ARTICLE pubs.acs.org/IECR
Growth Behavior of Aragonite under the Influence of Magnetic Field, Temperature, and Impurity Steven S.S. Wang, Meng-Chun Chang, Huan-Chieh Chang, Ming-Hui Chang, and Clifford Y. Tai* Department of Chemical Engineering, National Taiwan University No. 1, Sec.4, Roosevelt Road, Taipei, Taiwan 106, Republic of China ABSTRACT: With and without magnetization, the growth rates of aragonite seeds were measured in a stirred-tank crystallizer in the presence of Fe2+ or Sr2+ acting as an impurity at two temperature levels, using the constant-composition technique. The concentration range of the impurity investigated was between 0 and 2 ppm, and the temperature settings were either 25 or 35 °C. The individual factors each had a distinct effect on the growth rate of aragonite, and the combined effects became complicated, especially in the presence of Sr2+, which changed the surface structure of the aragonite seed dramatically.
1. INTRODUCTION Scale prevention using a magnetic water treatment device (MWTD) in the cooling water system has been a controversial issue for a long time, because effective and ineffective cases have been reported. The situation is similar to that judged from the results of laboratory-scale studies on the nucleation and crystal growth of calcite, which is the major component of scale.1 In order to explore the mechanism of the magnetic effect, the nucleation and growth behaviors of calcite and aragonite (i.e., the most common polymorphs of CaCO3) have been studied in our laboratory using the constant-composition method, which is the most suitable technique for controlling the pH and solution composition and for studying the crystallization of sparingly soluble salts, such as calcium carbonate2,3 and calcium fluoride.4,5 To investigate the magnetic effect on the crystal growth of the CaCO3 polymorphs, the experimental work started by measuring the crystal growth rate of calcite at 25 °C in the absence and in the presence of a magnetic field;6 then, the seed crystals were replaced by aragonite.7 In addition, nucleation of CaCO3 was investigated in the same apparatus. From the experimental facts on nucleation and crystal growth, a mechanism was proposed to explain the effect of a magnetic field.7 The growth experiments of calcite were further conducted in a simulated cooling-water environment (i.e., at higher temperature and in the presence of impurity).8 Then, the MWTD was incorporated into the growth system to observe its effect on the growth of calcite seeds in the same environment.9 The first work published in our laboratory on investigating the magnetic effect dealt with the growth behavior of calcite.6 In the absence of a magnetic field, the growth rates of calcite seeds on the order of 10111010 m/s were measured at 25 °C and within the metastable region. When the supersaturated solution was magnetized with an MWTD, the growth rate was reduced by an order of magnitude; it even stopped growing at lower supersaturations. The growth experiment was carried out further using aragonite as the seed crystal.7 This was provoked by the fact that the magnetic field favors the formation of aragonite at room temperature, at which there is almost no aragonite formed without magnetization.10,11 The effect of the magnetic field on aragonite growth was similar to that on aragonite formation, r 2011 American Chemical Society
i.e., the aragonite seed did not grow at room temperature (25 °C) but grew at a rate that was close to that of calcite when applying a magnetic field. From the experimental facts regarding the magnetic effect on nucleation and crystal growth, Chang and Tai7 proposed a cluster transformation mechanism to explain the effect of the magnetic field. Although there is no instrument available for identifying the cluster structure, they intuitively suggested that clusters of different structure existed in the supersaturated solution, both with and without magnetization. The clusters would form calcite crystals upon nucleation or incorporate into the crystal lattice of calcite by seeding. When the solution was subject to magnetization, the calcite-like clusters would transform to aragonite-like clusters with an affinity to aragonite seed. Meanwhile, the aragonite would form upon nucleation. To simulate the calcite growth under the condition of cooling water, the growth experiments were further conducted at higher temperatures of up to 40 °C and in the presence of different impurities.8,12 When the growth rates of the calcite and aragonite seeds were measured under the influence of a magnetic field at temperatures above room temperature (25 °C), the synergetic effects were significant but with adverse effects on the different seeds. Again, the cluster-transformation mechanism was used to explain the growth behavior of the two polymorphs. Tai et al.12 conjectured that the two types of clusters—i.e., calcite-like and aragonite-like clusters—were in equilibrium at temperatures above room temperature, with more aragonite existing at higher temperatures. When the magnetic field was applied, the time needed for the calcite to stop growing and for the aragonite to reach a steady growth—meaning that all the clusters had transformed—was shorter at higher temperatures. As far as the impurity effect was concerned, it was rather difficult to predict. Earlier, a few studies on the impurity effect had been reported. In 1993, Sobbides and Koutsoukos13 conducted a growth experiment of calcium carbonate polymorphs, including calcite, aragonite, and vaterite, in seawater containing Mg2+ at a pH of 8.5 and Received: July 26, 2011 Accepted: December 12, 2011 Revised: December 8, 2011 Published: December 12, 2011 1041
dx.doi.org/10.1021/ie2016015 | Ind. Eng. Chem. Res. 2012, 51, 1041–1049
Industrial & Engineering Chemistry Research
ARTICLE
Figure 1. Schematic diagram of constant-composition crystallization system with a magnetic water treatment device (MWTD) incorporated into the design.
a temperature of 25 °C. They reported that aragonite would grow on the three seeds, but the growth rates would be different in the order that follows: aragonite > vaterite > calcite. Meanwhile, Katz and his co-workers2,14,15 investigated the effects of cationic impurities, including Fe3+, Fe2+, and Cu2+, on calcite growth, which was inhibited by the three cations. Recently, two cations, Fe2+ and Sr2+, were chosen from the two groups, based on the atomic radius being smaller and larger than that of Ca2+, respectively, for a further study of impurities in our laboratory8 The former, which favors the formation of aragonite,16 reduced the calcite growth rate in accordance with the result reported by Herzog et al.,14 while the Sr2+, which favors the formation of calcite,16 accelerated the calcite growth rate. Tai et al.8 postulated that the adsorption of different impurity species on the crystal surface brings forth the adverse effect. Then, the impurity effect was investigated in the presence of the magnetic field.9 The calcite did not grow without adding an impurity. When a small amount of either impurity (i.e, 0.5 ppm) was added to the solution, the calcite growth rate jumped to a significant value. Then, the growth rate decreased and increased with a further increase in the Fe2+ and Sr2+ concentration, respectively. Liu9 used the cluster transformation mechanism again to explain the sudden jump of the growth rate at the impurity concentration of 0.5 ppm. In the absence of impurity, all of the calcite-like clusters existing in the solution had transformed to aragonite-like clusters after magnetization, resulting in no growth of calcite; however, the presence of either impurity would cause the two types of clusters to be in equilibrium. Therefore, the growth rates were measurable, even at the low concentration of 0.5 ppm. As for the adverse effect of the two impurities at higher concentrations, they postulated that the adsorption of different impurity species on the crystal surface brought forth the difference. For the case of Fe2+, the adsorption of more FeCO3 at higher impurity concentrations blocked more growth sites, thereby reducing the growth rate as suggested by Herzog et al.14 On the other hand, the adsorption of more Sr2+, which favors the formation of calcite at a higher impurity concentration, caused more nuclei to form on the seed surface, thus accelerating the growth rate. In addition, the equilibrium constant between the two types of clusters in the
presence of an impurity might be a function of the impurity concentration and may cause the difference in the growth rate. In this study, aragonite growth rates under the influence of a magnetic field, temperature, and an impurity will be measured in a stirred tank, using the constant-composition technique. The type of magnetic device and impurity, along with the temperature level chosen, will be the same as those used in the previous study of calcite, i.e., Descal-A-Matic DC-1 for the magnetic device, Fe2+ and Sr2+ for the impurity, and 25 and 35 °C for the two temperature levels.9 First, in the absence of the magnetic field, the aragonite growth rate data will be collected at the two temperatures and at various concentrations of each impurity. Then, the growth experiment will be repeated under the same operation conditions, but in the presence of a magnetic field. Meanwhile, the microstructure of the seed crystals will be examined for the cases of different growth environments in order to explain the effects of the three factors.
2. EXPERIMENTAL SECTION 2.1. Preparation of Aragonite Seed Crystals. The procedure of preparing aragonite seeds was the same as that reported by Chang and Tai.7 Here, we will briefly describe the curing step, which is crucial for the reproducibility of the growth rate data. A chunk of natural aragonite from Morocco was crushed and ground into small particles, then sieved to give a sample with an average size of 774 μm using 16- and 24-mesh sieves. The sieved seeds were rinsed with ethyl alcohol and deionized water before being stored in a sealed bottle. Before the growth experiment, the required amount of seeds were cured in the growth apparatus with the magnetic water treatment device (MWTD) incorporated for about 5 h to make sure that the seed surface was covered with acicular aragonite, which will be described later. The following solution conditions for curing were chosen: pH 9.0; ionic strength, I = 0.018 M; relative supersaturation, ) = 5.54; and temperature, T = 298 K. σ = 0.73; R (aCa2+/aCO2 3 2.2. Experimental Apparatus and Procedure. The experimental apparatus shown in Figure 1 and the procedures adopted in this study are similar to those reported by Chang and Tai,7 except for the case of adding impurity. The crystallizer, a 6-L 1042
dx.doi.org/10.1021/ie2016015 |Ind. Eng. Chem. Res. 2012, 51, 1041–1049
Industrial & Engineering Chemistry Research
ARTICLE
Figure 2. Photographs of aragonite seed at different stages: (a) aragonite chunk, (b) surface of ground aragonite, (c) surface of seeds after curing, and (d) surface of seeds after growth.
agitated vessel, was fitted with a jacket connected to a constant water bath for temperature control and a stainless-steel mesh at the outlet to prevent seed crystal from going through the circulation loop. At the beginning of an experiment, a supersaturated solution prepared following the procedure stated in the previous study6 was circulated at a rate of 2 L/min through the MWTD (Descal-A-Matic, Model DC-1) before flowing back to the vessel for 20 h. This premagnetization step was necessary for obtaining a constant growth rate of aragonite seeds, as indicated by Chang and Tai.7 During the premagnetization period, the pH, temperature, and Ca2+ potential became steady; then, a batch of 10 g aragonite seed with an average size of 774 μm was added into the vessel. When the seeds started to grow, the solution concentration and pH would decrease, because of the deposition of solute causing the release of H+. Nevertheless, they were maintained constant using two autotitrators: one for feeding CaCl2 and Na2Co3 solutions in stoichiometric ratio simultaneously and the other for adding NaOH solution. The titration volume of the reactant was recorded automatically to give the titration curve, which was used to calculate the growth rate, according to the following equation derived by Tai et al.:3 G¼
dva LM 2þ ½Ca a ½Ca2þ 0 3W dt
ð1Þ
of the titration curve, which should be a straight line for steady growth. The concentration of the titration solution [Ca2+]a was 0.1 M and the [Ca2+]0 differed with the degree of supersaturation. To ensure the steady growth, an experimental run typically lasted for 1 h. Then, the seed crystals were filtered, washed with deionized water and alcohol, and air-dried for examination via scanning electronic microscopy (SEM). The growth rates of aragonite were also measured in the absence of the magnetic field. In this case, the circulating loop of the crystallizer was removed and the premagnetization step was skipped. The other steps of the experimental procedure were essentially the same as those in the presence of the magnetic field. 2.3. Correction of Temperature-Related Constants. The equation for estimating the relative supersaturation (σ) is the same as that used by Chang and Tai7 in the study of calcite: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðaCa2þ ÞðaCO2 Þ 3 1 σ ¼ Ksp sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2Þ γ2 ½Ca2þ ½CO2 3 ¼ 1 Ksp The solubility product (Ksp) of aragonite is different from that of calcite. It can be expressed as a function of temperature:17
where G is the growth rate, L the mean crystal size, M the molecular weight of CaCO3, W the weight of seed crystals, [Ca2+]a the Ca2+ concentration of the titration, and [Ca2+]0 the Ca2+ concentration of the original solution; va is the titration volume, and dva/dt is the slope
log Ksp ¼ 171:9733 0:077993T þ þ 71:595 log T 1043
2903:293 T ð3Þ
dx.doi.org/10.1021/ie2016015 |Ind. Eng. Chem. Res. 2012, 51, 1041–1049
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Growth Rates of Two Batch Seeds Under the Influence of Magnetic Field and Fe2+a Growth Rate ( 1010 m/s) [Fe2+] = 0.5
[Fe2+] = 1.0
[Fe2+] = 1.5
[Fe2+] = 2.0
seed batch
ppm
ppm
ppm
ppm
Batch I
5.37
5.35
5.00
5.07
Batch II
5.25
4.98
5.12
5.28
variation
2.2
6.9
2.4
4.1
(%)
The other operating conditions were as follows: σ = 1.25, pH 9.0, I = 0.018 M, R = 5.54, T = 25 °C.
a
Figure 3. (a) XRD pattern of natural ground aragonite. (b) JCPDS XRD standard of aragonite.
where T is the absolute temperature (given in Kelvin). On the other hand, the activity coefficient (γ) varies little with temperature in the range studied (i.e., 2540 °C, according to the Davies equation).12 The calculated values of 0.588 and 0.580 were used for temperatures of 25 and 40 °C, respectively. In addition, the constants in the mass-action equation also should be corrected for temperature.18 They are not presented here because there are too many to list.
3. RESULTS AND DISCUSSION 3.1. Surface Structure of Aragonite Seed. The photographs of aragonite seed at different stages of the experiment are shown in Figure 2; here, Figure 2a shows a chunk of natural aragonite that is used as the starting material, Figure 2b shows the surface of the ground aragonite, Figure 2c shows the surface of the seeds after curing, and Figure 2d shows the seed surface after growth. The surface of the ground natural aragonite is rather flat, with some crystal dust adhering to it, as shown in Figure 2b. After the curing process, as shown in Figure 2c, the surface of the aragonite seeds was covered with acicular particles of micrometer size, which is the most common form of aragonite. After the growth process, the acicular particles grew larger, as shown in Figure 2d. Thus, the linear growth rate measured in this experiment is the increment of solute volume deposited on the acicular crystals divided by the surface area of the uncured crystals. To ensure the structure of seed crystal, the ground particles were analyzed via X-ray diffraction (XRD), as shown in Figure 3,
together with the JCPDS file card of aragonite. Note that the peaks appearing in Figures 3a and 3b match each other. 3.2. Reproducibility of Growth Rate Data. As shown in Figure 2d, the acicular aragonite on the seed surface grew larger during the growth experiment, causing variation in the growth rate for continuous use of seeds. To overcome this problem, the aragonite seed was replaced by a batch of freshly cured sample after a series of experiments (i.e. about five runs of the experiment). In this way, the surface area for growth should not change too much, since the growth rate is low. Nevertheless, the reproducibility of the growth rate data should be checked using a different batch of fresh seed. The reproducibility has been reported by Tai et al.12 in the absence of impurity. The difference between two batches of seed ranges between 3.8% and 11.2% for various supersaturations. In this study, the reproducibility of the aragonite growth rate was checked again when the impurity Fe2+ was present in the solution. The growth rates were measured at various Fe2+ concentrations, ranging from 0.5 ppm to 2.0 ppm, for two batches of seed crystals, as shown in Table 1. The difference between the two sets of data is
