A microfluidic study of acetate conversion kinetics in a microbial electrolysis cell: The role of age, concentration and flow on biofilm permeability

Abstract

This work addresses the need for kinetic studies to identify potentially exploitable mechanisms involved in generating current from flow-based bioelectrochemical systems (BES). Unlike most kinetic studies, which focus on electron transport, we focus on chemical mass transport by controlling the relevant experimental parameters. This is accomplished using a microfluidic 3-electrode setup that recorded output current (I) from a mature Geobacter sulfurreducens electroactive biofilm (EAB) while accurate control is applied over acetate concentration ([Ac]) and flow rate (Q). Additionally, the flow mode (tangential and perpendicular) is controlled to apply expansive or compressive sheer forces against the EAB. A detailed analysis of the effects of the control variables on the current is based on data collected for nearly 1 year, the longest timeframe for a microfluidic BES experiment to date. All experimental parameters affect output, but we find that age is the dominant factor. After nearly 1 year, current densities were as high as 29.5 A m−1, which is higher than in any reported 3-electrode experiment on G. sulfurreducens EAB. We conclude that flow-based deacidification of the EAB led to increases to outputs during early growth stages, whereas at later times, the increases were related to improved EAB permeability to acetate. Additionally, after 5 months each flow mode provokes complementary kinetic properties based on measurements of apparent enzyme/substrate affinity (KM(app)) and maximum current (Imax) values. Therefore, in addition to providing fundamental insights into BES functionality, these findings also open the door to practical applications and offer a road map to further experimental optimization.