^dCentre for Mined Land Rehabilitation, Sustainable Minerals Institute, The University of Queensland, Australia

Being a versatile metal nickel is employed in several industrial processes, with extensive amounts of industrial nickel-rich wastes produced worldwide. At present, industrial sludges are to the greatest extent being landfilled, without any economical or environmental benefit. The possibility to recover valuable metals from materials that would otherwise be disposed is of particular interest. Likewise, it is crucial to develop alternative approaches to reduce the environmental pressure of nickel related industry and for the green production of profitable nickel compounds. The existence of nickel hyperaccumulator plant species offers the possibility to investigate new approaches of nickel recovery from industrial waste. Hyperaccumulator plants are capable to accumulate between 0.1 and 3% nickel in their dry biomass and their application for commercial metal recovery from mineral rich substrates has been defined as phytomining. At present, phytomining researches have mainly focused on nickel extraction from naturally nickel rich soils. Therefore, this study aims to investigate the possibility of nickel transfer from industrial waste into plants biomass, to support its recovery from bio-ores. For this purpose, the nickel hyperaccumulator plant Odontharrena chalcidica (form. Alyssum murale) has been cultivated on five substrates derived from nickel-rich galvanic sludge. Different waste materials have been identified from industries operating in nickel plating processes, containing 80 to 150 g kg^-1 Ni, and converted into artificial substrates (technosols) to support plant growth. A greenhouse pot experiment was conducted for three months on five waste-derived substrates and a natural ultramafic soil control. Fertilized and unfertilized replicates were included to assess differences in plants biomass and nickel accumulation. Pore water samples collection was carried out to monitor microelements and nutrients availability and define possible risk of metal leaching. Results showed that a nickel content between 1 and 2.5% was achieved in the dry biomass of the unfertilized O.chalcidica, while lower concentrations were obtained in the fertilized replicates (up to 2%). Nickel accumulation from three waste-derived substrates resulted to be higher or comparable to the control natural soil. Fertilization resulted to be effective in improving plant biomass for all the matrices, although due to waste toxicity all the replicates growing on waste substrates showed a biomass reduction of 20 to 90% compared with the ultramafic control. The pot experiment results demonstrated the possibility to transfer nickel from waste-derived technosols into plant biomass and thus the feasibility of phytomining application on galvanic sludges. Further studies should focus on improving biomass production with substrate amelioration aimed at reducing waste phytotoxicity.