Article pubs.acs.org/Langmuir
Evidence for Carbonate Surface Complexation during Forsterite Carbonation in Wet Supercritical Carbon Dioxide John S. Loring,†,* Jeffrey Chen,† Pascale Bénézeth, ‡ Odeta Qafoku,† Eugene S. Ilton,† Nancy M. Washton,† Christopher J. Thompson,† Paul F. Martin,† B. Peter McGrail,† Kevin M. Rosso,† Andrew R. Felmy,† and Herbert T. Schaef† †
Pacific Northwest National Laboratory, Richland, Washington 99352 United States Géosciences Environnement Toulouse (GET), CNRS, UMR 5563, 14 Avenue Edouard Belin, 31400 Toulouse, France
‡
S Supporting Information *
ABSTRACT: Continental flood basalts are attractive formations for geologic sequestration of carbon dioxide because of their reactive divalent-cation containing silicates, such as forsterite (Mg2SiO4), suitable for long-term trapping of CO2 mineralized as metal carbonates. The goal of this study was to investigate at a molecular level the carbonation products formed during the reaction of forsterite with supercritical CO2 (scCO2) as a function of the concentration of H2O adsorbed to the forsterite surface. Experiments were performed at 50 °C and 90 bar using an in situ IR titration capability, and postreaction samples were examined by ex situ techniques, including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), focused ion beam transmission electron microscopy (FIB-TEM), thermal gravimetric analysis mass spectrometry (TGA-MS), and magic angle spinning nuclear magnetic resonance (MAS NMR). Carbonation products and reaction extents varied greatly with adsorbed H2O. We show for the first time evidence of Mg-carbonate surface complexation under wet scCO2 conditions. Carbonate is found to be coordinated to Mg at the forsterite surface in a predominately bidentate fashion at adsorbed H2O concentrations below 27 μmol/m2. Above this concentration and up to 76 μmol/m2, monodentate coordinated complexes become dominant. Beyond a threshold adsorbed H2O concentration of 76 μmol/m2, crystalline carbonates continuously precipitate as magnesite, and the particles that form are hundreds of times larger than the estimated thicknesses of the adsorbed water films of about 7 to 15 Å. At an applied level, these results suggest that mineral carbonation in scCO2 dominated fluids near the wellbore and adjacent to caprocks will be insignificant and limited to surface complexation, unless adsorbed H2O concentrations are high enough to promote crystalline carbonate formation. At a fundamental level, the surface complexes and their dependence on adsorbed H2O concentration give insights regarding forsterite dissolution processes and magnesite nucleation and growth.
1. INTRODUCTION Continued reliance on carbon intensive energy sources to sustain global economies is creating an international urgency to develop strategies and technologies to manage greenhouse gas emissions.1 In geologic carbon sequestration (GCS), CO2 is captured and pumped as a fluid deep into underground rock formations where it (1) is contained by a low permeability caprock, (2) dissolves into formation waters, and in some cases (3) reacts with host rock to form solid carbonates in a process called mineral trapping.2,3 Reservoirs that are best suited for mineral trapping are those in volcanic flood basalts, a rock type © 2015 American Chemical Society
dominating certain provincial domains in the CO2 producing countries of the United States, India, and China.4−8 In the basalt−CO2−H2O system, mineral trapping occurs through dissolution of primary components (i.e., plagioclase, pyroxene, olivine, and glassy mesostasis), which releases divalent cations (Mg2+, Ca2+, and Fe2+) that react with CO2 rich fluids to precipitate stable carbonate minerals. Subsurface conditions in Received: March 23, 2015 Revised: June 9, 2015 Published: June 16, 2015 7533
DOI: 10.1021/acs.langmuir.5b01052 Langmuir 2015, 31, 7533−7543
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Langmuir
example, the dependence of mineral carbonation rate, extent, and product on dissolved H2O concentration in scCO2 has been examined not only with forsterite, but also other olivines,21 serpentines,20 pyroxenes,22 and simple metal hydroxides.23 A common characteristic in most of these systems is that the carbonation reaction occurs at the interface between the mineral surface and an adsorbed water film that separates the mineral surface from the scCO2 fluid, and often a threshold in mineral reactivity is observed with dissolved H2O concentration. Hence, gaining insights into the properties of and chemistry in adsorbed H2O films is fundamental for understanding mineral carbonation reactions in water-bearing scCO2, and in low water fluids in general. Difficulties pertain to measuring or defining quantities such as pH, ionic strength, and ion mobility in quasi 2-dimensional adsorbed H2O layers. Furthermore, experimental characterization of adsorbed H2O film chemistry under in situ conditions remains challenging at scCO2 pressures and temperatures. The objective of the present study is to understand CO2 speciation in adsorbed water films from their earliest stages of interaction onward during carbonation of forsterite in wet scCO2. Specific goals are to measure the concentration of adsorbed H2O on the forsterite surface at the threshold for continuous carbonation, to determine the water and CO2 speciation at the mineral surface prior to the threshold, and to characterize the carbonate and SiO2 precipitates formed beyond the threshold. In this study, the reaction of forsterite with variably wet scCO2 is investigated at 50 °C and 90 bar using a newly developed in situ IR titration capability,24 and postreaction samples are examined ex situ by SEM, XRD, thermal gravimetric analysis mass spectrometry (TGA-MS), MAS NMR, X-ray photoelectron spectroscopy (XPS), and focused ion beam transmission electron microscopy (FIBTEM). We show for the first time evidence for the formation of Mg-carbonate surface complexes at the water-film/forsterite interface under scCO2 conditions. We speculate on its importance in the ultimate nucleation and growth of product metal carbonate phases.
GCS dictate that injected carbon dioxide will be supercritical (scCO2) and initially reside in pore spaces in two-phase equilibrium with variable amounts of incompletely displaced remaining water. The solubility of water in scCO2 is small but significant,9 and importantly dissolved water has been shown to increase the reactivity of scCO2 toward geologic material.10−13 In contrast to aqueous dominated systems, mineral dissolution, nucleation, and growth reactions in water bearing scCO2 fluids are just beginning to be investigated. As part of a comprehensive strategy for GCS-supporting science, it is necessary to understand the fundamental geochemistry occurring under low water conditions for robust prediction of the fate of stored CO2. Forsterite (Mg2SiO4), the magnesium end member of olivine, has been the focus of several studies under wetscCO2 conditions, in part due to its high reactivity at laboratory time scales and availability as a discrete natural phase or a pure synthetic material. Loring et al. (2011)14 used in situ infrared (IR) spectroscopy to study forsterite carbonation in scCO2 as a function of water content at 50 °C and 184 bar. In that study, while no carbonation was detected in the anhydrous fluid, when water was dissolved into the scCO2, it was found to adsorb to forsterite surfaces as a discrete liquid water film on the order of Ångstroms- to nanometers-thick, and a carbonation reaction occurred. A threshold in dissolved H2O concentration was reported, above which the forsterite continuously carbonated but below which there was only limited reactivity. The authors suggested that this threshold was related to the amount of adsorbed H2O needed to dissolve and transport ions. Kwak et al. (2011)15 also identified a dissolved H2O threshold for continuous carbonation in an ex situ magic angle spinning (MAS) nuclear magnetic resonance (NMR) study of forsterite exposed to wet scCO2 at 80 °C and 76 bar. They suggested that below the threshold, water is a reactant in limited supply that becomes consumed as a hydrated magnesium carbonate solid phase, which stops carbonation. Above the threshold, H2O has an apparent catalytic role in the formation of magnesite, supporting continuous carbonation. Kerisit et al. (2012)16 used molecular dynamics (MD) simulations to show that H2O readily displaces CO2 at the forsterite surface and that adsorbed H2O amounts up to three monolayers thick are energetically favorable. Schaef et al. (2012)17 used in situ X-ray diffraction (XRD) to demonstrate both hydrated and anhydrous magnesium carbonate precipitation during the reaction of forsterite with wet scCO2 at 50 °C and 90 bar. They showed that the relative amounts of each carbonate product depended on the concentration of dissolved H2O in the scCO2. Felmy et al. in 2012,18 and later Qafoku et al. in 2014,19 followed forsterite reactivity in H2O-saturated scCO2 at 35−80 °C and 92 bar using ex situ techniques including scanning electron microscopy (SEM). They documented for the first time magnesite formation at temperatures as low as 35 °C and suggested that it precipitated homogeneously within adsorbed H2O layers. Thompson et al. (2013)20 studied forsterite carbonation at 35 °C in wet scCO2 (100 bar) using in situ IR spectroscopy. They observed the dissolved H2O concentration threshold at this condition and reported for the first time evidence of the bicarbonate anion dissolved in the adsorbed H2O film. While forsterite continues to be a useful case-study mineral, the processes controlling carbonation in wet-scCO2 are likely similar over a wider range of GCS relevant phases that are rich in divalent cations and suitable for mineral trapping. For
2. EXPERIMENTAL SECTION 2.1. Materials, Preparation and Characterization. Synthetic forsterite was prepared using the procedure of Saberi et al. (2007).25 Calcination was carried out in a platinum crucible at 850 °C for 6.5 h. The Brunauer−Emmett−Teller (BET) surface area was 26.7 ± 0.1 m2/g. The IR spectrum of this forsterite is similar to that reported in Loring et al. (2011). XRD analysis confirmed the presence of crystalline forsterite, along with a small amount (
