strong north-south going currents do not only fail to deposit material carried from other parts of the lake but also, to a large extent, prevent sedimentation of the local atmospheric inputs.
Conclusions We have determined sedimentation rates as well as anthropogenic and natural fluxes of As, Pb, Zn, and Cd for Green Bay and northern Lake Michigan sediments. The inputs of solids and trace elements of Green Bay sediments are largely governed by rivers and runoff flowing into the bay. For the northern Lake Michigan stations, aerosol fallout may be the dominant source of solids and several trace elements. This is consistent with the water circulation and the remoteness of river outfalls. Two of these stations receive even less Pb, Zn, and Cd than the atmospheric inputs, with one station showing no recent input as evidenced by the Pb-210 activities which are essentially constant and equal to supported Pb-210 levels throughout the core. A third Lake Michigan station accumulates Pb, Zn, Cd, and Pb-210 at rates indicating that the atmosphere is the major source not only of P b but also of Zn and Cd.
(9) Kemp, A. L. W.; Williams, J. D. H.; Thomas, R. L.; Gregory, M. L. Water, Air. Soil Pollut. 1978,10, 381-402. (10) Edgington, D. N.; Robbins, J. A. Enuiron. Sci. Technol. 1975, 10,266-74. (11) Winchester, J. W.; Nifong, G. D. Water,Air, Soil Pollut. 1971, I , 50-64. (12) Bertrand, G.; Lang, J.;Ross, J. “The Green Bay Watershed, Past, Present, Future”; Technical Report No. 229, University of Wisconsin Sea Grant College Program, 1976. (13) Moore, J. R.; Meyer, R. P.; Morgan, C. L. “Investigation of the Sediments and Potential Manganese Nodule Resources of Green Bay, Wisconsin”; Technical Report WIS-SG-73-218, University of Wisconsin Sea Grant College Program, 1973. (14) Wickham, J. T.; Gross, D. L.; Lineback, J. A.; Thomas, R. L. Enuiron. Geol. Notes Ill. State Geol. Suru. 1978,84,10-22. (15) Koide, M.; Bruland, K. W. Anal. Chim. Acta 1975,75,1-19. (16) Robbins, J. A.; Edgington, D. N. Geochim. Cosmoshim. Acta 1975,39,285-304. (17) Gross, D. L., Illinois Geological Survey, personal communication, 1980. (18) Van Loon, J. C.; Lichwa, J.; Ruttan, D.; Kinrade, J. Water,Air, Soil Pollut. 1973,2,473-82. (19) Perkin-Elmer Cora.. Norwalk. CT. “Instruction Manual for Atomic Absorption Spectrophotometer Model 305A”, 1973. (20) Edgington, D. N.; Callender, E. Earth Planet. Sci. Lett. 1970, 8,97-100. (21) U S . Environmental Protection Agency “Report on the Degree of Pollution of Bottom Sediments, Green Bay, Wisconsin”; Region V, Great Lakes National Program Office, 1977. (22) Krezoski. J. R.: Mozlev. S. C.: Robbins. J. A. Limnol. Oceanom. 1978,23,1011-16. (23) Robbins, J. A.: Krezoski. J. R.: Mozlev, S. C. Earth Planet. Sci. Lett. 1977,36, 325-33. (24) Robbins, J. A.; Edgington, D. N. Proc. Fed. Conf. Great Lakes, 2nd 1975,378-90. (25) Robbins, J. A.; Edgington, D. N.; Kemp, A. L. W. Quat. Res. ( N . Y . )1978,10,256-78. (26) Hodge, V.; Johnson, S. R.; Goldberg, E. D. Geochem. J. 1977, 12.7-20. (27) ‘Turekian,K. K.; Nozaki, Y.; Benninger, L. K. Annu. Reu. Earth Planet Sci. 1977,5,227-55. (28) Dolske, D. A.; Sievering, H. J. Great Lakes Res. 1980,6,18494. (29) Kemp, A. L. W.; Dell, C. I.; Harper, N. S. J . Great Lakes Res. 1978,4,276-87. (30) Durham, R. W.; Joshi, S. R., submitted to Chem. Geol. (31) Kemp, A. L. W.; Harper, N. S. J. Great Lakes Res. 1976, 2, 324-40. (32) Sato, G. K.; Mortimer, C. H. “Lake Currents and Temperatures Near the Western Shore of Lake Michigan”; Special Report No. 22, Center for Great Lakes Studies, University of Wisconsin-Milwaukee, 1975. (33) Anderson, M. A.; Holm, T. R.; Iverson, D. G.; Stanforth, R. R. “Input, Deposition and Conversion of Arsenic in an Aquatic Environment”; EPA report, Grant No. R804881010,1980. (34) Cahill, R. A., Illinois Geological Survey, personal communication, 1980. (35) Fingleton, D. J.; Robbins, J. A. J. Great Lakes Res. 1980, 6, 22-37. (36) Eisenreich, S. J. Water, Air, Soil Pollut. 1980,13, 287-301. I
Acknowledgment We thank C. H. Mortimer and D. N. Edgington for their interest in this work, M. Koide for supplying Pb-120 standards and low-activity carrier lead, and R. Grunewald for use of his counting equipment. Appreciation is further extended to R. A. Cahill for supplying the Lake Michigan cores, to M. C. Birchall, captain of the Canadian survey ship Limnos, and to project leader R. L. Thomas. The Green Bay cores were taken with the assistance of M. A. Anderson and T. Holm from RIV Neeskay operated by Captain R. Popp. Literature Cited (1) Walters, L. J., Jr.; Wolery, T. J.; Myser, R. D., Proc. Conf. Great Lakes Res. 1974,17,219-34. (2) Shimp, N. F.; Schleicher, J. A,; Ruch, R. R.; Heck, D. B.; Leland, H. V. Enuiron. Geol. Notes (Ill. State Geol. Suru.) 1971,41. (3) , , Frve. J. C.: Shima. N. F. Enuiron. Geol. Notes (Ill. State Geol. Sur;.)’1973,’60, 12: ’ (4) . , Krishnaswamv. S.: Lal. D.: Martin, J. M.: Meybeck, M. Earth Planet, Sci. Let”t: 1971,11, 407-14. (5) Koide. M.: Soutar, A.; Goldberg, E. D. Earth Planet. Sci. Lett. 1972,14,442-6. (6) Bruland, K. W.; Bertine, K.; Koide, M.; Goldberg, E. D. Enuiron. Sci. Technol. 1974,8,425-32. (7) Christensen, E. R.; Scherfig, J.; Koide, M. Enuiron. Sci. Technol. 1978, 12,1168-73. ( 8 ) Bertine, K. K.; Mendeck, M. F. Erzuiron. Sci. Technol. 1978,12, 201-7.
Received for reuiew July 9,1980. Accepted January 6,1981.
A Cylindrical Pb02 Diffusion Tube for Separating SO2 from an Airstream David J. Kaplan* and David M. Himmelblau Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 787 12
Chikao Kanaoka Department of Chemical Engineering, The University of Kanazawa, Kanazawa, Japan
Diffusion denuders or strippers have been used in a number of aerosol studies for removing SO2 from a gas stream (1-4). The diffusion-denuder principle has also been applied in diffusion driers for removing water vapor from an airstream containing aerosols and for removing water vapor from an airstream containing SOz, as in some commercial SO2 analyzers. For SO2 mixed with an airstream containing sulfatebearing aerosols, such as in the stack exhaust from a coal-fired 558
Environmental Science & Technology
power plant, it is necessary to separate the SO2 from the airstream if one is interested in obtaining an accurate measure of the total amount of sulfate in the aerosols. If this separation is not done, SO2 will continue to react in the aerosols while they are in the collection device to form additional sulfate. Moreover, if a filter is used as the collector, SO2 may react with the filter material to form sulfate. Therefore, it is necessary to use a stripper upstream of the collection point. 0013-936X/81/0915-0558$01.25/0 @ 1981 American Chemical Society
An SO2 stripper containing PbO2 powder held behind a fine tubular mesh was developed and tested. Its active length was 60 cm, and its inner diameter was 1.27 cm. Tests using 35S as a radioactive tracer showed that the axial variation of the SO2 concentration in the stripper agreed well with that predicted by the theory of Gormley and Kennedy. At a flow rate of 420 cm3/min the stripper had an input-to-output SO2 concentration reduction factor of -lo6. Measurements indi-
cated that the usable life of the stripper a t an input concentration of 1ppm SO2 was several hundred hours. Several different types of SO2 strippers were fabricated and tested, and the PbO2 powder type was found to be far superior in terms of efficiency and absorptive capacity. These qualities make the stripper a very useful tool in separating SO2 from aerosols in sampling applications or as a reduction terminator in Son-aerosol kinetic studies.
Strippers have also been used to measure the amount of stripped-out material. Hickey and Hendrickson ( 5 ) have summarized the results of several studies in which PbOz cylinders (generally PbOz-gum tragacanth mixtures applied to a cylindrical form) were used to measure the amount of SO2 in air. They found from their own experiments that the smaller the grain size of the PbO2 used the higher the rate of absorption of SO2 and the higher the critical loading or absorptive capacity of PbO2 reacted. It should be noted that in these studies there was no flow of air through the cylinders; these were static studies in which only the absorption and the reaction were studied. The stripper described here was developed in connection with an experimental apparatus used to measure the rate of SO2 oxidation in aerosols. In the experiment aerosols consisting of salt-solution droplets were allowed to react with SO2 tagged with radioactive 35Sin a flowing-gas system. After flowing through the reactor, the aerosols passed through the stripper which terminated the reaction by removing SO2 from the gas phase and from the droplets. Finally, the droplets were collected on a filter, and the total amount of sulfate formed in the aerosols was measured by counting in a scintillation counter the 35Scollected on the filter. In the aerosol experiment the requirements on the stripper performance were severe: (1)The stripper had to have the highest possible efficiency or ratio of input to output SO2 concentration for a fixed length and flow rate so that the background count from the 35S leaving the stripper would be negligible. It was estimated that an efficiency of -lo6 was required. (2) The SO2 cumulative absorptive capacity of the stripper had to be as high as possible. In other words, it was required that, for given input conditions, the concentration vs. distance characteristic of the stripper had to remain essentially constant over as long a time as possible (at least several hundred hours or several experimental runs). This requirement was necessary because the terminating time of the reaction had to remain constant. (3) The inner surface of the stripper had to be fairly smooth in order not to cause turbulence in the airflow that might have resulted in significant (>lo%) losses of aerosol particles. (4) The PbOz in the wall of the stripper had to be firmly secured. Because the wall became increasingly radioactive as 35S02reacted with the wall material, shedding of even nanomoles of wall material would have led to collection of radioactive material on the collection filter and thus produced an unacceptably high background count rate. The goal, therefore, was to develop a stripper which would satisfy the above four requirements. The stripper reported in this paper differed from the strippers developed by Durham et al. ( 4 ) and by Smith et al. (6) in that their strippers were made by electroplating PbOz onto lead foil. Their strippers had efficiencies as high as ours and had stripper characteristic curves which agreed well with theory, but they probably did not have absorptive capacities as high as ours. These workers u ~ e d as ~ ~a radioactive S tracer to measure the amount of SO2 diffused to the walls. The wall material was then cut into strips to measure the axial variation of SO2 in their strippers. In our approach the SO2 gas con-
centration in the stripper was measured directly, since we could not section our wall material and analyze for 35Sas in the above-mentioned studies. Several papers have treated the theory of removal from a gas mixture of a reactive gas flowing through a cylindrical tube in which diffusion of one component takes place to a reactive wall (7-9). Gormley and Kennedy (8)considered the situation in which a gas flows in a cylindrical tube with a constant input concentration, no, of the reactive gas at the input face of the tube. The velocity profile was assumed to be fully developed, and the wall was assumed to be a perfect sink for the reactive gas, so that the partial pressure of the reactive gas at the wall was zero. For these boundary conditions they solved the three-dimensional steady-state diffusion plus convection equation for the axial dependence of the partial pressure of the reactive gas. They found that the concentration, n, of the reactive gas averaged over any given cross section at a distance x from the axial coordinate origin located at the center of the input face of the tube was given approximately by
+
n/no = 0.8191e-7.314h 0.0975e-44.6h+ 0.0325e-114h
+ . ..
(1)
where h = x / ( K ~ ~K )=, 2Q/(m4D), Q is the volumetric flow rate of the gas mixture in the tube, u is the tube radius, and D is the diffusivity of the reactive gas component through the gas mixture at a given temperature and pressure. Our experimental conditions were the same as those used by Gormley and Kennedy (8) in their treatment of the problem, and our results agreed well with eq 1. Experimental Section Several different types of cylindrical SO2 strippers were tested: (1)PbOz powder behind mesh, (2) anodized lead foil, (3) PbOz deposited electrochemically from P b ( N 0 3 ) ~solution onto stainless-steel foil, (4) PbOz paste using Elmer's glue, (5) PbOz paste using gum tragacanth, (6) LiOH impregnated into filter paper, (7) PbOz-alcohol slurry deposited on a glass tube, (8) MnOz of various mesh sizes behind mesh, and (9) activated charcoal packed behind mesh. Design and Construction of the Stripper. Figure 1 is a scale drawing of the stripper shown in cross section. The inlet pieces (1, 2, and 9) were made of No. 316 stainless steel, a material fairly inert to S02. All pieces downstream of the point where the PbO2 wall began did not need to be made of materials especially inert to S02, since SO2 was to be stripped out of the air. The length of the inlet tube ( based on the requirements of the aerosol experiment) was 27.3 cm. The tube was just long enough to allow a fully developed parabolic velocity profile to develop. The outlet tube tapered down to 0.25-in. 0.d. to match the diameter of the aerosol counter sampling inlet located just downstream of the stripper in our aerosol experiment. The most difficult task in fabricating the stripper was the construction of the stainless-steel mesh cylinder used to hold back the PbOz powder. Because of the extreme fineness of the PbOz powder (average particle size,
