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Analyst, Febmary, 1976, Vol. 101, pp. 91-95
91
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An Improved Digestion Method for the Extraction of Mercury from Environmental Sampies Haig Agemian and A. S. Y. Chau Canada Centre for Inland Waters, Watev Quality Branch, P.O. Box 5050, 867 Lakeshore Road, Burlington, Ontario, L7R 4A6, Canada
An improved digestion procedure for the extraction of mercury from environmental material is reported. The method involves the digestion of the sample at 60 "C with sulphuric acid - nitric acid (2 l ) , containing a trace amount of hydrochloric acid, and subsequent oxidation with permanganate and persulphate solutions. With this procedure mercury is successfully recovered from organic matter and resistant inorganic forms such as mercury(I1) sulphide. Unlike digestion with aqua regia, this procedure is simple and safe, and is applicable to the digestion of a large number of samples simultaneously. The method can be adapted to the automated cold-vapour and flame atomic-absorption techniques and is therefore ideal for routine monitoring.
+
Agemian and Chaul reported a method for the determination of mercury in sediments, in which a comprehensive manual digestion method developed by Iskandar et aL2 was adapted to the automated cold-vapour atomic-absorption technique. The extraction technique involved the digestion of the sediment with sulphuric acid - nitric acid (2 1) at 60 "C and subsequent oxidation of the organomercury compounds with permanganate and persulphate solutions. Complete recovery2 of several organomercury compounds was obtained by using this technique. Jacobs and Keeneys showed that this procedure does not adequately dissolve mercury( 11) sulphide (cinnabar), which may be formed under reducing conditions. They suggested treating the sediment with boiling aqua regia. However, we found that this treatment is hazardous, impractical and unnecessary for routine analysis of a large number of samples. Moreover, Jonasson et aL4 have reported that the presence of excessive amounts of hydrochloric acid should be avoided as the consequent generation of chlorine5 may cause the loss of volatile mercury chlorides. They proposed the use of a solution of nitric acid plus a trace amount of hydrochloric acid, which technique enables mercury sulphides to be satisfactorily dissolved, and is suitable for application to geological samples. Agemian et aL6 showed the adaptability of both sulphuric acid - nitric acid (2 1) and hydrochloric acid - nitric acid (1 9) digestion mixtures to the automated cold-vapour atomic-absorption technique for mercury determination. They compared these extraction methods for a number of rock, soil and sediment samples and obtained identical results, presumably owing to the absence of cinnabar in the samples analysed. Samples that contain substantial amounts of this mercury compound are not commonly analysed in this laboratory. However, in this study, the intention was to obtain a comprehensive method that is applicable to all types of samples, including those containing mercury in forms such as mercury(I1) sulphide. Cinnabar is resistant to attack by sulphuric and nitric acids.* However, the inclusion of the minimum4 amount of hydrochloric acid promotes its rapid decomposition. Thus, a slight modification of the digestion method of Iskandar et aZ.,2by the addition of a trace amount of hydrochloric acid, enables large amounts of mercury(I1) sulphide, at levels much higher than are found in natural samples, to be recovered. The proposed method is simple, rapid and adaptable to the determination of all organic and inorganic forms of mercury in a large number of samples by the automated cold-vapour atomic-absorption technique. The method can be suited to the digestion of biological (fish)' or geological (rocks, soils and sediments) samples by using the procedure with or without the inclusion of hydrochloric acid. The extraction medium is especially suitable for sediments and soils as its oxidising nature enables the large amounts of organic matter frequently found in these samples to be oxidised;
+
+
+
View Article Online
92
AGEMIAN AND CHAU : AN IMPROVED DIGESTION METHOD FOR THE
A n a l y s t , VoZ. 101
Published on 01 January 1976. Downloaded by University of Central Florida on 28/10/2014 13:37:00.
also, the strongly acidic medium, in the presence of a trace amount of hydrochloric acid, extracts inorganic forms of mercury including that found in cinnabar.
Experimental Apparatus Samples were digested in 100-ml calibrated flasks in a temperature-controlled shaker bath (Precision Scientific Co. , Model 75). The equipment used for the automated cold-vapour analysis consisted of ( a ) an automatic sampler (Technicon AutoAnalyzer I1 sampler with 20-1/5 cam) ; ( b ) a proportionating pump (Carlo Erba, Model 08-59-10202); (c) Technicon AutoAnalyzer tubing of specified dimensions ; ( d ) a gas separator, as used by Agemian and Chaul; ( e ) a mercury monitor (Pharmacia Fine Chemicals) ; and (f) a strip-chart recorder (Hewlett-Packard, Model 7101B). The system is similar to that used by Agemian and co-workers.l,s For high levels of mercury, a Perkin-Elmer 503 atomic-absorption spectrophotometer, equipped with an Intensitron lamp, was used with an air - acetylene flame. Reagents High-purity certified reagents were used for all analyses. Sulphuric acid, 36 N. Nitric acid, 16 N. Hydrochloric acid, 12 N. Potassium permanganate solution, 6 p e r cent. m/V. Potassium persulphate solution, 5 per cent. m/V. Tin(II)sulphate solution, 10 p e r cent. m/V in 2 N sulphuric acid. H y d r o x y l a m m o n i u m sulphate (6 per cent. m/V) - sodium chloride (6 per cefzt. m/V) solution. Procedure Determine the water content of the wet sediment by drying it overnight t o constant mass at 110 "C. Weigh a representative sample of wet sediment, equivalent to 0.1-2 g of the dry mass, into a 100-ml calibrated flask. Wash the sediment down to the bottom of the flask with mercury-free distilled water, place the flask in an ice - water bath and slowly add 15 ml of sulphuric acid - nitric acid (2 1). After cooling, add 2 ml of hydrochloric acid. Shake the mixture and let it stand for 5 min. After expelling the acid fumes from the flask, place it in a shaking water-bath at a temperature of 50-60 "C and digest for 2 h. Then allow the flask to cool for 30 min and carefully add 10 ml of potassium permanganate solution while cooling in an ice - water bath. If the colour does not persist for 15 min, add a further amount of potassium permanganate solution. After 30 min, add 5 ml of potassium persulphate solution, with gentle stirring, and allow the mixture to stand overnight. If all of the permanganate is reduced, as witnessed by the absence of the purple colour, add potassium permanganate solution until the colour persists. Add 10 ml of hydroxylammonium sulphate - sodium chloride solution and stir the mixture gently until the solution becomes clear and all of the precipitated manganese(1V) oxide has dissolved. Make the solution up to volume and centrifuge an aliquot a t 2500 rev min-l for 5 min. Transfer an aliquot of the clear supernatant liquid into a glass sample cup and place it in the automatic sampler for analysis. Use a cam designed for 20 samples per hour and a sample to wash ratio of 1 : 5 , corresponding to a sampling time of 30 s and a wash time of 2.5 min. For concentrated solutions of mercury use the flame technique. The linear concentration range in solution is 0.0002-0.006 mg 1-l for the non-flame detection system1 and 10-300 mg 1-1 for the flame detection system.8 For sample concentrations of 0-1-2 g per 100 ml, the non-flame detection system has a range of 0.01-6 mg kg-l and the flame method 5-300 mg k g l of mercury in the sediment. Further dilution of the extracts could extend the upper limit of both detection systems. Treat all standards exactly as for the above samples.
+
Results and Discussion Sulphuric acid is used extensively for the extraction of mercury from biological materials, either alone7,Bs10 or with nitric acid.ll It has also been used separatelyll-l3 or together with
View Article Online
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February, 1976
93
EXTRACTION OF MERCURY FROM ENVIRONMENTAL SAMPLES
nitric acid2J4J5 for sediments and rocks. Iskandar et aL2 also showed that a mixture of sulphuric and nitric acids is necessary for the adequate digestion of organic matter in sediments and soils and for the subsequent liberation of mercury. As already indicated, the presence of hydrochloric acid is necessary for the decomposition of cinnabar. Initially, an attempt was made to use large amounts of hydrochloric acid, as suggested by Jacobs and K e e n e ~ but , ~ the procedure proved to be very impractical and difficult and gave rise to low recoveries of mercury from sediments. The use of a fume hood in which to perform the digestion is essential when hydrochloric acid is heated, even to 60 "C: Further, on adding potassium permanganate in order to oxidise the organomercurials, violent frothing of the solution occurs, with evolution of chlorine, which makes the analysis very difficult. In addition, the decompositions of the permanganate to manganese(1V) oxide caused by the hydrochloric acid, renders it ineffective as an oxidant for organomercurials, and a low recovery of mercury may therefore result. We found that as little as 5 m l of hydrochloric acid caused the rapid decomposition of 25 ml of a 5 per cent. m/V potassium permanganate solution. Table I shows the effect of excess of hydrochloric acid on the recovery of mercury from sediments. The low results obtained (third column) are attributed to the loss of mercury due to violent frothing of the solution and possible volatilisation, and also to the reduced efficiency of oxidation of the organic matter by permanganate owing to its decomposition by hydrochloric acid. The analyses were performed by using the automated method reported by Agemian and coworkers.lt6 The samples examined were ordinary samples as analysed in our laboratory and therefore did not give any indication of the presence of cinnabar. Comequently, the method of Iskandar et aL2 gave results for recovery that are similar to those given by the proposed met hod. TABLEI EXTRACTION OF MERCURY FROM SEDIMENT SAMPLES BY USING DIFFERENT EXTRACTION MEDIA
Results are expressed in pg kg-1. Extraction medium 7
Sample Silt Silty clay Clay Clay Clay
.. .. .. .. ..
.. .. ..
.. ..
-
HISO, - HNO, (2 I)* 310 600 970
€I,S04 - HNO, - HC1 (10 5 2)t 320 600 960
1000
1000 1600
+
+ +
1600
H,SO, (2
- HNO,
- HCf
+ 1 + 1): 250 450
600 800 1400
* Method of Iskandar et al., t Present method.
:Contains an excessive amount of hydrochloric acid.
I n order to check the efficiency of the method, sediment samples were spiked with mercury(I1) sulphide powder and the recovery of the mercury was determined. As only milligram amounts could reliably be weighed, the analysis had to be performed by the flame atomic-absorption method; the results obtained are given in Table I1 and show a satisfactory recovery of 100 & 5 per cent. of mercury added as mercury(I1) sulphide. The small amount of hydrochloric acid added in the digestion procedure causes the decomposition of the cinnabar to become visually apparent, the disappearance in a few seconds of the bright red colour indicating its complete dissolution.
TABLE I1 RECOVERY OF
MERCURY FROM MERCURY(II) SULPHIDE BY THE PROPOSED METHOD
Mercury added (as HgS)/g Recovery, per cent. . .
.. ..
..
..
0.0018 102
0.007 1 98
0.0103 105
0.0222 95
0.029 1 101
The levels of the spikes used for the recovery tests are much higher than would be found in any real samples, the mercury contents16 of most uncontaminated solid earth materials being in the range 10-500 ng k g l . Based on l-g samples, the results shown in Table I1 correspond
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94
AGEMIAN AND CHAU: AN IMPROVED DIGESTION METHOD FOR THE
Analyst, Vol. 101
to levels of lo00 mg k g 1 and above, which are much higher than those reported for most contaminated samples; Konradl' reported levels of 700-800 mg k g l for samples from chloralkali plants in Wisconsin. Therefore, the satisfactory recovery of amounts of mercury that are much larger than those encountered in highly contaminated samples indicates that the proposed method is adequate. As confirmed by Jacobs and Keeney,a use of the method of Iskandar et aL2 did not permit the recovery of any of the metcury introduced as mercury(I1) sulphide, which was completely resistant to the sulphuric acid - nitric acid mixture. Table I11 shows the effect of hydrochloric acid on the recovery of mercury from geological samples that contain large amounts of sulphide. The results given in this table show the superiority of the proposed method, using hydrochloric acid, for determining low levels of mercury. The precisions of the two methods are similar and the coefficient of variation6
TABLE I11 EXTRACTION OF MERCURY
FROM GEOLOGICAL SAMPLES THAT HAVE HIGH SULPHIDE CONTENTS
Results are expressed in mg kg-l; all samples were analysed in triplicate. Extraction medium , Sample Sphalerite .. Copper concentrate Pyrite concentrate Ore head Tailing . . .. Tailing . . .. Lead concentrate Ore head Copper concentrate Oxidised tailing
.. ..
..
..
..
..
-
HSSO, HNO, (2 1)s 11.1
.. .. .. ..
..
+
0.30 0.19 29.7 16.8 8.6 32.6 6.2
6.2 0.9
HSSO, - HNO, - HCl (10 6 2)t 12.6 0.31 0.33 29.3 16.6 8.7 36.7 6.7
+ +
6.2 0.9
I
Difference, per cent. 11.2 3.2
+ + +42-4
- 1.4 - 1.8
+ 1.1
+9.0 +4-6 0 0
* Method of Iskandar et al.' t Present method.
varies with the concentration of mercury as follows: 14, 2 and 2 per cent. for 0.1, 0.6 and 2 mg kg-1 of mercury, respectively, levelling off at values of about 2 per cent. above this concentration range. Thus, from Table I11 it can be seen that the first, third, seventh and eighth samples statistically show higher levels of mercury by the proposed method. This evidence is consistent with the hypothesis that in these samples some of the mercury is in the form of mercury(I1) sulphide. In the remaining samples there is again, apparently, no mercury(I1) sulphide in spite of the high sulphide content. Therefore, the above results show that use of the proposed method is advantageous and convenient for the safe digestion of a large number of samples. In addition, the proposed method is directly applicable, without modification, to the digestion of organic and biological tissues, We have not shown recoveries for organomercury compounds because both Iskandar et aL2 and Jacobs and KeeneyS have shown that with their methods the mercury in these compounds can be successfully recovered. Their methods represent the two extremes of the proposed method, that is, our method is intermediate between the two digestion techniques and was found to give similar recoveries.
Conclusion A digestion method applicable to biological and geological samples for the extraction of mercury has been shown to enable mercury to be satisfactorily recovered from mercury(I1) sulphide. The method permits the decomposition of all forms of mercury and is simple, rapid, safe and adaptable to the automated cold-vapour and flame atomic-absorption techniques for the determination of mercury. The authors thank Y. K. Chau for his comments on the original manuscript, I. R. Jonasson and K. I. Aspila for supplying the geological samples and S. Scott for her secretarial help.
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EXTRACTION O F MERCURY FROM ENVIRONMENTAL SAMPLES
95
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References 1. Agemian, H., and Chau, A. S. Y., Analytica Chim. Acta, 1975, 75, 297. 2. Iskandar, I. K., Syers, J. K., Jacobs, L. W., Keeney, D. R., and Gilmour, J . T., Analyst, 1972, 97, 388. 3. Jacobs, L. W., and Keeney,‘D. R., Envir. Sci. Technol., 1974, 8, 267. 4. Jonasson, I. R., Lynch, J. J., and Trip, L. J., Geol. Surv. Pap. Can., 1973, Paper 73-21. 5. Lingane, J. J., “Analytical Chemistry of Selected Metallic Elements,” Reinhold Publishing Corporation, New York, 1966. 6. Agemian, H., Aspila, K. I . , and Chau, A. S. Y . ,Analyst, 1975, 100, 253. 7. Uthe, J . F., Armstrong, F. A. J., and Stainton, M. P., J. Fish. Res. Bd Can., 1970,27, 806. 8. “Analytical Methods for Atomic Absorption Spectrophotometry,” Perkin-Elmer Corp., Norwalk, Connecticut, 1973. 9. Polley, D., and Miller, V. L., Analyt. Chem., 1966, 27, 1162. 10. Thorpe, V. A., J . Ass. Off.Analyt. Chem.. 1971, 54, 206. 11. Jeffus, M. T., and Elkins, T. S., J . Ass. 08. Analyt. Chem., 1970, 53, 1172. 12. Hatch, W. R., and Ott, W. L., Analyt. Chem., 1968, 40, 2085. 13. Head, P. C., and Nicholson, R. A., Analyst, 1973, 98, 63. 14. Cranston, R. E., and Buckley, D. E., Envir. Sci. Tcchnol., 1972, 6, 274. 16. Melton, J. R., Hoover, W. L., and Howard, P. A., Proc. Soil Sci. SOC.Am., 1971, 35, 850. 16. “Mercury in the Environment,” Prof.Pap. U.S. Geol. Surv., 1970, Paper 713. 17. Konrad, J . G., Research Report 74, Wisconsin Department of Natural Resources, Madison, Wis., 1971. Received March 24th, 1976 Accepted August 27th, 1976
Analyst, Febmary, 1976, Vol. 101, pp. 91-95
91
Published on 01 January 1976. Downloaded by University of Central Florida on 28/10/2014 13:37:00.
An Improved Digestion Method for the Extraction of Mercury from Environmental Sampies Haig Agemian and A. S. Y. Chau Canada Centre for Inland Waters, Watev Quality Branch, P.O. Box 5050, 867 Lakeshore Road, Burlington, Ontario, L7R 4A6, Canada
An improved digestion procedure for the extraction of mercury from environmental material is reported. The method involves the digestion of the sample at 60 "C with sulphuric acid - nitric acid (2 l ) , containing a trace amount of hydrochloric acid, and subsequent oxidation with permanganate and persulphate solutions. With this procedure mercury is successfully recovered from organic matter and resistant inorganic forms such as mercury(I1) sulphide. Unlike digestion with aqua regia, this procedure is simple and safe, and is applicable to the digestion of a large number of samples simultaneously. The method can be adapted to the automated cold-vapour and flame atomic-absorption techniques and is therefore ideal for routine monitoring.
+
Agemian and Chaul reported a method for the determination of mercury in sediments, in which a comprehensive manual digestion method developed by Iskandar et aL2 was adapted to the automated cold-vapour atomic-absorption technique. The extraction technique involved the digestion of the sediment with sulphuric acid - nitric acid (2 1) at 60 "C and subsequent oxidation of the organomercury compounds with permanganate and persulphate solutions. Complete recovery2 of several organomercury compounds was obtained by using this technique. Jacobs and Keeneys showed that this procedure does not adequately dissolve mercury( 11) sulphide (cinnabar), which may be formed under reducing conditions. They suggested treating the sediment with boiling aqua regia. However, we found that this treatment is hazardous, impractical and unnecessary for routine analysis of a large number of samples. Moreover, Jonasson et aL4 have reported that the presence of excessive amounts of hydrochloric acid should be avoided as the consequent generation of chlorine5 may cause the loss of volatile mercury chlorides. They proposed the use of a solution of nitric acid plus a trace amount of hydrochloric acid, which technique enables mercury sulphides to be satisfactorily dissolved, and is suitable for application to geological samples. Agemian et aL6 showed the adaptability of both sulphuric acid - nitric acid (2 1) and hydrochloric acid - nitric acid (1 9) digestion mixtures to the automated cold-vapour atomic-absorption technique for mercury determination. They compared these extraction methods for a number of rock, soil and sediment samples and obtained identical results, presumably owing to the absence of cinnabar in the samples analysed. Samples that contain substantial amounts of this mercury compound are not commonly analysed in this laboratory. However, in this study, the intention was to obtain a comprehensive method that is applicable to all types of samples, including those containing mercury in forms such as mercury(I1) sulphide. Cinnabar is resistant to attack by sulphuric and nitric acids.* However, the inclusion of the minimum4 amount of hydrochloric acid promotes its rapid decomposition. Thus, a slight modification of the digestion method of Iskandar et aZ.,2by the addition of a trace amount of hydrochloric acid, enables large amounts of mercury(I1) sulphide, at levels much higher than are found in natural samples, to be recovered. The proposed method is simple, rapid and adaptable to the determination of all organic and inorganic forms of mercury in a large number of samples by the automated cold-vapour atomic-absorption technique. The method can be suited to the digestion of biological (fish)' or geological (rocks, soils and sediments) samples by using the procedure with or without the inclusion of hydrochloric acid. The extraction medium is especially suitable for sediments and soils as its oxidising nature enables the large amounts of organic matter frequently found in these samples to be oxidised;
+
+
+
View Article Online
92
AGEMIAN AND CHAU : AN IMPROVED DIGESTION METHOD FOR THE
A n a l y s t , VoZ. 101
Published on 01 January 1976. Downloaded by University of Central Florida on 28/10/2014 13:37:00.
also, the strongly acidic medium, in the presence of a trace amount of hydrochloric acid, extracts inorganic forms of mercury including that found in cinnabar.
Experimental Apparatus Samples were digested in 100-ml calibrated flasks in a temperature-controlled shaker bath (Precision Scientific Co. , Model 75). The equipment used for the automated cold-vapour analysis consisted of ( a ) an automatic sampler (Technicon AutoAnalyzer I1 sampler with 20-1/5 cam) ; ( b ) a proportionating pump (Carlo Erba, Model 08-59-10202); (c) Technicon AutoAnalyzer tubing of specified dimensions ; ( d ) a gas separator, as used by Agemian and Chaul; ( e ) a mercury monitor (Pharmacia Fine Chemicals) ; and (f) a strip-chart recorder (Hewlett-Packard, Model 7101B). The system is similar to that used by Agemian and co-workers.l,s For high levels of mercury, a Perkin-Elmer 503 atomic-absorption spectrophotometer, equipped with an Intensitron lamp, was used with an air - acetylene flame. Reagents High-purity certified reagents were used for all analyses. Sulphuric acid, 36 N. Nitric acid, 16 N. Hydrochloric acid, 12 N. Potassium permanganate solution, 6 p e r cent. m/V. Potassium persulphate solution, 5 per cent. m/V. Tin(II)sulphate solution, 10 p e r cent. m/V in 2 N sulphuric acid. H y d r o x y l a m m o n i u m sulphate (6 per cent. m/V) - sodium chloride (6 per cefzt. m/V) solution. Procedure Determine the water content of the wet sediment by drying it overnight t o constant mass at 110 "C. Weigh a representative sample of wet sediment, equivalent to 0.1-2 g of the dry mass, into a 100-ml calibrated flask. Wash the sediment down to the bottom of the flask with mercury-free distilled water, place the flask in an ice - water bath and slowly add 15 ml of sulphuric acid - nitric acid (2 1). After cooling, add 2 ml of hydrochloric acid. Shake the mixture and let it stand for 5 min. After expelling the acid fumes from the flask, place it in a shaking water-bath at a temperature of 50-60 "C and digest for 2 h. Then allow the flask to cool for 30 min and carefully add 10 ml of potassium permanganate solution while cooling in an ice - water bath. If the colour does not persist for 15 min, add a further amount of potassium permanganate solution. After 30 min, add 5 ml of potassium persulphate solution, with gentle stirring, and allow the mixture to stand overnight. If all of the permanganate is reduced, as witnessed by the absence of the purple colour, add potassium permanganate solution until the colour persists. Add 10 ml of hydroxylammonium sulphate - sodium chloride solution and stir the mixture gently until the solution becomes clear and all of the precipitated manganese(1V) oxide has dissolved. Make the solution up to volume and centrifuge an aliquot a t 2500 rev min-l for 5 min. Transfer an aliquot of the clear supernatant liquid into a glass sample cup and place it in the automatic sampler for analysis. Use a cam designed for 20 samples per hour and a sample to wash ratio of 1 : 5 , corresponding to a sampling time of 30 s and a wash time of 2.5 min. For concentrated solutions of mercury use the flame technique. The linear concentration range in solution is 0.0002-0.006 mg 1-l for the non-flame detection system1 and 10-300 mg 1-1 for the flame detection system.8 For sample concentrations of 0-1-2 g per 100 ml, the non-flame detection system has a range of 0.01-6 mg kg-l and the flame method 5-300 mg k g l of mercury in the sediment. Further dilution of the extracts could extend the upper limit of both detection systems. Treat all standards exactly as for the above samples.
+
Results and Discussion Sulphuric acid is used extensively for the extraction of mercury from biological materials, either alone7,Bs10 or with nitric acid.ll It has also been used separatelyll-l3 or together with
View Article Online
Published on 01 January 1976. Downloaded by University of Central Florida on 28/10/2014 13:37:00.
February, 1976
93
EXTRACTION OF MERCURY FROM ENVIRONMENTAL SAMPLES
nitric acid2J4J5 for sediments and rocks. Iskandar et aL2 also showed that a mixture of sulphuric and nitric acids is necessary for the adequate digestion of organic matter in sediments and soils and for the subsequent liberation of mercury. As already indicated, the presence of hydrochloric acid is necessary for the decomposition of cinnabar. Initially, an attempt was made to use large amounts of hydrochloric acid, as suggested by Jacobs and K e e n e ~ but , ~ the procedure proved to be very impractical and difficult and gave rise to low recoveries of mercury from sediments. The use of a fume hood in which to perform the digestion is essential when hydrochloric acid is heated, even to 60 "C: Further, on adding potassium permanganate in order to oxidise the organomercurials, violent frothing of the solution occurs, with evolution of chlorine, which makes the analysis very difficult. In addition, the decompositions of the permanganate to manganese(1V) oxide caused by the hydrochloric acid, renders it ineffective as an oxidant for organomercurials, and a low recovery of mercury may therefore result. We found that as little as 5 m l of hydrochloric acid caused the rapid decomposition of 25 ml of a 5 per cent. m/V potassium permanganate solution. Table I shows the effect of excess of hydrochloric acid on the recovery of mercury from sediments. The low results obtained (third column) are attributed to the loss of mercury due to violent frothing of the solution and possible volatilisation, and also to the reduced efficiency of oxidation of the organic matter by permanganate owing to its decomposition by hydrochloric acid. The analyses were performed by using the automated method reported by Agemian and coworkers.lt6 The samples examined were ordinary samples as analysed in our laboratory and therefore did not give any indication of the presence of cinnabar. Comequently, the method of Iskandar et aL2 gave results for recovery that are similar to those given by the proposed met hod. TABLEI EXTRACTION OF MERCURY FROM SEDIMENT SAMPLES BY USING DIFFERENT EXTRACTION MEDIA
Results are expressed in pg kg-1. Extraction medium 7
Sample Silt Silty clay Clay Clay Clay
.. .. .. .. ..
.. .. ..
.. ..
-
HISO, - HNO, (2 I)* 310 600 970
€I,S04 - HNO, - HC1 (10 5 2)t 320 600 960
1000
1000 1600
+
+ +
1600
H,SO, (2
- HNO,
- HCf
+ 1 + 1): 250 450
600 800 1400
* Method of Iskandar et al., t Present method.
:Contains an excessive amount of hydrochloric acid.
I n order to check the efficiency of the method, sediment samples were spiked with mercury(I1) sulphide powder and the recovery of the mercury was determined. As only milligram amounts could reliably be weighed, the analysis had to be performed by the flame atomic-absorption method; the results obtained are given in Table I1 and show a satisfactory recovery of 100 & 5 per cent. of mercury added as mercury(I1) sulphide. The small amount of hydrochloric acid added in the digestion procedure causes the decomposition of the cinnabar to become visually apparent, the disappearance in a few seconds of the bright red colour indicating its complete dissolution.
TABLE I1 RECOVERY OF
MERCURY FROM MERCURY(II) SULPHIDE BY THE PROPOSED METHOD
Mercury added (as HgS)/g Recovery, per cent. . .
.. ..
..
..
0.0018 102
0.007 1 98
0.0103 105
0.0222 95
0.029 1 101
The levels of the spikes used for the recovery tests are much higher than would be found in any real samples, the mercury contents16 of most uncontaminated solid earth materials being in the range 10-500 ng k g l . Based on l-g samples, the results shown in Table I1 correspond
View Article Online
Published on 01 January 1976. Downloaded by University of Central Florida on 28/10/2014 13:37:00.
94
AGEMIAN AND CHAU: AN IMPROVED DIGESTION METHOD FOR THE
Analyst, Vol. 101
to levels of lo00 mg k g 1 and above, which are much higher than those reported for most contaminated samples; Konradl' reported levels of 700-800 mg k g l for samples from chloralkali plants in Wisconsin. Therefore, the satisfactory recovery of amounts of mercury that are much larger than those encountered in highly contaminated samples indicates that the proposed method is adequate. As confirmed by Jacobs and Keeney,a use of the method of Iskandar et aL2 did not permit the recovery of any of the metcury introduced as mercury(I1) sulphide, which was completely resistant to the sulphuric acid - nitric acid mixture. Table I11 shows the effect of hydrochloric acid on the recovery of mercury from geological samples that contain large amounts of sulphide. The results given in this table show the superiority of the proposed method, using hydrochloric acid, for determining low levels of mercury. The precisions of the two methods are similar and the coefficient of variation6
TABLE I11 EXTRACTION OF MERCURY
FROM GEOLOGICAL SAMPLES THAT HAVE HIGH SULPHIDE CONTENTS
Results are expressed in mg kg-l; all samples were analysed in triplicate. Extraction medium , Sample Sphalerite .. Copper concentrate Pyrite concentrate Ore head Tailing . . .. Tailing . . .. Lead concentrate Ore head Copper concentrate Oxidised tailing
.. ..
..
..
..
..
-
HSSO, HNO, (2 1)s 11.1
.. .. .. ..
..
+
0.30 0.19 29.7 16.8 8.6 32.6 6.2
6.2 0.9
HSSO, - HNO, - HCl (10 6 2)t 12.6 0.31 0.33 29.3 16.6 8.7 36.7 6.7
+ +
6.2 0.9
I
Difference, per cent. 11.2 3.2
+ + +42-4
- 1.4 - 1.8
+ 1.1
+9.0 +4-6 0 0
* Method of Iskandar et al.' t Present method.
varies with the concentration of mercury as follows: 14, 2 and 2 per cent. for 0.1, 0.6 and 2 mg kg-1 of mercury, respectively, levelling off at values of about 2 per cent. above this concentration range. Thus, from Table I11 it can be seen that the first, third, seventh and eighth samples statistically show higher levels of mercury by the proposed method. This evidence is consistent with the hypothesis that in these samples some of the mercury is in the form of mercury(I1) sulphide. In the remaining samples there is again, apparently, no mercury(I1) sulphide in spite of the high sulphide content. Therefore, the above results show that use of the proposed method is advantageous and convenient for the safe digestion of a large number of samples. In addition, the proposed method is directly applicable, without modification, to the digestion of organic and biological tissues, We have not shown recoveries for organomercury compounds because both Iskandar et aL2 and Jacobs and KeeneyS have shown that with their methods the mercury in these compounds can be successfully recovered. Their methods represent the two extremes of the proposed method, that is, our method is intermediate between the two digestion techniques and was found to give similar recoveries.
Conclusion A digestion method applicable to biological and geological samples for the extraction of mercury has been shown to enable mercury to be satisfactorily recovered from mercury(I1) sulphide. The method permits the decomposition of all forms of mercury and is simple, rapid, safe and adaptable to the automated cold-vapour and flame atomic-absorption techniques for the determination of mercury. The authors thank Y. K. Chau for his comments on the original manuscript, I. R. Jonasson and K. I. Aspila for supplying the geological samples and S. Scott for her secretarial help.
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February, 1976
EXTRACTION O F MERCURY FROM ENVIRONMENTAL SAMPLES
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