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Morphology controls on calcite recrystallization Frank Heberling, Leonie Paulig, Zhe Nie, Dieter Schild, and Nicolas Finck Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04011 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on October 5, 2016
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Morphology controls on calcite recrystallization
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Authors:
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Frank Heberling*, (
[email protected]) Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
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Leonie Paulig, (
[email protected]) Institute for Geography and Geoecology, Karlsruhe Institute of Technology, Kaiserstraße 12, 76131 Karlsruhe, Germany.
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Zhe Nie, (
[email protected]) College of Chemistry and Molecular Engineering, Peking University, 5 Yiheyuan Road, 100871 Beijing, China, and Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
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Dieter Schild (
[email protected]) Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
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Nicolas Finck (
[email protected]) Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
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(*: corresponding author)
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TOC-Graphics:
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Abstract
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Environmental- and Geoscientists working in different fields regard the reactivity of calcite and corresponding changes in its trace elemental- or isotopic composition from diametrically opposed points of view. As one extreme, calcite based environmental remediation strategies rely on the fast recrystallization of calcite and the concurrent uptake and immobilization of pollutants. Paleo-ecological investigations denote the other extreme, and rely on the invariability of calcite composition over geological periods of time. We use long-term radiotracer experiments to quantify recrystallization rates of seven types of calcite powder with diverse morphology and particle size distribution. On the one hand our results demonstrate the long-term meta-stability of calcite with equilibrated crystal surfaces even at isotopic dis-equilibrium. On the other hand, we document the extremely high reactivity and interfacial free energy of freshly ground, rough calcite. Our results indicate that bulk calcite recrystallization is an interfacial free energy driven Ostwald-ripening process, in which particle roughness effects dominate over the effect of crystal habitus and particle size. We confirm that the dynamic equilibrium exchange of crystal constituents between kink sites involves an activation barrier of about 25 kJ/mol. At room temperature the equilibrium exchange is limited to a near surface region and proceeds at a rate of (3.6± 1.4) ·10-13 mol/(m2·s).
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Introduction
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The process we refer to as recrystallization is at room temperature generally understood as solution mediated dissolution – re-precipitation process [1]. As a consequence of recrystallization, minerals can change their trace elemental- or isotopic composition. For sparingly soluble salts like calcite or barite this process is not restricted to the near surface region of the crystals (one to a few monolayers), but has been demonstrated to transform large parts of the bulk crystals [2-5]. Recrystallization provides an important pathway for the immobilization of cationic and anionic pollutants (e.g. Pb2+, Cd2+, Ra2+, CrO42-, SeO42-) and is correspondingly an important aspect of environmental remediation strategies [6].
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Sectoral- and other zoning patterns in calcite have been reported for the trace elemental- [7, 8] as well as for the isotopic composition [9] of natural calcite. Isotopic zoning even seems to prevail in calcitic cleft filling material, in contact with groundwater for > 400 million years [10, 11]. Zoning patterns in natural crystals, in contrast to statements in the previous paragraph, are one example for the long term metastability of non-equilibrium configurations in crystals over geologic periods of time. Meta-stability of calcite composition in turn is, as mentioned above, an essential pre-requisite for the extraction of relevant information about the conditions during crystal formation from the geologic record.
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In order to assess the potential of calcite to sequester trace elements and to judge whether calcite may conserve a certain isotopic or trace elemental composition over geologic periods of time, even in contact with groundwater, it is crucial to know whether bulk calcite recrystallization is a spontaneous process, which occurs just due to the dynamic nature of the solubility equilibrium, or a process, which requires an additional driving force. As an approach to these issues we investigate the recrystallization behaviour of seven calcite powders with diverse characteristics, covering a range of average particle sizes from 90 nm to 10.8 µm and corresponding specific surface areas from 23.8 m2/g to 0.2 m2/g. The morphologies ACS Paragon Plus Environment
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include rhombohedra, scalenohedra, prisms, and rounded/irregular particles. The objectives of the study are to investigate the variability of recrystallization rates of calcite as a function of morphology and particle size and to get first insights into the involved rates, processes and driving forces from a macroscopic perspective.
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Materials and Methods
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We use calcite powder from three sources: natural Iceland Spar, commercial precipitated calcium carbonate (PCC) products from Schäfer Kalk (Schäfer PRECAL), and calcium carbonate (suprapur) from Merck, Germany. Prior to the experiments powders are characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, gas adsorption (N2-BET), X-ray powder diffraction, and zeta-potential measurements. Details about materials and methods used in this study are presented in the electronic supplementary information (ESI) document.
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Solutions are pre-equilibrated with Merck calcite and atmospheric CO2 for two months prior to the experiments in order to establish bulk equilibrium conditions (negligible surface effects), SI(calcite) = 0.0 ± 0.1. After equilibration the calcite saturated solution, CSS, was separated from the solid by filtration.
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Batch-type recrystallization experiments
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For the recrystallization experiments 5 g/L dry calcite powder and a spike of 116 Bq/mL 45Ca are added to 40 mL CSS. Reported results are average and standard deviation of triplicate experiments. Calcite free reference experiments are used to test for 45Ca adsorption at container walls, which turned out to be negligible. At each sampling time a 1.3 mL aliquot of homogeneous suspension is withdrawn from the experiments and centrifuged at 4000 rpm for 30 min. 1 mL of the supernatant is added to 10 mL Perkin Elmer Ultima Gold XR Liquid Scintillation Cocktail. The 45Ca (radio-)activity concentration is measured in a Perkin Elmer Tricarb Liquid Scintillation Counter for 30 min in an energy range from 0 to 2000 keV. During the first 250 days of the recrystallization experiments, the suspensions are continuously agitated on a roller mixer (Stuart SRT9). After 250 days and 600 days solids are sampled and characterized by SEM and XRD (cf. ESI). Experiments are performed at room temperature, (23 ± 3) °C. The amount of recrystallized calcite, nrec , is calculated from the measured 45Ca-activity concentrations based on the heterogeneous and homogeneous recrystallization models [5]. Equations are derived in the ESI. According to the heterogeneous model nrec is calculated as:
∙ ∙ ( )
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() =
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According to the homogeneous model nrec is calculated as:
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() =
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()
()
− 1 ∙ ∙ ( )
(1).
(2)
W is the mass of water in the system. m denotes molalities of ions in solution. m0 is the initial molality ACS Paragon Plus Environment
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and m(t) the molality at time, t. mtot is the total molality of Ca2+ in the aqueous solution. As experiments proceed at close to equilibrium conditions mtot is assumed to remain constant throughout the experiment. It is further assumed that calcite with natural isotopic composition (>96% 40Ca) and 45Ca-calcite form an ideal solid solution and have identical solubility products, and that calcium isotope fractionation effects [12] can be neglected at close to equilibrium conditions. The expected homogeneous equilibrium 45Caactivity in solution at the experimental solid to liquid ratio (5 g/L) is 1.98 Bq/mL.
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To calculated recrystallization rates, r (mol/(m2 s)), the difference in nrec between two sampling times is divided by the time span in between, Δt, and normalized by the initial surface area, A, of the calcite powder: ri = (nrec(ti)- nrec(ti-1))/(Δti A). The calculated amounts of recrystallized calcite and recrystallization rates are repoted in the ESI (Figs.: S6, S9, S13, S17, S21, S25,S29). The initial recrystallization rates (Figure 1, right panel) represent the first calculated rate. Long term rates represent the average of at the last 4 to 7 constant rate values.
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pH-jump measurements
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pH-jump-measurements are applied in order to quantify the effect of the calcite surfaces on the solution composition. With pH-jump measurements we denote the simultaneous monitoring of pH and calcium activity, a(Ca2+), in 50 mL CSS after addition of 500 mg of calcite powder, using a pH- and a Ca-sensitive electrode (as described in the ESI) recording values every 10 s. Note that activity in this context no longer denotes radioactivity concentration as in the case of 45Ca, but effective concentration representing ionicstrength effects. Only the effect recorded for the most reactive calcite powder, ground Cc, turned out to be reproducible and significantly greater than electrode drift. The spontaneous increase in pH and a(Ca2+) is interpreted as spontaneous calcite dissolution due to increased interfacial free energy. We use PhreeqC to calculate an apparent supersaturation state of the solution. Apparent as it is only supersaturated relative to a fictive bulk equilibrium state in the absence of surface effects. This apparent supersaturation allows us to estimate the interfacial free energy of the calcite powder based on the particle size of the calcite powder and the Schindler equation [13] (cf. below and ESI).
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Results and Discussion
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Most relevant details about the particle characteristics are listed in Table 1. For full details, please refer to the ESI. The progression of 45Ca uptake is depicted in the left panel in Figure 1. Common for all calcite powders is a relatively fast initial recrystallization and uptake of 45Ca. On the long-term, 45Ca uptake is observed to: 1) cease completely within uncertainty (for ground calcite and fine grained PCC) or 2) proceed with a constant long-term rate. Initial and long-term recrystallization rates are depicted on the right hand side of Figure 1. Interesting details are revealed by looking at the transition between initial and final rates. For the samples aged and fresh coarse calcite, 45Ca activity concentration reaches a plateau after one to three weeks with recrystallization rates decreasing to zero. The amount of calcite recrystallized at that stage is equivalent to about 10 monolayers for fresh- and three monolayers for aged coarse calcite. After about two months, recrystallization reinitiates and proceeds at a relatively constant rate. Both initial- and long-term rates are higher for the fresh coarse calcite compared to the aged coarse calcite. The total amount of coarse calcite recrystallized after 600 days corresponds to ACS Paragon Plus Environment
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around 1% for both samples. SEM micrographs indicate that recrystallization processes on coarse calcite samples are limited to small features at the surfaces of the crystals. No changes in particle size distribution and general morphology are observable.
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Table 1: Initial characteristics and transformation of the calcite powders during the recrystallization experiments. The first line indicates the label, the specific surface area, and the source of the calcite (Cc) powders. SEM micrographs in the second and third line are taken before (2nd row) and after (3rd row) the experiments. Experiments ran 250 days for ground calcite and fine grained PCC (precipitated calcium carbonate) and 600 days for the other powders. Inserts in the PCC images highlight the morphological changes discussed in the text. They have arbitrary scales. Particle sizes were analyzed before the experiment and after 250 and 600 days of recrystallization. The corresponding changes in particle size distribution are indicated in the box-plots in the last row.
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: area of SEM micrographs
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Figure 1: 45Ca-uptake and recrystallization rates. The left plot in Figure 1 shows an overview over the decay corrected 45Ca activity concentration data. The insert highlights the details of the experiments with the two most reactive calcite powders: ground calcite and fine grained PCC. Note that all experiments start at 116 Bq/mL. The thin horizontal line in the insert indicates the expected equilibrium 45Ca-activity, 1.98 Bq/mL. The right plot shows the corresponding recrystallization rates. Detailed plots about the recrystallization progress and the recrystallization rates for all powders are presented in the ESI.
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For rhombohedral-, scalenohedral-, and prismatic PCC we observe a smooth transition from initial-, fast towards slow long-term recrystallization. Rhombohedral- and prismatic PCC show an unexpected decrease in particle size between 0 and 250 days. This can be attributed to de-agglomeration of particle aggregates. The particle size distribution of scalenohedral PCC remains constant at this stage. At later stages the particle size distribution does not change significantly for scalenohedral- and rhombohedral PCC, while for prismatic PCC a slight particle growth is indicated. It is obvious that scalenohedral- and rhombohedral PCC develop more euhedral crystallites with smoother faces and sharper edges during recrystallization (see inserts in the SEM images in Table 1). Only for prismatic PCC a transformation of the crystal habitus is observed. During the long term reaction, particle morphology changes noticeably from irregular particles with indications of prism surfaces towards rhombohedral particles.
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The fine grained PCC exhibits a transformation from rounded-, irregular particles towards rhombohedra. Fast initial recrystallization directly merges to zero net-recrystallization. The final 45Ca-activity concentration coincides within uncertainty with the expected equilibrium value of 1.98 Bq/mL (uncertainty < 0.1%), strongly suggesting the formation of a homogeneous bulk solid in equilibrium with the aqueous solution during a 3-4 months reaction period.
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Ground calcite is the most reactive calcite powder investigated. The initial rate is about one order of magnitude higher compared to other calcite powders and is comparable with precipitation rates in surface controlled precipitation experiments [14, 15]. During fast initial recrystallization the 45Ca-activity decreases significantly below the expected equilibrium value (cf. Figure 1 and ESI), thus strongly suggesting that heterogeneous recrystallization must at least partially play a role during this stage. At later stages 45Ca is released back into solution finally reaching a constant steady state activity, which is significantly higher than the expected equilibrium value. This in turn demonstrates that 45Ca-uptake in ACS Paragon Plus Environment
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this system is only partially reversible. Assuming that the initial uptake reaction was fully heterogeneous, the measured release of 45Ca back into solution can be explained by a homogenization of 7 % of the initially recrystallized calcite (0.5% of total calcite). This amount corresponds to about two monolayers. In the rest of the initially recrystallized calcite, 45Ca is irreversibly bound. Obviously, in the ground calcite system the backward reaction is limited to a near surface region. Starting from completely irregular particles, left by the grinding process, ground calcite develops euhedral rhombohedral features during recrystallization (Figure 1). Ground calcite exhibits to some extent the expected particle size evolution from small towards larger particles. We interpret these results in a way that the forward reaction is driven by interfacial free energy. Once this driving force is consumed, further recrystallization (45Ca release) is limited to a narrow near surface region.
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pH-jump measurements
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The observations from the recrystallization experiments clearly point towards recrystallization being an interfacial free energy driven process. To test this, and to quantify the associated driving forces, we perform pH-jump measurements. In these we monitor pH and Ca2+ activity in calcite equilibrium solutions during the addition of calcite powder using pH- and Ca-sensitive electrodes (cf. ESI). It turned out that reproducible results, with signals clearly above electrode drift can only be obtained for highly reactive ground calcite (Figure 2). Ground calcite has an increased solubility relative to bulk calcite. Addition of ground calcite to a previously equilibrated solution leads to rapid dissolution of a small portion (< 0.1% at our experimental conditions) of ground calcite causing the documented increase in pH and Ca2+ activity. By modelling the recorded changes in solution composition (cf. ESI), an apparent supersaturation state (SIapp = log10((a(Ca2+) a(CO32-))/KSP), where a denotes activities of aqueous species and KSP = 10-8.48 is the solubility product of bulk calcite) relative to the solubility of bulk calcite is calculated. For freshly ground calcite the maximum SIapp reaches values up to 0.8 (Figure 2), indicating that at the experimental conditions freshly ground calcite can even be less stable than vaterite (log10(KSP(vaterite)) – log10(KSP(calcite)) = 0.57 < SIapp_max) and aragonite (log10(KSP(aragonite)) – log10(KSP(calcite)) = 0.14
