“Is phyto-mining commercially viable? “ asks Andrew Harris from University of Sydney
While there are numerous successful phytoremediation examples, to date there have been no commercial phyto-mining successes.
However, recent research conducted by the US Department of Agriculture suggests that crops or timber grown on nickel-rich soils typically yield around $50–$100 per hectare per year. A phyto-mining crop grown on the same land could produce an annual yield of 400 kg of nickel per hectare, worth more than $2000 even at today’s depressed market price.
This yield could be increased to over $3000 by selling the by-product energy generated when burning the plants to create the nickel rich ash. Much research remains to be carried out, particularly in the field of increasing metal uptake by plants either through genetic manipulation or the addition of reagents to the soil, and the potential leaching of metals during induced hyperaccumulation.
Furthermore an optimum method for recovering the metals once sequestered by the plants has yet to be determined.
Despite this, researchers in the US, UK, Australia and New Zealand signed an agreement to develop nickel phyto-mining technologies using two patented Alyssum hyperaccumulator species.
With these plants, soils containing as little as 0.05 wt% nickel are able to produce a profitable nickel harvest. In countries like Indonesia natural levels of 0.5% nickel are quite common. With proper soil management, this would allow a ‘phyto-mine’ to operate for centuries.
“The most astonishing example of a hyperaccumulator is the tree Sebertia acuminata from Asia, which produces a deep green sap containing 25% nickel by weight.”
The advantages of phyto-mining
Phyto-mining offers several advantages over conventional mining. They include:
- The possibility of exploiting ore bodies or mineralized soils otherwise uneconomic to develop.
- Its environmental impact is minimal when compared with the erosion caused by open-cut mining.
- The operation would be visibly indistinguishable from a commercial farming operation.
- A ‘bio-ore’ has a higher metal content than a conventional ore and thus needs less space for storage.
Because of its low sulphur content, smelting a ‘bio-ore’ does not contribute significantly to acid rain.
Special plants make phyto-mining feasible
Plant scientists have known for a long time that certain metals are essential for survival. Indeed, early prospectors in Europe used weeds known to accumulate metals as indicator plants to identify likely ore bodies. More recently it has been determined that some plant species are able to hyperaccumulate certain metals, up to concentrations several hundreds of times those found in non-hyperaccumulating plants. It is thought that this provides a measure of protection for the plant from insects and others herbivores.
Hyperaccumulator plants have two common characteristics:
- They show a bio-concentration factor, defined as the ratio of metal concentration in plant shoots to that in the soil, greater than one. In some cases bio-concentration factors up to 100 have been observed. The bio-concentration factor is a measure of the ability of a plant to take up and transport metals to the shoots, which are the parts that can be most easily harvested.
- They possess an enhanced tolerance (known as hypertolerance) to metals both at the cellular level and in the environment. This indicates a strong mechanism for coping with high metal concentrations. For example, a ‘normal’ plant will accumulate between 10–100 mg/kg nickel on a dry weight basis, however a nickel hyperaccumulator will accumulate this metal to a concentration greater than 1000 mg/kg.
The most astonishing example of a hyperaccumulator is the tree Sebertia acuminata from Asia, which produces a deep green sap containing 25% nickel by weight.
There have been several hundred metal hyperaccumulators identified since the 1970s. Most research to date has focussed on the phytoextraction of nickel because it has the highest number of known hyperaccumulators (some 300 species) and looks most likely to achieve commercialisation. Other likely targets for phyto-mining are the precious metals, gold, platinum and palladium and thallium.
ANDREW T. HARRIS
Andrew Harris is head of the Laboratory for Sustainable Technology and a lecturer in the department of chemical engineering at the University of Sydney; his research is focused on development of technologies that maximise resource and energy usage and minimise environmental impact.