compared to 0.1 A/m2 in the anaerobic experiment. The presence of oxygen in the system may produce a background current as a result of oxygen reduction. The combination of oxygen reduction and copper reduction resulted in higher current densities.
Cathodic efficiency was calculated from the decrease in copper concentration and the produced current. In the anaerobic experiment, the cathodic efficiency was 84%. This means, that more electrons were recorded as current than electrons were consumed for copper reduction. Apparently, there was another electron sink present in the catholyte. Even though the catholyte was continuously flushed with nitrogen gas, there may have been some trace concentrations of oxygen in the system, which could take up part of the electrons. The cathodic efficiency of the aerobic experiment was significantly lower: 43%. This lower efficiency under aerobic conditions compared to anaerobic conditions further indicates that besides copper, also oxygen was reduced at the cathode.
Open circuit cathode potential was +0.08 V vs SCE at a copper concentration of 0.93 g/L, corresponding to a value of +0.321 V vs NHE. At open circuit, no losses occur at the electrode and the measured potential should be equal to the theoretical potential. The theoretical potential of the reaction Cu2+ + 2 e- Cu is +0.34 V vs NHE. A copper concentration of 0.93 g/L would then result in a cathode potential of +0.285 V vs NHE. So, the measured open circuit potential was somewhat higher than the theoretical cathode potential. This may be an indication of some oxygen present even though the catholyte was continuously flushed with nitrogen gas. Under aerobic conditions, open circuit cathode potential was even higher: +0.12 V vs SCE (+0.361 V vs NHE). As the open circuit potential for oxygen reduction at pH=3 is and pO2=0.2 bar is +1.04 V vs NHE, oxygen is a more preferred electron acceptor. The higher measured open circuit cathode potential may thus be the result of a mixed potential of both copper and oxygen reduction.
6.3.5 Implications
A high removal efficiency of 99.88% combined with high energy production makes this new process of interest for copper recovery. The obtained power densities are comparable to results achieved with other well-performing cathode systems using oxygen (Ter Heijne et al., 2007; Cheng et al., 2006; Freguia et al., 2008). This is surprising, considering that the standard potential of Cu2+/Cu is considerably lower than that of oxygen reduction: +0.286 V vs NHE for Cu2+ reduction compared to +0.805 V vs NHE for oxygen reduction. Besides, the use of the bipolar membrane leads to an additional potential loss. Despite these drawbacks, the system achieved a cell voltage of 0.25 V at the maximum power density under aerobic
6
conditions, which corresponds to a voltage efficiency of 43% using the maximum cell voltage based on the thermodynamic cathode potential for copper (Ecell=0.585 V), or 23% using the maximum cell voltage based on the thermodynamic cathode potential for oxygen reduction (Ecell=1.09 V).
The high performance of the MFC with Cu2+ reducing cathode could be a result from a number of factors: (i) mass transfer of Cu2+ is faster as copper has high solubility at acidic conditions (we used 16 mM), while mass transfer limitations of oxygen occur as a result of low oxygen solubility of only 0.20 mM when catholyte was saturated with air. We see that only below 0.2 g/L of Cu2+ (Figure 4B) that there is a limitation that might be attributed to mass transfer limitations, (ii) the oxygen reduction reaction has a high overpotential, while the overpotential for Cu2+ reduction is much lower, and (iii) copper might function as a catalyst for the oxygen reduction reaction. Considering the low cathodic efficiency under aerobic
conditions, the increased current under aerobic conditions was clearly a result of additional oxygen reduction. This additional current was however much higher than expected from the performance of the blank experiment.
Expressing the cathode overpotential in terms of the current at which cathode potential was zero, we find for the blank, anaerobic, and aerobic experiment a current density of 0, 1.5, and 3.8 A/m2. This difference between the current produced by the anaerobic and aerobic Cu2+ reduction reaction cannot be explained by the catalytic activity of the carbon cathode for oxygen reduction, as the blank produced no current at a cathode potential of 0 V vs SCE. This would be an indication that copper is somehow involved as a catalyst.
This new metallurgical MFC combines electricity production with copper recovery. As the metallurgical MFC is still in early stage of development, it cannot be directly compared to the current recovery methods like electrowinning (Jergensen, 1999) and precipitation with sulfides (Bijmans et al., 2009), however, some main points of attention will be addressed here to position the metallurgical MFC within the field of copper recovery.
First, compared to electrowinning, the metallurgical MFC has the advantage of electricity production instead of electricity consumption, and the high removal efficiency leading to final copper concentrations <1.2 mg/L. It should be noted however, that electrowinning occurs at current densities of several orders of magnitudes higher than reported in this study, and it should be investigated to what extent the energy losses in the metallurgical MFC increase when operating at similar current densities as electrowinning.
Secondly, an organic source is needed to provide the anodic microorganisms with the necessary energy. These organics are not always available on sites where mining and