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The synergistic effect of orthophosphate and polymer on the precipitation of calcium carbonate
Desalination 255 (2010) 143–147
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Desalination j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / d e s a l
The synergistic effect of orthophosphate and polymer on the precipitation of calcium carbonate Xiaoshuang Yin ⁎, Wenzhong Yang, Yongming Tang, Ying Liu, Jintang Wang College of Science, Nanjing University of Technology, 5 XinMoFan Road, Nanjing, 210009, China
a r t i c l e
i n f o
Article history: Received 9 July 2009 Received in revised form 28 December 2009 Accepted 7 January 2010 Available online 2 February 2010 Keywords: Calcium carbonate Orthophosphate Polymer Synergistic effect Inhibition
a b s t r a c t The inhibition properties of polymer acrylic acid/2-acrylamido-2-methylpropanesulfonic acid/2-hydroxypropyl acrylate (AA/AMPS/HPA) and orthophosphate on calcium carbonate precipitation were studied by measuring the induction period and inhibition efficiency. Employing the classical homogeneous nucleation theory, the interfacial tension between calcium carbonate and supersaturated solutions was estimated in the absence and presence of orthophosphate and AA/AMPS/HPA. It was observed that the induction period increased with increasing orthophosphate concentrations up to 5 × 10−5 M and then decreased slightly. The length of induction period increased with increasing AA/AMPS/HPA concentrations up to 20 mg L−1. Good synergistic effects between orthophosphate and AA/AMPS/HPA polymer were obtained. It is therefore possible to assume that orthophosphate and AA/AMPS/HPA are adsorbed on the calcium carbonate crystal and thus yield an obviously elevation of interfacial tension. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the water treatment industry, the deposition of calcium carbonate is still one of the most persistent problems. Attention has been focused on the use of water-soluble additives preventing or drastically retarding the formation of calcium carbonate. Such additives can have a marked effect on the nucleation and growth rate [1,2]. Among others, phosphonate-based compounds and polymeric inhibitors are important classes of chemical additives. In the phosphonate-based water treatment program, phosphonates undergo some reversion so that there will be some orthophosphate. It's found that the presence of a very small amount of orthophosphate sometimes influences the morphology of final crystal in CaCO3 crystallization process [3–5] and/or retards the growth rate by adsorption [6–8]. It's believed that the adsorption reaction is a chemisorption process [9]. Through adsorption the calcium carbonate crystal growth active sites are blocked [3,6]. Electrostatic interactions may be important in the adsorption of phosphate on the calcium carbonate surface [6,10]. Acrylic acid (AA)-based copolymers as crystal growth inhibitors have been used extensively for several decades [11–16]. It has been also reported that substituting the carboxyl groups with bulky groups, for example, 2-acrylamido-2-methylpropanesulfonic acid(AMPS),
⁎ Corresponding author. Tel.: +86 25 83587442; fax: +86 25 83587443. E-mail address: [email protected] (X. Yin). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.01.001
results in increased inhibitory power of the copolymer in preventing the precipitation of calcium phosphate [17,18]. The efficiency of AA/ AMPS-based copolymers may be affected by the operating conditions, i.e., temperature, pH, coexisting additives and relative supersaturation [18,19]. Synergistic effects can occur when chemical additives are used in admixture. Such effects can enhance the performance of each additive. Synergistic effects between phosphonate and polyphosphate have been studied previously [20–22]. But only very limited data is available concerning the synergistic effects between inorganic anion and polymeric inhibitor on the crystallization of calcium carbonate. In the investigation reported here, the effects of orthophosphate and polymeric inhibitors, acrylic acid/2-acrylamido-2-methylpropanesulfonic acid/2-hydroxypropyl acrylate (AA/AMPS/HPA), were investigated on the nucleation of calcium carbonate. The synergistic mechanism was also discussed. 2. Experimental section 2.1. Materials Calcium chloride (CaCl2, Beijing Chemical Importing Corporation, >99.5 ), sodium carbonate (Na2CO3, Tianjin Chemical Importing Corporation, >99.9 ), sodium bicarbonate (NaHCO3, Tianjin Chemical Importing Corporation, >99.9 ) and sodium orthophosphate (Na3PO4, Beijing Chemical Importing Corporation, >99.5 ) were of analytical grade. The polymer (AA/AMPS/HPA, 30 wt.%, MW 2000) was obtained from Rohm & Hass Corp.
144
X. Yin et al. / Desalination 255 (2010) 143–147
2.2. Procedure and measurement 2.2.1. Inhibition test [19] The solutions containing 4.7 mM Ca2+ and 9.4 mM HCO3− were prepared in different glass conical flasks. Different dosage levels of AA/ AMPS/HPA and orthophosphate were added. The pHs were buffered to 9.0 ± 0.1 by borax. The solutions were placed in a temperaturecontrolled water bath to maintain constant temperature at 80 °C for 10 h. After being cooled to room temperature, the solutions were filtered through member filter. The filtered solutions were diluted and analyzed for calcium concentration by EDTA titration [19]. The scale inhibition efficiency can be defined as [19] h
i h i Ca2+ − Ca2+ 1 0 × 100 ηð%Þ = 4:7− Ca2+ 0
ð1Þ
where 4.7 is the initial molar calcium concentration, [Ca2+]0 and [Ca2+]1 are the final molar calcium concentration in the absence and presence of additives after 10 h at 80 °C, respectively. 2.2.2. Determination of induction periods For the purpose of further investigating the possible inhibition mechanism of orthophosphate and AA/AMPS/HPA on the calcium carbonate precipitation, other experiments were carried out as described in detail elsewhere[23]. 200-mL scale formation solution was used in each experiment. CaCO3 precipitated spontaneously by mixing two solutions as rapidly as possible in order to avoid local supersaturation (100 mL brine 1 containing calcium ions and 100 mL brine 2 containing carbonate ions, orthophosphate and/or AA/AMPS/ HPA). The solution was stirred at 200 rpm by a Teflon-coated magnetic stirring bar in a vessel thermostat at 20 °C. The reaction was monitored by withdrawing aliquots of the suspension solution which was quickly filtered through a membrane filter. The calcium concentration of filter solution was then analyzed as described in chemical static test before. The induction period, tind, is defined as the time elapsed between the creation of supersaturation and first appearance of a new phase. The induction time may be determined by monitoring the variations in the solution turbidity, conductivity or concentration of one of the crystal ions [23–25]. For this research it was determined by monitoring the variations in the calcium concentrations. The time period elapsed from the mixing of the solution up to the first change in calcium concentration was defined as the induction period. The induction period was measured five times for each run. A typical example of this method was shown in Fig. 1. The maximum of standard deviation of the induction period is 8 s. 2.2.3. Calculation of the saturation index The degree of mineral saturation can be quantified via the saturation ratio (SR), which is the ratio of the ionic activity products divided by the thermodynamic solubility product. The SR for CaCO3 can be expressed as [23,25] SR =
αCa2+ αCO2− 3
KspðCaCO3 Þ
:
ð2Þ
The impacts of orthophosphate and AA/AMPS/HPA in the solution on the activity of Ca2+ were generally ignored due to their low concentrations compared to calcium ions. Table 1 represents the solution prime compositions used in the experiments and SI values of saturation solutions. 2.2.4. SEM measurement The deposits formed in the inhibition test were characterized by a scanning electron microscope (SEM, JSM-5900) after drying at 105 °C for 24 h. 3. Results and discussion 3.1. Inhibition test Table 2 represents the scale inhibition efficiency of the combination of AA/AMPS/HPA and orthophosphate. The increments of inhibition
Table 1 Initial solution concentration and saturation index at 20 °C. No.
Ca2+ (mM)
Na+ (mM)
Cl− (mM)
CO2− 3 (mM)
SI
1 2 3 4
4.70 3.31 1.57 0.78
9.40 6.62 3.14 1.56
9.40 6.62 3.14 1.56
4.70 3.31 1.57 0.78
3.19 2.72 2.44 1.94
Table 2 The synergistic effect between AA/AMPS/HPA and orthophosphate. −5 PO3− M) 4 (10
AA/AMPS/HPA(mg L−1) 0
The saturation index (SI) of the solution with respect to CaCO3 is defined as the logarithm value of the saturation ratio [23,25]. SI = log ðSRÞ = log
Fig. 1. Variations in the calcium concentration vs. time to illustrate the determination of the induction period of the calcium carbonate nucleation for a typical run at 20 °C.
cCa2+ cCO2− fCa2+ fCO2− 3
KspðCaCO3 Þ
3
! ð3Þ
where ci and fi are, respectively, molar concentration and ionic activity coefficients for species i. Ionic activity coefficients are obtained from the modification of the Debye–Hückel equation.
0 0.8
1.7
1.6
4.2
5
4.8
8
3.6
20
−2.3
2
8
12
20
4.7 8.1 (+2.1) 9.2 (+0.7) 10.4 (+2.1) 8.6 (+0.7) 8.0 (+6.1)
43.4 54.5 (+9.4) 60.8 (+ 15.8) 65.7 (+ 17.5) 59.2 (+ 14.2) 50.3 (+9.2)
45.9 63.1 (+15.4) 72.4 (+22.3) 75.6 (+24.9) 68.8 (+18.3) 65.9 (+22.3)
42.2 66.8 (+ 22.9) 80.4 (+ 34.0) 83.4 (+ 36.4) 75.2 (+ 29.4) 73.1 (+ 33.2)
X. Yin et al. / Desalination 255 (2010) 143–147
145
efficiency (Δη) are represented in the brackets. Δη is calculated from the following equation [19]: Δη = ηC −ðηA + ηP Þ
½4
where ηA, ηP and ηC are the inhibition efficiencies of AA/AMPS/HPA, PO3− and the combination of AA/AMPS/HPA and PO3− 4 4 , respectively. Δη numbers vs. orthophosphate concentrations for various concentrations of AA/AMPS/HPA are represented in Fig. 2. The maximum of standard deviation of the inhibition efficiency with certain additives is 6%. These results suggest that PO3− alone does not provide any 4 significant calcium carbonate inhibition. The calcium carbonate inhibition efficiency of AA/AMPS/HPA is greatly enhanced by orthophosphate. Good synergistic effect is obtained between AA/ AMPS/HPA and orthophosphate on inhibiting calcium carbonate precipitation, especially when AA/AMPS/HPA is in the range of 12 to 20 mg L−1 and orthophosphate is in the range of 1.6 × 10−5 M to 8 × 10−5 M.
Fig. 3. The orthophosphate concentration dependence of the induction period for calcium carbonate in various saturation states.
3.2. Calcium carbonate precipitation procedure with orthophosphate The orthophosphate concentration dependence of the induction periods with different degree of CaCO3 saturation is presented in Fig. 3. It is clear that higher values of induction period are obtained at lower SI values. The induction period increases with increasing orthophosphate concentration and reaches maximum at a certain concentration, followed by a decrease in all cases. The decreasing of induction period may be regarded as the form of phosphate precipitation. The rate of nuclei formation is an important parameter in the nucleation process. The steady-state nucleation rate ( J, cm−3 s−1) can be expressed as [25–28]: " J = Aexp −
# βσ 3 ν2m NA f ðθÞ ðRT Þ3 ð2:3SI Þ2
nucleation f(θ) = 1), T is the absolute temperature (K) and R is the gas constant (8.314 J mol−1 K−1). If the induction period is related to the time needed for the formation of a critical nucleus in the system, it can be assumed to be inversely proportional to the rate of nucleation, tind ∝J−1 and be related to the saturation index in Eq. (6) at a given temperature [23,25,27]: −2
log tind = bðSIÞ
−c
ð6Þ
where c is a constant, b is the slope of the linear log tind vs. (SI)−2 plot and a function of temperature and interfacial tension in Eq. (7)
ð5Þ b=
βσ 3 v2m NA f ðθÞ ð2:3RT Þ3
ð7Þ
where A is a frequency constant (cm−3 s−1), β is a geometric factor (about 16π/3 for a spherical nucleus), σ is interfacial tension (J m−2), νm is the molar volume (cm3 mol−1), NA is Avogadro's number (6.02 × 1023 mol−1), f(θ) is contact angle factor (in homogeneous
σ = 2:3RT
Fig. 2. Δη numbers vs. orthophosphate concentrations for various concentrations of AA/AMPS/HPA.
Fig. 4. Effect of supersaturation on induction period of CaCO3 nucleation in the presence 3− −5 of PO3− ; c.1.6 × 10−5; d.8 × 10−5; e.2 × 10−4. 4 . c(PO4 )/(M): a.0; b.0.8 × 10
Therefore the interfacial tension can be calculated from
b
1 = 3
ν2m NA βf ðθÞ
:
ð8Þ
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X. Yin et al. / Desalination 255 (2010) 143–147
Table 3 Calculated results of the interfacial tensions between calcium carbonate and supersaturation solutions based on Eq. (8). c(PO3− 4 )/ (10−5 M) 0 0.8 1.6 8 20
Value Standard Value Standard Value Standard Value Standard Value Standard
deviation deviation deviation deviation deviation
Intercept(c)
Slope(b)
R2
σ/(mJ m−2)
−1.57 0.05 −1.60 0.07 −1.61 0.05 −1.54 0.01 −1.50 0.03
12.7 0.2 15.4 0.1 15.8 0.1 14.9 0.2 11.6 0.1
0.969
52.7
0.990
56.2
0.996
56.7
0.978
55.6
0.995
51.1
A linear plot of log tind vs. (SI)−2 is presented in Fig. 4 for calcium carbonate nucleation. The correlation coefficients of the plotted straight lines and the interfacial tension calculated are listed in Table 3 using Eq. (8) assuming a spherical nucleus. The interfacial tension between the crystal and supersaturated aqueous solution is a fundamental parameter in understanding and modeling the rate of both nucleation and crystal growth. These calculations indicate that the addition of orthophosphate results in an increase of the interfacial tension and maximum value is obtained at PO3− 4 concentration of 1.6×10−5 M. A higher value of interfacial tension means a lower value of nucleation rate as expected from the classical nucleation equation (Eq. (5)). However, a further increase in orthophosphate concentration causes a decrease of the interfacial tension. 3.3. Calcium carbonate precipitation procedure with AA/AMPS/HPA The nucleation of CaCO3 can be effectively inhibited by AA/AMPS/ HPA at temperature 20 °C through extending the length of induction period. Prolonged induction period as a function of AA/AMPS/HPA concentration is graphically shown in Fig. 5. It is found that the length of induction period increases with increasing AA/AMPS/HPA concentrations. 3.4. Calcium carbonate precipitation procedure with the combination of AA/AMPS/HPA and orthophosphate
Fig. 5. AA/AMPS/HPA concentration dependence of the induction period for calcium carbonate in various saturation states.
lattice increases the crystal interfacial tension and thus hinders further precipitation. The observation of SEM further confirms that orthophosphate and AA/AMPS/HPA have great influence on the crystallization of CaCO3. 4. Conclusion The experimental data of chemical static test and induction periods for nucleation of calcium carbonate can be interpreted by the classical homogeneous nucleation theory. The interfacial tension between calcium carbonate and supersaturated solution was estimated. Calcium carbonate crystal was affected by orthophosphate and AA/AMPS/HPA. Good synergistic effect can be attained by orthophosphate together with AA/AMPS/HPA on inhibiting calcium carbonate precipitation. It is believed to be due to the following mechanism: the adsorbed orthophosphate and AA/AMPS/HPA poison CaCO3 crystal growth on active sites. The incorporation of orthophosphate and AA/AMPS/HPA into the lattice increases CaCO3 crystal interfacial tension.
The interfacial tension for calcium carbonate in the presence of the combination of AA/AMPS/HPA and orthophosphate at a given concentration can be estimated by the same method described before. The estimated interfacial tension as a function of orthophosphate concentration in the presence and absence of AA/AMPS/HPA is graphically shown in Fig. 6. It is clear that in the case of orthophosphate and AA/AMPS/HPA combination, the interfacial tension increases sharply. The increment of interfacial tension is greater than that in the case of AA/AMPS/HPA or orthophosphate separately. This strongly verifies the results about good synergistic effect between orthophosphate and AA/AMPS/HPA in the chemical static test before. 3.5. SEM analysis Fig. 7 shows the SEM images of CaCO3 particles obtained in the absence and presence of orthophosphate and AA/AMPS/HPA. The CaCO3 crystals precipitated from pure supersaturated calcium carbonate solution exhibit mainly rhomboidal crystals. In the presence of orthophosphate and AA/AMPS/HPA, the shape of CaCO3 change strongly. It is therefore possible to assume that the phosphate ions adsorb on the crystal surface, blocking the growth of active sites. That the incorporation of orthophosphate or AA/AMPS/HPA into the
Fig. 6. Interfacial tension as a function of orthophosphate concentration in the presence and absence of AA/AMPS/HPA.
X. Yin et al. / Desalination 255 (2010) 143–147
147
Fig. 7. SEM images of CaCO3 particles obtained in the absence and presence of orthophosphate and AA/AMPS/HPA. (a) and (d) without any additive; (b) and (e) with 5×10−5 M PO3− 4 ; (c) and −1 (f) with 5×10−5 M PO3− AA/AMPS/HPA. 4 and 12 mg L
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[16] P. Shakkthivel, T. Vasudevan, Desalination 197 (2006) 179–189. [17] Z. Amjad, J. Pugh, J. Zibrida, R. Zuhl, Mater. Performance 36 (1997) 32–38. [18] Z. Amjad, Water Soluble Polymers: Solution Properties and Applications, Plenum Press, New York, 1998. [19] C.Q. Lu, Y.L. Han, Y.J. Ma, Chinese J. Appl. Chem. 13 (1996) 83–85. [20] L.S. Spinelli, A.S. Aquino, R.V. Pires, E.M. Barboza, A.M.T. Louvisse, E.F. Lucas, J. Pet. Sci. Eng. 58 (2007) 111–118. [21] X. Kang, Y. Zhao, Technology of Water Treatment 31 (2005) 24–26. [22] R. Wang, Z.Q. Shen, J. Ding, Petrochem. Technol. 28 (1999) 615–617. [23] S. He, J.E. Oddo, M.B. Tomson, J. Colloid Interface Sci. 174 (1995) 327–335. [24] S.F.E. Boerlage, M.D. Kennedy, I. Bremere, G.J. Witkamp, J.P. Van der Hoek, J.C. Schippers, J. Membr. Sci. 179 (2000) 53–68. [25] S. He, J.E. Oddo, M.B. Tomson, J. Colloid Interface Sci. 162 (1994) 297–303. [26] J. Gómez-Morales, J. Torrent-Burgués, R. Rodríguez-Clemente, J. Cryst Growth 169 (1996) 331–338. [27] O. Söhnel, J.W. Mullin, J. Colloid and Interface Sci. 123 (1988) 43–50. [28] A.E. Nielsen, Kinetics of Precipitation, Pergamon Press, New York, 1964.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / d e s a l
The synergistic effect of orthophosphate and polymer on the precipitation of calcium carbonate Xiaoshuang Yin ⁎, Wenzhong Yang, Yongming Tang, Ying Liu, Jintang Wang College of Science, Nanjing University of Technology, 5 XinMoFan Road, Nanjing, 210009, China
a r t i c l e
i n f o
Article history: Received 9 July 2009 Received in revised form 28 December 2009 Accepted 7 January 2010 Available online 2 February 2010 Keywords: Calcium carbonate Orthophosphate Polymer Synergistic effect Inhibition
a b s t r a c t The inhibition properties of polymer acrylic acid/2-acrylamido-2-methylpropanesulfonic acid/2-hydroxypropyl acrylate (AA/AMPS/HPA) and orthophosphate on calcium carbonate precipitation were studied by measuring the induction period and inhibition efficiency. Employing the classical homogeneous nucleation theory, the interfacial tension between calcium carbonate and supersaturated solutions was estimated in the absence and presence of orthophosphate and AA/AMPS/HPA. It was observed that the induction period increased with increasing orthophosphate concentrations up to 5 × 10−5 M and then decreased slightly. The length of induction period increased with increasing AA/AMPS/HPA concentrations up to 20 mg L−1. Good synergistic effects between orthophosphate and AA/AMPS/HPA polymer were obtained. It is therefore possible to assume that orthophosphate and AA/AMPS/HPA are adsorbed on the calcium carbonate crystal and thus yield an obviously elevation of interfacial tension. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the water treatment industry, the deposition of calcium carbonate is still one of the most persistent problems. Attention has been focused on the use of water-soluble additives preventing or drastically retarding the formation of calcium carbonate. Such additives can have a marked effect on the nucleation and growth rate [1,2]. Among others, phosphonate-based compounds and polymeric inhibitors are important classes of chemical additives. In the phosphonate-based water treatment program, phosphonates undergo some reversion so that there will be some orthophosphate. It's found that the presence of a very small amount of orthophosphate sometimes influences the morphology of final crystal in CaCO3 crystallization process [3–5] and/or retards the growth rate by adsorption [6–8]. It's believed that the adsorption reaction is a chemisorption process [9]. Through adsorption the calcium carbonate crystal growth active sites are blocked [3,6]. Electrostatic interactions may be important in the adsorption of phosphate on the calcium carbonate surface [6,10]. Acrylic acid (AA)-based copolymers as crystal growth inhibitors have been used extensively for several decades [11–16]. It has been also reported that substituting the carboxyl groups with bulky groups, for example, 2-acrylamido-2-methylpropanesulfonic acid(AMPS),
⁎ Corresponding author. Tel.: +86 25 83587442; fax: +86 25 83587443. E-mail address: [email protected] (X. Yin). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.01.001
results in increased inhibitory power of the copolymer in preventing the precipitation of calcium phosphate [17,18]. The efficiency of AA/ AMPS-based copolymers may be affected by the operating conditions, i.e., temperature, pH, coexisting additives and relative supersaturation [18,19]. Synergistic effects can occur when chemical additives are used in admixture. Such effects can enhance the performance of each additive. Synergistic effects between phosphonate and polyphosphate have been studied previously [20–22]. But only very limited data is available concerning the synergistic effects between inorganic anion and polymeric inhibitor on the crystallization of calcium carbonate. In the investigation reported here, the effects of orthophosphate and polymeric inhibitors, acrylic acid/2-acrylamido-2-methylpropanesulfonic acid/2-hydroxypropyl acrylate (AA/AMPS/HPA), were investigated on the nucleation of calcium carbonate. The synergistic mechanism was also discussed. 2. Experimental section 2.1. Materials Calcium chloride (CaCl2, Beijing Chemical Importing Corporation, >99.5 ), sodium carbonate (Na2CO3, Tianjin Chemical Importing Corporation, >99.9 ), sodium bicarbonate (NaHCO3, Tianjin Chemical Importing Corporation, >99.9 ) and sodium orthophosphate (Na3PO4, Beijing Chemical Importing Corporation, >99.5 ) were of analytical grade. The polymer (AA/AMPS/HPA, 30 wt.%, MW 2000) was obtained from Rohm & Hass Corp.
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X. Yin et al. / Desalination 255 (2010) 143–147
2.2. Procedure and measurement 2.2.1. Inhibition test [19] The solutions containing 4.7 mM Ca2+ and 9.4 mM HCO3− were prepared in different glass conical flasks. Different dosage levels of AA/ AMPS/HPA and orthophosphate were added. The pHs were buffered to 9.0 ± 0.1 by borax. The solutions were placed in a temperaturecontrolled water bath to maintain constant temperature at 80 °C for 10 h. After being cooled to room temperature, the solutions were filtered through member filter. The filtered solutions were diluted and analyzed for calcium concentration by EDTA titration [19]. The scale inhibition efficiency can be defined as [19] h
i h i Ca2+ − Ca2+ 1 0 × 100 ηð%Þ = 4:7− Ca2+ 0
ð1Þ
where 4.7 is the initial molar calcium concentration, [Ca2+]0 and [Ca2+]1 are the final molar calcium concentration in the absence and presence of additives after 10 h at 80 °C, respectively. 2.2.2. Determination of induction periods For the purpose of further investigating the possible inhibition mechanism of orthophosphate and AA/AMPS/HPA on the calcium carbonate precipitation, other experiments were carried out as described in detail elsewhere[23]. 200-mL scale formation solution was used in each experiment. CaCO3 precipitated spontaneously by mixing two solutions as rapidly as possible in order to avoid local supersaturation (100 mL brine 1 containing calcium ions and 100 mL brine 2 containing carbonate ions, orthophosphate and/or AA/AMPS/ HPA). The solution was stirred at 200 rpm by a Teflon-coated magnetic stirring bar in a vessel thermostat at 20 °C. The reaction was monitored by withdrawing aliquots of the suspension solution which was quickly filtered through a membrane filter. The calcium concentration of filter solution was then analyzed as described in chemical static test before. The induction period, tind, is defined as the time elapsed between the creation of supersaturation and first appearance of a new phase. The induction time may be determined by monitoring the variations in the solution turbidity, conductivity or concentration of one of the crystal ions [23–25]. For this research it was determined by monitoring the variations in the calcium concentrations. The time period elapsed from the mixing of the solution up to the first change in calcium concentration was defined as the induction period. The induction period was measured five times for each run. A typical example of this method was shown in Fig. 1. The maximum of standard deviation of the induction period is 8 s. 2.2.3. Calculation of the saturation index The degree of mineral saturation can be quantified via the saturation ratio (SR), which is the ratio of the ionic activity products divided by the thermodynamic solubility product. The SR for CaCO3 can be expressed as [23,25] SR =
αCa2+ αCO2− 3
KspðCaCO3 Þ
:
ð2Þ
The impacts of orthophosphate and AA/AMPS/HPA in the solution on the activity of Ca2+ were generally ignored due to their low concentrations compared to calcium ions. Table 1 represents the solution prime compositions used in the experiments and SI values of saturation solutions. 2.2.4. SEM measurement The deposits formed in the inhibition test were characterized by a scanning electron microscope (SEM, JSM-5900) after drying at 105 °C for 24 h. 3. Results and discussion 3.1. Inhibition test Table 2 represents the scale inhibition efficiency of the combination of AA/AMPS/HPA and orthophosphate. The increments of inhibition
Table 1 Initial solution concentration and saturation index at 20 °C. No.
Ca2+ (mM)
Na+ (mM)
Cl− (mM)
CO2− 3 (mM)
SI
1 2 3 4
4.70 3.31 1.57 0.78
9.40 6.62 3.14 1.56
9.40 6.62 3.14 1.56
4.70 3.31 1.57 0.78
3.19 2.72 2.44 1.94
Table 2 The synergistic effect between AA/AMPS/HPA and orthophosphate. −5 PO3− M) 4 (10
AA/AMPS/HPA(mg L−1) 0
The saturation index (SI) of the solution with respect to CaCO3 is defined as the logarithm value of the saturation ratio [23,25]. SI = log ðSRÞ = log
Fig. 1. Variations in the calcium concentration vs. time to illustrate the determination of the induction period of the calcium carbonate nucleation for a typical run at 20 °C.
cCa2+ cCO2− fCa2+ fCO2− 3
KspðCaCO3 Þ
3
! ð3Þ
where ci and fi are, respectively, molar concentration and ionic activity coefficients for species i. Ionic activity coefficients are obtained from the modification of the Debye–Hückel equation.
0 0.8
1.7
1.6
4.2
5
4.8
8
3.6
20
−2.3
2
8
12
20
4.7 8.1 (+2.1) 9.2 (+0.7) 10.4 (+2.1) 8.6 (+0.7) 8.0 (+6.1)
43.4 54.5 (+9.4) 60.8 (+ 15.8) 65.7 (+ 17.5) 59.2 (+ 14.2) 50.3 (+9.2)
45.9 63.1 (+15.4) 72.4 (+22.3) 75.6 (+24.9) 68.8 (+18.3) 65.9 (+22.3)
42.2 66.8 (+ 22.9) 80.4 (+ 34.0) 83.4 (+ 36.4) 75.2 (+ 29.4) 73.1 (+ 33.2)
X. Yin et al. / Desalination 255 (2010) 143–147
145
efficiency (Δη) are represented in the brackets. Δη is calculated from the following equation [19]: Δη = ηC −ðηA + ηP Þ
½4
where ηA, ηP and ηC are the inhibition efficiencies of AA/AMPS/HPA, PO3− and the combination of AA/AMPS/HPA and PO3− 4 4 , respectively. Δη numbers vs. orthophosphate concentrations for various concentrations of AA/AMPS/HPA are represented in Fig. 2. The maximum of standard deviation of the inhibition efficiency with certain additives is 6%. These results suggest that PO3− alone does not provide any 4 significant calcium carbonate inhibition. The calcium carbonate inhibition efficiency of AA/AMPS/HPA is greatly enhanced by orthophosphate. Good synergistic effect is obtained between AA/ AMPS/HPA and orthophosphate on inhibiting calcium carbonate precipitation, especially when AA/AMPS/HPA is in the range of 12 to 20 mg L−1 and orthophosphate is in the range of 1.6 × 10−5 M to 8 × 10−5 M.
Fig. 3. The orthophosphate concentration dependence of the induction period for calcium carbonate in various saturation states.
3.2. Calcium carbonate precipitation procedure with orthophosphate The orthophosphate concentration dependence of the induction periods with different degree of CaCO3 saturation is presented in Fig. 3. It is clear that higher values of induction period are obtained at lower SI values. The induction period increases with increasing orthophosphate concentration and reaches maximum at a certain concentration, followed by a decrease in all cases. The decreasing of induction period may be regarded as the form of phosphate precipitation. The rate of nuclei formation is an important parameter in the nucleation process. The steady-state nucleation rate ( J, cm−3 s−1) can be expressed as [25–28]: " J = Aexp −
# βσ 3 ν2m NA f ðθÞ ðRT Þ3 ð2:3SI Þ2
nucleation f(θ) = 1), T is the absolute temperature (K) and R is the gas constant (8.314 J mol−1 K−1). If the induction period is related to the time needed for the formation of a critical nucleus in the system, it can be assumed to be inversely proportional to the rate of nucleation, tind ∝J−1 and be related to the saturation index in Eq. (6) at a given temperature [23,25,27]: −2
log tind = bðSIÞ
−c
ð6Þ
where c is a constant, b is the slope of the linear log tind vs. (SI)−2 plot and a function of temperature and interfacial tension in Eq. (7)
ð5Þ b=
βσ 3 v2m NA f ðθÞ ð2:3RT Þ3
ð7Þ
where A is a frequency constant (cm−3 s−1), β is a geometric factor (about 16π/3 for a spherical nucleus), σ is interfacial tension (J m−2), νm is the molar volume (cm3 mol−1), NA is Avogadro's number (6.02 × 1023 mol−1), f(θ) is contact angle factor (in homogeneous
σ = 2:3RT
Fig. 2. Δη numbers vs. orthophosphate concentrations for various concentrations of AA/AMPS/HPA.
Fig. 4. Effect of supersaturation on induction period of CaCO3 nucleation in the presence 3− −5 of PO3− ; c.1.6 × 10−5; d.8 × 10−5; e.2 × 10−4. 4 . c(PO4 )/(M): a.0; b.0.8 × 10
Therefore the interfacial tension can be calculated from
b
1 = 3
ν2m NA βf ðθÞ
:
ð8Þ
146
X. Yin et al. / Desalination 255 (2010) 143–147
Table 3 Calculated results of the interfacial tensions between calcium carbonate and supersaturation solutions based on Eq. (8). c(PO3− 4 )/ (10−5 M) 0 0.8 1.6 8 20
Value Standard Value Standard Value Standard Value Standard Value Standard
deviation deviation deviation deviation deviation
Intercept(c)
Slope(b)
R2
σ/(mJ m−2)
−1.57 0.05 −1.60 0.07 −1.61 0.05 −1.54 0.01 −1.50 0.03
12.7 0.2 15.4 0.1 15.8 0.1 14.9 0.2 11.6 0.1
0.969
52.7
0.990
56.2
0.996
56.7
0.978
55.6
0.995
51.1
A linear plot of log tind vs. (SI)−2 is presented in Fig. 4 for calcium carbonate nucleation. The correlation coefficients of the plotted straight lines and the interfacial tension calculated are listed in Table 3 using Eq. (8) assuming a spherical nucleus. The interfacial tension between the crystal and supersaturated aqueous solution is a fundamental parameter in understanding and modeling the rate of both nucleation and crystal growth. These calculations indicate that the addition of orthophosphate results in an increase of the interfacial tension and maximum value is obtained at PO3− 4 concentration of 1.6×10−5 M. A higher value of interfacial tension means a lower value of nucleation rate as expected from the classical nucleation equation (Eq. (5)). However, a further increase in orthophosphate concentration causes a decrease of the interfacial tension. 3.3. Calcium carbonate precipitation procedure with AA/AMPS/HPA The nucleation of CaCO3 can be effectively inhibited by AA/AMPS/ HPA at temperature 20 °C through extending the length of induction period. Prolonged induction period as a function of AA/AMPS/HPA concentration is graphically shown in Fig. 5. It is found that the length of induction period increases with increasing AA/AMPS/HPA concentrations. 3.4. Calcium carbonate precipitation procedure with the combination of AA/AMPS/HPA and orthophosphate
Fig. 5. AA/AMPS/HPA concentration dependence of the induction period for calcium carbonate in various saturation states.
lattice increases the crystal interfacial tension and thus hinders further precipitation. The observation of SEM further confirms that orthophosphate and AA/AMPS/HPA have great influence on the crystallization of CaCO3. 4. Conclusion The experimental data of chemical static test and induction periods for nucleation of calcium carbonate can be interpreted by the classical homogeneous nucleation theory. The interfacial tension between calcium carbonate and supersaturated solution was estimated. Calcium carbonate crystal was affected by orthophosphate and AA/AMPS/HPA. Good synergistic effect can be attained by orthophosphate together with AA/AMPS/HPA on inhibiting calcium carbonate precipitation. It is believed to be due to the following mechanism: the adsorbed orthophosphate and AA/AMPS/HPA poison CaCO3 crystal growth on active sites. The incorporation of orthophosphate and AA/AMPS/HPA into the lattice increases CaCO3 crystal interfacial tension.
The interfacial tension for calcium carbonate in the presence of the combination of AA/AMPS/HPA and orthophosphate at a given concentration can be estimated by the same method described before. The estimated interfacial tension as a function of orthophosphate concentration in the presence and absence of AA/AMPS/HPA is graphically shown in Fig. 6. It is clear that in the case of orthophosphate and AA/AMPS/HPA combination, the interfacial tension increases sharply. The increment of interfacial tension is greater than that in the case of AA/AMPS/HPA or orthophosphate separately. This strongly verifies the results about good synergistic effect between orthophosphate and AA/AMPS/HPA in the chemical static test before. 3.5. SEM analysis Fig. 7 shows the SEM images of CaCO3 particles obtained in the absence and presence of orthophosphate and AA/AMPS/HPA. The CaCO3 crystals precipitated from pure supersaturated calcium carbonate solution exhibit mainly rhomboidal crystals. In the presence of orthophosphate and AA/AMPS/HPA, the shape of CaCO3 change strongly. It is therefore possible to assume that the phosphate ions adsorb on the crystal surface, blocking the growth of active sites. That the incorporation of orthophosphate or AA/AMPS/HPA into the
Fig. 6. Interfacial tension as a function of orthophosphate concentration in the presence and absence of AA/AMPS/HPA.
X. Yin et al. / Desalination 255 (2010) 143–147
147
Fig. 7. SEM images of CaCO3 particles obtained in the absence and presence of orthophosphate and AA/AMPS/HPA. (a) and (d) without any additive; (b) and (e) with 5×10−5 M PO3− 4 ; (c) and −1 (f) with 5×10−5 M PO3− AA/AMPS/HPA. 4 and 12 mg L
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