Phyto remediation-using plants to remove toxins

Phytoremediation-using plants to remove toxic chemicals from soil/water and Air-

Some plants can absorb and store toxins; other plants can break down nasty chemicals and turn them into less harmful  substances. Plants grown for phytoremediation can also prevent toxins from moving from a contaminated site into other areas.

The fact is that many many of these toxins can be removed from your property by using specific kinds of plants that like to eat specific kinds of toxins.  EPA recommends them, and the average home owner can afford to purchase, plant and dispose of it without tearing up the property or destroying the home value by landing an environmental contamination notation on their property card.

Do some additional reading, of course, depending on what you have on your property, and in the case of heavy metal poisoning, pull the plants up at the end of the season, and dispose of them at the landfill.  Do not plow them under or let the leaves fall to the ground, that would be defeating the whole purpose.

Scroll Down For the detailed explanations of the different kinds of techniques.  But for anyone wanting to know which plants will pull what horrible stuff up out of the ground here in Jacksonville, stick with me.

There are many plants that can be used for bioremediation, but I’m only going to talk about attractive plants that can grow in our area and which toxic waste they pull up.  This is by no means an exhaustive list;  for now I’m sticking to ornamentals that are fairly common and relatively easy to grow.

Hydrangeas are popular ornamental plants grown for their large clumps of flowers, which can be pink, blue, purple, creamy white or somewhere in between. You might already have some growing in your yard, and never knew that they draw aluminum out of the soil.
http://en.wikipedia.org/wiki/Hydrangea & Compact pink hydrangea

Another attractive flowering shrub that removes aluminum from the soil is Melastoma affine, also known by the common name Blue Tongue.  It has leathery dark green foliage (evergreen in hot climates) and purple flowers, followed by sweet purple fruits that stain your tongue blue, hence the name.

For removing lead from soil, most members of the Brassica family will do the trick: Kale, mustard greens, collards, broccoli, and so forth.  However, in this usage obviously you aren’t going to want to eat them, so you can go for ornamental varieties of kale and cabbage, which are frilly and colorful, and generally too tough to eat, anyway.

Water Hyssop (Bacopa monnieri) removes not only lead, but mercury, cadmium and chromium from bogs and wetlands, and makes a lovely ground cover for muddy shores. It has small succulent leaves, and dainty white flowers.
http://en.wikipedia.org/wiki/Water_hyssop

The water hyacinth naturally absorbs pollutants from water, including cadmium, chromium, mercury, lead, zinc, cesium, strontium-90, uranium, and pesticides. It is extremely fast growing and has lovely blossoms, mostly lavender to pink.  It originated in South America but is now an invasive species all over the place.
http://en.wikipedia.org/wiki/Water_hyacinth

TREES

 If you’re interested in trees, the common mulberry tree (M. rubra) has been shown to release chemicals  that support the growth of  bacteria that break down PCBs, and willow trees absorb cadmium, zinc, and copper.

But as far as I can tell, the undefeated champion of phytoremediation has got to be the sunflower.  Yep, sunflowers, which are so cheap and easy to grow that they’re popular as a beginner garden project for little kids.

Sunflowers absorb lead, arsenic, zinc, chromium, copper, and  manganese,  and were successfully used to clean up uranium and strontium-90 from contaminated soil in the Ukraine after the Chernobyl disaster, the worst nuclear power plant accident in history.

Springfield isn’t radioactive…. yet, so we’ll just focus on the fact that sunflowers are not only awesome at sucking filthy rubbish out of the ground, but they’re just awesome.  Anybody can grow them; all they really need is sunshine and some water.  There’s a staggering variety of them; sunflowers can be one foot tall, or five feet tall or even grow over 20 feet tall. Sunflowers come in a range of colors from creamy palest yellow through bright yellow, gold, orange, red and rich dark burgundy.  They can be more than one color; the blossoms can be smaller than the palm of your hand or bigger than a dinner plate.

I would love to see Springfield transformed into a waving forest of sunflowers, and if it helps avert the need for having even MORE stuff torn up, even better.

Miss Janice Price, aka DeadGirlsDontDance

What is Phytoremediation

Phytoremediation is the use of living green plants for in situ risk reduction and/or removal of contaminants from contaminated soil, water, sediments, and air. Specially selected or engineered plants are used in the process. Risk reduction can be through a process of removal, degradation of, or containment of a contaminant or a combination of any of these factors. Phytoremediation is an energy efficient, aesthically pleasing method of remediating sites with low to moderate levels of contamination and it can be used in conjuction with other more traditional remedial methods as a finishing step to the remedial process.

One of the main advantages of phytoremediation is that of its relatively low cost compared to other remedial methods such as excavation. The cost of phytoremediation has been estimated as $25 – $100 per ton of soil, and $0.60 – $6.00 per 1000 gallons of polluted water with remediation of organics being cheaper than remediation of metals. In many cases phytoremediation has been found to be less than half the price of alternative methods. Phytoremediation also offers a permanent in situ remediation rather than simply translocating the problem. However phytoremediation is not without its faults, it is a process which is dependent on the depth of the roots and the tolerance of the plant to the contaminant. Exposure of animals to plants which act as hyperaccumulators can also be a concern to environmentalists as herbivorous animals may accumulate contaminate particles in their tissues which could in turn affect a whole food web.

How Does It Work?

Phytoremediation is actually a generic term for several ways in which plants can be used to clean up contaminated soils and water. Plants may break down or degrade organic pollutants, or remove and stabilize metal contaminants. This may be done through one of or a combination of methods. The methods used to phytoremediate metal contaminants like lead and mercury, are slightly different to those used to remediate sites polluted with organic contaminants.

Metal  
Phytoextraction
Rhizofiltration
Phytostabilisation

Organic
Phytodegradation
Rhizodegradation
Phytovolatilisation

Phytoremediation of metal contaminated sites

Phytoextraction (Phytoaccumulation)
Phytoextraction is where plant roots suck up metal contaminants from the soil and translocate them to the parts of the plant that is above the soil. Different plants have different abilities to suck up and/or survive various metals, so many different plants may be used. Especially in places that are polluted with more than one type of metal. There are certain species called Hyperaccumulator plants that absorb much higher amounts of pollutants than most other species. These species are used on many sites due to their ability to thrive in highly polluted areas

Once the plants have grown and absorbed the metal they are harvested and disposed of safely. This process is repeated several times to reduce contamination to acceptable levels.

In some cases it is possible to actually recycle the metals through a process known as phytomining, though this is usually reserved for use with precious metals. Metal compounds that have been successfully phytoextracted include zinc, copper, and nickel, but there is promising research being completed on lead and chromium absorbing plants.

Understanding How It Works:

(Uptake, Translocation, and Accumulation in Shoot)
Metal contaminants in the soil: are absorbed by the roots (uptake), move into the shoot (translocation), and are stored in the shoot (accumulation).

Harvest the Shoot and Recover Metal
A plant that contains metal contaminants can be harvested and destroyed, allowing for the recovery of the metals.

Rhizofiltration
Rhizofiltration is similar to Phytoextraction but is used to clean up contaminated groundwater rather than polluted soils. The contaminants are either adsorbed onto the root surface or are absorbed by the plant roots. Plants used for rhizoliltration are not planted directly in the site but have to be acclimated to the pollutant first.

Plants are hydroponically grown in clean water rather than soil, until a large root system has developed. Once a large root system is in place the water supply is substituted for a polluted water supply to acclimatise the plant. After the plants become acclimatised they are planted in the polluted area where the roots uptake the polluted water and the contaminants along with it. As the roots become saturated they are harvested and disposed of safely. Repeated treatments of the site can reduce pollution to suitable levels as proven at Chernobyl where sunflowers were grown in radioactively contaminated pools.

Phytostabilisation
Phytostabilisation is the use of certain plants to immobilize poisons in soil and water. To prevent the contamination from spreading and moving throughout the soil and groundwater, they are absorbed and accumulated by roots, absorbed onto the roots, or held in the rhizosphere (this is the area around roots which works like a small chemistry lab with microbes and bacteria and micro organisms that are secreted by the plants.) This reduces or even prevents migration into the groundwater or air, and also reduces the bioavailibility of the contaminant thus preventing spread through the food chain. This technique can also be used to re-establish a plant community on sites that have been completely deadly to plants due to the high levels of metal contamination. Once a community of these tolerant plants gets rooted and growing, even wind erosion and leaching of the soil contaminants are also reduced.

Understanding How It Works:

Direct Transformation by Exudates
Organic contaminants in the soil are: absorbed by the plant roots and broken down into their component parts by “exudates” in the plant root system.

Phytoremediation of organic polluted sites

Phytodegradation (Phytotransformation)
Phytodegradation is the breakdown of organic contaminants by metabolic processes driven by the plant. Ex planta metabolic processes hydrolyse organic compounds into smaller units that can be absorbed by the plant. Some contaminants can be absorbed then broken down by plant enzymes. These smaller pollutant molecules may then be used as metabolites by the plant as it grows, thus becoming incorporated into the plant tissues. Plant enzymes have been identified that breakdown ammunition wastes, chlorinated solvents such as TCE (Trichloroethane), and other plants which break down organic herbicides.

Understanding How It Works:

(Uptake, Translocation and Metabolism)
Organic contaminants in the soil:are absorbed by the roots (uptake), travel up the shoot to the leaves (translocation), where they are broken down into their component parts (metabolism) and stored in the leaves.

Rhizodegradation
Rhizodegradation (also called enhanced rhizosphere biodegradation, phytostimulation, and plant assisted bioremediation) is the breakdown of organic contaminants in the soil by soil dwelling microbes that like the root systems of certain plants. There are soil dwelling microbes that digest fuels and solvents, producing harmless products through a process known as Bioremediation. Plant root byproducts such as sugars, alcohols, and organic acids act as carbohydrate sources for the soil micro plants and will enhance microbial growth and activity. Some of these compound may also act as chemically attractive signals for fuel eating microbes. The plant roots also loosen the soil and transport water to the rhizosphere stimulating this helpful microbial activity

Understanding How It Works:

Microbially Mediated (plant assisted microbial biodegradation)
Organic contaminants in the soil are broken down by microbes that live in the soil near the plant roots.

Phytovolatilization
Phytovolatilization is where plants suck up contaminaints that are water soluble and release them into the atmosphere as they release the water from their leaves. A, the toxic material may become modified as the water travels along the plant’s vascular system from the roots to the leaves.  Then the contaminants evaporate into the air surrounding the plant. There are varying degrees of success with plants as phytovolatilizers with one study showing poplar trees to convert and disperse up to 90% of the TCE they absorb.

Understanding How It Works:

(Uptake, Translocation and Volatilization)
Organic contaminants in the soil: are absorbed by the roots (uptake), travel up the shoot to the leaves (translocation), andare released into the air (volatilization).

Hydraulic control of Pollutants
Hydraulic control is the term given to the use of plants to control the migration of subsurface water through the rapid uptake of large volumes of water by the plants. The plants are effectively acting as natural hydraulic pumps which—when a dense root network has been established near the water table— can transpire up to 300 gallons of water per day. This fact has been utilised to decrease the migration of contaminants from surface water into the groundwater (below the water table) and drinking water supplies. There are two such uses for plants:

Riparian corridors
Riparian corridors and buffer strips are the simultaneous use of many aspects of phytoremediation along the banks of a river or the edges of groundwater plumes. They are basically long stretches that act as a filter and processing system of plants that breakdown, contain or extract the pollution.
Pytodegradation, phytovolatilization, and rhizodegradation are used to control the spread of contaminants and to remediate polluted sites.
Riparian strips are used along the banks of rivers and streams:
Buffer strips are the use of such applications along the perimeter of landfills.

Vegetative cover
Vegetative cover is the name given to the use of plants as a cover or cap growing over landfill sites. The standard caps for such sites are usually plastic or clay. Plants used in this manner are not only more aesthically pleasing they may also help to control erosion, leaching of contaminants, and may also help to degrade the underlying landfill.

Where has Phytoremediation Been Used?

Location  Application Pollutant Medium plant(s)
Ogden, UT  Phytoextraction & Rhizodegradation  Petroleum & Hydrocarbons  Soil & Groundwater  Alfalfa, poplar, juniper, fescue
Anderson, ST  Phytostabilisation  Heavy Metals  Soil  Hybrid poplar, grasses
Ashtabula, OH  Rhizofiltration  Radionuclides  Groundwater  Sunflowers
Upton, NY  Phytoextraction  Radionuclides  Soil  Indian mustard, cabbage
Milan, TN  Phytodegradation  Expolsives waste  Groundwater  Duckweed, parrotfeather
Amana, IA  Riparian corridor, phytodegradation  Nitrates  Groundwater  Hybrid poplar

Pro’s & Con’s of Phytoremediation
As with most new technologies phytoremediation has many pro’s and cons. When compared to other more traditional methods of environmental remediation it becomes clearer what the detailed advantages and disadvantages actually are.

Advantages of phytoremediation compared to classical remediation

Quote

It is more economically viable using the same tools and supplies as agriculture

It is less disruptive to the environment and does not involve waiting for new plant communities to recolonise the site

Disposal sites are not needed

It is more likely to be accepted by the public as it is more aesthetically pleasing then traditonal methods

It avoids excavation and transport of polluted media thus reducing the risk of spreading the contamination

It has the potential to treat sites polluted with more than one type of pollutant

Disadvantages of phytoremediation compared to classical remediation

Quote

It is dependant on the growing conditions required by the plant (ie climate, geology, altitude, temperature)

Large scale operations require access to agricultural equpment and knowledge

Success is dependant on the tolerance of the plant to the pollutant

Contaminants collected in senescing tissues may be released back into the environment in autumn

Contaminants may be collected in woody tissues used as fuel

Time taken to remediate sites far exceeds that of other technologies.  Of course this is balanced out in any project involving governmental or public bodies, as they generally have more paperwork and process than those required by a private firm on private property.  

Text by Janice Price and Stephen Dare

A partial list of resources used to write this article that may be of interest:

http://www.clu-in.org/live/default.cfm#Phytotechnologies
http://deoracle.org/learning-objects/phytoremediation-organic-contaminants.html
http://lewdpunkzine.blogspot.com/2009/04/sunflowers-love-heavy-metal.html
http://www.scielo.cl/fbpe/img/ejb/v6n3/a06/bip/index.html
* EPA Contaminated Site Clean-up Information – http://www.epa.gov/region4/waste/npl/nplfln/jaxashfl.htm
* EPA Superfund Jacksonville Ash Site Information – http://www.epa.gov/region4/waste/npl/nplfln/jaxashfl.htm
* A Citizen’s Guide to Phytoremediation – http://www.clu-in.org/download/citizens/citphyto.pdf
* Root turnover: an important source of microbial substrates in rhizosphere remediation of recalcitrant contaminants. – http://www.ncbi.nlm.nih.gov/pubmed/11999069?dopt=Abstract