water[3](Figure1). This technology is more suitable for: (a) large sites with shallow contaminants where only “polishing treatment” is required; and (b) the sites where vegetation is used as a final cap and closure of the site[4].
Figure 1 Basic concepts of phytoremediation
2. Mechanisms
There are seven mechanisms of phytoremediation: phytoextraction, rhizofiltration, phytovolatilization, phytostabilization phytodegradation, hydraulic control and rhizodegradation. They can be described as follows:
Phytoextraction
This is also known as phytoaccumulation. It aims to utilize plant roots in order to uptake and translocate the metal contaminants in the soil into the above ground portions of the plants[5].
The plants absorb, concentrate, and precipitate the toxic metals from the soils into shoots, leaves, etc. (Fig. 1)[6].
Figure 2 Phytoextraction of Ni from contaminated soil
There are some plants, called hyper-accumulators, which are capable of accumulating extremely large amount of metals especially nickel, zinc and copper[7]. These plants need to be either incinerated or composted to recycle the metal after have been used for some time[5].
Rhizofiltration
It is the adsorption or precipitation onto plant root surfaces, or absorption into contaminants which present in the soil solution in the root zone. Although rhizofiltration looks similar with phytoextraction, the main function of rhizofiltration is to remediate the contaminated groundwater rather than the soil by removing inorganics and metals. The plants are first raised in greenhouses with their roots in water until a large root system has been completed. At this stage, the original water source is replaced by contaminated for acclimatization. Once the saturation of contaminants in the roots has been reached, the plants are harvested. In the study of removal ability, sunflower, Indian mustard, tobacco, corn and etc. have been used to investigate the removal of lead from water. Among those plants, sunflower has the greatest ability[5, 6].
Phytovolatilization
This involves the uptake and transpiration of contaminants by plants, with release of the contaminants in vapour form to the atmosphere. It looks like a natural air-stripping pump system. This natural ability of volatilization enables the plants to volatilize the volatile organic compounds in parts of refinery site, vinyl chloride as well as inorganics and etc[6, 8].
Phytostabilization
This process is to use plants for immobilizing contaminants in the soil and groundwater. The roots absorb and accumulate the contaminants, provide adsorption or precipitation within the
rhizosphere (root zone) in order to reduce the mobility of contaminants. As contaminant migration to the groundwater or air has been minimized, the bioavailability for their entry into the food chain can be lowered. Metal-tolerant species can be used to restore vegetation to those metal-contaminated sites. Those species not only can decrease the potential migration of contaminants but also prevent the leaching of contaminants to groundwater[5].
Phytodegradation
This is also called phytotransformation. It is the breakdown of contaminants through metabolic processes or the effect of constituents (e.g. enzymes) produced by the plants. The complex organic contaminants are degraded into simple molecules and these molecules are then incorporated into plant tissues[5].
Hydraulic Control
Hydraulic control is employed by plant canopies on the control of water table and the soil field capacity. Phreatophytic trees and plants are commonly used due to the ability to transpire large amount of water and thereby influence the water balance at the site. The increased transpiration decreases the tendency of contaminants to move towards groundwater water or alleviates the migration of contaminants from the site in groundwater plumes[6]. There is something to be noted that trees must be rooted into a shallow water table aquifer in order to successfully prevent plume migration[9].
Rhizodegradation
This is often referred to as phytostimulation or planted-assisted bioremediation/degradation. It can be achieved by breaking down the contaminants in the soil within the rhizosphere through microbial activities. During the microbial activities, organic contaminants such as fuels and solvents can be biodegraded by microorganisms into harmless products. The nutrients for the microorganisms are provided by the exudates produced by the plant[5].
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3. Major influencing factors
(a) Soil composition and quality: Soils with high concentration of contaminants soils tends to have a poor physical conditioning which is not able to promote the growth of vegetation and rhizosphere microorganisms. Common limitations are the deficiencies in moisture-holding capacity, aeration, nutrient and permeability and so on. Thus, adjustments are required to improve the quality of soil before planting. A common adjustment is the amendment on pH of soil by adding sulphur or lime[9].
(b) Selection of plant: Plants are selected by taking the concerns of the target contaminants to be polluted and the remedial objectives for redevelopment such as time frame and risk management. Moreover, the climate for the plants to be adapted into is also very important. The ability of the plant acclimating to the soil and the depth of plant’s root structure also cannot be neglected. The selection and use of plant species must be done carefully in order to avoid the introduction of non-native species into the areas which are new to that species[10].
4. Advantages
Relatively low cost: Comparing with other treatment technologies such as thermal treatment, chemical extraction, some ex-situ technologies and so on, phytoremediation is relatively inexpensive as it only uses plants. [5, 11]. Besides, there is no extraction cost as it is an in-situ treatment.
Safe and passive: Phytoremediation is driven by solar energy and there is no chemical usage
Feasible for large varieties of contaminants: It can be used on the major contaminants produced in oil refinery-petroleum hydrocarbon as well as the other minor contaminants such as VOCs, TCE and even heavy metals and etc[12].
Mitigation of soil erosion: The establishment of vegetation can effectively improve the soil structure and resulting in reducing the soil erosion.
Preventing migration of contaminants: Phytoremediation avoids excavation and transport of polluted media as the contaminants are destroyed in place. Therefore, it can lower the risk of spreading the contamination[13].
Aesthetically pleasing: The use of green plants can contribute a more eye-pleasing and natural green environment.
5. Disadvantages
Relatively shallow clean-up of soil with low contaminant concentration: Treatment is limited to soils less than 1 m from surface for grasses, less than 3 m for shrubs, less than 6 m for deep-rooting trees and groundwater less than 3 m from the surface. Besides, it is not effective for contaminated site with high concentration [5, 14].
Slow process: The whole process is relatively slow compared with other technologies as three to five growing seasons are needed to achieve remediation goals[5].
Site specific and critical plant selection: The optimization of plant growth and the contaminants uptake depends on the characteristics of the site and the plant species that are selected[5].
Potential food chain contamination: Contaminants may enter the food chain through animals which eat the plants or borne fruits[14].
Production of residual waste: Using phytoremediation may relocate contaminants from the subsurface to the plant, thereby creating residual waste to be disposed of[5].
By looking at the features of phytoremediation in various aspects, we may find some drawbacks and limitations. Although the overall performance of phytoremediation is still not effective as soil vapor extraction and other technologies, its trade-off such as low cost, practicality and environmental-friendliness indicates that it can be a promising solution for remediation in oil refinery, especially in developing countries. Also, many studies on phytoremediation are still being carried out, so it can certainly be combined with a lot of technologies for future improvement and ultimately be widely applied in oil refineries all over the world.
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