Introduction to Phytoremediation What is Phytoremediation? How does Phytoremediation work? Various mechanisms Potential of Phytoremediation as a remediation technology Limitations Benefits About Petroleum Hydrocarbon What is Petroleum Hydrocarbon? Sources and uses How does it get into the environment? Risks Posed by Petroleum Hydrocarbons Phytoremediation as a solution to Petroleum Hydrocarbon Contamination Considerations Depth of Pollution Method Phytostimulation
Plant Selection Natural Revegetation using native plants 2. Establishment of Non-native plants Operations and Maintenance Cost Fertilizer Irrigation Aeration Harvest and Disposal Conclusion Introduction to Phytoremediation Phytoremediation is a word formed from the Greek prefix “phyto” meaning plant, and the Latin suffix “remedium” meaning to clean or restore (Mcgraw hill). It is a bioremediation process which uses various types of plants to remove, transfer, stabilize, or even destroy contaminants in the soil and groundwater.
Plants can be used to remove many kinds of pollution such as explosives, metals, pesticides, radioactive material and oil. Plants have the potential to prevent groundwater, wind and rain from carrying pollutants away to other locations. Phytoremediation works the best when there is low to medium pollution because high pollutants can kill the plants or damage their root systems. When plants take in water and nutrients through their roots they also remove pollutants from the contaminated soil, groundwater or stream.
When the chemical pollutants are inside of the plant they can be stored in the roots, stems or leaves, converted into less harmful chemicals by the plant, or changed into gasses that the plant releases into the air as it transpires (Environmental Protection Agency). There are several different mechanisms of Phytoremediation. When plants release natural substances through their roots and supply nutrients to microorganisms in the soil that enhance biological degradation it is called Rhizosphere biodegradation.
Rhizosphere biodegradation is one of the most popular mechanisms involved in Petroleum Hydrocarbon removal (Mcgraw hill). Phyto-stabilization is a process when chemical compounds produced by the plant cause the contaminants to be immobilized rather than degraded. Phyto-accumulation (also called phyto-extraction) is a method that is mainly used to counteract pollution by metals, the plant absorb pollutants in the soil along with water and nutrients and it ends up in their shoots and leaves (The Center for Public Environmental Oversight ).
The plants are then harvested and either taken to have the metal they contain recycled or they are disposed of carefully so they do not re-introduce the pollutants into the environment. Rhizofiltration is a hydroponic system for treating water streams, it is similar to phyto-accumulation but the plants are raised in a greenhouse with their roots submerged in water. This system can be used for ex-situ groundwater treatment, where groundwater is pumped to the surface to irrigate the plants. Plants raised in a greenhouse utilize artificial soil such as sand with vermiculite or perlite.
They are grown until the roots are saturated with contaminants and then they are properly disposed of. During Phyto-volatilization plants take up organic contaminants and release them into the air via their leaves. Phyto-degradation is when the plant literally destroys the contaminants within the plant tissues. During Hydraulic Control trees indirectly remediate by controlling groundwater movement. Trees act as natural pumps and have roots that can reach down towards the water table where they can develop strong, dense root masses that take up large quantities of water.
As an example a cottonwood tree can absorb 350 gallons of water a day. Trees are generally left where they are when they are planted for phytoremediation purposes (The Center for Public Environmental Oversight ). Before a crop is planted for the purpose of phytoremediation a risk assessment must be completed in order to determine what human health hazards and ecological risks are preexisting at the site. It must be ensured that phytoremediation is the right remedial process to use to counteract the contaminant.
Phytoremediation will only be used if it offers risk reductions or cost savings over excavating and landfilling contaminated material. Numerous benefits of phytoremediation have been discovered or hypothesized. It can be less invasive and destructive that other technologies and actually has the potential to rebuild an environment by promoting biodiversity at a site that may have been previously uninhabitable to animals as a result of human activity. Some studies have indicated that implementing phytoremediation can save 50 to 80 percent as opposed to traditional technologies (Environmental Protection Agency).
The added vegetation at phytoremediation sites has proven to help reduce erosion by wind and water as well as enhance the general aesthetics of an area that previously would have been dead and brown due to contamination (T. V. A. Yu. Muratova). Phytoremediation is not completely without limitations, if the soil is too contaminated crops may not be able to grow and survive, for it to be successful the contamination must not be too severe and also must be shallow enough for the plant roots to reach unless the contamination is physically brought to the plant.
If a non-native species is used as a phytoremediation crop the consequences of introducing the plants into the environment may be unknown or unexpected (K. V. Nedunuri). Phytoremediation also can take a lot longer than traditional techniques; it can take several growing seasons to establish a steady crop and for contaminates to be reduced. It is apparent from the risks and benefits of phytoremediation that the success of the process is dependent on many factors including contaminant types, concentrations, and depths; contaminated media; selection of appropriate vegetation; plant growth and survival; and site climate.
As with any other remediation process a thorough examination of all potential occurrences and outcomes must be evaluated before the first seed is planted and phytoremediation is chosen as a solution to the polluted site (Ndimele). About Petroleum Hydrocarbon Petroleum Hydrocarbon is produced by the decomposition of buried plants and animals that have been subjected to high temperatures and pressures for millions of years. It is a thick liquid that consists of combustible hydrocarbons. It is most commonly referred to as crude oil or petroleum (Harmon).
Petroleum products are defined as common soil contaminants that are carcinogenic and may contain potentially toxic compounds (Salmi Nur Ain Sanusi). A broad family of several hundred compounds come from Petroleum hydrocarbon, or crude oil, such as gasoline, diesel fuel and kerosene, they are all mixtures of compounds with varying constitutes and molecular complexity (T. V. A. Yu. Muratova). Petroleum Hydrocarbons are some of the most common toxic organic substances that can be in the environment (Ali Daryabeigi Zand). Soils contaminated with Petroleum Hydrocarbon pose a great threat to all life that surrounds them.
This substance can get into the soil as a result of petroleum extraction, transportation, refining, consumption and accidents like leakage from tanks and ruptured pipelines (Ali A. Besalatpour). For hundreds of years oil products have been disposed of in the environment with the assumption that environment would absorb and deal with the pollutants on its own. Now we know that is not the case agricultural scientists have been hustling to figure out how they can clean the contaminated environments that are affected by this substance so that they no longer pose a threat to living organisms.
Though pollution by petroleum hydrocarbons is a problem on a global scale, it has the largest effect on countries that produce, transport and refine oil. Russia, for example, has a major ecological problem associated with their oil-refining factories. The refineries were renovated and left behind large oil-sludge pits they were liquidated and now remediation is needed to make the environment and soil clean again (T. V. A. Yu. Muratova). It is estimated that between 1. 7 and 8. million metric tons of crude oil are released into the worlds water every year, with 90 percent of that being deliberate waste disposal into the environment (Ndimele). Oil spillage from non-point discharge sources (run-off and boat bile) pose just as much of a threat to public health and the environment as direct spillage (Ndimele). Petroleum Hydrocarbons pose a threat to humans who come into contact with contaminated soil or ground water and therefor it is very important to find low-cost effective technologies to clean up polluted soils.
Phytoremediation as a solution to Petroleum Hydrocarbon Contamination Remediation technologies are definitely required to fix soils that have been contaminated with Petroleum Hydrocarbon (Salmi Nur Ain Sanusi). The use of plant based systems to clean sites that have been contaminated with organic and inorganic substances alike is a process that dates back hundreds of years. While biological, chemical and physical methods can all be used to xtract Petroleum Hydrocarbon from soil, phytoremediation has been shown to be tremendously effective in a number of field and greenhouse studies (Salmi Nur Ain Sanusi). Something to consider when planting a crop for phytoremediation is the depth of the contaminant, if the pollutants are below the root zone it will be difficult, if not impossible, for the roots to really suck up the pollutant. Often removal of petroleum hydrocarbons is attributed to the microorganisms living in the rhizosphere under the influence of plant roots in a process known as Phytostimulation (Ali Daryabeigi Zand).
Using multicomponent systems, including plants, mineral fertilizers, and plant growth promoting microorganisms, has been studied in vegetative experiments in order to stimulate phytoremediation of contaminated soil (A. D. A. Yu. Muratova). In Phytostimulation the rhizosphere microorganisms benefit from the root exudates and in turn the plants benefit from the metabolic detoxification of compounds that could potentially be toxic brought about my microbial communities.
The microbial community also benefits the plant through recycling of mineral nutrients as well as the supplying of vitamins, amino acids, auxins, cytokinins, and gibberellins all of which stimulate plant growth (Ali Daryabeigi Zand). To establish stable vegetation cover and achieve a maximum reduction in petroleum hydrocarbon concentration, plants should be chosen carefully so that they provide as much root surface area as possible.
They should be tolerant to petroleum hydrocarbon and should preferably be native to the contaminated site. Large, healthy plants with extensive root systems are capable of pumping more nutrients through the soil system than smaller weaker plants (Ali A. Besalatpour). Weathering is another issue to consider when planting to eradicate Petroleum Hydrocarbon contaminated soil. Petroleum Hydrocarbons are subject to weathering such as leaching, evaporation and even biodegradation (A. D. A. Yu. Muratova).
If hydrocarbon contamination has had a lot of time to undergo weathering the contaminant may be lost to the environment and any attempt at phytoremediation could essentially be a lost cause. In terms of reproduction, growth and survival under environmental stress native plants are often times the best path to choose (K. V. Nedunuri). Though the selection of vegetation is dependent upon many variables, such as climate, soil, root system and ability to withstand contamination; plant selection also depends on the project goal.
Grasses and Legumes have extensive root systems that work well for rhizodegredation of hydrocarbons in shallow soils and Willow trees can be used when the project calls for a change in ground water flow and contaminant migration by transpiring a lot (T. V. A. Yu. Muratova). Natural Revegetation is when a crop is planted for the purpose of growing wild, planting native vegetation and allowing it to grow in its natural habitat is almost always the best way to ensure the plants will be able to thrive in the environment they are placed in.
The introduction of a non-native species of plant to a contaminated site could have repercussions that were never expected (K. V. Nedunuri). The plant could become noxious and infest the area in or around the site and could potentially lead to an issue even bigger than the original pollutant being combated. The cost of keeping up with a phytoremediation crop is, in general, relatively cost effective. After plants have been selected and planted, further care for the crop is often called for.
These activities include fertilization, irrigation, weeding, pruning, mowing, and replanting vegetation that does not survive. Plants also need to be constantly inspected and watched for changes to ensure that they are healthy and properly filtering contaminants. Fertilization can be vital for two main reasons; contaminated soil may not have the right amount of minerals and other nutrients that are needed for optimal growth and function of the plant being grown, and because often times phytoremediation relies on the microorganisms in the soil, they also benefit from the excess nutrients in the fertilizer.
Irrigation is necessary because water is essential to both plant and microbe growth. Aeration is important when growing for phytoremediation because oxygen is very important in the biodegradation of hydrocarbons; microbial degradation of petroleum hydrocarbons is significantly increased when oxygen is present (Ndimele). The ecological exposure that is a possibility when contaminants build up in a plants biomass are of a real concern to some phytoremediation applications and therefore require harvesting and then immediately disposing of the crop.
However, uptake and accumulation of hydrocarbons in plants is not something that is normally seen or projected and so there is never really any immediate rush to dispose of the crops (Environmental Protection Agency). Conclusion In conclusion if phytoremediation is being considered as the solution to the clean-up of a site contaminated with Petroleum Hydrocarbon its success will essentially rely on the understanding of the crop being grown and the polluted site being grown on.
Depth of the contaminant and the plant that is being chosen, with specificity to the root zone, will also play a major role in the eventual stabilization of an unhealthy site. The method of phytoremediation that is chosen must be one that is known to have the ability to deal with the dense structure of Petroleum Hydrocarbon. The amount of weathering the Petroleum Hydrocarbons in the soil has gone through will also have a hand in the outcome of the phytoremediation process, too much weathering previously will wash the pollutant further into the environment and out of reach of the plant.
Weathered hydrocarbons are more resistant to rhizodegredation and the vegetation alone may not break down the contaminants. Highly contaminated soils are more difficult to work with because they are generally to toxic for plant growth and root stimulation. Phytoremediation is a field that is growing fast and soon the challenges that stand in the way of removing Petroleum Hydrocarbons or any other soil contaminant from the environment will be a thing of the past as phytoremediation expands and grows into the future. Glossary
Phytoremediation- the treatment of environmental problem (bioremediation) through the use of plants Bioremediation- the treatment of pollutants or waste (as in an oil spill, contaminated groundwater, or an industrial process) by the use of microorganisms (as bacteria) that break down the undesirable substances. Groundwater- water within the earth especially that supplies wells and springs Petroleum Hydrocarbon- Any of a number of solvents refined from cured petroleum and used to lower the viscosity of oils and resins.
Ex-situ- Ex-situ conservation means literally, “off-site conservation”. It is the process of protecting an endangered species of plant or animal outside of its natural habitat Pipelines- a line of pipe with pumps, valves, and control devices for conveying liquids, gases, or finely divided solids Rhizosphere- oil that surrounds and is influenced by the roots of a plant Leaching- to subject to the action of percolating liquid (as water) in order to separate the soluble components (such as toxins in soil)
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