Thus far, microbial fuel cells (MFCs) have been used to convert carbon-based substrates to electricity. However, sulfur compounds are ubiquitously present in organic waste and wastewater. In this study, a MFC with a hexacyanoferrate cathodic electrolyte was used to convert dissolved sulfide to elemental sulfur. Two types of MFCs were used, a square type closed to the air and a tubular type in which the cathode compartment was open to the air. The square-type MFCs demonstrated a potential-dependent conversion of sulfide to sulfur. In the tubular system, up to 514 mg sulfide L(-1) net anodic compartment (NAC) day(-1) (241 mg L(-1) day(-1) total anodic compartment, TAC) was removed. The sulfide oxidation in the anodic compartment resulted in electricity generation with power outputs up to 101 mW L(-1) NAC (47 W m(-3) TAC). Microbial fuel cells were coupled to an anaerobic upflow anaerobic sludge blanket reactor, providing total removals of up to 98% and 46% of the sulfide and acetate, respectively. The MFCs were capable of simultaneously removing sulfate via sulfide. This demonstrates that digester effluents can be polished by a MFC for both residual carbon and sulfur compounds. The recovery of electrons from sulfides implies a recovery of energy otherwise lost in the methane digester.
Abstract. The aim of this study was to identify the microbial communities that are actively involved in the assimilation of rhizosphere-C and are most sensitive in their activity to elevated atmospheric CO 2 in a temperate semi-natural low-input grassland ecosystem. For this, we analyzed 13 C signatures in microbial biomarker phospholipid fatty acids (PLFA) from an in-situ 13 CO 2 pulse-labeling experiment in the Giessen Free Air Carbon dioxide Enrichment grasslands (GiFACE, Germany) exposed to ambient and elevated (i.e. 50% above ambient) CO 2 concentrations. Short-term 13 C PLFA measurements at 3 h and 10 h after the pulselabeling revealed very little to no 13 C enrichment after 3 h in biomarker PLFAs and a much greater incorporation of new plant-C into fungal compared to bacterial PLFAs after 10 h. After a period of 11 months following the pulselabeling experiment, the 13 C enrichment of fungal PLFAs was still largely present but had decreased, while bacterial PLFAs were much more enriched in 13 C compared to a few hours after the pulse-labeling. These results imply that new rhizodeposit-C is rapidly processed by fungal communities and only much later by the bacterial communities, which we attributed to either a fungal-mediated translocation of rhizosphere-C from the fungal to bacterial biomass or a preferential bacterial use of dead root or fungal necromass materials as C source over the direct utilization of fresh rootexudate C in these N-limited grassland ecosystems. Elevated CO 2 caused an increase in the proportional 13 C enrichment (relative to the universal biomarker 16:0) of the arbuscular mycorrhizal fungal biomarker PLFA 16:1ω5 and one grampositive bacterial biomarker PLFA i16:0, but a decrease in the proportional 13 C enrichment of 18:1ω9c, a commonly used though questionable fungal biomarker PLFA. This sugCorrespondence to: K. Denef (karolien.denef@ugent.be) gests enhanced fungal rhizodeposit-C assimilation only by arbuscular mycorrhizal fungal species under elevated CO 2 .
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