Homogeneous Precipitation of Mixed Anhydrous Ca− Mg and Ba− Sr

Homogeneous Precipitation of Mixed Anhydrous Ca-Mg and Ba-Sr Carbonates by Enzyme-Catalyzed Reaction Srecˇo D.

Sˇ kapin†

and Ivan

Sondi*,‡

Jozˇ ef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia, and Ru]er Bosˇ kovic´ Institute, Bijenicˇ ka cesta 54, 10000 Zagreb, Croatia Received May 3, 2005;

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 5 1933-1938

Revised Manuscript Received June 8, 2005

ABSTRACT: A modified homogeneous precipitation process involving catalytic decomposition of urea by the enzyme urease was used to precipitate mixed Ca-Mg and Ba-Sr carbonate particles in a reacting solution of their salts at room temperature. The structure and morphology of the obtained precipitates were investigated using scanning electron microscopy (SEM) and X-ray diffraction (XRD). There are distinct differences in the formation and the nature of these solids. It was found that magnesium ions were only slightly incorporated into the calcite structure, making low-magnesium calcite, while the presence of barium and strontium ions in the reacting solution resulted in the formation of equimolar solid solutions of Ba-Sr carbonate. However, the presence of magnesium ions in the reacting solution at early stages of the precipitation process governs the formation of nonaggregated and nanosized calcite particles characterized by high specific surface area (SSA). Finally, this study has proven the recently renewed mechanism in which some of carbonate colloids could be formed through aggregation of preformed nanosized crystalline particles. Introduction Carbonates are some of the most abundant minerals in nature and, as such, have been of considerable interest in geo- and biosciences, as well as in materials research. Recently, carbonates have been commonly used in many industrial applications, and there is need for development of carbonate materials of novel morphological, physical, and chemical properties by simple synthetic procedures in a cost-effective manner. The control of the crystal formation and development of different morphologies and physicochemical properties of carbonate precipitates has been developed to a remarkable degree by using organics during the precipitation process.1-7 It is well established that protein macromolecules are intimately involved in the regulation of growth, crystal morphology, and colloid stability of the obtained inorganic precipitates.8-12 Less attention has been paid to the role of specific proteins, enzymes, on the precipitation of inorganic precipitates. Recently, Bang and co-workers 13 have described microbiological precipitation of calcium carbonate particles induced by the Bacillus pasteurii urease enzyme. Ureases are multi-subunit, nickel-containing enzymes that occur in nature in many bacteria, several species of yeast, and a number of plants.14,15 In particular, urease extracted from a soil organism (Bacillus pasteurii) consists of three different subunits with two nickel atoms in individual active sites.16 Urease catalyzes the hydrolysis of urea to form ammonia and carbon dioxide. This process causes increases in pH and therefore has also been used in precipitation of different inorganic hydroxide particles.17-19 Our previous studies have described a conceptually new method for the precipitation of different types of * Corresponding author. Tel: +385 +385 1 4680 242. E-mail: [email protected]. † Joz ˇ ef Stefan Institute. ‡ Ru]er Bos ˇ kovic´ Institute.

1

4680

124.

Fax:

anhydrous carbonates of simple composition effected through catalytic decomposition of urea by the urease enzyme in aqueous solutions containing different alkaline earth salts.20,21 In particular, the chemical and biological precipitations of carbonate particles of simple composition such as calcium carbonate polymorphs have been extensively investigated. However, few studies have dealt with mixed alkaline earth carbonates. Among them, magnesium calcite and mixed Ba-Sr carbonates from the aragonite group are omnipresent in nature. Thus, the aim of the present work was to carry out a systematic investigation on precipitation of complex anhydrous mixed Ca-Mg and Ba-Sr carbonates by the enzymatic activity of urease in aqueous solutions containing urea, magnesium, calcium, strontium, and barium salts. Experimental Section Materials. Inorganic chemicals and urea were reagent grade and used without further purification. Stock solutions of CaCl2, MgCl2, BaCl2, and SrCl2 were freshly prepared and filtered through 0.22 µm Millipore membranes before use. Urease enzyme (lot 21K7038, molecular weight 470 000 Da, Sigma) was fractionated from crude Jack Bean (Canavalia ensiformis) meal extract with activity of 45 units per milligram of dry weight. Preparation and Characterization of the Precipitates. Reacting solutions containing magnesium and calcium or barium and strontium chloride salts, urea, and urease were kept in tightly stopped Erlenmeyer flasks at room temperature from 10 min to 72 h. Solutions of alkaline earth salts and urea were purged by nitrogen bubbling before urease was injected and the precipitation process started. Equimolar concentrations of metal chlorides in reacting solutions were kept constant at 0.2 mol dm-3, and that of urea was kept at 0.5 mol dm-3, while the concentration of urease enzyme was 0.5 mg cm-3. To evaluate the agitation effects on the particle forming process, selected experiments were carried out under magnetic stirring. All experiments were performed at 25 °C. The onsets of precipitation were determined by the appearance of milky clouds, whiting, in reacting solutions.

10.1021/cg050197c CCC: $30.25 © 2005 American Chemical Society Published on Web 07/09/2005

1934

Crystal Growth & Design, Vol. 5, No. 5, 2005

Figure 1. Scanning electron micrographs (SEM) of calcite precipitates obtained by aging solutions containing 0.5 mol dm-3 urea, 0.2 mol dm-3 CaCl2, 0.2 mol dm-3 MgCl2, and 0.5 mg cm-3 Canavalia ensiformis urease at 25 °C for (a) 10 min, (b) 60 min, and (c) 72 h and (d) particles shown in panel c at higher magnification. The size and the morphology of obtained precipitates were examined by scanning electron microscopy (SEM; JXA 840A, JEOL), while their chemical composition was determined by energy-dispersive X-ray analyses (EDAX) using TRACOR (NORAN TRACOR Series II, Tracor Northern) software. The crystalline phases of carbonate precipitates were analyzed by X-ray powder diffraction (XRD) using D4 Endeavor, Bruker AXS, and identified according to the JCPDS powder diffraction files. The software TOPAS 2.1 (Bruker AXS) with the basic parameters approach was used for Rietveld refinement. The diffraction peaks on the XRD patterns were coded as follows: C-calcite, A-aragonite, N-nesquehonite, and SB-mixed Ba-Sr carbonate. Specific surface area measurements (SSA) of the freezedried powder of carbonate solids were made by single-point nitrogen adsorption using a Micromeritics FlowSorb II 2300 instrument.

Sˇ kapin and Sondi

Figure 2. X-ray diffraction (XRD) patterns of calcium carbonate precipitates obtained by aging solutions containing 0.5 mol dm-3 urea, 0.2 mol dm-3 CaCl2, 0.2 mol dm-3 MgCl2, and 0.5 mg cm-3 Canavalia ensiformis urease at 25 °C for (a) 10 min, (b) 60 min, and (c) 72 h.

Results Using homogeneous precipitation processes, we studied formation of anhydrous mixed Ca-Mg and Ba-Sr carbonates in two reacting systems containing (i) urea, urease, magnesium, and calcium chloride and (ii) urea, urease, and strontium and barium chlorides, respectively. In both cases, the onset of precipitation was noted after 5 min of reaction time. The pH of the reacting solutions changed from 6.4 at the beginning to 9.5 at the end of the precipitation process. The specifics of these two mentioned processes are as follows. Ca1-xMgxCO3 System. The effect of the reaction time on the formation of Ca-Mg carbonate precipitates is demonstrated by scanning electron micrographs displayed in Figure 1. Submicrometer, uniform, nearly spherical, and nonaggregated particles having modal diameter of ∼200 nm were obtained after 10 min of reaction time (Figure 1a). The X-ray diffraction data (Figure 2, pattern a) of this precipitate are characteristic of the calcite crystal structure. Energy-dispersive X-ray analysis (EDAX) revealed that this solid contained ∼2 mol % of Mg (Figure 3). In addition, on the basis of the relation between the interplanar spacing d(104) and the composition of the Ca-Mg carbonates,22 it was also estimated that calcite contained about 2 mol % of Mg.

Figure 3. EDAX spectrum of calcium carbonate precipitate obtained by aging solutions containing 0.5 mol dm-3 urea, 0.2 mol dm-3 CaCl2, 0.2 mol dm-3 MgCl2, and 0.5 mg cm-3 Canavalia ensiformis urease at 25 °C for 10 min.

Specific surface area (SSA) of this precipitate was 36 m2 g-1. On aging, particles removed from the mother liquor after 30 min remained uniform and spherical, while their size was increased to ∼ 400 nm (Figure 1b). As expected, SSA of this solid decreased to 10 m2 g-1. When the reaction proceeded for a longer period of time, the nature of the particles changed in a profound manner. Thus, two fractions of calcite particles were developed in a solution after 60 min of reaction time; one was characterized by particles of a few hundred nanometers and the second by much larger micrometersized particles (Figure 1c). The SSA of the latter precipitate was 1.4 m2 g-1. It is evident at much higher magnification that the larger particles are built up of much smaller subunits (Figure 1d). Although of different size, the particles retained the calcite crystal

Precipitation of Ca-Mg and Ba-Sr Carbonates

Crystal Growth & Design, Vol. 5, No. 5, 2005 1935

Figure 4. Scanning electron micrographs (SEM) of precipitates obtained by aging solutions containing 0.5 mol dm-3 urea, 0.2 mol dm-3 CaCl2, 0.2 mol dm-3 MgCl2, and 0.5 mg cm-3 Canavalia ensiformis urease at 25 °C for 72 h.

Figure 6. X-ray diffraction (XRD) patterns of calcium carbonate precipitates obtained by aging solutions containing 0.5 mol dm-3 urea, 0.2 mol dm-3 CaCl2, 0.2 mol dm-3 MgCl2, and 0.5 mg cm-3 Canavalia ensiformis urease at 25 °C at gentle magnetic stirring for (a) 10 min, (b) 60 min, and (c) 24 h.

Figure 5. Scanning electron micrographs (SEM) of calcite precipitates obtained by aging solutions containing 0.5 mol dm-3 urea, 0.2 mol dm-3 CaCl2, 0.2 mol dm-3 MgCl2, and 0.5 mg cm-3 Canavalia ensiformis urease at 25 °C with gentle magnetic stirring for (a) 10 min, (b) 60 min, and (c) 24 h.

structure as shown by XRD patterns (Figure 2, pattern b). EDAX analysis revealed that this solid also contained ∼2 mol % of Mg, indicating no significant differences in chemical composition of precipitates obtained during different reaction times. However, a discontinuity in the precipitation process appeared in a reacting solution after 72 h of reaction time, when besides calcite, large needlelike particles were precipitated (Figure 4a,b). The XRD pattern of the latter precipitate is characteristic of nesquehonite, a water-bearing carbonate (Mg(HCO3)(OH)‚2H2O) (Figure 2, pattern c). Applying gentle mixing with a magnetic bar during the precipitation process resulted in formation of particles of different morphology and crystal structure. Most of the particles grow to ∼3 µm in size during the first 60 min of the precipitation process (Figure 5a,b). Continuous aging of this reacting system produced, besides calcite, needlelike particles of aragonite (Figure 5c). Indeed, XRD patterns of the latter solid are char-

Figure 7. Scanning electron micrographs (SEM) of Ba-Sr carbonate precipitates obtained by aging solutions containing 0.5 mol dm-3 urea, 0.2 mol dm-3 BaCl2, 0.2 mol dm-3 SrCl2, and 0.5 mg cm-3 Canavalia ensiformis urease at 25 °C for (a) 10 min and (c) 24 h and (b and d) particles shown in panels a and c at higher magnification.

acteristic of a mixture of calcite and aragonite (Figure 6, patterns b and c). Ba1-xSrxCO3 System. Scanning electron micrographs, displayed in Figure 7, demonstrate the effect of aging time on the formation and the morphology of mixed Sr-Ba carbonates. Approximately 10 min after reacting components were mixed, polydispersed spherical particles were formed (Figure 7a). At much higher magnification, these particles are composed of much smaller nanosized subunits (Figure 7b). However, if the reaction proceeded for longer periods of time, spherical particles were transformed into needlelike clusters (Figure 7c,d). The XRD patterns displayed in Figure 8 do not show any significant differences in crystal structures of precipitates obtained at different reaction times. Broad peaks correspond to the solid solution of both SrCO3 and BaCO3 solids. Indeed, observed patterns

1936

Crystal Growth & Design, Vol. 5, No. 5, 2005

Figure 8. X-ray diffraction (XRD) patterns of Ba-Sr carbonate precipitates obtained by aging solutions containing 0.5 mol dm-3 urea, 0.2 mol dm-3 BaCl2, 0.2 mol dm-3 SrCl2, and 0.5 mg cm-3 Canavalia ensiformis urease at 25 °C for (a) 10 min, and (b) 24 h.

matched that of the orthorhombic crystal structure with a space group of Pmcn like SrCO3 and BaCO3 compounds. Rietveld analysis, displayed in Figure 9, showed that the best fit between observed and calculated patterns was obtained when the occupancy for the metal site was 0.54 for Ba and 0.46 for Sr, confirming the results of the EDS analysis (Figure 10). Finally, there were no significant differences in precipitation of SrBa carbonates in reaction solutions under the influence of gentle magnetic stirring, as observed for the Ca-Mg system. Discussion Results of the present investigation have highlighted the differences in the formation of complex mixed anhydrous Mg-Ca and Ba-Sr carbonate solids in reacting solutions containing alkaline earth salts, urea, and urease at room temperature. The roles of different conditions, reaction time, and gentle magnetic stirring offer a possibility for speculation. As expected, despite the same concentration of magnesium and calcium ions in the reacting solution, lowmagnesium calcite, characterized by a lower solubility product then magnesite, was precipitated. There was no significant incorporation of magnesium in the crystal lattice of the calcite structure. However, in comparison with our previous study,20 the presence of magnesium ions in the reacting solution prevented formation of an amorphous phase. This phase was obtained in a reacting system containing calcium salt only at the early stage of the precipitation process.20 In the present study, uniform, spherical, and nanosized particles were formed (Figure 1a,b). While the mechanism of this phenomenon is still unresolved, it would appear that magnesium ions, together with macromolecules of urease, control the precipitation process. It has been observed that magnesium is considered to be the principal modifier of calcite morphology in many natural environments.23 Recently, Meldrum and Hyde24 showed that the mag-

Sˇ kapin and Sondi

nesium ions, in combination with organic additives, affect calcite morphologies by mechanisms such as adsorption on specific crystal faces altering the nucleation and growth process and inhibiting crystal growth. This may result in the formation of nanosized calcite particles. Indeed, a molecular dynamics simulation on the inhibiting effect of magnesium ions on calcite crystal growth proved this finding.25 These crystallites were not aggregated, which resulted in high specific surface area (SSA). So far, there are no reports in the literature that describe precipitation of calcium carbonates of high specific surface area in such a simple and cost-effective manner. Therefore, results of this study offer possibilities for the precipitation of new types of nanosized carbonate particles of high performance and their use in a multitude of industrial applications. Another somewhat unique observation is that on continuous aging of the original reacting mixtures the formation of larger spherical particles took place. These particles are built up of much smaller nanosized subunits, indicating the importance of the aggregation mechanism in their formation (Figure 1c,d).The significance of the aggregation processes in the formation of colloid particles from preformed nanocrystallites has been observed by Tezˇak and co-workers in the late 1960s.26 This finding has long remained neglected. Only recently, this mechanism was shown to be quite common in the formation of colloidal particles that show crystalline characteristics.27-29 On continuous aging of the reacting solution, an exotic crystalline phase, nesquehonite, appeared. It is clear that continuous aging of the reacting solution resulted in higher pH and Mg2+/Ca2+ molar ratio, establishing conditions for the precipitation of the nesquehonite phase. This study has also shown that the size and shape of calcium carbonate precipitate were sensitive to magnetic stirring during the aging process. A comparison with particles produced without stirring show them to be elongated and micrometer-sized (Figure 5a,b). In addition, aging of the reacting solution under stirring for longer periods of time induced formation of needlelike aragonite particles. It is very difficult to recognize the role of single factors such as presence of magnesium ions and the appearance of mechanical energy and a low magnetic field generated by magnetic stirring on the formation of these precipitates. It can be summarized that the agitation destabilizes the calcite crystal structure, while the presence of magnesium ions and induced magnetic field govern the formation of the aragonite phase. Indeed, there are literature data that magnetic treatment and magneto-hydrodynamic factors influence the calcium carbonate polymorph phase equilibrium and govern the formation of the aragonite phase.30,31 In contrast with the Ca-Mg carbonate system, spherical particles of solid solution of equimolar SrCO3 and BaCO3 components were precipitated at the early stage of the precipitation process. As expected, these particles of distinct crystallinity are also built up of much smaller nanosized subunits, which on prolonged aging grow into needlelike clusters. As described previously, it would seem that two unique mechanisms are involved in the formation of these precipitates.21 First, the process starts with the formation of nanosized particles, which

Precipitation of Ca-Mg and Ba-Sr Carbonates

Crystal Growth & Design, Vol. 5, No. 5, 2005 1937

Figure 9. Experimental (black line) and calculated (red line) XRD profiles for the Rietveld refinement of the Ba1-xSrxCO3-based sample precipitated from the system aged for 2 h. The lower trace represents the difference between the calculated and experimental data.

concentration of reactants, and on pH, should play an important role in the mechanism of particle aggregation. Further extensive investigations directed to better understanding of electrokinetic properties and interactions of anhydrous carbonate particles within reaction mixtures should contribute to clarification of these mechanisms. Acknowledgment. This work was supported by the Ministry of Science and Technology of the Republic of Croatia and the Republic of Slovenia under the Grant BI-HR/04-05-026. The authors gratefully acknowledge Dr. Velimir Pravdic´ for useful comments. References

Figure 10. EDAX spectrum of mixed Ba-Sr carbonate precipitates obtained by aging solutions containing 0.5 mol dm-3 urea, 0.2 mol dm-3 SrCl2, 0.2 mol dm-3 BaCl2, and 0.5 mg cm-3 Canavalia ensiformis urease at 25 °C for 10 min.

rapidly aggregate to uniform particles stabilized by the protein. During the second process, on continuous aging of the reacting solution, the preformed nanosized subunits act as nucleating agents for the subsequent growth of needle-form structures. While the mechanisms of these processes are not completely understood, they indicate the role of the precipitation processes at the nanoscale in the formation of colloidal carbonate particles. According to the study of Gauckler and coworkers,32 it is possible to control the stability and interparticle forces of colloid particles within suspension by means of time-delayed decomposition of urea by urease enzyme. In this respect, the changes in electrokinetic properties of formed particles (e.g., isoelectric point), which depends on the reaction time, on the

(1) DeOliveira, D. B.; Laursen, R. A. J. Am. Chem. Soc. 1997, 119, 10627. (2) Berkovitch-Yellin, Z.; van Mil, J.; Addadi, L.; Idelson, M.; Lahav, M.; Leiserowitz, L. J. Am. Chem. Soc. 1985, 107, 3111. (3) Xu, G.; Yao, N.; Aksay, I. A.; Groves, J. T. J. Am. Chem. Soc. 1998, 120, 11977. (4) Orme, C. A.; Noy, A.; Wierzbicki, A.; McBride, M. T.; Grantham, M.; Teng, H. H.; Dove, P. M.; DeYoreo, J. J. Nature 2001, 411, 775. (5) Hosoda, N.; Kato, T. Chem. Mater. 2001, 13, 688. (6) Dickinson, S. R.; McGrath, K. M. Cryst. Growth Des. 2004, 4, 1411. (7) Wang, T. X.; Co¨lfen, H.; Antonietti, M. J. Am. Chem. Soc. 2005, 127, 3246. (8) Meldrum, F. C. Int. Mater. Rev. 2003, 48, 187. (9) Falini, G.; Albeck, S.; Weiner, S.; Addadi, L. Science 1996, 271, 67. (10) Mann, S.; Archibald, D. D.; Didymus, J. M.; Douglas, T.; Heywood, B. R.; Meldrum, F. C. Reeves, N. J. Science 1993, 261, 1286. (11) Addadi, L.; Weiner, S. Angew. Chem., Int. Ed. Engl. 1992, 31, 153. (12) Aizenberg, J.; Lambert, G.; Weiner, S.; Addadi, L. J. Am. Chem. Soc. 2002, 124, 32. (13) Stocks-Fisher, S.; Galinat, J. K.; Bang, S. S. Soil Biol. Biochem. 1999, 31, 1563. (14) Sumner, J. B. J. Biol. Chem. 1926, 69, 435.

1938

Sˇ kapin and Sondi

Crystal Growth & Design, Vol. 5, No. 5, 2005

(15) Blakeley, R. L.; Treston, A.; Andrews, R. K.; Zerner, B. J. Am. Chem. Soc. 1982, 104, 612. (16) Benini, S.; Rypniewski, W.; Wilson, K. S.; Miletti, S.; Ciurli, S.; Mangani, S. Structure 1999, 7, 205. (17) Unuma, H.; Kato, S.; Ota, T.; Takahashi, M. Adv. Powder Technol. 1998, 9, 181. (18) Kara, F.; Sahin, G. J. Eur. Ceram. Soc. 2000, 20, 689. (19) Simpson, R. H.; Habeger, C.; Rabinovich, A.; Adair, J. H. J. Am. Ceram. Soc. 1998, 81, 1377. (20) Sondi, I.; Matijevic´, E. J. Colloid Interface Sci. 2001, 238, 208. (21) Sondi, I.; Matijevic´, E. Chem. Mater. 2003, 15, 1322. (22) Goldsmith, J. R.; Graf, D. L. Am. Mineral. 1958, 43, 84. (23) Davis, K. J.; Dove, P. M.; Wasylenki, L. E.; De Yoreo, J. J. Am. Mineral. 2004, 89, 714. (24) Meldrum, F. C.; Hyde, S. T. J. Cryst. Growth 2001, 231, 544.

(25) de Leeuw, N. H. J. Phys. Chem. B 2002, 106, 5241. (26) Petres, J. J.; Dezˇelic´, Gj.; Tezˇak, B. Croat. Chem. Acta 1969, 41, 183. (27) Park, J.; Privman, V.; Matijevic´, E. J. Phys. Chem. B 2001, 105, 11630. (28) van Hyning, D. L.; Zukoski, C. F. Langmuir 1998, 14, 7034. (29) van Hyning, D. L.; Klemperer, W. G.; Zukoski, C. F. Langmuir 2001, 17, 3128. (30) Knez, S.; Pohar, C. J. Colloid Interface Sci. 2005, 281, 377. (31) Kobe, S.; Drazˇic´, G.; McGuiness, P. J.; Strazˇisˇar, J. Acta Chim. Slov. 2001, 48, 77. (32) Gauckler, L. J.; Graule, Th.; Baader, F. Mater. Chem. Phys. 1999, 61, 78.

CG050197C