Environ. Sci. Technol. 2008, 42, 2753–2759
Formation of Carbon Dioxide Hydrate in Soil and Soil Mineral Suspensions with Electrolytes RHEO B. LAMORENA AND WOOJIN LEE* Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-Dong, Yuseong-Gu, Daejon 305-701, Korea
Received September 1, 2007. Revised manuscript received December 8, 2007. Accepted December 10, 2007.
We have identified the effects of solid surface (soil, bentonite, kaolinite, nontronite, and pyrite) and electrolyte (NaCl, KCl, CaCl2, and MgCl2) types on the formation and dissociation of CO2 hydrate in this study. The hydrate formation experiments were conducted by injecting CO2 gas into the soil suspensions with and without electrolytes in a 50 mL pressurized vessel. The formation of CO2 hydrate in deionized water was faster than that in aqueous electrolyte solutions. The addition of soil suspensions accelerated the formation of CO2 hydrate in the electrolyte solutions. The hydrate formation times in the solid suspensions without electrolytes were very similar to that in the deionized water. We did not observe any significant differences between the hydrate dissociation in the solid suspension and that in the deionized water. The pHs of clay mineral suspensions decreased significantly after CO2 hydrate formation and dissociation experiments, while the pH of the soil suspension slightly decreased by less than pH 1 and that of pyrite slightly increased due to the dissolution of CO2 forming carbonic acid. The results obtained from this research could be indirectly applied to the fate of CO2 sequestered into geological formations as well as its storage as a form of CO2 hydrate.
Introduction Clathrate hydrates found in natural environments are special types of crystalline inclusion compounds that cage suitable guest molecules physically combined with water. The water molecules rearrange in the presence of solutes to form cages or networks with regular pentagonal and hexagonal faces that can trap at least one guest-solute molecule (1–3). This natural phenomenon (i.e., clathrate hydrate formation) provides an attractive idea for capturing and storing greenhouse gases in a long-term stable geological formation under high-pressure and low-temperature conditions. The hydrate formation can be described by a simple gas reaction equation under the appropriate hydrate forming conditions (2): CO2 · 6H2O S CO2 (g) + 6H2O
(1)
However, it becomes complicated in the presence of electrolytes and solid (sediment or soil mineral) surfaces. The presence of electrolytes can disrupt the original structure and chemical properties of water as well as the solid surfaces (4). The presence of solids can also alter the thermodynamic conditions (e.g., pressure and temperature) for stability (5, 6). * Corresponding author phone: 82-42-869-3624; fax: 82-42-8693610; e-mail:
[email protected]. 10.1021/es702179p CCC: $40.75
Published on Web 01/31/2008
2008 American Chemical Society
The equilibrium conditions for the formation and dissociation of hydrates can be influenced by the interlayer structure and surface chemistry of the host sediments which, in turn, affect the affinities of carbon dioxide and water molecules to the solid surfaces (7). Recent molecular level computer simulations by Titiloye and Skipper (8) have demonstrated the structure of methane and its transport in smectite clays. They observed that, under imposed pressure and temperature conditions, a stable methane hydrate interlayer complex was formed. The transport of these interlayer species through the smectite clay was attributed to a complex interaction between hydrophobic and hydrophilic hydration. The parallel behavior of methane estimated by the numerical simulations might be applied to CO2 molecules forming CO2 hydrates in the similar geochemical environments. We can infer from the numerical simulations on methane that the interaction of carbon dioxide molecules with sediments may also be significantly dependent on the characteristics of solid surfaces (i.e., surface charge and surface area, etc.) in the sediment suspensions. The stacking layers of clay minerals form interlayers where water molecules are adsorbed and hydrogen bonds develop. The surfaces of clay minerals with stacked silicate sheets usually carry a net negative charge developed by the isomorphic substitution and/or deprotonation of surface functional group. The geochemical behavior of the soil mineral surfaces is significantly influenced by its response to changes in pH-dependent environmental conditions (9, 10). Soil mineral surfaces additionally behave differently in a soil mineral-water-electrolyte system (11, 12). It has been shown that some reactive sites on the clay mineral surfaces are involved in promoting clathrate hydrate formation by providing a well-ordered hydrate lattice and/or nucleation sites (13, 14). Knowledge on the CO2 hydrate formation kinetics as well as dissociation of the hydrate is essential to identify the reaction mechanisms regarding the formation stability and potential leakage rates from the hydrate. Understanding the hydrate formation and dissociation at different types of solid surfaces is also important in evaluating the interaction between host sediments and injected CO2, which could influence the long-term stability of the surrounding geologic sediments after CO2 sequestration. However, very limited studies have been conducted to investigate the formation and dissociation of CO2 hydrates in a system consisting of hydrate-forming gas, hydrate, electrolyte, and sediment. Few studies have focused on the effect of different types of soil mineral surfaces on the formation and dissociation of CO2 hydrates (15, 16). Further characterization study needs to be conducted to identify the interactions of hydrates with host soil minerals and electrolytes. In this study, we have investigated the effects of solid surfaces and electrolytes on the formation and dissociation of CO2 hydrate in the soil and soil mineral suspensions. NaCl, KCl, CaCl2, and MgCl2 were used as representative electrolytes, and soil minerals (bentonite, kaolinite, nontronite, and pyrite) and soil were used as solid surfaces frequently found in the natural geological environments.
Experimental Section Materials. The gas hydrate former used in the experiment was a commercial 99.9% grade compressed CO2 (Sam-O Gas Co., Korea). Bentonite (Na(Al, Mg)6(Si4O10)3(OH)2 · nH2O, Kyoungju, Korea) and kaolinite (Al2Si2O5(OH)4, Pusan, Korea) were obtained from Korea Bentonite Co. (Seoul, Korea) and Duckyu Ceramics (Sungnam, Korea), respectively. Nontronite (Na0.3Fe3+2Si3AlO10(OH)2 · 4H2O, Cheney, WA) and pyrite (FeS2, VOL. 42, NO. 8, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Zacatecas, Mexico) were purchased from Ward’s International (New York). The soil sample was collected from the subsurface near the W1-2 building at the Korea Advanced Institute of Science and Technology, Daejeon, Korea. The soil samples collected from the first 20–40 cm of the surface were brought to the laboratory without any efforts to maintain the original redox state. No sample pretreatment with acid and reductant was conducted to control the redox state of soil minerals and soil in this study. Pyrite and nontronite were ground with ceramic mortar and pestle. All soil and soil minerals were screened with sieves; the fraction of particles smaller than 250 µm was collected and dried at ambient temperature (25 ( 0.5 °C). The soil and soil mineral samples used in this study were analyzed to investigate their identity, purity, and crystallinity by X-ray diffraction (XRD), the diffractometer being equipped with Cu KR radiation and a RINT2000 wide-angle goniometer (3–90° 2θ, scan speed of 8° 2θ min-1). The XRD patterns of soil minerals were in good agreement with those described in Joint Committee on Powder Diffraction Standards (JCPDS; see Figure S1(a), Supporting Information). The diffractograms of clay, silt, sand, and soil were shown in Figure S1(b). The surface and pore areas of soil and soil minerals were determined by an ASAP 2000 (Micromeritics) using nitrogen adsorption. The soil is composed of 66.9% sand, 18.5% silt, and 14.6% clay (see Table S1(a,b), Supporting Information). The following chemicals used were reagent or higher grade chemicals: NaCl (99.5%, Junsei, Japan), CaCl2 (95%, Junsei, Japan), MgCl2 (98%, Sam-Chun, Korea), KCl (99%, Sam-Chun, Korea), and ethylene glycol (99.5%, Sam-Chun, Korea). The gas, soil, soil minerals, and chemicals were used without further purification. An exact amount of electrolyte (NaCl, KCl, CaCl2, and MgCl2) was added to deionized water (18 MΩ · cm) to gravimetrically prepare 3.5% electrolyte solution. Soil and soil minerals (10 g of each solid) were added into 1 L solutions containing the electrolytes to prepare the solid suspensions. Experimental Setup. The hydrate formation and dissociation experiments were conducted using a cylindrical 304 stainless steel pressurized vessel (50 cm3) reactor (see Figure S2, Supporting Information). It has two tempered sight ports on opposite sides to allow visual observation during the hydrate formation and dissociation experiments. The vessel was immersed in a temperature-controlled liquid bath maintained by a refrigerated liquid circulator (Model WCL212, Daihan, Korea). The bath liquid was a mixture of ethylene glycol and tap water. The pressure of the vessel was continuously monitored by a pressure gauge with a fullscale accuracy of (1% (Daewon, Korea) in the range of working pressure between 0 and 200 bar. The reaction temperature in the vessel was monitored by a bimetal thermometer ((1% full-scale accuracy, Sigma, Korea). Both pressure gauge and thermometer were installed laterally to the pressurized vessel. Experimental Procedure. CO2 hydrate formation experiments were conducted under isochoric and isothermal conditions (50 cm3 and 273.4 K). Monitoring of hydrate formation conditions and measurement of hydrate formation time were conducted by following experimental steps below. The reaction vessel was filled with 25–30 mL of solid suspension with 3.5% electrolyte, leaving 20-25 mL of headspace. The reactor was purged with CO2 to remove air from the headspace and dissolved air in the suspension. The temperature of the vessel was first lowered to a desired working temperature (273.4 K). The vessel was pressurized to 35 bar by introducing 99.9% pure CO2 gas into the solid suspension and left undisturbed for 30 min to reach a temporal equilibrium (i.e., in both pressure and temperature) static system. CO2 gas was then released to adjust the pressure of the vessel at 30 bar. The reaction vessel was shaken for 2754
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complete mixing of the system for 5–10 min and stood undisturbed to reach the temporal equilibrium for 30 min. The mixing/equilibration routine was repeated several times until the pressure drop of the vessel was not significant (less than 1 bar) to ensure CO2 dissolution in the aqueous phase. This indicates the start of the hydrate formation period. The running time for the hydrate formation period (i.e., from the observation of no significant pressure drop to the first observation of thin- layered CO2 hydrate ( soil > bentonite > pyrite. The result contrasts with that in the solid suspensions with 3.5% NaCl. The order of fast hydrate formation in the solid suspensions with 3.5% CaCl2 was kaolinite > nontronite > bentonite > soil > pyrite and that with 3.5% MgCl2 was kaolinite > soil > bentonite > nontronite > pyrite. Kaolinite has been known as a bad promoter for hydrate formation compared to the bentonite. However, our results showed that CO2 hydrate formation seems to be accelerated on the kaolinite surfaces in the electrolyte solutions containing large divalent cations (Ca2+ and Mg2+) and polarizable monovalent cation (K+). The large cations readily allow the distortion of their hydration spheres than the small cation (Na+) does so that the small hydrated cation can approach closer to the kaolinite surfaces forming a strong binding on the surface sites. The cation bound on the kaolinite surfaces could compete with available water and CO2 molecules for hydrate nucleation sites. Large and stable hydrated cations, however, could be weakly bound onto the kaolinite surfaces so that they could be easily removed from the hydrate nucleation sites by the available hydrate forming molecules. The hydrate formation in bentonite suspensions with electrolytes was relatively slow among the clay mineral suspensions, although bentonite has the highest total (internal + external) surface area. The effect of nontronite on the formation time seems to be comparable to that of bentonite and kaolinite. Nontronite is a 2:l clay containing a high content of Fe(III) as well as a high surface area (90.7 m2/g), but it does not swell as much as bentonite does (25). Nontronite showed shorter hydrate formation time in KCl and CaCl2 solutions than bentonite did. The hydrate formation times of soil, bentonite, and nontronite with MgCl2 are almost similar to each other. Clay minerals with high surface area are more effective than pyrite and soil for the fast formation of CO2 hydrates. The soil suspensions with electrolytes showed a different degree of hydrate formation times which is due mainly to the complex composition of soil. Pyrite showed the longest hydrate formation time in most colloidal suspensions with electrolytes. The results described here showed that CO2 hydrate formation was significantly affected by the presence of solid surface and its availability. The hydrate formation time can be determined by the promoting effect of the solid surfaces and inhibiting effect of the electrolytes in the solid suspensions. Uchida et al. have reported similar observations that equilibrium conditions of gas hydrates are fully dependent on the presence of both clay mineral surfaces and electrolytes (27). Effect of the Dissolution of Gaseous CO2 in Solid Suspensions on the Hydrate Formation. pHs of the solid suspensions were measured before and immediately after each experimental run (columns 2 and 4) and are listed in Table 1. The control experiments (columns 1 and 3) using solid suspensions without CO2 were conducted by measuring the pH of the suspensions before and after each experimental run. The pH of control samples did not change significantly during the experimental run. Therefore, we can conclude that the pH drop or slight increase in the solid suspensions is due mainly to the dissolution of gaseous CO2 in the solid suspensions. The pH values of bentonite, kaolinite, and nontronite significantly decreased after the experiments due to the dissolution of CO2 hydrates into the solid suspensions and subsequent formation of carbonic acid. However, the
TABLE 1. pH of Colloidal Suspensions before and Immediately after Each Experimental Run pH beforea colloidal suspension KCl
bentonite kaolinite nontronite pyrite soil CaCl2 bentonite kaolinite nontronite pyrite soil deionized water bentonite kaolinite pyrite soil
afterb
controlc sampled control sample 9.45 8.92 5.85 3.22 4.33 7.78 7.65 6.51 3.08 4.66 10.03 8.72 3.14 4.80
9.49 9.02 5.77 3.53 4.47 7.80 7.60 6.62 2.78 4.74 10.31 9.12 3.53 5.43
8.63 8.48 5.72 3.00 4.39 7.22 7.08 6.31 2.91 4.54 8.61 8.21 3.03 4.42
4.33 4.95 4.75 3.60 4.35 4.58 4.70 4.10 3.56 4.08 4.21 4.53 3.95 4.42
a pH before starting the experimental run (no CO2 pressurization). b pH after finishing the run (for the after sample case, CO2 was pressurized and then depressurized). c CO2 was not pressurized into the control. d Sample was not pressurized before the run.
pH of soil suspensions slightly decreased by less than pH 1 and that of pyrite suspensions slightly increased. The initial pHs of soil suspensions were in the range of 4.33–5.43, and the slight pH decrease in the soil suspensions after running the experiments would be caused by the strong pH buffer capacity of soil under the carbonic acid forming condition by the dissolution of CO2. The initial pHs of pyrite suspensions were acidic (pH 2.78–3.53) resulting from the dissolution of iron and sulfur species from the pyrite. The slight pH increase in the pyrite suspensions would be also due to the CO2 dissolution resulting in the formation of carbonic acid under the acidic condition. Two possible events affecting the surface charge of solid surfaces can explain the CO2 hydrate formation in the CO2 saturated suspension system: (1) dissolution of the CO2 gas and (2) CO2 gas molecules combining with structured water (28). The pH change in solid suspensions due to the CO2 dissolution can affect the charge of the solid surfaces. In clay mineral suspensions, the surface and edge of clay mineral particles are mostly surrounded by oxy and hydroxy ion species. A low suspension pH could change the surface characteristics by promoting a positive edge to attract more anions (9). The exact values and ranges of point zero charge (pzc) of bentonite, kaolinite, pyrite, and nontronite are 2.5 (29), 4.0–5.0 (29), 1.2–2.5 (30), and ∼6.5 (31), respectively. Although we did not monitor the pH of solid suspensions during the experiment, kaolinite and nontronite could be positively charged during the hydrate formation because the measured pH values after experimental runs were lower than the pzc values of these soil minerals. The pzc of soil sample was not measured in this experiment. H+, Na+, Ca2+, and Mg2+ in bentonite suspension may enter the soil mineral interlayer, which can release other cations in the interlayer into the bulk water via isomorphic substitution. The altered ion-exchange processes on the negatively charged clay-water interface may affect the combination of CO2 and structured water (hydrate precursors), which ultimately affect the hydrate formation kinetics (28). However, the extent of influence by the processes due to isomorphic substitution was not measured and investigated in this research. Hydrate Dissociation. The dissociation of CO2 hydrate in solid suspensions with electrolytes is shown in Figure 4a,b. The time from the y axis to the line drawn parallel to the y axis indicates the hydrate formation time (hydrate formation VOL. 42, NO. 8, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. CO2 hydrate formation and dissociation in solid suspensions with 3.5% KCl (a) and with 3.5% MgCl2 (b). The time from y axis (x ) 0) to the vertical line drawn parallel to the y axis at the first point of each sample indicates the hydrate formation time (hydrate formation period). After the first point, the dissociation of hydrate starts. period). The dissociation of CO2 hydrates started immediately after the hydrate formation period and continued until no hydrate was observed. The temperature of the reaction vessel was increased by 1 or 2 K at each sampling point. The increase of pressure in the reaction vessel after the hydrate formation period is due mainly to the CO2 released from the hydrate entrapment. The exact amount of gaseous CO2 released from the CO2 hydrates was not measured in this experiment. In contrast to the results from the CO2 hydrates formation, the types of solid surfaces and electrolytes did not significantly affect the hydrate dissociation kinetics. No significant differences in the dissociation kinetics were observed among the different types of solid suspensions. One research group has reported a similar observation that the effects of sediment surfaces on the hydrate dissociation appeared to be statistically insignificant (15). The pressure in the reaction vessel increased as the temperature of the reaction vessel increased. This shows that the dissociation of CO2 hydrates can be controlled by temperature changes within the vessel. To maintain CO2 sequestration sites safely after the sequestration 2758
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as a form of CO2 hydrates and to predict the stability of geochemical environments exactly, the temperature and pressure of the geologic environments in addition to the characteristics of the sediments and surrounding water should be monitored regularly. The experimental results from this research can provide an insight into the formation and dissociation of CO2 hydrates in the geological formations and basic knowledge for the geochemical interaction at the water and solid surface interface. Furthermore, results from this investigation can provide an insight into the application of hydrate inclusions to other green house gases, methane and nitrous oxide, and gaseous pollutant such as sulfur dioxide.
Acknowledgments We thankfully acknowledged Dr. Yeotaek Seo at Korea Institute of Energy Research and Dr. Jeoungyun Choi at Korea Advanced Institute of Science and Technology for the helpful discussions. This work was fully supported by grants from the Basic Research Program of the Korea Science and
Engineering Foundation (Grant R01-2006-000-10727-0), the Korea Ministry of Construction and Transportation (Grant 07-UR-B04), and the Korea Research Foundation (Grant KRF2007-211-C00045).
Supporting Information Available Physical soil properties (Table S1), cation/anion characteristics used in the study (Table S2), and schematic diagram, diffractograms, and hydrate photographs (Figures S1-S3). This material is available free of charge via the Internet at http://pubs.acs.org.
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