Imagine what your home city, town or village would look like if plants and trees grew money. Chances are, empty buildings would be replaced by gardens, deforestation would no longer be a problem, and hedge fund managers would switch to managing actual hedges. But what if we drove this to the extreme and used all our land to grow money, and none to grow food? This glimpse of a different future is perhaps not such a nice vision after all, but gives you some idea about the potential and the problems with using plants to extract certain metals from the soil, processes called phytoremediation and phytomining.
1. The main picture shows you some fronds of fern, probably Dryopteris filix-mas, a popular garden plant in the UK, lying on a glove box, together with some acids that can bind dissolved metal ions. While there are many chemical reactions which can be carried out in air, some compounds are only safe and stable when handled in an atmosphere excluding oxygen and water. Such an atmosphere can be created in a sealed box, normally filled with nitrogen or argon gas. The handling and manipulation of reagents, reactions and equipment in such a glove box is possible by using rubber gloves integrated into its walls, hence the name. Glove boxes also have air locks so materials can be taken in and out without introducing oxygen and moisture. While awkward to use and expensive to purchase and maintain, glove boxes are an important tool in modern chemical synthesis. They are also commonly used by researchers working with hazardous substances such as viruses, but there the aim is to contain the dangerous material, rather than protect it from oxygen and moisture.
Plants have only limited options when it comes to protecting themselves from hazards as they cannot just move away. Most plants cannot tolerate high concentrations of toxic elements in the soil and water, so they become poisoned and die, or simply do not germinate on/spread to such land. Toxic elements include manganese, iron, cobalt, nickel, copper, zinc, molybdenum, cadmium, mercury (all of these are (transition) metals), but also arsenic and selenium (main group elements which are not considered metallic). Some of these elements have biological functions required for normal growth, but become poisonous at higher concentrations.
Unlike the Dryopteris in the picture, the Chinese ladder brake, Pteris vittata, is a fern which has been found to have a much higher tolerance for the toxic element arsenic. In fact, it accumulates arsenic in its leaves without becoming poisoned, making it a hyperaccumulator of arsenic. For a teaching resource from the Florida Center for Instructional Technology at the University of South Florida, follow this link, and for a scientific review, have a look at Environ. Sci. Technol., 2009, 43, 8488–8495). This fern has been grown on contaminated land in the hope that the the plants can eventually be harvested and removed, thereby helping to clean up areas where nothing else will grow.
This process is called phytoremediation (see a page hosted by the Missouri Botanical Garden for a nice collection of resources), and other plants have been identified that are hyperaccumulators for the “heavy” metals listed above – some examples thought to hyperaccumulate are shown in the photo below and include the sunflower, Helianthus annuus, the sneezeweed, Helenium autumnale, as well as ferns. As we have mentioned in one of our previous posts, sunflowers have been planted on contaminated land near Fukushima because it is hoped that they will not just help to brighten up deserted land but also hyperaccumulate radioactive elements from the soil. Similar efforts have been made near the site of the Chernobyl nuclear accident, in this case using plants in the mustard family, Brassica. Bioremediation using mushrooms has also been trialled.
While phytoremediation sounds like a neat way of using plants to clean up such wasteland, it is often not enough. Plants often only hyperaccumulate one element, whereas there may be a mixture of compounds present. In addition, they can only be used for light contamination in the top layers of soil, while deeper layers of contamination cannot be reached unless roots are very deep. While plants are growing, access to sites needs to be controlled, so these compounds do not end up in the food chain. Then the plants will need to be harvested, removed and processed, meaning further costs and potential risks. It is also not a fast process, and several generations of plants may be needed to achieve some remediation. On the other hand, where contamination is light, this can make use of land and labour that has no other possible uses. You can find a bit more detail about these considerations via this link.
Another aspect of using hyperaccumulators is phytomining (for a slightly light-hearted look at this, try this link), where plants are trialled for the extraction of valuable and precious metals, most often gold, from soil disturbed by abandoned mines, or even just normal soil. While this sounds promising, gold needs to be made soluble by the addition of other compounds, which can be toxic and harmful in their own right. If you are interested in a scientific review, have a look at the Journal of Geochemical Exploration, 2013, 128, 42-50.
2. How and why plants hyperaccumulate is not fully understood yet (Plant Science, 2011, 180, 169-181). Plants that tolerate high metal concentrations may simply have evolved to benefit from the reduced competition for resources on land where other plants struggle to survive, or they may have found accumulations of metal in their leaves a good defence against herbivores. It is also possible that plants need microorganisms in the soil to supply the compounds (ligands) that bind metal ions, although some may be of plant origin. Interactions with such ligands are called ionic bonding and rely on the electrostatic attraction of ions with opposite charges. Most plants can only access metals in the soil when these are dissolved in water, as they can take up aqueous solutions through their root system. If a metal is in its elemental state or locked up in compounds that are not water soluble, it cannot be taken up by hyperaccumulators.
Luckily, in the presence of the right additives, it is quite easy to ionise metals and dissolve them in water, and they often become bound by charged ligands, such as the deprotonated forms of citric, aspartic and malonic acid shown in Scheme 1, which have been found in nickel hyperaccumulators (Phytochemistry, 2012, 81, 80-89). While these acids occur naturally and are harmless, other metal ions may require more dangerous additives – gold phytomining can be made more successful by the addition of cyanides, for example. Many of these metals also have a biological function and the amino acid histidine (also shown in Scheme 1) is known to bind copper ions in living organisms, so it seems likely that a whole cocktail of potential ligands is available in many biological systems, but only hyperaccumulators survive in the long run.
Many ligands have more than one site for binding to metal ions and can either form several ionic bonds with the same ion, or connect several ions as shown for the trinickel dicitrate complex in Scheme 2. Experimentally, it has been observed that forming several bonds between one ligand and a metal ion is favoured over forming bonds of the same kind with several ligands and this observation has been called the chelate effect. To understand the chelate effect, we need to take a brief detour into the Laws of Thermodynamics. The Second Law states that the entropy of an isolated system increases in the course of a spontaneous change. Entropy can be thought of as a measure of disorder, and at first sight, linking this to the chelate effect is difficult, because the complex of interest starts out as a metal ion plus charged ligand, both able to move around in solution (more disorder), and ends up more ordered, as a single complex. However, each ion is surrounded by solvent molecules which are weakly attracted to the charges, so solvent mobility is reduced. On chelation, these solvents are released and there is a net increase in independent molecules when compared with ligands that have only one binding site. So, even though the ligand binding itself might be predicted to be unfavourable, the entropy of the solution does increase, making chelation entropically favourable and hence a spontaneous process. Chelating ligands are also used in medicine and the deprotonated form of ethylenediaminetetraacetic acid, edta, shown in Scheme 3, is used to treat lead poisoning and works by capturing the lead(II) ions in a complex which can then be excreted.
3. Selective and controlled/reversible binding of metal ions is also the target of many groups working in synthetic coordination chemistry, with a range of applications including in medicine (Coord. Chem. Rev., 2010, 254, 1686-1712), sensing (Chem. Soc. Rev., 2013, 42, 1500-1524) and the treatment of nuclear waste (Acc. Chem. Res., 2010, 43, 19-29).
Macrocycles are a group of compounds that can fit the bill, and different groups can be incorporated into their structure to provide a binding site that is very specific to the compound of interest. Scheme 4 shows a macrocycle that preferentially binds the radioactive ions thallium and uranium, potentially activating them to new chemical reactions (Chem. Sci., 2014, 5, 756-765). However, macrocycles do not just capture metal ions, but can also be potential targets for pharmaceuticals in their own right (Nature Reviews Drug Discovery 2008, 7, 608-624). This field is vast and beyond the scope of this post, but shows perhaps that capturing and controlling metal ions continues to be valuable to both plants and other organisms, including us.
Contributors: Natalie Fey (research, images and writing), Jenny Slaughter (photos, editing), Ben Mills (photos), Emma Kastrisianaki-Guyton (research).