RESUMEN
The decomposition of marine plankton in two-chamber, seawater-filled microbial fuel cells (MFCs) has been investigated and related to resulting chemical changes, electrode potentials, current efficiencies, and microbial diversity. Six experiments were run at various discharge potentials, and a seventh served as an open-circuit control. The plankton consisted of a mixture of freshly captured phytoplankton and zooplankton (0.21 to 1 mm) added at an initial batch concentration of 27.5 mmol liter(-1) particulate organic carbon (OC). After 56.7 days, between 19.6 and 22.2% of the initial OC remained, sulfate reduction coupled to OC oxidation accounted for the majority of the OC that was degraded, and current efficiencies (of the active MFCs) were between 11.3 and 15.5%. In the open-circuit control cell, anaerobic plankton decomposition (as quantified by the decrease in total OC) could be modeled by three terms: two first-order reaction rate expressions (0.79 day(-1) and 0.037 day(-1), at 15 degrees C) and one constant, no-reaction term (representing 10.6% of the initial OC). However, in each active MFC, decomposition rates increased during the third week, lagging just behind periods of peak electricity generation. We interpret these decomposition rate changes to have been due primarily to the metabolic activity of sulfur-reducing microorganisms at the anode, a finding consistent with the electrochemical oxidization of sulfide to elemental sulfur and the elimination of inhibitory effects of dissolved sulfide. Representative phylotypes, found to be associated with anodes, were allied with Delta-, Epsilon-, and Gammaproteobacteria as well as the Flavobacterium-Cytophaga-Bacteroides and Fusobacteria. Based upon these results, we posit that higher current efficiencies can be achieved by optimizing plankton-fed MFCs for direct electron transfer from organic matter to electrodes, including microbial precolonization of high-surface-area electrodes and pulsed flowthrough additions of biomass.
Asunto(s)
Fenómenos Fisiológicos Bacterianos , Fuentes de Energía Bioeléctrica , Reactores Biológicos , Diseño de Equipo , Fuentes de Energía Bioeléctrica/microbiología , Electricidad , Electrodos , Transporte de Electrón , Cinética , Plancton/crecimiento & desarrollo , Agua de MarRESUMEN
In the present study, the adsorption of U(VI) by a natural iron-rich sand in the presence of citrate was studied over a range of citrate concentrations and pH values. Adsorption of U(VI) on the iron-rich sand decreased in the presence of increasing concentrations of citrate. Adsorption of citrate to the sand was weak under most conditions studied. Several explanations for the adsorption behavior of U(VI) and citrate were investigated, including aqueous complexation of U(VI) by citrate, competition of U(VI) and citrate for adsorption sites, and extraction of Fe and Al from the sorbent surface by citrate (surface alteration). Although aqueous complexation of U(VI) by citrate may still play a significant role, both competitive adsorption and aqueous complexation proved to be inadequate explanations of the adsorption behavior. Both physical surface alteration (i.e., loss of surface area) and chemical surface alteration (i.e., change in the chemical composition of the sand surface) were investigated, with chemical surface alteration controlling the bulk of U(VI) adsorption. Considering these results, remediation schemes that involve organic complexing agents should address the possibility of surface alteration affecting radionuclide adsorption and mobility.
Asunto(s)
Ácido Cítrico/química , Modelos Químicos , Contaminantes Radiactivos del Suelo/análisis , Uranio/química , Adsorción , Aluminio/química , Concentración de Iones de Hidrógeno , Hierro/química , Espectrometría de Masas , Conteo por Cintilación , Dióxido de SilicioRESUMEN
The rates of reduction of carbon tetrachloride (CT) and nitrobenzene (NB) by iron-oxide coated gold electrodes were studied to gain insight into the processes that control reduction of groundwater contaminants by zerovalent metal permeable reactive barriers. Fe(III)-oxide films were deposited on gold electrodes with a small fraction of the Fe(III) electrochemically reduced to Fe(II) to investigate the role of Fe(II) in the reduction of the CT and NB. Mass transport to the surface of the oxide film was controlled through use of a well-defined flow-through system similar to a wall-jet electrode. The factors affecting the overall reduction rate were investigated by varying the Fe(II) content in the iron-oxide, controlling mass transport of the electroactive species to the oxide surface, and varying the thickness of the oxide film. The rates of reduction of CT and NB were found to be independent of Fe(II) content in the iron-oxide and were only slightly dependent on the rate of transport to the surface of the oxide under a few sets of reaction conditions. Conversely, the rates of reduction were greatly dependent on the thickness of the oxide film, with the reduction rate decreasing as the oxide thickness increased. Evidence suggests that the location of the reduction reaction for CT and NB is at the gold surface and supports a barrier model for the system studied, in which the oxide film physically impedes direct contact of the electroactive species and the gold electrode, increases the diffusion path length, and creates adsorption sites.