|Collection Mémoires et thèses électroniques|
|AccueilÀ proposNous joindre|
Table des matières
Dans le chapitre précédent, la présence de deux types de colmatage, soit protéique et minéral, a été démontrée. La présence ou non de ces colmatages sur les CEM dépendait des conditions du milieu. Ce chapitre, rédigé sous forme d’article a comme but la confirmation de la nature des colmatages formés sur la CEM ainsi que la caractérisation et la description morphologique du colmatage minéral.
L’objectif de ces travaux était l’étude de l’effet du pH du concentrat et la composition minérale d’une solution de protéines de lactosérum traitée par ÉD conventionnelle sur le colmatage des MEC. Une attention particulière a été portée aux possibles mécanismes responsables de la formation du colmatage. Les membranes utilisées pour la déminéralisation des diluats contenant CaCl2 et CaCl2 + Na2CO3 ont présenté un colmatage sur les deux surfaces quand le pH du concentrat avait été maintenu à 12. Ces dépôts étaient composées de carbonate de calcium et/ou d’hydroxyde de calcium. Le colmatage minéral avait précipité sous forme d’agrégat de petits cristaux rhomboédriques quand le diluat contenait seulement CaCl2, tandis que celui avec CaCl2 + Na2CO3 présentait une forme plus sphérique et une texture plus fine. Seule la membrane du traitement avec un diluat contenant CaCl2 + Na2CO3 et un concentrat à pH 2 a présenté un colmatage protéique.
The aim of this work was to study the effect of the concentrate solution pH value and of the composition in calcium, carbonate and protein of the diluate solution to be treated by conventional electrodialysis on the fouling of cation-exchange membranes (CEM). It appeared that after demineralization of solutions containing CaCl2 and CaCl2 + Na2CO3 using a concentrate solution maintained at a pH of 12, a mineral fouling appeared on both sides of the CEM. The nature of the deposits was identified as calcium hydroxide and/or carbonate on both surfaces of the CEM. The mineral fouling presented an aggregation-like crystal following a carnation-like pattern of aggregates of small rhombohedral crystals with CaCl2 added alone, while CaCl2 + Na2CO3 added simultaneously yielded a more smooth spherical crystal. Protein fouling was detected only on the CEM surface in contact with the diluate, after demineralization of a solution containing CaCl2 + Na2CO3 using a concentrate pH value of 2.
Effectiveness and performance of electromembrane processes applied to bio-food systems may be influenced by parameters such as pH, ionic strength, temperature, nature and concentration of salts (Bazinet et al., 1996; 2001a). Under certain conditions, they may influence the formation of a fouling in ion-exchange membranes, which is among the most important limitations in ED processes. Fouling is the accumulation of undesired solid materials at the membrane-solution phase interfaces (Chong and Sheikholeslami, 2001). Its build-up causes an increase in resistance (Bleha et al., 1992), which deteriorates the performance of the process and can eventually lead to membrane integrity alteration. Process interruptions to perform cleaning or membrane replacement can be extremely expensive, so research in order to avoid or prevent fouling is justified.
Numerous studies have been done on the identification of species causing fouling (Korngold et al., 1970; Bleha et al., 1992; Linstrand et al., 2000b). The greater part of works on membrane fouling has focused on anion-exchange membranes (AEM), since most colloids present in natural water are negatively charged, hence making them more susceptible to fouling than cation-exchange membranes (CEM), (Korngold et al., 1970; Lindstrand et al., 2000a; 2000b). Many authors have reported the formation of calcium and magnesium hydroxide as well as calcium carbonate at the surface and inside the membranes used in the chlor-alkali industry (Momose et al., 1991). Such research has allowed the development of improved AEM with lower fouling tendency. However, recent studies on fouling of cation-exchange membranes (CEM) have reported the formation of a calcium carbonate and calcium hydroxide fouling layer (Bazinet et al., 2001b; 2005a). Furthermore, the presence of a protein deposit was described on certain CEM after skim milk electroacidification (Bazinet et al., 2001b). However, no work aimed specifically to investigate protein fouling of CEM during electrodialysis has been found.
This study was part of a broader research project aimed at understanding factors leading to fouling nature and formation, and its relation with the composition of the demineralized solution and the treatment conditions during conventional and bipolar membrane electrodialysis of food and nutraceutical products (Bazinet et al., 1998a; Ayala-Bribiesca et al. submitted-a). In the first part of this study, deposits, presumably of a protein-nature were found on certain CEM, while mineral fouling was observed on others (Ayala-Bribiesca et al., submitted-a). To complement such, our objectives in the present work were to identify the nature and to characterize the morphology of CEM fouling during conventional electrodialysis demineralization of a whey protein isolate solution with different compositions in calcium and carbonate, and using different pH conditions for the concentrate solution. The integrity of the membranes was analysed by means of membrane parameters and microscopic membrane surface elemental analysis and mapping.
Neosepta CMX-S cation-exchange membranes from Tokuyama Soda Ltd (Tokyo, Japan) used in a previous study (Ayala-Bribiesca et al., submitted-a) for model salt and protein diluate solution demineralizations were analyzed.
CaCl2·2H2O, NaCl and KCl were obtained from MAT Laboratory (Quebec, Quebec, Canada). NaOH 1.0N and HCl 1.0N were obtained from Fisher Scientific (Nepean, Ontario, Canada). Na2CO3 was obtained from EMD (EMD Chemicals, Gibbstown, New Jersey, USA). BiPRO whey protein isolate (WPI) was obtained from Davisco Foods International (Eden Prairie, Minnesota, USA). The average concentration of protein (92.7g), moisture (5.0g), sodium (600.0mg), potassium (120.0mg), calcium (120.0mg), phosphorous (25.0mg) and magnesium (15.0mg), expressed per 100g of BiPRO WPI, as is, was obtained from the manufacturer (Davisco Foods International).
The electrodialysis cell was a MicroFlow type cell (ElectroCell AB, Täby, Sweden) with two Neosepta CMX-S cationic membranes and two Neosepta AMX-SB anion-exchange membranes (Tokuyama Soda Ltd., Tokyo, Japan) defining three closed loops, as previously described (Figure 13) (Ayala-Bribiesca et al., submitted-a). Each closed loop was connected to a separate external plastic reservoir, allowing continuous recirculation. The membranes tested, with an effective surface of 10 cm2, were both in contact with the model salt and protein solution (i.e. diluate) on one side and with the pH-controlled KCl solution (i.e. concentrate) on the other side.
Original CMX-S membrane characteristics were measured on unused membranes freshly cut from the sheet and then compared to the membranes used for the demineralization procedure. Electrodialysis was carried out three times in 100-minute consecutive batches to ensure the fouling or not of the membranes. In order to maintain the system under the initial current density of 13mA/cm2, the voltage of the power supply (model HPD 60-5SX, Xantrex, Burnaby, BC, Canada) was adjusted accordingly at the beginning of each batch. The voltage necessary to stabilize such initial current density was then kept constant and the current was allowed to drop freely until the end of each demineralization batch.
The concentrate solution pH value (pH(concentrate)) was maintained constant during the demineralization process at 2, 7 or 12 by the addition of HCl and/or NaOH. The diluate solution pH value was adjusted at 6.5 with HCl 0.1M. Each pH(concentrate) value was used for each mineral condition, where the model diluate solution (800 mg/l KCl and 32.67g/l WPI) was enriched with CaCl2 (800mg/l), Na2CO3 (1000mg/l) or CaCl2 + Na2CO3 (800 and 1000 mg/l, respectively) (Ayala-Bribiesca et al., submitted-a).
After the three consecutive 100-minute demineralization batches the ED stack was dismounted and the cation-exchange membranes were photographed with a binocular microscope equipped with a digital camera. Membrane characteristics were determined by measuring electrical conductivity and thickness. Electron-microscopy images, coupled to X-ray elementary mapping, were taken from both sides of the membranes to complete the physical and chemical measurements. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra of the membrane surface in contact with the diluate solution were collected to detect the presence of a protein fouling.
The thickness of the membrane was measured using a Mitutoyo Corp. IDC type digimatic indicator with absolute encoder (Model ID-C112 EB, Kanagawa-Ken, Japan), specially devised for plastic film thickness measurements, with a resolution of 1 μm and a range of 12.7-0.001 mm. The indicator was equipped with a 10 mm diameter flat contact point. The membrane thickness value was averaged from ten measurements at different locations on the effective surface region of the membrane.
The membrane electrical conductivity was calculated from the measured membrane thickness and its electrical resistance, obtained from membrane conductance. The conductance was measured using a specially designed cell (conductivity clip) with a distance between the 1cm2-electrodes of 0.5 cm (Laboratoire des Matériaux Echangeurs d'Ions, Créteil, France) and a YSI conductivity meter (Model 35, Yellow Springs Instrument Co., Yellow Springs, OH) as previously described by Bazinet and Araya-Farías (2005a), and according to Lteif et al. (1999) and Lebrun et al. (2003). The electrical resistance was then calculated (Equation 1) from the measured conductance G measurement using the relationship R = 1 / G.
Where Rm is the transverse electric resistance of the membrane in ohms (Ω), Rm+s is the resistance of the membrane and the solution measured together in ohms, Rs is the resistance of the solution in ohms, Gm is the conductance of the membrane in Siemens, Gm+s is the conductance of the membrane and the solution measured together and Gs is the conductance of the solution. Finally, membrane electrical conductivity was calculated (Equation 2) according to the method used by Lteif et al. (1999).
Where κ is the membrane electrical conductivity in mS/cm, l is the thickness of the membrane in cm and A is the effective electrode area in cm2.
Photographs were taken at 40X magnitude on a binocular microscope (Laborlux S, Ernst Leitz Wetzlar GmbH, Wetzlar, Germany) equipped with a colour video camera (model hyper HAD, CCD-IRIS/RGB, Sony, Toronto, Ontario, Canada). The images were processed with Matrox Inspector software (version 3.1, Matrox Electronics Systems Ltd, Dorval, QC, Canada)
Images were taken on uncoated sample at 20 kV with an electron microscope (S-3000 N, Hitachi, Japan). Surface elemental analysis and mapping were performed on an X-ray energy-dispersive spectrometer (EDS) (INCA, Oxford Link Isis, Oxford Instruments, Concord, MA). The EDS conditions were 20 kV accelerating voltage and 14.7 mm working distance. Elements of interest were potassium, sodium, phosphorous, sulphur, calcium, chlorine, oxygen and carbon.
Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR) spectra of the CEM surface in contact with the diluate were taken in order to confirm or discard a protein-nature fouling layer. The sample was placed in a Nicolet 560 FTIR-Spectrometer (Nicolet Instrument Corp. Madison, WI, USA) covering the complete surface of the zinc selenide ATR crystal (Nicolet Instrument Corp.) and a delay of 20 minutes was allowed for the dehumidification of the optical bench. Once the spectra collection (Table 5) was finished, spectra were processed using Omnic 5.1a software (Nicolet Instrument Corp.), provided with the apparatus. The vapour spectrum was subtracted from the membrane spectra, followed by an automatic baseline correction in the 1700 cm-1 to 1500 cm-1 wavenumber region. An automatic smoothing process was applied before determining the absorbance peaks of each spectrum.
Thickness measurements were useful to identify the membranes without any evident alteration from those which integrity had been altered after the electrodialysis procedure. Thickness values measured were comparable to the control membrane (165.2 ± 2.1 µm) for all conditions (161.9 ± 3.7 µm), except for those containing CaCl2 and demineralized at a pH(concentrate) of 12 (Table 6). For membranes from such conditions, the thickness values were considerably higher with respect to the control membrane: 207.2 ± 7.8µm and 197.4 ± 9.2 µm, with and without Na2CO3 respectively. Furthermore, both membranes were severely deformed and the presence of a fouling was clearly visible (Ayala-Bribiesca et al., submitted-a). Such results may lead to the hypothesis that a certain deformation occurs when calcium salts, presumably, foul and eventually dehydrate the CEM, after which the membrane would deform. This mineral fouling would be enhanced by the highly basic conditions (pH(concentrate) = 12) of the concentrate solution, and would explain the absence of such deformation at the other pH(concentrate) values studied. The thickness value obtained for the control membrane was comparable to those reported in a previous study (Bazinet and Araya-Farías, 2005a).
The membrane electrical conductivity varied according to the mineral composition of the model solution and the pH(concentrate) value. After three consecutive 100-minute demineralizations, only CEM from condition with CaCl2 + Na2CO3 added and at a pH(concentrate) of 2 presented an electrical conductivity similar to the control CEM (0.97 ± 0.04 mS/cm) (Table 7). For the other conditions with CaCl2 added at a pH(concentrate) of 2 and with CaCl2 + Na2CO3 at a pH(concentrate) of 7, conductivity values decreased of about 0.40 mS/cm with respect to the control CEM (Table 7). Furthermore, at a pH(concentrate) of 12 with addition of CaCl2 and CaCl2 + Na2CO3, the electrical conductivity values dramatically dropped of about 0.80 mS/cm with respect to the control. On the other hand, for all conditions with only Na2CO3 added, regardless of the pH(concentrate) value, a rise in membrane conductivity of about 0.40 mS/cm, versus the control was observed (Table 7).
Table 7 CEM electrical conductivity (in mS/cm) after three consecutive 100-minute-demineralization batches.
During ED with addition of CaCl2, whatever the pH, calcium ions would have replaced membrane counter-ions with higher conductivity. This is in accordance with literature reporting that conductivity varies with the limiting ionic conductivity of counter-ions (Lteif et al., 1999; Lebrun et al., 2003). In addition, at basic pH(concentrate) conditions with CaCl2 added, the low solubility of calcium salts with respect to sodium or potassium and the formation of a fouling would explain the lower conductivity values obtained for membrane fouled with addition of calcium salts. However, at a pH(concentrate) of 2, for CaCl2 + Na2CO3, the high concentration of Na+ counterbalanced for the low mobility of calcium, which would explain the similar conductivity value as the control. With addition of Na2CO3, the high concentration of Na+ in the solutions increased their concentration as counter-ions in the CEM and consequently the electrical conductivity of the membranes.
Photos of CEM at x40 magnification (Figure 14) served as a rapid method to identify conditions where fouling had occurred and to confirm results observed in our previous work (Ayala-Bribiesca et al., submitted-a). Both sides of the membrane (i.e. in contact with the diluate and in contact with the concentrate) could be simultaneously seen on the same photograph given their translucent nature and that the light source was placed under the membrane. This allowed to observe both sides of the CEM and, therefore, determine the presence or not of foreign structures on the membrane surfaces.
Figure 14. Optical microscope photographs (x40) of cation-exchange membranes after three consecutive 100-minute-demineralization batches.
The control CEM presented a regular surface, through which the characteristic woven structure of the membrane was observed. After the demineralization procedure, most membranes presented comparable surface characteristics (i.e. transparency, appearance of the woven structure, absence of foreign structures) to those of the control CEM (not shown), except for those on which a deposit could be clearly seen. Apparently, the visibility of the membrane woven texture increased as membrane was used, even if no crystals or gel-like deposits were visible. Membranes presenting a deposit were those from conditions at pH(concentrate) of 2 with CaCl2 + Na2CO3 added, and at pH(concentrate) of 12 with CaCl2 added, with or without Na2CO3. An irregular and translucent gel-like deposit (Figure 14) was observed on the CEM after demineralization with CaCl2 + Na2CO3 and when a pH(concentrate) of 2 was maintained. As expected, this presumably protein deposit would be the one observed in the previous study (Ayala-Bribiesca et al. submitted-a), where it was shown that the CEM surface in contact with the diluate for the same demineralization condition presented a whitish deposit. For conditions where a pH(concentrate) of 12 was maintained, an opaque dot-shaped deposit resembling mineral crystals was observed for condition with only CaCl2 added. CEM from treatment at pH(concentrate) of 12 when CaCl2 + Na2CO3 were added presented a darker membrane whose structure resulted more evident.
Mineral fouling on the surface of the cationic membrane depended on the mineral composition of the diluate solution and on the pH(concentrate) value. SEM confirmed the presence of deposits on membranes from conditions at a pH(concentrate) of 12 with CaCl2 added, with or without Na2CO3, on both surfaces of the CEM, while for condition at a pH(concentrate) of 2 with CaCl2 + Na2CO3 added, the gel-like deposit appeared only on the CEM surface in contact with the diluate solution (Figure 15 and Figure 16).
No deposits were observed for the remaining mineral conditions treated at a pH(concentrate) of 2 nor any condition treated at a pH(concentrate) of 7. For these membranes without evident fouling, results were similar to those obtained by optical microscopy, obtaining comparable images to those of the control CEM (not shown). Besides the characteristic woven structure, membranes also presented a slightly cracked surface on both sides. This alteration would be mainly due to membrane drying out prior to the electron microscopy. In the case of conditions where a pH(concentrate) value of 12 was maintained, it was possible to identify, for mineral conditions with CaCl2 added, with or without Na2CO3, the presence of crystals on both sides of the membrane. Fouling on these CEM would consist of calcium precipitating as a carbonate or hydroxide at the membrane interface with the concentrate (Ayala-Bribiesca et al., submitted-a). For both conditions, crystals were rather small and distributed in a seldom way for the surface in contact with the diluate. Crystals on the concentrate side of the membrane were considerably larger and were present in higher quantity than on the diluate side. The crystal morphology seemed to be similar within the same membrane, but different according to the mineral condition of the diluate (i.e. presence or absence of Na2CO3). This way, CaCl2 added alone yielded an aggregation-like crystal (Figure 17) following a carnation-like pattern of aggregates of small rhombohedral crystals, while CaCl2 + Na2CO3 added simultaneously yielded a more smooth spherical crystal with a flaky texture (Figure 18). These different crystalline forms might suggest different growth conditions of the crystals (Kang et al., 2005) and/or a difference in their mineral composition (Kim et al., 2005).
For conditions where mineral fouling formed, crystals were found on both surfaces of the CEM (in contact with the diluate and in contact with concentrate), agreeing with the results reported by Bazinet and Araya-Farías (2005a), where fouling was also observed on both surfaces in contact with the concentrate and the diluate. Probably, once the membrane being severely fouled, minerals would also precipitate at the membrane-diluate interface, since they would still be attracted by the electric field but they could no longer cross the fouled membrane, increasing their concentration beyond their solubility threshold at the local pH conditions.
Figure 15. Scanning electron micrograph (x350) of diluate side of cation-exchange membranes after three consecutive 100-minute-demineralization batches.
Figure 16. Scanning electron micrograph (x350) of concentrate side of cation-exchange membranes after three consecutive 100-minute-demineralization batches.
Figure 17. Scanning electron micrograph (x2500) of crystals located on the concentrate side of CEM from condition at pH(concentrate) of 12 and only CaCl2 added.
ATR-FTIR surface analysis was performed to confirm the nature of the protein-presumed deposits. In order to identify the presence of a protein-nature deposit on the CEM surface in contact with the diluate, amide I and II band regions, in the wave number around 1640 and 1535 cm-1, respectively, were identified and related to the presence of protein (Pihlajamäki et al., 1996).
As expected from the membrane images, and the tendency of protein to preferentially foul AEM (Ayala-Bribiesca et al., submitted-a), very weak absorbance intensities in the amide I and II band regions were found in the overall lot of CEM membranes. Due to the low absorbance obtained, further deconvolution of spectra was not applied and only absorbance intensities higher than 0.003 were taken into account for analysis and interpretation. Under these criteria, no protein fouling was detected on CEM surfaces in contact with the diluate, except for the condition with CaCl2 and Na2CO3 added and a pH(concentrate) of 2 (Figure 19), which presented a slight whitish veil, as reported in our previous study (Ayala-Bribiesca et al., submitted-a), and a gel-like deposit: This condition presented a relatively higher absorbance intensity (0.0110 at 1623 cm-1), belonging to the amide I band region with respect to the control membrane (0.0045 at 1631 cm-1). Furthermore, this condition was the only one to show a peak absorbing in the amide II band region (0.0031 at 1541 cm-1) versus the control membrane (absorbance in the amide II band region < 0.0005) and the other conditions (absorbance in the amide II band region < 0.0013). These results confirm the protein-nature of the deposit identified with the optical microscope images for this particular condition.
Hence, two possible hypotheses for the presence of protein were suggested. The gel-like protein deposit would have precipitated a) over the AEM, eventually filling certain regions of the diluate compartment and entering in contact with the CEM, as previously observed in a previous study (Ayala-Bribiesca et al. submitted-a), and/or b) precipitating directly at the CEM interface with the diluate solution. This protein precipitation could be related to the simultaneous presence of Ca2+ and CO3 2- ions at an acidic pH, since it did not occur when they were added alone or at other pH(concentrate) values for the same mineral conditions. These results agree with those reported by Bazinet et al. (2003), where a slight protein fouling formed on cationic membranes when a medium with a very low pH was present on the opposite side of the membrane, in this case the concentrate. However, this did not occur for the other two mineral conditions demineralized with a pH(concentrate) of 2. Ions present, more specifically calcium and carbonate together, could have certain influence on this protein precipitate. Furthermore, in such work (Bazinet et al., 2003), both Ca2+ and CO3 2- ions were present in the streams treated by ED, supporting the second hypothesis.
Identification of elements on the CEM surfaces (Table 8) allowed confirming the presence of mineral fouling and led to the identification of its nature, when coupled to the mapping images. Although X-ray mapping was performed for both surfaces of each CEM, only images of the surfaces from conditions presenting a fouling (Figure 20) were included in this paper.
Table 8. Elementary mapping for CEM surface in contact with the diluate and the concentrate. Data expressed as relative atomic %.
Information obtained through EDS mapping was in accordance to the results previously discussed for SEM images. Composition of mineral fouling depended mainly on the mineral composition of the diluate. Potassium and phosphorous (Table 8) did no show any apparent differences between the control membrane and those employed for demineralization. This was also the case for sulphur and chlorine (Table 8), except for CEM from condition with only CaCl2 added and pH(concentrate) of 12, for which a significant reduction in their atomic ratio was found. Sulphur and chlorine constitute intrinsic components of the membrane, so the reduction in their atomic ratio could be explained by the masking of the membrane surface by a fouling layer. Sodium also presented similar values to the control CEM, except for a general reduction in the case of conditions at pH(concentrate) of 2 and 7 when CaCl2 had been added, with or without Na2CO3. This tendency would be normal, since calcium would have replaced some of the sodium ions as counter-ions in the CEM (Bazinet et al., 2003). As expected for calcium determination, conditions where only Na2CO3 was added presented similar calcium content to the control CEM, since it was only present in trace amounts in the WPI. Conditions where CaCl2 was added, alone and with Na2CO3, at a pH(concentrate) of 2 and 7 presented a content of calcium with an increase with respect to the control CEM. Higher concentration of calcium in the diluate solution for these conditions readily explained its increase, since it would, as mentioned before, compete with other ions as counter-ion for the membrane ionic sites. Finally, for conditions at pH(concentrate) of 12 where CaCl2 was added, with and without Na2CO3, a considerable amount of calcium was identified by the surface analysis. Furthermore, mapping images for these conditions (Figure 20, columns 3 and 5, respectively) show its arrangement on the membrane to be directly related to the regions corresponding to the crystals previously identified.
Figure 20. Elemental maps of CEM surfaces in contact with the diluate and/or concentrate from selected mineral and pH(concentrate) conditions.
Oxygen was comparable to the control for all membranes, except for the same conditions that presented considerably higher calcium content. Once again, oxygen was identified as a component of the crystals observed (
Figure 20, columns 3 and 5). However, between these 2 conditions, the relative presence of oxygen was slightly higher for condition for which only CaCl2 was added with a pH(concentrate) of 12, suggesting a fouling composition of calcium hydroxide, since no carbonate was added in this condition. Finally, carbon presence on the CEM surface was comparable to the control CEM for all conditions, except for conditions at pH(concentrate) of 12 where CaCl2 was added, with and without Na2CO3, the same two conditions where mineral fouling was identified on the diluate side of the membrane. However, carbon followed an opposite behaviour to oxygen, since its relative concentration was markedly reduced for both mineral-fouled conditions. This could be explained by the accumulation of deposits on the membrane surface, hiding the carbon of the membrane and reducing its relative atomic ratio with respect to the other elements determined.
Potassium presence was equivalent for all conditions and comparable to the control CEM. Sodium content of the membranes was also close to the control and to the results obtained for the diluate side of the CEM, except for conditions at pH(concentrate) of 12 where CaCl2 was added, with or without Na2CO3, where a slight increase was detected. This increase could be due to the presence of a mineral scaling, combining several ionic species. No sodium was detected for condition with a pH(concentrate) of 12 and Na2CO3 added alone. No particular explanation is given for this case, since ash analysis (Ayala-Bribiesca et al., submitted-a) did detect sodium in the membrane (0.43 mg sodium / g dry membrane). Sodium on the surface of the membrane was probably under the detection limit of the X-ray mapping. As expected, no phosphorous was found for control CEM, although traces of this element, possibly as a phosphate, were found for conditions with a pH(concentrate) of 12 and CaCl2 added, with and without Na2CO3. For these two conditions, phosphorous would have its origin from its intrinsic presence in the WPI and, given the basic pH, would have precipitated on the CEM. Sulphur content of this side of the membranes did not show any difference with respect to the control CEM, except for condition with pH(concentrate) of 12 and CaCl2 + Na2CO3 added. Reduction in sulphur detection would have been due, as previously mentioned, to a surface masking of the membrane by a fouling structure. For calcium, it appeared that its presence in the CEM surface in contact with the concentrate of condition with pH(concentrate) of 12 and only Na2CO3 added was relatively high when compared to the control and the other conditions with only Na2CO3 added. This could imply that calcium from the WPI would have precipitated on the membrane-concentrate solution interface as it migrated and entered in contact with the solution at pH(concentrate) of 12, starting the formation of a mineral fouling. Since only a limited amount of calcium was present, the diluate side of the membrane did not present any calcium deposits (Figure 20, column 6). Mapping determination of calcium confirmed the results of CEM ash analysis during the first part of this study (Ayala-Bribiesca et al., submitted-a), which showed an extremely high concentration of calcium for conditions with a pH(concentrate) value of 12 CaCl2 added alone (120 mg Ca / g of dry membrane) and CaCl2 + Na2CO3 added simultaneously (65 mg Ca / g of dry membrane), as compared to the average of the other seven conditions (6.92 ± 6.14 mg Ca / g of dry membrane). Contrary to membranes from conditions with CaCl2 and a pH(concentrate) of 12 (i.e. presenting a mineral fouling), no loss of membrane integrity was detected for CEM for pH(concentrate) of 12 and Na2CO3 added alone. Oxygen relative concentrations were comparable to the control membrane except for conditions with a pH(concentrate) of 12, which showed a considerably higher amount of oxygen. Carbon resulted in similar values to the control CEM for all conditions except those with a pH(concentrate) of 12, for which a marked reduction was observed. Furthermore, and according to images from the mapping (Figure 20, columns 2 and 4), some of the carbon detected overlaps with the crystals found, strongly suggesting a differentiating criterion between calcium carbonate and calcium hydroxide. This way, as shown by mapping images (Figure 20, columns 2 and 4), mineral fouling on CEM from condition with CaCl2 + Na2CO3 added would contain CaCO3, while mineral fouling from condition with only CaCl2 added would be composed of mainly Ca(OH)2, since no carbon was detected.
These results contrast to those reported by Bazinet and Araya-Farías (2005), where the deposits formed during their experiments were identified as Ca(OH)2, either there was presence or not of Na2CO3. This difference can be explained by the fact that traces of magnesium were present as part of the intrinsic composition of the WPI. These traces, however, would not have been abundant enough to appear on the x-ray mapping results. As reported by (Momose et al., 1991), CaCO3 precipitation can be induced with very low concentrations of magnesium at pH just highly above neutrality.
Mineral fouling was formed on CEM surface in contact with the concentrate when demineralization took place at a pH(concentrate) of 12, especially when CaCl2 was added, with or without Na2CO3. The mineral composition of the deposits was found to be, respectively, Ca(OH)2 and CaCO3 (with or without Ca(OH)2). This partly explained the different morphological appearance observed in the specific conditions. Furthermore, for both conditions, membranes were found to be severely deformed when the ED stack was dismounted. Membrane deformation would, then, pose a highly basic pH as a limiting parameter when calcium ions, and possibly other divalent ions such as magnesium, are relatively abundant in the diluate solution. Conditions where the pH(concentrate) value was maintained at 2 and 7 pH allowed to avoid both mineral fouling and membrane deformation. However, as previously mentioned, a pH(concentrate) of 2 may lead to the precipitation of protein when CaCl2 and Na2CO3 are simultaneously present. Conditions permitting the integrity preservation of membranes, like the use of a pH(concentrate) of 7, would spare the dismount of the ED stack and membrane replacement. Given the high cost of ED membranes, prevention stands as the ideal solution. Hence, a neutral pH could result in a good choice to prevent CEM integrity alteration during the demineralization of liquid foodstuffs containing protein and divalent cations.
In this study, it was shown that, according to the composition of the diluate solution and the pH value of the concentrate, different fouling deposits appeared on the surfaces of the CEM. Hence, a slight protein layer was found on the CEM surface in contact with the diluate when CaCl2 + Na2CO3 were simultaneously present and demineralization was done at a pH(concentrate) of 2. In addition to protein fouling on the diluate-facing side of the CEM, a mineral one was also found for both mineral conditions with calcium added and demineralized at a pH(concentrate) of 12. The presence of crystals on this surface of CEM indicate that pH conditions on the other side of the membrane actively affect the local pH conditions at the membrane-solution interface on the opposite side, precipitating Ca2+ as a salt or a hydroxide. Otherwise, salts may have begun to precipitate in the interior of the membrane once membrane was fouled at its interface with the concentrate in an autocatalytic membrane integrity alteration. Furthermore, pores would have been blocked, possibly by membrane dehydration, as previously discussed. Therefore, since the electric field kept attracting cations towards the CEM, their relative concentration at the membrane-solution interface would have been high enough to begin precipitation. A combination of both phenomena could also have occurred.
In order to complete the evaluation of the impact of pH on protein fouling and its relation with the presence of calcium and carbonate during ED treatment, the analysis of anion-exchange membranes used during these experiments is currently in progress. Further research should continue in order to confirm whether the protein fouling observed on the CEM came from cross contamination from the protein precipitating on the surface of the AEM or it actually formed on the CEM. Moreover, works on the impact of the presence of magnesium on the formation and nature of the fouling of ED membranes including crystal-characterization analysis will be carried out in a close future.
© Erik Ayala Bribiesca, 2005