CC BY
175
F. Alimi*’1’2, M. Tlili1, M. Ben Amor1, G. Maurin2 , C. Gabrielli2
INFLUENCE OF MAGNETIC FIELD ON CALCIUM CARBONATE PRECIPITATION IN THE PRESENCE OF FOREIGN IONS
1 Laboratoire d’entartrage et de Physicochimie de I’Eau, Centre des Recherches et Technologies
des Eaux, BP 273, 8020 Soliman, Tunisie, [email protected]
2 Laboratoire Interfaces et Systeme Electrochimie (LISE), UPRI5 du CNRS, Universite Pierre et
Marie Curie, Tour 22, 5e etage, 4 Place Jussieu, 75252 Paris Cedex 05 - France
1. Introduction
Calcium carbonate precipitation has been the subject of numerous investigations because of its importance in several industrial processes. Its scale deposition at the liquid/solid interface of flow systems induces important damage in the domestic or industrial installations using natural waters (pipe blocking, membrane clogging, efficiency decay of heaters or heat exchangers ...). The main reaction involved in the precipitation of CaCO3 is governed by the following equilibrium called calcocarbonic equilibrium:
Ca2+ + 2HCO- ^ CaCO3(s) + CO2 + H2O (1)
Depending on the temperature, ion concentration, pH, and flow rate of the solution, equilibrium Eq. 1 can be shifted in the forward or backward direction. Three crystalline forms of CaCO3 exist: calcite, which has a cubic shape; vaterite, which has a spherulite, hemispherical flowers, or lenses morphology; and aragonite, which is recognized as needles. Each CaCO3 variety is stable depending on thermodynamic conditions.
Various methods are used to prevent scaling, for example water decarbonation using electrochemical processes, seeding or acid addition. However these chemicals are generally deleterious for human health and their use is forbidding in drinking water.
Thus, environmental protection and economic considerations are two strong motivations for developing various types of physical antiscale methods (ultrasonic, ultraviolet radiation, electric and magnetic treatment). Magnetic methods attract a special attention due to their ecological purity, safety, and simplicity. This technique was widely employed on the treatment of irrigation water [1], industrial water [2] and in numerous other cases.
Natural water contains particles charged in the form of positive and negative ions. Taking into account this fact, various studies related to the effectiveness of the magnetic field (MF) on the calcium carbonate precipitation in the presence of foreign ions were done. The effect of a static magnetic field on the precipitation of CaCO3 was studied in the presence of Mg2+, Fe2+ and SO4- [3-5]. The found results showed that the MF affects the zeta potential, the pH, and the speed of precipitation. The presence of Fe2+ maintains the zeta potential positive, however the presence of SO;|- make it negative. The effect of Fe2+ and SO4- on
the zeta potential of CaCO3 probably results from their specific adsorption on CaCO3 surface. While the effect of Mg2+ is less clear. For Herzog et al. [6], it is the addition of particles of ferric hydroxide in the water which is responsible for the MF effect on CaCO3 precipitation.
In a previously study [7], the effect of a magnetic treatment on a calcocarbonic water was largely tested. It was observed that the effectiveness of the magnetic treatment on the precipitation of calcium carbonate is depending on various parameters such as pH of solution, hardness of water, flow rate and time of treatment. The results indicated that the MF enhanced the precipitation of CaCO3 in bulk-solution instead of heterogeneous one and decreases its solubility. It was advanced that the main magnetic effects concern the associations of ionic species which are present in the solution and which are involved in the nucleation process of CaCO3 precipitation.
* Corresponding author: e-mail [email protected], Phone: +00 216 23 783 226, Fax: +00 216 79 412 802
© Alimi F., Tlili M., Ben Amor M., Maurin G., Gabrielli C., Электронная обработка материалов, 2009, № 1, С. 65-72.
The properties of aqueous ionic solutions have been extensively researched in recent years since it has been shown that the presence of ions modifies the properties of water. In this reason, the application of a permanent MF on a more complex system remains very interesting. In this paper the experimental results on the effect of a MF on the precipitation process of calcium carbonate scale from calcocarbonic pure water
containing Mg2+, SOf or Na+/Cl" are reported.
2. Experimental
The synthetic treated water was prepared by adding Mg2+, SO^- and Cl-/Na+ to a calcocarbonic pure water (water containing only Ca2+, CO32- and HCO- ions), prepared by dissolving 0.4 g.dm-1 reagent
grade CaCO3 in deionized water, by bubbling carbon dioxide during a full day. The solutions, containing one of the foreign ions at the desired concentration, were prepared by dissolving Na2SO4, MgCl2 or NaCl in bidistilled water and added to the calcocarbonic solution just before treatment. Three level of ionic strength
(IS) are tested for each ion: 0.015, 0.0195 and 0.027 M, the reference solution is calcocarbonic pure water (IS=0.012M). The initial pH was adjusted at 7 and the temperature was maintained at 30°C.
The treatment of a volume of V = 0.50 dm3 of prepared water, through the permanent magnetic device, was applied in a pipe made of Tygon material. The magnetic device was described in details in the paper of Gabrielli et al. [8]: a series of 5 pairs of permanent magnets with north and south faces facing each other are associated alternately. The magnetic circuit of each pair was closed with a U shaped iron yoke. The field strength was about 0.16 T in the air gap. The choice of Tygon is due to that this material gives the higher amount of CaCO3 precipitated by homogeneous way [7, 9]. The total time of treatment tT was maintained for 15 minutes and the flow rate was moderate in order to have a laminar regime (0.74 dm3 min-1), the degassing test is for about 90 minutes. For every case of IS, two tests were made: treatment with circulation in the presence and absence of MF. All the experiments were replicated 3 times and average values were calculated (~3%). After each test the tubing was thoroughly cleansed, first, by recirculating a low-concentration acid solution for about 0.5 h, after which deionised water was recirculated for 0.5h. After each experimental series the tubing was replaced. The effect of MF on calcium carbonate precipitation from a pure calcocarbonic solution was, also, tested and three level of IS were chosen: 0.009, 0.012 and 0.015M. In this part, three types of pipe material were tested: Tygon, PVC and W PTFE (White PTFE).
The experiment apparatus of magnetic treatment and of degassing technique employed in this study were described in previous articles [7].
a b
Fig. 1. Effect of hardness of solution on (a) total and (b) homogeneous precipitation of CaCO3 for a various
material pipe of treatment
3. Results and Discussion
3.1. Effect of hardness
As illustrated in fig. 1, the total and the homogeneous precipitation ratio are increased, in the presence of MF, with increasing the water hardness and it’s also influenced by the nature of the pipe material,
especially for the total precipitation ratio. This effect could be attributed to the composition of the material. As reported by Parker and Ranney [10], Barcelona et al. [11] and by Curran and Tomson [12], the pipe could release solid particle on solution due to the erosion mechanism occurring at the polymer/water interface. The amount of these particles depends significantly on the nature of pipes. For these researchers, it was shown that Tygon leach substantial quantities of constituents and that PTFE do not appear to leach any constituents. These particles, in the step of degassing test, will support the nucleation of calcium carbonate, so, the amount of homogeneous precipitation increase with the number of constituents.
In presence of MF, this effect is amplified by the magnetohydrodynamic effect especially for the homogeneous amount of precipitation (fig. 2). The most important effect was observed for the Tygon pipe. This result is in agreement with the results observed by Amire and Dadkhah [13] who suggested that MF increased the amount of impurities leaded by the Tygon pipe in solution and with the observation cited before [10-12].
Homogeneous precipitation (WMF/W0MF)
a b
Fig. 2. Effect of hardness of solution on (a) total and (b) homogeneous precipitation of CaCO3 for a various material pipe (WMF: With Magnetic Field, WoMF: Without Magnetic Field)
3.2 Efficiency of magnetic field in the presence offoreign ions
The results obtained in absence of magnetic field and in presence of foreign ions, on the induction time, total amount of precipitation and the homogeneous amount of precipitation are presented on table 1.
Table 1. Effect of foreign ions on tind, total and homogeneous precipitation ratio of CaCO3
Ionic Strength / M Ion Induction time (tind) / min Total precipitation (TP) / % Homogeneous precipitation (TH) / %
0.012 10 72.5 22.8
0.015 NaCl 12 55 21.2
MgCl2 13 50 20.1
Na2SO4 13 53.6 19.5
0.0195 NaCl 16 52.5 20.4
MgCl2 20 35 17
Na2SO4 16 47.5 15.7
0.027 NaCl 16 50 18.6
MgCl2 26 17.5 13.3
Na2SO4 17 44 11.1
The results presented in this table showed that NaCl slow down nucleation and decrease the rate of total precipitation. The more the quantity of this salt is added, the more this effect is accentuated until rea-
ching an IS of 0.02M after which NaCl loosed its effect. For the homogeneous precipitation, the addition of NaCl hasn’t any influence.
For the sulfate, the application of treatment, haven’t great effect on the total precipitation some either the IS (-50%) but show a higher effect on the homogeneous precipitation. this amount decreased from 19.5% 11%for 0.015 and 0.027 M, respectively.
The presence of Mg2+ in solution prolongs the induction period of CaCO3, which also increased with increasing Mg2+ concentration as observed by Tai and Chien [14]. Pokrovsky [15] showed, also, that the induction period of CaCO3 increases with increasing Mg2+/Ca2+ activity ratio in solution. The increase in tind is due to an increase in the crystal-solution interfacial tension at higher Mg2+/Ca2+ ratio and due to an increase in the activity of Mg2+ at lower Mg2+/Ca2+ region. In our study the tind passed from 10 min, in absence of Mg2+, to 13 min for Mg2+/Ca2+=0.25 and reach 26 min for Mg2+/Ca2+=1.25.
The presence of MF causes further changes in the determined parameters. This effect depends on the nature and the concentration of the foreign ions used. Fig. 3 showed that MF reduces the nucleation time of CaCO3 some either the composition or the IS of treated solutions. Moreover, the degree of MF effect on tind is water composition-dependent. Indeed, with respect to reference solution (pure calcocarbonic water), the effect of MF on tind is more pronounced in presence of foreign ions until an IS value about 0.02 M. Beyond this IS value, MF effect on tind become less important. Also, it should be noted that MF effect on nucleation
time is more important in the presence of Mg2+ or SO42- ions.
□,□12 0,016 0,020 0,024 0,028
Fig. 3. Effect of MF on induction time of CaCO3 in presence of foreign ions
Fig. 4. Effect of MF on total precipitation ratio of CaCO3 in the presence of foreign ions For the TP of calcium carbonate in the presence of foreign ions, fig. 4 presents the obtained results versus the tests made without magnetic field. This curves showed that TP decrease in the case of NaCl and this reduce is more important for the higher IS. The opposite effect is observed in the case of magnesium. While, in the presence of sulfate, the effect of MF is almost constant whatever the IS.
strength/M
Fig. 5 Effect of MF on homogeneous precipitation ratio of CaCO3 in the presence of foreign ions
Counts
Position [°2 Theta]
Counts
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1000
0
1500
500
0:
1000
500
0
1000
500
0
1000
500
0:
500
S10-3S04 "—-—“—j AaJ Li ^ . . . ^A ,
A10-3S04 Lj IV A A !
S2,510-3S04 ^ A— Jl-A. _aA. A
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S510-3S04 L. .,JJ U JL. ..A A
A510-3S04 , | j|
10 20 30 40 50 GO
Position [°2 Theta]
a
b
c
Fig. 6. X-ray diffraction patterns for CaCO3 precipitated in presence of (a) NaCl, (b) sulfate and (c) magnesium with and without magnetic treatment
In fig. 5 is presented the effect of MF treatment on homogeneous precipitation. It’s shown that MF treatment favoured the precipitation of calcium carbonate in the bulk of the solution by homogeneous process. The presence of foreign ions inhibits his effect and a contradictory effect is registered for higher IS (>0.024 M) in the presence of Mg2+ and SO42- ions.
3.4 Effect of magnetic field on the morphology of CaCO3 formed in the presence offoreign ions
In fig. 6 are reported the X-ray diffraction patterns for the CaCO3 precipitated by degassing test, in the bulk of solution, in presence of NaCl, Na2SO4 or MgCl2 for treated and untreated solutions.
It was shown that in the presence of NaCl, MF don’t influence on the morphology of CaCO3 precipitate either the concentration of NaCl added. In presence of magnesium, with application of MF, the amount of formed aragonite was increased and the precipitation of vaterite and calcite were inhibited. When the IS is high (IS>0.02), only aragonite was precipitated. For the sulfate, the pattern of the deposit from the treated water shows that MF inhibits the formation of calcite for the high IS and favoured aragonite in the other case.
In this paper, it has been proved that magnetic water treatment has effect on calcium carbonate crystallization by increasing the total precipitate amount and by favoured its formation in the bulk-solution instead of its incrustation on the walls. It was, also, proved that water composition and its IS has a noteworthy effect on CaCO3 precipitation. Magnetic filed effectiveness is foreign ions-dependent, this effect is inhibiting with the presence of NaCl and encouraged in the presence of magnesium whereas the sulfate has no effect. These observations are not valid for the obtained results of homogeneous precipitate. The main difference is that the homogeneous precipitation is inhibited with IS. The morphology of CaCO3 formed with MF is influenced by the nature of foreign ions, MF has as effect to favour the precipitation of vaterite in the case of sulfate and NaCl and the precipitation of aragonite in the presence of magnesium.
This effect is explained by the magnetically modified hydration of ions and solid surfaces and Lo-rentz force effects on ions and dispersed particles. Which effect would prevail depends on the magnetic treatment regime and on the composition of the treated system [16].
Bush et al. [17] have assumed that the Lorenz forces exerted on charged species induce local convection movements in the liquid which could contribute to accelerate associations between ions or colloidal particles. According to these authors, magnetohydrodynamic (MHD) phenomena induce eddy currents which flattened the fluid velocity profile in the tube. This effect would result in a larger velocity gradient, along the walls. In addition, the streaming potential along the walls which is velocity dependent should increase with the magnetic field. This phenomenon, by changing the surface charge, could throw out of balance the water composition equilibrium in the vicinity of the tubing walls [17-19].
MF effects were also often explained in terms of magnetically induced changes in the hydration of ions or solid surfaces. According to Lungader Madsen [20-21] MF induces faster proton transfer from hydrogen carbonate to water, due to proton spin inversion in the field of diamagnetic salts. The increased formation of CO32- ions would explain the beneficial effect of MF on the amount of precipitate. Another explanation proposed by Higashitani et al. [22] is related to the specific influence of MF on the hydration of CO32-ions which could directly modify the polymorph phase equilibrium during the precipitation. The same phenomenon could also affect the dehydration process of the ionic pair associations and the hydrated calcium carbonate forms which act as precursors for the formation of the anhydrous forms which constitute the calcium carbonate precipitate.
4. Conclusion
Permanent magnetic treatment (0.16 T) of duration 15 min of calcocarbonic solution in the presence of Na2SO4, MgCl2 or NaCl affects properties of CaCO3 formed. It appears in changes of induction period, total and homogeneous precipitation ratio of calcium carbonate formed by degassing test of treated solution. The effects depended on the kind of the foreign ions present in the treated water. This effect is, also, influenced by the hardness of water and by the nature of the pipe material. Finally, MF influence the morphology of calcium carbonate precipitate by increasing the precipitation of vaterite and aragonite in the presence of magnesium and by suppressed the formation of calcite.
Acknowledgements
This research was supported by the CMCU international program « Projet de Reseau Elargi CMCU (04PR01) intitule Gestion des Ressources en Eaux Non Conventionnelles. Procedes de Traitement Pour Po-tabilisation et Reutilisation ».
REFERENCES
[1]. Bogatin, J., Bondarenko, N.Ph., Gak, E.Z., Rokhinson, E.E., Ananyev I.P. Magnetic treatment of irrigation water: Experimental results and application conditions, Environ. Sci. Techno., 1999, vol. 33, pp.1280-1285.
[2]. Szkatula, A., Balanda, M., Kopec M. Magnetic treatment of industrial water. Silica activation, The Eur. Phys. J. AP, 2002, vol. 18, pp. 41-49.
[3]. Chibowski, E., Hotysz, L. and Szczes A. Adhesion of in situ precipitated calcium carbonate in the presence and absence of magnetic field in quiescent conditions on different solid surfaces, Water Res., 2003, vol. 37, pp. 4685-4692.
[4]. Chibowski E., Hotysz L., Szczes A. Time dependent changes in zeta potential of freshly precipitated calcium carbonate, Colloids and Surfaces A: Physicochem. Eng. Aspects, 2003, vol. 222, pp. 41-54.
[5]. Hotysz, L., Chibowski, E., Szczes A., Influence of impurity and magnetic field on the properties of freshly precipitated calcium carbonate, Water Res., 2003, vol. 37, pp. 3351-3366.
[6]. Herzog, R.E., Shi, Q., Patil, J.N., Katz J.L. Magnetic Water Treatment: The Effect of Iron on Calcium Carbonate Nucleation and Growth, Langmuir, 1989, vol. 5, pp. 861-867.
[7]. Alimi F., Tlili M., Gabrielli C., Maurin G.,Ben Amor M. Effect of a magnetic water treatment on homogeneous and heterogeneous precipitation of calcium carbonate, Water Res., 2006, vol. 40, pp. 1941-1950.
[8]. Gabrielli C., Jaouhari R., Maurin, G., Keddam M. Magnetic water treatment for scale prevention, Water Res., 2001, vol. 35, pp. 3249-3259.
[9]. Alimi F., Tlili M., Gabrielli C., Maurin G.,Ben Amor M. Influence of magnetic field on calcium carbonate precipitation, Desalination, 2007, vol. 206, pp. 163-168.
[10]. ParkerL.V., Ranney T.A. Sampling Trace-Level Organics with Polymeric Tubings. Special Report 96-3 US Army Corps of Engineers Cold Regions Research & Engineering Laboratory February 1996.
[11]. Barcelona M.J., Helfrich J.A., Garske E.E. Sampling tubing effects on ground water samples, Analytical Chemistry, 1985, vol. 57, pp. 460-464.
[12]. Curran C.M., Tomson M.B. Leaching of trace organics into water from five common plastics, Ground Water Monitoring Review, 1983, vol. 3, pp. 68-71.
[13]. Amiri M.C., Dadkhah A.A. On reduction in the surface tension of water due to magnetic treatment, Colloid Surface A, 2006, vol. 278, pp. 252-255.
[14]. Tai C.Y., Chien W.C. Interpreting the effects of operating variables on the induction period of CaCl2-Na2CO3 system by a cluster coagulation model, Chem. Eng. Sci., 2003, vol. 58, pp. 3233-3241.
[15]. Pokrovsky O.S. Precipitation of calcium and magnesium carbonates from homogeneous supersaturated solutions, J. Cryst. Growth, 1998, vol. 186, pp. 233-239.
[16]. Kozic V., LipusL.C. Magnetic Water Treatment for a Less Tenacious Scale, J. Chem. Inf. Comput. Sci., 2003, vol. 43, pp. 1815-1819.
[17]. Busch K.W., Gopalakrishnan S., Busch M.A., Tombacz E. Magnetohydrodynamic aggregation of cholesterol and polystyrene latex suspensions, J. Colloid Interf. Sci., 1996, vol. 183, pp.528-538.
[18]. Martemianov S., Sviridov A. Study of near wall hydrodynamics and mass transfer under magnetic field influence. In: Alemany, A., Marty, Ph., Thibault, J.P. (Eds). Transfer phenomena in magnetohydrodynamic and electroconducting flows. Kluwer Academic Publishers, 1999, pp. 229-240.
[19]. Knez S., Pohar C. The magnetic field influence on the polymorph composition of CaCO3 precipitation from carbonized aqueous solutions, J. Colloid Interf. Sci., 2005, vol. 281, pp. 377-388.
[20]. Lungader Madsen H.E. Crystallization of calcium carbonate in magnetic field ordinary and heavy water, J. Cryst. Growth, 2004, vol. 267, pp. 251-255.
[21]. Lungader Madsen H.E. Influence of magnetic field on the precipitation of some inorganic salts // J. Cryst. Growth, 1995, vol. 152, pp. 94-100.
[22]. Highashitani K., Okuhara K., Hatade S. Effects of magnetic fields on stability of nonmagnetic ultrafine colloidal particles, J. Colloid Interf. Sci. 1992, vol. 152, pp. 125-131.
Received 25.08.08
Summary
The study of magnetic effect on the precipitation of calcium carbonate was widely studied. The main objective was to prevent the mineral scales deposits on the walls of industrial or domestic equipments. Magnetic water treatment for scale prevention has been around and has been reported as being effective in many instances. This investigation focused on MF effect on CaCO3 in presence of foreign ions. Because of their abundance in natural waters Mg2+, SO42-, Na+ and Cl" were chosen for this study. It was shown that MF application reduces the induction time (tind) some either the water composition or the ionic strength (IS). With respect to reference solution, the effect of MF on tind is more pronounced in presence of foreign ions until IS= 0.02 M. Beyond this value, MF effect on tind become less important. Moreover, it was shown that MF effect on total and homogeneous precipitation ratios decreases with IS increasing. This inhibitory effect of magnetic treatment, with IS, is water composition-dependent. The presence of NaCl has the most effect on total precipitation whereas MgCl2 and Na2SO4 inhibit the precipitation of calcium carbonate in bulk of the solution.
