Hydrothermal processing of nickel containing biomining or bioremediation biomass

The feasibility of recovering nickel and producing biofuels from nickel containing biomining or bioremediation biomass by a hydrothermal process has been investigated. Experiments were concerned with the reactions of nickel and biomass containing solutions (model solutions and an extract of Berkheya coddii) in hot compressed water between 200 and 375°C at 25 MPa. We found that in this temperature range nickel is soluble in aqueous solutions containing histidine, an amino acid present in high concentrations in nickel accumulators. The thermal decomposition products of the biomass reduce the histidine complexed nickel ions to metallic nickel. This reduction proceeds on metallic surfaces, and the nickel is deposited on the surface of the metal as a nickel or nickel/char layer. No added reducing agent such as hydrogen is required. An extract of the nickel bioaccumulator B. coddii and total biomass of B. coddii behaved similar to our model solutions. Based on our results we propose a three-step hydrothermal process for the recovery of nickel and biofuel from nickel containing biomass.