Sorbent contactor for carbon dioxide capture from water

Abstract

A contactor for selective carbon capture from water includes a micro- or nano-structured support and a bicarbonate-selective ligand chemically bonded to the support. The bicarbonate-selective ligand includes one or both of a guanidinium and an ammonium functional group. The support may be a high surface-area material selected from a group of hollow fibers, non-hollow fibers, solid rods, macro-particles, micro-particles, nano-particles, and membranes. A carbon capture module includes a housing defining an internal volume and a plurality of the contactors contained therein. Each contactor extends between first and second opposite longitudinal ends of the housing. A method of selectively capturing carbon from water including a salt and a carbon-containing species dissolved therein includes providing a housing including a plurality of the contactors disposed therein. The water including the dissolved carbon-containing species is fed through the housing such that the contactor adsorbs the dissolved carbon-containing species from the water.

Description

SORBENT CONTACTOR FOR CARBON DIOXIDE CAPTURE FROM WATERCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 650,928, filed May 23, 2024, the disclosure of which is incorporated by reference in its entirety.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[0002] This invention was made with government support under Contract No. DE- AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention.FIELD OF THE INVENTION

[0003] The present invention relates to carbon dioxide capture, and more particularly to devices and methods for capturing carbon dioxide from salt water and other applications.BACKGROUND OF THE INVENTION

[0004] Climate change is presently a major challenge caused in part by greenhouse gases including increased carbon dioxide (CO2) emissions. To confront this issue, the reduction of CO2 emissions and the removal of CO2 from the atmosphere are equally important as the concentration of atmospheric CO2 continues to rise above record-high levels. Carbon capture from concentrated sources has gained significant attention. However, to achieve net-negative emissions in the long term, capturing CO2 directly from air and / or seawater will likely play a major role. Seawater is a critical natural sink of CO2 and has absorbed about 40% of anthropogenic CO2 since the beginning of the industrial era with an effective CO2 concentration of 2.1 mmol kg-1, or 0.095 kg m-3in seawater, which is a factor of 140 times larger than in the atmosphere. Every year, approximately 9 Gt of anthropogenic CO2 is absorbed by the oceans, resulting in a significant influx of CO2 from the atmosphere. Over the past few decades, this process has been responsible for natural carbon sequestration. The increasing concentration ofanthropogenic CO2 contributes to seawater acidification which is detrimental to aquatic life. The vast quantity of CO2 emissions that enter the oceans is comparable to the amount retained in the atmosphere. Therefore, finding effective methods to remove this greenhouse gas from the oceans could greatly complement other negative emissions technologies, alleviating the environmental impact caused by CO2. CO2 capture and separation from seawater provides an alternative and unique approach to direct air capture and helps to maintain a healthy seawater ecosystem as well as leads to negative overall emissions.

[0005] Selective CO2 capture from seawater, however, faces significant technical and economic challenges, as the seawater contains CO2 in the form of dissolved gaseous CO2 and HCO3 and / or CO32, at a seawater pH of ~8.1 with the presence of high concentrations of competing ions. Although a few technologies have been explored to extract CO2 from seawater, there is currently no established technology commercially available for CO2 capture and separation from seawater on a gigaton scale. Recently, electrochemical approaches have been reported in literature for CO2 capture from seawater. Although electrochemical processes have shown great promise and are suitable for small-scale applications including CO2 capture with conversion, they tend to be energy-intensive and expensive, which prevents their commercial large scale deployment for processing gigantic amounts of water in the ocean. Hence, extensive research and development is needed to address the impending climate change crisis and discover energy-efficient, inexpensive materials and scalable processes for selective CO2 separation at a large scale.

[0006] Presently there is no well-established sorbent developed for CO2 separation from seawater. Extraction of CO2 from seawater necessitates precise separations tailored to specific targeted ionic species, and under challenging conditions such as high ionic strength, variable pH, and complex multi-component environments. In the pH levels (~8.1) found in seawater, the dissolved CO2 mostly stays in the form of a bicarbonate state (HCO3 ). The selective capture of bicarbonate ions (~2 mM) from seawater, amidst abundant sulfate (~30mM), chloride (-500 mM), and other competing species, presents a significant challenge. Additionally, release of the captured bicarbonate ions without extensive energy inputs is another obstacle to overcome in developing this technology. In light of these challenges, the need continues to exist for energy efficient and effective large-scale materials, systems, and methods for the capture of carbon from saltwater.SUMMARY OF THE INVENTION

[0007] A contactor for selective carbon capture from water is provided. The contactor includes a micro- or nano-structured support and a bicarbonate-selective ligand chemically bonded to the support. The bicarbonate-selective ligand includes one or both of a guanidinium functional group and an ammonium functional group.

[0008] In particular embodiments, the support is formed of a polymer, silica, ceramic, or carbon material.

[0009] In particular embodiments, the support has a surface area including hydroxyl groups bonded thereto.

[0010] In particular embodiments, the functional group of the bicarbonate-selective ligand is guanidinium.

[0011] In particular embodiments, the functional group of the bicarbonate-selective ligand is an ammonium.

[0012] In certain embodiments, the functional group of the bicarbonate-selective ligand is a quaternary ammonium.

[0013] In particular embodiments, the bicarbonate-selective ligand comprises a monomer or polymer including the guanidinium and / or ammonium functional group.

[0014] In certain embodiments, the monomer or polymer includes an acrylate or methacrylate unit.

[0015] In certain embodiments, the monomer or polymer is covalently bonded to the support.

[0016] In particular embodiments, the guanidinium or ammonium fimctional group is covalently bonded to the support.

[0017] In some embodiments, a contactor for selective carbon capture includes a high surface-area material selected from a group of hollow fibers, non-hollow fibers, solid rods, macro-particles, micro-particles, nano-particles, and membranes. A bicarbonate-selective ligand is chemically bonded to the surface area of the high surface-area material. The bicarbonate-selective ligand includes one or both of a guanidinium and an ammonium fimctional group.

[0018] A method of selectively capturing carbon from water including a salt and a carbon-containing species dissolved therein is also provided. The method includes providing a housing including a plurality of the contactors as described above disposed therein. The method fiirther includes feeding the water including the dissolved carbon-containing species through the housing, such that the contactor adsorbs the dissolved carbon-containing species from the water.

[0019] In particular embodiments, the carbon-containing species include one or more of gaseous carbon dioxide, carbonate ions, and bicarbonate ions.

[0020] In particular embodiments, the method further includes feeding a pH-swing solution through the housing, such that the carbon-containing species is desorbed from the contactor to release carbon dioxide into the pH-swing solution, thereby regenerating the contactor. The method also further includes the step of collecting the carbon dioxide from the pH-swing solution.

[0021] In certain embodiments, the pH-swing solution has a pH of less than 6.5.

[0022] In specific embodiments, the pH-swing solution has a pH in a range of approximately 4.5 to 6.5.

[0023] In certain embodiments, the pH-swing solution is a dilute, inorganic or organic acid.

[0024] In certain embodiments, the method includes an adsorption mode and a desorption mode. The desorption mode includes the steps of feeding the pH-swing solution through the contactor and collecting the carbon dioxide from the pH-swing solution. The adsorption mode and the desorption mode are performed sequentially in that order a plurality of times.

[0025] A carbon capture module for selective carbon capture is also provided. The carbon capture module includes a housing defining an internal volume. A plurality of the contactors as described above are contained within the internal volume. The housing has first and second opposite longitudinal ends, and the plurality of contactors extend between the first and second ends.

[0026] In particular embodiments, the internal volume of the housing is divided into a shell side and a lumen side, wherein the shell side includes an inlet and an outlet, and the lumen side includes an inlet and an outlet.

[0027] In particular embodiments, a seal is formed between the plurality of contactors adjacent both the first end and the second end of the housing.

[0028] These and other features of the invention will be more fully understood and appreciated by reference to the description of the embodiments and the drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1A is a schematic diagram of a chemical mechanism for binding of bicarbonate ions to an ammonium ligand;

[0030] Figure IB is a schematic diagram of a chemical mechanism for binding of bicarbonate ions to a guanidinium ligand;

[0031] Figure 2 is a schematic diagram of a chemical process for synthesis of a contactor having a guanidinium-based ligand in accordance with embodiments of the disclosure;

[0032] Figure 3 is a schematic diagram of a chemical process for synthesis of a contactor having an ammonium-based ligand in accordance with other embodiments of the disclosure;

[0033] Figure 4 is a schematic diagram of an alternative chemical process for synthesis of a contactor having a guanidinium-based ligand in accordance with yet other embodiments of the disclosure;

[0034] Figure 5 is a schematic diagram of another alternative chemical process for synthesis of a contactor having a guanidinium-based ligand in accordance with yet other embodiments of the disclosure;

[0035] Figure 6 is a schematic diagram of a carbon capture module for selective carbon capture in accordance with embodiments of the disclosure;

[0036] Figure 7 is a schematic diagram of a carbon capture system including the carbon capture module of Figure 6;

[0037] Figure 8 is a schematic diagram of an adsorption mode of the carbon capture system of Figure 7 performing a method of selectively capturing carbon from water including a salt and a carbon-containing species dissolved therein;

[0038] Figure 9 is a schematic diagram of a desorption mode of the carbon capture system of Figure 7 performing the method of selectively capturing carbon;

[0039] Figure 10 is a schematic diagram of chemical structuresa contactor in accordance with embodiments of the disclosure;

[0040] Figure 11 is a schematic diagram of chemical structuresa contactor in accordance with other embodiments of the disclosure;

[0041] Figure 12 is a schematic diagram of chemical structures of a contactor in accordance with yet other embodiments of the disclosure;

[0042] Figure 13 is a schematic diagram of chemical structures of a contactor in accordance with yet other embodiments of the disclosure;

[0043] Figure 14 is a schematic diagram of chemical structures of a contactor in accordance with yet other embodiments of the disclosure;

[0044] Figure 15 is a schematic diagram of chemical structures of a contactor in accordance with yet other embodiments of the disclosure;

[0045] Figure 16 is a schematic diagram of chemical structures of a contactor in accordance with yet other embodiments of the disclosure;

[0046] Figure 17 is a schematic diagram of chemical structures of a contactor in accordance with yet other embodiments of the disclosure;

[0047] Figure 18 is a schematic diagram of chemical structures of a contactor in accordance with yet other embodiments of the disclosure;

[0048] Figure 19 is a graph of experimentally observed changes in carbon dioxide concentration for a control and for contactors in accordance with embodiments of the disclosure under various conditions, with the initial carbon dioxide concentration in the solution shown in the left-hand-side bar and the carbon dioxide concentration in the solution after adsorption by the contactor in the right-hand-side bar;

[0049] Figure 20 is a graph of solution pH as a function of time for contactor regeneration performed after adsorption Cycle 1 in Figure 19;

[0050] Figure 21 is a graph of solution pH as a function of time for contactor regeneration performed after adsorption Cycle 2 in Figure 19;

[0051] Figure 22 is a graph of solution pH as a function of time for contactor regeneration performed after adsorption Cycle 3 in Figure 19;

[0052] Figure 23 is a graph of solution pH as a function of time for contactor regeneration performed after adsorption Cycle 4 in Figure 19;

[0053] Figure 24 is a graph of solution pH as a function of time for contactor regeneration performed after adsorption Cycle 5 in Figure 19; and

[0054] Figure 25 is a graph of solution pH as a function of time for contactor regeneration performed after adsorption Cycle 6 in Figure 19.DETAILED DESCRIPTION OF THE CURRENT EMBODIMENTS

[0055] A contactor for selective carbon capture from salt-containing water, a carbon capture module including the contactor, and a method of selectively capturing carbon from salt-containing water are provided. The salt-containing water may be, for example, but not limited to seawater, brackish water, industrial wastewater, or other body or source of water having a salinity at least greater than approximately 0.5 ppt or greater than approximately 0.1 g / kg. In various embodiments, the contactor generally includes a polymer sorbent grafted with fimctional groups / ligands that selectively capture bicarbonate ions and release CO2 under external stimuli such as pH swings. The present functionalized polymer grafted membrane contactor (FPGMC) functions as a CO2 sorbent and has a high surface area support backbone at which adsorption and desorption can occur at varying pH, making the process energyefficient and cost-effective. The sorbent can bind to dissolved gaseous CO2 and / or HCO3 and / or CO32ions formed from dissolved CO2 gas in water. The utilization of a high surface area support also allows for the incorporation of a high quantity of ligand-grafted polymers. This, in turn, facilitates the capture of a large volume of bicarbonate ions for enhanced sorbent performance. The FPGMC thereby offers an energy-efficient and cost-effective solution, since the polymer-based process is readily scalable, inexpensive, and requires significantly less capital investment and operating cost than other methods.

[0056] More particularly, the contactor includes a micro- or nano-structured support. In some embodiments, the support is formed of a high surface-area material that may be, but is not limited to, hollow fibers, non-hollow fibers, solid rods, macro-particles, micro-particles, nano-particles, and membranes. A high surface-area material may one that has a pore size in a range of approximately 20 nm to 100 nm, optionally in a range of approximately 30 nm to 100 nm, optionally in a range of approximately 40 nm to 100 nm, optionally in a range of approximately 50 nm to 100 nm, optionally in a range of approximately 60 nm to 100 nm, optionally in a range of approximately 70 nm to 100 nm, optionally in a range of approximately 80 nm to 100 nm, optionally in a range of approximately 90 nm to 100 nm, optionally in arange of approximately 20 nm to 90 nm, optionally in a range of approximately 20 nm to 80 nm, optionally in a range of approximately 20 nm to 70 nm, optionally in a range of approximately 20 nm to 60 nm, optionally in a range of approximately 20 nm to 50 nm, optionally in a range of approximately 20 nm to 40 nm, optionally in a range of approximately 20 nm to 30 nm, optionally in a range of approximately 30 nm to 90 nm, optionally in a range of approximately 40 nm to 80 nm, optionally in a range of approximately 50 nm to 70 nm. The support may be formed of a polymer, silica, ceramic, or carbon material. For example, the support may be a polymer fiber, a ceramic fiber, a glass fiber, glass wool, or glass beads. In specific non-limiting embodiments, the support is a hollow fiber membrane (HFM) material formed of a polymer such as polyvinylidene fluoride (PVDF). Hollow fiber membranes may also be produced, for example, by spinning other polymer materials such as cellulose acetate, polytetrafluoroethylene (PTFE), polysulfone (PSU), or polyethersulfone (PES). The surface area of the support includes or is modified to include a tether for attaching a bicarbonateselective ligand as described in greater detail below. The tethers are formed of a fimctional group and are preferably formed of hydroxyl (-OH) groups that are chemically bonded to the surface of the support material.

[0057] The contactor includes a bicarbonate-selective ligand chemically bonded to the support via the hydroxyl groups on the surface of the support. The bicarbonate-selective ligand may include one or both of a guanidinium and an ammonium fimctional group. In various embodiments, the fimctional group is guanidinium or guanidinium-based structure, an ammonium, and / or a quaternary ammonium. In specific non-limiting embodiments, the bicarbonate-selective ligand includes guanidine or an alkyl tertiary amine such as a triethyl amine. Exemplary bicarbonate-selective ligands are illustrated by the following ligand structures (L-i) through (L-xvi):2-methyl-1 H-imidazol-3-ium(L-iv)(2)-W-(1-aminoethylidene)hydroxylammonlum(L-v)(Z)-M-(amino(phenyl)methylene)hydroxylammonium(L-vi)bis(dirnethylamino)rnethaniminium (L-xii)benzenaminium (L-xiii)

[0058] These exemplary ligands have various pKa values. The higher the pKa value, the more strongly basic, and conversely the more weakly acidic, the ligand is. Ligands having a higher pKa value may be more effective in binding bicarbonate ions due to being a stronger base and weaker acid. Of the ligand structures (L-i) through (L-xvi), the guanidinium-based structures have the highest pKa values. Also, alkyl tertiary amines may have less selectivitytoward bicarbonate ions than guanidine, primarily due to their capacity to engage in electrostatic and hydrogen bonding interactions with bicarbonate ions. The chemical mechanism for bonding a bicarbonate ion to an ammonium ligand functional group is shown in Figure 1 A. In the ligand structure, the R group is the linker chain that connects the fimctional group to a polymer backbone and / or to the surface of the support. Similarly, the chemical mechanism for bonding a bicarbonate ion to a guanidinium ligand fimctional group is shown in Figure IB. In the ligand structure, the R group is again the linker chain that connects the fimctional group to a polymer backbone and / or to the surface of the support. In some embodiments, the guanidinium or ammonium fimctional group is covalently bonded directly to the support, such as bonding directly to the oxygen of the hydroxyl group on the support surface by substitution with of the hydroxyl group. In other embodiments, the bicarbonateselective ligand is a monomer or polymer including the guanidinium and / or ammonium fimctional group, and the monomer or polymer is covalently bonded to the support via the oxygen of the hydroxyl group. Thus, in these embodiments, the guanidinium and / or ammonium fimctional group is indirectly bonded to the support. In some of these embodiments, a plurality of the fimctional groups may be bonded to a single polymer strand that forms a backbone for the fimctional groups. In specific non-limiting embodiments, the monomer or polymer includes an acrylate or methacrylate unit.

[0059] The contactor may be synthesized by various methods. With reference first to Figure 2, in some embodiments the contactor is synthesized by a parallel approach using SN2 chemistry (bimolecular nucleophilic substitution) to add either a guanidinium-based polymer or a tertiary amine-based polymer to a support that is by way of example a PVDF fiber having hydroxyl groups bonded to the surface. Turning next to Figure 3, another method of synthesizing a contactor having a PVDF fiber functionalized with a tertiary amine polymer is shown. In yet other embodiments, the contactor is synthesized by UV functionalization of, for example, a PVDF fiber, by first modifying with benzophenone followed by surface-initiatedpolymerization with a ligand-bearing monomer as shown in Figure 4. And in yet other further embodiments, the contactor is synthesized by surface modification of, for example, a PVDF fiber, with Easaqua 401 followed by installation of a ligand-derived, amine-bearing polymer as shown in Figure 5. By way of example, functionalization with a guandinium-based polymer (poly-guanidinium (PGA)) is shown. These methods are discussed in greater detail in the Examples section below.

[0060] With reference now to Figure 6, a carbon capture module 10 for selective carbon capture includes a housing 12 defining an internal volume 14. The housing 12 may be, for example, a cylinder having first and second opposite longitudinal ends 16, 18. A plurality of the contactors 20 are contained within the internal volume 14 and generally extend in parallel between the first and second ends 16, 18. A seal 22 is formed between the plurality of contactors 20 and the housing wall within the internal volume 14 adjacent both the first end 16 and the second end 18 of the housing 12. The seal 22 may be formed, for example, by a resin or epoxy. The plurality of contactors 20 divide the internal volume 14 of the housing 12 into a shell side 24 and a lumen side 26. The lumen side 26 has an inlet 28 and an outlet 30 that provides for flow through the contactors 20. The shell side 24 also has an inlet 32 and an outlet 34 that may be in the form of side ports, and that provide for flow that passes over the surface area of the contactors 20.

[0061] Turning next to Figures 7-9, a carbon capture system 40 includes the carbon capture module 10. A source 42 of salt water (water including at least one salt dissolved therein) is connected to the inlet 32 of the shell side 24 of the module 10 as well as the inlet 28 of the lumen side 26. The salt water also includes a carbon-containing species dissolved therein. The carbon-containing species is one or more of gaseous carbon dioxide, carbonate ions, and bicarbonate ions. A pump 44 provides a flow of the salt water to the inlets 28, 32, and valves 46, 48 control flow to the inlets 28, 32. When the salt water is pumped to the inlets 28, 32, the valves 46, 48 are kept open, and when the flow of salt water is stopped, the valves 46, 48 maybe closed. A source 50 of a pH-swing solution is provided in a loop between the outlet 30 of the lumen side 26 of the module 10 and the inlet 28 of the lumen side 26. The pH-swing solution has a pH of less than 7, and typically has a pH of less than approximately 6.5. In some embodiments, the pH-swing solution has a pH in a range of approximately 4.5 to 6.5, alternatively between 5.0 and 6.5, alternatively between 5.5 and 6.5, alternatively between 6.0 and 6.5, alternatively between 4.5 and 5.0, alternatively between 4.5 and 5.5, alternatively between 4.5 and 6.0. The pH-swing solution may be a dilute inorganic acid or a dilute organic acid. In specific embodiments, the pH-swing solution is a mineral acid such as hydrochloric acid, nitric acid, or sulfuric acid. A separator 52 is also provided in the loop between the outlet 30 of the lumen side 26 and the source 50 of the pH swing solution. The separator 52 has a gas side 54 and a liquid side 56. The liquid side 56 is connected to the source 50 of pH swing solution, and a pump 58 provides for flow of liquid from the liquid side 56 of the separator 52 to the source 50 and subsequently to the inlet 28 of the lumen side 26 of the module 10. Valves 60, 62, 64 control the flow of liquid through the loop. When the pH-swing solution is to be fed through the module 10 and separator 52, the valves 60, 62, 64 are kept open, and when no pH- swing solution is to be fed through the module 10 and separator 52, the valves 60, 62, 64 are closed. The separator 52 also has an outlet 66 for desorbed carbon dioxide gas, and a valve 68 controls flow of gas through the outlet 66. A valve 70 is also provided at the outlet 34 of the shell side 24 of the module 10.

[0062] In exemplary embodiments, a method of selectively capturing carbon from water may be performed with the carbon capture system 40. The method includes providing the carbon capture module 10 including the plurality of contactors 20. In an adsorption mode shown in Figures 7 and 8, the valves 46, 48 are opened and the valves 62, 64 are closed. The salt water including dissolved carbon-containing species is pumped from the source 42 to the inlets 28, 32 of the carbon capture module 10. The pressure exerted on the salt water by the pump 44 feeds the salt water through both the shell side 24 and the lumen side 26 of the internalvolume 14 of the housing 12. As the salt water passes through and over the contactors 20, the bicarbonate-selective ligands of the contactors adsorb the dissolved carbon-containing species from the salt water. The salt water exiting the outlets 30, 34 of the carbon capture module 10 is essentially free of carbon-containing species (carbon dioxide, bicarbonate, carbonate), or at least has a reduced level of carbon-containing species relative to the source of salt water. Once the contactors 20 are fully loaded with adsorbed carbon-containing species, the carbon capture system 40 may switch to a desorption mode as shown in Figures 7 and 9. In the desorption mode, the valves 60, 62, 64 are opened and the valves 46, 48 are closed. The pH-swing solution is pumped from the source 50 to the inlets 28, 32 of the carbon capture module 10 by the pump 58, and is fed through the lumen side 26 and the shell side 24 of the housing 12. An advantage of the present bicarbonate-selective ligand such as a guanidine-based ligand is that it allows for easy regeneration through a mild pH swing, which off-gases the bound bicarbonate as carbon dioxide and ion exchanges the guanidinium or ammonium ligand to an inorganic material. As such, the pH-swing solution causes the carbon-containing species to be desorbed from the contactors 20, which releases the carbon-containing species such as carbon dioxide into the pH-swing solution. The carbon-containing species are released solely by the pH of the pH- swing solution, avoiding the use of electrochemistry to adjust the pH and hence lowering the energy use of the system 40. The desorption of the carbon-containing species regenerates the contactors 20 for further adsorption in a subsequent stage. The pH-swing solution including the carbon-containing species is led through the outlets 30, 34 of the housing 12 to the separator 52. In the separator, carbon dioxide gas is released from the pH-swing solution and is collected via the gas outlet 66 of the separator 52 by opening valve 68. Once all of the carbon dioxide gas from the carbon capture module 10 is collected, the system 40 may switch back to the adsorption mode. At this time, the adsorption mode and the desorption mode described above may be repeated, and these two modes may be performed sequentially in that order several times.EXAMPLES

[0063] The present method is farther described in connection with the following laboratory examples, which are intended to be non-limiting.

[0064] In a first example, PVDF fiber was chemically modified through an SN2 type approach where surface hydroxyl groups were activated before displacing with poly- guanidinium (PGA) as shown schematically in Figure 2. To synthesize the starting polymer, 7V-(3-aminopropyl)methacrylamide (1 eq) was mixed with a succinimide reversible additionfragmentation chain-transfer (RAFT) agent (0.003 eq) and AIBN initiator (0.00055 eq) and stirred at 70°C in DMF overnight. For purification, the polymer was transferred to a dialysis bag with a MW cutoff of 500 Da and dialyzed against water for 4 days. To quatemize the polymer and generate the bicarbonate ligand, ethyl carbamidothioate (3 eq) was added to the polymer (1 eq) in water. The reaction was stirred at 40°C in DMF overnight. The resulting polymer was dialyzed for purification and contained a mixture of primary amines and guanidinium functional groups. Simultaneously, the PVDF was activated through conversion of the surface hydroxyl groups to tosyl-groups. To complete this alteration, the PVDF fibers were first treated with 2.5M LiOH by stirring overnight at room temperature to add additional -OH groups while deprotonating existing -OH groups. The fibers were then transferred to a solution of tosyl chloride in tetrahydrofuran (150 mg mL1) and stirred overnight. Subsequently, the fibers were rinsed thoroughly in methanol (MeOH) to remove excess tosyl chloride before transferring to a solution of 100 mg of the amine-guanidinium polymer in 20 mL of water. Again, the fiber was stirred overnight to complete the addition of the polymer. Preliminary testing revealed that the fibers were capable of bicarbonate capture with selectivity over chloride ions.

[0065] In a second example, PVDF fiber was modified to include a tertiary amine polymer as shown schematically in Figure 3. The polymer backbone was synthesized in the same manner as described above in the first example for the guanidinium analog. However, toquatemize the polymer, and generate the bicarbonate ligand, bromoethane (3 eq) was added to the polymer (1 eq) in place of the guanylating agent. The reaction was stirred at 40°C in DMF overnight. After polymerization, the polymer was attached in a method in which PVDF was utilized as the nucleophile through amination. To accomplish this conversion, the PVDF was aminated with 3-(ethoxy dimethylsilyl)propan-l -amine by adding an excess of the silanated primary amine to the PVDF fiber in ethanol (EtOH) overnight. The PVDF was rinsed thoroughly in EtOH after reaction completion to remove any of the excess silane amine. 500 mg of the tertiary ammonium polymer was then added in 90 mL EtOH along with aminated PVDF and stirred at 55 °C overnight to complete the addition of the polymer to the PVDF surface. After, the fibers were removed and rinsed with pure ethanol to remove any unbound polymer before drying in a vacuum oven at 80°C overnight.

[0066] One significant aspect of the surface modification is the grafting density. Grafting density refers to the amount of polymer installed on the surface of the support and is directly correlated to the theoretical binding capacity. To calculate this value, two experiments were conducted in tandem: gel permeation chromatography (GPC) and thermogravimetric analysis (TGA). GPC was performed in aqueous conditions to identify the molecular weight (Mw) of the ligand-derived polymer. For the PGA (first example), the experimental Mw was 4,100 g / mol, indicating that for each polymer installed there are approximately 20 potential binding sites for bicarbonate anions. To identify the amount of polymer installed on the surface of the support (PVDF), a small piece of the support was weighed through a “heat-and-hold” methodology on the TGA through each phase of the contactor preparation. Using the TGA in place of a standard microbalance ensures that the weight is recorded accurately and is not influenced by residual solvent or water. In this experiment, the sample was heated to 100 °C and held at that temperature for 30 minutes. The mass of each sample was recorded at 30 minutes, after weight stabilization. The initial tosylation phase shows a decrease in mass, likely due to the removal of the hydrophilic coating installed by Arkema, the manufacturer. However,the mass increased during the polymer installation phase. The mass change corresponds to a grafting density of 0.02 chains / nm2. With both the grafting density and Mw, a theoretical binding capacity of 0.4 bicarbonate anions / nm2was calculated.

[0067] While utilization of the polymer or polymer support as nucleophiles seems a promising method for support functionalization, an alternative technique is UV polymerization directly onto the fiber, making the functionalization process more efficient. In a third example, synthesis of a guanidinium acrylate monomer utilizing UV polymerization was performed as shown schematically in Figure 4. A small molecule guanidinium monomer was synthesized by first deprotonating V-(3-aminopropyl)methacrylamide hydrochloride by dissolving in water (0.5 g mL1) and adding sodium hydroxide pellets to the solution until the pH was adjusted to 13. The basic solution was extracted with copious amounts of dichloromethane (4 x 250 mL) which was then dried over NaiSCh and evaporated to yield the intermediate V-(3- aminopropyl)methacrylamide. The methacrylamide (1 eq) was then added dropwise to a solution containing triethylamine (1.1 eq) and ethyl carbamidothioate (1.1 eq) in acetonitrile; 1 mL of water was necessary to dissolve the ethyl carbamidothioate. The reaction was allowed to stir at room temperature overnight after which the solvent was removed by rotary evaporation. After isolation of the reagent, a column was run on a combiflash system with 120g of silica gel and an eluent of 20% MeOH in ethyl acetate. The product was collected at the second peak from the column. To prepare the fiber for UV polymerization, the PVDF fiber surface had to first be charged with a radical. For this process, several methods for radical generation are possible, including the chemical addition of an initiator to the surface, exposure to high-intensity electron beams, or potentially exposure to high-strength UV light. For example, one pathway is to add benzophenone, a common UV polymerization initiator, onto the surface of the PVDF. To complete this addition, the PVDF fibers are first added to a concentrated solution of benzophenone in acetonitrile (ACN). The fiber is then removed and allowed to air dry before transferring to a glass vessel under inert environments. The coatedpolymer is then irradiated for 20 min with UV light and rinsed overnight in MeOH to remove any excess benzophenone. The addition of benzophenone allows for direct polymerization to the surface of the PVDF resulting in a strong bond between the polymer and the surface of the fiber. These UV-functionalized fibers should contain large chain lengths. The initiator fimctionalized fibers are then dipped in a concentrated solution of a guandininium acrylate monomer and transferred to an inert vessel. The coated fiber is subsequently irradiated with UV light and then washed to remove any excess polymer or residual monomer.

[0068] The first three examples described above for functionalization of PVDF fiber, or other supports, rely on a significant density of hydroxyl groups on the surface of the support. However, as an alternative to hydroxyl groups, a functional polyisocyanate may be used as shown schematically in Figure 5. In the method shown, Easaqua 401 is a polymer with three isocyanate groups as well some density of acrylates. Preliminary experiments showed that reacting a PVDF hollow fiber membrane with Easaqua 401 resulted in a new and intense signal for isocyanate groups in the FTIR spectra. This signal is significantly larger than the hydroxyl peak observed in the initial PVDF fiber received from the manufacturer. Further reaction of the polyisocyanate PVDF with PGA produced a ligand-installed PVDF (PVDF-PGA) with a strong absorbance in the regions correlated with the PGA bare polymer. Furthermore, reduction of the isocyanate peak in the FTIR spectra at 2263 cm'1indicated consumption of all the binding groups on PVDF backbone.

[0069] Exemplary chemical structures of fimctionalized PVDF fibers are shown in Figures 10-18, with specific examples of corresponding acrylate-based polymers shown underneath. While acrylates are shown by way of example due to their compatibility with RAFT and UV polymerizations, the chemical structure of the base polymer is not limited to acrylates and may vary depending on such factors as the ligand used in the polymerization.

[0070] After generation of the various fimctionalized contactor fibers described above, the fibers were incorporated into a PVDF tube / cylinder to form a module. Loose hollow fibermembranes (20 total, ~14 inch length) were drawn through a 1 foot length, Schedule 80 PVDF (0.957 inch ID, 1-5 / 16 inch OD) pipe. LOCTITE® EA 9462™ structural adhesive was applied at both ends to create a seal between the hollow fibers and PVDF to prevent leakage of solvent. Upon curing of the adhesive, the exposed fibers outside of the cured adhesive were cut off at the interface of the adhesive, creating the final custom-made, hollow-fiber-membrane- containing module (total membrane surface area « 0.014 m2).

[0071] The sodium bicarbonate (NaHCO3) adsorption performance was then evaluated in different compositions of water containing NaHCO3and NaCl. All solutions in this study contained 200 ppm NaHCOa in deionized water. This concentration of NaHCO3is the same concentration seen in ASTM seawater (-200 ppm). When converted to a gaseous state, this NaHCOa concentration corresponds to -104 ppm CO2 (2.38 mmol). Solutions containing 3.42 and 34.2 mmol NaCl in addition to 200 ppm NaHCOa were also prepared. These correspond to 1:1 and 10:1 ratios of NaCl:NaHCO3on a mass basis. The full compositions of these solutions used in this analysis are shown in Table 1 below.

[0072] Table 1 : Water compositions containing NaHCOa and NaCl.

[0073] Once prepared, 100 mL of solution was inserted into the hollow-fibermembrane-containing module. Once loaded, the module was mounted to a vortex mixer and allowed to gently shake for 24 hours. The solution was then drained from the module and evaluated for changes in pH, conductivity, and CO2 concentration via Total Inorganic Carbon (TIC) analysis. TIC analysis was done with a CM5330 Acidification Module and CM5017 CO2Coulometer. Samples of 10 mL were loaded into the cell at 25 °C and met with 10 mL of 2M HC1. Ambient air was used for the carrier gas flowing at 100 mL / min.

[0074] Samples for each cycle of adsorption at the starting and ending points were measured in triplicate and are shown in Figure 19. The left and right bars directly above each cycle (Control, 1, 2, 3...6) represent the starting and ending solutions, respectively, of the adsorption experiments in ppm of CO2, or mg CO2 / L. The tabulated values of these TIC measurements for each cycle are shown in Table 2 below. The cycle labeled “Control” represents as-received PVDF membranes in unfimctionalized form. Cycles 1, 2, 3, and 4 represent experiments using NaHCO3in deionized water. Cycles 5 and 6 represent experiments using 1:1 and 10:1 mass ratios of NaCl:NaHCO3.

[0075] Table 2: Changes in CO2 concentration for all cycles before and after adsorption.

[0076] As shown in Figure 19, there is little to no observable change in CO2 concentration (-3.36 ppm) for the Control cycle with unfunctionalized fibers. When guanidium fimctionalized fibers are introduced to the system, there are sizable changes in CO2 concentration as seen in Cycles 1, 2, 3, and 4 in the left- and right-hand side bars. Notably,decreases of 38.54, 12.07, 22.50, and 23.97 ppm of CO2 were measured for these adsorption experiments. These fibers thus demonstrated the ability to adsorb a significant amount of CO2 in repeated cycles of adsorption and regeneration. In cycle 5, NaCl was introduced into the system at a 1:1 mass ratio with NaHCO3, and a 15.85 ppm decrease was observed for CO2 concentration. The concentration of NaCl with respect to NaHCO3was then increased to a 10: 1 mass ratio in cycle 6, and 10.55 CO2 ppm decrease was observed. These six cycles with guanidium fimctionalized fibers represent a system capable of adsorbing NaHCO3for repeated cycles of adsorption and regeneration with and without the presence of NaCl. The pH of the solutions at the starting and ending points for all cycles were measured and are tabulated in Table 3 below. Changes in conductivity at the starting and ending points for all cycles were measured and are tabulated in Table 4 below.

[0077] Table 3: Changes in pH for all cycles before and after adsorption.

[0078] Table 4: Changes in conductivity for all cycles before and after adsorption.

[0079] For the regeneration process, a 500 mL solution of HC1 in deionized water was prepared with a pH of 4.5. To pump the solution through the shell and lumen side of the hollow fibers, two Masterflex® Digital Gear Pumps (Model No. 75211-70, 36-3600 rpm, 0.1 HP) and MICROPUMP GA Series high performance suction-shoe pump heads (model: GA- T23.PFS.A) were acquired from Avantor™. The pumps were operated at 100 mL / min for 70 to 100 minutes for each regeneration experiment. The pH versus time plots for all regeneration experiments performed in the six cycles are shown in Figure 20 (Cycle 1), Figure 21 (Cycle 2), Figure 22 (Cycle 3), Figure 23 (Cycle 4), Figure 24 (Cycle 5), and Figure 25 (Cycle 6). Upon turning on the pumps, the pH of the solution started to increase from 4.5 and towards neutrality as NaHCO3ions desorbed from the fibers and back into solution. The pH was intermittently adjusted back to 4.5 when the solution approached a pH of 7 to ensure the solution stayed in an acidic regime and continued desorbing NaHCO3ions off the fibers. After 70 to 100 minutes, the pH of the solution began to stabilize, indicating all available NaHCO3ions had been desorbed and the fibers were fully regenerated for another cycle of adsorption.

[0080] The above description is that of current embodiments of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which are to be interpreted in accordance with the principles of patent law including the doctrine of equivalents. This disclosure is presented for illustrative purposes and should not be interpreted as an exhaustive description of all embodiments of the invention or to limit the scope of the claims to the specific elements illustrated or described in connection with these embodiments. For example, and without limitation, any individual element(s) of the described invention may be replaced by alternative elements that provide substantially similar functionality or otherwise provide adequate operation. This includes, for example, presently known alternative elements, such as those that might be currently known to one skilled in the art, and alternative elements that may be developed in the future, such as those that one skilled in the art might, upon development, recognize as an alternative. Further, the disclosed embodiments include a plurality of features that are described in concert and that might cooperatively provide a collection of benefits. The present invention is not limited to only those embodiments that include all of these features or that provide all of the stated benefits, except to the extent otherwise expressly set forth in the issued claims. Any reference to claim elements in the singular, for example, using the articles “a,” “an,” “the” or “said,” is not to be construed as limiting the element to the singular.

Claims

CLAIMSWhat is claimed is:

1. A contactor for selective carbon capture from water, the contactor comprising: a micro- or nano-structured support; a bicarbonate-selective ligand chemically bonded to the support; wherein the bicarbonate-selective ligand includes one or both of a guanidinium and an ammonium functional group.

2. The contactor of claim 1, wherein the support is formed of a polymer, silica, ceramic, or carbon material.

3. The contactor of claim 1, wherein the support has a surface area including hydroxyl groups bonded thereto.

4. The contactor of claim 1, wherein the functional group of the bicarbonate-selective ligand is guanidinium.

5. The contactor of claim 1, wherein the functional group of the bicarbonate-selective ligand is an ammonium.

6. The contactor of claim 5, wherein the functional group of the bicarbonate-selective ligand is a quaternary ammonium.

7. The contactor of claim 1, wherein the bicarbonate-selective ligand comprises a monomer or polymer including the guanidinium and / or ammonium functional group.

8. The contactor of claim 7, wherein the monomer or polymer includes an acrylate or methacrylate unit.

9. The contactor of claim 7, wherein the monomer or polymer is covalently bonded to the support.

10. The contactor of claim 1, wherein the guanidinium or ammonium fimctional group is covalently bonded to the support.

11. A contactor for selective carbon capture, the contactor comprising:a high surface-area material selected from a group of hollow fibers, non-hollow fibers, solid rods, macro-particles, micro-particles, nano-particles, and membranes; a bicarbonate-selective ligand chemically bonded to the surface area of the high surface-area material; wherein the bicarbonate-selective ligand includes one or both of a guanidinium and an ammonium functional group.

12. A method of selectively capturing carbon from water, wherein the water includes a salt and a carbon-containing species dissolved therein, the method comprising the steps of: providing a housing including a plurality of the contactors of claim 1 disposed therein; feeding the water including the dissolved carbon-containing species through the housing, wherein the contactor adsorbs the dissolved carbon-containing species from the water.

13. The method of claim 11 , wherein the carbon-containing species include one or more of gaseous carbon dioxide, carbonate ions, and bicarbonate ions.

14. The method of claim 11 , fiirther comprising the steps of: feeding a pH-swing solution through the housing, wherein the carbon-containing species is desorbed from the contactor to release carbon dioxide into the pH-swing solution, thereby regenerating the contactor; and collecting the carbon dioxide from the pH-swing solution.

15. The method of claim 14, wherein the pH-swing solution has a pH of less than 6.5.

16. The method of claim 14, wherein the pH-swing solution has a pH in a range of approximately 4.5 to 6.5.

17. The method of claim 14, wherein the pH-swing solution is a dilute, inorganic or organic acid.

18. The method of claim 14, comprising an adsorption mode and a desorption mode; the adsorption mode including the step of feeding the water through the contactor;the desorption mode including the steps of feeding the pH-swing solution through the contactor and collecting the carbon dioxide from the pH-swing solution; wherein the adsorption mode and the desorption mode are performed sequentially in that order a plurality of times.

19. A carbon capture module for selective carbon capture, the carbon capture module comprising: a housing defining an internal volume; a plurality of the contactors of claim 1 contained within the internal volume; the housing having first and second opposite longitudinal ends, wherein the plurality of contactors extend between the first and second ends.

20. The carbon capture module of claim 19, wherein the internal volume of the housing is divided into a shell side and a lumen side, wherein the shell side includes an inlet and an outlet, and the lumen side includes an inlet and an outlet.

21. The carbon capture module of claim 19, wherein a seal is formed between the plurality of contactors adjacent both the first end and the second end of the housing.

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