Preconcentration of aluminium by micellar-enhanced ultrafiltration

173

Analytica Chimica Acta, 276 (1993) 173-179 Elsevier Science Publishers B.V., Amsterdam

Preconcentration of aluminium by micellar-enhanced ultrafiltration Miguel de la Guardia, Empar Perk-Cardells

and Angel Morales-Rubio

Department of Analytical Chemistry, University of Valencia, 50 Dr. Moliner Street, 46100 Butjassot, Valencia (Spain)

Alessandra Bianco-Prevot

and Edmondo Pramauro

Dipartimento di Chimica Analitica, Universitcidi Torino, 10125 Turin (Italy) (Received 24th July 1992; revised manuscript received 23rd October 1992)

Traces of aluminium were preconcentrated in water samples by forming a complex with lumogallion, and successfully accumulated in the micellar phase obtained from cationic or non-ionic surfactants and filtered through 10000 molecular weight cut-off ultrafiltration membranes. Studies indicated that, at pH 5.9, with 1 x 10m3 M lumogallion and 2 X lo-* M cetyltrimethylamonium bromide, quantitative retention of aluminium present at pg ml-’ concentration levels was achieved. A micellar-enhanced fluorimetric procedure was employed to analyse the permeate solutions, whereas nitrous oxide-acetylene flame atomic emission spectrometry was used to determine a1uminium in the retentate. The experimental parameters that influence the micellar-enhanced ultrafiltration (MEUF) efficiency were investigated and the analytical procedures were optimized. The MEUF of trace levels of aluminium allows for either preconcentration of the analyte or enhancement of the emission signals. Keywords: Atomic emission spectrometry; Fluorimetry; Aluminium; Micellar-enhanced tion; Ultrafiltration; Waters

Micellar-enhanced ultrafiltration (MEUF) is a separation technique based on the principles of ultrafiltration membranes but improved by the use of micellar assemblies, which permit the removal of dissolved molecular species [l-8]. In MEUF, a surfactant at concentrations greater than the critical micelle concentration (c.m.c.1 is added to the samples. The interactions between micelles and dissolved species results in the binding of organic molecules or ions within or at the surface of the surfactant aggregates. These bound species are retained together with their host aggregates, as most micelles do not pass through Correspondence to: M. de la Guardia, Department of Analytical Chemistry, University of Valencia, 50 Dr. Moliner Street, 46100 Bujassot, Valencia (Spain). 0003-2670/93/$06.00

ultrafiltration; Preconcentra-

membranes having a molecular weight cut-off (MWCO) of ca. 10000. This basic idea has been applied to the removal of organic molecules from water, such as phenols, benzene, chlorinated hydrocarbons, l,l,l-trichloro-2,2-bis(p-chorophenyljethane (DDT), aliphatic alcohols, chloroaromatic herbicides and aniline derivatives [S-11]. Inorganic ions, such as iron, copper, calcium, zinc, cadmium and chromate [12,13], have been also removed from aqueous solutions using surfactants possessing an opposite charge to the species considered (anionic surfactants have been employed for cations and cationic surfactants for the removal of anions). A new approach to metal ion concentration by MEUF is based on the use of functional chelating micelles, which can increase the

0 1993 - Elsevier Science Publishers B.V. All rights reserved

174

rejection percentage and/or improve the selectivity of the separations [14-161. Based on published reports, it can be concluded that MEUF is an interesting means with which to decontaminate waste waters and to remove inorganic species from liquid baths employed in industrial areas. However, little attention has been paid to the analytical applications of MEUF as a preconcentration technique for analytes to be determined in dilute samples or to improve the selectivity of the analytical methods. The aim of this work was to study the separation of trace amounts of aluminium from aqueous solution by the formation of a complex with lumogallion (LG), concentrated in the retentate by MEUF in the presence of cetyltrimethylammonium bromide (CTAB). This separation system was succesfully employed to preconcentrate the analyte and to improve the sensitivity of its determination by flame atomic emission spectrometry.

EXPERIMENTAL

Apparatus and reagents An S-43-70 ultrafiltration stirred cell with a volume of 70 ml (Spectrum) was used to carry out the ultrafiltration experiments. Spectra Por-Cl0 disc ultrafiltration membranes (43 mm diameter) with an MWCO of 10000 were employed. These membranes are composed of solubilized and cast cellulose material (mainly cellulose acetate and hydrolysed c$ulose acetate) and have pore sizes of about 25 A. In all the experiments, a constant nitrogen pressure of 3 atm was applied. A preliminary washing treatment of the membranes with 30 ml of water is necessary to eliminate the wetting agents incorporated in the cellulose filter. A Perkin-Elmer LS 50 single-beam, luminescence spectrometer, equipped with l-cm pathlength quartz cells, was employed for the fluorescence measurements of the permeate solutions. The experimental conditions are summarized in Table 1. The spectrometer is equipped with a 1% attenuator of the emission light, which permits the emission signals to be reduced in order to avoid saturation of the detector in the study of

M. de la Guardia et aL /Anal. Chim. Acta 276 (1993) 173-l 79 TABLE 1 Experimental conditions employed in fluorescence measurements Parameter

Bxperimental value

498 nm 552 nm Atem.) 7nm Slitiexc.) 7nm Slitfern. 1.5 x lo-’ M Lumogallion concentration 1.26x1O-2 M Brij-35 concentration 5 (acetic acid-acetate buffer) PH

Next.)

highly fluorescent systems. In this work in all instances the fluorescence of the aluminiumlumogallion complex was using this attenuator. A Varian SpectrAAflame atomic absorption spectrometer, equipped with a laminar burner for nitrous oxide-acetylene, was employed to carry out emission measurements of the retentate solutions under the experimental conditions indicated in Table 2. A Gilson P-2 Minipuls peristaltic pump was used for experiments carried out at a fixed carrier flow-rate. The reagents employed, provided by Panreac and Probus (Spain), unless indicated otherwise, were of analytical-reagent grade. An aluminium stock standard solution was prepared by dissolving 1.0000 g of Al metal (Merck) in a minimum amount of HCI (1 + 1) and diluting to 1 1 with 1% (v/v> HCl. Lumogallion (LG) (ICN Pharmaceuticals) was employed to complex aluminium. A 3.9 X low3 M

TABLE 2 Experimental conditions used to measure the atomic emission of aluminium Parameter

Experimental value

Wavelength C,H, flow-rate N,O flow-rate Burner height a Slit width Aspiration flow-rate Pump flow-rate

391.1 nm 5.5 1min-’ 9.71min-1 10 mm 0.5 nm 7mlmin-’ 7mlmin-’

a The burner height is the distance below the focal point of the optical system.

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M. de In Guardiaet al,/Anal. Chim.Acta 276 (1993) 173-179

stock solution was prepared in distilled, deionized water. A 0.05 M stock standard solution of N-acetylN,N,N-trimethylammonium bromide (CTAB) (Merck) was prepared in distilled, deionized water. A 0.05 M stock standard solution of Brij-35 (Aldrich) was prepared. Both surfactants were used as received. Uitrafiltation

experiments

To samples containing Al at concentrations I 6 mg l-i, were added appropriate amounts of LG, CTAB and acetic acid-acetate buffer, in order to obtain final concentrations of 0.02 M CI’AB and 1 x 10e3 M LG and a pH 2 5. This solution was heated at 75°C for 60 minutes, cooled, 30 ml were placed in the ultrafiltration cell, and ultrafiltration was carried out at a constant nitrogen pressure (3 atm) until 25 ml of permeate had been collected. The retentate was analysed directly by flame atomic emission spectrometry and the rejection factor was evaluated. The mass balance of the analyte was also checked by analysing the permeate solution by micellar-enhanced fluorescence, after addition of appropriate amounts of LG and the non-ionic surfactant Brij-35. Micellar-enhanced fluorimetric analysis

To determine the concentration of Al in the permeate solutions, LG and Brij-35 were added to filtered samples to give final concentrations of 1.5 x lo-’ M LG and 1.26 X lo-’ M surfactant. The samples were then heated at 75°C for 1 h and cooled. The fluorescence of these solutions was measured against a calibration graph obtained from aluminium standards (in the concentration range 8-40 pg 1-l) prepared under similar conditions. Flame atomic emission analysis

The retentate solutions were introduced directly into a nitrous oxide-acetylene flame by means of a peristaltic pump working at the same carrier flow-rate as used for the direct aspiration of aqueous solutions. The Al emission of these solutions was measured at 396.1 rmr. Standard solutions containing 3-12 mg 1-i Al and the same

LG and surfactant concentrations mentioned were employed.

as previously

RESULTS AND DISCUSSION

Fluorescence complex

of

the

aluminium-lumogallion

Ishibashi and Kina [17] reported that LG forms a complex with Al that can be used for the spectrophotometric and fluorimetric determination of traces of this metal. Previously, it was reported that the AI-LG complex interacts with micelles and that a fluorescence enhancement of 200% can be obtained in the presence of micelles composed of ethylene oxide condensates with fatty alcohol-type surfactants [18]. Based on that work, two surfactants, CTAB and Brij-35, were selected to obtain an appropriate micellar medium to conduct the ultrafiltration of water solutions containing AI in the presence of LG. The ultrafiltration of the Al solution results in concentration of analyte in the retentate. The permeate is of little analytical interest. However, in order to determine accurately the rejection coefficient of the analyte (R), the aluminium concentration in the permeate (C,) and in the initial solution (C,) must be known. The rejection factor is calculated from the equation R = 1 - (C,/C,) C, can be determined fluorimetricaly. During the ultrafiltration in the presence of CI’AB it was found that LG is quantitatively retained, whereas in the presence of Brij-35 1% of initial LG passes through the membranes. Hence it is necessary to add LG to the permeate solutions in order to ensure the complexation of the unretained Al, prior to the analysis of the permeate solutions. On the other hand, if the retentate concentration is not very high, only surfactant monomers pass through the ultrafiltration membrane. Therefore, a concentration of Brij-35 higher than the c.m.c. must be added to the permeate solutions in order to reconstitute the micellar medium. A typical calibration line obtained for the analysis of the permeate solutions provides the

M. de la Guardia et al /Anal. Chim Acta 276 (1993) 173479

176

equation 1, = 3.77 + 0.77C &g 1-l) with a regression coefficient of 0.9998 (see Fig. 1). The detection limit of the miccllar-enhanced fluorescence determination of AI with LG, in the presence of Brij 35, corresponds to 0.46 pg I-’ (established for a confidence level of 95%). The relative standard deviation, determined .for ten independent measurements of a solution containing 32 pg 1-l Al, is 1%. It was confirmed (results not shown) that the presence of CTAB concentrations from 8 X lo-’ to 1.2 x 10e4 M does not affect the fluorescence of the complex. Hence it is not necessary to compensate for the presence of CTAB monomers in the permeate solutions. The concentration of LG (between 1 X lo-’ and 1.6 x 10m5 M) does not affect the fluorescence measurements of the complex in the presence of Brij-35 micelles. Hence the analysis of permeate solutions obtained in the presence of this non-ionic surfactant (in which a small amount of LG passes through the membrane) can be performed without modifying the LG concentration added. Micellar-enhanced ultrafiltrationof aluminium Using the described procedure, aqueous solutions containing 6 mg I-’ Al were filtered in both CTAB and Brij-35 micellar media. Different ul-

II..(a.u.)

I 5 0

5hO

540

560

5i?O 6bO

Se?0 640 h

bm)

Fig. 1. Relative intensity (1,) of fluorescence emission (arbitrary units) obtained for several standard solutions of Al in the presence of LG in a Brij-35 micellar medium. Inset: calibration graph obtained using the experimental conditions reported in Table 1.

R 1.00 -

O.SS

0.94 1 0.98 0.92

o.900'o

r

Oh

1.0

1.5 2.0 LG 10" (mol

L-l;

Fig. 2. Effect of the lumogailicm concentration on the MEUF ofAl.pH=5;[Al]=6mg1~‘;[CTAB]=0.02M,R=rejection factor.

trafiltration parameters, such as surfactant concentration, LG concentration and pH, were varied in order to attempt to obtain quantitative retention of the analyte. MEUF of aluminium in the presence of CT&. Figure 2 indicates that, working at pH 5 in the presence of 0.02 M CTAB, the rejection factor (R) tends to 1 when the LG concentration is 2 1 x 10m3 M. It is convenient to use this concentration in order to avoid the consumption of an excess of ligand. The influence of the ligand on the preconcentration performances is very important. A rejection factor of 0.33 was found in the absence of ligand. This effect, which probably implies an electrostatic repulsion due to the surfactant absorbed in the membrane, has also been observed previously [ 141. It appears evident that in order to achieve quantitative preconcentration of Al in the retentate, it is necessary to ensure complex formation. This step is favoured by heating the solution. In fact, a rejection factor of only 0.84 was found in the presence of 1 X 10T3 M LG in unheated solutions, indicating that complexation is not complete without heating. Previous studies on MEUF separations of cations indicated that an increase in pH enhances the retention of these species. However, a high pH values also favours the hydrolysis of matrix components and the precipitation of hydroxides

M. de la Guardia et al./AnaL Chim. Acta 276 (1993) 173-179

R

1.0 -

0.9 0.9 0.7-

0.64

Y--

3

7

)

7

5

8

($

t

PH Fig. 3. Effect of pH on the MEUF of Al. [CTAEI]= 0.02 M, WI = 6 mg I-‘; [I-G] = 1 x low3 M; R = rejection factor.

can occur, which reduces the filtration rate and the selectivity of the process. Figure 3 indicates that, at pH 5, the rejection factor for Al is > 0.97 and at pH 5.9 the retention of this analyte become quantitative. At pH 5.9 in the presence of 1 X 10V3 M LG, the retention of Al was quantitative for CTAB concentrations r 7.5 x 10B3 (see Fig. 4). In an experiment carried out with 5 X 10m3 M CTAB, AI was quantitatively recovered in the retentate but, during the filtration step, the formation of a precipitate was noted. This could be due to the limited amount of micellized CTAB which is not able to solubilixe the Al-LG complex. Therefore, a surfactant concentration of about lo-* M is recommended in order to obtain a good Al separation with low reagent consumption.

R

1.00

0.99

0.1 5

0.010

0.015

0. O.d20 CTAB (mol I

:

Fig. 4. Effect of CTAEI concentration on the MEUF of Al. pH=5.9; [Al]=6 mg 1-l; [LG]=lX10-3 M, R=rejection factor.

177

MEUF of aluminium in the presence of Braj-35. Previous studies, carried out in 1 X lo-* M solutions of Brij-35 demonstrated that, starting from a 1 x 10e3 M concentration of LG, a concentration of 1.58 x lo-’ M of this ligand passes through the membrane. The viscosity of the retentate solution is higher than that obtain in CTAB. These two unfavourable aspects dictated the use of CI’AB. However, when working under the same experimental conditions as established previously for CTAB (pH 5.9, 1 X 10T3 M LG and 1 x lo-* M surfactant) the rejection factor obtained using Brij-35 was 0.98, which indicates that a good recovery of the analyte also resulted in this instance. Flame atomic emission of aluminium in the presence of surfactant As the aim of this study was to develop a sensitive procedure for the determination of trace amounts of Al after preconcentration by MEUF, the final step was the measurement of the flame emission of Al in the retentate. Previous work showed that flame atomic emission in a nitrone oxide-acetylene flame provides a higher sensitivity that atomic absorption measurements and can be applied to the determination of trace levels of Al both in aqueous solutions [19] and in oil-in-water emulsions [20]. The effects of the flame parameters, such as the burner height and the acetylene flow-rate, on the emission of Al solutions, were studied. Figure 5 shows the slope of the calibration lines obtained for an aqueous solution of Al and micellar solutions in the presence of LG. Both exhibit similar variations of the signal as a function of the burner height. Only for solutions containing 2 x 10m3 M LG and 7 x lo-* M Brij-35 was a strong reduction of the sensitivity (of the Al determination) observed. This could be due to the high viscosity of these solutions, which reduces the aspiration flow. Alternatively, a 5.5 1 min-’ acetylene flow-rate and a 9.7 1 min-’ nitrous oxide flow-rate provided the best emissions values for all the standards assayed. The effect of surfactant concentration on the aspiration flow-rate of the retentate solutions into the flame emission spectrometer was studied. As

M. de la Guardia et al. /Anal. Chbn. Acta 276 (I 993) 173-l 79

178 0.04

B

0.03

2 + 0.02 25 & 0 0.01 ii;

Brij-35 4 8 1’0 ’ 12 to* C (mol L-‘) Fig. 7. Effect of the surfactant concentration on the emission intensity (arbitrary units) obtained for the determination of Al by flame atomic emission spectrometry. Inset: same effects studied with an imposed carrier flow-rate of 7 ml mitt-‘. [Al]=6 mgl-‘. O1 8

0.00

5

10

15

20 h

I

(mm)

Fig. 5. Effect of the burner height on the sensitivity obtained by flame atomic emission spectrometry for different types of Al solutions. A = Aqueous standard; 0 = standards containing 1 x lo-’ M LG and 2x lo-* M Brij-35; n = standards containing 2X 10m3 M LG and 7X lo-* M Brij-35; o = standards containing 1 x 10m3 M LG and 2x lo-* M CTAB; l = standards containing 2x10d3 M LG and 7x10-’ M CTAB.

shown in Fig. 6, the aspiration flow-rate of 6 mg 1-i Al in the presence of increasing concentrations of CTAB or Brij-35 tends to decrease as the surfactant concentration increases. The effect of surfactant concentration on the emission signal obtained for Al solutions is shown in Fig. 7. CTAB increases the sensitivity slightly. On the other hand, a strong reduction in the aspiration flow-rate in the Brij-35 system, as discussed previously, negatively affects the sensitivity. In order to correct for this effect, it is necessary to use a peristaltic pump, which imposes a carrier transport flow, independently of the rheological features of the solutions. The results of this approach can be seen in the inset in Fig. 7.

Brij-35

mP r

4m '6

m4

’ 6

s

s lb v 12 8 10' C (mol L-‘)

Fig. 6. Effect of the surfactant concentration on the aspiration flow-rate of Al solutions into the flame.

’

Analytical performance of the developed procedure CT’AB was selected for the MEUF preconcen-

tration of Al solutions, which provides advantages in the flame atomic emission of Al. An LG concentration of the order of 6 X 10m3 M and a CTAB concentration of 0.12 M in the AI standards are recommended for the determination of Al in samples previously concentrated by MEUF. A typical calibration graph obtained for these standards (in the range 3-12 mg 1-i) corresponds to the equation E = 0.161+ 0.037C (mg 1-i) with r = 0.9996. The high value of the intercept is due to the influence of both LG and the surfactant on the emission background; it can be corrected by adjusting the zero instrument for a blank solution containing LG and CTAB in the absence of aluminium. Using the experimental conditions previously established, a detection limit of 0.5 mg 1-i of Al (for a confidence level of 95%) with a relative standard deviation of 0.3% for ten independent measurements of a 6 mg 1-l Al solution was achieved. Analytical application emSon method

\ 0

’ e

of

the

MEUF-flame

Aqueous solutions containing 1 mg 1-l of Al were treated as indicated under Experimental. The MEUF of 30 ml of sample was performed until a retentate volume of 5 ml had been ob-

179

M. de la Guardia et al. /Anal. Chim. Acta 276 (1993) 173-179

tamed, thus providing a sixfold preconcentration factor. The analysis by flame atomic emission of these retentates provided concentration values of 6.3 f 0.3 mg l-l, which are comparable to the expected result (6 mg 1-r). A 16-fold preconcentration factor was obtained by ultrafiltering 80 ml of the aqueous solution until to obtain 5 ml of retentate. In this instance the analysis of a water sample (containing 0.2 mg 1-r Al) after MEUF gave a concentration of 3.9 f 0.2 mg l-l, of the same order as the 3.2 mg 1-l value expected for a 16-fold preconcentration. From the above results, and taking into account the detection limit of the flame atomic emission of Al for CTAB solutions, samples containing Al concentrations of the order of 30 pg 1-l could still be analysed. Conclusions MEUF of aqueous Al solutions, in the presence of LG and CTAB at pH 5-5.9, provides a quantitative preconcentration of Al. This surfactant-rich matrix can be exploited to improve the analytical detection limit of the flame atomic emission spectrometric determination of Al. When cationic surfactants are used, the accumulation of uncomplexed metal ions in the retentate is possible, as some electrostatic repulsion from the membrane surfactant system occurs. However, taking into account that the atomic spectrometric analysis is less sensitive to the presence of interferences, the determination of Al in the CI’AR surfactant-rich retentate can be performed without problems. MEUF of metal complexes provides an alternative means of preconcentrating trace elements, comparable to that obtained by liquid-liquid extraction but avoiding the problems related to the handling of large volumes of organic solvents. Studies are in progress in order to develop a multi-elemental preconcentration method based on the use of non-specific ligands and MEUF, which could be employed as a preliminary step for analyses by atomic spectrometry.

A. Morales-Rubio acknowledges an FPI grant from the Ministerio de Educacidn y Ciencia.

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