WHERE THERE'S MUCK
There is a new way of dealing with hazardous toxic waste - and it has great green credentials.
Sunday 30 March 1997
"Where there's muck there's brass" is a saying that could have been coined for environmental clean-up. Dealing with hazardous existing waste in the US alone using conventional technologies is projected to cost at least $400 billion. But what if there was a real alternative that was cheaper, less disruptive and which came with impeccable green credentials? Well, there is.
Bioremediation and phytoremediation are the scientific names for processes which use bacterial micro-organisms and plants to deal with pollutants ranging from toxic metals and TNT to oil and chemical weapons. While bioremediation is a catch-all term for the use of bacterial clean-up agents, the use of plants splits four ways.
Phytoextraction is the process by which metal-accumulating plants concentrate metals from the soil into the harvestable parts of the plant above ground, where it can be removed, dried and burned to metal-rich ash, a process that the University of Maryland plant researcher, Rufus Chaney, compares to harvesting hay. "Burning allows recovery and recycling of the metals," he says. "The ash is similar to commercial ore and could be sold as 'Bio-ore'," he suggests.
The term rhizofiltration refers to clean-up operations where plant roots absorb and concentrate toxic metals from polluted effluents, a process used particularly with water pollution. It is also possible for plants to immobilise metal residues in soil, a process called phytostabilisation. Phytovolatolisation, by contrast, is where plants take up pollutants (particularly selenium and mercury) and then release them into the atmosphere - a pollution solution via dilution which is perhaps more pleasing as a rhyme than as an answer to the problem.
Proper studies using plants in environmental clean-up began in the late 1980s, but it was not until the early 1990s that an American company, Phytotech, took up the baton proffered by academic research and turned it into a corporation. The basic knowledge underpinning phytoremediation goes back even further. It was known in the Soviet Union in the 1950s that semi-aquatic plants like water hyacinth and duckweed drew up toxic metals such as lead and cadmium from contaminated waters, while the fact that plants such as the wild herb alpine pennycress thrived on zinc- and nickel-rich soils was used in the past by prospectors in the Alps and America's Rocky Mountains to help find ore deposits.
It is now thought that certain plants develop a liking for large doses of metal in their diet as its presence in stems and leaves protects them against certain fungal diseases and chewing insects. But how plants actually extract, store and tolerate the metal is still something of a mystery. A recent discovery by researchers at the Plant Gene Expression Center in Albany, California may have revealed the key in a gene for heavy-metal tolerance - parents of Metallica fans please note - which was found in a yeast, and dubbed "hmt1". Many plants produce molecules called peptides that bind metals for storage in vacuoles, the cell compartments in which plants either keep things they need or dump things they don't want. But metal-loving plants also use organic acids (such as citric acid) to bind high levels of metals. The hmt1 gene appears to prompt the manufacture of a protein that pumps more bound metals into vacuoles. If researchers can work out how to duplicate the hmt1 gene's metalworking activities inside high-yield crops, these could be used as super metal scavengers. Initial trials with tobacco have not been entirely successful, but the Albany team still predicts that gene-altered metal guzzling plants will be in use within a decade.
Just as old-time miners used metal-accumulator plants to strike it rich, biotech companies are also seeing dollar signs. The bioremediation market in North America and Europe is projected to be worth at least $1bn a year by the and of the century, while the use of phytoremediation against toxic metal contamination is expected to be worth around $400m a year as companies and government agencies move away from present methods such as excavation of soil for dumping in landfills or costly chemical processing that either removes metals or fixes them in the soil so they do not spread.
The earning potential of bio- and phytoremediation is huge, but British companies have been painfully slow to grab a slice of the action. It has been in America that the new technology has been most welcomed with hard cash and research effort, spurred on by tough environmental legislation. The irony of this is that the first ever successful commercial demonstration of phytoremediation's potential, though it took place in America, was a European Union-funded project under the control of two British academics, Alan Baker and Steve McGrath. The Pig's Eye Landfill in Minnesota was the battlefield, and alpine pennycress the weapon - a plant which can tolerate a zinc level in its leaves up to 60 times higher than that which would kill most other plants. Bioengineered pennycress is now expected to quadruple the rate of metal uptake, cutting the time the plant would take to cleanse totally the Pig's Eye site from a present estimate of 16 years to as little as four.
Despite this pioneering British triumph, there are few British counterparts to match the rash of companies which have followed Phytotech into a boom industry. Throughout the US, phytoremediation is being used at a growing number of sites, pitching Indian mustard against lead in New Jersey, sunflowers against uranium-contaminated water in Ohio, and bulrush against selenium in California. Indian mustard (Brassica juncea) is also tackling the environmental devastation around Chernobyl. In addition to accumulating up to an amazing 60 per cent of its dry root weight as lead, Brassica has shown a tolerance for radionuclides such as strontium, caesium and uraniam. Strontium was a particular favourite, showing in the plant's roots at concentrations 12 times higher than in the soil.
While plants are a powerful weapon against inorganic pollutants such as toxic metals, organic nasties such as oil are grist for microbial attack. Just as certain plants have adapted to dealing with metallic soils, so some natural bacteria have learnt to take advantage of the energy derived from oil and other hydrocarbons. Oil is, after all, a natural product, and what's good enough to power a car is also good enough to power a microbe. What's more, once the oil-loving bacteria have finished extracting the energy from hydrocarbons, the formerly complex chains of molecules have been broken down into two harmless products, carbon dioxide and water. Even better, some bioremediating bacteria actually leave by-products that are commercially useful, such as sulphite loving bugs that create methane. The basic processes used by bioremediating bacteria are also those behind fermentation, whose useful products include beer and wine.
Hydrocarbons present one of the two major challenges which are being dealt with by bioremediation, the other being dangerous military leftovers from the Cold War such as chemical weapons. It was, in fact, a military problem which led to the foundation of the modern bioremediation industry. When US forces in the Korean conflict of the early 1950s found their uniforms disintegrating in the humid climate, Howard Worne was commissioned by the US government to look into the situation and found that the culprit wasn't a new weapon but a micro-organism which could break down fabrics previously been considered non-biodegradable. Worne began an investigation to see whether other micro-organisms with similar powers existed, and eventually isolated one which could degrade phenol, a common organic pollutant. His work was taken up by others, and the continuing search for more and more pollutant-busting bacteria has spurred on bioremediation since.
The search has taken scientists to some strange places. Until recently, the most famous thing to come out of a whale's gut was Jonah, but Oregon State university toxicologist Morrie Craig might be about to change that. Intrigued by the tolerance of Alaska's bowhead whales to the large concentrations of oil and other industrial pollutants which have accumulated in their food chain, Craig began investigating the 1,000 or so species of bacteria that live in the leviathan's gut, and found himself staring at a bioremedial bonanza. He found some bacteria which digested hard-to-break-down oil carcinogens like naphthalene and anthracene, and others which made short work of PCBs (polychlorinated biphenyl), industrial pollutants which have long been linked to cancer. Furthermore, the whale bacteria proved to be anaerobic, capable of converting the pollutants to non-toxic substances in the absence of oxygen, in contrast to aerobic bioremediating bacteria found in seawater which need oxygen to function. This would make the whale bacteria particularly useful in tackling oil that has seeped underground.
Craig's gut instincts have also hit pay-dirt with a bacterium found in the stomachs of sheep and goats which has the ability to break down TNT, a common contaminant at munitions sites. Craig's discovery was timely, as other bacteria had only managed to break TNT down into other toxic substances. TNT is also facing the squeeze from a white rot fungus (WRF) which last year degraded over 97 per cent of TNT in the soil at one US test site. Another weapons material, RDX, was bioremediated completely at the same site.
Recent research at America's Oak Ridge National Laboratory has also found amoeba-associated bacteria packing a powerful punch against weapons material, TNT and napalut, as well as removing mercury from contaminated soil. And The Pentagon has found two new techniques to deal with 1,800 tonnes of mustard gas at a Diamyland base and 1,400 tonnes of the nerve gas VX in Indiana, in which bioremediation finishes off a process of treatment.
A more unexpected triumph for bioremediation came in 1993 when an Australian researcher, David Bourne, came across a member of the Sphingamonas bacterium which destroyed the poison in one of the commonest blue-green alga, microcystis. Also known as cyanobacteria, blue-green algae are being increasingly linked with cancer in humans, and until now they have proved difficult to deal with. Sphingamonas makes three enzymes which break down the most powerful toxin produced by microcystis into harmless amino acids, holding out hope for dealing with deadly algal blooms that are an increasing hazard on summer waterways.
Not all pollutants have fallen to the mean green treatment - 50 per cent breakdown is the best figure so far for dioxins, for example. But scientists are now working hard to fill in the gaps in their knowledge caused by the absence of comprehensive theoretical models explaining the interactions within variable combinations of bacteria/ contaminant/soil systems in the field. Sometimes, though, bioremediation seems simplicity itself, as the Australians proved in 1994. Take one phenol- polluted site at a Melbourne herbicide factory. Spread animal manure. Wait one year. Phenol gone. Where there's muck...
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