Feature

The Copper Flowers of Zaïre

Metal-tolerant plants from Zaïre in Central Africa can indicate the presence of minerals and ancient artefacts below the surface.

R.R.Brooks

Shaba Province in Zaïre, south-central Africa, contains some of the world's most extensive areas of copper/cobalt mineralisation. The outcrops have a total surface expression of about 20 km² and are spread over a 20,000 km² area of what is known as the Shaban Copper Arc.

The outcrops are easy to recognise because their phytotoxic soils are devoid of trees and covered with a highly specialised sparse flora containing a large number of endemic species, some of which are confined to even a single small outcrop of a few hectares in area.

The copper/cobalt flora has been studied by Massey scientists for the past 15 years. It contains over 50 species confined entirely to areas of mineralisation and over 40 species that are able to hyperaccumulate either copper or cobalt. The term "hyperaccumulator" refers to plants able to accumulate over 1000 mg/g (0.1%) of specific heavy elements in their dried tissue. Of the Zaïrean hyperaccumulators, 15 are specific to copper, 17 to cobalt, and nine hyperaccumulate both elements. The copper plants of Zaïre, especially the hyperaccumulators, have importance in the fields of mineral exploration, archaeological studies, the green remediation of polluted soils, and "biomining" -- the extraction of metals from uneconomic ores.

Mineral Exploration

The copper/cobalt plants are almost invariably geobotanical indicators of mineralisation, and have been used for over 60 years in mineral exploration. The so-called "copper flower", Becium homblei, was used by prospectors in the 1950s to map copper mineralisation over the Copper Belt in Zambia, since it was found almost exclusively in soil containing over 0.01% copper

Other plants, such as Haumaniastrum katangense, have also been used in mineral exploration with some success. This plant has been found not only over copper deposits but also over soils contaminated by emissions from the smoke stack of the great copper smelter at Lubumbashi in Shaba Province.

Geobotanical prospecting relies purely on visual observation of the distribution of plants. Biogeochemical prospecting involves chemical analysis of the plant material to detect mineralisation in the soil. The geobotanical method can actually be used to gauge the copper/cobalt content of the substrate because of the narrow range of elemental concentrations that specific plants will tolerate.

For example, in the Shaban Copper Arc, Uapaca robynsii is found in soils with 0.025-0.035% copper. Below this level the plant cannot grow because of competition from the local forest. Above 0.035% copper, the Uapaca is replaced by another indicator, Loudetia simplex, which in turn is superseded by Xerophyta retinervis growing in soils containing 0.5-1.2% copper and 0.03-0.15% cobalt.

Copper Flowers and Archaeology

The relationship between vegetation and archaeological remains might seem to be tenuous. However there is indeed a link, via the presence of mineralisation in the soil. The term "phytoarchaeology" has been used to describe this relationship. A good example of this is again found in Zaïre.

During the pre-colonial period in what is now Shaba Province, the 14th-century Kabambian culture had mastered the art of copper smelting and used to fashion copper crosses (croisettes) used as currency. Their furnaces were usually backed onto old termite mounds situated near streams for the water supply and near stands of trees to supply charcoal. These sites were often far removed from copper deposits. The ancient artisans brought copper ores to the smelting sites and usually stayed there for a few seasons until the local supply of charcoal had been exhausted.

With the passage of years, the furnaces on their termite mounds weathered down to ground level. There was no longer any visible sign of their presence until the sites were colonised by a copper-tolerant plant community.

Modern archaeologists have been able to identify the sites from their carpet of metal-tolerant plants far away from known mineralisation, and have uncovered numerous artefacts at depth, including the famous croisettes.

Green Remediation

A new technology has been developed in recent years, based on the principle that a crop of a hyperaccumulator species might be grown on poisoned soils and harvested, reducing the heavy metal burden. The term "green remediation" has been coined to describe this new technology.

The process of green remediation begins by growing a crop of the hyperaccumulator for one season. Many of these species are annuals and this growing period is therefore appropriate, with the further advantage that frost-tender plants can still be grown in cool areas.

At the end of the season, the crop is harvested and burnt to produce a "bio-ore", which could contain up to 10% of the polluting metal. It is possible to recoup some of the costs of the process by selling the bio-ore to smelting companies that are willing to take small quantities of ore.

Burning the plant material to produce the bio-ore is much more environmentally acceptable than roasting industrial ores because of the absence of sulphur in the bio-ore.

Suitable hyperaccumulators used in green remediation should have the following characteristics:

• the plant should have an appreciable biomass
• it should be able to tolerate a cool climate, since temperate zones have the most urgent need for remediation
• it must be easy to grow

Unfortunately there are few, if any, plants that possess all three of these characteristics, and a compromise must therefore be adopted.

Experiments are now under way in Maryland (US), Rothamsted (England) and elsewhere to test the procedure in the field. If hyperaccumulators of sufficient biomass cannot be found, a possible alternative would be to use bioengineering to introduce the hyperaccumulation gene into non-accumulators with a high biomass. Such a strategy would of course only be practical in the long term.

To date, green remediation has been centred around the extraction of cadmium and zinc from agricultural land, and there has been little demand for copper or cobalt extraction. This is probably because areas of cadmium or zinc pollution are far greater than sites involving copper. However, about ten years ago experiments were conducted at Texas A & M University in which an attempt was made to extract cobalt 60 from radioactively polluted soils using the copper/cobalt hyperaccumulator Haumaniastrum katangense. The results of the experiments are unknown.

Robert Brooks is Emeritus Professor of Geochemistry, Department of Soil Science, Massey University.