For the polymeric adsorbent, the binding energy (11) is probably lower than that of C,S so that the adsorbent has not only a low phenol capacity but also a capacity which depends strongly on the flow rate of the waste stream. We measured the virgin capacity of Amersorb XE-340 as recieved under similar experimental conditions and found a phenol capacity of 0.24 lb/ft3. This is much lower than the capacity of C,S (8.1 lb/ft3) at the same flow rate. Carbon-sulfur surface compounds have been studied as solvent-regenerable adsorbents for the removal of phenol from aqueous solution in a continuous, packed column. Results of cycling experiments show that at high flow rates (10-13 bed volumes/h), C,S has a virgin capacity of 8.1 lb/ft3 with an influent concentration of 2000 ppm a t room temperatures. Regenerations of the adsorbent with 2.0 bed volumes of 2propanol recover 90%of its capacity (7.3 lb/ft3) at room temperature. Parallel studies on a conventional activated carbon, Filtrasorb 300, under similar experimental conditions show that the activated carbon has an initial capacity of 5.6 lb/ft3 and the capacity changes gradually to an “equilibrium” value of 4.1 lb/ft3 after several cycles. The difference in regenerability between C,S and the activated carbon is attributed to the difference in surface groups. There are two kinds of adsorption sites for phenol. High-energy adsorption sites for the activated carbon are associated with oxygen surface complexes and cannot be easily regenerated. By replacing the carbonoxygen surface groups with carbon-sulfur complexes, the binding energy is lowered to that of lower-energy carbonoxygen sites. In addition, the binding energies of these sites are probably higher than those of polymeric adsorbents so that significantly higher phenol capacities than those of polymeric adsorbents are observed at lower influent concentrations and
higher flow rates while ease of solvent regeneration is maintained. Acknowledgment We express our appreciation to Dr. J. M. Longo for valuable discussions. Special thanks are extended to Mr. W. J. Bowling for his technical assistance. Literature Cited (1) Chang, C. H.; Savage, D. W.; Longo, J. M. J. ColloidInterface Sci.,
in press. (2) Puri, B. R. ACS Symp. Ser. 1975, No. 8, 212. (3) Barton, S. S.; Harrison, B. H. Carbon 1975,13, 283. (4) Chang, C. H. Carbon, in press. (5) Mattson, J. S.; Mark, H. B., Jr.; Malbin, M. D.; Weber, W. J., Jr.; Crittenden, J. C. J. Colloid Interface Sci. 1969,31, 116. (6) Buelow, R. W.; Carwell, J. K.; Symons, J. M. J. Am. Water Works Assoc. 1973,65, 57. (7) Buelow, R. W.; Carwell, J. K.; Symons, J. M. J. Am. Water Works Assoc. 1973,65, 195. (8) Pahl, R. H.; Mayhan, K. G.; Bertrand, G. L. Water Res. 1973,7, 1309. (9) Fox, C. R. Hydrocarbon Process. 1978,269. (10) Paleos, J. J . Colloid Znterfuce Sci. 1969,31, 7. (11) Farrier, D. S.;Hines, A. L.; Wang, S.W. J . Colloid Interface Sci. 1979,69, 233. (12) Simpson, R. M., presented at the 3rd International Symposium of the Institute of Advanced Sanitation Research, April 13, 1972. (13) Chang, C. H.; Kleppa, 0. J. Carbon, in press. (14) Neely, J. W., presented at the 176th National Meeting of the American Chemical Society, Miami Beach, FL, paper no. ENVR72, Sept 11-17,1978. (15) Modell, M.; deFilippi, R. P.; Krukonis, V., presented at the 176th National Meeting of the American Chemical Society, Miami Beach, FL, paper no. ENVR-203, Sept 11-17,1978. Received for review March 24,1980. Accepted November 3,1980.
Determination of Mercury Emissions from a Municipal Incinerator Ronny Dumarey,” Ronald Heindryckx, and Richard Dams Institute for Nuclear Sciences, Rijksuniversiteit Gent, Proeftuinstraat 86, B-9000 Gent, Belgium
A cold-vapor atomic absorption system is described for the determination of volatile mercury in the stack gases of a municipal solid-waste incinerator. The gases are washed in a K2Cr207/HN03 solution. By oxidation of the mercury in the washing bottle, a 99% collection efficiency is obtained. Total collection is guaranteed by adding a gold-coated sand absorber to the sampling train. After sampling, aliquots of the diluted absorbing solution are analyzed according to the reductionaeration method with SnC12. The released mercury is concentrated on a gold absorber. After thermal desorption from the gold, the mercury is measured in a modified atomic absorption spectrometer. The detection limit of the combined method is 1 ng. Calibration is achieved by injection of elemental mercury onto the gold. Generally, the reproducibility is better than 3%. The mercury concentrations found vary from 1 2 1 to 163 pg of Hg per m3 of flue gas. Introduction Recently, the increasing application of incinerators for the combustion of municipal and industrial waste has become an important source of air pollution. It contributes not only to the overall burden of gaseous pollutants but also to the particulate pollution by organics and heavy metals. Particulate emissions by incinerators have been shown to be significant contributors to the Cd, Sb, Zn, and, possibly, Ag, In, and Sn levels in urban aerosols ( I ) . Mercury is a special 206
Environmental Science & Technology
case. Owing to its high volatility, it is largely vaporized during the combustion process and emitted as elemental mercury vapor. This paper describes a sampling and an analytical technique for the determination of volatile mercury in the stack gases of a so-called “wet incinerator”, located in Gent, Belgium. This type of incinerator, in which the flue gases are cooled with water, is typical for many others in use in Europe and North America. The mercury is collected from the gas stream in a bubbler containing a strongly oxidizing solution. Several absorbing solutions with a varying composition are tested. Noncollected mercury, if any, is absorbed by amalgamation on a gold absorber placed immediately after the bubbler. A known fraction of the solution is analyzed with the reduction-aeration technique according to Hatch and Ott (2).The released elemental mercury vapor is concentrated on gold. After thermal desorption from the gold, the mercury content is measured by cold-vapor atomic absorption spectrometry. The mercury-emission values found for the incinerator studied are compared to estimated mercury outputs from incinerators in the United States. The latter values are based on the very sparse data on mercury in municipal solid waste (4). Experimental Section Incinerator Description. The refuse to be handled can be divided into two major fractions: the residual materials 0013-936X/81/0913-0206$01.OO/O @ 1981 American Chemical Society
from the neighboring municipal compostation plant (e.g., plastics and metals) and the coarse waste from industrial plants, hospitals, restaurants, and stores. After weighing, both fractions are stored together in a dumping bunker (1350-m3 capacity). The materials are carried to both of the furnaces through a large hopper using an electrohydraulic grab. Each of the furnaces has a combustion capacity of at least 5.5 metric tons h-l(2500 kcal k g l of material). After preliminary drying, the waste is deposed on a four-stage rocking grate. The most suitable furnace temperature is between 950 and 1050 "C. A complete combustion is obtained by insufflation of primary and secondary air. Depending on the composition of the material, additional fuel is injected. After the combustion process, the hot residues are dropped into a water-quench tank. Following quenching, the ashes are carried by a drag conveyor to a silo. The combustion gases leave the oven a t a temperature of -1000 "C and enter a cooling tower. The gases are first cooled to 400 "C with water and consecutively to -300 "C with air. The water is pumped out of a nearby river and is injected a t a rate of 10 m3 h-l. After cooling, the larger fly-ash particles are eliminated by two consecutive electrofilters for each furnace. The fly-ash content after cleaning should not exceed 100 mg m-3 (stp) at a COz concentration of 7%.Finally, the gases are emitted in the atmosphere through a 65-m high chimney (i.d., 2 m) at a velocity of -25 m s-l. This corresponds to a wet flue-gas emission of 131 850 m3 h-1 (stp) at an average combustion capacity of 6.12 metric tons of waste per hour and per furnace. These values were determined in separate investigations on the emissions. Sampling. Sampling is performed in the horizontal shaft after the electrofilters where the temperature is -250 "C. The combustion gases are drawn through a stainless-steel tube (1.6-m length; 4-mm i.d.) with a vacuum membrane pump. The sampling tube is placed in a stainless-steel protection tube. Preliminary laboratory experiments have shown that mercury losses due to absorption onto the stainless-steel tube do not occur at temperatures above 150 "C. The whole probe can be pushed into the shaft to different depths. Fly-ash is collected on a quartz-fiber filter (Gelman C$19 mm) mounted on top of the tube. The sampling probe is shown in Figure 1. Figure 2 illustrates the total sampling setup. The mercury is collected by oxidation in an absorption bottle (250-mm height; 40-mm i.d.) made of glass and filled with 75 mL of a 4% KzCr207/20% " 0 3 solution. An excellent dispersion of the gases is obtained by using a fritted glass candle. The collection of nonabsorbed mercury is guaranteed by subsequent amalgamation on a gold-coated seasand absorber. This absorber is described elsewhere ( 3 ) .Organic substances, possibly interfering with the amalgamation step, should be eliminated by an adequate absorber. Therefore the chromatographic absorber Tenax GC is used. Preliminary laboratory experiments showed that Tenax does not absorb significant amounts of mercury. All connections are performed with Swagelok stainless-steel fittings. The sampling rate is -2 L min-l. The total volume is measured with a calibrated dry-gas meter. A manometer placed before the pump measures the underpressure in the sampling line and permits one to check any leakages. After sampling, the bottle, as well as the absorber tube, is capped with stoppers in order to prevent contamination. Analysis and Apparatus. Reduction-Aeration. The absorbing solution is quantitatively transferred to a 500-mL volumetric flask and diluted with deionized water. Depending on the mercury concentration, a further dilution may be required. The most suitable range for determinations is 1-5 ng of Hg per milliliter. Five milliliters of the diluted solution is transferred to the reduction-aeration vessel, shown in Figure 3. A quartz tube,
reduciq unit
Figure 1. Sampling probe.
IO
Figure 2. Total sampling setup.
filled with the gold absorber, is connected to the outlet of the vessel by using Tygon tube. One milliliter of hydroxylamine hydrochloride (1.5%) and 1mL of tin(I1) chloride (30%) are added to the solution. The vessel is closed immediately with a glass stopper. The elemental mercury is aerated for 1 min with purified nitrogen a t a flow rate of 0.2 L min-l and is collected on the gold absorber. The thermal analysis of the mercury-loaded absorber tube is described further. After aeration, the vessel is emptied and rinsed with 1%HN03 and deionized water. At least five determinations are performed on each sample. Thermal Desorption. The thermal-desorption setup is shown in Figure 4. The nitrogen carrier gas is dried by a silica-gel scrubber and purified from mercury by an activated charcoal scrubber. The flow is adjusted to 0.1 L min-l. The desorption temperature of 800 "C is obtained by using a Nichrome heating coil of -1 m (6Q m-l) wrapped around the tube. The coil is connected to a 14-A, 220 V, ac input variable transformator. The mercury vapor, desorbed for 2 min from the sampler (A), is carried to a second permanent gold absorber (B) to avoid small variations in flow rate caused by differences in absorber packing. It also eliminates possible residual interfering compounds. After desorption of the sample, the sampling tube is removed and the gas supply is directly connected to the permanent absorber. The mercury desorbed from this tube is swept into the optical cell of a modified Coleman MAS-50 mercury analyzer system ( 3 ) .Output signals from the spectrophotometer are fed to a recorder and a digital integrator for peak-area measurements. Calibration. Calibration is performed by injection of known volumes of mercury-saturated air with a gastight 05-mL syringe. The mercury vapor is contained~abovea metallic mercury pool in a closed flask maintained a t constant Volume 15, Number 2, February 1981
207
Table 1. Influence of the Composition of the Absorbing Solution on the Collection Efficiency rinsrng solution
connection to Au-absorber
absorber % K~Cr207 % HNO3
4 4 4 2 4 6
10.5 15.8 21.0 15.8 15.8 15.8
% Hg In soh
efflclency % Hg on gold absorber
93 96 99 95 96 97
Table II. Influence of Sampling Duration on Collection Efficiency -inlet
N -gas
2
vol, L
40.0 69.6 99.8
spherical jcint
3- way valve
Flgure 3. Reduction-aeration vessel.
Flgure 4. Analytical setup.
temperature and ambient pressure. The equilibrium concentration in 1mL of air a t 25 OC and atmospheric pressure (101, 325 Pa) is 19.86 ng. For calibration, different volumes of mercury vapor are injected through a septum, directly onto the permanent gold absorber (B). The same thermal-desorption procedure is then used as for the samples. Reagents. (1)Absorbing solution: Dissolve 4 g of KzCrz07 (analytical grade) in 50 mL of deionized water, add 30 mL of 65% H N 0 3 (suprapur), and dilute to 100 mL. (2) Hydroxylamine hydrochloride (1.5%): Dissolve 0.75 g of NH20HaHCl (analytical grade) in 50 mL of water. (3) Tin(I1) chloride (30%): Dissolve 15 g of SnClz (analytical grade) in 5 mL of 12 N HCl at boiling temperature. Dilute with water to 50 mL. (4)Ausand absorber: For preparation see ref 3. (5)Tenax GC, 60-80 mesh (Chrompack): a 50-mm section in a 8-mm i.d. quartz tube is used.
Results and Discussion Choice of a Suitable Absorber. The use of a gold absorber, as described for ambient air ( 3 ) ,is not suitable for stack-gas sampling because the mercury concentrations are far too high to permit an efficient sampling during longer periods. A 208
Environmental Science & Technology
absorber duratlon, mln
19 34 52
% Hg In soh
efflclency % Hg on gold absorber
99 99 99
flue-gas volume of only 2-5 L may be sampled, corresponding to periods of 1-3 min. Such grab sampling might be less representative. A solution of 2% KMn04 and 10% H2S04 as absorber (5) cannot be used either because a considerable precipitate is formed causing mercury losses by coprecipitation. This deposit is soluble only in HC1. Thus, a very stable HgCL2complex is formed which interferes with the reductionaeration step. The permanganate solution also suffers from a very limited stability. Since a KzCr207/HN03 mixture has already been used for the conservation of diluted Hg2+ solutions (6),this mixture was tested as absorbing liquid. Efficiency of the Absorbing Solution. Table I gives the collection efficiency for mixtures with varying K2Cr207 and H N 0 3 content. A solution of 4%K2Cr207 in 20% HNO3 absorbs more than 99% of the mercury. A complete collection is obtained by using a gold absorber placed after the absorption flask. As shown in Table 11,no influence of sampling duration on collection efficiency could be found. The influence of sampling rate on the collection efficiency has not been tested since the highest flow rate obtainable was only 2 L min-l. On the other hand, this flow rate was the lowest limit for a proper working of the gas meter. Nevertheless, we suspect that the collection efficiency decreases to some extent with increasing flow rate. Interferences. With the KzCrz07/HN03 solution, no precipitate is formed during sampling. Absorption of Hg2+ ions on the wall of the bubbler is negligible because of the strongly oxidizing properties of the solution (6). The interference of organic products on the collection efficiency, the storage, and the analysis was experimentally investigated and found to be insignificant. During sampling, foreign materials could possibly be collected on the gold. This results in deactivation of the absorber and nonspecific light absorption during the subsequent analysis. Since the MAS-50 spectrometer has no possibilities for background correction, those interfering products must be removed. This can be achieved by placing a tube filled with Tenax GC absorber between the bubbler and the gold. This hydrophobic porous polymer collects most of the organic compounds without any mercury losses. For each absorbing solution, the mercury blank must be determined. The blank value, which generally amounts to -1 ng mL-1, is taken into account for the calculation of the total mercury concentration.
Limit of Detectability. The limit of detectability of the combined reduction-aeration and thermal-desorption technique is 1 ng of Hg. When one uses the above-mentioned procedure for a sampling period of 30 min a t a flow rate of 2 L min-1 and for a 5-mL fraction of the absorbing solution analyzed, this corresponds to a concentration of 250 ng m-3. The reproducibility of the sampling could not be tested since it was not possible to effectuate a parallel sampling. On the other hand, sequential sampling resulted in very fluctuating results, probably due to the inhomogeneity of the incinerated materials. The reproducibility of the combined reduction-aeration and thermal-desorption technique was checked with different mercury standard solutions. Thus, a reproducibility better than 3% was found. Sampling Results. Table I11 gives the results of 16 sampling periods spread over 4 days. For each run, -60 L of flue gas was sampled for 30 min. Both of the furnaces were working. Simultaneous flow-rate measurements were not performed. Therefore, the nominal values mentioned above are used for further calculations. The average mercury concentration is 146 f 40 pg m-3. Assuming that the flow rate is 131 850 m3 h-l, this value corresponds to an average of 19.3 g of Hg per hour for both furnaces with a range of 12.2-32.7 g. According to Low and Gordon ( 4 ) ,the municipal solid waste in the United States has an average calculated mercury concentration of 1.2 ppm with 0.66 and 1.90 ppm as limits. In the present case, for a combustion capacity of 6.12 metric tons per hour and per furnace, this corresponds to an average emission of 14.7 g of Hg per hour, assuming that all of the mercury is in the vapor phase. As compared with the value found, the calculated mercury emissions are -30% lower, as shown in Table IV. This difference can most probably be explained by a different composition of the incinerated materials. Calculated for continuously working, the incinerator studied has an average annual emission rate of 168.6 kg of mercury. The ash stored in the silo was not analyzed since the inhomogeneity of this ash did not allow collection of a representative sample. Three prefilters from the sampling probe were analyzed by direct pyrolysis (7). For a sample of -180 L of flue gas, the mercury content on the filter was less than 1ng. This can be explained by the high temperature of the stack gases (250-300
Table 111. Results (pg of Hg m-3) sampling run
day
1 2 3 4 jif s a a
1
2
164 113 122 171 142 f 29
163 126 205 147 163 f 30
3
4
157 248 156 103 105 93 131 137 162 f 61 121 f 29
s = standard deviation of the mean value.
Table IV. Comparison between the Expected ( 4 ) and the Experimentally Found Mercury Emissions (g h-’)
X min max
calcd
exptl
15
19f5 12 33
a 23
O C ) which probably prevents condensation or absorption of mercury on fly-ash particles.
Acknowledgment We gratefully acknowledge the willing cooperation and technical information given by P. De Vos, Director of the Municipal Incinerator Plant of Gent, as well as the technical assistance by M. Nagels. Literature Cited (1) Greenberg, R. R.; Zoller, W. H.; Gordon, G. E. Enuiron. Sei.
Technol. 1978,12, 566. ( 2 ) Hatch, W. R.; Ott, W. L. Anal. Chem. 1968,40, 2085. (3) Dumarey, R.; Heindryckx, R.; Dams, R.; Hoste, J. Anal. Chim. Acta 1979,107, 159. (4) Law, S . L.; Gordon, G. E. Enoiron. Sci. Technol. 1979,13, 432. (5) Air Pollution Control Directorate, Canada, Nov 1979, Report EPS 1-AP-76-1. (6) Feldman, C. Anal. Chem. 1974,46, 99. ( 7 ) Dnmarey, R.; Heindryckx, R.; Dams, R. Anal. Chim. Acta 1980, 116, 111. Received for review April 21,1980. Accepted September 29,1980. One of the author.s (R.D.) is indebted to the “Interuniuersitair Instituut uoor Kernwetenschappen-IIK W” for financial support.
Volume 15,Number 2,February 1981 209
