No abstract

In methanogenic microbial electrolysis cells (MMCs), CO2 is reduced to methane using a methanogenic biofilm on the cathode by either direct electron transfer or evolved hydrogen. To optimize methane generation, we examined several cathode materials: plain graphite blocks, graphite blocks coated with carbon black or carbon black containing metals (platinum, stainless steel or nickel) or insoluble minerals (ferrihydrite, magnetite, iron sulfide, or molybdenum disulfide), and carbon fiber brushes. Assuming a stoichiometric ratio of hydrogen (abiotic):methane (biotic) of 4:1, methane production with platinum could be explained solely by hydrogen production. For most other materials, however, abiotic hydrogen production rates were insufficient to explain methane production. At −600 mV, platinum on carbon black had the highest abiotic hydrogen gas formation rate (1600 ± 200 nmol cm–3 d–1) and the highest biotic methane production rate (250 ± 90 nmol cm–3 d–1). At −550 mV, plain graphite (76 nmol cm–3 d–1) performed similarly to platinum (73 nmol cm–3 d–1). Coulombic recoveries, based on the measured current and evolved gas, were initially greater than 100% for all materials except platinum, suggesting that cathodic corrosion also contributed to electromethanogenic gas production.

Eu 2+ -as well as Ce 3+ -doped Ba[Li 2 (Al 2 Si 2 )N 6 ] and its related Mg-substituted compounds Ba[(Mg 2-x Li x )(Al 4-x Si x )N 6 ]:Eu 2+ (x = 0-2) with x = 1.6, 1.8 have been synthesized by metathesis reactions in tantalum ampules. Crystal structures were solved and refined from single-crystal X-ray diffraction data. All three compounds crystallize in tetragonal space group P4/ncc (no. 130) (Z = 4, Ba[Li 2 (Al 2 Si 2 )N 6 ]:Eu 2+ : a = 7.8282(4), c = 9.9557(5) Å, R1 = 0.0144, wR2 = 0.0366). Their crystal structures, exhibiting the novel framework topology whj, consist of a highly condensated anionic tetrahedra network of disordered (Li/Mg)N 4 and (Al/Si)N 4 units, connected to each other by common edges and corners. The degree of condensation (i.e. atomic ratio (Al,Li,Mg,Si):N) is κ = 1. The Ba 2+ -position is coordinated eightfold by N 3-in form of a truncated square pyramid. Upon doping with Eu 2+ narrow-band emission in the green to yellow spectral range is observed (λ em = 532 -562 nm, fwhm ∼1962 cm -1 ). Ce 3+ -doped crystals of Ba[Li 2 (Al 2 Si 2 )N 6 ] show blue emission (λ em = 468; 507 nm). According to the tunability of the narrow-band green emission, application in LED-backlight LCDs appears promising.

High current densities in microbial electrolysis cells (MECs) result from the predominance of various Geobacter species on the anode, but it is not known if archaeal communities similarly converge to one specific genus. MECs were examined here on the basis of maximum methane production and current density relative to the inoculum community structure. We used anaerobic digester (AD) sludge dominated by acetoclastic Methanosaeta, and an anaerobic bog sediment where hydrogenotrophic methanogens were detected. Inoculation using solids to medium ratio of 25% (w/v) resulted in the highest methane production rates (0.27 mL mL−1 cm−2, gas volume normalized by liquid volume and cathode projected area) and highest peak current densities (0.5 mA cm−2) for the bog sample. Methane production was independent of solid to medium ratio when AD sludge was used as the inoculum. 16S rRNA gene community analysis using pyrosequencing and quantitative PCR confirmed the convergence of Archaea to Methanobacterium and Methanobrevibacter, and of Bacteria to Geobacter, despite their absence in AD sludge. Combined with other studies, these findings suggest that Archaea of the hydrogenotrophic genera Methanobacterium and Methanobrevibacter are the most important microorganisms for methane production in MECs and that their presence in the inoculum improves the performance.

Methane is the primary end product from cathodic current in microbial electrolysis cells (MECs) in the absence of methanogenic inhibitors, but little is known about the archaeal communities that develop in these systems. MECs containing cathodes made from different materials (carbon brushes, or plain graphite blocks or blocks coated with carbon black and platinum, stainless steel, nickel, ferrihydrite, magnetite, iron sulfide, or molybdenum disulfide) were inoculated with anaerobic digester sludge and acclimated at a set potential of -600 mV (versus a standard hydrogen electrode). The archaeal communities on all cathodes, except those coated with platinum, were by Methanobacterium (median 97% of archaea). Cathodes with platinum contained mainly archaea most similar to Methanobrevibacter. Neither of these methanogens were abundant (<0.1% of archaea) in the inoculum, and therefore their high abundance on the cathode resulted from selective enrichment. In contrast, bacterial communities on the cathode were more diverse, containing primarily δ-Proteobacteria (41% of bacteria). The lack of a consistent bacterial genus on the cathodes indicated that there was no similarly selective enrichment of bacteria on the cathode. These results suggest that the genus Methanobacterium was primarily responsible for methane production in MECs when cathodes lack efficient catalysts for hydrogen gas evolution.