Phytoremediation – nature’s cleaners
The environment can become polluted from man’s activities either through unintentional release events, such as happened in 1986 in the Chernobyl nuclear power plant accident, or as a consequence of an industrial process, e.g. run-off from mine spoil heaps, or inadequately controlled industrial processes that discharge into the atmosphere or into aquatic systems.
In the western world particularly, concern about the environment and for the consequences of pollution both on human health and the biosphere in general has been rising steadily since the early 1970’s. This has produced political pressure both to restrict emissions of pollutants into the environment, but also to remediate existing pollution hot spots. Remediation is the term given to cleaning up a particular environmental compartment (e.g. soil or water body) to reduce the levels of specific contaminants to acceptable concentrations. Phytoremediation is one remediation mechanism that can be employed to clean up the environment for a range of pollutants. It involves the use of plants that can tolerate and even thrive in certain types of polluted environments to reduce the specific pollution burden of the site.
Pollution – an older problem than you thought
The history of metal extraction from ore bodies can be traced back into man’s distant history: the Bronze Age (2500-600 BC) was named for a technological shift that occurred when man learned to use metals to fashion tools and other implements. Mining activities in this period caused the first anthropogenic pollution event: heavy metal pollution due to ancillary metals in the ore bodies. So, mankind has been utilising nature and generating pollution for thousands of years. It has taken a long time, but modern man is now very concerned about the damage we may be doing to sensitive ecosystems and wants to repair the harm. Living systems tolerate pollutants differently; concentrations of pollutant elements that would kill one plant species may have no effect on another. At these early sites of man’s modification of the natural world, plants that tolerate heavy metals thrived and proliferated whilst others that could not died back. This is the plant kingdom’s equivalent of the survival of the fittest.
Thriving on poison
Many pollutant chemicals, heavy metals, for instance, can be fatal to living systems if their concentrations are high enough. So how is it that what kills one plant will permit another to flourish? Well, there are a number of different mechanisms that have been identified that permit certain plants to exist in circumstances that would kill many others:
1. Rhizoshere biodegredation. This is a synergistic process where the plant releases natural substances into the soil (through its root system) that microorganisms need. In turn, the microorganisms break down the pollutant that would otherwise harm the plant.
2. Phyto-stabilization. Another strategy that plants use is to trap (or imobilse) contaminats within the structure of the plant such that they cannot interfere with vital functions.
3. Phyto-accumulation. A variation on the above; contaminants are taken up with nutrients and water and end up in the plant’s shoots and leaves.
4: Phyto-volatilization. Organic contaminants are taken up by plants from soil/water and released to atmosphere through their leaves.
5. Phyto-degradation. Some plants can metabolize and destroy contaminants within plant tissues turning pollution into plant tissue.
6. Hydraulic Control. Trees as natural pumps through their roots which reach the water table. A poplar tree, for example, pulls 30 gallons of water per day out of the water table; and a cottonwood can absorb up to 350 gallons per day. The pollutant can be removed from the ground water by this process.
However, it is also the case that much of our body burden of heavy metals such as cadmium and lead is derived from plants in our diet (or in animal diets) which can bioaccumulate these toxic elements which are naturally present in soils.
Using nature to clean up our mess
Once scientists had understood how plants were able to survive in toxic environments, it was a logical step to search for those plants that could be used to remediate contaminated environments. It must be remembered that the plant itself may become "a health hazard" depending on just how it handles the contaminant.
In the aftermath of the Chernobyl Nuclear Power Plant accident in 1986, large areas of the surrounding farmlands were contaminated with a wide array of radionulclides. Of particular concern was 137Cs, which is a gamma emmiting radionuclide with a half-life of 32 years. Both sunflowers and rape seed plants have been used to remediate contaminated lands. The oils produced were found to be free of radioactive contamination (which did not transfer into the seeds used to produce the oil) meaning that a commercial product could be produced from (relatively) highly contaminated lands. The plants took up the Cs, thereby remediating (to a certain extent) the soil. With a knowledge of how the radionuclide partitioned within the plant, the contaminated vegetation could be appropriately stored.
Bioremediation also holds out great promise for the natural, cost effective resolution of the problem of aquifers contaminated with arsenic; a problem estimated to affect some 200 000 000 people in South East Asia alone.
The content of this article was based, in part, on material from the following internet sources: