Methanogenic Biocathode Microbial Community Development and the Role of Bacteria

The cathode microbial community of a methanogenic bioelectrochemical system (BES) is key to the efficient conversion of carbon dioxide (CO₂) to methane (CH₄) with application to biogas upgrading. The objective of this study was to compare the performance and microbial community composition of a biocathode inoculated with a mixed methanogenic (MM) culture to a biocathode inoculated with an enriched hydrogenotrophic methanogenic (EHM) culture, developed from the MM culture following pre-enrichment with H₂ and CO₂ as the only externally supplied electron donor and carbon source, respectively. Using an adjacent Ag/AgCl reference electrode, biocathode potential was poised at −0.8 V (versus SHE) using a potentiostat, with the bioanode acting as the counter electrode. When normalized to cathode biofilm biomass, the methane production in the MM- and EHM-biocathode was 0.153 ± 0.010 and 0.586 ± 0.029 mmol CH₄/mg biomass-day, respectively. This study showed that H₂/CO₂ pre-enriched inoculum enhanced biocathode CH₄ production, although the archaeal communities in both biocathodes converged primarily (86–100%) on a phylotype closely related to <i>Methanobrevibacter arboriphilus</i>. The bacterial community of the MM-biocathode was similar to that of the MM inoculum but was enriched in Spirochaetes and other nonexoelectrogenic, fermentative Bacteria. In contrast, the EHM-biocathode bacterial community was enriched in Proteobacteria, exoelectrogens, and putative producers of electron shuttle mediators. Similar biomass levels were detected in the MM- and EHM-biocathodes. Thus, although the archaeal communities were similar in the two biocathodes, the difference in bacterial community composition was likely responsible for the 3.8-fold larger CH₄ production rate observed in the EHM-biocathode. Roles for abundant OTUs identified in the biofilm and inoculum cultures were highlighted on the basis of previous reports

Zero-Valent Iron Enhances Biocathodic Carbon Dioxide Reduction to Methane

Methanogenic bioelectrochemical systems (BESs), which convert carbon dioxide (CO₂) directly to methane (CH₄), promise to be an innovative technology for anaerobic digester biogas upgrading. Zero-valent iron (ZVI), which has previously been used to improve CH₄ production in anaerobic digesters, has not been explored in methanogenic biocathodes. Thus, the objective of this study was to assess the effect of biocathode ZVI on BES performance at 1 and 2 g/L initial ZVI concentrations and at various cathode potentials (−0.65 to −0.80 V versus SHE). The total CH₄ produced during a 7-day feeding cycle with 1 and 2 g/L initial ZVI was 2.8- and 2.9-fold higher, respectively, than the mean CH₄ production in the four prior cycles without ZVI addition. Furthermore, CH₄ production by the ZVI-amended biocathodes remained elevated throughout three subsequent feeding cycles, despite catholyte replacement and no new ZVI addition. The fourth cycle following a single ZVI addition of 1 g/L and 2 g/L yielded 123% and 231% more total CH₄ than in the non-ZVI cycles, respectively. The higher CH₄ production could not be fully explained by complete anaerobic oxidation of the ZVI and utilization of produced H₂ by hydrogenotrophic methanogens. Microbial community analysis showed that the same phylotype, most closely related to <i>Methanobrevibacter arboriphilus</i>, dominated the archaeal community in the ZVI-free and ZVI-amended biocathodes. However, the bacterial community experienced substantial changes following ZVI exposure, with more <i>Proteobacteria</i> and fewer <i>Bacteroidetes</i> in the ZVI-amended biocathode. Furthermore, it is likely that a redox-active precipitate formed in the ZVI-amended biocathode, which sorbed to the electrode and/or biofilm, acted as a redox mediator, and enhanced electron transfer and CH₄ production. Thus, ZVI may be used to increase biocathode CH₄ production, assist in the start-up of an electromethanogenic biocathode, and/or maintain microbial activity during voltage interruptions

Inhibitory Effect of Furanic and Phenolic Compounds on Exoelectrogenesis in a Microbial Electrolysis Cell Bioanode

The objective of this study was to systematically investigate the inhibitory effect of furfural (FF), 5-hydroxymethylfurfural (HMF), syringic acid (SA), vanillic acid (VA), and 4-hydroxybenzoic acid (HBA), which are problematic lignocellulose-derived byproducts, on exoelectrogenesis in the bioanode of a microbial electrolysis cell. The five compound mixture at an initial total concentration range from 0.8 to 8.0 g/L resulted in an up to 91% current decrease as a result of exoelectrogenesis inhibition; fermentative, nonexoelectrogenic biotransformation pathways of the five compounds were not affected. Furthermore, the parent compounds at a high concentration, as opposed to their biotransformation products, were responsible for the observed inhibition. All five parent compounds contributed to the observed inhibition of the mixture. The IC₅₀ (i.e., concentration resulting in 50% current decrease) of individually tested parent compounds was 2.7 g/L for FF, 3.0 g/L for HMF, 1.9 g/L for SA, 2.1 g/L for VA and 2.0 g/L for HBA. However, the parent compounds, when tested below their respective noninhibitory concentration, jointly resulted in significant inhibition as a mixture. Catechol and phenol, which were persistent biotransformation products, inhibited exoelectrogenesis only at high concentrations, but to a lesser extent than the parent compounds. Exoelectrogenesis recovery from inhibition by all compounds was observed at different rates, with the exception of catechol, which resulted in irreversible inhibition

Photodegradation of Veterinary Ionophore Antibiotics under UV and Solar Irradiation

The veterinary ionophore antibiotics (IPAs) are extensively used as coccidiostats and growth promoters and are released to the environment via land application of animal waste. Due to their propensity to be transported with runoff, IPAs likely end up in surface waters where they are subject to photodegradation. This study is among the first to investigate the photodegradation of three commonly used IPAs, monensin (MON), salinomycin (SAL) and narasin (NAR), under UV and solar irradiation. Results showed that MON was persistent in a deionized (DI) water matrix when exposed to UV and sunlight, whereas SAL and NAR could undergo direct photolysis with a high quantum yield. Water components including nitrate and dissolved organic matter had a great impact on the photodegradation of IPAs. A pseudosteady state kinetic model was successfully applied to predict IPAs’ photodegradation rates in real water matrices. Applying LC/MS/MS, multiple photolytic transformation products of IPAs were observed and their structures were proposed. The direct photolysis of SAL and NAR occurred via cleavage on the ketone moiety and self-sensitized photolysis. With the presence of nitrate, MON was primarily degraded by hydroxyl radicals, whereas SAL showed reactivity toward both hydroxyl and nitrogen-dioxide radicals. Additionally, toxicity tests showed that photodegradation of SAL eliminated its antibiotic properties against <i>Bacillus subtilis</i>

Effect of Alkyl Side Chain Location and Cyclicity on the Aerobic Biotransformation of Naphthenic Acids

Aerobic biodegradation of naphthenic acids is of importance to the oil industry for the long-term management and environmental impact of process water and wastewater. The effect of structure, particularly the location of the alkyl side chain as well as cyclicity, on the aerobic biotransformation of 10 model naphthenic acids (NAs) was investigated. Using an aerobic, mixed culture, enriched with a commercial NA mixture (NA sodium salt; TCI Chemicals), batch biotransformation assays were conducted with individual model NAs, including eight 8-carbon isomers. It was shown that NAs with a quaternary carbon at the α- or β-position or a tertiary carbon at the β- and/or β′-position are recalcitrant or have limited biodegradability. In addition, branched NAs exhibited lag periods and lower degradation rates than nonbranched or simple cyclic NAs. Two NA isomers used in a closed bottle, aerobic biodegradation assay were mineralized, while 21 and 35% of the parent compound carbon was incorporated into the biomass. The NA biodegradation probability estimated by two widely used models (BIOWIN 2 and 6) and a recently developed model (OCHEM) was compared to the biodegradability of the 10 model NAs tested in this study as well as other related NAs. The biodegradation probability estimated by the OCHEM model agreed best with the experimental data and was best correlated with the measured NA biodegradation rate

Biodegradation of Veterinary Ionophore Antibiotics in Broiler Litter and Soil Microcosms

Ionophore antibiotics (IPAs) are polyether compounds used in broiler feed to promote growth and control coccidiosis. Most of the ingested IPAs are excreted into broiler litter (BL), a mixture of excreta and bedding material. BL is considered a major source of IPAs released into the environment as BL is commonly used to fertilize agricultural fields. This study investigated IPA biodegradation in BL and soil microcosms, as a process affecting the fate of IPAs in the environment. The study focused on the most widely used IPAs, monensin (MON), salinomycin (SAL), and narasin (NAR). MON was stable in BL microcosms at 24–72% water content (water/wet litter, w/w) and 35–60 °C, whereas SAL and NAR degraded under certain conditions. Factor analysis was conducted to delineate the interaction of water and temperature on SAL and NAR degradation in the BL. A major transformation product of SAL and NAR was identified. Abiotic reaction(s) were primarily responsible for the degradation of MON and SAL in nonfertilized soil microcosms, whereas biodegradation contributed significantly in BL-fertilized soil microcosms. SAL biotransformation in soil microcosms yielded the same product as in the BL microcosms. A new primary biotransformation product of MON was identified in soil microcosms. A field study showed that MON and SAL were stable during BL stacking, whereas MON degraded after BL was applied to grassland. The biotransformation product of MON was also detected in the top soil layer where BL was applied