METHODS FOR SAMPLING AND
INORGANIC ANALYSIS OF COAL
U.S. Geological Survey Bulletin 1823
Edited by D.W. Golightly and F.O. Simon
THE DETERMINATION OF MERCURY IN WHOLE COAL BY COLD VAPOR ATOMIC ABSORPTION SPECTROMETRY
By Philip J. Aruscavage and Roosevelt Moore
Concentrations of mercury in coal are determined by cold-vapor atomic absorption spectrometry. After decomposition of a 100-mg sample of pulverized coal in a mixture of perchloric and nitric acids, Hg II is reduced to the metal by stannous chloride. The mercury metal is removed from the solution in a stream of nitrogen and subsequently amalgamated with gold chips contained within a quartz tube that is located on the axis of a cylindrically wound induction coil. Rapid induction heating of the gold amalgam then releases the mercury, which is subsequently swept into a quartz cell where the peak absorption of Hg 1 253.652 nm is measured. The determination limit of the method is 10 ng/g. The typical precision for measurements of concentrations of mercury is 5- to 10-percent relative standard deviation. Good accuracy is observed for concentrations of mercury determined for the National Bureau of Standards reference materials 1632,1633, and 1633a.
Although numerous methods exist for the determination of mercury in whole coal and related materials, the methods incorporating cold-vapor atomic absorption spectrometry (AAS) are used in many laboratories (Babu, 1975; Doolan, 1982; Swaine, 1985). The differences between the various cold-vapor AAS methods principally involve the processes used for decomposing, evolving, and collecting the mercury vapor prior to AAS measurements.
In the method recommended by the American Society for Testing and Materials (ASTM, 1984), a coal sample is decomposed in an oxygen bomb that contains nitric acid to collect the mercury. The resulting solution of mercury in nitric acid is treated with solutions of permanganate and hydroxylamine hydrochloride. Finally, stannous chloride solution is added to evolve the mercury vapor that is directly measured by the cold-vapor AAS technique.
In the method of Doolan (1982), a coal sample is heated in a high-temperature (1,250° to 1,450° C) oxygen-rich atmosphere within a combustion-tube furnace. The liberated mercury is collected in a permanganate-sulfuric acid solution and finally is evolved, through reduction by a stannous chloride solution, into the cell of a cold-vapor AAS. Ebdon and others (1982) used nonoxidative pyrolysis of coal at 800° C to remove mercury prior to its determination by atomic fluorescence spectrometry.
In a procedure used earlier in U.S. Geological Survey laboratories (Flanagan and others, 1982), mercury was removed from pulverized coal samples by nonoxidative pyrolysis in nitrogen at 900° C, and the evolved combustion products were passed through a gas-washing bottle that contained a stannous chloride solution. Then, the mercury was amalgamated with gold and finally released by induction heating and determined by cold-vapor AAS. Although good results were obtained for most anthracite and bituminous coal samples, erratic results were commonly observed for mercury in subbituminous coal. Also, subbituminous coals produced contamination that was difficult to remove from equipment. Because of these problems, a wet oxidation procedure was found to be more appropriate. A simple decomposition with a mixture of perchloric and nitric acids in a TeflonTM digestion vessel gave adequate decomposition of coal samples. This conclusion is supported by comparisons of measured mercury concentrations with certified values for National Bureau of Standards (NBS) reference coal standards. Mercury losses from "spiked" coal samples also were negligible. The addition of hydrofluoric acid to the decomposition solution did not cause an increase in the concentration of mercury found in the whole-coal samples. This observation indicates that mercury did not remain in the undecomposed silicate materials. The addition of dichromate or permanganate to the digestion solution is unnecessary if nitric acid is added to the hot digestion solution prior to the addition of distilled water and if the solutions do not stand for more than 2 to 3 h.
In the procedure described here, a 100-mg pulverized coal sample is decomposed by a solution of perchloric and nitric acids in a TeflonTM (PFA) container. The dissolved mercury is then reduced with a stannous chloride solution and removed from the solution in a stream of nitrogen. The free mercury is collected on 5g of gold chips, which then is inductively heated to drive the mercury into a 30-cm-long quartz cell where the absorption at 253.7 nm is measured.
EXPERIMENTAL EQUIPMENT AND METHODS
The arrangement for the cold-vapor AAS instrument is shown in figure 14. Individual apparatus and equipment items are listed here.
1. TeflonTM (PFA) digestion vials, 33 mL.
2. Aeration flask, PyrexTM gas washing bottle with coarse-fritted cylinder, 250-mL capacity.
3. Quartz tube, 30-cm long by 7-mm diameter, drawn out in the center to approximately 3-mm diameter.
4. One-way check valves.
5. Three-way stopcocks, borosilicate glass.
6. Two-stage pressure regulator (MathesonR model 8-580) and needle-valve flow regulator for nitrogen gas.
7. Flowmeter, precision ball-float rotameter with a range of 0.1 to 1 L/min.
8. Induction furnace, LECOR model 521, with five-turn copper coil of 3-cm diameter.
9. Mercury monitor system, LDCR model 1235, with dual gas quartz cell 30-cm long by 7.5-mm diameter and cell volume of 13.7 cm3.
10. Strip chart recorder, Perkin-ElmerR model 56, 1- to 10-mV range.
11. Hot plate.
All chemical reagents are of reagent-grade or higher purity. Doubly-distilled water is used in the preparation of all solutions.
1. Perchloric acid, 72 percent.
2. Hydrochloric acid, 12 N.
3. Nitric acid, 14 N.
4. Gold chips, approximately 5 g, 99.99-percent pure, 0.5-mm diameter.
5. Mercury metal, high purity.
7. 5 percent (w/v) SnCl2 in 12 N hydrochloric acid.
8. Nitrogen gas, 99-percent pure.
9. Anhydrous magnesium perchiorate (AnhydroneTM)
1. Prepare an aqueous stock solution containing 1.00 mg/mL mercury and 10 percent nitric acid by dissolving high-purity metallic mercury in nitric acid.
2. Prepare a solution of 10 m g/mL mercury in 5 percent nitric acid by diluting an aliquot of the 1 mg/mL stock solution. This solution should be freshly prepared each month.
3. Freshly prepare a solution of 0.10 m g/mL mercury in 5 percent nitric acid each day from the 10 m g/mL mercury solution. Six different aliquots of the 0.10 m g/mL solution are pipeted into the aeration flask to effect calihration of the instrument.
Coal standards NBS 1630, 1632, 1633, and 1633a are used to verify the accuracy of this analytical method.
Dissolution of Samples
Weigh 100 mg of air-dried, pulverized (100 mesh) coal into a 25-mL screw-cap TeflonTM digestion container. Add 5 mL of concentrated perchloric acid and 5 mL of concentrated nitric acid, and then heat the sample-acid mixture on a hot plate at 150° C until 2 to 3 mL of solution remains (3 to 4 h). Add 1 mL of concentrated nitric acid to the hot solution, and finally add distilled water until the volume of the solution is approximately 25 mL. Immediately close the container with the screw cap, and mix the contents.
Determination of Mercury
Transfer the solution from the TeflonTM container to the aeration flask (fig. 14) and adjust the volume to approximately 100 mL with distilled water. Add 6 mL of 5 percent (w/v) SnCl2 solution and close the flask. Begin the flow of nitrogen (delivery pressure equals 34.5 kPa (5 psi); flow rate equals 0.5 L/min) into the aeration flask. The evolved mercury is collected on the gold chips within approximately 3 min. After the collection process is completed, turn the first three-way stopcock (fig. 14) to the position that allows the nitrogen flow to bypass the aeration flask and to pass directly over the gold chips, thus removing water vapor from the system (30 s). After the system has dried, turn the second three-way stop-cock to direct the gas flow through the absorption cell. Power from the RF generator is then applied to the induction coil for 15.0 s to heat the gold chips, thus releasing the amalgamated mercury into the nitrogen stream. The mercury subsequently is transported into the cell of the mercury monitor system, where the peak absorption of HgI 253.652 nm is measured and recorded on a strip chart recorder. The peak absorption signal for each sample is used to extrapolate the mercury concentration from a calibration curve established by separately pipeting 0, 5, 10, 15, 20, and 25 ng of mercury from a 0.10 m g/mL mercury solution into the aeration flask, adding the reagents, and recording the corresponding peak absorption signal.
NBS coal standards 1630, 1632a, and 1633 and a U.S. Geological Survey "in-house" coal standard are used as control standards in each measurement sequence. The detection limit of the method, based on twice the standard deviation for 10 determinations of mercury in a blank, is 1 ng of Hg, which is equivalent to 10 ng/g in a 100-mg sample. The sensitivity of the method is 1.1 mV/ng of mercury. The long-term precision of the method, estimated from repeated measurements of mercury concentrations in these standards over several years, is 5- to 10-percent relative standard deviation for the concentration range from 50 to 500 ng/g. Both the accuracy and precision of the method (table 25) are comparable to those for the methods using oxygen-bomb combustion (ASTM, 1984) or high-temperature oxidation in a tube furnace (Doolan, 1982). The procedure is simple and rapid and is applicable to a wide variety of coal samples.
American Society for Testing and Materials (ASTM), 1984, D3684-78(1983) Standard test method for total mercury in coal by the oxygen bomb combustion--atomic absorption method, in 1984 annual book of ASTM standards, petroleum products, lubricants, and fossil fuels, sect. 5, v. 05.05: Gaseous fuels, coal, and coke: Philadelphia, ASTM, p. 470-473.
Babu, S.P., 1975, Trace elements in fuel, in Advances in chemistry series 141: Washington, D.C., American Chemical Society, 216 p.
Doolan, K.J., 1982, The determination of traces of mercury in solid fuels by high-temperature combustion and cold-vapor atomic absorption spectrometry: Analytica Chimica Acta, v. 140, no. 1, p. 187-195.
Ebdon, L., Wilkinson, J.R., and Jackson, K.W., 1982, Determination of mercury in coal by non-oxidative pyrolysis and cold-vapour atomic-fluorescence spectrometry: Analyst (London), v. 107, p. 269-275.
Flanagan, F.J., Moore, Roosevelt, and Aruscavage, P.J., 1982, Mercury in geologic reference samples: Geostandards Newsletter, v. 6, no. 1, p. 25-46.
Swaine, D.J., 1985, Modern methods in bituminous coal analysis: trace elements: CRC Critical Reviews in Analytical Chemistry, v. 15, no. 4, p.315-346.
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