A microfluidic study of the kinetics of bio-electrochemical oxidation of acetate by a Geobacter sulfurreducens biofilm

This work addresses the need for kinetic studies to better understand the mechanisms that are responsible for the outputs from flow-based bioelectrochemical systems (BES). Unlike most kinetic studies, which focus on electron transport, here we consider reaction kinetics with a focus on chemical mass transport. To achieve this, we used a precision microfluidic reactor which accurately controlled acetate concentration ([Ac]), flow rate (Q), and flow direction (tangential and perpendicular), while output current (I) from a mature Geobacter sulfurreducens electroactive biofilm (EAB) was measured. A detailed analysis of the effects of the control variables on the current (I) were evaluated over a long timeframe, between 1 month to nearly 1 year. The results indicate that all experimental parameters have effects on outputs, but overall, age is the dominant factor. After nearly 1 year, current densities were as high as 29.5 A m-1, which is higher than any other 3-electrode experiment on G. sulfurreducens EAB. The main mechanisms for high outputs at early stages appeared to be related to EAB deacidification, whereas for the old EAB, increases were related to significantly higher acetate permeability due to structural changes in the EAB. Additionally, each flow mode exhibits complementary kinetic properties based on a quantitative analysis of apparent enzyme/substrate affinity (KM(app)) and maximum current (Imax) values. Therefore, in addition to providing valuable insights to the biosensors and bioelectronics community, these findings open the door to practical approaches for optimization of BES device design and operation.