Bio-geo-electrochemical systems and their applications

Abstract

Some groups of microorganisms use poorly soluble, redox-reactive minerals as terminal electron acceptors. Collectively known as ‘dissimilatory metal-reducing bacteria (DMRB), these microbes significantly influence geochemical reactions with global implications [1]. Enzymatic reduction of poorly crystalline iron minerals by the DMRB Shewanella oneidensis MR-1 produces a variety of nanoparticulate biogenic minerals, such as magnetite and pyrite, with unique morphologies and potential for industrial applications. Research presented here investigates the use of applied electrical potential to expand the range of biogenic particles produced by DMRB under a variety of controlled laboratory conditions. MR-1 was grown in a chemically defined medium in continuous flow bioreactors under conditions that prepared them for growth in 2 types of experimental reaction chambers. Steady state operating conditions were monitored via sensors within the bioreactor vessel.Once obtained, cells were transferred from continuous flow reactors to electrochemical batch reactors instrumented with a three-electrode system under potentiostatic control. High surface area carbon felt or polished mild steel electrodes served as working surface in these reactors, which were continuously purged with sterile nitrogen gas and agitated with a magnetic stir rod. Cells from continuous flow bioreactors were also introduced to small gradient chambers containing a chemically defined medium containing poorly crystalline hydrous ferric oxide (HFO) and low melting temperature agar as a solidifying agent. Electrochemical activity of bacterial cultures was continuously controlled and monitored in both types of experimental reactors. An observed 300 millivolt decrease in open circuit potential was likely due to the consumption of oxygen as a terminal electron acceptor. Electrode surface area was directly related to the observed current production from active electrochemical cells, with an approximate 430 % increase between high surface area carbon felt and a bare titanium wire. Inoculum conditioning was also observed to influence the electrochemical properties of the microorganisms over the duration of the experiment.