The phytomining and environmental significance of hyperaccumulation of thallium by Iberis intermedia from Southern France

Unusual hyperaccumulation (>500 mu g/g dry mass) of toxic Tl has been determined in Iberis intermedia (Brassicaceae) from southern France. This species contained up to 3,070 mu g/g (0.31%) Tl in the whole-plant dry matter. Pot trials with Iberis showed that it could tolerate nearly 2,000 mu g/g available Tl in the substrate compared with about 20 mu g/g for the metal-tolerant grass Arrhenatherum elatius. This unusually high accumulation of Tl has significance for animal and human health, phytoremediation of contaminated soils, and phytomining for Tl. It was determined that three crops of Iberis would be sufficient to phytoremediate to a non-toxic level, a soil containing 10 mu g/g Tl, and the production of a saleable plant ash (bio-ore) would probably pay for the cost of the operation. Phytoming (growing a "crop" of a metal over ores subeconomic for conventional mining) could be a viable option since it has been calculated that a net return of $1,200/ha (twice the return from a crop of wheat) would be possible with a biomass yield of 10 t/ha containing 0.08 percent Tl in dry matter. The break-even point (net yield of $500/ha) would require 170 mu g/g (0.017%) Tl in dry matter. Such a project would, however, require a large area to be able to afford economies of scale.