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.

Electrochemical and Microbial Dissection of Electrified Biotrickling Filters

“…Consequently, a precise vertical mapping of the denitrification activity and contribution to N-TN RR in the granule bed can hardly be achieved. Nevertheless, considering the prevailing autotrophic growth conditions in the electrified biotrickling filters and the resulting limited biomass growth, the observed j seems reasonable when being compared with similar e-clamp experiments with granules from Geobacter enrichment biofilms cultivated in acetate-fed fixed bed electrodes [~2 μA mg −1 during CV recorded with 1 mV s −1 (Quejigo et al, 2021 )].…”

“…(2) Although the granule sampling was designed to be minimally invasive, it cannot be prevented that the biofilm integrity is disturbed during sampling so that cells may have lost contact with other cells or the granules. Consequently, some redox sites are not wired to the metabolism by actively participating in EET and show oxidation and reduction (Peng et al, 2016 ; Quejigo et al, 2021 ). Therefore, the ratio between rates of EET, intracellular electron transfer, metabolism, and especially their joints were changed, leading to apparent non-turnover signals.…”

“…(3) Single granules were subjected to different mass transfer regimes when comparing CV experiments with e-clamp (i.e., radial diffusion) and their embedding within the bed electrode (i.e., mainly planar diffusion). Thus, potential limitations due to, for instance, substrate supply and counter ion transport were decreased, leading to different kinetics (Quejigo et al, 2021 ).…”

“…The in-house developed granule sampling unit consisted of 2 cylinders made of polyvinyl chloride (PVC), the outer cylinder of the sampling unit was made of acrylic glass and fixed with glue to the PP tube. The sampling window was 12 × 8 mm, and the sampling pocket of the inner cylinder had a depth of 5 mm (Quejigo et al, 2021 ). At the bottom of the reactor, a PVC cap with a tube connector as an effluent port and a titanium mesh (5 × 5 cm) were integrated to prevent clogging of this port by granules.…”

“…Granules were extracted for performing cyclic voltammetry using the sampling units, fixed with the grab hook of the e-clamp, and directly transferred to four-neck round-bottom flasks with an integrated three-electrode setup ( Supplementary Figure S1 ) (Quejigo et al, 2021 ). The e-clamp was pierced through a silicone stopper and represented the working electrode, 250 ml medium was purged for 25 min with N 2 /CO 2 (80%:20%) before the e-clamp was inserted.…”