Inhibition of Calcium Sulfate Scale Formation during Pulsation Dead

It was shown that dead-end ultrafiltration conducted in the presence of 16 mg/L inhibitor with a periodic pressure release to zero ensures an enhanced...
0 downloads 0 Views 140KB Size
Download PDF
Ind. Eng. Chem. Res. 2007, 46, 2243-2248

2243

Inhibition of Calcium Sulfate Scale Formation during Pulsation Dead-End Ultrafiltration A. Kavitskaya,* M. Skilskaya, D. Kucheruk, and V. Goncharuk Institute of Colloid and Water Chemistry, Ukrainian National Academy of Sciences, 42 Vernadsky pr., KieV-142, 03680 Ukraine

Comparative effectiveness studies were conducted using sodium carboxy-methylcellulose (Na-CMC) as an inhibitor of calcium sulfate scaling on membranes under different hydrodynamic conditions of dead-end ultrafiltration. It was shown that dead-end ultrafiltration conducted in the presence of 16 mg/L inhibitor with a periodic pressure release to zero ensures an enhanced capacity of membranes by a factor of 1.3 while the time of stable operation increases about 2-fold as compared with dead-end ultrafiltration under a constant pressure. In this case, the initial capacity of membranes can be fully recovered by washing with water. 1. Introduction In recent years, substantial progress has been achieved in the membrane technology of treatment, concentration, and separation of various aqueous systems. However, the problem of scaling on membranes1-4 remains urgent. This is primarily true for dead-end ultrafiltration (UF), as can be seen, for example, from data available.2 Hence, a rapid reduction of the capacity of membranes was observed while using dead-end ultrafiltration (XIGA, concept based on NORIT X-Flow 8 in dead-end UF membrane element) before reverse osmosis for water treatment aimed at removing suspended matter and turbidity. To achieve the required capacity of the installation, reverse flushing of membranes was performed periodically after every 15 min of their operation, and after every 6 h of operation chemical reagents were added in the process of flushing. Methods contributing to reduction/prevention of scale formation on membranes are generally based on the processes of extraction, for example, suspended matter and changes of colloid-chemical properties of compounds capable of scale formation in such a way that these compounds cease to pose any danger as scaling admixtures. During the membrane treatment of waters containing hard salts, especially calcium sulfate, CaSO4‚2H2O scale can be formed on membranes. The mechanism of scale formation on membranes has been the subject of studies for a long time and a great variety of studies have been devoted to this issue.5-13 At present, the slowing down or complete prevention of scale formation in the technology of membrane separation of waters containing hardly soluble compounds, especially calcium sulfate, is ensured by introducing inhibitors of scale formation or special additives blocking the active nuclei germs of crystals or protecting the surface of membranes from pollution.14-18 Our earlier studies17,18 showed that the process of dead-end UF of highly concentrated solutions of calcium sulfate in the absence of crystalline scale on the surface of membranes can be implemented by using acid polyelectrolyte (PE)-sodium carboxy-methylcellulose (Na-CMC) as an inhibitor of scale formation. Addition of 16 mg/L Na-CMC makes it possible to conduct dead-end UF with CaSO4 concentrations in the filtered solution as high as 7.75 g/L. In this case, CaSO4‚2H2O scale is not present on membranes, but under these conditions * To whom correspondence should be addressed. Tel.: + 38 (044) 424-3575. Fax: + 38 (044) 423-8224. E-mail: [email protected].

of dead-end ultrafiltration, one can observe a gradual reduction of the capacity of membranes, which reaches 28% after 3 h of dead-end UF. Flushing with water enables us to recover the initial capacity of membranes to the level of 92%, that is, 8% of the working capacity of membranes is irretrievably lost. The process characteristics can be significantly improved by using membranes premodified with Na-CMC.18 However, this requires an additional stage of membrane modification making the dead-end UF technology more complex. The purpose of the present study is to carry out comparative estimation of the capacity of UAM-100 membrane and the quality of separation products under different hydrodynamic conditions of dead-end UF without and with addition of NaCMC. 2. Experimental Section 2.1. Membranes: Laboratory Setup. Membranes UAM100 were used as an object of studies. The choice of this type of membrane is related to the availability of sufficiently full information regarding the chemical nature of its surface and porous structure. This enables us to interpret experimental data in interrelation with properties of the membrane. We used ultrafiltration membranes on the basis of cellulose acetate UAM100 (produced by Polimersintez, Russia). Specific capacity of UAM-100 in terms of distilled water at temperature 20 ( 2 °C and pressure 0.15 MPa is within the limits 6-16.2 L/m2 h in accordance with specifications of the manufacturer. The membranes used in these studies are porous films on the basis of cellulose acetate with asymmetric porous structure. UAM-100 are hydrophilic membranes with the surface featuring a small negative charge. The last feature is determined by the presence of carboxyl groups in the polymer composition. According to data,19 the porous structure of membranes has the following characteristics: minimum radius of pores, 2.1 nm; maximum, 3.7 nm; number of pores, 1.84 × 1010/cm2; area of pores, 0.49%. To eliminate the effect of structure variation under the impact of pressure, before each experiment on solutions of calcium sulfate, we performed “shrinkage” of membranes under the working pressure P ) 400 kPa. Shrinkage was conducted on distilled water to obtain the time-constant capacity. The membranes used in experiments after shrinkage had the capacity within the limits G0 ) 21.0-25.0 L/m2‚h. Two samples of UAM-100 were characterized by an enhanced capacity: 26.9 and 33.2 L/m2‚h.

10.1021/ie0609315 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/14/2007

2244

Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007

The experiments were conducted in a laboratory dead-end UF cell FM-02-1000. The working volume of the cell was 1 dm3. The area of the membrane was 9.5 × 10-3 m2. Pressure in the cell was created by using a cylinder of compressed nitrogen. Solution in the cell was mixed by magnetic agitator. The mixing intensity was constant in all experiments and amounted to 350 rpm. The use of dead-end UF cell and the customary method of investigations enabled us to estimate the efficiency of Na-CMC action under conditions when the concentration of solution subjected to separation in the process of UF varies (depending on the UF conditions, it either rises or drops). 2.2. Solutions of Calcium Sulfate. The experiments were conducted on calcium sulfate solutions. Solutions were obtained by mixing equivalent amounts of salts Na2SO4 and CaCl2 of chemically pure grade. A Na2SO4 salt weight was dissolved in distilled water in a measuring flask having volume of 0.5 L. The 0.5 L solution of CaCl2 was separately prepared in the same way. Next, they were mixed and the concentration of calcium ions was immediately determined. In all the experiments, concentration of calcium ions in the solution was 1.12 and 1.24 g/L. The supersaturation degree (n) of the given solutions amounted to 1.06 and 1.16. Calculation of n was conducted with due regard for the variation of calcium sulfate solubility caused by the presence of a background (sodium chloride). The temperature of solutions during experiments was constant and was equal to 21 ( 1 °C and pH 6.5. 2.3. Inhibitor. For inhibitor of calcium sulfate scaling, we applied a commercial product Na-CMC of 75/400 type. The Na-CMC additive was introduced into the feed solutions as a 0.2% solution. Na-CMC is a strong acid polyelectrolyte (PE). Acidic properties of Na-CMC are determined by COOH groups contained in macromolecules of polyelectrolyte. PE has both stabilizing and flocculating properties. It belongs to flocculants of anionic type. The process of flocculation is characterized by fast aggregation of particles when small dosages of PE are added. This process is actually finished at the time the solution is mixed with polymer. The mechanisms of processes of flocculation and stabilization under the impact of anionic polyelectrolytes are identical and can be explained by adsorption of macromolecules on the surface of solid particles.20 In this case, the concentration of polyelectrolyte plays an important part. Given the shortage of PE, the latter does not stabilize the system because it is unable to bind all the particles or the majority of them. In our case, the system stabilization can be ensured by such PE concentration that would result in formation of a network of associated molecules of the polymer in the water. This network should prevent particles from drawing together and aggregating and, consequently, it should stabilize the system. Taking into account the above statement and also the results of earlier studies,17,18 the concentration of Na-CMC in this study amounted to 4 mg/L and 16 mg/L. In experiments with addition of Na-CMC, the feeding of polymer was performed at the time of mixing the salt solutions of Na2SO4 and CaCl2. 2.4. Method of Conducting Dead-End Ultrafiltration under the Constant Pressure (P ) Constant). Solutions of calcium sulfate had been prepared directly before conducting dead-end ultrafiltration after the shrinkage of membranes. Capacity of membranes and concentration of calcium ions were determined periodically every 0.13-0.17 h, while the retention coefficient of calcium ions was determined at the beginning and after completion of dead-end ultrafiltration. The analysis of calcium ions was conducted by the method of emission flame

photometry. The concurrent control was performed by the method of chelatometry. The relative error of determination by both methods did not exceed 2% with confidence probability of 95%. The relative variation of the capacity of membranes in time was estimated by the index ∆G ) [(Gp - Ghr)/Gp] × 100%, where Gp is capacity on the solution of calcium sulfate after 0.13 h and Ghr is capacity during dead-end UF. Once the experiment was finished, the cell was disassembled and visual inspection of the membrane surface was conducted. Next, filtering of distilled water was conducted under the working mode of operation, that is, at P ) 400 kPa until the time-constant capacity G′0 was achieved. Comparison of G′0 with G0 made it possible to conclude that the recovery of the capacity of membranes is possible after their operation on calcium sulfate solutions. 2.5. Method of Conducting Dead-End Ultrafiltration in the “Pulsation” Mode (P * Constant). The difference between the technique used at P * constant and the technique presented in section 2.4 is as follows. After each hour of dead-end UF in the “pulsation” mode, there was a sharp drop of pressure to zero. Before the release of pressure, the filtrate was sampled for analysis. After the release of pressure, a sample was taken from the solution in the cell. Duration of the operation was 2 min. Next, the pressure was raised to 400 kPa and dead-end UF was continued. Concentration of calcium ions was determined in these samples. It enabled us to calculate the retaining ability of membranes before each pressure drop. 3. Results and Discussion 3.1. Dead-End UF of Calcium Sulfate Solutions without Addition of the Inhibitor at P ) Constant and P * Constant. Figure 1a-c displays the kinetics of dead-end UF under constant pressure on membranes having the initial capacity G0. The table in the Supporting Information (SI), item 1 (I-IV), shows the relative variation of the capacity of membranes ∆G during deadend UF, the retention coefficient of calcium ions R (initial Rin and the appropriate value during the dead-end UF), concentration of calcium ions Cc in the solution subjected to separation after the completion of dead-end UF, and the capacity of membranes after their flushing with water G′0. A common feature for deadend UF of calcium sulfate solutions with initial degree of supersaturation 1.06 is the presence of crystalline CaSO4‚2H2O scale on membranes. Kinetics of the process (Figure 1) and the data from the SI table, item 1 (I-IV), indicate a significant influence of the initial capacity of membranes. As can be seen from Figure 1a, the pattern of variation of process indicators is identical for membranes having close values of the initial capacity: gradual reduction of the capacity and rise of the concentration of calcium ions in the filtrate. Relative reduction of capacity ∆G for these membranes lies within the limits 9.4%-14.3% and 9.6%-16.8%, while the retaining ability declines from 41.1% to 26.6% and from 39.2% to 21.3%. In this case, the concentration of calcium ions in the volume of solution subjected to separation increases from 1.12 to 1.58 and 1.50 g/L after 3.5 h of dead-end UF (see SI table, item 1 (III, IV)). Visual inspection of membranes after completion of the process showed that the surface of both membranes was covered with a scale consisting of relatively big crystals. According to data,21 the scale consisting of big crystals can be formed if a small number of germs of nuclei emerge in a unit volume of the solution. This scale can be easily removed from the surface of membranes by washing with water. It is the evidence of weak adhesion of the scale with the surface of hydrophilic membranes UAM-100. However, as can be seen from data in the SI table,

Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007 2245

Figure 2. Time variation of the capacity of membranes (curve 1) and concentration of calcium ions (curve 2) in the filtrate without inhibitor at P * constant: 1, 24.6 L/m2‚h; 2, 1.12 g/L.

Figure 1. Time variation of the capacity of membranes (curves labeled 1 or 1′) and concentration of calcium ions (curves labeled 2 or 2′) in the filtrate without inhibitor at P ) constant: (a) 1, 23.1 L/m2‚h; 2, 1.12 g/L; 1′, 24.7 L/m2‚h; 2′, 1.12 g/L; (b) 1, 26.9 L/m2‚h; 2, 1.12 g/L; (c) 1, 33.2 L/m2‚h; 2, 1.12 g/L.

items I, III, IV, the initial capacity is not recovered. This is evidently determined by partial blocking of pores with microcrystals and, possibly, with crystallization in the porous space of membranes. In case of using samples of the membrane with a higher initial capacity (Figure 1b, c), kinetics of the process is different. This is mainly determined by the impact of concentration polarization (CP). Under the invariable hydrodynamic conditions of separation, the impact of CP increases with the growing initial capacity

of membranes. Hence, CP plays a decisive role in high-capacity UAM-100 membranes (see Figure 1c). As can be seen from Figure 1b, the pattern of the process for the given membrane is similar to the above case during about 2 h of dead-end UF (Figure 1a). Later on, the capacity actually remains unchanged. However, we can observe a reduction of the concentration of calcium ions in the filtrate. After completion of dead-end UF (3.5 h), the concentration of calcium ions in the volume of solution subjected to separation is practically the same as that in the filtrate (SI table, item I, III), 0.88 g/L, that is, the concentration of solution subjected to separation is reduced from 1.12 g/L to 0.88 g/L. The retaining ability of the membrane drops to zero. Visual inspection of the membrane revealed the presence on the membrane surface of a scale with well-perceptible small crystals. This scale uniformly covers the entire surface and can be easily washed away with water. In this case, the initial capacity of membranes in fact is fully recovered after washing with water (SI table, item I, III). For the membrane having G0 ) 33.2 L/m2‚h (Figure 1c), the process of dead-end UF proceeds against the background of intensive scale formation. Sharp drop of the retention coefficient to zero is observed just after 2 h (SI table, item I, IV). Since the relative reduction of the capacity of the membrane with the progress of UF declines from 13.5% to 10.5%, one can suggest that the formed scale has a rather loose structure. This scale is broken during its washing away from the surface of membrane. As in the previous case, the initial capacity of the membrane is recovered after the washing. Almost full recovery of the initial capacity makes it possible to suggest that the scale is formed only on the surface of membranes and that it has a weak adhesion with the surface because of the hydrophilic nature of the UAM-100 surface. From the results obtained, it follows that the higher the initial capacity of membranes, the lower Rin, the more vigorous salt crystallization in the volume of solution undergoing separation, and the faster scale formation. All these observations indicate the decisive role played by the concentration polarization in the process of dead-end UF. An enhanced capacity of membranes after release of the working pressure is intrinsic to kinetics of filtration in the pulsation mode (Figure 2). The subsequent gradual reduction of capacity after 3 h amounts to only 10%. In this case, R ) 31.5% and Cc ) 1.43 g/L (see Table, item 2). However, as in the case of P ) constant, scale on the membrane is present, but in much smaller amounts. It features fine crystalline structure and irregular distribution across the surface. According to the

2246

Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007

Figure 3. Time variation of the capacity of membranes (curve 1) and concentration of calcium ions (curve 2) in the filtrate with addition of 4 mg/L Na-CMC at P ) constant: 1, 21.3 L/m2‚h; 2, 1.12 g/L.

results of washing membranes with water, this scale can be easily removed from the surface. Big losses of the initial capacity (7.7% as compared with 4.5% at P ) constant) are probably caused by partial blocking of pores with the scale that takes place as the pressure rises. Nevertheless, comparison of results at P ) constant (after 3 h ∆G is up to 12.3%, after 3.5 h, ∆G increases almost up to 17% while the retention coefficient of calcium ions is reduced almost 2-fold) with the results obtained at P * constant indicates that even without inhibitor the pulsation filtration, where periodic removal of the scale from the surface of membranes takes place, improves the process characteristics. The obtained results indicate a reduced effect of the concentration polarization in the pulsation mode of deadend UF. 3.2. Dead-End Ultrafiltration of Calcium Sulfate Solutions in the Presence of 4 mg/L Inhibitor at P ) Constant and P * Constant. Figure 3 illustrates the kinetics of dead-end UF in the presence of 4 mg/L Na-CMC at P ) constant. The SI table, item 3, presents the results characterizing the process under the given conditions. Comparing the obtained data with the data from the table, items I and III, one can see that the presence of PE visibly improves the process. Retaining ability of membranes increases up to 46.8%, ∆G amounts to 12% after 4 h, while the concentration of a solution subjected to separation increases by a factor of 1.67. In this case, the initial capacity of membranes fully recovers. The obtained results can be explained by the action of PE as an inhibitor of calcium sulfate scaling. One can suggest that the introduction of Na-CMC to the solution subjected to separation initially causes binding of calcium ions as a result of cationic exchange. Later on, as concentration proceeds, nuclei germs and microcrystals emerge and the system transits from homogeneous into heterogeneous state, and adsorption of macromolecules takes place on the surface of emerging microcrystals. In this case, we observe hydrophilization of the surface and blocking of further growth of crystals.22 Crystallization of salt in the volume of a solution subjected to separation slows down. The presence of insignificant crystalline scale on the surface of a membrane indicates the shortage of Na-CMC for complete prevention of calcium sulfate crystallization. In this case, in our view, a reverse hydrophilic layer is formed on the membrane during dead-end UF that exists only during this process. Under the impact of normal flux, this layer is consolidated. That is why a high retention coefficient (46.8%) evidently is determined by both steric factor (aggregation of microcrystals) and consolidation of the layer.

Figure 4. Time variation of the capacity of membranes (curve 1) and concentration of calcium ions (curve 2) in the filtrate with addition of 4 mg/L Na-CMC at P * constant: 1, 23.5 L/m2‚h; 2, 1.24 g/L.

Figure 5. Time variation of the capacity of membranes (curve 1) and concentration of calcium ions (curve 2) in the filtrate with addition of 16 mg/L Na-CMC at P ) constant: 1, 25.0 L/m2‚h; 2, 1.12 g/L.

In case of dead-end UF in the pulsation mode (Figure 4), the retention coefficient achieves the value of only 42% at a visibly higher capacity reduction (SI table, item 4). It can be explained by a higher concentration of the solution subjected to separation (n ) 1.16 for initial solution). In addition, reduction of R under the given conditions can also be caused by the growing concentration of the solution subjected to separation. In the pulsation mode, as in the case of P ) constant, a crystalline scale was detected on the membrane. However, it is barely visible thus confirming that a PE concentration of 4 mg/L is insufficient for complete inhibition of calcium sulfate scaling. 3.3. Dead-End Ultrafiltration of Calcium Sulfate Solutions in the Presence of 16 mg/L Inhibitor at P ) Constant and P * Constant. As the PE concentration increases to 16 mg/L during the dead-end UF at P ) constant (Figure 5), a high retention coefficient of 49% is achieved, while the solution concentration is doubled. In this case, a crystalline scale on the membrane is not present. According to the mechanism of PE action, one can suggest that with a higher content of Na-CMC, a dense network of associated molecules of the polymer is formed in water that hampers drawing together and aggregation of particles stabilizing the system. Losses (8%) of the initial capacity of membrane following its washing make it possible to suggest the existence of an ultrathin irreversible layer which

Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007 2247

Figure 6. Time variation of the capacity of membranes (curve 1) and concentration of calcium ions (curve 2) in the filtrate with addition of 16 mg/L Na-CMC at P * constant: 1, 21.3 L/m2‚h; 2, 1.12 g/L.

partially blocks pores of the membrane. Appearance of such a layer may be caused by continuous (3 h) action of the normal flux providing consolidation of the layer. The high retention coefficient of 49% shows high compactness of the layer, while the reduction of capacity to 28% after 3 h (SI table, item 5) is evidence of both its high hydrodynamic resistance and the influence of concentration polarization. As can be seen from Figure 6, conducting the dead-end UF in the pulsation mode provides for a significant stabilization of the process. Relative reduction of the capacity of a membrane after 1 h (before the release of pressure to zero) does not exceed 8% (SI table, item 6). The retention coefficient lies within the limits 41.0%-42.7%. These indicators are maintained for 4 h. During this time, the solution is concentrated by a factor of 1.7. After completion of the dead-end UF, no scale on the membrane was detected. Full recovery of the initial capacity of the membrane after washing with water for 15 min indicates the absence of the irreversible layer on the membrane. Under the given conditions, the retention coefficient is significantly lower than at P ) constant. However, in our view, this is compensated in full measure by the absence of scale formation on the membrane, the increased time of stable operation, the duration of which is almost doubled, an increase of the membrane working capacity by a factor of 1.3, and full recovery of the initial capacity of the membrane. 4. Conclusions (1) The initial capacity of membranes (volume flux) was shown to influence the rate of CaSO4‚2H2O scale formation on membranes. According to data of the kinetics of dead-end UF of calcium sulfate solutions in the absence of inhibitor, and taking into account the quality of the filtrate and concentrate, the visual assessment of the surface of membranes, and the results of washing, we drew a conclusion that a decisive role in determining the process characteristics belongs to concentration polarization. (2) It was established that at P ) constant the introduction of the inhibitor with concentration 4 mg/L slows down the process of salt crystallization in the volume of solution subjected to separation. In these conditions, a partially reversible layer is formed on membranes, while in case of the pulsation mode of dead-end UF, the layer formed on membranes is fully reversible. The results were obtained indicating the shortage of the polymer

for stabilization of calcium sulfate solution and the contribution of heterogeneous crystallization to the scaling process. (3) It was shown that, during the dead-end UF at P ) constant, the crystalline scale of calcium sulfate on the membrane is not present provided the Na-CMC concentration is increased to 16 mg/L. However, the formation of an irreversible ultrathin layer that partly blocks pores prevents the recovery of the initial capacity of the membrane after its washing. (4) It was established that in the pulsation mode facilitating to reduce consolidation of the layer formed on the membrane during dead-end UF, the relative reduction of the initial capacity during 4 h does not exceed 8%. Scaling on the membrane is not present. As compared with the results obtained at P ) constant (see Conclusions section, item 3), the time of stable operation of the membrane almost doubles, while its working capacity increases by a factor of 1.3. The initial capacity of the membrane is fully recovered after its washing with water. Supporting Information Available: Table of dead-end UF under various conditions (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Drioli, E.; Romano, M. Progress and new perspectives on integrated membrane operations for sustainable industrial growth. Ind. Eng. Chem. Res. 2001, 40, 1277-1300. (2) van Hoof, S. C. J. M.; Minnery, J. G.; Mack, B. Dead-end ultrafiltration as pretreatment to Seawater Reverse Osmosis. Desalin. Water Reuse 2002, 11/3, 44-48. (3) Gottberg, A.; Vaccaro, G. Kuwait’s giant membrane plant starts to take shape. Desalin. Water Reuse 2003, 13/2, 30-34. (4) Pervov, A. G.; Andrianov, A. P.; Efremov, R. V.; Kozlova, Yu. V. New trends in development of modern nanofiltration systems for conditioning of high quality drinking water: Review. Crit. Technol. Membr. 2005, 1 (25), 18-34. (5) Eriksson, P. K.; Petersen, R. J. ReVerse osmosis for water desalination: membranes, modules and system design considerations; Filmtec Corporation Matarvattensymposium: Stockholm, November 1988. (6) Himelstein, W. D.; Amjad, Z. Membranes maintenance. Water Waste Treat. 1985, 28 (3), 30-34. (7) Karelin, F. N. Desalination of water by reVerse osmosis (in Russ.); Stroiizdat: Moscow, 1988. (8) Okazarki, M.; Kimura, S. Scale formation on reverse osmosis membranes. J. Chem. Eng. Jpn. 1984, 17 (2), 145-151. (9) Logan, D. P.; Kimura, S. Control of gypsum scale of reverse osmosis membranes. Desalination 1985, 54, 321-331. (10) Pervov, A. G. Sulfate scale formation in reverse osmosis desalination plants (in Russ.). Tr. VNIVODGEO 1987, 41-51. (11) Borden, J.; Gilron, J.; Hasson, D. Analysis of RO flux decline due to membrane surface blockage. Desalination 1985, 54, 321-331. (12) Barba, D.; Brandani, V.; Giacomo, G. CaSO4 scale conditions of membranes predicted by multi-ion thermodynamic model. Desalination 1985, 54, 227-237. (13) Starov, V. M.; Churaev, N. V.; Dorokhov, V. M. Effect of association of ions in the zone of concentric polarization and precipitation of crystals on selectivity of reverse osmosis membrane (in Russ.). Khim. Tekhnol. Vody 1986, 8 (2), 67-78. (14) Weynen, M. P. C.; Van Rosmalen, G. M. The influence of various polyelectrolytes on the precipitation of gypsum. Desalination 1985, 54, 239-261. (15) Amjad, L. Applications of antiscalants to control calcium sulfate scaling in reverse osmosis system. Desalination 1985, 54, 263-276. (16) Lipman, J. L.; Hatch, R. T. Protecting RO membranes with polymers. Water Technol. 1985, 7 (5), 45-49. (17) Kavitskaya, A. A.; Knyazkova, T. V. Membrane filtration of calcium sulfate solution. J. Water Technol. 1992, 14, 3-8.

2248

Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007

(18) Kavitskaya, A. A. Possibilities of the dead-end ultrafiltration in hard water treatment. Desalination 2004, 168, 341-346. (19) Svyatchenko, V. V.; Bil’dyukevich, A. V. Porous structure of industrial and experimental ultrafiltration membranes (in Russ.). Zh. Prikl. Khim. 1991, 64 (7), 1571-1573. (20) Kulskii, L. A. Theoretical principles and technology of water conditioning (in Russ.); Naukova Dumka: Kiev, Ukraine, 1980. (21) Khamskii, E. V. OVersaturated solutions (in Russ.); Nauka: Leningrad, 1975.

(22) Belikova, M. I.; Neiman, O. V. Adsorptive modification of products of calcium sulfate crystallization (in Russ.). Kolloidn. Zh. 1973, 35 (8), 1037-1041.

ReceiVed for reView July 17, 2006 ReVised manuscript receiVed December 28, 2006 Accepted January 9, 2007 IE0609315