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Assessing the effect of zinc on the crystallization of calcium carbonate
Desalination 220 (2008) 394–402
Assessing the effect of zinc on the crystallization of calcium carbonate S. Ghizellaouia*, M. Euvrardb a
Département de Chimie, Université de Mentouri de Constantine, Route de Ain El Bey, 25000 Constantine, Algéria Tel./Fax +213 31 81 88 16; email: [email protected] b UTINAM (Equipe Matériaux et Surfaces Structurés), CNRS 6213, UFR Sciences et Techniques F-25030 Besançon Cedex, France Received 31 January 2007; accepted 4 February 2007
Abstract Scaling, often due to the precipitation of calcium carbonate, is a current problem in economical and technical life. To study this phenomenon, some experimental procedures have been developed. They are based for example on electrochemical precipitation, use of nanofiltration and rapid controlled precipitation (RCP). This latter technique appears as an efficient one because it permits to study the early stage of crystallization. An other technique could be considered as a promising route among all of them because it permits to follow in real time and in situ, the electrocrystallization of calcium carbonate. Some morphometric characteristics of crystals like: size, diameter, shape and also other parameters as the rate of covering surface of the electrode could be quantified. Experiments were carried out with this procedure and RCP test in order to study the effect of zinc on the precipitation of calcium carbonate. Experiments were carried out with the water of Hamma (locality situated near the city of Constantine) which has a high power of scaling. The process occurs in two steps in control water: spontaneous crystallization followed by growth of crystals. In presence of zinc (0.05, 0.1 and 10 mg/L), we observed a reduction of nucleation and a high inhibition of growth of crystal. At lower concentration, i.e. 10 μg/L, the crystallization still occurred but crystals were smaller than in control water. These results were in good agreement with those obtained previously. Moreover, the analysis by SEM revealed a change of morphology in presence of zinc and a change of crystalline varieties. Keywords: Scaling; Calcium carbonate; Zinc; Electrocrystallization; Visualization; Image analysis
*Corresponding author. Presented at the conference on Desalination and the Environment. Sponsored by the European Desalination Society and Center for Research and Technology Hellas (CERTH), Sani Resort, Halkidiki, Greece, April 22–25, 2007. 0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.02.044
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1. Introduction Scaling may cause serious problems in water supply networks and in industrial field. They are mainly due to the deposit of CaCO3 which can lead to damages of the water-pipings. As example, cooling processes in factories do not work well when they are fouled with these deposits [1–4]. Various experimental procedures have been developed in order to study scaling [5,6]. In general they are based on the acceleration of the precipitation of CaCO3 by the increase of the supersaturation. Some of these techniques are of chemical types using components such as Ca(OH)2, NaOH and Na2CO3 [7] whereas others are of electrochemical type using techniques like: chronoamperometry [8], chronoelectrogravimetry [9], and impedancimetry [10–13]. One of them permits to visualize the formation of crystals in situ and in function of time [11,14]. In this paper, we wish to present a study performed to quantify the scaling power of a drinking water coming from the source of Hamma (a locality very close to the city of Constantine and situated in the Eastern part of Algeria) and the effect of Zn on it. Electrocrystallization, conducted in presence and in absence of Zn permits to follow in situ the formation and growth of crystals; results were compared with those obtained with RCP. Finally, the role of the ions Zn in affecting the interface will be discussed. 2. Material and methods 2.1. Method of electrocrystallization The experimental assembly is composed of an electrochemical cell and a video set up (Fig. 1). This method consists in covering a metallic polarized surface with calcium carbonate. The application of a negative potential involves on the surface of metal primarily the reduction of oxygen according to the reaction:
Electrochemical cell Objective x 20 Video tube
Video recorder
Light
Camera
Monitor
Computer image analysis
Fig. 1. Experimental assembly.
O2 + 2H2O + 4e− → 4OH−
(1)
In the vicinity of the working electrode, the presence of the ions hydroxides involves an increase in the pH locally and the ions hydrogenocarbonates can then be transformed into ions carbonates according to the reaction: HCO3− + OH− → CO32− + H2O
(2)
The increase in the content of CO32− then involves the precipitation of calcium carbonate on the surface of the electrode according to the reaction: Ca2+ + CO2− 3 → CaCO3(s)
(3)
2.1.1. Electrochemical cell The electrochemical cell is composed of: • a removal cathodic plug comprising a 1 cm2 circular electrode of stainless steel, • a PMMA plate which contains the conductive glass coated with a tin oxide film acting as a window and a counter electrode, • a silver wire that was pretreated with a diluted solution of hydrochloric acid, working as an Ag/AgCl electrode.
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2.1.2. The video set up The optical assembly contained a video tube, a long working distance objective, a lighting process and a camera (Sonny SSC-Dc38p). This assembly (Fig. 1) permits to observe on the monitor, the state of the surface of the working electrode. The high power (1000) makes possible to study the growth of particles (as soon as they reach 1 µm). Micrometric screws fixed on the video tube allow the control of the total surface area of the working electrode. The evolution of the surface was filmed and recorded (recorder Sony SL-UE 710B) during experiments. Moreover, an image analysis system (Aries Software ESILAB) permits the quantification of crystals over different periods of time. Therefore we might obtain the following data, • number of crystals • mean characteristics of the crystals as size (mean diameter) • rate of covering of the metallic electrode. 2.1.3. Experimental conditions The following experimental conditions were applied during the experiments: • The stainless steel working electrode polished to 0.8 and 0.02 µm, rinsed in an ultrasonic tank and then immediately used for the experiment. • A potential of −1 V/Ag/AgCl was applied: this potential was chosen because it was in the middle of the diffusion plateau of oxygen on the cathodic polarization curve (between −0.7 and −1.2 V) • The flow rate was 40 mL/min. 2.2. Test of rapid controlled precipitation According to Lédion [5], in order to characterize the scaling power of water, the experimental procedure consists in carrying water to a degree of supersaturation of about 20 to 30 K’s.
To obtain it, the water was shaked to reach a pH sufficiently high by degasification of CO2. The maximum value is reached when the equilibrium is reached between dissolved CO2 and atmospheric CO2; it is governed by the law of Henry. Thus, when a degree of a supersaturation from 20 to 30 K’s was reached, the phenomenon of germination–growth of calcium carbonate is promoted. By following the pH and the resistivity in reference and doped waters, the effect of Zn was studied. A higher pH and a lower resistivity are significant of a delay of the crystallization. 2.3. Chemicals We chose the water of Hamma which feeds the town of Constantine with drinking water and which has a high scaling power and quite mineral-bearing. The composition of water is given in the Table 1. Table 1 Hamma water composition Parameter
Value
T (°C) pH EC (mS/cm) TOC (mg/L) CO2 (mg/L) HCO−3 (mg/L) Cl− (mg/L) SO42− (mg/L) PO43− (mg/L) NO−3 (mg/L) NO−2 (mg/L) NH4+ (mg/L) TH mg/L CaCO3 Ca2+ (mg/L) Mg2+ (mg/L) Na+ (mg/L) K+ (mg/L)
31–33 6.95–7.94 0.9–1.30 3.84–7.5 16.21–37.05 320–432 128–150 120–212 0.006–0.02 3.90–7 – – 475–600 124–131 30.48–43.2 84–115 2.22–13
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t = 5 min
t = 20 min
397
t = 60 min
Fig. 2. Water of Hamma.
Tests were carried out with the water of Hamma (Reference water) and after additions of Zn at concentrations of 0.01, 0.05, 0.1 and 10 mg/L in the form of acetate Zn(CH3COO)2, 2H2O. The analysis given in Table 1 shows that Hamma water have a low organic matter content (TOC: 3.84–7.5 mg/L), a rather high salts content (EC 0.9–1.30 mS/cm) and an high hardness (TH 475–600 mg/L CaCO3). 3. Results 3.1. Electrocrystallization The electrochemical procedure permits to obtain qualitative and quantitative pieces of information. 3.1.1. Qualitative results The first crystals obtained from the water of Hamma appeared as soon as the potential was
t = 5 min
Fig. 3. Water of Hamma doped with 10 mg/L of Zn.
applied; then they grew at the surface of the electrode. So, it is characteristic of spontaneous nucleation followed by crystal growth (Fig. 2). Tests carried out after the addition of Zn indicated that it acted on the nucleation and the growth of calcium carbonate crystals when its concentration in water was of 0.05, 0.1 and 10 mg/L of Zn (Fig. 3). The analysis by SEM gave complementary pieces of information. In reference water (Fig. 4), we observed the presence of calcite whatever the enlargements (500×, 1500×, 3500×). When 10 µg/L of Zn was added, we noted a change in the morphology of some crystals; white tasks were observed on the calcium carbonate crystals (Fig. 5); this crystalline form is characteristic of aragonite. A change of morphology was also observed when Zn was added at a concentration of 0.05 mg/L. Crystals presented lengthened forms and also various sizes (Fig. 6).
t = 20 min
t = 60 min
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Fig. 4. Calcite deposits obtained with the water of Hamma.
White tasks
Fig. 5. Calcium carbonate obtained after addition of 10 µg/L of Zn.
Lastly, for the maximum used concentration on Zn (10 mg/L) we noticed that Zn precipitated in the mixed form of Zn and Ca carbonate. The size of the particles was of about 0.1 to 0.5 µm (Fig. 7).
3.1.2. Quantitative results The image analysis permits to obtain quantitative pieces of information about the effect of Zn on the electrocrystallization of calcium carbonate. Fig. 8 shows the surface coverage of the electrode when reference water and doped waters were studied. When Zn was added at a concentration of 0.01 mg/L, the surface coverage was similar to this of reference water; it is significant of spontaneous nucleation. For higher concentrations, the presence of Zn leaded to a high decrease of the coverage of calcium carbonate. After 60 min of polarization, the number of crystals was quantified; the values are presented in Table 2. The results indicated that the numbers of crystals were close in the three cases. Nevertheless, as soon as the concentration of Zn reached 0.05 mg/L, the quantification of the crystals was delayed of a few minutes.
Fig. 6. Calcium carbonate crystals obtained after addition of 0.05 mg/L of Zn.
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Table 2 Number of crystals (/600 µm2)
Fig. 7. Calcium carbonate crystals obtained after addition of 10 mg/L of Zn.
Type of experiment
Number of crystals/cm2
Reference water With 0.01 mg/L of Zn With 10.0 mg/L of Zn
170 180 150
Indeed, whereas, the number of crystals were similar, the growth of crystals was very low in presence of Zn; this suggests that it may precipitate or adsorb on the surface of the crystals and avoid their development. 3.2. Test of rapid controlled precipitation
Moreover, the mean morphometric characteristics of crystals were quantified and presented in Tables 3 to 5. The morphometric characteristics of crystals over different periods of time were assessed. The crystals presented similar shape factors after 10 min for the reference water and doped water with Zn. (Tables 3–5). For longer periods of time, the shape factors decreased in doped waters; the presence of Zn altered the crystals which were more lenghtened. The results point out that the presence of zinc blocked the crystallisation of calcium carbonate. 40 35
Coverage (%)
30 25 Hamma Zn (10 mg/L) Zn (0.1 mg/L) Zn (0.05 mg/L)
20 15 10 5 0 0
10
20
30 40 Time (min)
50
Fig. 8. Rate of covering of the working electrode.
60
Test rapid controlled precipitation (RCP) was carried out for different concentrations of Zn: 0.05, 0.1, 0.2, 0.5, 1, 1.5 mg/L [15]. This technique was used in order to compare the results with those obtained with the method of electrocrystallization. Compared to reference water, it must be noted that the decrease of pH in treated water was delayed as shown on Figs. 9 and 10; this is significant of a delay of the precipitation. Moreover, the reduction is similar for the two curves; it may be due to a difference of the kinetics of precipitation. As shown on Fig. 11, the effectiveness was equal to 100%, when the water contained 1 mg/L of zinc. The curves Res = f(t) revealed the precipitation of Ca2+ and CO2− 3 ions by a increase of the resistivity; it was higher for reference than for treated waters. Tests carried out with Zn at different concentrations revealed that a high effect of Zn on the crystallization of calcium carbonate; a plateau was reached for a concentration of 0.5 mg/L. (Fig. 12). For an addition of 100 µg/L, the effectiveness reached 50%. The inhibiting effect was observed for the same concentration during the tests of electrocrystallization.
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Table 3 Mean morphometric characteristics of crystals (reference water)
Size (µm) Shape factor Surface area (µm2) Covering (%)
2 min
5 min
10 min
20 min
40 min
3.7 0.3 10.5 1.5
4.6 0.3 17.0 4.3
5.1 0.5 20.5 11.5
6.0 0.5 28.5 19.6
Formation of aggregates
4. Discussion Our results, using complementary experimental procedures, show a high effect of Zn on the crystallization of calcium carbonate. It may modify the crystallization of calcium carbonate namely by decreasing the step of growth of crystals. Experiments show that at a concentration of 1.5 ⋅ 10−4 M, the sizes of crystals did not reach 3 µm. It may be due to the précipitation of Zn or its adsorption on the crystals of calcium carbonate. Other studies are in agreement with these results. The study carried out by Meyer [16] showed the effect of zinc on the growth rate of
60 min
30.3
34.7
calcite and proved that this element is a good inhibitor. There is a reduction of the growth rate of 80% for 2 ⋅ 10−7 M; in our studies, we quantified an important effect for close concentration i.e. 7 ⋅ 10−7 M. Concerning the mechanism of inhibition, the effect of Zn was studied and its adsorption on crystals of calcite was revealed. The interaction of dissolved zinc on calcite was namely studied by Sawsan et al. [17]; the authors showed that the adsorption of zinc on the mineral followed the Freundlich model [18]. Moreover, Glasner and Weiss [19] showed that there was a coprecipitation with calcite.
Table 4 Mean morphometric characteristics of crystals (water with 10 mg/L of Zn)
Size (µm) Shape factor Surface area (µm2) Covering (%)
2 min
5 min
10 min
20 min
40 min
60 min
1.0 0.2 0.8 0.4
1.6 0.3 2.0 0.6
2.0 0.4 3.6 0.7
1.4 0.4 4.7 1.5
2.0 0.4 5.9 2.3
2.1 0.4 20.6 2.6
Table 5 Mean morphometric characteristics of crystals (water with 0.1 mg/L of Zn)
Size (µm) Shape factor Surface area (µm2) Covering (%)
2 min
5 min
10 min
20 min
40 min
60 min
1.0 0.2 1.2 0.3
1.2 0.3 2.4 0.5
1.3 0.2 2.0 0.8
1.4 0.3 2.3 1.8
1.5 0.3 2.5 3
1.5 0.3 2.4 3.5
S. Ghizellaoui, M. Euvrard / Desalination 220 (2008) 394–402 Resistivity (Ohms.cm) 1110
pH 8.8 8.6
1060
8.4 8.2
1010
8.0 7.8 7.6 7.4 7.2 0
10
20
30
40
50
60
70
pH H 960 pH Zn 0.1 Résist H 910 Résist Zn 0.1 860 80 90 100
401 Resistivity (Ohms.cm)
pH
1060
8.7 8.5 8.3 8.1 7.9 7.7 7.5 7.3 7.1 6.9
1010 pH H 960 pH Zn 1 Résist H Résist Zn 1 910
0
10
20
30
40
50
60
70
80
90
860 100
Time (min)
Time (min)
Fig. 9. Curves RCP (pH and resistivity) after addition of 0.1 mg/L of Zn.
Fig. 11. Curves RCP (pH and resistivity) after addition of 1 mg/L of Zn. 120
Resistivity (Ohms.cm)
pH H pH Zn 0.2 Résist H Résist Zn 0.2
pH 8.8 8.6
1110
100 Efficiency (%)
A recent study carried out by Dmitry et al. [20] pointed out that 2 mg/L of zinc was able to avoid bulk precipitation of CaCO3 in waters of moderate hardness. Finally, the inhibiting effect of zinc by various chemical processes and its application for water of Hamma is significant according to the study of Ghizellaoui et al. [21]: • For the test of polyethylene the efficiency reached 100% for 1 mg/L of Zn at ambient temperature. • For the test of potentiality of scaling at 5 mg/L of Zn we revealed the inhibiting effect of Zn with smaller size of the crystals and with a more significant number in the bulk.
80 60 40 20 0 0
0.5
1 1.5 Conc (mg/L) of Zn
2
Fig. 12. Evolution of the efficiency according to the concentration of Zn.
• Lastly a chemical softening in presence of lime showed that the abatement of calcium was pronounced at 5 mg/L for Zn.
1060
8.4 8.2
1010
8.0 960
7.8 7.6
910
7.4 7.2 0
10
20
30
40
50
60
70
80
90
860 100
Time (min)
Fig. 10. Curves RCP (pH and resistivity) after addition of 0.2 mg/L of Zn.
5. Conclusion This study, using to complementary experimental procedures, reveals the effect of Zn on the crystallization of calcium carbonate. Experiments were carried out with the water of Hamma (locality situated near the city of Constantine) which has a high power of scaling. The electocrystallization test showed that the nucleation was spontaneous and followed by
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growth of crystals. In presence of Zn (0.05, 0.1 and 10 mg/L), we observed a high inhibition of growth of crystals. At lower concentration, i.e. 10 μg/L, the crystallization still occurred but crystals were smaller than in control water. Moreover, the analysis by SEM revealed a change of morphology of crystals in presence of zinc and a change of crystalline varieties. Tests of controlled rapid precipitation, carried out with Zn at different concentrations revealed a high effect of Zn on the crystallization of calcium carbonate; a very important effect was reached for a concentration of 0.5 mg/L. Moreover, for a concentration of 100 µg/L of Zn, the effectiveness reached 50%. So, the results obtained with these experimental procedures are in agreement. The mechanisms by which Zn acts may be of two kinds: adsorption on crystals of calcite and/or coprecipitation with hydroxyde and carbonate ions. References [1]
[2] [3] [4]
M. Euvrard, F. Membrey, C. Filiatre, C. Pignolet and A. Foissy, J. Cryst. Growth, 291 (2006) 428–435. M. Jefferies and D. Comstock, Desalination, 139 (1) (2001) 341. M. Euvrard and C. Filiatre, J. Eur. Hydro., 30 (1) (1999) 35–46. S.J. Meyer and G.M. Graham, J. Petrolium. Eng., 35 (2002) 95.
[5] J. Ledion, B. François and J. Vienne, J. Eur. Hydro., 28 (1) (1998) 15–35. [6] R. Rosset, M. Zidoune, C. Gabrielli, M. Keddam, G. Maurin and H. Perrot, C.R. Acad. Sci., Paris, 322 (IIb) (1996) 335–341. [7] S. Ghizellaoui, Ph.D. Thesis, Université Mentouri Constantine, 2006. [8] J. Lédion, P. Leroy and J.-P. Labbé, TSM l’eau, 80 (7–8) (1985) 323–328. [9] A. Khalil, P. Sassiat, C. Colin, C. Meignem, C. Garnier, C. Gabrielli, M. Keddam and R. Rosset, C.R. Acad. Sci. Paris, 314 (II) (1992) 145–149. [10] A. Khalil, C. Colin, C. Garnier, M. Keddam and R. Rosset, C.R. Acad. Sci., Paris, 316 (II) (1993) 19–24. [11] M. Euvrard, C. Filiatre and E. Crauzas, J. Cryst. Growth, 216 (2000) 466–474. [12] J.Y. Gal, J.C. Bollinger, H. Tolosa and N. Gache, Talanta, 43 (1996) 1497. [13] L. Swinney, J. Stevens and R. Peters, Ind. Eng. Chem. Fundam., 21 (1982) 31. [14] M. Euvrard, F. Membrey, C. Filiatre and A. Foissy, J. Cryst. Growth, 265 (2004) 322–330. [15] S. Ghizellaoui, J. Lédion, S. Ghizellaoui and A. Chibani, Desalination, 166 (2004) 315–327. [16] H. Meyer, J. Cryst. Growth, 66 (1984) 639–646. [17] J.F. Sawsan, A. Godelitsas and A. Putnis, J. Cryst. Growth, 273 (2005) 535–545. [18] P. Balaz, A. Alacova and J. Briancin, Chem. Eng. J., 114 (2005) 115–121. [19] A. Glasner and D. Weiss, J. Inorg. Nucl. Chem., 42 (5) (1980) 655–663. [20] L. Dmitry, Q. Yang, D. Hasson and R. Semiat, Desalination, 183 (2005) 289–300. [21] S. Ghizellaoui, M. Euvrard, J. Lédion and A. Chibani, Desalination, 206 (2007) 185–197.
Assessing the effect of zinc on the crystallization of calcium carbonate S. Ghizellaouia*, M. Euvrardb a
Département de Chimie, Université de Mentouri de Constantine, Route de Ain El Bey, 25000 Constantine, Algéria Tel./Fax +213 31 81 88 16; email: [email protected] b UTINAM (Equipe Matériaux et Surfaces Structurés), CNRS 6213, UFR Sciences et Techniques F-25030 Besançon Cedex, France Received 31 January 2007; accepted 4 February 2007
Abstract Scaling, often due to the precipitation of calcium carbonate, is a current problem in economical and technical life. To study this phenomenon, some experimental procedures have been developed. They are based for example on electrochemical precipitation, use of nanofiltration and rapid controlled precipitation (RCP). This latter technique appears as an efficient one because it permits to study the early stage of crystallization. An other technique could be considered as a promising route among all of them because it permits to follow in real time and in situ, the electrocrystallization of calcium carbonate. Some morphometric characteristics of crystals like: size, diameter, shape and also other parameters as the rate of covering surface of the electrode could be quantified. Experiments were carried out with this procedure and RCP test in order to study the effect of zinc on the precipitation of calcium carbonate. Experiments were carried out with the water of Hamma (locality situated near the city of Constantine) which has a high power of scaling. The process occurs in two steps in control water: spontaneous crystallization followed by growth of crystals. In presence of zinc (0.05, 0.1 and 10 mg/L), we observed a reduction of nucleation and a high inhibition of growth of crystal. At lower concentration, i.e. 10 μg/L, the crystallization still occurred but crystals were smaller than in control water. These results were in good agreement with those obtained previously. Moreover, the analysis by SEM revealed a change of morphology in presence of zinc and a change of crystalline varieties. Keywords: Scaling; Calcium carbonate; Zinc; Electrocrystallization; Visualization; Image analysis
*Corresponding author. Presented at the conference on Desalination and the Environment. Sponsored by the European Desalination Society and Center for Research and Technology Hellas (CERTH), Sani Resort, Halkidiki, Greece, April 22–25, 2007. 0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.02.044
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1. Introduction Scaling may cause serious problems in water supply networks and in industrial field. They are mainly due to the deposit of CaCO3 which can lead to damages of the water-pipings. As example, cooling processes in factories do not work well when they are fouled with these deposits [1–4]. Various experimental procedures have been developed in order to study scaling [5,6]. In general they are based on the acceleration of the precipitation of CaCO3 by the increase of the supersaturation. Some of these techniques are of chemical types using components such as Ca(OH)2, NaOH and Na2CO3 [7] whereas others are of electrochemical type using techniques like: chronoamperometry [8], chronoelectrogravimetry [9], and impedancimetry [10–13]. One of them permits to visualize the formation of crystals in situ and in function of time [11,14]. In this paper, we wish to present a study performed to quantify the scaling power of a drinking water coming from the source of Hamma (a locality very close to the city of Constantine and situated in the Eastern part of Algeria) and the effect of Zn on it. Electrocrystallization, conducted in presence and in absence of Zn permits to follow in situ the formation and growth of crystals; results were compared with those obtained with RCP. Finally, the role of the ions Zn in affecting the interface will be discussed. 2. Material and methods 2.1. Method of electrocrystallization The experimental assembly is composed of an electrochemical cell and a video set up (Fig. 1). This method consists in covering a metallic polarized surface with calcium carbonate. The application of a negative potential involves on the surface of metal primarily the reduction of oxygen according to the reaction:
Electrochemical cell Objective x 20 Video tube
Video recorder
Light
Camera
Monitor
Computer image analysis
Fig. 1. Experimental assembly.
O2 + 2H2O + 4e− → 4OH−
(1)
In the vicinity of the working electrode, the presence of the ions hydroxides involves an increase in the pH locally and the ions hydrogenocarbonates can then be transformed into ions carbonates according to the reaction: HCO3− + OH− → CO32− + H2O
(2)
The increase in the content of CO32− then involves the precipitation of calcium carbonate on the surface of the electrode according to the reaction: Ca2+ + CO2− 3 → CaCO3(s)
(3)
2.1.1. Electrochemical cell The electrochemical cell is composed of: • a removal cathodic plug comprising a 1 cm2 circular electrode of stainless steel, • a PMMA plate which contains the conductive glass coated with a tin oxide film acting as a window and a counter electrode, • a silver wire that was pretreated with a diluted solution of hydrochloric acid, working as an Ag/AgCl electrode.
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2.1.2. The video set up The optical assembly contained a video tube, a long working distance objective, a lighting process and a camera (Sonny SSC-Dc38p). This assembly (Fig. 1) permits to observe on the monitor, the state of the surface of the working electrode. The high power (1000) makes possible to study the growth of particles (as soon as they reach 1 µm). Micrometric screws fixed on the video tube allow the control of the total surface area of the working electrode. The evolution of the surface was filmed and recorded (recorder Sony SL-UE 710B) during experiments. Moreover, an image analysis system (Aries Software ESILAB) permits the quantification of crystals over different periods of time. Therefore we might obtain the following data, • number of crystals • mean characteristics of the crystals as size (mean diameter) • rate of covering of the metallic electrode. 2.1.3. Experimental conditions The following experimental conditions were applied during the experiments: • The stainless steel working electrode polished to 0.8 and 0.02 µm, rinsed in an ultrasonic tank and then immediately used for the experiment. • A potential of −1 V/Ag/AgCl was applied: this potential was chosen because it was in the middle of the diffusion plateau of oxygen on the cathodic polarization curve (between −0.7 and −1.2 V) • The flow rate was 40 mL/min. 2.2. Test of rapid controlled precipitation According to Lédion [5], in order to characterize the scaling power of water, the experimental procedure consists in carrying water to a degree of supersaturation of about 20 to 30 K’s.
To obtain it, the water was shaked to reach a pH sufficiently high by degasification of CO2. The maximum value is reached when the equilibrium is reached between dissolved CO2 and atmospheric CO2; it is governed by the law of Henry. Thus, when a degree of a supersaturation from 20 to 30 K’s was reached, the phenomenon of germination–growth of calcium carbonate is promoted. By following the pH and the resistivity in reference and doped waters, the effect of Zn was studied. A higher pH and a lower resistivity are significant of a delay of the crystallization. 2.3. Chemicals We chose the water of Hamma which feeds the town of Constantine with drinking water and which has a high scaling power and quite mineral-bearing. The composition of water is given in the Table 1. Table 1 Hamma water composition Parameter
Value
T (°C) pH EC (mS/cm) TOC (mg/L) CO2 (mg/L) HCO−3 (mg/L) Cl− (mg/L) SO42− (mg/L) PO43− (mg/L) NO−3 (mg/L) NO−2 (mg/L) NH4+ (mg/L) TH mg/L CaCO3 Ca2+ (mg/L) Mg2+ (mg/L) Na+ (mg/L) K+ (mg/L)
31–33 6.95–7.94 0.9–1.30 3.84–7.5 16.21–37.05 320–432 128–150 120–212 0.006–0.02 3.90–7 – – 475–600 124–131 30.48–43.2 84–115 2.22–13
S. Ghizellaoui, M. Euvrard / Desalination 220 (2008) 394–402
t = 5 min
t = 20 min
397
t = 60 min
Fig. 2. Water of Hamma.
Tests were carried out with the water of Hamma (Reference water) and after additions of Zn at concentrations of 0.01, 0.05, 0.1 and 10 mg/L in the form of acetate Zn(CH3COO)2, 2H2O. The analysis given in Table 1 shows that Hamma water have a low organic matter content (TOC: 3.84–7.5 mg/L), a rather high salts content (EC 0.9–1.30 mS/cm) and an high hardness (TH 475–600 mg/L CaCO3). 3. Results 3.1. Electrocrystallization The electrochemical procedure permits to obtain qualitative and quantitative pieces of information. 3.1.1. Qualitative results The first crystals obtained from the water of Hamma appeared as soon as the potential was
t = 5 min
Fig. 3. Water of Hamma doped with 10 mg/L of Zn.
applied; then they grew at the surface of the electrode. So, it is characteristic of spontaneous nucleation followed by crystal growth (Fig. 2). Tests carried out after the addition of Zn indicated that it acted on the nucleation and the growth of calcium carbonate crystals when its concentration in water was of 0.05, 0.1 and 10 mg/L of Zn (Fig. 3). The analysis by SEM gave complementary pieces of information. In reference water (Fig. 4), we observed the presence of calcite whatever the enlargements (500×, 1500×, 3500×). When 10 µg/L of Zn was added, we noted a change in the morphology of some crystals; white tasks were observed on the calcium carbonate crystals (Fig. 5); this crystalline form is characteristic of aragonite. A change of morphology was also observed when Zn was added at a concentration of 0.05 mg/L. Crystals presented lengthened forms and also various sizes (Fig. 6).
t = 20 min
t = 60 min
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Fig. 4. Calcite deposits obtained with the water of Hamma.
White tasks
Fig. 5. Calcium carbonate obtained after addition of 10 µg/L of Zn.
Lastly, for the maximum used concentration on Zn (10 mg/L) we noticed that Zn precipitated in the mixed form of Zn and Ca carbonate. The size of the particles was of about 0.1 to 0.5 µm (Fig. 7).
3.1.2. Quantitative results The image analysis permits to obtain quantitative pieces of information about the effect of Zn on the electrocrystallization of calcium carbonate. Fig. 8 shows the surface coverage of the electrode when reference water and doped waters were studied. When Zn was added at a concentration of 0.01 mg/L, the surface coverage was similar to this of reference water; it is significant of spontaneous nucleation. For higher concentrations, the presence of Zn leaded to a high decrease of the coverage of calcium carbonate. After 60 min of polarization, the number of crystals was quantified; the values are presented in Table 2. The results indicated that the numbers of crystals were close in the three cases. Nevertheless, as soon as the concentration of Zn reached 0.05 mg/L, the quantification of the crystals was delayed of a few minutes.
Fig. 6. Calcium carbonate crystals obtained after addition of 0.05 mg/L of Zn.
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399
Table 2 Number of crystals (/600 µm2)
Fig. 7. Calcium carbonate crystals obtained after addition of 10 mg/L of Zn.
Type of experiment
Number of crystals/cm2
Reference water With 0.01 mg/L of Zn With 10.0 mg/L of Zn
170 180 150
Indeed, whereas, the number of crystals were similar, the growth of crystals was very low in presence of Zn; this suggests that it may precipitate or adsorb on the surface of the crystals and avoid their development. 3.2. Test of rapid controlled precipitation
Moreover, the mean morphometric characteristics of crystals were quantified and presented in Tables 3 to 5. The morphometric characteristics of crystals over different periods of time were assessed. The crystals presented similar shape factors after 10 min for the reference water and doped water with Zn. (Tables 3–5). For longer periods of time, the shape factors decreased in doped waters; the presence of Zn altered the crystals which were more lenghtened. The results point out that the presence of zinc blocked the crystallisation of calcium carbonate. 40 35
Coverage (%)
30 25 Hamma Zn (10 mg/L) Zn (0.1 mg/L) Zn (0.05 mg/L)
20 15 10 5 0 0
10
20
30 40 Time (min)
50
Fig. 8. Rate of covering of the working electrode.
60
Test rapid controlled precipitation (RCP) was carried out for different concentrations of Zn: 0.05, 0.1, 0.2, 0.5, 1, 1.5 mg/L [15]. This technique was used in order to compare the results with those obtained with the method of electrocrystallization. Compared to reference water, it must be noted that the decrease of pH in treated water was delayed as shown on Figs. 9 and 10; this is significant of a delay of the precipitation. Moreover, the reduction is similar for the two curves; it may be due to a difference of the kinetics of precipitation. As shown on Fig. 11, the effectiveness was equal to 100%, when the water contained 1 mg/L of zinc. The curves Res = f(t) revealed the precipitation of Ca2+ and CO2− 3 ions by a increase of the resistivity; it was higher for reference than for treated waters. Tests carried out with Zn at different concentrations revealed that a high effect of Zn on the crystallization of calcium carbonate; a plateau was reached for a concentration of 0.5 mg/L. (Fig. 12). For an addition of 100 µg/L, the effectiveness reached 50%. The inhibiting effect was observed for the same concentration during the tests of electrocrystallization.
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Table 3 Mean morphometric characteristics of crystals (reference water)
Size (µm) Shape factor Surface area (µm2) Covering (%)
2 min
5 min
10 min
20 min
40 min
3.7 0.3 10.5 1.5
4.6 0.3 17.0 4.3
5.1 0.5 20.5 11.5
6.0 0.5 28.5 19.6
Formation of aggregates
4. Discussion Our results, using complementary experimental procedures, show a high effect of Zn on the crystallization of calcium carbonate. It may modify the crystallization of calcium carbonate namely by decreasing the step of growth of crystals. Experiments show that at a concentration of 1.5 ⋅ 10−4 M, the sizes of crystals did not reach 3 µm. It may be due to the précipitation of Zn or its adsorption on the crystals of calcium carbonate. Other studies are in agreement with these results. The study carried out by Meyer [16] showed the effect of zinc on the growth rate of
60 min
30.3
34.7
calcite and proved that this element is a good inhibitor. There is a reduction of the growth rate of 80% for 2 ⋅ 10−7 M; in our studies, we quantified an important effect for close concentration i.e. 7 ⋅ 10−7 M. Concerning the mechanism of inhibition, the effect of Zn was studied and its adsorption on crystals of calcite was revealed. The interaction of dissolved zinc on calcite was namely studied by Sawsan et al. [17]; the authors showed that the adsorption of zinc on the mineral followed the Freundlich model [18]. Moreover, Glasner and Weiss [19] showed that there was a coprecipitation with calcite.
Table 4 Mean morphometric characteristics of crystals (water with 10 mg/L of Zn)
Size (µm) Shape factor Surface area (µm2) Covering (%)
2 min
5 min
10 min
20 min
40 min
60 min
1.0 0.2 0.8 0.4
1.6 0.3 2.0 0.6
2.0 0.4 3.6 0.7
1.4 0.4 4.7 1.5
2.0 0.4 5.9 2.3
2.1 0.4 20.6 2.6
Table 5 Mean morphometric characteristics of crystals (water with 0.1 mg/L of Zn)
Size (µm) Shape factor Surface area (µm2) Covering (%)
2 min
5 min
10 min
20 min
40 min
60 min
1.0 0.2 1.2 0.3
1.2 0.3 2.4 0.5
1.3 0.2 2.0 0.8
1.4 0.3 2.3 1.8
1.5 0.3 2.5 3
1.5 0.3 2.4 3.5
S. Ghizellaoui, M. Euvrard / Desalination 220 (2008) 394–402 Resistivity (Ohms.cm) 1110
pH 8.8 8.6
1060
8.4 8.2
1010
8.0 7.8 7.6 7.4 7.2 0
10
20
30
40
50
60
70
pH H 960 pH Zn 0.1 Résist H 910 Résist Zn 0.1 860 80 90 100
401 Resistivity (Ohms.cm)
pH
1060
8.7 8.5 8.3 8.1 7.9 7.7 7.5 7.3 7.1 6.9
1010 pH H 960 pH Zn 1 Résist H Résist Zn 1 910
0
10
20
30
40
50
60
70
80
90
860 100
Time (min)
Time (min)
Fig. 9. Curves RCP (pH and resistivity) after addition of 0.1 mg/L of Zn.
Fig. 11. Curves RCP (pH and resistivity) after addition of 1 mg/L of Zn. 120
Resistivity (Ohms.cm)
pH H pH Zn 0.2 Résist H Résist Zn 0.2
pH 8.8 8.6
1110
100 Efficiency (%)
A recent study carried out by Dmitry et al. [20] pointed out that 2 mg/L of zinc was able to avoid bulk precipitation of CaCO3 in waters of moderate hardness. Finally, the inhibiting effect of zinc by various chemical processes and its application for water of Hamma is significant according to the study of Ghizellaoui et al. [21]: • For the test of polyethylene the efficiency reached 100% for 1 mg/L of Zn at ambient temperature. • For the test of potentiality of scaling at 5 mg/L of Zn we revealed the inhibiting effect of Zn with smaller size of the crystals and with a more significant number in the bulk.
80 60 40 20 0 0
0.5
1 1.5 Conc (mg/L) of Zn
2
Fig. 12. Evolution of the efficiency according to the concentration of Zn.
• Lastly a chemical softening in presence of lime showed that the abatement of calcium was pronounced at 5 mg/L for Zn.
1060
8.4 8.2
1010
8.0 960
7.8 7.6
910
7.4 7.2 0
10
20
30
40
50
60
70
80
90
860 100
Time (min)
Fig. 10. Curves RCP (pH and resistivity) after addition of 0.2 mg/L of Zn.
5. Conclusion This study, using to complementary experimental procedures, reveals the effect of Zn on the crystallization of calcium carbonate. Experiments were carried out with the water of Hamma (locality situated near the city of Constantine) which has a high power of scaling. The electocrystallization test showed that the nucleation was spontaneous and followed by
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growth of crystals. In presence of Zn (0.05, 0.1 and 10 mg/L), we observed a high inhibition of growth of crystals. At lower concentration, i.e. 10 μg/L, the crystallization still occurred but crystals were smaller than in control water. Moreover, the analysis by SEM revealed a change of morphology of crystals in presence of zinc and a change of crystalline varieties. Tests of controlled rapid precipitation, carried out with Zn at different concentrations revealed a high effect of Zn on the crystallization of calcium carbonate; a very important effect was reached for a concentration of 0.5 mg/L. Moreover, for a concentration of 100 µg/L of Zn, the effectiveness reached 50%. So, the results obtained with these experimental procedures are in agreement. The mechanisms by which Zn acts may be of two kinds: adsorption on crystals of calcite and/or coprecipitation with hydroxyde and carbonate ions. References [1]
[2] [3] [4]
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