Plants join the battle to clean up contaminated land

As concern over the contamination of land by an army of pollutants such as oil, uranium, and arsenic increases, a process using plants is emerging as the new eco-ally that can cut the cost of clean-up, and restore much larger sites than has been possible with traditional remediation methods.

Different ecotypes of <i>Thlaspi caerulescens

Different ecotypes of Thlaspi caerulescens

Zinc is accumulated in the epidermal cells in the leaves of <i>Thlaspi caerulescens
Degraded land contaminated with arsenic (Photo courtesy of IACR Rothamsted).
Cadmium concentration in the topsoils of England and Wales (values in mg/kg soil) (Photo courtesy of IACR Rothamsted).
Known as phytoremediation, the technology uses plants such as Indian mustard, poplars, sunflowers, and ferns, and works in a variety of ways to remove pollutants from the soil. Currently, 400 species are known which can be used in phytoremediation, some of which ‘hyperaccumulate’ contaminants into plant material which can then be harvested, whilst others act as siphons, removing contaminants from the soil and then venting them into the atmosphere. A further group of plants enable large molecules, such as hydrocarbons from oil, to be broken down.

To find out more about this revolutionary technology, Helen André interviewed Professor Steve McGrath, of the Institute of Arable Crops Research, Rothamsted in England, who, in 1990, was a member of a British team of scientists that carried out the first ever field demonstration of phytoremediation.

Name: Professor Steve McGrath
Position: Programme Leader for soil protection and remediation at IACR-Rothamsted

How widely-used is the technology at the moment?
I’d say it was at an early stage – it is what we call a developing technology for remediation – and it has only really been thought about for about ten years, with the first research starting in about 1990.

In the States people have tried to use it in the field. In particular, there’s one version of phytoremediation which uses chemicals to enhance the up-take by plants. That was put on a commercial scale quite early on, and so some of the best examples of phytoremediation in the field have taken place in America. Elsewhere, I think it is basically still in the development stage, where people are trying to work out the systems and work out which plants or which crops to use, and try to suit them for the type of contamination. There are many developments in different sub-types of remediation which are taking place right at the moment.

Could you tell me a little bit about your current research.
We’re actually looking at several things. The first is natural hyperaccumulation. Hyperaccumulation is

"Hyperaccumulation is the natural ability of some plant take up huge amounts of metals."
a natural ability of some plant species – normally wild species, and normally quite unusual or even rare species that grow on naturally metal-rich areas of the Earth, which have evolved to take up huge amounts of metals. We went into the field with some of the first tests of these wild plants in about 1990, and did a few field seasons of that. But what we really need is more information about how they work, because if you are going to improve the system you need to know what the mechanisms are and how you might be able to either tweak those mechanisms, or move them into other species that might be suitable for the region or the climate or the harvesting regime.

We’ve been working on the basic understanding of this phenomenon called hyperaccumulation for a few years, particularly in relation to natural hyperaccumulators of zinc and cadmium. Other researchers are looking at hyperaccumulation of nickel, and then there are recent developments, for example, the hyperaccumulator of arsenic that has just been discovered, and we are working on that in collaboration with some American partners at the moment

Outside our research there are other developments such as making plants that transform mercury, by genetic engineering processes, which is happening in the States, with a view to them being used at some stage - if there is a public acceptance, of course - for the remediation of mercury-affected sites. So on the mechanistic side there is a lot going on, and it focuses on different contaminants and understanding particularly how the system works.

One of the things we have done recently, is to discover a massive accumulator of cadmium. It is a natural population of perhaps the best studied species for zinc, cadmium and nickel accumulation, called Thlaspi caerulescens This is almost the model species for looking at hyperaccumulation. It grows in different places in Europe. In particular, we discovered the population of this species growing in the Southern mountains in France has an extraordinary ability to take up cadmium quite naturally. This is interesting for two reasons. Firstly, we would like to know why a plant is taking up so much of a non-essential metal – it is completely non-essential, unlike zinc for which there’s a certain requirement; and secondly, it is accumulating up to 1.4% cadmium in the dry matter of the leaves, which is absolutely unknown to biology – it is absolutely massive. How it does this, and how it actually takes it up, is a matter of some great interest which we’re looking into at the moment.

What type of plant is Thlaspi caerulescens?
It’s called Alpine pennycress. It is a rosette type of plant and it has quite thick leathery leaves, and flower stalks that come up from the rosettes that usually have white or pink flowers on them. The plant is relatively small, so at the moment, scientists are looking at how it actually achieves this massive hyperaccumulation, and then either breed it, or try to engineer these traits into other plant species that might be of higher biomass, or they might be more suited to the local climate or to the harvesting system that you would want to employ in a particular area. So the whole idea is to try to improve the efficiency by this sort of general process of breeding or selection, or just by searching for more efficient plants.

Other than natural hyperaccumulation, we have also been looking at the system I mentioned earlier from America where they use chemicals to induce up-take of toxic metals by plants. The work there focussed a lot on lead, and we’ve been looking to see if it is effective and environmentally acceptable. So, for example, to make lead come off the soil particles, which is where it is normally strongly bound, you spray on releasing agents – things like the chelating agent EDTA which used to be used in washing powders. This releases the lead into solution. You rely on growing fairly big plants – quite normal plants – to suck up all the solution and take up the lead, but the problem is normal plants don’t have the tolerance to high levels of metals like lead.

We have been focussing on what happens in the soil after spraying on these releasing chemicals. It turns out that it may not be very acceptable to do this in the open because they can start the metals moving down through the soil. If you can’t control the water cycle, metals are likely to go into nearby water bodies or water supplies. If this process is going to be used it would have to be done in a completely contained way. However, if you are going to go to the bother of digging it up and putting it in either a concrete bed or digging up a huge area or dumping it on some plastic that’s going to be very expensive. You probably wouldn’t do this for large areas – which is what phytoremediation of metals can be very useful for. So, that technology, we think, needs a bit of a rethink.

The third area which has become important more recently is the phytoremediation of groups of organic compounds which can be broken down in amongst the roots of the plants – in the rhizosphere. It is a good area to break down contaminants in land because the plants produce photosynthates which leak out of the plant roots, providing energy for the microbes. These conditions stimulate the natural microbes in the soil around the roots to break down the organic contaminants. How you do that in contaminated land, and how you get the maximum stimulation of degradation is what we have been focussing on, in order to grow plants and supply the microbes with the right conditions. We have been looking at a group of compounds called polynuclear aromatic hydrocarbons – which are quite persistent pollutants, normally derived from the combustion of fuels such as coal and oil.

There are other researchers around the world who have been using phytoremediation, for example,

"They act as solar-driven pumps to pump the water back up and out into the atmosphere and disperse the pollutants."
with the more mobile organic compounds – there’s a compound called TCE (trichloroethelene), which has been used in various industries including the degreasing or engineering industries. It normally forms plumes moving underneath the soil surface. Poplars, for example, have been used to pump the water out of the ground, drawing back these contaminants to pump them through the plant and dilute them in the air, and they are then blown away. This represents another interesting development in this whole area of phytoremediation which is growing almost daily. They act as solar-driven pumps to pump the water back up and out into the atmosphere and disperse the pollutants.

Over what timescale does phytoremediation take place?
It really depends on the situation. If the soil is not far above the acceptable limit for the contaminant of concern, it can be done in one growing cycle. That is to say, assuming that the conditions are appropriate for plant growth, something like four to six months. On the other hand, if the gap between the levels and what is acceptable is large, it could take some years. However, unless there is pressure to re-use the land quickly, it is a low cost method that is well suited to being used over longer time periods.

Is there any danger from plants releasing pollutants into the atmosphere?
This, of course, is the big question. Whenever there is a pollutant that can go into vapour phase, or can be aspirated up into the atmosphere, you always have to do very careful analysis and risk assessment. Essentially, the key is the amount of dilution. For example, if the plants are pumping out mercury, or an organic compound, you would have to convince the appropriate authorities that the rate that was coming out per day, and the dispersion per day was resulting in incredibly low levels by dilution in the atmosphere, so that there is no chance of any build-up being toxic either to animals or to local people using the land.

There’s a case in point, in California. There is a natural problem of selenium, an essential element that is toxic in excess. There is a selenium-rich area on the west side of the central valley in California. The drainage in that area is very rich in selenium because the element is quite soluble, and they’ve had health problems in wildlife due to high selenium concentrations. They’ve used plants that take up large amounts of selenium, that, when combined with the associated microbes in the soil and on the leaves, volatilise selenium into the air. The selenium is then transported quite some distance, some coming down on the east side of the valley, which is actually deficient in selenium. Researchers have also been taking hay of a high selenium crop and making it into pellets which can be used as a supplement to animal feed.

Other than cost, what other particular advantages of phytoremediation are there?
Cost is the biggest one. The aesthetic side of it is good. Normally you have healthy plants with flowers and quite nice colours on the site at the time of remediation, which makes it look attractive. Phytoremediation appears to get a very high customer or public acceptance. People who live nearby, or would pass the site are normally in favour.

Secondly, in some cases phytoremediation is the only possible method. So, for example, if you have large areas that are contaminated there’s no way that you are going to spend the millions of dollars per acre to clean them up. The technology is low-cost and extensive – you are basically growing

"Phytoremediation appears to get a very high customer or public acceptance."
plants and using standard equipment to sow and to harvest the plants. For example, agricultural phosphates in the past were contaminated with cadmium. With phytoremediation you could take out that cadmium which is built up from quite a long period of phosphate fertiliser use, or indeed, to make phosphate fertiliser use sustainable. You could think about growing crops that accumulate cadmium say once every ten years to take out any residue which has been built up, and then carry on using the land in a sustainable way. If you can’t get the cadmium out then it starts to affect the food quality, then you would have to stop that use of the land.

Is there any problem of pollutants getting into the food chain?
Well, I think there are several issues here. If you are using hay or grain of a normal crop there might be some temptation for that to get out into the food chain in some way because it would look like normal fodder or grain, and there is a chance that might get out. Natural hyperaccumulators, however, don’t look like they should be eaten, and in fact, some of them are rather unpalatable. Studies show that insects basically don’t like them, they’re deterred from eating them by all the metal that’s present. So, the accumulation might be a natural protection system that they had in the first place, so that the predators and grazers are deterred.

Is it possible to recycle metals taken up by hyperaccumulators?
It depends very much on the value of the metals. There are a number of metals which are of high value and the dry hay material can be taken to metal processing plants and recycled.

"A lot of research is being carried out into the biomining of nickel..."
The best example is probably nickel, where nickel is quite a valuable metal; it is accumulated by quite a large number of natural hyperaccumulators in different parts of the world. A lot of research is being carried out into the biomining of nickel, where large areas exist that are affected by diffuse nickel contamination or from spoils from mining. It has already been shown that the economics are beneficial for drying the material and taking it away and smelting it back into nickel.

For other metals that are of lesser value, this becomes a bit less economic. At one extreme, if you can get one part per million of gold into a plant it is actually worthwhile. However, if you can get 1000 ppm or more of lead into a plant, lead is not of very high value, and there isn’t a huge market for it, so it is unlikely that you would want to actually recycle the metal.

However, there’s another aspect to this. Those metals that are in between, metals like zinc, for example, could be viable for recycling. You would need to convince someone with a zinc smelter that this is worthwhile and that it is not negative economically. The hay certainly burns very easily if you put it into a pyrometallurgical smelter, so that the material can be easily recycled into the zinc product, even though the actual value of that zinc might not be huge.

The whole area of recycling is the way to go, but it still needs a lot more development, and a lot more study of the economics and of the physical processes before we can get there. It is preferable to the other alternative which is to take the plant material containing high levels of contaminants and put it into a safe landfill. You have then taken the contaminants from an area where it is at relatively low concentrations but dispersed over large areas and concentrated it into a small amount of material which you then have to contain.

Where do you see this technology going in the future?
Improvements of efficiency is the key, to produce a package which is quicker, more efficient, and more convenient for growth in different regions. I think all those developments will take place, and so then we’ll have a much bigger package of basic materials or technologies that we can apply in different regions and for specific problems. I think that it will develop, along with the whole area of using plants to stimulate the degradation of organic compounds of various types. These will be either the accidentally man-made ones, or ones that are used purposefully, such as pesticide compounds which we would like to contain, and degrade naturally in the environment.

Relevant Links

Institute of Arable Crops Research, at Rothamsted
European Union Phytoremediation Projects
US and International Markets for Phytoremediation, 1999-2000 Executive Summary
Assessment of Phytoremediation as an In-Situ Technique for Cleaning Oil-Contaminated Sites
Technology Evaluation Report: Phytoremediation requires Adobe Acrobat
Adobe Acrobat
Missouri Botanical Garden's phytoremediation website
Bioremediation and Phytoremediation glossary
Phytoremediation bibliography
Remediation Technologies Development Forum
International Journal of Phytoremediation
Community Lead and Phytoremediation Research Lab
EPA Citizens' Guide to Phytoremediation
Tree Tec Environmental Corporation


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