Environ. Sci. Technol. 2001, 35, 4556-4561
Sorption Mechanisms of Zinc to Calcium Silicate Hydrate: Sorption and Microscopic Investigations F E L I X Z I E G L E R , † R E T O G I E R EÄ , ‡ A N D C . A N N E T T E J O H N S O N * ,† Department of Water Resources and Drinking Water, EAWAG, Swiss Federal Institute of Environmental Science and Technology, U ¨ berlandstrasse 133, CH-8600 Du ¨ bendorf, Switzerland, and Earth and Atmospheric Sciences, Purdue University, West Lafayette, Illinois 47907-1397
Knowledge of the binding mechanisms of heavy metals to cement minerals is essential for the prediction of the longterm leachability of secondary building materials and cementstabilized wastes containing heavy metals. In this study, the sorption of Zn(II) to calcium silicate hydrate (C-S-H(I)) in pre-equilibrated aqueous suspensions has been investigated as a function of time (up to 87 d), pH (11.7, 12.48, and 12.78) and Zn(II) concentration (4.8-4800 µM). Electron probe microanalysis (EPMA) was performed in order to determine where Zn(II) was bound. At high Zn(II) concentrations (>1000 µM), the precipitation of β2-Zn(OH)2 (pH 12) was observed. Surface precipitation could not be discerned. At lower concentrations, it was found that the sorption process was initially very rapid with over 50% sorbed within 30 min but that the sorption continued more slowly to at least 87 d. The data could be interpreted in terms of the Freundlich isotherm up to a Si:Zn(II)sorbed atomic ratio of approximately 6:1. Zinc was observed by EPMA to incorporate into the C-S-H(I) particles but did not appear to substitute for Ca or Si. The incorporation of Zn(II) in the interlayer of C-S-H(I) or sorption to internal surfaces of crystalline appear to be the most probable mechanisms for the observed Zn(II) sorption to C-S-H(I).
hydrate (C-S-H), the most abundant component of hydrated portland cement (∼60 wt %). There was quite an early interest in the chemical reactions of Zn(II) with cement minerals because Zn(II) is known to interact with the cement clinker grains during hydration so as to retard setting. Lieber and Gebauer (5) demonstrated, with X-ray diffraction, the intermediate formation of the crystalline calcium zincate (Zn2Ca(OH)6‚2H2O). After some days of reaction, the calcium zincate was no longer detectable, but a new solid Zn(II) phase could not be detected. With the aid of microprobe analysis, they concluded that Zn(II) was incorporated into a calcium silicate hydrate (C-S-H) gel. Later Poon et al. (6) investigated Zn(II) release from a cementitious matrix using leaching tests, scanning electron microspcopy, and X-ray diffraction and concluded that Ca(OH)2 was the major phase involved in the fixation. Further research suggested that Zn(II) precipitated as calcium zincate at the surface of cement grains (7-10). In agreement with ref 5, there is recent evidence to suggest a more intimate interaction of Zn(II) with C-S-H. Preliminary investigations of aqueous solutions in equilibrium with suspensions of C-S-H containing Zn(II) have found that dissolved Zn(II) concentrations are directly related to the solid-phase concentration at a given pH value (11, 12). Such a correlation is typical for adsorption and precipitation at surfaces or for Zn(II) incorporation in a solid-solution in C-S-H (13). In the former case, the metal cation stays at the surface, and in the other, it is incorporated into the bulk of the mineral. Using spectroscopic techniques, Moulin et al. (14) have found that structural retention by Si-O-Zn bonds is involved in the sorption of Zn(II) to C-S-H gel and that Zn(II) is probably linked to SiO4 tetrahedra at the end of the silica chains in C-S-H. Recent extended X-ray fine structure (XAFS) measurements (15) are in agreement with the aforementioned studies and suggest that Ca is not in the second atomic shell surrounding Zn(II) but that Zn-O-Si bonds may be involved. The aim of this work is to further elucidate the binding mechanism between Zn(II) and C-S-H. A combination of equilibration studies in aqueous suspensions and microscopic investigations using electron probe microanalysis (EPMA) have been used to determine whether Zn(II) is bound at the surface or incorporated within C-S-H particles.
Experimental Methods Introduction Interest in the mobility and fate of heavy metals in cements has increased over the past years because of the use of wastes as secondary materials in cement production and because of the utilization of cement to immobilize heavy metal and radionuclide-containing wastes (1, 2). An understanding of the immobilization mechanism in the cement matrix is essential for the long-term assessment of the use or hazards of such materials. In addition to the precipitation of discrete minerals in cement porewater, there appears to be a strong interaction between cement components and waste ions that further depresses aqueous concentrations (3, 4). Sorption to surfaces and incorporation into cement minerals are two mechanisms that have been suggested for metal uptake. This paper reports on an investigation of the interaction between a relatively soluble metal cation, Zn(II), and calcium silicate * Corresponding author e-mail:
[email protected]; telephone +41-1-8235486. † EAWAG. ‡ Purdue University. 4556
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Materials. All chemicals were at least of p.a. grade. Solutions were generally prepared using boiled ultrapure water (Barnstead Nanopur). To prevent a CO2 contamination, all procedures using alkaline solutions and solids were performed in a glovebox in an argon atmosphere with pCO2 < 1 ppm. The synthesis of C-S-H(I) was performed after Atkins et al. (16) to achieve a C-S-H(I) with a Ca:Si ratio of 1. For the subsequent experiments, only the 12) and at different equilibration times. The preliminary kinetic experiments were equilibrated between 5 min and 87 d. The sorption isotherm experiments were equilibrated for 4, 28, 56, and 87 d. Certain experiments were conducted without addition of the C-S-H(I) suspension or only in an electrolyte solution at the same pH value (without Ca and Si addition). To determine the appropriate equilibration times and the sorption behavior as a function of time, some kinetic experiments were carried out prior to the VOL. 35, NO. 22, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Sorption isotherms of Zn on C-S-H(I) after 87 d of equilibration at pH 12.48 (2) and pH 12.78 (b). The closed symbols indicate the data points undersaturated, and the open symbols supersaturated with respect to CaZn2(OH)6‚2H2O. The diagonal dashed line indicates the position of the pH 11.7 data. effective sorption experiments (pH 11.7, [Zn]tot ) 19 µM). After equilibration, the solution pH was measured in a 20-mL aliquot. The remaining sample was filtered and acidified (0.3 mL of concentrated HNO3), and the dissolved Zn, Ca, and Si concentrations were measured. All experiments were carried out in duplicate. The solid-phase samples were freeze-dried and analyzed by X-ray powder diffraction (XRPD) to determine possible changes in their mineralogical composition (Scintag XDS 2000 diffractometer, Cu KR, 1.5406 nm, 2000 W, 45 kV, 40 mA). EPMA. For the microprobe analysis, the samples were prepared in the same way as the sorption experiments in a 20× larger quantity. All samples were produced at pH 11.7 and at initial Zn(II) concentrations of 19 µM (equilibration times: 30 min, 24 h, and 28 d) and 190 µM (equilibration times: 30 min and 24 h). For the element distribution maps,
aliquots of the Zn-treated C-S-H(I) samples were embedded in epoxy resin (Araldit, Ciba Geigy). The embedded samples were polished and subsequently coated with about 200 Å of carbon (Balzers Union Carbon Evaporation Device CED 010). Semiquantitative element distribution maps were generated using a JEOL Superprobe JXA-8800L (WDS mode, 100 nA, 15 keV, 140 ms/pixel); quantitative analyses were performed with a CAMECA SX-50 (30 s/element). Only the largest particles (around 50 µm) were analyzed in order to ensure that we are looking at equatorial sections of particles. The system was calibrated using a zincite (99.99 wt % ZnO) and a pyroxene (5.36 wt % Na2O, 16.38 wt % CaO, 56.88 wt % SiO2) standard prior to the measurements. The background for each peak was determined by measuring for 15 s at suitable distances to the front and back of the peaks. The surfaces were coated with powder (200 Å carbon) prior to analysis by a JEOL SEM. To examine the possible formation of surface precipitates, the particle surfaces of unembedded C-S-H(I) samples each equilibrated for 30 min with 19 µM, 190 µM, and no Zn(II) were examined in the secondary electron (SE) and the backscattered electron (BSE) mode. Thermodynamic Equilibrium Calculations. Thermodynamic equilibrium calculations were performed with the aid of the computer programs MQV40TIT (ref 20, based on ref 21). Stability constants were taken from refs 22-25. The values were corrected for an ionic strength of 0.1 M in accordance with the experimental conditions by using activity coefficients calculated by the Davies equation.
Results and Discussion Sorption Kinetics. The sorption of Zn(II) to C-S-H(I) is initially fast (Figure 1a): more than 50% of the total Zn(II) in the system is already sorbed to the solid phase after 30 min of equilibration at pH 11.7. After this rapid uptake, the sorption rate gradually diminishes. At a concentration of 19 µM, approximately 83% and 90% of Zn(II) is sorbed after 24 and 96 h, respectively. After 4 d of equilibration, the sorption of Zn(II) continues at a slow rate, as samples equilibrated for up to 87 d show (Figure 1a; 26). At pH 11.7, the sorption rate appears to be independent of the initial Zn(II) concentration between 4.8 and 48 µM and between 4 and 87 d of equilibration. At higher pH values (Figure 1b), the same two-
FIGURE 4. Semiquantitative Zn distribution maps of C-S-H(I) particles after 30 min of equilibration at [Zn]tot ) (a) 19 and (b) 190 µM. Brighter areas indicate higher Zn concentrations. 4558
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step sorption process is observed, but the rate is lower than at pH 11.7, less is sorbed, and the sorption rate appears to be dependent on Zn(II) concentration. Sorption Isotherms. Figure 2a shows the sorption isotherm of Zn(II) to C-S-H(I) at a pH value of 11.7 and an equilibration time of 4 d. At low Zn concentrations, dissolved concentrations are directly correlated to the solid-phase concentrations of Zn(II). The solid line represents a linear fit of the data in the double logarithmic plot. The slope has a value of 0.67, which corresponds to the exponent n in the Freundlich adsorption isotherm:
[Zn(II)sorb] ) m[Zn(II)aq]n
(1)
where m is a constant. After 87 d of equilibration, the isotherm (Figure 2b) does not differ strongly from the isotherm after 4 d. The linear range of the isotherm moves slightly toward lower dissolved Zn(II) concentrations (for measurements with the same total concentration). The linear fit of the data in the double logarithmic diagram gives a value of 0.75 for the exponent n in the Freundlich equation. Experiments made at higher pH values (12.48 and 12.78) result in similar sorption isotherms to those determined at pH 11.7 and can be interpreted using the Freundlich equation (eq 1). Values of n are 0.56 and 0.55, respectively. A comparison of isotherms after 87 d of equilibration and under different pH conditions (Figure 3) shows that the affinity of Zn(II) to C-S-H(I) decreases with increasing pH. It should be noted that Si: Zn(II) ratios of up to 6:1 are observed to fit the Freundlich isotherm. At higher Zn concentrations, the solid-phase concentrations begin to increase, and the relationship between aqueous and solid-phase concentrations breaks down. In the experiments carried out at pH 11.7, dissolved Zn(II) concentrations are supersaturated with respect to the calculated solubility of ZnO. In the experiments with the two highest total Zn concentrations, the aqueous concentrations are quite close to the solubility of β2-Zn(OH)2 (Figure 2). X-ray diffication analyses confirm the precipitation of this crystalline Zn(II) phase. To test the role of the C-S-H(I) particles in this precipitation reaction, analogous experiments were made with the same solution composition but in the absence of C-S-H(I). At low Zn(II) concentrations, the total added Zn(II) concentration is found in solution as no sorption occurs (data not shown). At high concentrations, however, the measurements are nearly identical to the data from the experiments with C-S-H(I) (Figure 2a), indicating that the C-S-H(I) particles do not appear to play an important role in the sorption of Zn. It is therefore likely that precipitation occurs in the bulk solution rather than at the surface. At high Zn(II) concentrations and pH above 12, the aqueous Zn(II) concentrations are almost independent of total Zn concentrations, as expected for the formation of a pure solid phase. The XRPD analyses of the corresponding solid samples show the presence of crystalline calcium zincate (CaZn2(OH)6‚2H2O). The total aqueous Zn(II) concentrations agree well with the solubility calculated by means of a recently published solubility product (Figure 3, 27). EPMA. In the samples with low total Zn(II) concentrations (19 µM), Zn(II) is initially observed to be predominantly at the surface of the C-S-H(I) particles. After 30 min of equilibration, Zn(II)-enriched rims with a thickness of a few micrometers appear to have formed (Figure 4a). At Zn(II) concentrations of 190 µM, similar Zn(II)-rich rims with concentrations of around 3 atom % are observed (Figure 4b). In addition, Zn-rich precipitates are observed. (It should be noted that at 190 µM, sorption begins to deviate from the Freundlich isotherm.) These observations are substantiated by SE and BSE images (shown in ref 26). After 30 min, the surface morphology of the 19 µM samples is identical to the
FIGURE 5. Quantitative point analysis across the individual C-SH(I) particles for [Zn]tot ) 19 µM; (a and b) after 30 min (shown in Figure 4), (c and d) after 24 h, and (e and f) after 28 d of equilibration. untreated C-S-H(I) samples, whereas precipitates are observed at Zn(II) concentrations of 190 µM at pH 11.7. Quantitative Zn(II), Ca, and Si measurements made across some C-S-H(I) particles illustrate the incorporation of Zn(II) in C-S-H(I). These are shown in Figure 5 for samples with a total Zn(II) concentration of 19 µM. After 30 min, the profiles reveal a relatively sharp decrease in the Zn concentrations from about 1 atom % to values below 0.2% within the first 5-10 µm from the rim borders. After 24 h, two differences can be observed: first, the concentrations at the borders are higher (around 1.5%), and second, the gradient is less pronounced. The concentrations within the first 10 µm from the border do not fall below 0.2%. After 28 d of equilibration, this Zn gradient has almost gone. The concentrations at the borders no longer appear to have increased, but the regions within the particles with very low Zn concentrations have nearly vanished. The rate of Zn(II) uptake may be can be interpreted as diffusion, and a rough estimate of the diffusion coefficient, D, can be made using the following simple equation:
D)
d2 t
(2)
where d is the depth of penetration and t is the time. For t ) 30 min and a depth of 5 µm, D is estimated to be 1 × 10-14 m s-1; for t ) 24 h and a depth of 15 µm, a value of 3 × 10-15 m s-1 is obtained. VOL. 35, NO. 22, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Quantitative point analysis across two individual C-S-H(I) particles for [Zn]tot ) 190 µM after 30 min showing Zn(II) concentration and the Ca:Si ratio. The incorporation Zn(II) in the C-S-H(I) particles does not appear to influence the Ca and Si content of C-S-H(I). The high Zn contents in the rims of the particles treated with 190 µM Zn(II) would allow a significant change in the Ca:Si ratio to be measured if an exchange mechanism were taking place. A Zn-Ca exchange at a 3 atom % Zn(II) concentration would lower the Ca:Si ratio by 0.15. This difference would be significant since the mean Ca:Si ratio is 0.91 ( 0.08. However, the Ca:Si ratio was not even observed to decrease at Zn concentrations of 5-6% (Figure 6). Mechanisms of Zn(II) Sorption to C-S-H(I). Adsorption at the surface appears to be the first step in the sorption process. This is shown by both the kinetic and the microscopic measurements. The microscopic measurements show, however, that Zn(II) is incorporated into the C-S-H(I) particles. Whether this is a diffusion process or a re-dissolution and precipitation process of the C-S-H(I), in which the Zn(II) becomes involved, cannot be proven with certainty without further experiments. It is interesting to note that at all the three pH values investigated, a maximum Si:Zn(II) ratio of approximately 6:1 is reached. This could be envisaged as a sorption process to this maximum capacity. The important question that remains to be answered is that of the binding mechanism within the C-S-H(I) at intermediate Zn(II) concentrations. There are a number of possibilities. First, Zn(II) could isomorphically substitute for Ca within the C-S-H(I), but this was not observed in our experiments. Second, the structure of C-S-H(I), especially the interlayer structure, is known for its ability to vary in composition without a change in the main structural properties (28). Zinc could be incorporated into the interlayers of the C-S-H(I) structure, where it may sorb as ZnO4 tetrahedra to Si-O sites from the SiO4 chains oriented toward the interlayer as suggested by refs 14 and 15. Both XAFS studies suggest Si-O-Zn bonds. Third, it is also possible that binding occurs at internal surfaces between ordered domains in the C-S-H(I) gels. The structure of C-SH(I) is known to have little or no more than a two-dimensional order, so the existence of internal surfaces and micropores cannot be excluded. Here also, Zn(II) could sorb at Si-O sites. A fourth possibility is the precipitation of Zn silicates, although it should be noted that their formation would cause the release of Ca either to solution or for uptake by the interlayers of the remaining C-S-H(I) gels, and this has not been observed. Also, the thermodynamic stability of the zinc silicates would have to be thermodynamically more stable than C-S-H(I) under the given conditions, and this would eventually lead to the disintegration of C-S-H(I) in the presence of high Zn(II) concentrations, which was not 4560
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observed in our experiments. Such a process would have to be regarded as a local “surface” precipitation reaction. Further work may be able to distinguish between these mechanisms, but all the indications are that binding at Si-O sites within the interlayer or at domain “surfaces” is the most likely process.
Acknowledgments The authors gratefully acknowledge the funding of the Swiss Federal Institute of Environmental Science and Technology (EAWAG). We thank Peter Baccini for helpful suggestions and discussions.
Literature Cited (1) Atkins, M.; Glasser, F. P. Waste Manage. 1992, 12, 105-131. (2) Spence, R. D., Ed. Chemistry and Microstructure of Solidified Waste Forms; Lewis Publishers: Boca Raton, FL, 1993. (3) Glasser, F. P. In Environmental Aspects of Construction with Waste Materials; Goumans, J. J. J. M., van der Sloot, H. A., Aalbers, Th. G., Eds.; Elsevier: Amsterdam, 1994; pp 77-86. (4) Cougar, M. D. L.; Scheetz, B. E.; Roy, D. M. Waste Manage. 1996, 16, 295-303. (5) Lieber, W.; Gebauer, J. Zem.-Kalk-Gips 1969, 22, 161-164. (6) Poon, C. S.; Clark, A. I.; Peters, C. J.; Perry, R. Waste Manage. Res. 1985, 3, 127-142. (7) Poon, C. S.; Clark, A. I.; Perry, R. Environ. Technol. Lett. 1986, 7, 461-468. (8) Cocke, D. L.; Mollah, M. Y. A. In Chemistry and Microstructure of Solidified Waste Forms; Spence, R. D., Ed.; Lewis: Boca Raton, FL, 1993; pp 187-242. (9) Yousuf, M.; Mollah, A.; Vempati, R. K.; Lin, T.-C.; Cocke, D. L. Waste Manage. 1995, 15, 137-148. (10) Cocke, D. L.; Yousuf, A. M.; Hess, T. R.; Lin, T.-C. Mater. Res. Soc. Symp. Proc. 1997, 432, 63-68. (11) Johnson, C. A.; Kersten, M. Environ. Sci. Technol. 1999, 33, 22962298. (12) Ludwig, C.; Ziegler, F.; Johnson, C. A. In Waste Materials in Construction; Goumans, J. J. J. M., Senden, G. J., van der Sloot, H. A., Eds.; Elsevier: Amsterdam, 1997; pp 459-468. (13) Stumm, W. Chemistry of the Solid-Water Interface; John Wiley & Sons: New York, 1992. (14) Moulin, I.; Stone, W. E. E.; Sanz, J.; Bottero, J.-Y.; Mosnier, F.; Haehnel, C. Langmuir 1999, 15, 2829-2835. (15) Ziegler, F.; Scheidegger, A. M.; Johnson, C. A.; Da¨hn, R.; Wieland, E. Environ. Sci. Technol. 2001, 35, 1550-1555. (16) Atkins, M.; Glasser, F. P.; Kindness, A. Cem. Concr. Res. 1992, 22, 241-246. (17) Brunnauer, S.; Emmet, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309-319. (18) American Public Health Association, American Water and Wastewater Association, and Water Environmental Federation. In APHA 4-118-4-119, 19th ed.; Eaton, A. D., Clesceri, L. S., Greenberg A. E., Eds.; 1995.
(19) Berner, U. R. Radiochim. Acta 1988, 44/45, 387-393. (20) Furrer, G. MQV40TIT, Version 4.0; Institute of Terrestrial Ecology (ITO ¨ ): Schlieren, Germany. 1995. (21) Westall, J. C. MICROQL; Report 86-02; Department of Chemistry, Oregon State University: Corvallis, OR, 1986. (22) Baes, C. F.; Mesmer, R. E. The Hydrolysis of Cations; John Wiley & Sons: New York, 1974. (23) Smith, R. M.; Martell, A. E. Critical Stability Constants. Vol. 4: Inorganic Complexes; Plenum Press: New York, 1976. (24) Smith, R. M.; Martell, A. E. Critical Stability Constants. Vol. 5: First Supplement; Plenum Press: New York, 1982. (25) Smith, R. M.; Martell, A. E. Critical Stability Constants. Vol. 5: Second Supplement; Plenum Press: New York, 1989.
(26) Ziegler, F. Heavy Metal Binding in Cement-Based Waste Materials: An Investigation of the Mechanism of Zn Sorption to Calcium Silicate Hydrate. Ph.D. Thesis, Swiss Federal Institute of Science and Technology, Zuerich, Switzerland, Dissertation ETH 13569, 2000. (27) Ziegler, F.; Johnson, C. A. Cem. Concr. Res. 2001, 31(9), 13271332. (28) Berner, U. R. Waste Manage. 1992, 12, 201-219.
Received for review October 13, 2000. Revised manuscript received August 20, 2001. Accepted August 21, 2001. ES001768M
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