Subscriber access provided by UCL Library Services
Article
Molecular Understanding of CO2 and H2O in Montmorillonite Clay Interlayer under CO2 Geological Sequestration Conditions Qi Rao, and Yongsheng Leng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09683 • Publication Date (Web): 11 Jan 2016 Downloaded from http://pubs.acs.org on January 13, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 33
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
Submitted to The Journal of Physical Chemistry C on October 3, 2015; revised version submitted on Dec.13, 2015
Molecular Understanding of CO2 and H2O in Montmorillonite Clay Interlayer under CO2 Geological Sequestration Conditions Qi Rao and Yongsheng Leng * Department of Mechanical and Aerospace Engineering
The George Washington University, Washington, DC 20052, USA ABSTRACT: Grand canonical Monte Carlo (GCMC) simulations are carried out to investigate supercritical carbon dioxide (scCO2)-water mixture in Na-montmorillonite clay interlayer under typical CO2 geological sequestration conditions (T = 323 K, P = 90 bar and T = 348 K, P = 130 bar). The stable clay interlayer distances at different relative humidity (RH) are determined based on the normal pressure and free energy curves of CO2-H2O-Na+ complex in montmorillonite clay interlayer. Simulation results show that stable monolayer hydrates (1W) with a basal spacing around 12 Å are formed at RH = 30-60%. As RH is increased to 70% and above, bilayer CO2H2O mixture with a basal spacing around 15-16Å (2W) are more stable. In general, CO2 intercalation process is strongly influenced by RH. While high relative humidity facilitates water molecules entering clay interlayer, it nonetheless decreases CO2 intercalations. The sorbed H2O concentrations from our simulations compare remarkably well with the in-situ infrared (IR) spectroscopy experimental data by Loring et al. [Langmuir (2014) Vol. 30, pp. 6120-6128], if the continuous experimental curve is considered as the “smear-out” of the stepwise curve from our simulations. However, the overall sorbed CO2 concentrations from our simulations are higher than the IR experimental results. We attribute these discrepancies in both sorbed H2O and CO2 concentrations (measured from experiments and simulations) to the complexity of hydrated clay particles in IR spectroscopy experiment, to which the hydration-heterogeneity model could provide a reasonable interpretation. Molecular dynamics (MD) simulations show that the hydration state of CO2 molecules is changed from the partial hydration in 1W to the full hydration in 2W with the increase in RH, and CO2 dimers are frequently seen in both 1W and *
Corresponding author, Tel.: 202-994-5964; e-mail:
[email protected].
1 ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
2W hydration states. CO2 dimers largely take the slipped parallel configurations, while the remaining dimers take the perpendicular T-shaped geometry. Further, sodium ions in interlayer tend to be fully hydrated by water molecules due to their relatively large hydration energy. Moreover, we find that CO2 molecules hardly migrate into the first hydration shell of sodium ions. The overall diffusion coefficients of CO2 molecules are larger than those of water molecules and sodium ions. This comparably high mobility of CO2 molecules in clay interlayer, together with the low probability of CO2 participation in the first hydration shell of Na+ ions, essentially prevents CO2 and Na+ from direct interactions in clay interlayers.
2 ACS Paragon Plus Environment
Page 2 of 33
Page 3 of 33
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
1. INTRODUCTION Carbon dioxide (CO2) emission from fossil fuel combustion is a major contributor to climate change.1 CO2 capture and sequestration (CCS) in onshore geological subsurface or in offshore sediments is considered to be a promising option to counteract the accumulation of CO2 in the atmosphere and mitigate climate change.2-7 In CCS, the captured CO2 is injected in a supercritical phase (scCO2) into deep geological formations.4 These formations include deep saline aquifers, depleted oil and gas reservoirs, and unminable coal seams overlain with sealing caprock and geologic traps that will prevent the CO2 from escaping.2, 8-10 Since swelling clays are important components in caprocks,11-15 there is a compelling need to understand the interfacial phenomena related to the structure and dynamics of CO2-geological fluids in naturally occurring clay minerals. Recent experimental studies demonstrated that CO2 can be intercalated into clay interlayer under typical geological sequestration conditions.12, 15-22 Experiments12, 16, 18, 20-21 showed that in sub-single hydration layer (2W) to wet scCO2 resulted in a decrease of basal spacing.12, 16, 20, 22 Potential collapse or expansion of the interlayer spacing depends on the initial hydration state of the clay and the property of scCO2. Different types of metal ions in the clay interlayer also influence the clay swelling behavior and the final equilibrium state.12, 16, 20-22 For example, clay minerals containing divalent cations have stronger tendency to swell but slower swelling rate than those containing monovalent cations.21 Moreover, the percent of water saturation (PWS) in scCO219 also plays a significant role in determining the equilibrium basal spacing.16, 19, 22 Here, for the convenience, we still use the relative humidity (RH) as an equivalent variable to represent PWS. These experimental findings revealed the complexity of CO2-water interaction in clay hydrate. There are several molecular simulation studies on the structure and dynamics of CO2-H2O mixtures between charged clay surfaces,13-14, 23-28 primarily due to the initial interest in the role of CO2 molecules intercalated into clay hydrates. Recent molecular dynamics (MD) simulation studies by Cygan et al.14 and Myshakin et. al.24 investigated the swelling behavior of clay minerals in the presence of CO2 and explored the distribution of various species in the clay interlayer. The simulation results confirmed that the degree of clay swelling is dependent on the 3 ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
initial water content in the clay interlayer. Further, it was found that hydrated metal ions exhibit strong tendency to adhere to the clay surfaces, resulting in hydrophobicity of the surfaces. Botan et al.23 performed grand canonical Monte Carlo (GCMC) and MD simulations to investigate the equilibrium configuration of clay minerals with binary water-CO2 mixture. The geological temperature and pressure conditions of T = 348 K, P = 25 bar and P = 125 bar were considered. Simulation results showed that the presence of CO2 inhibits the diffusion of all the mobile species in clay interlayer. Lee et. al.27 employed the first-principle MD simulation to explore the hydrated Ca-montmorillonite exposed to scCO2 and CO2-SO2 mixtures under geologic storage conditions. They showed that in
