Phytomining for nickel, thallium and gold

Abstract

The technique of phytomining involves growing a crop of a metal-hyperaccumulating plant species, harvesting the biomass and burning it to produce a bio-ore. The first phytomining experiments were carried out in California using the Ni-hyperaccumulator Streptanthus polygaloides and it was found that a yield of 100 kg/ha of sulphur-free Ni could be produced. We have used the same technique to test the phytomining potential of the Ni-hyperaccumulators Alyssum bertolonii from Italy and Berkheya coddii from South Africa. The effect of different fertiliser treatments on growth of Alyssum bertolonii was established in situ in Tuscany and showed that the biomass of the plant could be increased by a factor of nearly 3 (4.5 t/ha to 12 t/ha) without significant loss of the Ni concentration (7600 mg/kg) in the plant. Analogous experiments have been carried out on Berkheya coddii where a biomass yield of over 20 t/ha can readily be achieved though the Ni concentration is not as high as in A. bertolonii. The total yield is, however, much greater. We have also been able to induce plants to hyperaccumulate Au by adding ammonium thiocyanate to the substrate. Up to 57 mg/kg Au (dry mass) could be accumulated by Indian mustard ( Brassica juncea). Unusual hyperaccumulation (>500 mg/kg dry mass) of Tl has been determined in Iberis intermedia and Biscutella laevigata (Brassicaceae) from southern France. The Iberis contained up to 0.4% Tl (4000 mg/kg) in the whole-plant dry matter and the Biscutella over 1.5%. This unusually high accumulation of Tl has significance for animal and human health, phytoremediation of contaminated soils, and phytomining for Tl. We calculate that using Iberis, a net return of $ US 1200/ha (twice the return from a crop of wheat) would be possible with a biomass yield of 10 t/ha containing 0.08% Tl in dry matter. The break-even point (net yield of $ US 500/ha) would require 170 mg/kg (0.017%) Tl in dry matter. A model of a phytomining operation and its economics is presented and its advantages and disadvantages discussed.