1273
Anel. Chem. 1993, 65, 1273-1276
Determination of a Trace Amount of Beryllium in Water Samples by Graphite Furnace Atomic Absorption Spectrometry after Preconcentration and Separation as a Beryllium-Acetylacetonate Complex on Activated Carbon Tadao Okutani, Yasuhiro Tsuruta, and Akio Sakuragawa' DeDartment of Industrial Chemistry, College - of. Science and Technology, Nihon University, 1-8-14, Kanda-Surugadai, Chiyoda-ku, Toyko 101, Japan
A rapid and simple preconcentration method by selective adsorption using activated carbon as an adsorbent and acetylacetoneas a complexingagent is described for the determination of a trace amount of beryllium by graphite furnace atomic absorption spectrometry. The beryllium-acetylacetonatecomplexis adsorbedeasily onto activated carbon at pH 8-10. The activated carbon which adsorbed the beryllium-acetylacetonate complex was separated and dispersed in pure water. The resulting suspension was introduced directly into the graphite furnace atomizer. The determination limit was 0.6 ng/L (S/N = 3), and the relative standard deviation at 0.25 pg/L was 3.0-4.070 (n= 6). Not only was there no interference from the major ions such as Na(I),K(I),Mg(II), Ca(II),Cl-, and Sod2- in seawater but there was also no interference from other minor ions. The proposed method was applied to the determination of nanograms per milliliter levels of beryllium in seawater and rainwater.
INTRODUCT10N Beryllium is lighter than aluminum, has a relatively high melting point, and is a major component of beryl (Be3Al~Si6018). Beryllium finds use in alloys, e.g., those of copper, aluminum, magnesium, nickel, zinc, and iron, in aircraft materials, and as a neutron reflector in nuclear reactors because the oxide shows good thermal conductivity in spite of ita high melting point and high electrical insulation. However, beryllium and ita compounds are very toxic, especiallyto the lung, skin, and eyes, and at high concentration can cause death.' For example, the toxic concentration (TC) of beryllium for a person is 0.1 mg/mS, the median lethal dose (LDw) of beryllium chloride for a rat, is 86 mg/kg, and the LDw of beryllium sulfate for a mouse is 80 mg/kg. In Japan, therefore, the allowable concentration of beryllium in air is established by regulation. To give an example, the average exposure value for 8 his 2 pg/m3or the momentary maximum value is 25 ~ g / m Consequently, ~ . ~ a reliable method for the determination of beryllium in environmental specimens is required. (1)Clesceri, L. S.; Greenberg, A. E.; Trussell, R. R. Standard Methods for theExarninationof Waterand Wastewater,17thed.;AmericanPublic Health Association: Washington, DC, 1989; pp 3-79. (2) Nanjo, M.; Hirano, S. Mukiohyouhisyokubunseki, 2nd ed.; Kyoritsushupan: Tokyo, 1980; Vol. 1. 0003-2700/93/0365-1273$04.00/0
Methods reported for the determination of beryllium include UV-visible spectrophotometry,3-5 gas chromatography,6 flame atomic absorption spectrometry (AAS),7-11 and graphite furnace (GF) AAS.12-1' The ligand acetylacetone (acac) reacts with beryllium to form a beryllium-acac complex and has been extensively used as an extracting reagent of beryllium.2 Indeed, the solvent extraction of beryllium as the acetylacetonate complex in the presence of EDTA has been used as a pretreatment method prior to AAS.8-10 Less than 1r g of beryllium can be separated from milligram levels of iron, aluminum, chromium, zinc, copper, manganese, silver, selenium, and uranium by this method. We have used activated carbon (AC) in order to selectively separate and concentrate trace amounts of metal ions. For example, selenium in water samples was determined by GFAAS after selectively adsorbing and separating ita 3-phenyl5-mercapto-1,3,4-thiadiazole-2-thione (bismuthiol-11) complex onto AC.'* This paper describes a method for the determination of -10-l2 g of beryllium in natural water samples. The proposed method is simple and precise.
EXPERIMENTAL SECTION Apparatus. A Shimadzu Model AA-680Gatomic absorption spectrometer,equipped with a Model GFA-4B graphite furnace, was used for the atomic absorption measurement of beryllium at the resonance line of 234.9 nm. The spectral bandwidth was set at 0.4 nm. A hollow-cathode lamp of beryllium was operated at 8 mA, and a deuterium lamp was used for background correction. A 10-pL aliquot of the sample was manually introduced into the graphite tube atomizer by Eppendorf micropipet. Measurements were made in the peak area mode. Reagents. Standard beryllium solutions were prepared by serial dilution of a 100pg/mL atomic absorption standard (Wako Pure Chemical Co. Ltd.) with 0.1 M "03. Suprapure acety(3) Matsubara, C. Bunseki Kagaku 1974,23, 878-883. (4) Yamaguchi, N.; Nishida, T.; Nishida, H. Bunseki Kagaku 1989,38, 48-51. ( 5 ) Nishida, H. Bunseki Kagaku 1990,39,805-809. (6) Tao, H.; Imagawa, T.; Miyazaki, A,; Bansho, K. Bunseki Kagaku 1987,36, 447-450. (7) Fleet, B.; Liberty, K. V.; West, T. S. Talanta 1970, 17, 203-210. (8)Terashima, S. Bunseki Kagaku 1973,22, 1317-1323. (9) Matsuzaki, K. Bunseki Kagaku 1975,24,442-446. (IO)Teraahima, S. Bunseki Kagaku 1982,31, 727-729. (11)Asami, T.; Fukuzawa, F. Soil Sci. Plant Nutr. 1985, 31, 43-53. (12) Shimomura, S.; Morita, H.; Kubota, M. Bunseki Kagaku 1986, 25, 539-543. (13) Nakajima, R. Bunseki Kagaku 1978,27, 185-188. (14) Paschal, D. C.; Bailey, G.G. A t . Spectrosc. 1986, 7, 1-3. (15) Schmidt, W. F.; Diotl, F. Fresenius' 2.Anal. Chern. 1987, 326, 40-42. (16) Ellis, W. G.;Hodge,V. F.;Darby,D. A.; Jones, C.L.At.Spectrosc. 1988, 9, 181-188. (17) Shan, X. Q.;Yian, Z.; Ni, Z. M. Anal. Chim. Acta 1989,217,271280. (18) Okutani, T.; Kubota, T.; Sugiyama, N.; Tsuruta, Y. Nippon Kagaku Kaishi 1991,375-379. 0 1993 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 85, NO. 9, MAY 1, 1993
Table I. Inetrumental Operating Conditions"
operating steps drying
temp ( O C ) 120 120 900
ashing
900 atomization
2800
time (s) 10 20 30 10 4
mode ramp step ramp step step
loot
* Wavelength, 234.9 nm; lamp current, 8 mA; Ar gas slow rate, 1.5 Limin except during atomization step.
lacetone (Wako Pure Chemical Co. Ltd.) was used. Activated carbon (Merck No. 2186, smaller than 300 Tyler mesh) was used after purification with acid.l9 All other reagents used were of analytical grade. Analytical Procedure. Ten milliliters of a 5% EDTA solutionwas added to 100-500 mL of a sample solutioncontaining less than 0.1 pg of beryllium, and 2 mL of a 5% acetylacetone solution was added to form the beryllium-acac complex. The pH of the solution was adjusted to 8-10 with 0.1 M sodium hydroxidesolution, 50 mg of AC was added, and the solution was stirred for a few seconds to facilitateadsorption of the berylliumacac complex onto the AC. The AC was separated from the aqueous phase using an 8-rm membrane filter (25 mm in diameter); the AC phase on the filter was washed with water and was then dispersed in 5 mL of pure water by using an ultrasonic agitator for -30 s. A 10-pL aliquot of the resulting suspension was directly injected into the graphite furnace. The peak area was measured under the conditions shown in Table I.
RESULTS AND DISCUSSION Analytical Conditions. The effect of the ashing temperature was investigated by varying only the ashing temperature, but keeping the other conditions shown in Table I constant. Beryllium did not volatilize up to ashing temperatures of 1100 "C; maximum and constant peak area values were obtained in the 800-1100 "C region. An ashing temperature of 900 "C was chosen since the smoke emitted from the AC was observed up to a temperature of 700 "C. The effect of the atomization temperature on the sensitivity was studied by varying the atomization temperature. An atomization temperature of 2800 "C was chosen because the peak area remained constant in the 2700-2900"C region. An argon gas flow of 1.5 L/min was used except during atomization, at which time the flow was interrupted. When the usual graphite tube (nonpyrolytically coated) was employed, a tailing of the AAS signals was observed. This difficulty can be overcome by using a commercially available pyrolytically coated tube (Shimazu Co. Ltd., No. 200-54525-085). The use of matrix modifiers such as magnesium nitrate,14 ammonium phosphomolybdate,l6 ascorbic acid,l6 and lanthanum nitrateZohas been reported for the determination of beryllium. We examined these modifiers, as well as rhodium nitrate, nickel nitrate, and aluminum nitrate. Though the use of these modifiers showed an increase in sensitivity for an aqueous solution containing beryllium, they did not result in either an increase in sensitivity or an improvement in reproducibility for the proposed method. Since the volatilization of beryllium was depressed in the presence of AC alone, they were not used in the present work. Effect of p H on t h e Adeorption of t h e BerylliumAcetylacetonate Complex onto Activated Carbon. Figure 1shows the effect of pH on the adsorption of the berylliumacac complex onto AC. The recovery increased with an increase in pH, and the optimum pH was set a t pH 9.0 since (19) Okutani,T., Oishi,Y.;Uchida, K.;Arai, N.NipponKagakuKaishi 1986, 853-858. (20) Welz, B. Atomic Absorption Spectrometry, 2nd ed.; V.C.H. Publishers: Deerfield Beach, FL, 1985; Chapter 10, pp 273, 274 (Skegg, C. English Translation).
2o
t
0 1
I
I
I
I
I
0
2
4
6
8
10
12
PH Flgurr 1. Effect of pH on adsorption of the beryllium-acac complex: (0)proposedmethod: (0)onlyAC; (0)AC and EDTA. Beryllium amount, 0.025 pg; AC amount, 50 mg.
'*; %
I
I
40
L
0
I
0
1
I
I
2 3 4 5Kacetybce~tane(ml)
5
I
I
Flgurr 2. Relatlonshlpbetween amount of acetylacetoneand recovery of beryllium: beryllium amount, 0.025 pg; AC amount 50 mg: pH 9.
the beryllium-acac complex was quantitatively recovered in the pH 8.0-10.0 region. When the analytical procedure was repeated without the addition of acac to the eample solution (i.e., the solution containing only beryllium and EDTA), there was little adsorption of beryllium onto AC, BB shown in Figure 1. Even though the beryllium-EDTA complex was formed at pH 9.0, it could not be adsorbed onto AC. The use of AC alone (Le., no EDTA) gave a recovery of -80% in the pH 7.0-9.0 region, but the reproducibility was poor. Consequently, preconcentration by the adsorption of the berylliumacac complex in the presence of EDTA onto AC was found to be most effective. Optimum Concentration of Acetylacetone for t h e Determination of Beryllium. The suitable amount of acac to achieve maximum adsorption of beryllium onto AC a t pH 9 as a beryllium-acac complex was examined. The result of this experiment is shown in Figure 2. Since beryllium was quantitatively recovered onto AC with a little acac and a constant recovery was obtained up to 3 mL of a 5% acac solution, in subsequent experirnenta, a fixed amount of 2 mL of 576 acac solution was used for the complexation of a trace amount of beryllium. For comparison, the AAS signals (peak area) of aqueous beryllium solutions containing various amounts of acac were measured by GFAAS. There was a decrease in peak area with an increase in the amount of acac. This may be due to the volatilization of the beryllium-acac complex at the ashing step since the sublimation point of the beryllium-acac complex is 270 "C. When AC was added to a sample solution containing acac, the decrease of peak area was not observed even though the amount of acac was increased. The volatilization is obviously depressed in the presence of AC, but the origin of the phenomenon is not clearly known at present.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 9, MAY 1, 1993
80
-
1275
4.0
c
1I: 20 0
‘a 3.0
e, . 0
0
8 2.0 I
20
I
40
I
I
I
60
80
100
Activated carbon (mg)
Flgwo 3. Relationship between amount of activated carbon and recovery of beryllium: (0)less than 300 mesh; (A)200-300 mesh; (0)more than 200 mesh. Beryllium amount, 0.025 pg; 5 mL of 5 % acetylacetone solution; pH 9.
Effect of AC Amount. Figure 3 shows the effect of the amount of AC on the adsorption of the beryllium-acac complex. As the particle size of AC was raised, the recovery of beryllium became lower. This is because an increase in particle size results in a decrease in surface area and then the beryllium-acac complexes are incompletely adsorbed onto the AC. Further, in such cases, since a suspension of ACadsorbing beryllium-acac complexesis not well dispersed and AC remains in the tip of pipet when AC suspensions are injected, the reproducibility is poor for the determination of beryllium. The beryllium-acac complexes are quantitatively recovered by the use of a fine mesh (the particle size is smaller than 300 Tyler mesh) and more than 40 mg of AC. When more than 60 mg of AC is employed for the analytical procedure, AC is not well dispersed into the water, and it is difficult to exactly take a definite amount of the AC suspension. Accordingly, in this work, 50 mg of AC (smaller than 300 Tyler mesh) is used for the determination of beryllium. Besides, the sensitivity of beryllium utilizing AC adsorption increased by -1.5 times compared to that of beryllium aqueous solution. This effect was seen regardless of the quantity of AC. Thorough mixing hastened formation of the berylliumacac complex, and the adsorption of the complex onto AC was complete within a few seconds after the addition of AC. Adsorption h o t herms. Freundlich’s adsorption isotherms are generally applicable in the case of monomolecule adsorption of a single species from a liquid to a solid phase. The equation can be expressed as
where q is the adsorption amount (ng/g),c is the solute amount in the solution (ng/mL), and K and l / n are arbitrary parameters. Under the above conditions, Freundlich’s adsorption isotherms were drawn and the results are shown in Figure 4. A t first, the procedure was carried out for a system without acac, i.e., an aqueous beryllium solution containing AC and EDTA. In this case, asteep adsorption isotherm was obtained (e),indicating that the adsorption ability was superior at high concentrations and inferior at low concentrations. When the procedure was repeated in the presente of acac, an adsorption isotherm with a gentle slope was obtained (Figure 4)showing efficient adsorption over the whole concentration range. The higher alevel of adsorption isotherm is, the better
1.o
0 -2.0
-1.0
0
1.0
2.0
log c I ng ml“
Flgure 4. Freundlich’sadsorption isotherms: (0)proposed method; (0) only AC; (0)AC and EDTA. AC amount, 50 mg; pH 9; c, solute
amount in the solution; q, adsorption amount.
Table 11. Freundlich’s Adsomtion Isotherm Parameters conditions K (ng/g) lln proposed method 1.8 x 10’ 0.75 absence of EDTA and acac 3.2 X lo2 0.5 presence of EDTA
6.8
1.1
the adsorption ability becomes.*l Table I1 summarizes the isotherm parameters. The value of l / n is usually less than 1. When the value is greater than 2, adsorption is difficult.21 The values for the three methods were less than 1, respectively. Consequently, both the proposed method and the method employing only AC result in good adsorption. Moreover, when the value of K is large, the adsorption ability is also higher. In the presence of AC alone (i.e., no EDTA and acac), however, beryllium could not be selectively and quantitatively adsorbed onto AC (Figure 1). Therefore, the proposed method using acac and EDTA is effective for the determination of trace amounts of beryllium. Calibration Curve. The calibration curve obtained with the propcsed analytical procedure for a 100-mL sample aliquot was linear up to 0.1 pg. The influence of the sample size was investigated by varying the volume of the sample supplemented with a known quantity of beryllium. A nearly quantitative recovery was obtained in the 100-500-mL range. Then, the use of a 500-mL sample aliquot can give a concentration factor of 100. Since the determination limit of beryllium was 0.3 ng (S/N = 3), as low as 0.6 ng/L could be determined by employing a 500-mL sample aliquot. The relative standard deviation of six measurements was 3.04.0% at 5 pg/L (25 ng/5 mL of an AC suspension) beryllium. Moreover, as previously described above,a calibration curve drawn by the proposed method did not agree with a calibration curve obtained for aqueous standard beryllium solutions. Therefore, calibration based on our procedure was used for the determination of beryllium in real samples, and the reproducibility obtained using the calibration curve was excellent. Effect of Diverse Ions. The effect of foreign ions usually found in seawater was examined when l(F9-1@12g~f beryllium was determined in these samples. The recovery of beryllium using the proposed procedure was measured in 100-mL samples containing 0.025 pg of beryllium and the matrix ions. (21)Hassler, J. W. Activated Carbon; Kyoritsushuppan: Tokyo,1973; pp 140, 321 (Oda, T.; Eguchi, Y. joint translation).
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ANALYTICAL CHEMISTRY, VOL. 85, NO. 9, MAY 1, 1993
Table 111. Effect of Foreign Ionss ion
amount (mg)
recovery ( % )
Na(1)
1300 50 150 50 0.1 1 1 10 20 10 20 2000 10 0.15 300 1.5 2.5 0.5
101 97 97 104 99 98 101 98 99 98 99 101 99 101 97 96 99 104
K(I) Mg(I1) Ca(I1) CO(I1) Sr(I1) Cu(I1) Al(II1) Fe(II1)
c1BrF-
SO? Si0,) BO%'-
PO4 I-
Beryllium amount, 0.025 wg; enrichment, 100 mL to 5 mL &e., concentration factor of 20). (1
The experimental results are given in Table 111. Not only was there no interference from the major ions in seawater such as Na(I), K(I), Mg(II), Ca(II), C1-, and Sod2-but there was also no interference from other minor ions (Table 111). Therefore, trace amounts of beryllium could be recovered quantitatively and selective using our proposed procedure. If EDTA solution as a masking agent was not added, the recovery of beryllium in the presence of 10 mg of Fe(II1) or Al(II1)was not quantitative. Potassium or sodium tartrate solution and sodium citrate solution as masking agenb for these ions were examined in place of EDTA, but the interferences of Fe(II1)
Table IV. Determination of Beryllium in Seawater and Rainwater sample (vol) seawater* (200 mL) June 1991 rainwater' (100 mL) June 1991
Beadded (ng)
Befound (ng)
*
0.76 0.033 1.77 3.23 0.83 0.030 1.81:3 3.375
1.0 2.5 1.0 2.5
rec (%)
RSD" (%)
4.3 101 96
(n = 7) 3.6
98 105
(n = 8)
*
Relative standard deviation. Tokyo Bay, Inage Harbor, Chiba prefecture. College of Science and Technology, Nihon University (Surugadai campus).
and Al(II1) could not be removed completely. Accordingly, 10 mL of 5 % EDTA solution was added. Determination of Beryllium in Water Samples. The beryllium contents in seawater and rainwater were determined by the proposed method. After suspended matter was removed by use of a 0.45-pm membrane filter (47 mm in diameter),a 100-or 200-mL sample aliquot was employed for the determination of beryllium according to the analytical procedure. The results obtained are listed in Table IV. In addition, the expected recoveries for each sample were obtained, as shown in Table IV. In summary, the proposed method is rapid and simple for the determination of ultratrace amounts of beryllium in surface water and seawater samples.
RECEIVED for review August 10, 1992. Accepted January 2, 1993.
