(27b) Evaluation of a Model Biological Fuel Cell System for Reduction Efficiency and Bioremediation Potential

     Bioelectrochemical systems and in particular microbial fuel cells (MFCs) have generated a large amount of interest and research in the past few years. These systems are largely focused on the oxidation of an inexpensive substrate, e.g. wastewater, to generate electrical energy. Such a process has the potential to eliminate several costs associated with treating wastewater, if operated in lieu of conventional treatment involving activated sludge and anaerobic digestion processes. Large uncertainties exist on the fundamental process governing the operation of these reactor systems. These include transport mechanisms for various species, mechanisms of biocatalysis, dominant members of a mixed community biocatalysis, etc. For these reasons, specific electrochemically active bacteria, Shewanella  spp. and Geobacter spp., have been used as model organisms due to the large amount of information already available for these species.

     One aspect of MFC systems currently under investigation is utilizing the reducing potential at the cathode to perform treatment as well. For example, denitrification or sulfate reduction may be employed at the cathode as part of a treatment system in tandem to the oxidation of wastewater. Of more interest is the potential of utilizing the cathode to reduce polluting heavy metals such as uranium or chromium. Oxidized forms of these metals are soluble and highly mobile. Upon reduction, these metals generally form insoluble precipitates, facilitating their immobilization and preventing their exposure to the environment.

     In this study, the focus of the MFC systems is aimed at catalyzing the reduction reaction occurring at the cathode. More specifically, several strains of Shewanella (S oneidensis MR-1, S. amazonensis SB2B, S. loihica PV-4, S. species W3-18-1, S. species ANA-3, and S. species MR-4) were evaluated as a cathodic biocatalyst for the reduction of hexavalent chromium, a common soil and groundwater pollutant. The aim of these experiments was to evaluate the efficiency and extent of chromium removal. While all species were able to provide some reduction of chromium, the speed and extent of reduction were different among species. For example, S. species W3-18-1 was able to reduce hexavalent chromium levels to less than 1 ppb, a level unattainable by the other species.

     Electron microscopy was also employed to evaluate morphological characteristics and biofilm characteristics. Several different biofilm types were observed. Spotty, clumped coverage was seen for MR-1, SB2B, ANA-3, and MR-4. A high degree of even, monolayer coverage was seen for W3-18-1.  One species, PV-4, did not show an appreciable amount of biofilm development. Moreover, very little attachment to the electrode surface was observed. Selected area elemental analysis was also conducted by energy dispersive spectroscopy to determine the spatial distribution of chromium precipitates located at the electrode and biofilm surfaces.

     The differences observed with these species of Shewanella operating as cathodic biocatalysts give some insight as to governing design aspects (i.e. biomass attachment, areas of chromium deposition, rate of reduction, etc.) associated with MFC performance as a hexavalent chromium bioremediation tool.

Key words: chromium, bioremediation, microbial fuel cell, Shewanella, cathodic biocatalyst