Capacitive deionization (CDI) with carbon aerogels has been shown to remove various inorganic species from aqueous solutions, though no studies have shown the electrosorption behavior of multisolute systems in which ions compete for limited surface area. Several experiments were conducted to determine the ion removal capacity and selectivity of carbon aerogel electrodes, using both laboratory and natural waters. Although carbon aerogel electrodes have been treated as electrical double-layer capacitors, this study showed that ion sorption followed a Langmuir isotherm, indicating monolayer adsorption. The sorption capacity of carbon aerogel electrodes was approximately 1.0-2.0 x 10(-4) equiv/g aerogel, with ion selectivity being based on ionic hydrated radius. Monovalent ions (e.g., sodium) with smaller hydrated radii were preferentially removed from solution over multivalent ions (e.g., calcium) on a percent or molar basis. Because of the relatively small average pore size (4-9 nm) of the carbon aerogel material, only 14-42 m2/g aerogel surface area was available for ion sorption. Natural organic matter may foul the aerogel surface and limit CDI effectiveness in treating natural waters.

Electrosorption of Inorganic Salts from Aqueous Solution Using Carbon Aerogels

Counter-electrode function in nanocrystalline photoelectrochemical cell configurations

Fundamental aspects and applications of electrochemical microfabrication

Previous studies have demonstrated that Geobacter species can effectively remove uranium from contaminated groundwater by reducing soluble U(VI) to the relatively insoluble U(IV) with organic compounds serving as the electron donor. Studies were conducted to determine whether electrodes might serve as an alternative electron donor for U(VI) reduction by a pure culture of Geobacter sulfurreducens and microorganisms in uranium-contaminated sediments. Electrodes poised at -500 mV (vs a Ag/AgCl reference) rapidly removed U(VI) from solution in the absence of cells. However, when the poise at the electrode was removed, all of the U(VI) returned to solution, demonstrating that the electrode did not reduce U(VI). If G. sulfurreducens was present on the electrode, U(VI) did not return to solution until the electrode was exposed to dissolved oxygen. This suggeststhat G. sulfurreducens on the electrode reduced U(VI) to U(IV) which was stably precipitated until reoxidized in the presence of oxygen. When an electrode was placed in uranium-contaminated subsurface sediments, U(VI) was removed and recovered from groundwater using poised electrodes. Electrodes emplaced in flow-through columns of uranium-contaminated sediments readily removed U(VI) from the groundwater, and 87% of the uranium that had been removed was recovered from the electrode surface after the electrode was pulled from the sediments. These results suggest that microorganisms can use electrons derived from electrodes to reduce U(VI) and that it may be possible to remove and recover uranium from contaminated groundwater with poised electrodes.

Remediation and Recovery of Uranium from Contaminated Subsurface Environments with Electrodes

An electrical double-layer model is developed to predict electrosorption of ions from aqueous solutions by carbon aerogel electrodes. The carbon aerogel electrodes are treated as electrical double-layer capacitors, and electrosorption is modeled using classical electrical double-layer theory. Because of the porous characteristics of the electrodes, the total capacity of the system is obtained by summing the contributions of the individual pores. The pore size distribution of the carbon aerogel is measured by the physical adsorption of N2 and CO2 as well as by mercury intrusion porosimetry. When a pore has a width smaller than a specific value (cutoff pore width), it does not contribute to the total capacity because of the electrical double-layer overlapping effect. This effect greatly reduces the electrosorption capacity for electrodes with significant numbers of micropores, such as carbon aerogel; thus, it is considered in the electrical double-layer model. The model in this study focuses on the electros...

Electrosorption of ions from aqueous solutions by carbon aerogel: An electrical double-layer model

Two-step reaction mechanisms involving adsorbed monovalent intermediate ions for the electrodeposition of iron and nickel as single metals can be combined to form a predictive model for the codeposition of iron-nickel alloys. Inhibition of the more noble nickel in the presence of iron is caused by preferential surface coverage of the adsorbed iron intermediate resulting from a difference between the two elements in Tafel constant for the electrosorption step. The role of hydrolyzed cations and surface pH is investigated and methods for evaluating the influence of pH are explored. The analysis shows that changes in surface pH with potential are not necessary for iron-rich (anomalous) deposits, but that variations in pH from one electrolyte to another may influence deposit composition. The tendency toward iron-rich deposits with increasing overpotential exists in all systems, however, and can be prevented only by decreasing the iron concentration of the bath. An extension of the analysis to account for transport limitations in baths with low iron concentration is developed and calculations with the model are presented to illustrate the effects of current density and electrolyte convection under conditions similar to those investigated experimentally in the literature.

Competitive adsorption effects in the electrodeposition of iron-nickel alloys

A flow(through porous electrode, made of reticulated vitreous carbon (RVC), has been designed to remove mercury from contaminated brine solutions. Experiments with a bench-scale reactor show that the mercury concentration of contaminated brine solutions can be reduced by as much as a factor of five thousand during a single pass through the electrode. The process is mass-transfer limited, and the results of the experiments are used to develop a general correlation for the dependence of the mass-transfer coefficient on the flow rate of electrolyte through RVC. In addition, the effect of counterelectrode placement on the cell resistance is examined, and the experimental data are compared to predictions from a mathematical model of the system. The model agrees favorably with the experimental results, and the benefits of upstream counterelectrode placement, indicated by the model, are verified.

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Experimental Investigation of a Porous Carbon Electrode for the Removal of Mercury from Contaminated Brine

A multisectioned flow-through porous electrode, consisting of alternating conducting and insulating sections, was designed, built, and tested for the electrosynthesis of D-arabinose in aqueous solution. The laboratory prototype, consisting of ten independent porous graphite slices each of 5 mm thickness, can be adapted by external electrical connection to allow both traditional and sectioned operations to be run in a single device. The experimental study in this work focuses on use of the multisectioned arrangement to improve current-distribution uniformity in the porous-electrode reactor. A model electrosynthesis system, consisting of oxidation of sodium gluconate to D-arabinose in competition with the parallel reaction of oxygen evolution, has been chosen for investigation. Measurements in the sectioned electrode show that the performance, expressed in terms of current efficiency for gluconate oxidation, increases significantly with an increase in the number of sections. A comparison of predicted theoretical performance with experimental measurements on the ten-section prototype has been used to extrapolate the performance of the sectioned electrode to configurations with larger numbers of sections. Extrapolation indicates that optimal performance in the 5 cm electrode can be achieved with fewer than 100 sections, a design that is feasible to construct with modern microfabrication technology

http://jes.ecsdl.org/content/146/8/2933.full.pdf

A multisectioned porous electrode for synthesis of D-arabinose

Industrial-scale electrochemical processes, whether for wastewater treatment by electroreduction of heavy-metal ions or for chemical production by electrosynthesis of organic precursors, require use of reactors with a large electroactive surface area in a limited volume. A theoretical analysis of the performance of multisectioned flow-through porous electrodes consisting of alternating microstructures of conducting ana insulating sections is presented. The potential benefits of the multisectioned technologies are examined in terms of selectivity for the production of an intermediate species in a typical charge-transfer reaction sequence. The analysis demonstrates the feasibility of using multisectioned electrodes to provide adequately uniform current distributions without the use of the membrane separators required in traditional configurations. Nevertheless, use of the multisectioned design solely to render the current distribution in the porous electrode more uniform may not be economically competitive when compared to standard electrode arrangements due to the significant ohmic penalty involved in the operation of the multisectioned reactors. More promising benefits of multisectioned constructions lie in the additional flexibility involved in operation with controllable spatial and time-varying current distributions. Raster pulse electrolysis is one technology of this type, and its potential interest for electrosynthesis is briefly discussed.

Michael Matlosz

Papers

Competitive adsorption effects in the electrodeposition of iron-nickel alloys

Experimental Investigation of a Porous Carbon Electrode for the Removal of Mercury from Contaminated Brine

A multisectioned porous electrode for synthesis of D-arabinose

Electrochemical Engineering Analysis of Multisectioned Porous Electrodes

Solving 1-D boundary-value problems with BandAid: A functional programming style and a complementary software tool