Abstract:

In a biofilm anode, anode-respiring bacteria (ARB) generate electrical current by transferring electrons to the anode. A microbial fuel cell and a microbial electrolytic cell (together, MXCs) utilize a biofilm anode for electrically harvesting energy useful for society. Mathematical modeling can prioritize governing factors for ARB in a biofilm anode and identify research needs.

The biofilm anode was modeled in four steps. First, the Nernst-Monod equation was developed for describing how the electrical current from ARB depends on electrical potential. The equation unified two important concepts for understanding bacteria: electrical current (a kinetics concept for describing movement of a charge per unit time) and electrical potential (a thermodynamic concept for describing energy required to move a charge).

Second, the solid conductive matrix of the biofilm anode was represented as a conductor that enables ARB to efficiently transfer electrons to the anode at long range. By representing the conductivity of the solid matrix using Ohm's law, the model described the electrical-potential gradient along the length of the biofilm anode. The model demonstrated that the biofilm anode is highly conductive, with a conductivity of at least 0.001 mS/cm.

Third, the Nernst-Monod equation was further tested by describing the kinetics of an aerobic bacterium, Pseudomonas putida . The applicability of the equation to aerobic respiration and anode respiration indicated that the two respirations share the situation in which their electron transport chain achieves thermodynamic equilibrium by transporting electrons rapidly and reversibly. This commonality suggests that the Nernst-Monod equation applies to many forms of respiration.

Fourth, a novel platform for biofilm modeling (PCBIOFILM) was created for describing simultaneous phenomena of electrical neutrality, the ARB half reaction, diffusion, migration, and acid-base chemistry. For handling complex computation, PCBIOFILM advanced the modeling methodology by clarifying the complementary roles of the proton condition and the electrical condition. The model demonstrated that migration plays a minor role and identified diffusion as the dominant transport mechanism that captures more than 85 percents of the impact. The four modeling achievements make significant impact on the field by clarifying and linking key phenomena of ARB in biofilm anodes.