Cathodes enhance Corynebacterium glutamicum growth with nitrate and promote acetate and formate production

AbstractThe industrially important Corynebacterium glutamicum can only incompletely reduce nitrate into nitrite which then accumulates and inhibits growth. Herein we report that cathodes can resolve this problem and enhance glucose fermentation and growth by promoting nitrite reduction. Cell growth was inhibited at relatively high potentials but was significant when potentials were more reductive (−1.20V with anthraquinone-2-sulfonate as redox mediator or −1.25V vs. Ag/AgCl). Under these conditions, glucose was consumed up to 6 times faster and acetate was produced at up to 11 times higher yields (up to 1.1mol/mol-glucose). Acetate concentrations are the highest reported so far for C. glutamicum under anaerobic conditions, reaching values up to 5.3±0.3g/L. Herein we also demonstrate for the first time formate production (up to 3.4±0.3g/L) by C. glutamicum under strongly reducing conditions, and we attribute this to a possible mechanism of CO2 bioreduction that was electrochemically triggered

Storage and handling of pretreated lignocellulose affects the redox chemistry during subsequent enzymatic saccharification

The decomposition of lignocellulose in nature, as well as when used as feedstock in industrial settings, takes place in a dynamic system of biotic and abiotic reactions. In the present study, the impact of abiotic reactions during the storage of pretreated lignocellulose on the efficiency of subsequent saccharification was investigated. Abiotic decarboxylation was higher in steam-pretreated wheat straw (SWS, up till 1.5% CO2) than in dilute-acid-catalysed steam-pretreated forestry residue (SFR, up till 3.2% CO2) which could be due to higher iron content in SFR and there was no significant CO2 production in warm-water-washed slurries. Unwashed slurries rapidly consumed O2 during incubation at 50\ua0\ub0C; the behaviour was more dependent on storage conditions in case of SWS than SFR slurries. There was a pH drop in the slurries which did not correlate with acetic acid release. Storage of SWS under aerobic conditions led to oxidation of the substrate and reduced the extent of enzymatic saccharification by Cellic\uae CTec3. Catalase had no effect on the fractional conversion of the aerobically stored substrate, suggesting that the lower fractional conversion was due to reduced activity of the lytic polysaccharide monooxygenase component during saccharification. The fractional conversion of SFR was low in all cases, and cellulose hydrolysis ceased before the first sampling point. This was possibly due to excessive pretreatment of the forest residues. The conditions at which pretreated lignocellulose are stored after pretreatment significantly influenced the extent and kind of abiotic reactions that take place during storage. This in turn influenced the efficiency of subsequent saccharification. Pretreated substrates for laboratory testing must, therefore, be stored in a manner that minimizes abiotic oxidation to ensure that the properties of the substrate resemble those in an industrial setting, where pretreated lignocellulose is fed almost directly into the saccharification vessel.[Figure not available: see fulltext.]

Regeneration of the power performance of cathodes affected by biofouling

© 2016 The Authors. Air cathode microbial fuel cells (MFCs) were used in a cascade-system, to treat neat human urine as the fuel. Their long-term operation caused biodeterioration and biofouling of the cathodes. The cathodes were made from two graphite-painted layers, separated by a current collector. The initial performance of the MFCs was reaching average values of 105.5 ± 32.2 μW and current of 1164.5 ± 120.2 μA. After 3 months of operation the power performance decreased to 9.8 ± 3.5 μW, whilst current decreased to 461.2 ± 137.5 μA. Polarisation studies revealed significant transport losses accompanied by a biofilm formation on the cathodes. The alkaline lysis procedure was established to remove the biomass and chemical compounds adsorbed on the cathode's surface. As a result, the current increased from 378.6 ± 108.3 μA to 503.8 ± 95.6 μA. The additional step of replacing the outer layer of the cathode resulted in a further increase of current to 698.1 ± 130 μA. Similarly, the power performance of the MFCs was recovered to the original level reaching 105.3 ± 16.3 μW, which corresponds to 100% recovery. Monitoring bacterial cell number on the cathode's surface showed that biofilm formed during operation was successfully removed and composed mainly of dead bacterial cells after treatment. To the best of the authors' knowledge, this is the first time that the performance of deteriorating cathodes, has been successfully recovered for MFCs in-situ. Through this easy, fast and inexpensive procedure, designing multilayer cathodes may help enhance the range of operating conditions, if a biofilm forms on their surface

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

© 2015 The Authors. 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

Microbial fuel cells: From fundamentals to applications. A review

© 2017 The Author(s) In the past 10–15 years, the microbial fuel cell (MFC) technology has captured the attention of the scientific community for the possibility of transforming organic waste directly into electricity through microbially catalyzed anodic, and microbial/enzymatic/abiotic cathodic electrochemical reactions. In this review, several aspects of the technology are considered. Firstly, a brief history of abiotic to biological fuel cells and subsequently, microbial fuel cells is presented. Secondly, the development of the concept of microbial fuel cell into a wider range of derivative technologies, called bioelectrochemical systems, is described introducing briefly microbial electrolysis cells, microbial desalination cells and microbial electrosynthesis cells. The focus is then shifted to electroactive biofilms and electron transfer mechanisms involved with solid electrodes. Carbonaceous and metallic anode materials are then introduced, followed by an explanation of the electro catalysis of the oxygen reduction reaction and its behavior in neutral media, from recent studies. Cathode catalysts based on carbonaceous, platinum-group metal and platinum-group-metal-free materials are presented, along with membrane materials with a view to future directions. Finally, microbial fuel cell practical implementation, through the utilization of energy output for practical applications, is described

Enhanced performance of hexavalent chromium reducing cathodes in the presence of Shewanella oneidensis MR-1 and lactate

Biocathodes for the reduction of the highly toxic hexavalent chromium (Cr(VI)) were investigated using Shewanella oneidensis MR-1 (MR-1) as a biocatalyst and performance was assessed in terms of current production and Cr(VI) reduction. Potentiostatically controlled experiments (?500 mV vs Ag/AgCl) showed that a mediatorless MR-1 biocathode started up under aerated conditions in the presence of lactate, received 5.5 and 1.7 times more electrons for Cr(VI) reduction over a 4 h operating period than controls without lactate and with lactate but without MR-1, respectively. Cr(VI) reduction was also enhanced, with a decrease in concentration over the 4 h operating period of 9 mg/L Cr(VI), compared to only 1 and 3 mg/L, respectively, in the controls. Riboflavin, an electron shuttle mediator naturally produced by MR-1, was also found to have a positive impact in potentiostatically controlled cathodes. Additionally, a microbial fuel cell (MFC) with MR-1 and lactate present in both anode and cathode produced a maximum current density of 32.5 mA/m2 (1000 ? external load) after receiving a 10 mg/L Cr(VI) addition in the cathode, and cathodic efficiency increased steadily over an 8 day operation period with successive Cr(VI) additions. In conclusion, effective and continuous Cr(VI) reduction with associated current production were achieved when MR-1 and lactate were both present in the biocathodes

Cr(VI) removal in bioelectrochemical systems with electrodes as electron donors

Hexavalent chromium (Cr(VI)) is a highly toxic and soluble substance present in a wide range of industrial effluents. An effective treatment method is the biochemical Cr(VI) reduction to the less toxic trivalent chromium (Cr(III)), and such a transformation has recently been demonstrated in bioelectrochemical systems. However, depending on the pH of the catholyte, a biocatalyst might be required in the cathode and also the process can be very much inhibited by Cr(III) products which tend to form on the electrode surface and deactivate it. Herein is demonstrated how an electrophilic bacterium, that is Shewanella oneidensis MR-1, can be used as a bacterial catalyst in Cr(VI) reducing cathodes of bioelectrochemical systems. Starting with potentiostatically controlled experiments (-500 mV vs. Ag/AgCl for 4 h), the abiotic cathode’s (AC) performance was shown to be affected by the Cr(VI)-reduction products that are deactivating the cathode and are severely inhibiting further Cr(VI) reduction. The presence of metal chelators like lactate delayed this deactivating effect and enhanced the system’s performance to a large extent; in the presence of 30 mM lactate, the AC delivered approximately 3 times more electrons to Cr(VI). In addition, approximately 15 times more electrons were delivered when 5 ?M of the electron shuttle riboflavin was also added in the AC. However, the presence of riboflavin did not have any effect in the absence of lactate. The MR-1 biocathode also exhibited an enhanced current production and Cr(VI) reduction, though the pre-treatment conditions were found to be important. When pre-treated in an aerated chamber with a poised electrode at +300 mV vs. Ag/AgCl, the MR-1 biocathode mediated 70% more electrons than the AC with 30 mM lactate, and only 39% more electrons when the electrode was initially poised at -500 mV vs. Ag/AgCl. Cr(VI) reduction was also enhanced, with a decrease in concentration over the 4 h operating period of 9 mg L-1 Cr(VI) in the aerobically pretreated MR-1 chamber, compared to only 1 and 3 mg L-1 in the AC without lactate and in the AC with 30 mM lactate respectively. On the other hand, when pre-treated anaerobically in the presence or absence of Cr(VI), the performance of the MR-1 biocathode was not much different than that of the AC with 30 mM lactate.The positive effect of lactate was further demonstrated in microbial fuel cell (MFC) cathodes, where maximum power densities produced were up to 44 times the power densities reported elsewhere for abiotic cathodes (8.8 mW m-2 vs. 0.2 mW m-2 at pH 7) and at similar levels to the power densities of biotic cathodes at pH 6 and 7. Considerable Cr(VI) reduction was also observed at alkaline pH abiotic cathodes and maximum power densities were 31 times the ones reported elsewhere for biotic cathodes at pH 8 (21.4 mW m-2 vs. 0.7 mW m-2). In MFCs, the presence of MR-1 enhanced the performance of pH 7 cathodes; in the presence of 30 mM lactate, the MR-1 biocathode bioelectrochemically reduced 3 times the amount of Cr(VI) reduced by the AC with the same amount of lactate. Compared to the results in the absence of an electrode, the MFC results suggested that different Cr(VI) reduction pathways could be utilised by MR-1 when the electron donor is in the poised electrode form rather than in the lactate form. In conclusion, effective and continuous Cr(VI) reduction with associated current production were achieved when MR-1 and lactate were both present in the biocathodes