Catalytic mechanism of activated carbon-assisted bioleaching of enargite concentrate

The catalytic mechanism of activated carbon-assisted bioleaching of enargite concentrate (enargite 37.4%; pyrite 47.3%) was investigated by employing microbiological, electrochemical and kinetic studies. By using moderately thermophilic microorganisms at 45 °C, the final Cu dissolution was improved from 36% to 53% at 0.2% (w/v) activated carbon. An excess activated carbon addition showed an adverse effect. The enargite mineral itself favored higher solution redox potential (E_h) for solubilization. However, the dissolution of co-existing pyrite, which also favors high E_h, immediately hindered enargite dissolution through the passivation effect. The surface of activated carbon functioned as an electron mediator to couple RISCs oxidation and Fe³⁺ reduction, so that elevation of the E_h level was controlled by offsetting microbial Fe³⁺ regeneration. As long as the E_h level was suppressed at <700 mV, the dissolution of pyrite was largely avoided, enabling a steady and continuous dissolution of the enargite mineral through the surface chemical reaction model. When the E_h-control by activated carbon becomes no longer sustainable and the E_h hits 700 mV, rapid pyrite dissolution was initiated and the surface chemical reaction of enargite dissolution came to an end. Arsenic species dissolved from enargite was constantly immobilized with an efficiency of 75–90% as amorphous ferric arsenate. However, the sudden initiation of pyrite dissolution also triggered the re-solubilization of ferric arsenate. Therefore, the sustainable E_h-controlling effect was shown to be critical to enable longer Cu dissolution from enargite as well as stabilization of As precipitates.