Environ. Sci. Technol. 1906, 20, 735-738
Schwarzenbach, R. P.; Giger, W.; Schaffner, C.; Wanner, 0. Environ. Sci. Technol. 1985,19, 322-327. Schweizer, C.; Tarradellas, J. Chimia 1980, 34, 509-519. Olie, K.; Vermeulen, P. L.; Hutzinger, 0. Chemosphere 1977,
of Germany, personal communication, 1985. (25) Harless, R. L.; Oswald, E. 0.;Wilkinson, M. K.; Dupuy, A. E., Jr., McDaniel, D. D.; Han, T.Anal. Chem. 1980, 52, 1239-1245.
6, 455-459. Buser, H. R.; Bosshardt, H. P.; Rappe, C. Chemosphere 1978, 7,165-172. Ballschmiter,K. University of Ulm, Ulm, Federal Republic
Received for review October 21, 1985. Accepted February 18, 1986.
Adsorption of Mercury Vapor on Particles Yoshio Otani,* Chikao Kanaoka, Chlyoki Usui, Saburo Matsui,? and Hltoshl Em1
Department of Chemical Engineering, Kanazawa University, 2-40-20 Kodatsuno, Kanazawa 920, Japan
w The adsorption of mercury vapor on particles was studied by using soot particles generated by incineration of sewage sludge (EP-ash) and activated carbon particles. Through the experiments, it was found that, at 298 K, the EP-ash has a fairly high adsorption capacity for mercury vapor in the order of lo4 g/g, which is between that of the ordinary soils and that of activated carbon particles. Furthermore, it was found that physical adsorption of mercury vapor on the studied particles at high temperature is described by Dubinin’s equation. On the basis of the equation, it was shown that the EP-ash physically adsorbs very little mercury vapor at high temperature, and therefore, most mercury in the EP-ash is chemically adsorbed or contained in a form of mercury compounds. Nevertheless, the total amount of mercury contained in the particles is very little compared to the total mercury in the exhaust gases so that most mercury behaves as a vapor even in the presence of particulate matter. Introduction It has been reported that mercury at a low concentration is released into the atmosphere together with combustion products resulting from the incineration of sewage sludge or municipal garbage. The mercury content in sewage sludge is usually 0.2-1.8 mg/kg dry matter, and most of the mercury is volatilized by the incineration to give mercury concentrations in the emission of about 0.3 mg/m3(N.m3) ( I , 2). Although there is no regulation for the mercury emission in waste gases, it is urgent to install effective collectors for mercury since it may cause serious diseases by accumulating in the human body. In incinerators where a large number of soot particles and reactive gases are generated at a high temperature, it is expected that there are strong interactions between mercury vapor and these substances. In order to develop an effective mercury control technique, it is necessary to conduct quantitative studies on the interaction of mercury vapor with particulate matter as well as reactive gases. In the previous paper (3),we investigated the existence of particulate mercury in a dust-free atmosphere at various mercury concentrations and found that mercury mostly exists as a vapor even at a concentration 200 times higher than the saturation at 273 K. In a dusty atmosphere, however, the situation would be more complicated because of mercury-particle interaction. In the present work, the interaction of mercury vapor and particles, i.e., the adsorption of mercury on particles, was studied with soot particles obtained from an electrostatic precipitator in a sewage sludge incineration plant ‘Present address: Department of Sanitary Engineering, Kyoto University, Yoshida-Honmachi, Sakyoku, Kyoto 606, Japan. 0013-936X/86/0920-0735$01.50/0
Table I. Chemical Compositions and Physical Properties of the Particles
ignition loss, % chemical component C,% H, % N, 70 SiOz,” % CaO,” %
KzO, % MgO, % A1203,
70
Fe, P P ~
Cd, P P ~ Pb, P P ~ geometric mean particle diameter, fim geometric standard deviation particle density, g/cm3 specific surface area (BET method), m2/g
heated
raw EP-ash 35.21
EP-ash 33.37
15.35 0.59 0.23 6.68 16.20 1.59 0.97 2.83 5.14 531 3470 1.6
16.05 0.36 0.15 7.49 7.60 2.06 1.06 3.16 5.41 1.6
52
1.8 17.8
1.2 716
1.8 2.47
-
uDeterminedby the method specified by JIS-R-2212.
activated carbon -b
-
-
(-),
measured.
not
(EP-ash) and also with activated carbon particles. Experimental Procedures Adsorption characteristics of mercury on particles were studied by the following experiments: (i) determination of mercury in solid particles (desorption method); (ii) determination of equilibrium adsorbed mass of mercury with packed bed of particles (adsorption method). In the experiments, EP-ash sampled from an electrostatic precipitator in a sewage sludge incineration plant (raw EP-ash) and the EP-ash heated at 773 K for 8 min in nitrogen atmosphere (heated EP-ash) were used with activated carbon particles as a reference adsorbent. The results of chemical composition analyses of the raw and heated EP-ashes and their physical properties are shown in Table I with those of the activated carbon particles. The details of experiments i and ii are described here. (i) Desorption Method. The mercury content in particles was measured by trapping thermally desorbed mercury in liquid absorbent. Particles were heated in nitrogen atmosphere by an infrared image furnace (Shinku-riko Co. Ltd.,Model RHL-E25). The desorbed mercury was entrained by nitrogen gas and collected in a gas washing bottle containing 200 cm3of 0.2 wt % KMnO, + 2 N H2S04( 4 ) . The collected mercury was then determined by a flameless atomic absorption spectrophotometer (Shimadzu, Model AA-640-13). The desorption method was examined by using activated carbon particles con-
0 1986 American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 7, 1986
735
Ha hollow
absorDtion
Table 111. Mercury Content in the Raw EP-asha
gas washing bottle
U
a;r
Figure 1. Experimentalsetup for the measurement of breakthrough curves.
Table 11. Experimental Conditions for the Measurement of Breakthrough Curves EP-ashes packed mass of particles, g average particle size, pm flow velocity of air, cm/s bed depth, cm bed diameter, cm bed, temperature, K mercury vapor concentration, mg/ (N.m3)
0.07 1.6 7.4 0.3
0.05 52 10.4 0.2
3.73 1.13 1.57 1.74 5.84 1.96 2.19 2.25 2.50 2.40 2.50 1.41 1.70 3.30 3.60
sample
mercury content, ppm
sample
mercury content, ppm
16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
2.75 3.60 3.35 3.48 2.70 2.00 2.76 2.77 2.00 1.20 3.47 1.87 1.47 2.93 2.13
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
1.41 2.44 1.80 2.83 1.73 1.73 2.67 2.54 2.67 1.93 1.28 2.83 1.55 2.90 3.34
- - - __ - -
-- - ---- - - --
"ppm = lo4 g/g. Heated E P - a s h a c t i v a t e d carbon Y
o
_-e-
._ c
#'
0 82
I
0.8 298 0.14-1.5
where Co and C are the inlet and outlet concentrations of mercury, and Qt is the total air volume flowing into the bed. The mercury content was also determined by the desorption method. The experimental conditions for the measurement of the breakthrough curves are shown in Table 11.
Results and Discussion The mercury contents in the EP-ash samples measured by the desorption method are shown in Table 111. The average mercury content in the particles is 2.42 X lo4 g/g with a standard deviation of 0.75 X lo4 g/g. The average Environ. Sci. Technol., Vol. 20, No. 7, 1986
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
activated carbon
taining known amounts of mercury since the desorption efficiency of mercury from particles depends upon the heating time and temperature. The mercury-containing activated carbon particles were prepared in the following manner. One gram of activated carbon particles was packed in a 8 mm diameter and 4 cm long bed, through which known volume of air containing mercury was passed. The total amount of mercury adsorbed in the particles was calculated by the total air volume passing through the bed and its mercury vapor concentration. When the temperature of particles was kept at 773 K for 8 min during desorption, the accuracy of the method was within 5% (standard deviation) over mercury contents from 3 X lo4 to 4.4 x 10-5 g/g. (ii) Adsorption Method. The equilibrium adsorbed mass of mercury in particles was determined by measuring breakthrough curves of the packed beds. The experimental setup is shown in Figure 1. Air containing a known concentration of mercury vapor was passed through the packed bed of particles, and the outlet concentration of mercury was continuously measured by introducing the exit gas directly to the flameless atomic absorption spectrophotometer. The equilibrium adsorbed mass of mercury, q, was obtained by using the following relationship.
736
sample
mercury content, pprn
-0
0.1 0.2 03 04 t o t a l oir volume flowing into bed, Qt [rn3-Nl
Flgure 2. Breakthrough curves for the heated EP-ash and activated carbon particles.
content agrees with the previously reported mercury contents in EP-ashes of sewage sludge incinerators (5, 6 ) . The breakthrough curves of the heated EP-ash are shown in Figure 2 with those of the activated carbon particles. The period of zero outlet concentration decreases in the following order: activated carbon, heated EP-ash, and raw EP-ash. For the same particles, breakthrough occurs more rapidly as the mercury concentration increases. The equilibrium adsorbed masses of mercury in the EP-ashes and the activated carbon particles measured by the adsorption method are plotted by hollow symbols in Figure 3, together with those of various soils measured by Fang (7). First, when the order of these sorbents is compared, it is noted that the EP-ashes, having an adsorption capacity of about g/g, are between activated carbon particles and ordinary soils. In the same figure, the adsorbed masses on the raw and heated EP-ashes measured by the desorption method are shown by solid symbols. The adsorbed masses on the heated EP-ash measured by both adsorption and desorption methods agree with each other within experimental error, whereas the adsorbed mass on the raw EP-ash measured by the adsorption method is lower than that measured by the desorption method since the raw EP-ash already contained an average of about 2.42 x g/g mercury. The adsorbed mass on the heated EP-ash is higher than that of the raw EP-ash, indicating that the sampled raw EP-ash is not thermally stable and its adsorption capacity may be changed by the heat treatment. Second, looking at concentration dependence of the isotherms, it is found that the heated EP-ash and the raw
I Heated EP-ash Adsorption method .pDesorption
method
Raw EP-ash
adsorption potential, A x I O - ~CJ/rnoll
Figure 4. Adsorbed masses of mercury on the activated carbon particles and EP-ashes as a function of the adsorption potential.
where 3.5 X < a < 5.4 X and /3 = 0.37. The Freundlich form of eq 5 is rewritten in the form of Dubinin's equation as Crng/rn3-Nl Figure 3. Comparison of adsorption isotherms of various adsorbents. mercury
concentration, C
EP-ash measured by the adsorption method have almost the same slope as that of the activated carbon. The agreement of the slopes probably indicates that mercury vapor is adsorbed on these particles by the same mechanism, Le., physical adsorption which is the main adsorption mechanism on the activated carbon. On the other hand, the slope of the raw EP-ash measured by the desorption method is smaller than that of the other adsorbents. The possible explanation for the weak concentration dependence of the raw EP-ash may be the result of mercury content of the raw EP-ash, which is most probably present in the form of mercury compounds. When the above considerations are taken into account and the Freundlich isotherm is applied to the data, physical adsorption of mercury vapor on the studied particles can be expressed by the following equations. For the raw EP-ash (by adsorption method) q = (3.5 X 10-6)C00~37
(2)
For the heated EP-ash q = (5.4 X 10-4)C00.37
(3)
For the activated carbon = (1.8x 10-2)c,0.37
(4)
where q is the adsorbed mass in g/g and Co is the equilibrium mercury concentration in g/ (N.m3). We have obtained the adsorption isotherms of mercury vapor on the raw EP-ashes at room temperature (298 K). In order to predict the adsorbed mass of mercury at elevated temperatures, we apply Dubinin's equation to the obtained isotherms, confining our consideration to the physical adsorption of mercury vapor. In addition, since the adsorption capacity of the raw EP-ash was found to be changed by the heat treatment, we assume that isotherm of the raw EP-ash at high temperature varies between that of the raw EP-ash a t room temperature and that of the heated EP-ash, following the same concentration dependence, i.e. q = ac/
(5)
9 = qo exp(-A/E)
(6)
In this equation, qo is a constant, E the characteristic energy of adsorption, and A the Polanyi's potential given by A = RT In (P,/Po) (7) where R is the gas content, T i s the temperature, and P, and Po are the saturated vapor pressure and the adsorption equilibrium pressure, respectively. qo and E are related to a and p in eq 5 by the following equations:
=)
P,M Qo = a (
E = RT/P
(9)
Substitution of a and /3 into eq 8 and 9 yields the constants of eq 6 as 8.2 X lo*
< qo < 1.3 X
E = 6.7 X lo3 (J/mol)
(g/g) for raw EP-ash (10)
To test the applicability of Dubinin's equation, the equilibrium adsorbed mass was measured at elevated temperatures. In the experiment, activated carbon particles were used because mercury vapor is physically adsorbed on both raw EP-ash and activated carbon particles and the equilibrium adsorbed mass of the raw EP-ash is changed by heat treatment. The experiment with the activated carbon can provide information on the temperature dependence of the mercury adsorption in case of physical adsorption. The equilibrium adsorbed mass measured at various temperature is plotted in Figure 4 as a function of the adsorption potential. The data for both raw and heated EP-ashes contain only those obtained at room temperature. In the figure, the curves are predicted lines by eq 6. The data obtained at various temperatures for the activated carbon particles are in fairly good agreement with the predicted line, showing that the form of eq 6 is applicable to the physical adsorption of mercury vapor. If we take representative values of the temperature, the concentration of mercury vapor, and the dust concentration at the outlet of the electrostatic precipitator as 473 K, 0.3 mg/(N.m3), and 0.2 g/(N.m3), respectively, then the Environ. Sci. Technol., Vol. 20, No. 7, 1986 737
adsorption potential is A = 5.3 X lo4 J/mol. By substitution of this value into eq 6 with the constants given in eq 10, the equilibrium adsorbed mass of mercury vapor is and 4.8 X g/g. Thus, the quantity between 3.0 X of mercury in the particles leaving the precipitator is only g for each N.m3 gas and 9.6 X between 6.0 X generated compared with the average mercury concentration 0.3 mg/(N-m3). This calculation result indicates that the raw EP-ash would contain very little mercury vapor a t high temperatures if the mercury is only physically adsorbed on the particles. However, since about 2.42 X lo4 g/g of mercury was detected in the raw EP-ash in average, it can be said that mercury is not adsorbed physically on the EP-ash but it is chemically adsorbed or contained in the form of chemical compounds. Therefore, if we assume that the average mercury content of the EP-ash does not change with temperature since the mercury compounds are more stable thermally than metal mercury, the amount of mercury a t the representative condition of the incinerator emission is 4.8 X lo-' g/ (N.m3). Although this predicted value is about 100 times higher than the amount of physically adsorbed mercury, it is still only 1/6M, of the total mercury content in the incinerator emission. Therefore, we may conclude that most mercury is in the vapor state even in the presence of soot particles. Conclusions
The adsorption experiments lead to the following conclusions: (1)The mercury content in the EP-ash sampled from an incineration plant is about 2.42 X lo4 g/g which agrees with the previously reported values for the sewage sludge incineration dust collected by an electrostatic precipitator.
738
Environ. Sci. Technol., Vol. 20, No. 7, 1986
(2) The EP-ash has a fairly high mercury adsorption capacity on the order of lo4 g/g compared with the ordinary soils. (3) Physical adsorption of mercury vapor at an elevated temperature can be predicted by Dubinin's equation. (4) Very little physically adsorbed mercury on the raw EP-ash occurs at a high temperature, and most mercury is chemically adsorbed or contained as chemical compounds in the EP-ash. Acknowledgments
We express our appreciation for the financial support and useful discussions by T, Kasakura, Assistant to the General Manager, Environmental System and Equipment Division, NGK Insulators, Ltd. Registry No. Hg, 7439-97-6; C, 7440-44-0.
Literature Cited (1) Suzuki, M.; Kubota, H.; Kanaya, K. Kougai to Taisaku 1985, 21, 69-76. (2) Kato, T. Nihon Gaishi Research Report-Kankyo Souchi Tokushu, NGK Insulators Ltd., Japan, 1976. (3) Otani, Y.; Emi, H.; Kanaoka, C.; Matsui, S. Enuiron. Sci. Technol. 1984, 18, 793-796. (4) Kitamura, S.; Kondo, M.; Takizawa, Y.; Fujii, M.; Fujiki, M. Suigin; Koudansha Press: Tokyo, 1976; p 84. (5) Hayashi, S. Sangyo Kougai 1980,16, 714-717. ( 6 ) Kurihara, S. Kagaku Kogyo 1980, I , 83-88. (7) Fang, S. C. Environ. Sci. Technol. 1978, 12, 285-288. (8) Astakhov, V. A.; Dubinin, M. M.; Romankov, P. G. Theor. O m . Khim. Tekhn. 1969,3, 292-295.
Received for review April 15,1985. Revised manuscript received February 20, 1986. Accepted April 8, 1986.
