CO2-permeable Membrane Research Articles

Development of membrane bioreactor for conversion of flue Gas-CO2 to C1 and C2 biomolecules

The capture and utilization of CO2 (CCU) from flue gases constitute a novel carbon feedstock for producing renewable chemicals and fuels. To lower the energy demand of CCU, Bio-Integrated Carbon Capture and Utilization (BICCU) was recently presented, where CO2-consuming microorganisms simultaneously desorb and convert CO2 captured from flue gas by methyl diethanolamine (MDEA) with H2 from water-electrolysis as the electron donor. The limiting factor is the microbial toxicity of MDEA, which restricts the capacity of the process. Furthermore, producing soluble molecules like the C2 platform molecule acetate is not possible without contaminating the product with capture agent. To resolve both challenges, we here propose a three-phase membrane system, where the MDEA and microorganisms are separated by a CO2-permeable membrane. Biological batch experiments with a mixed mesophilic culture confirmed the increased capacity of the BICCU-process for producing both methane and acetate when introducing a CO2-permeable membrane to the system to alleviate microbial inhibition by the capture agent and avoid contamination of soluble bioproducts. When using O2-rich flue gas from a biogas combustion engine, the CO2 conversion rate decreased by 18.0 % compared to a clean 20:80 CO2:N2 gas mixture, yet anaerobic metabolisms were still active in the mixed inoculum. Abiotic experiments furthermore showed that the microbial CO2 consumption maintained the gradient across the membrane and enhanced the CO2-flux by a factor of 3.1 ± 0.3. The membrane-based BICCU concept is expected to hold great promise particularly for producing acetate from flue gas-CO2, as the in situ separation eliminates the need for costly downstream separation.

Carrier-driving CO2 separation by amine-rich membranes with intercalated graphene oxide

Amine-rich facilitated transport membranes (FTMs) attract great interest in intensifying the membrane-based CO2 separation processes. The high-molecular-weight polyvinylamine (PVAm) polymers containing fixed-site carriers of the amino groups were used to prepare highly CO2-permeable membranes. The sterically hindered PVAm polymers of poly(N-methyl-N-vinylamine) and poly(N-isopropyl-N-vinylamine) were obtained by functionalization of PVAm to provide superior CO2 solubility. By loading the mobile carriers of amino acid salt (AAS) and CO2-philic graphene oxide (GO), the prepared FTMs render enhanced CO2 permeance and CO2/N2 selectivity. The d-spacing of 8.8 Å and the ultramicropores of 3.5 Å from GO nanosheets provide the combination of both selective surface flow and molecular sieving mechanisms to achieve improved CO2 permeance and CO2/N2 selectivity. In addition, the intercalation of GO hinders N2 transport through the membrane due to a longer pathway, while the mobile carriers of AAS introduced into the PVAm matrix facilitate CO2 transport through the selective layer. Therefore, the CO2/N2 selectivity of the prepared FTMs was significantly enhanced to 171 based on the intensified carrier-driving transport mechanism. It can be concluded that amine-rich membranes based on both fixed and mobile carriers of the amino groups together with intercalated GO can synergistically improve the CO2/N2 separation performance, and be potentially applied for CO2 capture from flue gas.

Simulation of the Membrane Process of CO2 Capture from Flue Gas via Commercial Membranes While Accounting for the Presence of Water Vapor.

Carbon capture and storage is one of the potential options for reducing CO2 emissions from coal-fired power plants while preserving their operation. Mathematical modeling was carried out for a one-stage membrane process of carbon dioxide capture from the flue gases of coal-fired power plants using commercial gas separation membranes. Our calculations show that highly CO2-permeable membranes provide similar characteristics with respect to the separation process (e.g., a specific area of membrane and a specific level of electrical energy consumption) despite the significant variation in CO2/N2 and H2O/CO2 selectivity. Regarding the development of processes for the recovery of CO2 from flue gas using membrane technology, ensuring high CO2 permeance of a membrane is more important than ensuring high CO2/N2 selectivity. The presence of water vapor in flue gas provides a higher driving force of CO2 transfer through the membrane due to the dilution of CO2 in the permeate. A cross-flow membrane module operation provides better recovery of CO2 in the presence of water vapor than a counter-current operation.

Catalytic effect of trifluoroacetic acid on the CO2 transport properties of organic-inorganic hybrid silica membranes

Developing silica membranes that are highly selective for CO2 has always been a challenge due to the small sizes of the pores and less amount of CO2 philic sites in a typical silica network structure. Herein, we describe the fabrication of silica (tetraethoxysilane) membranes functionalized with 3-aminopropyltriethoxysilyl (APTES) and trifluoroacetic acid (TFA). An interaction generated among primary (NH2) amines and TFA was identified, which was then also revealed by the reversible nature of CO2 adsorption/desorption — an opposite trend from observations when using another catalyst (HCl). The resultant TEOS-APTES (TFA) membranes demonstrated CO2 permeance of 3.8 × 10−7 mol m − 2 s − 1 Pa−1 and CO2/N2 selectivity of 35 at 50 ⁰C via the effect of surface diffusion. This is attributed to the increased microporosity and structural variations affected by TFA, which enhanced molecular sieving and controls the CO2-philic sites (-NHCOCF3) via interaction with amines. This novel approach would be effective for the energy-efficient fabrication of highly CO2-permeable membranes.

Two-stage membrane-based process utilizing highly CO2-selective membranes for cost and energy efficient carbon capture from coal flue gas: A process simulation study

Membrane technology for CO2 capture has become an attractive strategy due to its cost and energy efficiency and low materials costs. In the past decade, membrane-based process designs for post-combustion power plant CO2 capture have been developed, utilizing existing highly CO2-permeable membranes with relatively low CO2/N2 selectivity (<50), and have obtained reasonably economic carbon capture. However, few membrane-based process designs were proposed for moderate to highly CO2-selective membranes (CO2/N2 selectivity of 50–300, and >300, respectively), which have vastly emerged in recent years, such as various facilitated transport membranes (FTMs). Herein, we proposed a two-stage membrane-base process design targeting economic carbon capture from coal-fired flue gas. This process design features the utilization of highly CO2-selective membranes for one-stage CO2 enrichment to 95% dry-base purity in the first stage and recycle of the remaining CO2 by a highly CO2-permeable membrane in the second stage, in order to achieve economic CO2 capture with 90% capture rate and >95% CO2 product purity. Through an integration-iteration membrane model and the Aspen Plus process simulation, a sensitivity study of operating pressures (feed and permeate pressures) and membrane properties (CO2 permeance and CO2/N2 selectivity) was conducted. Critical CO2/N2 selectivity of 300–400 was found for the highly CO2-selctive membranes to meet the demand for cost and energy efficient results. The lowest possible membrane area of 4.8 × 105 m2 and fractional energy of 19.3% were obtained, which is comparable to or even more attractive than reported membrane-based process designs. This work provides a new membrane process design option for highly CO2-selective membranes and gives insights on the influence of membrane performance and operation condition.

Poly(poly(ethylene glycol) methyl ether acrylate) micelles for highly CO2 permeable membranes

Herein, we report highly CO2-permeable membranes obtained by blending in situ self-assembled micellar-structured poly(poly(ethylene glycol) methyl ether acrylate) (PPEGMEA) with poly(ether-block-amide) (Pebax). PPEGMEA with different molecular weights were prepared by varying the concentration of benzoyl peroxide (BPO) in a Pebax/PEGMEA/BPO mixture during radical polymerization. Significantly, the Pebax/PPEGMEA (30/70 w/w) blend membrane comprised highly CO2-philic PPEGMEA micelles, which appreciably increased the d-spacing of the Pebax matrix. Consequently, the blend membrane containing a high molecular weight of PPEGMEA exhibited an unprecedented CO2 permeability enhancement of 1054% compared to the pristine Pebax membrane while maintaining good CO2 selectivity relative to H2, O2, N2, CO, and CH4 because of the enriched polyethylene glycol moieties. This excellent separation performance was maintained for 100 h, validating the good long-term separation performance of the membrane. Furthermore, the membrane exhibited good mechanical strength and plasticization tolerance, resulting in excellent CO2 separation performance under equimolar CO2/N2 or CO2/CH4 mixed gas conditions (CO2 permeability of 1250 Barrer and CO2/N2 and CO2/CH4 selectivity of 31.5 and 12.7, respectively), up to a CO2 partial pressure of 10 atm. Our simple but effective in situ micelle-induced blending approach is a significant step toward realizing high-performance CO2 separation membranes.

Multiobjective Optimization Based on “Distance-to-Target” Approach of Membrane Units for Separation of CO2/CH4

The effective separation of CO2 and CH4 mixtures is essential for many applications, such as biogas upgrading, natural gas sweetening or enhanced oil recovery. Membrane separations can contribute greatly in these tasks, and innovative membrane materials are being developed for this gas separation. The aim of this work is the evaluation of the potential of two types of highly CO2-permeable membranes (modified commercial polydimethylsiloxane and non-commercial ionic liquid–chitosan composite membranes) whose selective layers possess different hydrophobic and hydrophilic characteristics for the separation of CO2/CH4 mixtures. The study of the technical performance of the selected membranes can provide a better understanding of their potentiality. The optimization of the performance of hollow fiber modules for both types of membranes was carried out by a “distance-to-target” approach that considered multiple objectives related to the purities and recovery of both gases. The results demonstrated that the ionic liquid–chitosan composite membranes improved the performance of other innovative membranes, with purity and recovery percentage values of 86 and 95%, respectively, for CO2 in the permeate stream, and 97 and 92% for CH4 in the retentate stream. The developed multiobjective optimization allowed for the determination of the optimal process design and performance parameters, such as the membrane area, pressure ratio and stage cut required to achieve maximum values for component separation in terms of purity and recovery. Since the purities and recoveries obtained were not enough to fulfill the requirements imposed on CO2 and CH4 streams to be directly valorized, the design of more complex multi-stage separation systems was also proposed by the application of this optimization methodology, which is considered as a useful tool to advance the implementation of the membrane separation processes.

Quantitation of transmembrane O 2 flux via RhAG in a neutral‐buoyancy assay

The central dogma of transmembrane gas flux had been that all gases cross all membranes by dissolving in the lipid phase of the membrane. Contradicting this simple view are three discoveries: (1) CO2-impermeable membranes; (2) CO2-permeable membrane proteins or “gas-channels”, namely, certain aquaporins (AQPs) and rhesus (Rh) proteins; and (3) a reduction in CO2-permeability when incorporating the CO2-impermeable Nicotiana tabacum AQP NtPIP2;1, into artificial membranes. The characterization of the CO2 and NH3 permeabilities of AQPs 0-9 and several rhesus (Rh) proteins shows that they can exhibit gas selectivity. Our laboratory has found that, during O2-offloading, ~55% of O2 exits mouse red blood cells (RBCs) via AQP1 and the Rh complex (i.e., RhAG + mRh). To characterize the mechanism by which RhAG and other proteins conduct O2, and to screen for novel O2 channels in a heterologous expression system, we have modified of our Neutral Buoyancy Assay (NBA), originally developed to assess transmembrane N2 fluxes. We inject a precise volume of N2 gas (number of gas molecules = nGas) into a Xenopus oocyte, which we place into a saline-containing pressure-resistant tube. We then impose sufficient pressure in the air phase above the air-water interface to collapse the injected bubble enough to maintain the oocyte at a depth of 5 cm. As N2 gas dissolves in the cytosol and ultimately diffuses into the extracellular fluid (ECF), the bubble shrinks, cell density increases, and the oocyte sinks. A camera/computer combination detects the sinking and decreases air-phase pressure enough to maintain neutral buoyancy at 5 cm depth. Calibration exercises allow us to compute the time course of ΔnGas, and thus gas efflux. Here we modify the NBA to quantify gas influx. When we raise [N2] in the ECF from [N2]o = 0.56 mM (room air at 1×ATA) to [N2]o = 2.06 mM (pre-equilibrating saline with 93% N2/7% O2 at 3×ATA) at constant [O2]o = 0.26 mM, N2 enters the cell during the NBA and nGas rises. If we now hold [N2]o constant at 2.06 mM but selectively increase [O2]o from 0.26 mM to 0.91 mM (pre-equilibrating saline with 78.96% N2/21% O2/0.04% CO2 at 3.5×ATA), nGas rises faster. Compared to control oocytes (injected with H2O rather than cRNA encoding RhAG), RhAG-expressing oocytes exhibit no significant difference in ΔnGas over 1000 s when [N2]o = 2.06 mM/[O2]o = 0.26 mM. Thus, RhAG appears not to be an N2 channel. However, when [N2]o = 2.06 mM/[O2]o = 0.91 mM, ΔnGas over 1000 s is significantly greater in RhAG vs control oocytes. Thus, RhAG is selective for O2 over N2. When we repeat the low-to-high [O2]o experiment with oocytes expressing NtPIP1;3 (reported to facilitate O2 flux in a spectrophotometric assay when expressed in yeast protoplasts), we also observe substantial increases in ΔnGas over 1000 s vs control oocytes. These data are consistent with our previous findings in RBCs that the Rh complex constitutes a major pathway for O2 flux across the plasma membrane, and also supports the NtPIP1;3 report from another laboratory. Although we specifically developed the NBA for N2 fluxes, here we demonstrate the versatility of the assay for measuring transmembrane fluxes of other gases.

A Novel System for Real-Time, In Situ Monitoring of CO2 Sequestration in Photoautotrophic Biofilms.

Climate change brought about by anthropogenic CO2 emissions has created a critical need for effective CO2 management solutions. Microalgae are well suited to contribute to efforts aimed at addressing this challenge, given their ability to rapidly sequester CO2 coupled with the commercial value of their biomass. Recently, microalgal biofilms have garnered significant attention over the more conventional suspended algal growth systems, since they allow for easier and cheaper biomass harvesting, among other key benefits. However, the path to cost-effectiveness and scaling up is hindered by a need for new tools and methodologies which can help evaluate, and in turn optimize, algal biofilm growth. Presented here is a novel system which facilitates the real-time in situ monitoring of algal biofilm CO2 sequestration. Utilizing a CO2-permeable membrane and a tube-within-a-tube design, the CO2 sequestration monitoring system (CSMS) was able to reliably detect slight changes in algal biofilm CO2 uptake brought about by light–dark cycling, light intensity shifts, and varying amounts of phototrophic biomass. This work presents an approach to advance our understanding of carbon flux in algal biofilms, and a base for potentially useful innovations to optimize, and eventually realize, algae biofilm-based CO2 sequestration.

Preliminary Performance and Cost Evaluation of Four Alternative Technologies for Post-Combustion CO2 Capture in Natural Gas-Fired Power Plants

The objective of this study is to assess the technical and economic potential of four alternative processes suitable for post-combustion CO2 capture from natural gas-fired power plants. These include: CO2 permeable membranes; molten carbonate fuel cells (MCFCs); pressurized CO2 absorption integrated with a multi-shaft gas turbine and heat recovery steam cycle; and supersonic flow-driven CO2 anti-sublimation and inertial separation. A common technical and economic framework is defined, and the performance and costs of the systems are evaluated based on process simulations and preliminary sizing. A state-of-the-art natural gas combined cycle (NGCC) without CO2 capture is taken as the reference case, whereas the same NGCC designed with CO2 capture (using chemical absorption with aqueous monoethanolamine solvent) is used as a base case. In an additional benchmarking case, the same NGCC is equipped with aqueous piperazine (PZ) CO2 absorption, to assess the techno-economic perspective of an advanced amine solvent. The comparison highlights that a combined cycle integrated with MCFCs looks the most attractive technology, both in terms of energy penalty and economics, i.e., CO2 avoided cost of 49 $/tCO2 avoided, and the specific primary energy consumption per unit of CO2 avoided (SPECCA) equal to 0.31 MJLHV/kgCO2 avoided. The second-best capture technology is PZ scrubbing (SPECCA = 2.73 MJLHV/kgCO2 avoided and cost of CO2 avoided = 68 $/tCO2 avoided), followed by the monoethanolamine (MEA) base case (SPECCA = 3.34 MJLHV/kgCO2 avoided and cost of CO2 avoided = 75 $/tCO2 avoided), and the supersonic flow driven CO2 anti-sublimation and inertial separation system and CO2 permeable membranes. The analysis shows that the integrated MCFC–NGCC systems allow the capture of CO2 with considerable reductions in energy penalty and costs.

Effect of surface ligands on gold nanocatalysts for CO2 reduction.

Nanoparticle catalysts display optimal mass activity due to their high surface to volume ratio and tunable size and structure. However, control of nanoparticle size requires the presence of surface ligands, which significantly influence catalytic performance. In this work, we investigate the effect of dodecanethiol on the activity, selectivity, and stability of Au nanoparticles for electrochemical carbon dioxide reduction (CO2R). Results show that dodecanethiol on Au nanoparticles significantly enhances selectivity and stability with minimal loss in activity by acting as a CO2-permeable membrane, which blocks the deposition of metal ions that are otherwise responsible for rapid deactivation. Although dodecanethiol occupies 90% or more of the electrochemical active surface area, it has a negligible effect on the partial current density to CO, indicating that it specifically does not block the active sites responsible for CO2R. Further, by preventing trace ion deposition, dodecanethiol stabilizes CO production on Au nanoparticles under conditions where CO2R selectivity on polycrystalline Au rapidly decays to zero. Comparison with other surface ligands and nanoparticles shows that this effect is specific to both the chemical identity and the surface structure of the dodecanethiol monolayer. To demonstrate the potential of this catalyst, CO2R was performed in electrolyte prepared from ambient river water, and dodecanethiol-capped Au nanoparticles produce more than 100 times higher CO yield compared to clean polycrystalline Au at identical potential and similar current.

Highly CO2-permeable membranes derived from a midblock-sulfonated multiblock polymer after submersion in water

To mitigate the effect of atmospheric CO2 on global climate change, gas separation materials that simultaneously exhibit high CO2 permeability and selectivity in gas mixtures must be developed. In this study, CO2 transport through midblock-sulfonated block polymer membranes prepared from four different solvents is investigated. The results presented here establish that membrane morphology and accompanying gas transport properties are sensitive to casting solvent and relative humidity. We likewise report an intriguing observation: submersion of these thermoplastic elastomeric membranes in liquid water, followed by drying prior to analysis, promotes not only a substantial change in membrane morphology, but also a significant improvement in both CO2 permeability and CO2/N2 selectivity. Measured CO2 permeability and CO2/N2 selectivity values of 482 Barrer and 57, respectively, surpass the Robeson upper bound, indicating that these nanostructured membranes constitute promising candidates for gas separation technologies aimed at CO2 capture.

Evaluation of Five Alternative CO2 Capture Technologies with Insights to Inform Further Development

The high cost of CO2 capture using amine solvents from combustion sources such as natural gas-fired power plants remains a barrier to the adoption of CO2 Capture and Storage (CCS) as a climate change mitigation measure. The objective of the work reported in this paper was to carry out a preliminary assessment of the potential of five alternative technologies suitable for post-combustion CO2 capture from natural gas derived exhaust gases:•CO2 permeable membranes•Molten Carbonate Fuel Cells•High-pressure solvent absorption from high-pressure exhaust gas from pressurised combustion / power generation•High-pressure solvent absorption supported by exhaust gas compression•Supersonic flow driven CO2 depositionThe results of the performance and cost evaluation for each technology are explained and the prospects for significant cost reduction compared to a state-of-the-art CO2 capture process are discussed. Recommendations for further technology development activity are summarised in the conclusion of the paper.

Process Investigation of a Solid Carbon-Fueled Solid Oxide Fuel Cell Integrated with a CO2-Permeating Membrane and a Sintering-Resistant Reverse Boudouard Reaction Catalyst

The process of a new type of carbon–air battery based on a solid oxide fuel cell (SOFC) integrated with a CO2-permeable membrane was investigated systematically. Solid carbon fuel was modified with an excellent reverse Boudouard reaction catalyst, FemOn (active component)–K2O (promoter). Al2O3 as a sintering inhibitor was also introduced to the catalyst–carbon mixture. Two preparation techniques for catalyst-loaded carbon fuel were tried. One technique was the direct mix of all catalyst components and carbon (method A), and the other technique was the premix of all catalyst components before introducing carbon (method B). The performance of carbon–air batteries with different catalyst–carbon mixing techniques was studied by an I–V polarization test. Both techniques produced a good maximum power density of approximately 300 mW cm–2 at 850 °C. The sintering of the catalyst at high temperatures was prohibited, for the most part, using an Al2O3 support (i.e., method B). The carbon–air battery could operate co...

A carbon-air battery for high power generation.

We report a carbon-air battery for power generation based on a solid-oxide fuel cell (SOFC) integrated with a ceramic CO2-permeable membrane. An anode-supported tubular SOFC functioned as a carbon fuel container as well as an electrochemical device for power generation, while a high-temperature CO2-permeable membrane composed of a CO3(2-) mixture and an O(2-) conducting phase (Sm(0.2)Ce(0.8)O(1.9)) was integrated for in situ separation of CO2 (electrochemical product) from the anode chamber, delivering high fuel-utilization efficiency. After modifying the carbon fuel with a reverse Boudouard reaction catalyst to promote the in situ gasification of carbon to CO, an attractive peak power density of 279.3 mW cm(-2) was achieved for the battery at 850 °C, and a small stack composed of two batteries can be operated continuously for 200 min. This work provides a novel type of electrochemical energy device that has a wide range of application potentials.

High-Frequency Spectrophotometric Measurements of Total Dissolved Inorganic Carbon in Seawater

A new spectrophotometric method was developed to achieve continuous measurements of total dissolved inorganic carbon (DIC) in seawater. It uses a countercurrent flow design and a highly CO2-permeable membrane (Teflon AF 2400) to achieve flow-through CO2 equilibration between an acidified sample and an indicator solution with a fast response time of ~22 s. This method improves the spatiotemporal resolution by more than 1 order of magnitude compared to the existing spectrophotometric method. The flow-through equilibration allows for continuous (~1 Hz) detection and real-time data smoothing. The method had a short-term precision of ± 2.0 μmol kg(-1) for a given flow-through sample. It achieved a field precision of ± 3.6 μmol kg(-1) and successfully captured high DIC variability down to minute scales. Measurements by the new method over the typical range of oceanic DIC showed good agreement with measurements made by an established method (mean differences -1.6 to 0.3 μmol kg(-1) with 1σ ± 6.0-6.7 μmol kg(-1)). This level of precision and accuracy is comparable to that of the existing spectrophotometric method. The characteristics of the new method make it particularly suitable for high-frequency, submerged measurements required for mobile observing platforms in the ocean. It can also be adapted for high-frequency, spectrophotometric measurements of seawater CO2 fugacity.

S-Trityl-L-cysteine Is a Reversible, Tight Binding Inhibitor of the Human Kinesin Eg5 That Specifically Blocks Mitotic Progression

Human Eg5, responsible for the formation of the bipolar mitotic spindle, has been identified recently as one of the targets of S-trityl-L-cysteine, a potent tumor growth inhibitor in the NCI 60 tumor cell line screen. Here we show that in cell-based assays S-trityl-L-cysteine does not prevent cell cycle progression at the S or G(2) phases but inhibits both separation of the duplicated centrosomes and bipolar spindle formation, thereby blocking cells specifically in the M phase of the cell cycle with monoastral spindles. Following removal of S-trityl-L-cysteine, mitotically arrested cells exit mitosis normally. In vitro, S-trityl-L-cysteine targets the catalytic domain of Eg5 and inhibits Eg5 basal and microtubule-activated ATPase activity as well as mant-ADP release. S-trityl-L-cysteine is a tight binding inhibitor (estimation of K(i,app) <150 nm at 300 mm NaCl and 600 nm at 25 mm KCl). S-trityl-L-cysteine binds more tightly than monastrol because it has both an approximately 8-fold faster association rate and approximately 4-fold slower release rate (6.1 microM(-1) s(-1) and 3.6 s(-1) for S-trityl-L-cysteine versus 0.78 microM(-1) s(-1) and 15 s(-1) for monastrol). S-trityl-L-cysteine inhibits Eg5-driven microtubule sliding velocity in a reversible fashion with an IC(50) of 500 nm. The S and D-enantiomers of S-tritylcysteine are nearly equally potent, indicating that there is no significant stereospecificity. Among nine different human kinesins tested, S-trityl-L-cysteine is specific for Eg5. The results presented here together with the proven effect on human tumor cell line growth make S-trityl-L-cysteine a very attractive starting point for the development of more potent mitotic inhibitors.

A long pathlength liquid-core waveguide sensor for real-time pCO 2 measurements at sea

An improved spectrophotometric pCO 2 sensor based on a long pathlength liquid-core waveguide is described for pCO 2 underway measurements. A low refractive index (RI) amorphous fluoropolymer (Teflon AF 2400) tubing, the heart of the sensor, served as both a CO 2-permeable membrane equilibrator and a long pathlength liquid-core waveguide spectrophotometric cell. By using absorbance ratios at three wavelengths and carefully preparing and storing indicator solution, good reproducibility and long-term stability were achieved. The sensor behaved closely to theoretical prediction. Two pronounced features of the sensor were fast response (approximately 2 min to reach 99% of full response) due to high permeability of the Teflon AF, and high precision (about ±2–3 μatm in the pCO 2 range of 200–500 μatm) due to the long pathlength. The temperature dependence of the sensor is also discussed. The sensor's measurements agreed well with the “showerhead equilibrator plus infrared detector” method during an underway survey of sea surface pCO 2 along a transect off the Georgia coast in December 2000. Besides underway mapping, the sensor shows broad applicability for measuring pCO 2 in different environments.

A carbon dioxide sensor based on fluorescence

The optical carbon dioxide sensor is prepared by covering a pH sensor based on fluorescence with a CO 2-permeable membrane and contacting the pH-sensitive membrane with a reservoir of hydrogen carbonate. As carbon dioxide diffuses across the membrane it causes a chnage in pH which is detected by measuring the change in fluorescence from the base form of the pH-sensitive fluorescentdye. The usable range of response depends on the concentration of hydrogen carbonate in contact with the membrane. The sensor also responds to sulfide and sulfite.

The CO 2 conductivity electrode, a fast-responding CO 2 microelectrode

A fast-responding CO 2 microelectrode system is presented; it consists of a double-lumen polyethylene catheter provided with a stainless steel catheter tip covered by a CO 2-permeable membrane, and connected with each lumen separately to a conductivity cell. The catheter is flushed with bidistilled water at constant flow rate and conductivity of inflowing and outflowing water is measured. The change of conductivity in the water leaving the catheter after exposure of the electrode to a medium containing CO 2 is related to the P CO 2 of the medium. High sensitivity and fast response, both depending on the flow rate of the carrier water, are shown to oppose each other. When cells are placed at a distance of 45 cm from the membrane, response time for 90% deflection is about 10 sec (at flow rates of 5 ml/min), but when located inside the electrode tip at a distance of within 10 mm from the membrane response time is reduced to about 4 sec. Hydrodynamical aspects concerning dispersion of CO 2 in the carrier water and reaction kinetics of CO; hydration are discussed.