Good to grow

Why mine for metals when we can cultivate them instead, asks Katia Moskvitch

ALAN BAKER squatted to get a closer look at the delicate white flowers that shouldn’t have been there. He knew that the soil in that part of England’s Peak District was laced with metals toxic to most plants. Yet here, in the desolate surroundings of an old lead mine, he had found spring sandwort flourishing. That was 45 years ago and the flowers that Baker spotted on his hike have guided his career as a plant scientist. Over the years, he and his colleagues have shown how some plants can take up certain metals in such large quantities that it is possible to “grow” a crop of metals. The idea could help regenerate landscapes blighted by mining and help farmers improve poor soils. And with metal prices soaring amid ravenous global demand for diminishing resources, it might even become a mining technique in its own right. Phytomining, as the approach is known, may be about to hit pay dirt. A US patent covering the idea is set to expire next year, and that could allow the technique to blossom, giving metal-loving plants plenty of scope to transform the landscape. When Baker came across the carpet of spring sandwort, he was a 20-year-old biology student at Imperial College London. Realising how unusual the find was – people have known for centuries that metals and plants don’t usually mix – he decided to combine his love for ecology with his childhood fascination with metals by doing a PhD on how plants cope in metal-rich environments. It was the early 1970s, and more and more countries were beginning to tackle the environmental consequences of mining. In the UK, researchers were intent on finding metal-tolerant grasses that could be grown on contaminated soils to conceal the underlying problem, says Baker. He wanted to try something different: to discover plants that wouldn’t just tolerate metal, but suck it up from the ground and lock it into leaves. On the other side of the Atlantic another young scientist, Rufus Chaney, had a similar idea. Chaney was based in Beltsville, Maryland, where he was working for the US Department of Agriculture (USDA) on using plants to decontaminate minefields and brownfield sites. After early experiments with farm crops like corn (which didn’t work), he began to scour the scientific literature. He came across research by the renowned geochemist Robert Brooks and his colleague Roger Reeves on the nickel-accumulating ability of some plants from the island of New Caledonia in the Pacific Ocean. Slowly Chaney unearthed more and more evidence that certain plant species just loved to take up metals. In 1979, he presented his findings at a conference in Los Angeles and put forward the idea of phytomining. Sitting in the audience was Baker, on his first conference trip abroad. The two researchers talked heavy metal over a beer and vowed to collaborate. While Chaney continued his research in the US, Baker criss-crossed the globe on the hunt for “hyperaccumulators” – plants that naturally absorb metals in large quantities. Wherever he went, he discovered new examples. On the islands of Palawan in the Philippines, for example, he stumbled upon an astonishing shrub. “The soils where it was growing were extremely enriched in nickel,” says Baker. “We cut the stem, and bright green liquid started pouring out of it.” The plant’s sap was a hefty 9 per cent nickel. Baker’s team called the plant Phyllanthus balgooyi. By 1992, Chaney thought that phytomining was ready for prime time. He and Baker started work on phytomining nickel with Scott Angle, then at the University of Maryland in College Park, and Yin-Ming Li, a plant breeder from USDA. They even found sponsors – a company called Viridian Environmental, backed by a family of lawyers in Houston, Texas, called the Nelkins. “They were looking for some idea relevant to the environment that might make money,” recalls Chaney. “ The shrub was astonishing – its bright green sap was a hefty 9 per cent nickel”

The Nelkins offered to fund large-scale  trials. In return, they got the rights to any ensuing patents. Phytomining is a fairly simple concept: locate soils rich in metal, grow plants that absorb those metals through their roots and concentrate them in the leaves, and then harvest them. To recover the metal, you burn the plants and process the ash in a smelter or refinery. The clever part – which is what the patents covered – is finding species that take up economically viable amounts of metal, and treating the soil to maximise the uptake. Dash for ash Phytomining can be made to work for a few metals, including cobalt, thallium and even gold. So far nickel has shown the greatest commercial potential. Nickel is used in stainless steel and other corrosion-free alloys, as well as in batteries (see diagram, below). There are around 400 known nickel hyperaccumulators, containing more than 1 milligram of nickel for every gram of dried foliage. The hardest-working of these are yellow alyssums, belonging to a genus of bushy perennial shrubs native to Turkey, the Balkans and south-eastern Europe. Some Alyssum species absorb such huge amounts of nickel that their leaves are toxic to animals. In 1996, Baker’s team gathered Alyssum seeds in Europe and, working with Richard Roseberg and Van Volk, both at Oregon State University in Klamath Falls, they planted them on a nickel-rich trial site in south Oregon. They worked out which fertilisers would adjust the soil chemistry to produce high yields of nickel, determined the herbicides needed to control grassy weeds, and bred improved plants that absorbed even more metal. The experiment was so successful that it caught the eye of Canadian mining firm Inco, now part of the Brazilian giant Vale. When the Ontario government ordered Inco to clean up land contaminated by its nickel refinery in Port Colborne, the company asked Viridian to do a trial run using Alyssum murale and Alyssum corsicum. At first the plants grew well,  though they succumbed to fungal disease in the first year due to poorly drained soils. Improvements in the drainage and planting, however, soon led to successful crops. With soil decontamination seeming to work, Inco wanted to test Alyssum’s phytomining potential. Viridian processed 500 kilograms of plant ash in an electric furnace and successfully extracted more than 100 kilograms of pure nickel within a few minutes. Phytomining had proven its mettle. For nickel, the process is commercially viable at soil concentrations as low as 0.1 per cent, says Chaney. In contrast, traditional mining requires ore that is at least 1 to 1.5 per cent nickel. With commercial backing and the mining industry’s interest, phytomining looked set to take off. But then everything stalled. In 2004, Viridian stopped all development. Inco wanted to expand the collaboration, but the Nelkins refused. No one knows why, and Jay Nelkin of Viridian didn’t reply to New Scientist’s interview requests. “They just sat on the patents, never conducted commercial phytomining after the trials and a two-year contract with Inco finished,” says Chaney. Although Viridian’s investment helped to prove that phytomining works, its grip on the patents held back industrial application of the technology. Yet it hasn’t stopped research. “As long as we are just doing experiments and not selling the biomass, there is no violation of Viridian’s patents,” says Chaney. And there’s much still to study. As traditional nickel sources decline, the mining industry is tapping into significant deposits in the tropics, in places such as Indonesia, the Philippines, Brazil, Madagascar and New Caledonia. The ore is often in regions covered by rainforest, and the vegetation and topsoil have to be removed before firms can collect the soil beneath for processing. “These operations are huge because ore grades are low, so they are clearing a great many hectares every year,” says Antony van der Ent, an environmental scientist at the University of Queensland in Brisbane, Australia. The impact is devastating, and restoring greenery to the landscape is difficult because the soil isn’t very fertile. Rehabilitation usually involves planting a mixture of native and exotic species, supported by significant amounts of fertiliser. But there is an alternative, says van der Ent: planting native hyperaccumulators. They would thrive in areas with soils rich in magnesium, iron and nickel, and provide an annual harvest of nickel to landowners as an added incentive. Van der Ent has redoubled his efforts to find tropical hyperaccumulators, hacking his way through the jungles of Borneo to find nickelrich soils and any plants that grow on them. The samples he took were preserved in liquid nitrogen and then taken to Japan for analysis using cutting-edge synchrotron X-ray absorption spectroscopy, which can reveal the chemical forms of nickel inside the tissues. Fields of metal Such high-tech techniques may help the poorest people. In Indonesia, van der Ent and his team talked to many farmers, a lot of them living in poverty. Crops don’t grow well on the country’s nickel-rich soils, which is why farmers turn to rainforest areas, where they use slash-and-burn techniques to clear plots to grow food. What if they were shown how to grow nickel hyperaccumulators on large areas of metalrich soils that are already cleared, logged or otherwise devoid of primary rainforest? Van der Ent believes that phytomining could become an alternative to traditional agriculture, or at least a source of extra income. Locals could become nickel farmers and save the rainforest on the side. The same principle could help hill farmers in Albania. The soil there is rich in nickel, poor in yield and riddled with Alyssum. For centuries farmers considered it a weed, but people in a village close to Tirana are now being paid by French researchers to let it grow on a field next to their crops. Plants found while hacking through the Borneo jungle were preserved in liquid nitrogen CENTRE FOR MINED LAND REHABILITATION/SMI/THE Aida Bani at the Agricultural University of Tirana hopes that Alyssum will extract the nickel and eventually improve the quality of soil, so that food crops can grow more successfully. Come harvest time in July, she will take the plants to her collaborators’ laboratory at the University of Lorraine in Nancy, France, for analysis. The trial has been running for five years and Bani says progress has been steady. “We’ve found that the amounts of phytoextracted nickel have improved from 22.6 to 105 kilograms per hectare,” says Bani. It’s all down to the right fertiliser, using herbicides to limit competing grasses and weeds, and cultivating Alyssum plants bred for their love of nickel. What the villagers haven’t been able to do yet is earn an income from the phytomined nickel itself, because of Viridian’s patents. But with those set to expire in 2015, everything could change. Already, extensive trials are being conducted in Cuba and Brazil. The Brazilian Agricultural Research Corporation (Embrapa) has been tasked by government to develop ways of restoring areas devastated by mining. Its researchers are busy spotting local hyperaccumulators, says Leide de Andrade of Embrapa. Among more than a hundred species of plants that were identified in a mining area of northern Brazil, 8 per cent turned out to be hyperaccumulators. In Indonesia, French mining giant Eramet also wants to use phytomining for biodiversity rehabilitation. Meanwhile, Mike Johnson, an adviser to a number of major mining companies including Rio Tinto, says that the prospect of using phytomining for land remediation is very attractive. Mining companies are notoriously conservative, though.  “They only change their existing methodology on the recommendation of people such as me, who sit on the interface between science and industry,” he says. He believes that the expiration of the patents could be a seminal moment and is prepared to look at proposals on behalf of Rio Tinto that help to improve land management after a mine closes. Baker, now professor of botany at the University of Melbourne, Australia, and Chaney are desperate to see the day the patents expire. Baker hopes that mining company Vale and others will then take up the technology. “In Australia, I worked with a number of gold mining companies, and they were quite intrigued by the phytomining idea,” he says. “But because the patents were locked up, they didn’t want to wait and quickly lost interest.” The pair’s original idea – to use plants for real mining – might take off, too. Bani’s colleagues in Nancy are setting up a phytomining start-up called Econick, partly funded by the French government. Among their many potential partners is French company Soléo Services, which wants to cultivate hyperaccumulators on industrial waste and extract the nickel for use. “Never has there been such a good opportunity, and we are determined to capitalise upon it,” says Baker.  ■