Plants as metal gatherers - who'd have thought it?

Having lived in South Africa for a while amongst the Johannesburg mine dumps, which were barren arsenic-riven, grooved huge mounds of red soil, with little growth in or around them .. I was really interested in reading more about these metal gatherers and reclamation plants – making an impact on lives around the world.

A creek in southern New Caledonia. Red colours reveal the richness of the ground in iron oxides and nickel.

I’ve written about the Eden Project before and how my mother and I have always enjoyed our visits, but more importantly we admire the Project’s concept of spreading the word on, amongst other things, reclamation ... just about everything within the site is reused, or reclaimed – they even made their own soil from the china clay pit spoils.

Having seen the barren dumps in South Africa, which in the late 1980s and early 1990s were flattened as they were reworked using improved technology extracting more gold from the spoil heaps, but leaving a bare landscape where the winds would blow the dust all over the place. I used to work in a new large retail store’s head office and we were dwarfed by the mine dumps around us .. but then suddenly the landscape changed and the building took on its own space on the landscape, relatively unnoticed before.

So having seen some dried up vegetation grasses and not much else on these dumps I was very interested to find out more about these metal grabbing plants. Any mine workings are likely to have very high residual concentrations of potentially toxic heavy metals such as copper, zinc, lead and cadmium and metalloids like arsenic, as well as having very low nutrient and organic matter content and they can be very acidic.

Painting of Bissoe Sulphuric Acid Works, Carnon Valley, Cornwall late 19th Century by Lamorna Birch

As an aside I didn’t know that arsenic occurred in the Periodic Table! .. now I know: apparently the Periodic Table contains metals and non metals .. and interlinking these are the metalloids, with their intermediate properties. For example – aluminium = metal; Silicon = metalloid; Phosphorous = non-metal.

The very harsh mine environment is the home of plants referred to as ‘metallophytes’, plants which have evolved metal tolerances and other adaptations. Charles Darwin, if he’d still been alive, would have been extremely interested in the evolutionary adaptation in plant populations.

Thrift below, Yorkshire fog grass to the right

In Cornwall some of the pasture grasses and certain plants have developed locally adapted populations tolerant to copper, lead and zinc – such as thrift, sea campion and the wonderfully named Yorkshire fog grass. In the Pennine ore fields running along the spine of England, where the rock is limestone and many of the old mines date back to Roman times, the vegetation has evolved and is much richer and can provide a continuous turf.

These metallophytes have particularly attracted renewed interest since the end of the War, but were known as indicator plants providing a powerful tool for mineral exploration .. the ‘copper flowers’ discovered in the Congo enabled early mineral prospectors to locate many of the major copper/cobalt mineral deposits in central Africa.

The pioneering work of the late Professor Tony Bradshaw, who collaborated with the Eden Project, has been central to mine-site rehabilitation and ecological restoration of the toxic wastes, thus enabling a sustainable plant cover to be established.

Plants adapt and evolve in ways we wouldn’t expect – some even restricting the uptake of toxic metals to their roots .. thereby limiting the potential impact on grazing animals and food chain transfers.

Alpine Pennycress - white
The Blue Sap of the Sebertia Acuminate

Some plants are not only highly metal-tolerant, but also exhibit metal uptake through the whole plant – these are known as hyperaccumulators. The most impressive of these is the Sebertia Acuminate, known locally as ‘Blue Sap’ of New Caledonia (Serpentine rich islands off north east Australia), as it bleeds, when slashed, a bright blue latex containing up to 25% nickel.

Alpine Pennycress (Thlaspi Caerulescens) in the UK has been used as a cropping plant, as it absorbs zinc and cadmium from the contaminated soils. While it's also found in the States - in Colorado, Wyoming and Montana. Alyssum as a genus has been used for years, but now forms the basis for a breeding programme for commercial nickel phyto-mining. Yellow Alyssum Murale has been used similarly on the barren serpentine soils of Oregon and northern California in the US. The dried plant material is then ashed which produces a high mineral content (upwards of 20%) – higher than any mineral source of the metal!

The Chinese Brake Fern similarly can be used for hyperaccumulating arsenic from soils and waste waters .. and is already being used commercially in its native China as well as the USA.

These plants with their unique gathering properties will continue to be studied and developed to enable us to clean up our environment by extracting valuable metal resources, with the least damage to a landscape already scarred by mineral extraction.

Today we need to do all we can to keep our world clean and safer – yet the world, unbeknownst to me, was already starting to do it on our behalf. It is extraordinary and wonderful this world of ours... – we just need a gold plant hyperaccumulator now!

Thank you Mr Postman for delivering this letter – my mother with her Cornish affiliations to the mining history will be really interested .. and we can discuss her visits to South Africa. She was cheerful today, but didn't want a story - however it's lovely seeing her happy ....

With thanks to Professor Alan Barker and the Eden Project 2009 spring publication for their informative article on this subject, which so inspired me.

Hilary Melton-Butcher
Positive Letters