Metal Recovery from Industrial and Mining Wastewaters

Biological sulphate-reduction is a microbial-mediated process where sulphate is reduced to sulphide, which can be used to recover metals as sulphidic precipitates. To date, this technology has been assessed at full scale to recover valuable metals such as Cu, Ni and Zn. Despite this, research gaps are still encountered in this technology for improving and expanding its scope. Accordingly, the present review discusses: (1) the state of the art of the sulphate-reduction process, (2) the substrate options available that can meet the needs of the process, (3) the bioreactor configurations and their suitability for metal recovery, (4) the principles and factors affecting metal sulphide-precipitation and (5) the basis and advances on modelling and control of the process. The high diversity and versatility of sulphate-reducing bacteria allows exploring the use of substrates and operational conditions that facilitate the recovery of metals in bioreactors. Due to the lack of organics on industrial and mining waste streams that can sustain sulphate-reducing bacteria, the selection of a degradable, cost-effective, available, and non-pollutant substrate becomes crucial for the process. Different bioreactor configurations have been tested for the removal of metals from waste streams upon variations of the several operational conditions, concentration and type of metals tested, but metal recovery is hardly reported. Sulphate-reduction modelling has been developed to predict sulphide-inhibition/toxicity, microbial competition, kinetic parameters, biofilm and granulation development, sulphide-equilibrium and for scale-up design. Physicochemical reactions such as sorption/desorption and precipitation/solubilisation are not included in sulphate-reduction models despite that they are highly important for metal recovery in these systems. Sulphide and pH control in sulphate-reducing bioreactors is inherently essential to achieve metal recovery and to avoid unnecessary electron donor addition and over production of sulphide.