Agromining of Thallium and Noble Metals

Thallium is a relatively toxic and valuable element, for which there are fast-growing hyperaccumulators that have the highest bioaccumulation coefficients (plant/soil concentration quotients) of any non-essential element. As with other elements, many Tl-hyperaccumulating plants are in the Brassicaceae family. In contrast, hyperaccumulation of the precious metals Au, Pd and Pt is not recorded by any plant species. To achieve uptake of these noble metals, chemicals must be added to soil to induce metal solubility, for which cyanide has proven repeatedly to be the most effective agent to promote uptake. However, cyanide does not specifically target the noble metals. Increased solubility and uptake of more toxic Cu and Ag can limit the uptake efficiency of a phytomining or agromining crop (a co-metallic effect). Worldwide, there are numerous soils with a high Tl burden (>1.5 mg kg−1) that are unsuitable for safe food production and have low value, and thus are ideal for agromining. Of the elements that could potentially be agromined, Tl has perhaps the greatest potential to be economically successful. Despite this promise, Tl has received relatively little attention. The geographical scope for noble metal uptake is in contrast, much more limited. Research is warranted on the discovery of new hyperaccumulators, the economics of recovering Tl and noble metals from biomass, and quantification of areas where agromining for these more valuable metals may be feasible.