Phytoextraction of gold and copper from mine tailings with Helianthus annuus L. and Kalanchoe serrata L.

To examine the feasibility of gold phytoextraction, and the corresponding uptake of copper that is induced at the same time, field and laboratory scale experiments were carried out using mine tailings from the Magistral mine in Sinaloa State, Mexico. The locally available plant species Helianthus annuus L. (sunflower) and Kalanchoe serrata L. (magic tower) were used in this work, in combination with the chemical amendments: sodium cyanide (NaCN), ammonium thiocyanate (NH₄SCN), ammonium thiosulphate (NH₄)₂S₂O₃, and thiourea SC (NH₂)₂] to promote gold uptake. The results show that for K. serrata, average copper concentrations were increased to above 4 mg/kg and gold concentrations to above 9 mg/kg in the dry matter of aerial tissues. For H. annuus average copper concentrations were increased to 118 mg/kg in roots, 141 mg/kg in stem and, 119 mg/kg in leaves while average gold concentrations were increased to 15 mg/kg in leaves, 16 mg/kg in roots and, 21 mg/kg in plant stems. Poor health of plants after treatment with chemicals to induce gold uptake could be a function of toxic concentrations of other trace elements such as copper in the plants. Our results confirm that phytoextraction technology can be used to recover precious metals from mine tailings, and that at the current market price for gold, this recovery may be economic. However, our results also highlight the differential response of plant species to copper and gold in the ground, and the importance of choosing the correct chemical to induce metal uptake.