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Marine microbial fuel cell as an online assimilated organic carbon biosensor

Quek, Soon Bee (2015) Marine microbial fuel cell as an online assimilated organic carbon biosensor. PhD thesis, Murdoch University.

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Over the past decades, the global concerns of limited freshwater supplies due to the excessive usage of water and increasing contamination of natural freshwater resources have encouraged research efforts to develop alternative technologies to provide clean water. In recent years, seawater desalination via reverse osmosis (RO) has been rapidly growing and is arguably becoming one of the most important technologies to provide freshwater supply. Currently, membrane fouling is debatably one of the major costs to the seawater desalination industry typically resulting in periodic cleanings and finally in total membrane replacement at non-regular intervals. One of the major types of RO membrane fouling is biofouling; which is caused by bacteria feeding on assimilable organic carbon (AOC) that are presence in seawater leading to bacterial growth on the RO membrane. Hence, AOC can be used as an indicator for the relative biofouling potential indicating that a good seawater monitoring system for AOC is crucial for early detection of biofouling potential.

At present, many biosensors for the monitoring of AOC in water are available. However, they tend to pose numerous problems such as high operation costs, inability to work online, time consuming, labour intensive, require specific pure culture or lack of sensitivity for early detection of biofouling potential. To overcome some of these drawbacks, a microbial fuel cell (MFC) was proposed as a new online biosensor for the monitoring of AOC in seawater to be used in seawater reverse osmosis (SWRO) plants. Therefore, this thesis investigates the potential application of a marine MFC (or potentiostat-controlled MFC, namely MEC) based biosensor for the monitoring of AOC in seawater.

All the MFC (or MEC)-biosensors in this study were conducted using a two-chamber MFC equipped with a cation exchange membrane, inoculated with microorganisms from marine sediment. Seawater was used for all the experiments. Acetate or yeast extracts was used as electron donor in the anode. In general, results showed that MFC-biosensor established could detect the presence of AOC under marine conditions by producing electrochemical signals such as current peaks, coulombs output and the change of anodic potentials. The electrochemical signals and AOC concentrations followed typical Michaelis-Menten model, where linear correlation only applied to a certain responsive range of low AOC concentrations and a finite maximum current were produced at saturation conditions. Nevertheless, the detection of higher levels of AOC was not of relevance in this study as determining the presence of low levels of AOC was the central concern of the study.

Initially a traditional MFC-biosensor, using a fixed external resistor and ferricyanide as catholyte, was established under marine condition. The MFC-biosensor reproducibly generated electrochemical signals (change of anodic potential and current peak) in response to defined AOC concentration. Although the linear correlation between electrochemical signals (change of anodic potential and current peak) and AOC concentrations were of satisfactory level (R2 > 0.98), the use of a simple MFC-biosensor that was continuously operating at a constant anodic potential was not practical or reliable. Moreover, the use of ferricyanide as catholyte required regular replacement.

It is important to control the anodic potential for accurate detection of current changes and the prevention of “false alarms”. To overcome the problems described above, a potentiostat-controlled marine MFC-biosensor (namely MEC-biosensor) was operated at a negative anodic potential (-300 mV (vs Ag/AgCl)) and produced useful signals within 14 days after inoculation. Results showed the electrochemical signals produced from the MEC-biosensor had sufficient reproducibility (standard deviations of 3 – 5 %) and could be of high precision (i.e. the AOC concentrations and electrochemically signals generated were highly correlated with R2 > 0.99).

The response of the MEC-biosensor should be rapid and highly sensitive towards AOC. By applying different anodic potentials, the performance of a MEC-biosensor could be optimised. The use of a positive anodic potential (+250 mV (vs Ag/AgCl)) lead to a faster establishment (2 - 4 times faster) of anodophilic bacteria, and a lower detection limit (2.5 μM of acetate) of the marine MEC-biosensor compared to the MEC-biosensor operated at negative anodic potentials (-250 mV (vs Ag/AgCl)), traditionally used in MFC. DNA analysis revealed that operating a MEC-biosensor at a higher anodic potential not only improved its performance but also altered the anodophilic biofilm communities. Electrochemically active bacteria such as from Shewanellaceae, Pseudoalteromonadaceae, and Clostridiaceae families were only present in the positive anodic potential MEC-biosensor while strictly anaerobic bacteria from the families of Desulfuromonadaceae, Desulfobulbaceae and Desulfobacteraceae were only present in the negative anodic potential MEC-biosensor.

It is well-known that response of a MFC is suppressed by dissolved oxygen. Dissolved oxygen is generally saturated in seawater and the use of a positive anodic potential for the MEC-biosensor only helps the MEC-biosensor to tolerate low concentration of dissolved oxygen. For practical application of the MEC-biosensor, it is essential to overcome the aforementioned problem to allow accurate monitoring of AOC in seawater under oxygen saturated condition. A hexacyanoferrate (HCF(III))-adapted anodophilic biofilm was developed to detect low levels of AOC in oxygen saturated seawater. As the HCF(III) was found to enable the development of an adapted biofilm that transferred electrons to HCF(III) rather than oxygen, the marine MFC-biosensor developed could, in principle, work in the presence of oxygen.

Although the signal suppression by the dissolved oxygen in a MFC-biosensor could be overcome by adapting anodophilic biofilm with HCF(III), the use of an external redoxmediator would hinder the MFC-biosensor’s practical application as it would need to be added constantly to maintain its concentrations. It was important to develop an alternative that enabled AOC monitoring in real plants using fully oxygenated seawater. An electrochemical deoxygenation cell was developed to remove dissolved oxygen from seawater prior to AOC detection using a MEC-biosensor. Using appropriate cathodic potentials (- 800 mV (vs Ag/AgCl)) in an electrochemical cell enabled a sufficiently fast dissolved oxygen removal without causing undesired reduction reactions that could lead to interference with the signal production of the MEC-biosensor. The coupling of an electrochemical cell with the MEC-biosensor has allowed real-time AOC monitoring in oxygen-saturated seawater.

Overall, it can be concluded that the MFC (or MEC)-biosensor can be effectively used as an online, rapid, and sensitive biosensor for the detection of AOC in seawater. The developed MFC-biosensor can be used as an early warning system that provides early detection and awareness of biofouling potential in seawater and to monitor the effectiveness of pre-treatment processes in seawater desalination plants that are based on reverse osmosis. A good AOC monitoring system could potentially assist in minimising biofouling which leads to potential savings on frequent cleaning of the RO membrane and replacement of RO membrane at an irregular interval. Nevertheless, further process optimisation and pilot scale trials in real RO systems is needed prior to making such biosensor available.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Engineering and Information Technology
Supervisor(s): Cord-Ruwisch, Ralf and Cheng, Liang
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