Water effect on physical–chemical and elastic parameters for a dense cellulose regenerated membrane: Transport of different aqueous electrolyte solutions

Journal of Membrane Science 352 (2010) 153–159

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Water effect on physical–chemical and elastic parameters for a dense cellulose regenerated membrane: Transport of different aqueous electrolyte solutions J.D. Ramos, C. Milano, V. Romero, S. Escalera, M.C. Alba, M.I. Vázquez, J. Benavente ∗ Departamento de Física Aplicada I, Grupo de Caracterización Electrocinética y de Transporte en Membranas e Interfases, Facultad de Ciencias, Universidad de Málaga, E-29071 Málaga, Spain

a r t i c l e

i n f o

Article history: Received 6 May 2009 Received in revised form 30 November 2009 Accepted 2 February 2010 Available online 6 February 2010 Keywords: Regenerated cellulose XPS Elasticity Membrane potentials Diffusion 1:1 and 2:1 electrolytes

a b s t r a c t Water effect in different physical–chemical and elastic parameters determined for a dense cellulose regenerated (RC) membrane is presented. Membrane samples characterization was carried out by obtaining XPS spectra, X-ray diffraction patterns and elasticity curves, and the significant differences determined when dry and wet samples are compared are basically associated to the water plasticization effect in this high hydrophilic material. Transport of different aqueous electrolyte solutions (NaCl, KCl, LiCl, MgCl2 and CaCl2 ) was study by measuring membrane potentials and diffusion, which allows the determination of effective membrane fixed charge concentration, ion transport numbers and permselectivity as well as diffusional permeability and diffusion coefficient of the different salts across the RC membrane. Differences associated to the kind of electrolyte (1:1 or 2:1 electrolytes) were obtained as a result of membrane–solute electrical interactions. Membrane dense structure significantly reduce both salt and water diffusional transport, being this latter determined by using tritiated water as permeate. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Regenerated cellulose (RC) is a material widely used for the manufacture of membranes for different separation processes such as dialysis, fractionation of polymer mixtures or ultrafiltration due to its rather good chemical and solvent resistance, while its hydrophilic character reduces the membrane fouling tendency [1,2]. RC membranes have an appropriate structure for medical applications, since they allow the permeation of small ions/molecules but reject the transport of proteins or macromolecules, which are basic requirements for hemodialysis and drug release systems [3,4]. However, RC membrane compaction under pressure is an important problem for its application in filtration processes, although the hydrophilicity of RC ultrafiltration membranes was reported to be a great advantage in pulp and paper mill filtration [5]. RC membranes can also present an electronegative character due to the oxidation of –CH2 OH groups to –COOH [6,7], which could play a notable role when the transport of electrolyte solutions is considered and differently affect the transport of ion/salt solutions depending on the kind of electrolytes [8,9]. In fact, the importance of water in the permeation of ionic penetrants into water-swollen cellulose membranes have already been studied

both experimental and theoretically [4,10] and differences for ion transport on the level of hydration of the cellulose network was indicated. In this context, cellulose–water interactions, swelling and adsorption phenomena are also of great interest. The main objective of this paper is to consider the effect of water on structural, elastic and chemical behaviour of a RC membrane as well as its electrochemical characterization by using aqueous solutions of different electrolytes (1:1 and 2:1 electrolytes). Electrochemical measurements allow the determination of membrane effective fixed charge, ion transport numbers and diffusion coefficients for NaCl, KCl, LiCl, MgCl2 and CaCl2 aqueous solutions, and differences related to both the type of electrolyte and solute–membrane interactions are indicated. Comparison of water and salt diffusion coefficients gives information on different contributions involved in this latter point. Another objective of the work is to illustrate the significant differences obtained for some characteristic material parameters when they are determined in dry and wet states, since this fact could be of interest when hydrophilic membranes are characterized under normal membrane working conditions (usually aqueous solutions). 2. Experimental 2.1. Membranes

∗ Corresponding author. Tel.: +34 952 13 19 29; fax: +34 952 13 2382. E-mail address: j [email protected] (J. Benavente). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.02.001

A flat dense cellulose membrane with a content of regener˜ S.A. (Burgos, ated cellulose of 0.06 kg/m2 from Cellophane Espanola,

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of a Shirley-type background taking into account the corresponding area sensitivity factor [13] for the different measured spectral regions. X-ray diffraction patterns were obtained with an X’Pert Pro (PANalytical) automated diffractometer using Ge(1 1 1)monochromated Cu K␣ radiation and an X’Celerator detector. The diffractograms were recorded between 5◦ and 45◦ (2) in 0.017◦ steps at 45 kV and 35 mA for 30 min. Dry and wet samples were placed on an aluminium support, adapting them to the goniometer in a –2 configuration. 2.3. Elastic measurements

Fig. 1. Membrane water uptake as a function of water immersion time.

Spain) was studied. Membrane in dry state will hereafter identify as RC-6(d), while after water immersion or aqueous electrolyte solutions contact will be named as RC-6(w). Membrane water uptake, Wm , was determined by Wm =

Mw − Md Md

where Md and Mw represent the mass average values of dry and wet samples, respectively. Due to the high hydrophilicity of cellulose and the laboratory atmospheric humidity (between 35% and 40%), prior to weight the dry membrane, five piece were introduced in a desiccator for 24 h, after which Md values were obtained; wet samples mass was determined after water immersion for different periods of time (0 ≤ t(h) ≤ 48) and their surfaces gently dried with paper previously to be weighted. Fig. 1 shows time dependence for the average water uptake, where the vertical bars correspond to the error interval. According to these results the RC-6 membrane shows a similar behaviour to hydrogels, in agreement with the highly hydrophilic cellulose character; although 8 h seems to be the time needed for complete membrane hydration, water and aqueous solution measurements were carried out after 48 h in contact with the membrane sample. 2.2. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) measurements Membrane surface chemical characterization was carried out by X-ray photoelectron spectroscopy or XPS analysis. XPS spectra were recorded with a Physical Electronics PHI 5700 spectrometer with a multi-channel hemispherical electroanalyzer and high-resolution spectra were recorded by a concentric hemispherical analyzer operating in the constant pass energy mode at 29.35 eV, using a 720 ␮m diameter analysis area. Mg K␣ X-ray was used as excitation sources (h = 1253.6 eV). Accurate ±0.1 eV binding energies were determined with respect to the position of the adventitious C 1 s peak at 284.8 eV, and the residual pressure in the analysis chamber was maintained below 5 × 10−7 Pa during data acquisition. Angle resolution XPS spectra (ARXPS) were recorded by using different take-off angles ( = 15◦ , 30◦ , 45◦ , 60◦ and 75◦ ), which allows the attainment of chemical information for depth ranging between 2.5 nm and 9.3 nm [11]. A PHI ACCESS ESCA-V6.0F software package was used for acquisition and data analysis [12]. Membranes were irradiated less than 20 min to minimize X-ray induced sample damage. Atomic concentration (A.C.) percentages of the characteristic membrane elements were determined after subtraction

Membrane elastic characterization was carried out with various samples of 2 cm length by using a force digital gauge (Mark-T, ES20 model) with a maximum tension of 100 N and length accurate of ±0.01 mm. Elastic measurements were carried out with dry and wet (after 72 h in distilled water) samples at room temperature and no macroscopic heating effect associated to sample elongation was considered. 2.4. Membrane potential, salt and water diffusion measurements Electrochemical and water diffusivity measurements were performed in a dead-end test cell. The membrane was clamped between two glass half-cells by using silicone rubber rings, and two magnetic stirrers were placed at the bottom of the half-cells to minimize the concentration–polarization at the membrane surfaces. Membrane potential and salt diffusion measurements were performed with aqueous solutions of diverse electrolytes (NaCl, LiCl, KCl, MgCl2 and CaCl2 ) at different concentrations, at room temperature (25.0 ± 0.5) ◦ C, standard pH (5.8 ± 0.2) and solution stirring rates of 525 rpm. - The electromotive force at the steady-state (Ecell ) between the solutions of different concentrations at both membrane sides caused by the concentration difference was measured with two reversible Ag/AgCl electrodes connected to a digital voltmeter (Yokohama 7552, 1 G input resistance). Measurements were carried out by keeping the concentration of the solution at one side of the membrane constant (Cc = 0.01 eq/l) and gradually changing the concentration of the solution at the other side (0.002 ≤ Cv (eq/l) ≤ 0.1). - Diffusion measurements were performed with the membrane separating a feed concentrated solution (Cf ) from a receiving diluted solution (Cr ), which initially was distilled water. Since for each electrolyte and temperature there is a given conductivity–concentration relationship, time variation of feed and receiving solution concentrations was determined by measuring conductivity changes ( f and  r , respectively) by means of a conductivity cell connected to a digital conductivity meter (Crison GLP 31). Conductivity versus concentration calibration curve was used for each electrolyte solution. Five different feed concentrations were measured (Cf = 0.005 eq/l, 0.01 eq/l, 0.02 eq/l, 0.05 eq/l and 0.1 eq/l). Water diffusion across the membrane was determined following a similar procedure than that indicated for electrolyte diffusion but using a radiotracer (TOH or tritiated water). Five different TOH feed concentrations ranging between 30 ␮l tritium/15 ml of distilled water and 240 ␮l tritium/15 ml distilled water were used. Time variation of the tritium activity in the donor and receiving chambers was determined at different time instances by taken samples of 50 ␮l which were analyzed in a Beckman LS6500 scintillation counter. All measurements were carried out with a stirring rate of 550 rpm in both semi-cells and they were performed at the

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Table 1 Atomic concentration percentages of the elements found on RC-6(d) and RC-6(w) membranes surfaces. Sample

RC-6(d)

RC-6(w)

C 1s O 1s Si 2p N 1s

62.3% 35.2% 2.2% 0.3%

63.2% 20.4% 15.8% 0.6%

Radioisotope Laboratory (Central Research Services, Málaga University) taken into account radiation safety protocols. 3. Results and discussion 3.1. Physicochemical and elastic characterizations of dry and wet membranes Chemical surface characterization of homogenous membranes is usually carried out by XPS measurement, which allows the determination of the atomic concentration percentages (A.C. %) of the elements present on the membrane surfaces by analyzing the recorded spectra. Table 1 shows the A.C. (%) of the elements founds on the surface of the RC-6(d) and RC-6(w) samples (take-off angle of 45◦ ). As can be observed, two non-characteristic cellulose elements, silicon and nitrogen, were found on the surface of both samples, which might be attributed to the membrane manufacture process or to environmental contamination [14,15], although they could also be purposed added to improve membrane functionality. In order to elucidate this point, it is important considered the decrease of the oxygen A.C. percentage in the wet sample and the large increase of silicon A.C. (%) exhibited by the wet sample, which should be caused by the re-orientation of hydrophobic silicon compounds to the surface. These values seems to indicate the addition of organic compounds with silicon (such as poly(methylhydrosiloxane)) to the cellulose material to improve membrane plasticity [16]. Moreover, the silicon re-orientation in the RC-6(w) sample was also confirmed by the angle resolution XPS spectra measurements shown in Fig. 2, where the silicon A.C. (%) deep profile results for dry and wet samples are indicated. As can be observed, a small reduction in the Si (%) values at the lowest angle (surface contamination) was obtained for RC-6(d) membrane, while the RC-6(w) sample shows a linear A.C. (%)-deep decrease. Fig. 3a shows a comparison of the C 1s core level spectra for both surfaces of dry and wet samples. Only slight differences exist if both surfaces of a given sample are compared (solid and dashed lines), but significant differences are observed when both samples are compared. For sample RC-6(d) three contributions at different binding energies (B.E.) are differentiated [17–19]: (i) –CH– and

Fig. 2. Variation of silicon atomic concentration percentage with take-off angle (deep profile). () dry RC-6(d) sample, () wet RC-6(w) sample.

–C–C– carbons (Ca ) at 285.0 eV (33% of total area); (ii) the band at a B.E. of 286.6 eV (Cb ) corresponds to –C–O– links (47% total area); (ii) the shoulder at 288.0 eV (Cc ) is associated to O–C–O links (20% total area). However, RC-6(w) spectra only shows a practically symmetric and well defined peak at a B.E. of 285.0 eV. Moreover, no changes exist for the O 1s core level of dry and wet samples as can be seen in Fig. 3b. These results confirm the re-organization of the different chemical elements in the wet sample, which causes the displacement of the more hydrophobic aliphatic carbon to the surface. In any case, a partial cleaning of the membrane surface as a result of water contact may also exist. Differences between dry and wet samples obtained by XPS analysis agree with previous infrared results, which showed the  (OH) tensions and the silicone alkyl organic groups in the wet sample [20]. The effect of water on the membrane crystal structure was analyzed by X-ray diffraction and Fig. 4 shows the diffraction patterns for RC-6(d) and RC-6(w) samples; for reason of clarity, the spectra for RC-6(d) membrane was shifted up without any scale modification. These diffractograms are similar to those reported by Ruan et al. [21] for a microporous regenerated cellulose membrane, and they correspond to the (1 1¯ 0), (1 1 0) and (2 0 0) planes of cellulose II crystal [22]. However, RC-6(w) diffraction pattern shows a shoulder at 26◦ and a decrease in the peak intensity for all planes, which is indicative of an amorphous phase and, consequently, a loose of membrane-material crystallinity.

Fig. 3. (a) C 1s core level spectra for dry (solid line) and wet (dotted line) samples, and indication of the different carbon contribution. (b) O 1s core level spectra for dry (solid) and wet (dashed) lines.

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late transport parameters with membrane structure. Particularly, membrane elastic characteristic could be of great interest for filtration applications [5]. 3.2. Aqueous electrolyte solutions and water transport characterizations

Fig. 4. X-ray diffraction patterns for RC-6(d) and RC-6(w) samples.

The elastic behaviour of membranes is not usually independently studied, although it is of great interest when measurements involving both hydraulic and osmotic pressure are performed. The elastic characterization of RC-6(d) and RC-6(w) samples was performed by determining the normal stress-strain curve. Fig. 5 shows normal tensile stress (F/S) versus strain (L/Lo ) for dry and wet samples (Fig. 5a and b, respectively), where significant differences in the values and shape of both curves can be observed. The shape of the curve for dry samples shows a deep increase at the lowest tensile stress but it strongly decreases at highest L/Lo values (elastic deformation); however, as expected, wet samples show a more gentle increase of F/S values with enlargement (plastic deformation), associated to the water embedded into the RC-6 membrane, which reduces ten times the values of the normal tensile stress. According to this results dry samples show higher elastic or Young modulus than wet samples, which were determined from the slopes at low strain, and the following average values were obtained: Y(dry) = (9.2 ± 0.4) × 108 N/m2 and Y(wet) = (2.0 ± 0.1) × 107 N/m2 , but only slight differences for breaking elongation values between both kinds of samples were obtained (L/Lo dry  = 0.46 ± 0.02 and L/Lo wet  = 0.58 ± 0.04). Differences found in the physicochemical and elastic parameters for dry and wet RC samples is important since aqueous solutions are involved in most of the membrane separation processes of liquid mixtures and its characterization in common membrane “working conditions” is necessary to in order to corre-

The electrical nature of membranes is an important point for their characterization since it can strongly affect the transport of electrolytes or charged species. The effective charge exhibited by a membrane might be a material property or to be related with the adsorption of ions from the solutions in contact. The effective charge affects the transport of ions or charged species across a membrane and its effect is usually characterized by determining the ion transport number, ti , which represents the ratio between the electric current transported by one ion with respect to the total current crossing the membrane (ti = Ii /IT , consequently i ti = 1), and for single salts: t+ + t− = 1. Ion transport number can be determined measuring the cell or concentration potential, Ecell , or electrical potential difference measured at both sides of a sample separating two solutions of the same electrolyte but different concentrations (c1 and c2 ) [23]: Ecell = −

  RT   + F

c2

t+ dc±

(1)

c1

where + is the cation stoichiometric number ( = + + − ), R and F are the gas and Faraday constants, respectively, and T is the thermodynamic temperature of the system. For an ideal cationexchange membrane t+ = 1 and Ec reaches the maximum value: Emax = −[(/+ ) (RT/F)] ln(cv /cf ), then, the cation transport number in the membrane for each pair of solutions can be obtained by [23]: t+ =

Ecell Emax

(2)

It may be indicated that the possible contribution of water transport across the membrane is not included in Eq. (1) and the transport number obtained is usually considered as an “apparent” transport number, which could slightly differ from real membrane transport number depending on membrane nature and structure [23]. Fig. 6 shows Ecell as a function of the logarithm of the concentration ratio of the electrolyte solutions in both half-cells for the different electrolytes studied (no activity values were considered for the range of concentration studied); for comparison, cell potentials for an ideal cation-exchanger membrane are also represented in Fig. 6 (solid line for 1:1 electrolytes and dotted line for 2:1 elec-

Fig. 5. Normal tensile stress (F/S) versus relative deformation (L/Lo ) for: (a) dry samples; (b) wet samples.

J.D. Ramos et al. / Journal of Membrane Science 352 (2010) 153–159

Fig. 6. Measured cell potential, Ecell , as a function of ln(Cv /Cf ) for the RC-6 membrane and the different electrolyte solutions: () NaCl, (♦) KCl, () LiCl, () MgCl2 and () CaCl2 .

157

Fig. 8. Apparent cation transport number, t+ ap , as a function of the average concentration Cavg . () NaCl, (♦) KCl, () LiCl, () MgCl2 and () CaCl2 .

of both lines (Cmax ) by using the following expression [24]: trolytes), where a slight deviation of the Ecell − ln(cv /cf ) linearity at low concentrations can be observed for concentrations higher than 0.02 eq/l (dot-dash-dot lines). However, due to Ecell values include both the electrical potential associated to the membrane itself (membrane potential) and the electrode potential (elec ), the membrane electrical behaviour cannot be observed from these values. Membrane potentials, ˚memb , were determined by subtracting the electrode contribution to measured Ecell values and their dependence with concentration ratio is shown in Fig. 7a for 1:1 electrolytes and Fig. 7b for 2:1 electrolytes. Membrane potentials show significant differences depending on the kind of electrolyte, but in both cases two lines with different slopes where obtained for variable concentration values lower or higher that 0.02 eq/l. These two different branches are attributed to the effect of the small negative membrane fixed charge, Xf , which causes a Donnan exclusion of the co-ion (anion) at solutions concentrations C lower than Xf , while at higher C values the charges in the solutions are able to screening the membrane fixed charge, then the Donnan exclusion can be neglected and the membrane potential corresponds to the different diffusion mobility of the ions in the membrane or diffusion potential [23]. Xf value for each electrolyte can be determined from the slope of the right-hand line (diffusion branch) and the intercept

Xf =

2 z Cmax U (1 − U 2 )

(1/2)

(3)

where parameter U is related to the diffusion branch slope. Xf values for the RC-6 membrane in contact with the different solutions are indicated in Table 2, where slight differences depending on the electrolyte can be seen. Cation transport numbers across the RC-6 membrane were determined using Eq. (2) and their variation with the average concentration, Cavg = (Cf + Cv )/2, for the different electrolyte solutions is shown in Fig. 8. This picture also shows two linear but clearly differentiates zones: at low concentrations, where the Donnan exclusion of the anion is important, significantly higher t+ values were obtained but they strongly decrease with the increase of average solution concentration (zone lw ); while lower t+ values were obtained for the highest concentration (zone h), but they slightly depend on Cavg . Then, the average cation transport numbers in zone h for each electrolyte, t+ , were determined and the values are also indicated in Table 2. As was already indicated, water transport can reduce the values obtained for cation transport numbers across the membrane and true values seem to increase the average value around 10% in the case of NaCl and KCl solutions but 15% for LiCl, CaCl2 and MaCl2 electrolytes.

Fig. 7. Membrane potential versus ln(Cv /Cf ). (a) () NaCl, (♦) KCl, () LiCl; (b) () MgCl2 , () CaCl2 .

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Table 2 Fixed charge concentration, Xf , average cation transport number, t+ , average cationic permselectivity, P(+), salt diffusion coefficient, Ds , and membrane/solution salt diffusion coefficient ration, Ds /Ds o , for RC-6 membrane and the different electrolytes. Electrolyte

NaCl

KCl

LiCl

MgCl2

CaCl2

Xf (M) t+  P(+) (%) Ds (m2 /s) Ds /Do

−1.0 × 10−2 0.54 ± 0.04 24.8 1.8 × 10−10 0.11

−1.4 × 10−2 0.61 ± 0.04 23.5 2.5 × 10−10 0.13

−1.6 × 10−2 0.44 ± 0.04 17.0 3.7 × 10−10 0.27

−0.9 × 10−2 0.43 ± 0.04 9.8 2.1 × 10−10 0.25

−0.8 × 10−2 0.45 ± 0.02 5.2 2.5 × 10−10 0.18

Differences among t+  values can not directly be related with the barrier effect of the membrane due to the different solution o ). For that reason, ion permselectivity, S(i), is transport numbers (t+ commonly used to quantify the effect of a membrane on the transport of different counter-ions. For negatively charged membranes, cationic permselectivity S(+) can be expressed by [23]: S(+) =

o t+ − t+ o 1 − t+

(4)

The RC-6 membrane cationic permselectivity for the studied electrolytes was determined by using Eq. (4) and the values are also given in Table 2. As can be observed the effect of the membrane charge on the transport of ions is strongly reduced when solutions with X +2 cations are measured. Salt or diffusional permeability across a membrane, Ps , can be obtained from salt diffusion measurements. Fick’s first low establishes the relation between the solute flux crossing the membrane at steady-state, Js , and the concentration difference which causes such flux, C = (Cf − Cr ): Js =

dn = Ps C = Ps (Cf − Cr ) dt

(5)

Eq. (5) can also be expressed as Sm dCr = Ps dt Cf − Cr Vo

(6)

Vo and Sm are the volume of the receiving cell and the membrane area, respectively. Taking into account the mass continuity: Cf o + Cr o = Cf t + Cr t = cte, where Cf o and Cr o indicate the initial concentrations and Cf t and Cr t at time t, the following expression is obtained [25]: ln([1 −





Sm 2Cr ] = −2 · Ps · t Cf (Vo · xw )

(7)

where xw represents the (wet) membrane thickness. Ps values for each electrolyte and concentration were obtained from the slopes of the corresponding straight-lines. Variation of Ps with feed concentration is shown in Fig. 9a for 1:1 electrolytes and in Fig. 9b for

2:1 electrolytes; practically constant Ps values for the whole range of concentrations were obtained for 2:1 solutions, but a significant increase in diffusional permeability for 1:1 electrolytes at low concentrations can be observed; this behaviour was already reported in the literature for other cellulose membranes and 1:1 salts [10,26] and it may be related to electrical membrane–solute interactions. It should be pointed out the relatively high Ps values for the RC6 membrane when compares with its low hydraulic permeability (Lp = 7 × 10−13 m/s Pa) evidences the pressure compaction during filtration process (compaction factor ≈ 15%) associated to the membrane elastic characteristics. The Ps –Cf curve obtained for 1:1 electrolytes is characteristic of single layers charged membranes according to Filipov et al. [27], and they obtained the following expression relating salt permeability with the fixed charge concentration and diffusion coefficient (Ds ):

 Ps =

Ds Xf 2Cf xw

 

1+



2Cf Xf

1/2 



−1

(8)

where represents the membrane partition coefficient ( = cmembr /csolt ), which can be taken as 1 for membranes with high water content as RC-6 membrane [26]), and xw is the wet membrane thickness. Dotted lines in Fig. 9a represent the fitted curves, which were obtained using the Xf values previously determined and the diffusion coefficients are also indicated in Table 2; in the case of the 2:1 electrolytes diffusion coefficient a was obtained using the average permeability by: Ds = Ps xw [1]. In all cases, diffusion coefficient or diffusivity across the membrane is one order of magnitude lower than in solution, which is attributed to the friction of solute and the cellulose chains due to the dense structure of RC-6 membrane. The membrane/free-solution diffusion coefficient ration, Ds /Ds o , was calculated and the values are also presented in Table 2. According to these results, the following sequence for the diffusional velocity of the solute across the membrane can be considered: LiCl > CaCl2 ≈ MgCl2 > KCl > NaCl; these sequence evidences the high influence of the solute hygroscopic character on diffusional transport across the RC-6 membrane, but it also can be

Fig. 9. Diffusional permeability versus feed concentration. (a) () NaCl, (♦) KCl, () LiCl; (b) () MgCl2 , () CaCl2 .

J.D. Ramos et al. / Journal of Membrane Science 352 (2010) 153–159

taken as an indication of its separation efficiency (represented by the higher/lower velocity of transport) for the studied electrolytes. Similar differences for the sequence of diffusion coefficients across pristine and cross-linked chitosan membranes and solute size were also reported [28]. Water permeability across the RC-6 membrane, Pw , was determined for five different feed activities by following a procedure similar to that indicated above for diffusional permeability, and very similar values were obtained for the different solution activity measured. Average water permeability and diffusion coefficient across the RC-6 membrane are: = (3.7 ± 0.4) × 10−6 m/s and Dw  = (3.2 ± 0.4) × 10−10 m2 /s, respectively, and both values are of the same order of magnitude than that obtained for electrolyte diffusion. The membrane/self-diffusion water coefficient ratio is: Dw /Dw o = 0.17, which is also in concordance with the values obtained for the electrolyte solutions and support the validity of the obtained results. 4. Conclusions Physicochemical and elastic characterizations of dry and wet samples of a dense regenerated cellulose (RC) membrane were carried out. Results show significant differences between both kinds of samples when membranes are manufacture from a high hydrophilic membrane-material as RC. Membrane elastic characterization may be considered for estimation of best filtration conditions. An electrochemical characterization of the RC-6 membrane was performed by measuring membrane potential and diffusional permeability with aqueous solutions of different 1:1 electrolytes (NaCl, KCl and LiCl) and 2:1 electrolytes (MgCl2 and CaCl2 ). These measurements allow the determination of the effective fixed charge concentration, ion transport numbers and permselectivity, as well as the diffusional permeability and diffusion coefficient in the membrane. These results show stronger effect of electrical interactions on ionic/diffusive transport in the case of 1:1 electrolytes due to the small negative effective fixed membrane charge, while solute hydration seems to favour diffusional transport in the case of 2:1 electrolytes. Solute–membrane interactions significantly affect the diffusive transport and they drastically reduce both water and salt diffusion coefficients (between 80% and 90%). Acknowledgements We thank to Comisión Interministerial de Ciencia y Tecnología (CICYT, Project MAT/2007-65065, Spain) for financial support. References [1] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1992. [2] L. Zhang, G. Yang, L. Xiao, Blend membranes of cellulose cuoxan/casein, J. Membr. Sci. 103 (1995) 65.

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