Phytomining for nickel, thallium and gold

The technique of phytomining involves growing a crop of a metal-hyperaccumu lating plant species, harvesting the biomass and burning it to produce a bi o-ore. The first phytomining experiments were carried out in California usi ng 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-hyperaccumulato rs 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 s ignificant loss of the Ni concentration (7600 mg/kg) in the plant. Analogou s experiments have been carried out on Berkheya coddii where a biomass yiel d of over 20 t/ha can readily be achieved though the Ni concentration is no t as high as in A. bertolonii. The total yield is, however, much greater. W e have also been able to induce plants to hyperaccumulate Au by adding ammo nium thiocyanate to the substrate. Up to 57 mg/kg Au (dry mass) could be ac cumulated by Indian mustard (Brassica juncea). Unusual hyperaccumulation (> 500 mg/kg dry mass) of T1 has been determined in Iberis intermedia and Bisc utella laevigata (Brassicaceae) from southern France. The Iberis contained up to 0.445 TI (4000 mg/kg) in the whole-plant dry matter and the Biscutell a over 1.5%. This unusually high accumulation of T1 has significance for an imal and human health, phytoremediation of contaminated soils, and phytomin ing for T1. We calculate that using Iberis, a net return of $ US 1200/ha (t wice the return from a crop of wheat) would be possible with a biomass yiel d of 10 t/ha containing 0.08% T1 in dry matter. The break-even point (net y ield of $ US 500/ha) would require 170 mg/kg (0.017%) T1 in dry matter. A m odel of a phytomining operation and its economics is presented and its adva ntages and disadvantages discussed. (C) 1999 Elsevier Science B.V. All righ ts reserved.