An Experimental Study of the Effect of Temperature upon Aerosol

(4) Pelon, W., Whitman, B. F., Beasley, T. W., Enuiron. Sci. Technol., 11,619 (1977). (5) Pelon, W., Whitman, B. F., Beasley, T. W., Lesley, D. E., Enuiron. Int., in press. (6) Rhim, J. S., Cho, H. Y., Rabstein, L., Gordon, R. J., Bryan, R. J., Gardner, M. B., Huebner, R. J., Nature (London), 239, 103 (1972). (7) Rhim, J. S., Cho, H. Y., Jaglekor, M. H., Huebner, R. J., J . Natl. Cancer Inst., 48,949 (1972). ( 8 ) Rhim, J. S., Creasy, B., Huebner, R. J., Proc. Natl. Acad. Sei. U.S.A., 68,2212 (1971). (9) Louer, J. C.. Lana, D. R., Smith. C. C.. Water Chlorination: En"iron. Impact H e d t h Eff., Proc. Conf., 2,433 (1978). (10) Page, T., Harris, R. H., Epstein, S. S., Science, 193,55 (1976).

(11) Harris, R. H., Page, T., Reiches, N. A., in "Origins of Human Cancer", Hiatt, H. H., Watson, J. D., Winsten, J. A., Ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y., 1977, p 309.

Receiued for review May 18, 1979. Accepted March 10, 1980. This project has been financed primarily with federal f u n d s from the Environmental Protection Agency under Grant N o . R800188, awarded by the Water Quality Division, Health Effects Research Laboratory, Cincinnati, Ohio. The contents d o not necessarily reflect the views and policies of the Environmental Protection Agency, nor does mention of trade names or commercial products constitute an endorsement or recommendation for use.

An Experimental Study of the Effect of Temperature upon Aerosol Charge State Hsu-Chi Yeh" and Yung-Sung Cheng Inhalation Toxicology Research Institute. Lovelace Biomedical and Environmental Research Institute, P.O.Box 5890, Albuquerque, N. Mex. 871 15 Average electrostatic charges (polarity ignored) on three sizes of monodisperse fused aluminosilicate spherical particles were measured as a function of heat treatment temperature. The range of temperatures studied was 45 to 1150 "C. Results show that there are two charge peaks a t about 600 and 1000 "C, respectively. The first peak is probably due to electron emission, while the second peak is attributed to positive ion emission. It was also found that the initial charge state has minimal effect on the final charge state for temperatures higher than 600 "C for the fused aluminosilicate particles studied here. Possible implications of the thermionic emissions are also discussed. The electrostatic charge of particles is an important property of aerosols that will influence aerosol behavior ( I ) . In many aerosol studies, a knowledge of the aerosol charge state is desired or necessary. Particles may acquire electrostatic charges by various mechanisms and processes. Aerosols are usually highly charged when produced by nebulization of , direct resuspension of a dry solutions or suspensions ( 2 , 3 ) by powder ( 4 ) , or by a vibrating orifice aerosol generator ( 5 ) . Particles also may be charged by exposing the aerosol to a unipolar or bipolar ionic atmosphere ( I ) , or by the electrical induction method (6). In the case of a radioactive aerosol, particles may gain charges due to the self-charging mechanism (7,8).One other mechanism by which an aerosol can acquire electrostatic charges is by thermionic emission when it is heated to a sufficiently high temperature. This process has not received adequate attention. Since the first discovery of thermionic emission by Thomas Edison in 1883 ( 9 ) , a large volume of work has been done, primarily in the field of vacuum tubes, involving the study of the emission of electrons from a metal surface a t high temperature under high vacuum and an applied electric field. Feeney et al. (IO) and Blewett and Jones ( I I ) reported techniques for producing sources of positive ions of various alkali metals from aluminosilicate, also under vacuum and the presence of an electric field. Only limited information exists concerning thermionic emissions from aerosol particles. Shuler and Weber (12)reported that electron concentration in rich hydrocarbon flames was found to be higher by one to two orders of magnitude than for a purely thermal flame ionization. They attributed this higher electron concentration to the thermionic emission of electrons from soot particles formed 726

Environmental Science & Technology

in the flame. Almost no reports exist concerning direct measurement of the aerosol charge state or changes in aerosol charge state as a function of temperature change. High-temperature heat treatments of aerosols are sometimes used in the laboratory to obtain a desired physicochemical form of the aerosol (13). Particles produced and released from power plants or internal combustion engines have also been subjected to high temperature. In this paper, experimentally observed changes in aerosol charge state with temperature are reported. M a t e r i a l s and M e t h o d s

Aerosol. Monodisperse fused aluminosilicate spherical particles (FAP) were chosen as the test particles for their ability to withstand temperatures up to about 1200 "C. The preparation of the monodisperse FAP involved: (1)treating an aqueous suspension of finely ground montmorillonite clay with 30% hydrogen peroxide solution to remove organic impurities, (2) packing exchange sites with sodium and removing excess sodium by running water dialysis, (3) aerosolizing using a nebulizer and passing aerosols through a heating column a t 1150 O C , (4) separating into monodisperse fractions using a Lovelace Aerosol Particle Separator, LAPS ( 1 4 ) , and ( 5 ) resuspending the desired monodisperse fraction and aerosolizing using a nebulizer. Details of the production procedures of the monodisperse FAP aerosol have been reported (13, 15). Charge Measurement. The aerosol charge was measured using a parallel plate electrical mobility spectrometer previously described (7). The spectrometer consisted of two parallel metal plates between which a voltage was applied. The aerosol was introduced into the spectrometer through a narrow inlet slot nozzle as a thin ribbon and sheathed with clean air on all sides. The electrical mobility, Z,, of an aerosol particle was calculated using the equation:

2, = 2 u h 2 / V x where u (centimeterslsecond) is the mean flow velocity inside the spectrometer, h (centimeters) is the half-interplate distance of the spectrometer, x (centimeters) is the distance from the aerosol inlet nozzle a t which a particle having electrical mobility 2, deposits on the plates, and V (statvolts) is the potential difference between the plates. The theoretical electrical mobility of a spherical particle is given by:

0013-936X/00/0914-0726$01 .OO/O @ 1980

American Chemical Society

I .o

(A)

T * 45'C 0.4

A

T = 600.C

,

T * 1,OOO'C

0.2 0

FAP d p ' 0 . 6 6 p m d o K r - 8 5 I

1.0

I

1

I

I

1

A-&

I

I

I

I

(B1

Figure 1. Schematic diagram of the experimental system. (Krypton-85 was used only as indicated in the text)

1

-

Z, = N,eC/6~pr,

'

T = 1,OOO'C

(2)

where N , is the number of charges on the particle, e is the esu), C is the slip correction, p electron charge (4.8 X (poise) is the fluid viscosity, and r p (centimeters) is the radius of the particle. By combining Equations 1 and 2, the number of charges can be written as:

N , = 12nprPuh2/eCVx

A

T245.C T~600.C

(3)

Let x = L , the total length of the plate; then for a monodisperse aerosol under a given sampling condition, Equation 3 indicates that those particles with number of charges (both positive and negative, i.e., ignoring polarity) less than N,will not be collected on the plates and will thus penetrate the spectrometer. By varying the applied potential on the plates and counting the number of particles which are not collected, we were able to determine the cumulated charge distribution (ignoring polarity of the charge) of the aerosol. A schematic diagram of the experimental setup is shown in Figure 1. The aerosol was produced by nebulization of a monodisperse FAP suspension using a Lovelace nebulizer and then passed through a heating column a t a given temperature. Immediately after the heating column, the aerosol was diluted with a large volume of dry clean air and drawn through an aerosol chamber This essentially brought the aerosol temperature down to room temperature. The aerosol sample was then drawn from the aerosol chamber, and the charge distribution was deterrnined by the electrical mobility spectrometer followed by an optical particle counter (Climet 208). Four series of experiments with three sizes (0.66, 1.41, and 2.04 pm diamettr) of monodisperse fused aluminosilicate spherical particles (density = 2.2 g/cm3) were conducted over the temperature range of 45 to 1150 "C. In one of the series, an s5Kr aerosol discharger was placed between the aerosol generator and the heating column, so that the aerosol was first discharged before entering the heating column. In each series, the particles were aerosolized by nebulization from the same aqueous suspension of FAP, with the same nebulizer and under identical experimental conditions, except for the temperature of the heating column. The transit time for particles to travel through the heating column ranged from 1 to 5 s depending on the heating column temperature (mass flow rate = constant). The potential difference applied between the plates ranged from 0 to 1 2 000 V (40 statvolts). The heating column temperature was measured a t the wall of the heating quartz tube. Assuming constant wall temperature and fully developed flow, the air stream temperature inside the heating tube can be estimated from standard heat transfer equations for laminar flow in a tube ( 1 6 ) .I t was estimated that by the time the air reached the midpoint of the heating tube, the temperature of the air a t the center of the tube would be 098 to 0.99T, ( T , is the tube wall tempera-

0.2

0

E4P dp = 1.41 pm w/o I 40

I

INpl, ELEMENTARY

-

Kr-85

I I20

I

I

80

-

I

160

CHARGE UNITS

A

Tz600.C

'

T = !,OOO*C

FAP dp: 2 04 pm w/o

Kr-85

(D)

-

-

A

T=45*C T

-

:600.C

T : 1,000.C FAP dp = 2 . 0 4 p m with Kr-85

I

lNpl

, ELEMENTARY

1 120

-

I

160

CWRGE UNITS

Figure 2. Cumulative charge distributions for fused aluminosilicate particles heat treated at different temperatures: (A) 0.66 pm; (8)1.41 pm; (C) 2.04 pm; (D)2.04 p m (discharging with an 85Kr discharger, before heat treatment)

ture). By the time the air reached the end point of the heating tube, the air stream temperature was essentially the same as the wall temperature.

Results and Discussion With the present method of charge measurement, the polarity of the charge associated with the particle could not be distinguished. Therefore, the charge being measured is lNpl, rather than N,. Also, it is assumed that the effect of rapid cooling after the heating column on the particle charge state is negligible. Figures 2A-D show the cumulative charge diStributions for particles heat treated a t T = 45, 600, and 1000 "C obtained Volume 14, Number 6, June 1980

727

(A) p-

12

I

I

z 3

I

I

I

FAP dp=O,66pm w/o K r - 8 5

I." I

I

I

400 T.

(B)

$Or

I

f l

I

1200

800

'C 1

I

I

I

I

1

FAP d p = l . l l p m w b K r - 8 5

E

1

I

I

800

400 T.

1200

Dc

W

/-I

2

0

IZ"

O

I 4

I-! 1

I#

I+1

I

I f f I f f I-I I-I I-I I-I 1 4 - 1 I 4 /-I

400

I

800 T,

1200

C '

Figure 3. Average particle charge vs. heat treatment temperature for fused aluminosilicateparticles: (A) 0.66 pm: (B) 1.41 pm: (C) 2.04 pm. [(+)and (-) indicate the sign of the dominant polarity of the charge at a given heat treatment temperature]

from the four series of experiments. As shown in the figures, the charge distribution changed with temperature, indicating that either charging or discharging due to the effect of temperature had taken place. At lower temperature, the aerosols probably were bipolarly charged, for clearly some fraction of the aerosols carried zero charge. But a t T = 1000 "C, these figures indicate that almost every particle carried some charges. This phenomenon suggests that the aerosols were unipolarly charged, due to thermionic emission. From the charge distribution data, the average charge (polarity ignored), IRpl,a t each heat-treated temperature can be calculated. The results of )R,l vs. T are shown in Figure 3. Two charge peaks were seen, one a t about 600 "C and the other a t about 1000 "C, for the temperature range studied herein with FAP aerosols. At T = 1150 "C, the average charge had been reduced to a level similar to those aerosols discharged by an S5Kr discharger a t 45 "C. From Figure 3C one notes that although the initial charge state of the aerosol had some effects on the final charge state after heat treatment a t moderately high temperature, the effect was negligible for heat treatment a t temperatures higher than 600 "C. This can also be seen in Figures 2C and 2D where similar charge distributions were obtained for aerosols treated a t 1000 "C. Since either electrons or ions can be emitted due to thermionic emission, it would be interesting to know which charge polarity dominates among the aerosols treated a t each temperature. This was done for both series with d, = 2.04 pm by drawing aerosol samples from the aerosol chamber into a Faraday cup and detecting the polarity of the current with an 728

Environmental Science & Technology

electrometer. Results for the dominant polarity of the charge are also indicated in Figure 3C as either (+) or (-) for positive and negative polarity, respectively. Both series, with or without passing aerosol through the Kr-85 discharger before entering the heating column, had the same sign of the dominant polarity a t each corresponding temperature. The sign of the dominant polarity changed from negative for T = 45-400 "C to positive for T = 500-750 "C, and then changed back to negative for T greater than 800 "C. These observations suggest that at moderately high temperatures, FAP particles may emit electrons, probably due to some impurities, but the intensity of this electron emission is not very strong, so that not all the particles carry positive charge. The thermionic emission of negative ions is possible, but it is highly unlikely a t this temperature range. For T greater than 800 "C, these particles may emit positive ions. The montmorillonite clay used in this study has the formula (A13 14Feo Ij2MgO GO)(Sip,)OZo(OH)4(Naol&ao 21) (171, in which the first parentheses represent octahedrally coordinated ions; the second, tetrahedrally coordinated ions; and the last, exchangeable cations. Among these constituents, the most probable positive ion emitted at high temperature would be Na+ due to its relatively low positive ion work function (about 3.8 eV for pure Na metal, which may be lower with the presence of impurities). Thermionic emission of Na+ ion from aluminosilicate in the temperature range 750-1100 "C has been reported (10).When the temperature reaches about 1150 "C or above, the thermal ionization of the air may reach a level adequate to discharge the particles. It has been suggested that, at T = 816 "C, the thermal ionization of air may start to become noticeable and, above 1127-1327 "C, the thermal ionization of air becomes very intense (18).This may explain the particle charge peak of FAP a t about 1000 "C, where the self-charging due to thermionic ion emission competes with discharging from thermal ionized air, a phenomenon similar to that of radioactive aerosols (8). A potential problem that may cause errors was that the particle size may change due to heating, resulting in an apparent change in particle charge state. To check this possibility, a point-to-plane electrostatic precipitator (19)was used to obtain samples of 1.41 pm diameter FAP particles heat treated a t T = 45 and 1150 "C. The samples were studied using a Hitachi 11-C transmission electron microscope. About 100 particles from each temperature run were sized. Results indicated that for T = 1150 "C, the particle diameter was reduced by 3%, as compared to those a t 45 "C. One should note that this 3% reduction in size is comparable to experimental error using the EM method. The geometrical standard deviations (a,) were found to be 1.13 and 1.12 for particles heat treated a t T = 45 and 1150 "C, respectively. Therefore, reheating monodisperse FAP particles caused no significant change in the particle size and size distribution. It is possible that the heating quartz tube may emit nuclei which may carry charge and subsequently become attached to FAP particles. To check this possibility of nuclei emission from a heated quartz tube, the same nebulizer without aqueous FAP suspension was used so that clean air, a t the same flow rate, was passed through the heating column. A sample was drawn from the chamber and the nuclei concentration was determined using a condensation nuclei counter (Environmental/One Model Rich 100). Another sample was drawn from the chamber into a Faraday cup, connected to an electrometer, to measure the current associated with nuclei flow. Neither detectable nuclei concentration nor measurable current was observed for T I1140 "C. At T = 1150 "C, nuclei concentration on the order of 103/cm3 was observed, but no current was observed. These results suggest that the contribution of nuclei emission, if any, to the measured particle charge state is negligible in our experiments.

At present the basic phenomena for electron and positive ion emissions from solids are not well understood except for pure metals. In the case of pure metals, the emission of electrons can be estimated from the Richardson equation, which is an exponential of electron work function and temperature. The estimation of emission of ions is more complicated. Values of the electron work function and the positive ion work function are influenced by the state and purity of the emitting surface, such as charges on the surface, electric field around the surface, amount and type of impurity, etc. A review of thermionic emission can be found in standard physics handbooks (20). Most aerosol particles rarely consist of a pure metal; thus, making a prediction of thermionic emission is almost impossible. Aerosol generation and preparation inevitably introduced some impuirities to the particles, including those originating from the residue of the distilled-deionized water (adjusted to pH 10)used in making the nebulizing suspension and resuspending monodisperse FAP from stainless steel foil. Since the impurities are unknown and very difficult to control, the average number of charges on FAP particles after heat treatment at different temperatures as observed herein should be used only as a relative reference number or trend and should not be considered as a definite value. As discussed previously, these observations suggest that the FAP particles will gain charges due to thermionic emission, even a t moderately high temperatures of about 600 “C. Therefore, when sampling high-temperature process streams from a coal-fired power plant or sampling from the exhaust of internal combustion engines, it is recommended that some sort of aerosol discharge device be used to avoid or minimize sampling bias and aerosol losses due to possible charge effects. Also, in designing cleanup devices, such as electrostatic precipitators, effects due to thermionic emission should be considered. I t is reasonable to assume that a t high temperatures the charging efficiency and thus the collection efficiency would be higher for negative corona than for positive corona if the particles have emitted positive ions and gained negative charges due to thermionic emission. Brown and Walker (21) reported that the removal efficiencies of alumina dust for an electrostatic precipitator operated a t 1650 OF (900 “C) and 100 psig were higher when a negative corona was used rather than a positive corona. This may partly be due to thermionic emission of the particles.

Ac k noic 1e d g m e n t s The authors are indebted to Dr. M. B. Snipes for supplying preseparated monodisperse fused aluminosilicate particles, to Dr. P. B. DeNee for the electron microscope work, to many colleagues for helpful suggestions and review of the manuscript, to Mr. Emerson E. Goff for preparing the illustrations, and to R. Allen and J. Latham for typing the manuscript. Literature Cited (1) Whitby, K. T., Liu, B. Y. H., “Aerosol Science”, Davies, C. N., Ed., Academic Press, New York, 1966, pp 59-86. (2) Chow, H. Y., Mercer, T. T., A m . Ind. Hyg. Assoc. J., 32,247-55 (1971). (3) John, W., Davis, J. W., Atmos. Enciron., 8, 1029-34 (1974). (4) Marple, V. A., Liu, B. Y. H., Rubow, K. L., A m . Ind. Hyg. Assoc. J., 39,26-32 (1978). (5) Liu, B. Y. H., Pui, D. Y. H., J . Aerosol Sci., 5,465-72 (1974). (6) Reischl, G., John, W., Devor, W., J . Aerosol Sci., 8, 55-65 (1977). (7) Yeh, H. C., Newton, G. J., Raabe, 0. G., Boor. D. R., J . Aerosol Sci., 7, 245-53 (1976). (8) Yeh, H. C., Newton, G. J., Teague, S.V., Health Phys., 35,500-3 (1978). (9) Sears, F. W., Zemansky, M. W., “University Physics”, AddisonWesley, Reading, Mass., 1964. (10) Feenev. R. K.. Savle. “ . W. E.. 11.,HooDer. . J. W.. Rec. Sci. Instrum.. 47, 964-?’(1976). (11) Blewett, J. P., Jones, E. J., Phys. Reu., 50,464-8 (1936). (12) Shuler, K. E., Weber, J., J . Chcm. Phys., 22,491-502 (1954). (13) Raabe, 0. G., “Fine Particles”, Liu, B. Y. H., Ed., Academic Press, New York, 1976, p p 57-110. (14) Kotrappa, P., Light, M. E., Reu. Sei. Instrum., 43, 1106-12 (1972). (15) Raabe, 0. G., Kanapilly, G. M., Newton, G. J., “Inhaled Particles 111”, Walton, W. H., Ed., Unwin Brothers, Surrey, England, 1971, pp 3-18. (16) Knudsen, J. G., Katz, D. L., “Fluid Dynamics and Heat Transfer”, McGraw-Hill, New York, 1958, pp 368-72. (17) Eliason, J. R., A m . Miner., 51,324-35 (1966). (18) Bush, J. R., Feldman, P. L., Robinson, M., J . Air Pollut. Control Assoc., 29,365-71 (1979). (19) Morrow, P. E., Mercer, T. T., A m . Ind. Hyg. Assoc. J., 25,8-14 (1964). (20) Condon, E. U., Odishaw, H., Ed., “Handbook of Physics”, McGraw-Hill, New York, 1958. (21) Brown, R. F., Walker, A. B., J . Air Pollut. Control Assoc., 21, 617-20 (1971).

Received for review January 8, 1980. Accepted March 17, 1980. Research was performed under U.S. Department of Energy Contract DE-AC04-76EV01013.

Volume 14, Number 6, June 1980

729