- Email: [email protected]
E-beam treatment of trichloroethylene-air mixtures: Products and rates
Radiat. Phys. Chem. Vol. 50, No. 3, pp. 283-291, 1997 © 1997ElsevierScienceLtd. All rights reserved Printed in Great Britain PlI: S0969-806X(97)00031-5 0969-806x/97 $17.oo+ o.oo
Pergamon
E-BEAM TREATMENT OF TRICHLOROETHYLENE-AIR MIXTURES: PRODUCTS AND RATES THEODORE MILL,t ~ M I N G G O N G SU, I C. C. DAVID YAO, I STEPHEN M. MATTHEWS 2 and FRANCIS T. S. W A N G 2 LSRI International, Menlo Park, CA 94205, U.S.A. and 2Lawrence Livermore National Laboratory, Livermore, CA 94551, U.S.A. (Received 22 January 199& accepted 5 February 1997) Abstract--Electron beam (E-beam) treatment of 3000 ppmv trichloroethylene (TCE) vapor in dry and wet air led to rapid, nearly quantitative, conversion of TCE to dichloroacetyl chloride, plus small amounts of phosgene. Higher E-beam doses, up to 110 kGy, led to oxidation of the initial products to CO, CO2, HCI and C12.The results parallel results found for photo- and Cl-atom initiated oxidation of TCE vapor, and are accounted for by an efficientCl-atom chain oxidation. Lack of effect of 28,000 ppmv water vapor (90% RH) on rates or products reflects a very high efficiency for the O-atom chain oxidation and the very slow reaction of vapor phase water with acyl halides. Irradiation experiments conducted with TCE dissolved in aerated and deaerated water at 10 and 300 ppm showed marked differences in radiolytic products from those found in the vapor phase. A preliminary cost estimate indicates that E-beam treatment of TCE vapor is very competitive with conventional activated carbon treatment and catalytic oxidation. © 1997 Elsevier Science Ltd
I. INTRODUCTION High-energy ionizing radiation, such as electron beams or bremsstrahlung, has been effectively demonstrated to destroy toxic organic compounds both in aqueous and vapor streams (Matthews et al., 1992; Minchner et al., 1991). The vapor treatment method has been tested at a superfund site as an environmentally acceptable technology for treatment of toxic compounds and hazardous wastes (Matthews et al., 1993; Matthews and Boegel, 1993; Waite et al., 1993). However, to successfully develop use of electron-beam (E-beam) technology to treat volatile organic compounds stripped from groundwater or other waste streams, it is important to demonstrate that the technology can convert organics to easily scavenged intermediates or to carbon oxides, H20 and heteroatom species such as HCI. We selected trichloroethylene (TCE) for this study because of the widespread occurrence of TCE in polluted groundwater. The objectives of the study were to determine the intermediates and products of TCE-air mixtures at different E-beam doses, to determine the effect of water vapor on the rates and products and to establish probable pathways for reactions. U.v. photolysis of TCE-air mixtures (Huybrechts and Meyers, 1966; Bertrand et al., 1968; Johnson tTo whom all correspondence should be addressed.
et al., 1992) gave rapid, efficient conversion of TCE to dichloroacetyl chloride (DCAC) followed in some cases by slower oxidation of DCAC to carbon oxides and HCI; some phosgene (PG) also forms. CI2C--CHCI + O2--,HCCI2C(O)CI + COC12. Oxidation of TCE in the vapor phase on an illuminated TiO2 surface also forms these same products, but in different proportions (Nimlos et al., 1993). Oxidation in the neat liquid phase gives both trichloroethylene oxide (TCEO) and DCAC (Mayo and Honda, 1968). Gehringer et al. (1988) reported the near-absence of dichloroacetic acid (DCA) as a product of E-beam oxidation of TCE in aqueous solution, even at low conversions. DCA accounts for less than 10% of oxidized aqueous TCE, possibly because the HO radical is more important than the Cl-atom and leads to different products. Mehnert et al. (1993) reported that electron beam irradiation of ~ 3000 ppmv TCE in air gave more chloroform than phosgene, in addition to DCAC. However their products account for only 20% of the oxidized TCE. The stoichiometry of TCE-air oxidations requires that no more than one-third of the chlorine in TCE can form HCI when carbon forms CO and CO2, unless other hydrocarbons are present. The remaining chlorine atoms must form C12, which could further oxidize to CIO, C102 or CIO~. 283
284
T h e o d o r e Mill et al. 2. RESULTS
2.1. Sample preparation and treatment
Samples of 3000 ppmv TCE were exposed to the electron beam in Tedlar (polyvinyl fluoride) sampling bags in triplicate for each dose in dry and wet air. Exposure doses ranged from 1.4 to 110 kGys. 21 TCE samples were diluted in the dry synthetic air and 24 samples were diluted in air also containing 28,000 ppmv water vapor (90% RH). Each hag was exposed individually to the electron beam. Two or three bags were required at each dose to separately measure organic products, CO and CO2, and chloride ion. Following irradiation of each series of three bags at the same dose, pure methanol was immediately added to one bag to convert DCAC and phosgene to the corresponding methyl esters. A second exposed sample bag was washed with either 10% or 50% methanol, or Ca(OH): solution to scavenge HC1 and C12. The third bag used for CO and CO2 analyses was either left untreated or a 50 ml aliquot of the vapor mixture in the bag was transferred to a borosilicate sampling bulb for later analysis.
_ _ ~ = ~ o~
~ o° o
~oo~-'-
~E
oq I
0
2.2. Oxidations in dry air 21 samples of 3000 ppmv TCE in dry air were exposed at seven doses from 1.4 to 110 kGy. Analyses were reproducible to < + 1% for CO, CO2 and TCE and + 3% for DCAC and PG (as methyl esters). Duplicate samples exposed to (avg) 30 and 53 kGy doses can be intercompared and averaged with respect to loss of TCE and formation of products. Almost all ( < 98%) TCE disappears with just a 3 kGy dose giving a 91% yield of dichloroacetyl chloride (DCAC) and 9% yields of phosgene (PG) and CO. No HCC13 or CC14 was detected above 2% of DCAC, see Mehnert et al. (1993). When all TCE is oxidized at a dose of 30 kGys, 78% of the DCAC is also oxidized further to CO, COCI, and CO2. DCAC and PG are still present after 53 kGy doses, but at the highest dose, 110 kGy, all organic products were oxidized to CO and CO,. Table 1 provides a listing of all products and the mass balances on carbon and chlorine. Carbon balances are quite good in all experiments, including the 110 kGy sample after correcting for background CO and CO2 produced by E-beam-induced oxidation of the Tedlar film at 110 kGy. Very good chloride ion and chlorine mass balances (Table I, columns 11 and 12) are found only at low doses where most chloride ion is derived from hydrolysis of DCAC and PG, little or no chlorine (C12) is present and most chlorine atoms remain bound to carbon in DCAC. At 30, 50 and 110 kGy doses, where most measured chloride ion is derived from HCI and C12, chlorine atom balances are still satisfactory, but fall below 90%. Figure 1 shows the disappearance of TCE, appearance and disappearance of DCAC and PG and
¢-q +1
o= . ~ ~ o 'q.
o
--
E
~1 v= -~
•
e[
o~°.~.~.~.
No
oo.r., r . . o ~ o
~'.e~°
~.
~
~
E~
E
+ G ~
o+~-~" .-~v+O + ~ o=
o
-~ o
~
0
~
~
.~,~
.~ ~,
.~
o
~g
_-
.o o ~ . o
. ~ ' ~~
.r.O ,.~ ~'~'0
285
E-beam treatment of trichloroethylene-air mixtures Radiolytic Product Yield From T C E T C E Vapor in Dry Standard Air Initial TCE Concentration - 3000 ppmv
200
~7 /
CO2 o
/
// / /
-
150
(3// ]7 /
"6
/ ///
E .o
TCE
/ "
1 / HCI
//
DCAC
• •
/:
0
100 I
//
~.
// ",~,/
eL
/ 50
~
,."
/
//
[]
/
/CI2
"~/
/
o
.,//
0
/
/
0 0.100
"T
1.000
10.000
100.000
Dose in kilograys - log scale Fig. 1. Oxidation of 3000 ppmv TCE in dry air (0.01 -= 0 dose).
atom balance (244 lamol total 122 Ixmol initial TCE.
appearance of CO and CO2 with E-beam doses up to 110 kGys. Both CO and CO2 build up with time, but CO levels out beyond the 30 kGy dose, while CO2 continues to increase. The initial sources of CO and CO2 may be different, but CO subsequently oxidizes to CO2. Figure 2 replots the data of Fig. 1 to show C and CI balances as a function of dose. The carbon
C) is based on
2.3. Oxidations in wet air 24 samples of 3000 ppmv TCE in wet (28,000 ppmv, 90% RH) air were exposed to the electron beam at six doses ranging from 3 to 70 kGy.
Carbon and Chlorine Balance T C E Vapor in Dry Standard Air Initial TCE Concentration - 3000 ppmv 400
o Chlorine b
a
Carbon balance •
,
l
a
n
350 300
E .o
250
-
-~
•
0 ~
co2:
E 200 I
o
0__,___.. 0
O
150 - ~TCE
DCAC
1oo ~- ~
50F 0.100
/ /
• •
.,,J"
. . . . . . . . . . . . .
.."
o 1.000
...-\
" S "v
10.000
-
-._
/ HCI o
o
co 100.000
Dose in kilograys - log scale Fig. 2. Carbon and chlorine atom balances in oxidation of 3000 ppmv TCE in dry air.
286
Theodore Mill et al.
atom balance (244 ~tmol total C) is based on 122 ~tmol initial TCE. 2.3. Oxidations in wet air 24 samples of 3000 ppmv TCE in wet (28,000 ppmv, 90% RH) air were exposed to the electron beam at six doses ranging from 3 to 70 kGy. The rates and patterns of loss of TCE and formation and loss of DCAC and PG were essentially the same as found in dry air experiments shown in Fig. 1. The results make clear that 90% RH does not change the gas phase chemistry of TCE oxidation to form DCAC or subsequent steps to form phosgene, CO and CO2. Carbon atom balances in wet air experiments are quite good at all doses except for two values at 4.2 and 62 kGy. As with dry air samples, chlorine atom balances are very good at low doses, where most chlorine is bound to carbon or hydrolyzed in the sampling process to HCI. After DCAC oxidizes to carbon oxides and PG ( > 30 kGy), chlorine atom balances once again fall below 90%. The possibility that C l 2 is lost by oxidation to C102 or C103 seems unlikely, because we found no one-electron reduced anions (C10:- or CIO3-) by ion chromatography. However, we did no control experiments with these chlorine oxides to determine their speciation on recovery from Ca(OH)2 solution. Irradiation of Cl2-air mixtures in Tedlar, followed by digestion of the Tedlar pieces in hot, methanolic KOH, gave no significant amount of chloride ion indicating that few Cl atoms were lost by attachment to the polymer. 2.4. Experiments in liquid water We conducted two sets of irradiation experiments with aqueous TCE. One set used oxygen saturated (1 raM) water with l0 ppm (0.076 mM) TCE and the other set used 300 ppm (2.28 raM) TCE in argonated water. Both sets of solutions were exposed to up to l0 kGy doses and then analysed for chloride ion and organic carbon. Table 2 summarizes the results. The oxidizing experiments show prompt proTable 2. E-beam treatment of aqueous TCE Chloride Chloride TCE ion carbon Exposure (kGy) (rag/l) (rag/l) (rag/I) Deoxygenated water with 300 ppm dissolvedTCE 0 300 0* 54.8 3.3 5.7 104 -6.7 0.44 150 -10
0.89
177
43.8
Oxygenated water with I0 ppm dissolvedTCE 0 10 0* 1.83 3.3 0 10.5 -6.7 0 10.5 1.12 10 0 10.5 -* Total availablechlorideis 243 mg/l. ' Total availablechlorideis 8.1 mg/I.
duction of all available chlorine as chloride ion at 0.3 kGy, and loss of 40% of the organic carbon, presumably as CO and CO2 at 1 kGy dose. A deoxygenated TCE solution produced 72% of chloride ion with l0 kGy dose, but lost only 20% of organic carbon. 3. DISCUSSION The initial process which converts TCE to DCAC in high yield is an efficient chlorine atom chain, initiated by electron attachment initially to 02 or N2, with subsequent energy or atom (X') transfer to TCE, followed by a short sequence to form a Cl-atom. The CI-atom initiated oxidation follows the pathway originally proposed by Huybrechts and Meyers (1966) for photo-oxidation of TCE. N~' or X + TCE-*TCE* or XCH1CICCI3
(i)
TCE* or XCHCICCI~CI" + products
(ii)
C I ' + TCE~CI2CHCCI~
(l)
CI2CHCCI~ + O2-*CI2CHCCI2OO"
(2)
2C12CHCCI2OO'-*2CI2CHCCI20' + 02
(3)
C12CHCCI20'-*CI2CHC(O)CI(DCAC) + Cl'.
(4)
The high combined yields of DCAC, PG and CO at low doses ( > 99% on reacted TCE) indicate that reactions (1)-(4) are very efficient, with each Cl-atom cycling through reactions (1)-(4) 20 to 50 times for every chain initiating step (ii). Chain terminating reactions involving CI2CHCCI20~ or Cl-atom could form peroxides or C12, but long chains minimize formation of these products. Phosgene, which accompanies formation of DCAC, arises either by C-C cleavage of C12CHCCI20~ in reaction (5) or by reverse addition of Cl-atom to TCE followed by C-C cleavage (reactions (6) and (7)). Bertrand et al. (1968) showed that CI-atom initiated oxidation of TCE led to formation of 89% DCAC and 11% PG even at low conversions, in good agreement with our results. Trichloroacetaldehyde could form via reactions (6) and (7) by a pathway analogous to that for DCAC and trichloroethylene oxide (TCEO) could form by O-atom addition to TCE or by addition-rearrangement of the radical adducts of CI2CHCCI20~ and TCE. However, we found no TCEO or its rearranged product, trichloroacetaldehyde, once again pointing to the dominance of the CI-atom chain process. Ci2CHCCI20'-*CI2CH" + COCI2
(5)
C I ' + CI2CCHCI~CI3CCHCI'
(6)
C13CCHCI" + O2~-*C12CO + HCI + CO + CI'. (7) With both E-beam and Cl-atom initiation, phosgene formation increases with conversion at the expense of
E-beam treatment of trichloroethylene-air mixtures DCAC, showing that oxidation of DCAC is the main source of phosgene at high conversions. C12CHCOCI + CI" or X'~CI2C'C(O)CI + HCI or HX
(8)
CI2CC(O)C1--, ~C12C(O)COCI
(9)
CI2C(O)COCI~COCI2 + COCI
(10)
C O C I ~ C O + CI'
(11)
C12C(O')COCI~CIC(O)COCI + CI'.
(12)
This sequence also predicts that some CO and oxalyl chloride (C1C(O)COCI) should form along with PG (reactions (11) and (12)). In fact, we find oxalyl chloride, but only in 1-2% yield based on oxidized DCAC; CO and PG formation are comparable only at doses below 3 kGys. At higher doses, CO2 is the dominant product at the expense of both PG and CO via multistep processes (13) and (14). 'COC1 + 1/20:--*---,CO2 + CI CO + 1 / 2 0 2 ~ C O 2 .
(13) (14)
Reactions (8)-(11) also predict that DCAC should oxidize through a Cl-atom chain similar to that for TCE. However, conversion of DCAC (in the near-absence of TCE) requires about 0.3 Gy/~tmol, compared with only 0.003 Gy/lamol for TCE or a factor of about 100 in efficiency (see Table 1). The most likely reason for this large decrease in efficiency is the much slower rate of H-atom removal from DCAC by Cl-atom compared with addition of Cl-atom to TCE, which we estimate to be a factor of >70, based on the relative reactivity of HO radical towards addition vs. abstraction (Atkinson, 1985). Mehnert et al. (1993) also examined the E-beam treatment of TCE in ordinary air at doses up to 43 kGy. They found chloroform (CF) as well as DCAC and PG. At 7 kGy absorbed dose, where products formed in highest yields, all TCE (2.28 mmol/m 3) was oxidized, but only 0.26 mmol/m 3 of DCAC, PG and CF, combined, formed, giving a 10 mol% yield of products, DCAC comprises 7% of oxidized TCE, CF is about 3% and PG is only 0.2%. Although we found no chloroform in our oxidation mixtures, its formation in 1-2% yield could have escaped detection. The large loss of products in the Mehnert study may have occurred during gc/ms analysis from wall reactions of these reactive compounds. In addition, chloroform might form by pyrolytic decomposition of DCAC with loss of CO in the gc/ms. But, even if this reaction were the source of chloroform, it would only increase the yield of DCAC from 7% to 10%. Wet air (90% RH) has almost 10 times as much water as TCE (mol/mol) in the vapor phase, but we found no evidence for participation by water-derived HO radical in the oxidation of TCE. Hydroxyl radical oxidation of TCE will give glyoxalyl chloride
287
(CHOCOCI) and PG, but we find about as much DCAC in wet air as dry air. We conclude that, although HO radical formation from energy transfer processes or via O-atom oxidation of water to HO radical may be important, HO addition to TCE cannot compete with the chlorine atom chain process. Moreover, reaction (15) is endothermic by almost 105 kJ/mol (Benson, 1974) and is much too slow to compete with Cl-atom addition to TCE. CI + H 2 0 ~ H C I + H O .
(15/-15)
We cannot distinguish HO radical from Cl-atom oxidation of DCAC to phosgene. In advanced stages of the oxidation process, HO radical may form in low concentrations, but reaction ( - 15) will quickly convert most HO to Cl-atom radical. Aqueous oxygenated solutions of TCE with half the concentration of vapor samples release all chlorine as chloride ion at low dose, although only a minor fraction of organic carbon is converted to CO and CO:, consistent with the results of Gehringer et al. (1988). We found only trace amounts of dichloroacetate ion, suggesting that aqueous oxidation takes another pathway, involving primarily HO radical addition and solvated electron attachment to form species such as HOCC12CHCI, HOCHC1CCI~ and CHCI---CCI', all of which can undergo further reaction with oxygen to lose all chlorine, leaving oxygenated two-carbon fragments. The three most important conclusions that emerge from this study are that (1) TCE vapor is very rapidly and efficiently oxidized in air with the E-beam to the more toxic PG and DCAC (Budavari, 1989) by the same Cl-atom chain process observed in photolytic oxidation, (2) further oxidation of DCAC and PG requires doses of > 60-70 kGy, and (3) water vapor has no effect on the efficiency or pathways for oxidation processes or on the stability of DCAC or PG. 4. E C O N O M I C CONSIDERATIONS
We estimate the cost for E-beam treatment of a waste vapor stream consisting of TCE at 3000 ppmv concentration and we compare this cost with that of two alternative treatment methods, catalytic oxidation and granulated activated charcoal (GAC). Our estimates are based on a cost study conducted by the Radian Corporation for E-beam treatment of soil extraction vapor at McClellan Air Force Base using an E-beam processing unit manufactured by Zapit Technology Inc., and on an analysis of current catalytic oxidation treatment costs at McClellan conducted by the MITRE Corporation (Anon., 1995). 4.1. Mcclellan A F B cost study The Radian cost estimates are based on benchscale, E-beam vapor-treatment tests conducted inflow-through mode to determine whether electron
288
Theodore Mill et al. Table 3. Unit dollar costs used for treatment cost estimates Electricity 0.075/kW h 25% Caustic solution 0.22/kg Labor 60/h Catalyst 2.20/kg Natural gas fuel 0.50/therm Granular carbon 3.25/kg
beam technology would provide suitable, costeffective treatment for soil extraction vapor as a replacement for the catalytic oxidation/caustic scrubbing treatment currently employed at McClellan Air Force Base, California. The E-beam tests were performed by Zapit Technology Inc. at the University of Tennessee Space Institute under subcontract to Radian Corporation. Zapit has integrated the electron beam sources, appropriate power supplies, a reaction chamber, and instrumentation and controls into a portable working pilot unit. Radian used the Zapit data to estimate cost effectiveness of E-beam treatment followed by caustic scrubbing at McClellan. The unit costs used in the Radian estimates are shown in Table 3. The cost effectiveness of the currently used catalytic oxidation/caustic scrubbing treatment is based on a recent analysis by MITRE Corporation of the systems in operation at McClellan AFB. These costs are compared to the estimated costs for E-beam treatment (Anon., 1995). 4.2. The Zapit Technology Inc. E-beam tests Two soil vapor extraction sites at McClellan AFB produce two vapor mixtures of up to 14 different VOCs with concentrations of 1353 ppmv (Mixture 1) and 3345 ppmv (Mixture 2). Simulated samples of these vapors were irradiated in flow-through mode
using a 175 keV electron beam at currents of 0-20 mA to obtain bench scale treatment data. The beam use efficiency during these measurements was 28%, however Zapit expects to increase the beam energy to 450 keV and obtain 50% use efficiency for production treatment. It was found that E-beam doses of 300 kGy on Mixture 1 and 500 kGy on Mixture 2 were required to oxidize the VOCs in the simulated samples to acid vapors that were then removed with a caustic scrubber. NOx was produced by the E-beam treatment and measured at the scrubber outlet at concentrations of 71 ppmv and 120 ppmv for the two mixtures respectively. 4.3. E-beam treatment cost estimates at McClellan AFB The Radian cost estimates and capital expenditures for the Zapit E-beam/scrubber treatment at both McClellan AFB sites are shown in Table 4 under the column headings Mixture 1 and Mixture 2. These estimates assume 24 h per day operation at a vapor flow rate of 22.7 m3/min (800 cfm). The E-beam provided by Zapit's 450 keV accelerator is assumed to treat the vapor with a 50% overall use efficiency. The electron beam power required to provide a 500 kGy dose to the vapor at the assumed flow rate and beam use efficiency is 450 kW. Capital costs for the Zapit system are based on the fabrication of a single unit. Zapit estimates that these costs can be reduced by 50% when units are manufactured through commercial production. The daily capital payback expenses are based on a 5-year amortization using an annual interest rate of 5%. The Radian estimates of daily costs and capital expenditures using the Zapit E-beam process for Mixture 1 are $1947/day and $1.41 million, and for Mixture 2, $2928/day with a $1.95 million capital expenditure.
Table 4. E-beam/scrubberdollar cost estimate Mixture 1 Mixture 2 3000 ppmv Electricity Caustic Labor Maintenancematerials Total O and M Capital Total
Capital expenditure
Electricity Caustic
Labor Natural gas Catalyst
Total O and M Capital Total
Capital expenditure
616/day 175 240 44 1075 872 1947/day 1.41 million
990/day 425 240 61 1716 1212 2928/day 1.95 million
TCE 74/day 38I 240 19 714 379 1093/day 611,000
Table 5. Cat-Ox/scrubberdollar cost estimate Mixture 1 Mixture 2 TCE 3000 ppmv 180/day 180/day 180/day 175 425 38I 240 240 240 55 55 55 9
9
9
659 228 887/day 368,000
909 228 I 137/day 368,000
865 228 1093/day 368,000
289
E-beam treatment of trichloroethylene-air mixtures 4.4. Cost estimate for E-beam treatment o f TCE vapor Our experiments demonstrate that an irradiation dose of 10 kGy quantitatively converts 3000 ppmv TCE vapor in air to DCAC and phosgene (PG), but complete conversion to inorganic products requires 110 kGy absorbed dose. Both DCAC and PG are toxic and cannot be released to the environment, however both these vapors can be effectively scrubbed with a caustic solution to form dichloroacetate, carbonate and chloride ions. We estimate the cost of treating a waste TCE vapor stream at 3000 ppmv concentration by E-beam exposure to 10 kGy with a Zapit type unit, followed by scrubbing with caustic solution. We use the Radian cost information to estimate the total daily vapor treatment costs at a flow rate of 22.7 m3/min (800 cfm) normalized to a 10 kGy E-beam exposure and 3000 ppmv scrubbing requirement. The E-beam power required to deliver this dose is 9 kW, assuming a 50% beam use efficiency. We also assume that a commercially produced unit, rather than a singly fabricated unit, provides the E-beam treatment. The daily treatment cost estimate of $1093 and capital expenditure estimate of $611,000 are shown in Table 4 under the column heading TCE 3000 ppmv.
4.7. Discussion o f cost estimates We have estimated the daily cost for treating a TCE vapor flow at concentration 3000 ppmv and flow rate 22.7 m3/min (800 cfm) using three different treatment methods. These are E-beam/caustic scrubbing, catalytic oxidation/caustic scrubbing, and adsorption on GAC. The daily treatment costs are summarized below. Treatment
Total daily cost ($)
E-beam/caustic scrubber Cat-Ox/caustic scrubber Adsorption on GAC
1093 1093 3162
These estimates indicate that the cost for electron beam treatment is comparable to the cost of catalytic oxidation treatment. The cost of treatment using TCE adsorption on granulated activated charcoal (GAC) is prohibitively high. The cost of just the G A C alone is close to three times higher than the total treatment cost using E-beam or Cat-Ox treatment. Daily GAC costs may be reduced somewhat by steam backflushing the spent G A C and its re-use; however, the use of G A C is a collection method rather than a destruction technology and requires subsequent indefinite storage of the backflushed VOCs. 5. EXPERIMENTAL METHODS
4.5. Catalytic oxidation cost estimates
5.1. Sample preparation
The MITRE Corporation analysis determined the McClellan AFB costs for catalytic oxidation/ caustic scrubber treatment of the two vapor mixtures assuming 24 h per day operation at a vapor flow rate of 22.7 m3/min (800 cfm). These cost determinations and capital expenditures are listed in Table 5 under the headings Mixture 1 and Mixture 2. We normalized the MITRE data to a 3000 ppmv scrubbing requirement to estimate the daily costs and capital expenditure for catalytic oxidation/caustic scrubber treatment of TCE vapor at 3000 ppmv concentration and 22.7 m3/min (800 cfm) flow rate. These estimates are listed in Table 5 under the heading TCE 3000 ppmv for comparison with the E-beam/scrubber costs listed in Table 4.
1 i Tedlar bags (polyvinyl fluoride with 1% epoxy, SKC West) were used as the reaction vessels for E-beam treatment of TCE because of their low permeability and reactivity towards TCE and its oxidation products and high transparency and resistance to electrons. Each bag was filled with synthetic, dry air and emptied twice as a cleaning procedure. The cleaned bag was filled with 1 1 of synthetic air (measured with a calibrated bubble meter) and spiked with 16 mg of TCE and, when used, 21 mg of water, by injection as liquids with a 25 or 100 lal syringe.
4.6. Treatment costs using GAC The G A C treatment costs are based on TCE adsorption data and G A C cost as listed in Table 3. TCE vapor in air at a concentration of 3000 ppmv and temperature 25°C (77°F) has a saturated adsorption on granulated activated charcoal (GAC) of approximately 54% by weight. This vapor, flowing at a rate of 22.7 m3/min (800 cfm), transfers TCE at a mass rate of 365 g/min. Assuming a 54*/0 weight adsorption coefficient of TCE on GAC, we find that a daily consumption of 973 kg of G A C is required to treat the vapor. The daily treatment cost for G A C at $3.25/kg is therefore $3162.
5.2. E-beam treatment Electron beam treatment of TCE samples was accomplished using a 250 W MINAC linear accelerator capable of generating 5.9 x 10-~3 j (3.7 MeV) electrons. The samples were subjected to electron beam treatment at Zapit Technology's test cell in Sunnyvale, CA. The distance from the electron source to the sample was controlled by a simple fixture at a distance of 69 cm that produced a relatively flat field. To monitor the beam dose deposited in each sample bag, l 0 radiochromic detector film strips were placed on the 1 I Tedlar bags. Nine film strips were placed on the front and one on the back near the center of each sample bag. Eight of the strips were placed equidistant along the edges of the sampling bag, the ninth in the front center and the tenth on the back off-center from the front center strip (see Fig. 3).
Theodore
290
I
Mill et al.
5.4. Analyses for chlorine and chloride ion , TEDLAR
I
\
E-BEAM SOURCE
/ FRONT VIEW
SIDE VIEW
Fig. 3. Placement of radiochromic film strips on Tedlar sampling bags.
This orientation of film strips was empirically developed to best represent the sample’s average absorbed dose. Each sample bag was individually exposed to the electron beam source for a predetermined time. A radiochromic optical film reader was used to measure the absorbed dose from the change in optical density caused by exposure to the electron beam. For high dose levels, the optical density was read at 510 nm, and for low dose levels, at 600 nm. The measured optical density was converted to the electron dose received by each film strip by comparison to known standard curves. The average of the doses given to the 10 film strips is the absorbed dose listed for each sample. The radiochromic film manufacturer states that the dose may be determined to within an accuracy of + 3%. The doses varied across the bags by + 30% from the average value, but by < 10% from front to back. 5.3. Analyses for TCE and products DCAC and PG were converted to the corresponding methyl esters (methyl dichloroacetate and dimethyl carbonate) by adding 100% methanol to the sampling bag, thoroughly shaking the bag to dissolve the contents and allowing at least 5 h to elapse before the GC analyses to ensure that all PG was converted to dimethyl carbonate (based on trial experiments with PG and methanol). Gas chromatography (GC) of TCE and the esters used an HP 5890 chromatograph; a flame ionization detector with a DB-1 capillary column (0.53 m x 30 m x 1.0 pm, Alltech Associates, Inc.) and Ccl, as internal standard. Carbon monoxide and carbon dioxide were analysed with an HP 5890 gas chromatograph with thermoconductivity detector (TCD). A Hayesep DB column (loo/l20 mesh, 0.91 m x 0.32 cm x 0.089 cm) was used for carbon dioxide analysis and a molecular sieve column (acid washed, 0.18 m x 0.32 cm x 0.22 cm) was used for carbon monoxide analysis. External standards of CO and CO2 gas in helium (Scott Gas) were used to calibrate the chromatograms.
Analyses for chloride ion and chlorine in oxidation mixtures used methanol-water mixtures to scavenge HCl and Cl*. Hypochlorite oxidizes methanol to give chloride ion as well, but the reaction may be slow, so there is some uncertainty in the chloride ion values at high doses, prompting a change to a pH 11 solution of Ca(OH), to scavenge HCl and C12. Chloride ion was then measured with a chloride-ion electrode before and after reduction of Cl, (as CaOCI) with 1 M NaHSO, solution. Chloride ion was measured in two independent ways: a pH/ISE meter with chloride-specific electrode (Orion Research Inc.) adjusted for ionic strength with 5 M NaNO, to 50 mM and an ion chromatograph (Dionex Corporation) with AS9 PIB column and conductivity detector. Chlorine (as hypochlorite ion) was analysed as chloride ion following reduction with 1 M Na,SO,. The sensitivity of this analysis was 2 ppm. To independently evaluate the reliability of the ion measurements for chloride ion and chlorine, we conducted a series of measurements with dry and wet (90% RH) air mixtures containing known amounts of Cl2 and/or HCl and, in some cases, with DCAC and PG present. Some gas mixtures were injected directly into Ca(OH), solutions and some were initially injected into a 1 1Tedlar bag and then extracted from the gas phase in the bag by injecting 10 ml of Ca(OH), solution into the bag and shaking it for 30 min. The measured chloride ion values were often lower by lO-20% compared to chloride ion values based on gas volumes of HCl, CIZ, DCAC or PG injected directly into the vials. Chloride ion values in HCl experiments are lower by 6--S%, but the differences increase to 15-20% when Cl, is injected, suggesting that hypochlorite ion interferes with chloride measurements using the ion-specific electrode. These experiments showed that storage of HCl and Cl, for up to 4 h in the Tedlar bags did not affect recovery efficiencies of chloride ion or Cl,. Similarly, 90-95% RH had no effect on recovery efficiencies of chloride and Cl,. Acknowledgements-The authors gratefully acknowledge the cooperation of Peter Schonberg and Neal Minahane of Zapit Technology for irradiation of TCE vapor samples. We thank a referee for bringing the Mehnert ef al. paper to our attention. This work was supported by a DOE contract between SRI and Lawrence Livermore National Laboratory.
REFERENCES Anon. (1995) Electron beam bench scale. Technical Report, Installation Restoration Program, McClellan Air Force Base (U.S. Air Force). Atkinson, R. (1985) Kinetics and mechanisms of gas phase reactions of hydroxyl radical with organic compounds. Chem. Rev. 85, 69.
E-beam treatment of trichloroethylene-air mixtures Benson S. (1974) Thermochemical Kinetics. Wiley, New York. Bertrand, L., Goldfinger, F. P. and Huybrechts, G. (1968) The point of attack of a chlorine atom on trichloroethylene. J. Phys. Chem. 72, 3926. Budavari S. (Editor) (1989) The Merck Index, llth edn. Merck and Co., Rahway, NJ. Gehringer, P., Proksch, E., Szinovatz, W. and Eschweiler, H. (1988) Radiation induced decomposition of aqueous trichloroethylene solutions. Appl. Radiat. Isot. 39, 1227. Huybrechts, G. and Meyers, L. (1966) Gas-phase chlorinesensitized oxidation of trichloroethylene. Trans. Faraday Soc. 62, 2191. Johnson M. D., Haag W. R., Blystone P. G. and Daley P. F. (1992) Destruction of organic contaminants in air using advanced ultraviolet flashlamps. Final Report for EPA Contract No. 818209-01-N0, EPA Risk Reduction Engineering Laboratory, Cincinnati, OH. Matthews, S. M. and Boegel, A. J. (1993) High energy irradiation of chlorinated hydrocarbons. Radiat. Phys. Chem. 42, 689. Matthews, S. M., Boegel, A. J,, Eccles, S. F., Homann, S. G., Rice, D. W., Loftis, J. A., Jovanovich, M. C., Caufield, R. A., Mincher, B. J., Meikrantz, D., Murphy, R. J.. Gresham, G. L. and Connoly, M. J. (1992)
291
High energy irradiation of chlorinated hydrocarbons. J. Rad. Nucl. Chem. 161, 253. Matthews, S. M., Boegel, A. J. and Loftis, J. A. (1993) Radiolytic decomposition of environmental contaminants and site remediation using an electron accelerator. Environ. Cleanup Costs Technol. 3, 459. Mayo, F. R. and Honda, M. (1968) Liquid phase oxidation of trichloroethylene. Preprints Div. Petrol. Chem., Am. Chem. Soc. 13, C5. Mehnert, R., Klenert, P. and Prager, L. (1993) Low-energy electron accelerators for industrial radiation processing. Radiat. Phys. Chem. 42, 525-529. Nimlos, M. R., Jacoby, W. A , Blake, D. M. and Milne, T. A. (1993) Direct mass spectrometric studies of the destruction of hazardous waste, 2. Gas-phase photocatalyric oxidation of trichloroethylene over TiO2. Era,iron. Sci. Technol. 27, 732. Minchner, B. J., Meikrantz, D. H., Murphy, R. J., Gresham, G. L. and Connolly, M. J. (1991) Gammaray induced degradation of PCBs and pesticides using spent reactor fuel. Appl. Radiat. Isot. 42, 110. Waite T. D., Kuruez C. N. and Cooper W. J. (1993) Operational experience of a full scale electron beam treatment system in Miami, Florida. Florida Proceedings, First Int. EPRI/NSF Sympos. Adv. Oxidation, Vol. 2.
Pergamon
E-BEAM TREATMENT OF TRICHLOROETHYLENE-AIR MIXTURES: PRODUCTS AND RATES THEODORE MILL,t ~ M I N G G O N G SU, I C. C. DAVID YAO, I STEPHEN M. MATTHEWS 2 and FRANCIS T. S. W A N G 2 LSRI International, Menlo Park, CA 94205, U.S.A. and 2Lawrence Livermore National Laboratory, Livermore, CA 94551, U.S.A. (Received 22 January 199& accepted 5 February 1997) Abstract--Electron beam (E-beam) treatment of 3000 ppmv trichloroethylene (TCE) vapor in dry and wet air led to rapid, nearly quantitative, conversion of TCE to dichloroacetyl chloride, plus small amounts of phosgene. Higher E-beam doses, up to 110 kGy, led to oxidation of the initial products to CO, CO2, HCI and C12.The results parallel results found for photo- and Cl-atom initiated oxidation of TCE vapor, and are accounted for by an efficientCl-atom chain oxidation. Lack of effect of 28,000 ppmv water vapor (90% RH) on rates or products reflects a very high efficiency for the O-atom chain oxidation and the very slow reaction of vapor phase water with acyl halides. Irradiation experiments conducted with TCE dissolved in aerated and deaerated water at 10 and 300 ppm showed marked differences in radiolytic products from those found in the vapor phase. A preliminary cost estimate indicates that E-beam treatment of TCE vapor is very competitive with conventional activated carbon treatment and catalytic oxidation. © 1997 Elsevier Science Ltd
I. INTRODUCTION High-energy ionizing radiation, such as electron beams or bremsstrahlung, has been effectively demonstrated to destroy toxic organic compounds both in aqueous and vapor streams (Matthews et al., 1992; Minchner et al., 1991). The vapor treatment method has been tested at a superfund site as an environmentally acceptable technology for treatment of toxic compounds and hazardous wastes (Matthews et al., 1993; Matthews and Boegel, 1993; Waite et al., 1993). However, to successfully develop use of electron-beam (E-beam) technology to treat volatile organic compounds stripped from groundwater or other waste streams, it is important to demonstrate that the technology can convert organics to easily scavenged intermediates or to carbon oxides, H20 and heteroatom species such as HCI. We selected trichloroethylene (TCE) for this study because of the widespread occurrence of TCE in polluted groundwater. The objectives of the study were to determine the intermediates and products of TCE-air mixtures at different E-beam doses, to determine the effect of water vapor on the rates and products and to establish probable pathways for reactions. U.v. photolysis of TCE-air mixtures (Huybrechts and Meyers, 1966; Bertrand et al., 1968; Johnson tTo whom all correspondence should be addressed.
et al., 1992) gave rapid, efficient conversion of TCE to dichloroacetyl chloride (DCAC) followed in some cases by slower oxidation of DCAC to carbon oxides and HCI; some phosgene (PG) also forms. CI2C--CHCI + O2--,HCCI2C(O)CI + COC12. Oxidation of TCE in the vapor phase on an illuminated TiO2 surface also forms these same products, but in different proportions (Nimlos et al., 1993). Oxidation in the neat liquid phase gives both trichloroethylene oxide (TCEO) and DCAC (Mayo and Honda, 1968). Gehringer et al. (1988) reported the near-absence of dichloroacetic acid (DCA) as a product of E-beam oxidation of TCE in aqueous solution, even at low conversions. DCA accounts for less than 10% of oxidized aqueous TCE, possibly because the HO radical is more important than the Cl-atom and leads to different products. Mehnert et al. (1993) reported that electron beam irradiation of ~ 3000 ppmv TCE in air gave more chloroform than phosgene, in addition to DCAC. However their products account for only 20% of the oxidized TCE. The stoichiometry of TCE-air oxidations requires that no more than one-third of the chlorine in TCE can form HCI when carbon forms CO and CO2, unless other hydrocarbons are present. The remaining chlorine atoms must form C12, which could further oxidize to CIO, C102 or CIO~. 283
284
T h e o d o r e Mill et al. 2. RESULTS
2.1. Sample preparation and treatment
Samples of 3000 ppmv TCE were exposed to the electron beam in Tedlar (polyvinyl fluoride) sampling bags in triplicate for each dose in dry and wet air. Exposure doses ranged from 1.4 to 110 kGys. 21 TCE samples were diluted in the dry synthetic air and 24 samples were diluted in air also containing 28,000 ppmv water vapor (90% RH). Each hag was exposed individually to the electron beam. Two or three bags were required at each dose to separately measure organic products, CO and CO2, and chloride ion. Following irradiation of each series of three bags at the same dose, pure methanol was immediately added to one bag to convert DCAC and phosgene to the corresponding methyl esters. A second exposed sample bag was washed with either 10% or 50% methanol, or Ca(OH): solution to scavenge HC1 and C12. The third bag used for CO and CO2 analyses was either left untreated or a 50 ml aliquot of the vapor mixture in the bag was transferred to a borosilicate sampling bulb for later analysis.
_ _ ~ = ~ o~
~ o° o
~oo~-'-
~E
oq I
0
2.2. Oxidations in dry air 21 samples of 3000 ppmv TCE in dry air were exposed at seven doses from 1.4 to 110 kGy. Analyses were reproducible to < + 1% for CO, CO2 and TCE and + 3% for DCAC and PG (as methyl esters). Duplicate samples exposed to (avg) 30 and 53 kGy doses can be intercompared and averaged with respect to loss of TCE and formation of products. Almost all ( < 98%) TCE disappears with just a 3 kGy dose giving a 91% yield of dichloroacetyl chloride (DCAC) and 9% yields of phosgene (PG) and CO. No HCC13 or CC14 was detected above 2% of DCAC, see Mehnert et al. (1993). When all TCE is oxidized at a dose of 30 kGys, 78% of the DCAC is also oxidized further to CO, COCI, and CO2. DCAC and PG are still present after 53 kGy doses, but at the highest dose, 110 kGy, all organic products were oxidized to CO and CO,. Table 1 provides a listing of all products and the mass balances on carbon and chlorine. Carbon balances are quite good in all experiments, including the 110 kGy sample after correcting for background CO and CO2 produced by E-beam-induced oxidation of the Tedlar film at 110 kGy. Very good chloride ion and chlorine mass balances (Table I, columns 11 and 12) are found only at low doses where most chloride ion is derived from hydrolysis of DCAC and PG, little or no chlorine (C12) is present and most chlorine atoms remain bound to carbon in DCAC. At 30, 50 and 110 kGy doses, where most measured chloride ion is derived from HCI and C12, chlorine atom balances are still satisfactory, but fall below 90%. Figure 1 shows the disappearance of TCE, appearance and disappearance of DCAC and PG and
¢-q +1
o= . ~ ~ o 'q.
o
--
E
~1 v= -~
•
e[
o~°.~.~.~.
No
oo.r., r . . o ~ o
~'.e~°
~.
~
~
E~
E
+ G ~
o+~-~" .-~v+O + ~ o=
o
-~ o
~
0
~
~
.~,~
.~ ~,
.~
o
~g
_-
.o o ~ . o
. ~ ' ~~
.r.O ,.~ ~'~'0
285
E-beam treatment of trichloroethylene-air mixtures Radiolytic Product Yield From T C E T C E Vapor in Dry Standard Air Initial TCE Concentration - 3000 ppmv
200
~7 /
CO2 o
/
// / /
-
150
(3// ]7 /
"6
/ ///
E .o
TCE
/ "
1 / HCI
//
DCAC
• •
/:
0
100 I
//
~.
// ",~,/
eL
/ 50
~
,."
/
//
[]
/
/CI2
"~/
/
o
.,//
0
/
/
0 0.100
"T
1.000
10.000
100.000
Dose in kilograys - log scale Fig. 1. Oxidation of 3000 ppmv TCE in dry air (0.01 -= 0 dose).
atom balance (244 lamol total 122 Ixmol initial TCE.
appearance of CO and CO2 with E-beam doses up to 110 kGys. Both CO and CO2 build up with time, but CO levels out beyond the 30 kGy dose, while CO2 continues to increase. The initial sources of CO and CO2 may be different, but CO subsequently oxidizes to CO2. Figure 2 replots the data of Fig. 1 to show C and CI balances as a function of dose. The carbon
C) is based on
2.3. Oxidations in wet air 24 samples of 3000 ppmv TCE in wet (28,000 ppmv, 90% RH) air were exposed to the electron beam at six doses ranging from 3 to 70 kGy.
Carbon and Chlorine Balance T C E Vapor in Dry Standard Air Initial TCE Concentration - 3000 ppmv 400
o Chlorine b
a
Carbon balance •
,
l
a
n
350 300
E .o
250
-
-~
•
0 ~
co2:
E 200 I
o
0__,___.. 0
O
150 - ~TCE
DCAC
1oo ~- ~
50F 0.100
/ /
• •
.,,J"
. . . . . . . . . . . . .
.."
o 1.000
...-\
" S "v
10.000
-
-._
/ HCI o
o
co 100.000
Dose in kilograys - log scale Fig. 2. Carbon and chlorine atom balances in oxidation of 3000 ppmv TCE in dry air.
286
Theodore Mill et al.
atom balance (244 ~tmol total C) is based on 122 ~tmol initial TCE. 2.3. Oxidations in wet air 24 samples of 3000 ppmv TCE in wet (28,000 ppmv, 90% RH) air were exposed to the electron beam at six doses ranging from 3 to 70 kGy. The rates and patterns of loss of TCE and formation and loss of DCAC and PG were essentially the same as found in dry air experiments shown in Fig. 1. The results make clear that 90% RH does not change the gas phase chemistry of TCE oxidation to form DCAC or subsequent steps to form phosgene, CO and CO2. Carbon atom balances in wet air experiments are quite good at all doses except for two values at 4.2 and 62 kGy. As with dry air samples, chlorine atom balances are very good at low doses, where most chlorine is bound to carbon or hydrolyzed in the sampling process to HCI. After DCAC oxidizes to carbon oxides and PG ( > 30 kGy), chlorine atom balances once again fall below 90%. The possibility that C l 2 is lost by oxidation to C102 or C103 seems unlikely, because we found no one-electron reduced anions (C10:- or CIO3-) by ion chromatography. However, we did no control experiments with these chlorine oxides to determine their speciation on recovery from Ca(OH)2 solution. Irradiation of Cl2-air mixtures in Tedlar, followed by digestion of the Tedlar pieces in hot, methanolic KOH, gave no significant amount of chloride ion indicating that few Cl atoms were lost by attachment to the polymer. 2.4. Experiments in liquid water We conducted two sets of irradiation experiments with aqueous TCE. One set used oxygen saturated (1 raM) water with l0 ppm (0.076 mM) TCE and the other set used 300 ppm (2.28 raM) TCE in argonated water. Both sets of solutions were exposed to up to l0 kGy doses and then analysed for chloride ion and organic carbon. Table 2 summarizes the results. The oxidizing experiments show prompt proTable 2. E-beam treatment of aqueous TCE Chloride Chloride TCE ion carbon Exposure (kGy) (rag/l) (rag/l) (rag/I) Deoxygenated water with 300 ppm dissolvedTCE 0 300 0* 54.8 3.3 5.7 104 -6.7 0.44 150 -10
0.89
177
43.8
Oxygenated water with I0 ppm dissolvedTCE 0 10 0* 1.83 3.3 0 10.5 -6.7 0 10.5 1.12 10 0 10.5 -* Total availablechlorideis 243 mg/l. ' Total availablechlorideis 8.1 mg/I.
duction of all available chlorine as chloride ion at 0.3 kGy, and loss of 40% of the organic carbon, presumably as CO and CO2 at 1 kGy dose. A deoxygenated TCE solution produced 72% of chloride ion with l0 kGy dose, but lost only 20% of organic carbon. 3. DISCUSSION The initial process which converts TCE to DCAC in high yield is an efficient chlorine atom chain, initiated by electron attachment initially to 02 or N2, with subsequent energy or atom (X') transfer to TCE, followed by a short sequence to form a Cl-atom. The CI-atom initiated oxidation follows the pathway originally proposed by Huybrechts and Meyers (1966) for photo-oxidation of TCE. N~' or X + TCE-*TCE* or XCH1CICCI3
(i)
TCE* or XCHCICCI~CI" + products
(ii)
C I ' + TCE~CI2CHCCI~
(l)
CI2CHCCI~ + O2-*CI2CHCCI2OO"
(2)
2C12CHCCI2OO'-*2CI2CHCCI20' + 02
(3)
C12CHCCI20'-*CI2CHC(O)CI(DCAC) + Cl'.
(4)
The high combined yields of DCAC, PG and CO at low doses ( > 99% on reacted TCE) indicate that reactions (1)-(4) are very efficient, with each Cl-atom cycling through reactions (1)-(4) 20 to 50 times for every chain initiating step (ii). Chain terminating reactions involving CI2CHCCI20~ or Cl-atom could form peroxides or C12, but long chains minimize formation of these products. Phosgene, which accompanies formation of DCAC, arises either by C-C cleavage of C12CHCCI20~ in reaction (5) or by reverse addition of Cl-atom to TCE followed by C-C cleavage (reactions (6) and (7)). Bertrand et al. (1968) showed that CI-atom initiated oxidation of TCE led to formation of 89% DCAC and 11% PG even at low conversions, in good agreement with our results. Trichloroacetaldehyde could form via reactions (6) and (7) by a pathway analogous to that for DCAC and trichloroethylene oxide (TCEO) could form by O-atom addition to TCE or by addition-rearrangement of the radical adducts of CI2CHCCI20~ and TCE. However, we found no TCEO or its rearranged product, trichloroacetaldehyde, once again pointing to the dominance of the CI-atom chain process. Ci2CHCCI20'-*CI2CH" + COCI2
(5)
C I ' + CI2CCHCI~CI3CCHCI'
(6)
C13CCHCI" + O2~-*C12CO + HCI + CO + CI'. (7) With both E-beam and Cl-atom initiation, phosgene formation increases with conversion at the expense of
E-beam treatment of trichloroethylene-air mixtures DCAC, showing that oxidation of DCAC is the main source of phosgene at high conversions. C12CHCOCI + CI" or X'~CI2C'C(O)CI + HCI or HX
(8)
CI2CC(O)C1--, ~C12C(O)COCI
(9)
CI2C(O)COCI~COCI2 + COCI
(10)
C O C I ~ C O + CI'
(11)
C12C(O')COCI~CIC(O)COCI + CI'.
(12)
This sequence also predicts that some CO and oxalyl chloride (C1C(O)COCI) should form along with PG (reactions (11) and (12)). In fact, we find oxalyl chloride, but only in 1-2% yield based on oxidized DCAC; CO and PG formation are comparable only at doses below 3 kGys. At higher doses, CO2 is the dominant product at the expense of both PG and CO via multistep processes (13) and (14). 'COC1 + 1/20:--*---,CO2 + CI CO + 1 / 2 0 2 ~ C O 2 .
(13) (14)
Reactions (8)-(11) also predict that DCAC should oxidize through a Cl-atom chain similar to that for TCE. However, conversion of DCAC (in the near-absence of TCE) requires about 0.3 Gy/~tmol, compared with only 0.003 Gy/lamol for TCE or a factor of about 100 in efficiency (see Table 1). The most likely reason for this large decrease in efficiency is the much slower rate of H-atom removal from DCAC by Cl-atom compared with addition of Cl-atom to TCE, which we estimate to be a factor of >70, based on the relative reactivity of HO radical towards addition vs. abstraction (Atkinson, 1985). Mehnert et al. (1993) also examined the E-beam treatment of TCE in ordinary air at doses up to 43 kGy. They found chloroform (CF) as well as DCAC and PG. At 7 kGy absorbed dose, where products formed in highest yields, all TCE (2.28 mmol/m 3) was oxidized, but only 0.26 mmol/m 3 of DCAC, PG and CF, combined, formed, giving a 10 mol% yield of products, DCAC comprises 7% of oxidized TCE, CF is about 3% and PG is only 0.2%. Although we found no chloroform in our oxidation mixtures, its formation in 1-2% yield could have escaped detection. The large loss of products in the Mehnert study may have occurred during gc/ms analysis from wall reactions of these reactive compounds. In addition, chloroform might form by pyrolytic decomposition of DCAC with loss of CO in the gc/ms. But, even if this reaction were the source of chloroform, it would only increase the yield of DCAC from 7% to 10%. Wet air (90% RH) has almost 10 times as much water as TCE (mol/mol) in the vapor phase, but we found no evidence for participation by water-derived HO radical in the oxidation of TCE. Hydroxyl radical oxidation of TCE will give glyoxalyl chloride
287
(CHOCOCI) and PG, but we find about as much DCAC in wet air as dry air. We conclude that, although HO radical formation from energy transfer processes or via O-atom oxidation of water to HO radical may be important, HO addition to TCE cannot compete with the chlorine atom chain process. Moreover, reaction (15) is endothermic by almost 105 kJ/mol (Benson, 1974) and is much too slow to compete with Cl-atom addition to TCE. CI + H 2 0 ~ H C I + H O .
(15/-15)
We cannot distinguish HO radical from Cl-atom oxidation of DCAC to phosgene. In advanced stages of the oxidation process, HO radical may form in low concentrations, but reaction ( - 15) will quickly convert most HO to Cl-atom radical. Aqueous oxygenated solutions of TCE with half the concentration of vapor samples release all chlorine as chloride ion at low dose, although only a minor fraction of organic carbon is converted to CO and CO:, consistent with the results of Gehringer et al. (1988). We found only trace amounts of dichloroacetate ion, suggesting that aqueous oxidation takes another pathway, involving primarily HO radical addition and solvated electron attachment to form species such as HOCC12CHCI, HOCHC1CCI~ and CHCI---CCI', all of which can undergo further reaction with oxygen to lose all chlorine, leaving oxygenated two-carbon fragments. The three most important conclusions that emerge from this study are that (1) TCE vapor is very rapidly and efficiently oxidized in air with the E-beam to the more toxic PG and DCAC (Budavari, 1989) by the same Cl-atom chain process observed in photolytic oxidation, (2) further oxidation of DCAC and PG requires doses of > 60-70 kGy, and (3) water vapor has no effect on the efficiency or pathways for oxidation processes or on the stability of DCAC or PG. 4. E C O N O M I C CONSIDERATIONS
We estimate the cost for E-beam treatment of a waste vapor stream consisting of TCE at 3000 ppmv concentration and we compare this cost with that of two alternative treatment methods, catalytic oxidation and granulated activated charcoal (GAC). Our estimates are based on a cost study conducted by the Radian Corporation for E-beam treatment of soil extraction vapor at McClellan Air Force Base using an E-beam processing unit manufactured by Zapit Technology Inc., and on an analysis of current catalytic oxidation treatment costs at McClellan conducted by the MITRE Corporation (Anon., 1995). 4.1. Mcclellan A F B cost study The Radian cost estimates are based on benchscale, E-beam vapor-treatment tests conducted inflow-through mode to determine whether electron
288
Theodore Mill et al. Table 3. Unit dollar costs used for treatment cost estimates Electricity 0.075/kW h 25% Caustic solution 0.22/kg Labor 60/h Catalyst 2.20/kg Natural gas fuel 0.50/therm Granular carbon 3.25/kg
beam technology would provide suitable, costeffective treatment for soil extraction vapor as a replacement for the catalytic oxidation/caustic scrubbing treatment currently employed at McClellan Air Force Base, California. The E-beam tests were performed by Zapit Technology Inc. at the University of Tennessee Space Institute under subcontract to Radian Corporation. Zapit has integrated the electron beam sources, appropriate power supplies, a reaction chamber, and instrumentation and controls into a portable working pilot unit. Radian used the Zapit data to estimate cost effectiveness of E-beam treatment followed by caustic scrubbing at McClellan. The unit costs used in the Radian estimates are shown in Table 3. The cost effectiveness of the currently used catalytic oxidation/caustic scrubbing treatment is based on a recent analysis by MITRE Corporation of the systems in operation at McClellan AFB. These costs are compared to the estimated costs for E-beam treatment (Anon., 1995). 4.2. The Zapit Technology Inc. E-beam tests Two soil vapor extraction sites at McClellan AFB produce two vapor mixtures of up to 14 different VOCs with concentrations of 1353 ppmv (Mixture 1) and 3345 ppmv (Mixture 2). Simulated samples of these vapors were irradiated in flow-through mode
using a 175 keV electron beam at currents of 0-20 mA to obtain bench scale treatment data. The beam use efficiency during these measurements was 28%, however Zapit expects to increase the beam energy to 450 keV and obtain 50% use efficiency for production treatment. It was found that E-beam doses of 300 kGy on Mixture 1 and 500 kGy on Mixture 2 were required to oxidize the VOCs in the simulated samples to acid vapors that were then removed with a caustic scrubber. NOx was produced by the E-beam treatment and measured at the scrubber outlet at concentrations of 71 ppmv and 120 ppmv for the two mixtures respectively. 4.3. E-beam treatment cost estimates at McClellan AFB The Radian cost estimates and capital expenditures for the Zapit E-beam/scrubber treatment at both McClellan AFB sites are shown in Table 4 under the column headings Mixture 1 and Mixture 2. These estimates assume 24 h per day operation at a vapor flow rate of 22.7 m3/min (800 cfm). The E-beam provided by Zapit's 450 keV accelerator is assumed to treat the vapor with a 50% overall use efficiency. The electron beam power required to provide a 500 kGy dose to the vapor at the assumed flow rate and beam use efficiency is 450 kW. Capital costs for the Zapit system are based on the fabrication of a single unit. Zapit estimates that these costs can be reduced by 50% when units are manufactured through commercial production. The daily capital payback expenses are based on a 5-year amortization using an annual interest rate of 5%. The Radian estimates of daily costs and capital expenditures using the Zapit E-beam process for Mixture 1 are $1947/day and $1.41 million, and for Mixture 2, $2928/day with a $1.95 million capital expenditure.
Table 4. E-beam/scrubberdollar cost estimate Mixture 1 Mixture 2 3000 ppmv Electricity Caustic Labor Maintenancematerials Total O and M Capital Total
Capital expenditure
Electricity Caustic
Labor Natural gas Catalyst
Total O and M Capital Total
Capital expenditure
616/day 175 240 44 1075 872 1947/day 1.41 million
990/day 425 240 61 1716 1212 2928/day 1.95 million
TCE 74/day 38I 240 19 714 379 1093/day 611,000
Table 5. Cat-Ox/scrubberdollar cost estimate Mixture 1 Mixture 2 TCE 3000 ppmv 180/day 180/day 180/day 175 425 38I 240 240 240 55 55 55 9
9
9
659 228 887/day 368,000
909 228 I 137/day 368,000
865 228 1093/day 368,000
289
E-beam treatment of trichloroethylene-air mixtures 4.4. Cost estimate for E-beam treatment o f TCE vapor Our experiments demonstrate that an irradiation dose of 10 kGy quantitatively converts 3000 ppmv TCE vapor in air to DCAC and phosgene (PG), but complete conversion to inorganic products requires 110 kGy absorbed dose. Both DCAC and PG are toxic and cannot be released to the environment, however both these vapors can be effectively scrubbed with a caustic solution to form dichloroacetate, carbonate and chloride ions. We estimate the cost of treating a waste TCE vapor stream at 3000 ppmv concentration by E-beam exposure to 10 kGy with a Zapit type unit, followed by scrubbing with caustic solution. We use the Radian cost information to estimate the total daily vapor treatment costs at a flow rate of 22.7 m3/min (800 cfm) normalized to a 10 kGy E-beam exposure and 3000 ppmv scrubbing requirement. The E-beam power required to deliver this dose is 9 kW, assuming a 50% beam use efficiency. We also assume that a commercially produced unit, rather than a singly fabricated unit, provides the E-beam treatment. The daily treatment cost estimate of $1093 and capital expenditure estimate of $611,000 are shown in Table 4 under the column heading TCE 3000 ppmv.
4.7. Discussion o f cost estimates We have estimated the daily cost for treating a TCE vapor flow at concentration 3000 ppmv and flow rate 22.7 m3/min (800 cfm) using three different treatment methods. These are E-beam/caustic scrubbing, catalytic oxidation/caustic scrubbing, and adsorption on GAC. The daily treatment costs are summarized below. Treatment
Total daily cost ($)
E-beam/caustic scrubber Cat-Ox/caustic scrubber Adsorption on GAC
1093 1093 3162
These estimates indicate that the cost for electron beam treatment is comparable to the cost of catalytic oxidation treatment. The cost of treatment using TCE adsorption on granulated activated charcoal (GAC) is prohibitively high. The cost of just the G A C alone is close to three times higher than the total treatment cost using E-beam or Cat-Ox treatment. Daily GAC costs may be reduced somewhat by steam backflushing the spent G A C and its re-use; however, the use of G A C is a collection method rather than a destruction technology and requires subsequent indefinite storage of the backflushed VOCs. 5. EXPERIMENTAL METHODS
4.5. Catalytic oxidation cost estimates
5.1. Sample preparation
The MITRE Corporation analysis determined the McClellan AFB costs for catalytic oxidation/ caustic scrubber treatment of the two vapor mixtures assuming 24 h per day operation at a vapor flow rate of 22.7 m3/min (800 cfm). These cost determinations and capital expenditures are listed in Table 5 under the headings Mixture 1 and Mixture 2. We normalized the MITRE data to a 3000 ppmv scrubbing requirement to estimate the daily costs and capital expenditure for catalytic oxidation/caustic scrubber treatment of TCE vapor at 3000 ppmv concentration and 22.7 m3/min (800 cfm) flow rate. These estimates are listed in Table 5 under the heading TCE 3000 ppmv for comparison with the E-beam/scrubber costs listed in Table 4.
1 i Tedlar bags (polyvinyl fluoride with 1% epoxy, SKC West) were used as the reaction vessels for E-beam treatment of TCE because of their low permeability and reactivity towards TCE and its oxidation products and high transparency and resistance to electrons. Each bag was filled with synthetic, dry air and emptied twice as a cleaning procedure. The cleaned bag was filled with 1 1 of synthetic air (measured with a calibrated bubble meter) and spiked with 16 mg of TCE and, when used, 21 mg of water, by injection as liquids with a 25 or 100 lal syringe.
4.6. Treatment costs using GAC The G A C treatment costs are based on TCE adsorption data and G A C cost as listed in Table 3. TCE vapor in air at a concentration of 3000 ppmv and temperature 25°C (77°F) has a saturated adsorption on granulated activated charcoal (GAC) of approximately 54% by weight. This vapor, flowing at a rate of 22.7 m3/min (800 cfm), transfers TCE at a mass rate of 365 g/min. Assuming a 54*/0 weight adsorption coefficient of TCE on GAC, we find that a daily consumption of 973 kg of G A C is required to treat the vapor. The daily treatment cost for G A C at $3.25/kg is therefore $3162.
5.2. E-beam treatment Electron beam treatment of TCE samples was accomplished using a 250 W MINAC linear accelerator capable of generating 5.9 x 10-~3 j (3.7 MeV) electrons. The samples were subjected to electron beam treatment at Zapit Technology's test cell in Sunnyvale, CA. The distance from the electron source to the sample was controlled by a simple fixture at a distance of 69 cm that produced a relatively flat field. To monitor the beam dose deposited in each sample bag, l 0 radiochromic detector film strips were placed on the 1 I Tedlar bags. Nine film strips were placed on the front and one on the back near the center of each sample bag. Eight of the strips were placed equidistant along the edges of the sampling bag, the ninth in the front center and the tenth on the back off-center from the front center strip (see Fig. 3).
Theodore
290
I
Mill et al.
5.4. Analyses for chlorine and chloride ion , TEDLAR
I
\
E-BEAM SOURCE
/ FRONT VIEW
SIDE VIEW
Fig. 3. Placement of radiochromic film strips on Tedlar sampling bags.
This orientation of film strips was empirically developed to best represent the sample’s average absorbed dose. Each sample bag was individually exposed to the electron beam source for a predetermined time. A radiochromic optical film reader was used to measure the absorbed dose from the change in optical density caused by exposure to the electron beam. For high dose levels, the optical density was read at 510 nm, and for low dose levels, at 600 nm. The measured optical density was converted to the electron dose received by each film strip by comparison to known standard curves. The average of the doses given to the 10 film strips is the absorbed dose listed for each sample. The radiochromic film manufacturer states that the dose may be determined to within an accuracy of + 3%. The doses varied across the bags by + 30% from the average value, but by < 10% from front to back. 5.3. Analyses for TCE and products DCAC and PG were converted to the corresponding methyl esters (methyl dichloroacetate and dimethyl carbonate) by adding 100% methanol to the sampling bag, thoroughly shaking the bag to dissolve the contents and allowing at least 5 h to elapse before the GC analyses to ensure that all PG was converted to dimethyl carbonate (based on trial experiments with PG and methanol). Gas chromatography (GC) of TCE and the esters used an HP 5890 chromatograph; a flame ionization detector with a DB-1 capillary column (0.53 m x 30 m x 1.0 pm, Alltech Associates, Inc.) and Ccl, as internal standard. Carbon monoxide and carbon dioxide were analysed with an HP 5890 gas chromatograph with thermoconductivity detector (TCD). A Hayesep DB column (loo/l20 mesh, 0.91 m x 0.32 cm x 0.089 cm) was used for carbon dioxide analysis and a molecular sieve column (acid washed, 0.18 m x 0.32 cm x 0.22 cm) was used for carbon monoxide analysis. External standards of CO and CO2 gas in helium (Scott Gas) were used to calibrate the chromatograms.
Analyses for chloride ion and chlorine in oxidation mixtures used methanol-water mixtures to scavenge HCl and Cl*. Hypochlorite oxidizes methanol to give chloride ion as well, but the reaction may be slow, so there is some uncertainty in the chloride ion values at high doses, prompting a change to a pH 11 solution of Ca(OH), to scavenge HCl and C12. Chloride ion was then measured with a chloride-ion electrode before and after reduction of Cl, (as CaOCI) with 1 M NaHSO, solution. Chloride ion was measured in two independent ways: a pH/ISE meter with chloride-specific electrode (Orion Research Inc.) adjusted for ionic strength with 5 M NaNO, to 50 mM and an ion chromatograph (Dionex Corporation) with AS9 PIB column and conductivity detector. Chlorine (as hypochlorite ion) was analysed as chloride ion following reduction with 1 M Na,SO,. The sensitivity of this analysis was 2 ppm. To independently evaluate the reliability of the ion measurements for chloride ion and chlorine, we conducted a series of measurements with dry and wet (90% RH) air mixtures containing known amounts of Cl2 and/or HCl and, in some cases, with DCAC and PG present. Some gas mixtures were injected directly into Ca(OH), solutions and some were initially injected into a 1 1Tedlar bag and then extracted from the gas phase in the bag by injecting 10 ml of Ca(OH), solution into the bag and shaking it for 30 min. The measured chloride ion values were often lower by lO-20% compared to chloride ion values based on gas volumes of HCl, CIZ, DCAC or PG injected directly into the vials. Chloride ion values in HCl experiments are lower by 6--S%, but the differences increase to 15-20% when Cl, is injected, suggesting that hypochlorite ion interferes with chloride measurements using the ion-specific electrode. These experiments showed that storage of HCl and Cl, for up to 4 h in the Tedlar bags did not affect recovery efficiencies of chloride ion or Cl,. Similarly, 90-95% RH had no effect on recovery efficiencies of chloride and Cl,. Acknowledgements-The authors gratefully acknowledge the cooperation of Peter Schonberg and Neal Minahane of Zapit Technology for irradiation of TCE vapor samples. We thank a referee for bringing the Mehnert ef al. paper to our attention. This work was supported by a DOE contract between SRI and Lawrence Livermore National Laboratory.
REFERENCES Anon. (1995) Electron beam bench scale. Technical Report, Installation Restoration Program, McClellan Air Force Base (U.S. Air Force). Atkinson, R. (1985) Kinetics and mechanisms of gas phase reactions of hydroxyl radical with organic compounds. Chem. Rev. 85, 69.
E-beam treatment of trichloroethylene-air mixtures Benson S. (1974) Thermochemical Kinetics. Wiley, New York. Bertrand, L., Goldfinger, F. P. and Huybrechts, G. (1968) The point of attack of a chlorine atom on trichloroethylene. J. Phys. Chem. 72, 3926. Budavari S. (Editor) (1989) The Merck Index, llth edn. Merck and Co., Rahway, NJ. Gehringer, P., Proksch, E., Szinovatz, W. and Eschweiler, H. (1988) Radiation induced decomposition of aqueous trichloroethylene solutions. Appl. Radiat. Isot. 39, 1227. Huybrechts, G. and Meyers, L. (1966) Gas-phase chlorinesensitized oxidation of trichloroethylene. Trans. Faraday Soc. 62, 2191. Johnson M. D., Haag W. R., Blystone P. G. and Daley P. F. (1992) Destruction of organic contaminants in air using advanced ultraviolet flashlamps. Final Report for EPA Contract No. 818209-01-N0, EPA Risk Reduction Engineering Laboratory, Cincinnati, OH. Matthews, S. M. and Boegel, A. J. (1993) High energy irradiation of chlorinated hydrocarbons. Radiat. Phys. Chem. 42, 689. Matthews, S. M., Boegel, A. J,, Eccles, S. F., Homann, S. G., Rice, D. W., Loftis, J. A., Jovanovich, M. C., Caufield, R. A., Mincher, B. J., Meikrantz, D., Murphy, R. J.. Gresham, G. L. and Connoly, M. J. (1992)
291
High energy irradiation of chlorinated hydrocarbons. J. Rad. Nucl. Chem. 161, 253. Matthews, S. M., Boegel, A. J. and Loftis, J. A. (1993) Radiolytic decomposition of environmental contaminants and site remediation using an electron accelerator. Environ. Cleanup Costs Technol. 3, 459. Mayo, F. R. and Honda, M. (1968) Liquid phase oxidation of trichloroethylene. Preprints Div. Petrol. Chem., Am. Chem. Soc. 13, C5. Mehnert, R., Klenert, P. and Prager, L. (1993) Low-energy electron accelerators for industrial radiation processing. Radiat. Phys. Chem. 42, 525-529. Nimlos, M. R., Jacoby, W. A , Blake, D. M. and Milne, T. A. (1993) Direct mass spectrometric studies of the destruction of hazardous waste, 2. Gas-phase photocatalyric oxidation of trichloroethylene over TiO2. Era,iron. Sci. Technol. 27, 732. Minchner, B. J., Meikrantz, D. H., Murphy, R. J., Gresham, G. L. and Connolly, M. J. (1991) Gammaray induced degradation of PCBs and pesticides using spent reactor fuel. Appl. Radiat. Isot. 42, 110. Waite T. D., Kuruez C. N. and Cooper W. J. (1993) Operational experience of a full scale electron beam treatment system in Miami, Florida. Florida Proceedings, First Int. EPRI/NSF Sympos. Adv. Oxidation, Vol. 2.