CaCO3 Precipitation and Polymorph Forms During CO2 Sequestration

Article pubs.acs.org/crystal

CaCO3 Precipitation and Polymorph Forms During CO2 Sequestration from the Atmosphere: Effects of the Basic Buffer Components Shichoon Lee,† Dong Hun Sin,‡ and Kilwon Cho*,‡ †

Department of Materials Science and Engineering, Jungwon University, Goesan 367-805, Republic of Korea Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea

‡

S Supporting Information *

ABSTRACT: CO2 sequestration and polymorph selection was achieved by CaCO3 precipitation via the reaction of calcium ions and atmospheric CO2 in a basic buffer, in a process that mimicked geological sedimentation. Precipitation proceeded in yield exceeding 80% in the presence of basic buffers at room temperature over 10 h. Calcite formed mainly during the early stages of precipitation, within less than 5 h, followed by needle-like aragonite precipitation between 5 and 10 h of aging. The aragonite polymorph selection increased in the presence of carbonic anhydrase and at high solution temperatures. We found that the deposited CaCO3 polymorphs depended on the rate of calcium ion consumption and precipitation as well as the ionic strength of the basic buffer and the solution pH. We developed a method for depositing high-purity aragonitic CaCO3 crystals in solutions with temperatures exceeding 60 °C in the presence of basic buffer, using CO2 from the atmosphere without the need for seed crystals or metal ions.

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INTRODUCTION Among several CO2 capture and storage systems (CCS), largescale ocean storage and geological disposal have been highly considered; however, leakage of the stored CO2 over years, with its attendant influence on ocean ecosystems, remains a concern, and the environmental risks associated with geological storage have not yet been examined.1 CO2 capture through CaCO3 mineral carbonation is one of the most reliable and environmentally safe long-term storage methods.2 Some dissolved CO2 reacts with calcium ions, mainly in the sea, to form CaCO3. Aqueous CO2 reacts with water to dissociate to bicarbonate ions, and the reaction proceeds further to form carbonate ions, CO32−, at high pH. The carbonate ions generated in an aqueous solution containing calcium ions precipitate as CaCO3. A high solution pH is a critical requirement for a high precipitation rate. Bases are indispensable to the formation of CaCO3 in solution by enhancing the generation of carbonate ions. Corals have specific transport mechanisms that increase the pH within a calcification cite during the biomineralization process.3,4 Ammonia gas, ammonium compounds,5−8 and an alkali solution9−13 are often added to control the solution pH during laboratory calcification experiments. Carbonate sources are typically added in the form of CO2 gas5,7,8 or a solution containing bicarbonate,9−13 and the systems are closed, thereby preventing the exchange of CO2 with the atmosphere. Some studies have examined the effects of the compositions and operating variables on the process of CaCO3 precipitation.11−13 Some studies have focused on enhancing the precipitation rate of CaCO3 through the use of biomimetic methods involving carbonic anhydrase (CA),14−16 an enzyme that is abundant in organisms and catalyzes the hydration and dehydration reaction of CO2 with water.17 CA increases the © 2015 American Chemical Society

hydration rate of CO2 and leads to rapid CaCO3 precipitation in solutions containing calcium ions. Various calcite and aragonite polymorphs frequently appear in organisms.18−20 Several factors, such as solutions, proteins, and additives on the polymorph selection of CaCO3 crystals, have been investigated through biomimetic routes.21−27 Diverse CaCO3 architectures are found in nature.28 Several systems were devised to emulate CaCO3 precipitation, and the effects of various operating variables, including the pH and temperature of a solution and the concentration of Ca2+ and carbonate ions, on the precipitation and polymorphs were examed.6−13,29−31 It is important to study the effects of operating variables including basic components in environments similar to those in which CaCO3 forms in nature. The solutions used in this study were exposed to air, and CaCO3 precipitation was induced by a reaction between calcium ions and carbonate ions derived from CO2 in the air. Here, we have characterized the extent of CaCO3 precipitation in solution under the conditions tested and the precipitate polymorphs obtained from different basic buffers and at different temperatures.

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EXPERIMENTAL SECTION

Materials and Chemical Reagents. The basic buffer contained 100 mM Tris(hydroxyethyl)aminomethane (Tris), 100 mM ammonium hydroxide (AMH), and 10 mg/mL polyethylenimine (PEI) with a molecular weight of 2000. PEI is a water-soluble and highly branched polymer with primary, secondary, and tertiary amine groups. The addition of 10 μg/mL bovine carbonic anhydrase (CA) from bovine Received: August 18, 2014 Revised: November 14, 2014 Published: January 5, 2015 610

DOI: 10.1021/cg501227z Cryst. Growth Des. 2015, 15, 610−616

Article

Crystal Growth & Design erythrocytes (Sigma-Aldrich) was found to affect the precipitation equilibrium. Buffer solutions were freshly prepared with Milli-Q water having a resistance of 18.2 MΩ. Preparation of CaCO3. The precipitation was achieved in a 10 cm Petri dish containing 50 mL of a 10 mM CaCl2 solution that had been covered by a plastic lid with seven 2-mm-diameter holes and then overlaid loosely with a paper tissue to avoid contamination. The precipitation reaction resulted from the reaction between calcium ions and CO2 dissolved from the solution/air interface under atmospheric conditions, at 25 ± 1 °C with stirring at 300 rpm. No CaCO3 seed crystals or carbonate sources were added to the solution. The precipitate solutions were subsequently decanted carefully, and the precipitates in the Petri dish were vacuum-dried and weighed. The amount of consumed calcium ions was measured using the ethylenediaminetetraacetic acid (EDTA) titration method (Supporting Information Figure 1). The aging time was defined as the time needed to stir the solution with a magnetic stirrer after the addition of all components. The pH change in each solution varied after precipitation as follows: 10.7 → 8.5 in 100 mM Tris buffer, 11.2 → 9.4 in 100 mM AMH buffer, and 10.3 → 9.5 in 10 mg/mL PEI buffer, after incubating for 24 h. The pH of the solution containing CA was reduced by 0.1 compared with the solutions containing no CA (Supporting Information Figure 2). The temperature-dependent precipitation experiments were carried out in a glass flask, and the temperature was controlled using a water circulator. The precipitated CaCO3 was deposited onto a silicon wafer cleaned with piranha solution (H2SO4/ H2O2 = 7/3) for further characterization. After precipitation, the wafer was removed, rinsed with water, and blown dry with nitrogen gas. Characterization. Field emission scanning electron microscopy (SEM, Hitachi S-4800) were operated at 3 kV operating voltage and 5 mm working distance. X-ray diffraction (XRD) studies, conducted over θ/2θ scans with Cu Kα 0.154 nm radiation and in the twodimensional (2D-XRD) mode (Pohang Accelerator Laboratory), were used to investigate the crystal structures of the precipitated CaCO3. Raman spectroscopy (Raman) was performed using helium neon with a laser wavelength of 532 nm (Reinshaw System 3000) capable of focusing on crystals of interest.

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Figure 1. CaCO3 precipitation was plotted to characterize the reaction between calcium ions in a CaCl2 solution and CO2 from the air, over differing aging times and in the presence of different basic buffers: (a) calcium ion consumption measured by titrating three solution samples and averaging the results; (b) CaCO3 precipitate was weighed, and the average values obtained from four tests; (c) calcium ion consumptions, varying temperature in the presence of PEI buffer.

RESULTS AND DISCUSSION Calcium carbonate was precipitated via a reaction between CO2, dissolved from the solution/air interface, with calcium ions present in solution. Figure 1a shows that the calcium ion consumption increased with aging and reached a maximum after 24 h aging. In the PEI buffered solution, 50 mg CaCO3 precipitated within 24 h aging time (Figure 1b), suggesting that nearly all calcium ions in the solution were consumed by the reaction with dissolved CO2. The greatest amount of CaCO3 precipitate was recovered from the PEI buffer. The precipitation yield was estimated by measuring the depletion of calcium ions from the solution using an EDTA titration method, as shown in Figure 1a. The two precipitation measurement methods yielded similar trends as a function of aging, although the EDTA method of measurement displayed a smaller range of error, as shown in Figure 1a and b. In the absence of basic buffer components, no precipitated crystals formed. These differences were attributed to the proton exchange capabilities of the buffer during CO2 hydration and the production of carbonate ions. Khalifah17,32 proposed a buffer-mediated protonation and deprotonation mechanism, and Kernohan33 recognized the importance of the inhibition or promotion functions of the buffer components during CO2 hydration. The protons in the basic buffer clearly promote the reactions that generate bicarbonate ions. The PEI-buffered solution, in particular, did not display a large drop in pH, indicating better proton exchange. The results obtained here indicated that the polymeric PEI, a well-known proton sponge

used as a reversible gellant material in the presence of CO2,34 provides a suitable basic buffer component for the precipitation of CaCO3. Calcium consumption increased rapidly in the presence of CA, particularly between the 5 and 10 h aging times, as shown in Figure 1a. The precipitate mass exceeded 40 mg in the PEI/ CA buffer, even after a 10 h aging time, corresponding to an 80% maximum precipitation yield, even though the ratio in the PEI buffer prepared without added CA was 55% as seen in Figure 1b. The mass of precipitate recovered from the AMH and Tris buffers upon the addition of CA was not as large as that recovered from the PEI buffer (Supporting Information Figure 3). These results indicated that even though CA increased the hydration reaction rate, the basic buffers mediated the dissociation of bicarbonate to form carbonate ions, and this mediation was essential for CaCO3 precipitation. The basic buffer-associated precipitation depended on the buffer species. Figure 1c shows the amounts of CaCO3 that precipitated in the presence of PEI buffer at various temperatures and aging times. At 60 °C the precipitation reaction reached completion within a 3 h aging time, whereas only about 30% of the calcium ions were consumed at 5 °C over a 24 h aging time. The higher 611


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Crystal Growth & Design

Figure 2. SEM images showing CaCO3 crystals precipitated at different aging times in 10 mM CaCl2 and basic buffers. The scale bar indicates 10 μm. (a) Tris, (b) AMH, (c) PEI, and (d) PEI/CA.

structures, as revealed in Supporting Information Figure 4b. In addition to the (104) face, most calcite crystals grown in the PEI buffer included another crystal growth plane (hkl), as reported in ref 27, suggesting that the polymeric basic buffer component PEI acted as a soluble additive during the crystal growth process and interacted with the specific face to modify the crystal morphology. Biominerals mainly possess acidic macromolecules that control the rate and direction of crystal growth.18,35 Interestingly, the basic polymeric buffer, PEI, also acted as an additive in the context of crystal growth in our work. Raman spectroscopy confirmed that the crystals formed in 100 mM Tris or in 100 mM AMH buffer displayed mixed morphologies consisting of rhombohedral calcite, characterized by peaks at 713 and 283 cm−1, and aragonite by peaks at 707 cm−1 (Figure 3a,b), respectively.36−38 Peaks at 707 and 207 cm−1 were obtained from CaCO3 crystals that formed in 10

solution temperature rapidly precipitated the CaCO3. Higher temperatures also increased the equilibrium constant for the reaction involving the consumption of bicarbonate to form carbonate (dissociation constant, Ka2),31 meaning that the generation of carbonate ions could be made more favorable at lower pH values by increasing the temperature. Raising solution temperature causes solubility of CO2 to be lower, but increases the association constant for the formation of solid CaCO3, KCaCO3, resulting in enhancing precipitation.31 In conclusion, the increased CaCO3 precipitation at higher temperatures can be mainly due to the promoted formation of CaCO3 rather than the increase in concentration of carbonate ions. The development of the precipitate morphology, in each buffer and for various aging times, was examined and is presented in Figure 2. Calcite morphologies dominated the early aging times, within 5 h, in all precipitates, and especially in the Tris buffer and AMH buffer. Needle-like aragonite polymorphs were found to overlay the calcite after 10 h aging, regardless of the buffers, as shown in Figure 2a−c. The morphological transition to a needle-like aragonite occurred between 5 and 10 h aging time. Mixed morphologies with the calcites on the bottom and the overlaid aragonites were definite. Aragonite needles formed in the presence of AMH buffer aggregated in 24 h aging time (Figure 2b-24h). Thick rosette aragonite appeared in the PEI buffer after 24 h aging time (Figure 2c). The crystals that formed in the Tris and AMH buffers assumed identical rhombohedral calcite morphologies. These crystals grew along the (104) plane (Supporting Information Figure 4a), similar to the growth patterns obtained from abiotic CaCO3 minerals grown without additives. The crystals grown in the PEI buffer, on the other hand, displayed distinct

Figure 3. Raman spectra of CaCO3 crystals deposited after a 10 h aging time in a 10 mM CaCl2 solution and the basic buffers. The large peaks at 522 cm−1 were attributed to the silicon wafer. (a) Tris, (b) AMH, (c) PEI, and (d) PEI/CA. 612


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Figure 4. CaCO3 crystal polymorphs, deposited after 5 and 10 h aging times in a 10 mM CaCl2 solution and in various basic buffers, were characterized using 2D-XRD and XRD techniques. The length of reciprocal lattice vector, q, displayed and crystal planes indexed on the 2Ddiffraction patterns. A and C denote aragonite and calcite, respectively. (a) Tris, (b) AMH, (c) PEI.

times were similar but more notable for calcite bands to those collected from the corresponding precipitates after 10 h (Supporting Information Figure 5). Some studies revealed that factors, such as temperature, solution pH, aging time, ionic strength, calcium ion concentration, the presence of additives, and the concentration of reagents, affected the morphology of the CaCO3 precipitate. Tai et al.13 used a constant composition technique in which the pH and composition were held constant in a solution containing CaCl2 and Na2CO3, and they reported that among several operating variables, the solution pH was the most crucial to the morphology of the precipitated CaCO3 at room temperature. The aragonite yield reached a maximum at pH 11, and was lower at lower pH values. Vaterite was dominant over the pH range 8.5−10, and calcite was dominant at pH values exceeding 12. In our study, the pH between 5 and 10 h ranged from 10.1 to 9.8 (PEI buffer), 10.2 to 9.7 (AMH buffer), and 9.6 to 9.4 (Tris buffer). Vaterite did not form in all buffers at room temperature under the experimental conditions. The solution pH was apparently not the decisive factor that determined the polymorphs that formed. The presence of additives can affect CaCO3 polymorphs, particularly additives such as magnesium ions, manganese ions,13 and PEI polymers,24,40 which promote the formation of aragonite. Our work suggested that the basic buffer components acted as additives that regulated the CaCO3 polymorphs. AMH and Tris buffer also produced aragonite polymorphs over a narrow range of aging times and

mg/mL PEI and PEI/CA buffers (Figure 3c,d) and were dominated by characteristic aragonite bands. The 2D-XRD patterns obtained from CaCO3 crystals that had been precipitated in different buffers over 5 and 10 h aging times indicated the presence of mixed polymorphs, as shown in Figure 4. The patterns are displayed on the length of reciprocal lattice vector, q, and each is indexed into the crystal plane. A calcite band corresponding to a (104) plane was apparent, even after aging for 5 h, whereas wide bands corresponding to aragonite crystals were obtained under all buffer conditions, particularly in crystals grown in PEI (Figure 4c), indicating the presence of amorphous components and small crystal domains.39 Calcite formation prevailed during the first 5 h aging. Aragonite bands were dominant by 10 h, and the bands became narrower, meaning the crystal domains became larger. The aragonite bands appeared connected and uniformly wide, whereas the calcite bands were discrete and scattered. These are attributable to a range of scattered rhombohedral shape of calcite and symmetric and fine acicular chrysanthemum crystals of the aragonite as seen in Figure 2. These results indicate that we can distinguish polymorphs of CaCO3 based on the 2DXRD patterns. Other definite calcite bands were observed among the crystals grown in the PEI buffer, corresponding to (110) and (113) planes, as well as to (104) and (102) planes, suggesting diverse crystal growth directions (Figure 4c). These findings are consistent with the SEM images shown in Figure 2 and XRD patterns as seen on the right side of Figure 4. The XRD patterns collected from the precipitates after 24 h aging 613


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PEI/CA buffer, without observing the calcite peak (Figure 5a). These results suggested that calcite dominated at the early

concentrations in the presence of atmospheric CO2 without the addition of metal ions. Calcite was found to be the major product at low concentrations of around 3 mM AMH and Tris buffer, as in the case of 0.3 mg/mL PEI buffer, suggesting that the buffer concentration influenced the polymorph (Supporting Information Figure 6). The ionic strength in these solutions may contribute to the appearance of aragonite polymorph at higher buffer concentrations over 10 h aging times. The ionic strength in a 100 mM AMH and Tris buffer is 0.12 M, based on the total calcium ion concentration. A 10 mg/mL PEI buffer has an ionic strength of 0.25 M, considering that the polyethylenimine polymer was composed of monomeric, amine-functionalized repeating units with a molecular weight of 42 g. Tai13 reported that the yield of aragonite increased as the ionic strength was raised to 0.11 M, and high-purity aragonite was obtained at higher ionic strengths. Aragonite formation was dominant in the PEI buffer due to the high ionic strength. Mixed polymorphs of calcite and aragonite were observed in all buffers at 10 h aging times. Although aragonite crystals were dominant in solutions of high ionic strength, it was difficult to form only aragonite at room temperature. The addition of CA to the PEI buffer induced the crystal morphologies to evolve differently, depending on the aging time (Figure 2d). The aragonite needles that formed in the PEI solution in the presence of CA appeared more dominant, even after 5 h aging times, as shown in Figure 2d. Aggregated aragonite morphology appeared after a 10 h aging time. Aragonite dominated the crystals formed during the early stages of aging in the PEI/CA buffer, unlike the morphology formed in the PEI-only buffer. The polymorphs and morphology were influenced by the rate at which the reactants were added because an induction period preceded the formation of the precipitate. Hu41 showed that the morphology could be changed from needle-like to chrysanthemum-like clusters by regulating the rate at which the carbonate source was added. Tai et al.13 reported that the aragonite yield could be tuned, depending on the ratio of calcium and carbonate ion concentrations. The formation of aragonite and the subsequent precipitation was enhanced by the rapid generation of CO32−. Reef corals in tropical shallow seawater experience strong wave agitation and tidal flows, which favor the formation of aragonite.28 Under our experimental conditions, the solutions were stirred and waves were produced, which favored the rapid conversion of inflowing CO2 into CO32−, thereby favoring the formation of aragonite polymorphs. In our study, CA increased the CO2 hydration rate during the early stages of aging, thereby increasing the carbonate ions and producing a higher proportion of aragonite crystals. In view of these factors, it was not surprising that the amount of aragonite precipitate that formed in the PEI/CA solution was larger than that which formed in any other dish. The slopes corresponding to the calcium ion consumption rate and the CaCO3 precipitation rate were steeper between 5 and 10 h aging times, as shown in Figure 1. These results indicated that the precipitation rates were higher during this period of time, so that the aragonite formation was preferred under the conditions. Grazing-incidence X-ray diffraction (GI-XRD) is a suitable tool for characterizing crystals based on the surface structures by reducing scattering from the substrate. A fixed grazing incidence angle (0.18°) below the critical angle (0.23°) of the silicon substrate was used to observe only a couple of distinctive aragonite bands from the precipitate formed in the

Figure 5. Characterization of a rosette CaCO3 deposited on the calcite crystals grown in the presence of PEI buffer. (a) GI-XRD pattern, 10 h aging time, (b) SEM images captured with tilted angle of 30°, and (c) viewed from bottom.

stages, followed by aragonite precipitation. In fact, we observed that many acicular aragonite crystals grew on the calcite crystals in the PEI and PEI/CA buffers (Figure 5b,c). Gómes-Morales et al.6 showed the dominance of calcite polymorph at early precipitation and low concentration of NH4HCO3 used as carbonate ion source by using the vapor diffusion technique. They interpreted that droplets first reached the critical supersaturation of the least soluble calcite polymorph. As vapor diffusion continues, the ionic activity product of Ca2+ and CO32− surpasses the solubility product of aragonite, leading to the nucleation and growth of aragonite polymorph. These results coincided with the appearance of calcite at early 5 h and aragonite dominance after 10 h aging in our experiments. Figure 6 shows the morphological changes that resulted from changes in the PEI-buffered solution temperature. At 60 °C,

Figure 6. Effect of the solution temperature on the CaCO3 morphology in CaCl2 and in PEI. (a) 60 °C, (b) 5 °C.

almost all crystals were acicular aragonites. At 37 °C and at room temperature, aragonite and calcite crystals appeared together, whereas spherulitic vaterites were only present at 5 °C (Figure 6a). These results agreed with the observation that aragonite formation was favored at high temperatures (exceeding 60 °C), and vaterite formation was favored at low temperatures around 5 °C, during sedimentation. Higher 614


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showing CaCO3 crystals deposited after 10 h of aging time at low concentration of basic buffers. This material is available free of charge via the Internet at http://pubs.acs.org.

temperatures are known to favor aragonite polymorph formation in seawater.11,13,29 Figure 1c shows that the most rapid calcium ion consumption occurred at 60 °C, suggesting that more aragonite crystals were deposited at high temperatures. At low temperatures, around 5 °C, vaterite prevailed.13,31 Figure 7 shows the XRD patterns obtained from the CaCO3 precipitated during a 10 h aging time in a PEI buffered solution

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 82-54-279-2270. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ABBREVIATIONS CA, carbonic anhydrase; Tris, Tris(hydroxyethyl)aminomethane; AMH, ammonium hydroxide; PEI, polyethylenimine; EDTA, ethylenediaminetetraacetic acid; SEM, scanning electron microscopy; XRD, X-ray diffraction; GI-XRD, grazingincidence X-ray diffraction

Figure 7. XRD patterns obtained from the CaCO3 crystals deposited at different solution temperatures over 10 h aging times in the PEI buffered solution. A, C, and V denote aragonite, calcite, and vaterite, respectively, (a) at 60 °C, and (b) at 5 °C.

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at different solution temperatures. At 60 °C, bands due to aragonites were dominant, although a calcite peak appeared at 2θ = 29.4 (the 104 plane). Interestingly, peaks corresponding to vaterite crystals alone appeared at 5 °C, and no peaks due to calcite were observed.

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CONCLUSIONS This study examined CaCO3 crystal formation from atmospheric CO2 in an open system exposed to the atmosphere. The system mainly produced calcite crystals with aragonite as the minor component. Vaterite was deposited only at low solution temperatures. Calcite crystals formed together with aragonite consecutively or concurrently in the process of precipitation at room temperature. The dominance of the aragonite and vaterite polymorphs depended on the temperature and the rate of calcium consumption and CaCO3 precipitation, rather than the use of a basic buffer or a high pH. The high CaCO3 precipitation rate increased the proportion of aragonite polymorph that formed. The open system provided a reliable approach to mimicking sedimentation and polymorph selection in nature. This system may be applicable to atmospheric CO2 capture. The selective harvesting of crystals with a desirable polymorph may be achieved by regulating the operating variables and using a suitable basic buffer.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

1. Amount of consumed calcium ions was measured by using master curve with ethylenediaminetetraacetic acid (EDTA) titration method 2. Solution pH change at varying concentrations of basic buffers with aging time. 3. Amounts of precipitated CaCO3 by the aging of calcium ions in solution and CO2 from the air at aging times and basic buffers. Each precipitated amount was the average of four tests. 4. (a) Synthetic rhombohedral calcite crystals in the AMH buffer. The crystals grew only (104) plane. (b) Synthetic calcite crystals grown in the PEI buffer showed the development of new crystal face (hkl). 5. XRD patterns collected from the precipitates after 24 h aging times. 6. Scanning electron microscope photographs 615


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CaCO3 Precipitation and Polymorph Forms During CO2 Sequestration

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