Environ. Sci. Technol. 3994, 28, 2249-2258
Radiolytic Degradation of 2,3,7,8-TCDD in Artificially Contaminated Soils Roger J. Hilarides and Klmberly A. Gray' Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, Indiana 46556
Joseph Gurzetta, Norma Cortellucci, and Christopher Sommer Technology Center, Occidental Chemical Corporation, Grand Island, New York 14072
This paper reports results of 2,3,7,8-tetrachlorodibenzop-dioxin (TCDD) destruction on artificially contaminated soil using cobalt-60 (60Co)y radiation. Important parameters (soil moisture content, surfactant type/concentration, equilibration, and radiation dose) and their optimum ranges are identified. Scavenger (N20, 0 2 , 2-propanol) studies were conducted to explore the consequences of controlling oxidative and reductive conditions in the soil, and reaction byproducts have been analyzed under selected reaction conditions. A standard soil (EPA SSM-91)was artificially contaminated with 2,3,7,8-TCDD to 100 ppb, and in the presence of 25% water and 2% surfactant (RA-40) and at a high irradiation dose (800 kGy), greater than 92% TCDD destruction was achieved, resulting in a final TCDD concentration of less than 7 ppb. Results of scavenger and byproduct studies and theoretical target theory calculations indicate that TCDD destruction is caused primarily by direct radiation effects and proceeds through reductive dechlorination.
Introduction There are approximately 500 000 t of soil and sediment in the United States that are contaminated with 2,3,7,8tetrachlorodibenzo-p-dioxin (TCDD) and require treatment ( I ) . The toxicity of 2,3,7,8-TCDDis well documented (2-8) and it is expected that EPA's stringent environmental standards for dioxin will remain in effect (9). Available dioxin destruction techniques include both thermal and nonthermal methods. Incineration is the most common and effective thermal treatment and has been used for dioxin destruction at several contamination sites ( I , 10, 11). The capital and operating costs of the incinerators coupled with poor public approval make incinerators a somewhat less attractive treatment technology. Nonthermal dioxin destruction techniques such as photolysis, chemical treatments, and radiolysis have been investigated (1,11-17). Although some of these methods have been applied successfully to the treatment of TCDD adsorbed to soils, none has been employed in site remediation ( I , 11). Crosby et al. (12) reported that TCDD could be degraded rapidly in an alcohol solution using artificial light or sunlight, but destruction was found to be negligible in soils. More recent photolysis studies have demonstrated destruction of TCDD on soils in the presence of organic solvents or surfactant/water mixtures (13-16, 19). Other studies have successfully degraded polychlorinated dibenzo-p-dioxin (PCDD) in nonaqueous solvents using UV light and sunlight (1420-22). Koester and Hites (23) reported photodegradation of PCDD in solutions and on silica gel; no destruction, however, occurred when bound to fly ash.
* To whom correspondence should be sent. 0013-936X/94/0928-2249$04.50/0
0 1994 Amerlcan Chemical Soclety
Ionizing radiation in the form of high-energy electron beams and y rays is also a potential nonthermal destruction technique. Theoretical and some empirical assessments suggest that these high-energy sources may be well suited to transforming dioxin to innocuous products. Electron beams have been shown to degrade chlorinated organic compounds in solutions and slurries (24,25),but have not been evaluated for the destruction of TCDD in either solution or soil. y radiolysis has been shown to be effective in the degradation of PCDD and polychlorinated biphenyls (PCBs) in organic solvents (28,291and in the disinfection of wastewaters (30, 31). Using a cobalt-60 POCO)y ray source, Fanelli and co-workers (17)destroyed TCDD in a variety of organic solvents (dioxane, acetone, and ethanol). In ethanol, 96% destruction was achieved at a dose of 300 kGy. Lower levels of destruction were obtained in the other solvents (75 %/dioxane and 84%/acetone). There are no published reports of soil irradiation using a 6oCoy ray source to degrade chlorinated dioxins. The objective of this research is to determine if it is possible to destroy radiolytically TCDD on soils using a 6oCoy ray source and to evaluate the factors controlling the rate and extent of TCDD degradation. The purpose of this paper is to report the influence of water content, surfactants, radical scavengers, and dose on the disappearance of TCDD on a model soil. Direct and indirect radiolytic interactions are discussed, byproducts are evaluated, and for these experiments, the predominant pathway of destruction is proposed.
Radiation Chemistry y Radiation. The spontaneous radioactive decay of 6oCoresults in the emission of a pair of y rays at energies of 1.17 and 1.33 MeV. The natural decay of isotopes such as 6oCo then is a reliable source of ionizing radiation requiring no direct, external power supply. y rays have high penetration lengths in materials of moderate density (i-e.,1g/cm3),and alength of 30.4 in. is required to absorb 90% of a y flux at 1.25 MeV (26, 27, 32). This fact illustrates that y rays penetrate much farther into a soil than other types of ionizing radiation such as electron beams or UV and visible light. y rays are high energy photons which interact with material by direct and indirect effects as shown in Figure 1. As y rays pass throughmatter,they deposit their energy in volumes called spurs creating a series of excited states and ionizations via compton scattering, the photoelectric effect, and pair production (26,32). The direct effect of this energy deposition is the production of primary products, a high-energy electron, and a resultant cation. When the energy of the ionizing radiation is absorbed by the target molecule itself (TCDD), shown in Figure 1along path a, either a TCDD radical cation-electron pair or an electronically excited TCDD molecule is formed. DirectEnviron. Scl. Technol., Vol. 28, No. 13, 1994 2249
Flgun 1. Conceptual diagram of direct versus indirect radiation effects. The direct effects result from the ionization of the target compound (electrons are emitted) as shown along path a. Along path b. a thermalized electron emitted by an ionized adjacent molecule interacts with the target compound to cause chemlcal transformation. In this caw. reductive dechlorlnation may result in the presence of a hydrogen donw. Natural organic material (NOM) may play this role. The indirect effects occur from the radiolysis of the solvent, whereby radicals are created which then diffuse to the target compound to cause a chemical transformation.
like effects also result from the interactions of the highenergy electrons as they are thermalized which is shown along path h. It is proposed that thermalized primary electrons may interact with the TCDD molecule, which in the presence of a hydrogen donor will then undergo reductive dechlorination. If ionizations occur in or adjacent to a molecule, referred to as a target, the resulting target transformation or destruction is considered due to a direct radiation "hit". An indirect effect of energy deposition by ionizing radiation is the production of secondary products. In general, indirect effects involve the absorption of ionizing radiation by the environment (e.g., the solvent or in this case water), generating reactive intermediates which may destroy the target molecule (34,35). In water at pH 7, the products created in the greatest proportion and at a ratio of approximately 1:l are the hydroxyl free radical ('OH) and the aqueous electron (e,) (34). Direct and indirect radiation effects occur simultaneously in a system and would probably both contribute, albeit to varying degrees, to the destruction of dioxin on soil. Very high doses would be expected in the case where target compound destruction was due predominantly to direct effects. In contrast, relatively lower doses would probably be required for effective destruction by indirect effects. For the degradation of TCDD hound to soil, the limited aqueous solubility of TCDD and the scavenging ability of soils may diminish the efficiency of indirect effects. Other factors such as soil conditions, organic and inorganic content, and pH will also affect the suite of prevailing radical species and influence the relative predominance of direct and indirect effects. The specific pathway by which TCDD is degraded by ionizing radiation is not fully understood (I7,36). Indirect pathways can be reductive or oxidative. Under reductive conditions, the free radical (e-aq)pathway of destruction is thought to proceed via reductive dechlorination (36), whereas under oxidative conditions ('OH), the dioxin bonds may be broken creating substituted phenols or 2250 Envlron. Sol. TBehnd.. Voi. 28. NO. 13. 1994
biphenyls (16). The initial transformation of TCDD hy direct radiation effects could also proceed through several routes. Ionization of the TCDD molecule will cause an electron to be ejected, leaving a TCDD radical cation. Another possible route is electron addition to TCDD from ionization of an adjacent molecule, yielding primarily reduced TCDD products such as lesser chlorinated dioxins. Since these reduced species have far greater solubilities in water than TCDD (19,37,38),the likelihood of further degradation by indirect action via the hydroxyl radical or the aqueous electron may be enhanced. The goal of this research is to determine the basic interaction between y radiation and matter in this system that results in TCDD destruction. With this understanding, the primary factors controlling the rate and extent of TCDD destructioncan be identified and possibly modified to achieve an effective alternative treatment process.
Experimental Methods
Soil Characterization. An artificially contaminated standard soil has been prepared. The standard soil is from the US. EPA's Synthetic Soil Matrix Blending System (SSM-91). and its constituents were selected by the U.S. EPA based on an extensive review of soil characteristics atSuperfundsitesandeasternU.S.soils. SSM-Slphysical and chemical characteristics are shown in Table 1 (45). The pH, density, and cation-exchange capacity were measured in our laboratory using EPA standard procedures, and total organic carbon (TOO was measured using a Dohrmann TOC boat sampler and COz analyzer. Soil Contamination. Dioxin (2,3,7,8-TCDD) was obtained from CambridgeIsotope Laboratories. Artificial soil contamination to approximately 100 ppb TCDD was accomplished by a mixing procedure using hexane. A 1.2mL aliquot of 50 (*5) ng/mL TCDD was added to 500 mL of hexane. Approximately600 g of soil was evenly spread in a shallow aluminum pan and sprayed with hexane until wetted. Then the TCDD and hexane solution was slowly
Table 1. SSM-91Physical and Chemical Characteristics soil component gravel (no. 9) sand silt clay bentonite kaolinite topsoil
% content 5.7 31.5 28.3 5.4 9.4 19.7
chemical characteristic
EPA reported
UND
cation-exchange capacity (mequiv/ 100 g) PH moisture content (%)
11 8.5 6 0.22
9 7.9 3 0.5 1.45
TOC (%) density (g/cm3)
poured onto the soil, and the slurry was continuously stirred until all the hexane evaporated. Homogeneous contamination to approximately 100 ppb was verified by subsamples with a standard deviation of 6.9 ppb for 10 subsamples. These results indicate that uniform soil contamination at a targeted level can be achieved successfully with this method. TCDD Analysis. Analysis for 2,3,7,8-TCDD was developed and conducted by the Dioxin Laboratory of Occidental Chemical Corporation, TechnologyCenter. The soil samples were extracted using a modified EPA IFB series jar extraction (46). A 1 g (dry weight) aliquot of sample was weighed; the TCDD was extracted from the soil, isolated using an alumina column, and concentrated in hexane to a 100-pL final volume. The extract and a five-level calibration were loaded onto a Hewlett Packard 5970B MSD coupled to a HP 5890 GC for HRGC/LRMS analysis. All results are surrogate recovery corrected with carbon labeled TCDD added in the extraction and cleanup steps. A more detailed discussion of this analytical procedure and the required calculations are provided in EPA Method 1613 and 8280 (47,48). Byproduct Analysis. Analyses for mono-, di-, and trichlorodibenzo-p-dioxins were also developed and conducted by Occidental’s Dioxin Laboratory. The analyses for 2,3,7,8-T4CDD, 2,3,7-trichlorodibenzo-p-dioxin (T3CDD), 2,7- and 2,3-dichlorodibenzo-p-dioxins (DCDDs), and 2-monochlorodibenzo-p-dioxin(MCDD) were performed by extraction and cleanup of an aliquot from a single sample vial. Extracts were concentrated in toluene with [W1-1,2,3,4-TCDD internal standard. A fraction of the toluene extract was diluted into hexadecane containing additional [l3C1-1,2,3,4-TCDDinternal standard. The remaining toluene extract was analyzed for mono- through tri-CDDs, while the hexadecane extract was independently analyzed for 2,3,7,8-TCDD. All results are corrected for recovery of a surrogate (i.e., [l3C1-2,3,7,8-TCDD)which was introduced prior to extraction and addition of the internal standard (i.e., [13C]1,2,3,4-TCDD). MCDD and DCDD recoveries were corrected based on [l3C1-2,7-DCDD,T3CDD was corrected based on 1,2,4-T3CDD(which was previously verified not to be formed during irradiation), and T4CDD recoveries were based on V3C1-2,3,7,8-TCDD. During analysis, it was noted that the monohomolog was lost more quickly than the dihomolog. Therefore, since MCDDs were corrected based on DCDD surrogate recoveries, the final corrected concentrations of MCDDs may be slightly lower than the actual levels. A more detailed discussion of these
procedures and required calculations is also provided in EPA Methods 1613 and 8280 (47,48). Experimental Setup. The samples were prepared in 7.4-mL silanized borosilicate glass vials with Teflon septa. Four grams of dry soil was added to the vial, and then Milli-Q (Millipore Corp.) water and surfactants were added as a weight percentage. Three surfactants were tested: cationic hexadecyltrimethyl ammonium chloride (CTAC1); nonionic alkoxylated fatty alcohol (Plurafac RA-40, BASF Corp.); and nonionic polyoxyalkylated fatty acid ester (Adsee 799, Witco Corp.). The vials were sealed and the contents were mixed by using a vortex shaker and inverting the vials. In scavenger and kinetics studies, soil or soil plus surfactant were added to the vials, the vials were sealed, and gas was sparged through the soil to displace the air in the sample. The gas entered and exited the vial via tygon tubing and syringes. Milli-Q water, saturated with the sparge gas, was then syringe injected into the vial. A second sparge was conducted using hydrated gas. Those samples containing alkaline 2-propanol (IPOH)were injected with 4% (w/w) IPOH solution at this point in the preparation. The samples were then shaken as described above. The sample vials were loaded into a 12-samplealuminum rack for irradiation. All irradiations were conducted on the Shepherd 109,lO 000 Ci, 6oCosource. The Shepherd 109 is a concentric, well-type source. Because of the symmetry of the 6oCosource, the dose is evenly distributed throughout the samples. Doses were measured using Fricke dosimetry (49),which is based on the oxidation of iron(I1) to iron(II1) in a standard solution of ferrous ammonium sulfate. These doses have been verified using National Bureau of Standards data for the Shepherd 109 irradiator. The dose rate decreases with time because of the 5.27-yr radioactive half-life of 6oCoand for the work reported herein ranged from 7.4 to 6.4 kGy/h. The dose is reported in units of kilograys (kGy) where 1gray = 100 rad and 1 gray = 1 J of energy deposited/kg of mass. All samples were done in triplicate, except samples from the kinetics study, which were not replicated. TCDD degradation was determined by loss of the parent compound (2,3,7,8-TCDD) relative to a set of unirradiated controls that accompanied every group of samples and replicated the conditions in the sample group.
Results and Discussion Soil Water Content. Initial experiments were conducted to screen the effects of two parameters thought to be important in the radiolytic destruction of TCDD: soil moisture content and surfactant addition. Water was hypothesized to have three possible effects depending on the pathway of destruction. First, the free radical attack of TCDD promoted by indirect radiation effects involves ionization of the solvent. Since the radiolysis of water at neutral pH produces ‘OH and e-aq in the greatest as well as equal proportions, reaction conditions can be simultaneously, strongly oxidizing and reducing or with appropriate scavengers either oxidative or reductive conditions can be selected. In addition, the choice of water as the solvent rather than typical organic solvents has practical advantages such as lower cost and environmental impact. Second, in the case of direct radiation effects, water may provide mass adjacent to TCDD for deposition of radiation energy. Third, since surfactants are thought Envlron. Scl. Technol., Vol. 28, No. 13, 1994 2251
70
60
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40
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10
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-.-
40
50
60
% MOISTURE CONTENT
-A-
75 kGy (10 hr)
150 kGy ( 2 0 hr)
--e--
450 kGy (60 hr)
Figure 2. Comparison of percent TCDD destruction versus soil moisture content for three irradiation doses (75,150, and 450 kGy). Standard deviations are shown as error bars and in some cases are smaller than the symbols.
to be a necessary soil amendment for the reasons discussed in the next section, some water addition is required for uniform mixing of the surfactant throughout the soil. From an engineering point of view, it was an important goal of these experiments to find minimum levels of water and surfactant for the maximum amount of destruction. By achieving this, the creation of an additional waste stream would be avoided, and treatment costs may be minimized (50). Moisture content of the contaminated soil was varied from its original state of 3 % -52 % (w/w) where two phases are clearly visible. In Figure 2, the degradation of TCDD over this range of soil moisture is shown for three radiation doses. At radiation doses of 75 and 150 kGy, essentially no dioxin destruction is observed at very low levels of moisture (3 %), indicating direct destruction is not significant. At a dose of 450 kGy and 3 % moisture, however, approximately 33 % TCDD destruction is observed, indicating that direct ionizations may be significant at high radiation doses. Yet, in comparing these results the same degree of destruction is achieved at one-sixth the dose (75 kGy) or much shorter irradiation times with the addition of a small amount of water (i.e., 25% moisture). At all three doses there appears to be an optimum moisture content around 25% beyond which no significant gain in degradation occurs. Furthermore, 25 % moisture is near the saturation point of 33% for this soil, at which point samples begin to display slurry-like qualities (45). Based on these considerations, 25 % moisture was selected as an optimum value for water addition, and all subsequent experiments were conducted at this level. Surfactants, Use of surfactants has been shown to improve significantly the photolytic destruction of TCDD in soil, yet its exact role is not fully known (15). On the basis of these findings and unpublished data in aqueous solutions, surfactant addition to soil systems was thought to be necessary for effective radiolytic TCDD destruction. Surfactant effects in this application are thought to be 2252
Envlron. Sci. Technol., Vol. 28, No. 13, 1994
complex and numerous (39, 40). Surfactants may (1) mobilize the dioxin molecule to a location where it is attacked more readily; (2) interact specifically in the degradation reaction, as a hydrogen donor, for instance; (3) increase the target size and modify material density, thus enhancing direct radiation effects; (4) provide electrostatic rate enhancement for attack by the aqueous electron (43, 44). Three surfactants, two nonionic and one cationic, have been chosen for the surfactant experiments. Witco Adsee 799 and BASF Corp. Plurafac RA-40, both nonionic surfactants, were selected based on successfuluse in dioxin bioremediation studies (41). The effects of hexadecyltrimethylammonium chloride (CTACl), a cationic surfactant, were investigated based on results of radiolytic destruction in aqueous solution. Two surfactant concentrations, 0.5% and 2 % , were chosen and tested on the basis of literature reports of soil studies (15, 41). In the experiments described herein, surfactants have not been used to solubilize the dioxin within micelles in the solvent phase. Rather, surfactants are thought to bring the dioxin into an interfacial phase at the soil surface, making it more available to either direct radiation attack or indirect radical interactions. In the case of direct radiation interactions, a surfactant is thought to modify target size and density to minimize the radiation dose required for effective destruction. This relationship between target size, material density, and radiation dose is described by target theory (33). In the case of indirect radiation interactions, rates for the reductive dechlorination of chlorophenols by the hydrated electron in solution are 1-2 orders of magnitude less than diffusion control (42). These reaction rates can be increased in heterogeneous cationic micellar suspensions by modifying electron diffusion kinetics through favorable electrostatic interactions (43,44). It is expected that slow kinetics for reductive dechlorination by the hydrated electron may also limit the effectiveness of radiolysis in soils. CTACl
A
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loo
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0.5% C
2% C
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NO S U R F
SURFACTANT TYPE/CONCENTRATION
Parameter study investigatlngsurfactant effectsfw homogeneously contamlnated soils at 25% moisture content. Three doses,three surfactants (ADSEE 799, Plurafac RA-40. and CTACI). and two surfactantconcentrations (0.5% and 2.0%) are investigated.Standard deviations are less than 8%. except for the 75 kGy, 0.5% Adsee which had a SD of 22%. Flgure 3.
then was used in this research to provide electrostatic rate enhancement for interactions with e, and to provide an interesting point of comparison with solution studies. In the bar graph shown in Figure 3, the effects of surfactant type and amount on the extent of TCDD destruction are compared to the destruction accomplished in the absence of surfactant at three radiation doses in soils with 25% moisture. In the absence of surfactant and under these experimental conditions, dioxin destruction does not appear to increase markedly with dose, but is instead limited to approximately 50-55% destruction. An increase in surfactant concentration from 0.5% to 2.0% improvedTCDD destruction. A t high radiation doses and in the presence of 2 % nonionic surfactant, 25% water, and air, approximately 86 % of TCDD was destroyed, and the apparent ceiling observed in the water-only system was lifted. Yet, in comparing the results of surfactant and wateronly systems, at radiation doses of 75 and 150 kGy, surfactant addition failed to enhance TCDD degradation. Significantlyhigher TCDD destruction was achieved only in the presence of 2% nonionic surfactants at radiation doses of 450 kGy, and under these conditions the nonionic surfactant RA-40 produced the best results with nearly 90% destruction. Although these data illustrate that surfactants increase the degree of TCDD destruction under certain conditions, at the lower radiation doses surfactant addition appeared to affect adversely destruction in comparison-io that observed in the absence of surfactant. A particular radiation dose. however. is a function of the amount of time a sample has been irradiated (see Figure 3 for corresponding irradiation times). Since high radiation
doses are achieved by exposing sample vials to they source for longer times, higher radiation doses also represent longer contact times between dioxinand surfactant. Lower destructions at lower doses then may occur because insufficient time has elapsed to bring the dioxin and surfactant into contact, resulting in an absence of the proposed surfactant effects and possibly an unknown inhibitory effect. Further investigation has determined that at 25% moisture the surfactant-soil system requires a minimum of 24 h to "equilibrate." All subsequent experiments included this 24-hcontactperiod,andinhibitionofTCDD degradation was never again observed in the presence of surfactant for this soil. Since these experiments were done in the presence of air,oxygen would he expected to scavenge the aqueous electron. In this scenario, the electrostatic enhancement proposed for the cationic surfactant would not be observed and was not. Scavenger Studies. In order to probe the possible routes of radical interaction in TCDD destruction and to explore the utility of controlling reaction conditions, scavenger studies were conducted. Sample vials were sparged with nitrogen gas to exclude oxygen, producing an initial 1:l proportion of 'OH and e-aq In the presence of air or oxygen, e, is scavenged, yielding superoxide radical and possibly other activated oxygen species (51). 0,
+ e-as + 0;-
(1)
Nitrous oxide (N@) also scavenges e-aq to produce an *OH and Oxidizing conditions. N,O
+ e g + H,O * N, + OH' + OH-
(2)
Envlron. SCi. Technol.. Vol. 28. No. 13. 1994 2253
The 'OH is scavengedby 2-propanol (IPOH) under alkaline conditions creating e-aqdominance and reducing conditions. (CH,),CHOH
+ OH'*
(CH,),C'OH
+ H,O
(3)
Scavenger experiments were performed at a single radiation dose (210 kGy; 30-h irradiation), 25% moisture, and 2 % surfactant concentration. A summary of these results is shown in Figure 4. For each of the systems, no surfactant, cationic (CTACl), and nonionic (RA-40) surfactant, there are only small differencesin theextentofTCDDdestructionunderthevarious scavenger conditions, with the possible exception of 02 in the absence of surfactant. With this same exception, surfactant addition enhanced significantly the extent of TCDD disappearance. Comparison of these results also suggests that, despite varying radical predominance, surfactant addition creates more uniform irradiation conditions for TCDD destruction than is observed in the no surfactant system. As observed among the data presented inFigure 3, RA-40 produced the greatest TCDD degradation. The degree and pattern of destruction measured in the presence of CTAC1, the cationic surfactant, indicate that electrostatic rate enhancement does not play an observable role in this soil system. Finally, the fact that the extent ofTCDD destruction was relatively insensitive to free radical conditions is compellingevidence that the predominant route of degradation in this soil is not via indirect radiation effects. Byproducts. Byproduct studies were conducted to evaluate the products and pathway of TCDD destruction from irradiation w i t h W o 7 radiation. Figure 5 illustrates byproduct data from the irradiation of 2,3,7,8-TCDD contaminated soil at 25% soil moisture, 2% RA-40 surfactant, and nitrogen sparge. Three irradiation times were used (75, 150, and 450 kGy), and eight replicates were analyzed for each dose. From these data in Figure 5, 2,3,7,8-TCDD appears to he undergoing stepwise
-
NOSURF. 80
reductive dechlorination from tetra to tri, then di-, and then monochlorodioxin. At 75 kGy, 84% of the initial TCDD concentration is accounted for, and at 150 kGy, 87% of the total carbon is present as various chlorinated species. Analysis for nonchlorinated dioxins and phenols or biphenyls has not yet been conducted, but it is expected that these species will close the mass balance on carbon. Volatile losses of dioxin are considered negligible because the irradiation vials are closed and the temperature increase throughout irradiation is less than 12 "C. Byproducts were also analyzed under selected scavenger conditions, and these data are presented in Table 2. All experiments were conducted at 25% water content and a radiation dose of 150 kGy. In the absence of surfactant addition (A), approximately 20 % TCDD loss was observed and only a small amount of T3CDD was detected. In contrast, with the addition of 2% RA-40 and under varied scavenger conditions (B-D), between 50% and 70% destruction was observed. Slightly less destruction occurred under oxidative conditions created with NzO addition, but the distribution of lesser chlorinated byproducts was similar among the surfactant systems. It is irreconcilable,based on the principlesof solution chemistry and indirect radiation effects, that the products of the radiolytic degradation of TCDD and a NzO sparge (oxidative conditions) are reductive species and that the extent of destruction under oxidative and reductive conditions are so similar (*5%). These results, however, are explained if the chemical transformation of TCDD is occurringprimarily through electron addition from directlike radiation effects. Moreover, these results illustrate the fact that for this soil, surfactant addition is necessary to enhance the effectiveness of direct effects for TCDD destruction. Kinetics. Kinetic studies were conducted to compare the rate of TCDD destruction, as a function of radiation dose, between surfactant (2% RA-40) and no surfactant soil systems containing 25% moisture and sparged with 2% RA-40
2%CTACl
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Comparisonsof bee radical conditions created by scavengers for soils with 25% moisture, 210 kGy dose, and 2% surfactantM no surfactant. Standard deviations are less than 9%. Flgure 4.
2254 Envlron. Scl. Technol., Vol. 28, NO. 13, 1994
-+-
TCDD
--W- XCDD
--A--DCDD
-
B -MCDD
-+-PCDDs
DOSE (kGy) Flgure 5. Byproduct data for three radiation doses in soil with 25% moisture content, 2 % RA-40 surfactant, and under a nitrogen sparge. Eight replicates have been averaged for each concentration listed, and the standard deviations are shown as error bars whlch in some cases are smaller than the symbols.
Table 2. Product Distribution from 7 Irradiation of TCDD on Soil under Varied Conditions of Radical Scavengers* product concentrations (ppb) B C D
product
control
A
T&DD T&DD DzCDD MlCDD
88.6
71.4 2.5
0.0 0.0 0.0
0.0 0.0
44.4 21.7 10.6 1.9
35.8 20.6 14.3 4.2
28.1 24.7 17.2 5.2
All samples (A-D) had 25 % water content and were irradiated to 150 kGy. The control is unirradiated. Samples B-D also had 2% RA-40 surfactant added. The following scavengers were used: A, Nz sparge; B, NzO sparge; C, Nz/2-propanol; D, Nz sparge.
In Figure 6, kinetic data are presented with open symbols to describe the residual TCDD concentration as a function of radiation dose. The surfactant data (open circles) were fit with a pseudo-first-order decay expression to yield an apparent initial rate constant of 0.0469 h-l ( R = 0.968). In comparison, TCDD destruction in the absence of surfactant (open squares) occurred at a lower rate and to a limited extent. A first-order fit of these data gave an initial rate constant (0.0094h-l) five times lower than that in the presence of surfactant. The solid symbols are data obtained in separate experiments performed at doses of 130, 210, 450, and 800 kGy but under comparable conditions. The solid line is an extrapolation to higher doses of the initial decay kinetics in the presence of RA40. Although the TCDD decay measured at higher doses does not fit this prediction exactly, the trend is reflected accurately. Lack of precise agreement is probably due to the fact that initial reaction conditions are not maintained over the long irradiation times required to achieve these high doses; some surfactant destruction may occur under the y flux and is being investigated. Both the data and initial kinetics do show, however, that very high doses and surfactant addition are required N2.
to achieve complete TCDD transformation. On the basis of these surfactant data, in order to degrade TCDD to a residual level of
