Handa Notebook Thesis

[Audio] Hello Everyone, I am Jasreen Kaur from the Department of Environment and Energy Engineering. First and foremost, I would like to thank Professor Jung Sokhee for allowing me to present this very interesting paper entitled a kinetic perspective on extracellular electron transfer by anode-respiring bacteria. Without further a due let's get started..

[Audio] This presentation is divided into four section which are the introduction and objectives, anode potential losses, EET mechanisms types used by ARB and last but not least the conclusion. At the end of the slides there will also be a section on appendix.

[Audio] Diving into part 1 which is the Introduction. In microbial fuel cells and electrolysis cells , anode-respiring bacteria oxidize organic substrates to produce electrical current. In order to develop an electrical current, A R B must transfer electrons to a solid anode through extracellular electron transfer. A R B use various E E T mechanisms to transfer electrons to the anode. The first mechanism is the direct electron transfer. Direct Electron Transfer between electron carriers in the bacteria and the solid electron acceptor. This mechanism is supported by the presence of outer-membrane cytochromes that can interact directly with the solid surface to carry out respiration. Bacteria using this mechanism require direct contact with the solid electron acceptor and, thus, cannot form a biofilm. The second E E T mechanism is the soluble electron shuttle. Soluble Electron Shuttle: a compound that carries electrons from the bacteria by diffusive transport to the surface of the metal oxide and is able to react with it, discharging its electrons. Then, this compound, in its oxidized state, diffuses back to the cells, which should be able to use the same compound repeatedly .hence the name shuttle. The third E E T mechanism is the solid component. Solid Component that is part of the extracellular biofilm matrix and is conductive for electron transfer from the bacteria to the solid surface. This mechanism is supported by the recent discovery of the possible role of cellular pili as nanowires, which are being characterized for their capability to conduct electrons. Other components may also be conductive and contribute in EET, such as extracellular cytochromes or bound electron mediators.

[Audio] Thus, coming to the objectives of this review paper. There are two main objective whereby the first one is that ARB must be able to perform a twofold task which are A to produce a high current density that minimizes anode materials and reactor size and B at a low anode potential or can be defined as being as close as possible to the redox potential of the substrate being oxidized, which translates into minimal anode potential losses and high energy output. The second objective is to evaluate how well each EET mechanism can produce a high current density more than 10 Ampere per meter square without a large anode potential loss.

[Audio] Second part is the anode potential losses. In this section is it divided into 4 parts which are the intracellular potential losses, rate of substrate utilization by ARB, rate of electron production by A R B and the extracellular potential losses and its mechanisms..

1. 2. 3. How to characterize anode potential (osses? figure 3 depicts the sequential losses of electrical potential that become conceptually important when addressing the function of an ARB biofilm and contribute to the anode potential loss For our analysis, we consider ARB that are located at a certain distance from the anode surface. The total anode potential loss (nan ode) is defined as the difference between the electron-donor potential and the anode FX)tential (nanode = Edonor — Eanode EET 3. Pot—Gs of the to — EET ts — The can refer to the energy Of electrons. 'Energy' and 'potential' interchangeably throughout the manuscript when referring to electrons After ARB release the electrons. two additional processes can result in losses- The first is the SET mechanism that transports electrons to the anode interface, which Will change the electron energy from to E_interface- The secorri is the reaction occurring at the anode interface, which can decrease the from E_interface to E _anode Because MXCs must be operated to minimize (low ancxie potential losses}, the total energy given to ARB to carry out intracellular and extracellular Frocesses iS minimized. Thus. the ARB community must manage energy efficiently by maximizing energy conversion to ATP (intracellular processes) and minirhi2ing losses due to extracellular processes (EET, anode interface reaction) that do not yÉId energy to the cell.

[Audio] Coming into Part 1 of section 2 the intracellular potential losses Two kinetic processes are involved in intracellular potential losses from E donor minus E O M these processes are common in all respiratory bacteria and are depicted in Figure. 5. From Figure. 5.1 First, bacteria oxidize the electron donor, producing intracellular reducing power in the form of an electron carrier such as N A D H. To generate energy for the cells, the electron carrier is oxidized by transferring its electrons into the membrane associated proteins that are part of the electron transport chain, which ultimately leads to the external electron acceptor. From Figure. 5.2 In the case of A R B, the electrons are transferred to O M proteins that initiate the EET process..

[Audio] Part 2 of section 2 is the rate of substrate utilization by A R B. The rate of substrate utilization in microbial processes are frequently modeled using the Monod relationship. This relationship explicitly describes the rate at which bacteria oxidize the substrate and produce the reduced intracellular electron carrier. When the substrate is rate-limiting for the entire process of metabolism and EET, the current density generated by A R B can be written as equation. 1 for a biofilm setting where j is current density obtained by A R B, j max is maximum current density of the ARB biofilm, S is substrate concentration in the liquid, K s,apparent is the apparent half-saturation substrate concentration in a biofilm..

[Audio] Part 3 is rate of electron production by A R B Once ARB have produced the reduced intracellular carrier, they initiate electron flow through the electron transport chain until the electrons reach membrane-bound cytochromes that, in turn, initiate EET. The rate of reduction of electron shuttles can be modeled by the Monod relationship expressed in terms of the concentration of the soluble shuttle The problem is that Because the electron acceptor for A R B is a solid anode, we cannot use the Monod relationship, because we cannot define an anode concentration. Thus, we transform the concentration of an electron acceptor into the anode potential, which is achieved using the Nernst Monod relationship developed to express the current density in an A R B biofilm. The formula is defined as below where R is the ideal gas constant, F is the Faraday constant, T is the temperature in kelvin and E k a is the potential at which the current density one over two maximum current density.

[Audio] The Nernst Monod relationship combines the Monod relationship typically used to calculate the rate of electron acceptor utilization with the Nernst equation for describing the anode potential availability as the electron acceptor for A R B Because enzyme electrode and bacteria electrode systems have similar kinetic processes, we can analyze the potential losses in the A R B biofilm depicted in Figure. 6 in a similar manner as is performed in enzyme electrochemistry. Nernst Monod equation as the baseline to distinguish intracellular potential losses from n anode. Thus, deviations from the Nernst Monod equation will help us determine extracellular potential losses due to EET and interface electron transfer..

[Audio] Moving to Part 4 which is the extracellular potential losses. Two kinetic processes are involved in extracellular potential losses. First, electrons are transported from the A R B's O M proteins example cytochromes to the surface of the anode by either electron shuttles Figure. 6.3a or by a solid conductive matrix Figure 6.3 b This reduces the electron energy from E O M to E interface Then, electrons are transferred to the electrode by interface electrode transfer Figure 6.4,reducing the electron energy from E interface to E anode Given that these potential losses occur on the outside of the cell, it is unlikely that these losses are associated with A R B energy conservation and growth The underlying mechanisms involved in these kinetic processes includes diffusive transport, conduction, and electrochemical reactions are known to dissipate energy as heat or as an increase in entropy, thus strengthening our assumption that A R B cannot recover this energy for growth In order to maximize their growth by capturing as much energy as they can, A R B must minimize extracellular potential losses. Hence, the next section will be talking about the first mechanism which is the direct contact mechanism.

[Audio] For the direct contact mechanism, a single layer of A R B colonizes the anode and directly transfers electrons to the electrode from a portion of its O M that is in contact with the anode. Direct contact should represent the lowest extracellular potential loss because electrons do not need to travel over a significant distance to reach the anode. After electrons have reached the surface of the electrode, an electrochemical reaction must occur that releases these electrons into the conductive anode. This reaction occurs between an O M protein and the electrode and is probably reversible, given the reversibility of electron transfer observed in cytochromes The rate at which a reversible reaction occurs at an electrode interface is often described by the Butler Volmer equation as follows.

[Audio] The Butler Volmer equation describes the final potential loss for all EET mechanisms; this reaction can occur between a protein and the anode or by a compound such as an electron shuttle and the anode. Although low extracellular potential losses are possible with direct contact, the total amount of biomass in contact with the anode is surely a limiting factor in achieving a high current density. If A R B need to be in direct contact with the electrode, a monolayer biofilm would be the highest amount of biomass active in EET; this would be equivalent to a biofilm thickness of In the case of direct transfer, j max is a function of the active biofilm thickness according to the following equation simplified.

[Audio] For a monolayer of A R B the biofilm thickness is 2 micro meter and concentration of active biomass us 2.8 times 10 to the power of negative to produce 15 ampere per meter square. Q max would have to be nearly 1, more than an order of magnitude higher than reported values of Escherichia coli respiring oxygen.

[Audio] Soluble Electron Shuttle Mechanism is the second mechanism under the extracellular potential losses. The use of electron shuttles allows A R B to be located away from the anode surface and to accumulate more than a monolayer of bacteria. Although shuttles allow more A R B to be active per anode surface area, the distance between A R B and the anode becomes a limiting factor due to diffusion limitations of the electron shuttles Transport of soluble electron shuttles is mainly carried out by diffusion through Fick's law, shown here modified to reflect current density calculations: The total current density obtained by A R B using electron shuttles can be limited by the diffusion of electron shuttles according to equation 5 Diffusion coefficients of organic molecules are relatively small, indicating that diffusion is an inherently slow process..

[Audio] A mathematical model based on electron shuttles also calculated a j max limitation is due to electron shuttle diffusion. Even at an extremely high electron shuttle concentration the calculated j max was nearly 0.57 ampere per meter square . Thus, experimental and estimated jmax, when using electron shuttles, are at least 20 times smaller than the higher values observed in the literature Electron-shuttle transport results in an inherent potential loss due to the concentration gradient needed for diffusive transport. The potential loss for an electron-shuttle reaction can be calculated by the Nernst equation.

[Audio] In order to maximize j max, the concentration gradient must be maximized, but the gradient causes a potential loss according to Equation. 6 Assuming that we have a concentration gradient that is 99% of the total shuttle concentration in order to maximize the diffusion rate for example 99% oxidized at the OM and 99% reduced at the anode interface, E O M minus E interface would be equal to 118 milli volt. This potential loss is observed in experimental data by Marsili, as well as in modeling results by Picioreanu as a deviation from the typical Nernst Monod curve. Loss of electron shuttles in the effluent of the M X C poses another challenge to its use by A R B. Given that electron shuttles must be soluble in order to ' shuttle,' most of them could be lost from MXC systems in the effluent water..

[Audio] The third mechanism is the solid conductive matrix mechanism The possibility that A R B are able to use a solid conductive or a semi-conductive material for electron transfer is mainly supported by recent findings of microbial ' nanowires' that appear to be responsible for EET Unlike soluble electron shuttles, the solid mechanism is not restricted by Fick's Law, but by the rate at which the solid conductive matrix is able to conduct electrons. Depending on the characteristic of the solid conductive matrix, its conductivity could be modeled using various equations that describe conductors and semi-conductors We have used Ohm's Law in our past modeling. based solely on bacterial nanowires. This calculation could help us evaluate whether nanowires are capable of transferring the observed high current densities. K bio determines the potential losses E O M minus E Interface associated with j. A high , K bio can benefit ARB by minimizing E O M minus E Interface and maximizing j across the conductive matrix. Obtaining nanowire conductivity and the nanowire density inside the ARB biofilm can allow us to calculate K bio.

[Audio] If A R B are able to maximize K bio , their extracellular potential losses would be minimal, and their voltammetric curve would resemble that of the Nernst Monod relationship. A R B would also be able to produce higher j max values, because their current production would be determined by their metabolic activity and not by their EET mechanism Recent studies have shown that A R B known to produce a solid conductive matrix can produce high current densities. obtained up to 8 ampere per meter square using G. sulfurreducens grown on graphite, A R B that are known to produce microbial nanowires and do not produce soluble electron shuttles These results, as well as others presented in Figure. 8, also show minimal potential losses, indicative of a high K bio ..

[Audio] Figure 8 shows the Comparison of LSCV of various A R B communities According to our modeling results for the LSCV in Figure . 8a, k bio must be higher than 0.5 in order to achieve these low potential losses at high current densities Similar results were reported by Srikanth , Marsili shown in Figure. 8b, and Fricke shown in Figure. 8c using G. sulfurreducens, an A R B known to produce nanowires. As explained before, the combination of high current densities combined with low extracellular potential losses is only possible if a solid conductive matrix is used for EET. These results not only confirm the presence of a solid conductive matrix, but they also satisfy the requirements of high current densities at low anode potential losses for MXCs..