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Morphometric characterization of calcium sulfate dihydrate (gypsum) scale on reverse osmosis membranes
Journal of Membrane Science 252 (2005) 253–263
Morphometric characterization of calcium sulfate dihydrate (gypsum) scale on reverse osmosis membranes Wen-Yi Shih, Anditya Rahardianto, Ron-Wai Lee, Yoram Cohen ∗ Department of Chemical Engineering, University of California, 5531 Boelter Hall, 405 Hilgard Avenue, Los Angeles, CA 90095-1592, USA Received 14 October 2004; received in revised form 2 December 2004; accepted 17 December 2004 Available online 6 February 2005
Abstract The axial development of calcium sulfate dihydrate (gypsum) scaling on selected reverse osmosis (RO) membrane surfaces was investigated experimentally in a plate-and-frame RO system. Scaling experiments with model solutions demonstrated progressive axial development of surface gypsum crystals along the membrane surface. The impact of surface crystallization was characterized via flux decline measurements and by both optical and high-resolution scanning electron microscopy (SEM) surface imaging. Surface coverage by gypsum scale (percent area covered and gypsum surface mass density) increased along the membranes (towards the membrane exit), consistent with the increase in concentration polarization. At the membrane channel entry, surface gypsum crystals were at their initial growth stage formed as primarily needle-like structures. With increasing axial distance from the entry, surface crystallization resulted in crystal structures that transitioned from the needle and plate-like structures, in the submicron size range, to partial rosettes to complete rosette structures in the mm size range. Significant differences were observed in the extent of surface scale coverage and surface crystal size among the membrane studied. Antiscalant addition to the feed solution led to progressive decline in the percent of area covered by scale with increasing antiscalant dosage and corresponding decrease in flux decline. Antiscalant addition resulted in perturbed surface crystal morphology of flattened and/or fused rosette crystal arms and to complete elimination of surface crystals at a sufficiently high antiscalant dosage. The present study demonstrates that gypsum scale development is affected by the formation of surface crystals on the membrane surface, thereby suggesting that there is merit to expanded research on the direct impact of surface topology and chemistry on surface crystallization of mineral salts. © 2005 Elsevier B.V. All rights reserved. Keywords: Reverse osmosis; Scale development; Scale morphology; Gypsum; Calcium sulfate dihydrate
1. Introduction In recent years, there has been a growing interest in the implementation of membrane desalination processes for the treatment and reclamation of high-salinity inland water sources (e.g., groundwater, agricultural drainage water and surface water). The development of low-pressure RO membranes has led to intensive efforts to develop RO desalination technology to conserve progressively dwindling water resources in various regions in the United States [1,2], drought-prone zones such as the Middle East [3,4], and in areas where freshwater supplies are scarce, such as Singa∗
Corresponding author. Tel.: +1 310 825 8766; fax: +1 310 206 4107. E-mail address: [email protected] (Y. Cohen).
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.12.023
pore [5]. Water product recovery for inland water reclamation by membrane desalination has to be sufficiently high (80%) to be economically feasible [6]. However, with increased recovery at the required rejection levels of >95%, the concentration of sparingly soluble mineral salt ions (e.g., calcium sulfate, barium sulfate, calcium carbonate and possibly other salts) on the feed side of the membrane increases and, depending on the feed composition, can often reach levels that are above saturation [7–9]. As a consequence, crystallization of these mineral salts may take place near and onto the membrane surfaces leading to mineral salt scaling of the membranes surface. The formation of scale on membrane surfaces results in flux decline and shortening of membrane life. Although various salts have been associated with membrane scale formation, calcium sulfate dihydrate (gypsum)
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and calcium carbonate are most commonly encountered in desalination of surface and ground waters. While the precipitation of calcium carbonate scale can often be minimized by adjusting the water feed (prior to RO desalination) to acidic conditions, calcium sulfate and barium sulfate precipitation is insensitive to pH. Both increased recovery and salt rejection lead to greater concentration of the retentate relative to the feed stream as expressed by the concentration factor, CF, below: CC 1 CF = [1 − RW (1 − RS )] = (1) CF 1 − RW in which CC and CF are the retentate and feed concentrations, respectively, RS is the fractional salt rejection (RS = 1 − CP /CF , where CP is the permeate concentration) and RW is the fractional product water recovery (RW = QP /QF , where QP and QF are the permeate and feed flow rates, respectively). As an illustration, it is instructive to consider the desalination of agricultural drainage water (AD water) and Colorado River water (CR water) with the typical compositions as given in Table 1. For example, for product water recovery in the range of 80–90% and rejection of 95% of the Colorado River raw water feed, CF values are in the range of 4.8–9.5, with estimated saturation indices (pH ∼ 7) of 0.48–1.1, 63–150 and 45–98, for calcium sulfate, barium sulfate and calcium carbonate, respectively. By acid-dosing the feed water to pH <7, calcium carbonate concentration can be lowered to a level below saturation, and only barite and gypsum would remain oversaturated. Clearly, at the above conditions surface scaling is likely to be an impediment to achieving the desired level of water recovery. It should be noted that, due to concentration polarization, CF value at the membrane surface would be even higher than the estimate provided by Eq. (1). In order to develop effective scale mitigation strategies, various studies have focused on evaluating the operational conditions under which membrane scale forms and the Table 1 Examples of the concentrations of major ions in groundwater in the California Buena Vista Water Storage District and Colorado River Water Ion
Concentration (mg/L) Buena Vista Storage District Water
Colorado River Water
Cations Na+ Ca2+ Mg2+ Ba2+
1150 555 60.7 0.17
108 82 32 0.13
Anions Cl− SO4 2− HCO3 −
210 1020 291
95 300 300
pH TDS (mg/L) Saturation index
7.7 5250 0.5
8.4 703 0.06
associated impact on flux decline. It is now accepted that mineral salt scaling occurs by both the deposition of bulk formed crystals onto the membrane surface and direct surface crystallization on the membrane surface (Fig. 1) [7,10,11]. The relationship between scale formation and flux decline has been investigated in a number of studies that focused on either calcium carbonate or calcium sulfate crystallization [7,9,10]. These studies have provided semiempirical interpretive models that allow one to describe flux decline as a function of the rate of scale buildup. It is now accepted that when scale formation is dominated by surface crystallization, flux decline is due to blockage by the laterally growing surface crystals [9]. Although various studies have investigated the morphology of surface scale crystals and percentage of total area scaled [7,9,12–21], the axial morphological development of surface scale crystals along the membrane has not been documented in the literature. At present, a measure of success for controlling membrane scaling due to calcium sulfate crystallization has been achieved by the addition of antiscalant additives to the RO feed. Antiscalants are typically polyelectrolytes such as polycarboxylates, polyacrylates, polyphosphonates and polyphosphates [12,13,22–31] with reported optimal molecular weights in the range of 1000–3500 [32]. It has been suggested that antiscalants adsorb onto formed crystals or associate/complex with incipient nuclei (or crystals), thereby inhibiting mineral salt crystallization [13]. Various studies have shown that, in supersaturated solutions of sparingly soluble salts, a significant delay in crystal nucleation and subsequent growth is observed in response to antiscalant treatment [15,23,33,34]. This delay, which is referred to as the crystallization “induction time”, occurs at remarkably low “threshold dosages” in the order of 1–10 ppm. Although it is accepted that antiscalants retard homogeneous crystallization and reduce membrane scale formation, there has not been direct evidence of antiscalant impact on the morphological development of surface crystals. In the present study, we report on the optical and SEM characterization of the axial development of calcium sulfate crystals on a selected number of reverse osmosis membranes and on the effect of antiscalant additives on surface crystal formation. The change in shape and size of surface scale elements and crystal aerial and mass density along the membrane are quantified and evidence is provided of the early growth stages associated with surface crystallization.
Fig. 1. Conceptual representation of membrane scaling mechanisms.
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Table 2 Reverse osmosis membrane characteristics Membrane type LFC-1 (polyamide) TFC-HR (polyamide) TFC-ULP (polyamide)
Manufacturer
Manufacturer specifications (flux, NaCl rejection, pressure)
RMS surface roughness (nm)
Hydraunatics Koch membrane systems Koch membrane systems
50.9 L/m2 /h,
65.5 66.5 54.2
99.5% for 1500 ppm feed, 1551 kPa 50.9 L/m2 /h, 99.5% for 2000 ppm feed, 1551 kPa 42.4 L/m2 /h, 98.5% for 2000 ppm feed, 689 kPa
2. Experimental 2.1. Materials and reagents Three aromatic polyamide composite low-pressure RO/NF membranes were selected [35] from two major U.S. manufacturers based on their low biofouling potential and relatively high calcium rejection of 94%–96%, ∼99%, ∼99% for the TFC-ULP, LFC-1 and TFC-HR membranes, respectively, with corresponding permeate flux of 35–85 L/m2 /h, 25–35 L/m2 /h and 32–50 L/m2 /h over the concentration range (<0.03 M based on calcium chloride solutions) and transmembrane pressures in the present study at 20 ◦ C. These membranes, Hydranautics LFC-1 (Oceanside, CA), Koch Membrane Systems TFC-ULP and TFC-HR (San Diego, CA), were stored in accordance to manufacturer specifications (Table 2). Ultra-pure deionized water was obtained by filtering distilled water through a Milli-Q water system (Millipore Corp., San Jose, CA). Inorganic salts used included calcium chloride dihydrate (certified A.C.S), magnesium sulfate (certified A.C.S), sodium chloride (USP/FCC granular), and sodium sulfate (certified A.C.S anhydrous), all obtained from Fisher Scientific (Pittsburgh, PA). The antiscalant used in the RO scale retardation experiments was Vitec 2000 (Avista Technologies, San Marcos, CA), selected due to its effectiveness as a gypsum-scale suppressant [32]. 2.2. Solution preparation Concentrations of calcium and sulfate in the model solution were set to be a factor of CF times their respective concentrations of the reference solution (Table 3). The reference solution (Table 3) was selected to mimic the compositions of major ions in Colorado River water (Table 1). It is noted that the saturation index with respect to gypsum (log SIG = log IAP − log KS , where IAP is the ion activity product and KS is the equilibrium constant for formation of gypsum) for the reference solution was 0.06. Gypsum crystal growth along the membrane surface was determined with a model solution of composition set at CF = 8 relative to the Table 3 Feed solution composition for RO membrane scaling studies Salt
Concentration (M)
CaCl2 MgSO4 Na2 SO4 Saturation index (SI) Concentration factor (CF)
1.64 × 10−2 1.05 × 10−2 1.45 × 10−2 1.01 8
reference solution, for which SIG was 1.01. It is noted that concentrations of the major ions, magnesium, sodium, and chloride, were adjusted to match the required CF level, while maintaining charge balance and total dissolved solids (TDS) level of the reference solution. Bicarbonate and barium were not included to avoid co-precipitation of calcite and barite.
2.3. Membrane test system Membrane scaling experiments were conducted using a laboratory plate-and-frame RO membrane recirculation unit (Fig. 2). This unit consists of two test cells (Industrial Research Machine Products, El Cajon, CA) arranged in parallel, each cell with membrane surface area of 19.76 cm2 (2.6 cm × 7.6 cm) with a channel height of 2.66 mm. The feed reservoir was a magnetically stirred 18 L polyethylene tank. A refrigerated recirculator (model 625, Fisher Scientific, Pittsburgh, PA) served to maintain a constant reservoir temperature. A positive displacement pump (Hydra-Cell, Wanner Engineering, Minneapolis, MN) was used to deliver up to 6.94 × 10−5 m3 /s of feed solution. Transmembrane pressure was adjusted using a back-pressure regulator (US Paraplate, Auburn, CA) and a digital flow meter (model 1000, Fisher Scientific, Pittsburgh, PA), interfaced with a PC, provided for continuous monitoring of permeate flux and accumulated volume of product water. Permeate conductivity was monitored using a conductivity meter (model WD-35607-30, Oakton Research, Vernon Hills, IL). Scaling runs in which surface crystallization was dominant were carried out by filtering the retentate stream to remove bulk crystals, using a 0.45 m polypropylene microfiltration cartridge (Flotrex PN pleated Filter, Osmonics, Minnetonka, MN). Scaling experiments in the absence of permeate flow were carried out with the membrane positioned on top of a 2.6 cm × 7.6 cm PMMA plate installed in the membrane channel to prevent water permeation through the membrane.
2.4. Gypsum scaling experiment The feed solution for the membrane scaling runs was set at CF = 8 relative to the reference solution (Table 3). For this feed solution (SIG = 1.01), no bulk crystals were observed for a period of 3 days, as determined by the dual-probe method of Shih et al. [33]. This relatively long time delay before the occurrence of crystallization ensured that bulk crystallization in the feed reservoir and in the membrane module was negligible during the 24-h scaling runs.
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Fig. 2. Schematic of laboratory reverse osmosis system.
Prior to each scaling experiment, the membranes (2.6 cm × 7.6 cm) were equilibrated in the RO cells by recirculating water through the system at a crossflow velocity of 0.11 m/s and permeate flow rate of about 3.1 mL/min for 2 h, followed by the addition of the model solution at CF = 8 (without calcium) for another 2 h at cross-flow velocity of 0.11 m/s and permeate flow rate of 2.4 mL/min to establish a baseline for permeate flux. At the end of the 4 h period, calcium chloride solution was added to the feed reservoir to initiate the membrane scaling experiment. All membrane-scaling experiments were carried out at the same cross-flow velocity of 0.11 m/s and initial permeate flow rate of 2.25 mL/min which was set by appropriately adjusting the transmembrane pressure. Runs with antiscalant were conducted by adding the antiscalant 5 min prior to the addition of calcium ions. The permeate and retentate streams were recirculated continuously in a total recycle mode for a period of 24 h. This ensured that ion concentrations in the feed solution remained essentially constant, given the negligible loss of calcium and sulfate ions due to crystallization at SIG = 1.01. After the completion of each 24 h run, the membranes were removed from the cells and stored dry for further analysis. All scaling runs to quantify axial development of gypsum scale were conducted with retentate filtration to ensure dominance of surface crystallization. In order to provide a reasonable comparison of membrane scaling propensity, all scaling runs were carried out at the same initial level of concentration polarization, CP, as estimated from the simple film model [36]:
CP =
C m − Cp J = exp Cb − C p k
(2)
where Cm , Cb and Cp refer to solute concentrations near the membrane, in the bulk, and in the permeate, respectively, J is the permeate flux and k the solute feed-side mass transfer coefficient. At high rejection, Eq. (2) can be simplified since it is reasonable to assume that for such a condition, Cm Cp and Cb Cp . Eq. (2), while adequate for characterizing the experimental conditions with respect to concentration polarization, is an approximation that does not account for axial increase of the concentration polarization layer along the flow channel [37–42].
3. Sample analysis Optical images of the dried membranes were obtained using a high-resolution digital camera (Nikon model D100, Nikon Corp., Japan) with a 28–105 mm lens and a 4+ macro-lens attachment. High-contrast imaging of the surface crystals was obtained using a low-angle dark-field illuminator (Model DF-200-1, Northeast Robotics, NH). The digital images were converted to a gray-scale representation and thresholded, using an image analysis software (Fovea Pro, Version 3.0, Reindeer Graphics, Asheville, NC), to detect crystal edges. The fraction of surface area covered by gypsum scale was determined by differentiating the pixels representing gypsum crystals from the pixels that represent the membrane background. Selected areas of the membranes were also imaged with a high-resolution scanning electron microscope (Hitachi S4700 Field Emission SEM, Japan) at 1–3 kV. SEM samples were first sputtered with gold at 13.3 Pa and 45 mA for 30 s. The morphology and dimensions of the surface gypsum crystals were determined from the above images by refining the contrast with Fovea Pro. Surface
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Fig. 3. Membrane scaling due to bulk and surface crystallization: (a) scaling without retentate filtration (significant contribution from bulk crystal deposition); (b) scaling with retentate filtration (dominated by surface crystallization).
Fig. 4. (a) Monoclinic gypsum crystal morphology (numbers in parenthesis represent crystal orientation); (b) SEM image of gypsum crystal on LFC-1 membrane in the absence of permeation.
Fig. 5. Flux decline for LFC-1, TFC-ULP and TFC-HR membranes. Experimental conditions: temperature = 20 ◦ C; feed solution at CF = 8; 0.11 m/s crossflow velocity, transmembrane pressure of 1380 kPa for TFC-ULP membrane and 999 kPa for TFC-HR and LFC-1 membranes, initial permeate flux (F0 ) of 33.7 L/m2 /h.
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roughness of native membranes were obtained by a Veeco Multimode atomic force microscope (Veeco Instruments, Inc., Woodbury, NY) operated in the tapping mode. In order to provide additional insight with respect to the axial development of gypsum scale, the crystal surface mass density along the membrane was determined for the scaled membranes (LFC-1), which had the highest level of scale coverage. The LFC-1 membrane was segmented into 1.4 cm × 2.6 cm strips for calcium analysis. Scaled membrane strips were analyzed for surface calcium precipitate following Method 3030 of the Standard Methods for the Examination of Water and Wastewater [43]. In this approach, each of the scaled membrane strips was separately acid-digested by concentrated HNO3 acid followed by ICP-mass spectroscopy analysis for calcium.
the formation of rosette structures. However, detailed investigation over a wider range of flux conditions is warranted to determine the relationship between permeate flux and the morphology of gypsum surface crystals. The buildup of gypsum scale leads to permeate flux decline as shown in Fig. 5 for three different commercial membranes. These flux decline experiments reveal that the scaling propensity of the three membranes (as quantified by flux decline) is different, despite the fact that all scaling runs commenced at the identical concentration polarization level of 1.86. The axial development of gypsum scale (SEM) was observable (Fig. 5) even for the relatively short axial length (7.6 cm) of the present membrane channel. It is apparent
4. Results and discussion Gypsum scale can form due to the coupled effect of lateral growth of crystals directly on the membrane surface and also due to deposition of bulk formed crystals onto the membrane surface as suggested by the morphology of surface crystals as illustrated in Fig. 3 for the LFC1 membrane. Gypsum crystal growth reported in the literature [7,12–17,19,21–24,26–28,30,44] is typically described as exhibiting mostly needle and platelet assemblies with monoclinic, prismatic structures (Fig. 4a) with water molecules between the calcium and sulfate ions in the unit cell. The crystal-covered LFC-1 membrane images in Fig. 3a suggest that, in the absence of retentate filtration, gypsum crystals found on the surface consist of slender platelets and rods in the size range of 50–400 m and remnants of rosettes, which are believed to have been fractured by bulk crystal deposition. On the other hand, scaling runs in which the retentate was filtered, and from hereon the focus of the proceeding discussion, demonstrate the formation of rosette arrangements consisting of gypsum needles originating from a core growth region on the membrane surface (Fig. 3b). The above results suggest that retentate filtration essentially eliminate gypsum crystal scale formation by deposition of bulk crystals. Clearly, with retentate filtration the formation of rosette structures is dominated by surface crystallization. It is worth noting that in the absence of permeation through the membrane (where a PMMA plate blocked the underside of the RO membrane), surface crystals were of a wide size distribution consisting of individual and clusters of CaSO4 ·2H2 O crystals (Fig. 4b) typical of those reported in the heat-exchanger literature [19–21,44]. It is emphasized that the above no-permeation scaling test was conducted with a cross-flow velocity also set at 0.11 m/s, and with the feed solution at a supersaturation level that matched the estimated initial CP level at the membrane surface (see Eq. (2)). In this particular case (Fig. 4b), surface growth centers were not observed and rosette structures did not form. The above behavior suggests that permeate flux may be a prerequisite for
Fig. 6. SEM images of early gypsum rods emanating from a TFC-ULP membrane surface.
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259
Fig. 7. Axial development of gypsum crystals from rod to rosette structures.
that the degree of scaling increases toward the channel exit. We note that the level of concentration polarization is also expected to increase along the membrane. Arguably, the buildup of scale in a non-uniform manner along the membrane can lead to local variations in the concentration field near the membrane surface. Since the level of concentration polarization increases as a function of channel distance from the channel entrance, the rate of crystallization is expected to increase correspondingly [37–42]. This behavior can be rationalized by the now well-accepted kinetic model for gypsum crystallization [7,10,14,18,19,45,46]: dm = k(Cm − Cs )n dt
(3)
in which k is the growth rate constant, Cm the concentration of gypsum at the membrane surface, Cs the solubility of calcium sulfate at the experimental conditions and n an kinetic order which varies from a value of 1 for diffusion-controlled crystallization to a value of 2 for surface reaction process. From Eq. (3) and given that CP increases axially along the membrane, it can be inferred that the rate of surface scale formation should increase (with increasing Cm ) as the membrane exit
Fig. 8. Example of axial profile of membrane surface mass density for gypsum on LFC-1 membrane.
region is approached, a behavior which is consistent with the scaled membrane images shown in Fig. 5. For the scaled LFC1, TFC-HR and TFC-ULP membranes, shown in Fig. 5, the percentage of membrane area covered by scale, after the 24 h scaling run, was 35, 37 and 17%, respectively, with a corresponding percent flux decline of 30, 25 and 15%. We note that, within experimental error (≤5%), the near match between percent flux decline and percent surface area covered by scale is in agreement with the suggestion of Gilron and Hasson [9] that flux decline is primarily due to membrane pore blockage. Inspection of the scaled membranes by scanning electron micrograph imaging revealed gypsum surface crystals that developed to complete rosette structures, consistent with earlier studies [7,9,47]. The gypsum rosette structures became more fully developed as the channel exit was approached with typical major axis in the range of 0.5–1.0 mm. In the entry region, however, SEM imaging showed that in their early development, i.e., in the axial positions where the rate of crystallization is low, gypsum crystals are in the form of thin needles and platelets type structures of sizes in the sub-micron range. A vivid illustration of the initial growth of surface crystals is shown in Fig. 6 for the TFC-ULP membrane at various entry regions. This membrane exhibited the lowest degree of gypsum scale coverage and thus crystals at an early growth stage that could be observed by SEM imaging. The crystal rods (Fig. 6) are clearly seen stemming from within the membrane surface region with an equivalent diameter ranging from about 100 to 500 nm. It is interesting to note that the gypsum rods are adjacent to one another, which is consistent with the subsequent multi-rod rosette structures that appear to be emanating from growth center sites on the surface. As one progresses along this TFC-ULP membrane from the entrance to the exit region (Fig. 7), gypsum crystals transform from rod-like structures, in the sub-micron size range to a mixture of platelets and semirosettes in the 400–700 m size range. This typical behavior is shown in Fig. 7 where it is also evident that radial growth
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of crystal formations becomes more pronounced towards the exit region. Initially, crystal arms appear tightly packed, growing at a small angle away from the membrane surface. Eventually, the gypsum rosettes mature with multiple crystal arms branching out from growth centers. At the downstream region of the channel, where concentration polarization level is highest, gypsum rosettes cluster and overlap one another. The surface mass density of the gypsum scale is therefore also higher as the membrane exit region is approached (Fig. 8). Extensive SEM imaging of the scaled membranes revealed crystals of similar morphology but with differences in size of structures that progressed from needle-like to platelet to partial-rosette and to finally complete rosettes. The observed projected areas of the various crystal structures, from the entrance region of the membrane to the exit, were of the order of 10−7 to 1.4 mm2 . A summary of the size of the different crystal formations, expressed in terms of projected surface for the three membranes, is provided in Fig. 9a–c. As the membrane exit is approached the average projected areas of all structures increased, consistent with the corresponding increase in concentration polarization. The TFC-ULP membrane appeared to have supported the smallest surface crystals formations compared to the LFC-1 and TFC-HR membranes. The largest rosette structures were encountered on the LFC-1 membrane. Clearly, significant differences do exist among the different
membranes with respect to the size of surface crystal formations. At present, we can only speculate that differences in surface topology and possibly surface chemistry among the membranes may be responsible for the observed differences. This suggests that studies with clearly defined surrogate surfaces are warranted in order to clarify the impact of surface properties on mineral salt scale formation. Mitigation of scale formation with the use of antiscalant additives is known to retard the onset of homogenous crystallization [12,13,17,23–28,30,31] and to reduce membrane scaling. The impact of antiscalants on local morphology of surface crystals is of interest for direct assessment of antiscalant effectiveness. A number of studies have reported that antiscalant adsorption onto growing facets of mineral salt crystals alters the crystal morphology relative to its pristine state [12–14,16,22–30]. In order to explore antiscalant impact on surface crystal growth, 24 h scaling runs were conducted in which the RO feed was dosed with a suitable commercial antiscalant. These scaling tests were carried out with the LFC-1 membrane which had the highest level of scale coverage. For antisclant dosage in the range of 1–5 ppm the flux decline was in the range of 20–3.5% relative to 30% flux decline for the antiscalant-free feed solution (Table 4). At an antiscalant dosage of 3 ppm and higher, flux decline was solely due to membrane compaction as observed in runs
Fig. 9. Axial variation of gypsum crystal size in terms of projected area: (a) all crystal structures; (b) partial rosettes; (c) complete rosettes. Indicated locations along the membrane correspond to those shown in Fig. 7.
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Fig. 10. Effect of antiscalant on surface crystal morphology on LFC-1 membrane. Table 4 Effect of antiscalant dosage on flux decline for the LFC-1 membrane Vitec 2000 dosage (ppm)
% Flux decline
0 0.5 1.0 2 3 5
30.1 19.6 8.3 5.1 3.5a 3.7a
a At 3 and 5 ppm, the flux observed flux decline is solely due to membrane compaction.
with NaCl and CaCl2 salt solutions. SEM images of selected areas of the scaled LFC-1 membrane (Fig. 10) are strikingly demonstrative of the impact of antiscalant action on the resulting surface crystal morphology. At 1 ppm antiscalant dosage the gypsum rosette arms appeared to be fused and flattened onto the membrane. At a higher antiscalant dosage of 2 ppm, the rosette arms appear thicker and do not bear any resemblance to the rosettes observed for the antiscalantfree case. At antiscalant dosage of 3 ppm and higher, surface crystals could not be observed at the present SEM resolution.
strated a clear progressive development of surface gypsum crystals. The size of surface crystals increased with axial position along the membrane, consistent with the corresponding increase in the degree of concentration polarization. Surface crystals at the early stages of gypsum development consisted of needles and plate-like structures in the sub-micron ranges, to mixtures of platelets and partial rosettes and complete rosettes in the mm size range. Rosette structures consisted of gypsum rods extending radially from growth centers with rosette structure overlapping as the surface crystal density increased towards the membrane exit. Significant differences were apparent in the extent of surface scale coverage and surface crystal size among the three membranes studied when scaled at the same level of initial concentration polarization. Mitigation of membrane mineral salt scaling by antiscalant addition to the feed demonstrated a significant reduction in surface crystals size and less defined facets of rosette needlelike arms with increased antiscalant dosage The results of the present study suggest that both membrane gypsum scaling and antiscalant action appear to directly affect surface crystallization. Clearly, results of the present study suggest the need for further studies on the impact of surface topology and chemistry on chemistry on surface crystallization.
5. Conclusions Acknowledgements Experimental investigation of axial development of calcium sulfate dihydrate (gypsum) scale on reverse osmosis membranes, in the surface crystallization regime, demon-
The present study was supported in part by the California Department of Water Resources, the California Energy
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Commission, the Metropolitan Water District of Southern California, and by an International Desalination Association (IDA) Research Fellowship to Ms. Wen-Yi Shih. The authors also acknowledge the support and encouragement of Mark D. Williams and Christopher J. Gabelich. Koch Membrane Systems and Hydranautics are acknowledged for their generous supply of membrane samples, and Avista Techonologies for the antiscalant samples. Acid-digestion of membranes followed by ICP-MS was performed by the Water Quality Lab of Metropolitan Water District of Southern California (La Verne, California).
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[18] R. Sheikholeslami, H.W.K. Ong, Kinetics thermodynamics of calcium carbonate and calcium sulfate at salinities up to 1.5 M, Desalination 157 (2003) 217. [19] S.H. Najibi, H. Muller-Steinhagen, M. Jamialahmadi, Calcium sulfate scale formation during subcooled flow boiling, Chem. Eng. Sci. 52 (1997) 1265. [20] A. Quddus, Effects of hydrodynamics on the deposition of CaSO4 scale on stainless steel, Desalination 142 (2002) 57. [21] O.D. Linnikov, Investigation of the initial period of sulfate scale formation. Part 1. Kinetics and mechanism of calcium sulfate surface nucleation at its crystallization on a heat-exchange surface, Desalination 122 (1999) 1. [22] M.E. Tadros, I. Mayes, Linear growth rates of calcium sulfate dihydrate crystals in the presence of additives, J. Colloid Interf. Sci. 72 (1979) 245. [23] P.G. Klepetsanis, P.G. Koutsoukos, Kinetics of calcium sulfate formation in aqueous media: effect of organophosphorus compounds, J. Cryst. Growth 193 (1998) 156. [24] M.P.C. Weijen, G.M.V. Rosmalen, The influence of various polyelectrolytes on the precipitation of gypsum, Desalination 54 (1985) 239. [25] M. Reddy, A.R. Hoch, Calcite crystal growth rate inhibition by polycarboxylic acids, J. Colloid Interf. Sci. 235 (2001) 365. [26] M.H.H. Mahmoud, M.M. Rashad, I.A. Ibrahim, E.A. Abdel-Aal, Crystal modification of calcium sulfate dihydrate in the presence of some surface-active agents, J. Colloid Interf. Sci. 270 (2004) 99. [27] B.R. Smith, Scale prevention by addition of polyelectrolytes, Desalination 3 (1967) 263. [28] D.H. Solomon, P.F. Rolfe, Polymers that inhibit the deposition of calcium sulfate, Desalination 1 (1966) 260. [29] D. Hasson, R. SEmiat, D. Bramson, M. Busch, B. Limoni-Relis, Suppression of CaCO3 scale deposition by antiscalants, Desalination 118 (1998) 285. [30] H.A. El Dahan, H.S. Hegazy, Gypsum scale control by phosphate ester, Desalination 127 (2000) 111. [31] M. Al-Shammiri, M. Safar, M. Al-Dawas, Evaluation of two different antiscalants in real operation at the Doha Research Plant, Desalination 128 (2000) 1. [32] S.W. Walinsky, B.J. Morton, J.J. O’neill, Proceedings of the Symposium on Crystal Growth From Solution, Am. Chem. Soc. Meeting, Washington, DC, 1983. [33] W.-Y. Shih, K. Albrecht, J. Glater, Y. Cohen, A dual-probe approach for evaluation of gypsum crystallization in response to antiscalant treatment, Desalination 169 (2004) 213. [34] A.G. Pervov, Scale formation prognosis and cleaning procedure schedules in reverse osmosis systems operation, Desalination 83 (1991) 77. [35] R.-W. Lee, J. Glater, Y. Cohen, C. Martin, K. Kovac, M.N. Milobar, D.W. Bartel, Low-pressure RO membrane desalination of agricultural drainage water, Desalination 155 (2003) 109. [36] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, Boston, 1997. [37] L. Song, M. Elimelech, Theory of concentration polarization in crossflow filtration, J. Chem. Soc., Faraday Trans. 91 (1995) 3389. [38] L. Song, S. Yu, Concentration polarization in cross-flow reverse osmosis, AICHE J. 45 (1999) 921. [39] D. Bhattacharyya, S.L. Back, R.I. Kermode, Prediction of concentration polarization and flux behavior in reverse osmosis by numerical analysis, J. Membr. Sci. 48 (1990) 231. [40] K. Madireddi, R.B. Babcock, B. Levine, J.H. Kim, M.K. Stenstrom, An unsteady-state model to predict concentration polarization in commercial spiral wound membranes, J. Membr. Sci. 157 (1999) 13. [41] S. Bhattarcharjee, J.C. Chen, M. Elimelech, Coupled model of concentration polarization and pore transport in crossflow nanofiltration, AICHE J. 47 (2001) 2733.
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[45] G.H. Nancollas, The growth of crystals in solution, Adv. Colloid Interf. Sci. 10 (1979) 215. [46] M. Okazaki, S. Kimura, Scale formation on reverse osmosis membranes, J. Chem. Eng. Jpn. 172 (1984) 145. [47] S. Lee, J. Kim, C.-H. Lee, Analysis of CaSO4 scale formation mechanism in various nanofiltration modules, J. Membr. Sci. 163 (1999) 63.
Morphometric characterization of calcium sulfate dihydrate (gypsum) scale on reverse osmosis membranes Wen-Yi Shih, Anditya Rahardianto, Ron-Wai Lee, Yoram Cohen ∗ Department of Chemical Engineering, University of California, 5531 Boelter Hall, 405 Hilgard Avenue, Los Angeles, CA 90095-1592, USA Received 14 October 2004; received in revised form 2 December 2004; accepted 17 December 2004 Available online 6 February 2005
Abstract The axial development of calcium sulfate dihydrate (gypsum) scaling on selected reverse osmosis (RO) membrane surfaces was investigated experimentally in a plate-and-frame RO system. Scaling experiments with model solutions demonstrated progressive axial development of surface gypsum crystals along the membrane surface. The impact of surface crystallization was characterized via flux decline measurements and by both optical and high-resolution scanning electron microscopy (SEM) surface imaging. Surface coverage by gypsum scale (percent area covered and gypsum surface mass density) increased along the membranes (towards the membrane exit), consistent with the increase in concentration polarization. At the membrane channel entry, surface gypsum crystals were at their initial growth stage formed as primarily needle-like structures. With increasing axial distance from the entry, surface crystallization resulted in crystal structures that transitioned from the needle and plate-like structures, in the submicron size range, to partial rosettes to complete rosette structures in the mm size range. Significant differences were observed in the extent of surface scale coverage and surface crystal size among the membrane studied. Antiscalant addition to the feed solution led to progressive decline in the percent of area covered by scale with increasing antiscalant dosage and corresponding decrease in flux decline. Antiscalant addition resulted in perturbed surface crystal morphology of flattened and/or fused rosette crystal arms and to complete elimination of surface crystals at a sufficiently high antiscalant dosage. The present study demonstrates that gypsum scale development is affected by the formation of surface crystals on the membrane surface, thereby suggesting that there is merit to expanded research on the direct impact of surface topology and chemistry on surface crystallization of mineral salts. © 2005 Elsevier B.V. All rights reserved. Keywords: Reverse osmosis; Scale development; Scale morphology; Gypsum; Calcium sulfate dihydrate
1. Introduction In recent years, there has been a growing interest in the implementation of membrane desalination processes for the treatment and reclamation of high-salinity inland water sources (e.g., groundwater, agricultural drainage water and surface water). The development of low-pressure RO membranes has led to intensive efforts to develop RO desalination technology to conserve progressively dwindling water resources in various regions in the United States [1,2], drought-prone zones such as the Middle East [3,4], and in areas where freshwater supplies are scarce, such as Singa∗
Corresponding author. Tel.: +1 310 825 8766; fax: +1 310 206 4107. E-mail address: [email protected] (Y. Cohen).
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.12.023
pore [5]. Water product recovery for inland water reclamation by membrane desalination has to be sufficiently high (80%) to be economically feasible [6]. However, with increased recovery at the required rejection levels of >95%, the concentration of sparingly soluble mineral salt ions (e.g., calcium sulfate, barium sulfate, calcium carbonate and possibly other salts) on the feed side of the membrane increases and, depending on the feed composition, can often reach levels that are above saturation [7–9]. As a consequence, crystallization of these mineral salts may take place near and onto the membrane surfaces leading to mineral salt scaling of the membranes surface. The formation of scale on membrane surfaces results in flux decline and shortening of membrane life. Although various salts have been associated with membrane scale formation, calcium sulfate dihydrate (gypsum)
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and calcium carbonate are most commonly encountered in desalination of surface and ground waters. While the precipitation of calcium carbonate scale can often be minimized by adjusting the water feed (prior to RO desalination) to acidic conditions, calcium sulfate and barium sulfate precipitation is insensitive to pH. Both increased recovery and salt rejection lead to greater concentration of the retentate relative to the feed stream as expressed by the concentration factor, CF, below: CC 1 CF = [1 − RW (1 − RS )] = (1) CF 1 − RW in which CC and CF are the retentate and feed concentrations, respectively, RS is the fractional salt rejection (RS = 1 − CP /CF , where CP is the permeate concentration) and RW is the fractional product water recovery (RW = QP /QF , where QP and QF are the permeate and feed flow rates, respectively). As an illustration, it is instructive to consider the desalination of agricultural drainage water (AD water) and Colorado River water (CR water) with the typical compositions as given in Table 1. For example, for product water recovery in the range of 80–90% and rejection of 95% of the Colorado River raw water feed, CF values are in the range of 4.8–9.5, with estimated saturation indices (pH ∼ 7) of 0.48–1.1, 63–150 and 45–98, for calcium sulfate, barium sulfate and calcium carbonate, respectively. By acid-dosing the feed water to pH <7, calcium carbonate concentration can be lowered to a level below saturation, and only barite and gypsum would remain oversaturated. Clearly, at the above conditions surface scaling is likely to be an impediment to achieving the desired level of water recovery. It should be noted that, due to concentration polarization, CF value at the membrane surface would be even higher than the estimate provided by Eq. (1). In order to develop effective scale mitigation strategies, various studies have focused on evaluating the operational conditions under which membrane scale forms and the Table 1 Examples of the concentrations of major ions in groundwater in the California Buena Vista Water Storage District and Colorado River Water Ion
Concentration (mg/L) Buena Vista Storage District Water
Colorado River Water
Cations Na+ Ca2+ Mg2+ Ba2+
1150 555 60.7 0.17
108 82 32 0.13
Anions Cl− SO4 2− HCO3 −
210 1020 291
95 300 300
pH TDS (mg/L) Saturation index
7.7 5250 0.5
8.4 703 0.06
associated impact on flux decline. It is now accepted that mineral salt scaling occurs by both the deposition of bulk formed crystals onto the membrane surface and direct surface crystallization on the membrane surface (Fig. 1) [7,10,11]. The relationship between scale formation and flux decline has been investigated in a number of studies that focused on either calcium carbonate or calcium sulfate crystallization [7,9,10]. These studies have provided semiempirical interpretive models that allow one to describe flux decline as a function of the rate of scale buildup. It is now accepted that when scale formation is dominated by surface crystallization, flux decline is due to blockage by the laterally growing surface crystals [9]. Although various studies have investigated the morphology of surface scale crystals and percentage of total area scaled [7,9,12–21], the axial morphological development of surface scale crystals along the membrane has not been documented in the literature. At present, a measure of success for controlling membrane scaling due to calcium sulfate crystallization has been achieved by the addition of antiscalant additives to the RO feed. Antiscalants are typically polyelectrolytes such as polycarboxylates, polyacrylates, polyphosphonates and polyphosphates [12,13,22–31] with reported optimal molecular weights in the range of 1000–3500 [32]. It has been suggested that antiscalants adsorb onto formed crystals or associate/complex with incipient nuclei (or crystals), thereby inhibiting mineral salt crystallization [13]. Various studies have shown that, in supersaturated solutions of sparingly soluble salts, a significant delay in crystal nucleation and subsequent growth is observed in response to antiscalant treatment [15,23,33,34]. This delay, which is referred to as the crystallization “induction time”, occurs at remarkably low “threshold dosages” in the order of 1–10 ppm. Although it is accepted that antiscalants retard homogeneous crystallization and reduce membrane scale formation, there has not been direct evidence of antiscalant impact on the morphological development of surface crystals. In the present study, we report on the optical and SEM characterization of the axial development of calcium sulfate crystals on a selected number of reverse osmosis membranes and on the effect of antiscalant additives on surface crystal formation. The change in shape and size of surface scale elements and crystal aerial and mass density along the membrane are quantified and evidence is provided of the early growth stages associated with surface crystallization.
Fig. 1. Conceptual representation of membrane scaling mechanisms.
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Table 2 Reverse osmosis membrane characteristics Membrane type LFC-1 (polyamide) TFC-HR (polyamide) TFC-ULP (polyamide)
Manufacturer
Manufacturer specifications (flux, NaCl rejection, pressure)
RMS surface roughness (nm)
Hydraunatics Koch membrane systems Koch membrane systems
50.9 L/m2 /h,
65.5 66.5 54.2
99.5% for 1500 ppm feed, 1551 kPa 50.9 L/m2 /h, 99.5% for 2000 ppm feed, 1551 kPa 42.4 L/m2 /h, 98.5% for 2000 ppm feed, 689 kPa
2. Experimental 2.1. Materials and reagents Three aromatic polyamide composite low-pressure RO/NF membranes were selected [35] from two major U.S. manufacturers based on their low biofouling potential and relatively high calcium rejection of 94%–96%, ∼99%, ∼99% for the TFC-ULP, LFC-1 and TFC-HR membranes, respectively, with corresponding permeate flux of 35–85 L/m2 /h, 25–35 L/m2 /h and 32–50 L/m2 /h over the concentration range (<0.03 M based on calcium chloride solutions) and transmembrane pressures in the present study at 20 ◦ C. These membranes, Hydranautics LFC-1 (Oceanside, CA), Koch Membrane Systems TFC-ULP and TFC-HR (San Diego, CA), were stored in accordance to manufacturer specifications (Table 2). Ultra-pure deionized water was obtained by filtering distilled water through a Milli-Q water system (Millipore Corp., San Jose, CA). Inorganic salts used included calcium chloride dihydrate (certified A.C.S), magnesium sulfate (certified A.C.S), sodium chloride (USP/FCC granular), and sodium sulfate (certified A.C.S anhydrous), all obtained from Fisher Scientific (Pittsburgh, PA). The antiscalant used in the RO scale retardation experiments was Vitec 2000 (Avista Technologies, San Marcos, CA), selected due to its effectiveness as a gypsum-scale suppressant [32]. 2.2. Solution preparation Concentrations of calcium and sulfate in the model solution were set to be a factor of CF times their respective concentrations of the reference solution (Table 3). The reference solution (Table 3) was selected to mimic the compositions of major ions in Colorado River water (Table 1). It is noted that the saturation index with respect to gypsum (log SIG = log IAP − log KS , where IAP is the ion activity product and KS is the equilibrium constant for formation of gypsum) for the reference solution was 0.06. Gypsum crystal growth along the membrane surface was determined with a model solution of composition set at CF = 8 relative to the Table 3 Feed solution composition for RO membrane scaling studies Salt
Concentration (M)
CaCl2 MgSO4 Na2 SO4 Saturation index (SI) Concentration factor (CF)
1.64 × 10−2 1.05 × 10−2 1.45 × 10−2 1.01 8
reference solution, for which SIG was 1.01. It is noted that concentrations of the major ions, magnesium, sodium, and chloride, were adjusted to match the required CF level, while maintaining charge balance and total dissolved solids (TDS) level of the reference solution. Bicarbonate and barium were not included to avoid co-precipitation of calcite and barite.
2.3. Membrane test system Membrane scaling experiments were conducted using a laboratory plate-and-frame RO membrane recirculation unit (Fig. 2). This unit consists of two test cells (Industrial Research Machine Products, El Cajon, CA) arranged in parallel, each cell with membrane surface area of 19.76 cm2 (2.6 cm × 7.6 cm) with a channel height of 2.66 mm. The feed reservoir was a magnetically stirred 18 L polyethylene tank. A refrigerated recirculator (model 625, Fisher Scientific, Pittsburgh, PA) served to maintain a constant reservoir temperature. A positive displacement pump (Hydra-Cell, Wanner Engineering, Minneapolis, MN) was used to deliver up to 6.94 × 10−5 m3 /s of feed solution. Transmembrane pressure was adjusted using a back-pressure regulator (US Paraplate, Auburn, CA) and a digital flow meter (model 1000, Fisher Scientific, Pittsburgh, PA), interfaced with a PC, provided for continuous monitoring of permeate flux and accumulated volume of product water. Permeate conductivity was monitored using a conductivity meter (model WD-35607-30, Oakton Research, Vernon Hills, IL). Scaling runs in which surface crystallization was dominant were carried out by filtering the retentate stream to remove bulk crystals, using a 0.45 m polypropylene microfiltration cartridge (Flotrex PN pleated Filter, Osmonics, Minnetonka, MN). Scaling experiments in the absence of permeate flow were carried out with the membrane positioned on top of a 2.6 cm × 7.6 cm PMMA plate installed in the membrane channel to prevent water permeation through the membrane.
2.4. Gypsum scaling experiment The feed solution for the membrane scaling runs was set at CF = 8 relative to the reference solution (Table 3). For this feed solution (SIG = 1.01), no bulk crystals were observed for a period of 3 days, as determined by the dual-probe method of Shih et al. [33]. This relatively long time delay before the occurrence of crystallization ensured that bulk crystallization in the feed reservoir and in the membrane module was negligible during the 24-h scaling runs.
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Fig. 2. Schematic of laboratory reverse osmosis system.
Prior to each scaling experiment, the membranes (2.6 cm × 7.6 cm) were equilibrated in the RO cells by recirculating water through the system at a crossflow velocity of 0.11 m/s and permeate flow rate of about 3.1 mL/min for 2 h, followed by the addition of the model solution at CF = 8 (without calcium) for another 2 h at cross-flow velocity of 0.11 m/s and permeate flow rate of 2.4 mL/min to establish a baseline for permeate flux. At the end of the 4 h period, calcium chloride solution was added to the feed reservoir to initiate the membrane scaling experiment. All membrane-scaling experiments were carried out at the same cross-flow velocity of 0.11 m/s and initial permeate flow rate of 2.25 mL/min which was set by appropriately adjusting the transmembrane pressure. Runs with antiscalant were conducted by adding the antiscalant 5 min prior to the addition of calcium ions. The permeate and retentate streams were recirculated continuously in a total recycle mode for a period of 24 h. This ensured that ion concentrations in the feed solution remained essentially constant, given the negligible loss of calcium and sulfate ions due to crystallization at SIG = 1.01. After the completion of each 24 h run, the membranes were removed from the cells and stored dry for further analysis. All scaling runs to quantify axial development of gypsum scale were conducted with retentate filtration to ensure dominance of surface crystallization. In order to provide a reasonable comparison of membrane scaling propensity, all scaling runs were carried out at the same initial level of concentration polarization, CP, as estimated from the simple film model [36]:
CP =
C m − Cp J = exp Cb − C p k
(2)
where Cm , Cb and Cp refer to solute concentrations near the membrane, in the bulk, and in the permeate, respectively, J is the permeate flux and k the solute feed-side mass transfer coefficient. At high rejection, Eq. (2) can be simplified since it is reasonable to assume that for such a condition, Cm Cp and Cb Cp . Eq. (2), while adequate for characterizing the experimental conditions with respect to concentration polarization, is an approximation that does not account for axial increase of the concentration polarization layer along the flow channel [37–42].
3. Sample analysis Optical images of the dried membranes were obtained using a high-resolution digital camera (Nikon model D100, Nikon Corp., Japan) with a 28–105 mm lens and a 4+ macro-lens attachment. High-contrast imaging of the surface crystals was obtained using a low-angle dark-field illuminator (Model DF-200-1, Northeast Robotics, NH). The digital images were converted to a gray-scale representation and thresholded, using an image analysis software (Fovea Pro, Version 3.0, Reindeer Graphics, Asheville, NC), to detect crystal edges. The fraction of surface area covered by gypsum scale was determined by differentiating the pixels representing gypsum crystals from the pixels that represent the membrane background. Selected areas of the membranes were also imaged with a high-resolution scanning electron microscope (Hitachi S4700 Field Emission SEM, Japan) at 1–3 kV. SEM samples were first sputtered with gold at 13.3 Pa and 45 mA for 30 s. The morphology and dimensions of the surface gypsum crystals were determined from the above images by refining the contrast with Fovea Pro. Surface
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Fig. 3. Membrane scaling due to bulk and surface crystallization: (a) scaling without retentate filtration (significant contribution from bulk crystal deposition); (b) scaling with retentate filtration (dominated by surface crystallization).
Fig. 4. (a) Monoclinic gypsum crystal morphology (numbers in parenthesis represent crystal orientation); (b) SEM image of gypsum crystal on LFC-1 membrane in the absence of permeation.
Fig. 5. Flux decline for LFC-1, TFC-ULP and TFC-HR membranes. Experimental conditions: temperature = 20 ◦ C; feed solution at CF = 8; 0.11 m/s crossflow velocity, transmembrane pressure of 1380 kPa for TFC-ULP membrane and 999 kPa for TFC-HR and LFC-1 membranes, initial permeate flux (F0 ) of 33.7 L/m2 /h.
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roughness of native membranes were obtained by a Veeco Multimode atomic force microscope (Veeco Instruments, Inc., Woodbury, NY) operated in the tapping mode. In order to provide additional insight with respect to the axial development of gypsum scale, the crystal surface mass density along the membrane was determined for the scaled membranes (LFC-1), which had the highest level of scale coverage. The LFC-1 membrane was segmented into 1.4 cm × 2.6 cm strips for calcium analysis. Scaled membrane strips were analyzed for surface calcium precipitate following Method 3030 of the Standard Methods for the Examination of Water and Wastewater [43]. In this approach, each of the scaled membrane strips was separately acid-digested by concentrated HNO3 acid followed by ICP-mass spectroscopy analysis for calcium.
the formation of rosette structures. However, detailed investigation over a wider range of flux conditions is warranted to determine the relationship between permeate flux and the morphology of gypsum surface crystals. The buildup of gypsum scale leads to permeate flux decline as shown in Fig. 5 for three different commercial membranes. These flux decline experiments reveal that the scaling propensity of the three membranes (as quantified by flux decline) is different, despite the fact that all scaling runs commenced at the identical concentration polarization level of 1.86. The axial development of gypsum scale (SEM) was observable (Fig. 5) even for the relatively short axial length (7.6 cm) of the present membrane channel. It is apparent
4. Results and discussion Gypsum scale can form due to the coupled effect of lateral growth of crystals directly on the membrane surface and also due to deposition of bulk formed crystals onto the membrane surface as suggested by the morphology of surface crystals as illustrated in Fig. 3 for the LFC1 membrane. Gypsum crystal growth reported in the literature [7,12–17,19,21–24,26–28,30,44] is typically described as exhibiting mostly needle and platelet assemblies with monoclinic, prismatic structures (Fig. 4a) with water molecules between the calcium and sulfate ions in the unit cell. The crystal-covered LFC-1 membrane images in Fig. 3a suggest that, in the absence of retentate filtration, gypsum crystals found on the surface consist of slender platelets and rods in the size range of 50–400 m and remnants of rosettes, which are believed to have been fractured by bulk crystal deposition. On the other hand, scaling runs in which the retentate was filtered, and from hereon the focus of the proceeding discussion, demonstrate the formation of rosette arrangements consisting of gypsum needles originating from a core growth region on the membrane surface (Fig. 3b). The above results suggest that retentate filtration essentially eliminate gypsum crystal scale formation by deposition of bulk crystals. Clearly, with retentate filtration the formation of rosette structures is dominated by surface crystallization. It is worth noting that in the absence of permeation through the membrane (where a PMMA plate blocked the underside of the RO membrane), surface crystals were of a wide size distribution consisting of individual and clusters of CaSO4 ·2H2 O crystals (Fig. 4b) typical of those reported in the heat-exchanger literature [19–21,44]. It is emphasized that the above no-permeation scaling test was conducted with a cross-flow velocity also set at 0.11 m/s, and with the feed solution at a supersaturation level that matched the estimated initial CP level at the membrane surface (see Eq. (2)). In this particular case (Fig. 4b), surface growth centers were not observed and rosette structures did not form. The above behavior suggests that permeate flux may be a prerequisite for
Fig. 6. SEM images of early gypsum rods emanating from a TFC-ULP membrane surface.
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259
Fig. 7. Axial development of gypsum crystals from rod to rosette structures.
that the degree of scaling increases toward the channel exit. We note that the level of concentration polarization is also expected to increase along the membrane. Arguably, the buildup of scale in a non-uniform manner along the membrane can lead to local variations in the concentration field near the membrane surface. Since the level of concentration polarization increases as a function of channel distance from the channel entrance, the rate of crystallization is expected to increase correspondingly [37–42]. This behavior can be rationalized by the now well-accepted kinetic model for gypsum crystallization [7,10,14,18,19,45,46]: dm = k(Cm − Cs )n dt
(3)
in which k is the growth rate constant, Cm the concentration of gypsum at the membrane surface, Cs the solubility of calcium sulfate at the experimental conditions and n an kinetic order which varies from a value of 1 for diffusion-controlled crystallization to a value of 2 for surface reaction process. From Eq. (3) and given that CP increases axially along the membrane, it can be inferred that the rate of surface scale formation should increase (with increasing Cm ) as the membrane exit
Fig. 8. Example of axial profile of membrane surface mass density for gypsum on LFC-1 membrane.
region is approached, a behavior which is consistent with the scaled membrane images shown in Fig. 5. For the scaled LFC1, TFC-HR and TFC-ULP membranes, shown in Fig. 5, the percentage of membrane area covered by scale, after the 24 h scaling run, was 35, 37 and 17%, respectively, with a corresponding percent flux decline of 30, 25 and 15%. We note that, within experimental error (≤5%), the near match between percent flux decline and percent surface area covered by scale is in agreement with the suggestion of Gilron and Hasson [9] that flux decline is primarily due to membrane pore blockage. Inspection of the scaled membranes by scanning electron micrograph imaging revealed gypsum surface crystals that developed to complete rosette structures, consistent with earlier studies [7,9,47]. The gypsum rosette structures became more fully developed as the channel exit was approached with typical major axis in the range of 0.5–1.0 mm. In the entry region, however, SEM imaging showed that in their early development, i.e., in the axial positions where the rate of crystallization is low, gypsum crystals are in the form of thin needles and platelets type structures of sizes in the sub-micron range. A vivid illustration of the initial growth of surface crystals is shown in Fig. 6 for the TFC-ULP membrane at various entry regions. This membrane exhibited the lowest degree of gypsum scale coverage and thus crystals at an early growth stage that could be observed by SEM imaging. The crystal rods (Fig. 6) are clearly seen stemming from within the membrane surface region with an equivalent diameter ranging from about 100 to 500 nm. It is interesting to note that the gypsum rods are adjacent to one another, which is consistent with the subsequent multi-rod rosette structures that appear to be emanating from growth center sites on the surface. As one progresses along this TFC-ULP membrane from the entrance to the exit region (Fig. 7), gypsum crystals transform from rod-like structures, in the sub-micron size range to a mixture of platelets and semirosettes in the 400–700 m size range. This typical behavior is shown in Fig. 7 where it is also evident that radial growth
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of crystal formations becomes more pronounced towards the exit region. Initially, crystal arms appear tightly packed, growing at a small angle away from the membrane surface. Eventually, the gypsum rosettes mature with multiple crystal arms branching out from growth centers. At the downstream region of the channel, where concentration polarization level is highest, gypsum rosettes cluster and overlap one another. The surface mass density of the gypsum scale is therefore also higher as the membrane exit region is approached (Fig. 8). Extensive SEM imaging of the scaled membranes revealed crystals of similar morphology but with differences in size of structures that progressed from needle-like to platelet to partial-rosette and to finally complete rosettes. The observed projected areas of the various crystal structures, from the entrance region of the membrane to the exit, were of the order of 10−7 to 1.4 mm2 . A summary of the size of the different crystal formations, expressed in terms of projected surface for the three membranes, is provided in Fig. 9a–c. As the membrane exit is approached the average projected areas of all structures increased, consistent with the corresponding increase in concentration polarization. The TFC-ULP membrane appeared to have supported the smallest surface crystals formations compared to the LFC-1 and TFC-HR membranes. The largest rosette structures were encountered on the LFC-1 membrane. Clearly, significant differences do exist among the different
membranes with respect to the size of surface crystal formations. At present, we can only speculate that differences in surface topology and possibly surface chemistry among the membranes may be responsible for the observed differences. This suggests that studies with clearly defined surrogate surfaces are warranted in order to clarify the impact of surface properties on mineral salt scale formation. Mitigation of scale formation with the use of antiscalant additives is known to retard the onset of homogenous crystallization [12,13,17,23–28,30,31] and to reduce membrane scaling. The impact of antiscalants on local morphology of surface crystals is of interest for direct assessment of antiscalant effectiveness. A number of studies have reported that antiscalant adsorption onto growing facets of mineral salt crystals alters the crystal morphology relative to its pristine state [12–14,16,22–30]. In order to explore antiscalant impact on surface crystal growth, 24 h scaling runs were conducted in which the RO feed was dosed with a suitable commercial antiscalant. These scaling tests were carried out with the LFC-1 membrane which had the highest level of scale coverage. For antisclant dosage in the range of 1–5 ppm the flux decline was in the range of 20–3.5% relative to 30% flux decline for the antiscalant-free feed solution (Table 4). At an antiscalant dosage of 3 ppm and higher, flux decline was solely due to membrane compaction as observed in runs
Fig. 9. Axial variation of gypsum crystal size in terms of projected area: (a) all crystal structures; (b) partial rosettes; (c) complete rosettes. Indicated locations along the membrane correspond to those shown in Fig. 7.
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Fig. 10. Effect of antiscalant on surface crystal morphology on LFC-1 membrane. Table 4 Effect of antiscalant dosage on flux decline for the LFC-1 membrane Vitec 2000 dosage (ppm)
% Flux decline
0 0.5 1.0 2 3 5
30.1 19.6 8.3 5.1 3.5a 3.7a
a At 3 and 5 ppm, the flux observed flux decline is solely due to membrane compaction.
with NaCl and CaCl2 salt solutions. SEM images of selected areas of the scaled LFC-1 membrane (Fig. 10) are strikingly demonstrative of the impact of antiscalant action on the resulting surface crystal morphology. At 1 ppm antiscalant dosage the gypsum rosette arms appeared to be fused and flattened onto the membrane. At a higher antiscalant dosage of 2 ppm, the rosette arms appear thicker and do not bear any resemblance to the rosettes observed for the antiscalantfree case. At antiscalant dosage of 3 ppm and higher, surface crystals could not be observed at the present SEM resolution.
strated a clear progressive development of surface gypsum crystals. The size of surface crystals increased with axial position along the membrane, consistent with the corresponding increase in the degree of concentration polarization. Surface crystals at the early stages of gypsum development consisted of needles and plate-like structures in the sub-micron ranges, to mixtures of platelets and partial rosettes and complete rosettes in the mm size range. Rosette structures consisted of gypsum rods extending radially from growth centers with rosette structure overlapping as the surface crystal density increased towards the membrane exit. Significant differences were apparent in the extent of surface scale coverage and surface crystal size among the three membranes studied when scaled at the same level of initial concentration polarization. Mitigation of membrane mineral salt scaling by antiscalant addition to the feed demonstrated a significant reduction in surface crystals size and less defined facets of rosette needlelike arms with increased antiscalant dosage The results of the present study suggest that both membrane gypsum scaling and antiscalant action appear to directly affect surface crystallization. Clearly, results of the present study suggest the need for further studies on the impact of surface topology and chemistry on chemistry on surface crystallization.
5. Conclusions Acknowledgements Experimental investigation of axial development of calcium sulfate dihydrate (gypsum) scale on reverse osmosis membranes, in the surface crystallization regime, demon-
The present study was supported in part by the California Department of Water Resources, the California Energy
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Commission, the Metropolitan Water District of Southern California, and by an International Desalination Association (IDA) Research Fellowship to Ms. Wen-Yi Shih. The authors also acknowledge the support and encouragement of Mark D. Williams and Christopher J. Gabelich. Koch Membrane Systems and Hydranautics are acknowledged for their generous supply of membrane samples, and Avista Techonologies for the antiscalant samples. Acid-digestion of membranes followed by ICP-MS was performed by the Water Quality Lab of Metropolitan Water District of Southern California (La Verne, California).
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