Chemically-induced uptake of gold by root crops: Its significance for phytomining

Five root crops (carrot, red beet, onion and two cultivars of radish) were grown in an artificial substrate consisting of 3.8 mg/kg (ppm) of elemental gold dispersed in sand. The possibility of using these crops for phytomining for gold was determined by separately adding the chelating agents ammonium thiocyanate and ammonium thiosulfate at respective rates of 1 and 2 g/kg to the substrate after a growing period of 9 weeks. The plants were harvested 10 days later and analysed for gold by atomic absorption spectrometry. In most cases, gold concentrations were higher in the roots than in the above-ground plant material. The highest mean gold concentrations were found in carrot roots (48.3 mg/kg dry weight), and roots of two radish cultivars (113 and 102 mg/kg dry weight). Because of the very high biomass yield of carrot (16 t/ha — dry weight), this plant showed the best potential for phytomining for gold. After allowing for reduced biomass yields over mine tailings, and taking into account the cost of the chelates as well as agrarian expenses and the cost of burning the crop, it was concluded that there was some potential of use of carrot to grow an economic crop of gold from mine tailings. None of the other four plants had potential for this purpose.