Phytoremediation: Cleaning the Soil with Flowers

According to the International Phytotechnology Society website, phytotechnology is defined as the science of using plants to solve environmental problems such as pollution, reforestation, biofuels, and landfilling. Phytoremediation, a subcategory of phytotechnology, uses plants to absorb pollutants from soils or from water.

The pollutants involved can include heavy metals, defined as any elements considered as a metal that may cause pollution or an environmental problem, and that cannot be further degraded. A high accumulation of heavy metals in a soil or water can be considered toxic to plants or animals.

Why Use Phytoremediation?

Other methodologies used to remediate soils polluted with heavy metals can cost $1 million US per acre, whereas phytoremediation was estimated to cost between 45 cents and $1.69 US per square foot, lowering the cost per acre to the tens of thousands of dollars.

How Does Phytoremediation Work?

Not every plant species can be used for phytoremediation. A plant that is able to take up more metals than normal plants is called a hyperaccumulator. Hyperaccumulators can absorb more heavy metals than is present in the soil in which they are growing.

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All plants need some heavy metals in small amounts; iron, copper, and manganese are just a few of the heavy metals which are essential to plant function. Also, there are plants that can tolerate a high amount of metals in their system, even more than they need for normal growth, instead of exhibiting toxicity symptoms. For example, a species of Thlaspi has a protein called a "metal tolerance protein". Zinc is heavily taken up by Thlaspi due to the activation of a systemic zinc deficiency response. In other words, the metal tolerance protein tells the plant that it needs more zinc because it "needs more", even if it doesn't, so it takes more up!

Specialized metal transporters within a plant can assist in the uptake of heavy metals also. The transporters, which are specific to the heavy metal to which it binds, are proteins which assist in the transport, detoxification, and sequestration of heavy metals within plants.

Microbes in the rhizosphere cling to the surface of plant roots, and some remediating microbes are able to break down organic materials such as petroleum and take heavy metals up and out of the soil. This benefits the microbes as well as the plant, as the process can provide a template and a food source for microbes that can degrade organic pollutants. The plants subsequently release root exudates, enzymes, and organic carbon for the microbes to feed upon.

History Of Phytoremediation

The "godfather" of phytoremediation and the study of hyperaccumulator plants may very well be R. R. Brooks of New Zealand. One of the first papers involving an unusually high level of heavy metal uptake in plants in a polluted ecosystem was written by Reeves and Brooks in 1983. They found that the concentration of lead in Thlaspi located in a mining area was easily the highest ever recorded for any flowering plant.

Professor Brooks' work on heavy metal hyperaccumulation by plants led to questions as to how this knowledge could be used to clean polluted soils. The first article on phytoremediation was written by scientists at Rutgers University about the use of specially-selected and engineered metal-accumulating plants used to clean polluted soils. In 1993, a United States patent was filed by a company called Phytotech. Titled "Phytoremediation of Metals", the patent disclosed a method to remove metal ions from soil using plants. Several species of plants, including radish and mustard, were genetically engineered to express a protein called metallothionein. The plant protein binds heavy metals and removes them so that plant toxicity does not occur. Due to this technology, genetically engineered plants, including Arabidopsis, tobacco, canola, and rice have been modified to remediate areas contaminated with mercury.

External Factors Affecting Phytoremediation

The main factor affecting a plant's ability to hyperaccumulate heavy metals is age. Young roots grow faster and take up nutrients at a higher rate than older roots, and age may also affect how the chemical contaminant moves throughout the plant. Naturally, the microbial populations in the root area affect the uptake of metals. Transpiration rates, due to sun/shade exposure and seasonal changes, can affect plant uptake of heavy metals as well.

Plant Species Used For Phytoremediation

Over 500 plant species are reported to have hyperaccumulation properties. Natural hyperaccumulators include Iberis intermedia and Thlaspi spp. Different plants accumulate different metals; for example, Brassica juncea accumulates copper, selenium, and nickel, whereas Arabidopsis halleri accumulates cadmium and Lemna gibba accumulates arsenic. Plants used in engineered wetlands include sedges, rushes, reeds, and cattails because they are flood tolerant and are able to uptake pollutants. Genetically engineered plants, including Arabidopsis, tobacco, canola, and rice, have been modified to remediate areas contaminated with mercury.

How are plants tested for their hyperaccumulative abilities? Plant tissue cultures are used frequently in phytoremediation research, due to their ability to predict plant response and to save time and money.

Marketability Of Phytoremediation

Phytoremediation is popular in theory due to its low establishment cost and relative simplicity. In the 1990's, there were several companies working with phytoremediation, including Phytotech, PhytoWorks, and Earthcare. Other large companies such as Chevron and DuPont were also developing phytoremediation technologies. However, little work has been performed recently by the companies, and several of the smaller companies have gone out of business. Problems with the technology include the fact that plant roots cannot reach far enough into the soil core to accumulate some pollutants, and the disposal of the plants after hyperaccumulation has taken place. The plants cannot be plowed back into the soil, consumed by humans or animals, or put into a landfill. Dr. Brooks led pioneering work on the extraction of metals from hyperaccumulator plants. This process is called phytomining and involves the smelting of metals from the plants.