Anal. Chem. 1980, 52, 672-676
672
Determination of Trace Elements in Seawater by Neutron Activation Ana Iy sis and ElectrochemicaI Separation Kari JPrstad and Brit Salbu" Department of Chemistry, University of Oslo, Oslo, Norway
The combination of neutron activation analysis and electrolysis at a constant, controlled potential has proved itself to be a useful multielement method for the determination of 28 elements in seawater. After freeze-drying and Irradiation, the samples are dissolved and electrolyzed for 1 h. The radioactive species deposited on the mercury cathode allow determination of 14 elements (Ag, As, Au, Cd, Co, Cr, Fe, Ga, Hg, La, Mo, Sb, Se, Zn). Another 14 elements (Ba, Br, Ca, Ce, Cs, Eu, Na, Rb, Sc, Sm, Sr, Th, U, Yb) are quantitatively determined by measuring the activities in the residual solution. To obtain a reproducible electrolysis, radioactlve tracers have been used to study the decrease of element concentrations in solution as a function of time of electrolysis, the influence of the initial element Concentration on the rate constant k , the effect of cathode material and of the pH in the solution.
The concentration of trace elements in seawater is generally very low, frequently in the ppb range or lower. Because of interfering effects of the matrix, this is below the sensitivity limit of most analytical methods. Adding the requirements of good precision and accuracy, the number of useful methods for analysis of trace elements in seawater is greatly reduced. Except for neutron activition analysis and anodic stripping voltammetry, determination cannot be made without a prcconcentration step. In multielemerit analysis of the above mentioned matrix, however, these two methods also suffer from disadvantages which reduce the applicability of the methods. The high content of salts in seawater considerably reduces the number of elements which can be determined by instrumental neutron activation analysis. Reports concerning a small number of elements have been published ( I , Z), showing that the high amount of 24Na (tip = 15.0 h) prevents the measurement of more short-lived nuclides. However, by introducing radiochemical separations, a higher number of elements can be determined ( 3 ) . Anodic stripping voltammetry is a convenient and highly sensitive method for analysis of heavy metals a t the pph level or lower. Application of this method to seawater and other samples of marine origin by using different types of electrodes has recently been reported ( 4 , 5 ) . The limitation of the method, however, is that only a few elements present in an electroactive form can be determined. T o reduce the disadvantages of the two separate methods mentioned, neutron activation analysis has been combined with electrodeposition on a mercury electrode to concentrate traces of reducible metals leaving interfering elements in solution. The present paper describes this technique applied to multielement analysis of seawater samples.
THEORY The electrolysis is performed at a constant, controlied potential. During electrolysis, the concentration of reducible species in solution will decrease exponentially with time, fnllowing the first-order kinetic expression 0003-2700/80/0352-0672$01 OO/O
(1)
ct = coe-kt
where ct represents the concentration at, time t = t , r o that a t time t = 0. The rate constant is
DA V6
k = hi-
where D = the diffusion coefficient which increases with the temperature and decreases with the viscosity of the solution, A = the area of the electrode, V = the volume of solution, and 6 = the thickness of the diffusion layer. k , will depend oil the geometry of the electrolytic cell, the material of the electrode, and the applied potential. A plot of log c, against the time of electrolysis ( t ) , should give a straight line with a negative slope equal to k . The half-life ( t l l L= In 2 / k ) of the electrolysis is the time required for the concentration co to decrease to one half of its initial value. The deposition of species on the electrode is given by
mt = coV(l - e
kt)
(3)
where m, represents the amount of the deposited elenient a t time t . For practical reasons, a rapid electrolysis is wanted, Le., a high value of k . For B given cell and potential, this is obtained by a high value of the diffusion coefficient ( D ) (Le., high temperature and low acid concentration), large area of the cathode ( A ) ,small volume of solution (13 and a thin diffusion layer (6). The latter is influenced by the stirring of the solution. A first-order rate law for a rapid deposition of bismuth from 10 M solutions has been reported by Fahland and Herrmain (6). They suggest, however, that a longer electrolysis time is required when very minute element concentrations are used. This seems to indicate a dependence of the slope K on the initial concentration, which contradicts the first-order kinetics. Therefore in the present work some variables influencing k have been studied with the aim of obtaining a reproducible electrolysis.
EXPERIMENTAL The controlled potential electrolysis was performed by means of an especially built potentiostat. In order to get as many elements as possible deposited on the cathode, the applied potential was chosen as -1.5 V as only a few elements were deposited at -1.0 V. The area of the cathode was 8 cm', the volume of electrolyte 25 cm3, and the temperature 22 2 "C. Both mercury and graphite were used as cathode material, while a platinum coil served as counterelectrode. The reference electrode, a saturated Ag/AgCl electrode,was placed inside a salt bridge which consisted of a glass tube equipped with glass frit. Figure 1 shows the electrolytic cell used. During the electrolysis, the solution was stirred at a constant rate using a magnetic stirrer. To investigate the influence of different parameters on the electrolysis, tracer experiments were performed. Salts (NaC1,KC1) were dissolved in distilled water to obtain an electrolyte similar to that of seawater. Owing to acid conservation of the water samples for investigation, the pH was adjusted to 1.5 by nitric
*
1980 American Chemical Society i_l_
. -
ANALYTICAL CHEMISTRY, VOL. 52, NO 4, APRIL. 1980 Reference electrode
(Ag/AgCl
,
I
0
I
---~ ,- 7----,-
-T
T---r
673 '-7
Stirrer Magnet'c
~
Figure 1. T h e electrolytic cell
Table I. The Elements Studied most abun-
element Ag As
Au Cd C0 C;r F'e Ga
Hg La MO Sb
Se Zn
nuclide llor'Ag
ha1f -1if e
250.4 d 26.3 h 98A~ 2.7 d I1'Cd-llSrnI11 53.5 h A 4.5 h 5.26 y 6nC0 51Cr 27.8 d 44.6 d 59Fe 72Ga 14.1 h 46.6 d 203Hg 1401,a 40.3 h g P M ~ - 9 9 m T c 66.2 h + 6.02 h 'Z2Sb 2.7 d ''%e 120.0 d 65Zn 243.8 d 76As
_ I _ _ _ _ _
dant y ray, keV 657.7 559.1 411.8 336.3 1332.5 320.1 1099.3 834.0 279.2 1596.2 140.5 564.1 264.5 1115.5
__
acid. In addition variable amounts of multielement standard solutions containing the different elements to be investigated were irradiated and added to the electrolyte as radioactive tracers. The results based on single element standard solutions were in full agreement with those obtained using a mixture of elements. Thus, no interference between elements seems to take place during the electrolysis. The radioactivity measurements were based on Ge(Li) y spectrometry. The elements studied, the corresponding radionuclides and the y energies used in the measurements are given in Table I. Peak location and calculation of peak areas were performed by means of GAMANI,, a computer program developed by Gunnink et al. (7) and adjusted to the CDC-3300 and CDCCYBER 74 computers at the [Jniversity of Oslo by Scheidemann (8). During the electrolysis, 0.5-mL samples were withdrawn from the electrolyte and the concentrations of the elements in solution were determined as a function of time. Each experiment lasted for 8 h. The electrolysis was performed using different initial concentrations of each element. Thus the decrease of element concentrations in the solution was studied as a function of time, and information on the influence of the initial concentration on the rate constant k was obtained. In addition the influence of cathode material and pH was investigated. T o control possible surface electrode effects using mercury as cathode, the electrode was in some cases renewed alter 8 h and the electrolysis was continued.
When using graphite ab the cathode, t,he mercury was replaced by a sheet of graphite having the same area and placed at the bottom of the cell using a holder especially constructed for this purpose. In acid solution (pH 1.5), evolution of hydrogen at the cathode may complicate the deposition of metals when the applied po. tential is -1.5 V. To study this, experiments were also carried out at pH 9.2 using a citrate ammonia buffer for a d j u s h g the PH. In order to control sorption effects on t,he wall of the electrolytic cell, desorption experiments using 6 N nitric acid were carried out a t the end of electrolysis. As the main aim of the present work wm to detelop a techiiqul: for use in trace element anal) of seawater, detailed studies concerning the electrolysis itself as the influence of applied potential and the effect of stirring will be published elsewhere.
RESIJLTS AND DISCIJSSION As seen from Figure 2 , which represents a t,ypical behavior, the concentration of metal ions in the solution decreased rapidly with time during the first hour of electrolysis. After about 1 h, a level was approached; t l , 2increased considerably; and only a minute amount (if metal was therefore deposited on the electrode. Such a level appears for all elements investigated, but its positioil depends not only on the element in question but also on its initial concentration. However, the material of the working electrode (mercury or graphite) does not seem t o have any influence. Renewing the mercury electrode after electrolyzing for 8 h did not change the slope of t,he above ment,ioned level. ' r h m , no saturation effects seem t o be involved. T h e level appears not t o be affected by the p l i in t h e solution, and sorption effects are negligible. T h e analysis of the experiments shows that the half-life of the electrolysis increases with decreasing initial coricentrat,ion of t h e different elements as indicated by Fahland and Herrmann (6). This effect appears t o be independent of the cathode material and pH. Thus, the rate constant for each element depends on its initial concentration and the fist-order kinetic expression given by Equation 1 is not valid when using the concentrations and experiment,al condi-tions described above.
674
ANALYTICAL CHEMISTRY, VOL. 52, NO. 4, APRIL 1980
Table 11. The Results of the Electrolysis Experiments
element
compound used iil preparation of the standard solution
Ag
&NO,
As
As203
Au
met./HNO,:HCl
Cd
met./HNO,
=
1:3
initial concentration, c,
final concentration, c,h
37.6 pg/L 15.2 pg/L 8.0 pg/L 333.6 pg/L 111.2 pg/L 17.6 pg/L 7.2 pgIL 0.6 pg/L 2.15 mg/L 716.4 pg/L 72.4 pg/L 57.2 pg/L 48.0 pg/L 24.0 pg/L 11.6 pg/L 11.14 mg/L 7.43 mg/L 178.4 pg/L 71.6 pg/L
6.4 uglL 6.8 pg/L 1.1pg/L 146.0 pg/L 60.8 pg/L 12.4 pg/L 5.2 pg/L 0.5 PgiL 344.0 pg/L 279.2 pg/L 15.6 pg/L 5.2 pgiL 11.6 pg/L 3.8 ug/L 0.6 ug/L 300.4 pg/L 141.2 pg/L 64.8 pg/L 20.0 pg/L
4.22 mg/L 2.81 mg/L 22.4 pg/L 0.24 pg/L
8 h >8 h >8h
1 2 min 1 7 rnin 25 min 27 min 28 rnin 30 min 39 min 11 min
1 4 min 20 min 26 min 6 8 21 36
min min min min
56 rnin 84 min 9 2 min 1 0 0 min
2.94 mg/L 1.96 mg/L 981.2 pg/L 8.0 PgIL
81.6 pg/L 9.6 MdL 5.2 FgIL 3.6 p d L 441.6 ;Lg/L 333.6 pg/L 186.4 pg/L 1.6 pg/L
1.02 mg/L 682.8 pg/L 341.2 pg/L
263.2 pg/L 176.8 pg/L 226.0 pg/L
20 min 24 min
>8 h 5 min 11 min 44 min 78 rnin
1 2 min 1 4 min 18 min 34 min
Hg
HgO
2.53 mg/L 1.68 mg/L 26.8 pg/L 0.14 pg/L
101.2 pg/L 114.8 pg/L 6.0 pg/L 0.02 pgIL
La
La203
3.70 mg/L 2.47 mg/L 1.23 mg/L 98.8 pg/L
1.49 mg/L 1 . 2 9 mg/L 694.8 pg/L 68.4 pg/L
>8h >8 h
63 min >8 h
Mo
MOO,
395.6 pg/L 263.6 pg/L 132.0 pg/L
183.2 pg/L 65.2 pgJL 47.6 pglL
1 2 min 18 min 36 min
Sb
Sb, 0
2.40 mg/L 1.60 mg/L 128.0 pg/L 51.2 pg/L
341.2 pg/L 240.4 pg/L 36.4 pgJL 25.2 pg/L
8 min 15 min
350.8 pg/L 117.2 pg/L 14.0 pg/L 0.6 pg/L 355.6 pg/L 226.0 pg/L 88.8 pg/L
280.8 pgJL 71.2 pg/L 10.8 pg/L 0.5 CrdL 30.0 pg/L 56.4 pgJL 1 7 . 6 pg/L
Se
Zn
3
SeO,
met./HNO,
T h e different elements and t h e initial concentrations studied, together with the obtained half-lives of the electrolysis, are given in Table 11. The reproducibility of the half-lives obtained from replicate electrolysis experiments is within 1 or 2 min. A semilog plot of the initial concentration in solution against the half-life for the first part of the electrolysis gives a straight
30 min 36 min >8 h >8 h
>8 h >8 h 1 6 min 29 min 46 min
line (least-square fitting) for each element as illustrated in Figure 3. Except in the cases of As, La, and Se, the element concentrations in solution were considerably reduced after 1-h electrolysis. For some elements, t h e described level was already reached. Owing to this fact a n d in order to obtain a rapid method of separation, this time of electrolysis (1 h) was
ANALYTICAL CHEMISTRY, VOL. 52. Nc? 4. APFIIL i 9 8 0
__
______.._____I_
___
...
_ I ~ _ _I__ _ ____ _
675 -
.........
Table 111. Data Obtained by Analysis of 4 Aliquots of a Multistandard Solution with Known Elt?rnetli Concentrations
concentraelement tion, P g / L -4s
7.86 17.79
AU
48.14
Cd
178.21 22.49 9.60 48.00 341.30 1.69 98.74 263.60 128.18 14.04 88.80
Ag
CO
Cr Fe Ga Hg La Mo Sb
Se Zn
parellei 1, PgiL
parallel 2, PdL
parallel 3, gglL
parallel 4,
mean value,
llgiL
DYiL
6.47 17.61 53.17 163.18 26.96 10.13 49.70 327.42 1.37 88.31 249.65 134.67 16.33 77.48
6.96 19.34 41.42 190.52 25.17
6.73 16.63 52.81 172.1 1 21.16 8.94 13.12 361.84 1.61 94.28 281.67 122.73 13.78 79.14
8.16 16.87 55.16 184.31 21.83 9.12 55.41 319.58 1.39 96.44 268.59 11 7.32 11.98 92.86
10.01
55.63 359.27 1.84
106.47 274.04 131.14 15.81 87.39
. A
0
5.0P
2
miix. 11t~vi:~l io11 fioni m ~ a 1 3 > i : > n c l . VaIIlt:,
3.13
177.53 : 23.78 .
'i
9.55 - 0.60 50.97 - '7.85 342.03 I. 2 2 . 4 5
1.55 : 0.29 . 10.09
96.38 268.49 126.47 16.22 84.21
...
:
i
18.84 9.15
:
1.44
i
8.65
. .-
Cd
Hg
Zn
i
i' i
1
0
:
'\
i
i
L
-
.
-
-
-
L---
c
a1
1
A
_ _ --,_ I
./ .
,;,
""
f~/~(mini
Figure 3.
1.1-
. . . , ....
,
,
A _ .
,.>. - .A
:cC"
i}il/lI
Influence of the initial concentration on the half-life of the electrolysis
centration
chosen. In the case of Zn, Cd, and Hg, the relationship between t h e amount of deposited metal (mlh) and t h e initial concentration (co) in the solution is shown in Figure 3 . Straight lines are also obtained for Ag, As, 4 u , Co, Cr, Fe, Ga. La, Mo, S b , and Se. T h e relation between t h e deposited amount of metal after electrolyzing for a given time (1h ) arid the initial concentration is characteristic for electrolysis using mercury as cathode under the conditions described in this paper, i.e., pH 1.5, temperature 22 2 "C, applied potential -1.5V, constant stirring, a n d the cell used. However, the relationship has t o be redetermined if any of t h e conditions are changed. Even though a first-order kinetic expression is not valid for the electrolysis as described above, the method is reproducible in trace element analysis. Preliminary studies without stirring of the solution during t h e electrolysis seem t o indicate a major influence of this variable both on t h e mentioned level and on the deviation from a first-order kinetic expression when low concentrations are involved. Further investigations of the stirring effect are now being carried out.
T h e described elect,rocheinical method combined with neutron activation analysis is a ver!; usefuil ~ I I for P tletermination of a great number of elements in wwnter. The samples are freeze-dried in quartz aniporilcs prior t o irridiatiog. After dissolving in dilute nitric acid, t h e irradiated sanipit. is rlectrolyzed for 1h. Then the activities d e p o s i t d < J ~ Ithe n!arcury electrode are measured and the initial conceri:r'ations uf the elements Ag, As: Au, Cd, ((0, C'r. Fe. Ga. Hg. l.,:~,Mi>.SI).%, and Zn are determined as de, b e d a i x w r . For aliquots withdrawn from the same seawater sample, the results obtained by the described method give a chemical yield of about 95% compared with amouri!s measured 1)y inst,rurnental neutron activation analysis for elernen t:. hawng long-lived isotopes (Ag, Co, Cr, Fe, Hg, Se, and Zn!. T h e accuracy and precision based CJII 4 a l q u o t s oi a rnultistandard solution with knilwi eienient ctrncentrations o f the 13 elements, together with the maxi m \ i m deviat.ion f r m i the mean value for ehch element are listed i n Table !TI. With t,he exception of Ag and Se. the accurxy of the method is within a standard devi;ttion f!-;>rn the tnie va!ut. T h e
*
Figure 4.
Amount of deposited metal as
a function
3f
the initial con-
APPLICATION TO SEAWATER ANAIiYSIS
Anal. Chem. 1980.
676
The determination limits ( n ~of) the present method are calculated using the GAMANL (7, 8) criterion of the least possible registerable peak area as described by Salbu (9) (Table IV). The determination limits are of the order 10-'-10-3 pg/L for 22 of the 28 elements investigated, which underlines the usefulness of this combined method. Considering the very low concentration of most of these 28 elements in seawater, emphasis should also be put on the fact that contamination problems during electrolysis are avoided as radioactive species only, deposited on the electrode or left in the solution, are being measured. Hence, technical reagents can be utilized. Thus, using the combination of neutron activation analysis and electrolysis as described in this paper, 28 elements in seawater samples may be determined quantitatively. The method has been applied to a study of samples of marine origin from a Norwegian lake, the results of which will be published elsewhere. This method may also be used in multielement analysis of other samples having high content of interfering elements which are not deposited on the electrode, Le., samples of biological and geological origin.
Table IV. Determination Limits of the Present Method Applied to Seawater Samples.
element
determination , limit, n ~pg/L
background level, counts/ channel
Elements deposited on the mercury electrode 0.30 0.11
Ag As AU Cd
0.001
0.21 0.04 0.17 20.96 0.04 0.05 0.04 0.42 0.06 0.68 14.16
co Cr Fe Ga
Hg
La Mo Sb Se Zn
2000 1800 2200 2700 3 00 3000 200 2200 3200 100
4300 1900 3200 500
Elements left in solution Ba Br Ca Ce
cs
Eu Na Rb sc
Sm Sr Th U Yb
2.25 0.12
46.92 0.07 0.02 0.03 17.48 0.57 0.002 0.03 18.36 0.03
0.12 0.14
52. 676-679
150 150 330 370
ACKNOWLEDGMENT The authors are indebted to W. Lund, Department of Chemistry, University of Oslo, for kindly providing the equipment for electrolysis. They express their sincere gratitude to A. C. Pappas for valuable discussions and for having read the manuscript.
40
200 90 25 40 420 150 200 4 20 150
LITERATURE CITED
precision varies from 6% (Cr) to 19% (Hg). Because of counting statistics, the reproducibility increases with concentration. The residual solution, after electrolyzing for 1 h, still contains high amounts of Na and Br. After radioactive decay-time of 2 weeks, however, the determination of additional elements, Le., Ba, Br, Ca, Ce, Cs, Eu, Na, Rb, Sc, Sm, Sr, T h , U, and Yb, is carried out by measuring the activities in the residual solution.
(1) Piper, D. Z.; Goles, G. G. Anal. Chim. Acta 1969, 4 7 , 560-563. (2) Bolter, E.; Turekian, K. K.; Schutz, D. F. Geochim. Cosmochim. Acta 1964, 28, 1459-1466. (3) Schutz, D. F.; Turekian, K . K. Geochim. Cosmochim. Acta 1965, 29, 259-313. (4) Florence, T. M. J . Nectroanal. Chem. 1972, 35, 237-245. (5) Lund, W.; Salberg, M. Anal. Chim. Acta 1975, 76, 131-141. (6) Fahland, J.; Herrmann, G. Z.Anorg. Allg. Chem. 1962, 376, 141-153. (7) Gunnink, R.; Levy, H. B.; Niday, E. UCID-15140. 1967. (8) Scheidemann, @, Department of Chemistry, University of Oslo, private communication, 1970. (9) Salbu, B.; Steinnes, E.; Pappas, A. C. Anal. Chem. 1975, 4 7 , 1011-1016.
RECEIVEDfor review October 12, 1979. Accepted December 12, 1979. Based on a thesis by Kari J$rstad.
Simultaneous Determination of Mercury and Cadmium in Biological Materials by Radiochemical Neutron Activation Ana Iys is Robert R. Greenberg Center for Analytical Chemistry, National Bureau o f Standards, Washington, D.C. 20234
A radiochemical procedure has been developed for the simultaneous determination of Hg and Cd in biological matrices. The procedure is based upon bomb dissolution followed by solvent extraction using Ni and Zn diethyldlthlocarbamates. Mercury is separated from Se allowing the use of the '03Hg isotope as well as the '"Hg isotope to quantify Hg.
Mercury and cadmium are two of the more common environmental pollutants. Even a t very low levels, these eleThis article not subject to
US. Copyright.
ments have been suspected of causing detrimental health effects. Mercury and cadmium both appear on the Environmental Protection Agency's Priority List of Toxic Substances ( I ) . The Safe Water Drinking Act allows a maximum Hg concentration of 2 pg/L, and marine organisms from water containing more than 0.05 pg/L may be harmful to human consumers (2). The maximum allowable Cd concentration in drinking water is 10 pglL and concentrations above 0.4 pg/L may be harmful to some types of aquatic life (2). Mercury and cadmium are commonly found in biological materials a t the ppm or sub ppm level. At these levels,
Published 1980 by the American Chemical Society
