(19) Hidy, G., “Characterizationof Aerosols in California (ACHEX), Final Report”, Vol IV, pp 3-4-3-18, Sept. 1974. (20) Tanner, R. L., Newman, L., J . Air Pollut. Control Assoc., 26,737 (1976). (21) Pierson, W. R., Hammerle, R. H., Brachaczek, W. W., Anal. Chem., 48,1808 (1976). (22) Cox, R. A,, Penkett, S. A., J . Chem. Soc. Faraday Trans. I , 65, 1735 (1972). (23) Liu, B.Y.H., Pui, D.Y.H., Aerosol Sci., 6,249 (1975). (24) Miguel, A., private communication, 1977. (25) Liepmann, H. W., Roshko, A,, “Elementsof Gas Dynamics”, p
55, Wiley, New York, N.Y., 1967. (26) Shames, I. H., “Mechanics of Fluids”, pp 437-442, 1962. (27) Davies, C. N., Aylward, M., Proc. Phys. Soc. London B , 64,889 (1951).
Received for review June 3, 1977. Accepted November 30, 1977. Presented at Division of Environmental Chemistry, 173rd Meeting, ACS, New Orleans, La., March 1977. Work supported i n part by N I E H S Grant PHS E S 00080-01 and by N S F I R A N N Grant ENV76-04179. The Pasadena Lung Association provided funds for instrumentation.
Development of Microwave Plasma Detoxification Process for Hazardous Wastes-Part 1 Lionel J. Bailin’ and Barry L. Hertzler Department of Chemistry, Lockheed Palo Alto Research Laboratory, 325 1 Hanover Street, Palo Alto, Calif. 94304
Donald A. Oberacker Solid and Hazardous Waste Research Division, Municipal Environmental Research Laboratory, U.S. Environmental Protection Agency, Cincinnati, Ohio 45268
A microwave-induced oxygen plasma was applied successfully for detoxification of pesticides and hazardous wastes. Materials were passed through a laboratory-size reactor to determine conversion efficiencies and product identities. Construction of an expanded-volume system followed which resulted in an increase in throughput from 1-5 g/h for the laboratory unit to 450-3200 g (1-7 lb) per h in the larger system. Substances treated were PCB’s, phenylmercuric acetate (PMA) solution, methyl bromide, malathion, a polyaromatic dye mixture, and Kepone. Detoxification of PMA yielded metallic mercury as a salable by-product. Treatment costs were computed which included electricity, oxygen, capital equipment, and labor. The design and construction of pilot equipment is underway for expansion to portable units of 5-14 kg (10-30 lb) per h.
Of the approximately 10 million tons of toxic and hazardous wastes that are generated yearly in the United States, it has been estimated that 10-20% will need special methods for disposal because of extreme difficulties in their treatment. These materials are made up in large part of pesticides that have been withdrawn from use, obsolete or below-specification toxic materials, industrial wastes from process streams, chemicals, explosives, and biological wastes, carcinogens, mutagens, teratogens, etc. Typical of the materials that must be managed or disposed of safely and effectively are chlorinated hydrocarbons, organophosphorus, organonitrogen, and organometallic compounds, which include many highly toxic, refractory, or extremely persistent wastes (1). Research on the decomposition of organic compounds by passage through a microwave discharge began at the Lockheed Palo Alto Research Laboratory in 1967. In a U.S. Army-supported program conducted during 1970-1972, the decomposition of toxic gas simulants was carried out in discharges containing helium and air in which nearly loOO? decomposition of selected organophosphonate materials was effected (2).The materials were passed through a small 1-5-g/h capacity laboratory-size reactor having a plasma volume of about 10 cm3. For commercial or plant-scale development of the process, it was obvious that large-capacity reactors would be required. When it was determined that larger size microwave power applicators could be obtained on a custom basis from micro-
wave hardware suppliers, the U.S. EPA, Solid and Hazardous Waste Research Division, Municipal environmental Research Laboratory, Cincinnati, Ohio, supported the following study to test the process on several toxic pesticides and wastes. E q u i p m e n t a n d Materials
Microwave Plasma System. For details of the microwave plasma process, ref. 2 may be consulted. The plasma operates at reduced pressures, up to several hundred torr. This permits the free electrons to be energized to temperatures much higher than that of the neutral gas, since at the lower pressures significantly fewer inelastic collisions occur to cool the reactive electrons. By operating under these nonequilibrium conditions, it is possible to maintain the free electrons at high temperatures without heating the bulk neutral gas, thereby conserving electrical energy. Since the plasma decomposition mechanism is principally electronic, rather than thermal, the microwave applicator-power coupling equipment can be maintained at relatively low temperatures. Thus, the materials of construction associated with furnaces or incinerator equipment are generally unnecessary. In addition, the systems are leak tight, which is a result of the requirement for working a t reduced pressures, thereby contributing to a high level of safety in operation. The microwave plasmas were produced in a laboratory-size resonant cavity, and by three dual-trough waveguide applicators. In the laboratory unit the applicator was a resonant cavity, Varian Associates, Model EC2DRS2, powered by a single 2.5-kW, 2450-MHz supply. The expanded-scale applicators and power supply hardware were supplied by Gerling Moore, Inc., Palo Alto, Calif. The laboratory-scale plasma used during the initial stages of the study to determine product identities and conversion efficiencies was essentially the same as that utilized previously for the decomposition of organophosphonate compounds, but required modifications in technique for the dropwise introduction of liquids. A gravity-feed pressure-equalized dropping funnel of approximately 100 cm3 capacity was installed at the input to the plasma reactor for this purpose. A method was also needed to increase the time for passage of the drops through the discharge. This was necessitated since the time of fall under vacuum through the plasma was too short, as evidenced by drops exiting the reactor without having reacted completely. The problem was solved by utilizing a hollow quartz mesh “basket” positioned
0013-936X/78/0912-0673$01.00/0 @ 1978 American Chemical Society
Volume 12, Number 6, June 1978
673
a t the center of the plasma zone. Quartz mesh fibers were loaded into the basket to serve as a contact area for the drops. The basket contained a number of holes to allow passage of the effluent products. The residence time of the drops was estimated to he '12 to 1 s, the time for reaction flashes to he completed in the plasma zone. In the expanded-scale system, the reactor tubes were fahricated from transparent quartz of about 50-mm o.d., and 1.5-mm wall thickness. Quartz Raschig rings and, in some instances, quartz wool plugs were used to fill sections of the reactor to increase residence time within the plasma zone. The liquid feed system was based on a 1-L pressure-equalized version of the unit used for the lahoratory-scale plasma tests. For relatively volatile solutions, atmospheric pressure was maintained above the solution to avoid vacuum pumping the solvent from the solution. Reduced pressures were obtained using a Welch DuoSeal Model 1397 oil-sealed 2-stage mechanical pump. Various trap configurations were installed between the reactor output and the pump for product collection. A photograph of typical system components is shown in Figure 1. During operation, a Narda Model B86B3 radiation monitor was used to monitor power leakage. Levels were less than 1 mW/cm2 in the immediate vicinity of the discharge tube. Analytical Systems. Mass spectrometric (MS) analysis of the gases leaving the reactor were performed on a Varian Associates Model 974-0002 residual gas analyzer (quadrupole mass spectrometer) with a range of 250 amu. A small quantity of the gas was continuously pumped past a variable-leak sampling valve. The gases bled into the mass spectrometer by the sampling valve were pumped from the system by an ion pump. Infrared spectra of solid and liquid effluents collected from the product receiver and traps were determined on a Perkin-Elmer 621 infrared spectrophotometer with a range of 4000-400 cm-' (2.5-25 f i ) . Materials to he analyzed were ground with KBr and compressed into pellets for scanning over the prescribed spectrum. Visible and ultraviolet spectra from 200 to 700 nm on solid and liquid effluents were obtained on a Cary Model 14 recording spectrophotometer using conventional procedures. A Finnegan Model 4021 GC/MS data system was used toward completion of the study for analysis of polyaromatic dye decomnosition products.
Figure 1. Expanded-scale microwave p lama system
Pesticides, Hazardous Wastes, and G;ases. The materials that were detoxified or decomposed by milxowave plasma are listed in Tahle I. Selections were m a d e ,m the basis of the extent of the environmental problems t h#atwere associated with these materials, EPA's interest, Imd the refractory characteristics of the materials. The rea ctant/carrier gases were the following: oxygen, 99.5% min purity, Fed. Spec. BB-0-925(a), Type I; argon, 99.995% niin purity, Mil-A18455B. The oxygen contained 0.5% maxi mum impurities, in which approximately 0.05% was nitrogen, t he remainder being argon and other gases in trace amounts.
-
Table I. Pesticides and Hazardous Wastes for Detoxification Tests Claaslllcallon
Organophosphorus pesticide Chlorinated hydrocarbon waste Brominated hydrocarbon rodenticide Heavy metal fungicide Chlorinated hydrocarbon pesticide Polyaromatic red dye mixture
674
Material
Form IeSIed
Pure liquid
purity) PCB's (polychlorinated
Liquid mixture
Monsanto
Commercial gas
Matheson Gas
Commercial aqueous methanol solution 1. Commercial powde!I 2. Laboratory aqueous dispersion 3. Laboratory methancti solution 1. Laboratory aqueous dispersion 2. Laboratory methyl ethy1 ketone sol,uiion
Troy Chemical Allied Chemical
biphenyls). Methyl bromide (99.5% min purity) Phenylmercuric acetate (30% PMA solids)
Kepone (80% active ingredient, 20% clay)
55.4% Xylene azo-0-
naphthol 18.9% 1-Methylaminanthraquinone 18.0% Sucrose 1.8% Graphite 5.9% Silica binder (KC103 oxidant excluded)
Environmental Science &Technology
Grade or type
M~nulanureror source
American
Malathion (95% min
uLV
Cyanamid
A roclor 1242 A roclor 1254
Troysan PMA-30
a0% Powder C8oncentrate, tf :chnical grade, Code
9406
Naval Weaoons Support Center, Crane, Ind.
u .S.Navy MIK
13 Mod 0
Mlarine Smoke and 111umination
Signal
E x p e r i m e n t a l Procedure
In general, the procedure for operation of both laboratory and expanded scale units was the same as that described in ref. 2 . Certain modifications were required as the result of differences in feed technique, however. For example, when a vacuum dropping funnel was used for introduction of a low volatility fluid, the entire system, including the section above the liquid, was evacuated to 1 torr. The pressure was then adjusted to about 10 torr by the addition of oxygen or argon. The microwave power was then turned on to start the plasma. Additional oxygen was introduced to obtain the desired pressure and flow rate in combination with regulation by the main throttle valve. The microwave power was set to the desired level with the tuning controls adjusted to give minimum reflected power. After obtaining a background MS scan (reactant gas flowing minus material to be detoxified), a needle valve at the bottom of the dropping funnel was opened to yield the desired feed rate. The gaseous effluent from the plasma was then sampled and analyzed by MS. For methyl bromide gas, the pesticide was fed directly, bypassing the funnel. Product traps were an ice water cooled receiver, followed by one or more liquid nitrogen (LN)or dry ice acetone traps. Results
Laboratory-Scale Plasma Reactor. Reactions in the laboratory system were carried out with the toxic substances mixed with oxygen or argon. Although it was well known that simpler organic compounds exposed to inert gas plasmas would react to form a variety of compounds and polymers ( 3 ) , nevertheless, argon, in addition to oxygen, was evaluated for comparison with the helium and air decomposition reactions previously studied. However, after observing the offensive mercaptan/sulfide compounds that resulted from the malathion-argon plasma reactions, the formation of carbonaceous flake deposits from methyl bromide-argon, plus assessing the probability for the formation of toxic methyl mercury compounds from PMA in argon, emphasis was directed toward utilization of oxygen as the sole reactant gas for use in the expanded scale system. Details of the reactions that led to these conclusions are given below. Malathion-Oxygen. Cythion ULV grade malathion was passed through a 200-250-W plasma a t 100-120 torr using the quartz basket technique. The reactions appeared t o occur spontaneously as the drops contacted the quartz fibers. With the exception of a white etch zone and a high viscosity water-white liquid that formed below the plasma zone, all the products were gases. Mass spectrometry indicated Con, CO, Son, and HzO as effluent gases. Infrared spectroscopy showed the liquid product to be phosphoric acid. Material balances indicated that metaphosphoric acid was the probable material from which conversion to orthophosphoric acid in moist air occurred in 1-2 days. Analysis for malathion in the liquid reaction product was carried out spectrophotometrically in the visible region ( 4 ) .Percent conversion was 99.98+% based on 0.016% malathion determined. P o l y c h l o r i n a t e d B i p h e n y l ( P C B ) - O x y g e n . Monsanto Aroclor 1242 liquid was passed through a 250-W plasma a t 100 torr. Mass balance showed no liquids attributable to the starting material. All the products of decomposition were gases. On the basis of control runs in the absence of the plasma reaction, percent conversion was calculated a t greater than 99.9%. Gas products were identified as COz, CO, Hz0, HC1, Clz, with minor amounts of ClzO and COClZ. The latter gases, chlorine oxide and phosgene, were not observed in the expanded scale plasma reactions; instead, hydrogen chloride was the principal C1-containing product. M e t h y l Bromide-Oxygen. Gaseous methyl bromide was
passed through a 300400-W, 50-torr plasma at 2 to 3 g/h. The products of reaction were COz, CO, HzO, HBr, and Brz. Oxides of bromine were found in the liquid nitrogen traps, but were not otherwise observed a t ambient temperatures. The extent of the reaction was determined by mass spectrometer, in which the ratios of the CH3Br ion signal intensities before and during the plasma reactions were compared. Decomposition was greater than 99%, which was the limit of precision of the mass spectrometer for this chemical system. Phenylmercuric Acetate-Oxygen. Commercial Troysan PMA-30 solution was passed through 225-280-W plasmas a t 120 torr. Mercury metal was observed as a metallic mirror on the glass tubing downstream from the discharge zone. Material balance indicated >99.9% decomposition to the metal. Mass spectrometry showed the products formed in addition to Hg were HzO, CO2, and CO. There was no evidence of dimethyl mercury or other volatile organomercurials. Malathion-Argon. Decomposition reactions were carried out a t 200-250 W, 100 torr, in pure argon. The resultant yellow-brown products were extremely offensive and malodorous, similar to mercaptan and disulfide compounds. Because of the potential for very high toxicity, further analysis was not undertaken. M e t h y l Bromide-Argon. Methyl bromide was mixed with argon and passed through 300-400-W plasmas a t 50 torr. The products of reaction estimated by mass spectrometer were Brz, HBr, methane, ethylene, and acetylene. Carbonaceous flake deposits were formed in the reactor tube. Quantitative analysis by MS showed that not less than 99% conversion had occurred. Expanded-Scale Plasma Reactor. The approach taken in the study was to obtain maximum throughput, with the objective of achieving low process costs. Generally, the total microwave power available, 4.2-4.7 kW, was applied to the discharge. This allowed the plasma to operate a t higher pressures, thereby permitting a maximum amount of oxygen to be used as the plasma gas for reaction with the pesticides and wastes. During the initial runs, the Series A microwave power applicator with a 2.7-L reactor volume was used for plasma decompositions of Aroclor No. 1242 PCB. It was determined that the liquid had been decomposed and that one of the reaction products-a black soot-like deposit that coated the product receiver-contained little or no PCB. After additional runs were carried out in which feed, pressure, and absorbed power were varied, it became apparent that the reactor was too large for the power available. The Series B and C applicators, having reactor volumes 1.5 and 0.6 L, respectively, were evaluated in turn. The results are detailed in Table I1 and are described in the following sections. Malathion liquid was drop-fed onto a porous, quartz wool bundle positioned at the top power input to the plasma zone. By this means, in a mechanism similar to that used in the laboratory-scale system, large numbers of smaller droplets were produced within the matrix of the wool, and propelled by the gas stream through the plasma. Products were S, SO2, COz, CO, and HzO, plus a liquid phosphoric acid. During the reaction, deposits of a dark yellow-brown sulfur product mixed with a clear water-white high viscosity liquid were formed, which flowed slowly down the sides of the reactor into the receiver. No carbonaceous or other products resembling the starting material were observed. Spectrophotometric analysis of liquids from the two reactions gave residues of 12 and 1 ppm malathion. Polychlorinated biphenyls yielded HC1, COz, CO, and HZO as determined by MS. No C 1 ~ 0or COC12 was observed. There was formation of some soot in the product receiver; infrared analysis gave no indication of PCB residues. I t was determined, however, that at throughput levels of about 1 kg (2 lb) Volume 12, Number 6, June 1978
675
Table II. Summary of Expanded-Scale Oxygen Plasma Reactions Pesticidelwasle
Run no.
Applicator series
Microwave power (kW)
Feed rate [glh Pressure range (Ib/h)] (torr)
Oxygen gas flow (Llh)
Malathion "Cythion" ULV Malathion "Cythion" ULV PCB Aroclor 1242 PCB Aroclor 1242 PCB Aroclor 1254 PMA Troysan PMA-30
31-16 31-46 31-8 31-10 31-62 31-88
B B 0 0 0 C
3.7 4.7 4.6 4.2 4.5 4.6
504 (1.1) 480 (1.1) 270 (0.6) 492 (1.1) 206 (0.4) 1020 (2.25)
28-46 28-30 17-35 19-36 13-25 120-140
36 1 480 323 395 360 960
PMA Troysan PMA-30
31-108
C
4.0
2380 (5.25)
100- 120
792
PMA Troysan PMA-30
31-1 10
C
4.3
2950 (6.5)
100- 120
792
Kepone 80120 20% methanol solution Kepone 80/20 10% solids, aqueous slurry Kepone 80/20 2- to 3-9 solid discs Red dye mixture 15.5% solids aqueous slurry
38-30
C
4.6
b
45-60
720
38-36
C
4.2
...
35-50
None
38-38
C
4.6
...
30-70
810
68-58
C
4.6
b
35-60
300
a
Reactor packing a
Wool Wool Wool Wool
plug plug plug plug
Solid rings Raschig rings Raschig rings Raschig rings Raschig rings Raschig rings Raschig rings Raschig rings
Conversion
(%I 99.9988 99.9999 >99 >99 >99
Complete, estd 99.996
Complete, estd 99.99
Complete, estd 99.99 >99b >99 >99 >99.999
Quartz. See text
per h in the B applicator system, complete reaction had not occurred. This was determined by infrared analysis of the black tar-like liquid products in the receiver trap, which indicated the presence of PCB starting material. consequently, the Series C applicator was tested next to determine its utility for increasing the level of throughput. Phenylmercuric acetate, Troysan PMA-30 solution, was passed through the Series C system in several runs to determine the effect of the shortened length of the applicator, as well as to determine the effect of quartz plugs and rings in the reactor tube. The reaction was considered complete if none of the methanol component was found by mass spectrometry in the effluent gas. Mass spectrometer sensitivity was 2-3 parts per thousand for CH30H, based on control runs performed in the absence of the plasma. The MS analysis showed that a t a throughput of 3600 g (8 lb) per h, small amounts of methanol were detected in the effluent. This indicated that maximum detoxification or destruction of PMA-30 would occur a t about 7 lb/h. The principal gases of the reaction were COz, CO, and H20. Volatile organomercurials were not detected by MS. Metallic mercury was deposited in the traps downstream from the plasma. Experiments were also performed to modify the residence time of feed material in the plasma zone. Quartz Raschig rings were tested to evaluate throughput under different packed bed conditions. For PMA-30, maximum throughput was defined as the feed rate that showed no methanol component in the plasma effluent as determined by mass spectrometer. The MS thus served as an endpoint indicator for the PMA-oxygen plasma reactions for determining the influence of the packings on throughput. The data are listed in Table 111. For the Kepone runs, a commercial mixture, Allied Chemical 80% powder concentrate, was used as starting material. Approximately 200 g was converted into aqueous slurries, methanol solutions, and solid presscakes. The presscakes were prepared by compressing 2-3-g batches in a circular die under 1000 psi pressure. The discs were positioned a t the top of the Raschig ring area in the plasma reactor tube before the plasma was initiated. It was observed visually that breakdown and decomposition of the solids occurred within 10-30 s, depending on the flow of oxygen and the pressure 676
Environmental Science & Technology
within the reactor. Solutions of 20% Kepone in methanol, after filtration to remove the clay particles, were gravity fed into the plasma from a 250-cm3 buret needle-valve feed system in which atmospheric pressure instead of reduced pressure was maintained over the solution. Dispersions of Kepone in water formed readily and were fed unfiltered from the same system. The gaseous reaction products from the solvent and slurry mixtures were COz, CO, HC1, and H,O; phosgene or chlorinated hydrocarbons were not detected. Because of the short reaction times for the Kepone presscakes, MS analysis of the gaseous effluents was not performed. Instead, the clay support powders that passed through the reactor from the aqueous slurry reactions were collected from the receiver and analyzed by infrared spectroscopy. No Kepone, hexachlorobenzene, or other chlorinated hydrocarbons were detected in the solid residues. Percentage conversions were estimated a t better than 99%. Because of the quantity of the starting material, the reactions were not maximized with respect to throughput. A polyaromatic dye composition comprised of two dyes, sucrose, carbon black, and silica, which is contained in the smoke component of U S . Navy MK 13 Mod 0 Marine Smoke and Illumination Signal, was introduced into the plasma as a solvent solution and an aqueous dispersion. The dye components were 55.4% xylene azo-@-naphthol and 18.9% 1methylaminoanthraquinone. The KC103 oxidant was omitted from the evaluation in this series. For the dye-solvent solution, a 15% solids methyl ethyl ketone (MEK) mixture was decomposed with no red coloration or deposits visible below the reactor. However, because the MEK oxygen demand would prevent development of high throughput, an aqueous vehicle was tested for use as part of the feed system. Since the red dye components were essentially hydrophobic, a nonionic surfactant, 2% TEC 1216E, based on solids, was added to yield a 15.5% hydrophilic slurry, density 1.03 g/cm3. The slurry was added a t rates from 2 to 8 cm3/min. The reactions were not maximized because of limitations in starting material supply, as was the case for Kepone. Based on weight % of starting material, the solid residue measured less than 0.2% in the receiver traps, or 99.8% conversion to gaseous products. Spectrophotometric comparisons in the visible region of a methylene dichloride solution of the unknown solid,
Table 111. Effect of Packed Bed on PMA-30 Conversion in Series C Reactor
Run no.
Packing of 45-mm 1.d. reactor Bed length Ring slze 0.d. X length (mm) (cm)
Oxygen flow Pressure (torr) (std L/min) Top Bottom
Microwave power ( k W )
Throughput (ib/h)
31-88
8x8
16
8-16
120
90
4.6
2.25
31-108 31-110 38-6 38-8 38-14
8x8 8x8 10 x 10 8 x 4 8x8
45 45 34 31 31
13.2 13.2 13.2 13.2-18.4 13.2-16.0
120 120 115 112 130
64 60 42 60 75
4.0 4.3 4.3 4.6 4.7
5.25 6.5 4.25 6.0 8.0
and known concentrations o f the initial dye mixture in the same solvent indicated that not more than 5 ppm of the starting dyes had passed through the plasma. Polyaromatic hydrocarbons were not detected above 2 ppm, using ultraviolet fluorescence, infrared, UV absorption spectrophotometry, and GC/MS. The
