Effect of Organic Matter on CO2 Hydrate Phase Equilibrium inPhyllosilicate SuspensionsTaehyung 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 phaseequilibria under diverse, heterogeneous conditions, to provide basicknowledge for successful ocean CO2 sequestration in offshore marinesediments. We investigated the effect of geochemical factors on CO2hydrate 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 Na-montmorillonite (Na-MMT)], and (iii) mixtures of them was measuredin the ranges of 274.5−277.0 K and 14−22 bar. Organic matterinhibited the phase equilibrium of CO2 hydrate by association withwater molecules. The inhibition effect decreased in the order: urea <glycine < glucose. Illite and kaolinite (unexpandable clays) barelyaffected the CO2 hydrate phase equilibrium, while Na-MMT(expandable clay) affected the phase equilibrium because of its interlayercations. The CO2 hydrate equilibrium conditions, in the illite and kaolinite suspensions with organic matter, were very similar tothose in the aqueous organic matter solutions. However, the equilibrium condition in the Na-MMT suspension with organicmatter changed because of reduction of its inhibition effect by intercalated organic matter associated with cations in the Na-MMT interlayer.

■ INTRODUCTION

The atmospheric concentration of carbon dioxide (CO2) isincreasing at an accelerating rate from decade to decadebecause of the continued burning of fossil fuels by humans. Thehigher concentration of CO2 in the atmosphere hassubstantially contributed to environmental problems, such asglobal warming and climate change.1,2 Geologic CO2sequestration has been accepted as a promising approach formassive reduction of atmospheric CO2 because fossil fuels willcontinue to be used as our primary energy sources for a while.3

Various options for geologic sequestration of CO2 have beenproposed, in which huge amounts of CO2 might be stored interrestrial (depleted oil and gas reservoirs, coal beds, and salineaquifers) and ocean (marine sediments) areas.3−5 Among thegeologic CO2 sequestration options, marine sediments havebeen highlighted along with the terrestrial sequestrationbecause of their tremendous capacity for CO2 storage.6

Additionally, it has been reported that marine sequestrationcould overcome the limitations of terrestrial CO2 sequestration:the absence of impermeable cap-rock structures and the risk ofbuoyant CO2 leakage by the formation of a CO2 hydrate layeron the vicinity of the storage site.7

CO2 hydrates are special, ice-like crystalline compoundscomposed of hydrogen-bonded water cages with CO2

molecules inside (guest molecules). These form only underhigh-pressure and low-temperature conditions, such as naturally

occur in deep-sea sediments, on continental margins, and inpermafrost regions.8,9 Natural gas hydrates are abundant indeep-ocean sediments, and these gas-hydrate-bearing sedimentsare expected to be potential CO2 sequestration sites.10 Formore practical storage of CO2 in marine sediments, CO2

hydrate formation kinetics and phase equilibrium conditionsin marine sediments have to be estimated.9,11 These kineticsindicate how fast CO2 molecules can be trapped in hydratestructures when CO2 is injected at proper storage sites.12−14

The phase equilibrium conditions of CO2 hydrates aresignificant indicators for evaluation of CO2 storage capacityand the stability of stored CO2 hydrates.7,9,11 The hydrateformation and dissociation conditions, in accordance with thehydrate phase equilibrium, determine the thickness and widthof the hydrate stability zone.15 Previous studies were conductedto evaluate the CO2 hydrate formation kinetics in the presenceof marine environmental factors, such as electrolytes, soilminerals, and organic matter common in marine sediments.These reports indicated a number of factors that cansignificantly affect the kinetics of CO2 hydrate formation.

6,16,17

However, the effects of such factors on CO2 hydrate phase

Received: November 16, 2013Revised: April 7, 2014Accepted: May 20, 2014

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equilibrium conditions have not yet been adequatelyinvestigated. Even if the concentration of dissolved organicmatter in the open ocean is low (34−80 μmol/kg−1),18 varioustypes of microorganisms and the organic matter generated bytheir microbial activities, have been found in marine sedi-ments.19 Gas hydrate induction times and formation rates areknown to be influenced by different types of organic matter inmarine environments,20,21 which makes the role of organicmatter important for the offshore CO2 sequestration process.22

It has been reported that most CO2 hydrates are formed in thepore space of marine sediments.23,24 Additionally, theconcentration of dissolved organic matter is known to behigher in the pore water of marine sediments because oforganic matter accumulation.25 Therefore, it is reasonable toexpect that the high concentration of dissolved organic matteraccumulated in pore water would significantly affect theformation of CO2 hydrate. According to the analysis of marinesediments obtained from one area in which gas hydrates areabundant [Ulleung Basin (UB), East Sea, Korea], sedimentsincluded as much as 10% of organic content.17 Thesephenomena triggered the curiosity of researchers to figure outwhether the presence of organic matter in marine sedimentsaffects the stability of CO2 hydrate, under marine sedimentconditions.7

There has been some research to investigate the effect ofindividual geochemical factors (i.e., organic matter and clayminerals) on the hydrate phase equilibrium.26,27 However, anexclusive understanding of each geochemical factor is notsufficient to successfully implement offshore CO2 sequestrationtechnologies. Therefore, the effects of coexisting geochemicalfactors possibly present in the marine sediments (i.e., potentialinteraction between dissolved organic matter and clay minerals)should be clearly understood. In this study, we investigated theeffect of geochemical factors, such as organic matter (glucose,glycine, and urea) and phyllosilicates [Na-montmorillonite(Na-MMT), illite, and kaolinite] on the stability of CO2hydrate by quantitative evaluation of the change in its phaseequilibrium. In addition, we also examined the phaseequilibrium of CO2 hydrate in mixtures of organic matter andphyllosilicates, to understand the effect of their complexinteraction on the stability of CO2 hydrate.

■ EXPERIMENTAL SECTIONMaterials. The CO2 gas used for gas hydrate formation in

the experiment was a commercial-grade (99.9%) compressedCO2 (Sam-O Gas Co., Korea). Illite,6 kaolinite,6 and Na-MMT28 (Changdong, South Korea) were selected asrepresentative phyllosilicates because of their high abundancein marine sediments. Methods of preparation and character-ization of phyllosilicate samples have previously been reportedin detail.6,14 Glycine [NH2CH2COOH), glucose (C6H12O5),and urea (NH2CONH2)] were selected (Sigma-Aldrich, St.Louis, MO) as representative forms of natural organic matterpossibly present in hydrate-bearing sediments.17 The gas wasused without further purification. An exact amount of organicmatter were added to 30 mL of deionized water (DIW, 18 MΩcm) to prepare 0.5 mol % of each organic matter solution. Thephyllosilicates (10 g each) were added to the prepared organicmatter solutions to make the phyllosilicate suspensions.Experimental Apparatus. The experimental setups were

designed to measure the hydrate dissociation temperature andpressure in the range of 274.5−277.0 K and 14−22 bar,respectively. These setups are graphically illustrated in Figure

S1 of the Supporting Information. A cell made of 304 stainlesssteel with a volume of 150 cm3 was equipped with a temperedglass window to allow for visual observation of hydrateformation and dissociation. During all of the experimentalprocedures, the temperature of the experimental system wascontrolled by a refrigerated liquid circulator (model WCL-212,Daihan, Korea) filled with a mixture of ethylene glycol and tapwater.14 Solutions or suspensions were completely mixed usinga polytetrafluoroethylene (PTFE)-coated magnetic bar and asubmersible magnetic stirring unit.29 Bimetal thermometers(±1% full-scale accuracy, 7Sigma, Korea) and pressuretransducers (±1% full-scale accuracy, Sensys, Korea) wereconnected to the pressurized vessel and a data acquisition unit(Agilent 34970A) with a response time of 20 s.14

Experimental Procedures. CO2 hydrate equilibriumexperiments were conducted by the temperature and pressuretrace method9 under isochoric conditions (150 cm3). Hydratedissociation temperature and pressure conditions weremeasured following the experimental steps below. An exactamount (30 mL) of solution (without 10 g of phyllosilicate) orsuspension (with 10 g of phyllosilicate) was put in the high-pressure vessel. This was submerged in the liquid circulator fortemperature stabilization. The vessel was flushed with CO2 gasseveral times, and then a vacuum pump was used to remove theresidual air molecules inside the reactor. CO2 was then suppliedcontinuously at 15 bar until the reactor was equilibrated toreach a temporal static equilibrium state. The temperature ofthe liquid circulator was then lowered, and the vessel was fullyagitated to initiate CO2 hydrate nucleation and formation. Thedissociation and formation procedures were repeated twice forthe memory effect,30 which can bring a sufficient amount ofCO2 hydrate to measure its phase equilibrium. The temperaturewas increased in steps of 0.4 K after the pressure was stabilizedat the certain point for more than 2 h. When CO2 hydrates startto dissociate, at every step, the temperature was kept constantfor 1 h and the stabilized pressure at a constant temperaturewas considered as a phase equilibrium condition. Theprocedure was repeated until the CO2 hydrates werecompletely dissociated. The gradient of the equilibriumtemperature versus pressure plot decreased as the procedureproceeded. The hydrate phase equilibrium experiments werealso carried out using different types of organic matter; withand without phyllosilicates, by following the experimental stepsabove. All of the experiments were conducted using deionizedwater to specifically understand the effect of organic matter,cations in clay minerals, and their interactions on CO2 hydratephase equilibrium.

Analytical Procedure. The basal spacing values of Na-MMT, with and without water and organic matter, weremeasured from the d(001) peak by a high-performance X-raydiffractometer (XRD, Bruker AXS D8 Advance) with Ni-filtered Cu Kα1,2 radiation. The samples were prepared in thesame ways as for the hydrate phase equilibrium experimentsand then freeze-dried. They were scanned between 5° and 10°at a scan speed of 2° min−1.

Computational Method. The solvation free energy ofthree organic matter (glucose, glycine, and urea) and theirbinding energy with sodium cation (Na+) were calculated toestimate the relative interaction intensity of each form oforganic matter with water and Na+ using density functionaltheory (DFT). The computational structures of the organicmatter and Na+ were generated via Material Studio, version 5.5.The setup charges for the organic matter and Na+ were 0 and

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+1, respectively. Geometric optimization of the preparedstructures 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 Equilibratedstructures were obtained from optimization at the B3LYP/lacvp** level and then used to calculate both solvation freeenergy and binding energy. The solvation free energies of theorganic matter were measured using the Poisson−Boltzmannsolvation model depicting the organic solute as a set of atomiccharges located in a cavity and immersed in a continuum waterbox with a high dielectric constant of 80.37. The solute−solventboundary was represented by the surface of the closet approachas a spherical probe with a radius of 1.40 Å. The binding energybetween the organic matter and Na+ was computed on the basisof eq 1

= + − −E E E Ei ibinding OM, Na OM, Na (1)

where Ebinding is the binding energy between organic matter andNa+, EOM,i is the energy of each form of organic matter, ENa isthe energy of Na+, and EOM,i−Na is the energy of the organicmatter linked with Na+.

■ RESULTS AND DISCUSSIONEffect of Organic Matter on CO2 Phase Equilibrium

Conditions. The experiments were conducted to evaluate thestability of CO2 hydrate in the presence of several forms oforganic matter, and the results are summarized in Table S1 ofthe Supporting Information. The experimental data are alsographically described in Figure 1. The phase equilibrium

conditions of CO2 hydrate in pure water were compared tothose in references previously reported33,34 to check the validityof our experimental procedures and results. Because ourexperimental data matched well those in earlier work, weconcluded that our experimental procedures and results werevalid.The effect of organic matter on the hydrate phase

equilibrium condition was manifested by a shift of the phaseequilibrium curve to the upper left region (higher pressure and

lower temperature conditions). Amino acids composed ofamine (−NH2) and carboxylic acid (−COOH) functionalgroups have been known to act as natural thermodynamicinhibitors and hinder hydrate formation.35 This is because asignificant fraction of the water molecules cannot be involved inCO2 hydrate formation because of strong hydrogen bondingbetween the dissolved organic matter and the water molecules,via a dipole−dipole interaction.9 Therefore, higher pressure,lower temperature, or both are commonly required toovercome the inhibition effect of amino acids on CO2 hydrateformation.29 In Figure 1, the CO2 hydrate equilibrium pressurewas lowest for the DIW sample and followed by urea < glycine< glucose. Although the inhibition effect strengthened as theconcentration of organic matter increased from 0.5 to 1.0 mol%, the order of inhibition by each form of organic matter wasnot changed by its concentration. This indicates that theinhibition effect of glucose on the CO2 hydrate phaseequilibrium is the greatest at all concentrations. Solvation freeenergy was calculated to estimate the interaction intensitybetween water molecules and water-soluble organic matterbased on the thermodynamic sequence in the process oforganic solute dissolution from gas to aqueous phase.35 Thesolvation free energy can be assumed to be the same as thewater-solute interaction energy when the concentration ofsolute is low enough to be neglected.36 Table 1 shows the

calculated solvation free energy of glucose, glycine, and urea(−24.89, −12.09, and −10.78 kcal/mol, respectively). It isknown that solutes with negative solvation free energy are morelikely to dissolve in water spontaneously and that systems withlower free energy are more stable via water−solute associa-tion.36,37 This phenomenon appropriately explains ourexperimental results for the effect of organic matter. Thesolvation free energy of glucose was the lowest among theselected examples of organic matter. This indicates that glucosehas the strongest interaction with water molecules and, moresignificantly, affects the CO2 hydrate phase equilibrium morethan the other forms of organic matter. Glucose has fivehydroxyl functional groups (−OH) that can effectively formhydrogen bonds with water molecules.38 In turn, glycine has astronger interaction with water molecules than urea, resulting ina higher phase equilibrium curve shift to the unstable upperregion (Figure 1). This is mainly due to the potential forfunctional 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 Equili-brium Conditions. The phase equilibrium conditions weremeasured in the presence of different types of phyllosilicateclays. The CO2 hydrate equilibrium conditions in the solidsuspensions are shown in Table S1 of the SupportingInformation and illustrated in Figure 2. There was noremarkable change of the phase equilibrium condition by theaddition of illite and kaolinite compared to the DIW control.Illite is a 2:1 phyllosilicate mineral, in which an octahedral sheetis sandwiched between two tetrahedral sheets.40 It is typically

Figure 1. CO2 hydrate phase equilibrium conditions of DIW andorganic matter solutions.

Table 1. Solvation Free Energy of Organic Matter

organic compound molecular weight (g/mol) solvation energy (kcal/mol)

glycine 75.07 −12.094glucose 180.16 −24.888urea 60.06 −10.780

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found in the sand and silt fractions of soils. It has stronglyassociated K+ ions located in the hexagonal holes between thetetrahedral sheets, which make illite unexpandable. Kaolinite isone of the most common 1:1 phyllosilicate clays, in which eachlayer has one tetrahedral (silicon) and one octahedral(aluminum) sheet.40 These two adjacent tetrahedral andoctahedral sheets are bound together by hydrogen bonding,which prevents expansion between the layers when kaolinite issaturated with water.40 For both kaolinite and illite, watermolecules generally cannot enter inside the layers, meaning thatCO2 hydrate formation might have occurred on the outersurface of these forms of clay.Unlike for illite and kaolinite, the phase equilibrium

condition of CO2 hydrate was significantly shifted to theunstable region in Na-MMT suspension. Na-MMT is one ofthe most common smectite group clays of which octahedrallayers are sandwiched between two tetrahedral layers.40 Itsinterlayer can accept and hold water molecules and can swell aswater molecules are intercalated inside the layer.41 Theinterlayers contain various types of cations on their surfaces,41

and the presence of interlayer cations is an important factorcontrolling the swelling behavior of Na-MMT clay.42 It hasbeen reported that the characteristics of hydrate formation inthe interlayer can be different from those in the bulk phasebecause of its narrow spacing and surface chemicalcomposition.43 Swelling of the Na-MMT interlayer wasconfirmed by XRD (Figure 3). The 2θ peak value for a driedNa-MMT sample decreased as the sample was saturated withwater and its interlayer was swollen by the intercalation ofwater molecules.44 This resulted in an increase in basal spacingfrom 10.88 to 12.97 Å, which implies that the CO2 hydratemight have formed predominantly in the Na-MMT interlayers.Several studies have focused on the gas hydrate formationphenomena in the inner space of clay layers, as affected bycapillary pressure and complex interactions among clay, water,and cations.11,44,45 Association of water molecules with soilmineral surfaces could reduce the water activity of a chemicalsystem, especially for the surfaces of clays, such as MMT, whichhave extensive surface area and distinct ionic double layers.11,42

It is widely known that cations are likely to interact with thedipoles of water molecules via electrostatic attraction, which is

stronger than hydrogen bonds or van der Waals force, resultingin a severe drop in water activity.9 CO2 hydrate formation wassignificantly inhibited by the interlayer cations of Na-MMT,leading to significant change in the phase equilibrium conditionof the CO2 hydrate in the Na-MMT suspension. The resultsindicated that the presence of non-expandable phyllosilicateminerals (illite and kaolinite) less significantly affects CO2hydrate stability than swelling phyllosilicate minerals (Na-MMT) in relation to ocean CO2 sequestration in deep-seasediments.

Effect of Organic Matter on CO2 Phase EquilibriumConditions in Phyllosilicate Clay Suspensions. Phaseequilibrium conditions of CO2 hydrate in the presence oforganic matter in phyllosilicate clay suspensions are shown inTable S1 of the Supporting Information and demonstrated inFigures 4 and 5. Prepared organic matter solutions were mixedwith clay mineral suspensions, and their phase equilibriumconditions were measured. The phase equilibrium conditions inclay mineral suspensions without organic matter were used ascontrols (Figure 3). The phase equilibrium conditions of CO2hydrate were made unstable by the addition of organic matterto illite and kaolinite suspensions (Figure 4). These results werevery similar to the effect of organic matter on phase equilibriumwithout any clay minerals, as shown in the previous section(Figure 1). This indicates that organic matter did not associatewith unexpandable clay minerals to affect the CO2 hydrateequilibrium conditions.The addition of organic matter to the Na-MMT suspension

shifted the CO2 hydrate equilibrium curve (Figure 5) to alower, more stable region (lower pressure and highertemperature) than for the Na-MMT suspension withoutorganic matter. It was expected that the addition of organicmatter to the Na-MMT suspension would inhibit theequilibrium pressure and temperature more severely, becauseboth organic matter and Na-MMT are known to haveinhibitory effects on the phase equilibrium of CO2 hydrates.However, the inhibition intensity diminished as more organicmatter was added to the Na-MMT suspension (Figure 5). Thisindicates that the organic matter may play a role in relaxing theintense inhibition effect of cations inside the Na-MMTinterlayer, thus significantly affecting the CO2 hydrateequilibrium condition. An increase of the basal spacing values

Figure 2. CO2 hydrate phase equilibrium conditions of DIW andphyllosilicate clay suspensions.

Figure 3. XRD patterns of dried Na-MMT (red line) and swollen Na-MMT by DIW (black line). The value (in Å units) refers to the basalspacing value of each sample.

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of Na-MMT by the addition of organic matter was confirmedby XRD. This showed that the organic matter was intercalatedwithin the interlayer of Na-MMT (Figure 6). The addition of0.5 mol % of glucose, glycine, and urea solutions increased thebasal spacing values of Na-MMT from 12.97 Å to 15.63, 15.04,and 14.06 Å, respectively. This indicates that the organic matterwas intercalated into the Na-MMT interlayer and possiblyaffected the CO2 hydrate formation and dissociation processesby associating with water molecules and cations in theinterlayer surface. It has been reported that organic mattercould attract and trap ionic species via electrostatic interactionand ionic bonding.46,47 Therefore, the inhibition effects ofcations and organic matter could be countervailed by suchinteractions, leading to more favorable equilibrium conditionsfor 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). Thisresult contrasts well with the phase equilibrium conditions of

CO2 hydrate in the organic matter solutions in the previoussection (Figure 1). Glucose destabilized the CO2 hydrate phaseequilibrium most intensively among the organic mattersolutions but most highly stabilized it among the Na-MMTsuspensions with organic matter. This implies that theassociation of glucose with the interlayer cations of Na-MMTwas stronger than that of the other forms of organic matter,resulting in the greatest change of the hydrate phaseequilibrium by cation inhibition. To evaluate the interactionintensity between the organic matter and representative cation(Na+), a computational method was adopted to calculate theirbinding energy. The computed binding energy between thethree 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. Becausethis is the energy required to decompose a whole or linkedstructure into its individual parts,48 the binding energy oforganic matter with Na+ indicates how strongly they areassociated. By their strong bonding, the forms of organic matterhaving higher binding energy with Na+ more effectively reducedthe inhibition effect of the cations. As a result, more water

Figure 4. CO2 hydrate phase equilibrium conditions of (a) kaolinite and (b) illite suspensions with organic matter.

Figure 5. CO2 hydrate phase equilibrium conditions of Na-MMTsuspensions with organic matter.

Figure 6. XRD patterns of Na-MMT samples saturated with DIW andintercalated with organic matter. The value (in Å units) refers to thebasal spacing value of each sample.

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molecules were available for the CO2 hydrate formation processon the inner surface of Na-MMT. These results suggest that thepresence of interlayer cations in Na-MMT could significantlyaffect the CO2 hydrate phase equilibrium conditions in marinesediments. However, the presence of abundant natural organicsin the sediments of some sea beds (e.g., UB, East Sea, Korea)might play a significant role in alleviating the inhibition effectby cations on phase equilibrium conditions.Environmental Implications for Offshore CO2 Seques-

tration in Marine Sediments. The storage of CO2 in marinesediments is needed, in addition to storage in terrestrialgeological formations to mitigate increases in the level ofanthropogenic CO2. For successful implementation of thetechnology, further investigation of the CO2 hydrate equili-brium in deep-sea sediments is necessary. The major gashydrate zones at prospective sites are known to have differenttypes and amounts of organic matter and clays in their marinesediments.26,49 In this study, we investigated the effect oforganic matter and phyllosilicates possibly present in marinesediments on the CO2 hydrate phase equilibrium. The resultsindicated that organic matter could play an independent role asa natural inhibitor of change in the phase equilibrium duringoffshore CO2 storage in marine sediments. Unexpandablephyllosilicates, such as illite and kaolinite, had no changes in theCO2 hydrate phase equilibrium, as previously reported.27 Somestudies have also reported that MMT hardly affects hydrateformation and phase equilibrium conditions unlike some othersoil minerals.26 However, our experimental results suggest adifferent reaction mechanism of hydrate formation and changein its phase equilibrium for Na-MMT suspensions. CO2hydrates formed in the inner surface of the Na-MMT interlayerand were significantly influenced by interlayer cations (Na+),which resulted in a shift of the equilibrium curve to an upperunstable (higher pressure and lower temperature) region. Theclay portion of marine sediments was overlooked and notsignificantly studied in previous studies, but our experimentalresults suggest a possibility that the hydrate formation processin marine sediments could occur in the inner surfaces ofphyllosilicate interlayers as long as the clay mineral swells as aresult of intrusion by water molecules. In this study, we firstdemonstrated that the effect of organic matter on the CO2hydrate phase equilibrium conditions can be retained orreduced in the phyllosilicate suspensions. These findingsimply that marine sediments with a high content ofunexpandable clays, such as kaolinite and illite, can betterserve as potential CO2 storage sites because of the moderatehydrate formation and phase equilibrium conditions comparedto 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 organicmatter, it might reduce the inhibition effect of the interlayercations in the expandable clays and alleviate the CO2 hydrateequilibrium conditions compared to sites with only unexpand-able clays. Our study suggests that CO2 hydrate phaseequilibrium conditions can be altered by marine geochemicalfactors (e.g., soil minerals and organic matter) that are

commonly found in real marine sediments. Within the salinityranges normally found in the ocean, there will be an additionalinhibition effect by electrolytes that could significantly affect thephase equilibrium of CO2 hydrate because of the disorderedstructural equilibrium of water molecules and decreased wateractivity.50 Therefore, the phase equilibrium of CO2 hydrateshould be further investigated, in relation to various geo-chemical factors, including electrolytes, clay minerals, andorganic matter in marine sediments, to provide a better basisfor successful offshore CO2 sequestration. Our lab-scale studiesmay have limited relevance to field-scale CO2 storageoperations; even so, our experimental results and findingsshould provide fundamental background knowledge helpful tobetter understand the hydrate phase equilibrium conditionsduring offshore CO2 sequestration processes in marinesediments and to better evaluate locations as potential marineCO2 storage sites.

■ ASSOCIATED CONTENT

*S Supporting InformationSchematic diagram of the experimental setup (Figure S1) andexperimental results of CO2 hydrate phase equilibriumconditions (Table S1). This material is available free of chargevia the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*Telephone: 82-42-350-3624. Fax: 82-42-350-3610. E-mail:woojin_lee@kaist.ac.kr.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This research was partially supported by the National ResearchFoundation of Korea (NRF) funded by the Ministry ofEducation (2012-C1AAA001-M1A2A2026588) and the Geo-Advanced Innovative Action (GAIA) Project funded by theKorean Ministry of Environment.

■ REFERENCES(1) Hansen, J. Defusing the global warning time bomb. Sci. Am. 2004,290, 68−77.(2) Climate Change 2001: The Scientific Basis: Contribution of WorkingGroup I to the Third Assessment Report of the Intergovernmental Panel onClimate Change; Houghton, J. T., Ding, Y., Griggs, D. J., Noguer, M.,van der Linden, P. J., Dai, X., Maskell, K., Johnson, C. A., Eds.;Cambridge University Press: New York, 2001.(3) White, C. M.; Strazisar, B. R.; Granite, E. J.; Hoffman, J. S.;Pennline, H. W. Separation and capture of CO2 from large stationarysources and sequestration in geological formationsCoalbeds anddeep saline aquifers. J. Air Waste Manage. Assoc. 2003, 53, 645−715.(4) Bergman, P. D.; Winter, E. M. Disposal of carbon dioxide inaquifers in the U.S. Energy Convers. Manage. 1995, 36, 523−526.(5) Blunt, M.; Fayers, F. J.; Orr, F. M., Jr. Carbon dioxide inenhanced oil recovery. Energy Convers. Manage. 1993, 34, 1197−1204.

Table 2. Calculated Binding Energy between Organic Matter and Na+ Ion

structure Na+ ion glycine glucose urea Na+ with glycine Na+ with glucose Na+ with urea

energy (hartrees) −162.08 −284.42 −687.18 −225.27 −446.59 −849.40 −387.43interaction between Na+ and glycine Na+ and glucose Na+ and ureabinding energy (kcal/mol) 57.04 84.21 48.95

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dx.doi.org/10.1021/es405099z | Environ. Sci. Technol. XXXX, XXX, XXX−XXXG