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Stability of supported liquid membranes containing longchain aliphatic amines as carriers
Journal of Membrane
Science,
Elsevier Science Publishers
55 (1991) 65-17 B.V., Amsterdam
65
Stability of supported liquid membranes containing longchain aliphatic amines as carriers R. Chiarizia* Chemistry (U.S.A.)
Division,
Argonne
National Laboratory,
9700 S. Cass Avenue,
Argonne,
IL 60439
(Received April 30,199O; accepted in revised form July 16, 1990)
Abstract The stability of liquid membranes containing as carriers three different commercially available long-chain aliphatic amines, Primene JM-T (primary), Amberlite LA-2 (secondary) and trilaurylamine (TLA, tertiary) has been investigated in experiments where the feed solution was synthetic Hanford site groundwater, the strip solution was 1 M NaOH and n-dodecane was the diluent. By using flat-sheet supports the following order of stability was measured: tertiary > secondary > primary. This is the reverse order of the interfacial tension lowering at a water-n-dodecane interface. The water solubility of the amines also seems to play an important role in determining the membrane stability. Experiments performed with much thicker hollowfiber supports have shown that a larger inventory of organic phase absorbed in the membrane pores ensures a remarkably higher membrane stability. The secondary amine provides the best overall performance and is the carrier of choice for a SLM process for groundwater decontamination from anionic species. Possible alternative uses of the primary and tertiary amines for the same decontamination process are also discussed. Keywords:
membrane
liquid membranes; support; membrane
facilitated transport, groundwater membrane stability; carrier: Primene JM-T, Amberlite LA-2, trilaurylamine
Celgard
Introduction
The application of supported membranes (SLM) for the removal of contaminants from Hanford site groundwater has been reported in previous work in this series [l-3]. We suggested that the pH of groundwater be lowered to 2 by addition of sulfuric acid. At pH 2, the carrier bis(2,4,4_trimethylpentyl)phosphinic acid, contained in the commercial extractant Cyanex 272 (registered trade mark of American Cyanamid Co), is very effective in selectively removing uranium (VI) from the contaminated groundwater. After passing through a first SLM module, containing Cyanex 272 in dodecane as the liquid membrane, where the uranium separation takes place, the acidified groundwater is purified from other anionic contaminants (nitrate, pertechnetate and *On leave from the Italian Alternative
0376-7388/91/$03.50
0 1991-
and Nuclear Energy Agency (ENEA).
Elsevier Science Publishers
B.V.
66
chromate anions) in a second SLM module containing a dodecane solution of a long-chain aliphatic amine as the liquid membrane. In Ref. [3] we investigated the use of the three commercially available amines Primene JM-T (primary), Amberlite LA-2 (secondary) and trilaurylamine (TLA, tertiary) as carriers. The following order of effectiveness for single contaminants was measured: for NO,: for TcO;: tertisecondary > primary>>tertiary; ary 2 secondary > primary; for HCrO; : secondary > primary > tertiary. Based on these findings, it was concluded that the carrier of choice for the simultaneous removal of the three contaminants is Amberlite LA-2. The primary amine, however, exhibited the unique property of also removing anionic U (VI) sulfato-complexes from the groundwater acidified with sulfuric acid. This fact led us to consider the possibility of an alternative SLM process, where uranium is removed together with the anionic contaminants in a single module containing the primary amine as carrier. The membrane experiments upon which the conclusions reached in Ref. [ 31 are based were short duration experiments involving the use of flat-sheet supports without consideration for the long-term membrane performance. The stability of liquid membranes is, however, of paramount importance for the practical application of SLM based separation processes. In Ref. [2] we reported a study of the stability of Cyanex 272-n-dodecane liquid membranes absorbed on hollow fibers for the uranium separation from groundwater. We demonstrated that acceptable stability performances were obtained with periodic reimpregnation of a conventional hollow-fiber module. Encouraging results were also obtained with a self-reimpregnating module. In this work we report our results on the stability of liquid membranes containing Primene JM-T, Amberlite LA-2 or TLA as the carrier. The aim of the work has been: (i) to obtain information on the stability of amine based SLMs under the conditions likely to be used in a process application for the decontamination of groundwater from anionic species; (ii) to get the experimental evidence needed for final selection of the most appropriate carrier for the above application. Experimental
Reagents Several liters of synthetic Hanford site groundwater (SGW) were prepared as previously reported [l-3]. The composition of the SGW, after addition of enough concentrated sulfuric acid to bring the pH value to ca. 2, has been reported [ l-3 1. Primene JM-T (Rohm and Haas), Amberlite LA-2 (Rohm and Haas) and trilaurylamine (TLA, Eastman Kodak) were described in Ref. [3]. All three amines were used as received because we assumed that the unpurified commercial products would most likely be used in a process application. Cyanex 272 was obtained from American Cyanamid Co. and used as re-
67
ceived. Solutions of the carriers were prepared using n-dodecane as diluent, following the same reasoning reported in Ref. [ 11. All other reagents were analytical grade products and were used without further purification. Membrane supports The flat-sheet membrane experiments were performed using as liquid membrane support Celgard 2500 polypropylene sheets 25 pm thick, with a 45% porosity and 0.04 pm pore size. The hollow-fiber modules were made with the same Enka Q-31 fibers and using the same procedure as reported in Ref. [ 21. The fibers were made of propylene, 200 pm thick (I.D. =0.6 mm, O.D. = 1 mm) with 75% porosity and 0.1 pm pore size. Interfacial tension measurements The interfacial tension measurements were performed at 23 + 0.5’ C using the DuNouy ring method and a Fisher Surface Tensiometer, Model 20. Initially, 20 ml of n-dodecane and 30 ml of water were equilibrated in a glass cell and the interfacial tension was measured. Small aliquots of concentrated stock solutions of amine in n-dodecane were introduced into the cell and the interface was stirred for several minutes by means of a motor driven small glass stirrer. The interfacial tension was then measured again at the new amine concentration in the organic phase. The stirring of the interface and the measurements were repeated at intervals until a constant value of interfacial tension was obtained. At this point a new aliquot of stock solution was added and the measuring procedure repeated. Each set of measurements (a y vs. log C curve) was repeated at least twice. The interfacial tension data at each amine concentration were reproducible within one dyne-cm-‘. The densities of the n-dodecane solutions of the carriers, needed for the correction of the apparent interfacial tension values provided by the instrument, were measured by weighing known volumes of the solutions at 23 2 0.5 ’ C. Permeation measurements The cell used with flat-sheet membrane supports was similar to that described in Ref. [3] for the HNO, transport experiments. The feed and strip compartments were stirred by means of magnetic stirring bars, located in 0.5 cm deep circular wells with a 3.5 cm diameter, and rotated by Micro-V ColeParmer magnetic stirrers at a speed of about 300 rpm. The membrane area was 16.8 cm2 and feed and strip solutions had a volume of 100 cm3. In these experiments the acid transport took place in “non-plateau” conditions, that is, in stirring conditions where the thickness of the aqueous diffusion layer and the aqueous resistance to mass transfer were not minimized. The permeation results have been used only for comparing the stability behavior of the different amines in strictly constant experimental conditions. The modules used in the stability experiments involving hollow-fiber supports contained ten ENKA Q-
68
31 fibers, each 6.9 cm long, with a total membrane area of 12.3 cm’. The modules were operated in a recirculating mode, as reported in Ref. [2]. Feed (35 cm3) and strip (50 cm”) were circulated through the lumen and on the shell side of the fibers, respectively, by means of calibrated peristaltic pumps. The feed flow rate was 17 ml/min, which corresponds to a linear velocity of 10 cmset-I. This value of feed linear velocity ensured that the modules were operated in the plateau region ( > 8 cm-set-’ ) as determined for a similar module
[al.
With both membrane configurations the feed was SGW at pH 2 and the strip was 1 M NaOH. The acid transport through the membrane was followed by potentiometrically measuring the H+ concentration in the feed solution. An Ag/AgCl-glass Orion 91-03 combined electrode was used with a Fisher ACCUMET model 750 Selective Ion Analyzer and a Cole-Parmer model 83767 30 strip-chart recorder. In this way, for each experiment, an electrode potential (mV) vs. time plot was obtained. Before and after each experiment the glass electrode was calibrated with solutions of known acidity. Results and discussion
Interfacial behavior of the amines Measurements of the interfacial tension lowering brought about by the three amines at a water-dodecane interface were performed because the interfacial behavior of the carrier in a supported liquid membrane plays a major role in determining the membrane stability. Danesi et al. have demonstrated [ 41 that, in order to maximize the lifetime of a SLM, it is essential to use organic liquid phases exhibiting a high organic-water interfacial tension. A similar conclusion was reached in Refs. [ 5 ] and [ 61: the authors concluded that longer membrane lifetimes could be achieved by using membrane solvents of higher interfacial tension, and of lower surface tension than the critical surface tension of the polymeric solid support. In particular, they suggested that aliphatic hydrocarbons of high boiling point (such as n-dodecane) are suitable membrane solvents. This consideration is very important for the present work, where amines are used as membrane carriers, because alkylammonium salts are much more soluble in aromatic than in aliphatic hydrocarbons. The choice of n-dodecane as a membrane solvent should ensure a better stability performance, besides the traditional advantage of being chemically more inert toward the polymeric support. There is, however, the risk that the alkylammonium salt can reach saturation point and locally precipitate in the membrane pores. In a recent study, Smolders and co-workers [ 71 reported that the main cause of liquid membrane deterioration is emulsion formation at the low salt concentration feed side due to lateral shear forces. In other words, the rapid flow of a water phase with a low salt concentration has the same effect on the interface in a SLM, at a microscope level, as the shaking of two immiscible phases in a test-tube. This hypothesis again emphasizes the role of the interfacial tension,
69
because the emulsion will form more easily for more interfacially active carriers. Some information is available on the interfacial behavior of amines and their salts, concerning, in general, purified substances in contact with aqueous solutions containing a high salt concentration. In Ref. [8], for example, the interfacial pressure of pure TLA has been investigated at the toluene-1 it4 NaCl interface. In the present study, water is used as the aqueous phase, and n-dodecane as the organic diluent. The results are reported in Fig. 1 in the form of interfacial pressure, 17, vs. the logarithm of the bulk concentration. The interfacial pressure is defined, for each amine concentration, as:
(1)
n=YO-Yi
that is, as the difference between the interfacial tension in the absence (y,,) and in the presence of the amine (yi). The value of y0was 48 2 0.5 dyne-cm-‘. The data show a significant difference in interfacial behavior among the three amines, with Primene JM-T being a stronger surface active agent than Amberlite LA-2 and, especially, stronger than TLA. The rise of the interfacial pressure with the bulk organic concentration can be explained by the adsorption of the molecules of the organic solute at the 50
I
I
’
I
’
I
’
A 40
7 E 0 x F s. z c
3o Primene JM-T 0 A
*O
-8
-6
-4
i
-2
0
Log c
Fig. 1. Interfacial pressure of Primene JM-T, Amberlite LA-2 and TLA at a water-n-dodecane interface.
70
water-organic interface, forming a monomolecular described by the Gibbs equation:
film. This phenomenon
dI7/d In C=nikT
is
(2)
where C is the bulk concentration of the surfactant and ni is the number of molecules adsorbed at the interface per unit area. For all the investigated systems, starting from very dilute solutions where no adsorption occurs (dZ7/d In C = 0)) a saturation concentration is eventually reached. The saturated interface is indicated by a constant slope in the Uvs. log C plot. In Fig. 1 the slope of the curve of 17 vs. log C reaches a constant value at a lower concentration of Primene JM-T ( z 10m6M) than with Amberlite LA-2 ( z 10p5M) and much lower than with TLA ( z 10W4M). From these considerations we can conclude that TLA, being the least surface active among the investigated amines, should give more stable liquid membranes. An interesting comparison can be made with the interfacial behavior of Cyanex 272, reported in Fig. 2, and measured by following the same procedure and under the same conditions as used for the amines. At very low concentrations the Uvalues for Cyanex 272 are somewhat higher, but at 0.1 A4 the 17values for Cyanex 272 and TLA are practically the same. This is due to the fact that the Cyanex 272 data are not aligned on a straight line, but show a curvature, which in turn, depends on the tendency of the phosphinic acid molecules to form a dimer in the organic phase, while only the monomer in equilibrium with the dimer is adsorbed at the interface. A similar behavior has been 40 _
! / 0 i._.,
-8 Fig. 2. Interfacial
A. -6
..____1__
-4 Log c
/
-2
0
pressure of Cyanex 272 at a SGW (pH = 2) -n-dodecane
interface.
71
reported for other aggregating species, such as TLAHCl and TLAHN03, in Ref. [ 81. The very similar nvalues of TLA and Cyanex 272 at the concentration of 0.1 Mare an indication that rather stable TLA liquid membranes should be obtained, because the stability studies performed previously [ 21 with a 0.1 M Cyanex 272 solution in n-dodecane provided very encouraging results. Membrane
stability experiments
To determine the relative stability of liquid membranes containing the three amines in n-dodecane, a series of experiments was conducted using the cell described in the experimental section (see also Ref. [ 31) . To shorten the time required for the experiments, the experimental conditions were chosen in such a way that a low stability was expected. 1 M NaOH was always used as the strip solution to have a high osmotic pressure gradient between strip and feed solution. The feed solution was always SGW at pH 2. Also, flat-sheet Celgard 2500 supports were used, because of their very small thickness (25 pm). It is known that very thin membrane supports give rise to shorter membrane lives [ 91, probably because of the very small inventory of organic phase absorbed in their pores, which enhances the effect of the loss of organic phase due to aqueous solubility, and because of the small length of the pores, which enhances the occurrence of water bridges between feed and strip solutions through pores devoid of organic phase. The experiments were performed by following (without interruption) the decrease of acidity in the feed with a glass electrode. Every day the feed solution was replaced with a fresh portion of SGW at pH 2, and a plot of electrode potential (mV) vs. time was recorded. The membrane was considered “failed” when a relatively fast and dramatic pH change was recorded in the feed. The sudden pH change, with the feed becoming strongly alkaline, was attributed to the diffusion of the 1 M NaOH strip solution into the feed through water bridged pores. The slope of the straight line portion of the experimental E (electrode potential, mV) vs. t plots was used to calculate the permeability coefficient P (cm-set-‘) by means of the equation: P_dE
2.303 V ___dt S A
(3)
Equation (3 ) was obtained by combining the permeability equation In [H+ ] = -$
Pt
(4)
(where A and V are membrane area and feed volume, respectively), which describes the straight portion of the experimental E vs. t plots [3], with the electrode equation: E=E,
+S log [H+]
(5)
72
where E, and S are the Nernst constants of the electrode. When the membrane failed, the value of P, calculated with eqn. (3), became exceedingly high, indicating NaOH diffusion from the strip solution. An example of experimental data obtained with a liquid membrane containing 1 A4 Primene JM-T is reported in Fig. 3, as d (mV), the variation of the feed glass electrode potential, or feed pH, vs. time. The membrane failed during the sixth day of continuous operation, as shown by the pH change from 2 to above 10. The stability data collected under the same experimental conditions, and following the same experimental procedure, are reported in Fig. 4 for the three amines at different concentrations. In the figure, the apparent sudden increase of the permeability coefficient due to NaOH diffusion from the strip is shown by an arrow pointing toward higher P values. Some considerations can be drawn on the data of Fig. 4. The stability of the Primene JM-T membranes strongly depends on the concentration of the carrier, going through a maximum at 0.6 M (8 days). This behavior indicates that more than one factor determines the stability. At very high concentration, the stability is low, probably because of the very low interfacial tension at the feedmembrane interface, which increases the tendency toward emulsion formation and consequently water penetration into the pores. The same mechanism probably operates at the strip interface, although interfacial tension measurements of the organic carrier solution in contact with the strip solutions have not been performed. At low carrier concentration, the very low stability of the membrane seems to indicate that the membrane solution inventory also plays an important role, probably through a solubility mechanism. With Amberlite LA-2, the concentration effect on the stability is less evident. The primary factor here seems to be the interfacial tension, which is higher for lower con-
500 0.0
I 1 .o
,
I
I
2.0
3.0
,
1 4.0
Time, hours Fig. 3. Stability data for a 1 MPrimene JM-T SLM. A (mV) =variation of glass electrode potential or feed pH (right scale) vs. time (hours), as function of days of continuous operation. Feed= SGW at pH 2; strip = 1, NaOH; membrane area = 16.8 cm’, volume of feed and strip = 100 cm3; magnetic stirring.
73
I
, I=
Prlmene JM-T II = Amberlfte LA~2 III = TLA Flat Sheet Support
I
I
10 20 Time, days Fig. 4. Permeability coefficient, P, vs. days of operation for SLMs containing amines at different concentrations. Same conditions as in Fig. 3. The arrows indicate an apparent sudden increase of P due to membrane failure.
centrations (see Fig. 1))rather than the organic phase inventory. It is known, in fact, that the aqueous solubility of the amines decreases in the order primary > secondary > tertiary [lo]. A strikingly good stability is shown by the TLA membranes, both at 0.6 and 0.1 M. In the former case the experiment was interrupted after 26 days of continuous operation without measuring any significant decline of the membrane performance. The good stability of the TLA membranes can probably be ascribed to a higher value of the interfacial tension. It has been previously mentioned that, based on a comparison with the membranes used for U (VI) removal from SGW, rather stable membranes were expected with TLA. An alternative explanation, which does not exclude the previous one, can be based on the local precipitation processed of TLA alkylammonim salts in the membrane pores. Such precipitation processes were hypothesized in Ref. [3] to explain the peculiar TLA permeability data reported in that study, and also the slow transport of HCrO, by TLA. The formation of solid or gelatinous precipitates in the pores of the membrane, when the saturation limit is reached in the liquid membrane phase, has the effect of slowing down the diffusion of the permeating species, but it might well be a convenient way for enhancing the stability of a membrane, by preventing the formation of emulsion with the aqueous phase and by acting as a barrier against water bridging in semi-devoid pores. Stability experiments were also performed using hollow-fiber modules under the conditions described in the experimental section. The procedure was the same as for the flat-sheet experiments. The concentration of the amine was always 0.2 M. The feed and strip solutions (SGW at pH 2 and 1 M NaOH, respectively) were circulated in the module without interruption and the de-
74
crease of acidity in the feed was periodically measured with the combined glass electrode immersed in the feed reservoir. The feed solution was replaced daily. Unlike the flat-sheet case, a complete failure of the membranes, evidenced by the diffusion of NaOH from the strip into the feed solution with a rapid increase of the feed pH into the alkaline range, was never observed for any of the three amines, at least in the time span of the experiments. This fact demonstrates the very important role of the membrane thickness in preventing a complete failure of the membrane with intermixing of the feed and strip solutions. The much larger thickness of the hollow fibers used in this work (200 pm) as opposed to the flat-sheet supports (25 pm), together with the much higher inventory of liquid membrane absorbed in the pores of the hollow fiber, made water bridging between fed and strip solution through the membrane pores devoid of organic phase practically impossible. As an example, the d (mV) or dpH vs. time plots, measured with 0.2 M Primene JM-T, are reported in Fig. 5, after 3, 9 and 23 days of continuous operation. The progressive deterioration of the membrane is not shown here by the slope of the straight portion of the curves [correlated to the permeability coefficient through eqn (3) 1, which does not change significantly from day 3 to day 23. It is best described, instead, by the variation of the electrode potential in the first portion of each experiment, that is, by the initial H+ flux. To compare the behavior of the three amines, the H+ (M cm-set-l) in the first 5 hr of each experiment has been calculated as:
J (5hr)
=‘+/$
(6)
The data are reported
in Fig. 6 as percentage
variation
of the initial
J,, hr)
vs. days of operation. For all three amines, when the stability experiments interrupted
after about 40 days, the modules were still operating
were without signs
I
300’ 0
5
10
15
Time, hours
Fig. 5. Stability data for a 0.2 A4 Primene JM-T SLM on hollow fibers. A(mV) =variation of glass electrode potential or feed pH (right scale) vs. time (hours) after 3,9 and 23 days of continuous operation. Feed= SGW at pH 2, 35 cm”; strip = 1 M NaOH, 50 cm’; Membrane area = 12.3 cm’: feed linear velocity (lumen side) = 10 cm-set-‘.
75
0
IO
20
30
40
Time, days
Fig. 6. Percentage of H’ flux (5 hr) as function of days of continuous SLMs containing 0.2 M amines. All other conditions as in Fig. 5.
operation
for hollow fiber
of diffusion of NaOH through water bridged pores, although the membrane performance had strongly deteriorated as shown by the decline of J,, hrj. [In the case of Primene JM-T the tendency of JC5hrjto rise in the last days of the experiment might be a first indication that the membrane was approaching a situation of complete failure]. The flux decline is particularly evident for Primene JM-T, confirming the previous result that the primary amine gives the least stable membrane because of low interfacial tension and relatively high solubility in water. However, the fact that the Primene JM-T membrane was still operating after about 30 days, although with a reduced efficiency for H+ transport, is in striking contrast with the extremely low stability (less than one day) measured with the same liquid membrane on flat-sheet supports. The secondary amine shows the best performance over a time span of 40 days. While a 0.2 M Amberlite LA-2 membrane on flat-sheet supports failed after 5 days, on a much thicker support the same carrier solution seems to retain its effectiveness for a much longer time. The relatively good performance of the tertiary amine is not surprising, based on the already high stability measured with flat-sheet supports. Summary and conclusions The stability experiments performed with n-dodecane solutions of Primene JM-T, Amberlite LA-2 and TLA, adsorbed on flat-sheet supports, have allowed us to determine that the following order of stability is exhibited by the liquid membranes: tertiary > secondary > primary This is the reverse of the order of amine solubility in water and of the interfacial tension lowering at a water-n dodecane interface. Both factors, solubility and interfacial tension, seem to be operative in determining the liquid membrane stability, together with other usual factors such as support material, pore
76
size, osmotic pressure gradient and flow rate, which have been kept constant in this work. Another factor that probably contributes to a higher stability is the solubility of the amine salts in the diluent of the membrane. The formation of solid or semi-solid amine salts in the membrane pores, while having a detrimental effect on the permeation speed, may have a positive effect on the membrane life. The experiments performed with hollow fibers having a much larger thickness than the flat-sheet supports have shown a much less pronounced difference in the stability performance of the three amines. The primary factor with hollow fibers seems to be the inventory of absorbed organic phase. Even the liquid membrane that gave the worst results on flat-sheet supports (0.2 M Primene JM-T) worked remarkably well in a hollow fiber configuration without reaching a complete failure in about 30 days of continuous operation, although its performance in terms of H+ flux measured after 5 hr had significantly deteriorated. Concerning the application of amines as carriers for the decontamination of groundwater from anionic species, the stability experiments involving hollow fibers in the experimental conditions likely to be used in a SLM process have shown that the best overall results are given by the secondary amine. Amberlite LA-2 seems to provide the best compromise among the different carrier properties that determine the membrane performance: high affinity for the anionic species to be removed [ 3 ] ; relatively high interfacial tension at a water-dodecane interface (this work); very low water solubility [lo]. These properties are reflected in the following features of the transport of anionic species by Amberlite LA-2: high permeability coefficients of the different transported species [ 31; very extended plateau of the permeability coefficients as function of the amine concentration [ 3 ] ; high initial flux of the transported species [ 3 ] ; moderate decline of the initial flux of the transported species as a function of time (this work). Based on the stability experiments with hollow fibers, however, the use of a single module, containing the primary amine as the carrier, for the simultaneous removal of the anionic contaminants and of U (VI) cannot be ruled out, if allowance is made in the process for a more frequent module reimpregnation with the carrier solution, or if a self-reimpregnating module is used. Also, the use of the tertiary amine as a carrier cannot be excluded, because of its good stability. Although this is not evident from the data of Fig. 6, it is reasonable to assume that the TLA membrane would eventually last longer than the Amberlite LA-2 membrane, as suggested by the data of Fig. 4. The main problem associated with the use of TLA membranes is the lower effectiveness of the l
l
l
l
l
l
l
77
tertiary amine in transporting nitric acid. This drawback could however be, compensated for by a less frequent module reimpregnation. A TLA based membrane could also be used in a combined process where the removal of nitrates is achieved mainly by biological means. Acknowledgements The author wishes to express his gratitude to Westinghouse Hanford Co. for the financial support provided. He also wishes to thank P.G. Rickert for help provided in the hollow-fiber module experiments, and Dr. E.P. Horwitz for stimulating discussions and for revising the manuscript. References 1 2 3 4
5 6 7
8
9 10
R. Chiarizia and E.P. Horwitz, Study of uranium removal from groundwater by supported liquid membranes, Solvent Extr. Ion Exch., 8( 1) (1990) 65. R. Chiarizia, E.P. Horwitz, P.G. Rickert and K.M. Hodgson, Application of supported liquid membranes for removal of uranium from groundwater, Sep. Sci. Technol., 25 (1990) xx R. Chiarizia, Application of supported liquid membranes for removal of nitrate, technetium(VI1) and chromium(V1) from groundwater, J. Membrane Sci., 55 (1991) 39-64. P.R. Danesi, L. Reichley-Yinger and P.G. Rickert, Life-time of supported liquid membranes: The influence of interfacial properties, chemical composition and water transport on the long-term stability of the membranes, J. Membrane Sci., 31 (1987) 117. H. Takeuchi, K. Takahashi and W. Goto, Some observations on the stability of supported liquid membranes, J. Membrane Sci., 34,19 (1987). H. Takeuchi and M. Nakano, Progressive wetting of supported liquid membranes by aqueous solutions, J. Membrane, Sci., 42 (1989) 183. A.M. Neplenbroek, D. Bargeman and C.A. Smolders, SLM degradation by emulsion formation, Proc. Int. Solvent Extr. Conf., ISEC’88, Moscow, July 18-24,1988, Vol. III, Vernadsky Institute of Geochemistry and Analytical chemistry of the USSR, Academy of Science, MOScow, p. 61. M. Pizzichini and R. Chiarizia and P.R. Danesi, Interfacial behavior of trilaurylamine and its chloride and nitrate salts at a water-organic diluent interface, J. Inorg. Nucl. Chem., 40 (1978) 669. D. Pearson, supported liquid membranes using Accurel fibers. Progress report No. 1, Warren Spring Lab., Report LR473 (ME)M, January 1984. Y. Marcus and A.S. Kertes, Ion exchange and solvent extraction of metal complexes, Wiley, New York, NY, 1969.
Science,
Elsevier Science Publishers
55 (1991) 65-17 B.V., Amsterdam
65
Stability of supported liquid membranes containing longchain aliphatic amines as carriers R. Chiarizia* Chemistry (U.S.A.)
Division,
Argonne
National Laboratory,
9700 S. Cass Avenue,
Argonne,
IL 60439
(Received April 30,199O; accepted in revised form July 16, 1990)
Abstract The stability of liquid membranes containing as carriers three different commercially available long-chain aliphatic amines, Primene JM-T (primary), Amberlite LA-2 (secondary) and trilaurylamine (TLA, tertiary) has been investigated in experiments where the feed solution was synthetic Hanford site groundwater, the strip solution was 1 M NaOH and n-dodecane was the diluent. By using flat-sheet supports the following order of stability was measured: tertiary > secondary > primary. This is the reverse order of the interfacial tension lowering at a water-n-dodecane interface. The water solubility of the amines also seems to play an important role in determining the membrane stability. Experiments performed with much thicker hollowfiber supports have shown that a larger inventory of organic phase absorbed in the membrane pores ensures a remarkably higher membrane stability. The secondary amine provides the best overall performance and is the carrier of choice for a SLM process for groundwater decontamination from anionic species. Possible alternative uses of the primary and tertiary amines for the same decontamination process are also discussed. Keywords:
membrane
liquid membranes; support; membrane
facilitated transport, groundwater membrane stability; carrier: Primene JM-T, Amberlite LA-2, trilaurylamine
Celgard
Introduction
The application of supported membranes (SLM) for the removal of contaminants from Hanford site groundwater has been reported in previous work in this series [l-3]. We suggested that the pH of groundwater be lowered to 2 by addition of sulfuric acid. At pH 2, the carrier bis(2,4,4_trimethylpentyl)phosphinic acid, contained in the commercial extractant Cyanex 272 (registered trade mark of American Cyanamid Co), is very effective in selectively removing uranium (VI) from the contaminated groundwater. After passing through a first SLM module, containing Cyanex 272 in dodecane as the liquid membrane, where the uranium separation takes place, the acidified groundwater is purified from other anionic contaminants (nitrate, pertechnetate and *On leave from the Italian Alternative
0376-7388/91/$03.50
0 1991-
and Nuclear Energy Agency (ENEA).
Elsevier Science Publishers
B.V.
66
chromate anions) in a second SLM module containing a dodecane solution of a long-chain aliphatic amine as the liquid membrane. In Ref. [3] we investigated the use of the three commercially available amines Primene JM-T (primary), Amberlite LA-2 (secondary) and trilaurylamine (TLA, tertiary) as carriers. The following order of effectiveness for single contaminants was measured: for NO,: for TcO;: tertisecondary > primary>>tertiary; ary 2 secondary > primary; for HCrO; : secondary > primary > tertiary. Based on these findings, it was concluded that the carrier of choice for the simultaneous removal of the three contaminants is Amberlite LA-2. The primary amine, however, exhibited the unique property of also removing anionic U (VI) sulfato-complexes from the groundwater acidified with sulfuric acid. This fact led us to consider the possibility of an alternative SLM process, where uranium is removed together with the anionic contaminants in a single module containing the primary amine as carrier. The membrane experiments upon which the conclusions reached in Ref. [ 31 are based were short duration experiments involving the use of flat-sheet supports without consideration for the long-term membrane performance. The stability of liquid membranes is, however, of paramount importance for the practical application of SLM based separation processes. In Ref. [2] we reported a study of the stability of Cyanex 272-n-dodecane liquid membranes absorbed on hollow fibers for the uranium separation from groundwater. We demonstrated that acceptable stability performances were obtained with periodic reimpregnation of a conventional hollow-fiber module. Encouraging results were also obtained with a self-reimpregnating module. In this work we report our results on the stability of liquid membranes containing Primene JM-T, Amberlite LA-2 or TLA as the carrier. The aim of the work has been: (i) to obtain information on the stability of amine based SLMs under the conditions likely to be used in a process application for the decontamination of groundwater from anionic species; (ii) to get the experimental evidence needed for final selection of the most appropriate carrier for the above application. Experimental
Reagents Several liters of synthetic Hanford site groundwater (SGW) were prepared as previously reported [l-3]. The composition of the SGW, after addition of enough concentrated sulfuric acid to bring the pH value to ca. 2, has been reported [ l-3 1. Primene JM-T (Rohm and Haas), Amberlite LA-2 (Rohm and Haas) and trilaurylamine (TLA, Eastman Kodak) were described in Ref. [3]. All three amines were used as received because we assumed that the unpurified commercial products would most likely be used in a process application. Cyanex 272 was obtained from American Cyanamid Co. and used as re-
67
ceived. Solutions of the carriers were prepared using n-dodecane as diluent, following the same reasoning reported in Ref. [ 11. All other reagents were analytical grade products and were used without further purification. Membrane supports The flat-sheet membrane experiments were performed using as liquid membrane support Celgard 2500 polypropylene sheets 25 pm thick, with a 45% porosity and 0.04 pm pore size. The hollow-fiber modules were made with the same Enka Q-31 fibers and using the same procedure as reported in Ref. [ 21. The fibers were made of propylene, 200 pm thick (I.D. =0.6 mm, O.D. = 1 mm) with 75% porosity and 0.1 pm pore size. Interfacial tension measurements The interfacial tension measurements were performed at 23 + 0.5’ C using the DuNouy ring method and a Fisher Surface Tensiometer, Model 20. Initially, 20 ml of n-dodecane and 30 ml of water were equilibrated in a glass cell and the interfacial tension was measured. Small aliquots of concentrated stock solutions of amine in n-dodecane were introduced into the cell and the interface was stirred for several minutes by means of a motor driven small glass stirrer. The interfacial tension was then measured again at the new amine concentration in the organic phase. The stirring of the interface and the measurements were repeated at intervals until a constant value of interfacial tension was obtained. At this point a new aliquot of stock solution was added and the measuring procedure repeated. Each set of measurements (a y vs. log C curve) was repeated at least twice. The interfacial tension data at each amine concentration were reproducible within one dyne-cm-‘. The densities of the n-dodecane solutions of the carriers, needed for the correction of the apparent interfacial tension values provided by the instrument, were measured by weighing known volumes of the solutions at 23 2 0.5 ’ C. Permeation measurements The cell used with flat-sheet membrane supports was similar to that described in Ref. [3] for the HNO, transport experiments. The feed and strip compartments were stirred by means of magnetic stirring bars, located in 0.5 cm deep circular wells with a 3.5 cm diameter, and rotated by Micro-V ColeParmer magnetic stirrers at a speed of about 300 rpm. The membrane area was 16.8 cm2 and feed and strip solutions had a volume of 100 cm3. In these experiments the acid transport took place in “non-plateau” conditions, that is, in stirring conditions where the thickness of the aqueous diffusion layer and the aqueous resistance to mass transfer were not minimized. The permeation results have been used only for comparing the stability behavior of the different amines in strictly constant experimental conditions. The modules used in the stability experiments involving hollow-fiber supports contained ten ENKA Q-
68
31 fibers, each 6.9 cm long, with a total membrane area of 12.3 cm’. The modules were operated in a recirculating mode, as reported in Ref. [2]. Feed (35 cm3) and strip (50 cm”) were circulated through the lumen and on the shell side of the fibers, respectively, by means of calibrated peristaltic pumps. The feed flow rate was 17 ml/min, which corresponds to a linear velocity of 10 cmset-I. This value of feed linear velocity ensured that the modules were operated in the plateau region ( > 8 cm-set-’ ) as determined for a similar module
[al.
With both membrane configurations the feed was SGW at pH 2 and the strip was 1 M NaOH. The acid transport through the membrane was followed by potentiometrically measuring the H+ concentration in the feed solution. An Ag/AgCl-glass Orion 91-03 combined electrode was used with a Fisher ACCUMET model 750 Selective Ion Analyzer and a Cole-Parmer model 83767 30 strip-chart recorder. In this way, for each experiment, an electrode potential (mV) vs. time plot was obtained. Before and after each experiment the glass electrode was calibrated with solutions of known acidity. Results and discussion
Interfacial behavior of the amines Measurements of the interfacial tension lowering brought about by the three amines at a water-dodecane interface were performed because the interfacial behavior of the carrier in a supported liquid membrane plays a major role in determining the membrane stability. Danesi et al. have demonstrated [ 41 that, in order to maximize the lifetime of a SLM, it is essential to use organic liquid phases exhibiting a high organic-water interfacial tension. A similar conclusion was reached in Refs. [ 5 ] and [ 61: the authors concluded that longer membrane lifetimes could be achieved by using membrane solvents of higher interfacial tension, and of lower surface tension than the critical surface tension of the polymeric solid support. In particular, they suggested that aliphatic hydrocarbons of high boiling point (such as n-dodecane) are suitable membrane solvents. This consideration is very important for the present work, where amines are used as membrane carriers, because alkylammonium salts are much more soluble in aromatic than in aliphatic hydrocarbons. The choice of n-dodecane as a membrane solvent should ensure a better stability performance, besides the traditional advantage of being chemically more inert toward the polymeric support. There is, however, the risk that the alkylammonium salt can reach saturation point and locally precipitate in the membrane pores. In a recent study, Smolders and co-workers [ 71 reported that the main cause of liquid membrane deterioration is emulsion formation at the low salt concentration feed side due to lateral shear forces. In other words, the rapid flow of a water phase with a low salt concentration has the same effect on the interface in a SLM, at a microscope level, as the shaking of two immiscible phases in a test-tube. This hypothesis again emphasizes the role of the interfacial tension,
69
because the emulsion will form more easily for more interfacially active carriers. Some information is available on the interfacial behavior of amines and their salts, concerning, in general, purified substances in contact with aqueous solutions containing a high salt concentration. In Ref. [8], for example, the interfacial pressure of pure TLA has been investigated at the toluene-1 it4 NaCl interface. In the present study, water is used as the aqueous phase, and n-dodecane as the organic diluent. The results are reported in Fig. 1 in the form of interfacial pressure, 17, vs. the logarithm of the bulk concentration. The interfacial pressure is defined, for each amine concentration, as:
(1)
n=YO-Yi
that is, as the difference between the interfacial tension in the absence (y,,) and in the presence of the amine (yi). The value of y0was 48 2 0.5 dyne-cm-‘. The data show a significant difference in interfacial behavior among the three amines, with Primene JM-T being a stronger surface active agent than Amberlite LA-2 and, especially, stronger than TLA. The rise of the interfacial pressure with the bulk organic concentration can be explained by the adsorption of the molecules of the organic solute at the 50
I
I
’
I
’
I
’
A 40
7 E 0 x F s. z c
3o Primene JM-T 0 A
*O
-8
-6
-4
i
-2
0
Log c
Fig. 1. Interfacial pressure of Primene JM-T, Amberlite LA-2 and TLA at a water-n-dodecane interface.
70
water-organic interface, forming a monomolecular described by the Gibbs equation:
film. This phenomenon
dI7/d In C=nikT
is
(2)
where C is the bulk concentration of the surfactant and ni is the number of molecules adsorbed at the interface per unit area. For all the investigated systems, starting from very dilute solutions where no adsorption occurs (dZ7/d In C = 0)) a saturation concentration is eventually reached. The saturated interface is indicated by a constant slope in the Uvs. log C plot. In Fig. 1 the slope of the curve of 17 vs. log C reaches a constant value at a lower concentration of Primene JM-T ( z 10m6M) than with Amberlite LA-2 ( z 10p5M) and much lower than with TLA ( z 10W4M). From these considerations we can conclude that TLA, being the least surface active among the investigated amines, should give more stable liquid membranes. An interesting comparison can be made with the interfacial behavior of Cyanex 272, reported in Fig. 2, and measured by following the same procedure and under the same conditions as used for the amines. At very low concentrations the Uvalues for Cyanex 272 are somewhat higher, but at 0.1 A4 the 17values for Cyanex 272 and TLA are practically the same. This is due to the fact that the Cyanex 272 data are not aligned on a straight line, but show a curvature, which in turn, depends on the tendency of the phosphinic acid molecules to form a dimer in the organic phase, while only the monomer in equilibrium with the dimer is adsorbed at the interface. A similar behavior has been 40 _
! / 0 i._.,
-8 Fig. 2. Interfacial
A. -6
..____1__
-4 Log c
/
-2
0
pressure of Cyanex 272 at a SGW (pH = 2) -n-dodecane
interface.
71
reported for other aggregating species, such as TLAHCl and TLAHN03, in Ref. [ 81. The very similar nvalues of TLA and Cyanex 272 at the concentration of 0.1 Mare an indication that rather stable TLA liquid membranes should be obtained, because the stability studies performed previously [ 21 with a 0.1 M Cyanex 272 solution in n-dodecane provided very encouraging results. Membrane
stability experiments
To determine the relative stability of liquid membranes containing the three amines in n-dodecane, a series of experiments was conducted using the cell described in the experimental section (see also Ref. [ 31) . To shorten the time required for the experiments, the experimental conditions were chosen in such a way that a low stability was expected. 1 M NaOH was always used as the strip solution to have a high osmotic pressure gradient between strip and feed solution. The feed solution was always SGW at pH 2. Also, flat-sheet Celgard 2500 supports were used, because of their very small thickness (25 pm). It is known that very thin membrane supports give rise to shorter membrane lives [ 91, probably because of the very small inventory of organic phase absorbed in their pores, which enhances the effect of the loss of organic phase due to aqueous solubility, and because of the small length of the pores, which enhances the occurrence of water bridges between feed and strip solutions through pores devoid of organic phase. The experiments were performed by following (without interruption) the decrease of acidity in the feed with a glass electrode. Every day the feed solution was replaced with a fresh portion of SGW at pH 2, and a plot of electrode potential (mV) vs. time was recorded. The membrane was considered “failed” when a relatively fast and dramatic pH change was recorded in the feed. The sudden pH change, with the feed becoming strongly alkaline, was attributed to the diffusion of the 1 M NaOH strip solution into the feed through water bridged pores. The slope of the straight line portion of the experimental E (electrode potential, mV) vs. t plots was used to calculate the permeability coefficient P (cm-set-‘) by means of the equation: P_dE
2.303 V ___dt S A
(3)
Equation (3 ) was obtained by combining the permeability equation In [H+ ] = -$
Pt
(4)
(where A and V are membrane area and feed volume, respectively), which describes the straight portion of the experimental E vs. t plots [3], with the electrode equation: E=E,
+S log [H+]
(5)
72
where E, and S are the Nernst constants of the electrode. When the membrane failed, the value of P, calculated with eqn. (3), became exceedingly high, indicating NaOH diffusion from the strip solution. An example of experimental data obtained with a liquid membrane containing 1 A4 Primene JM-T is reported in Fig. 3, as d (mV), the variation of the feed glass electrode potential, or feed pH, vs. time. The membrane failed during the sixth day of continuous operation, as shown by the pH change from 2 to above 10. The stability data collected under the same experimental conditions, and following the same experimental procedure, are reported in Fig. 4 for the three amines at different concentrations. In the figure, the apparent sudden increase of the permeability coefficient due to NaOH diffusion from the strip is shown by an arrow pointing toward higher P values. Some considerations can be drawn on the data of Fig. 4. The stability of the Primene JM-T membranes strongly depends on the concentration of the carrier, going through a maximum at 0.6 M (8 days). This behavior indicates that more than one factor determines the stability. At very high concentration, the stability is low, probably because of the very low interfacial tension at the feedmembrane interface, which increases the tendency toward emulsion formation and consequently water penetration into the pores. The same mechanism probably operates at the strip interface, although interfacial tension measurements of the organic carrier solution in contact with the strip solutions have not been performed. At low carrier concentration, the very low stability of the membrane seems to indicate that the membrane solution inventory also plays an important role, probably through a solubility mechanism. With Amberlite LA-2, the concentration effect on the stability is less evident. The primary factor here seems to be the interfacial tension, which is higher for lower con-
500 0.0
I 1 .o
,
I
I
2.0
3.0
,
1 4.0
Time, hours Fig. 3. Stability data for a 1 MPrimene JM-T SLM. A (mV) =variation of glass electrode potential or feed pH (right scale) vs. time (hours), as function of days of continuous operation. Feed= SGW at pH 2; strip = 1, NaOH; membrane area = 16.8 cm’, volume of feed and strip = 100 cm3; magnetic stirring.
73
I
, I=
Prlmene JM-T II = Amberlfte LA~2 III = TLA Flat Sheet Support
I
I
10 20 Time, days Fig. 4. Permeability coefficient, P, vs. days of operation for SLMs containing amines at different concentrations. Same conditions as in Fig. 3. The arrows indicate an apparent sudden increase of P due to membrane failure.
centrations (see Fig. 1))rather than the organic phase inventory. It is known, in fact, that the aqueous solubility of the amines decreases in the order primary > secondary > tertiary [lo]. A strikingly good stability is shown by the TLA membranes, both at 0.6 and 0.1 M. In the former case the experiment was interrupted after 26 days of continuous operation without measuring any significant decline of the membrane performance. The good stability of the TLA membranes can probably be ascribed to a higher value of the interfacial tension. It has been previously mentioned that, based on a comparison with the membranes used for U (VI) removal from SGW, rather stable membranes were expected with TLA. An alternative explanation, which does not exclude the previous one, can be based on the local precipitation processed of TLA alkylammonim salts in the membrane pores. Such precipitation processes were hypothesized in Ref. [3] to explain the peculiar TLA permeability data reported in that study, and also the slow transport of HCrO, by TLA. The formation of solid or gelatinous precipitates in the pores of the membrane, when the saturation limit is reached in the liquid membrane phase, has the effect of slowing down the diffusion of the permeating species, but it might well be a convenient way for enhancing the stability of a membrane, by preventing the formation of emulsion with the aqueous phase and by acting as a barrier against water bridging in semi-devoid pores. Stability experiments were also performed using hollow-fiber modules under the conditions described in the experimental section. The procedure was the same as for the flat-sheet experiments. The concentration of the amine was always 0.2 M. The feed and strip solutions (SGW at pH 2 and 1 M NaOH, respectively) were circulated in the module without interruption and the de-
74
crease of acidity in the feed was periodically measured with the combined glass electrode immersed in the feed reservoir. The feed solution was replaced daily. Unlike the flat-sheet case, a complete failure of the membranes, evidenced by the diffusion of NaOH from the strip into the feed solution with a rapid increase of the feed pH into the alkaline range, was never observed for any of the three amines, at least in the time span of the experiments. This fact demonstrates the very important role of the membrane thickness in preventing a complete failure of the membrane with intermixing of the feed and strip solutions. The much larger thickness of the hollow fibers used in this work (200 pm) as opposed to the flat-sheet supports (25 pm), together with the much higher inventory of liquid membrane absorbed in the pores of the hollow fiber, made water bridging between fed and strip solution through the membrane pores devoid of organic phase practically impossible. As an example, the d (mV) or dpH vs. time plots, measured with 0.2 M Primene JM-T, are reported in Fig. 5, after 3, 9 and 23 days of continuous operation. The progressive deterioration of the membrane is not shown here by the slope of the straight portion of the curves [correlated to the permeability coefficient through eqn (3) 1, which does not change significantly from day 3 to day 23. It is best described, instead, by the variation of the electrode potential in the first portion of each experiment, that is, by the initial H+ flux. To compare the behavior of the three amines, the H+ (M cm-set-l) in the first 5 hr of each experiment has been calculated as:
J (5hr)
=‘+/$
(6)
The data are reported
in Fig. 6 as percentage
variation
of the initial
J,, hr)
vs. days of operation. For all three amines, when the stability experiments interrupted
after about 40 days, the modules were still operating
were without signs
I
300’ 0
5
10
15
Time, hours
Fig. 5. Stability data for a 0.2 A4 Primene JM-T SLM on hollow fibers. A(mV) =variation of glass electrode potential or feed pH (right scale) vs. time (hours) after 3,9 and 23 days of continuous operation. Feed= SGW at pH 2, 35 cm”; strip = 1 M NaOH, 50 cm’; Membrane area = 12.3 cm’: feed linear velocity (lumen side) = 10 cm-set-‘.
75
0
IO
20
30
40
Time, days
Fig. 6. Percentage of H’ flux (5 hr) as function of days of continuous SLMs containing 0.2 M amines. All other conditions as in Fig. 5.
operation
for hollow fiber
of diffusion of NaOH through water bridged pores, although the membrane performance had strongly deteriorated as shown by the decline of J,, hrj. [In the case of Primene JM-T the tendency of JC5hrjto rise in the last days of the experiment might be a first indication that the membrane was approaching a situation of complete failure]. The flux decline is particularly evident for Primene JM-T, confirming the previous result that the primary amine gives the least stable membrane because of low interfacial tension and relatively high solubility in water. However, the fact that the Primene JM-T membrane was still operating after about 30 days, although with a reduced efficiency for H+ transport, is in striking contrast with the extremely low stability (less than one day) measured with the same liquid membrane on flat-sheet supports. The secondary amine shows the best performance over a time span of 40 days. While a 0.2 M Amberlite LA-2 membrane on flat-sheet supports failed after 5 days, on a much thicker support the same carrier solution seems to retain its effectiveness for a much longer time. The relatively good performance of the tertiary amine is not surprising, based on the already high stability measured with flat-sheet supports. Summary and conclusions The stability experiments performed with n-dodecane solutions of Primene JM-T, Amberlite LA-2 and TLA, adsorbed on flat-sheet supports, have allowed us to determine that the following order of stability is exhibited by the liquid membranes: tertiary > secondary > primary This is the reverse of the order of amine solubility in water and of the interfacial tension lowering at a water-n dodecane interface. Both factors, solubility and interfacial tension, seem to be operative in determining the liquid membrane stability, together with other usual factors such as support material, pore
76
size, osmotic pressure gradient and flow rate, which have been kept constant in this work. Another factor that probably contributes to a higher stability is the solubility of the amine salts in the diluent of the membrane. The formation of solid or semi-solid amine salts in the membrane pores, while having a detrimental effect on the permeation speed, may have a positive effect on the membrane life. The experiments performed with hollow fibers having a much larger thickness than the flat-sheet supports have shown a much less pronounced difference in the stability performance of the three amines. The primary factor with hollow fibers seems to be the inventory of absorbed organic phase. Even the liquid membrane that gave the worst results on flat-sheet supports (0.2 M Primene JM-T) worked remarkably well in a hollow fiber configuration without reaching a complete failure in about 30 days of continuous operation, although its performance in terms of H+ flux measured after 5 hr had significantly deteriorated. Concerning the application of amines as carriers for the decontamination of groundwater from anionic species, the stability experiments involving hollow fibers in the experimental conditions likely to be used in a SLM process have shown that the best overall results are given by the secondary amine. Amberlite LA-2 seems to provide the best compromise among the different carrier properties that determine the membrane performance: high affinity for the anionic species to be removed [ 3 ] ; relatively high interfacial tension at a water-dodecane interface (this work); very low water solubility [lo]. These properties are reflected in the following features of the transport of anionic species by Amberlite LA-2: high permeability coefficients of the different transported species [ 31; very extended plateau of the permeability coefficients as function of the amine concentration [ 3 ] ; high initial flux of the transported species [ 3 ] ; moderate decline of the initial flux of the transported species as a function of time (this work). Based on the stability experiments with hollow fibers, however, the use of a single module, containing the primary amine as the carrier, for the simultaneous removal of the anionic contaminants and of U (VI) cannot be ruled out, if allowance is made in the process for a more frequent module reimpregnation with the carrier solution, or if a self-reimpregnating module is used. Also, the use of the tertiary amine as a carrier cannot be excluded, because of its good stability. Although this is not evident from the data of Fig. 6, it is reasonable to assume that the TLA membrane would eventually last longer than the Amberlite LA-2 membrane, as suggested by the data of Fig. 4. The main problem associated with the use of TLA membranes is the lower effectiveness of the l
l
l
l
l
l
l
77
tertiary amine in transporting nitric acid. This drawback could however be, compensated for by a less frequent module reimpregnation. A TLA based membrane could also be used in a combined process where the removal of nitrates is achieved mainly by biological means. Acknowledgements The author wishes to express his gratitude to Westinghouse Hanford Co. for the financial support provided. He also wishes to thank P.G. Rickert for help provided in the hollow-fiber module experiments, and Dr. E.P. Horwitz for stimulating discussions and for revising the manuscript. References 1 2 3 4
5 6 7
8
9 10
R. Chiarizia and E.P. Horwitz, Study of uranium removal from groundwater by supported liquid membranes, Solvent Extr. Ion Exch., 8( 1) (1990) 65. R. Chiarizia, E.P. Horwitz, P.G. Rickert and K.M. Hodgson, Application of supported liquid membranes for removal of uranium from groundwater, Sep. Sci. Technol., 25 (1990) xx R. Chiarizia, Application of supported liquid membranes for removal of nitrate, technetium(VI1) and chromium(V1) from groundwater, J. Membrane Sci., 55 (1991) 39-64. P.R. Danesi, L. Reichley-Yinger and P.G. Rickert, Life-time of supported liquid membranes: The influence of interfacial properties, chemical composition and water transport on the long-term stability of the membranes, J. Membrane Sci., 31 (1987) 117. H. Takeuchi, K. Takahashi and W. Goto, Some observations on the stability of supported liquid membranes, J. Membrane Sci., 34,19 (1987). H. Takeuchi and M. Nakano, Progressive wetting of supported liquid membranes by aqueous solutions, J. Membrane, Sci., 42 (1989) 183. A.M. Neplenbroek, D. Bargeman and C.A. Smolders, SLM degradation by emulsion formation, Proc. Int. Solvent Extr. Conf., ISEC’88, Moscow, July 18-24,1988, Vol. III, Vernadsky Institute of Geochemistry and Analytical chemistry of the USSR, Academy of Science, MOScow, p. 61. M. Pizzichini and R. Chiarizia and P.R. Danesi, Interfacial behavior of trilaurylamine and its chloride and nitrate salts at a water-organic diluent interface, J. Inorg. Nucl. Chem., 40 (1978) 669. D. Pearson, supported liquid membranes using Accurel fibers. Progress report No. 1, Warren Spring Lab., Report LR473 (ME)M, January 1984. Y. Marcus and A.S. Kertes, Ion exchange and solvent extraction of metal complexes, Wiley, New York, NY, 1969.