Cathode potential and anode electron donor evaluation for a suitable treatment of nitrate-contaminated groundwater in bioelectrochemical systems

Abstract

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Microbial electrochemical systems for nitrate removal from diluted streams

“…The DNRA pathway is generally not considered in most cathodic denitrification studies due to the absence or very low concentrations of ammonium usually detected within the denitrifying cathodes [82,96]. However, our results indicate that it is in fact happening in mixed culture denitrifying cathodes, which corroborates a previous publication that reported detection of ammonium in denitrifying microbial fuel cell (MFC) inoculated with non-adapted microbial community [70].…”

“…Thus, a higher electromotive force will be achieved at lower cathodic potentials [96]. However, although potentially higher energy is gained by the cells for their metabolism when low cathodic potentials are applied, the quality of the effluent (in terms of intermediate products) may be negatively affected when the cathode is operated at potentials lower than -0.23 V vs SHE [82]. Alternatively, if a specialised microbial community is present at the cathode, the application of more negative cathodic potentials might enable microbial electrosynthesis of hydrogen and/or reduced organic compounds such as acetate [113][114][115], which could potentially be further used by autotrophic and heterotrophic denitifiers respectively, thus improving nitrate removal rates.…”

“…This means that, when attempting to generate acetate bioelectrochemically, applied cathodic potentials should be lower than -0.28 V. Although this acetate produced in the reactor could be further used as electron/carbon donor for heterotrophic denitrification, the very low potential applied is likely also providing great energy for the reduction of nitrate, thus possibly enabling the denitrifiers to outcompete acetogens. Interestingly, a recent study on cathodic denitrification showed that nitrate reduction rates improved with decreasing potentials only until reaching -0.4 V vs SHE [82], which indicates the biofilm on the referred study had reached its maximum denitrification capacity at potentials which would in practice still not enable acetate or hydrogen production. In fact, as indicated in Figure 17, an applied cathodic potential of approximately -0.45 V vs SHE was required to…”

“…Overall, a bioelectrochemical autotrophic denitrification is advantageous over traditional heterotrophic denitrification due to less biomass formation (avoiding clogging of the reactors), and due to the fact that no external organic matter is required in the process, which eliminates the need of supply and on-site storage of chemicals and avoids further carbon contamination into watercourses [21,22]. In fact, a few studies have demonstrated cathodic denitrification by combining it with an abiotic anode compartment [81][82][83], in which electrons will be generated abiotically by electrochemical water splitting at a counter electrode (i.e., the anode electrode), with the simultaneous production of oxygen, as exemplified in Figure 5. The electrons generated from the anodic oxygen evolution are transferred to the cathode through an external circuitry where they are used for microbialmediated denitrification.…”

“…In fact, several cathodic denitrification studies (either in MFC or MEC operation modes) have used a similar flat, rectangular frame configuration [53,54,82,95,131,132]. In this configuration, cathode and anode electrodes are placed in parallel and generally separated by a CEM to enable ion transport (i.e.…”