Phytomining of valuable metals: status and prospective-a review

Precious metals accumulated to high concentration spread across tailing dumps, mineralised soil and at many metal-processing facilities across the African continent. Currently, about 71% of the world's deposits of platinum group metals (PGMs) are found in South Africa. The country produces about 60% of the world's output of three of these precious metals namely; platinum, palladium and rhodium. Traditional mining technology is grossly insufficient to quantitatively recover all metals economically. Therefore, low concentrations of these valuable metals are often wasted away as leftovers – posing environmental menace to the general ecosystem via dissolved 'associated-toxic metal leaching' and run-offs. In the past few decades, the concept of phytomining to recover low concentration precious metals from mine tailings, has been investigated by various researchers inMexico, New Zeeland, Caledonia, Canada, United Kingdom and Australia. This review presents a synopsis of the reported precious metal phytomining studies developed on the laboratory scale, and/or in greenhouse or pilot scale as well as in the field, in the last two and half decades. A summary of work done so far as relating to the techno-economic assessment of precious metal phytomining is also presented. We have also taken the opportunity to specifically discuss the applications of the phytoextracted plant-based metal in catalysis and related fields – an area with great potential to support the catalyst market and its adjacent chemicals and pharmaceutical industries but has been less explored for its possible coupling with phytomining activities so as to augment the economic prospects of this mining method.