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Precipitation of calcium carbonate from magnetically treated sodium carbonate solution
Colloids and Surfaces A: Physicochem. Eng. Aspects 225 (2003) 63 /73 www.elsevier.com/locate/colsurfa
Precipitation of calcium carbonate from magnetically treated sodium carbonate solution Emil Chibowski a,*, Lucyna Holysz a, Aleksandra Szczes´ a, Marcin Chibowski b a
b
Department of Interfacial Phenomena, Faculty of Chemistry, Maria Curie-Sklodowska University, 20-031 Lublin, Poland Department of Numerical Analysis, Faculty of Mathematics and Natural Science, Catholic University of Lublin, 20-950 Lublin, Poland Received 27 July 2002; accepted 20 March 2003
Abstract Using a commercial ‘magnetizer’ with two S /S poles of the magnet 0.1 T each, the magnetic field (MF) effect on the in situ precipitated calcium carbonate have been investigated via MF treatment of the Na2CO3 solution prior to the precipitation. The MF was applied perpendicularly to Na2CO3 solution flowing in a silicone tube. In the second series, the MF was applied in quiescent conditions, where a plastic tube with the solution was placed between the poles. The S /S arrangement of the magnetizer poles caused MF strength close to zero in the middle of the tube. The MF treatment time was 5, 20, and 70 min. After that calcium carbonate was precipitated from equimolar volumes of Na2CO3 and CaCl2 (8 10 3 M) solutions at 20 8C or 30 8C and the light absorbance at 543.2 nm and pH of the suspension were recorded as a function of time, up to 30 min. Within this time in most cases the light absorbance dropped to zero. For each tested system 3 /4 replicas of the experiment were made and then average results were taken. It was found that MF effect on in situ precipitated calcium carbonate from Na2CO3 treated in the flowing conditions generally appeared in a slower nucleation and sedimentation rates of the precipitate and also in some differences in pH changes of the suspensions after the solutions mixing. When CaCO3 was precipitated from MF-treated Na2CO3 solution in quiescent environment the maximum absorbance of the light was reduced relative to the reference system by 15, 22, and 37%, for 5, 20, and 70 min MF treatment time (at 30 8C), respectively. The reduction of the absorbance might result from smaller number of the crystals and a larger their size. # 2003 Elsevier B.V. All rights reserved. Keywords: Magnetic field treatment; Calcium carbonate; In situ precipitation
1. Introduction
* Corresponding author. Tel.: /48-81-537-5651; fax: /4881-533-3348. E-mail address: [email protected] (E. Chibowski).
Investigations of magnetic field (MF) treatment effects on the dispersed systems and water properties are of increasing interest. In the Internet search (using google.com) on the web site are looked up about 337 000 entries on ‘magnetic treatment’, 141 000 entries on ‘magnetic water
0927-7757/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0927-7757(03)00133-X
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treatment’, 90 200 entries dealing with ‘magnetic scale treatment’, and 3150 entries on ‘magnetic calcium carbonate treatment’. Many of them originate from various companies offering magnetizers for the scale preventing or treatment, and rather small number of the entries deals with papers on fundamental studies of MF effects on properties of the studied systems. Although presence of some MF effects on the properties of precipitated calcium carbonate seems to be now well documented [1 /10], this issue is still a controversial one [11,12]. While some authors [4] maintain that after magnetic treatment up to 50% reduction of the scale formation took place in a recirculating system, others [13] have found only minor or non-influence of the field. At present most frequently the following MF effects are investigated and/or have been proved experimentally. However, some of the statements are in contradiction each other. For example: / nucleation rate increases */more crystals of smaller size and irregular shapes form [10], / nucleation frequency is suppressed but the particle growth is accelerated [1,5,6]. The MF effect after the solutions treatment can last up to 120 h [1] or 200 h [8], which is called ‘memory effect’, / zeta potential of the precipitated particles is changed [2,10,14], / increases conductivity and pH of the solutions [3,15,16], / at the beginning pH of the slurry decreases, which affects the scale growth [4,5], / particles of calcium carbonate crystallized in magnetic field consist mainly of aragonite and vaterite [8,9], / free radicals and/or reactive oxygen species (ozone, superoxide, hydroxyl radicals, singled oxygen) and others are created, which are stable for days [17], / an increase in dissolved oxygen concentration takes place [18], / the impurity Fe2 or Zn2 ions, if present, can influence nucleation [19,20]. Coey and Cass [8] have found that even weak magnetic field (B :/0.1T) influenced aragonite/
calcite ratio in the precipitated CaCO3 in groundwater from a well sunk in limestone, which contained 132 mg l1 of Ca2 and some amounts of other element ions (Na, Mg, K, Fe, Mn, and Zn) as well as anions NO 3 ; NO2 ; and Cl : The groundwater was drawn through a stack of Teflon-coated ferrite ring magnet or by bypass to omit the magnet. They also used a commercial mineral water in a 500 cm3 bottle on the neck of which the ring magnet was fitted. They have found no systematic effect of the water flow speed through the magnets (from 0.04 to 1.2 m s 1). They tested in total more than 100 samples heating them in an open 500 cm3 beakers at 80 8C for different time of the water incubation (0 /200 h) prior to the heating. The aragonite content in the precipitate increased during first tens incubationhours (ca. 40 h) reaching a maximum. Statistical analyses of 32 pairs of the samples showed that at the 99.9% probability and 3.4 confidence level, magnetic treatment increases the amount of aragonite in the carbonate deposits. Investigating 20 samples of the well water after MF treatment in 14 of them aragonite content has increased, in 3 decreased, and in 3 has not changed. In case of the mineral water for 12 samples the numbers were 11, 0, 1, respectively. Szkatula et al. [21] have conducted large-scale experiments on magnetic treatment on a heating installation using two identical 25 kW heat exchangers, as well as on three blocks of 1 GW power plant. They used a stack of cylindrical strontium-ferrite permanent magnets placed in a ferromagnetic pipe with the field amplitude of 1.5 kOe. The construction of the magnet device caused that the water flowing inside it periodically changed its flow from 1.0 to 1.6 m s 1, as well as the magnetic field profile also changed along the device. After 4-month experiment the authors found significant differences in the water being magnetically treated relative to that which has not been treated, as well as in the amount and hardness of the scale deposited. The scale was hard in the loop of MF-untreated water and soft in the loop where the water first flew through the magnetic device. Moreover, the total deposited amounts of the scale were 190 g and 7 g, respectively, and the scale distribution inside the
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pipes was quite different. The deposit was mostly amorphous after MF treatment and possessed essentially different composition. From the literature it results that efficiency of magnetic field treatment to a high degree depends on many physicochemical parameters, such as the temperature, pH, the ionic composition, the flow rate, as well as some geometrical factors of the treated system, which are difficult to be well established [22]. Lately Baker and Judd [11] have reviewed the literature on MF treatment for the protection of hard scale formation. From the subject literature it results that still more experimental evidences have to be gathered to put forward and to verify the hypotheses dealing with mechanisms of the field interactions and to recognize the parameters affecting MF treatment [11,12], which so far are often controversial, as quoted above. Therefore, it seemed to us interesting to use a commercial ‘magnetizer’ to test whether such a MF affects in situ precipitated calcium carbonate in the laboratory scale, especially at well-defined and controlled systems and conditions. In this paper CaCO3 was precipitated at 20 or 30 8C from magnetically treated Na2CO3 [1], both in flowing and quiescent conditions, and changes in the light absorbance and pH as a function of time were recorded. As the reference system each time the calcium carbonate was also precipitated without MF in the same conditions and using the same solutions.
2. Experimental section Calcium carbonate was precipitated at 20 or 30 8C from equimolar volumes of CaCl2 and Na2CO3 solutions having concentration of 8/ 103 M [1,6]. The diluted solutions (0.3 dm3) were prepared from 0.1 M stock solutions of calcium chloride and sodium carbonate. The salts were of p.a. grade from POCH Gliwice, Poland, and were used as received. Water used was doubly distilled and then deionized using Millipore Q-Plus 185 system. The magnetic field originated from two S /S type permanent magnets, 0.1T each, obtained
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kindly from the Magnetizer Group Inc., USA. Because from the literature [1,6] it resulted that magnetic field affects more sodium carbonate Na2CO3 than calcium chloride CaCl2 solution, therefore the former solution (0.3 dm3) was treated with MF. The solution circulated (with volume speed 4 dm3 min 1) in a silicone tube (3 m long, 15 mm of the outer and 10 mm of the inner diameters) for 5, 20 or 70 min, using Masterflex I/P pump, Model 77601-10 (Cole-Palmer Instr. Comp. USA). The magnetic field was applied perpendicularly to the solution flow. After the MF-treatment of Na2CO3, equimolar volumes (25 cm3) of Na2CO3 and CaCl2 solutions were mixed up instantly and absorbance of the light (at l/543.2 nm) and pH were measured as a function of time up to 30 min. The absorbance was measured using an Avantes (the Netherlands) UVVIS-NIR fiber optic spectrometer DH-2000 and the first reading was taken after ca. 3 min since the moment of MF had ceased. All experiments were replicated 3 /4 times and the average values together with the standard deviations were calculated. In second series of experiments the treatment with magnetic field has been conducted in quiescent conditions, i.e. Na2CO3 solution in plastic tube was placed between the poles of the same magnetizer as used above and treated for 5, 20, and 70 min at 30 8C. Then calcium carbonate was precipitated by simultaneous mixing with equimolar volume of CaCl2 solution and the absorbance was recorded up to 30 min. Prior to these experiments CaCO3 had been precipitated from non-circulated and MF-untreated Na2CO3 as the reference system. Besides the absorbance measurements also deposition of in situ precipitated calcium carbonate was investigated. For this purpose in 50 cm3 beakers glass plate (5 cm2) were placed on their bottom and 10 cm3 of Na2CO3 (MF-treated or untreated) were mixed with 10 cm3 of CaCl2. The suspensions were left for 2 h at 30 8C, then the glass plates were carefully dipped into distilled water to remove non-adhered particles and dried at 30 8C. After that photograph were taken using Nikon optical polarization microscope and the pictures were analyzed as for the number and size of the crystals deposited.
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3. Results and discussion The magnetic field effect on in situ calcium carbonate precipitation is often investigated via measurements of the light absorbance [1,5,6]. From Highashitani et al’s paper [1] and others [6], it results that MF effect is more pronounced if prior to the calcium carbonate precipitation the solution of Na2CO3, but not CaCl2, has been treated with MF field. The MF field irradiation can be realized in static conditions or on the solution circulating in the tube, as well as the field may alter (e.g. rotate) or pulse [7]. As was mentioned above, even weak magnetic field (B :/ 0.1 T) influenced aragonite/calcite ratio in the precipitated CaCO3 in groundwater from a well sunk in limestone as well as commercial mineral water [8]. In our studies we also have used the magnetic field B /0.1 T, but it originated from two permanent magnets of commercial, so called, magnetizer (Magnetizer Co., USA) with south poles (S) fitted around the silicone tube in which Na2CO3 circulated for a defined time, i.e. 5, 20, or 70 min at 20 8C. The magnet poles arrangement caused that in the middle of the tube the MF field strength was close to zero. Three to four replicas of the experiment were taken for each system and the average values were calculated. For each system first precipitation of CaCO3 from magnetically non-treated Na2CO3 solution has been conducted to get the reference results. From the measured absorbances of the precipitated CaCO3 suspensions as a function of time it was seen that MF effect appeared first of all in the kinetic of precipitation. Therefore in Fig. 1 these are shown differences between the absorbance of MF treated and untreated systems (DA ) taken at the same time after both solution mixing. As can be seen all DA curves in Fig. 1 cross DA /0 around 7.5 min. It means that MF treated and untreated suspensions behave in the same way. However, very soon on the time scale again significant DA values appears, which means that MF effect still exists. While this first stage can be considered as primarily nucleation during first minutes and then the crystal growth, so the second one is surely sedimentation of the precipitated particles. It is characteristic that
Fig. 1. Differences in the light absorbances (l /543 nm), DA, of CaCO3 suspensions between MF treated and untreated systems in flowing conditions versus time at 20 8C. The MF treatment time of Na2CO3 solution was 5, 20 and 70 min. The differences are also shown for the CaCO3 suspension precipitated after 1 h from 70 min Na2CO3 treated solution.
5 and 70 min MF treatment causes opposite changes during the nucleation stage (positive and negative DA , respectively), while 20 min MF treatment has only minor influence. Maximum effect during nucleation appears after 2.5 /3.5 min since the solutions mixing. But, while in the suspension obtained from 5 min-MF-treated Na2CO3 solution DA is positive, which means that the nucleation is faster than in the MF untreated solution, so in 70 min-MF-treated Na2CO3 solution the nucleation is shower (negative DA ). Moreover, if CaCO3 was precipitated 1 h after 70 min-MF treatment the affect was still present, and in the second stage even more pronounced than if the suspension was precipitated immediately after the field had ceased. In the sedimentation stage the MF effect is reversed, i.e. if nucleation stage was faster (i.e. 5 min-MF treated), then the sedimentation rate is slower with a minimum after 10 min (Fig. 1). Such behavior indicates that the precipitated crystals were smaller. Contrary, in case of 70 min MF treated Na2CO3 solution; the nucleation was slower but sedimentation faster, indicating that the crystals were larger. It should be mentioned that the total precipitated amount in grams of CaCO3 in all the systems studied should be the same. These results, as for the nucleation stage, are
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in agreement with those found in [1,5,6]. They also throw a light why some authors observed an increased nucleation rate [10], while others suppressed nucleation [1,5,6]. From our experiments it is clearly seen that MF effect depends strongly on the kinetic conditions and the treatment time. Simultaneously with the absorbance measurements pH changes in the suspensions were recorded. Again, to better visualize MF effects in Fig. 2 are presented DpH, which are differences between pH of the MF-treated and untreated suspensions measured at the same time. As can be seen, except for 20 min treated Na2CO3 the DpH are negative during first 10 min. Thus, the magnetic field causes decrease in pH relative to the reference system by 0.05 /0.1 pH units. At the same time in case of 20 min MF treated solution the suspension pH is 0.15 /0.1 units higher than in non-treated suspension. After 10 min since the precipitation of CaCO3 the pH differences increase, and they depend on MF treatment time. From the very beginning the largest and positive differences in pH appeared in 20 min MF treated suspensions while the effect in the nucleation stage was the smallest. So, it may be concluded that initial pH value has an important impact on the nucleation of CaCO3. Two principal reactions taking place during precipitation of CaCO3 are following: Na2 CO3 H2 O l 2Na 2OH H2 CO3 (high initial pH)
(1)
Fig. 2. Differences in pH for the same systems as presented in Fig. 1.
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2Na 2OH H2 CO3 Ca2 2Cl 0¡ CaCO3 2H2 O2Na 2Cl (pH decrease)
(2)
In consequence the pH decreases. Obviously there are accompanying secondary reactions, which influence the suspension pH too [23], among others, surface hydrolysis reactions: CO3 H2 O l HCO3 OH (below pHpzc )
Ca H2 O l CaOHH
(above pHpzc )
(3) (4)
The reactions are determined by H and OH concentration in the solution, i.e. pH of the suspension. With decreasing pH of the suspension an increased solubility of CaCO3 takes place. The isoelectric point (iep) for calcite occurs at pH 8.2 / 8.4 [23]. At pHs above the iep negative species (/ HCO3 and CO2 3 ) dominate in the solution, thus also causing net negative surface charge of calcite, and below pHiep Ca2 and CaOH are present in an excess over negative ones in the solution. So, for calcite principal potential determining ions in solution are: Ca2, CO3 ; HCO3 ; and the secondary ones are H and OH . The neutral sites on the calcium carbonate surface are of course also present, /CaOHo and /CO3Ho [23], and they were found by spectroscopic methods (XSP and LEED) [24]. The above mentioned relatively small changes in the suspension pH may still be significant for the precipitation process as observed via differences in the absorbance [3,4]. Parsons et al. [4] found that using 0.7 T magnet pH of the treated system (at 60 8C) dropped by 0.5 pH units (from 8 to 7.5) in CaCl2 and NaHCO3 mixed solutions. The authors [4] obtained 48% reduction in the scale formation, but there was practically no MF effect if pH was controlled to the initial 8 or 8.5. The effect of magnetic field on the pH can be connected with these secondary reactions. From our other studies on influence of impurity ions on CaCO3 precipitation we concluded that MF affects hydration shell of the ions [25,26]. Because the hydration energy of anion is lower than cations, it explains why MF 2 field effect is a larger on CO ion 3 than Ca solution [27].
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Using Surfer computer program it is possible to present isoabsorbance contour diagrams on pH and time domain, which is called fingerprinting technique [26]. The contours were obtained with a help of the Kriging option of the program, which is a geostatical girding method. The method attempts to express trends that are suggested in the date, i.e. the absorbances on the pH and time domain. We have found that better visualization of the contours was obtained taking ln(t) instead of time. Similarly Prescott et al. [28] applied logarithm of conductivity to present the electrophoretic fingerprints. In Fig. 3 there are shown overlapped isoabsorbances for 5 min-MF treated (broken lines) and MF-untreated (solid lines) suspension of CaCO3, and in Fig. 4 similar fingerprints for untreated and 70 min MF-treated Na2CO3 for CaCO3 precipitated 1 h after the field had ceased. It can be seen in Fig. 3 for 5 min-MFtreated system shifted isoabsorbances relative to those for MF-untreated suspension. In the reference system the absorbances around 10 min are practically pH independent (vertical position) while those of MF treated system are clearly pH dependent. At this time in the untreated system the absorbance values were 0.16 /0.10, while in the MF treated they were 0.28 /0.18. More pronounced differences in the isoabsorbances can be seen in Fig. 4 for 70 min-MF-treated system. The differences can be even better depicted by plotting numerically interpolated profiles of the absorbances vs. time at characteristic pH for the MF-treated and -untreated systems. In case of 5 min-MF-treated system this pH value was 10.3 (see Fig. 3), and it was 10.25 for 20 min-MFtreated (the isoabsorbances contours are not presented here). Thus obtained cuts of the absor-
Fig. 3. Isoabsorbances of light (l /543 nm) for MF-untreated (solid lines) and 5 min-MF-treated in flowing conditions systems (broken line) presented on pH and time domain.
Fig. 4. Isoabsorbances of light (l /543 nm) for MF-untreated (solid lines) and 70 min-MF-treated in flowing conditions systems (broken line) presented on pH and time domain. The calcium carbonate was precipitated 1 h since the field has ceased.
bances versus time are shown in Fig. 5. Here is clearly seen how the absorbances of MF treated systems would be shifted relative to those of untreated reference system keeping the same pH value. The differences are especially large in the stage of the particles sedimentation. In the MF treated systems CaCO3 sediments much slower, particularly in 5 min treated. In the case of 70 minMF-treated system the cuts of the isoabsorbances taken at pH 10.16 (Fig. 6) show differences in the nucleation stage. In the sedimentation stage significant shift is seen for CaCO3 precipitated 1 h after the field has ceased. Very rapid CaCO3 sedimentation took place in 10th minute both in MF-untreated and treated systems, if calcium carbonate was precipitated immediately after the treatment. However, if it was precipitated after 1h, the sedimentation rate was slower, what appeared in a higher absorbances at the same time (Fig. 6).
Fig. 5. Numerically interpolated absorbances vs. time profiles for CaCO3 suspensions at characteristic values of pH (see Fig. 3): 5 min MF at pH 10.3, 20 min MF for pH 10.25.
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The absorbance differences during the nucleation stage at the same pH indicate for a significant role of the initial pH for MF effect. This was also found by Parsons [4]. In the light of the results presented in Figs. 3 /6 it is seen that fingerprinting technique used for the results presentation is a useful one and delivers more information on MF effects. In the second series CaCO3 was precipitated from Na2CO2 solution, which was MF treated in quiescent conditions at 30 8C (which was almost the room temperature at the time of experiments). The results obtained are presented in Fig. 7. As can be seen, the maximum absorbance of the light
is smaller and the sedimentation stage occurs more rapidly in the suspensions obtained from MF treated Na2CO3 than in the reference suspension. Moreover, the longer MF treatment time the lower the absorbance is. These results suggest that less calcium carbonate crystals (and probably bigger size) precipitated from the MF-treated solutions. These results are in agreement with those obtained by Highashitani et al. [1], and Coey and Cass [8], but disagree, for example, with Wang et al’s. [5] results. To better depict the kinetics of MF effect, in Fig. 8 are plotted differences in the light absorbance between MF treated and untreated systems, DA , on the time scale. In all three systems MF effect expressed by DA as a function of time is clearly seen. The longer MF treatment time in quiescent environment the larger the effect is. In the case of 5- and 20 min-MF-treated systems the minimum in DA (the maximum MF effect) occurs practically at the same time (ca. 3 min), and that for 70 min treated system the maximum MF effect appears after 5 min since the moment of precipitation. Oshitani et al. [7] have found a maximum treatment effect of MF in quiescent conditions to occur after 20 min of the treatment. In our experiments already 5 min treated system shows the differences. Because all the DA values are negative it means that at the same time in case of MF treated Na2CO3 solution the nucleation rate of calcium carbonate is slower and this effect is the
Fig. 7. Changes in the light absorbances (l/543 nm) of calcium carbonate suspensions obtained from 0, 5, 20, and 70 min MF-treated Na2CO3 solution in quiescent conditions as a function of time since the moment of the precipitation.
Fig. 8. Differences in the light absorbances (l /543 nm), DA, of CaCO3 suspensions between MF treated and untreated systems in quiescent conditions versus time at 30 8C. The MF treatment time of Na2CO3 solution was 5, 20 and 70 min.
Fig. 6. Numerically interpolated absorbances vs. time profiles for CaCO3 suspensions at characteristic values of pH (see Fig. 4): 70 min MF at pH 10.16, after 1 h at pH 10.09.
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largest in the case of 70 min treated solution and appears within 5 min since the moment of CaCO3 precipitation. Then, the MF effect vanishes within next 5 min, except for 5 min-MF-treated system where small positive DA values can be seen up to 20 min (Fig. 8). To learn about MF effect in quiescent conditions on the crystal size and their adhesion, calcium carbonate was precipitated from: 0, 5, 20 or 70 min-MF-treated Na2CO3 in beakers on the bottom of which 5 cm2 glass plates were placed. After 2 h since the precipitation and incubation at 30 8C the non-adhered particles were removed from the surface by slow dipping of the plates into deionized water for three times. Then, after drying them photographs were taken with a help of optical polarization microscope (Nikon, Japan). The photographs for the reference system (MFuntreated), 5 min and 70 min-MF-treated samples are shown in Fig. 9a, b, c at 200 /, and in Fig. 9a?, b?, c? at 400 / magnification, respectively. The 400 / photographs are the fragments, which are marked in Fig. 9a?, b?, c? by the frames. The photograph for 20 min MF treatment was similar to that for 5 min and therefore is not presented. On the photographs in Fig. 9a, b, c it can be seen a decreasing number of the crystals in MF-treated systems, especially that 70 min-MF-treated. This observation is in accordance with the literature data [1] and the absorbance changes (Figs. 7 and 8). Then, using computer program ‘Lucia Image Analysis Systems’ (Nikon) the number of the objects and their distribution were analyzed on the pictures in Fig. 9a?, b?, c?. Results of the analysis are plotted in Fig. 10a, b, c, where the histograms show percentage distribution of the crystal contour field areas and Gauss’ distribution curves. There are clearly visible differences in the contour-field-area distributions among the samples. In case of MF-untreated sample (Fig. 10a) there is a broad range of the crystal size and of similar population in the range of 10/40 mm2. The maximum of ca. 14% appears around 25 mm2. If the sample was 5 min-MF-treated (Fig. 10b) less large populations of the crystal size are seen. Moreover the maximum appearing around 20 mm2 amounts 32%, which is roughly twice of that for the untreated sample. If the sample was 70
min-MF-treated (Fig. 10c), the maximum on Gauss’ curve still occur around the same size as for 5 min treated sample but it amounts ca.16% and the populations of the crystals between 10 /60 mm2 are almost the same. From this figure it can be seen that the largest amount of 10 /20 mm2 crystals precipitated in the 5 min-MF-treated sample, which stand for 37% of the total amount. It should bee stressed that the total amount (weight) of the precipitated calcium carbonate should be the same in all the three samples, and it also reflects in the total contour- field-area counted, which is 2667, 2221, and 2875, for 0, 5, and 70 min MF treated samples, respectively. Above analysis of the MF effect was done for only one glass plate with the deposited CaCO3 of each experiment. In the paper to follow a more rigorous statistical analysis will be carried out using 15 samples for each kind of the experiments. Comparing the MF effects obtained in following conditions (Fig. 1) with those obtained in quiescent conditions (Fig. 8) different run in DA changes as a function of time can be seen. In the flowing conditions two stages can be distinguished, which were considered as nucleation and sedimentation and the MF effect causing the increase or reduction in the nucleation rate depended on the treatment time. On the time scale for 5 min- and 70 min-MF-treated systems DA changes run as a mirror-like reflection (Fig. 1). However, in quiescent conditions for all three treatment times (5, 20 and 70 min) DA values run in the same way as a function of time (Fig. 8), and the effect increases with the treatment time increase. One can conclude that some hydrodynamic flow effect concurs with magnetic field effect. Hence, the resulting effect depends on the duration of the liquid flow and MF irradiation. But, more experiments would be needed to verify this hypothesis.
4. Conclusions Magnetic field originating from two S/S poles 0.1T each of a commercial magnetizer applied perpendicularly to the Na2CO3 solution flow for 5, 20, and 70 min influences the nucleation stage of
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Fig. 9. Photographs of CaCO3 precipitated at 30 8C from 0-, 5-, and 70 min-MF-treated Na2CO3 solutions in quiescent conditions: a), b), c) /200/, and a?), b?), c?) /400 / magnifications.
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a larger size (faster sedimentation rate), which was found indeed taking the photographs of the crystals deposited on the glass plates. The calculated differences in the absorbance occurring after the same time since the moment of CaCO3 precipitation showed essential differences in the MF effect on the kinetics of nucleation and sedimentation, which depended on the treatment conditions (flowing or quiescent) as well as the treatment time. In flowing conditions a concurrent with MF the hydrodynamic flow effect is suggested, but this need further verification. Our results, similarly like those of other authors, support some of MF effects results reported in the literature but disagree with some others. It means that there are factors, for example intensity and time of the hydrodynamic flow, that compete with MF effects. Sometimes, it may be difficult to evaluate them, and hence the apparent MF effects as obtained in different laboratories can contradict each other.
Acknowledgements
Fig. 10. Histograms of the calcium carbonate crystal size distributions for MF-untreated sample a), 5 min-MF-treated sample b), and 70 min-MF-treated sample c).
CaCO3 precipitation process. In the case of 20 min- and 70 min-RF-treated systems the precipitating crystals are smaller (slower sedimentation rate, positive DA values), but they are probably larger in 5 min-RF-treated system (negative DA values). The applied MF field also causes changes in the suspension pH. The use of so called fingerprinting technique for the results presentation helps for better visualization of MF effects. Using the same magnetizer there are also clearly visible MF effects in the case of Na2CO3 solution being treated in quiescent environment for 5, 20, and 70 min. However, here the longer MF treatment time the lower is the absorbance maximum, which is reduced by 17, 24, and 39%, respectively. This should be due to fewer amounts of the crystals precipitated during the same time but of
We very much appreciate the financial support of the project from KBN, grant No. 3 T09B 089 18.
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[20] P.P. Coetzee, M. Yacoby, S. Howell, Water SA 22 (1995) 333. [21] A. Szkatula, M. Balanda, M. Kopec, Eur. Phys. J. AP 18 (2002) 41. [22] S. Parsons, Antiscale Magnetic Treatment, http:// www.cranfield.ec.uk/sims/water/magnets.htm, taken on 14/03/01. [23] S.L. Stipp, M.F. Hochella, Jr., Geochim. Cosmochim. Acta 55 (1991) 1723. [24] M. Colic, D. Morse, Colloids Surf. A 154 (1999) 167. [25] L. Holysz, E. Chibowski, A. Szczes´, Wat. Res., submitted for publication. [26] E. Chibowski, L. Holysz, A. Szczes´, M. Chibowski, Water Sci. Technol., submitted for publication. [27] Y. Marcus, Biophys. Chem. 51 (1994) 111. [28] J.H. Prescot, S. Shiau, R.L. Rowell, Langmuir 9 (1993) 2071.
Precipitation of calcium carbonate from magnetically treated sodium carbonate solution Emil Chibowski a,*, Lucyna Holysz a, Aleksandra Szczes´ a, Marcin Chibowski b a
b
Department of Interfacial Phenomena, Faculty of Chemistry, Maria Curie-Sklodowska University, 20-031 Lublin, Poland Department of Numerical Analysis, Faculty of Mathematics and Natural Science, Catholic University of Lublin, 20-950 Lublin, Poland Received 27 July 2002; accepted 20 March 2003
Abstract Using a commercial ‘magnetizer’ with two S /S poles of the magnet 0.1 T each, the magnetic field (MF) effect on the in situ precipitated calcium carbonate have been investigated via MF treatment of the Na2CO3 solution prior to the precipitation. The MF was applied perpendicularly to Na2CO3 solution flowing in a silicone tube. In the second series, the MF was applied in quiescent conditions, where a plastic tube with the solution was placed between the poles. The S /S arrangement of the magnetizer poles caused MF strength close to zero in the middle of the tube. The MF treatment time was 5, 20, and 70 min. After that calcium carbonate was precipitated from equimolar volumes of Na2CO3 and CaCl2 (8 10 3 M) solutions at 20 8C or 30 8C and the light absorbance at 543.2 nm and pH of the suspension were recorded as a function of time, up to 30 min. Within this time in most cases the light absorbance dropped to zero. For each tested system 3 /4 replicas of the experiment were made and then average results were taken. It was found that MF effect on in situ precipitated calcium carbonate from Na2CO3 treated in the flowing conditions generally appeared in a slower nucleation and sedimentation rates of the precipitate and also in some differences in pH changes of the suspensions after the solutions mixing. When CaCO3 was precipitated from MF-treated Na2CO3 solution in quiescent environment the maximum absorbance of the light was reduced relative to the reference system by 15, 22, and 37%, for 5, 20, and 70 min MF treatment time (at 30 8C), respectively. The reduction of the absorbance might result from smaller number of the crystals and a larger their size. # 2003 Elsevier B.V. All rights reserved. Keywords: Magnetic field treatment; Calcium carbonate; In situ precipitation
1. Introduction
* Corresponding author. Tel.: /48-81-537-5651; fax: /4881-533-3348. E-mail address: [email protected] (E. Chibowski).
Investigations of magnetic field (MF) treatment effects on the dispersed systems and water properties are of increasing interest. In the Internet search (using google.com) on the web site are looked up about 337 000 entries on ‘magnetic treatment’, 141 000 entries on ‘magnetic water
0927-7757/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0927-7757(03)00133-X
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treatment’, 90 200 entries dealing with ‘magnetic scale treatment’, and 3150 entries on ‘magnetic calcium carbonate treatment’. Many of them originate from various companies offering magnetizers for the scale preventing or treatment, and rather small number of the entries deals with papers on fundamental studies of MF effects on properties of the studied systems. Although presence of some MF effects on the properties of precipitated calcium carbonate seems to be now well documented [1 /10], this issue is still a controversial one [11,12]. While some authors [4] maintain that after magnetic treatment up to 50% reduction of the scale formation took place in a recirculating system, others [13] have found only minor or non-influence of the field. At present most frequently the following MF effects are investigated and/or have been proved experimentally. However, some of the statements are in contradiction each other. For example: / nucleation rate increases */more crystals of smaller size and irregular shapes form [10], / nucleation frequency is suppressed but the particle growth is accelerated [1,5,6]. The MF effect after the solutions treatment can last up to 120 h [1] or 200 h [8], which is called ‘memory effect’, / zeta potential of the precipitated particles is changed [2,10,14], / increases conductivity and pH of the solutions [3,15,16], / at the beginning pH of the slurry decreases, which affects the scale growth [4,5], / particles of calcium carbonate crystallized in magnetic field consist mainly of aragonite and vaterite [8,9], / free radicals and/or reactive oxygen species (ozone, superoxide, hydroxyl radicals, singled oxygen) and others are created, which are stable for days [17], / an increase in dissolved oxygen concentration takes place [18], / the impurity Fe2 or Zn2 ions, if present, can influence nucleation [19,20]. Coey and Cass [8] have found that even weak magnetic field (B :/0.1T) influenced aragonite/
calcite ratio in the precipitated CaCO3 in groundwater from a well sunk in limestone, which contained 132 mg l1 of Ca2 and some amounts of other element ions (Na, Mg, K, Fe, Mn, and Zn) as well as anions NO 3 ; NO2 ; and Cl : The groundwater was drawn through a stack of Teflon-coated ferrite ring magnet or by bypass to omit the magnet. They also used a commercial mineral water in a 500 cm3 bottle on the neck of which the ring magnet was fitted. They have found no systematic effect of the water flow speed through the magnets (from 0.04 to 1.2 m s 1). They tested in total more than 100 samples heating them in an open 500 cm3 beakers at 80 8C for different time of the water incubation (0 /200 h) prior to the heating. The aragonite content in the precipitate increased during first tens incubationhours (ca. 40 h) reaching a maximum. Statistical analyses of 32 pairs of the samples showed that at the 99.9% probability and 3.4 confidence level, magnetic treatment increases the amount of aragonite in the carbonate deposits. Investigating 20 samples of the well water after MF treatment in 14 of them aragonite content has increased, in 3 decreased, and in 3 has not changed. In case of the mineral water for 12 samples the numbers were 11, 0, 1, respectively. Szkatula et al. [21] have conducted large-scale experiments on magnetic treatment on a heating installation using two identical 25 kW heat exchangers, as well as on three blocks of 1 GW power plant. They used a stack of cylindrical strontium-ferrite permanent magnets placed in a ferromagnetic pipe with the field amplitude of 1.5 kOe. The construction of the magnet device caused that the water flowing inside it periodically changed its flow from 1.0 to 1.6 m s 1, as well as the magnetic field profile also changed along the device. After 4-month experiment the authors found significant differences in the water being magnetically treated relative to that which has not been treated, as well as in the amount and hardness of the scale deposited. The scale was hard in the loop of MF-untreated water and soft in the loop where the water first flew through the magnetic device. Moreover, the total deposited amounts of the scale were 190 g and 7 g, respectively, and the scale distribution inside the
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pipes was quite different. The deposit was mostly amorphous after MF treatment and possessed essentially different composition. From the literature it results that efficiency of magnetic field treatment to a high degree depends on many physicochemical parameters, such as the temperature, pH, the ionic composition, the flow rate, as well as some geometrical factors of the treated system, which are difficult to be well established [22]. Lately Baker and Judd [11] have reviewed the literature on MF treatment for the protection of hard scale formation. From the subject literature it results that still more experimental evidences have to be gathered to put forward and to verify the hypotheses dealing with mechanisms of the field interactions and to recognize the parameters affecting MF treatment [11,12], which so far are often controversial, as quoted above. Therefore, it seemed to us interesting to use a commercial ‘magnetizer’ to test whether such a MF affects in situ precipitated calcium carbonate in the laboratory scale, especially at well-defined and controlled systems and conditions. In this paper CaCO3 was precipitated at 20 or 30 8C from magnetically treated Na2CO3 [1], both in flowing and quiescent conditions, and changes in the light absorbance and pH as a function of time were recorded. As the reference system each time the calcium carbonate was also precipitated without MF in the same conditions and using the same solutions.
2. Experimental section Calcium carbonate was precipitated at 20 or 30 8C from equimolar volumes of CaCl2 and Na2CO3 solutions having concentration of 8/ 103 M [1,6]. The diluted solutions (0.3 dm3) were prepared from 0.1 M stock solutions of calcium chloride and sodium carbonate. The salts were of p.a. grade from POCH Gliwice, Poland, and were used as received. Water used was doubly distilled and then deionized using Millipore Q-Plus 185 system. The magnetic field originated from two S /S type permanent magnets, 0.1T each, obtained
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kindly from the Magnetizer Group Inc., USA. Because from the literature [1,6] it resulted that magnetic field affects more sodium carbonate Na2CO3 than calcium chloride CaCl2 solution, therefore the former solution (0.3 dm3) was treated with MF. The solution circulated (with volume speed 4 dm3 min 1) in a silicone tube (3 m long, 15 mm of the outer and 10 mm of the inner diameters) for 5, 20 or 70 min, using Masterflex I/P pump, Model 77601-10 (Cole-Palmer Instr. Comp. USA). The magnetic field was applied perpendicularly to the solution flow. After the MF-treatment of Na2CO3, equimolar volumes (25 cm3) of Na2CO3 and CaCl2 solutions were mixed up instantly and absorbance of the light (at l/543.2 nm) and pH were measured as a function of time up to 30 min. The absorbance was measured using an Avantes (the Netherlands) UVVIS-NIR fiber optic spectrometer DH-2000 and the first reading was taken after ca. 3 min since the moment of MF had ceased. All experiments were replicated 3 /4 times and the average values together with the standard deviations were calculated. In second series of experiments the treatment with magnetic field has been conducted in quiescent conditions, i.e. Na2CO3 solution in plastic tube was placed between the poles of the same magnetizer as used above and treated for 5, 20, and 70 min at 30 8C. Then calcium carbonate was precipitated by simultaneous mixing with equimolar volume of CaCl2 solution and the absorbance was recorded up to 30 min. Prior to these experiments CaCO3 had been precipitated from non-circulated and MF-untreated Na2CO3 as the reference system. Besides the absorbance measurements also deposition of in situ precipitated calcium carbonate was investigated. For this purpose in 50 cm3 beakers glass plate (5 cm2) were placed on their bottom and 10 cm3 of Na2CO3 (MF-treated or untreated) were mixed with 10 cm3 of CaCl2. The suspensions were left for 2 h at 30 8C, then the glass plates were carefully dipped into distilled water to remove non-adhered particles and dried at 30 8C. After that photograph were taken using Nikon optical polarization microscope and the pictures were analyzed as for the number and size of the crystals deposited.
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3. Results and discussion The magnetic field effect on in situ calcium carbonate precipitation is often investigated via measurements of the light absorbance [1,5,6]. From Highashitani et al’s paper [1] and others [6], it results that MF effect is more pronounced if prior to the calcium carbonate precipitation the solution of Na2CO3, but not CaCl2, has been treated with MF field. The MF field irradiation can be realized in static conditions or on the solution circulating in the tube, as well as the field may alter (e.g. rotate) or pulse [7]. As was mentioned above, even weak magnetic field (B :/ 0.1 T) influenced aragonite/calcite ratio in the precipitated CaCO3 in groundwater from a well sunk in limestone as well as commercial mineral water [8]. In our studies we also have used the magnetic field B /0.1 T, but it originated from two permanent magnets of commercial, so called, magnetizer (Magnetizer Co., USA) with south poles (S) fitted around the silicone tube in which Na2CO3 circulated for a defined time, i.e. 5, 20, or 70 min at 20 8C. The magnet poles arrangement caused that in the middle of the tube the MF field strength was close to zero. Three to four replicas of the experiment were taken for each system and the average values were calculated. For each system first precipitation of CaCO3 from magnetically non-treated Na2CO3 solution has been conducted to get the reference results. From the measured absorbances of the precipitated CaCO3 suspensions as a function of time it was seen that MF effect appeared first of all in the kinetic of precipitation. Therefore in Fig. 1 these are shown differences between the absorbance of MF treated and untreated systems (DA ) taken at the same time after both solution mixing. As can be seen all DA curves in Fig. 1 cross DA /0 around 7.5 min. It means that MF treated and untreated suspensions behave in the same way. However, very soon on the time scale again significant DA values appears, which means that MF effect still exists. While this first stage can be considered as primarily nucleation during first minutes and then the crystal growth, so the second one is surely sedimentation of the precipitated particles. It is characteristic that
Fig. 1. Differences in the light absorbances (l /543 nm), DA, of CaCO3 suspensions between MF treated and untreated systems in flowing conditions versus time at 20 8C. The MF treatment time of Na2CO3 solution was 5, 20 and 70 min. The differences are also shown for the CaCO3 suspension precipitated after 1 h from 70 min Na2CO3 treated solution.
5 and 70 min MF treatment causes opposite changes during the nucleation stage (positive and negative DA , respectively), while 20 min MF treatment has only minor influence. Maximum effect during nucleation appears after 2.5 /3.5 min since the solutions mixing. But, while in the suspension obtained from 5 min-MF-treated Na2CO3 solution DA is positive, which means that the nucleation is faster than in the MF untreated solution, so in 70 min-MF-treated Na2CO3 solution the nucleation is shower (negative DA ). Moreover, if CaCO3 was precipitated 1 h after 70 min-MF treatment the affect was still present, and in the second stage even more pronounced than if the suspension was precipitated immediately after the field had ceased. In the sedimentation stage the MF effect is reversed, i.e. if nucleation stage was faster (i.e. 5 min-MF treated), then the sedimentation rate is slower with a minimum after 10 min (Fig. 1). Such behavior indicates that the precipitated crystals were smaller. Contrary, in case of 70 min MF treated Na2CO3 solution; the nucleation was slower but sedimentation faster, indicating that the crystals were larger. It should be mentioned that the total precipitated amount in grams of CaCO3 in all the systems studied should be the same. These results, as for the nucleation stage, are
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in agreement with those found in [1,5,6]. They also throw a light why some authors observed an increased nucleation rate [10], while others suppressed nucleation [1,5,6]. From our experiments it is clearly seen that MF effect depends strongly on the kinetic conditions and the treatment time. Simultaneously with the absorbance measurements pH changes in the suspensions were recorded. Again, to better visualize MF effects in Fig. 2 are presented DpH, which are differences between pH of the MF-treated and untreated suspensions measured at the same time. As can be seen, except for 20 min treated Na2CO3 the DpH are negative during first 10 min. Thus, the magnetic field causes decrease in pH relative to the reference system by 0.05 /0.1 pH units. At the same time in case of 20 min MF treated solution the suspension pH is 0.15 /0.1 units higher than in non-treated suspension. After 10 min since the precipitation of CaCO3 the pH differences increase, and they depend on MF treatment time. From the very beginning the largest and positive differences in pH appeared in 20 min MF treated suspensions while the effect in the nucleation stage was the smallest. So, it may be concluded that initial pH value has an important impact on the nucleation of CaCO3. Two principal reactions taking place during precipitation of CaCO3 are following: Na2 CO3 H2 O l 2Na 2OH H2 CO3 (high initial pH)
(1)
Fig. 2. Differences in pH for the same systems as presented in Fig. 1.
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2Na 2OH H2 CO3 Ca2 2Cl 0¡ CaCO3 2H2 O2Na 2Cl (pH decrease)
(2)
In consequence the pH decreases. Obviously there are accompanying secondary reactions, which influence the suspension pH too [23], among others, surface hydrolysis reactions: CO3 H2 O l HCO3 OH (below pHpzc )
Ca H2 O l CaOHH
(above pHpzc )
(3) (4)
The reactions are determined by H and OH concentration in the solution, i.e. pH of the suspension. With decreasing pH of the suspension an increased solubility of CaCO3 takes place. The isoelectric point (iep) for calcite occurs at pH 8.2 / 8.4 [23]. At pHs above the iep negative species (/ HCO3 and CO2 3 ) dominate in the solution, thus also causing net negative surface charge of calcite, and below pHiep Ca2 and CaOH are present in an excess over negative ones in the solution. So, for calcite principal potential determining ions in solution are: Ca2, CO3 ; HCO3 ; and the secondary ones are H and OH . The neutral sites on the calcium carbonate surface are of course also present, /CaOHo and /CO3Ho [23], and they were found by spectroscopic methods (XSP and LEED) [24]. The above mentioned relatively small changes in the suspension pH may still be significant for the precipitation process as observed via differences in the absorbance [3,4]. Parsons et al. [4] found that using 0.7 T magnet pH of the treated system (at 60 8C) dropped by 0.5 pH units (from 8 to 7.5) in CaCl2 and NaHCO3 mixed solutions. The authors [4] obtained 48% reduction in the scale formation, but there was practically no MF effect if pH was controlled to the initial 8 or 8.5. The effect of magnetic field on the pH can be connected with these secondary reactions. From our other studies on influence of impurity ions on CaCO3 precipitation we concluded that MF affects hydration shell of the ions [25,26]. Because the hydration energy of anion is lower than cations, it explains why MF 2 field effect is a larger on CO ion 3 than Ca solution [27].
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Using Surfer computer program it is possible to present isoabsorbance contour diagrams on pH and time domain, which is called fingerprinting technique [26]. The contours were obtained with a help of the Kriging option of the program, which is a geostatical girding method. The method attempts to express trends that are suggested in the date, i.e. the absorbances on the pH and time domain. We have found that better visualization of the contours was obtained taking ln(t) instead of time. Similarly Prescott et al. [28] applied logarithm of conductivity to present the electrophoretic fingerprints. In Fig. 3 there are shown overlapped isoabsorbances for 5 min-MF treated (broken lines) and MF-untreated (solid lines) suspension of CaCO3, and in Fig. 4 similar fingerprints for untreated and 70 min MF-treated Na2CO3 for CaCO3 precipitated 1 h after the field had ceased. It can be seen in Fig. 3 for 5 min-MFtreated system shifted isoabsorbances relative to those for MF-untreated suspension. In the reference system the absorbances around 10 min are practically pH independent (vertical position) while those of MF treated system are clearly pH dependent. At this time in the untreated system the absorbance values were 0.16 /0.10, while in the MF treated they were 0.28 /0.18. More pronounced differences in the isoabsorbances can be seen in Fig. 4 for 70 min-MF-treated system. The differences can be even better depicted by plotting numerically interpolated profiles of the absorbances vs. time at characteristic pH for the MF-treated and -untreated systems. In case of 5 min-MF-treated system this pH value was 10.3 (see Fig. 3), and it was 10.25 for 20 min-MFtreated (the isoabsorbances contours are not presented here). Thus obtained cuts of the absor-
Fig. 3. Isoabsorbances of light (l /543 nm) for MF-untreated (solid lines) and 5 min-MF-treated in flowing conditions systems (broken line) presented on pH and time domain.
Fig. 4. Isoabsorbances of light (l /543 nm) for MF-untreated (solid lines) and 70 min-MF-treated in flowing conditions systems (broken line) presented on pH and time domain. The calcium carbonate was precipitated 1 h since the field has ceased.
bances versus time are shown in Fig. 5. Here is clearly seen how the absorbances of MF treated systems would be shifted relative to those of untreated reference system keeping the same pH value. The differences are especially large in the stage of the particles sedimentation. In the MF treated systems CaCO3 sediments much slower, particularly in 5 min treated. In the case of 70 minMF-treated system the cuts of the isoabsorbances taken at pH 10.16 (Fig. 6) show differences in the nucleation stage. In the sedimentation stage significant shift is seen for CaCO3 precipitated 1 h after the field has ceased. Very rapid CaCO3 sedimentation took place in 10th minute both in MF-untreated and treated systems, if calcium carbonate was precipitated immediately after the treatment. However, if it was precipitated after 1h, the sedimentation rate was slower, what appeared in a higher absorbances at the same time (Fig. 6).
Fig. 5. Numerically interpolated absorbances vs. time profiles for CaCO3 suspensions at characteristic values of pH (see Fig. 3): 5 min MF at pH 10.3, 20 min MF for pH 10.25.
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The absorbance differences during the nucleation stage at the same pH indicate for a significant role of the initial pH for MF effect. This was also found by Parsons [4]. In the light of the results presented in Figs. 3 /6 it is seen that fingerprinting technique used for the results presentation is a useful one and delivers more information on MF effects. In the second series CaCO3 was precipitated from Na2CO2 solution, which was MF treated in quiescent conditions at 30 8C (which was almost the room temperature at the time of experiments). The results obtained are presented in Fig. 7. As can be seen, the maximum absorbance of the light
is smaller and the sedimentation stage occurs more rapidly in the suspensions obtained from MF treated Na2CO3 than in the reference suspension. Moreover, the longer MF treatment time the lower the absorbance is. These results suggest that less calcium carbonate crystals (and probably bigger size) precipitated from the MF-treated solutions. These results are in agreement with those obtained by Highashitani et al. [1], and Coey and Cass [8], but disagree, for example, with Wang et al’s. [5] results. To better depict the kinetics of MF effect, in Fig. 8 are plotted differences in the light absorbance between MF treated and untreated systems, DA , on the time scale. In all three systems MF effect expressed by DA as a function of time is clearly seen. The longer MF treatment time in quiescent environment the larger the effect is. In the case of 5- and 20 min-MF-treated systems the minimum in DA (the maximum MF effect) occurs practically at the same time (ca. 3 min), and that for 70 min treated system the maximum MF effect appears after 5 min since the moment of precipitation. Oshitani et al. [7] have found a maximum treatment effect of MF in quiescent conditions to occur after 20 min of the treatment. In our experiments already 5 min treated system shows the differences. Because all the DA values are negative it means that at the same time in case of MF treated Na2CO3 solution the nucleation rate of calcium carbonate is slower and this effect is the
Fig. 7. Changes in the light absorbances (l/543 nm) of calcium carbonate suspensions obtained from 0, 5, 20, and 70 min MF-treated Na2CO3 solution in quiescent conditions as a function of time since the moment of the precipitation.
Fig. 8. Differences in the light absorbances (l /543 nm), DA, of CaCO3 suspensions between MF treated and untreated systems in quiescent conditions versus time at 30 8C. The MF treatment time of Na2CO3 solution was 5, 20 and 70 min.
Fig. 6. Numerically interpolated absorbances vs. time profiles for CaCO3 suspensions at characteristic values of pH (see Fig. 4): 70 min MF at pH 10.16, after 1 h at pH 10.09.
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largest in the case of 70 min treated solution and appears within 5 min since the moment of CaCO3 precipitation. Then, the MF effect vanishes within next 5 min, except for 5 min-MF-treated system where small positive DA values can be seen up to 20 min (Fig. 8). To learn about MF effect in quiescent conditions on the crystal size and their adhesion, calcium carbonate was precipitated from: 0, 5, 20 or 70 min-MF-treated Na2CO3 in beakers on the bottom of which 5 cm2 glass plates were placed. After 2 h since the precipitation and incubation at 30 8C the non-adhered particles were removed from the surface by slow dipping of the plates into deionized water for three times. Then, after drying them photographs were taken with a help of optical polarization microscope (Nikon, Japan). The photographs for the reference system (MFuntreated), 5 min and 70 min-MF-treated samples are shown in Fig. 9a, b, c at 200 /, and in Fig. 9a?, b?, c? at 400 / magnification, respectively. The 400 / photographs are the fragments, which are marked in Fig. 9a?, b?, c? by the frames. The photograph for 20 min MF treatment was similar to that for 5 min and therefore is not presented. On the photographs in Fig. 9a, b, c it can be seen a decreasing number of the crystals in MF-treated systems, especially that 70 min-MF-treated. This observation is in accordance with the literature data [1] and the absorbance changes (Figs. 7 and 8). Then, using computer program ‘Lucia Image Analysis Systems’ (Nikon) the number of the objects and their distribution were analyzed on the pictures in Fig. 9a?, b?, c?. Results of the analysis are plotted in Fig. 10a, b, c, where the histograms show percentage distribution of the crystal contour field areas and Gauss’ distribution curves. There are clearly visible differences in the contour-field-area distributions among the samples. In case of MF-untreated sample (Fig. 10a) there is a broad range of the crystal size and of similar population in the range of 10/40 mm2. The maximum of ca. 14% appears around 25 mm2. If the sample was 5 min-MF-treated (Fig. 10b) less large populations of the crystal size are seen. Moreover the maximum appearing around 20 mm2 amounts 32%, which is roughly twice of that for the untreated sample. If the sample was 70
min-MF-treated (Fig. 10c), the maximum on Gauss’ curve still occur around the same size as for 5 min treated sample but it amounts ca.16% and the populations of the crystals between 10 /60 mm2 are almost the same. From this figure it can be seen that the largest amount of 10 /20 mm2 crystals precipitated in the 5 min-MF-treated sample, which stand for 37% of the total amount. It should bee stressed that the total amount (weight) of the precipitated calcium carbonate should be the same in all the three samples, and it also reflects in the total contour- field-area counted, which is 2667, 2221, and 2875, for 0, 5, and 70 min MF treated samples, respectively. Above analysis of the MF effect was done for only one glass plate with the deposited CaCO3 of each experiment. In the paper to follow a more rigorous statistical analysis will be carried out using 15 samples for each kind of the experiments. Comparing the MF effects obtained in following conditions (Fig. 1) with those obtained in quiescent conditions (Fig. 8) different run in DA changes as a function of time can be seen. In the flowing conditions two stages can be distinguished, which were considered as nucleation and sedimentation and the MF effect causing the increase or reduction in the nucleation rate depended on the treatment time. On the time scale for 5 min- and 70 min-MF-treated systems DA changes run as a mirror-like reflection (Fig. 1). However, in quiescent conditions for all three treatment times (5, 20 and 70 min) DA values run in the same way as a function of time (Fig. 8), and the effect increases with the treatment time increase. One can conclude that some hydrodynamic flow effect concurs with magnetic field effect. Hence, the resulting effect depends on the duration of the liquid flow and MF irradiation. But, more experiments would be needed to verify this hypothesis.
4. Conclusions Magnetic field originating from two S/S poles 0.1T each of a commercial magnetizer applied perpendicularly to the Na2CO3 solution flow for 5, 20, and 70 min influences the nucleation stage of
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Fig. 9. Photographs of CaCO3 precipitated at 30 8C from 0-, 5-, and 70 min-MF-treated Na2CO3 solutions in quiescent conditions: a), b), c) /200/, and a?), b?), c?) /400 / magnifications.
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a larger size (faster sedimentation rate), which was found indeed taking the photographs of the crystals deposited on the glass plates. The calculated differences in the absorbance occurring after the same time since the moment of CaCO3 precipitation showed essential differences in the MF effect on the kinetics of nucleation and sedimentation, which depended on the treatment conditions (flowing or quiescent) as well as the treatment time. In flowing conditions a concurrent with MF the hydrodynamic flow effect is suggested, but this need further verification. Our results, similarly like those of other authors, support some of MF effects results reported in the literature but disagree with some others. It means that there are factors, for example intensity and time of the hydrodynamic flow, that compete with MF effects. Sometimes, it may be difficult to evaluate them, and hence the apparent MF effects as obtained in different laboratories can contradict each other.
Acknowledgements
Fig. 10. Histograms of the calcium carbonate crystal size distributions for MF-untreated sample a), 5 min-MF-treated sample b), and 70 min-MF-treated sample c).
CaCO3 precipitation process. In the case of 20 min- and 70 min-RF-treated systems the precipitating crystals are smaller (slower sedimentation rate, positive DA values), but they are probably larger in 5 min-RF-treated system (negative DA values). The applied MF field also causes changes in the suspension pH. The use of so called fingerprinting technique for the results presentation helps for better visualization of MF effects. Using the same magnetizer there are also clearly visible MF effects in the case of Na2CO3 solution being treated in quiescent environment for 5, 20, and 70 min. However, here the longer MF treatment time the lower is the absorbance maximum, which is reduced by 17, 24, and 39%, respectively. This should be due to fewer amounts of the crystals precipitated during the same time but of
We very much appreciate the financial support of the project from KBN, grant No. 3 T09B 089 18.
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