Speciation of nickel extracted from hyperaccumulator plants by water leaching

Agromining combines agronomic and hydrometallurgical processes to extract metals from secondary resources using hyperaccumulator plants (HA) and produce high value compounds. Agromining has been primarily developed for nickel (Ni) recovery from ultramafic soils. Most often, the dry HAs, containing Ca 1 wt% of Ni are burnt to concentrate Ni in ashes (15-20 wt%) and remove organic matter; then the ashes are treated to give Ni metal, oxide or salts. This work was undertaken to investigate the recovery of Ni without any combustion stage in eco-friendly conditions.Ni extraction was tested with two HAs, the former grown in a temperate latitude (A. murale) the latter in a tropical climate (R. bengalensis), using water as an extracting agent in batch reactor, at two solid to liquid (S/L) ratios (4 and 8%). Ni extraction yield was higher from the former HA (90%) than from the latter (50%). Ni concentration in the leachates increased with the S/L ratio. Therefore column experiments were run with A. murale to work with the highest possible S/L ratio. Other main cations (K, Ca, Mg) and dissolved organic compounds were solubilized as well. Knowing that Ni is mainly chelated by malate and citrate in the plant tissues, low molecular weight carboxylic acids (LMWCAs) were analyzed in the leachates; they accounted for 14% of the dissolved organic carbon.The composition of the leachate at thermodynamic equilibrium was computed using the Chess software, dedicated to the simulation of the equilibrium of complex solutions, including gases, minerals, organics and colloids. Results showed that Ni was almost entirely complexed with LMWCAs. Further simulations were conducted to elucidate why Ni could not be simply recovered by precipitation of Ni hydroxide from the leachate. This behavior was interpreted by the contribution of a strong Ni ligand preventing the hydroxide precipitation. Calculations enabled us to obtain the minimum value of the complexation constant.The evidence that Ni can be extracted from HAs by water leaching and a better understanding of the equilibrium of the leaching liquor are important stages for further Ni recovery and purification.