Application of supported liquid membranes for removal of nitrate, technetium (VII) and chromium (VI) from groundwater

Journal of Membrane Science, 55 (1991) 39-64 Elsevier Science Publishers B.V.. Amsterdam

39

Application of supported liquid membranes for removal of nitrate, technetium (VII) and chromium (VI) from groundwater R. Chiarizia* Chemistry

Division, Argonne

National

Laboratory,

9700 South Cuss Avenue,

Argonne,

IL 60439 (U.S.A.) (Received

April 30,199O;

accepted

July 2,199O)

Abstract The separation of nitrate, pertechnetate and chromate ions from synthetic Hanford site groundwater was studied by liquid-liquid extraction and by supported liquid membranes, SLMs. Three different commercially available long-chain aliphatic amines, Primene JM-T (primary), Amberlite LA-2 (secondary) and trilaurylamine (TLA, tertiary), were investigated as membrane carriers. n-Dodecane was used as diluent and polypropylene membranes were used as the support. Sodium hydroxide was used as the stripping agent. The basic strength of the three amines in dodecane was studied by biphasic potentiometric titrations using a glass electrode. Flat-sheet membrane experiments, where the removal of nitric acid from the feed solution was followed by using either a glass electrode or a nitrate electrode, showed that the secondary and primary amines are more effective than the tertiary one. A detailed study of the permeation of nitric acid through SLMs as function of the concentration of the three amines in the liquid membrane was performed. Information was obtained the nitrate-alkylammonium

on the diffusion coefficients of nitric acid in the aqueous phase and of salt in the membrane phase. A similar investigation was performed

with HTcO,. In this case the secondary and tertiary amines proved to be better secondary amine also performed better than the other two as a carrier for H,CrO,. amine showed the unique feature of removing U (VI) as an anionic sulfato-complex, nitrate, pertechnetate and chromate ions, from the synthetic groundwater. Keywords:

liquid membranes;

membrane

carrier:

Primene

facilitated

JM-T,

transport;

Amberlite

groundwater;

Celgard

carriers. The The primary together with

membrane

support;

LA-2, trilaurylamine

Introduction In previous studies [ 1,2] we reported on the application of a supported liquid membrane (SLM) system for removal of uranium from contaminated groundwater. We suggested that the pH of groundwater be lowered to 2 by addition of sulfuric acid. We demonstrated that, 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. ) , was very effective in selectively removing uranium (VI ) from synthetic Hanford site groundwater, and *On leave from the Italian

0376.7388/91/$03.50

Alternative

0 1991-

and Nuclear Energy Agency

(ENEA).

Elsevier

B.V.

Science

Publishers

40

that the separated uranium (VI) could be stripped and concentrated in a solution containing 1-hydroxyethane-l,l-diphosphonic acid, HEDPA. After passing through a SLM module, in which the uranium separation took place, the pH of the groundwater had not been significantly changed by UOZ2+-H+ exchange with the phosphinic acid. As a consequence, advantage could be taken of the still acidic pH of the groundwater to remove, in a second SLM module, other anionic contaminants in the form of acids. The contaminants of interest are nitrate, pertechnetate and chromate anions (Table 1). For this purpose a basic membrane carrier is needed that is capable of reacting with nitric, pertechnetic and chromic acids at the feed side of the membrane to form membrane soluble salts. After diffusing through the liquid membrane, these salts can be released at the strip side of the membrane, where an alkaline stripping solution ensures that the free carrier is regenerated. Among several neutral and basic carriers, including tributyl phosphate, trioctylphosphine oxide and bifunctional organophosphorus extractants, Danesi et al. [3] demonstrated that primary and tertiary amines in n-dodecane effectively removed HNO, from a 1 it4 NaN03 feed solution. The use of a trilaurylamine (TLA)-n-dodecane liquid membrane to remove HNO, from a high ionic strength feed was also reported by Dworzak and Naser [ 41, In this case, the study was performed for a pilot-scale development of a SLM process involving an industrial membrane module. The use of a secondary amine, Amberlite LA-2, was suggested by Kreevoy and Nitsche [ 51 for a liquid membrane based extraction of nitric acid from water. In this paper the authors suggested the use of trioctyl phosphate (TOP) as diluent. The extraction of HNOB by the secondary amine in TOP was enhanced to such an extent that the feed did not need to be acidified in order to have effective removal of nitrate ions. In Ref. [ 61 Kreevoy et al. proposed, as more effective carriers in TOP, a quaternary ammonium phenoxide or sulfonamidate, either of which is a stronger base than the amines. These carriers, however, are not commercially available. A liquid membrane containing Primene JM-T in decalin was used in Refs. [7] and [8] as a device to minimize the nitric acid concentration in the intermediate compartment of a double liquid membrane system, during the removal of TABLE 1 Concentrations of some contaminants in Hanford groundwater” Contaminant

Low

High

Nitrate (ppm) Chromate (ppb) Technetium-99 (pCi/l)

45.1 51 906

1460 431 29,100

Drinking water standards 45

50 900

“Personal communication fromK.M. Hodgson, Westinghouse Hanford Co., Richland, WA, U.S.A.

41

actinides from simulated acidic nuclear wastes or nitric acid digested biological samples. The use of amines as carriers for the transport of species other than nitric acid has also been reported. In Refs. [ 91 and [lo], for example, TLA was used for Cd(II)-Zn(I1) and Fe(III)-Co(II)-Ni(I1) membrane separations from solutions containing high concentrations of Cl- ions. Alamine 336, a C&C,, tertiary amine, has been used for uranium separation from vanadium and molybdenum in a SLM based process applied to hydrometallurgical leach liquors containing sulfuric acid at a pH of ca. 1 [ 11,121. The removal of chromium as dichromate from acidic solutions using Alamine 336 [ 131 or TLA [ 141 as membrane carrier has also been investigated. In the cited work the membrane was used in the liquid surfactant membrane configuration, but the chemistry of the system applies as well to the SLM configuration. More recently, tri-n-octylamine (TOA) [ 151 and a quaternary alkylammonium salt ( Aliquat 336) [ 161 have also been used as carriers for Cr (VI ) anions. An extended investigation on the recovery of dichromate ions from plating bath liquors by means of amine containing SLMs in the aromatic diluent Escaid 350 has been reported in Refs. [ 171 and [ 181. Here it was found that Alamine 336 was an unsuitable carrier: its performance deteriorated rapidly, apparently due to oxidative attack on the amine by the dichromate solution. Better results were obtained using Primene JM-T, which showed a higher resistance to oxidation. The problem of chemical degradation of the carrier is, however, of little importance in our proposed application to groundwater decontamination, because the concentration of chromate ( x500 ppb) and the acidity of the feed are much more favorable than the conditions used in Refs. [17] and [18] (>lg/latpH ~1). Very little is known about the transport of HTc04 through liquid membranes containing amines as carriers. It can be inferred, however, from solvent extraction literature that the removal of HTcO, from acidified groundwater should not give rise to any particular problem. From 0.01 N H,SO, solutions, technetium distribution ratios between 10 and 10’ are reported with primary, secondary and tertiary amines in xylene [ 191. The aim of this research effort has been to establish chemical systems and conditions for SLM processes for groundwater decontamination. In particular, we wanted to find optimum extraction and stripping conditions to be used in an SLM system to remove technetium (VII), chromium (VI) and nitrate from groundwater following uranium removal by SLMs. For this purpose, commercially available long-chain aliphatic amines have been considered, belonging to primary, secondary and tertiary types. Experimental Synthetic groundwater Several liters of synthetic Hanford site groundwater (SGW) were prepared using the reagents listed in Table 2. After addition of enough concentrated

42

TABLE 2 Reagents used for the preparation

of synthetic

Reagent

Moles per liters of SGW

Na HSO,

0.014 0.0034 0.0028 0.012 0.0016 0.0009 0.0004

Mg SO, Mg(NG& Ca(NO,), NaCl Na,SiO, UO,(NO,),

groundwater

(SGW)

TABLE 3 Composition

of SGW at pH 2

Constituent

Molarity

Calcium Magnesium Sodium Silicon Chloride Sulfate-bisulfate Nitrate Uranium Sum of molarities

0.012 0.0062 0.017 0.0009 0.0016 0.017 0.030 0.0004 0.094

sulfuric acid to bring the pH value to ca. 2, the mixture had the composition reported in Table 3. The solution simulates the composition of contaminated groundwater acidified to pH E 2 using sulfuric acid. In the experiments involving technetium aliquots of the above solution were spiked with a 0.04 M HN03 solution of “Tc, obtained from Argonne National Laboratory ( ANL) stocks. The initial concentration of technetium in the SGW was ca. 3.5 x 1O-5 M. In a few experiments, the SGW solution was spiked with the same 223Ustock solution as used in Ref. [ 111. The initial uranium concentration in the SGW was 1.1 x lop4 M. In the experiments involving chromium, the SGW was made lo-” M in Na,CrO, by dissolving in it weighed amounts of Na,CrO,*H,O. Reagents Primene JM-T, a long-chain primary alkylamine from t-C,,H,,NH, to tC22H45NH2,was obtained from Rohm and Haas. Its neutralization equivalent, measured by biphasic potentiometric titration with standardized NaOH, was

43

336&5. Amberlite LA-Z, a mixture of variously branched secondary amines containing 24 C atoms, was also obtained from Rohm and Haas. Its equivalent weight was measured as being 370 +-10. Trilaurylamine, TLA, (C,,H,,),N, MW 522, was obtained from Eastman Kodak. Its equivalent was also checked by potentiometric titration. All three amines were used as received, on the assumption that the unpurified commercial products would most likely be used in a process application. Solutions of the carriers were prepared using n-dodecane as diluent, following the same reasoning reported in Ref. [ 11. Trioctyl phosphate (TOP) was obtained from ANL stocks. Cyanex 923, a mixture of C, and C, trialkylphosphine oxides, was obtained from American Cyanamid Company. All other reagents used in this work 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 45% porosity and 0.04 pm pore size. Viscosity measurements Viscosity measurements were performed on n-dodecane solutions of the carriers on an RGI V-2100 falling ball viscosimeter, calibrated with n-dodecane, using a stainless steel ball with a density of 8.02 g/cm3. The viscosimeter and the carrier solution were thermostated at 25 ? 0.1 ‘C. Biphasic titrations Biphasic titrations of the amine solutions were performed following the method described in Ref. [ 201 for acidic organophosphorus compounds. Thus 10 ml of low2 and 10-l M or 1 ml of 1 M solutions of the three amines were equilibrated with 10 ml of 1 M NaNO, in a titration cell containing a magnetic stirrer and a semi-micro Ag/AgCl-glass Orion 91-03 combined electrode, previously calibrated against buffer standards. The mixture was vigorously stirred and a titrated (0.1 or 1 M HN03) solution was progressively added, reading the pH values on a Fisher ACCUMET Selective Ion Analyzer, Model 750. For each addition the pH reading was taken after several minutes, that is, when a constant pH value was reached. Duplicate experiments showed very good reproducibility of the titration curves. The biphasic titrations of the amines were also performed with H,SO, using as aqueous phase 1 M Na,SO,. Distribution ratio measurements The distribution of Tc (VII), Cr(V1) and U(V1) between amine solutions and SGW spiked with “Tc or 233U,or lop3 M in Na2Cr04, was measured using standard solvent extraction and liquid scintillation (“Tc and 233U) or induc-

44

tively coupled plasma-atomic emission spectroscopy (ICP-AES ) (Cr ) techniques. A United Technologies Packard TRI-CARB Liquid Scintillation Analyzer was used. All measurements were performed at 25 + 1 “C, at a phase ratio of 1. The two phases, contained in screw-top tubes, were contacted by vortex mixing for 2 min, which was enough to attain equilibrium. The phases were then separated after centrifugation and aliquots were withdrawn for analysis. In the case of uranium, the same technique described in detail in Ref. [l] was used, in order to have distribution ratio values unaffected by the small non-uranium activity in the “‘U stock solution. All distribution ratios were reproducible within ? 5%. Permeation measurements (a) Technetium(VII) and uranium(VI) transport experiments The same miniaturized membrane cell used in Ref. [l] was employed in this work. The flat-sheet support was impregnated with the carrier solution by immersion for a few hours. All the experiments were performed with aqueous feed and strip solutions stirred at 250 rpm with the stirring equipment used previously [ 11. It was demonstrated in Ref. [ 211 that this cell and stirring apparatus gave a “plateau” region (i.e. constant permeability) for stirring speeds higher than 200 rpm. In the plateau region, the thickness of the aqueous diffusion layer and the aqueous resistance to mass transfer were minimized. The membrane area available for mass transfer was always 1.71 cm2 and the volumes of the feed and strip solutions were always 4 cm3. The permeation of the radionuclides through the SLM was monitored by periodically sampling the feed and/or strip solution. The data were plotted on semilogarithmic scales as feed activity vs. time. From the slope of the straight line, a value for P, the permeability coefficient (cm-sec- ’ ) was calculated. (b) Chromium(VI, and HNO, transport experiments The membrane cell used in this case had a geometry similar to that previously mentioned, but it was much larger, with a membrane area of 18 cm2 and feed and strip volumes equal to 70 cm3. The solutions in the feed and strip compartments were mechanically stirred by means of motor driven glass stirrers having 2 x 1 cm rectangular blades. Feed and strip compartments had teflon covers with holes for the stirrer shafts and for electrodes. A plateau region was identified for this cell and stirring apparatus at stirring speeds higher than 500 rpm (see next section). In the case of chromium (VI) experiments, the feed solution was periodically sampled and analyzed by ICP-AES. In the HNO, transport experiments the acid concentration in the feed phase was followed potentiometrically. Either a Ag/AgCl-glass Orion 91-03 combined electrode or a nitrate electrode (Orion

45

93-07) coupled with a double junction reference electrode (Orion 90-02) was used with a Fisher ACCUMET Model 750 Selective Ion Analyzer, and a ColeParmer 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. Similarly, the nitrate electrode, which contained a daily replaced 0.04 M (NH,),SO, solution in the outer chamber, was periodically calibrated with 0.01 M H,SO, solutions containing known concentrations of nitrate ions in the range 2 x 10-2-10-5M. The nitrate electrode response was linear up to at least 10W4M N03-, with a slope of 56? 1 mV. In all cases, duplicate and triplicate permeation experiments showed that the transport data were reproducible within 10%. All the permeability coefficients reported in the following sections have an uncertainty interval of t 10% when not otherwise specified. Results and discussion Basic strength of the amines The basic strength of amines can be characterized by their ability to react with acids. It is generally recognized [22] that long-chain aliphatic amines, dissolved in a water-immiscible diluent, react with mineral acids according to: nH++A”-+nB*(BH),A

(I)

where the bar indicates organic phase species. If the mineral acid is HNOB, the equilibrium constant of eqn. (1)) assuming the ratio of the activity coefficients of the organic phase species to be constant, becomes K=

-

LBHNOz1

[Bl [H+ 1 [NO,- 1 ~:mvo~

(2)

By titrating a n-dodecane solution of an amine with HNO,, at the 50% neutralization point we have [BHNO,] = [B] . It follows that: 1 K= [H+

1 [NO,- 1 Y&ZNO~

(3)

If the aqueous phase contains a constant 1 M NOB-, by including the aqueous activity coefficients in the constant, it follows that:

log K=PHF,o%

(4)

are rigorously valid only at infinite dilution of the amines, Equations (l)-(4) where aggregation processes of the alkylammonium salts are negligible. It is well known that the complete lack of solvating power in aliphatic hydrocarbons facilitates the formation of aggregates and their growth [ 191. The aggregation processes of the alkylammonium salts in the case of nitrate can be represented as

46

m BHNO,*

K,

(BHNO,),;

(5)

The size of the aggregates and the equilibrium constant of reaction (5 ) in aliphatic diluents are generally very large. For example, for TLA HN03 in ndodecane, in equilibrium with 1 M NaNO,, the values log I&=20.3 and log KdO= 108 are reported in Ref. [ 231 for reaction (5 ). The aggregation of the alkylammonium salt has two main effects on the acid-base reaction (1):(i) the equilibrium is shifted to the right, that is, toward more mineral acid extraction or higher distribution ratio of the acid, and (ii) because the aggregation is strongly dependent on the concentration of the amine, the K value obtained through eqn. (4) also will be concentration dependent, and will be higher for higher amine concentrations. In spite of the last consideration, the approach followed in eqns. (l)- (4) can still be useful for a comparison among different amines, if the same experimental conditions are used, and bearing in mind that the reaction constant K defined by eqn. (4) is only an apparent constant, Kapp,relative to a specific amine concentration. The pHSO%values obtained in HN03 biphasic titrations of n-dodecane solutions of the three amines used in this work, using 1 it4 NaNO, as aqueous phase, are reported in Table 4. The data show that Primene JM-T is an effective extractant for HN03 in the whole concentration range explored, while TLA is the least effective. The previous finding is in agreement with the generally reported order of extraction constants of acids according to reaction (1)) that is, primary > secondary > tertiary [ 221. This order is the same as the degree of steric hindrance on the nitrogen atom, which, more than electronic density effects, seems priTABLE

4

pH,,% for the biphasic titration of amines in n-dodecane 1 M NaNO, or 1 M Na,SO,)”

lB1 (M)

Primene JM-T

Amberlite

PH,,,~

PHW

LA-2

with HNO, and H,SO,

(aqueous phase

TLA PH,,,

Titrant = HNOsb 0.01 0.1

6.38 7.20

1

7.95

1

7.08

4.85 5.50 6.40 Titrant = H2S0,C 4.74

2.93 3.75 5.05 3.23

“The initial aqueous phase volume was always 10 cm3; the initial organic phase volume was 10 cm3 for 0.01 M and 0.1 M amine, 1 cm3 for 1 M amine. bThe titrant concentration was 0.1 M HNO, for 0.01 MB, 1 M HNO, for 0.1 and 1 MB. “The titrant concentration was always 1 M H,SO,. dAll pHsoB values are t 0.05 pH unit.

47

marily to determine the energy of formation and the stability constants of alkylammonium salts, and consequently the extraction constants of acids [ 221. Since the acidified synthetic groundwater contains a significant amount of sulfate-bisulfate anions, biphasic titrations of the amines have also been performed with H2S04, using a 1M Na2S0, as the aqueous phase. The pHsO%obtained from H,SO, biphasic titrations of 1 M amine solutions are reported in Table 4, together with the HN03 results. Also in the H&SO, case, the extraction constant sequence primary > secondary > tertiary amine is followed. A direct comparison between the pHsO%values for HN03 and H,SO, is not possible, due to the different stoichiometry of the extraction reaction and to the sulfate-bisulfate equilibria taking place both in the aqueous and in the organic phase [ 191. The values of pHsO%for HNOB in Table 4 agree well with literature data. Grinstead [24], for example, who titrated several 0.1 M amines in toluene using 1 M Cl- as the aqueous phase, reports log KaP,,values in the range 6% 8.2 for primary amines (7.1 for Primene JM-T), in the range 4.6-7.2 for secondary amines, and in the range 2.0-U for tertiary amines (3.8 for trioctylamine and trihexylamine, which are very similar to TLA). Regarding the extraction of H2S04, the results of our biphasic titrations agree with the finding, reported in Ref. [ 251, that 0.1 M Primene JM-T begins to extract H,SO, at an acid concentration of ca. 10W8M, while 0.1 M TLA requires an acid concentration of ca. 10e4 M for the extraction to start. Membrane experiments A. Transport of HNO, The permeation of acids through liquid membranes containing amines as carriers dissolved in a water-immiscible solvent can be described by the same laws derived for the transport of metal species. In a study of HN03 permeation through SLMs containing TLA in diethylbenzene, Cianetti and Danesi [ 261 demonstrated that the transmembrane flux of the acid could be expressed as:

(6) of the permeating species, M see-’ cm, with dt A = membrane area and V= volume of feed solution; d, = d,/D, = thickness of aqueous diffusion layer/aqueous diffusion coefficient, cm-‘-set; d,=d,/ D,= membrane thickness/diffusion coefficient of the permeating species in the membrane, cm-’ -set; [B] e= membrane equilibrium concentration of amine not bound to HNO,; K= conditional equilibrium constant of the biphasic neutralization reaction (1) .

where J=d [H+l i=flux ’

48

Equation (6) was obtained under the following conditions: - The composition of the strip solution was such that equilibrium (1) was completely shifted to the left at the membrane-aqueous strip interface (very low distribution ratio of the acid). - The interfacial chemical reactions were fast. - The acid transport occurred at the steady state and the concentration gradients were linear. One more simplifying assumption had to be introduced to obtain eqn. (6), and that is that the contribution of polymerized HNO,-TLA species to the overall transport of HNO, was negligible. However, it was demonstrated in Ref. [ 261 that, out of all possible polymeric species present in the membrane, the monomer of the alkylammonium salt was mainly responsible for the facilitated transport of HN03. From eqn. (6) the behavior of the membrane in two limiting cases can be predicted. When the HNOB feed concentration is so low that the amount of free amine in the membrane is practically equal to its analytical concentration, [El, the integration of the flux equation gives: ln

[H+l

[ITI+],-

Apt

(7)

-v

where [H+ ] ,, is the initial HN03 feed concentration and P is the permeability coefficient, cm-set-‘, a time independent quantity equal to:

J

p=--

K61

(8)

[I-f+] -K[ii]A,+A,

In these conditions the acid transport can be controlled by either aqueous or membrane diffusion processes (or both) depending on the values of d,, d,, K and [B] . Equation (8) is the same as the permeability equation used in the case of uranium (VI ) transport by a Cyanex 272 SLM [ 11, the only difference being that the extent of the U(VI)-carrier reaction at the feed phase was expressed by means of the U (VI) distribution ratio. The permeability coefficient can be determined as in Ref. [ 1 ] from the slope of the straight line obtained from a semilogarithmic [H+ ] vs. t plot. When, at the other extreme, the feed concentration of HNO, is large enough to convert practically all the amine into a nitrate alkylammonium salt [ [ii], in eqn. (6) very small], it follows that:

=- LB1

(9)

The integration of eqn. (9) gives [El+]=

[H’lo-~$t

(10)

0

In these conditions the flux of HNO, is constant with time and is controlled

only by diffusion processes occurring in the membrane. The acid feed concentration decreases linearly with time, and the slope of a plot of [H + ] vs. t allows the calculation of d, and, thus, of the membrane diffusion coefficient of the permeating species. The slope of the straight line obtained by plotting [H+ ] vs. t can be correlated to an initial membrane permeability coefficient, PO, defined as

El -- J po= [H+],-d,[H+l,

(11)

Equations (7) and (10) allow us to predict the shape of a HN03 permeation experiment when the data are plotted in the form log [H+ ] vs. t. Such a shape is described in Fig. 1. The curve can be divided into three regions, A, B and C. In region A, where the feed concentration of HNO, is still high, eqn. (10) applies, and log [H+ ] declines with time following a curve. In region B, the feed H+ concentration is low enough to have the data follow eqn. (7). Here the log [H+ ] vs. t plot is described by a straight line, and a permeability coefficient can be calculated from its slope. Region C describes the end of the experiment, where a condition of equilibrium is slowly approached. In this region the driving force for the acid extraction is strongly reduced because the concentration of acid in the feed has become very small. The HN03 concentration at which region C appears will be determined by the value of the equilibrium constant K of reaction ( 1) , and will be lower for higher K values. These predictions, based mostly on the equations derived in Ref. [26], have been fully verified by our membrane experiments described in the following paragraphs. The cell used for the membrane experiments had first to be hydrodynamically characterized. For this purpose, HNOB permeation experiments were per-

Fig. 1. General shape of a HNOB permeation experiments in the form feed log[H+] (arbitrary units).

vs. time

50

formed as a function of stirring speed (rpm) using 1 M Primene JM-T as carrier and an initial feed HN03 concentration equal to lop2 M, and 0.1 M NaOH as the stripping solution. The feed H+ concentration was followed with a glass electrode and each experiment was recorded as an E (electrode potential, mV) vs. t curve. The slope of the straight portion of the experimental curve (region B discussed above), dE/dt, was used to calculate the permeability coefficient. It can be demonstrated that, by combining eqn. (7) with E=Eo+S

log[H+]

(12)

where E, and S are the Nernst constants characteristic of the electrode, one obtains P_dE __2.303 V dt S A

(13)

The initial permeability was calculated for each experiment from the H + flux measured in the first 10 min of transport, after converting the E values into H+ concentrations by means of a calibration curve obtained with solutions of known acidity. The data are reported in Fig. 2 as P and P,, vs. rpm. As expected, P,, was practically independent of rpm, because, for a relatively high HNO, concentration, the flux is controlled only by membrane diffusion [see eqn. (9) 1. The P values reached a constant value at rpm values higher than 500. At this stirring rate the aqueous diffusion layer is minimized. All other experiments performed with the same cell have been conducted at 600 rpm. The permeation of HNO, through SLMs containing Primene JM-T, Amberlite LA-2 or TLA was followed at different carrier concentrations, always using lo-’ M HN03 ad feed and 0.1 M NaOH as strip solution and a glass nitrate electrode to monitor [NO,- ] in the feed solution. Some representative results for selected amine concentrations are reported in Fig. 3 as feed [NO,- ] /

P

-

.PO 0

200

400

600

800

rw

Fig. 2. Dependence of P and PO of HNO, on the stirring speed. Feed= lo-’ M HNO,; strip = 0.1 M NaOH; membrane = 0.1 M Primene JM-T in n-dodecane on Celgard 2500; membrane area = 18 cm’, volume of feed and strip = 70 cm3.

51

[NO,-], vs. t. It appears from the data that the permeation of HNOB with Primene JM-T is strongly dependent on the carrier concentration, going from very fast at 1 M to very slow at 0.1 M. TLA, on the other hand, removes HNO, rather slowly, the best concentration being in the range 0.2-0.6 M. The low efficiency of TLA as a carrier for HNOB is not surprising based on the much lower values of the biphasic neutralization reaction constants reported in Table 4. With Amberlite LA-2 a consistently fast HN03 removal was measured in the whole concentration range. From the data of Fig. 3 and other data not reported in the figure, the permeability coefficients were calculated for all the systems investigated. The P values for the three amines are reported in Fig. 4 as functions of the amine concentration. It appears from the data that, with the primary and secondary amine, the loo

F

10-l

-

b”

gg

,o

m

-

lo-'

1o-3

1

2

3

5

4

Time, hours

Fig. 3. Feed [NO,-]/ [NO,-], vs. time (hr) curves for SLMs containing different amines at different concentrations as carrier. [NO,- ] was measured by nitrate electrode. All other conditions as in Fig. 2.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

[Amine],!

Fig. 4. Dependence of P, permeability coefficient of HNOs, on the molar concentration of Primene JM-T. Amberlite LA-2 and TLA in n-dodecane.

52

same limiting value, P= (5.65 20.05) x lop3cm-set-‘, is reached. Amberlite LA-2, however, reaches the limiting value at a much lower concentration and is therefore a better carrier for nitric acid. The reason for this behavior is not fully understood. It might be due to the higher solubility of Primene JM-T in the aqueous phase, or simply to the extreme complexity of the equilibria taking place in the organic phase and mentioned previously. The TLA values are always much lower than for the other two amines, except for very low carrier concentration. This may be an indication that a local precipitation of the nitrate-alkylammonium salt takes place in the pores of the membrane, reducing the speed of permeation. The limiting permeability value is related to the aqueous diffusion coeffrcient of the permeating species by the relation: 1 D, p=d,=d,

(14)

Equation (14) is obtained from eqn. (8)) where a high carrier concentration leads to d, being negligible as compared to K[B]d,. By assuming for d, the thickness of the aqueous diffusion layer, the value of 4.8 x 1O-3cm used in Ref. [ 1 ] and determined previously for similar cells and stirring apparatus [27], the value 2.7~ 10d5 cm’-set-l is obtained for the aqueous diffusion coefficient of HN03. A calculated value for D, of HN03 at infinite dilution can be obtained [ 281 by means of the equation: (15) where 1z1 I and I z2 I are the absolute charges of the H+ and NOB- ions and Do are the diffusion coefficients of H+ and NOB- at infinite dilution, 9.3 X 10e5 cm2-set-’ and 1.90x 10e5 cm2-set-l, respectively [29]. The value of DENo3 calculated with eqn. (15) is 3.16 x lop5 cm2-set-l, which is in good agreement with our value of 2.7 x lop5 cm2-see-l considering that the calculated value refers to infinite dilution. The initial parts of the experiments of Fig. 4 have been used to evaluate the initial permeability coefficients, PO.The data are reported in Fig. 5 as a function of the amine concentration. It appears from the figure that, for all three amines, the data are not aligned on straight lines as predicted by eqn. ( 11)) but go through a maximum which appears earliest for TLA. A direct proportionality between PO and the amine concentration exists only in a limited concentration range, up to ca. 1 M for Primene JM-T and up to ca. 0.1 M for the other two amines. This behavior can be ascribed to a non-constant value of d,, that is, to a membrane diffusion coefficient, D,,which decreases for higher amine concentrations. A decrease of D, with increasing concentration can be caused by a change in the viscosity of the organic membrane phase. The vis-

53

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 [Amine],g

Fig. 5. Dependence of P,, initial permeability coefficient of HNOs, on the molar concentration Primene JM-T, Amberlite LA-2 and TLA in n-dodecane.

TLA/

oL 0.0

’

of

1

I p.5

1.0

1.5

[Amine],y

Fig. 6. Viscosity, q (cP) vs. molar concentration n-dodecane.

of Primene JM-T, Amberlite LA-2 and TLA in

cosity data reported in Fig. 6 for the three amines in n-dodecane seem to support this hypothesis, at least in part. The increase of viscosity at higher concentrations is particularly evident with TLA, as is the decline of I’,, in Fig. 5. On the other hand, the much lower P,, values for TLA at high concentration, and the maximum exhibited at a much lower concentration than with the other two amines, again seem to indicate that local precipitation processes of the TLA alkylammonium salt occur in the membrane, lowering the average diffusion coefficient of the permeating species. From the direct proportionality regions between PO and amine concentration, indicated by the straight lines shown in Fig. 5, the following membrane diffusion coefficients have been calculated, using eqn. (11)) for the three amines: Primene JM-T

D, = (5.7 2 1) x 10e8 cm’-set-’

Amberlite LA-2

D, = (2.1+ 0.3) x 10e7 cm’-set-’

TLA

D,= (1.0 % 0.2) X 10e7 cm2-set-l

These values refer to low carrier concentrations, before viscosity or precipita-

54

tion effects start to show up. They are all lower than the 4.0 x 10 -’ value reported in Ref. [l]for the U (VI)-Cyanex 272 organic species, probably because of a contribution by aggregated nitrate-alkylammonium salts to the overall membrane diffusion of nitrate containing species. From the data shown in Figs. 4 and 5, obtained with experiments lasting a few hours, it appears that Amberlite LA-2 should be the carrier of choice for HNO, removal, in spite of Primene JM-T being a better nitric acid extractant. The secondary amine shows higher permeability values at low concentration, a more extended plateau of the permeability as a function of the carrier concentration, and a substantially higher value of the initial permeability in the concentration range 0.2-0.6 M. B. Removal of HNO, from SG W

A series of membrane experiments has been performed using SGW as the feed. The three amines at various concentrations were used as carriers. All other experimental conditions were the same as in the two previous sections. The results obtained by measuring the decrease of the feed [H+ ] by a glass electrode are reported in Fig 7 for some selected amine concentrations. The values of the P and P, permeability coefficients are shown in Table 5. A comparison of these permeability values with those of Figs. 4 and 5 shows that the POvalues obtained with SGW are always lower than those measured with lo-” M HN03, while the opposite is true for the P values. In other words, the removal of acid (as HNO, + H2S04) from SGW proceeds initially at a lower rate as compared to the removal of nitric acid from a lo-* M HNO,>feed, and becomes faster when most of the H+ has been removed. The higher P values measured with a feed made of SGW can be explained by considering that the

Time, hours Fig. 7. Feed [H+ ] / [H+ 1, vs. time (hr) curves for SGW as feed, with SLMs containing amines at different concentrations as carrier. All other conditions as in Fig. 2.

different

55 TABLE 5 Permeability [Amine] (M) 0.1

0.2 0.6 1

coefficients

for acid removal from SGW by SLMs containing

P (cm-set-‘)

(+ 10%)

Pa (cm-see-‘)

I

II

III

N.D? N.D.b 1.1x10-~ 1.1 x 10-z

6.1 x 1O-3 7.5x 1o-3 1.0x10-* 1.1x10-~

9.0x 1.2x 1.4x 1.4x

I 10-4 10-a 10-a 10-3

10-5 1o-4 1o-4 1o-3

amines”

(*20%) II

7.3 x 1.5x 8.0X 1.1 x

“I = Primene JM-T, II = Amberlite LA-2, III =TLA. bThe experiment was not followed long enough to determine

different

3.0x 4.5x 5.7x 5.7 x

III 10-4 10-4 10-4 10-4

2.2 x 10-4 2.4x 1O-4 1.5 x 1o-4 1.4x 10-4

a P value.

SGW solutions contain much less H+ (1.3 x lop2 M, obtained by titration of SGW at pH 2 with standard base) than NO,(3 x lop2 M) and HSO,- + SOb2- (1.7 x 10e2 M). Consequently, when, for example, 90% of H+ has been removed by the SLM as HNOB and/or H2S04, most of the anions are still in the feed. Their presence at a still relatively high concentration contributes to shift to the right the acid-amine reaction at the feed side, accelerating the acid removal and giving rise to higher P values than in the experiments where the feed originally contained only HN03. A visual inspection of the curves of Fig. 7 shows, in analogy with the data of Fig. 3, that Primene JM-T is very ineffective at a 0.1-0.2 M concentration for acid removal from SGW. In contrast, the acid permeation with Amberlite LA2 is always fast, with the O-2-0.6 Mconcentration range giving the best results, while with TLA it is always relatively slow, being fastest at 0.2 M TLA. Again it is possible to draw the conclusion that Amberlite LA-2 at a concentration of 0.2-0.6 A4 is the best carrier for acid removal from SGW. In the experiments of Fig. 7, H+, the concentration of which was monitored by a glass electrode, was removed from SGW as either HN03 or H2S04. Since we are interested in the decontamination of SGW from nitrates and not from sulfates, it is important to know what fraction of the total H+ is transported by the SLM into the strip solution as HN03. With this aim, the experiments of Fig. 7 were repeated, using this time a nitrate electrode to follow the removal from SGW of only nitrate ions (the nitrate electrode employed in this work has a high specificity for N03- over SOd2- ). If the results of these experiments vs. t,one would observe that in the best case wereplottedas [NO,-]/[NO,-1, only about 40% of the initial nitrates of the SGW are removed, because SGW contains much more NO,- ions than H+ ions. In other words, the removal of nitrate ions can proceed only as long as the co-transported H+ ions are present in the feed. A more significant way to represent the data is to plot them as the residual fraction of the removable nitrates vs. time. This fraction can be expressed as:

56

1_

lstrip [H+ lo

tNo3-

(16)

where [NOs-],trip is the concentration of nitrates in the strip solution, obtained by mass balance, and [H + ] o, the initial feed concentration of H+, is equal to the highest concentration of nitrates that can be removed. Figure 8 reports the results of the experiments obtained with 1 A4 Primene JM-T and 0.6 M Amberlite LA-2 or TLA, together with the results obtained when a glass electrode was used. One can see from the figure that the removal of nitrates follows quite closely the removal of total acid. For Amberlite LA-2, for example, when 90% of H+ has been removed, about 75% of removable nitrate has left the feed. Similar results are obtained with Primene JM-T, while for TLA the two curves are almost coincident. We conclude, therefore, that hydrogen ion is removed from SGW mainly as nitric acid and that amine based SLMs are, indeed, effective in removing nitrates from SGW even in the presence of large quantities of sulfate-bisulfate anions. For a complete removal of nitrates, however, it is necessary that the feed acidity be continuously monitored and adjusted. When the pH of the feed reaches a value between 3 and 4, more H&SO, should be added to the feed in order to provide more hydrogen ions to be co-transported with the residual nitrate ions. C. Use of membrane diluent modifiers We mentioned in the introduction that the equilibrium constant of the biphasic reaction of HNO, with Amberlite LA-2 was substantially increased by

0

50 100 Time, minutes

150

Fig. 8. Fraction of [H+] and fraction of removable [NO,-] residual in the SGW vs. time (min), with SLMs containing 1 M Primene JM-T, 0.6 M Amberlite LA-2 or 0.6 M TLA as carrier. All other conditions as in Fig. 2.

using trioctyl phosphate (TOP) as diluent [5]. This is due to the fact that a more polar diluent is more compatible with the polar alkylammonium salt formed by the neutralization reaction. Based on this consideration, we decided to study the behavior of amine based SLMs where the diluent was n-dodecane with 30% of TOP or Cyanex 923. These different liquid membrane formulations were used in experiments where the feed was 10e2 M HN03, the strip solution was 0.1 M NaOH and the nitrate removal was followed by a nitrate electrode. All other conditions were the same as in the other membrane experiments reported above. The concentration of the amine in the membrane was 0.2 M for Primene JM-T, and 0.1 M for Amberlite LA-2 and TLA. The results (which, for brevity, are not reported here) showed that TOP has some accelerating effect on the transport of HNO, by the primary and tertiary amine (P values higher by a factor 2)) while only a slight acceleration was measured with the secondary amine. With Cyanex 923 present in the membrane, practically the same nitrate permeation curves were obtained with all three amines, that is, the HN03 transport was strongly decelerated with Amberlite LA-2 and strongly accelerated with Primene JM-T and TLA. This is a quite remarkable diluent effect. The basicity of the diluent modifier present at a high concentration in the membrane becomes the primary factor in determining the permeation of HN03, completely masking the basic strength difference among the three amines. This effect is exhibited by the dodecaneCyanex 923 mixture only, because the basicity of the P=O group of phosphine oxides is much higher than that of alkyl phosphates. In spite of the measured acceleration effects, however, liquid membranes containing TOP or Cyanex 923 cannot be used in practice. During the experiments described above, the liquid membrane in several cases failed in a few hours. The membrane failure was indicated by interdiffusion of the strip and feed solutions, with the nitrate feed concentration going through a minimum and eventually reaching about 50% of the initial value. The lack of stability of these modified liquid membranes seems to exclude their use for any practical application, in spite of their interesting chemistry. D. Transport of technetium(VII) The distribution ratio of Tc (VII) as TcO,- anion was measuredusing SGW at pH 2 spiked with “Tc as the aqueous phase, and n-dodecane solutions of the three amines at different concentrations as the organic phase. The amine solutions were not preequilibrated with the aqueous phase, to have data more representative of the actual conditions present at the feed-membrane interface. The data are reported in Table 6. The order of Tc (VII) extraction, tertiary > secondary> primary, is the same as reported in Ref. [ 191 for similar amines and from a chloride medium. This order is the reverse of the order observed in nitric acid extraction. The highest Tc extraction is shown by the amine with the lowest basic strength.

58

TABLE 6 Tc(VI1) distribution ratio between preequilibrated solutions)

Primene 0.1

JM-T

Amberlite

4.2 2.2 1.8

0.2 0.4

SGW at pH 2 and n-dodecane

LA-2

of amines

(non-

TLA 230 216 105

48.1 47.0 34.3

Feed = SGW I = 0.2 M Primene II = 0.2 M Amberlite III = 0.2 M TLA

solutions

i I

3L

,n I”

21

0

1

2

3

Time. hours

Fig. 9. “Tc feed activity (c.p.m./50 ~1) vs. time (hr). Feed=SGW at pH 2; strip=O.l M NaOH; membrane = 0.2 M Primene JM-T, Amberlite LA-2 or TLA in n-dodecane on Celgard 2500; membrane area = 1.71 cm”; volume of feed and strip = 4 cm”; stirring speed= 250 rpm.

No simple explanation can be found for the negative dependence of the Tc distribution ratios on the amine concentration. It is probable that this behavior depends on the fact that the solutions were not preequilibrated. The membrane experiments involving Tc (VII ) were performed using SGW as the feed, 0.1 M NaOH as the strip, and SLMs containing the three amines at different concentrations in n-dodecane. The hydrodynamic conditions were the same as in Ref. [ 11, since the same miniaturized cell was used. Figure 9 reports some typical permeation data, as “Tc activity in the feed vs. time, for a 0.2 M concentration of the three amines. The data show a decline of the feed activity with time over almost two orders of magnitude. The permeation process is described only by eqn. (7 ) (no region A of Fig. 1 is present), because the initial feed concentration of pertechnetate ions was always very low (3.5 x lo-” M). The deviation from linearity shown by the data after about 2 hr is probably due to non-permeating impurities present in the “Tc stock

59

[Amine], M

Fig. 10. Dependence of “Tc permeability coefficient, P, on the molar concentration JM-T, Amberlite LA-2 and TLA.

of Primene

solution. An alternative explanation could be that HN03 and H,SO, are also removed from the feed, progressively reducing the concentration of H+ ions needed for the permeation of HTcO,. This explanation should not apply to the TLA data, however, because with this carrier HTcO, is removed much faster than HNO, and H,SO,. The order of the Tc (VII) permeability coefficients, evaluated from the slopes of the straight lines of Fig. 10, is the same as the order of the distribution ratios. With all three amines, however, the permeation of Tc (VII) proceeds rather rapidly. The results of a complete investigation of the Tc(VI1) permeation speed as a function of the carrier concentration are reported in Fig. 10 as Tc (VII) permeability coefficient vs. amine molarity. Contrary to the HNO, case illustrated in Fig. 4, a limiting permeability value, extending for over two orders of magnitude, was found for TLA and Amberlite LA-2. The same limiting permeability value was exhibited by Primene JM-T membranes only for concentrations approaching 1 M. From the limiting permeability, (1.75 ? 0.05) x 10e3 cm-set-‘, a value for the diffusion coefficient of HTcO, can be calculated, using eqn. (14) and d, ~4.8 x 10e3 cm. The D, value, 8.4 x lop6 cm’-see-‘, is lower than the value calculated previously for the much smaller HNO,-species by a reasonable amount. E. Transport of chromium( VI) Cr(V1) species exist in solution as Cr04’- ions at pH> 6, as HCrO*- in equilibrium with dichromate ions Cr,072- between pH 2 and 6, and mainly as H2Cr0, at pH < 1 [ 301. For simplicity we will refer to Cr (VI) as HCr04-, but the results of our measurements apply to the dichromate ions as well. As in the Tc (VII ) case, distribution ratios of Cr (VI ) were measured between SGW at pH 2, lop3 M in Cr (VI), and organic solutions containing 0.2 M amines in n-dodecane. The results, reported in Table 7, show that, similarly to the Tc (VII) case, the order of Cr (VI) extraction is tertiary > secondary > primary,

60

although the difference of the distribution ratios is less pronounced. Again, the extraction order is the reverse of the basic strength order. Membrane experiments with the same organic solutions used as the liquid membranes, SGW at pH 2 and 1O-3 M in Cr(V1) as feed and 0.1 M NaOH as strip, were performed using the same cell as for HN03 permeation, with a stirring speed of 600 rpm. The results reported in Fig. 11, as feed Cr(V1) concentration vs. time, show that the secondary amine is much more effective in transporting HCrO,- [PHCro4-= (1.2 2 0.1) x lo-” cm-set-‘1 than the primary [PHCro4-= (4.520.1) x 10e4 cm-set-‘1 and, especially, than the tertiary [PHCro4-= (1.7 + 0.1) x lop4 cm-set-l]. In other words, TLA, which shows the highest distribution ratio for Cr(VI), gives rise to the lowest permeation rate. The unexpectedly low P value with TLA is very likely due to the poor solubility of the chromate salt of the amine in the diluent. The low solubility of the chromate-Alamine 336 salt in aliphatic hydrocarbons is the main TABLE I Cr (VI) distribution ratio between SGW at pH 2 and 0.2 M solutions of amines in n-dodecane (non-preequilibrated solutions) Amine

K d,Cr

Primene JM-T Amberlite LA-2 TLA

1.21 12.0 31.5

;‘;; 5 E

i

Amberlite

i

t

1

2

3

Time, hours

Fig. 11. Feed Cr(V1) concentration vs. time (hr). Feed=SGW at pH 2; strip=O.l M NaOH; membrane = 0.2 M Primene JM-T, Amberlite LA-2 or TLA in n-dodecane on Celgard 2500; membrane area= 18 cm’; volume of feed and strip = 70 cm3; stirring speed= 600 rpm.

61

reason why the authors of Ref. [ 181 used the aromatic diluent Escaid 350 in their study. Although a complete investigation of the dependence of PHCr04-on the concentration of the amines has not been performed, it is reasonable to assume that, at least for Amberlite LA-2, the PHCro4_measured from the data of Fig. 11 is the limiting permeability coefficient value. In this hypothesis, the average diffusion coefficient of the Cr(V1) containing species in the aqueous phase, calculated as in the Tc (VII) case, is equal to 5.8 x 10 -’ cm2-see-‘. This value, somewhat lower than for HTcO,, may reflect the contribution to the overall diffusion of the larger Cr20T2- anion in equilibrium with HCrO,-. In spite of the limited number of data points obtained with the SGW-HCrO,system, we can conclude this section by emphasizing that, on the basis of shorttime membrane experiments, Amberlite LA-2 is again the best carrier for Cr(V1) removal from SGW at pH 2. F. Transport of uranium(VI) Uranium (VI ) is quite strongly complexed by SOd2- ions (log p values equal to 1.81, 2.5 and 3.7 are reported [31] for the 1: 1, 1: 2 and 1: 3 complexes at ionic strength equal to 1). In SGW at pH 2, U (VI) exists therefore mainly in the form of anionic complexes. We decided to determine whether anionic sulfato-complexes of U(V1) would be removed from SGW at pH 2 by the three amines in the same way and with the same co-transport mechanism as TcO,-, HCrO,- and NO,-. A first indication on this possibility was given by the measurement of the distribution ratio of uranium between SGW at pH 2 and 0.2 M solutions of the amines in n-dodecane. The data, reported in Table 8, show that Primene JMT is indeed capable of extracting uranium from SGW, while the other two amines exhibit rather low distribution ratio values. This finding agrees with the general statement reported in Ref. [ 191 that, while primary and secondary amines are generally poor extractants of metal species from aqueous chloride or nitrate solutions as compared to the tertiary amines, from sulfate solutions their extractive capacity is of a comparable magnitude or frequently even higher. The different extraction behavior of the three amines fully explains the TABLE 8 U(V1) distribution ratio between SGW at pH 2 and 0.2 M solutions of amines in n-dodecane (non-preequilibrated solutions) Amine

&,u

Primene JM-T Amberlite LA-2 TLA

33.7 0.11 0.88

62

0

1

2

3

4

5

Time. hours

Fig. 12. Feed uranium activity/uranium strip=O.l M NaOH f0.02 M HEDPA; TLA in n-dodecane

activity at time zero vs. time (hr). Feed= SGW at pH 2; membraneE0.2 M Primene JM-T, Amberlite LA-2 or

on Celgard 2500. All other conditions

as in Fig. 9.

permeability experiments reported in Fig. 12, where SGW at pH 2 spiked with 233Uwas used as feed and a 0.1 M NaOH + 0.02 M HEDPA solution was used as strip solution (HEDPA was used in mixture with NaOH to prevent the precipitation of uranium oxide on the strip side of the membrane. An alternative strip solution could be 0.1 M Na,CO,). The data show that uranium (VI) is quite efficiently removed from SGW only by the primary amine [P= (5.1 I-0.1) ~10~~ cm-set-l], while Amberlite LA-2 and TLA show low P values [ (1.1 -t 0.1) x 10m5 and (2.5 & 0.1) X lo-” cm-set-‘, respectively]. Consequently, a simultaneous decontamination of SGW from U(V1) as well from Cr (VI), Tc (VII ) and NO,- can be accomplished only using Primene JMT as carrier. Summary

and conclusions

We have demonstrated that a supported liquid membrane system consisting of a n-dodecane solution of the commercially available primary amine, Primene JM-T, secondary amine, Amberlite LA-2 and tertiary amine, TLA, can be used for the removal of nitrate, pertechnetate and chromate ions from synthetic groundwater at pH 2. The basic strength of the three amines was characterized by biphasic titrations and the following order of basic strength was established: primary > secondary > tertiary The behavior of the amines with respect to the species to be removed was studied by permeation experiments involving flat-sheet supports. The following orders of effectiveness for the single contaminants were measured:

63

for N03-

secondary > primary >> tertiary

for TcO,-

tertiary 2 secondary > primary

for HCrO,-

secondary > primary > tertiary

Based on these findings, it appears that the carrier of choice for the simultaneous removal of the three contaminants is Amberlite LA-2. On the other hand, Primene JM-T shows the unique property of also removing U (VI) anionic sulfato-complexes from a synthetic groundwater acidified with H,SO,. An alternative process might be, therefore, the simultaneous removal of U (VI ) , Cr (VI), Tc (VII) and NO,- species from acidified groundwater in a single module containing a primary amine as carrier, if sufficient stability of such a liquid membrane could be demonstrated. Acknowledgements The author wishes to express his gratitude to Westinghouse Hanford Co. for the financial support provided. He also wishes to thank E.A. Huff for the Cr (VI) analyses, and Dr. E.P. Horwitz for stimulating discussions and for kindly 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. P.R. Danesi, C. Cianetti and E.P. Horwitz, Acid extraction by supported liquid membranes containing basic carriers, Solvent Extr. Ion Exch., l(2) (1983) 299. W.R. Dworzak and A.J. Naser, Pilot-scale evaluation of supported liquid membrane extraction, Sep. Sci. Technol., 22 (2-3) (1987) 677. M.M. Kreevoy and C.I. Nitsche, Progress toward a solution to the nitrate problem, Environ. Sci. Technol., 16 (1982) 635. M.M. Kreevoy, A.T. Kotchevar and C.W. Aften, Decontamination of nitrate polluted water, Sep. Sci. Technol., 22(2-3) (1987) 361. R. Chiarizia and P.R. Danesi, A double liquid membrane system for the removal of actinides and lanthanides from acidic nuclear wastes, Sep. Sci. Technol., 22 (2-3) (1987) 641. R. Chiarizia, P.R. Danesi, E.P. Horwitz and P.K. Tse, Application of a double liquid membrane for actinide determination in biological samples, Proc. Int. Solvent Extr. Conf., 1988, Moscow, July 18-24, Vol. IV, Vernadsky Institute of Geochemistry and Analytical Chemistry of the USSR, Academy of Sciences, Moscow, p. 10. P.R. Danesi, R. Chiarizia and A. Castagnola, Transfer rate and separation of Cd(I1) and Zn (II) chloride species by a trilaurylammonium chloride-triethylbenzene supported liquid membrane, J. Membrane Sci., 14 (1983) 161. R. Chiarizia and A. Castagnola, Transfer rate and separation of Fe (III), Co (II) and Ni (II 1 chloride species by a supported liquid membrane, Solvent Extr. Ion Exch., 2 (1984) 479.

64 11

12 13 14 15

16

17 18

W.C. Babcock, R.W. Baker, E.D. Lachapelle and K.L. Smith, Coupled transport membranes. II. The mechanism of uranium transport with a tertiary amine, J. Membrane Sci., 7 (1980) 71; Coupled transport membranes. III. The rate limiting step in uranium transport with a tertiary amine. J. Membrane Sci., 7 (1980) 89. W.C. Babcock, D.T. Friesen and E.D. Lachapelle,

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from vanadium and uranium fro molybdenum, J. Membrane Sci., 26 (1986) 303. T. Kitagawa, Y. Nishikawa, J.W. Frankenfeld and N.N. Li, Wastewater treatment by liquid membrane process, Environ. Sci. Technol., ll(6) (1977) 602. A.M. Hochhauser and E.L. Cussler, Concentrating chromium with liquid surfactant membranes, AIChE Symp. Ser., 71(152) (1976) 136. M. Teramoto, N. Tono, N. Ohnishi and H. Matsuyama, Development of a spiral-type flowing liquid membrane module with high stability and its application to the recovery of chromium and zinc, Sep. Sci. Technol., 24(12-13) (1989) 981. R. Molinari, E. Drioli and G. Pantano, Stability and effect of diluents in supported liquid membranes for Cr(III), Cr(V1) and Cd(H) recovery, Sep. Sci. Technol., 24(12-13) (1989) 1015. D. Pearson, Supported liquid membranes for metal extraction from dilute solutions, in: D.S. Flett (Ed.), Ion Exchange Membranes, Ellis Horwood, Chichester, 1983, pp. 55-73.

21

D. Pearson, Supported liquid membranes using Accurel fibres. Progress report no. 1, Warren Spring Lab., Stevenage, Hertfordshire, U.K., 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. C. Sella and D. Bauer, Diphasic acido-basic properties of.organophosphorus acids, Solvent Extr. Ion Exch., 6 (1988) 819. P.R. Danesi, R. Chiarizia, P.G. Rickert and E.P. Horwitz, Separations of actinides and lan-

22

thanides from acidic nuclear wastes by supported liquid membranes, Solvent Extr. Ion Exch., 3 (1985) 111. V.S. Shmidt, Amine Extraction, Israel Program for Scientific Translations, Jerusalem, 1971.

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E. Hogfeldt, From Nernst to synergism, in: S. Alegret (Ed.), Developments in Solvent Extraction, Ellis Horwood, Chichester, 1988. R.R. Grinstead, Base strength of amines in liquid-liquid extraction systems, in: D. Dyrssen, J.O. Liljenzin and J. Rydberg (Eds.), Solvent Extraction Chemistry, North-Holland, Amsterdam, 1967, p. 426. J.C. Blazquez, J.M. Madariaga and M.C.A. Sandino, The extraction of sulfuric acid by Primene-JM-T dissolved in toluene. E.m.f. and batch studies at 25”C, Proc. Int. Solvent Extr. Conf., 1986, Munich, September 11-16, Vol. III, DECHEMA, Frankfurt a. M., p. 839. C. Cianetti and P.R. Danesi, Facilitated transport of HNO, through a supported liquid membrane containing a tertiary amine as carrier, Solvent Extr. Ion Exch., 1 (1983) 565.

27

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S. Kotrly and L. Sucha, Handbook Horwood,

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