Enhancement in copper extraction from chalcopyrite by re-inoculation of different acidophilic, moderately thermophilic microorganisms

• Potential of re-inoculation for improving chalcopyrite bioleaching was studied.
• Re-inoculation efficacy depended on strain selection and inoculation time/amount.
• Re-inoculation into the early stage was superior to that at the advanced stage.
• Re-inoculation into the early stage improved Fe/S oxidation and copper leaching.
• Early re-addition of L. ferriphilum showed the best enhancement in copper extraction.

The feasibility and optimization of strategies of microbial re-inoculation were investigated to enhance chalcopyrite bioleaching. The application of microbial re-inoculation consisted of the re-addition of cultures of Acidithiobacillus caldus, Ferroplasma thermophilum or Leptospirillum ferriphilum (with different inoculum concentrations) into the two defined different bioleaching systems that separately represented the early and middle stages of chalcopyrite bioleaching. Changes in the bioleaching performance and microbial community structure after re-inoculation were compared to that in the control bioleaching experiment without re-inoculation. Results of re-addition of pre-grown microbial cultures into the early stage indicated that the re-inoculated strain survived and/or grew in the leaching environments and meanwhile synergistically promoted the growth of other microorganisms, then accelerated the total iron/sulfur oxidation compared to the unamended control. Finally, all these factors resulted in a significant enhancement of copper extraction. Moreover, a higher re-inoculation concentration exerted a more significant improvement in copper leaching from chalcopyrite. In all experiments, re-inoculation with L. ferriphilum into the early stage showed the best enhancement in copper leaching, which significantly shortened the incubation time and improved the maximum copper extraction. Re-inoculation with iron or sulphur oxidisers into the middle stage could not or slightly improve the final leaching level of copper. These results demonstrate that re-inoculation can be a useful step to improve the bioleaching kinetics and level of chalcopyrite; however its efficacy is influenced by the functional strain selection and procedures of culture re-inoculation (including re-addition time and inoculum concentration).