Determination of Elemental Mercury in an Emission Source Having a High Sulfur Dioxide Concentration by Amalgamation with Gold and Ultraviolet Spectrophotometry Charles M. Baldeck and G. William Kalb TraDet Laboratories, Inc., 930 Kinnear Road, P.O. Box 5093, Columbus, Ohio 43212
Howard L. Crist Quality Assurance & EnvironmentalMonitoring Laboratory, National EnvironmentalResearch Center, EnvironmentalProtection Agency, Research Triangle Park, N.C. 277 1 1
This paper describes the use of gold as a collection medium for the quantitative separation of elemental mercury from stack gases in the presence of 6-8% SO2. A slightly modified Environmental Protection Agency sampling train with the impingers altered to hold 20-30 grams of gold chips was used. The gold amalgam is heated in an induction coil and the liberated mercury is absorbed in 3 % KMn04 solution; particulates on the filter are digested in "Os. Both SOlutions are reduced, aerated, and the absorbance is measured at 253.7 nm by an ultraviolet spectrophotometer. Collection of mercury on gold eliminates several interferences such as SO2, H2S, and organic substances which may be present in stack gases and react with liquid absorbing reagents inhibiting their quantitative retention of mercury. The collection efficiency of the filter and amalgamators was, for most runs, in excess of 95 %. The retention of mercury on three gold amalgamators between the sampling rates of 0.3 and 0.7 cubic feet per minute is well above 95% at sampling times of 5, 10, and 15 minutes.
The designation of mercury as a Hazardous Pollutant in 1971 ( 1 ) has stimulated interest in its determination from a variety of sources. Mercury is a natural constituent of many metal-bearing ores; a number of copper, zinc, and lead ore concentrate samples from different areas of the United States were analyzed for mercury. T h e results indicated t h a t approximately 14% of the samples were in excess of 25 ppm mercury, with a few samples exceeding 100 ppm ( 2 ) . In view of the large amount of ores processed in t h e United States annually, it is evident t h a t metal smelters constitute a potentially important source of mercury emissions to the environment. T o properly assess the role of the smelter in contributing to the total mercury pollution burden, it is necessary t o have a procedure adequate to test stacks for mercury in the presence of several percent SOs. Numerous studies have been published describing various methods of mercury determination in water, biological, coal, rock, and ambient air samples (3-9). T h e use of gold as a collector of mercury has been reported by several investigators (9-11). However, t h e (1) Fed. Regist.. 36 (62), 5931, March 31, 1971. (2) Source Sample and Fuels Analysis Branch, Quality Assurance and Environmental Monitoring Laboratory, unpublished data, EPA, Research Triangle Park, N.C. 27711. (3) W. R. Hatch and W. L. Ott, Anal. Chem., 40, 2085 (1968). (4) Y. Chau and H. Saitoh, Environ. Sci. Techno/., 4, 839 (1970). (5) R. Thomas, R. Hagstrom, and E. Kuchar, Anal. Chem., 44, 512 (1972). (6) A. 0. Rathje. Amer. lnd. Hyg. Ass. J., 30., 126 (1969). (7) G. W. Kalb, At. Absorption Newslett., 9, 84 (1970). (8) A. E. Moffitt, Jr., and R. E. Kupel. Amer. lnd. Hyg. Ass. J., 32, 614 (1971). (9) D. H. Anderson, J. H. Evans, J. J. Murphy, and W. W. White, Anal. Chem., 43, 1511 (1971). (10) J. V. O'Gorman. N. H. Suhr, and P. L. Walker, Jr., Appl. Spectrosc., 26, 44 (1972). (11) I. 0. Joensuu, Appl. Spectrosc., 25, 526 (1971).
1500
literature is not as replete with reports of the determination of mercury in stack emission gases with high concentrations (6-8%) of sulfur dioxide. T h e officially promulgated EPA reference methods 101 and 102, for t h e determination of mercury emissions from mercury ore processing facilities and mercury cell chloralkali plants, are based on drawing the gas through a series of impingers containing a liquid oxidizing (acidic IC1) solution, with subsequent analysis of the solution by flameless atomic absorption (12). Such methods, which rely on t h e quantitative oxidation of mercury for entrapment, are subject to serious limitations when applied to sources where SO;? concentrations greater than 200-300 ppm are present in the gas being sampled (13).In high SO2 environments, such as are observed at smelters and some power plants, the oxidizing reagents are quickly reduced by t h e SO;?,greatly limiting the usable sampling time. Our efforts to install SO;?scrubbing solutions in front of the oxidizing reagents were not successful; these solutions absorbed mercury and were soon depleted by the high SO2 levels present. An alternate method of mercury collection, one which does not depend on the oxidation of mercury for entrapment, is needed for stack sampling under these conditions. Studies have recently been published describing very sensitive and specific methods for mercury sampling in ambient air a t relatively low sampling rates and total sample volumes (200 cm:i/min, 0.288 m3 in 24 hr) using silver wool ( 2 4 ) or activated charcoal (15) as the collecting media. These methods avoid the problems of variable blank levels and poor reproducibility associated with t h e use of liquid oxidizing media when used for very low mercury levels. While these procedures are reported to be free of interferences from substances such as S O P ,H;?S,and NO2 a t levels likely to be encountered in ambient air, they are not suited to isokinetic source or "stack" sampling applications where much higher sampling rates (0.75 ft3/min) and total sample volumes (40-100 ft3) must be employed to obtain a representative sample and where relatively high concentrations of SO2, H2S, NO,, H20, hydrocarbons, and particulates may also be present. Previous work by this laboratory (16) has shown t h a t elemental mercury can be quantitatively collected from a stack gas sample by direct amalgamation onto gold (silver (12) Fed. Regist., 38 (66), 8835-8845, April 6, 1973. (13) D. R. Patrick, "Sampling Stack Effluents for Mercury and Beryllium." unpublished report, Office of Air Quality Planning and Standards, EPA, Research Triangle Park, N.C. 2771 1 (1973). (14) S. J . Long, D. R. Scott, and R. J. Thompson, Anal. Chem., 45, 2227 (1973). (15) F. P. Scaringelli, J. C. Puzak, B. I. Bennett, and R. L. Denny, Anal. Chem., 46, 278 (1974). (16) G. W. Kalb and C. Baldeck. "The Development of the Gold Amalgamation Sampling and Analytical Procedure for Investigation of Mercury in Stack Gases." EPA Publication APTD-1171 (EPA-R2-72-071A), NTIS No. PB 210-817, Research Triangle Park, N.C. 2771 1, June 1972.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 11. SEPTEMBER 1974
THEGBOWETEG
HLPTEO F l L T t F
\
PROBE
ALYE
SREENBURG S1,llTH IliPINLiER S C R U B B E R SOLUTION h1001F
i
I
ED
GREENBURG SMTH IIsIPINGER
VPCUlJ@; LINE TO DRY GGS METER pu"Ip 4NC O R I F I C E
Figure 1. Configuration of the sampling train
is r a p i d l y attacked b y sulfides). This paper d e s c r i b e s t h e use of g o l d , w h i c h is n o t a f f e c t e d by the corrosive or reduci n g p r o p e r t i e s of s u b s t a n c e s s u c h as Sop, HzS, and sulfuric a c i d mist, for the q u a n t i t a t i v e collection of elemental m e r c u r y f r o m ore s m e l t e r s t a c k gases of 6-8% SO2 c o n t e n t . The p r o c e d u r e d e s c r i b e d h e r e utilizes a slightly m o d i f i e d EPA s a m p l i n g train (17). w i t h the i m p i n g e r s adapted to hold gold c h i p s w h i c h collect the volatile m e r c u r y as an amalg a m . The m e r c u r y is then r e l e a s e d b y i n d u c t i o n h e a t i n g of t h e gold and c o l l e c t e d in an a c i d i c K M n 0 4 solution for subs e q u e n t a n a l y s i s b y u l t r a v i o l e t s p e c t r o p h o t o m e t r y using a direct vaporization procedure. This s t u d y was c o n d u c t e d in three p h a s e s . An initial field i n v e s t i g a t i o n was m a d e at a n o r e smelter utilizing the gold amalgamation p r o c e d u r e of Kalb and B a l d e c k f o r coll e c t i o n of m e r c u r y in coal fired p o w e r plants (16). U s e of this method resulted in an u n a c c e p t a b l y short sampling time due t o the h i g h e r m e r c u r y c o n c e n t r a t i o n s in t h e s m e l t e r s t a c k g a s e s (18). This field s t u d y was then followed b y a l a b o r a t o r y i n v e s t i g a t i o n of methods to i n c r e a s e the s a m p l i n g p e r i o d and o p t i m i z e the a n a l y t i c a l d e t e r m i n a t i o n of t h e m e r c u r y collected (19). The r e s u l t i n g p r o c e d u r e w a s then t e s t e d d u r i n g a second, n i n e - w e e k field s t u d y at a z i n c s u l f i d e o r e s m e l t e r . The r e s u l t s p r e s e n t e d in this paper w e r e o b t a i n e d o n a p p l i c a t i o n of the o p t i m i z e d p r o c e d u r e d u r i n g the second field s t u d y .
Figure 2. Amalgamator
Reagents. All chemicals were ACS reagent grade and distilled water was used for all rinses and dilutions. Glassware was rinsed before use with 1%SnClz in 21,9% HC1 followed by 1:3 HNO:%-H?O and finally with water. A 3% w/v solution of acidic K M n 0 4 was prepared fresh daily by dissolving 30 grams of KMn04 in 800 ml of diluting to 1000 ml with H20, H20, adding 100 ml of concd "03, then filtering to remove solids. A 10% solution of NHZOH-HCI was prepared by dissolving 50 grams of the reagent in 500 ml of H20. A solution of 20% SnCla in 50% HC1 was prepared by dissolving 100 grams of t h e salt in 250 ml of concd HCI, diluting to 500 ml, and adding a few pieces of metallic tin. T h e SnC12 rinse was prepared by diluting this solution 1:20 as needed. A stock solution of 1000 ppm mercury was prepared by dissolving 1.3535 grams of HgC12 in 50 ml of concd " 0 3 and diluting to 1000 ml with H20. A 1-ppm mercury standard solution was prepared fresh daily by diluting to 1000 ml. 1.00 ml of the stock solution and 50 ml of concd "03 Blanks were run periodically on the distilled water and on each lot of reagents. A blank was run on the 3% KMn04 solution daily or whenever a fresh solution was prepared. S a m p l i n g T r a i n P r e p a r a t i o n . T h e first impinger contained 250 ml of distilled H20 and was t h e only one with the GreenburgSmith tip; the second was empty, and the next three to five positions were amalgamators. For several runs one or two impingers containing 250 ml of water and sufficient KMnOo to last for the sampling period-this required excess solid K M n 0 4 in the solution which resulted in high blank determinations-were placed behind the series of amalgamators to serve as an aid in verifying the collection efficiency of the gold. T h e last impinger always contained silica gel. For this study, up to nine impingers were used in the train. By compressing the insulation in the sample box. up to seven impingers were fitted in the ice compartment and one or two mcire were taped to the outside of the box as required. Amalgamators were constructed from modified GreenhurgSmith impingers (without the tip), altered as shown in Figure 2, by forming indentations (to support a small quartz wool plug and the gold chips) and attaching a 28/15 Pyrex ball joint to the bottom of the vertical tube. The interchangeability of impingers and amalgamators permitted the use of a combination of amalgamation and wet absorption techniques to verify the collection efficiencies of the amalgamators, and allowed the standard EPA particulate sampling equipment to be used for mercury collection without major alterations. T h e amalgamators were prepared for each run by inserting a small plug of quartz wool through the top and pushing it into place against the supporting indentations. A minimum of the quartz wool was used to avoid excessive pressure drop in the system. Gold chips were prepared by cutting up a 0.007-inch thick sheet of the pure metal into approximately Yls-inch squares. For each run the gold chips were placed in small crucibles and fired overnight in an oven at 600-700 "C. At the start of each run the gold was removed from the oven, allowed to cool, then weighed out
EXPERIMENTAL S a m p l i n g Site. T h e zinc sulfide ore was roasted in air in a fluid bed roaster a t 900 "C, volatilizing the sulfur as SO2 which was carried through a waste heat boiler, a cyclone, and two electrostatic precipitators (to remove particulates). The gas was then carried through a 3%-foot diameter horizontal steel cross-over duct to t h e acid recovery plant where SO:! was converted to H2SO4. Approximately seventy stack gas samples were obtained from a sampling port located in the cross-over duct. T h e gas stream at this point contained 6-8% SO2 from the roasting process. A p p a r a t u s . Samples were taken a t various sampling rates (both isokinetic and non-isokinetic). sampling times, and with different sampling train configurations, using a standard Research Appliance Company Model 2343 "Staksamplr" console and sample hox equipped with a S-foot glass lined probe heated to 250 O F . T h e sampling train, shown in Figure 1. was based on the EPA particulate sampling train ( 1 7 ) . T h e impingers were immersed in a n ice hath and the sample box with the pre-weighed glass fiber filter was kept at a temperature of 250 "F to prevent moisture condensation. An umbilical cord connected the impinger train with the meter box containing the dry gas meter and vacuum pump. In the laboratory, mercury vapor absorbance measurements were made with a Laboratory Data Control UV Monitor equipped with a 30-cm dual path cell and connected to a Perkin-Elmer Model 165 strip chart recorder having a full scale response time of less than 1 sec. (17) Fed. Regisf.,36 (247), 24888-24890, December 23, 1971. (18) G. W. Kalb, "The Adaptation of the Gold Amalgamation Sampling and Analytical Procedure for the Analysis of Mercury in Stack Gases to High SO2 Environments Observed in Smelters," EPA Publication EPA-R2-720718, NTlS No. PB 21 1-215, Research Triangle Park, N.C. 27711, June
1972 (19) C. Baldeck and G. W. Kalb, "The Determination of Mercuryb Stack Gases of High SO2 Content by the Gold Amalgamation Technique," € P A Publication EPA-R2-73-153, NTlS No. PB 220-323, Research Triangle Park, N.C. 2771 1, January 1973.
ANALYT'ICAL CHEMISTRY, VOL. 46, NO 0
11, S E P T E M B E R 1974
1501
INDUCTION FURNACE
GO
BUBBLERIITH INTERCHANGEABLE BCTTOhl SECTION
KMnOp SOLUTION ... _ .... _
___
BUBBLER WITH INTERCHANGEABLE BOTTOM SECTION
N2
Figure 3. Apparatus used for vaporizing mercury from amalgama-
Figure 4. Apparatus used for analyzing the samples
tors (Table II), and poured into the amalgamator on top of the plug using a plastic funnel to prevent spillage. The amalgamator was held in a vertical position and tapped gently to help settle the chips. The train was then assembled in the sample box using a minimum of silicone grease on each of the ground glass fittings. After assembly, the sampling train was tested for leaks by plugging the filter holder inlet and pulling a 15-inch Hg vacuum. A leakage rate of less than 0.02 ft3/min was considered acceptable ( 2 7 ) . The ice compartment was filled with ice and water, the probe inserted into the sampling port and connected to the filter, and the probe and filter heaters were turned on. After allowing the probe and sample box temperatures to equilibrate, the pump was started and the flow adjusted to give the desired sampling rate. Sampling times of 5-15 minutes were used. No attempt was made to traverse the duct. Sample Recovery and Preparation. Samples from each part of the train were taken separately during the sample recovery procedure to account for all the mercury collected and its distribution in the train. The probe and filter holder were washed into a 16ounce jar containing 25 ml of 3% K M n 0 4 solution. The filter was placed in a clean container and marked with the run number. T h e water in the first impinger was transferred to a sample container and KMn04 added until the violet color persisted. T h e addition of the KMn04 to the water from the first impinger was necessary to preserve the sample and to avoid interference by SO2 in the mercury analysis step. The empty impinger was rinsed into 25 ml of 3 % KMn04. Moisture condensed in originally empty impingers located in front of the amalgamators often contained significant amounts of mercury. Each amalgamator "shell" and the impinger connector leading into it were also rinsed into a separate jar containing 25 ml of 3% KMn04. The KMn04 backup solutions were transferred to sample jars, using a minimum of a 1%NHzOH.HC1 rinse solution as required to remove permanganate stain. T h e mercury on each amalgamator was volatilized into 50 mi of 3% KMn04-10% HNO:, solution using the apparatus shown in Figure 3. The lower portion of the trapping chamber consisted of a closed tube of about 100 ml capacity with a standard taper fitting allowing the tubes to be used as interchangeable sample holders. The amalgamator was centered in the coil of a Leco Model 521 induction furnace and connected to the nitrogen supply and the trapping bubbler with two female ball-joint adapters and clamps. The nitrogen flow was set a t 0.5 l./min, and heating was commenced a t a variable transformer setting of 60% and increased by 59" each minute until the gold was glowing. This was done to avoid introducing a large "spike" of mercury into the trapping bubbler and to ensure complete volatilization of the mercury. Laboratory tests of mercury purged from aqueous solutions a t a carrier gas flow rate of 1 l./min into 40 mi of the 3% KMn04-10% " 0 3 solution showed no bypass of the trapping solution for 100 micrograms of mercury; an amount several times higher than that collected on any amalgamator during this study. T h e amalgamators were fired in reverse of their order in the train to minimize possible contamination of successive samples. After firing the series of amalgamators for a run, the trapping chamber and sample tubes were cleaned, using the rinse procedure described above followed by an acetone rinse. Tygon tubing connecting the amalgamator and trapping bubbler adsorbed and desorbed mercury from the gas stream passing through it; therefore a glass butt-to-butt connection was used. 1502
Analysis. Each of the solutions obtained from the sample preparation procedure was analyzed separately. The total volume of each sample was measured in the laboratory prior to analysis. T h e solutions into which the mercury was volatilized from the amalgamators were each diluted to 100 mi in volumetric flasks. Suitable aliquots of each solution were taken for analysis by the direct vaporization procedure. A schematic diagram of the vaporization apparatus is shown in Figure 4. T h e mercury vapor absorbance was measured a t 254 nm using an LDC UV Monitor equipped with a 30-cm dual path cell and connected to a strip chart recorder. Both Nz and compressed air were investigated as carrier gases. With the use of a Mg(C104)Z drying tube, no differences were observed; without the drying tube, the compressed air appeared to volatilize more water from the vaporization chamber than the nitrogen, resulting in a higher water vapor background. T h e air flow was adjusted to approximately 1.4 l./min and the three-way stopcock set to bypass the chamber. The standard or sample was placed in an interchangeable sample holder, diluted to approximately 50 ml with distilled water, 3 ml of 10% NH20H.HC1 solution were added, and the tube was swirled until the permanganate color disappeared. T h e tube was then attached to the apparatus, 1 ml of the 20% SnC12 solution was injected with a syringe through the ampoule stopper, and the stopcock was turned to vaporize the reduced mercury from the sample through the drying tube and the optical cell. Standards were run before and after each sample to prepare a working curve of peak absorbance us. total micrograms of mercury. All samples were run in duplicate and the total amount of mercury in each sample was calculated (after blank correction) from the sample volume and the size of the aliquot. Two analytical methods for dissolving the filter particulates were investigated. In the first method, half of each filter was boiled in 10 ml of concd HNO:, for 10 minutes. The filter was then macerated and the mixture diluted to 100 ml. In the second method, the other half of each filter was digested with 2 mg of ammonium metavanadate, 5 ml concd "03 and 10 ml of 70% HCIO? in a Bethge was collected and withdrawn from the apparatus (20). The "03 reaction flask; the filter was left to digest in the HC104 for 10 minutes. The "03 and HCIOI solutions were then combined and diluted to 100 ml. After cooling, the solutions resulting from both methods were analyzed for mercury. Both methods dissolved all visible particulates and gave the same results within experimental error. Because of its simplicity, the first method was used for particulate samples in this study. The total time required for train preparation, sample preparation, and analysis was approximately 2 hours per run, exclusive of sampling time. RESULTS AND DISCUSSION Sampling Train Configuration. T h e first impinger contained water to help remove H&04 mist and cool the gas stream. A significant amount of condensate was always observed in the second (originally dry) impinger, whether the first impinger was dry or contained water. An empty impinger was therefore used in the second position to re(20) "The Wet Chemical Oxidation of Organic Compositions Employing Perchloric Acid," The G.Frederick Smith Chemical Co., Inc.. 867 McKinley Ave.. P.O. Box 23344. Columbus, Ohio 43223, 1965.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 11, SEPTEMBER 1974
Table I. Average Collection Efficiency for One Amalgamator Gold chips, gram
Mercury collected,
10 20 30
94.6 95.3 99.3
Std dev,
5%
S
il.5 +2.9 fl.3
“Escape Fraction,” (1 - 7 )
move moisture, and the first amalgamator placed a t the third position from the filter. In this third position, there was normally very little or no visible moisture condensation on the gold under the sampling conditions encountered in this study. Collection Efficiency us. Amount of Gold. Laboratory experiments to determine the relationship between the weight of gold chips in an amalgamator and its collection efficiency were performed to determine the amount of gold needed to obtain essentially quantitative removal of elemental mercury from the gas stream. An amalgamator was connected between the vaporization chamber (Figure 4) and the trapping bubbler (Figure 3). Mercury standards in the range of 15-70 micrograms, volatilized by SnC12 reduction from aqueous solutions in the vaporization chamber, were passed through gold chips in the amalgamator and then collected in 50 ml of 3% KMn04 solution in the trapping bubbler. Analysis of the KMn04 trapping solution showed the amount of mercury passing uncollected through the amalgamator. The results, shown in Table I, are average collection efficiencies per amalgamator, as measured under laboratory conditions. Calculation of Theoretical Efficiency for a Series of Collectors. If the amalgamator train can be considered as a series of collectors, each of which removes the same fraction of the total mercury entering it a t any given flow rate, and if it can be assumed that the collection properties are not altered during the sampling process (neglecting any threshold or carryover effects), a theoretical relationship can be derived between the ratio of mercury found in any two adjacent amalgamators and the collection efficiency of the series. For n collectors connected in series, the fraction of the total mercury entering the train which escapes through the nth collector, f,,,, is given by: fa,, =
(1 - Y),
where r is the fraction of entering mercury trapped by a single collector. The fraction of the total mercury captured by the nth collector, f 99.9 96.6 99.9 99.0 >99.9 97.6 >99.9 94.5 94.9 95.1 84.7 91.1 97.6 89.1 >99.9 99.7 >99.9 ... 100.0 9 8 . 8 >99.9 ... >99.9 , . . >99.9 99.9 1 99.8 >99.9 >99.9 ...
A KMnOI backup iinpinger was not
deposited on the gold when the amalgamators were fired in the induction coil. This did not affect the accuracy of that determination, however. but did cause a significant decrease in collection efficiency if the same amalgamator was used again for another sample without first cleaning t h e gold. Heating the gold to 600-700 “C in a refractory oven for two hours between runs, to remove any contaminant picked up from the stack or from the quartz wool plug on firing, was a satisfactory cleaning procedure. Table I1 shows the data for several runs (55-62) using a series of three amalgamators, each containing 20 grams of gold. Runs 55-59, taken under “normal” stack conditions, showed about 95-99% of the mercury collected ahead of t h e KMnOJ backup solutions and “escape fraction” ( t J t l ) values of
