Multiple Metals Recovery In Stackable Self-driven MFC-MFC Systems

Heavy metal ions of Cr(VI), Cu(Ⅱ) and Cd(Ⅱ) are extensively present in electroplating wastewater, which is normally treated by chemical, physical, biological and electrochemical processes. However, the drawbacks of these methods include occupying large areas, high dosages of reagents and secondary pollution. Environmental friendly and effective approaches for treating these species in the wastewater’s are still in great need.Bio-electrochemical systems (BESs) including microbial fuel cells (MFCs) and microbial electrolysis cells (MECs), are believed to be one clean energy technology. Due to the different electrode potentials of Cr(Ⅵ), Cu(Ⅱ) and Cd(Ⅱ) under standard conditions, Cr(VI) and Cu(Ⅱ) may be potentially used as final electron acceptors in the cathodes of MFCs, respectively (MFCsCr and MFCscu) which are then stacked in serial or parallel to drive MECs using Cd(Ⅱ) as a final electron acceptor (MECscd). In this self-driven system, three heavy metals are recovered simultaneously with no consumption of any external power.Compared to that carbon rod as cathodes in MECs cannot proceed Cd(Ⅱ) reduction (-0.502Ⅴ) inside regardless of stackable systems, titanium sheet as cathodes in MECs successfully forward Cd(Ⅱ) reduction in MECs with simultaneous Cr(VI) reduction in MFCscr and Cu(Ⅱ) reduction in MFCsCu.Serial connection increased voltage output whereas parallel configuration improved loop current, either of which further resulted in improvements of Cr(Ⅵ) reduction (56%) in MFCsCr, Cu(Ⅱ) reduction (46%) in MFCsCu and Cd(Ⅱ) reduction (28%) in MECs. Under the condition of halving working volume in MFCscr, voltage reverse in the stacked MFCscr and MFCscu was decreased, which led to the improvement in reductions of all the three ions (Cr(Ⅵ):62%, Cu(Ⅱ):53%, Cd(Ⅱ):44%) compared to these with no working volume decrease. Using stainless steel mesh as MECs cathode electrode, the removal of heavy metal ions was not as effective as titanium sheet(Cr(Ⅵ):45%, Cu(Ⅱ):46%, Cd(Ⅱ):33%). Similarly, all of Cr(Ⅵ), Cu(Ⅱ) and Cd(Ⅱ) at an identical low concentration of5mg/L were completely reduced under the condition of half working volume in MFCscr stacked with MFCscu and MECscd, which satisfied the national discharge standard.levels. This study provides an efficient approach to simultaneously reduce Cr(Ⅵ), Cu(Ⅱ) and Cd(Ⅱ), and recycle of electroplating wastewater with concomitant utilization of energy generated in MFCs.