Nickel phytomining from industrial wastes: Growing nickel hyperaccumulator plants on galvanic sludges

Nickel (Ni) is used in numerous industrial processes, with large amounts of Ni-rich industrial wastes produced, which are largely sent to landfill. Nickel recovery from waste materials that would otherwise be disposed is of particular interest. Nickel phytomining represents a new technology in which hyperaccumulator plants are cultivated on Ni-rich substrates for commercial metal recovery. The aim of this study was to investigate the possibility of Ni transfer from industrial waste into plant biomass, to support recovery processes from bio-ores. Different industrial galvanic sludges (containing 85-150 g kg^-1 Ni) were converted into artificial substrates (i.e. technosols) and the Ni hyperaccumulator Odontarrhena chalcidica (formerly Alyssum murale) was cultivated on these Ni-rich matrices. A greenhouse pot experiment was conducted for three months including an ultramafic soil control and testing fertilized (NPK) and unfertilized replicates. The results showed that fertilization was effective in improving plant biomass for all the substrates and that O. chalcidica was capable of viably growing on technosols, producing a comparable biomass to O. chalcidica on the control (ultramafic soil). On all technosols, O. chalcidica achieved Ni shoot concentrations of more than >1000 mg Ni kg ^-1 and maximum Ni uptake was obtained from one of the technosols (26.8 g kg ^-1 Ni, unfertilized; 20.2 g kg ^-1 Ni, fertilized). Nickel accumulation from three of the technosols resulted to be comparable with the control ultramafic soil. This study demonstrated the feasibility of transferring Ni from toxic waste into the biomass of Odontarrhena chalcidica and that phytomining from galvanic sludge-derived technosols can provide similar Ni yields as from natural ultramafic soils.