Journal of Colloid and Interface Science 303 (2006) 164–170 www.elsevier.com/locate/jcis

Calcium sulfate precipitation in the presence of water-soluble polymers Maria G. Lioliou, Christakis A. Paraskeva, Petros G. Koutsoukos ∗ , Alkiviades C. Payatakes Institute of Chemical Engineering and High-Temperature Chemical Processes—Foundation for Research and Technology, P.O. Box 1414, GR-26500, Patras, Greece Department of Chemical Engineering, University of Patras, GR-26500, Patras, Greece Received 21 June 2006; accepted 22 July 2006 Available online 26 July 2006

Abstract The effect of four different polymers on the precipitation of calcium sulfate was investigated in the present work. The degree of inhibition was estimated from measurements of the calcium ion activity and from specific solution conductivity measurements in the supersaturated solutions during the course of the precipitation process. The effects of polyacrylic acid (PAA, three different polymers with average molecular weight 2000, 50,000, and 240,000, respectively) and of a co-polymer of PAA with polystyrene sulfonic acid (PSA, average molecular weight PAA2 > PSA > PAA3. The presence of the polymers in the supersaturated solutions resulted in modification of the precipitated gypsum crystals morphology. © 2006 Elsevier Inc. All rights reserved. Keywords: Calcium sulfate dihydrate; Gypsum; Spontaneous precipitation; Inhibition; Polyacrylic acid; Polystyrene sulfonate

1. Introduction The formation of tenaciously adhering calcium sulfate scale in a number of processes from water desalination to heat exchangers and processes involving heating of water is a persistent problem [1]. Although six different calcium sulfate crystal forms are known to exist [2], three different salts are usually encountered in natural formations and scale precipitates: calcium sulfate dihydrate (CaSO4 ·2H2 O, CSD), calcium sulfate hemihydrate (CaSO4 ·1/2H2 O, CSH) and anhydrous calcium sulfate * Corresponding author. Fax: +30 2610997579.

E-mail address: [email protected] (P.G. Koutsoukos). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.07.054

(CaSO4 , CSA). Both the CSA and the CSH salts may undergo further dehydration via phase transformation to the anhydrous form [3]. Despite the fact that considerable research has been going on during the past decades on the formation of calcium sulfate in aqueous media there is still large uncertainty concerning the mechanism of formation of this salt because of the largely variable conditions of the solutions in which the salt formation takes place, including temperature, pH, ionic strength and composition and the presence of foreign ions and or water soluble compounds. A large number of the published studies agree on the fact that the formation of the calcium sulfate nuclei is initiated on solid substrates. These substrates may be either metallic surfaces of heat exchangers or crystals of the same or different substrates [4–9].

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Among the most important objectives of the mechanistic investigations has been the possibility to control the formation of the various forms of calcium sulfate. The main effort to this end has been focused in the use of water soluble inhibitors which may act either as threshold inhibitors which block the development of the supercritical nuclei [10], or as retarders of the growth of the calcium sulfate crystals [11–16]. Several investigations have been carried out on the influence of trace amounts of polymeric scale inhibitors on both the precipitation and crystal habit modification of calcium sulfate forms [12,17–19]. Polymers containing carboxylic groups such as carboxymethyl cellulose (CMC), polymethacrylic acid (PMA), and polyacrylic acid (PAA) were found to be particularly effective as CaSO4 ·2H2 O growth inhibitors [12]. In the case of polymers in solution, there is general agreement that inhibition of the formation or the growth of the salt nuclei is effected by adsorption of these molecules on the active growth sites. Polymers tend to adsorb on solids from solutions due both to van der Waals [20] and/or electrostatic interactions [21,22]. The length of the polymer chains therefore as well as the functional groups present, which through ionization regulate the electrostatic charges of the polymers, are of primary importance for the investigation of the role of the respective polymeric additives in the crystal growth of calcium sulfate. In the present work, we have addressed the problem of the effect of PAA on the scale formation of gypsum (CSD) using polymers of markedly different molecular weight. Moreover, in order to compare the relative importance of the presence of the carboxylic groups, a co-polymer of PAA with sulfonated polystyrene was tested. The sulfonate groups are more strongly ionized in comparison to the carboxylic groups and are expected to promote stronger electrostatic interactions between the polymer and the surface of the calcium containing crystals which form in the supersaturated solutions. The effect of the water soluble polymeric additives was investigated in experiments in which calcium sulfate precipitation took place spontaneously from unstable supersaturated solutions, past the lapse of measurable induction time characteristic of the time frame needed for the formation of the supercritical nuclei and the subsequent initiation of the precipitation process. In order to accentuate the effect of the additives, the supersaturated solutions conditions selected for the test experiments in the absence of polymeric additives, yielded spontaneous precipitation with practically zero induction time.

The supersaturated solutions were prepared directly in the reactor by mixing equal volumes of equimolar calcium chloride and sodium sulfate solutions. The master variable used to monitor the process of CSD spontaneous precipitation was the specific conductivity of the solution. The conductivity and temperature of the reacting solution were monitored during the crystallization process by a D/A converter attached to a computer unit with the appropriate software. The progress and the extent of the crystal growth process both in the presence and in the absence of the inhibitors tested, was characterized by the decrease of the solution conductivity as a function of time. In the experiments done in the presence of polymeric inhibitors, the additives were introduced in the sulfate solution to avoid complexation [23], conformational changes [24], and even precipitation of calcium-polymer salts [25]. The polymers tested are summarized in Table 1. The homogeneity of the supersaturated solutions was ensured by magnetic stirring at ca. 250 rpm. Past the establishment of the solution supersaturation the recording of the solution conductivity was initiated. The formation of calcium sulfate crystals was detected by a decrease of the solution conductivity due to the reduction of calcium concentration. The time lapse between the preparation of the supersaturated solutions and the appearance of the inflection point in the conductivity– time profile, was defined as the induction time, τ . The point of inflection was considered as the point in which the slope of the curve changed and was determined by the intersection of the two tangents drawn: one at the initial period (flat) and one at the dropping part of the curve. During the course of the precipitation process samples were withdrawn from the reactor and were filtered through membrane filters (0.22 µm, Millipore). The filtrates were analyzed for calcium by EDTA complexometric titrations [26]. The combination of the chemical analyses of the solution composition with the solution conductivity allowed for the construction of calibration curves. The recordings of the solution conductivity as a function of time were thus converted into calcium vs time profiles. These curves showed a profile similar to that corresponding to decreasing conductivity as a function of time, which allowed for the calculation of the initial rates of calcium sulfate precipitation. The calcium– time profiles were fitted according to fourth-order polynomial and the [d[Ca2+ ]/dt]t→0 was taken as the value for the initial rate in each experiment. This correlation allowed for the quantitative measurements of the rates of crystallization. The values reported are the mean of three different measurements.

2. Experimental

3. Results and discussion

Crystal growth experiments were carried out in a 0.250dm3 double-walled Pyrex vessel thermostated at 25.0 ± 0.2 ◦ C by water circulation from a constant-temperature bath. Stock calcium chloride and sodium sulfate solutions were prepared from the respective crystalline solids (Merck, pro analisi). The solutions were filtered through membrane filters (0.22 µm, Millipore) and standardized by atomic absorption spectrometry (Perkin Elmer A Analyst 300) and ion chromatography (Dionex) for calcium and sulfate respectively.

In all experiments of the present work, the pH of the supersaturated solutions was about 5.0 and it was not adjusted. It is established that pH over a wide range (3.0–8.0) does not affect the kinetics of spontaneous precipitation of CSD [9,27,28]. Three types of PAA polymers of different molecular weights and one polysulfonic acid polymer were tested as inhibitors of the calcium sulfate precipitation. The first three polymers are characterized by the same binding capacity with respect to calcium but different conformation and adsorption properties with

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Table 1 Polymeric inhibitors tested in the present work Polymer

K752

PAA

K702

K798

Acronym Chemical type M.W.

PAA1 Poly(acrylic acid)

PAA2 Poly(acrylic acid)

PAA3 Poly(acrylic acid)

PSA Poly(acrylic acid/sulfonic acid/sulfonated styrene)

2000

50,000

240,000

300

PAA3

Concentration (ppm) Ind. time (min)

6 10

20 30

30 36

40 50

PSA

Concentration (ppm) Ind. time (min)

6 27

20 60

40 125

Note. Total calcium, Cat = 120 mM; total sulfate, St = 120 mM, 25 ◦ C.

Fig. 3. Spontaneous precipitation of CSD in the presence of 6 ppm of PAA. Total calcium, Cat = 120 mM, total sulfate, St = 120 mM, 25 ◦ C; (2) blank; (!) PAA1 (M.W. 2000); (P) PAA2 (M.W. 50,000); (1) PAA3 (M.W. 240,000).

Fig. 2. Variation of the induction times preceding the spontaneous precipitation of CSD as a function of the concentration of polymers; total calcium, Cat = 120 mM, total sulfate, St = 120 mM, 25 ◦ C. ( ) PAA1; (1) PAA2; (!) PAA3; (P) PSA.

talline substrate. It is thus possible that the low MW chains adsorb flat on the surface blocking the active sites of the critical nuclei which cannot grow further. The higher the MW the mode of adsorption may be such that the presence of loops and trains on the solid substrate [22] do allow for a larger number of unblocked active sites. The efficiency of the PAAs tested with respect to the threshold inhibition of CSD, decreased with increasing MW. The order found was PAA1 > PAA2 > PAA3 in agreement with previous reports [12,29]. In all cases, it was found that an increase in polymer concentration results in an increase of the induction time values. This means that the duration of the induction period is increased by increasing the amount of the additive in the reacting solution. Induction times for CSD spontaneous precipitation in the presence of 6 ppm of PAA increased from ∼20 to more than 110 min for MW decrease from 240,000 (PAA3) to 2000 (PAA1). The effect of the polymers on the spontaneous precipitation of CSD may be seen from the desupersaturation curves shown in Fig. 3. PSA, inhibited also the spontaneous precipitation of CSD at concentration levels exceeding 20 ppm, as may be seen in the concentration–time profile shown in Fig. 4. The precipitation was significantly suppressed in the presence of 60 and 100 ppm of the polymer. The PSA inhibitor contained hydrophobic phenyl groups attached to the polymer chain. In

Fig. 4. Spontaneous precipitation of CSD in the presence of PSA: (2) blank; (1) 6 ppm; (!) 20 ppm; () 60 ppm; (
) 100 ppm; total calcium, Cat = 120 mM, total sulfate, St = 120 mM, 25 ◦ C.

general, hydrophobic groups have been reported to be detrimental for the performance of an inhibitor. The presence of the phenyl group may, therefore, explain the relatively lower inhibition capacity of the PSA co-polymer in comparison to the PAA. It should be noted however that sulfonic acid groups are present in the PSA molecules. Sulfonic anions may replace the sulfate ions in the calcium sulfate lattice, enhancing, thus, the inhibiting capacity of the additive. In the present work however, it was found that PSA did not perform better than the PAA polymers, containing –COOH functional groups. Superior performance of PSA may have been anticipated because of the more acidic character of the sulfonic acid group compared to the carboxylic acid group. It may therefore be suggested that the presence of hydrophobic aromatic nuclei in PSA prevailed affecting the conformation of the polymeric chains at the gypsum/water interface yielding weaker interactions and therefore lower affinity of the polymer for the crystalline material. These results are in agreement with earlier reports which have attributed the weaker interactions between the crystal surfaces and the aromatic sulfonate containing polyelectrolytes to the fact that they do not favor the flat on conformation of the ad-

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(a)

(b)

(c)

(d)

(e) Fig. 5. Scanning electron micrographs of CSD crystals precipitated spontaneously; total calcium, Cat = 120 mM, total sulfate, St = 120 mM, 25 ◦ C; (a) no additive, bar = 40 µm; (b) 6 ppm PAA1, bar = 30 µm; (c) 6 ppm PAA2, bar = 30 µm; (d) 6 ppm PAA3, bar = 30 µm; (e) 6 ppm PSA, bar = 60 µm.

sorbed molecules, which is more likely for the carboxyl group containing PAAs [31]. The crystal habit of the precipitated CSD crystals was affected by presence of the polymers, as may be seen in the scanning electron photographs shown in Fig. 5. The habit of the CSD crystals precipitated in the absence of additives (Fig. 5a) was the well-known thin, elongated calcium sulfate crystals as a result of rapid growth on the [111] faces. The presence of

polyacrylic acid polymers in the reacting solutions enhanced the agglomeration of the crystals, retarded the growth of the [111] faces and as a result more plate-like crystals were obtained. In general, due to their adsorption on the crystal surface, PAAs do change the surface charge of the crystals, enhancing agglomeration. The presence of high molecular weight PAA (PAA3) resulted in even higher agglomeration as may be seen in Fig. 5d. In this case, the precipitated crystals consisted of

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Table 3 Affinity constants as calculated for the polymers tested for calcium sulfate crystal growth

Fig. 6. Plot of ratio R0 /(R0 − Ri ) as a function of the inverse of inhibitor concentration for (1) PAA1; (!) PAA2; (P) PAA3; (
) PSA.

rather deformed crystals, covered by sponge-like mass of tiny crystallites. The kinetic data obtained in the presence of the polymers were fitted to a kinetic model based on the assumption of the Langmuir isotherm [38]. Assuming that the rate of crystal growth in the absence of an additive is R0 and the limiting rate in its presence is bR0 and defining the ratio Cm /Cads as the fraction of the occupied sites, θ , the overall rate measured in the presence of an inhibitor, Ri , is given by Ri = R0 − θ R0 (1 − b).

(2)

Substituting θ and rearranging: R0 1 kdes 1 (3) = + . R0 − Ri 1 − b kads (1 − b) C The ratio kads /kdes is the affinity constant of the additive for the particular substrate and the parameter b is a measure of the effectiveness of the inhibitor present at infinite concentration in the case of monolayer coverage or at concentrations lower than the one corresponding to monolayer coverage (0 < b < 1). 0 as a function of C1 yielded satisfacPlots of the ratio R0R−R i tory linear fits for all the polymers used in the present work, as shown in Fig. 6. The intercept of the fitted lines for PAA1 and PAA2 were found to be 1 for the other two polymers (PAA3 and PSA) may be interpreted as the inability of these additives to completely inhibit crystal growth of calcium sulfate. The inverse of the slopes of the fitted lines yield the affinity constants of the polymers used for calcium sulfate crystal surfaces, which are given in Table 3. The affinities calculated for PAA1 and PAA2 were 1.96 × 106 and 1.19 × 106 , respectively, while the affinities for PAA3 and PSA were 1.33 × 105 and 2.48 × 105 . It is interesting to point out the small difference between the values for the first two PAAs in spite of the difference in their molecular weights, while PAA3 gave a value of affinity constant lower by one order of magnitude. This difference may be ascribed to the chain

Polymer

Acronym

Affinity constant

K752 PAA K702 K798

PAA1 PAA2 PAA3 PSA

1.96 × 106 1.19 × 106 1.33 × 105 2.48 × 105

length of the polymer which affected the respective surface conformation. It should also be mentioned that the experiments were conducted at pH approximately 5.0. At this pH value the higher molecular weight polymer is probably ionized to a lower extent, in comparison to the lower MW PAAs. Lower extent of ionization would in turn affect the conformation and/or the extent of adsorption of PAA on the newly formed gypsum nuclei. The presence of the aromatic ring in PSA structure apparently influenced its hydrophobic character and the conformation of the molecule at the solid–solution interface, resulting in reduced inhibition activity. Finally it should be noted that despite the fact that the present work was done at acidic pH, sufficiently high to have the polyelectrolytes in ionized form, increasing the solution pH is expected to increase the efficiency of the inhibitors tested because of the increase of the degree of ionization of the soluble polyelectrolytes [28,31,39]. Acknowledgments The authors wish to acknowledge financial support by the General Secretariat for Research and Technology, Ministry of Development, through PENED Program Contract M413/2002. References [1] Z. Amjad, J. Colloid Interface Sci. 123 (1988) 523. [2] J. Glater, J.L. York, K.S. Campbell, in: K.S. Spiegler, A.D.K. Laird (Eds.), Principles of Desalination, Part B, second ed., Academic Press, New York, 1980, pp. 627–678. [3] P.G. Klepetsanis, P.G. Koutsoukos, J. Colloid Interface Sci. 143 (2) (1991) 299. [4] D. Hasson, J. Zahavi, Ind. Eng. Chem. Fundam. 9 (1) (1970) 26. [5] O.D. Linnikov, Desalination 128 (2000) 35. [6] J.S. Gill, G.H. Nancollas, J. Cryst. Growth 48 (1980) 34. [7] G.H. Nancollas, W.P. Klima, Mater. Performance 21 (1982) 9. [8] G.H. Nancollas, W.F. Klima, Paper 81, Corrosion/81, National Association of Corrosion Engineers Conference, Toronto, Ontario, 1981. [9] S.T. Liu, G.H. Nancollas, J. Colloid Interface Sci. 44 (1973) 422. [10] S. He, J.E. Oddo, M.B. Tomson, J. Colloid Interface Sci. 162 (2) (1994) 297. [11] B.R. Smith, A.E. Alexander, J. Colloid Interface Sci. 34 (1970) 81. [12] E.R. McCartney, A.E. Alexander, J. Colloid Interface Sci. 13 (1958) 383. [13] G.H. Nancollas, W. White, F. Tsai, L. Maslow, Corrosion 35 (1979) 304. [14] M.E. Tadros, I. Mayes, J. Colloid Interface Sci. 72 (1979) 245. [15] M.P.C. Weijnen, G.M. van Rosmalen, J. Cryst. Growth 79 (1986) 157. [16] M.P.C. Weijnen, G.M. van Rosmalen, P. Bennema, J.J.M. Rijpkema, J. Cryst. Growth 82 (1987) 509. [17] R.A. Kuntz, Nature 211 (1966) 406. [18] L.W. Jones, Corrosion 17 (1961) 232. [19] Z. Amjad, Desalination 54 (1985) 263. [20] P. Somasundaran, T.W. Healy, D.W. Fuerstenau, J. Phys. Chem. 68 (1964) 3562.

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