Electrochemistry and microbiology of microbial fuel cells treating marine sediments polluted with heavy metals

Abstract

The industrial contamination of marine sediments with chromium, copper and nickel in Penang, Malaysia was addressed with bio-remediation, coupled with power generation, using in situ sediment microbial cells (SMFCs) under various conditions. The efficiency of aerated sediment microbial fuel cells (A-SMFCs) and non-aerated sediment microbial fuel cells (NA-SMFCs) was studied. The A-SMFCs generated a voltage of 580.5 mV between 50 and 60 days, while NA-SMFCs produced a voltage of 510 mV between 60 and 80 days. The cell design point for A-SMFCs was 2 kΩ, while for NA-SMFCs it was 200 Ω. In both SMFCs, the maximum current values relating to forward scanning, reverse scanning and oxidation/reduction peaks were recorded on the 80^th day. The anode showed maximum additional capacitance on the 80^th day (A-SMFC: 2.7 F cm⁻²; and NA-SMFC: 2.2 F cm⁻²). The whole cell electrochemical impedance using the Nyquist model was 21 Ω for A-SMFCs and 15 Ω for NA-SMFCs. After glucose enrichment, the impedance of A-SMFCs was 24.3 Ω and 14.6 Ω for NA-SMFCs. After 60 days, the A-SMFCs reduced the maximum amount of Cr(VI) to Cr(III) ions (80.70%) and Cu(II) to Cu(I) ions (72.72%), and showed maximum intracellular uptake of Ni(II) ions (80.37%); the optimum remediation efficiency of NA-SMFCs was after 80 days toward Cr(VI) ions (67.36%), Cu(II) ions (59.36%) and Ni(II) ions (52.74%). Both SMFCs showed highest heavy metal reduction and power generation at a pH of 7.0. SEM images and 16S rRNA gene analysis showed a diverse bacterial community in both A-SMFCs and NA-SMFCs. The performance of A-SMFCs showed that they could be exercised as durable and efficient technology for power production and the detoxification of heavy metal sediments. The NA-SMFCs could also be employed where anaerobic fermentation is required.