Article pubs.acs.org/est
Forsterite [Mg2SiO4)] Carbonation in Wet Supercritical CO2: An in Situ High-Pressure X-ray Diffraction Study Herbert Todd Schaef,* Bernard P. McGrail, John L. Loring, Mark E. Bowden, Bruce W. Arey, and Kevin M. Rosso Pacific Northwest National Laboratory, Richland, Washington 99352, United States S Supporting Information *
ABSTRACT: Mechanisms controlling mineral stabilities in contact with injected supercritical fluids containing water are relatively unknown. In this paper, we discuss carbonation reactions occurring with forsterite (Mg2SiO4) exposed to variably wet supercritical CO2 (scCO2). Transformation reactions were tracked by in situ high-pressure X-ray diffraction in the presence of scCO2 containing dissolved water. Under modest pressures (90 bar) and temperatures (50 °C), scCO2 saturated with water converted >70 wt % forsterite to a hydrated magnesium carbonate, nesquehonite (MgCO3·3H2O), and magnesite (MgCO3) after 72 h. However, comparable tests with scCO2 at only partial water saturation showed a faster carbonation rate but significantly less nesquehonite formation and no evidence of the anhydrous form (MgCO3). The presence and properties of a thin water film, observed by in situ infrared (IR) spectroscopy and with isotopically labeled oxygen (18O), appears to be critical for this silicate mineral to carbonate in low water environments. The carbonation products formed demonstrated by temperature and water-content dependence highlights the importance of these kinds of studies to enable better predictions of the long-term fate of geologically stored CO2.
150 °C and 150 bar in the presence of either pure water or simulated brines. Through varying particle size and solution to solid ratios, they demonstrated precipitation of magnesite as high as 57 wt %; amorphous silica was also observed to precipitate. Others, including King et al..5 have observed the precipitation of hydrated magnesium carbonates such as hydromagnesite (Mg5(CO3)4(OH)2·4H2O) along with the anhydrous form, magnesite, when reacting natural olivine with carbonated salt solutions at 200 °C. Daval et al.6 pointed out that a number of aqueous-based dissolution experiments are conducted at temperatures >100 °C, which are above thermal limits considered relevant to the majority of geologic storage sites for CO2. However, there are some instances, such as with Giammar et al.,7 where carbonation experiments were conducted below this threshold.
1. INTRODUCTION Global warming is viewed by many as an anthropogenicdominated phenomenon that can be mitigated through a combination of conservation efforts, alternative energy sources, and the development of technologies capable of managing CO2 emitted from the burning of fossil fuels. One option receiving attention is long-term geologic storage of CO2 in deep basalt formations. Due in part to their abundance of carbonateforming metal cations, basalts have the reactive potential to permanently trap CO 2 as stable carbonate minerals. 1 Experimental studies reacting basaltic rocks with water and CO2 to form carbonate minerals have provided a basis to conduct field demonstrations in the United States and Iceland.2,3 Most studies on basalts in the context of geologic sequestration of CO2 have focused on reactions between CO2-equilibrated aqueous solutions and discrete mineral components with the highest mineral trapping potential, such as forsterite (Mg2SiO4). For example, Garcia et al.4 reported dissolution of olivine grains (20−80 μm) and subsequent precipitation of magnesite (MgCO3) in closed-batch reactors at © 2012 American Chemical Society
Special Issue: Carbon Sequestration Received: Revised: Accepted: Published: 174
March 22, 2012 May 17, 2012 May 21, 2012 May 21, 2012 dx.doi.org/10.1021/es301126f | Environ. Sci. Technol. 2013, 47, 174−181
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Table 1. Experimental Parameters Used during the in Situ High-Pressure X-ray Diffraction Study Reacting Wet scCO2 with Forsterite carbonate quantification experiment
mass (mg)
CO2 (g)
temp (°C)
#1 #2 #3 #4 #5
3.9 4.6 5.1 4.3 3.9
2.6 2.1 2.6 2.9 2.6
EXP #6 EXP #7
4.5 5.8
1.6 1.6
EXP EXP EXP EXP EXP
a
H2O (μL)
% sat
pressure (bar)
duration (hours)
carbonate (XRD)
wt% TGA-MS
wt% XRD
50 50 50 50 50
1.0a 2.5a 4a 50
0 33 67 82 1185b
90 92 90 101 90
72 72 72 72 74
3 10 19 36 70c
75 75
2.5a 20a
47 319
91 90
161 71
none none nesquehonite nesquehonite magnesite nesquehonite magnesite magnesite
0 0 6 18 18 50 20 77
14 72
H218O. bAbove that which is required to saturate CO2 at 50 °C and 90 bar. cThis value represents a combination of magnesite and nesquehonite.
with the host formation and caprock, eventually becoming a water-bearing scCO2 phase. Mineral stabilities under these conditions are typically unknown. Researchers are beginning to focus on the more poorly understood reactivity of waterbearing scCO2 with minerals, and the potential impacts to reservoir and caprock systems.10,12−20 Loring et al.14 used in situ infrared (IR) spectroscopy to examine synthetic forsterite during exposure to dry and wet scCO2. Their material, calcinated at 850 °C, is also described in Chen and Navrotsky.21 Under dry conditions, the in situ IR technique identified forsterite and no reaction products. Moreover, Loring et al.14 observe carbonation in combination with a thin water film on forsterite grains when the scCO2 contained 55% of the water needed for saturation. Kwak et al.16 report crystalline hydrated/hydroxylated Mg-carbonates on the surface of their forsterite grains when scCO2 was under saturated with water (74%), indirect evidence of water forming a thin film. Continued development of hydrated carbonate phases appears to occur only when enough water exists to maintain a thin water film. In some instances, as discussed by White et al.,22 the intermediate hydrated carbonates transform into anhydrous forms, releasing structural water for further reactions. These studies have thus put forth the concept that a minimal concentration of dissolved water in the scCO2 is required before carbonation proceeds. Similarly, McGrail et al.17 describe a moisture threshold for steel corrosion in the presence of wet scCO2. Their experimental work showed reactions in liquid CO2 are limited when water concentrations fell below 600 ppm. In this work, we use a newly developed high-pressure X-ray diffraction (HXRD) capability that allows in situ characterization of forsterite carbonation at reservoir conditions. High surface area chemically pure synthetic forsterite was selected to better understand reaction mechanisms and the role water plays in the carbonation process. Oxygen-18 labeled water (H218O) was used in these experiments as a chemical probe for use in determining reaction processes related to wet scCO2.
Their work shows magnesite formation occurring in conjunction with forsterite dissolution in static batch tests with moderate CO2 pressures (1−100 bar) at 95 °C. More recently, Felmy et al.8 successfully demonstrated coprecipitation of magnesite, nesquehonite, and the formation of an amorphous Si rich layer during the carbonation of synthetic forsterite at 35 and 50 °C in the presence of scCO2 (100 bar) and free water. A similar amorphous layer was reported by Giammar et al.7 at 35 °C, but in the absence of any detectable crystalline carbonate. An overall reaction for the carbonation of forsterite in an aqueous-dominated system can be written as follows:7 Mg 2SiO4 (s) + 2HCO3−(aq) + 2H+(aq) ↔ 2MgCO3(s) + H4SiO4 (aq)
(1)
Here, CO2 dissolution, carbonic acid formation, and deprotonation are implied. However, this reaction is typically considered a multiple-step process involving the precipitation of intermediate hydrated Mg carbonate phases as in either of the following reactions: Mg 2SiO4 (s) + 2HCO3−(aq) + 2H+(aq) + 3H 2O(l) ↔ 2MgCO3 · 3H 2O(s) + H4SiO4 (aq)
(2)
5Mg 2SiO4 (s) + 8HCO3−(aq) + 8H+(aq) + 12H 2O(l) ↔ 2Mg5(CO3)4 (OH)2 · 4H 2O(s) + 5H4SiO4 (aq)
(3)
In the preceding examples, nesquehonite (MgCO3·3H2O, eq 2) or hydromagnesite (Mg5(CO3)4(OH)2·4H2O, eq 3) are precipitated, but numerous other forms of hydrated magnesium carbonates exist.9 These hydrated phases often transition into the anhydrous form magnesite (MgCO3) above ∼60 °C, according to the following: MgCO3 · 3H 2O(s) ↔ MgCO3(s) + 3H 2O(l)
(4)
Mg5(CO3)4 (OH)2 · 4H 2O(s) + HCO3−(aq) + H+(aq) ↔ 5MgCO3(s) + 6H 2O(l)
2. MATERIALS AND METHODS Synthetic forsterite used in this study was obtained from Chen and Navrotksy21 and has a calcination temperature of 800 °C and a Brunauer−Emmett−Teller (BET) surface area of 31.3 m2/g. Based on thermogravimetric analysis, the material contains 0.44 mols of H2O per mole of Mg2SiO4. This amount of water adsorbed on the sample would contribute less than 10 ppmw water to the CO2 under our test conditions. Surface characterization by X-ray photoelectron spectroscopy (XPS) indicates a less than pure crystalline material, with evidence of a
(5)
Reactions shown above rely on traditional aqueous mediated systems involving mineral dissolution and carbonate precipitation. However, under actual geologic storage conditions, most of the rock will be exposed to water-bearing scCO2 where nonaqueous-dominated types of mineral dissolution and precipitation reactions should be considered.10 Gaus11 pointed out that injected CO2 will reside as a buoyant fluid in contact 175
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thin silica-poor coating. Characterization by TGA-MS indicated a weight loss of 2.93% between 25 and 600 °C, with 1.47% weight loss occurring before 300 °C. Additionally, a small amount of CO2 was observed through an increase in ion current (m/z) of mass 44 between 300 and 400 °C, indicating a trace of carbonate in the sample. Experiments were conducted using a high-pressure reactor semitransparent to X-rays, which is described in detail by Schaef et al.,20,23 and the experimental procedures (sample loading, reactor pressurization, data acquisition, and processing) are summarized in the Supporting Information (SI). The apparatus and procedure for conducting in situ high-pressure infrared spectroscopy (IR) measurements of mineral carbonation reactions has been described in detail previously14 and is briefly summarized in the SI.
3. RESULTS Experiments were conducted at 50 and 75 °C, at between 90 and 101 bar, and with varying concentrations of water for extended periods of time. Testing conditions are listed in Table 1 and include the amounts of CO2, H216O, or H218O, forsterite, and the identification of secondary crystalline phases by in situ HXRD. Calculations used to determine water solubility in scCO2 were based on Spycher et al.24 Tests performed at 50 °C consisted of scCO2 that was 33% (EXP #2), 67% (EXP #3), or 82% (EXP #4) water saturated. EXP #5 was also conducted at 50 °C and included an amount of water corresponding to 1185% in excess of the amount required for saturation. Tests performed at 75 °C used scCO2 that was 47% (EXP #6) water saturated or fully saturated with a 300% excess (EXP # 7). Results are presented as graphs (three-dimensional or stacked X-ray tracings) and tabulated peak positions in ° 2θ. 3.1. 50 °C Experiments. Forsterite exposed to dry scCO2 (EXP #1) or scCO2 at 33% water saturation (EXP #2) at 92 bar, 50 °C, and after 72 h showed no detectable carbonation reaction by in situ HXRD (Table 1). Depending on peak positions and types of carbonation, detection limits on the order of 5 wt % can be expected to occur with this technique. Peak profiles and intensities remained constant with no variations throughout the tests, and observed reflections were assigned only to forsterite, based on PDF #7-0074. In contrast, forsterite reacted at 50 °C with scCO2 at 67% water saturation at 90 bar (EXP #3), 82% H218O saturation at 101 bar (EXP #4), and 1185% excess water required for saturation at 90 bar (EXP #5) resulted in secondary crystalline-phase products. Data from these experiments are presented in Figure 1 as threedimensional graphs of in situ HXRD patterns depicting relative peak intensities collected over time. Reflections assigned to forsterite are clearly visible after pressurization (time = 0). The data from EXP #3 (Figure 1a) show that initial exposure to scCO2 and H218O results in the immediate appearance of new peaks on the X-ray tracings at 17.380, 27.619, and 34.359° 2θ. These reflections match nesquehonite (PDF # 20-0669), a hydrated magnesium carbonate (MgCO3·3H2O). Comparison of patterns collected at 3.5 and 72 h of testing were similar, indicating carbonate progression had discontinued early on in the testing. Quantitative analysis obtained from the final pattern indicates that ∼6 wt % nesquehonite precipitated during 72 h of reaction. The in situ HXRD results from EXP #4 (Figure 1b) at 82% water saturation also show reflections matching nesquehonite that increase in intensity with time. The maximum peak intensity growth occurs during the initial 36 h of testing. Also, reflections corresponding to forsterite decrease with time,
Figure 1. In situ HXRD graphs depicting conversion of forsterite into nesquehonite when exposed to scCO2 at 50° C: (a) 67% water saturation at 90 bar (EXP #3), (b) 82% water saturation at 101 bar (EXP #4), and (c) 1185% in excess of water saturation at 90 bar (EXP #5).
which signifies that this crystalline phase is dissolving. Quantitative analysis of the pattern collected at the end of the experiment after 72 h of reaction indicates a mixture of 82 wt % forsterite and 18 wt % nesquehonite. Furthermore, quantitative comparison of the results from EXP #3 (Figure 1a) and EXP #4 (Figure 1b) shows that increasing water saturation levels from 67% to 82%, respectively, resulted in a ∼3X increase in carbonate precipitation. In situ HXRD patterns collected during the exposure of forsterite with scCO2 with a 1185% excess in water required for saturation at 90 bar (EXP #5, Figure 1c) also revealed nesquehonite as the only crystalline-phase product. Increased exposure times produced more intense nesquehonite reflections, with the maximum peak intensity growth occurring during the first 36 h of reaction. Forsterite reflections were observed to decrease with time. During quantitative analysis of the HXRD patterns, the nesquehonite peak around 32°2θ begins to fit poorly at longer times, as if there were an additional contribution from another phase. Magnesite fit rather well, resulting in a combination of 50 wt % nesquehonite and 18 wt % magnesite. However, this is the only reliable magnesite peak in the scan range, and its overlap with a nesquehonite reflection prohibits unambiguous identification. 176
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Figure 2. SEM microphotographs of (a) unreacted synthetic forsterite, (b) microstructure of reacted forsterite (EXP #2, 33% saturation, 50 °C), (c) nesquehonite growth on surface of forsterite (EXP #3, 67% saturation, 50 °C), (d) nesquehonite rods (EXP #4, 82% saturation, 50 °C), (e) large nesquehonite crystals (EXP #5, 1185% excess water required for saturation, 50 °C), and (f) individual magnesite crystals (EXP #7, 319% saturation, 75 °C).
unreacted forsterite with the outer layers increasing in Si concentrations relative to Mg. Similar features were observed after exposure of forsterite to scCO2 at 82% water saturation (EXP #4, Figure 2d). Comparable layered structures were visible in the cross sections of these reacted grains as well (Figure S2b). Unlike the morphologies at the lower water concentrations, the nesquehonite precipitates that formed under conditions with excess water (1185% excess water required for saturation) appear porous and multisized, with some particles maintaining a thickness of 30 μm (Figure 2e). TGA-MS data for the forsterite sample that was reacted with scCO2 at 33% water saturation (EXP #2) indicate an overall mass loss of 10 wt % between 55 and 442 °C, with less than half of the loss occurring after 270 °C. There were no detectable releases of H216O or H218O based on the mass to charge ratios (m/z) at 18 and 20, respectively. Two separate releases of m/z = 44 due to CO2 were observed starting at 300 °C, peaking at ∼330 °C, and again at 410 °C, which likely indicates thermal decomposition of an amorphous carbonate phase.26 The TGAMS results are presented in Figure S1 for forsterite that was
SEM images of un- and postreacted forsterite are shown in Figure 2 and illustrate relevant solid-phase morphologies of carbonation as a function of water content. The microstructure of unreacted forsterite consists of tiny spheres measuring ∼100 nm (Figure 2a). After exposure of forsterite to scCO2 at 33% water saturation (EXP #2), the forsterite microstructure evolves into much larger elongated structures (Figure 2b). Increasing the amount of water dissolved into scCO2 to 67% saturation (EXP #3) produced well developed crystals consistent with nesquehonite.20,25 Examples of surface coatings are shown in Figure 2c, where well-formed needle-shaped crystals are clearly visible. These precipitates were typically 1−2 μm long and randomly distributed on the forsterite grains. The nesquehonite crystals often appeared as if originating from within the forsterite grain, with growth typically occurring along the b-axis. Cross sectioning areas of reacted grains absent of visible carbonate growth with the FIB-SEM technique revealed the presence of a layered structure (Figure S2a). Although difficult to obtain chemistry on these small regions, limited EDX analysis indicated an interior Mg and Si ratio typical of 177
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and/or carbonation products.14 A band at 1432 cm−1 and a shoulder at 1520 cm−1 are due to the ν3 asymmetric C−O stretching mode of a magnesium carbonate precipitate.14 In the spectrum collected at the end of the experiment after the water was removed from the supercritical fluid, the intensities of the O−H stretching and HOH bending modes are decreased by about 50% due to the desorption of the thin-film water. The fact that these bands do not completely disappear after the system was dried is evidence that the precipitated magnesium carbonate contains structural waters. Likewise, based on the shapes and wavenumber positions of the ν3 asymmetric C−O stretching bands,30 the precipitate contains similar traits associated with a carbonate solid having a stoichiometry analogous to nesquehonite. Coupled with TGA-MS results, there is strong evidence for the presence of an amorphous hydrated carbonate phase. 3.2. 75 °C Experiments. In situ HXRD was used to follow the reaction of forsterite with scCO2 at 75 °C, 91 bar, and 47% water saturation (EXP #6) for 161 h. Magnesite is an expected carbonation product at this experimental temperature.9 However, its carbonate mineral’s primary reflection is at 32.631° 2θ, which overlaps with the 32.316° 2θ reflection for forsterite. It is thus difficult to verify minor amounts of magnesite formation based on in situ HXRD alone. Still, by the end of the experiment, it was apparent that a shoulder had developed on the forsterite 32.316° 2θ reflection, attributed to magnesite precipitation. Following depressurization, thermal decomposition of the postreacted sample indicated a mass loss of 7.4% coinciding with an increase in m/z = 44 due to CO2 between 380 and 480 °C. Over the same temperature interval, pure magnesite has a mass loss of 51.9 wt %,31 which is similar to 50.39 wt % obtained from a crushed magnesite standard analyzed in our laboratory. Based on these TGA-MS results, we estimate that the postreacted forsterite contained 14 wt % magnesite. In EXP #7, forsterite was reacted for 71 h with wet scCO2, also at 75 °C and 90 bar, but with three times more water used than required to saturate the scCO2. The in situ HXRD results for this experiment are shown in Figure S4. Immediately following pressurization, all observable reflections were assignable to forsterite. However, after 6 h of reaction, a low intensity reflection appeared on the HXRD tracings positioned at 15.360° 2θ, matching the 100% reflection for hydromagnesite, Mg(CO3)4(OH)2·4H2O, (PDF# 25-0513). This weak reflection remained visible during the next 4 h of testing before disappearing into the background. Throughout the duration of the experiment, the forsterite reflections at 17.400, 22.940, 23.920, 36.600, and 29.880° 2θ decreased in intensity, whereas reflections at 25.560 and 32.400° 2θ strengthened with time. These latter two reflections are positioned closely to peaks for magnesite, PDF 8-0479. Furthermore, the dominant forsterite reflections (22.884, 35.714, and 36.526° 2θ) were not detected during the later ∼20 h of the experiment, indicating significant forsterite dissolution had occurred. The pattern collected after 71 h of reaction was strikingly different compared to earlier patterns; reflections positioned at 32.560, 35.800, and 42.995° 2θ were easily visible and corresponded to magnesite. Depressurization allowed for a more detailed characterization of the postreacted sample from EXP #7 by XRD. The resulting pattern clearly indicates a broad amorphous hump between 16 and 28° 2θ, assignable to SiO2, an expected product from forsterite dissolution.5 Intense reflections positioned at 32.6,
reacted at 50 °C with scCO2 at 67% water saturation (EXP #3), 82% water saturation (EXP #4), and 1185% excess water required for saturation (EXP #5). In contrast to the data from EXP #2 at 33% water saturation, the results at the higher water concentrations where nesquehonite was identified by in situ HXRD show distinct weight loss steps corresponding to the dehydration of nesquehonite. During decomposition (50 to 270 °C), pure nesquehonite experiences ∼34.81% decrease in weight due to H2O loss, followed by an additional ∼36.31% decrease in weight due to the release of CO2 starting after ∼300 °C.27 In the TGA-MS results, mass changes related to water loss (55−270 °C) ranged from a low of 6.6 wt % (EXP #3) to a high of 24.2 wt % (EXP #5), while the changes associated with CO2 loss (>300 °C) were similar and ranged from 7.0% (EXP #3) to 21.3% (EXP #5). Based on a sharp increase in ion current (m/z), the thermal decomposition of the reacted forsterite begins around 55 °C with a sharp increase in m/z = 18 due to H216O, peaking at ∼95 °C and to a lesser extent at 128 °C. A small inflection in mass 18, most pronounced for forsterite reacted with scCO2 with a 1185% excess of water required for saturation (EXP #5), is visible at 190 °C prior to reaching steady-state conditions that are generally maintained at higher temperatures. Similar, although much less intense, are increases in the baseline for mass 20 (indicating H218O) observed for samples reacted at 67% and 82% water saturation conditions (EXP #3 and EXP #4, respectively). These small releases at 50 and 95 °C are attributed to the presence of H218O within the nesquehonite structure. The ratio obtained for H218O and H216O (m/z = 20 and m/z = 18, respectively), between the temperature interval 50 and 140 °C, was 10× higher for EXP #3 (0.5) than the value obtained from EXP #4. Insignificant changes in the baseline of the ion current (m/z) at 18 and 20 were observed between 300 and 850 °C, indicating no further release of H216O or H218O. Inflections for m/z = 44 corresponded to a bimodal release of CO2 (C16O16O) from the nesquehonite structure starting at ∼320 °C, with maxima occurring at 420 °C and to a lesser extent at 475 °C. At identical temperatures, slight baseline elevations were observed for m/z = 46 due to C16O18O for forsterite reacted with scCO2 at 67% and 82% H218O saturation conditions (EXP #3 and EXP #4, respectively). Based on the integrated peak area between 320 and 440 °C, m/z = 46 was ∼3% for EXP #3 of the m/z = 44 peak. It is noteworthy that the percentage for EXP #4 (∼1.5%) was similar to that for the forsterite sample reacted with scCO2 at 1185% excess water conditions in EXP #5, where H216O was used. No detectable releases of C16O16O or C16O18O occurred at temperatures above 520 °C in any of the reacted samples. An in situ IR spectroscopic experiment was conducted to follow the reaction of forsterite with scCO2 at 50 °C, 100 bar, and 30% water saturation.14 Spectral data collected every 3 h for 24 h are presented in Figure S3. The negative-going bands at about 1052 and 912 cm−1 are due to the SiO stretching modes of forsterite 28 and indicate the dissolution of the mineral. Based on the decrease in absorbance of these bands, ∼2% of the forsterite transformed into a carbonate phase and amorphous SiO2. The broad peak centered at about 1052 cm−1 is assigned to the SiO stretching modes of a highly polymerized silicate phase,29 consistent with the precipitation of amorphous SiO2. A broad O−H stretching band at 3380 cm−1 and a shoulder due to a HOH bending mode at 1640 cm−1 indicates the presence of thin liquid-like water films on the forsterite reactant 178
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35.8, and 43.0° 2θ are matches for magnesite (PDF #8-0479). Minor reflections consistent with forsterite were still detectable in the unpressurized sample, an indication of incomplete carbonation. Particle morphology, obtained by SEM, revealed the carbonation in EXP #7 had formed large rounded structures, measuring 10−50 μm in diameter. Closer examination shows these structures were composed of small rhombohedrals, a characteristic crystal habit of magnesite. Wellformed magnesite particles, measuring around 0.5 μm appeared as discrete particles as seen in Figure 2f. Thermal decomposition of the postreacted sample over the temperature interval 100 to 850 °C resulted in a mass loss equal to 37.26 wt %. Evolution of CO2 gas (m/z = 44) began at 380 °C, peaked at 520 °C, and was complete by 580 °C. Additionally, small concentrations of C16O18O (m/z = 46), equal to ∼8% of the C16O2 (m/z = 44) mass, began evolving at 450 °C and ending at 530 °C. Dehydration of the sample was negligible, as indicated through minimal changes in the baseline of m/z = 18 and m/z = 20 (H216O and H218O, respectively), with slightly elevated readings occurring in unison during the peak degassing as observed with m/z = 44.
to scCO2 and free water at 80 °C and 97 bar. Felmy et al.8 studied a synthetic forsterite in contact with a free water phase and scCO2 at 35 °C and 100 bar and identified a mixture of magnesite and nesquehonite as carbonation products. Additionally in this latter study, NMR characterization of the associated amorphous silicate phase indicated it was porous and therefore did not exhibit passivating traits. Both of these recent studies suggest unique interactions between the forsterite surface, scCO2, and water that promotes carbonation. One plausible reaction mechanism involves the formation of water films on the hydrophilic surface of forsterite that are thicker as the water concentrations in the scCO2 increase.14,16 Molecular dynamic simulations of scCO2 in contact with excess water required for saturation predict the formation of a water film (>2 nm) on the forsterite surface that contains properties similar to bulk water.32 In fact, the large welldeveloped prismatic needles of nesquehonite (EXP #5) are similar to those obtained from direct precipitation form aqueous solutions (Figure 2e).9,33,34 Likewise, the perfectly formed magnesite crystals (EXP #7) appear as clusters or mounds deposited on the forsterite surface, features consistent with direct precipitation from solution. Therefore, based on our experimental observations and MD simulations by Kerisit et al.,32 we believe that carbonation in our experiments with a free water phase occur within a thick bulk-like water film associated with the forsterite surface. Intermediate phases are commonly observed during magnesium carbonate precipitation, where hydrated phases are kinetically favored although they are thermodynamically unstable. Mechanisms controlling the transformation of hydrated compounds to energetically preferred magnesite include shrinkage (dehydration) and dissolution−precipitation solvent-mediated transformation.9 In both instances, the hydration properties of Mg2+ are considered the dominate factor. Felmy et al.8 propose a reduction in Mg2+ dehydration or more kinetically favorable conditions for the transformation occurring in low water environments. Given that there are only a few instances in the literature reporting magnesite precipitation at temperatures lower than 80 °C,20,35 these findings are significant and suggest reactions occurring between hydrated mineral surfaces and scCO2 are not well understood. In comparison with the reaction of forsterite with scCO2 at water concentrations below saturation produce less carbonation overall, exclusively nesquehonite at 50 °C, and exclusively magnesite at 75 °C. At 50 °C, reactions at 67% (EXP #3) and 82% (EXP #4) water saturations produce nesquehonite at ∼6 wt % and ∼18 wt %, respectively, whereas magnesite was not detected. At 75 °C and 47% water saturation (EXP #6), magnesite comprised 20 wt % of the reacted sample, nearly 4× less than observed with the oversaturated experiment. Crosssectional imaging of reacted forsterite grains from EXP #3 and EXP #4 (Figure S2) revealed chemically distinct zones with the outmost layer enriched in silica. Consistent with observations by others,4,6 it appears the formation of a passivating silica rich layer is limiting carbonate precipitation in these low water containing experiments. Further limitations in water concentrations (EXP #2, 33% water saturation) produced X-ray tracings with no obvious indicators of mineral dissolution (forsterite) or carbonate precipitation. Still, this does not exclude the possibility that a carbonation reaction occurred, since a carbonate precipitate under these conditions could be amorphous or the amount of a crystalline carbonate phase could be below the HXRD
4. DISCUSSION Our experimental work has shown that the carbonation reaction of forsterite with water-containing scCO2 often involves reaction intermediates, results in variable carbonate products, and increases in rate and extent with increasing water concentration (Figure 3). Carbonation rates are fastest in the
Figure 3. Forsterite carbonation at 50 °C and 90−101 bar when exposed to scCO2 containing water at levels below and above that which is required to saturate the scCO2.
presence of a free water phase, with reaction products consisting of nesquehonite and magnesite at 50 °C (EXP #5) or exclusively magnesite at 75 °C (EXP #7). At 50 °C, the rate of nesquehonite precipitation is initially fast but appears to plateau near ∼50 h and coincides with an uptick in the rate of magnesite formation, suggesting nesquehonite is an intermediate under these conditions and transforms into magnesite by dehydration.9 At 75 °C (EXP #7), an intermediate hydromagnesite phase was identified in the in situ HXRD tracings, but the ultimate carbonation product was magnesite. Previous studies have identified nesquehonite and magnesite as carbonation products, as well as reaction intermediates, in the reaction of forsterite with scCO2 and a free water phase. Kwak et al.13 identified dypingite (Mg5(CO3)4(OH)2·5H2O) by NMR as a transitional step when exposing natural forsterite 179
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detection limit. In fact, in (EXP #2), carbonation was observed through a bimodal loss of CO2 in TGA-MS data, although the temperature intervals for these mass losses were inconsistent with nesquehonite. Based on in situ IR spectroscopic measurements at nearly identical conditions (Figure S3), evidence of forsterite dissolution and both carbonate and amorphous silica formation were observed. It is also important to note amorphous layers similar to these are observed during carbonation of silicate minerals in an aqueous-dominated system that are passivating.4,6 The existence of a thin film of water on the forsterite surface in contact with scCO2 that is under-saturated with water (30%) is supported by our in situ IR spectroscopic experiment. To better understand how the presence of water films impact carbonation, we utilized oxygen-18 (18O) enriched water to track water film formation and behavior. Based on oxygen fractionation studies by Rosenbaum, 36 oxygen isotope exchanges are rapid between aqueous CO2 and a free water phase. However, due to the lack of a polarizable fluid medium in which ion exchange is facile, oxygen isotope exchange is relatively slow for water that is dissolved in CO2 (i.e., no free water phase). Hence, if a liquid-like water film forms on the surface prior to carbonation, then the O18 in the water will quickly exchange with O16 from the scCO2 fluid and not be concentrated in a carbonate phase during slow precipitation. 18 O enriched water was used in our previous study on brucite reactivity with wet scCO2, where 18O was found incorporated into precipitated carbonates, which suggests that water films are not formed on this reactive mineral. A similar approach was used to establish the existence and characteristics of a water film on forsterite grains exposed to wet scCO2 (under saturated with H218O). As shown by thermal decomposition of samples from EXP #3 (Figure S1a), and to a lesser extent in EXP #4, 18O is retained in the precipitate as H218O. Furthermore, 18O only appears in the carbonate product from EXP #3. In contrast to IR experimental results, these observations suggest the absence of water films on the forsterite surface. However, it is plausible water films are present that are insufficiently thick to exhibit bulk water properties or facilitate isotopic exchanges as suggested by Kwak et al.16 Molecular dynamic simulations by Kerisit et al.32 show the formation of 1/ 4 and 1/2 monolayers of water on the hydrophilic forsterite surface in the presence of scCO2 under saturated with H2O. When the fluid is 100% saturated, thicker water films form (∼2 nm) and access to the mineral surface by CO2 is possibly limited. Likewise, MD simulations of scCO2 and water mixtures indicate water cluster formation within the scCO2 is energetically favorable and ultimately leads to phase separation at high H2O concentrations.37 One reaction path would include dissolution of CO2 into the H218O film to form H2C18O16O2 allowing carbonation reactions to proceed while incorporating the 18O within the nesquehonite structure. As dissolved water concentrations increase, thicker water films develop on the forsterite surface accelerating isotopic exchanges between the 18 O in the water film and the C16O2 in the gas phase through the development of HC18O16O2−. This is consistent with TGAMS data, where 10× the amount of 18O in the form of structural water was measured in EXP #3 compared to EXP #4. Additionally, the rapid carbonation observed in the under saturated experiments can be contributed to quicker diffusion of CO2 into the water film compared to slower diffusion kinetics expected with thicker water layers.
5. ENVIRONMENTAL IMPLICATIONS Through utilization of high surface area synthetic forsterite, new insights into mechanisms controlling silicate mineral stability during exposure to water-bearing scCO2 fluids were observed. Experiments conducted under reservoir conditions and laboratory time scales clearly illustrate the importance of water on silicate carbonation processes and products. Whereas the formation of both anhydrous and hydrated carbonates was significant, the broader reaching implications are associated with overall silicate mineral stabilities as related to the host formation, pore water contents, and maintaining cap rock seal integrity. This research shows the importance for the inclusion of supercritical geochemistry processes into predictive models for the long-term fate and subsurface storage of CO2.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information on the experimental procedures for high-pressure XRD, high-pressure IR, FIB-SEM, and TGA-MS analysis; graphs of TGA-MS data for EXP #3, EXP #4, and EXP #5; imaged cross sections of reacted grains from EXP #3 and EXP #4; stacked HXRD graphs collected during EXP #7 (75 °C and 90 bar). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel: 509-371-7102; fax: 509-371-6354; e-mail: Todd.schaef@ pnl.gov; mail: Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box 999, MSIN K6-81, Richland, WA 99352 USA. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Carbon Sequestration Initiative, a Laboratory Directed Research and Development program at Pacific Northwest National Laboratory (PNNL), and the U.S. Department of Energy Office of Fossil Energy. Part of this work was performed at EMSL, a national scientific user facility at PNNL that is managed by the DOE’s office of Biological and Environmental Research. PNNL is operated for DOE by Battelle Memorial Institute under Contract DE-AC0676RLO-1830.
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REFERENCES
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