Enhanced Performance of Hexavalent Chromium Reducing Cathodes in the Presence ofShewanella oneidensisMR-1 and Lactate

Abstract

Biocathodes for the reduction of the highly toxic hexavalent chromium (Cr(VI)) were investigated using Shewanella oneidensis MR-1 (MR-1) as a bio-catalyst 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 2 under aerated conditions in the presence of lactate, received 5.5 and 1.7 times more electrons for Cr(VI) reduction over a 4-hour 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/m 2 (1,000 Ω 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. INTRODUCTIONHexavalent chromium (Cr(VI)) is a highly toxic, mutagenic and carcinogenic substance that is present in the effluent streams of a wide range of industrial processes, including electroplating, leather tanning and wood preserving 1 . It is highly soluble and, because of its long history of use and often its disposal after inappropriate or no treatment, it has become one of the most abundant inorganic contaminants in groundwater. Ideally Cr(VI) should be removed from groundwater in the natural environment, and because of the inherent dangers to health the stringent guideline limit of 50 μg/L has been issued for total chromium concentration in drinking water 1,2 . Removal options include ion-exchange, adsorption and electrodialysis 3 ; however in many of these applications chromium keeps its toxic hexavalent state. Reduction of Cr(VI) to the considerably 3 less toxic trivalent form Cr(III) and its subsequent precipitation at neutral pH could be considered a more effective remediation strategy 4 .A possible method for Cr(VI) reduction using a microbial fuel cell (MFC) has recently been proposed, where Cr(VI) was used as an oxidant in the cathode to generate an electrical energy output 5,6 . At low pH values where H + is abundant, cathodic Cr(VI) reduction has been demonstrated at relatively fast rates and without the use of a catalyst 5,6 . In the neutral pH region, however, the reaction kinetics are slower due to low H + availability:CrO 4 2-+ 8H + + 3e -= Cr 3+ + 4H 2 O (1) At neutral pH, Cr(VI) reduction takes place under lower cathode potentials and is also severely inhibited by Cr(III) oxyhydroxide monolayers w...

Evaluating hexavalent chromium reduction and electricity production in microbial fuel cells with alkaline cathodes

“…However, as demonstrated elsewhere, this was not the case when oxygen was supplied in the MR-1-inoculated anode (Rosenbaum et al 2010;Xafenias et al 2013). In addition, this would not explain the slow recovery (or the absence of any recovery) observed after Cr(VI) depletion.…”

“…In a previous study (Xafenias et al 2013), we demonstrated that lactate can act as a Cr(III)-chelating agent, enhancing the performance of neutral pH cathodes. However, neutral pH will rarely be the case of Cr(VI)-contaminated environments, and therefore, the effectiveness of this approach has to be tested at different pH ranges.…”

“…Therefore, in order to study Cr(VI) reduction at different pH values, all catholytes contained 200 mM lactate which is a known metal chelator that can form chelates with Cr(III) (Hamada et al 2005;Puzon et al 2005) and thus enhance the Cr(VI) electroreduction process by limiting Cr(III) electrodeposition on the cathode (Xafenias et al 2013). The reason we used lactate among other metal chelators is that it occurs naturally in the environment, it exhibits low environmental toxicity (Bowmer et al 1998), it is easily biodegradable by bacteria of environmental interest like Shewanella (Pinchuk et al 2011) and can be used as an electron donor for Cr(VI) bioremediation (Brodie et al 2011;Alam et al 2006).…”

“…Passivation of the electroactive components responsible for anodic current production by Cr(III) precipitates on the bacterial cell surface could be a reason for the reduced electroactivity of MR-1 in the anode. However, recent findings suggesting that MR-1 remained electroactive during Cr(VI) reduction and could utilise the electrode as electron donor during bioelectrochemical Cr(VI) reduction (Xafenias et al 2013) and could indicate differences between the electron transfer pathways of anodic and cathodic current production. Uncomplexed Cr(III) formed during Cr(VI) reduction is known to have toxic effects on Shewanella, as it accumulates within the cytoplasm and probably binds with the DNA, affecting the cell's functionality (Bencheikh-Latmani et al 2007).…”

Impact of Fe(III) as an effective electron-shuttle mediator for enhanced Cr(VI) reduction in microbial fuel cells: Reduction of diffusional resistances and cathode overpotentials

“…However, since the Cr(VI) species (Cr 2 O 7 2-or CrO 4 2-) are electronegative and mutually exclusive against the negatively charged cathodes, the reduction of Cr(VI) and the electrochemical reaction kinetics are usually slow. The development of biocathodes based on the electrotrophic mediation between cathode electrode and Cr(VI) [12][13][15][16][17][18][19] appears to be a promising method for the reduction of Cr(VI) in…”