Comparison of digestion procedures for the determination of mercury in soils by cold-vapour atomic absorption spectrometry

Analytlca Chcmwa Acta, 209 (1988) 147-156 Elsevier Science Publishers B.V., Amsterdam -

147 Printed in The Netherlands

COMPARISON OF DIGESTION PROCEDURES FOR THE DETERMINATION OF MERCURY IN SOILS BY COLD-VAPOUR ATOMIC ABSORPTION SPECTROMETRY

W. VAN DELFT* and G. VOS State Institute for Quality Control of Agricultural Products, P.O. Box 230, NL-6700 AE Wageningen (The Netherlands) (Received 1st April 1987)

SUMMARY Five digestion procedures were investigated for the determination of mercury in soils by coldvapour atomic absorption spectrometry. These methods included three acid leaching procedures in open systems and two acid digestion procedures in closed decomposition vessels. The closed vessels were heated in a conventional laboratory oven or a laboratory microwave oven. In the open systems, digestion with concentrated acids at elevated temperatures led to considerable losses of (organo)mercury compounds, while digestion at ambient temperature gave incomplete oxidation of the sample matrix. To prevent any losses of mercury and to obtain complete oxidation, the use of a closed decomposition system at elevated temperatures appeared to be necessary. The results obtained with these closed systems were in good agreement with those obtained by neutron activation analysis. Heating in a microwave oven appeared to be a considerable improvement over conventional thermal heating.

International cooperation to stop or decrease the pollution of the environment by toxic trace elements like cadmium, lead, arsenic and mercury is still growing. Extensive research is being conducted to establish the extent of the contamination and to study the behaviour of these elements in the food chain. To support those studies, reliable methods of analysis must be available. Methods for the determination of traces of mercury were reviewed by Chilov [ 11. Absorption of ultraviolet radiation by mercury is the basis of direct spectrophotometric and atomic absorption spectrometric (8.8.5. ) procedures which have generally replaced the classical dithizone (spectrophotometric) procedure. Neutron activation analysis requires expensive equipment, access to an irradiation source and may be time-consuming. Because of the lower sensitivity and the longer irradiation times required, non-destructive neutron activation is suitable only for higher mercury contents. In the earlier literature concerning the determination of mercury in different matrices by 8.8.5. [ 21, the major part deals with the optimization and appli-

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cation of the cold-vapour technique introduced by Hatch and Ott [ 31. In most of these methods, the mercury is removed from the sample solution, after reduction to the metal with tin(I1) sulphate or chloride, by passing a stream of air or inert gas through it. The mercury vapour is carried into the absorption measuring cell and is then recirculated through the system or directly passed to exhaust. A disadvantage of the use of sodium tetrahydroborate as reductant is the interference caused by the presence of metals like nickel, copper and lead [4,5]. When tin(I1) sulphate or chloride is used, metals which are less easily reduced than mercury have little or no influence on the analysis. The interference by noble metals can be considerable [ 2,6,7]. In practice, however, these metals are seldom present in soil in interfering amounts. Preceding the determination of total mercury in soils by cold-vapour a.a.s., sample decomposition has to be done in such a way that the mercury present in the sample is completely brought into solution and that there are no losses of mercury. The latter is of particular importance because of the volatile character of mercury and its compounds. In top-soil, mercury is mainly bound to organic material. Many different acids or acid mixtures have been used for the digestion of soil samples followed by determination of mercury by cold-vapour a.a.s.; nitric acid [ 2,5,8-111, nitric acid/sulfuric acid [ 2,6,12-151, nitric acid/hydrochloric acid [ 2,7,8,11,16,17], often followed by oxidation with potassium permanganate or peroxodisulfate, because acid digestion alone has been shown to give incomplete recovery [ 13,141. In some methods, bromine is used to stabilize the solution; the excess of bromine is reduced with hydroxylammonium chloride just before the solution is used for cold-vapour a.a.s. However, analysis for mercury of sample solutions with high bromide concentrations can result in much decreased peak heights [ 6,181. The acid digestion procedures differ in the time and temperature used as well as whether the digestion is done in an open system under reflux, or under pressure in a closed decomposition vessel. In the literature, opinions expressed about the different digestion procedures is often contradictory. Other digestion procedures are impractical and timeconsuming, and require a large amount of glassware and other equipment. In the study reported in this paper, five digestion procedures were investigated. The results obtained for different soil samples were compared with the data obtained by destructive neutron activation. In addition, recovery experiments were conducted, samples were spiked with mercury(I1) nitrate as well as with the volatile methylmercury chloride. Suitable reference samples are not available, because they are in general pretreated in such a way that only the stable non-volatile mercury compounds are present [ 19,201. The analysis of reference samples alone, therefore, can be no criterion in the judgement of a digestion procedure for the determination of mercury in soils.

149 EXPERIMENTAL

Because of the volatility of most mercury compounds, analytical problems arise directly after sampling. To prevent loss of mercury the sample must be dried at a temperature as low as possible. Iskandar et al. [ 131 reported losses, particularly of organomercury compounds which were added to sediment, after drying at 60°C for 16 h of 0.2% mercury (added as mercury(I1) chloride), 16.8% (added as methylmercury chloride), 11.7% (added as phenylmercury hydroxide), and 2.3% (added as phenylmercury acetate). The samples investigated in this study were made available by the Institute for Soil Fertility (Haren, The Netherlands). Samples were pretreated according to the directions published by this Institute [ 211, which included drying at 40” C, breaking up of the mass and sieving through an aluminium sieve (2 mm); only the fraction passing through the sieve was used. All chemicals were of the highest purity available. All solutions were prepared with Millipore Mini-Q water. For the recovery experiments described in this study, the samples were spiked with aqueous solutions of mercury (II) nitrate and methylmercury chloride. Before use, all glassware was soaked in (1+ 1) nitric acid (at least overnight) and then rinsed several times with MilliQ water. Neutron activation analysis The neutron activation analyses were done by the Department of Analytical Chemistry, Division of Technology for Society, TN0 (Delft) by order of the Institute for Soil Fertility. The procedure has been described in detail by Tjioe et al. [ 221. The dried, pulverized and sieved samples were sealed in quartz vials and irradiated for 12-24 h; the thermal neutron flux was 1.0~ 1013 n cmP2 s-‘. After irradiation, the samples were digested in a half-closed system with concentrated sulfuric acid and 30% hydrogen peroxide. The volatile elements (arsenic, selenium, antimony, mercury and gold) were separated by distillation with hydrobromic acid. The distillates were passed through Dowex 2 x 8 anionexchanger columns, and mercury was separated from bromide, arsenic, selenium and antimony by sequential elution with hydrobromic acid, 4 M hydrochloric acid containing 0.2% hydrogen peroxide and 3% sodium sulfate solutions, respectively. Digestion procedures Method 1. Open digestion with nitric acid. Method IA, a 0.5-g portion of soil was transferred to a long-necked glass digestion tube, marked at a volume of 40 ml. Subsequently, 7 ml of concentrated nitric acid was added and the sample was digested for 3 h at 95-100’ C in a VA0 automatic wet digestion device [ 231

150

(Hans Kurner Analysentechnik, Rosenheim, F.R.G.). After cooling to ambient temperature, the digest was diluted with water to the mark and homogenized. Method 1B was similar to Method lA, but after cooling to ambient temperature, 20 ml of water and 1 ml of 1% (w/v) hydroxylammonium chloride solution were added to the digest. After a few minutes, the mixture was diluted to the mark with water and mixed. Method 2. Open digestion with hydrochloric acid. A l-g portion of soil was transferred to a lOO-ml Erlenmeyer flask. After the addition of 30 ml of 3 M hydrochloric acid, the flask was heated on a boiling water bath for 3 h. After cooling to ambient temperature, the digest was transferred to a lOO-ml volumetric flask, diluted to the mark with water and mixed. Method 3. Open digestion with nitric acid, sulfuric acid and potassium peroxodisulfate. A l-g portion of soil was transferred to a lOO-ml volumetric flask, and 10 ml of concentrated nitric acid, 10 ml of concentrated sulfuric acid and 10 ml of a saturated potassium peroxodisulfate solution were added slowly (and carefully). The mixture was allowed to stand for 1 h at ambient temperature, with occasional swirling. The digest was diluted to the mark with water and mixed. Method 4. Closed digestion with nitric acid in a teflon bomb. A 0.2-g portion of soil was transferred to the 23-ml teflon vessel of a stainless-steel decomposition device (Eriks, Alkmaar, The Netherlands); 3 ml of concentrated nitric acid was added, the teflon lid was placed in position and the bomb was screwed together in the usual way. The bomb was heated for 3 h at 140’ C in a laboratory oven (type T5042E, Heraeus) , equipped with a thermostat, and then cooled to ambient temperature in order to prevent losses of mercury when opening. The digest was transferred to a 50-ml volumetric flask. After addition of 1.25 ml of 10% (w/v) hydroxylammonium chloride solution, the mixture was diluted to the mark with water and mixed. Method 5. Closed digestion with nitric acid and hydrochloric acid in a microwaue oven. A 0.5-g portion of soil was transferred to a special 120-ml teflon vessel. After the addition of 10 ml of water, 6 ml of concentrated hydrochloric acid and 2 ml of concentrated nitric acid, a pressure relief valve was installed and the vessel was closed with a teflon screw cap (see later). Up to 12 vessels were placed on a rotating turntable in the microwave oven for 1 min at 30% power, 4 min at 80% power, and 10 min at 100% power (600 W ). A laboratory microwave oven (model MDS-81, CEM Corp., Indian Trail, NC) was used. After cooling to ambient temperature, the digest was transferred to a 50-ml volumetric flask, diluted to the mark with water and mixed. Determination of mercury by cold-vapour atomic absorption spectrometry A mercury monitor (Laboratory Data Control, Riviera Beach, FL) was used. The system contains a measuring cell and a reference cell, each 30 cm long and 0.75 cm in diameter. The absorbance was measured at 253.7 nm and recorded

151 with a Kipp (BD 140) chart recorder. A glass reduction/aeration apparatus was used, comprising a 3-way stop-cock, a gas inlet and outlet tube for passing nitrogen through a 12-ml test tube, with a 14/23 joint which held the sample solution. Between the gas outlet and the measuring cell, a drying tube filled with magnesium perchlorate was inserted. Behind the measuring cell, a scrubber filled with potassium permanganate in diluted sulfuric acid was installed to trap mercury vapour. Connections were made with silicone rubber tubing. After sediment in the sample solution had settled, 2 ml of the supernatant solution was pipetted into a test tube, 0.5 ml of reducing solution (5 g of tin (II) chloride and 3 g of hydroxylammonium chloride in 100 ml of 1 M sulfuric acid) was added, and the test tube was installed in the system as quickly as possible. The 3-way stopcock was turned after 30 s so that a stream of nitrogen (7.5 ml min-‘) passed from the reference cell through the solution and drying tube into the measuring cell. After the recorder pen had passed its maximum deflection and had returned to about half the maximum deflection, the test tube was replaced by an empty tube to prevent rapid saturation of the magnesium perchlorate in the drying tube. When the recorder pen had returned to the baseline, the nitrogen was by-passed by turning the stopcock; the empty tube was removed and the gas inlet washed. The apparatus was then ready for the next sample. Each measurement was made at least in duplicate. Average peak heights were converted to concentration units with the aid of a calibration graph, which was linear over the range O-4 ng of mercury. Sample solutions containing more than 4 ng of mercury were diluted with a blank solution. Generally, the absorbance of a blank solution was indistinguishable from the baseline noise. RESULTS AND DISCUSSION

Twenty-two soil samples including marine and river clay, peat and reclaimed moorland soil were digested at least in duplicate according to the above digestion procedures. Subsequently, the mercury content was also determined by cold-vapour a.a.s. The mercury content of the samples was also determined by the neutron activation method. The results of these experiments are listed in Table 1. The results of the recovery experiments for Methods 1B and 4 are listed in Table 2. The different samples were spiked with mercury (II) nitrate or methylmercury chloride before digestion. For the digestion with nitric acid (Method 1A) and subsequent determination of the mercury content by cold-vapour a.a.s., low results were obtained for all soil samples analysed. This confirms the observations of Iskandar et al. [13]and Ure and Shand [14] who found, in contrast to Jones and Hinesly [ 91, that nitric acid alone gave incomplete oxidation and low recovery. Iskandar et al. [ 131 also reported frothing difficulties when aerating the test solution. The addition of hydroxylammonium chloride directly after digestion

152 TABLE 1 Average (n> 2) mercury contents (mg kg-‘) of soil samples determined by destructive neutron activation analysis and cold-vapour atomic absorption spectrometry after the application of various digestion procedures (Methods l-5) Type of soil

River clay 1 2 3 4 5 6 7 8 9 10 11 12 Marine clay 13 14 15 16 17 Peat 18 19 20 21

Neutron activation analysis

Mercury content (mg kg-‘) 1A

1B

2

3

4

5

4.9

0.96

0.15 0.20 0.08 0.38 0.06 0.15 0.05 0.18 0.57 0.12 1.14

0.84 0.15 0.29 0.07 0.33 0.06 0.14 0.05 0.17 0.59 0.12 1.14

0.80 0.14 0.18 0.06 0.35 0.06 0.12 0.06 0.17 0.48 0.12 1.03

0.88 0.15 0.20 0.08 0.43 0.07 0.16 0.05 0.18 0.59 0.14 1.16

0.95

0.17 0.23 0.08 0.42 0.08 0.19 0.07 0.22 0.61 0.15 1.23

0.83 0.14 0.19 0.06 0.31 0.06 0.13 0.05 0.15 0.53 0.11 1.13

0.89

2.0 1.5 3.4 10.1 1.5 7.6 2.5 4.6 2.9 1.7 5.0

0.17 0.66 0.11 1.15

2.5 2.9 1.2 1.6 2.4

0.05 0.07 0.12 0.26 0.11

0.05 0.05 0.09 0.26 0.07

0.05 0.06 0.11 0.30 0.09

0.04 0.06 0.10 0.28 0.09

0.05 0.06 0.11 0.27 0.09

0.05 0.07 0.10 0.31 0.08

0.04 0.08 0.12 0.33 0.08

20.3 53.7 21.1 13.0

1.06 0.26 0.11 0.04

0.52 0.15 0.06 0.04

0.70 0.15 0.06 0.03

0.89 0.16 0.08 0.03

0.34 0.22 0.08 0.04

0.93 0.26 0.10 0.04

0.91 0.21 0.09 0.04

0.16 0.02

0.13 0.03

0.12 0.02

0.22 0.02

0.20 0.02

0.21 0.03

0.18 0.02

0.99

0.97

0.99

0.88

0.99

1.05

Organic matter (%)

Reclaimed moor 22 25.3 23 8.8 IAEA Soil 5

0.16 0.21 0.06 0.42 0.07 0.15

(Method 1B ) decreased the frothing problem. This also gave higher mercury recoveries, but, particularly for the peat and reclaimed moorland soil samples, the results were still low. Further oxidation of the digest with potassium permanganate and dichromate gave no improvement. The results of the recovery experiments listed in Table 2 showed that considerable losses of organomercury occurred during digestion with nitric acid. For samples with a high organic matter content (peat and reclaimed moorland soil), the losses were indepen-

153 TABLE 2 Recovery (% ) of mercury(I1) (mean +s.d., ns4) Type of soil

River and marine clay Peat and reclaimed moor

nitrate and methylmercury chloride added to different soils

Method 1B

Method 4

Hg(N0312

CH,HgCl

Hg(NO&

CH,HgCl

102f6 82f6

79+8 81f8

105+_7 105k7

98k7 107f4

dent of whether the samples were spiked with mercury (II) nitrate or methylmercury chloride. The results for digestion with hydrochloric acid (Method 2) are similar to those obtained by digestion with nitric acid (Method 1B). Both methods must be rejected because losses of mercury are very likely to occur when the samples are heated with concentrated acids in an open system. This is particularly the case with organic soils, such as peat and reclaimed moorland soil samples. Melton et al. [ 121 introduced a simple and fast method in which the soil samples were leached at ambient temperature with a mixture of equal volumes of concentrated nitric acid, concentrated sulfuric acid and saturated potassium peroxodisulfate solution. Because permanganate appeared to be unable to oxidize some organomercury compounds, peroxodisulfate was used. The authors also reported signal suppression by frothing when digests of organic soils such as peat were aerated. Ure and Shand [ 141 reported incomplete oxidation. The results listed in Table 1 (Method 3) also show that in this study unacceptably low results were obtained for this method. Signal suppression by frothing is indeed the cause of the extreme low mercury content found in sample 18 (peat ) . Addition of mercury(I1) nitrate or methylmercury chloride resulted in a recovery of 105% or 51%, respectively. Because volatilization of mercury compounds is not very likely at ambient temperature, incomplete oxidation of organically bound mercury must be the cause of the low results obtained with this method. Applications of a closed teflon vessel (bomb) for the decomposition of different matrices and subsequent determination of trace elements are widespread. In most cases, nitric acid or a mixture of nitric and hydrochloric acids is used for the digestion. Total decomposition of the siliceous matrix with hydrofluoric acid is not necessary for mercury determination in soils. Because of its large ionic radius, mercury does not form natural silicate minerals [ 71. The results listed in Table 1 show that the values for the mercury contents obtained after the digestion with nitric acid in the teflon bomb (Method 4) are in good agreement with those obtained by neutron activation. Comparison by linear regression of the results of the bomb digestion/cold-vapour 8.8.8. method with

154

those obtained by neutron activation yielded a plot with a slope of 0.929 2 0.013. The results of the recovery experiments for Method 4 are given in Table 2. For all types of soils analysed, the recovery of both inorganic and organic mercury was between 90 and 110%. This shows that no losses of mercury occur when samples are heated with concentrated acid in a closed system. One of the major disadvantages of the use of the bomb method is the limitation of the sample weight to 0.1-0.2 g of organic matter. This limitation requires extra attention to the homogeneity of the samples. For replicate analysis (n= 5) of sample 18 (peat) and the finely ground reference soil sample (IAEA Soil 5), the standard deviations were 7.5% and 7.1%, respectively, for a sample weight of 0.2 g. The results for all samples analysed with a mercury content greater than 0.1 mg kg- ’ (n = 15 ) showed an average standard deviation of 7.9%. The repeatability of the method, therefore, is satisfactory, which indicates that sufficiently homogeneous samples can be obtained for at least some types of soil. Weighing wet samples combined with a separate determination of the moisture content can increase considerably the standard deviation of the method [ 161. Analytical applications of microwave dissolution have been reported [24271. They show a significant improvement over conventional thermal heating. The microwave action generates an improved sample/acid contact that results in more extensive and efficient dissolution. The oven used here has a variable power range to 100% full power (600 W) in 1% increments and a variable time range of < 99 h. A maximum of three sequential steps of varying power and time intervals can be programmed. The teflon-coated cavity has a variablespeed exhaust system, which eliminates the problem of acid fumes attacking the electronics of the oven and damaging the magnetron. The magnetron is also protected electronically against reflected microwave energy. Up to twelve teflon sample vessels can be placed on the rotating turntable. A patented relief valve is installed between the vessel and the screw cap. It responds to both temperature and pressure inside the vessel. A collection vessel is connected to the sample vessels through teflon tubes. When the valves release, vapours are vented and condensed in the collection vessel. The programmable power range and the construction of the vessels with a relief valve allow some control over the regulation of the pressure inside the vessel. Generally the digestion was done in a closed system, preventing any losses of (organo)mercury. With the applied power/time program, the valves released only occasionally at the very end of the digestion step. The mercury contents obtained (Table 1, Method 5) are in good agreement with those obtained with the bomb digestion procedure (Method 4) and by the neutron activation method. The results of the two methods correlated well, giving a linear graph with a slope of 0.941? 0.018. In spite of the larger sample used compared to the bomb digestion procedure (0.5 vs. 0.2 g), only a twelfth of the time required for the bomb digestion was needed for the microwave dissolution.

155

Conclusions For the determination of mercury in soils, accurate results can be obtained by digestion in a closed system followed by cold-vapour a.a.s. detection. Digestions in a closed system can be done by using a teflon-lined digestion bomb with conventional heating or by the application of microwave dissolution. The soil mercury concentrations found by these methods are in close agreement with those obtained by neutron activation analysis. Complete recoveries are obtained for both organic and inorganic mercury. The application of a microwave oven appears to be a considerable improvement over conventional heating, especially because of the short time required for complete digestion. Digestion with concentrated acids in open systems results in considerable losses of (organo)mercury compounds, particularly for samples with a high organic matter content. These losses are caused by volatilization of (organo)mercury compounds at elevated temperatures or by incomplete oxidation of the sample matrix at ambient temperature. For the reference sample (IAEA soil 5) satisfactory results were obtained by all applied methods. This indicates that the analysis of reference samples can lead to erroneous conclusions about the selection of a method for the determination of mercury. The analysis of a wide range of natural samples by different methods is considered to be the best criterion for validating such a method. The authors are indebted to Beun de Ronde B.V. (Abcoude, The Netherlands) for placing the microwave digestion equipment at our disposal for several weeks. The Institute for Soil Fertility (Haren, The Netherlands) is gratefully acknowledged for the provision of soil samples, and for the determination of the organic matter content of these samples. The authors are indebted to Mrs. Hovens, Mr. Teeuwen, Mr. Horstman, Mr. Lammers and Mr. Keukens for their contributions to this study.

REFERENCES

2 3 4 5 6 7 8 9 10

S. Chilov, Talanta, 22 (1975) 205. A. M. Ure, Anal. Chim. Acta, 76 ( 1975 ) 1 (and references therein ) . W. R. Hatch and W. L. Ott, Anal. Chem., 40 (1968) 2085. N. Bernth and K. Vendelbo, Analyst, 109 (1984) 309. K. W. Riley, At. Spectrosc., 6 (1985) 76. G. Lindstedt, Analyst, 95 (1970) 264. A. Bartha and K. Ikrenyi, Anal. Chim. Acta, 139 (1982) 329. S. H. Omang and P. E. Paus, Anal. Chim. Acta, 56 (1971) 393. R. L. Jones and T. D. Hinesly, Soil Sci. Sot. Am., Proc., 36 (1972) 921. H. Nagase, Y. Ose, T. Sato, T. Ishikawa and K. Mitani, Int. J. Environ. Anal. Chem., 7 ( 1980) 285.

156 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

R. Breder, Fresenius’ Z. Anal. Chem., 313 (1982) 395. J. R. Melton, W. L. Hoover and P. A. Howard, Soil Sci. Sot. Am., Proc., 35 (1971) 850. I. K. Iskandar, J. K. Syers, L. W. Jacobs, D. R. Keeney and J. T. Gilmour, Analyst, 97 (1972) 388. A. M. Ure and C. A. Shand, Anal. Chim. Acta, 72 (1974) 63. M. D. K. Abo-Rady, Fresenius’ Z. Anal. Chem., 299 (1979) 187. R. L. Lutze, Analyst, 104 (1979) 979. H. B. MacPherson, A. P. Mykytink, S. S. Berman and D. S. Russel, Can. J. Spectrosc., 27 (1982) 132. S. H. Omang, Anal. Chim. Acta, 63 (1973) 247. V. Cheam and A. S. Y. Chan, Analyst, 109 (1984) 775. B. Griepink, H. Muntau and E. Colinet, Fresenius’ Z. Anal. Chem., 318 (1984) 490. Institute for Soil Fertility, Haren, The Netherlands, Report 5-81,1981, (in Dutch). P. S. Tjioe, J. J. M. de Goey and J. P. W. Houtman, J. Radioanal. Chem., 37 (1971) 511. G. Knapp, Fresenius’ Z. Anal. Chem., 274 (1975) 271. L. B. Fisher, Anal. Chem., 58 (1986) 263. R. T. White and C. E. Douthit, J. Assoc. Off. Anal. Chem., 68 (1985) 766. R. Blust, A. van der Linden and W. Decleir, At. Spectrosc., 6 (1985) 163. R. A. Nadkarni, Anal. Chem., 56 (1984) 2233.