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Evaluation of Structural Properties of Novel Activated Composite Membranes Containing Organophosphorus Extractants as Carriers Maria Oleinikova,† Maria Mun˜oz,*,† Juana Benavente,‡ and Manuel Valiente*,† Departamento de Quı´mica Analı´tica, Facultad de Ciencias, Universidad Autonoma de Barcelona, E-08193, Spain, and Departamento de Fı´sica Aplicada, Facultad de Ciencias, Universidad de Ma´ laga, E-29071, Spain Received April 2, 1999. In Final Form: August 10, 1999 This paper reports the results obtained by studying the structure of activated composite membranes (ACM) containing organophosphorus extractants as carriers. The porous structure of membrane samples containing different concentrations of di(2-ethylhexyl)phosphoric acid (DEHPA) and di(2-ethylhexyl)dithiophosphoric acid (DTPA) from 0 to 7 × 10-3 mmol‚cm-2 was characterized by gas adsorption method (BET). The results obtained by the BET technique indicate that both the total pore volume and the specific surface area of ACM samples decrease with the concentration of the carrier in the casting solution. The carrier in the membrane is mainly located in pores having large and medium diameters. The chemical structure of the carriers studied does not influence their distribution in the ACM. These results correlate well with those obtained by using different spectroscopic techniques to evaluate the properties of ACM samples. Thus, a quantitative correlation has been obtained between the carrier content in the membrane and its initial concentration in the modifying solution for both organophosphorus extractants studied by using the ICP technique. Dependencies of this type can serve as a sort of calibration curve to prepare ACM samples with a desired carrier content. X-ray microanalysis (EDS) was used as a nondestructive method for determination of carrier concentration in ACM. Both EDS and impedance spectroscopy (IS) methods were applied to evaluate the changes of internal membrane structure in the course of preparation of ACM samples with different carrier content.
Introduction Membrane processes have made a great impact on separation science. The elegance and simplicity of selective transport of a gas, ion, or complex molecule through a thin barrier produces a clean nonpolluting separation step. Recent reviews1,2 describe the development of new membranes, their structure and function, and their applications in various fields, including reverse osmosis, ultrafiltration, electrodialysis, and biotechnology. The challenge that today faces materials and membrane science is to be able to tailor the required membrane properties to tune them to the chemical properties of system under separation. Selective separation of different metals from hydromineral sources and hydrometallurgical effluents has become important during the past decades. The extraction properties of a wide spectrum of metal selective compounds have been described in numerous publications.3 The behavior of those extractants has also been studied in different processes, such as facilitated transport of metal species through liquid membranes. In systems of this type ions can be transported across the membrane against their concentration gradient. The driving force is provided by the chemical species, other than the transported metal ion, which are present on the two opposite sides of the membrane. A mobile carrier, dissolved in a waterimmiscible organic solvent, is responsible for the facilitated * Corresponding author. † Universidad Autonoma de Barcelona. ‡ Universidad de Ma ´ laga. (1) Scott K. Handbook of Industrial Membranes, 1st ed.; Elsevier: Oxford, 1995. (2) Membrane Handbook; Ho, W. S. W., Sirkar, K., Eds.; Van Nostrand Reinhold: New York, 1992. (3) Metals Handbook; Boyer, H. E., Gall, T. L., Eds.; American Society for Metals: OH, 1992.
transport. When the organic solution of an extractant is absorbed on a thin microporous film, we have a supported liquid membrane (SLM). Among many advantages of SLMs are the high diffusion rates and selectivity, low operating costs and energy consumption, the possibility of using expensive extractants, and some others.4 At the same time, a short lifetime caused by the loss of membrane liquid (ML) out of the pores of the support membrane is the major reason SLMs are not yet implemented into largescale industrial separation processes.5-7 Several ways to improve the stability of SLMs have been discussed in numerous publications.8-12 Recently a new class of SLMs, activated composite membranes (ACM), containing organophosphorus extractants for cation transport with enhanced stability have been synthesized in our laboratory13 and successfully tested for their ability to separate and concentrate different metal species.14 The transport properties of ACM systems containing di(2-ethylhexyl)phosphoric (DEHPA) and di(4) Danesi, P. R. Sep. Sci. Technol. 1984-85, 19, 857. (5) Danesi, P. R.; Reichley-Yinger, L.; Rickert, P. G. J. Membr. Sci. 1987, 67, 117. (6) Neplenbroek, A. M.; Bargeman, D.; Smolders, C. A. J. Membr. Sci. 1992, 67, 133. (7) Zha, F. F.; Fane, A. G.; Fell, C. J. D. J. Membr. Sci. 1995,107, 75. (8) Wienk, M.; Stolwijk, T.; Sudholter, E.; Reinhouldt, D. J. Am. Chem. Soc. 1990, 112, 797. (9) Schow, A. J.; Peterson, R. T.; Lamb, J. D. J. Membr. Sci. 1996, 111, 291. (10) Levin, G.; Bromberg, L. J. Appl. Polym. Sci. 1993, 48, 335. (11) Neplenbroek, A. M.; Bargeman, D.; Smolders, C. A. J. Membr. Sci. 1992, 67, 149. (12) Kemperman, A. J. B.; Rolevink, H. H. M.; Bargeman, D.; Van den Boomgaard, Th.; Strathmann, H. J. Membr. Sci. 1998, 138, 43. (13) Benavente, J.; Oleinikova, M.; Mun˜oz, M.; Valiente, M. J. Electroanal. Chem. 1998, 451, 173. (14) Oleinikova, M.; Gonzalez, C.; Mun˜oz, M.; Valiente, M. Polyhedron, in press.
10.1021/la9903898 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/28/1999
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(2-ethylhexyl)dithiophosphoric (DTPA) acids as carriers toward polyvalent metal ions have been investigated. Highly selective transport of Zn2+ versus Cu2+, Ni2+, Ca2+, Mn2+, Mg2+, Fe3+, and Al3+ as well as high membrane stability confirms the possibility of applying ACM systems in industrial processes. The carrier content and its bulk distribution in ACMs have been determined by different destructive and nondestructive techniques such as extraction of the carrier from ACM followed by its direct analysis by inductively coupled plasma spectroscopy (ICP), and X-ray energy dispersion spectroscopy (EDS) of ACM. The impedance spectroscopy (IS) technique has been applied for measuring the electrical resistance of a given membrane under different working conditions in situ (i.e., in contact with solutions at different salt concentrations), since it permits separate evaluation of electrical contribution of both the membrane and the electrolyte solution.13 The properties of membranes that render them useful for different industrial processes include high porosity and surface area as well as the physical and chemical nature of the internal adsorptive surfaces. With this in mind, the BET method (Brunnauer et al.) was applied in this study to evaluate the internal structure of ACMs and to follow its variation with ACM preparation conditions. This work presents a comparative study of DEHPAand DTPA-containing ACMs. The selection of carriers was dictated, on the one hand, by structural similarity between DEHPA and DTPA and, on the other hand, by dramatic difference in DTPA selectivity versus DEHPA due to substitution of two oxygen atoms (in DEHPA) with sulfur (in DTPA).15 The results on membrane structure obtained in this investigation by gas adsorption porosimetry (BET) also provide a better understanding of and deeper insight into our previous observations13,16 about membrane morphology and its correlation with membrane manufacturing conditions. Experimental Section Membranes. The nonwoven fabric (Hollytex 3329, France) was used as a support to manufacture reinforced polysulfone (PS) membranes, which were obtained by the phase inversion technique as described elsewhere.17 PS casting solution (15 mass %) was prepared by dissolving Udel P-3500 PS (Union Carbide Co., USA) in A.R. grade N,N-dimethylformamide (DMF) (Fluka, Germany) by vigorous agitation for 12-14 h at 25 °C. A thin top layer of polyamide containing DEHPA or DTPA of different concentrations (from 50 to 1000 mM) was obtained by interfacial polymerization.18 DEHPA (99%) was purchased from Carlo Erba R.S. (Milano, Italy). DTPA was synthesized from phosphorus pentasulfide (Aldrich, Germany) and 2-ethylhexanol (Aldrich, Germany) as described elsewhere.19 Further purification of DTPA was carried out by following a previously reported procedure.20 The purity of the final product determined by potentiometric titration of samples dissolved in water-ethanol mixture with 0.05 M NaOH appeared to be 95 ( 0.2%. Aqueous amine solution (1,3-phenylenediamine, 98%, Merck, Germany) was mixed with water-immiscible organic solution of multifunctional acid chloride (1,3,5-benzenetricarbonyl trichloride, 98%, Aldrich, Germany) containing DEHPA or DTPA. Excess solution was washed off the surface of the membrane with water. Finally, the polyamide
Langmuir, Vol. 16, No. 2, 2000 717 Table 1. Carrier Content in ACM Samples Prepared by Using Different Casting Solutions init carrier concn in casting solution, mM 0 50 200 400 500 700 1000
carrier concn in ACM, mmol/cm2 DEHPA 0 1.58 × 10-3 2.70 × 10-3 3.70 × 10-3 4.88 × 10-3 7.14 × 10-3
DTPA 0 2.63 × 10-4 1.16 × 10-3 2.53 × 10-3 3.08 × 10-3 5.72 × 10-3
top layer containing carrier was dried in an oven at 60 °C for 10 min. The membrane, so prepared, is made of two layers containing carrier. Procedures. The pore-structure characterization of the ACMs containing organophosphoric acids as a carrier has been carried out by using traditional gas adsorption method (BET). The measurements of the carrier content in the membrane by means of ICP technique were also done. To complete these data, X-ray microanalysis (EDS) was applied to determine the distribution of the carrier in the membrane. Electrical properties of the membrane were evaluated by systematic impedance measurements with different contacting solutions of NaCl. ICP Spectroscopy. The total amount of immobilized organophosphonic acids was evaluated by the quantitative extraction of the carrier from the membranes using ethanol (Carlo Erba, Italy, 95%). After concentration of the ethanol solution by evaporation, the residual solution was dissolved in aqueous NaOH solution and the total concentration of organophosphonic acids in the aqueous phase was determined by analyzing the phosphorus concentration by the ICP technique using an ARL Model 3410 spectrophotometer provided with a minitorch. The emission line used was 213.618 nm. Calibration solutions were prepared by sequential dilution of H3PO4 solutions (Panreac, Spain, 85%) of known concentrations.21 X-ray Microanalysis. The organophosphonic acids distribution in the membranes was evaluated by X-ray microanalysis carried out on a JSM-6300 scanning electron microscope supplied with an Oxford (England) energy-dispersive X-ray spectrometer. Flat membranes and their cross sections were coated with carbon prior to determination of the elemental composition of the membrane material. The relative accuracy of the X-ray microanalysis was around 1%. A GaP sample was used as a standard for the quantitative determination of P. Gas Adsorption Porosimetry. The quantitative characterization of the pore structure of ACM membranes was performed by measuring the nitrogen adsorption at 77 K over the relative pressure range from 0.01 < P/Po < 0.99 by using an automated volumetric apparatus ASAP 2000 (Micromeritics, USA). Surface areas were calculated using BET theory, a molecular area for N2 of 0.162 nm2, and the relative pressure range of 0.05-0.3. The porosimetry data were treated by calculation of the relative pore volume for each mean pore diameter determined for a given interval of pore sizes scanned. Impedance Spectroscopy. Electrical impedance was measured by using an impedance analyzer (Solartron 1260) connected to the measuring cell with a pair of platinum electrodes22 for 100 different frequencies in the range of 10-107 Hz at a maximum voltage of 0.01 V. Each half-cell was filled with NaCl solution of the same concentration, and measurements were carried out at six different concentrations ranging from 10-3 to 5 × 10-2 M at constant temperature t ) (25.0 ( 0.4) °C and pH ) (6.5 ( 0.4).
Results and Discussion (15) Muraviev, D.; Oleinikova, M.; Valiente, M. Langmuir 1997, 13, 4915. (16) Ariza, M. J.; Rodriguez-Castellon, E.; Rico, R.; Benavente, J.; Oleinikova, M.; Mun˜oz, M. In Ceramics. Getting into 2000, volume D, Proceedings of the CIMTEC’98 Symposium; Vincenzini P., Ed., in press. (17) Mulder, M. Basic Principles of Membrane Technology; Kluwer Academic Publishers: Dordrecht, 1992. (18) Cadotte, J. E. J. Macromol. Sci.sChem. 1981, A-15, 727. (19) Handley, T. H.; Dean, J. A. Anal. Chem. 1962, 34, 1312. (20) Levin, I. S.; Sergeeva, V. V.; Tarasova, V. A.; Varentsova, V. I.; Rodina, T. F.; Vorsina, I. A.; Kozlova, N. E.; Kogan, B. I. Zh. Neorg. Khim. 1973, 18, 1643 (in Russian).
Results obtained by determination of the amount of extractant incorporated into the membranes by impregnation using casting solutions of different extractant concentrations in ACM-DTPA and ACM-DEHPA systems are collected in Table 1. As seen, the quantity of both (21) Ken-an Li; Muralidharan, S.; Freiser, H. Solvent Extr. Ion Exch. 1986, 4, 739. (22) Benavente, J.; Ramos-Barrado, J. R.; Martinez, M.; Bruque, S. J. Appl. Electrochem. 1995, 25, 68.
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Figure 1. Relative heights of phosphorus peak measured by EDS technique in membrane samples with different carrier content plotted versus carrier concentration in casting solution: (O) DEHPA and (b) DTPA.
DTPA and DEHPA incorporated into the thin polyamide film of ACM is directly proportional to extractant concentration in the casting solution. This allows use of the relation between the content of extractant in ACM and that in the casting solution as a sort of calibration curve for the determination of the concentration of extractant in the casting solution to be used for manufacturing an ACM with a desired carrier content. The higher content of carrier observed for the DEHPA-based membranes in the whole range of extractant concentration studied can be explained by the higher tendency of DEHPA to form dimers in the organic phase (in the casting solution) in comparison with DTPA.23 This difference in extractant behavior is attributed, in turn, to the difference in the strength of organophosphoric acids reflected in the respective pKa values, which are equal to 3.51 (DEHPA)24 and -1.25 (DTPA).25 At the same time, the use of the ICP technique for analysis of the carrier content in ACM requires complete removal of extractant from the membrane under study (by using extraction with an appropriate solvent), which leads to the irreversible destruction of the membrane sample. In this context, the search for nondestructive methods applicable to solving this problem is of particular importance. The validity of the results obtained by determining the carrier content in ACM using the ICP technique (which can also be classified as an indirect method) is confirmed by those obtained by the direct determination of the phosphorus in the ACM using the scanning electron microscopy measurements of the X-ray signal energy distribution along the membrane surface. Figure 1 shows the relative heights (Hrel ) Hi/Hmax) of phosphorus peak measured by the EDS technique by scanning the surface of freshly prepared membranes, plotted versus the carrier concentration in the casting solution. As seen, the data presented in Figure 1 can be satisfactorily approximated by a single line (for both DEHPA- and DTPA-based (23) Lewin, I. S.; Sergeeva, V. V.; Rodina, T. F.; Yukhin, Yu. M.; Varentsova, V. I.; Vorsina, I. A.; Balakireva, N. A.; Bukhovskaya, I. A.; Novoseltseva, L. A.; Tishenko, L. I.; Frid, O. M.; Kogan, B. I.; Marinkina, G. A. In Proceedings of the International Solvent Extraction Conference; London, 1974. (24) Preston, J. S.; Du Preez, A. C. MINTEK, Report No. M.378; The Council of Mineral Technology, ISBN 086999851X, 1988. (25) Rodina, T. F.; Varenzova, V. I.; Lewin, I. S.; Kolyshev, A. N. Izv. Sib. Otd. Acad. Nauk SSSR, Ser. Khim. Nauk 1973, 6, 14.
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membranes), whose shape resembles a Langmuir-like pattern. Comparison of the results presented in Table 1 with those shown in Figure 1 indicates a nonhomogeneous distribution of the carrier in the ACM samples under study. Indeed, although the bulk content of the carrier linearly increases with the concentration of the casting solution (see Table 1), its concentration in the surface (polyamide) membrane layer of ∼1 µm depth (which can actually be examined by EDS method26) reaches a certain limiting value at the extractant concentration of ∼400 mM. These results are consistent with our previous data,13 which indicate that the maximum carrier concentration is observed in the polyamide membrane layer. From this one can conclude that saturation of this layer proceeds faster than that of the remaining membrane volume (the polysulfone layer). The faster saturation of the polyamide layer can be explained by the chemical interaction of the acidic extractant with the polyamide matrix27 upon incorporation of the extractant into the polymeric support. This hypothesis is confirmed by recently reported results16 obtained by studying DEHPA-ACM samples using X-ray photoelectron spectroscopy (XPS) technique. At the same time, the comparison of the results shown in Table 1 and in Figure 1 demonstrates the better applicability of the ICP technique for evaluation of the total carrier content in ACM. This conclusion evidently follows from a wider linearity range between the measured phosphorus concentration and the initial carrier content in the casting solution observed by using ICP technique (see Table 1) versus the results obtained by EDS method (see Figure 1). Nevertheless, the results of ACM examination by using the EDS technique are of particular interest as they confirm the importance of studying the distribution of the carrier in the pore volume of the polymeric support. This information can be obtained by evaluating the pore redistribution in the course of ACM preparation by using casting solutions of different concentration. The characterization of the ACM pore structure was carried out using the traditional gas adsorption method. The full description of a porous solid phase may require many parameters such as porosity, density, surface area, pore volume, pore size (pore size distribution), pore connectivity, pore shapes, and some others. The desired (necessary and sufficient) information about the pore structure of an object under study depends on the area of its application. For example, for adsorbents, the surface area and the pore size are the most relevant parameters. In this work the total pore volume and the distribution of absolute pore volumes versus absolute pore sizes were chosen as the main factors, making it possible to follow the variation of structural features of different ACM samples in comparison with those of the carrier-free support. Figure 2 shows a comparison of the total pore volume of ACM samples prepared by using different casting solutions plotted versus carrier concentration in the casting solution. Figure 3 presents the results obtained by the determination of the specific surface area by using BET technique28 for freshly manufactured ACM samples. Comparison of the results shown in Figures 2 and 3 indicates that both the total pore volume and the specific surface area of ACM samples nonlinearly decrease with (26) Goldstein, J. I.; Newbury, D.; Echlin, P.; Joy, D.; Romig, A. D., Jr.; Lyman, C. E.; Fiori, C.; Lifshin, E. Scanning Electron Microscopy and X-ray Microanalysis, 2nd ed.; Plenum Press: New York, 1992. (27) The obvious reason for this interaction is the formation of hydrogen bonds between either POOH (in DEHPA) or PSSH (in DTPA) and nitrogen atoms of the polyamide skeleton. (28) Allen, T. Particle Size Measurements, 4th ed.; Chapman and Hall: London, 1990.
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Figure 2. Total pore volume of ACM samples prepared by using different casting solutions plotted versus carrier concentration in casting solution: (1) absence of carrier; (O) DEHPA; (b) DTPA.
Figure 4. Differential pore volume vs concentration of carrier in casting solution determined for different pore diameters: (a) 1000 Å; (b) (O) 560 Å; (1) 305 Å, (3) 170 Å; (9) 100 Å; (0) 55 Å.
Figure 3. BET surface area of ACM samples prepared by using different casting solutions plotted versus carrier concentration in casting solution: (1) absence of carrier; (O) DEHPA; (b) DTPA.
the concentration of the carrier in the casting solution. It is interesting to note that the general trend in variation of the pore structure is nearly identical for both DEHPAand DTPA-based ACM samples. The nonlinear pattern of pore size decrease observed for membranes containing different quantities of the carrier supports our previous observation on predominating localization of the carrier in nonporous polyamide layer of the membrane.29 Figure 4 shows the variation of the differential pore volume (determined for different pore diameters) with concentration of the carrier in the casting solution. The pore distribution in ACM samples changes dramatically in comparison with the carrier-free support. Another conclusion, which follows from the results presented in Figure 4, concerns the redistribution of pore sizes in different ACM samples. The increase of the carrier concentration in the casting solution leads to the gradual disappearance of pores of small diameters in ACM. The (29) OleiniKova, M.; Mun˜oz, M.; Benavente, J.; Valiente, M. Anal. Chim. Acta, in press.
ACM samples prepared by using extractant solutions with concentration of 200 mM and more are essentially characterized by the presence of only large pores of ∼1000 Å diameter while smaller pores appear to be almost completely filled with the carrier solution. At the same time, the data of Figure 4 make it also possible to conclude that most of the carrier in ACM samples is located in the pores of large and medium diameters. This conclusion is consistent with the results shown in Figure 5, where the pore size distribution versus average pore diameter dependencies are shown for the carrier-free support and different ACM samples. The most substantial decrease in the relative pore content is observed for the pores of >300 Å diameter. From the above results one can conclude that the most dramatic changes are observed in the most dense (and less porous) polyamide membrane layer, whose thickness does not exceed 104 Å.17 This leads, in turn, to the conclusion that the results of evaluation of the carrier content obtained by using the EDS technique must also be sensitive to the alteration of the porous structure of ACMs. The confirmation of this hypothesis obviously follows from the results presented in Figure 6, where the specific surface area of ACM samples (see Figure 3) is plotted versus relative phosphorus content (see Figure 1). The results shown in Figure 6 are of particular interest as they demonstrate a possible route of evaluation of the membrane morphology by using a nondestructive (unlike the BET method30) EDS technique. Nevertheless, we
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Figure 5. Pore size distribution vs average pore diameter determined for different ACM samples: (b) absence of carrier; (O) 50 mM DEHPA; (1) 200 mM DEHPA; (3) 500 mM DEHPA; (9) 1000 mM DTPA.
Figure 6. BET surface area of different ACM samples vs relative heights of phosphorus peak determined by EDS method: experimental data shown in Figure 3 for (O) DTPA, (b) DEHPA; data calculated from respective dependence shown in Figure 3 for (0) DTPA, (9) DEHPA.
would like to emphasize that this point requires additional experimental confirmation and we intend to follow this in our further studies. Electrical characterization of ACM samples under experimental conditions similar to those used in membrane processes (i.e., in contact with electrolyte solution of different concentrations) was performed by impedance spectroscopy (IS) measurements. Figure 7 shows the real and imaginary parts of the impedance versus frequency, respectively, for DEHPA and DTPA samples in contact with electrolyte solution of the same concentration (C ) 5 × 10-3 M NaCl). Two different relaxation processes can clearly be observed in Figure 7: one at low frequency (fmax ≈ 2 × 103 Hz) corresponds to the membrane and the other at high frequency (fmax ≈ 106 Hz) represents the contribution of the electrolyte solution layer between the electrodes and the membrane surfaces. Important differences were obtained for parameters directly related to the membrane (30) To obtain BET results of acceptable reproducibility, one needs to finely cut the ACM sample under study prior to the gas absorption measurements.
Figure 7. Real impedance (a) and imaginary impedance (b) parts vs frequency, for (O) 700 mM DEHPA and (4) 700 mM DTPA samples in contact with 5 × 10-3 M NaCl solution; (b) 700 mM DEHPA sample in contact with 10-2 M NaCl solution.
electrical resistance (Zreal) and capacitance (-Zimg) for both membrane samples; however no differences exist for the electrolyte contribution. The analysis of the impedance curves allows the determination of electrical resistance of the membrane samples at different electrolyte concentrations.31,32 For comparison, Zreal data obtained for DEHPA sample in contact with 10-2 M NaCl solution are also presented in Figure 7a, which demonstrates the dependence of membrane electrical resistance, Rm, on the salt concentration. A strong decrease in Rm values can be attributed to the effect of the electrolyte embedded in the membrane matrix.22 To determine the membrane matrix resistance (without any electrolyte contribution) Rm values were fitted to the following expression: Rm(C) ) R0 - aCb, where R0 is the membrane matrix resistance at CNaCl ) 0, a material characteristic, which does not depend on the NaCl concentration, and a and b are two empirical parameters.13 Membrane matrix resistance values R0 determined for different ACM samples are collected in Table 2. As seen, R0 values increase with carrier concentration in the (31) Macdonald, J. R.; Johnson, W. B. In Impedance Spectroscopy; Macdonald, J. R., Ed.; Wiley: New York, 1987; p 12. (32) Jonscher, A. K. Dielectric Relaxation in Solids; Chelsea Dielectric Press: London, 1983.
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Table 2. Electrical Resistance of Membrane Matrix Values, R0, for ACM Samples Prepared by Using Different Casting Solutions init carrier concn in casting solution, mM 0 50 100 200 400 500 700 1000
R0, Ω‚m2 DEHPA
DTPA
28.19 32.74 39.11 52.76 78.24
28.19
133.30
68.04 101.22 121.24 159.60
values determined for both DEHPA- and DTPA-based ACMs are plotted versus carrier concentration in the membrane phase (mmol/m2), recalculated according to the results of BET measurements (see Figure 3). The last results indicate the alternative possibility of using a nondestructive technique (IS) to follow the changes of the internal structure (or at least some parameters directly related to the structure) of activated composite membrane. Moreover, the IS technique allows in situ control of changes in the membrane properties that are of particular importance for industrial applications of ACM-based separation processes. Conclusions
Figure 8. Electrical resistance of membrane matrix vs carrier concentration in membrane phase (mmol/m2), recalculated according to BET measurements (see Figure 3): (b) DEHPA; (O) DTPA.
membrane, and they are practically independent of the type of carrier used. These results may indicate that membrane matrix resistance increases mainly due to the changes in geometrical factors of the membrane structure, namely by gradual loading of the upper membrane layers at higher carrier concentrations in the casting solution used for the membrane manufacturing. It is interesting to note that the results of impedance measurements appear to be also sensitive to the variation of the ACM sample morphology. This conclusion evidently follows from the results shown in Figure 8, where the R0
The following conclusions can be derived from the results obtained in this work: 1. ICP technique allows establishment of a quantitative correlation between carrier content in the membrane and carrier initial concentration in the modifying solution for different organophosphorus extractants. Correlations of this type can serve as a sort of calibration curve to prepare ACM samples with a desired carrier content. 2. X-ray microanalysis can be used as a nondestructive method for carrier determination in ACM. The wider linearity range observed for ICP data versus the results obtained by EDS method makes the first technique more applicable for evaluation of total carrier content in the membrane sample. 3. The results of BET studies indicate that both the total pore volume and the specific surface area of ACM samples decrease with concentration of the carrier in the casting solution and the main quantity of the carrier in the membrane is located in the pores of large and medium diameters. The chemical structure of the carriers studied does not influence its distribution in the ACM. 4. Both EDS and impedance spectroscopy can be used as nondestructive techniques to evaluate changes of the internal membrane structure in the course of preparation of ACM samples with different carrier content. Acknowledgment. The authors thank the Comisio´n Interministerial de Ciencia y Technologı´a (CICYT) (Project No. MAT 97-0970-C03). M.O. is a recipient of a fellowship from CIRIT (Comisio´n de Ciencia y Tecnologı´a de Catalunya). LA9903898
