Environ. Sci. Technol. 2000, 34, 3452-3455
Method To Simultaneously Improve PCB Radiolysis Rates in Transformer Oil and To Close the Chlorine Mass Balance BRUCE J. MINCHER* Idaho National Engineering and Environmental Laboratory, P.O. Box 1625, Idaho Falls, Idaho 83415-7111 RANDY C. CURRY University of Missouri, Columbia, Columbia, Missouri 65211 RICHARD BREY Idaho State University, Pocatello, Idaho, 83209
The addition of alkaline 2-propanol to isooctane and transformer oil solutions of PCBs was found to increase the rate of PCB dechlorination by γ-radiolysis. Simultaneously, it facilitates the closure of the chlorine mass balance. The chlorine liberated from PCBs irradiated in alkaline 2-propanol-spiked solutions precipitated as an inorganic salt in a stoichiometric amount. This was demonstrated for an individual PCB congener (PCB 155) in isooctane and for Aroclor 1260 in transformer oil. Radiolysis rates are reported in terms of dose constants, d (kGy-1), the exponential rate constant for PCB decomposition with respect to absorbed dose. These are the highest decomposition rates yet reported for the radiolytic treatment of PCB-contaminated transformer oil.
Introduction The radiation chemistry of the PCBs has recently been reviewed (1). The radiolytic decomposition of PCBs in nonpolar solvents is the result of reductive dechlorination by electron capture (2) and by electron transfer from radical anions produced in solution (3). The PCBs are high electronaffinity compounds that are able to capture solvated electrons at diffusion-limited rates. They also successfully compete with geminate recombination for radiolytic electrons. Thus, PCBs have higher decomposition rates than would be predicted based on the free-ion yields for nonpolar solvents (2). Each PCB congener decomposes at a rate related to its affinity for electrons, as measured by that congener’s lowest unoccupied molecular orbital energy (LUMO) (2). The result is dechlorination, and the concentration change is exponential over the entire range, which has been investigated (4, 5). Furthermore, it has been shown that the dosedependent pseudo-first-order rate constant (dose constant) for PCB decomposition in irradiated isooctane increases as the initial concentration decreases (6). The predicted products of PCB dechlorination are less chlorinated PCB congeners and chloride ion. Since less chlorinated congeners are not necessarily less toxic congeners, complete dechlorination of all PCBs may be necessary for treatment. Continued irradiation results in biphenyl or * Corresponding author telephone: (208)533-4449; fax: (208)5334369; e-mail:
[email protected]. 3452
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biphenyl-solvent adducts and, theoretically, a stoichiometric amount of chloride ion (4, 7). Unfortunately, the chlorine liberated by radiolysis cannot be recovered as an anion in nonpolar solutions (2). This inability to close the chlorine mass balance is a factor that has hampered the adoption of radiolysis as a PCB treatment. Since PCB dechlorination occurs by electron capture, an important factor affecting radiolysis rates is the presence of electron-scavenger species. The nature and concentration of scavengers overwhelms PCB concentration effects (6). Among these is dissolved oxygen. Several investigators have shown that radiolysis rates are increased by removing oxygen (4, 8, 9). It has also been shown that hydraulic oils containing electron-capturing impurities have lower PCB decomposition rates than clean oils (6, 10). Improvements in radiolysis rates can only improve the profitability of a proposed treatment process. A final factor hampering the adoption of radiolysis is the lack of a figure-of-merit by which absorbed doses required for desired amounts of PCB destruction may be predicted. The G value normally used in radiation chemistry is not useful for the scaling of dose requirements (11). This paper will present a method for increasing the rate of PCB radiolysis in transformer oils. The same technique simultaneously collects the PCB chlorine stoichiometrically, demonstrating that hazardous chlorinated byproducts are not created. The effect is first shown for an individual PCB congener in isooctane and then for Aroclor 1260 in transformer oil. Furthermore, the use of the empirically determined exponential decay constant (dose constant, d) is discussed as an appropriate figure-of-merit for process dose calculations (11).
Experimental Section Irradiations. Samples were γ-ray-irradiated at the Advanced Test Reactor at the Idaho National Engineering and Environmental Laboratory (INEEL). Spent nuclear fuel was the source, and dose rates varied to a maximum of 35 kGy h-1. The samples were batch irradiated in small volumes contained in septum-sealed vials, with radiochromic film dosimetry. Experience with the system has shown that dose rate effects are not measurable for these ranges. Sample preparation, irradiation, and dosimetry have been described previously (4, 12). Analytical Methods. Two nonpolar solvents were used in these experiments. The first, reagent-grade isooctane, was used for the individual congener measurements. Individual PCB congeners were identified and quantified on a HewlettPackard 5995 GC/MS, operated in electron impact mode. Analytical quality control was based on EPA Method 680. The second solvent was Shell Diala A transformer oil (Ultra Scientific, North Kingston, RI), which was used for the Aroclor 1260 experiments. Aroclor 1260 was analyzed using a HewlettPackard 5890 series II gas chromatograph. Since Aroclors are mixtures of PCB congeners, irradiation of Aroclor PCBs results in an altered congener distribution favoring lesschlorinated molecules. Quantifying the new congener mixture is problematic due the varying ECD response factors for differing levels of chlorination. For the purposes of reporting Aroclor concentration decreases here, all ECD peaks within an appropriate retention time window were considered to be Aroclor 1260. This window (8-20 min) included the retention times of the expected byproduct congeners. These were compared to a true Aroclor 1260 standard. This may underestimate the true total PCB concentration. However, when the results of this technique were used to calculate 10.1021/es991095b CCC: $19.00
2000 American Chemical Society Published on Web 06/29/2000
predicted amounts of chloride ion produced in irradiated 2-propanol-transformer oil mixtures, they agreed with the amount of chloride ion actually measured. Thus, any error in Aroclor quantification is probably small. This is discussed in more detail in the Results and Discussion section. Chloride ion in isooctane and transformer oil was measured by back-extraction with a pure (18 MΩ) water followed by analysis with a Dionex 2010i ion chromatograph with a AS4A separator column. The standard 2 mmol dm-3 sodium carbonate/0.75 mmol dm-3 sodium bicarbonate eluent with 25 mmol dm-3 sulfuric acid background suppression was used. Chloride eluted at about 1.6 min, free of interference. The chloride that precipitated from irradiated isooctane or transformer oils spiked with alkaline 2-propanol was readily soluble in water and was quantified in the same fashion. Chlorine as an unknown species in unspiked postirradiation isooctane was quantified by neutron activation analysis (NAA). Stable 37Cl is 24% of naturally occurring chlorine. It captures thermal neutrons to become 38Cl, which decays by a characteristic γ-emission with a 37-min half-life. The chlorine concentration was determined by comparison of sample γ-ray spectra to spectra of known NaCl concentrations following activation. Neutrons were generated at Idaho State University (I.S.U.) with a 10 MeV linear accelerator. Bremsstrahlung were directed onto a 3.8-L deuterium oxide target, surrounded by a 25 cm thick paraffin cube encased with 10 cm of graphite. Neutrons were generated via the 2H(γ,n)1H reaction and moderated by the paraffin and graphite block. Reaction Rates. The dose constant (d) has often been used as a figure-of-merit to describe PCB radiolysis (2, 4-6, 8, 11). Although PCB dechlorination may be the result of a combination of electron capture reactions, the result is an exponential concentration decrease over the entire soluble concentration range. It may be modeled with an effective first-order rate constant, with respect to absorbed dose rather than time. This is reported in units of reciprocal dose (kGy-1). It should be cautioned that the dose constant is not a fundamental rate constant, rather it is dependent on process conditions. It does serve well for process calculations where it is preferred to the G value because it is a rate constant, rather than merely a rate at some point in the irradiation. Absorbed dose requirements may be calculated using a modified first-order rate equation:
C ) Coe-dD where C is the concentration to be arrived at due to treatment, Co is the initial concentration, and D is the absorbed dose required. For exponential reactions such as PCB dechlorination, the two figures-of-merit are related as shown:
Go ) dCo Thus, for an exponential reaction, the rate equals the product of the rate constant and the concentration.
Results and Discussion Chlorine Mass Balance. Figure 1 shows the decomposition of 268 mg kg-1 PCB 103 (2,2′,4,5′,6-pentachlorobiphenyl) in isooctane, irradiated to a maximum absorbed dose of 120 kGy. It can be seen that PCB 103 decomposition is exponential. The dose constant is found to be 0.054 kGy-1. This was used to calculate the initial decomposition rate (Go) for this concentration (Co). The resultant Go is 0.04 µmol J-1. Figure 2 shows the amount of chloride that should have been produced if the PCB 103 were stoichiometrically dechlorinated, producing five chloride ions per PCB. Also shown is the amount of free chloride ion actually found in
FIGURE 1. Exponential decrease in PCB 103 concentration when irradiated in isooctane. The slope of the line is the dose constant.
FIGURE 2. Chloride ion production in irradiated isooctane containing PCB 103. Curve a shows the amount of chloride predicted for stoichiometric dechlorination. Curve b shows the actual free chloride found by ion chromatography. the irradiated isooctane. While large amounts of free chloride ion are found in irradiated polar solvents under similar conditions (2), it is apparent from Figure 2 that only trivial amounts of free chloride ion are recovered in isooctane. This is the case even though essentially all the PCB 103 has been decomposed at 120 kGy. Since less-chlorinated PCB byproducts and solvent-adducts have been shown to account for only a small percentage of this chlorine deficit, most PCB chlorine is unaccounted for (2). This is a typical result and suggests that unknown organochlorine species, possibly even PCB polymers, are produced. Closure of the chlorine mass balance in irradiated PCB oils is crucial to demonstrating that the reaction byproducts are not hazardous organochlorine compounds. Arbon showed that PCB carbon remained in the bulk solution following irradiation (2), not being lost as volatile compounds. Presumably the PCB chlorine also remains in solution. To demonstrate this, neutron activation analysis (NAA) was performed on irradiated isooctane solutions of PCBs. A 1000 mg kg-1 solution of PCB 54 (2,2′,6,6′-tetrachlorobiphenyl) was irradiated to a maximum of 500 kGy. The high absorbed dose was chosen to completely dechlorinate the PCB and its byproducts. The solution was then sparged with argon to remove any volatile species formed. Furthermore, to eliminate the possibility that chloride ion had formed but had attached to active sites on glass (due to the inability of nonpolar solutions to solvate it), the solutions were transferred to new glass prior to NAA. The results are shown in Table 1. It can be seen that unidentified chlorine VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Neutron Activation Analysis of Chlorine in γ-Ray-Irradiated Isooctane Solution of PCB 54 absorbed dose (kGy)
chlorine (mg L-1)
0 93 198 500
592 637 540 572
TABLE 3. Radiolysis of 1250 mg kg-1 Aroclor 1260 in Transformer Oil Using Alkaline 2-Propanol (IPA) Spiking and an Absorbed Dose of 207 kGy % IPA
final concn (mg kg-1)
d (kGy-1)
Go (µmol J-1)
0 10 30 50
740 35 9.3 12
0.003 0.017 0.024 0.022
0.01 0.06 0.08 0.08
TABLE 2. Increase in Radiolytic Decomposition Rate of PCB 155 When Alkaline 2-Propanol Is Added to Isooctane at 10% by Volume absorbed dose (kGy)
PCB 155 10% IPA
concn (mg kg-1) pure isooctane
0 92.6 151 198 500
1016 1.4
946 146 72 28 0.08
