Possible Errors Caused Prior to Measurement of Mercury in Natural Waters with Special Reference to Seawater Katsuhiko Matsunaga, Shigeki Konishi', and Masakichi Nishimura" Department of Chemistry, Faculty of Fisheries, Hokkaido University, Hakodate, 041 Japan
Taking seawater as an example, analytical problems in the mercury determination of natural waters are presented, with a special reference to errors caused prior to measurement of mercury. A mercury concentration of 0.5 ppb decreases rapidly even in an acidified solution, but the presence of sodium chloride prevents the adsorption loss onto the bottle wall. Therefore, if a seawater sample is acidified to 0.2 M with sulfuric acid a t sampling, the mercury is stable for at least 60 days. Polyethylene bottles are unreliable because of mercury contamination. Grlass bottles contaminated with mercury can be cleaned by heating a t 500 OC or rinsing with diluted hydrofluoric acid. When there is no contamination and no mercury loss, the apparent mercury concentration of an acidified seawater sample increases for a couple of weeks due to a change in forim of the mercury species in the seawater, and then reaches a constant value which is consistent with the value determined after digestion with a mixture of nitric acid and sulfuric acid, or a mixture of nitric acid, potassium permanganate, and persulfate. The level of 5-6 rig of Hg/L determined with the considerations described above seems to be the hase-line concentration of mercury in the oceans. Although determinations of mercury in the oceans have been made by many investigators, the reported values are widely scattered, e.g., 2-2800 ng/L. The scattering of the values seems to be attributed to problems in analytical techniques, i.e., contamination, loss of Hg during storage, and inadequate pretreatment of a sample, rather than the diversity of the nature of seawater in the oceans. Similar problems occur in determinations of mercury in not only seawater but also river, lake, and rain waters. Some possible errors caused prior to measurement of mercury in natural waters were examined, seawater being taken as an example.
Procedure for Measurement of Mercury The procedure adopted ( 1 )is outlined. Add SnC12 to a water sample which has been acidified or pretreated. Pass Nz through the solution and collect the vaporized Hg on porous silver metal particles in a small glass tube. Transfer the silver on a small boat, and place the boat in a quartz tube held in an electric furnace whose temperature is kept a t 500 "C. Introduce the Hg vapor into an absorption cell by passing N2. Record the absorbance peak a t 253.5 nm by using an atomic absorption spectrophotometer. Add NHZOH-HCl prior to the addition of SnClL, when KMn04 and K2S20s are used a t the pretreatment. In all cases, the blank of used reagents was determined (2) and subtracted. Loss o f Mercury during Storage of W a t e r S a m p l e s A number of studies, including a literature review ( 3 ) have , been reported on the loss of mercury during storage under various conditions 14-7). Some of the papers, however, have shown contradictory results, because the result is primarily affected by the original concentration of Hg, and secondarily by the acidity and salt concentration of the sample. Figure 1 shows that the Hg concentration decreased rapidly
'
Present address, Sanyo Hydrographic Survey Co., Ltd., Tokyo, Japan. 0013-936X/79/0913-0063$01.00/0
@ 1979 American Chemical Society
even in 0.2 M H2S04without NaC1, and almost no mercury was detectable after a 5-day storage period. This fact means that if Hg in a freshwater sample is not immediately determined after sampling, an incorrectly low value is obtained even if the sample has been kept acidic, and if an acidified standard solution of very diluted Hg concentration is used some hours or some days after preparation, it results in an apparently high value for a water sample. The authors have found that the addition of NaCl is very effective in preventing the adsorption loss of Hg, and preliminarily reported it (8). If a solution was made 0.2 M with H2S04and more than 3% with NaC1, no change of Hg concentration was observed for a t least 60 days. This finding was traced and reconfirmed by Ambe and Suwabe (9).The effect of salt is favorable to seawater samples, because seawater itself contains more than 3% salt. Thus, if only H#O4 is added to a seawater sample a t the time of sampling, the loss of Hg during storage is prevented. Acid and NaCl are both necessary for prevention of adsorption. Unacidified seawater lost one-half of its Hg after 5 days of storage in a glass bottle. Weiss et al. ( 5 ) also showed that NaCl was useful in preventing the loss of Hg.
Contamination of Mercury f r o m Vessels Contamination from vessels as well as from reagents is an important factor in trace analyses. The latter could be corrected, but the former varies from one vessel to another. Polyethylene bottles are commonly used for sampling and storage of natural waters, because of their durability and relatively low metal content. Five l - L polyethylene bottles which had been used for various purposes in the laboratory were washed with H N 0 3 and then pure water. One liter of seawater was placed in each bottle and acidified to 0.2 M with H2S04.After a 2-month storage period, the Hg concentration was determined. As seen in the first column of Table I, the apparent Hg concentrations were 10 to 420 ng/L, compared to 5.2 ng/L for the same water which had been stored in carefully cleaned, uncontaminated glass bottles. After the first series of determinations, the used bottles were rewashed with HN03, and used again for the second seawater sample. The Hg concentrations determined after 1 month are given in the second column in Table I, and those for the third determination with the same bottles are recorded in the last column. These experiments show that washing with "03 and successive uses of the same bottle are not effective in preventing Hg contamination; therefore, used polyethylene bottles introduce a serious error in the determination of trace amounts of Hg. Hg contamination from unused polyethylene bottles was generally less than from used ones, but irregular contaminations were still observed, especially in the samples stored for long periods of time (Table 11). I t may be concluded that polyethylene bottles should not be used for sampling and storage of natural waters to be analyzed for Hg. Unused glass bottles washed with "03 showed no contamination after 2-months storage with acidified seawater samples, except that one of the ten bottles examined gave 9 ng of Hg/L compared to a standard Hg value of 5 ng/L. The conclusion that a glass bottle is better than a polyethylene bottle in keeping seawater samples from being contaminated with Hg is consistent with the results reported by Bothner and Robertson (IO).They also concluded that the contamination Volume 13, Number 1, January 1979
63
'
O
0
°
20
C
60
40
I,
0
Standing t i m e , days
Figure 1. Change of mercury concentration in a glass bottle during
10
20
30
"
I
60
Standing time, days
storage
Figure 2. Change of mercury concentration in acidified seawater during
Initial Hg, 0.5 ppb in 0.2 M H2S04. % NaCI: (V)none; ( 0 )1.0; (A)2.0; (0) 2.8; ( 0 )3.0;( X ) 4.0
( 0 )0.1 M H2S04;( X ) 0.2M H2S04;(0)heated with H2S04 and HNO3
storage
~~~
Table 111. Cleaning of Contaminated Glass Bottles
Table 1. Contamination from Used Polyethylene Bottles Washed with HN03 no. of bottle
1 2 3 4 5
glass bottle
1st use
Hg found in seawater samples, nglL 2nd use
10 420 10 360 110 5.2
10 10 14 45 31 5.4
cleaning method for contam. glass bottles a
9 9 28 >50 a
1 2 3 4 5 6 a
washed bet,ore use
with with with with with with
HN03 HN03 HN03 hot H N 0 3 hot H N 0 3 hot H N 0 3
Ha found in seawater samples, nglL after after 30-day 50-day storage storage
5.2 10 6.2 4.9 5.8 4.8
13 11 8.8 5.2 11 9.2
Determined by using carefully cleaned glass bottles.
of samples in polyethylene containers may come from leaching of Hg from container surfaces, from passage of Hg vapor from ambient air through the container wall into the solution, or from both sources. If a bottle is contaminated once with Hg, the Hg is barely removed with "03, as shown in Table 111. Each 1-L bottle was filled with a neutral aqueous solution containing 50 ng of Hg(I1) and allowed to stand for a couple of weeks. All the Hg was adsorbed onto the wall of the bottle during this period, based on studies which showed that such a trace amount of Hg in a neutral solution would be completely adsorbed onto the wall after a short period. Hg contamination of 50 ng on the wall of a bottle is not unusual. The pure water commonly used in the authors' laboratory contains Hg of a few nanogramsb. If a bottle is used to store 1L of such water more than 10 times, the bottle will easily be contaminated with as much as 50 ng of Hg. Five 1-L seawater samples containing 5.0 ng of Hg and acidified to 0.2 M with HzSO4 were placed in the 1-L glass bottles, which had been intentionally contaminated with 50 64
Environmental Science & Technology
Contaminated with 50 ng of Hg.
*
19 10 10 5.1 5.1 Originally contained 5.0 ng of HgIL.
4.9
Table II. Contamination from Unused Polyethylene Bottles for Seawater Containing 5.2 ng of Hg/L a
no. of bottle
washed with H 2 0 washed with H N 0 3 washed with hot H N 0 3 heated at 500 OC for 5 h rinsed with 10% HF
3rd use
Hg found in seawater sample, nglL
ng of Hg and then cleaned by several methods shown in Table 111. After 30- or 80-days storage, the Hg was determined, and the average values are shown in the table. The results indicate that washing with nitric acid is not satisfactory, but even a contaminated glass bottle can be used if it is heated a t 500 "C or if it is rinsed with 10%H F before use. If a bottle is cleaned once, the same bottle can be repeatedly used for sampling without further special cleaning.
Change of Analytical Values during Storage Judging from the above experiments, if an acidified seawater sample is kept in a cleaned glass bottle, there must be no change in the Hg concentration during storage, because there is no contamination and no adsorption loss. As shown in Figure 2, however, the apparent Hg value of an acidified seawater sample increased slowly, and then reached a constant value which seems to be the total Hg of the water sample. This result means that about 60% of the total Hg in seawater exists in a form which is readily reduced to metallic Hg by Sn(II), but 40% of the total is in another form which is resistant to reduction. The latter form of Hg changes to the easily reducible form during storage with the addition of HzS04. Carr and Wilkniss (11) reported an apparently similar observation, but the nature of their water was very different from the authors' sample. The former was typical estuarine water of 5% salinity and contained as much as I mg/L suspended particles, and the increase of Hg value during storage was thought to be attributed to dissolution of Hg from the particulate matter. In our case, the increase cannot be explained by such a simple dissolution mechanism since a normal seawater sample was used. Pretreatment of Seawater S a m p l e It has been frequently mentioned that a different value for Hg concentration is obtained by different pretreatments of natural water samples. Three different pretreatments were tried using the cleaned glass bottles (Table IV). (A) Five milliliters of concentrated HZS04 was added to a 500-mL
Table IV. Digestion Method for Seawater Hg In
method A B
sample I 7.3 f 1.20 7.7 f 1.4
C
ground of the analysis (12).By our determinations, a 5-6-ng/L level may be a reliable value for the base line of Hg in unpolluted oceans, which is roughly l / ~ oor '/loo lower than the concentrations reported in the literature, except for several recent reports of relatively low values (13-17).
a
seawater, nglL sample II
8.3f 0.5 7.6 f 1.5
Literature Cited
To 500 mL of seawater were added the following: (A) 5 mL of HzSO.4; allow to stand for >20 days; (5)5 mL of HzSO4 and 10 mL of " 0 3 : heat for 4 h; (C) 5 rnL of H2SO4, 5 mL of HN03, 4 rnL of 5 % KMn04, and 4 mL of 5 % K2S208; heat for 2 h. Standard deviation. E
seawater sample immediately after sampling, and stored for more than 20 days. (B) T o sample A, 10 mL of concentrated "03 was added and the solution was heated for 4 h in a boiling water hath. (C) T o sample A, 5 mL of concentrated HNO3 and 4 mL each of 5% K M n 0 4 solution and 5% K2S208 solution was added; the sample was heated for 2 h in boiling water. Among thle three digestion methods, no significant differences were found in the analyzed Hg values a t a 95% confidence level, and therefore each of the three methods seems to give total mercury. From the point of view of reagent contamination, the simplest method (A) is recommended for seawater.
Mercury Concentration in t h e Ocean Using the precautions stated above, the determination of Hg in seawater has been carried out by the authors. Part of the data was already reported without a description of the back-
(1) Nishimura, M., Matsunaga, K., Konishi, S., Runseki Kagaku, 24, 656 (1975). (2) Uchino, E.. Konishi, S., Nishimura, M., ibid., 27,457 (1978). (3) Jenne, E. A,. Avotins, P., J . Enuiron. Qual., 4,427 (1975). (4) Sanemasa, I., Deguchi, T., Urata, K., Tomooka, J . , Nagai, H., A d . Chim. Acta, 87,479 (1976). ( 5 ) Weiss, H. V., Shipman, W. H., Guttman, M. A , , ibid., 81, 211 (1976). (6) Carron, J., Agemian, H., ibid., 92,61 (1977). (7) Mahan, K. I., Mahan, S. E., Anal. Chem., 49,662 (1977). (8) Nishimura, M., Konishi, S., Kaiyo Kagaku (Mar. Sci., J p n . ) , 8, 820 (1976). (9) Ambe, M., Suwabe, K., Anal. Chim.Acta, 92,55 (1977). (10) Bothner, M. H., Robertson, D. E., Anal. Chem., 47, 592 (1975). (11) Carr, R. A,, Wilkniss, P. E.; Entliron. Sei. Techno/., 7, 62 (1973). (12) Matsunaga, K., Nishimura, M., Konishi, S., Nature (London), 258,224 (1975). (13) Burton, J. D., Leatherland, T. M., ibid., 231,440 (1971). (14) Leatherland, T. M., Burton, J. D., McCartney, M. J., Culkin, F., ibid., 232,112 (1971). (16) Fitzgerald, W. F., Hunt. C. D., J . Rech. Atmos., 8,629 (1974). ( 1 6 ) Fitzgerald, W. F., Adc. Chem. Ser., No. 147,99 (1975). (17) Fitzgerald, W. F., Lyons, W. B., Limnol. Oceanogr., 20, 468 (1976).
Receiced for reuieu: April 24, 1978. Accepted J u l y 27, 1978
Distribution of Polychlorinated Biphenyls (PCB) in Estuarine Ecosystems. Testing the Concept of Equilibrium Partitioning in the Marine Environment S. P. Pavlou" and R. N. Dexter URS Company, Fourth and Vine Building, Seattle, Washington 98121
w
Spatial and temporal trends in the chlorobiphenyl concentrations observed in various marine components of Puget Sound between 1973 and 1977 are presented. The distribution and accumulation characteristics are discussed in terms of the physical chemical processes that control their flow throughout the ecosystem. For the low levels detected in seawater, the data suggest that uptake is predominantly governed by equilibrium partitioning of the chemicals between suspended phases and ambient water. In 1972 we initiated a project to study the distribution and bioaccumulation characteristics of polychlorinated biphenyls (PCBs) in Puget Sound, Wash. These studies were intended to provide a data base for adopting appropriate criteria for regional enforcement, as well as an estimate of the potential hazard to the ecosystem ( I ) . Throughout the course of the study, it was realized that in order to assess the impact of these chemicals on the marine biota, it was necessary to obtain some quantitative information on the mechanism of accumulation based mainly on field residue data which reflect the concentrations commonly encountered in the marine environment. For the low levels detected in seawater, the data suggest that accumulation in 1 he various marine components, including biological uptake, is predominantly governed by equilibrium partitioning (1-3). 0013-936X/79/091:~-0065$01.OO/O @ 1979 American Chemical
Society
The material presented in this paper includes (a) a general discussion on the characteristics of the distribution and fluxes of PCBs in Puget Sound, (b) an evaluation of the partitioning mechanism in suspended phases and lower trophic level biota, and (c) a discussion of the implications of the equilibrium partitioning concept to the general distribution of persistent organic compounds in the marine environment.
Distribution of PCBs i n Puget Sound The field program completed during these studies has provided us with a sufficient data base to obtain a fairly coherent and comprehensive description of the distribution of PCBs in Puget Sound. The regions investigated have been presented in a previous report (2, Figure 1).A complete presentation of the PCB data, together with supporting hydrographic and biological measurements, is available elsewhere (3-6). These publications also contain a detailed discussion on sampling and analytical methodology. Characteristics of the Spatial Distribution. A summary of the mean PCB levels in Puget Sound is presented in Table I. The large standard deviation exhibited in certain subregions results from the existence of spatial variability within the region. Nevertheless, the data as presented in the table facilitate the delineation of regional trends. In general, the values for all sample types correlate well with areas of increased industrial and municipal activity. The water, susVolume 13, Number 1, January 1979 65
