Article pubs.acs.org/crystal
Effect of Ultrasonic Treatment on Early Growth during CaCO3 Precipitation R. Martijn Wagterveld,*,†,‡ Henk Miedema,† and Geert-Jan Witkamp†,‡ †
Wetsus, Centre of Excellence for Sustainable Water Technology, P.O. Box 1113, 8900 CC Leeuwarden, The Netherlands Laboratory for Process Equipment, Delft University of Technology, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands
‡
S Supporting Information *
ABSTRACT: The present study focuses on the effect of ultrasound (42 kHz, 7.1 W cm−2) on the free drift precipitation of CaCO3 from a clear, supersaturated solution. To delineate the way ultrasound exerts its effect, we applied different treatment periods (time windows). Applying ultrasound during the first 10 min of the experiment did not result in any significant effect which rules out an influence on primary nucleation. The application of ultrasound starting later in the experiment enhanced precipitation of CaCO3. The dominant mechanism responsible for enhanced precipitation is deaggregation during the early growth phase (nuclei to crystals conversion regime). This effect is attributed to shear induced by micromixing and/or shear/stress induced by (supersonic) shockwaves, as a result of cavitation. With ultrasound applied, online pH and scattering measurements displayed a reduction in induction time and an increase in the volumetric precipitation rate. Scanning electron microscopy analysis shows that ultrasound increases the total number of particles that has, in addition, a more uniform size distribution compared with the untreated experiment. Consequently the available surface area for growth is higher resulting in a higher volumetric precipitation rate. With and without ultrasound the formed polymorph was predominantly vaterite with small amounts of calcite.
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INTRODUCTION Calcium carbonate is one of the most abundant minerals on earth, and its scaling propensity is a problem in many industrial water treatment processes. Moreover it is an important raw material in a wide range of industries. Controlling the formation of calcium carbonate is therefore of great interest, and as a result the crystallization of CaCO3 from clear solution (spontaneous precipitation) has been investigated extensively.1−15 In the initial phase of precipitation, or nucleation, clusters of calcium carbonate obtain a critical size. In the past decade more evidence has pointed to the existence of a two-step nucleation mechanism16 in which the nucleation of a crystal occurs within stable mesoscopic clusters of dense liquid. These prenucleation phases were also found in calcium carbonate experiments.17,18 Calcium carbonate nucleation, in this case the vaterite polymorph, consists of several steps. Prenucleation clusters are formed (∼1 nm) and aggregate to nanoparticles with a size distribution of around 30 nm.18 These particles aggregate and grow out at the expense of others. The nanocrystalline domains share the same three-dimensional (3D) orientation, resulting in the development of a single crystalline structure.18,19 Mostly, some time elapses before a measurable amount of the newly formed material is detected, the induction time, or induction period, and it marks the ability of the solution to stay in a metastable state. This induction time is heavily dependent on the measurement method and various methods have been used by others.4−7,20,21 The application of ultrasound during crystallization and precipitation processes receives increasing attention. Ultra© 2012 American Chemical Society
sound can help controlling the course of these processes and is also referred to as sonocrystallization.22 The positive effects obtained in sonocrystallization are usually ascribed to effects caused by cavitation, a phenomenon occurring when a liquid is exposed to high power ultrasound. Cavitation is the interaction of (acoustic) pressure waves with cavities (microbubbles), caused by the rupture of liquid in the negative pressure cycle. Microscopic bubbles grow and collapse under the varying pressure field inside the treated liquid. Several effects can occur during this process: the formation of radicals, the generation of shockwaves and microjets, creation of local hotspots of high pressure and temperature, micromixing, macromixing and rise of bulk temperature.23 Several authors already investigated the effect of ultrasound on the precipitation of calcium carbonate from solution24−30 but the results are not unambiguous.30 Previously the authors reported on the effect of ultrasound on the growth phase of calcium carbonate.31 The volumetric crystal growth rate of calcite in a constant composition experiment was increased by ultrasonic treatment. High speed recordings and scanning electron microscopy (SEM) revealed that ultrasound caused the suspended crystals to deagglomerate, disaggregate, and accelerate. Also particle breakage and attrition occurred in these experiments. This created a larger specific surface area available for growth, leading to larger volumetric growth rates.32 Received: May 1, 2012 Revised: July 19, 2012 Published: August 2, 2012 4403
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light entering the photodiode and reduce the noise, the sensor, including flow-through cuvette, is placed in an electrically grounded metal box. The current generated by the scattered laser light is amplified (AD711, Analog Devices) and measured with dedicated LabVIEW software (LabVIEW 2010, NI PCI-6251 DAQ). In order to avoid any possible influence or damage by the ultrasonic irradiation, the pH electrode is placed in a glass cell in a bypass stream. The pH electrode is equipped with a temperature sensor. A positive displacement membrane pump was used to pump the reactor liquid with 1.0 dm3 min−1 through the bypass.
The present study aims to shed light on the basic processes underlying the effects of ultrasound. A prerequisite to pursuit that goal requires a firm definition of terms. We therefore distinguish three stages in precipitation. The first one is primary nucleation, either homogeneous or heterogeneous. In this work this stage is also referred to as “nucleation”, characterized by high supersaturation. The second one is the “early growth” stage when secondary nucleation can take place and crystals grow out to detectable size. During this stage the supersaturation starts to decrease. The last stage is that of “late growth” during which formed crystals continue to grow and the supersaturation is reducing to saturation. In order to delineate the way ultrasound exerts its effect, we applied different treatment periods (time windows). Calcium carbonate formation is monitored by three independent parameters: pH, light scattering, and SEM.
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Experimental Procedures. Free drift experiments were started by simultaneously adding 500 cm3 of CaCl2 and 500 cm3 NaHCO3 solution to the reactor. The experiments were performed in an open reactor. Before addition, the pH was adjusted to the desired value (8.79) by adding NaOH to the former solution, and the ionic strength of the mixture was adjusted to 0.1 by adding KCl to the latter solution. The mixture (see Table 1 for composition) was stirred at 400 rpm,
EXPERIMENTAL SECTION
Chemicals. Only analytical grade reagents, grade A glassware, and high quality water (Milli-Q Reagent Water System, resistivity > 18 MΩ·cm) were used throughout the experiments. Calcium chloride (CaCl2·2H2O), sodium bicarbonate (NaHCO3), and sodium hydroxide (NaOH) were obtained from VWR (Amsterdam, The Netherlands), and potassium chloride (KCl) was from Sigma-Aldrich (Zwijndrecht, The Netherlands). Experimental Setup. A schematic representation of the experimental setup used is presented in Figure 1. A double-walled
Table 1. Solution Composition and Conditions in the Reactor (After Mixing)a [CaCl2] (mM)
[NaHCO3] (mM)
[KCl] (mM)
[NaOH] (mM)
S (−)
4.0
2.0
86.0
0.2
2.10
a
Supersaturation ratio S is defined in eq 2, with respect to the vaterite polymorph at 298 K, pH 8.79 and an ionic strength of 0.10. Calculations based on software: Visual Minteq v3.0; model: Davies.
and the temperature of the solution was maintained at 298 ± 0.1 K by circulating cooling water around the reactor. The nucleation and subsequent growth could now be monitored by a drop in pH and a rise in voltage of the static light scattering sensor. The newly formed crystals were allowed to grow for some time, and samples were taken by filtering 25 cm3 of solution over a 0.2 μm filter (Isopore, Millipore). The dried crystals were characterized by scanning electron microscopy (Jeol JSM-6480LV). Raman spectroscopy (Horiba Jobin-Yvon LabRAM HR) was performed on the dried crystals to confirm the obtained polymorph. During the experiments, aliquots of solution were rapidly removed, filtered through a 0.2 μm filter, dried, and analyzed for the calcium content with inductivecoupled plasma spectrometry (Optima 3000XL, Perkin-Elmer) to measure the degree of supersaturation. In all cases the applied ultrasonic frequency was 42 150 Hz and the intensity 7.1 W cm−2 or 17 W dm−3 (real output power predetermined by measuring the adiabatic temperature rise in time using a similar well isolated glass reactor filled with 1 dm3 of water). All experiments started with the same initial conditions as given in Table 1. The fundamental dimensionless driving force for precipitation in electrolyte solutions is defined as follows:34
Figure 1. Experimental setup used consisting of (1) double walled thermostatted glass reactor, (2) floating magnetic stirrer bar, (3) pump, (4) ultrasonic transducer, (5) light scattering sensor, (6) pH electrode with integrated temperature sensor, (7) sampling port.
Δμ = v ln S (1) RT −1 Here Δμ (J) is the change in chemical potential, R (J K ) is the gas constant, T (K) is the absolute temperature, v (−) is the number of ions in the formula unit (v = 2 for CaCO3), and S (−) is the supersaturation ratio, which is expressed in terms of activities:
thermostatted glass reactor, equipped with a floating magnetic stirrer bar (Nalgene) to minimize any grinding effects, a bypass loop, and an ultrasonic transducer were used. The ultrasonic transducer was part of a dedicated home-built system that could be controlled precisely in terms of shape, frequency, and amplitude of the alternating current for driving the transducer. Free drift experiments33 were conducted and nucleation/growth were determined online by recording pH (CPS11D, Endress+Hausser, buffer accuracy ±0.02 pH) and static light scattering using a home-built sensor. This sensor consists of 3 mW 850 nm diode laser (LDM850/RLJ, Roithner-laser) coupled into a quartz flow-through cuvette (Hellma) and a photodiode (SFH213FA, Osram) placed at an angle of 90°. To prevent ambient
⎛ IAP ⎞1/ v ⎟⎟ S = ⎜⎜ ⎝ K sp ⎠ 4404
(2)
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where IAP is the ion activity product and Ksp is the thermodynamic equilibrium solubility product. The supersaturation is pH- and temperature-dependent and polymorph specific.34 Not included in the supersaturation ratio is the sizedependent solubility, defined by the Gibbs−Thomson (also known as Gibbs−Kelvin or Kelvin) relation.34 This relation states that small crystals have a higher solubility than large crystals. As a result of supersaturation, created at the start of every experiment, critical clusters are being formed, a process called (primary) nucleation, followed by the cluster outgrowth and/or agglomeration and eventually resulting in detectable crystals (growth). Apart from SEM measurements, pH recordings are at the heart of this study and the conclusions drawn. A critical reflection on the precision and repeatability of the procedure followed is therefore mandatory. The precision of the pH electrode itself is 0.01 pH unit (n = 10). In addition, we determined the repeatability of the pH recording during the precipitation experiments employing two types of methods (criteria) and using two independent data sets (pH vs time) recorded under identical experimental conditions. The first method calculates the mean absolute error (ideally, |ΔpH| = 0) of the difference between the pH recorded at time t during the first experiment and the pH recorded at the corresponding time t during the duplicate experiment. For the blank (1) and ultrasound experiment (2), |ΔpH| (±s.d. with n = 500) is 0.009 ± 0.005 and 0.020 ± 0.017, respectively. The second method relies on the calculated correlation coefficient (ideally, R2 = 1) of the two pH data sets (pH vs pH). For the blank (1) and ultrasound experiment (2), R2 = 0.995 and 0.979, respectively. These results make us conclude that the pH recordings during the experiments are highly repeatable.
Figure 2. Free drift experiments performed under five different conditions: (A) pH (−) vs time (s) measurements, and (B) scattering (A.U.) vs time (s) measurements. The bars just above the time axis represent the ultrasonic treatment periods corresponding with the following time windows: 1. Blank: No ultrasonic treatment applied; 2. Ultrasound: Complete ultrasonic treatment (t = 0−4500 s); 3. Ultrasonic treatment applied during the first 10 min only (t = 0−600 s); 4. Ultrasonic treatment applied from 10 to 20 min only (t = 600− 1200 s); 5. Ultrasonic treatment applied from 30 to 40 min only (t = 1800−2400 s).
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RESULTS Free drift experiments were performed to investigate the effect of ultrasound on spontaneous precipitation of calcium carbonate. The ultrasonic treatment was started at different moments in time and applied during several treatment periods (see bars above the time axes in Figure 2). Five regimes can be distinguished: 1. Blank: No ultrasonic treatment applied. In this experiment the ultrasonic source is in the solution, but there is no driving voltage. 2. Ultrasound: Complete ultrasonic treatment, starting when adding the CaCl2/NaHCO3 mixture until the end of the experiment after 75 min (t = 0−4500 s). 3. Ultrasonic treatment applied during the first 10 min only (t = 0−600 s). 4. Ultrasonic treatment applied from 10 to 20 min only (t = 600−1200 s). 5. Ultrasonic treatment applied from 30 to 40 min only (t = 1800−2400 s). Experiments 1−5, as described above, can be compared by the pH profiles (see S2, Supporting Information) as shown in Figure 2A, since precipitation of CaCO3 leads to a decrease in pH under the chosen conditions. In addition, particles form and cause scattering of incident light. The amount of scattering per unit of time depends on both the size and number of particles present. The online measurement of scattering is therefore a (qualitative) measure of CaCO3 precipitation. The scattering profiles of experiments 1−5 can be found in Figure 2B. It shows the average of scattered light intensity (of 1 s, sampling
frequency 100 s−1) of the incident light measured at an angle of 90°. Scanning electron microscope images (Figure 3) provide more information on the size and habit of the precipitated crystals. Ultrasound during Entire Experiment (0−4500 s). From the onset of the experiments calcium carbonate is formed, releasing protons (see S2, Supporting Information). The induction time marks the interval between the onset of the experiment and the moment this pH decline becomes measurable. Apart from the initial deviation due to a slow response of the pH-electrode, note that until 1000 s the pH in both the blank and ultrasound experiment remains fairly constant. This observation indicates that possible (enhanced) CO2 exchange (see S3, Supporting Information) does not have a measurable effect on pH. We therefore conclude that CO2 exchange can safely be neglected in our experiments. After the induction time has elapsed, the steepness of the pH profile is related to the volumetric precipitation rate (the amount of CaCO3 precipitated per unit of time). Comparing the pH profiles of the blank and ultrasound experiments, experiments 1 and 2 as described above, reveals that the complete ultrasonic treatment decreases the induction time and increases the volumetric precipitation rate (Figure 2a), with much faster dropping of the supersaturation. 4405
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Figure 3. Scanning electron microscope (SEM) images of samples taken at t = 2250: (A) Blank experiment (1), (B) ultrasound experiment (2), complete treatment; samples taken at t = 4500: (C) blank experiment (1), (D) ultrasound experiment (2), complete treatment. Sample volume: 25 cm3, filtered over a 0.2 μm membrane filter (visible as background).
Figure 4. Particle size distribution (PSD) of blank and ultrasound experiments (complete treatment), based on the samples of Figure 3, presented as size-frequency histograms with a logarithmic x-axis. (A) Frequency and (B) relative frequency PSD at time t = 2500 s of blank experiment (1) (gray line; from Figure 3A) and ultrasound experiment (2), complete treatment (black line; from Figure 3B); (C) frequency and (D) relative frequency PSD at time t = 4500 s of blank experiment (1) (gray line; from Figure 3C), and ultrasound experiment (2), complete treatment (black line; from Figure 3D).
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the Gibbs−Thomson relation) and the recrystallization of one polymorph to form another (Ostwald’s rule of stages).34 Figure 3C,D shows the SEM images of the blank and ultrasound experiment respectively at the end of the experiment. In both images some crystals of the calcite polymorph are visible (cubic shaped, polymorph confirmed with Raman spectroscopy,35 S5 Figure 2b, Supporting Information). The edges of the vaterite crystals, especially those in the ultrasound experiment, are more ragged compared with the crystals halfway in the experiment. This points at dissolution of the edges.34 The total number of crystals at the end of the ultrasound experiment is also a bit less than halfway in the experiment, as can be derived from the frequency PSD (Figure 4C). The crystals have a wider distribution with both more larger and smaller crystals (the distributions becomes more bimodal). Probably the supersaturation of vaterite is so low (supersaturation ratio, S = 1.20, at the end of the experiment) that small vaterite crystals dissolve while large crystals still grow (Gibbs−Thomson effect).36 Also, dissolved vaterite will reprecipitate to form the calcite polymorph while this polymorph is still supersaturated (supersaturation ratio, S = 2.30) at the end of the experiment (t = 4500 s). The blank experiment on the other hand does not show a drop in scattering before t = 4500. At the end of this experiment crystals are larger compared with crystals halfway. The relative PSD profiles of the blank experiment are similar (only the crystals grew larger) and the total number of crystals did increase for this experiment. The same drop in scattering is also present in the blank experiment though, but takes place after t = 4500 and, consequently, is not shown in Figure 2. To decipher the effect of ultrasound on precipitation, notably to study the stage during which ultrasound exerts its effect, the treatment periods were varied, as described above for experiments 3−5. Ultrasound during Primary Nucleation (0−600 s). To investigate the effect of ultrasound on the initial phase of precipitation, the formation of clusters or nuclei, ultrasound is applied during the first 10 minutes of the experiment only (t = 0−600 s), regime 3. The assumption is made that the process of nucleation starts upon the first instance of supersaturation, in this case time t = 0. Because of the small size of a critical nucleus, the actual initial process of nucleus formation cannot be determined. Later processes in time can be followed however by looking at the pH and scattering profiles during the precipitation experiments. If ultrasound increases the kinetics of (critical) nucleus formation, this should result in a shorter induction time, visible in the pH and scattering profiles. Additionally, if ultrasound increases the concentration of possible nucleation sites this should be noticeable as an increase in volumetric precipitation rate and possibly a shorter induction time. The latter is valid when the number of particles influences the time of detection, which seems to be the case for both the pH and scattering measurements. As Figure 2 shows, both the pH and scattering profile of experiment 3 show no clear difference with the blank experiment. This finding makes us conclude that ultrasound has no measurable effect during the first 10 min of CaCO3 precipitation. Ultrasound during Early Growth (600−1200 s). In contrast to the unchanged profiles of experiment number 3, when ultrasound is applied a little later, from t = 600−1200 s (regime 4) there is a clear effect on both the pH and scattering profile compared with the blank experiment, Figure 2. The treatment is applied when, in the blank experiment, there is no
A similar time course as seen in the pH-profile was observed during the scattering response, Figure 2B. Here, the initial high levels of scattering are caused by bubbles created during mixing. Over time these bubbles disappear from the solution leading to a decrease in scattering as observed over the first minutes. Note that in case of the ultrasound experiment the bubbles disappear much faster. As seen in the pH measurement, during the induction time the level of scattering in both the blank and ultrasound experiment does not change. The ultrasound experiment deviates from the blank experiment by a shorter induction time (now defined as the time the scattering profile starts to increase). This confirms that the reduced induction time, as seen in the pH measurement, is related to enhanced precipitation and not to possible CO2 exchange. The ultrasound experiment also shows a faster increase in scattering, which might be seen as an increased volumetric precipitation rate, and a larger amount of overall scattering. This last observation can be caused by larger particles, more particles or a combination of both. Experiments performed under the same conditions showed some deviation in the induction time, but the precipitation profiles (steepness and overall scattering) were very similar and repeatable (S4 Figure 1, Supporting Information). Figure 3A,B shows the scanning electron microscope (SEM) images of the blank and ultrasound experiment (complete treatment), respectively, of samples taken halfway in the experiment, at t = 2250 s. Before this moment in time, growth had started in both experiments. The ultrasound experiment has a lower pH, and thus lower supersaturation, but also a much higher level of scattering than the blank experiment. The precipitated polymorph appears to be vaterite in both cases (hexagonal shaped, polymorph confirmed with Raman spectroscopy,35 S5 Figure 2a, Supporting Information). Figure 4A,B shows the particle size distribution (PSD) and relative particle size distribution of these images, respectively, presented as size-frequency histograms with a logarithmic xaxis. The SEM images show a representative portion assuming a uniform particle distribution in both the reactor and on the filter. The frequency PSD shows the amount of particles with a certain size range; the relative frequency PSD is normalized with the total number of particles. The frequency particle size distribution (Figure 4A) shows that there are many more particles present in the case of the ultrasound experiment compared with the blank. However, the relative size distribution of Figure 4B shows that the sizes of the particles in both experiments are similar. The particles in the blank experiment are even slightly larger. The higher amount of scattering in the ultrasound experiment is thus predominantly caused by an increase in the number of particles in this particular case. The higher volumetric precipitation rate is therefore ascribed to the larger number of particles, implying an inherent larger total surface area available for growth. Interestingly the scattering profile of the ultrasound experiment, Figure 2B, shows a slight decrease toward the end. This suggests that particles dissolve. However, evidence obtained from the simultaneously recorded pH profiles, points to the conclusion of calcium carbonate growth, displayed by the continuous pH decrease (Figure 2A). Several mechanisms, or a combination of those mechanisms, can be responsible for the decline in scattering while the pH decreases: agglomeration of crystals (resulting in less particles), scaling on the walls of the experimental equipment, dissolution of small crystals resulting in extra growth of large crystals (Ostwald ripening, driven by 4407
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not expected that radicals change the course of calcium carbonate crystallization directly.30 Nishida25 noticed an enhanced effect on the precipitation kinetics and attributes this to the enhanced mixing, especially macrostreaming. They concluded that there was no direct effect of cavitation on the formed product. This corresponds well with the results in this study. The conditions as used by Nishida are also close to the CO2 equilibrium so any effect of CO2 would have been minor. Interestingly it is also claimed that CaCO3 can nucleate on nanobubbles, as produced in high frequency ultrasound (MHz range), resulting in hollow spheroid nanoparticles.27 These conditions have not been reproduced yet and most of the work is done at lower ultrasonic frequencies (kHz). Keeping the temperature constant by actively cooling rules out possible bulk temperature effects. In some reported experiments the temperature is not controlled,26,28 which led to extreme temperature rises, especially in small volumes, causing much higher supersaturations and different precipitation pathways. In other temperature controlled experiments, only the cooling liquid temperature is maintained constant.25 In that case the temperature inside the reactor might still rise when ultrasound is applied. The work in which the generation of aragonite by ultrasonic treatment, as opposed to vaterite in blank experiments, is described,26,30 can most certainly be ascribed to rising bulk temperatures as is also recognized as such by Price et al.30 To rule out an effect of bulk temperature in our experiments, the reactor vessel is cooled actively. This does not prevent the existence of local hotspots during bubble collapse. However, although these hotspots are said to lead to an increased nucleation rate,26 the absence of a measurable effect when ultrasound is applied from t = 0−600 s (regime 3) shows that this phenomenon is not applicable for the chosen experimental conditions. Another possible explanation for enhanced primary nucleation: cluster segregation due to large pressure gradients in the vicinity of a collapsing bubbles44 seems to be ruled out by these results. Although it is often assumed that ultrasound leads to a larger number of nucleation sites,22,26,30 our study does not validate the conclusion that ultrasound has an effect during primary nucleation. Breakage of crystals, as described previously,32 could also lead to more and smaller particles. However, when particles are too small (
