Costas Tsouris

Separation of CO2 from Flue Gas: A Review

Abstract: As a result of human activity, approximately 7 Gt of carbon are emitted to the earth’s atmosphere each year. A large portion of this carbon is in the form of gaseous CO2, and approximately 30% of this CO2 comes from fossil fuel power plants. In addition to rising levels of atmospheric CO2, the earth’s temperature is increasing. Since CO2 can act as a trap for heat (similar to the glass in a greenhouse), reduction of CO2 emissions is an important area of research. Separation and sequestration of CO2 are near-term goals for emissions reduction. Better fuel efficiency (in power production, transportation, and other areas) can be considered a mid-term goal. An acceptable long-term goal for reducing emissions is using alternate power sources such as nuclear, solar, and wind power. Because separation and sequestration are short-term goals, they are critical and challenging steps for researchers. Methods that are reviewed in this paper include absorption using solvents or solid sorbents, pressure- and temperature-swing adsorption using various solid sorbents, cryogenic distillation, membranes, and several novel and emerging technologies. Upon completion of this review, it was concluded that the most promising current method for CO2 separation is liquid absorption using monoethanolamine (MEA). While this method is currently most promising, the development of ceramic and metallic membranes for membrane diffusion should produce membranes significantly more efficient at separation than liquid absorption. The other methods investigated in this report are either too new for comparison or appear unlikely to experience significant changes to make them desirable for implementation.

Mesoporous Carbon for Capacitive Deionization of Saline Water

Self-assembled mesoporous carbon (MC) materials have been synthesized and tested for application in capacitive deionization (CDI) of saline water. MC was prepared by self-assembly of a triblock copolymer with hydrogen-bonded chains via a phenolic resin, such as resorcinol or phloroglucinol in acidic conditions, followed by carbonization and, in some cases, activation by KOH. Carbon synthesized in this way was ground into powder, from which activated MC sheets were produced. In a variation of this process, after the reaction of triblock copolymer with resorcinol or phloroglucinol, the gel that was formed was used to coat a graphite plate and then carbonized. The coated graphite plate in this case was not activated and was tested to serve as current collector during the CDI process. The performance of these MC materials was compared to that of carbon aerogel for salt concentrations ranging between 1000 ppm and 35,000 ppm. Resorcinol-based MC removed up to 15.2 mg salt per gram of carbon, while carbon aerogel removed 5.8 mg salt per gram of carbon. Phloroglucinol-based MC-coated graphite exhibited the highest ion removal capacity at 21 mg of salt per gram of carbon for 35,000 ppm salt concentration.

Recovery of Uranium from Seawater: A Review of Current Status and Future Research Needs

The recovery of uranium (U) from seawater has been investigated for over six decades in efforts to secure uranium sources for future energy production. The majority of the research activities have focused on inorganic materials, chelating polymers, and nanomaterials. Previous studies of uranium adsorption from aqueous solutions, mainly seawater, are reviewed here with a focus on various adsorbent materials, adsorption parameters, adsorption characterization, and marine studies. Continuous progress has been made over several decades, with adsorbent loadings approaching 3.2 mg U/g adsorbent in equilibrium with seawater. Further research is needed to improve first, the viability including improved capacity, selectivity, and kinetics, and second, the sorbent regeneration for multicycle use. An overview of the status of the uranium adsorption technology is provided and future research needs to make this technology commercially competitive are discussed.

Microbubble generation for environmental and industrial separations

Small gas bubbles are used in many environmental and industrial processes for solid-liquid separations or to facilitate heat and mass transfer between phases. Typically, smaller bubbles are preferred in treatment techniques due to both their high surface area-to-volume ratio and their increased bubble density at a fixed flow rate. This study examines some of the factors that affect the size of bubbles produced in the processes of electroflotation, dissolved air flotation, and a relatively new method known as electrostatic spraying. The effect of voltage, current and ionic strength was studied in electroflotation, the effect of pressure was studied in dissolved air flotation and the effect of voltage, capillary dimensions and flow rate was studied in electrostatic spraying. In electroflotation, the flow rate of gas produced increased as a function of voltage and current. Flow rate also increased as the ionic strength of the aqueous medium was increased. However, no clear trends in bubble size as a function of these parameters were evident. The bubbles produced in dissolved air flotation showed a decrease in size as saturation pressure was increased; however, the differences were insignificant at high pressures. Bubble size in electrostatic spraying decreased as voltage was increased. Finally, this study compares the three methods of bubble production in terms of average bubble diameter, bubble size distribution and power consumed during production. Dissolved air flotation produced the largest average bubble diameters, while electroflotation produced the smallest average bubble diameters. In terms of bubble size distribution, dissolved air flotation produced the most narrow distribution, electrostatic spraying produced the widest distribution, and electroflotation produced an intermediate distribution. In terms of power consumption, the pilot-scale dissolved air flotation system maximized surface area production, electroflotation produced an intermediate value, and electrostatic spraying of air produced the least surface area as a function of power consumed.

Seawater Uranium Sorbents: Preparation from a Mesoporous Copolymer Initiator by Atom-Transfer Radical Polymerization†

The world s oceans, where uranium is found quite uniformly at a concentration of 3.3 mgL , present an alternative source of uranium to terrestrial mining for nuclear fuel. Environmental concerns associated with mining will undoubtedly increase as reserves are depleted, thus increasing the utility of more environmentally friendly feedstocks. Hence, before terrestrial resources become scarce, the development of sorbents designed for seawater extraction is of strategic importance to guarantee future uranium resources. From the first inorganic adsorbents, which showed poor selectivity and mechanical resistance, to the most recent polyethylene-fiberbased sorbents containing amidoxime–carboxylic acid copolymers, and more recently layered metal sulfides and metal– organic frameworks, interest in uranium seawater extractions has continuously increased among governments worldwide. Because the concentration of uranium in the oceans is relatively low, maximization of the adsorption properties of sorbents, for example, through changes in their surface area and pore structure, can greatly improve the kinetics of uranium extraction and the adsorption capacity simultaneously. To facilitate the uptake of uranyl ions with fast kinetics, various sorbents containing the amidoxime group, such as hydrogels, particles and beads, membranes, macroporous fibers, and composites, have been prepared by suspension polymerization, radiation-induced grafting, and even sonochemical functionalization. However, silica beads and most carbon materials have a relatively small accessible surface area for the growth of large polymers or a low number of surface sites available for the grafting of functional groups. Thus, the design of substrates with large numbers of accessible reactive sites for the grafting of polymeric surface groups is necessary for the development of materials with improved uranium-adsorption capacity. Recently, porous polymers based on divinylbenzene (DVB) have been developed for applications in separations and catalysis. For example, the copolymerization of p-styrene sulfonate with divinylbenzene led to a catalytically active porous polymer. This method has the additional advantage that polymers can be obtained with controlled porosity and high surface areas without porogens. It is thus timeand cost-effective, as well as more environmentally friendly than the templated synthesis of carbonaceous materials. Motivated by these findings, we report herein nanoporous polymers based on the vinylbenzyl chloride (VBC) monomer and the DVB cross-linking agent. As well as a well-developed nanoporous structure of microand mesopores, the obtained polymers contain large numbers of accessible chlorine species, which can be used as initiators for atom-transfer radical polymerization (ATRP). These materials are the first examples of ATRP initiators in which the initiator species is located within the framework of the mesoporous support. The accessible framework and surface chlorine species were used to grow polyacrylonitrile chains, which were then converted into polyamidoxime for uranium adsorption from seawater with tailorable adsorption and surface properties. Three copolymer monoliths were synthesized by freeradical polymerization; that is, the monomer 4-vinylbenzyl chloride was cross-linked by divinylbenzene with 2,2’-azobisisobutyronitrile (AIBN, 98%) as the initiator to give copolymers hereafter referred to as p(xDVB-VBC) (in which x stands for the molar ratio of DVB to VBC). By varying the ratio of the monomer and the cross-linking reactant, it was possible to adjust the pore structure, that is, the surface area and pore volume (Figure 1). Since these adjustments arose from changes in the DVB to VBC ratio, the initiator concentration (i.e. the amount of chloride substituents present) was also varied. The nitrogen isotherms measured at 196 8C for the samples show that nonporous materials as well as materials with tailorable mesopore volumes can be [*] Dr. Y. Yue, Dr. R. T. Mayes, Dr. P. F. Fulvio, Dr. X.-G. Sun, Prof. Dr. S. Dai Chemical Sciences Division, Oak Ridge National Laboratory Oak Ridge, TN 37831 (USA) E-mail: dais@ornl.gov

Hierarchical ordered mesoporous carbon from phloroglucinol-glyoxal and its application in capacitive deionization of brackish water

Templated carbon materials have recently received tremendous attention due to energy storage and separations applications. Hierarchical structures are ideal for increased mass-transport throughout the carbon material. A new ordered mesoporous carbon material has been developed using glyoxal which exhibits a hierarchical structure with pore sizes up to 200 nm. The hierarchical structure arises from the cross linking reagent and not from the standard spinodal decomposition of a secondary solvent. The carbon material was studied for potential application as a capacitive deionization (CDI) electrode for brackish water. Results indicate that the hierarchical structure provides a pathway for faster adsorption kinetics when compared to standard resorcinol-formaldehyde CDI electrodes.

Electrosorption capacitance of nanostructured carbon aerogel obtained by cyclic voltammetry

Abstract Cyclic voltammetry experiments at various electrolyte solution concentrations (0.001–0.1 M) and scan rates (1 to 5 mV s −1 ) have been performed to study the electrical double layer (edl) formation in nanostructured carbon aerogel. The results show that carbon aerogel is a good edl capacitor and can be further divided into mesoporous and microporous capacitors. According to the experiments, the mesoporous capacitor shows a fast charging/discharging response and is only minimally affected by the electrolyte concentration and scan rate. Therefore, the specific capacitance of the mesoporous capacitor is found to be constant over a wide range of applied electrical potentials. On the other hand, the microporous capacitor shows a slow charging/discharging response and its capacitance strongly depends on the electrolyte concentration and potential. Unlike previous experiments, in which only a flat minimum was observed at the point of zero charge (pzc), in the current study, a deep minimum is observed near the pzc at low electrolyte concentration if a slow scan rate is used. This unique feature is a result of edl overlapping in the micropores and is consistent with the predictions by the Gouy–Chapman model employed in this study. Based on this behavior, a new approach is suggested for pzc measurements of solid porous materials for which a large portion of the surface area is in the micropore region.

Preparation of activated mesoporous carbons for electrosorption of ions from aqueous solutions

Mesoporous carbon with a narrow pore size distribution centered at about 9 nm, which was prepared by self assembly of block copolymer and phloroglucinol-formaldehyde resin via the soft-template method, was activated by CO2 and potassium hydroxide (KOH). The effects of activation conditions, such as the temperature, activation time, and mass ratio of KOH/C, on the textural properties of the resulting activated mesoporous carbons were investigated. Activated mesoporous carbons exhibit high BET specific surface areas (up to ∼ 2000 m2 g−1) and large pore volumes (up to ∼ 1.6 cm3 g−1), but still maintain a highly mesoporous structure. Heat treatment of mesoporous carbons by CO2 generally requires a moderate to high extent of activation in order to increase its BET surface area by 2–3 times, while KOH activation needs a much smaller degree of activation than the former to reach an identical surface area, ensuring high yields of activated mesoporous carbons. In addition, KOH activation allows a controllable degree of activation by adjusting the mass ratio of KOH/C (2–8), as evidenced by the fact that surface area and pore volume increase with the mass ratio of KOH/C. The electrosorption properties of activated mesoporous carbons were investigated by cyclic voltammetry in 0.1 M NaCl aqueous solutions. Upon activation, the electrosorption capacitance of activated mesoporous carbons was greatly enhanced.

Understanding long-term changes in microbial fuel cell performance using electrochemical impedance spectroscopy.

Changes in the anode, cathode, and solution/membrane impedances during enrichment of an anode microbial consortium were measured using electrochemical impedance spectroscopy. The consortium was enriched in a compact, flow-through porous electrode chamber coupled to an air-cathode. The anode impedance initially decreased from 296.1 to 36.3 Omega in the first 43 days indicating exoelectrogenic biofilm formation. The external load on the MFC was decreased in a stepwise manner to allow further enrichment. MFC operation at a final load of 50 Omega decreased the anode impedance to 1.4 Omega, with a corresponding cathode and membrane/solution impedance of 12.1 and 3.0 Omega, respectively. An analysis of the capacitive element suggested that most of the three-dimensional anode surface was participating in the bioelectrochemical reaction. The power density of the air-cathode MFC stabilized after 3 months of operation and stayed at 422 +/- 42 mW/m(2) (33 W/m(3)) for the next 3 months. The normalized anode impedance for the MFC was 0.017 kOmega cm(2), a 28-fold reduction over that reported previously. This study demonstrates a unique ability of biological systems to reduce the electron transfer resistance in MFCs, and their potential for stable energy production over extended periods of time.

Uranium recovery from seawater: development of fiber adsorbents prepared via atom-transfer radical polymerization

A novel adsorbent preparation method using atom-transfer radical polymerization (ATRP) combined with radiation-induced graft polymerization (RIGP) was developed to synthesize an adsorbent for uranium recovery from seawater. The ATRP method allowed a much higher degree of grafting on the adsorbent fibers (595–2818%) than that allowed by RIGP alone. The adsorbents were prepared with varied compositions of amidoxime groups and hydrophilic acrylate groups. The successful preparation revealed that both ligand density and hydrophilicity were critical for optimal performance of the adsorbents. Adsorbents synthesized in this study showed a relatively high performance (141–179 mg g−1 at 49–62% adsorption) in laboratory screening tests using a uranium concentration of ∼6 ppm. This performance is much higher than that of known commercial adsorbents. However, actual seawater experiment showed impeded performance compared to the recently reported high-surface-area-fiber adsorbents, due to slow adsorption kinetics. The impeded performance motivated the investigation of the effect of hydrophilic block addition on the graft chain terminus. The addition of a hydrophilic block on the graft chain terminus nearly doubled the uranium adsorption capacity in seawater, from 1.56 mg g−1 to 3.02 mg g−1. The investigation revealed the importance of polymer chain conformation, in addition to the ligand and hydrophilic group ratio, for advanced adsorbent synthesis for uranium recovery from seawater.