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Advantages of phytoremediation over other remediation methods

Phytoremediation is process of removing toxins, heavy metals, organic pollutants and other waste using specifically selected or genetically engineered plants. Soil microorganisms (living in rhizosphere) are contributing to the phytoremediation process.

Plants absorb different chemicals from the ground and transform them into valuable nutrients (used for their own growth) or less harmful substances that could evaporate through their leaves. Trace elements are essential for the plant grown. They are found in the soil as well as heavy metals. Due to high similarity between essential and toxic (metal) elements, plant will absorb them all. Heavy metals will be bioaccumulated in plants tissue. Some plants contain 1000 times more heavy metals than surrounding soil. Best results in soil remediation will be achieved in the 3-6 feet wide area around the root. Besides being active in absorbing and transforming pollutants from the ground, plants can accelerate bioremediation by increasing the number and activity of the soil microorganisms. Constant exchange of nutrients and oxygen between the root and soil result in increased activity of microorganisms that are responsible for faster degradation of the soil contaminants. Carbohydrates, amino acids and mucigel (gelatinous substance that facilitate root penetration through the ground during the growth) are the most important nutrients for the microbial growth. On the other hand, after organic pollutants are degraded by soil microorganism, plant will have enough nutrients (C, N, P, K and S) for its own growth.

Phytoremediation in real life examples:

A mustard green (Brassica juncea) is used to decrease the amount of lead in the children’s park in Boston. Plants were removed and safely disposed after they absorbed 45% of the lead from the ground. Pumpkin vine (Cucurbita pepo maxima) is used for cleaning the old Magic Marker factory in New Jersey. After Chernobyl nuclear catastrophe, sunflowers (Helianthus annuus) were used to absorb radioactive waste.

Large plant species are used when big amount of water (and associated contaminants) need to be absorbed from the ground. Single willow tree (Salix alba) can transpire 5,000 gallons of water in a summer day. One hectare of saltwater cordgrass (Spartina alterniflora) will evapo-transpirate (sum of evaporation and transpiration) 21,000 gallons of water per day. All heavy metals and various pollutants will be absorbed with the water and metabolized (or accumulated) in the plant.

Halophytes are interesting group of plants that could be used in phytoremediation as well. These plants could tolerate high level of the salt in the ground. In 2 years long project of phytoremediation of the gas and oil contaminated soil in Oklahoma, they managed to reduce the level of salt for 65%. After excess salt was removed, ground was colonized by the plants that lived there before “ecological disaster”.

List of pollutants that could be easily removed from the ground using phytoremediation: petroleum hydrocarbons, polycyclic aromatic hydrocarbons, chlorinated pesticides, polychlorinated biphenyls, trichloroethylene, explosives (TNT, DNT), organophosphate insecticides (diazanon and parathion), surfactants (detergents)…

List of plants used in phytoremediation:

Alfalfa is living in symbiosis with hydrocarbon-degrading bacteria. Arabidopsis can transform Hg into a gaseous state thanks to inserted bacterial gene. Bamboo family can accumulate silica in its stalk and N in its leaves. Bladder campion can accumulate Zn and Cu. Brassica juncea can accumulate Se, S, Pb, Cr, Cd, Ni, Zn, and Cu. Buxaceae and Euphorbiaceae can accumulate Ni. Compositae family is living in symbiosis with Arthrobacter bacteria and accumulates Cs and Sr. Ordinary tomato and alpine pennycress can accumulate Pb, Zn and Cd. Poplar is used for the absorption of the atrazine (pesticide).

Phytoremediation is cost effective because planting doesn't require specific equipment or technology (same methods are used in agriculture). This type of remediation is less expensive than conventionally used chemical, physical or thermal methods. It’s also less expensive than bioremediation. Removal of the pollutants from the top 15 centimeters of the contaminated soil using plants will cost between 2,500 to 15,000 dollars per hectare, compared to 7,500 to 20,000 dollars when microorganisms are used. Other advantage of phytoremediation over conventional methods is that it’s happening in situ. Contaminants are simply immobilized and transformed or stored within the plant after extraction. Conventional methods are more complicated since extracted contaminated material needs to be stored and transported to the landfill where it will be incinerated. Also, conventional methods are less eco-friendly.