Diclofenac Transport through Stagnant Sandwich and Supported

transport (counter transport reached at 1.75 vs 26.3 h) for the SLM system than for ... diclofenac transport in the SLM from the feed to the strip was...
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Ind. Eng. Chem. Res. 2006, 45, 9115-9121

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Diclofenac Transport through Stagnant Sandwich and Supported Liquid Membrane Systems Raffaele Molinari,* Angela Caruso, Pietro Argurio, and Teresa Poerio Department of Chemical Engineering and Materials, UniVersity of Calabria Via P. Bucci, 44/A, I-87030 Rende (CS), Italy

The removal of a drug, diclofenac (DCF), from aqueous media by using a supported liquid membrane (SLM) and a stagnant sandwich LM (SSwLM) has been investigated. The two systems were compared in terms of flux and stability. Optimal chemical conditions were determined by means of liquid-liquid extraction tests. In the transport tests, the obtained results indicated a higher initial flux J0 (2.4 vs 1.3 mmol/h‚m2) and a faster transport (counter transport reached at 1.75 vs 26.3 h) for the SLM system than for the SSwLM system. The diclofenac transport in the SLM from the feed to the strip was quite complete after 3 h, whereas in the SSwLM, only 39% of the initial drug was recovered in the strip phase after 30 h. Higher system stability was obtained using the SSwLM system (>120 vs 47.5 h). The overall results obtained in this work showed that liquid membrane systems can effectively be used to remove pharmaceuticals present at low concentration in wastewaters. Introduction The presence of pharmaceutically active compounds (PhACs) in the aquatic environment has recently assumed increased importance. Many studies have been performed to determine the types of pharmaceuticals, their concentrations, and the causes of this presence. Particularly, Heberer detected more than 80 compounds, from various prescription classes (analgesics, antiinflammatories, antibiotics, antiepileptic drugs, etc.), up to the microgram per liter level in sewage, surface water, and groundwater.1 Several investigations have shown evidence that some substances of pharmaceutical origin are not removed during wastewater treatment and also are not biodegraded in the environment.2,3 The occurrence of pharmaceutical residues in the environment might also be caused by agricultural applications of large amounts of PhACs as veterinary drugs and feed additives in livestock breeding or by pharmaceutical manufacturing discharges. On this basis, the demand for the development of efficient systems for removing these compounds from water has assumed a great research interest. Membrane operations are increasingly employed in many industrial sectors (e.g., textile, agro-food, paper, pharmaceutical, milk/cheese) as important alternative technologies to the classical processes of separation such as distillation, crystallization, solvent extraction, and precipitation. Particular types of membranes that have been the focus of numerous studies in the past decades are the liquid membranes. They consist of an organic solvent immobilized in the pores of a hydrophobic microfiltration membrane. This liquid membrane (LM) phase contains an extractant agent (carrier) that binds with high selectivity to one or a class of components in the feed phase, transporting it (or them) to the receiving phase (strip) through the membrane. The target molecule present in the feed forms a complex with the carrier that is soluble in the organic phase of the liquid membrane. After diffusion in the acceptor phase, the molecule has a form that prevents its return through the membrane. * To whom correspondence should be addressed. Tel.: +39 0984 496699. Fax: +39 0984 496655. E-mail: [email protected].

Of great importance is the specific nature of the carriersolute interactions allowing highly selective separations. The choice of the carrier will be made, therefore, considering the solute to be transported, as well as the experimental conditions.4 Depending on the mode in which the carrier is retained in the membrane, different types of liquid membranes can be distinguished, for example, supported liquid membranes (SLMs), “sandwich” liquid membranes (SwLMs), emulsion liquid membranes (ELMs), among others. SLMs have been studied in the transport of various metal ions,5-15 as well as different molecules of biological interest such as amino acids,16,17 sugars,18-20 proteins,21 and numerous drugs, including penicillin G,22 penicillin V,23 and eritromicin.24 Despite the many advantages of supported liquid membranes (e.g., easy membrane preparation, high selectivity of the process with a suitable carrier molecule, low operating costs), they are not yet used at large scale in industrial applications because of the low stability, or lifetime, of the membrane. Common problems causing instability of liquid membranes are (i) loss of the organic phase due to membrane dissolution, formation of droplets of emulsion, pressure differences through the membrane, and solvent evaporation; (ii) chemical characteristics of the support; and (iii) method of preparation. Many studies have shown that one of the principal causes of instability is the loss of the LM phase (carrier and/or solvent) from the pores of the support, followed by its substitution by aqueous solutions, influencing both the flow and the selectivity.25, 26 A possible alternative to minimize these problems is the use of stagnant sandwich liquid membranes (SSwLMs).27,28 In this configuration, the liquid membrane is an organic phase confined between two hydrophilic membranes that separates the feed and strip phases. The present article analyzes the different behaviors of SLMs and SSwLMs for the separation, from aqueous solutions, of a pharmaceutical molecule, diclofenac, in aqueous solution using tributyl phosphate (TBP) as the carrier. Diclofenac is a pharmaceutically active component, a nonsteroidal antiinflammatory drug, that has commonly been detected in wastewaters.1,2

10.1021/ie0607088 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/03/2006

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Figure 1. Structural formula of sodium diclofenac.

Experimental Section Reagents. Diclofenac is a sodium salt of (2-[(2,6-dichlorophenyl)amino]benzeneacetic acid (Figure 1), MW ) 318.1 g/mol, and was obtained from Sigma. Diclofenac solutions were prepared by dissolving it in ultrapure water (Elix 5, Millipore). The pHs of the aqueous phases were maintained at the optimal operating values by using (i) phosphate buffer (50 mM) prepared with sodium dihydrogen phosphate monohydrate (MW ) 137.99 g/mol, purity ) 99%, Fluka), (ii) citrate buffer (50 mM) prepared with citric acid monohydrate (MW ) 210.1 g/mol, purity ) 99%, Sigma), and (iii) droplets of sulfuric acid (H2SO4, 96% w/w solution in water, Carlo Erba) and sodium hydroxide (NaOH, analytical purity, Merck) at suitable concentrations. The extractant tested as a carrier in L-L extraction and membrane transport tests was tributyl phosphate (TBP, MW ) 266.32 g/mol, purity ) 97%), obtained from Aldrich. It was selected after tests with various types of possible carriers. The organic solvent used to dissolve the carrier was n-decane (MW) 142.29 g/mol, purity ) 95%), supplied by Merck. Flat-sheet microfiltration hydrophobic membranes of polypropylene (Accurel, manufactured by Membrana, thickness ) 142 µm, pore size ) 0.2 µm, porosity ) 70%) were used in the SLM tests. In the SSwLM experiments, two different hydrophilic supports were tested: polyethersulfone (PES) ultrafiltration membranes IRIS 10 (cutoff ) 10 kDa) and IRIS 30 (cutoff 30 kDa) from Tech-Sep. Equipments and Methods. Diclofenac concentration measurements were carried out using the spectrophotometric technique. Absorbance readings at a wavelength of 276 nm were performed using a recording spectrophotometer (UV-1601, Shimadzu Corporation - Analytical Instruments Division). A pH meter (WTW Inolab terminal level 3) with a SenTix 81 glass pH electrode (WTW) was used for pH measurements. Total organic carbon (TOC) measurements were carried out with two systems: an LT 100-1 thermostat, an LASA 100 photometer, and analytical kits LCK 380 and LCK 381 (depending on the concentration of total carbon estimated in the sample to analyze) from Dr. Lange and a TOC-V CSN analyzer (Shimadzu). The membrane surfaces (Iris 10 and Iris 30) were observed using an FEI QUANTA 200 scanning electron microscope (SEM). The images were processed with a Scion Image program (from Meyer Instruments, Inc., and Scion Corporation). Some of the experiments were repeated several times, giving agreement of the data to within 5-8%. L-L Extraction Test Procedure. All L-L extraction tests were carried out in a test tube by mixing 5 mL of aqueous diclofenac solution ([diclofenac]in) 20 mg/L) with 5 mL of organic phase at 25 °C temperature. The organic phase was prepared by dissolving the carrier (30% v/v) in n-decane. In each extraction step, to ensure sufficient contact time, the two phases were stirred three times with a tube stirrer at 10-min intervals. After a resting period of at least 1 h to leave time for the phases to separate and equilibrium to be reached, the

diclofenac concentration in the aqueous phases was measured. In these L-L extraction tests, the equilibrium time was selected on the basis of our previous results for copper extraction, for which 10 min was the equilibrium time.29 In the present work, a triplicate time was chosen. The data from L-L extraction tests were used to calculate the extraction percentage, E (%), from the following equation

E (%) )

(nTC)org (nTC)org + (nTC)aq (nTC)org

(nTC)aq ×

[

(nTC)org (nTC)aq

× 100 )

+1

]

× 100 )

Kd × 100 (1) Kd + 1

where TC is the target component (in this study diclofenac) and Kd is the partition coefficient, defined as Kd ) (nTC)org/ (nTC)aq and equal to Kd ) [TC]org/[TC]aq when Vaq ) Vorg. Method of Preparation of Liquid Membranes and Scheme of the Experimental System. The supported liquid membrane was prepared by dipping the solid support into the liquid membrane organic phase overnight (even though 1 h is already enough). Then, it was wiped with a soft paper cloth to eliminate the excess solution and inserted into the permeation cell. SLM permeation tests were performed in two different permeation cells: The first one had an active membrane surface area of 4 cm × 6 cm ) 24 cm2, whereas the second one had an active surface area of 2.4 cm × 2.4 cm ) 5.8 cm2. The SSwLM system was prepared by placing the organic solution, sonicated for 20 min to eliminate the presence of air bubbles, inside a chamber with a volume of 58 µL and an initial thickness of 100 µm. The chamber was obtained by positioning an appropriately cut commercial triacetate sheet between two hydrophilic membranes, so that the film acted as a spacer containing the organic liquid phase. This sandwich was assembled in the smaller permeation cell with an active surface area equal to 2.4 cm × 2.4 cm ) 5.8 cm2. All experiments on diclofenac transport through the different membrane systems were performed in the laboratory plant schematized in Figure 2, with the SLM or the SSwLM placed between the feed and strip compartments. The global system worked in batch mode, but each module element in the loop worked in continuous flow with a tangential co-current flow for the aqueous phases. The setup consisted of two loops in which the feed and strip phases were separately recirculated with equal flow rates (Q) of 75 mL/min. The feed and strip reservoirs had a volume of 75 mL and were immersed in a thermostatic water bath at 25 °C, so that the process took place under isothermal conditions, at the same temperature as used in L-L extraction tests. At established time intervals, samples were withdrawn from the feed and strip reservoirs, and the diclofenac concentrations were measured. The feed and strip pHs were adjusted to the operating value by adding some drops of 1 M H2SO4 in the feed and 1 M NaOH in the strip. Two peristaltic pumps (Masterflex Easy Load), splined to the same shaft operated by an electric engine (Masterflex Console Drive), generated equal flows. To compare the SLM and SSwLM fluxes, the following mass balance equation for diclofenac in the feed was used

JDCF )

(

)

VF d[TC] × Sexpm dt

Feed

(2)

where TC is the target component (in this study, diclofenac);

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Figure 2. Scheme of the apparatus for a flat-sheet SLM or SSwLM operating in continuous recirculation flow: (1, 2) compartments with feed and strip phases, (3, 4) peristaltic pumps, (5, 6) membrane cell elements, (7) membrane, (8) feed-phase loop, (9) strip-phase loop, (10) thermostatic water bath, (11, 12) manometers.

VF is the feed-phase volume; and Sexp is the membrane surface exposed to the feed and strip phases (it is the installed membrane area needed in practical applications), which is different from the effective membrane surface (Seff) over which transport actually occurs because of membrane surface porosity (m): Seff ) Sexpm. Results and Discussion To find the chemical conditions to be employed in the two membrane systems, tests on L-L extraction at different carrier concentrations and the chemical behavior of diclofenac at various pHs were performed. L-L Extraction Tests. Tributyl phosphate (TBP) is a cationic carrier; in acidic solution, it is positively charged because of protonation

(Car)org + (H+)aq h (Car+)org

(3)

where Car represents the carrier; Car+ represents the protonated carrier; and the subscripts aq and org indicate the aqueous and organic phases, respectively. The positively charged carrier binds the carboxylic group of diclofenac, forming a neutral complex according to the following equilibrium complexation reaction

(ArCH2COO-) aq + (Car+)org h (ArCH2COOCar)org

(4)

where ArCH2COO- represents diclofenac and ArCH2COOCar represents the neutral complex formed. In alkaline solution the neutral complex ArCH2COOCar releases the drug by means of the following equilibrium decomplexation reaction

(ArCH2COOCar)org + (OH-)aq h (ArCH2COO-)aq + H2O + (Car)org (5) From eqs 3-5 it can be observed that, in the liquid membrane systems, both H+ and the drug move from feed to strip, i.e., the mechanism is a co-coupled transport. The results obtained using TBP at different volume percentages in the range of 1.72-70% (Figure 3) show the following optimal conditions to be employed in the transport tests of

Figure 3. Extraction percentage vs equilibrium pH in L-L extraction tests with TBP at various volume percentages in n-decane (Vaq ) Vorg ) 5 mL, Cf,in ) 20 mg/L).

diclofenac in the liquid membranes: feed phase, aqueous solution of diclofenac at a pH of about 6.0; strip phase, ultrapure water at a pH of around 11; organic phase, 30% v/v TBP in n-decane. In the L-L tests, the analyses of the samples withdrawn from the three aqueous phases every 10 min confirmed that equilibrium was definitely reached within the time needed to obtain phase separation. The choice of the optimal carrier concentration in the organic phase was made by taking into account the following considerations: (i) Operating at 50-70% v/v carrier concentration, solution clouding was observed. (ii) Operating at 1.72% v/v carrier concentration means a 1:1 ratio between the numbers of moles of carrier and diclofenac, but in an SLM application, this low carrier concentration results in a molar deficiency, because of the lower volume of organic phase compared to the volume of the aqueous phases. Thus, a carrier concentration of 30% v/v was chosen as the optimal carrier concentration in the organic liquid membrane (LM) phase. Diclofenac Behavior at Various pH Values. Upon analysis of the diclofenac UV absorbance at two pH values (6.0 and 2.7), it was noticed (Figure 4) that the two solutions, both at an initial concentration of 20 mg/L, exhibited absorbance peaks at the same wavelengths, but of smaller intensity for the acid solution. The data reported in Table 1 indicate the tendency for the initial concentration to decrease as the pH decreases for the wavelength of 276 nm. This behavior is due to the formation of insoluble diclofenic acid, as shown in the following equi-

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Figure 5. Feed concentration vs time in various tests of diclofenac transport in an SLM with and without buffers. (Feed phase: Cin ) 20, 10, 5 mg/L; V ) 75 mL; pH 6.0. Strip phase: Cin ) 0 mg/L, V ) 75 mL, pH 11. Organic phase: 30% v/v TBP in n-decane, Q ) 75 mL/min, CBuffer ) 50 mM.)

Figure 4. Absorbances at various wavelengths of diclofenac solutions (20 mg/L) at pH 6.0 and pH 2.7. Table 1. Variation of Diclofenac Concentration with pHa diclofenac concentration (mg/L)

a

initial pH

after 3 h

after 24 h

6.2 4.9 4.1 3.2 2.6 2.2

21.1 ( 0.3 20.1 ( 0.2 5.25 ( 0.1 3.01 ( 0.2 2.53 ( 0.2 2.04 ( 0.1

20.7 ( 0.2 19.8 ( 0.3 3.24 ( 0.2 1.49 ( 0.1 1.21 ( 0.1 1.22 ( 0.2

Initial concentration ) 20 mg/L.

Figure 6. Strip concentration vs time in various tests of diclofenac transport in an SLM with and without buffers. (Feed phase: Cin ) 20, 10, 5 mg/L; V ) 75 mL; pH 6.0. Strip phase: Cin ) 0 mg/L, V ) 75 mL, pH 11. Organic phase: 30% v/v TBP in n-decane, Q ) 75 mL/min, CBuffer ) 50 mM.)

librium reaction

ArCH2COO- + H3O+ h ArCH2COOH + H2O

(6)

However, this behavior did not cause operating problems for the liquid membrane, because the strip and feed pHs previously found were in the range 5-11. Transport Tests of Diclofenac in the SLM System. Transport SLM tests were performed using aqueous feed solutions at diclofenac concentrations equal to 20, 10, and 5 mg/L and at an initial pH of approximately 6.0 in the permeation cell with an active membrane surface area equal to 24 cm2. During transport tests, the pHs of the aqueous phases were controlled by using phosphate or citrate buffers (50 mM). Only one test was carried out without pH control, to evaluate the influence of the buffer on diclofenac transport across the membrane system. The results, reported in Figures 5 and 6 as normalized concentrations ([diclofenac]/[diclofenac]feed,in) of the feed and strip phases as a function of the time, show that the decrease of the feed concentration and the increase of the strip concentration follow approximately complementary trends, except in the unbuffered transport test. In the latter test, it was not possible to maintain the feed pH at acceptable values. Indeed, the feedphase pH decreased sharply to values lower than 5, as a result of acidification upon contact with the organic phase, which caused diclofenic acid formation. The experimental data on diclofenac concentration in the feed versus the time, Cf(t), show an exponential decay that is fitted by the equation

Cf(t) ) a · e-bt

(7)

where a and b are constants. The good quality of the fit was evidenced by the calculation of the regression factor (R2), which was always greater than 93%; it gave the values of the a and b constants. The decrease of the diclofenac concentration in the feed is due to both transport through the membrane and accumulation on the membrane support. To obtain the contribution of transport alone, the diclofenac concentration in the feed was corrected by a mass balance. Because the feed and strip volumes are equal, the mass balance is expressed as

Cf′(t) ) Cf,in - Cs(t)

(8)

where Cf,in is the initial concentration in the feed [Cf(t) at t ) 0 min] and Cs(t) is the value of the diclofenac concentration in the strip at the same time t. Differentiation of eq 7 and substitution into eq 2 gives the flux J as a function of time

J(t) ) -(dCf′(t)/dt)(VF/Sexp) ) (VF/Sexp)a′be-bt

(9)

where VF ) 7.500 × 10-5 m3; Sexp ) 2.400 × 10-3 m2; a′ is the constant a divided by the drug molecular weight; and m is assumed equal to 1, which means that the flux is referred to the exposed surface. Use of this equation, with respect to the simple interpretation of the experimental data, has the advantage of reducing errors from the scattering of single experimental points by fitting the entire concentration range.

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Figure 7. Diclofenac concentration versus time for the four feed-strip pairs sequentially injected in the SLM test with initial membrane washing. (Feed phase: Cin ) 20 mg/L, V ) 75 mL, pH 6.0, phosphate buffer. Strip phase: Cin ) 0 mg/L, V ) 75 mL, pH 11, phosphate buffer. Organic phase: 30% v/v TBP in n-decane, Q ) 75 mL/min, CBuffer ) 50 mM, active surface membrane area ) 24 cm2.)

On the basis of eq 9, the initial flux J(t ) 0) ) J0 was calculated, and this parameter was used to compare the different transport tests. The obtained data showed that, as the initial concentration of diclofenac in the feed was decreased, J0 also decreased (1.3, 0.84, and 0.49 mmol/h‚m2 for 20, 10, and 5 mg/L initial feed concentrations, respectively) but not linearly, indicating a saturation effect of the carrier. In all tests, for times greater than 150-180 min, the system showed destabilization upon passage of the solution from one side of the liquid membrane to the other. Through TOC analyses of the two phases, it was found that passage of the organic phase from the membrane phase to both aqueous solutions occurred, probably because of the not negligible solubility (6 g/L at 20 °C) of TBP in the aqueous phases, which caused system destabilization. To minimize this problem, a simple approach was tested: The newly prepared SLM, already inserted in the cell, was washed with ultrapure water on both sides before contacting with feed and strip phases. The purpose was to eliminate the presence of surplus of organic phase on the membrane surfaces that was not removed by the soft paper cloth and could be the first cause of destabilization. A significant increase in stability was observed, giving transport for over 30 h, by replacing three consecutive feed and strip phases (Figure 7). The initial flux J0 was higher than that obtained in the tests without washing (1.7 vs 1.3 mmol/h‚m2). Better results for SLM performance were obtained by using phosphate buffer (50 mM), feed solution at a diclofenac concentration of 20 mg/L, feed pH approximately equal to 6.0, and washing of the SLM with ultrapure water before starting the transport test. The poorer results obtained by using the citrate buffer (see Figure 6) were due to its competition with the drug, because its carboxylic groups bind with the carrier, causing a decrease of available carrier molecules for drug transport. Some SLM tests were also carried out working with these latter operating conditions using the permeation cell with the lower active membrane surface area (5.8 cm2). The results, reported in Figure 8, show that, for the cell with the smaller membrane surface area, the general trend of J0 vs time is about the same as that observed for the higher-surfacearea membrane. However, a higher initial diclofenac flux (J0 ) 2.4 vs 1.7 mmol/h‚m2) and greater system stability (47.5 vs 24.2 h) were obtained using the cell with the lower membrane area, probably because of the differences in the fluid dynamics in the two cases. (The tangential velocity on the membrane surface was higher in the case of the smaller cell, as the flow rate was the same in both systems.)

Figure 8. Comparison of the SLM initial flux (J0) using two permeation cells with different surface membrane areas and changing the feed and strip pairs in time. (Feed phase: Cin ) 20 mg/L, V ) 75 mL, pH 6.0. Strip phase: Cin ) 0 mg/L, V ) 75 mL, pH 11. Organic phase: 30% v/v TBP in n-decane, Q ) 75 mL/min, CBuffer ) 50 mM.)

Figure 9. Behavior of the feed and strip concentrations in time during SSwLM diclofenac transport tests with Iris 10 and Iris 30 membranes. (Feed phase: Cin ) 20 mg/L, V ) 75 mL, pH 6.0. Strip phase: Cin ) 0 mg/L, V ) 75 mL, pH 11. Organic phase: 30% v/v TBP in n-decane, Q ) 75 mL/ min, CBuffer ) 50 mM.)

Transport Tests of Diclofenac in the SSwLM System. Transport tests in the SSwLM system were carried out using the Iris 10 and Iris 30 membrane supports. The membrane supports were previously characterized by measurements of the water flux at 2 bar: the values obtained were 60.4 and 100.4 L/h‚m2, respectively. By elaborating the SEM images of the active surface of both membranes using the Scion Image program, practically the same surface porosity was obtained (14.5% ( 0.5%), as well as pore size estimates of 66 nm for the Iris 10 membrane and 97 nm for the Iris 30 membrane. In the two SSwLM systems, the influence of the membrane pore size on the transport performance (flux and stability) was evaluated. The two systems were also compared in terms of counter transport time, meaning the time when transport against the drug concentration gradient (uphill transport) occurs. The results of the SSwLM transport tests showed a flux of 1.3 mmol/h‚m2 for the Iris 10 support and a time needed to obtain the counter transport of 1580 min (26.3 h) (Figure 9). For the Iris 30 membrane, a flux of 0.62 mmol/h‚m2 was obtained, and a lower time, 495 min (8.25 h), was needed to achieve counter transport. The stability in both the SSwLM systems was greater than 120 h. The lower time to reach counter transport for the Iris 30 support compared to the Iris 10 support, 8.25 vs 26.3 h, does not agree with the previously reported values of the flux or with the general trend reported in Figure 9. However, consider the following two points: (i) A mass balance for each type of membrane, when counter transport was achieved, expressed as the total diclofenac concentration (cFEED + cSTRIP), because VF ) VS, gave values of about 18.5 and 11.2 mg/L for the Iris 10 and Iris 30 supports, respectively. (ii) The total diclofenac

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Table 2. Mass Balance in Washing the SSwLM Made with the Iris 30 Membrane with a Solution at pH 11 at the End of a Runa mass of diclofenac (mg) DCFIN DCFOUT DCFACC DCFWASH-FEED DCFWASH-STRIP DCFTOTAL WASH

1.50 1.02 0.480 (calculated) 0.377 0.083 0.460 (measured)

a Feed and strip phases: C ) 0 mg/L, V ) 75 mL, pH 11. Organic in phase: 30% v/v TBP in n-decane, Q ) 75 mL/min, CBuffer ) 50 mM.

concentrations at a transport time of 1500 min were 18.4 and 13.6 mg/L for the Iris 10 and Iris 30 membranes, respectively. Considering these two points, one can see that a higher drug amount “disappeared” in the second system and that it was released slowly in the strip, as indicated by the higher value (13.6 mg/L) for point ii than the 11.2 mg/L value for point i. To determine where (feed and/or strip sides) the diclofenac “loss” occurred, the SSwLM system made with the Iris 30 membrane was washed at the end of the run with a solution of pH 11, which is the value required for diclofenac release. A quite complete recovery of the “missing” diclofenac was observed (Table 2), in percentages of 82% and 18% for the feed and strip sides, respectively. Taking into account these results, the sharp decrease in Figure 9 of the diclofenac concentration for the Iris 30 support can be interpreted as an easier penetration of the drug into the pores on the feed side of the support. This behavior should give a higher flux than the Iris 10 support (or at most, the same, if we consider carrier saturation), because more target molecules should be in contact with the carrier in the liquid membrane. However, the higher amount of diclofenac permeating across the feed side of the Iris 30 support accumulated at the interface and probably gave two effects: (i) saturation of the carrier present at the interface and (ii) precipitation of the drug, because the initial value of 20 mg/L in the feed was close to the solubility limit. The experimental results indicate a predominance of effect ii, which means that the transport in the liquid membrane is not blocked but it is slowed. Considering diclofenac accumulation in the Iris 30 membrane pores, the comparison between the two SSwLM systems should be made not in terms of the achievement of counter transport, but by comparing the two flux values previously reported. Thus, the better SSwLM support was the Iris 10 membrane, characterized by a higher flux and lower diclofenac accumulation. Comparison between the SLM and SSwLM Systems. The SLM and SSwLM tests were compared in terms of initial flux J0, system stability, and counter transport time. The system stability was measured as the time when a significant change of the volumes of the two aqueous phases was observed. This was caused by the exiting of the LM phase from the pores of the support, followed by its substitution by the aqueous phase. In Table 3, the values of the initial flux J0, estimated using the previously introduced exponential equation (eq 9), and system stabilities are reported. A significant difference of the fluxes and stability between the two systems can be observed. In particular, a comparison of the concentrations vs time in the two best system configurations (SLM and SSwLM with the Iris 10 support) is reported in Figure 10. The results in Table 3 evidence a higher initial diclofenac flux J0 using the SLM system (2.4 vs 1.3 mmol/h‚m2). In addition, the comparison of the concentration trends reported in Figure 10 shows that a very fast transport was obtained using the SLM system. Indeed, uphill transport of the drug occurred at 1.75 and 26.3 h for the SLM

Table 3. Values of a, b, Regression Factor R2, and Initial Flux J0 Estimated Using Eq 6 for All SLM and SSwLM Separation Tests and System Stabilitya test SLM phosphate 20 SLM phosphate 20 with washing SLM phosphate 20 with washing 5.76 cm2 SLM phosphate 10 SLM phosphate 5 SLM citrate 20 SLM unbuffered SSwLM Iris 10 phosphate 20 5.76 cm2 SSwLM Iris 30 phosphate 20 5.76 cm2

a (mg/L)

b (t-1)

R2 (%)

J0 (mmol/h‚m2]

19.7

0.011

96.4

1.3

3.0

20.6

0.014

99.5

1.7

24.2

20.4

0.005

98.4

2.4

47.5

11.4

0.013

97.9

0.84

3.0

0.015

97.6

0.49

3.0

20.2

0.009

98.4

1.1

3.0

19.5

0.006

93.5

0.64

3.0

20.7

0.003

93.2

1.3

>120

21.0

0.001

93.8

0.62

>120

5.41

system stability (h)

a Feed phase: C ) 20, 10, 5 mg/L; V ) 75 mL; pH 6.0. Strip phase: in Cin) 0 mg/L, V ) 75 mL, pH 11. Organic phase: 30% v/v TBP in n-decane, Q ) 75 mL/min).

Figure 10. Comparison of concentrations in the feed and strip phases vs time in the best SLM system (with washing) and the best SSwLM system (with the Iris 10 support). (Feed phase: Cin ) 20 mg/L, V ) 75 mL, pH 6.0. Strip phase: Cin ) 0 mg/L, V ) 75 mL, pH 11. Organic phase: 30% v/v TBP in n-decane, Q ) 75 mL/min, CBuffer ) 50 mM, active surface membrane area ) 5.76 cm2).

and SSwLM systems, respectively. Moreover, the diclofenac transport from the feed to the strip in the SLM was quite complete after 3 h, whereas in the SSwLM, only 39% of the initial drug was present in the strip phase after 30 h. The evidenced difference in the transport rate of diclofenac in the two studied systems might be due to the influence of various parameters characterizing the diclofenac transport through the liquid membrane systems, such as the nanostructures of the supports used. For example, the pore size of the polypropylene support is higher than that of the Iris 10 membrane (about 200 nm vs 66 nm), permitting easier drug transport in the SLM. Another important factor is the thickness of the membrane system, which is 140 µm for the SLM and 390 µm for the SSwLM, meaning lower transport resistance in the first case. The SSwLM system did not show destabilization in 120 h of testing. At this time, the SSwLM system was stopped not for stability problems, but for excessive system slowness.

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The higher system stability obtained using the SSwLM system is due to its specific configuration, which permits minimization of the loss of the organic phase entrapped between the two hydrophilic supports. Therefore, the best liquid membrane system in terms of drug flux was the SLM, whereas considering system stability, the best system was the SSwLM. Conclusion The obtained results show that liquid membrane systems can be efficiently used to remove pharmaceuticals present at low concentration in wastewaters. Experimental data from SLM tests, performed at various initial feed concentrations, showed that the decrease in concentration corresponds to the decrease in J0 (1.3, 0.84, and 0.49 mmol/h‚m2 for initial feed concentrations of 20, 10, and 5 mg/ L, respectively). Better results for the SLM system were obtained using phosphate buffer (50 mM), an aqueous feed solution of diclofenac at a concentration of 20 mg/L, an initial pH of approximately 6.0, and washing of the SLM with ultrapure water before the transport tests. In addition, a higher diclofenac flux across the SLM (J0 ) 2.4 vs 1.7 mmol/h‚m2) was obtained by operating with the smaller membrane (5.76 cm2). This result can be ascribed to the different fluid dynamics, which also gave rise to greater system stability (47.5 vs 24.2 h). An important factor strongly influencing the transport tests in the SSwLM system was diclofenac accumulation in the pores of the Iris 30 membrane. In terms of this phenomenon, the better SSwLM support was the Iris 10 membrane, characterized by a higher flux and lower diclofenac accumulation. A comparison of the experimental results obtained for the two liquid membrane systems showed a higher efficiency in the transport of the diclofenac for the SLM system than for the SSwLM. Indeed, a higher diclofenac initial flux J0, 2.4 vs 1.3 mmol/h‚m2, was obtained using the SLM system. In addition, very fast transport was obtained using the SLM system, with faster counter transport of the drug (1.75 and 26.3 h) and quite complete drug transport in 3 h in the SLM. Higher system stability was obtained using the SSwLM system (>120 vs 47.5 h). This result is encouraging in terms of improving SSwLM system performance by choosing adequate hydrophilic supports and operating parameters that ensure an acceptable flux coupled with high stability. Acknowledgment The financial support of the MIUR (Italian Research Ministry), within the PRIN 2004 program, is gratefully recognized. Literature Cited (1) Heberer, T. Occurrence, fate, and removal of pharmaceutical residues in the aquatic environment: A review of recent research data. Toxicol. Lett. 2002, 5, 131. (2) Ternes, T. A. Occurrence of drugs in German sewage treatment plants and rivers. Water Res. 1998, 32, 3245. (3) Zwiener, C.; Glauner, T.; Frimmel, F. H. Biodegradation of pharmaceutical residues investigated by SPE-GC/ITD-MS and on-line derivatization. HRC-J. High Res. Chromatogr. 2000, 23, 474. (4) Molinari, R.; Poerio, T.; Cassano, R.; Picci, N.; Argurio, P. Copper(II) removal from wastewaters by a new synthesized selective extractant and SLM viability. Ind. Eng. Chem. Res. 2004, 43, 623. (5) Zhang, B.; Gozzelino, G. Facilitated transport of Fe(III) and Cu(II) through supported liquid membranes. Colloids Surf. A 2003, 215, 67. (6) Shampisur, M.; Kazemi, S. Y.; Azimi, G.; Madaeni, S. S.; Lippolis, V.; Garau, A.; Isaia, F. Selective transport of silver ion through a supported

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ReceiVed for reView June 5, 2006 ReVised manuscript receiVed September 22, 2006 Accepted September 25, 2006 IE0607088