Environ. Sci. Technol. 2010, 44, 7937–7943
Effect of Surface-Exposed Chemical Groups on Calcium-Phosphate Mineralization in Water-Treatment Systems Z V I S T E I N E R , † H A N N A R A P A P O R T , * ,‡ Y O R A M O R E N , † A N D R O N I K A S H E R * ,† Department of Desalination and Water Treatment, Zuckerberg Institute for Water Research, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer campus 84990, Israel, and Department of Biotechnology Engineering, Ben Gurion University of the Negev, Beer-Sheva 84105, Israel
Received June 2, 2010. Revised manuscript received September 13, 2010. Accepted September 14, 2010.
Calcium-phosphate-scale formation on reverse osmosis (RO) membranes is a major limiting factor for cost-effective desalination of wastewater. We determined the effects of various organic chemical groups found on membrane surfaces on calciumphosphate scaling. Langmuir films exposing different functional groups were equilibrated with a solution simulating the ionic profile of secondary effluent (SSE). Surface pressure-area (Langmuir) isotherms combined with ICP elemental analyses of the interfacial precipitate suggested acceleration of calciumphosphate mineralization by the surface functional groups in the order: PO4 > COOH ∼ NH2 > COOH:NH2 (1:1) > OH > ethylene glycol. Immersion of gold-coated silicon wafers selfassembled with different alkanethiols in SSE solution showed formation of a hydroxyapatite precipitate by X-ray diffraction and ATR-IR analysis. Data showed diverse influences of functional groups on mineralization, implying low calcium-phosphate scaling for uncharged surfaces or surfaces coated with both positively and negatively charged groups. This information is valuable for understanding scaling processes, and for designing of novel low-scaling membranes for water desalination.
Introduction A major drawback in membrane-based separations is foulingsthe accumulation and precipitation of solids or buildup of gel-like layers on the membrane surface. Fouling causes a decline in permeate flux, resulting from additional resistance and reduced transmembrane pressure, decreased salt rejection, and increased operational costs. Different types of membrane fouling are observed: (a) inorganic scaling by sparingly soluble salts and in some cases, by dissolved silica (1); (b) particulate and colloidal fouling (2); (c) organic fouling, caused by organic matter such as humic acids (3-5), and (d) biofouling, caused by microorganisms adhering to the membrane surface and forming a biofilm (6-8). In all types of fouling, concentration polarization causes high solute concentrations at the membrane-water interface, which may * Address correspondence to either author. Phone: 972-86563531 (R.K.); 972-86479043 (H.R.). Fax: 972-86596889 (R.K.); 972-86472983 (H.R.). E-mail:
[email protected] (R.K.);
[email protected] (H.R.). † Jacob Blaustein Institutes for Desert Research. ‡ Department of Biotechnology Engineering. 10.1021/es101773t
2010 American Chemical Society
Published on Web 09/27/2010
lead to precipitation on the membrane even when their concentration in the feedwater is well below saturation. It has been shown that scaling, organic fouling, and biofouling coexist and may affect each other on membrane surfaces (9-11). There is evidence that biofilms influence the properties and extent of scale precipitates on membrane surfaces. In a long-term piloting study Mekorot company found (12, 13) that fouling by reverse osmosis (RO) concentrate of tertiary effluent appears to be a combination of biofouling and calcium-phosphate precipitation. Recently, Koren (14) showed that flux decline resulting from CaSO4 scaling is accelerated in the presence of biofilm. Treatment of tertiary water by RO is currently considered as an important supplementary source for drinking water. Treated municipal wastewater contains significant concentrations of sparingly soluble salts such as calcium carbonate and calcium phosphate. Concentration of the latter varies over a wide range, depending on the wastewater source. At Shafdan, Israel’s largest wastewater-reclamation plant, total phosphorus concentration ranges between 0.9 and 2.8 ppm (12). Calcium-carbonate scaling is controlled by using antiscalants, or by slight acidification of the feed. On the other hand, calcium phosphate scaling on membranes is almost unavoidable due to a lack of suitable antiscalants (15). Ning and Troyer describe colloidal fouling of RO membranes by calcium phosphate nanoparticles passing through MF/UF pretreatment in wastewater reclamation plant (16). Autopsy of the membranes showed 20% organic matter and 80% inorganics, primarily calcium phosphate, hence interactions between colloidal calcium phosphate and organic matter might have an important role in phosphate scaling on RO. Calcium-phosphate mineralization has been extensively studied in the context of bone tissue buildup. It has been suggested to be a complex process involving the formation of amorphous calcium phosphate (ACP) followed by transformation to hydroxyapatite, possibly through an autocatalytic phase transition that involves dissolution and renucleation (17-19). The effects of organic-inorganic interfacial interactions on calcium-phosphate mineralization were reviewed by Sato (20) by Xu et al. (21) and by many others, particularly in the context of bioinspired mineralization (22, 23). In an earlier study, Tanahashi and Matsuda evaluated the rate of apatite formation on gold surfaces, self-assembled with alkane thiols carrying different functional groups (24). They found that in a simulated body fluid, the growth rate of apatite depends on the exposed functional groups, following the order PO4H2 > COOH . CONH2 ∼ OH > NH2 . CH3, with the phosphate group inducing the fastest mineralization. A recent study by our group showed that calcium-phosphate mineralization on a monolayer of acidic β-sheet peptides equilibrated with simulated body fluid, begins in the amorphous phase that is transformed into octacalcium phosphate and apatite over time (25). In the present study, we elucidate the effects of surfaceexposed organic functional groups on calcium-phosphate precipitation in wastewater desalination. Using model surfaces such as Langmuir films and self-assembled monolayers (SAMs), we study mineralization in a model solution with an ionic profile simulating secondary wastewater effluents (named SSE solution). Understanding of the chemical interactions between scale-forming minerals and organic molecules would enable the development of more efficient ways to tackle scaling on RO membranes in water treatment processes. VOL. 44, NO. 20, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Experimental Section Chemicals. We purchased 1-octadecanol, stearic acid, 1,2dipalmitoyl-sn-glycero-3-phosphate (DPGP), octadecylamine, triethylene glycol monooctadecyl, 1,4-piperazinediethanesulfonic acid (PIPES), 11-mercapto-1-undecanol, 1-dodecanethiol, potassium phosphate monobasic and ethanol from Sigma-Aldrich (St Louis, MO). Sodium chloride, sodium sulfate, and magnesium chloride hexahydrate were purchased from Frutarom (Haifa, Israel). Sodium hydrogen carbonate, chloroform, and methanol were purchased from Bio-Lab (Jerusalem, Israel). Hydrochloric acid was purchased from Gadot (Netanya, Israel). Calcium chloride dihydrate was purchased from Carlo Erba Reagents (Rodano, Italy). The 11-mercaptoundecanoic acid, and 11-amino-1-undecanethiol, were purchased from Asemblon (Redmond, WA). Preparation of SSE Solution. A model solution, simulating the yearly average mineral content of the Tel Aviv region wastewater reclamation plant (12; Shafdan, Israel) was used. The solution was prepared by consecutively dissolving 17.1 mg KH2PO4, 460 mg NaCl, 2.5 mL 1 M HCl, 1370 mg MgCl2 · 6H2O, 1645 mg CaCl2 · 2H2O, and 663.5 mg Na2SO4 in 500 mL deionized water. The pH was elevated to 7.0 by slowly adding 835 mg NaHCO3 and then 20 mL of 0.5 M, pH 6.9, PIPES buffer. Water was then added to a final volume of 1 L. Final ion concentrations in SSE solution were: 448 mg/L Ca2+, 164 mg/L Mg2+, 623 mg/L Na+, 5 mg/L K+, 606 mg/L HCO3-, 451 mg/L SO42-, 1639 mg/L Cl-, 12 mg/L PO43-, 1211 mg/L PIPES buffer, with total dissolved solids (TDS) of 5160 mg/L. Before immersion and monolayer equilibration experiments, the solution was filtered through a 0.22 µm PVDF filter, and stored at room temperature. Dynamic Light Scattering. Light scattering of the SSE solution was measured 1, 5, and 8 days after preparation. Spectra were collected using a CGS-3 light-scattering spectrometer equipped with an LSE-5003 digital correlator (ALV, Langen, Germany). The laser power was 20 mW at the He-Ne laser line (632.8 nm). The laser beam was passed through a quartz cell holding the solutions. Scattering intensity was detected 20 times for 10 s each time at angles of 90-150° with respect to the incident beam, and calculated by ALV/ LSE 5003 correlator. The correlograms were Laplace-inverted with the program CONTIN (26). Light-scattering intensities were normalized to laser power. Particles detectable by the instrument are at a size range of 0.5 nm to 1 µm. However, nanoparticles smaller than 5 nm may not be detected at low concentrations due to sensitivity limitation. Langmuir Isotherms. A mini-trough (KSV Instruments, Helsinki, Finland) was filled with deionized water or with SSE solution and kept at a constant 25 °C. A solution of 0.1-0.3 mg/mL analyte in chloroform (except DPGP which was dissolved in a methanol:chloroform 1:4 v/v mixture) was then spread over the subphase. Film compression (Langmuir isotherm) was initiated after 15 min, to allow the solvent to evaporate and the analyte molecules to arrange in an equilibrium state. Similar experiments were carried out on monolayers equilibrated for 16 or 30 h (where in various cases mineralization could be detected by a naked eye). During the isotherm experiment, two barriers, placed on either side of the trough, compressed the monolayer while moving toward the center of the trough at a constant rate of 3 mm/min. Changes in surface pressure (reduction in surface tension) were constantly monitored using a Wilhelmy plate and presented as a function of the mean molecular area. At the end of the compression, the film was collected from the surface with a spatula, dissolved in 3 mL of 20 mM HCl solution, PVDF-filtered (0.22 µm) and analyzed for the chemical composition of the minerals with a Varian 720-ES ICP optical emission spectrometer (Varian Inc., Walnut Creek, CA). 7938
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Brewster Angle Microscopy (BAM). A Brewster angle microscope(EP3SW-BAM,NFT,Gottingen,Germany)mounted on the Langmuir film balance was used to observe the structures at the interface, in situ. The light source of the BAM was a frequency-doubled Nd:YAG laser with a wavelength of 532 nm and 50 mW primary output power in a collimated beam. The BAM images were recorded with a CCD camera. The scanner objective was a Nikon super long working distance objective with nominal 20× magnification and a diffraction-limited lateral resolution of 1 µm. The images were corrected to eliminate side ratio distortion originating from the microscope’s nonperpendicular line of vision. Preparation of Self-Assembled Monolayers of Alkanethiols on Gold. Gold-coated silicon surfaces were prepared by coating silicon wafers (one side polished, 330 µm thick) with 10 nm titanium (99.995%, Kurt J. Lesker) followed by 30 nm gold (99.999%, Kurt J. Lesker) by evaporation. Both metals were evaporated thermally at a pressure equal to 2 × 10-6 bar using a thermal evaporator (Odem Ltd., Rehovot, Israel). Prior to coating, the silicon substrates were cleaned in acetone, then methanol, and then isopropanol in an ultrasonic bath (Bendeline Sonorex, London, England). Next, these were subjected to oxygen plasma cleaning for 5 min (0.4 mbar of oxygen pressure in chamber). Prior to self-assembly, the gold-coated silicon wafers were cleaned twice for 10 min in the ultrasonic bath in toluene, acetone, and ethanol. The wafers were then dried with N2 gas and placed in an ozone generator (Bioforce Nanosciences, Ames, IA) for 30 min. After cleaning, the wafers were immersed in 1 mM alkanethiol with various different terminal groups in ethanol solution for 24 h at room temperature. The wafers were then transferred to 1 mM dodecanethiol in ethanol for additional 24 h. At the end of the second immersion step, the wafers were dried in N2 gas and stored in a drybox. Contact Angle Measurements. Static contact angle of water under air for the different SAMs was measured using OCA 20 (Dataphysics Products, Filderstadt, Germany). Water drop size was 0.3 µL. At least four measurements were taken for each sample. The contact angle was extracted by SCA 20 software. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed with an ESCALAB 250 (Thermo Fisher Scientific Inc., Waltham, UK) using an Al X-ray source and a monochromator. General survey spectra and high-resolution spectra of elements were recorded. Binding-energy measurements for the elements were corrected for the charging effect with reference to the C1s peak at 284.6 eV. Long-term Immersion Experiments. SAMs of alkanethiols on gold surfaces (1 cm2) were immersed in 50 mL SSE solution for 10 or 20 days at room temperature. The wafers were then dried under N2 flow and stored under vacuum. The formed precipitate was studied by Hyperion microscope connected to a VERTEX 70 FT-IR spectrometer at a resolution of 4 cm-1 (Bruker Optics Inc., Ettlingen, Germany). X-ray diffraction (XRD) of the precipitate was performed on D8 Discovery (Bruker AXS Inc. Madison, WI) to study the crystal structure.
Results and Discussion The bulk concentrations of secondary effluents in Shafdan wastewater treatment plant are 85 mg/L Ca2+, 32 mg/L Mg2+, 314 mg/L Cl-, 4.6 mg/L PO43-, and 899 mg/L TDS, based on previous publications (12, 13) by Mekorot. SSE solution concentrations (see Experimental Section) are 5.2-fold higher than the bulk values of Shafdan secondary effluents in order to mimic a stage of 80-85% recovery in desalination, as well as concentration polarization occurring at the membranesolution interface. The concentration factor for phosphate
FIGURE 1. Langmuir isotherms of the various amphiphilic compounds at 25 °C on subphase of water (solid gray line), on SSE equilibrated before compression for 15 min (dash dotted black line), of SSE equilibrated before compression for 16 h (solid black line) or 30 h (only for stearic acid, dashed gray line). Amphiphilic compounds were stearic acid (A), 1-octadecanol (B), octadecylamine (C), 1,2-dipalmitoyl-sn-glycero-3-phosphate (DPGP; D), 1:1 molar ratio mix of stearic acid and octadecylamine (E), and triethylene glycol monooctadecyl (F). Insert in graph A presents Brewster angle microscopy (BAM) images of stearic acid film taken at 25.7 Å2/molecule over water (top), over SSE with 15 min equilibration (middle), and 16 h equilibration (bottom). A tangent line drawn at the steepest slope of the curve (dotted red line in graph C) is used for determination of limiting area per molecule. ion is lower than 5.2 since part of the phosphate is removed by the coagulation and filtration commonly applied as pretreatment at the Shafdan pilot plant. SSE solution contains a buffer to maintain the pH at 7.0. The solution was oversaturated with respect to hydroxyapatite [Ca5(PO4)3(OH)] and close to saturation for calcium carbonate (scaling tendency of 0.94), as calculated by OLI stream analyzer 2.0 software (OLI Systems, Morris plains, NJ). Sulfate minerals were far from saturation and not expected to precipitate. Possible aggregation in the solution was evaluated by particle size measurements using dynamic light scattering, showing virtually no particles with sizes of 0.5 nm to 1 µm after one day. Hence, no extensive homogeneous mineralization is expected during mineralization experiments, except for nanoparticles smaller than 5 nm (see Experimental Section). Measurements taken after five and eight days showed a small increase in particle concentration in a sample that was kept in a closed flask and a significant increase in particle concentration in a sample that was left open (data not shown). The latter observation is attributed to an increase in pH up to 8.2 due to CO2 release from the solution. The stability measurements therefore showed that the solution remains essentially stable at room temperature for at least 24 h, even if kept open to the atmosphere. However, the solution remained stable for longer times when kept in a sealed bottle. Variations in the concentrations of dissolved ions in sealed SSE were monitored by ICP analysis of aliquots taken at specified time intervals over the course of 11 days. No
detectable variations in ion concentrations were observed (data not shown), indicating that no appreciable precipitation occurred in the solution. Interactions between hydrophilic groups of five amphiphilic molecules and the model solution at the liquid-air interface were studied using Langmuir films. The molecules are a phospholipid, a fatty acid, amine- and hydroxylterminated fatty acid derivatives, and triethylene glycol monooctadecyl (Figure 1). These compounds were spread over deionized water or the SSE solution and surface pressuremolecular area (π-A) isotherms were recorded shortly after film formation or after equilibration for 16 and 30 h with the solution. Langmuir isotherms of the molecules at the air-water and air-SSE solution interfaces were analyzed with respect to the area per molecule corresponding to onset in surface pressure that is, the area per molecule at which the first increase of ∼0.5-1 mN/m in surface pressure is observed. The limiting area per molecule, defined by the intersection of molecular area axis with a tangential line drawn at the steepest slope in the isotherm (see for example, dotted line in Figure 1C) denotes the state at which molecules are most closely packed. Beyond the limiting area per molecule, further compression leads to film collapse corresponding to the state at which the monolayer is converted into a multilayer (27). The area per molecule corresponding to the onset of surface pressure, the limiting area per molecule and the surface pressure at the collapse point are expected to be different on VOL. 44, NO. 20, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Limiting Area Per Molecule, Collapse Surface Pressure and Compressibility in the Region of Maximum Slope of the Various Amphiphilic Films Extracted from Figure 1, and P/Ca Concentration Ratio Determined by ICP Analysis of the Mineralized Film Collected after Completion of Each Langmuir-Isotherm Measurement on SSE Solution
experimenta
substance
limiting area % increase of minimum P/Ca (w/w) in per molecule limiting area Π at collapse compressibility mineralized (Å2/molecule) (compared to water) (mN/m) (m/N) filmc
stearic acid
water SSE 0 h SSE 16 h SSE 30 h
21 28 35 37
33% 67% 76%
52 44 48 35
0.9 9.2 8.7 6.5
0.04 0.36 0.43
1-octadecanol
water SSE 0 h SSE 16 h
19 19 21
0% 10%
52 48 53
0.9 2.1 2.7
0.01 0.13
octadecylamine
water SSE 0 h SSE 16 h
11 19 29
53%b
53 ∼57 45
3.0 2.4 5.2
0.01 0.38
DPGP
water SSE 0 h SSE 16 h
32 41 59
28% 84%
52 ∼47 ∼35
1.2 6.7 10.0
0.03 0.42
triethylene glycol monooctadecyl
water SSE 0 h SSE 16 h
26 26 ∼34
0% 31%
37 37 ∼37
13.1 11.1 17.8
0.01 0.02
stearic acid: octadecylamine 1:1 mix
water SSE 0 h SSE 16 h
17 19 26
57 60 60
1.7 2.4 7.2
0.01 0.21
37%b
a SSE 0 h, SSE 16 h, SSE 30 h, isotherm experiments over SSE as subphase and time of pre-equilibration. Water isotherm experiments over water as subphase. b Percent increase in limiting area per molecule for octadecylamine and for octadecylamine:stearic acid mix after 16 h equilibration was calculated relative to SSE 0 h rather than to water (see text). c P/Ca(hydroxyapatite) ) 0.46; P/Ca(SSE solution) ) 0.01. ICP measurement error bar is 2.6% at a confidence level of 99%.
water and on SSE due to interactions with ions in the latter (Figure 1; Table 1). An additional measure for the possible effect of mineralization at the interface is the minimum compressibility (k) of the monolayer systems (Figure 1), that is, the normalized change of the mean molecular area as a function of applied surface pressure. This was calculated in the region of steepest slope of the isotherm (28): k)-
1 ∆A A ∆π
( )
T
(1)
∆A/∆π is the inverse maximum slope of the isotherm around the region of the limiting area per molecule, T is the temperature. It was recently shown that Langmuir isotherms of β-sheet peptide monolayers on a simulated body fluid solution display a different curve to that of peptide on water (25). These differences were interpreted as an indication of the peptide monolayer becoming mineralized by calcium phosphate in the course of incubation. According to the π-A isotherms (Figure 1) all the amphiphilic compounds exhibited an onset in surface pressure at higher area per molecule when measured after 16 h of pre-equilibration relative to isotherms taken after 15 min equilibration. This appears to result from accumulation of ions from the SSE solution on the interface, where they interact with the hydrophilic end groups of the monolayers exposed to the solution. In all cases, the limiting area per molecule (Table 1) was larger for the SSE solution than for water; it was significantly higher for monolayers of negatively charged terminal groups of phosphatidic and carboxylic acids (Figure 1D and A, respectively) compared to the other groups. The phospholipid DPGP (Figure 1D) showed an increase in limiting area per molecule from 32 Å2 in water to 59 Å2 (84%) when equilibrated for 16 h with SSE. The stearic acid (Figure 7940
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1A) increased from 21 Å2 in water to 35 Å2 in SSE (67%). Phosphatidic acid and stearic acid monolayers (Figure 1D and A, respectively) showed a significant increase compared to water even after 15 min equilibration time, from 32 Å2 in water to 41 Å2 in SSE (28%) for the former and from 21 Å2 to 28 Å2 (33%) for the latter. In the case of the positively charged octadecylamine (Figure 1C), an increase from 19 Å2 in SSE solution after 15 min of equilibration to 29 Å2 in SSE after 16 h equilibration (53% increase) was observed. This increase is attributed to SSE since octadecylamine seems to partially dissolve in deionized water, showing a limiting area per molecule (Figure 1C) that is smaller than the projected area of a methylene chain (∼18-20 Å2). Uncharged terminal groups of triethylene glycol (Figure 1F) showed a 31% increase (26 Å2 to ∼34 Å2) in the limiting area per molecule on SSE after 16 h equilibration as compared to water. An equimolar mixture of stearic acid and octadecylamine (Figure 1E) exhibited a 37% increase in the limiting area per molecule on SSE after 16 h equilibration as compared to isotherms taken right after film formation on SSE (19 Å2 and 26 Å2, respectively). Interestingly, octadecanol monolayer (Figure 1B) showed a significant increase in the onset in surface pressure after 16 h incubation, indicative of adsorption of ions to the film. However, subsequent compression probably released these ions from the film, as the limiting area per molecule eventually showed almost the same value for water and for SSE after 16 h, of 19 Å2 and 21 Å2, respectively. In summary, after 16 h incubation on SSE the increase in limiting area per molecule varied between the monolayers, from 84% for DPGP and 67% for stearic acid, to 53% for the positively charged octadecylamine, and down to 31% for triethylene glycol monooctadecyl and almost no increase for octadecanol.
An increase by approximately 1 order of magnitude in the compressibility (eq 1) of the stearic acid film (Table 1) indicated a significant change in molecular packing due to interactions with the SSE. These results were supported by BAM measurements providing visualization of the morphological changes of the stearic acid films on water and on the SSE solution (Figure 1A inset). Image brightness, determined by the amount of light reflected from the surface, suggests that adsorption and accumulation of SSE ions at the interface, as well as mineral formation, were already in progress in the film equilibrated for only 15 min. The highly bright images obtained after 16 h equilibration indicate significant thickening of the film, as indeed in this case the film was visible to the naked eye. DPGP film compressibility increased by 2 orders of magnitude when equilibrated for 15 min on SSE solution as compared to water (Table 1), indicating a pronounced effect of the SSE solution and ionic interactions on the molecular packing. The compressibility of this film further increased (nearly doubled) after 16 h of equilibration. Uncharged films of 1-octadecanol and triethylene glycol showed a relatively small change in isotherm shape, suggesting limited interactions with the solution. Octadecylamine did not change its compressibility on SSE solution compared to water; however, the increase in limiting area and collapse pressure suggested that its solubility had decreased dramatically in this case. In general, longer equilibration time tended to increase the compressibility of the films. Surface pressure at the collapse point decreased significantly after long equilibration times over SSE solution as compared to water for stearic acid (52-35 mN/m), octadecylamine (53-45 mN/m) and DPGP (52-35 mN/m; see Table 1). Collapse at a lower pressure may be explained by lower ability of the organic-mineral phase to tolerate the applied compression force without undergoing irreversible deformation compared to the more elastic properties of the pure organic film on water. ICP analyses of the interfacial films collected at the end of each Langmuir isotherm experiment (Table 1) indicate a significant increase in phosphorus to calcium weight ratio (P/Ca) after 16 h equilibration, following the order: DPGP > stearic acid ∼ octadecylamine > stearic acid:octadecylamine mixture > octadecanol > triethylene glycol monooctadecyl. Noteworthy, P/Ca ratio of the mineralized DPGP monolayer after 16 h of equilibration were very close to the theoretical ratio of hydroxyapatite (0.46). Other common calciumphosphate minerals such as octacalcium phosphate (P/Ca ) 0.58) and dicalcium phosphate (P/Ca ) 0.77) have higher P/Ca ratios than those measured here. Equilibrating stearic acid with SSE solution for 30 h showed P/Ca ratios similar to those found for DPGP after 16 h (0.43 and 0.42, respectively). Octadecylamine and stearic acid showed comparable P/Ca ratios (0.38 and 0.36, respectively) after 16 h of equilibration. Phosphorus content of the stearic acid-octadecylamine mixture was less than a third of the content found in stearic acid and octadecylamine alone. The P/Ca ratio increased for octadecanol and triethylene glycol monooctadecyl, but to a significantly lower extent. P/Ca ratios obtained by ICP were consistently lower than the calculated values for hydroxyapatite. This may result either from presence of SSE solution with the film collected, or from isomorphic substitutions of phosphate by carbonate in the mineral. Calcium-carbonate coprecipitation did not appear to be significant, as was evident by XRD analysis (Figure 2B). Nevertheless, the P/Ca values were close to pure hydroxyapatite, especially in the case of DPGP, suggesting that hydroxyapatite is the mineral formed at the interface of SSE and the film. SAMs of alkane thiols on gold were prepared and used to study mineralization on solid surfaces in SSE solution. The
FIGURE 2. Characterization of SAMs on gold surfaces after immersion in SSE solution. (A) ATR-IR spectra taken under a microscope of surfaces terminated with methyl, hydroxyl, amine, and carboxylic acid functional groups after 10 days of immersion in SSE solution. (B) XRD pattern of SAMs on gold surfaces terminated with hydroxyl, amine and carboxyl functional groups after 10 days (left) or 20 days of immersion (right) in SSE solution.
TABLE 2. Static Contact angle and XPS Surface Elemental Composition of SAMs of Functionalized-Alkanethiols on Gold-Coated Silicon Wafers functionalized thiol used for self-assembly
contact angle (deg)a
Au (%)b
C (%)b
O (%)b
S (%)b
HS-(CH2)11-CH3 HS-(CH2)11-NH2 HS-(CH2)11-OH HS-(CH2)10-COOH
106° ( 1° 63° ( 3° 45° ( 3° 42.5° ( 3°
55.7 32.7 52.2 41.8
40.3 50.1 40.5 45.2
0.9 9.3 4.9 11.1
3.0 4.1 2.3 2.0
N (%)b 3.1
a Contact angle values are the average of five surfaces. A minimum of four different water drops were used to characterize each surface. b Percent determined by XPS.
preparation of SAMs on gold was based on the study by Prime and Whitesides (29), including immobilization of four alkanethiols with different functional end groups (Table 2) on gold, followed by a second immobilization step of dodecanethiol to ensure maximal coverage of the gold by alkanethiols. The success of preparation of SAMs on gold was evaluated by measuring the static contact angle of a water drop under air and by XPS analysis (Table 2). The results show that the monolayers had been formed successfully. The observed water drop contact angles were higher (lower wettabilities) than values found in the literature (24, 30), VOL. 44, NO. 20, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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probably due to the second immobilization step with the hydrophobic dodecanethiol. For crystallization experiments, SAMs of alkanethiols were immersed in SSE solution for 10 and 20 days. At the end of the immersion period, the gold-coated silicon wafers were dried by N2 flow and the surfaces were characterized by ATRIR and XRD. ATR-IR was measured under a microscope, focusing on locations with apparently crystalline precipitates (Figure 2). Phosphates generate the most intense bands in the 1000-1200 cm-1 region (31-33). The IR peak at 1059 cm-1 found in NH2- and COOH-terminated samples may be attributed to carbonate-substituted apatite (34). The peak at 1179 cm-1 was associated with poorly crystalline hydroxyapatite (32). A small peak at 1466 cm-1 was attributed to asymmetric stretch of the carbonate CO bond (35). It should be noted that the 1059/1466 peak-intensity ratio remained essentially constant in all samples and that the IR peak intensities could not be quantitatively related to the extent of mineralization since the IR spectra were taken on specific crystalline locations. In summary, peaks attributed to hydroxyapatite were found in the IR spectra (Figure 2) of SAMs terminating with amine or carboxylic acid after 10 days. The same peaks were also found for the hydroxyl-terminating SAMs after 20 days of immersion but not after 10 days. No phosphate-IR absorption peaks appeared for the methylterminating SAM. XRD was performed for the SAMs of alkanethiols after mineralization experiments in SSE solution (Figure 2B). A diffraction peak observed at 2θ ) 31.7° is characteristic of hydroxyapatite. In general, the XRD data supported the ATRIR data, and confirmed that the major mineral precipitating on the SAMs in SSE solution is hydroxyapatite. Noteworthy, attempts made to wash the wafer with clean water or ethanol at the end of the immersion period resulted in partial dissolution of some of the mineral precipitates and decreased the reproducibility of the experiments. Langmuir isotherms (Figure 1) and ICP measurements of the film formed above SSE solution (Table 1) showed that charged surfaces induce calcium-phosphate mineral formation, compared to uncharged surfaces. These results were in agreement with data obtained in the long immersion experiments of gold-coated silicon wafers in SSE solution (Figure 2). Over all, quantitative assessment of calcium phosphate mineralization by Langmuir isotherms and ICP analysis showed acceleration by surface-exposed functional groups at the following order: PO4 > COOH ∼ NH2 > COOH: NH2 (1:1) > OH > ethylene glycol. ICP analyses of films collected from the SSE interface after long immersion experiments indicated virtually no difference between mineralization influenced by the positively charged octadecylamine and the negatively charged stearic acid. This observation was quite surprising since data found in the literature on calcium-phosphate mineralization in solutions simulating human blood plasma (Simulated body fluid; SBF) indicate accelerated precipitation only on negatively charged surfaces. Sato (20) previously studied hydroxyapatite crystallization on Langmuir films of arachidic acid and octadecylamine above SBF solution and found that arachidic acid promotes hydroxyapatite crystallization, whereas octadecylamine inhibits it. Similar data were obtained by Tanahashi and Matsuda (24), measuring hydroxyapatite mineralization on SAMs on gold surfaces by XPS and quartz crystal microbalance in long immersion experiments using SBF solution. One possible mechanism for induced nucleation in the case of negatively charged surfaces involves accumulation of calcium cations near the surface, followed by phosphate ions that adhere to the calcium and form nucleation sites. Another mechanism is based on premineralized nanoparticles that adhere to the surface. Onuma et al. studied 7942
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nucleation of calcium phosphate on SAMs, and found that nanodots of 5-10 nm diameter are initially arranged twodimensionally on the surface, followed by random adherence of 20-30 nm particles (36). Zhu et al. (37) investigated apatite deposition on SAMs having aromatic linkers terminating with NH2, OH and COOH. Measurements were performed using a quartz crystal microbalance in a 1.5× concentrated SBF solution at 50 °C at different pH values. They found that at pH values lower than 7.4, nucleation occurs mostly on the negatively charged surfaces while at pH values of 7.4-7.6 nucleation occurs mostly on the positively charged amine surfaces. Under pH 7.4-7.6 the solution is oversaturated and apatite precipitates spontaneously in solution. The authors (37) proposed a twostep mechanism for mineralization on positively charged surfaces in which small, negatively charged calciumphosphate particles are formed in solution and adhere to the positively charged surface, and in a second step a thin film is formed by progressive growth of the microparticles. This mechanism may also be applicable to the current study; although dynamic light scattering showed no significant nucleation in the SSE solution (see above), it does not rule out the formation of nanoparticles smaller than 5 nm. Since the SSE solution is oversaturated, aggregation of calcium phosphate may occur at the nanometer scale, generating negatively charged colloids that could adhere to the positively charged amine surface and form nucleation sites. The fact that calcium-phosphate precipitation rate decreases when both carboxylic acid and amine are present on the surface, may be taken into consideration in designing low-scaling surfaces. Results from the current study show that calcium-phosphate scaling rate is dramatically reduced on surfaces having equal concentrations of positively charged amine and negatively charged carboxyl groups. Interestingly, the presence of equal surface concentrations of positively and negatively charged moieties reduced organic fouling as well, as was recently shown for polymeric hydrogels (38). Hence, surface functionalization by mixed positively and negatively charged groups constitutes a general approach to reducing both calcium-phosphate scaling as well as organic fouling when membrane-based wastewater treatment is considered. Reduction of scale formation on mixed ammonium and carboxylate surfaces may be explained by the formation of an internal neutral salt in which the influence of negative and positive charges on scaling is minimized.
Acknowledgments We are indebted to the German-Israeli BMBF-MOST Water Research Program (grant WT0902; R.K. and H.R.) for financial support. MSc scholarship (Z.S.) was supported by the Max Efrimzon fund and the Roy Zuckerberg foundation. We thank Dr. Jack Gilron (Ben-Gurion University of the Negev, Israel) for discussions on chemical composition of secondary wastewater effluents, Mr. Costa Abarbanel for the gold coatings, Dr. Sofia Kolusheva, Dr. Sharon Vanounou, Dr. Natalia Froumin, Dr. Ludmila Katz and Vladimir Vaiser (BenGurion University of the Negev) for their assistance with the analytical equipment.
Supporting Information Available Stability measurements of SSE solution by dynamic light scattering; full sized BAM images of films on Langmuir experiment; detailed XPS surface elemental analyses with specific bond attributions of self-assembled monolayers on gold-coated silicon wafers. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Lee, S.; Lee, C. H. Scale formation in NF/RO: mechanism and control. Water Sci. Technol. 2005, 51, 267–275.
(2) Kilduff, J. E.; Mattaraj, S.; Zhou, M. Y.; Belfort, G. Kinetics of membrane flux decline: The role of natural colloids and mitigation via membrane surface modification. J. Nanopart. Res. 2005, 7, 525–544. (3) Tang, C. Y.; Kwon, Y. N.; Leckie, J. O. Fouling of reverse osmosis and nanofiltration membranes by humic acidsEffects of solution composition and hydrodynamic conditions. J. Membr. Sci. 2007, 290, 86–94. (4) Li, Q. L.; Elimelech, M. Organic fouling and chemical cleaning of nanofiltration membranes: Measurements and mechanisms. Environ. Sci. Technol. 2004, 38, 4683–4693. (5) Violleau, D.; Essis-Tome, H.; Habarou, H.; Croue, J. P.; Pontie, M. Fouling studies of a polyamide nanofiltration membrane by selected natural organic matter: an analytical approach. Desalination 2005, 173, 223–238. (6) Flemming, H. C. Reverse osmosis membrane biofouling. Exp. Therm. Fluid Sci. 1997, 14, 382–391. (7) Flemming, H. C.; Schaule, G.; Griebe, T.; Schmitt, J.; Tamachkiarowa, A. Biofoulingsthe Achilles heel of membrane processes. Desalination 1997, 113, 215–225. (8) Baker, J. S.; Dudley, L. Y. Biofouling in membrane systemssA review. Desalination 1998, 118, 81–89. (9) Contreras, A. E.; Kim, A.; Li, Q. L. Combined fouling of nanofiltration membranes: Mechanisms and effect of organic matter. J. Membr. Sci. 2009, 327, 87–95. (10) Hong, S. K.; Elimelech, M. Chemical and physical aspects of natural organic matter (NOM) fouling of nanofiltration membranes. J. Membr. Sci. 1997, 132, 159–181. (11) Seidel, A.; Elimelech, M. Coupling between chemical and physical interactions in natural organic matter (NOM) fouling of nanofiltration membranes: implications for fouling control. J. Membr. Sci. 2002, 203, 245–255. (12) Glueckstern, P.; Priel, M.; Gelman, E.; Perlov, N. Wastewater desalination in Israel. Desalination 2008, 222, 151–164. (13) Priel, M.; Gelman, E.; Glueckstern, P.; Balkwill, A.; David, I.; Arviv, R. Optimization of wastewater desalination. Desalin. Water Treat. 2009, 7, 71–77. (14) Koren, O. The effect of biofouling on scale formation in RO membranes. M.Sc. thesis, Department of Desalination and Water Treatment, Ben-Gurion University of the Negev, Israel, 2008. (15) Greenberg, G.; Hasson, D.; Semiat, R. Limits of RO recovery imposed by calcium phosphate precipitation. Desalination 2005, 183, 273–288. (16) Ning, R. Y.; Troyer, T. L. Colloidal fouling of RO membranes following MF/UF in the reclamation of municipal wastewater. Desalination 2007, 208, 232–237. (17) Boskey, A. L.; Posner, A. S. Conversion of amorphous calcium phosphate to microcrystalline hydroxyapatitesPh-dependent, solution-mediated, solid-solid conversion. J. Phys. Chem. 1973, 77, 2313–2317. (18) Posner, A. S.; Betts, F. Synthetic amorphous calcium-phosphate and its relation to bone-mineral structure. Acc. Chem. Res. 1975, 8, 273–281. (19) Yin, X. L.; Stott, M. J. Biological calcium phosphates and Posner’s cluster. J. Chem. Phys. 2003, 118, 3717–3723. (20) Sato, K. Inorganic-organic interfacial interactions in hydroxyapatite mineralization processes. In Biomineralization I: Crystallization and Self-Organization Process; Topics in
(21) (22) (23) (24)
(25)
(26) (27) (28) (29) (30) (31) (32) (33)
(34) (35) (36)
(37)
(38)
Current Chemistry; Springer-Verlag: Berlin, 2007; Vol. 270, pp 127-153. Xu, A.-W.; Ma, Y.; Colfen, H. Biomimetic mineralization. J. Mater. Chem. 2007, 17, 415–449. Addadi, L.; Weiner, S. Control and design principles in biological mineralization. Angew. Chem., Int. Ed. Engl. 1992, 31, 153–169. Weiner, S.; Addadi, L.; Wagner, H. D. Materials design in biology. Mater. Sci. Eng., C 2000, 11, 1–8. Tanahashi, M.; Matsuda, T. Surface functional group dependence on apatite formation on self-assembled monolayers in a simulated body fluid. J. Biomed. Mater. Res. 1997, 34, 305– 315. Segman-Magidovich, S.; Grisaru, H.; Gitli, T.; Levi-Kalisman, Y.; Rapaport, H. Matrices of acidic beta-sheet peptides as templates for calcium phosphate mineralization. Adv. Mater. 2008, 20, 2156–2161. Provencher, S. W. ContinsA general-purpose constrained regularization program for inverting noisy linear algebraic and integral-equations. Comput. Phys. Commun. 1982, 27, 229–242. Birdi, K. S., Ed. Handbook of Surface and Colloid Chemistry; 3rd ed.; CRC Press: Boca Raton, FL, 2009. Isenberg, H.; Kjaer, K.; Rapaport, H. Elasticity of crystalline betasheet monolayers. J. Am. Chem. Soc. 2006, 128, 12468–12472. Prime, K. L.; Whitesides, G. M. Self-assembled organic monolayerssModel systems for studying adsorption of proteins at surfaces. Science 1991, 252, 1164–1167. Sigal, G. B.; Mrksich, M.; Whitesides, G. M. Effect of surface wettability on the adsorption of proteins and detergents. J. Am. Chem. Soc. 1998, 120, 3464–3473. Fowler, B. O. Infrared studies of apatites 0.1. Vibrational assignments for calcium, strontium, and barium hydroxyapatites utilizing isotopic-substitution. Inorg. Chem. 1974, 13, 194–207. Pleshko, N.; Boskey, A.; Mendelsohn, R. Novel infrared spectroscopic method for the determination of crystallinity of hydroxyapatite minerals. Biophys. J. 1991, 60, 786–793. Rey, C.; Shimizu, M.; Collins, B.; Glimcher, M. J. Resolutionenhanced fourier-transform infrared-spectroscopy study of the environment of phosphate ion in the early deposits of a solidphase of calcium-phosphate in bone and enamel and their evolution with age 0.2. Investigations in the Nu-3 Po4 domain. Calcif. Tissue Int. 1991, 49, 383–388. Ou-Yang, H.; Paschalis, E. P.; Boskey, A. L.; Mendelsohn, R. Two-dimensional vibrational correlation spectroscopy of in vitro hydroxyapatite maturation. Biopolymers 2000, 57, 129–139. Kaczorowska, B.; Hacura, A.; Kupka, T.; Wrzalik, R.; Talik, E.; Pasterny, G.; Matuszewska, A. Spectroscopic characterization of natural corals. Anal. Bioanal. Chem. 2003, 377, 1032–1037. Onuma, K.; Oyane, A.; Kokubo, T.; Treboux, G.; Kanzaki, N.; Ito, A. Nucleation of calcium phosphate on 11-mercaptoundecanoic acid self-assembled monolayer in a pseudophysiological solution. J. Phys. Chem. B 2000, 104, 11950–11956. Zhu, P. X.; Masuda, Y.; Yonezawa, T.; Koumoto, K. Investigation of apatite deposition onto charged surfaces in aqueous solutions using a quartz-crystal microbalance. J. Am. Chem. Soc. 2003, 86, 782–790. Chen, S. F.; Jiang, S. Y. A new avenue to nonfouling materials. Adv. Mater. 2008, 20, 335–338.
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