Electro-osmotic-based catholyte production by Microbial Fuel Cells for carbon capture

Iwona Gajda,John Greenman,Chris Melhuish,Carlo Santoro,Baikun Li,Pierangela Cristiani,Ioannis Ieropoulos

doi:10.1016/j.watres.2015.08.014

Abstract

In Microbial Fuel Cells (MFCs), the recovery of water can be achieved with the help of both active (electro-osmosis), and passive (osmosis) transport pathways of electrolyte through the semi-permeable selective separator. The electrical current-dependent transport, results in cations and electro-osmotically dragged water molecules reaching the cathode. The present study reports on the production of catholyte on the surface of the cathode, which was achieved as a direct result of electricity generation using MFCs fed with wastewater, and employing Pt-free carbon based cathode electrodes. The highest pH levels (>13) of produced liquid were achieved by the MFCs with the activated carbon cathodes producing the highest power (309 μW). Caustic catholyte formation is presented in the context of beneficial cathode flooding and transport mechanisms, in an attempt to understand the effects of active and passive diffusion. Active transport was dominant under closed circuit conditions and showed a linear correlation with power performance, whereas osmotic (passive) transport was governing the passive flux of liquid in open circuit conditions. Caustic catholyte was mineralised to a mixture of carbonate and bicarbonate salts (trona) thus demonstrating an active carbon capture mechanism as a result of the MFC energy-generating performance. Carbon capture would be valuable for establishing a carbon negative economy and environmental sustainability of the wastewater treatment process.

Highlights

Addressing water scarcity and sanitation problems requires new methods of purifying water at lower costs and with less energy, whilst at the same time minimising the use of chemicals and their impact on the environment (Shannon et al, 2008)
The droplets had been dripping into the collection syringes, avoiding accumulation directly onto the electrode surface, the steady state of power generation had not been negatively affected by the catholyte
A previous study suggested that the catholyte accumulation is a function of Microbial Fuel Cells (MFCs) performance (Gajda et al, 2014a) it was attempted to plot the current level vs volume of accumulated liquid (Fig. 3)

Summary

Introduction

Addressing water scarcity and sanitation problems requires new methods of purifying water at lower costs and with less energy, whilst at the same time minimising the use of chemicals and their impact on the environment (Shannon et al, 2008). Platinum (Pt) for example, has been widely used in chemical fuel cells due to its high catalytic performance at low pH (Erable et al, 2009), due to its high cost, in addition to relatively fast deactivation in the presence of pollutants such as sulphur, other alternatives are being explored Some of these approaches include chemical (Haoran et al, 2014), enzymatic (Santoro et al, 2013a), microbial catalysts (Erable et al, 2012) or Non-Pt electrode modification to improve the performance (Ghasemi et al, 2011; Lefebvre et al, 2009; Santoro et al, 2012, 2013b). The 4-electron pathway appears to be predominant on noble metal catalysts (Kinoshita, 1988), whilst the peroxide pathway is more common on carbon based electrodes (Kinoshita, 1988)

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