Phosphate reuse in a microbial fuel cell

Abstract

The growing concern over global energy supplies has spurred much research into sustainable energy sources such as biomass energy. The microbial fuel cell (MFC) is a promising technology that generates electricity from the biologically mediated breakdown of organic substrates. These include acetate, glucose or even wastewater, allowing the use of readily available and sustainable energy sources. A prime limitation in MFC application is the development of a pH gradient when a membrane is used to separate the anode and cathode compartment. This pH gradient creates a barrier to charge transport, causing a loss of voltage and power density. Phosphate buffer solution (PBS) is commonly added to MFCs to control pH changes during operation. However, the cost and disposal issues of PBS remain a source of concern. A 3-chamber glucose-fed MFC with an anion exchange membrane (AEM) and cation exchange membrane (CEM) was used to investigate the extraction of PBS from used MFC anolyte, with a view to recycling PBS for subsequent batches. In proof-of-concept experiments, the middle compartment of the MFC was used to contain 0.05 M PBS. During MFC operation, the ions in the middle compartment were separated into the cathode and anode chambers, due to electrokinetic force. This allowed for the transfer of phosphate into the anolyte, which did not contain PBS in the beginning. A maximum voltage of 0.0293 V and power density of 0.794 W/m2 using a 1 kΩ external resistance was attained during operation of the MFC. At the same time, phosphate concentration at the anolyte increased 5-7 times. The pH change at the anode registered a marginal drop of 3.4%. The phosphate migration across the AEM was found to be mainly diffusion-driven, but electrokinetic force achieved an increase of 25% anode phosphate concentration. However, the rate of phosphate accumulation was found to be insufficient to establish significant buffer capacity at the anode. Nevertheless, these results demonstrate the potential for MFCs to perform ion removal similar to electrodialysis, but without the need for external power sources.