"Last summer U. S., Russian, and Ukrainian defense ministers planted sunflowers on the site of a former Soviet missile silo, celebrating the Ukraine's nuclear-free status. Little did they know that the sunflowers were also decontaminating the soil, removing heavy metals and toxins accumulated in the area." --Alice W. Wilke "Saying It With Flowers" E. Magazine
This occurrence is one of many that has influenced the development of a process now known as Phytoremediation. Phytoremediation is a technological process using vegetation for removal and recovery of contaminated soils, sludges, ponds, streams, and other forms of wastes. Through recent experimentation this solution has proven itself to be a very time and cost efficient method.
There are two major applications of phytoremediation to remove toxic metals. They are called phytoextraction and rhizofiltration. Phytoextraction is the use of high biomass metal accumulating plants to remove toxic metals from the soil, whereas rhizofiltration is the use of similar plants to remove toxic metals and radionuclides from water.
In phytoextraction, the water and the metals move through the soil toward the roots of the plants, where the extraction flow of the plants takes them into the plants' tissue. The water and metals then move up into the stem and leaves, where the water evaporates. The heavy metals remain in the plant. As time goes by older plants are harvested by traditional agricultural methods. The image at the right shows lead being absorbed into a plant using phytoextraction (Taken from Phytotech, Inc.).
In the above mentioned article, rhizofiltration was the process used. As you can see in the image at the left (Taken from E. Magazine), the plants are placed in styrofoam rafts so that only their roots are submerged. The sunflowers absorb the metals and radionuclides into their roots and release clean water. As with phytoextraction, the plants are also harvested.
When the plants are harvested they are landfilled or burned according to toxic waste standards. Sometimes the harvested plants are recycled, as in the cases with heavy metals. The plants are put through a metal smelter, burning all the organic matter off, leaving the metal ore. This can be sold back to companies and used again.
So how do plants do it? The secret lies within the plants' natural disposal sites, the vacuoles. Within the vacuoles the metals may be held in organic complexes, which prevent the metals from damaging the plant or disrupting its biochemistry. The metal packets find their way into roots, shoots, and leaves, where they are stored over the lifetime of the plant. The push in research is to understand the genetics of this miraculous process. Genetic engineers are already breeding hyperaccumulators; plants that are engineered or selectively bred to be even more efficient metal scavengers.
Phytoextraction and rhizofiltration are very inexpensive compared with the conventional technology. Normally, companies have to contain, excavate, and relocate or bury the soil for treatment. This method on average costs ten times more than phytoremediation methods.
The cost associated for this method of site remediation depends on a number of factors including the density of soil, area of site contaminated, transportation and landfill costs. The same equipment is used in phytoremediation as are common in agricultural practices. In some cases, the costs of phytoremediation can be equated to the local costs to plant crops. Phytoremediation also lacks the need for the removal of large masses of soil. In fact, no soil need be removed, just the plants. This decreases the disposal mass from 30,000 tons, for a sample 10 acre site with the extraction method, to less than 5%, or 1400 tons. This results in tremendous savings when compared to the extraction method. A sample 10 acre site may cost between $3.5-4.5 million for the traditional extraction method, where as, the same site would only cost $1.0-1.2 million for phytoremediation. These savings typically average about 75-85% over the cost of the conventional method. In addition to the economic benefits, phytoremediation is less environmentally destructive than the traditional method due to the fact that the soil is not removed and the metals may be reclaimed for the plant residue. The promise of this technology has created a tremendous market opportunity in the toxic metal remediation industry, a market estimated at $34 billion, over the next five years. It also promises a bright future for the emerging $200 billion radioactive contamination treatment market.
The effectiveness of this process is remarkable: "The submerged sunflowers removed 95% of the uranium decreasing levels below EPA standards within the first 24 hours" according to Alice W. Wilke ( "Saying It With Flowers" E. Magazine). This makes phytoremediation an even more immediate means of removing the hazardous threat when compared to the excavation/burial method.
In conclusion, the more experimentation conducted, the more phytoremediation proves to be a growing trend. It is hard for a staunch advocate of industry to refuse environmental compliance when it is made inexpensive and very effective.
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