Article pubs.acs.org/est
Effect of Organic Matter on CO2 Hydrate Phase Equilibrium in Phyllosilicate Suspensions Taehyung Park, Daeseung Kyung, and Woojin Lee* Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Republic of Korea S Supporting Information *
ABSTRACT: In this study, we examined various CO2 hydrate phase equilibria under diverse, heterogeneous conditions, to provide basic knowledge for successful ocean CO2 sequestration in offshore marine sediments. We investigated the effect of geochemical factors on CO2 hydrate phase equilibrium. The three-phase (liquid−hydrate−vapor) equilibrium of CO2 hydrate in the presence of (i) organic matter (glycine, glucose, and urea), (ii) phyllosilicates [illite, kaolinite, and Namontmorillonite (Na-MMT)], and (iii) mixtures of them was measured in the ranges of 274.5−277.0 K and 14−22 bar. Organic matter inhibited the phase equilibrium of CO2 hydrate by association with water molecules. The inhibition effect decreased in the order: urea < glycine < glucose. Illite and kaolinite (unexpandable clays) barely affected the CO2 hydrate phase equilibrium, while Na-MMT (expandable clay) affected the phase equilibrium because of its interlayer cations. The CO2 hydrate equilibrium conditions, in the illite and kaolinite suspensions with organic matter, were very similar to those in the aqueous organic matter solutions. However, the equilibrium condition in the Na-MMT suspension with organic matter changed because of reduction of its inhibition effect by intercalated organic matter associated with cations in the NaMMT interlayer.
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INTRODUCTION The atmospheric concentration of carbon dioxide (CO2) is increasing at an accelerating rate from decade to decade because of the continued burning of fossil fuels by humans. The higher concentration of CO 2 in the atmosphere has substantially contributed to environmental problems, such as global warming and climate change. 1,2 Geologic CO 2 sequestration has been accepted as a promising approach for massive reduction of atmospheric CO2 because fossil fuels will continue to be used as our primary energy sources for a while.3 Various options for geologic sequestration of CO2 have been proposed, in which huge amounts of CO2 might be stored in terrestrial (depleted oil and gas reservoirs, coal beds, and saline aquifers) and ocean (marine sediments) areas.3−5 Among the geologic CO2 sequestration options, marine sediments have been highlighted along with the terrestrial sequestration because of their tremendous capacity for CO2 storage.6 Additionally, it has been reported that marine sequestration could overcome the limitations of terrestrial CO2 sequestration: the absence of impermeable cap-rock structures and the risk of buoyant CO2 leakage by the formation of a CO2 hydrate layer on the vicinity of the storage site.7 CO2 hydrates are special, ice-like crystalline compounds composed of hydrogen-bonded water cages with CO 2 molecules inside (guest molecules). These form only under high-pressure and low-temperature conditions, such as naturally © 2014 American Chemical Society
occur in deep-sea sediments, on continental margins, and in permafrost regions.8,9 Natural gas hydrates are abundant in deep-ocean sediments, and these gas-hydrate-bearing sediments are expected to be potential CO2 sequestration sites.10 For more practical storage of CO2 in marine sediments, CO2 hydrate formation kinetics and phase equilibrium conditions in marine sediments have to be estimated.9,11 These kinetics indicate how fast CO2 molecules can be trapped in hydrate structures when CO2 is injected at proper storage sites.12−14 The phase equilibrium conditions of CO2 hydrates are significant indicators for evaluation of CO2 storage capacity and the stability of stored CO2 hydrates.7,9,11 The hydrate formation and dissociation conditions, in accordance with the hydrate phase equilibrium, determine the thickness and width of the hydrate stability zone.15 Previous studies were conducted to evaluate the CO2 hydrate formation kinetics in the presence of marine environmental factors, such as electrolytes, soil minerals, and organic matter common in marine sediments. These reports indicated a number of factors that can significantly affect the kinetics of CO2 hydrate formation.6,16,17 However, the effects of such factors on CO2 hydrate phase Received: Revised: Accepted: Published: 6597
November 16, 2013 April 7, 2014 May 20, 2014 May 20, 2014 dx.doi.org/10.1021/es405099z | Environ. Sci. Technol. 2014, 48, 6597−6603
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S1 of the Supporting Information. A cell made of 304 stainless steel with a volume of 150 cm3 was equipped with a tempered glass window to allow for visual observation of hydrate formation and dissociation. During all of the experimental procedures, the temperature of the experimental system was controlled by a refrigerated liquid circulator (model WCL-212, Daihan, Korea) filled with a mixture of ethylene glycol and tap water.14 Solutions or suspensions were completely mixed using a polytetrafluoroethylene (PTFE)-coated magnetic bar and a submersible magnetic stirring unit.29 Bimetal thermometers (±1% full-scale accuracy, 7Sigma, Korea) and pressure transducers (±1% full-scale accuracy, Sensys, Korea) were connected to the pressurized vessel and a data acquisition unit (Agilent 34970A) with a response time of 20 s.14 Experimental Procedures. CO2 hydrate equilibrium experiments were conducted by the temperature and pressure trace method9 under isochoric conditions (150 cm3). Hydrate dissociation temperature and pressure conditions were measured following the experimental steps below. An exact amount (30 mL) of solution (without 10 g of phyllosilicate) or suspension (with 10 g of phyllosilicate) was put in the highpressure vessel. This was submerged in the liquid circulator for temperature stabilization. The vessel was flushed with CO2 gas several times, and then a vacuum pump was used to remove the residual air molecules inside the reactor. CO2 was then supplied continuously at 15 bar until the reactor was equilibrated to reach a temporal static equilibrium state. The temperature of the liquid circulator was then lowered, and the vessel was fully agitated to initiate CO2 hydrate nucleation and formation. The dissociation and formation procedures were repeated twice for the memory effect,30 which can bring a sufficient amount of CO2 hydrate to measure its phase equilibrium. The temperature was increased in steps of 0.4 K after the pressure was stabilized at the certain point for more than 2 h. When CO2 hydrates start to dissociate, at every step, the temperature was kept constant for 1 h and the stabilized pressure at a constant temperature was considered as a phase equilibrium condition. The procedure was repeated until the CO2 hydrates were completely dissociated. The gradient of the equilibrium temperature versus pressure plot decreased as the procedure proceeded. The hydrate phase equilibrium experiments were also carried out using different types of organic matter; with and without phyllosilicates, by following the experimental steps above. All of the experiments were conducted using deionized water to specifically understand the effect of organic matter, cations in clay minerals, and their interactions on CO2 hydrate phase equilibrium. Analytical Procedure. The basal spacing values of NaMMT, with and without water and organic matter, were measured from the d(001) peak by a high-performance X-ray diffractometer (XRD, Bruker AXS D8 Advance) with Nifiltered Cu Kα1,2 radiation. The samples were prepared in the same ways as for the hydrate phase equilibrium experiments and then freeze-dried. They were scanned between 5° and 10° at a scan speed of 2° min−1. Computational Method. The solvation free energy of three organic matter (glucose, glycine, and urea) and their binding energy with sodium cation (Na+) were calculated to estimate the relative interaction intensity of each form of organic matter with water and Na+ using density functional theory (DFT). The computational structures of the organic matter and Na+ were generated via Material Studio, version 5.5. The setup charges for the organic matter and Na+ were 0 and
equilibrium conditions have not yet been adequately investigated. Even if the concentration of dissolved organic matter in the open ocean is low (34−80 μmol/kg−1),18 various types of microorganisms and the organic matter generated by their microbial activities, have been found in marine sediments.19 Gas hydrate induction times and formation rates are known to be influenced by different types of organic matter in marine environments,20,21 which makes the role of organic matter important for the offshore CO2 sequestration process.22 It has been reported that most CO2 hydrates are formed in the pore space of marine sediments.23,24 Additionally, the concentration of dissolved organic matter is known to be higher in the pore water of marine sediments because of organic matter accumulation.25 Therefore, it is reasonable to expect that the high concentration of dissolved organic matter accumulated in pore water would significantly affect the formation of CO2 hydrate. According to the analysis of marine sediments obtained from one area in which gas hydrates are abundant [Ulleung Basin (UB), East Sea, Korea], sediments included as much as 10% of organic content.17 These phenomena triggered the curiosity of researchers to figure out whether the presence of organic matter in marine sediments affects the stability of CO2 hydrate, under marine sediment conditions.7 There has been some research to investigate the effect of individual geochemical factors (i.e., organic matter and clay minerals) on the hydrate phase equilibrium.26,27 However, an exclusive understanding of each geochemical factor is not sufficient to successfully implement offshore CO2 sequestration technologies. Therefore, the effects of coexisting geochemical factors possibly present in the marine sediments (i.e., potential interaction between dissolved organic matter and clay minerals) should be clearly understood. In this study, we investigated the effect of geochemical factors, such as organic matter (glucose, glycine, and urea) and phyllosilicates [Na-montmorillonite (Na-MMT), illite, and kaolinite] on the stability of CO2 hydrate by quantitative evaluation of the change in its phase equilibrium. In addition, we also examined the phase equilibrium of CO2 hydrate in mixtures of organic matter and phyllosilicates, to understand the effect of their complex interaction on the stability of CO2 hydrate.
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EXPERIMENTAL SECTION Materials. The CO2 gas used for gas hydrate formation in the experiment was a commercial-grade (99.9%) compressed CO2 (Sam-O Gas Co., Korea). Illite,6 kaolinite,6 and NaMMT 28 (Changdong, South Korea) were selected as representative phyllosilicates because of their high abundance in marine sediments. Methods of preparation and characterization of phyllosilicate samples have previously been reported in detail.6,14 Glycine [NH2CH2COOH), glucose (C6H12O5), and urea (NH2CONH2)] were selected (Sigma-Aldrich, St. Louis, MO) as representative forms of natural organic matter possibly present in hydrate-bearing sediments.17 The gas was used without further purification. An exact amount of organic matter were added to 30 mL of deionized water (DIW, 18 MΩ cm) to prepare 0.5 mol % of each organic matter solution. The phyllosilicates (10 g each) were added to the prepared organic matter solutions to make the phyllosilicate suspensions. Experimental Apparatus. The experimental setups were designed to measure the hydrate dissociation temperature and pressure in the range of 274.5−277.0 K and 14−22 bar, respectively. These setups are graphically illustrated in Figure 6598
dx.doi.org/10.1021/es405099z | Environ. Sci. Technol. 2014, 48, 6597−6603
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lower temperature conditions). Amino acids composed of amine (−NH2) and carboxylic acid (−COOH) functional groups have been known to act as natural thermodynamic inhibitors and hinder hydrate formation.35 This is because a significant fraction of the water molecules cannot be involved in CO2 hydrate formation because of strong hydrogen bonding between the dissolved organic matter and the water molecules, via a dipole−dipole interaction.9 Therefore, higher pressure, lower temperature, or both are commonly required to overcome the inhibition effect of amino acids on CO2 hydrate formation.29 In Figure 1, the CO2 hydrate equilibrium pressure was lowest for the DIW sample and followed by urea < glycine < glucose. Although the inhibition effect strengthened as the concentration of organic matter increased from 0.5 to 1.0 mol %, the order of inhibition by each form of organic matter was not changed by its concentration. This indicates that the inhibition effect of glucose on the CO2 hydrate phase equilibrium is the greatest at all concentrations. Solvation free energy was calculated to estimate the interaction intensity between water molecules and water-soluble organic matter based on the thermodynamic sequence in the process of organic solute dissolution from gas to aqueous phase.35 The solvation free energy can be assumed to be the same as the water-solute interaction energy when the concentration of solute is low enough to be neglected.36 Table 1 shows the
+1, respectively. Geometric optimization of the prepared structures was performed using Jaguar incorporated in Maestro, version 3.1, by adopting the Becke three-parameter functional (B3) combined with the correlation functional of Lee, Yang, and Parr (LYP), using the lacvp** basis set.31,32 Equilibrated structures were obtained from optimization at the B3LYP/ lacvp** level and then used to calculate both solvation free energy and binding energy. The solvation free energies of the organic matter were measured using the Poisson−Boltzmann solvation model depicting the organic solute as a set of atomic charges located in a cavity and immersed in a continuum water box with a high dielectric constant of 80.37. The solute−solvent boundary was represented by the surface of the closet approach as a spherical probe with a radius of 1.40 Å. The binding energy between the organic matter and Na+ was computed on the basis of eq 1 E binding = EOM, i + E Na − EOM, i − Na
(1)
where Ebinding is the binding energy between organic matter and Na+, EOM,i is the energy of each form of organic matter, ENa is the energy of Na+, and EOM,i−Na is the energy of the organic matter linked with Na+.
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RESULTS AND DISCUSSION Effect of Organic Matter on CO2 Phase Equilibrium Conditions. The experiments were conducted to evaluate the stability of CO2 hydrate in the presence of several forms of organic matter, and the results are summarized in Table S1 of the Supporting Information. The experimental data are also graphically described in Figure 1. The phase equilibrium
Table 1. Solvation Free Energy of Organic Matter organic compound
molecular weight (g/mol)
solvation energy (kcal/mol)
glycine glucose urea
75.07 180.16 60.06
−12.094 −24.888 −10.780
calculated solvation free energy of glucose, glycine, and urea (−24.89, −12.09, and −10.78 kcal/mol, respectively). It is known that solutes with negative solvation free energy are more likely to dissolve in water spontaneously and that systems with lower free energy are more stable via water−solute association.36,37 This phenomenon appropriately explains our experimental results for the effect of organic matter. The solvation free energy of glucose was the lowest among the selected examples of organic matter. This indicates that glucose has the strongest interaction with water molecules and, more significantly, affects the CO2 hydrate phase equilibrium more than the other forms of organic matter. Glucose has five hydroxyl functional groups (−OH) that can effectively form hydrogen bonds with water molecules.38 In turn, glycine has a stronger interaction with water molecules than urea, resulting in a higher phase equilibrium curve shift to the unstable upper region (Figure 1). This is mainly due to the potential for functional groups of glycine (charged −NH3+ and −COO− groups39), which are more reactive than those of urea (two −NH2 groups), to form hydrogen bonds with water molecules. Effect of Phyllosilicate Clays on CO2 Phase Equilibrium Conditions. The phase equilibrium conditions were measured in the presence of different types of phyllosilicate clays. The CO2 hydrate equilibrium conditions in the solid suspensions are shown in Table S1 of the Supporting Information and illustrated in Figure 2. There was no remarkable change of the phase equilibrium condition by the addition of illite and kaolinite compared to the DIW control. Illite is a 2:1 phyllosilicate mineral, in which an octahedral sheet is sandwiched between two tetrahedral sheets.40 It is typically
Figure 1. CO2 hydrate phase equilibrium conditions of DIW and organic matter solutions.
conditions of CO2 hydrate in pure water were compared to those in references previously reported33,34 to check the validity of our experimental procedures and results. Because our experimental data matched well those in earlier work, we concluded that our experimental procedures and results were valid. The effect of organic matter on the hydrate phase equilibrium condition was manifested by a shift of the phase equilibrium curve to the upper left region (higher pressure and 6599
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Figure 3. XRD patterns of dried Na-MMT (red line) and swollen NaMMT by DIW (black line). The value (in Å units) refers to the basal spacing value of each sample. Figure 2. CO2 hydrate phase equilibrium conditions of DIW and phyllosilicate clay suspensions.
stronger than hydrogen bonds or van der Waals force, resulting in a severe drop in water activity.9 CO2 hydrate formation was significantly inhibited by the interlayer cations of Na-MMT, leading to significant change in the phase equilibrium condition of the CO2 hydrate in the Na-MMT suspension. The results indicated that the presence of non-expandable phyllosilicate minerals (illite and kaolinite) less significantly affects CO2 hydrate stability than swelling phyllosilicate minerals (NaMMT) in relation to ocean CO2 sequestration in deep-sea sediments. Effect of Organic Matter on CO2 Phase Equilibrium Conditions in Phyllosilicate Clay Suspensions. Phase equilibrium conditions of CO2 hydrate in the presence of organic matter in phyllosilicate clay suspensions are shown in Table S1 of the Supporting Information and demonstrated in Figures 4 and 5. Prepared organic matter solutions were mixed with clay mineral suspensions, and their phase equilibrium conditions were measured. The phase equilibrium conditions in clay mineral suspensions without organic matter were used as controls (Figure 3). The phase equilibrium conditions of CO2 hydrate were made unstable by the addition of organic matter to illite and kaolinite suspensions (Figure 4). These results were very similar to the effect of organic matter on phase equilibrium without any clay minerals, as shown in the previous section (Figure 1). This indicates that organic matter did not associate with unexpandable clay minerals to affect the CO2 hydrate equilibrium conditions. The addition of organic matter to the Na-MMT suspension shifted the CO2 hydrate equilibrium curve (Figure 5) to a lower, more stable region (lower pressure and higher temperature) than for the Na-MMT suspension without organic matter. It was expected that the addition of organic matter to the Na-MMT suspension would inhibit the equilibrium pressure and temperature more severely, because both organic matter and Na-MMT are known to have inhibitory effects on the phase equilibrium of CO2 hydrates. However, the inhibition intensity diminished as more organic matter was added to the Na-MMT suspension (Figure 5). This indicates that the organic matter may play a role in relaxing the intense inhibition effect of cations inside the Na-MMT interlayer, thus significantly affecting the CO2 hydrate equilibrium condition. An increase of the basal spacing values
found in the sand and silt fractions of soils. It has strongly associated K+ ions located in the hexagonal holes between the tetrahedral sheets, which make illite unexpandable. Kaolinite is one of the most common 1:1 phyllosilicate clays, in which each layer has one tetrahedral (silicon) and one octahedral (aluminum) sheet.40 These two adjacent tetrahedral and octahedral sheets are bound together by hydrogen bonding, which prevents expansion between the layers when kaolinite is saturated with water.40 For both kaolinite and illite, water molecules generally cannot enter inside the layers, meaning that CO2 hydrate formation might have occurred on the outer surface of these forms of clay. Unlike for illite and kaolinite, the phase equilibrium condition of CO2 hydrate was significantly shifted to the unstable region in Na-MMT suspension. Na-MMT is one of the most common smectite group clays of which octahedral layers are sandwiched between two tetrahedral layers.40 Its interlayer can accept and hold water molecules and can swell as water molecules are intercalated inside the layer.41 The interlayers contain various types of cations on their surfaces,41 and the presence of interlayer cations is an important factor controlling the swelling behavior of Na-MMT clay.42 It has been reported that the characteristics of hydrate formation in the interlayer can be different from those in the bulk phase because of its narrow spacing and surface chemical composition.43 Swelling of the Na-MMT interlayer was confirmed by XRD (Figure 3). The 2θ peak value for a dried Na-MMT sample decreased as the sample was saturated with water and its interlayer was swollen by the intercalation of water molecules.44 This resulted in an increase in basal spacing from 10.88 to 12.97 Å, which implies that the CO2 hydrate might have formed predominantly in the Na-MMT interlayers. Several studies have focused on the gas hydrate formation phenomena in the inner space of clay layers, as affected by capillary pressure and complex interactions among clay, water, and cations.11,44,45 Association of water molecules with soil mineral surfaces could reduce the water activity of a chemical system, especially for the surfaces of clays, such as MMT, which have extensive surface area and distinct ionic double layers.11,42 It is widely known that cations are likely to interact with the dipoles of water molecules via electrostatic attraction, which is 6600
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Figure 4. CO2 hydrate phase equilibrium conditions of (a) kaolinite and (b) illite suspensions with organic matter.
Figure 6. XRD patterns of Na-MMT samples saturated with DIW and intercalated with organic matter. The value (in Å units) refers to the basal spacing value of each sample.
Figure 5. CO2 hydrate phase equilibrium conditions of Na-MMT suspensions with organic matter.
CO2 hydrate in the organic matter solutions in the previous section (Figure 1). Glucose destabilized the CO2 hydrate phase equilibrium most intensively among the organic matter solutions but most highly stabilized it among the Na-MMT suspensions with organic matter. This implies that the association of glucose with the interlayer cations of Na-MMT was stronger than that of the other forms of organic matter, resulting in the greatest change of the hydrate phase equilibrium by cation inhibition. To evaluate the interaction intensity between the organic matter and representative cation (Na+), a computational method was adopted to calculate their binding energy. The computed binding energy between the three forms of organic matter and Na+ are shown in Table 2. The binding energies of glucose, glycine, and urea with Na+ were 84.21, 57.04, and 48.95 kcal/mol, respectively. Because this is the energy required to decompose a whole or linked structure into its individual parts,48 the binding energy of organic matter with Na+ indicates how strongly they are associated. By their strong bonding, the forms of organic matter having higher binding energy with Na+ more effectively reduced the inhibition effect of the cations. As a result, more water
of Na-MMT by the addition of organic matter was confirmed by XRD. This showed that the organic matter was intercalated within the interlayer of Na-MMT (Figure 6). The addition of 0.5 mol % of glucose, glycine, and urea solutions increased the basal spacing values of Na-MMT from 12.97 Å to 15.63, 15.04, and 14.06 Å, respectively. This indicates that the organic matter was intercalated into the Na-MMT interlayer and possibly affected the CO2 hydrate formation and dissociation processes by associating with water molecules and cations in the interlayer surface. It has been reported that organic matter could attract and trap ionic species via electrostatic interaction and ionic bonding.46,47 Therefore, the inhibition effects of cations and organic matter could be countervailed by such interactions, leading to more favorable equilibrium conditions for CO2 hydrate than expected. The addition of glucose to the Na-MMT suspension showed the highest level of alleviation of equilibrium conditions, followed by glycine and urea (DIW < Na-MMT + glucose < Na-MMT + glycine < Na-MMT + urea < Na-MMT). This result contrasts well with the phase equilibrium conditions of 6601
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Table 2. Calculated Binding Energy between Organic Matter and Na+ Ion structure energy (hartrees) interaction between binding energy (kcal/mol)
Na+ ion
glycine
−162.08 −284.42 Na+ and glycine 57.04
glucose
urea
−687.18 −225.27 Na+ and glucose 84.21
Na+ with glycine
Na+ with glucose
Na+ with urea
−849.40
−387.43
−446.59 Na+ and urea 48.95
commonly found in real marine sediments. Within the salinity ranges normally found in the ocean, there will be an additional inhibition effect by electrolytes that could significantly affect the phase equilibrium of CO2 hydrate because of the disordered structural equilibrium of water molecules and decreased water activity.50 Therefore, the phase equilibrium of CO2 hydrate should be further investigated, in relation to various geochemical factors, including electrolytes, clay minerals, and organic matter in marine sediments, to provide a better basis for successful offshore CO2 sequestration. Our lab-scale studies may have limited relevance to field-scale CO2 storage operations; even so, our experimental results and findings should provide fundamental background knowledge helpful to better understand the hydrate phase equilibrium conditions during offshore CO2 sequestration processes in marine sediments and to better evaluate locations as potential marine CO2 storage sites.
molecules were available for the CO2 hydrate formation process on the inner surface of Na-MMT. These results suggest that the presence of interlayer cations in Na-MMT could significantly affect the CO2 hydrate phase equilibrium conditions in marine sediments. However, the presence of abundant natural organics in the sediments of some sea beds (e.g., UB, East Sea, Korea) might play a significant role in alleviating the inhibition effect by cations on phase equilibrium conditions. Environmental Implications for Offshore CO2 Sequestration in Marine Sediments. The storage of CO2 in marine sediments is needed, in addition to storage in terrestrial geological formations to mitigate increases in the level of anthropogenic CO2. For successful implementation of the technology, further investigation of the CO2 hydrate equilibrium in deep-sea sediments is necessary. The major gas hydrate zones at prospective sites are known to have different types and amounts of organic matter and clays in their marine sediments.26,49 In this study, we investigated the effect of organic matter and phyllosilicates possibly present in marine sediments on the CO2 hydrate phase equilibrium. The results indicated that organic matter could play an independent role as a natural inhibitor of change in the phase equilibrium during offshore CO2 storage in marine sediments. Unexpandable phyllosilicates, such as illite and kaolinite, had no changes in the CO2 hydrate phase equilibrium, as previously reported.27 Some studies have also reported that MMT hardly affects hydrate formation and phase equilibrium conditions unlike some other soil minerals.26 However, our experimental results suggest a different reaction mechanism of hydrate formation and change in its phase equilibrium for Na-MMT suspensions. CO2 hydrates formed in the inner surface of the Na-MMT interlayer and were significantly influenced by interlayer cations (Na+), which resulted in a shift of the equilibrium curve to an upper unstable (higher pressure and lower temperature) region. The clay portion of marine sediments was overlooked and not significantly studied in previous studies, but our experimental results suggest a possibility that the hydrate formation process in marine sediments could occur in the inner surfaces of phyllosilicate interlayers as long as the clay mineral swells as a result of intrusion by water molecules. In this study, we first demonstrated that the effect of organic matter on the CO2 hydrate phase equilibrium conditions can be retained or reduced in the phyllosilicate suspensions. These findings imply that marine sediments with a high content of unexpandable clays, such as kaolinite and illite, can better serve as potential CO2 storage sites because of the moderate hydrate formation and phase equilibrium conditions compared to sites with large amounts of expandable clays, such as MMT, and in which the amount of organic matter is also relatively low. However, if the storage site contains a large amount of organic matter, it might reduce the inhibition effect of the interlayer cations in the expandable clays and alleviate the CO2 hydrate equilibrium conditions compared to sites with only unexpandable clays. Our study suggests that CO2 hydrate phase equilibrium conditions can be altered by marine geochemical factors (e.g., soil minerals and organic matter) that are
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ASSOCIATED CONTENT
S Supporting Information *
Schematic diagram of the experimental setup (Figure S1) and experimental results of CO2 hydrate phase equilibrium conditions (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 82-42-350-3624. Fax: 82-42-350-3610. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was partially supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2012-C1AAA001-M1A2A2026588) and the GeoAdvanced Innovative Action (GAIA) Project funded by the Korean Ministry of Environment.
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REFERENCES
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dx.doi.org/10.1021/es405099z | Environ. Sci. Technol. 2014, 48, 6597−6603
