Redox Potential Heterogeneity in Fixed-Bed Electrodes Leads to Microbial Stratification and Inhomogeneous Performance

Abstract

Bed electrodes provide high electrode area-to-volume ratios represent a promising configuration for transferring bioelectrochemical systems close to industrial applications. Nevertheless, the intrinsic electrical resistance leads to poor polarization behavior. Therefore, the distribution of Geobacter spp. and their electrochemical performance within exemplary fixed-bed electrodes are investigated. A minimally invasive sampling system allows characterization of granules from different spatial locations of bed electrodes. Cyclic voltammetry of single granules (n=63) demonstrates that the major share of electroactivity (134.3 mA L^-1 ) is achieved by approximately 10 % of the bed volume, specifically that being close to the current collector. Nevertheless, analysis of the microbial community reveals that Geobacter spp. dominated all sampled granules. These findings clearly demonstrate the need for engineered bed electrodes to improve electron exchange between microorganisms and granules.

Keywords: 3D printing; cyclic voltammetry; electrochemistry; electrode materials; electron transfer.

Figure 1

Chronoamperogram of bedBES1 polarized at +200 mV: 1) Inoculation with secondary Geobacter spp. enrichment biofilm; 2) granular sampling during batch operation; 3) change in reactor operation from batch to continuous mode; 4) acetate concentration increase from 10 mM to 20 mM; 5) granular sampling during continuous operation (see Figure S1 for bedBES2).

Figure 2

Exemplary cyclic voltammograms (CV) of granules sampled from bedBES1 longitudinal axis under a) batch and b) continuous operation. The CV analyses were performed with the e‐Clamp and granules from the middle pocket of the top (black lines), middle (red lines), and bottom sampling unit (blue lines).

Figure 3

Redox potential recorded during one batch cycle at the top (black), middle (red), and bottom (blue) redox sensor distributed over the longitudinal axis of bedBES1. The green dotted line represents the current production of bedBES1 polarized at +200 mV.

Figure 4

Exemplary cyclic voltammograms (CV) of graphite granules extracted from bedBES1 transversal axis during a) batch and b) continuous operation. The CV analysis was performed directly after sampling granules with the e‐Clamp from the inner (black), middle (red), and outer (blue) pockets of the top sampling unit. The scan rate was 1 mV s⁻¹ (3^rd scans are shown).

Figure 5

Microbial community composition within bedBES1 determined with TRFLP for a) the bacterial (restriction enzyme HaeIII) and b) the methanogenic community (restriction enzyme MwoI). Granules from all pockets of the top (S 1.1, S 1.2, and S 1.3 for inner, middle, and outer sampling pocket, respectively), middle (S 2.1, S 2.2, and S 2.3), and bottom (S 3.1, S 3.2, and S 3.3) sampling unit were analyzed. Sampling was conducted at day 33 and day 62 for obtaining results for batch and continuous operation, respectively. No bacterial DNA could be extracted from sampling pocket S 3.3 during batch cultivation.

Figure 6

a) Scheme of a fixed‐bed electrode bioelectrochemical system (bedBES). b) Picture showing a section of the constructed bedBES with the three minimally invasive sampling units (top, middle, bottom) at different heights of the longitudinal axis of the bedBES. Each sampling unit provided access to granules in three different points of the transversal axis of bedBES by rotating the middle cylinder of the sampling units in order to have an open configuration of the sampling units. Thereby, the granules fell through the sampling windows of the outer and middle cylinder into the sampling pockets. Subsequently, the inner cylinder of the sampling unit was pulled allowing a minimally invasive sampling of granules by using the e‐Clamp. c) e‐Clamp inserted into a bioelectrochemical reactor allowing cyclic voltammetry of single granules.