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as ‘‘carrier molecules’’ for CO2. The acidic solution converted the HCO3 and CO32 into dissolved CO2 which then bubbled out of solution.3,4 In contrast, the extraction of CO2 from seawater described in this article does not involve the transport of HCO3 or CO32 across the AEMs (see Fig. 2b). Natural seawater has a DIC concentration (typically 2.2mM–2.5mM) that is over 200 times smaller than the concentration of Cl ions (typically 546 mM),14 and therefore any process that relied on the transport of HCO3 ions across and AEM would be very inefficient since over 99.5% of the applied energy would be used to transport Cl ions, assuming similar transport properties for HCO3 and Cl ions through the membrane. Instead, the experiments described in this article acidify and basify incoming seawater in alternating compartments of the BPMED unit, using the H+ and OH ions that are produced on opposite sides of the BPMs. In the acidic compartments, the DIC in the input seawater is converted into dissolved CO2 via the reactions CO32 + 2H+ $ HCO3 + H+ $ CO2 + H2O, and this CO2 is extracted by vacuum stripping. As shown in Fig. 2, as OH ions flow into the basic solution, anions are transported from the basic solution to acidic solution, and anions with the highest concentration (for example, Cl) will carry a proportionally large fraction of this current across the AEM. Assuming that the fraction of current carried across the AEM by a given anion depends linearly on its concentration, one can show that for the normalized current densities used in our experiments (approximately 0.4 mA cm2 lpm1 as shown in Fig. 3a), only 0.07% of the DIC in the seawater input to the basified compartment is transported across the AEM into the acidic compartment. This demonstrates that the extraction of CO2 from seawater described here does not rely on the transport of HCO3 or CO32 across the ion-selective membranes, but rather results from the acidification of the seawater input to the acidified compartments. Results and discussion Experiments were performed under steady-state conditions to explore the effect of varying the acid solution output pH for different solution flow rates. Acid solution flow rates of 3.1 lpm, 3.6 lpm, 4.1 lpm, and 6 lpm were investigated in the experiments described below, with base solution flow rates equal to the acid solution rate in all cases. For a given flow rate, a constant current value was chosen to achieve a desired pH value for the output acid solution. Generally, three to five constant-current experi- ments were run for each acid flow rate value, yielding acid output pH values in the range 2.9–6.3. In addition to using seawater (35.95 g of Instant Ocean! sea salt per litre of DI water) as an input solution to the BPMED unit, we also tested a solution with a sea-salt concentration two times higher than the seawater solution (71.9 g of Instant Ocean! sea salt per litre of DI water, labelled as ‘‘RO’’ in Fig. 3). Although the concentrations of various ions in real-world RO brines vary depending on the feed solution and process conditions,29,30 the ‘‘RO’’ solution is meant to provide a representative solution to test the use of RO brine as an input to the BPMED unit. Before starting the measurements on seawater and RO brine solutions, we first performed a series of control experiments. The first experiments compared the measured rate of CO2 extraction for input solutions of 0.5M NaCl with: (1) a solution of 0.5M NaCl/2.5mM NaHCO3 (a solution with salt and bicarbonate ion concentrations very similar to that of seawater), and (2) an Instant Ocean! seawater solution (35.95 g of Instant Ocean! sea salt per litre of DI water). The CO2 extraction rate for the 0.5M NaCl solutions was too small for our system to detect, while the extraction rates for the 0.5M NaCl/2.5mM NaHCO3 and seawater solutions were very similar to the rates shown below in Fig. 3c for seawater solutions. In addition, for each of these input solutions, gas samples from the output ports of the membrane contactors were analysed using gas chromatography. The 0.5M NaCl/2.5mM NaHCO3 and seawater solutions each produced a pronounced CO2 peak that was absent in the 0.5M NaCl data, verifying that CO2 is extracted from seawater solu- tions by our system. Finally, we performed a control experiment clearly demonstrating that the observed CO2 extraction does not depend of the transport of HCO3 or CO32 across the anion exchange membranes from the basic solution to the acidic solu- tion. This experiment used 0.5M NaCl (with no HCO3 or CO32 ions) as the basic solution and 0.5M NaCl/50mM NaHCO3 as the acidic solution. Feeding these solutions into the BPMED system resulted in significant CO2 extraction from the acid solution, demonstrating that this process does not rely on the transport of HCO3 or CO32 ions from the basic solution to acidic solution. We note that because no degassing of the input solution is performed prior to acidification and vacuum stripping, some N2 and O2 are extracted along with the CO2. As described in the Procedure section above, the rates of N2 and O2 extraction would be included in the baseline offset determined for the flow meter by measuring the flow meter reading with the solution flow and vacuum pump on, but no current applied to the BPMED stack. This means that while the gas extracted by our system as described will include some O2 and N2 mixed in with the CO2, the measured rate of extraction (shown in Fig. 3c) only considers the rate of CO2 extraction. If desired, the N2 and O2 could be eliminated from the extracted gas mixture by adding a vacuum stripping stage prior to the BPMED unit. Once confirming the principle of operation of our system via these control experiments, we moved on to characterize the performance of this system for CO2 extraction from seawater and RO brine solutions. Fig. 3 shows measured values of normalized current density (total current per total active membrane area per acid solution flow rate in units of mA cm2 lpm1) (Fig. 3a), efficiency of CO2 extraction (Fig. 3b), CO2 flow rate (slpm) (Fig. 3c), and energy (kJ mol1 (CO2)) (Fig. 3d) as functions of the pH of the output acid solution for constant-current, constant-flow-rate experiments for seawater input at flow rates of 3.1 lpm, 3.6 lpm, 4.1 lpm, and 6 lpm and RO brine input at a flow rate of 3.6 lpm. From Fig. 3a we see that the normalized current density required to acidify seawater to a given pH value is the same for different flow rates in the range 3.1 lpm–6 lpm. This makes sense, since normalizing the total applied current by the total membrane area and volumetric flow rate means that the normalized current density (as defined here) required to achieve a given pH should only depend on the ionic composition of the input solution. The data from Fig. 3a support this assertion, both in the fact that the seawater data at all flow rates collapse onto a single curve, and the fact that the ‘‘RO’’ solution requires View Online This journal is a The Royal Society of Chemistry 2012 Energy Environ. Sci. Downloaded by University of Oxford on 26 March 2012 Published on 06 February 2012 on http://pubs.rsc.org | doi:10.1039/C2EE03393CPDF Image | Co2 Extration from Seawater
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