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Wet deposition of mercury and methylmercury from the atmosphere
249
The Science of the Total Environment, 60 (1987) 249261 Elsevier Science Publishers B.V., Amsterdam ~ Printed in The Netherlands
WET DEPOSITION OF MERCURY THE ATMOSPHERE*
RIAZ AHMEDI,
KARL MAY and MARKUS
AND METHYLMERCURY
FROM
STOEPPLER$
Institute of Applied Physical Chemistry (ZCH-4), Nuclear Research Centre, Jiilich (KFA), Boz 1913, D-5170 Jiilich (Federal Republic of Germany)
P.O.
ABSTRACT The wet deposition of mercury and methylmercury was monitored for a period of 1 year at selected areas in the Federal Republic of Germany. These areas represented an industrial zone, an urban area, a rural area close to the industrial zone and other rural areas. The wet deposition of mercury was very much dependent on the collection area; the wet deposition of methylmercury (MM) was not area dependent. Maximum mercury was found as follows: industrial area > urban area > rural area close to the industrial zone > other rural areas. Methylmercury concentrations at all areas were almost uniform within analytical error. Methylmercury as a percentage of total mercury was 5- 6% for rural areas and decreased to < 1% for industrial areas. Wet deposition of mercury increased in summer. Based on the experimental data the present burdens caused by the wet deposition of mercury and methylmercury are evaluated.
INTRODUCTION
Mercury and its compounds have been known for a long time and have been extensively used for the benefit of mankind. Mercury is a liquid metal used extensively in industry, and, because it is volatile, an appreciable amount escapes into the atmosphere as vapour. In addition to its use in industry, emissions of mercury vapour also arise from naturally occurring ores of mercury. Organomercury compounds are well known as bactericides and in the past have been extensively used for seed dressing to protect them from insects and bacteria. The input of mercury and its compounds into the environment has increased through rapid industrialization. This input of mercury and its compounds into the atmosphere, through different localized sources, is of great significance because of its toxicity [l-3]. Mercury belongs to the a priori toxic metal group consisting of Cd, Pb and Hg, with great significance and priority in ecochemistry and ecotoxicology [4-6]. Metals and metalloids are characterized by special ecochemical features. They are not biodegradable, but undergo a biogeochemical cycle during which transformations into more or less toxic species occur [5]. They are accumulated by organisms and cause increased *Dedicated to the memory of Professor Dr Hans Wolfgang Niirnberg. tpresent address: NCD-PINSTECH, P.O. Nilore, Islamabad, Pakistan, iAuthor to whom correspondence should be addressed.
250
toxic effects in man and mammals after long-term exposure. The most important source of uptake is food and the significant pathways of toxic metals into food chains is through the atmosphere from which the metals are introduced into ecosystems by dry and wet deposition. Wet deposition through rain and snow is of particular importance because it provides favourable conditions for uptake by vegetation and water. A considerable amount of work has been carried out on the analysis of Cd, Pb, Ni, Cu, Zn, etc. in wet deposition [779], but there are hardly any reliable data available for mercury in rain water and hardly any measurements have been made for the methylmercury (MM) content of rain water. The reasons for this may be the difficulties associated with the reliable analysis of mercury at the very low levels found in aqueous systems [lo, 111. However, in this institute several studies have been undertaken for the analysis of mercury and MM [12-171, and a sensitive and reliable method for their analysis at very low levels has recently been developed [18, 191. Detailed studies have been described for the analysis of mercury and MM in rain water [20]. With the development of very sensitive and reliable methods for the analysis of mercury, particularly in rain water, it seemed appropriate to begin a further systematic investigation into the mercury and MM concentrations in rain water. This paper describes the collection of rain water samples from polluted and relatively unpolluted areas of the Federal Republic of Germany covering a period of 1 year and their analysis for total mercury and methylmercury and is an addition to the institutes extended and ongoing studies on wet precipitation in the F.R.G. [7-g]. EXPERIMENTAL
Apparatus Measurements were made using an atomic absorption spectrometer (Model 400, Perkin Elmer, Bodenseewerk, F.R.G.). A mercury vapour lamp (0.2 A/15 V, wave-length 253.7 nm and slit width 2.0nm) was used as the light source. Mercury was reduced in a 1 1 Pyrex glass vessel with 10% SnCl, + 20% H, SO,. An SE 120 recorder (Goertz Metrawatt, F.R.G.) was used. The automatic heating and gas flow control system used was of our own design (constructed by Dr Beerwald, Bochum, F.R.G.). Rain water samples were filtered using a SM16511 Sartorius system with a membrane filter of 0.45 pm pore size. A UV lamp of 150 W was used for UV irradiation. Digestion vessels were made of quartz glass. Chemicals All acids were Merck suprapure, other reagents were of Merck p.a. grade. Mercury determinations were carried out on all reagents.
251
Purification
of sampling bottles and labware
The sampling bottles and other labware were cleaned by filling with IO-20% HNO, and then allowed to stand for 2-3 days. They were again washed and allowed to stand for a further 2-3 days, after which mercury contamination was determined. Those containers showing some mercury contamination were again soaked in dilute nitric acid and heated at 1OO’C for 2-3 days, washed, followed by a further soaking in dilute nitric acid, heated and analyzed for mercury. This process was repeated until the glassware was clean. Rain water sampling bottles were cleaned with aqua regia to remove any sulfides of mercury adsorbed on the walls; they were then cleaned with dilute nitric acid and blank values determined before use. The 0.45 pm membrane filters were soaked in 1: 1 HNO,, the mercury content of which was subsequently determined. If any mercury was found to be present, the filters were again soaked in acid, and the mercury content determined until the acid was free from mercury. Sampling and sample handling Samples were collected in 2.5 1 brown glass bottles with a glass funnel 19 cm in diameter. These bottles were placed in a dark container (Fig. 1) to avoid any exposure to light. Sampling bottles were placed more than 1 m above ground level to avoid any contamination from soil particles and dust by splashing during heavy rain fall. Before collection of rain water, 25 ml of cont. HCl was
Fig. 1. Rain water sampler (brown glass bottle with glass funnel on top and a cover against light).
252
Stolbeq 0
*
Werth
? Bmsfeldhammer
t Leversbarh
Fig. 2. Sampling stations for rain water collection.
added to each bottle to avoid any loss of mercury or MM during the collection period. This amount of HCI stabilizes mercury and MM during the collection period (161; HNO, was not used because it interferes with the separation of MM from mercury [19]. Sampling locations are shown in Fig. 2. For the analysis of total mercury the samples were either filtered through a 0.45pm membrane filter to separate the suspended dust particles, and the mercury content of the filtrate and residue were determined separately after appropriate digestion, or a larger volume (- 250 ml) of unfiltered rain water was taken and total mercury determined after digestion. Large volumes were taken to avoid homogeneity problems associated with rain water containing suspended dust particles. Rain water samples containing high concentrations of mercury were diges-
253
ted with 40% HNO, + 10% HClO, in quartz glass flasks under slight pressure at 2OO’C. Filtrates with residue were digested with 10ml cont. HNO, + 2.5 ml HClO, under slight pressure in quartz glass vessels at 2OO’C [17]. Rain water samples with low concentrations of mercury (< 20 ngl-‘) were decomposed in the presence of 20% HCl + 5%HNO, with UV irradiation for 46 h, and acid concentrations were decreased to 4% HCl + 1% HNO, for very fresh samples with collection periods of not more than 1 week. For all the digestions, blank digestions with acids only were also undertaken and the values subtracted from those of the samples. Analysis
of mercury and methylmercury
For the analysis of total and ionic mercury, the cold vapour atomic absorption spectrometry (CVAAS) method was used. Mercury was reduced with 10% SnCl, + 20% H,SO, under N, gas at a flow rate of 2.62.51min’, which removed the reduced mercury vapour and deposited it on gold wool for 1 min. The N, flow rate was then increased to 5&100mlmin~i and the gold wool heated to 7OW3OO’C. Mercury deposited on the gold wool was vapourized and then measured in the cuvette at 253.7 nm [19]. Detection levels of ionic mercury by this method are < 0.1 ng ll’. The linearity range is from 0.0 to 10 ng of mercury [16]. The relative standard deviation is commonly < 5%. For the analysis of MM it was first separated from inorganic mercury by passing the rain water sample, adjusted with 5% HCl, through an anion exchange column previously conditioned with 6NHCl [19]. Ionic mercury is retained by the column and MM passes through. The filtrate’was analyzed for ionic mercury, then UV irradiated for 10min and analyzed for MM [16]. An overview of the analysis method for mercury and MM is given in Fig. 3. RESULTS
AND DISCUSSION
Evaluation
of deposition
data
From the concentrations of mercury and MM and the measured amount of rain water, the wet deposition of mercury and methylmercury per unit area can be evaluated:
where E, is the deposition over time period t, M is total rain water precipitated in time t, C is the concentration of metal, and F is the upper, open area of the funnel. For the calculation of daily average wet deposition (pgm-’ day-‘) the wet deposition amounts were added and divided by the number of days in a year (collection period). Since it did not rain continually, the actual deposition values during rainfall will be higher than the average values.
254
Samp1ir.g in glass with
qlas
bottles funnels
with
1
%
HCl
on top
v Filtration
0.5
,um
filter 20 8 HCl + 5 % HNOj
Digestion filter
of
at 200 OC
with
10 ml cont.
Digestion
with
HN03
+ 2.5 ml
40 % HNOj
+ 10 %
cont.
HClO
Add
5 % HCl
4
anion
exchange
J with
treated
6 N HCl
Filtrate
/
MM
Fig. 3. Scheme for overall trace analysisof Hg and methylmercury by cold vapour atomic absorption spectrometry (CVAAS).
Rainfall or precipitation
amounts
Rain water samples were collected from five locations which more or less represented a polluted area (Stolberg-Binsfeldhammer) an unpolluted area (Leversbach) and an urban area (Essen). Average daily wet precipitation from
255
25
233
2 0.. 1 b-5 r-
15 r.Y" -E 10.
0 5.
1
StalbergStowerg- LeversbachGusten Elnsfeldhammer Werth
Fig. 4. Average daily wet precipitation
?”
from Dec. 84 to Nov. 85 at sampling stations
Dec. 84 to Nov. 85 at all five collecting locations is shown in Fig. 4. Four of the collecting stations, Stolberg-Binsfeldhammer, Stolberg-Werth, Leversbach and Gusten, are all within a radius of 15 km (Fig. 2), therefore the average wet precipitation values are approximately the same. Average daily precipitation amounts for different collection periods over 1 year for these four collecting points were also similar. Atmospheric
distribution
In the atmosphere the emitted heavy metals may be bound to small dust particles with diameters between 0.1 and maximal 5ym or exist as gaseous organic species. The small particles form aerosols and have only a slight
25
1
20
05
0 0
StolbergBinsfeldhammer
Lever&h
Fig. 5. Average daily wet deposition smelter, Stolberg-Binsfeldhammer).
25 km Gusten
of mercury as a function of distance from a strong source (lead
256 12 0 !
Fig. 6. Average daily wet deposition of methylmercury source (lead smelter, Stolberg-Binsfeldhammer).
as a function
of distance
from a strong
point
tendency for sedimentation, therefore they may be transported by the wind significant distances from the point sources. A certain amount of the emitted heavy metals may enter into the upper troposphere as small aerosol particles and thus may be transported over long distances. Therefore, the results here for heavy metals may also be applicable to the long distance transport of other air pollutants [al]. Some studies were carried out to determine the distribution of mercury and MM from a pollution point source as a function of distance from the point source. Figure 5 illustrates the amount of mercury analyzed in precipitation as a function of the distance from a point source (lead smelter in Stolberg). It is clear from Fig. 5 that only after a distance of 4 km does the amount of mercury approach normal values. From this, however, one cannot rule out the possibility of mercury transport over larger distances. In the Stolberg area the pattern of deposition may be due to the emission of large particles from sources such as steel mills, and excess sulfur in the atmosphere which deposits in the immediate neighbourhood as large particles. The mercury is deposited adsorbed on sulfur particles or large particles impregnated with sulfur. The dissolution of heavy metals in small aerosols and thus transport over longer distances is also dependent upon the acidity of the rain and the wind direction [7, 8, 221. The distribution of MM in the atmosphere as a function of distance from the point source is shown in Fig. 6; it can be seen that the distribution is independent of the strength of the point source. This may be because less MM is emitted or none or only minimal conversion of inorganic mercury takes place. There should be more emissions of MM from mercury-polluted areas because it has been reported [23] that inorganic mercury may be converted to organic mercury in soil from which it can escape into the atmosphere. Also, MM may be transported and distributed in the atmosphere rapidly. Whatever the reasons, it is clear that the distribution of MM in the atmosphere is almost uniform.
257
250
50
Leversbach Gusten Stoiberg- StolbeqBlnsfeldhammer Werth
Fig. 7. Average daily wet deposition
Wet deposition
from Dec. 84 to Nov. 85
of mercury and methylmercury
Figure 7 shows the average daily wet deposition of mercury for 1 year at various locations. Maximum mercury deposition is in the location of a strong emission source (Stolberg-Binsfeldhammer), followed by an urban area (Essen), areas close to strong emission sources (Stolberg-Werth), followed by other rural areas. Figure 7 also shows the average daily wet deposition of MM at different locations. It is evident that the amounts of methylmercury found in different areas are nearly the same. There is not very much difference in concentration between the strong emission sources and others. Thus again one can conclude that the distribution of MM in the atmosphere is quite uniform. Concentrations
of mercury and methylmercury
in rain water
Rain water samples were collected from all five collecting stations from Dec. 84 to Nov. 85 and analyzed for total mercury and MM. Stolberg-Binsfeldhammer rain water samples contained an average mercury content of 1140nglV’ (range 18&3890ngl-‘), and an average MM content of 4.3ngl-’ (range 2.1-9.4ngl -‘) over a period of 1 year. Maximum average mercury and MM concentrations were found in March-April followed by May-June; minimum values were found in January and August-September. Rain water samples from Essen contained, on average, 77 ng Hg 1ml (range 53.9-122.0) and 2.4 ng MM 1-l (range 1.4-4.3) over a period of 1 year. Average daily mercury and MM concentrations were maximum in June and minima were in December-January and August-September. Stolberg-Werth rain water samples contained, on average, 69.5 ng Hg 1-l (range 31.4-114) and 3.6ngMM 1-l (range 1.2-7.2) over a period of 1 year. Average daily mercury and MM were maximum in May-June and minima were in December-January and August-September.
258
Giisten rain water samples contained, on average, 47.5 ng Hg 1-l (range 15.569) and 2.9 ng MM 1-l (range 1.5-4.0) for the year. Average daily mercury and MM were maximum in June. Rain water samples from Leversbach contained, on average, 410.0 ng Hg 1 ’ (range 31.4-61.4) and 2.4ngMM 111 (range 1.G3.6) over a period of 1 year. Average daily mercury and MM were maximum in JuneJuly and minima in December-January and August-September. In general, concentrations of mercury and MM in wet deposition increased appreciably in summer compared with winter at all collection stations. Generally the maximum mercury concentrations were found in May, June and July and minima in December, January and August-September. This was true for nearly all the five collecting stations. Comparison
between polluted and unpolluted areas
Among the rain water collection locations, Stolberg-Binsfeldhammer represents a relatively polluted area and Leversbach (Eifel) is assumed to be relatively unpolluted. Rain water was collected from these areas under similar conditions and for the same time periods. Collection time periods were increased or decreased so as to collect sufficient water for the analysis of mercury and MM and for comparative analytical purposes.
1%
Months
‘985
Fig. 8. Average daily wet precipitation, Leversbach for different time periods.
and wet deposition
of mercury
and methylmercury
at
259
Fig. 9. Average daily wet precipitation, and wet deposition Stolberg-Binsfeldhammer for different time periods.
of mercury and methylmercury
at
Figure 8 shows the average daily wet precipitation, and the average daily wet deposition of mercury and MM for different time periods for Leversbach, an unpolluted rural area. It can be seen that the deposition of mercury and MM is very much dependent on the amount of wet precipitation, since it is normal that more rain water should wash out more mercury and more MM. Although there were some emissions of mercury for April-May and October, on the whole wet deposition of mercury and methylmercury follows the pattern of wet precipitation. For the strong point source, which may be termed a relatively polluted area (Stolberg-Binsfeldhammer), the average daily wet precipitation, and the average daily wet deposition of mercury and methylmercury for different time periods are illustrated in Fig. 9. It is apparent that the deposition of mercury and MM are no longer dependent on wet precipitation; rather they indicate certain strong emission periods for mercury and MM. Here much mercury was found in the months of Feb.-March followed by MayJune and October, and similarly, maximum MM was found in MayJune. The deposition of mercury does not follow the pattern of wet precipitation, but, to some extent, the deposition of methylmercury does.
260
Ratio of methylmercury
to total mercury
Amounts of MM found compared with total mercury for 1 year are given in Table 1. The ratio of methylmercury to total mercury does not remain constant; the amount of MM remains nearly constant and the amount of total mercury varies. For polluted areas such as Stolberg-Binsfeldhammer, the percentage of methylmercury is < 1% of the total mercury; for urban areas (Essen) the MM percentage of total mercury is 3.1% and for other areas it is nearly 6%. The ratio of MM to total mercury might also be used as an indicator of mercury pollution. A decreasing MM percentage indicates a more mercury-polluted area. From Table 1 it is clear that a lower percentage of MM indicates a polluted area (Stolberg-Binsfeldhammer) and an increase in MM percentage exactly balances the decrease in total mercury pollution. TABLE 1 MM percentage of total Hg of the average daily wet deposition
for 1 year from Dec. 84 to Nov. 85
Sample No.
Sampling station
% MM of total Hg
1 2 3 4 5
Stolberg-Binsfeldhammer Essen Stolberg-Werth Leversbach Giisten
0.37 3.11 5.06 6.00 6.10
Total annual wet deposition of mercury and MM It is difficult to estimate from these very limited data the annual deposition of mercury and methylmercury in the Federal Republic of Germany, but a rough estimate (on the low side) may be made. Based on data also including the industrial zones (representing 5% of the whole territory, i.e. 12.40 x 10gm2) the annual deposition of mercury is 20 tons year’ over the whole F.R.G. (total territory 248 x 10gm2). It may be estimated that during the last decade or so this amount of mercury has been added yearly to the soil, vegetation and inland water sources. As the amount of MM is nearly constant for all the collecting locations, a reasonably good estimate for MM deposition can be calculated. The annual deposition of MM in the F.R.G. is 0.5 tons year-‘. This deposition of MM may seem low but its toxicity is very high. ACKNOWLEDGEMENT
One of the authors (R.A.) acknowledges financial support from the Federal Ministry of Science and Technology (BMFT), F.R.G. through International Bureau KFZ-Karlsruhe. This paper is dedicated to the memory of Prof. Dr. H.W. Nurnberg, late
261
director of the Institute of Applied Physical Chemistry who was always the driving force in extended studies on trace metals in the environment and particularly in precipitation. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
16 17 18 19 20 21 22 23
L. Friberg, G.F. Nordberg and B. Vouk (Eds), Handbook on the Toxicity of Metals, Elsevier/ North Holland Biomedical Press, Amsterdam, 1985. M.R. Greenwood and R. Von Burg, in E. Merian (Ed.), Metalle in der Umwelt, Verlag Chemie, Weinheim, 1984, p. 511. D. Taylor, Mercury as an Environmental Pollutant. A Bibliography, Imperial Chemical Industries Ltd., Brixham, 1975, p. 269. B. Venugopal and T.D. Lucky, Metal Toxicity in Mammals, Vol. 1,2, Plenum press, London, New York, 1978. E. Merian (Ed.), Metalle in der Umwelt, Verlag Chemie, Weinheim, 1984. E.I. Hamilton, Sci. Total Environ., 3 (1974) 3385; 5 (1976) 162. H.W. Niirnberg, P. Valenta, V.D. Nguyen, M. Godde and E. Urano de Carvalho, Fresenius Z. Anal. Chem., 317 (1984) 314323. H.W. Niirnberg, P. Valenta and V.D. Nguyen, Jahresbericht Kernforschungsanlage, Julich, 1982/83, pp. 4143. H.W. Niirnberg, V.D. Nguyen and P. Valenta, Umweltforschung, KFA, Julich, 1985, pp. 27-39. F.H. Frimmel, in H. Kienitz, R. Bock, W. Fresenius, W. Huber and G. Tolg (Eds), AnalytikerTaschenbuch, Band 1, Springer-Verlag, Berlin, New York, 1980, pp. 391402. AQC Committee, Analyst, llO(2) (1985) 103111. K. May, K. Reisinger, R. Flucht and M. Stoeppler, Jahrb. vom Wasser, 55 (1980) 65-76. K. May and M. Stoeppler, in G. Muller (Ed.), Proc. Int. Conf. Heavy Metals in the Environment, Vol. 1, CEP Consultants, Edinburgh, 1983, pp. 241-244. R. Ahmed, Ph. D. Thesis, Islamabad, 1982. K. May, R. Ahmed, B. Torres, K. Reisinger and M. Stoeppler, in T.D. Lekkas (Ed.), Proc. Int. Conf. Heavy Metals in the Environment, Vol. 2, CEP Consultants, Edinburgh, 1985, pp. 513-515. R. Ahmed and M. Stoeppler, Jill-Spez. March (1986) pp. l-52. K. May and M. Stoeppler, Fresenius Z. Anal. Chem., 317 (1984) 248251. K. May, K. Reisinger, B. Torres and M. Stoeppler, Fresenius’ Z. Anal. Chem., 320 (1985) 646. K. May, M. Stoeppler and K. Reisinger, Int. J. Environ. Anal. Chem., in press, R. Ahmed and M. Stoeppler, Fresenius’ Z. Anal. Chem., in press. L. Mart, Tellus, 35B (1983) 131. V.D. Nguyen, P. Valenta and H.W. Nurnberg, Sci. Total Environ., 12 (1979) 151-167. J.H. Weber, K. Reisinger and M. Stoeppler, in T.D. Lekkas (Ed.), Proc. Int. Conf. Heavy Metals in the Environment, Vol 2, CEP Consultants, Edinburgh, 1985, pp. 516512.
The Science of the Total Environment, 60 (1987) 249261 Elsevier Science Publishers B.V., Amsterdam ~ Printed in The Netherlands
WET DEPOSITION OF MERCURY THE ATMOSPHERE*
RIAZ AHMEDI,
KARL MAY and MARKUS
AND METHYLMERCURY
FROM
STOEPPLER$
Institute of Applied Physical Chemistry (ZCH-4), Nuclear Research Centre, Jiilich (KFA), Boz 1913, D-5170 Jiilich (Federal Republic of Germany)
P.O.
ABSTRACT The wet deposition of mercury and methylmercury was monitored for a period of 1 year at selected areas in the Federal Republic of Germany. These areas represented an industrial zone, an urban area, a rural area close to the industrial zone and other rural areas. The wet deposition of mercury was very much dependent on the collection area; the wet deposition of methylmercury (MM) was not area dependent. Maximum mercury was found as follows: industrial area > urban area > rural area close to the industrial zone > other rural areas. Methylmercury concentrations at all areas were almost uniform within analytical error. Methylmercury as a percentage of total mercury was 5- 6% for rural areas and decreased to < 1% for industrial areas. Wet deposition of mercury increased in summer. Based on the experimental data the present burdens caused by the wet deposition of mercury and methylmercury are evaluated.
INTRODUCTION
Mercury and its compounds have been known for a long time and have been extensively used for the benefit of mankind. Mercury is a liquid metal used extensively in industry, and, because it is volatile, an appreciable amount escapes into the atmosphere as vapour. In addition to its use in industry, emissions of mercury vapour also arise from naturally occurring ores of mercury. Organomercury compounds are well known as bactericides and in the past have been extensively used for seed dressing to protect them from insects and bacteria. The input of mercury and its compounds into the environment has increased through rapid industrialization. This input of mercury and its compounds into the atmosphere, through different localized sources, is of great significance because of its toxicity [l-3]. Mercury belongs to the a priori toxic metal group consisting of Cd, Pb and Hg, with great significance and priority in ecochemistry and ecotoxicology [4-6]. Metals and metalloids are characterized by special ecochemical features. They are not biodegradable, but undergo a biogeochemical cycle during which transformations into more or less toxic species occur [5]. They are accumulated by organisms and cause increased *Dedicated to the memory of Professor Dr Hans Wolfgang Niirnberg. tpresent address: NCD-PINSTECH, P.O. Nilore, Islamabad, Pakistan, iAuthor to whom correspondence should be addressed.
250
toxic effects in man and mammals after long-term exposure. The most important source of uptake is food and the significant pathways of toxic metals into food chains is through the atmosphere from which the metals are introduced into ecosystems by dry and wet deposition. Wet deposition through rain and snow is of particular importance because it provides favourable conditions for uptake by vegetation and water. A considerable amount of work has been carried out on the analysis of Cd, Pb, Ni, Cu, Zn, etc. in wet deposition [779], but there are hardly any reliable data available for mercury in rain water and hardly any measurements have been made for the methylmercury (MM) content of rain water. The reasons for this may be the difficulties associated with the reliable analysis of mercury at the very low levels found in aqueous systems [lo, 111. However, in this institute several studies have been undertaken for the analysis of mercury and MM [12-171, and a sensitive and reliable method for their analysis at very low levels has recently been developed [18, 191. Detailed studies have been described for the analysis of mercury and MM in rain water [20]. With the development of very sensitive and reliable methods for the analysis of mercury, particularly in rain water, it seemed appropriate to begin a further systematic investigation into the mercury and MM concentrations in rain water. This paper describes the collection of rain water samples from polluted and relatively unpolluted areas of the Federal Republic of Germany covering a period of 1 year and their analysis for total mercury and methylmercury and is an addition to the institutes extended and ongoing studies on wet precipitation in the F.R.G. [7-g]. EXPERIMENTAL
Apparatus Measurements were made using an atomic absorption spectrometer (Model 400, Perkin Elmer, Bodenseewerk, F.R.G.). A mercury vapour lamp (0.2 A/15 V, wave-length 253.7 nm and slit width 2.0nm) was used as the light source. Mercury was reduced in a 1 1 Pyrex glass vessel with 10% SnCl, + 20% H, SO,. An SE 120 recorder (Goertz Metrawatt, F.R.G.) was used. The automatic heating and gas flow control system used was of our own design (constructed by Dr Beerwald, Bochum, F.R.G.). Rain water samples were filtered using a SM16511 Sartorius system with a membrane filter of 0.45 pm pore size. A UV lamp of 150 W was used for UV irradiation. Digestion vessels were made of quartz glass. Chemicals All acids were Merck suprapure, other reagents were of Merck p.a. grade. Mercury determinations were carried out on all reagents.
251
Purification
of sampling bottles and labware
The sampling bottles and other labware were cleaned by filling with IO-20% HNO, and then allowed to stand for 2-3 days. They were again washed and allowed to stand for a further 2-3 days, after which mercury contamination was determined. Those containers showing some mercury contamination were again soaked in dilute nitric acid and heated at 1OO’C for 2-3 days, washed, followed by a further soaking in dilute nitric acid, heated and analyzed for mercury. This process was repeated until the glassware was clean. Rain water sampling bottles were cleaned with aqua regia to remove any sulfides of mercury adsorbed on the walls; they were then cleaned with dilute nitric acid and blank values determined before use. The 0.45 pm membrane filters were soaked in 1: 1 HNO,, the mercury content of which was subsequently determined. If any mercury was found to be present, the filters were again soaked in acid, and the mercury content determined until the acid was free from mercury. Sampling and sample handling Samples were collected in 2.5 1 brown glass bottles with a glass funnel 19 cm in diameter. These bottles were placed in a dark container (Fig. 1) to avoid any exposure to light. Sampling bottles were placed more than 1 m above ground level to avoid any contamination from soil particles and dust by splashing during heavy rain fall. Before collection of rain water, 25 ml of cont. HCl was
Fig. 1. Rain water sampler (brown glass bottle with glass funnel on top and a cover against light).
252
Stolbeq 0
*
Werth
? Bmsfeldhammer
t Leversbarh
Fig. 2. Sampling stations for rain water collection.
added to each bottle to avoid any loss of mercury or MM during the collection period. This amount of HCI stabilizes mercury and MM during the collection period (161; HNO, was not used because it interferes with the separation of MM from mercury [19]. Sampling locations are shown in Fig. 2. For the analysis of total mercury the samples were either filtered through a 0.45pm membrane filter to separate the suspended dust particles, and the mercury content of the filtrate and residue were determined separately after appropriate digestion, or a larger volume (- 250 ml) of unfiltered rain water was taken and total mercury determined after digestion. Large volumes were taken to avoid homogeneity problems associated with rain water containing suspended dust particles. Rain water samples containing high concentrations of mercury were diges-
253
ted with 40% HNO, + 10% HClO, in quartz glass flasks under slight pressure at 2OO’C. Filtrates with residue were digested with 10ml cont. HNO, + 2.5 ml HClO, under slight pressure in quartz glass vessels at 2OO’C [17]. Rain water samples with low concentrations of mercury (< 20 ngl-‘) were decomposed in the presence of 20% HCl + 5%HNO, with UV irradiation for 46 h, and acid concentrations were decreased to 4% HCl + 1% HNO, for very fresh samples with collection periods of not more than 1 week. For all the digestions, blank digestions with acids only were also undertaken and the values subtracted from those of the samples. Analysis
of mercury and methylmercury
For the analysis of total and ionic mercury, the cold vapour atomic absorption spectrometry (CVAAS) method was used. Mercury was reduced with 10% SnCl, + 20% H,SO, under N, gas at a flow rate of 2.62.51min’, which removed the reduced mercury vapour and deposited it on gold wool for 1 min. The N, flow rate was then increased to 5&100mlmin~i and the gold wool heated to 7OW3OO’C. Mercury deposited on the gold wool was vapourized and then measured in the cuvette at 253.7 nm [19]. Detection levels of ionic mercury by this method are < 0.1 ng ll’. The linearity range is from 0.0 to 10 ng of mercury [16]. The relative standard deviation is commonly < 5%. For the analysis of MM it was first separated from inorganic mercury by passing the rain water sample, adjusted with 5% HCl, through an anion exchange column previously conditioned with 6NHCl [19]. Ionic mercury is retained by the column and MM passes through. The filtrate’was analyzed for ionic mercury, then UV irradiated for 10min and analyzed for MM [16]. An overview of the analysis method for mercury and MM is given in Fig. 3. RESULTS
AND DISCUSSION
Evaluation
of deposition
data
From the concentrations of mercury and MM and the measured amount of rain water, the wet deposition of mercury and methylmercury per unit area can be evaluated:
where E, is the deposition over time period t, M is total rain water precipitated in time t, C is the concentration of metal, and F is the upper, open area of the funnel. For the calculation of daily average wet deposition (pgm-’ day-‘) the wet deposition amounts were added and divided by the number of days in a year (collection period). Since it did not rain continually, the actual deposition values during rainfall will be higher than the average values.
254
Samp1ir.g in glass with
qlas
bottles funnels
with
1
%
HCl
on top
v Filtration
0.5
,um
filter 20 8 HCl + 5 % HNOj
Digestion filter
of
at 200 OC
with
10 ml cont.
Digestion
with
HN03
+ 2.5 ml
40 % HNOj
+ 10 %
cont.
HClO
Add
5 % HCl
4
anion
exchange
J with
treated
6 N HCl
Filtrate
/
MM
Fig. 3. Scheme for overall trace analysisof Hg and methylmercury by cold vapour atomic absorption spectrometry (CVAAS).
Rainfall or precipitation
amounts
Rain water samples were collected from five locations which more or less represented a polluted area (Stolberg-Binsfeldhammer) an unpolluted area (Leversbach) and an urban area (Essen). Average daily wet precipitation from
255
25
233
2 0.. 1 b-5 r-
15 r.Y" -E 10.
0 5.
1
StalbergStowerg- LeversbachGusten Elnsfeldhammer Werth
Fig. 4. Average daily wet precipitation
?”
from Dec. 84 to Nov. 85 at sampling stations
Dec. 84 to Nov. 85 at all five collecting locations is shown in Fig. 4. Four of the collecting stations, Stolberg-Binsfeldhammer, Stolberg-Werth, Leversbach and Gusten, are all within a radius of 15 km (Fig. 2), therefore the average wet precipitation values are approximately the same. Average daily precipitation amounts for different collection periods over 1 year for these four collecting points were also similar. Atmospheric
distribution
In the atmosphere the emitted heavy metals may be bound to small dust particles with diameters between 0.1 and maximal 5ym or exist as gaseous organic species. The small particles form aerosols and have only a slight
25
1
20
05
0 0
StolbergBinsfeldhammer
Lever&h
Fig. 5. Average daily wet deposition smelter, Stolberg-Binsfeldhammer).
25 km Gusten
of mercury as a function of distance from a strong source (lead
256 12 0 !
Fig. 6. Average daily wet deposition of methylmercury source (lead smelter, Stolberg-Binsfeldhammer).
as a function
of distance
from a strong
point
tendency for sedimentation, therefore they may be transported by the wind significant distances from the point sources. A certain amount of the emitted heavy metals may enter into the upper troposphere as small aerosol particles and thus may be transported over long distances. Therefore, the results here for heavy metals may also be applicable to the long distance transport of other air pollutants [al]. Some studies were carried out to determine the distribution of mercury and MM from a pollution point source as a function of distance from the point source. Figure 5 illustrates the amount of mercury analyzed in precipitation as a function of the distance from a point source (lead smelter in Stolberg). It is clear from Fig. 5 that only after a distance of 4 km does the amount of mercury approach normal values. From this, however, one cannot rule out the possibility of mercury transport over larger distances. In the Stolberg area the pattern of deposition may be due to the emission of large particles from sources such as steel mills, and excess sulfur in the atmosphere which deposits in the immediate neighbourhood as large particles. The mercury is deposited adsorbed on sulfur particles or large particles impregnated with sulfur. The dissolution of heavy metals in small aerosols and thus transport over longer distances is also dependent upon the acidity of the rain and the wind direction [7, 8, 221. The distribution of MM in the atmosphere as a function of distance from the point source is shown in Fig. 6; it can be seen that the distribution is independent of the strength of the point source. This may be because less MM is emitted or none or only minimal conversion of inorganic mercury takes place. There should be more emissions of MM from mercury-polluted areas because it has been reported [23] that inorganic mercury may be converted to organic mercury in soil from which it can escape into the atmosphere. Also, MM may be transported and distributed in the atmosphere rapidly. Whatever the reasons, it is clear that the distribution of MM in the atmosphere is almost uniform.
257
250
50
Leversbach Gusten Stoiberg- StolbeqBlnsfeldhammer Werth
Fig. 7. Average daily wet deposition
Wet deposition
from Dec. 84 to Nov. 85
of mercury and methylmercury
Figure 7 shows the average daily wet deposition of mercury for 1 year at various locations. Maximum mercury deposition is in the location of a strong emission source (Stolberg-Binsfeldhammer), followed by an urban area (Essen), areas close to strong emission sources (Stolberg-Werth), followed by other rural areas. Figure 7 also shows the average daily wet deposition of MM at different locations. It is evident that the amounts of methylmercury found in different areas are nearly the same. There is not very much difference in concentration between the strong emission sources and others. Thus again one can conclude that the distribution of MM in the atmosphere is quite uniform. Concentrations
of mercury and methylmercury
in rain water
Rain water samples were collected from all five collecting stations from Dec. 84 to Nov. 85 and analyzed for total mercury and MM. Stolberg-Binsfeldhammer rain water samples contained an average mercury content of 1140nglV’ (range 18&3890ngl-‘), and an average MM content of 4.3ngl-’ (range 2.1-9.4ngl -‘) over a period of 1 year. Maximum average mercury and MM concentrations were found in March-April followed by May-June; minimum values were found in January and August-September. Rain water samples from Essen contained, on average, 77 ng Hg 1ml (range 53.9-122.0) and 2.4 ng MM 1-l (range 1.4-4.3) over a period of 1 year. Average daily mercury and MM concentrations were maximum in June and minima were in December-January and August-September. Stolberg-Werth rain water samples contained, on average, 69.5 ng Hg 1-l (range 31.4-114) and 3.6ngMM 1-l (range 1.2-7.2) over a period of 1 year. Average daily mercury and MM were maximum in May-June and minima were in December-January and August-September.
258
Giisten rain water samples contained, on average, 47.5 ng Hg 1-l (range 15.569) and 2.9 ng MM 1-l (range 1.5-4.0) for the year. Average daily mercury and MM were maximum in June. Rain water samples from Leversbach contained, on average, 410.0 ng Hg 1 ’ (range 31.4-61.4) and 2.4ngMM 111 (range 1.G3.6) over a period of 1 year. Average daily mercury and MM were maximum in JuneJuly and minima in December-January and August-September. In general, concentrations of mercury and MM in wet deposition increased appreciably in summer compared with winter at all collection stations. Generally the maximum mercury concentrations were found in May, June and July and minima in December, January and August-September. This was true for nearly all the five collecting stations. Comparison
between polluted and unpolluted areas
Among the rain water collection locations, Stolberg-Binsfeldhammer represents a relatively polluted area and Leversbach (Eifel) is assumed to be relatively unpolluted. Rain water was collected from these areas under similar conditions and for the same time periods. Collection time periods were increased or decreased so as to collect sufficient water for the analysis of mercury and MM and for comparative analytical purposes.
1%
Months
‘985
Fig. 8. Average daily wet precipitation, Leversbach for different time periods.
and wet deposition
of mercury
and methylmercury
at
259
Fig. 9. Average daily wet precipitation, and wet deposition Stolberg-Binsfeldhammer for different time periods.
of mercury and methylmercury
at
Figure 8 shows the average daily wet precipitation, and the average daily wet deposition of mercury and MM for different time periods for Leversbach, an unpolluted rural area. It can be seen that the deposition of mercury and MM is very much dependent on the amount of wet precipitation, since it is normal that more rain water should wash out more mercury and more MM. Although there were some emissions of mercury for April-May and October, on the whole wet deposition of mercury and methylmercury follows the pattern of wet precipitation. For the strong point source, which may be termed a relatively polluted area (Stolberg-Binsfeldhammer), the average daily wet precipitation, and the average daily wet deposition of mercury and methylmercury for different time periods are illustrated in Fig. 9. It is apparent that the deposition of mercury and MM are no longer dependent on wet precipitation; rather they indicate certain strong emission periods for mercury and MM. Here much mercury was found in the months of Feb.-March followed by MayJune and October, and similarly, maximum MM was found in MayJune. The deposition of mercury does not follow the pattern of wet precipitation, but, to some extent, the deposition of methylmercury does.
260
Ratio of methylmercury
to total mercury
Amounts of MM found compared with total mercury for 1 year are given in Table 1. The ratio of methylmercury to total mercury does not remain constant; the amount of MM remains nearly constant and the amount of total mercury varies. For polluted areas such as Stolberg-Binsfeldhammer, the percentage of methylmercury is < 1% of the total mercury; for urban areas (Essen) the MM percentage of total mercury is 3.1% and for other areas it is nearly 6%. The ratio of MM to total mercury might also be used as an indicator of mercury pollution. A decreasing MM percentage indicates a more mercury-polluted area. From Table 1 it is clear that a lower percentage of MM indicates a polluted area (Stolberg-Binsfeldhammer) and an increase in MM percentage exactly balances the decrease in total mercury pollution. TABLE 1 MM percentage of total Hg of the average daily wet deposition
for 1 year from Dec. 84 to Nov. 85
Sample No.
Sampling station
% MM of total Hg
1 2 3 4 5
Stolberg-Binsfeldhammer Essen Stolberg-Werth Leversbach Giisten
0.37 3.11 5.06 6.00 6.10
Total annual wet deposition of mercury and MM It is difficult to estimate from these very limited data the annual deposition of mercury and methylmercury in the Federal Republic of Germany, but a rough estimate (on the low side) may be made. Based on data also including the industrial zones (representing 5% of the whole territory, i.e. 12.40 x 10gm2) the annual deposition of mercury is 20 tons year’ over the whole F.R.G. (total territory 248 x 10gm2). It may be estimated that during the last decade or so this amount of mercury has been added yearly to the soil, vegetation and inland water sources. As the amount of MM is nearly constant for all the collecting locations, a reasonably good estimate for MM deposition can be calculated. The annual deposition of MM in the F.R.G. is 0.5 tons year-‘. This deposition of MM may seem low but its toxicity is very high. ACKNOWLEDGEMENT
One of the authors (R.A.) acknowledges financial support from the Federal Ministry of Science and Technology (BMFT), F.R.G. through International Bureau KFZ-Karlsruhe. This paper is dedicated to the memory of Prof. Dr. H.W. Nurnberg, late
261
director of the Institute of Applied Physical Chemistry who was always the driving force in extended studies on trace metals in the environment and particularly in precipitation. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
16 17 18 19 20 21 22 23
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