Microwave Decomposition of Toxic Vapor Simulants Lionel J. Bailin" and Merle E. Sibert Lockheed Palo Alto Research Laboratory, Palo Alto, Calif. 94304
Leonard A. Jonas Edgewood Arsenal, Aberdeen Proving Ground, Md. 21010
Alexis T. Bell University of California, Berkeley, Calif. 94720
An electrical discharge technique is described whereby the destruction of toxic gas simulants was carried out in a microwave plasma. Nearly 100% decomposition of dimethyl and diisopropyl methylphosphonates in helium and air was effected. Residence times ranged from 0.132.4 sec. Identification of reaction products indicates reduction of the compounds in the main to phosphoric acid or its precursor, P205, and dealkylated phosphonic acids. The method holds promise for the controlled destruction of pesticides, defoliants, and toxic chemical vapors in large volume plasma applicators. The removal of toxic contaminants or harmful gases from buildings, contained atmospheres, or process exhausts has become an environmental problem of consequence. Existing techniques have dealt with removal via scrubbing towers or adsorption on activated carbon, followed by long-term hydrolytic processing to destroy the toxic components. Catalytic dealkylation procedures as carried out by Baier and Weller ( I ) and Sanyal and Weller ( 2 ) have also shown promise, except that temperatures on the order of 220°C or higher were required. Research on the decomposition of toxic vapor or their simulants in contained atmospheres by passage through a microwave discharge began a t the Lockheed Palo Alto Research Laboratory in 1967. At that time it was well known that microwave discharges could be used to promote a variety of chemical reactions ( 3 ) . The initiation of these reactions occurred through the creation of free radicals which resulted from collisions between reactant molecules and the free electrons present in the plasma. It was therefore considered reasonable that this approach could be applied to the scission or destruction of bonds in compounds, which, for various reasons, were considered objectionable. As an example, the nerve gas, or G-agent simulant dimethyl methylphosphonate 'was exposed to a discharge produced in a commercial, laboratory-size microwave cavity. Nearly 100% decomposition of the compound resulted. Although a simulant was used which was structurally similar to the actual nerve gas, rather than the highly toxic material itself, it became apparent that a new, promising technique was available for the destruction of toxic vapors and liquids. Microwave P l a s m a Discharge
In the present context, the term microwave plasma denotes an ionized gas produced via microwave induced electron reactions with neutral gas molecules. Its generation may be envisioned as follows: In a gas under reduced pressure, a few low-energy electrons resulting from cosmic ray or other ionizing phenomena are accelerated by the microwave electromagnetic field until they gain sufficient energy to become involved in inelastic collisions with the gas molecules. These collisions can result in the genera254
Environmental Science & Technology
tion of additional electrons and charged ions. If their production rate is greater than the loss rate, gaseous breakdown, i.e., plasma generation, occurs. When microwave frequencies are used to induce ionization, the microwave field in the cavity oscillates so rapidly that the force on the electrons will change direction before they can travel far, and, therefore, the plasma will not be swept out of the discharge region ( 4 ) . Consequently, electrons and ionized gas particles can be generated to remain within the discharge cavity, and thus contribute to chemical reactions in the plasma. In 1965, Fehsenfeld ( 5 ) reported on their evaluations of various microwave cavities for the production of discharges in gases from 1 p to 1 atm pressure. When compared with arc or electrode plasmas, the cavities showed many favorable characteristics, such as: Production of high ionization levels and molecular dissociation without excess heating of the contained gas. Construction of reaction vessels which are simple, free from contamination and less subject to damage because of the absence of internal electrodes. Production of little electrical interference. Absence of high voltages which can be easily contacted. In recent applications of the technique to chemical reactions, Bell (6) described models for high-frequency electric discharge reactors; and Wightman (7) reviewed the chemical effects of microwave discharges. Since no applications of a microwave discharge to the decomposition of toxic materials have been reported, the systems described below are presented in detail in order to illustrate the method and feasibility of this approach. E q u i p m e n t and M a t e r i a l s
Microwave Plasma System. The microwave discharge system consisted of a simulant-carrier gas supply, a discharge tube reaction zone, and vapor and liquid sampling systems. The schematic diagram is shown in Figure 1. The microwave power source was a Varian PPS-2.5A unit, which has an output frequency of 2450 MHz and delivers up to 2.5 kW. A Microlite 287-microwave leakage detector (Crystal Mfg. Co., Oklahoma City, Okla.) was used to monitor power leakage. Levels of leakage greater than 1-3 mW/cm2, which is the lower limit of sensitivity of the apparatus, were not detected in the immediate vicinity of the discharge tube. Analytical System. A Varian Model 1720-5 gas chromatograph was used to analyze the gaseous and liquid products. Separation of individual components was accomplished by a 6 ft X Y4 in. column made of Teflon tubing and packed with 60-80 M Chromosorb W supporting a 20% loading of Carbowax 20 M. Helium was used as the carrier gas. Liquid samples were injected into the chromatograph directly, whereas gaseous samples were introduced by means of an all-glass sampling valve similar to that described by Golz and Moffatt (8). The decomposi-
tion products resulting from the plasma discharge were analyzed both by nuclear magnetic resonance and mass spectroscopy. The NMR equipment used was a Varian HA 100, operating a t 40.5 MHz and employing 85% as an external reference in the detection of P 31. The M S equipment used was a Perkin Elmer-Hitachi RMU-6E single focusing, magnetic sector mass spectrometer, with a solid probe introduction system. The spectra were obtained at a 70-eV excitation. Toxic Vapor Simulants and Gases. The two nontoxic simulants studied were diqethyl methylphosphonate (DMMP), whose formula is CH3P(O)(OCH3)2,and diisopropyl methyllphosphonate (DIMP), whose formuia is CH3P(0)(i-OC3H7)2 obtained from Edgewood Arsenal, Md., and Mobil Chemical Co. These materials were studied because of their stability toward thermal decomposition, their relative safety for kinetic studies, and because of their structural similarity to numerous chemical warfare agents. Purities were approximately 98% or greater. Helium, grade A, was used as a carrier gas because of its known plasma-forming characteristics and the fact that it is routinely used in gas chromatography, thereby simplifying the analysis. Air was used as the second carrier gas for simulating conditions in buildings which might be contaminated with toxic agents. Dry air, dewpoint -59"C, was used as standard.
Experimental Procedure Simulant Delivery. The carrier gas, either helium or air, is passed via flowmeter into the vessel containing the simulant liquid. The gas impinges onto the surface of the simulant. Since the surface is of constant area, the flow rate remains constant. In the case of runs made with air, it was desired to reduce the effective concentration of simulant to that anticipated in a contaminated environment. To accomplish this, an aspirator device was employed on the exit side of the supply vessel, thereby providing for an auxiliary air flow. The device was not utilized on runs with the helium carrier. The flow system was calibrated for both DMMP and DIMP by bringing the supply vessel to an appropriate temperature and determining the amlount of material transferred to the liquid nitrogen trap. For DMMP, the temperature was 35°C; for DIMP, 51°C. These temperatures were selected in order to achieve reasonable material flow. Plasma Discharge. The entire system is evacuated to its minimum pressure, about 20 p . With liquid nitrogen in the trap condensers and the bypass trap open, a flow of helium or air plus simulant is established through the system. Pressure is adjusted to the desired level by regulation of the main vacuum valve. With all water and air cooling to the cavity on, the microwave power source is activated. Power is set to the desired level, and the tuning controls adjusted to give minimum reflected power. From the supply vessel, the carrier gas-simulant mixture passes directly through the microwave cavity where the plasma is initiated and sustained. A Tesla coil is used to ignite the glow discharge. Initially, flow is diverted to a bypass trap. As soon as steady state conditions have been achieved, usually 1-2 min, the flow is diverted to the sampling trap. From the trap, the remaining gas stream passes to the vacuum pump by way of a vacuum valve for regulation of total pressure. Upon completion of a run, flow is again diverted to the bypass trap. That part of the system encompassing the sampling trap is back-filled with helium, and sampled for analysis. Analytical. 130th the liquid products which condensed in the liquid nitrogen trap and the gaseous products accumulated in the sample loop were analyzed by gas chroma-
tography. The solid products which collected in the reaction tubes were scraped from the tube sides for NMR and mass spectrometric analysis. Some of the solid materials were also examined by infrared spectroscopy.
Results In a parametric study to determine conditions which would permit rapid development of a steady state within the plasma, the simulants were exposed to discharges a t differing initial concentrations, flow rates, pressures, and power levels. Each variable influenced the volume of the plasma and, consequently, the residence time of the simulant in the discharge. These, in turn, influenced the extent of conversion or decomposition of the simulants. It will also be seen that the nature of the carrier gas influenced significantly the extent of conversion. Helium Discharge Decomposition of DMMP. Runs were carried out at power levels of 50-200 W, pressures of 3-55 torr, and concentrations of 0.06-0.31 g/l. Residence times were 1.5-2.4 sec. Data are listed in Table I. Yellow solids and small amounts of a black tarry deposit were found in the reaction tube. The yellow material hydrolyzed in laboratory air to give a viscous liquid with an acetylene like odor. The liquid products which collected in the cold trap were identified by gas chromatography to be largely trimethylphosphite, (CH30)3P, and methanol, CH30H, with small amounts, less than 0.5%, of residual DMMP. Infrared spectra indicated the solids were inorganic, possibly polymeric, phosphonate residuals. There was little evidence for the formation of major gaseous products as indicated by material balance. The color of
u
I
t
/------CGCCOILNh Figure 1.
Schematic of microwave discharge system
Table I. DMMP-He Decomposition Run no.
53-1 49-1 49-6 49-5 49-4 49-2 49-3 49-7 49-8 53-3
Power, W
50 100 100 150 150 150 200 200 180 200
PresDMMP sure, D M M P concn, Plasma P, flow, F, C vol, V, torr g/hr g j l He cm3
55 4 5 4 5 5 3 8 10 10
2.95 2.80 2.00 0.80 1.40 4.20 2.40 1.20 1.40 1.75
0.25 0.21 0.15 0.06 0.10 0.31 0.18 0.09 0.10 0.13
4.8 6.5 6.5 7.0 7.0 7.3 8.8 8.8 8.5 8.6
Residence time, t, sec
Conversion,
1.5 1.8 1.8 1.9 1.8 1.9 2.4 2.4 2.2 2.3
95.4 96.5 97.5 99.1 98.8 96.2 99.1 99.5 99.2 98.0
%
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255
the plasma varied with the concentration of simulant. At low concentrations, the color was an intense violet similar to that of a pure helium discharge. At high concentrations, the color appeared lighter, which was the likely result of color dilution by the yellow-white discharge produced by the simulant. Helium Discharge Decomposition of DIMP. Runs were carried out a t power levels of 130-210 W, pressures of 9-17 torr, and concentrations of 0.017-0.137 g/l. Residence times varied from 0.26 to 1.3 sec. Data are listed in Table 11. In general, solids were the predominant reaction products. However, relatively high levels of a liquefiable gas were also produced. This material vaporized a t slightly above liquid nitrogen temperature, such that when the trap containing the condensate was removed from its liquid nitrogen bath, the pressure increased rapidly resulting in physical separation of the trap parts. The material was not phosphine, PH3, since there was no evidence of burning in air. Although a definitive analysis of the gas was not made, it is reasonable to suspect that it was propylene, CHsCH=CHz, which has a boiling point of -47°C. Mass spectrometry of the liquid and solid residues indicated the presence of large amounts of and significant, but lesser, quantities of methyl phosphonic acid, CH3P(O)(OH)z. Air Discharge Decomposition of DMMP. Runs were carried out at power levels of 140-510 W, pressures of 29-62 torr, and concentrations of 0.0108-0.0128 g/l. Residence times were 0.12-0.15 sec. Data are presented in Table 111. The basic reactions were complicated by the action of the discharge on the air carrier which resulted in the production of nitrogen oxides. The latter were the only observed gaseous products. Solids were deposited in the reaction tube and liquids were collected in the liquid nitrogen trap. The initial runs in this series indicated that the conversions in air were significantly lower than comparable runs with the helium carrier. Percent decomposition was in the 75-85% range. Increasing the power input to 500-600 W resulted in a major increase in conversion, with residual DMMP being reduced to 1-3%. This was in accord with the preliminary work performed during 1967, in which power levels of about 750 W were employed. Since the weight of the products, exclusive of nitrogen oxide, was higher by more than a factor of two than the DMMP input, it was apparent that reactions had occurred between the DMMP and either the air or the products formed as a result of the action of the discharge in the air. Discharges carried out a t higher pressures yielded products which appeared to differ somewhat from those obtained a t lower pressures. No solids were evident, but a black, viscous liquid deposited on the walls of the reaction tube. Liquids collected in the cold trap appeared similar to those produced in the lower pressure range. Mass spectrometry of the solid products indicated methylphosphonic acid, CHsP(O)(OH)z, plus either methyl methylphosphonic acid, CH3P(O)(OH)(OCHs) or methylphosphorus acid, (CH30)P(OH)z, as the probable reaction products in small quantity. The black liquids contained carbonlike particulate matter. The major component in the liquid product was identified as phosphoric acid, HaPo*, plus a small amount of methanol. Air Discharge Decomposition of DKMP. Runs were carried out at power levels of 125-160 W, pressures of 11-82 torr, and concentrations of 0.0052-0.125 g/l. Residence times were from 0.13-0.49 sec. Data are given in Table IV. The efficiency of decomposition in air was significantly higher than in helium, even a t moderate power levels. Residual DIMP levels down to 0.5% were deter256
Environmental Science & Technology
mined. There were no apparent vapor products besides the usual nitrogen oxides. Mass spectrometric analysis of products collected in the discharge tubes a t moderate power levels indicated the presence of phosphoric acid and methylphosphonic acid. Chromatographic analysis of the liquid products yielded four peaks, with isopropanol indicated as a probable major component. Several additional runs were made a t 300-350 W for the principal purpose of determining the effect of higher power levels on the extent of conversion. Efficiencies were estimated a t 99.9+% as the result of finding no evidence of residual DIMP in the trap liqids. Finally, to investigate the possibility that thermal energy contributed to the DIMP decomposition, several runs were made during which the sample collection chamber was heated via tapes to 150-200°C with the DIMP-He or -air mixtures flowing a t normal rates. These experiments showed no evidence of the formation of decomposition products.
Discussion Decomposition Mechanisms. The decomposition of DMMP and DIMP in a microwave discharge involves a complex series of reactions. The primary step in each case involves a collision between the reactant and either a free electron or a reactive species produced by the action of the discharge on the carrier gas. Thus, when helium is used as the carrier gas, helium metastables or ions are
Table II. DI MP-He Decom position Run no.
60-10 60-11 63-4 56-10 56-3 56-13 56-8 63-5
Pressure, DlMP Power, P, flow, F, W torr g/hr
135 130 140 140 150 200 210 210
17 14 9 9 10 11 9 9
1.27 1.27 1.30 1.30 1.75 1.28 1.50 1.50
DlMP Resiconcn, Plasma dence V, time, vol, C, cm3 t, sec g/1 H e
Conversion,
5.6 5.5 3.6 3.6 3.6 4.5 3.8 3.8
76.5 62.4 94.7 92.7 80.4 92.8 87.6 91.7
0.017 0.017 0.086 0.085 0.115 0.084 0.132 0.137
0.27 0.26 0.86 0.85 0.85 1.1 1.2 1.3
% '
Table Ill. DMMP-Air Decomposition Run no.
Power W
Pressure, P torr
DMMP flow, F g/hr
78-3 78-2 78-10 78-13 78-14 80-3 80-4
140 160 150 250 370 390 510
30 36 62 29 30 34 36
1.28 0.919 0.919 1.28 1.28 0.92 0.92
ResiD M M P Plasma dence concn, C, vol, V, time, g j l air cm3 t, sec
0.0128 0.0108 0.0108 0.0128 0.0128 0.0108 0.0108
3.7 3.5 2.8 4.0 3.1 2.8 3.1
0.14 0.15 0.12 0.13 0.12 0.12 0.13
Conv,er-
sion, %
72.1 76.7 82.9 84.4 87.8 78.1 97.9
Table IV. DIMP-Air Decomposition Run no.
69-4 69-6 69-5 67-1 67-2 67-3 69-1
Power, W
125 130 145 150 150 160 160
PresResisure, D l M P DlMP Plasma dence P, flow, F, concn, C, VOI,V, time, torr g/hr gllair cm3 t, sec
82 50 70 16 25 11 22
0.44 1.04 0.84 1.34 1.47 1.25 1.41
0.0052 0.0245 0.0158 0.048 0.026 0.125 0.033
3.0 3.1 3.6 3.8 4.1 3.6 3.4
0.13 0.26 0.24 0.49 0.26 0.13 0.29
Conversion,
%
94.0 98.9 99.5 90.7 97.4 99.5 91.1
produced. These species carry 19 and 25 eV, respectively, which can be released upon collision with a reactant molecule. When air is the carrier gas, atomic oxygen becomes the primary reactive species. Through the action of electron collisions and collisions with other species, free radicals and atoms are produced from the simulant. These new species then further attack the reactant and combine to form the final products. In the discussion given below, we attempt to provide a plausible explanation for the formation of the observed decomposition products. Dimethyl Methylphosphonate-He Plasma Reactions. The formation of trimethylphosphite can be derived from transfer of a methyl free radical (after P-C bond scission) to the oxygen atom associated with the P-0 bond. Methanol may kle derived from rupture of the P-0 bond followed by recombination of the resulting CHsO radical with a hydrogen atom produced from C-H bond scission. The black tarry deposit is associated with the formation of fully reduced carbon particles dispersed in the polyphosphonate or PzO!j residue, which hydrolyzes in laboratory air producing the observed viscous liquid. Small amounts of acetylene, methane, and, possibly, ethane may also be produced, absorbed in the reaction tube deposits, and released upon hydrolysis, thus contributing to the acetylenic odor. The scheme in Figure 3 which follows describes these reactions. Dimethyl Methylphosphonate-Air Plasma Reactions. I t may be observed by comparison of data from Tables I and I11 that higher power inputs were required for high efficiency conversioins of DMMP in air than in helium. This appears to be of importance, since it implies that an inert gas may be preferred for this simulant if lower power levels are specifietd. Identification of products from the DMMP-air plasina reaction indicates the following possible reactions in Figure 3. Diisopropyl Methylphosphonate-He Plasma Reactions. Interpretation of the analytical data indicates reactions differing somewhat from those listed for the DMMP-He plasma. The principal reactions involve the probable formation of phosphoric acid, or PzOs, which hydrolyzes to give H3P04, frele carbon, isopropanol, isopropyl methylphosphonic acid, and propylene. The reaction scheme can be written as given in Figure 4. Comparing the decomposition data in Tables I and I1 for the two simulants in helium indicates that the amount of decomposition of DIMP was significantly less than that of DMMP. Inspection of Table I1 also reveals that a moderate increase in power to 300 W and the use of lower concentrations should result in 99-100% conversion. Diisopropyl Methylphosphonate-Air Plasma Reactions. The decomposition efficiency in air was observed to be much higher than in helium, as can be determined by inspection of Tables I1 and IV. Residual DIMP levels down to 0.5% were obtained in air. Apparently, DIMP is more easily attacked by the products of an air discharge than is DMMP. The analytical data indicate the following reactions in Figure 4. Effects of Operating Conditions on Conversion. A systematic investigation of the effects of discharge power, gas pressure, reactant concentration, and reactant flow rate on the extent of DMMP and DIMP conversion were not performed as a part of this study. Nevertheless, some preliminary conclusions can be drawn from the existing data. For example, a comparison of runs 78-3, 78-13, and 78-14 (Table 111) which were carried out a t constant pressure, DMMP flow rate, and DMMP concentration, shows that increasing power results in an increasing conversion of DMMP. A similar conclusion is drawn by comparing runs 78-2, 80-3, and 110-4. Runs 78-2 and 78-10 illustrate that
\
\
t
I
f:-I
"3
OH
OCH3
"3C-i-O
on
I
OR
*
CH,O-i*O
CHIOH
OH
Figure 2. DMMP decomposition mechanisms
Figure 3. DIMP decomposition mechanisms
increased pressure also contributes to an increase in conversion but has a smaller effect than power. Large-Scale Applicators. The plasma discharges described above vary in volume from 3-9 cm3, and as such may be considered as laboratory-size reactors. Larger applicators must be built if commercial or plant-scale uses are to be considered. In this regard, a large-scale microwave generator, utilizing a Z2O-cm3 reactor was described by Bosisio et a1 (9); and Asmussen et a1 (10) designed and operated a microwave plasma cavity of 170 cm3. At the Lockheed Palo Alto Research Laboratory, a 2-liter capacity microwave plasma applicator, based on the resonant cavity principle, is in the design stage preliminary to fabrication. The generation of these large plasma volumes will be aided by the microwave excitation of carrier gases other than He and air, such as Ar and Ne, with an admixture of 0 2 or HZ as reactive components for the decomposition process. With regard to material throughput in this system, if the residence time of a substance to be decomposed is on the order of 2 sec.-i.e., the same order of magnitude as that obtained in the laboratory model, then its throughput will be of the order of 1 liter per sec. However, the potential effects of other factors, such as product accumulation and removal, treatment of solids and slurries, and elimination of possible toxic hazards will require additional engineering evaluation. These topics are scheduled for investigation, and will be covered in future publications.
Summary The use of a microwave discharge has been shown to be a highly effective technique for the decomposition of the toxic gas simulants, dimethyl and diisopropyl methylphosphonates. The extent of decomposition was found to Volume 9, Number 3, March 1975
257
vary with the nature of the carrier gas, flow rate, simulant concentration in the carrier gas, power input, and total pressure. Reaction products were identified by gas chromatography, NMR, and mass spectyoscopy. Reactions in helium produced phosphoric acid, or its precursor, PzO5, various phosphonic acids, and, probably, free carbon, alcohols, and their alkenes. Oxides of nitrogen were produced when air was used as the carrier gas. The decomposition of dimethyl methylphosphonate in a helium discharge yielded trimethylphosphite as a major product. The equivalent phosphite, diisopropyl methylphosphite, was not observed during decomposition of diisopropyl methylphosphonate in a helium discharge. This may be explained by postulating differing reaction mechanisms. When air is used as the carrier gas, the decomposition of diisopropyl methylphosphonate is favored over that of dimethyl methylphosphonate. With modification of the plasma chamber size as exemplified by large plasma applicators, the technique may be utilized for large-scale decomposition of waste insecticides, defoliants, G-agents, and materia!s not readily amenable to hydrolysis or standard incineration procedures. Literature Cited (1) Baier, R. W., Weller, S. W., “Feasibility of Catalytic Methods for Air Purification,” Progress Reports 1-12, Contract DA-
18-108-CML-6671 (A), Philco Corp., Aeronutronic Div., 196163. (2) Sanyal, S. K., Weller, S. W., “Mechanisms of Air Purification,” Progress Reports 1-9, Contract DAA-15-67-C-0675, State University of New York, Buffalo, N.Y ., 1967-69. (3) McTaggart, F. K., “Plasma Chemistry in Chemical Discharges,” Elsevier, Amsterdam, 1967. (4) MacDonald, A. D., “Microwave Breakdown in Gases,” p 71, John Wiley, New York, N.Y., 1966. (5) Fehsenfeld, F. C., Evenson, K. M., Broida, H. P., “Microwave Discharge Cavities Operating at 2450 MHz,” Rev. Sci. Instr., 36 (3), 294-8 (1965). (6) Bell, Alexis T., “Model for High Frequency Electric Discharge Reactors,” Chemical Engineering Symposium Series No. 112, “Engineering, Chemistry, and Use of Plasma Reactors,” 67, 1-11 (1971). (7) Wightman, J. P., “Chemical Effects of Microwave Discharges,” Proc. IEEE, 4-11, January 1974. (8) Goltz, H . L., Moffatt, J. B., J . Chromatogr. Sci., 8 , 596-9 (1970). (9) Bosisio, R. G., Weissfloch, C. F., Wertheimer, M. R., “The Large Volume Microwave Plasma Generator.” J . Microwave Poler, 7,325-46 (1972). (10) Asmussen. J.. Mallavaruu, R.. Hamann, J: R.. Park, H. C., “The Design of a Microwave Plasma Cavity,” Proc.’ IEEE; 109-17, 1974. Received for review May 13, 1974. Accepted November 15, 1974. This work was carried out under Contract DAAA 15-70-C-0488 from Edgewood Arsenal, APG, M d . 21010, and the Lockheed Independent Deuelopment Program. Mention of commercial products is for identification only and does not constitute endorsement by the U.S. Government.
NOTES
Zinc Availability as Influenced by Application of Fly Ash to Soil Melvin G. Schnappinger, Jr.,’ David C. Martens,” and Carl 0. Plank Department of Agronomy, Virginia Polytechnic Institute and State University, Blacksburg, Va. 24061
Fifteen samples of fly ash were collected from power plants located in nine states. The fly ash ranged from 12.9-378.2 ppm in total Zn and from 1.5-93.0 ppm in acid extractable Zn. Six of these samples were selected for investigations to determine the effect of fly ash on the availability of soil Zn. Incorporation of an acidic fly ash sample into Frederick silt loam and Westmoreland silty clay loam corrected Zn deficiency of corn plants ( Z e a m a y s L.) grown in the greenhouse on the two soils. Dry weight and Zn uptake data indicated approximately equal availability of Zn in the acidic fly ash and in ZnSOr. 7 & 0 . Application of fly ash samples with relatively high titratable alkalinities decreased the dry weight of corn plants grown in the greenhouse on the Frederick soil. The decrease was attributed partly to accentuation of Zn deficiency a t the higher levels of alkalinity established in the soil by application of the fly ash. Results of this research suggest that Zn availability should be considered when soils in which plants are to be grown are used for disposal of fly ash. Present address, Agricultural Division, Ciba-Geigy Corp., Newton, Conn. 06470. 258
Environmental Science & Technology
Removal of fly ash from flue gases by mechanical collectors or electrostatic precipitators prior to discharge of the gases into the atmosphere has decreased particulate air pollution from power generating plants, but has created a solid waste management problem ( I , 2 ) . Over 78.1 million metric tons of fly ash were produced in the USA during the 3-year period from 1970 through 1972 and, of this, only 10.5% was either sold for commercial purposes or removed from plant sites a t no cost to the utility ( 3 ) . Disposal of the nonutilized fly ash represents a considerable expense to the electric power industry. The amount of fly ash utilization in agriculture is negligible as compared to that of other industries ( 3 ) . Agricultural consumption of fly ash probably is contingent on experimental results relative to the availability of elements in the material. The purpose of this investigation was to study the effect of fly ash application on the availability of soil Zn. Corn ( Z e a m a y s L.) plants, which frequently require Zn fertilization for normal growth; and soils, which supply inadequate Zn, were used in the investigation. This procedure was followed to allow evaluation of Zn availability on the basis of both plant growth and Zn uptake by plants.
