Improving gold phytoextraction in desert willow (Chilopsis linearis) using thiourea: a spectroscopic investigation

Environmental context. The conventional methods used for the extraction of gold from mine tailings and runoff are costly and often require harsh chemical treatment. Using plants to extract gold is more environmentally friendly and economically feasible. Plants are especially appealing because they can uptake low levels of gold and accumulate them in their tissues, whereas conventional methods are less effective at extracting gold at low levels. Thiourea has been proposed as an alternative gold chelator that could help in gold phytomining. It is less toxic than cyanide, which is the chemical commonly used to dissolve gold from mine ores. Abstract. Phytomining, the use of plants to recover noble metals, is developing as a feasible option to extract gold from mine tailings. In this study, thiourea (TU) was used to increase gold availability and to enhance gold accumulation by the desert plant Chilopsis linearis. Seedlings of C. linearis were grown in a hydroponic solution containing 25 μM Au and TU at 25, 50, 100, 200, and 400 μM. After two weeks of growth, the concentration of Au, micro- and macronutrients was determined using inductively coupled plasma–optical emission spectroscopy. In addition, X-ray absorption spectroscopy was used to determine the oxidation state and the coordination of the Au atom within the plant tissues. The effect of TU on plant growth was determined as well. The results of the present study demonstrated that TU at 25 μM was able to increase the Au uptake by C. linearis plants grown in hydroponics without any toxic effect. However, the translocation to stem and leaves was better at 100 and 200 μM of TU, respectively. The addition of TU to hydroponic solutions did not affect the uptake of Ca, Mg, P, and K. However, TU induced an increase in uptake of S, Fe, Cu, and Zn and a decrease in Mn uptake. When Au was chelated with TU, the plant transformed 64% of the Au–TU complex to Au(0) and the other 36% remained in the ionic form.