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A comparison of large-scale electron beam and bench-scale 60Co irradiations of simulated aqueous waste streams
Radiation Physics and Chemistry 65 (2002) 367–378
A comparison of large-scale electron beam and bench-scale 60 Co irradiations of simulated aqueous waste streams * c, Charles N. Kurucza,*, Thomas D. Waiteb, Suzana E. Otano William J. Cooperd, Michael G. Nickelsend a
Department of Management Science, University of Miami, P. O. Box 248237, Coral Gables, FL 33124, USA b Department of Civil and Architectural Engineering, University of Miami, Coral Gables, FL 33124, USA c Department of Biomedical Engineering, University of Miami, Coral Gables, FL 33124, USA d Department of Chemistry, University of North Carolina at Wilmington, Wilmington, NC 28403, USA
Abstract The effectiveness of using high energy electron beam irradiation for the removal of toxic organic chemicals from water and wastewater has been demonstrated by commercial-scale experiments conducted at the Electron Beam Research Facility (EBRF) located in Miami, Florida and elsewhere. The EBRF treats various waste and water streams up to 450 l min1 (120 gal min1) with doses up to 8 kilogray (kGy). Many experiments have been conducted by injecting toxic organic compounds into various plant feed streams and measuring the concentrations of compound(s) before and after exposure to the electron beam at various doses. Extensive experimentation has also been performed by dissolving selected chemicals in 22,700 l (6000 gal) tank trucks of potable water to simulate contaminated groundwater, and pumping the resulting solutions through the electron beam. These large-scale experiments, although necessary to demonstrate the commercial viability of the process, require a great deal of time and effort. This paper compares the results of large-scale electron beam irradiations to those obtained from bench-scale irradiations using gamma rays generated by a 60Co source. Dose constants from exponential contaminant removal models are found to depend on the source of radiation and initial contaminant concentration. Possible reasons for observed differences such as a dose rate effect are discussed. Models for estimating electron beam dose constants from bench-scale gamma experiments are presented. Data used to compare the removal of organic compounds using gamma irradiation and electron beam irradiation are taken from the literature and a series of experiments designed to examine the effects of pH, the presence of turbidity, and initial concentration on the removal of various organic compounds (benzene, toluene, phenol, PCE, TCE and chloroform) from simulated groundwater. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Electron beam irradiation; Gamma irradi; Liquid waste treatment; Aqueous waste streams; Large-scale irradiation; Benchscale irradiation; Dose rate; Toxic organic chemical removal
1. Introduction The effectiveness of using high energy electron beam irradiation for the removal of toxic organic chemicals from water and wastewater has been demonstrated by large-scale experiments conducted at the Electron Beam Research Facility (EBRF) located at the Miami Dade *Corresponding author. Fax: +1-305-284-2321. E-mail address: [email protected] (C.N. Kurucz).
Water and Sewer Authority Central District Plant in Miami, Florida and elsewhere. The EBRF, described in detail elsewhere (Kurucz et al., 1991, 1995a, b), utilizes a 1.5 MV, 75 kW electron accelerator to treat various waste and water streams at 450 l min1 (120 gal min1) with doses up to 8 kGy. Many experiments have been conducted by injecting toxic organic compounds into various plant feed streams, such as chlorinated secondary municipal wastewater and raw wastewater, and measuring the concentrations of compound(s) before
0969-806X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 2 ) 0 0 3 3 7 - 7
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and after exposure to the electron beam at various doses (Nickelsen et al., 1992; Cooper et al., 1992). Extensive experimentation has also been performed by dissolving selected chemicals in 22,700 l (6000 gal) tank trucks of potable water to simulate contaminated groundwater, and pumping the resulting solutions through the electron beam (Cooper et al., 1993a, b; Nickelson et al., 1998). Large-scale experiments are required to demonstrate feasibility and to establish engineering parameters for full-scale commercial applications of the process. However, they require a great deal of time and effort. A typical experiment requires five to eight people to run and preparation time is often quite extensive. For example, tanker experiments require large quantities of chemicals to be added to the tanker after it is filled. In experiments designed to determine the effects of suspended solids on the removal efficiency of target compounds, as much as 340 Kg (750 lb) of kaolin clay are added. Because of these difficulties and the relative lack of large-scale electron beam testing facilities, much of the experimentation on the use of ionizing radiation utilizes bench-scale gamma irradiators. Bench-scale experiments are much easier to perform and can save a great deal of effort when attempting to determine the feasibility of using high-energy electrons to treat specific chemical wastes. This paper compares the results of electron beam and gamma experiments and discusses the feasibility of predicting the results of large-scale electron beam irradiations utilizing data from bench-scale gamma (60Co) tests. Possible reasons for observed differences, including the effects of differences in dose rates that result from performing electron beam irradiations versus 60 Co irradiations, are also presented.
2. Methods and materials 2.1. Irradiation facilities As mentioned previously, electron beam experiments were performed using a 1.5 MeV, 50 mA insulated core transformer (ICT) accelerator at the Miami EBRF. Aqueous streams of contaminated water are exposed to a scanned beam in a waterfall approximately 127 cm wide and 0.44 cm thick. Doses of 0–8 kGy at flow rates up to 450 l min1 (120 gal min1) are obtained by varying beam current between 0 and 50 mA. The vertical spread of the beam is approximately 15 cm, including scatter, after passing through primary and secondary titanium window foils. At this point the average speed of the falling water in front of the beam is approximately 1.5 m s1. This yields a residence time of approximately 0.1 s and a dose rate of up to 80 kGy s1.
Gamma irradiation studies were conducted using a 5000 Curie 60Co g-source (Gamma Cell 150-A) located at the University of Miami Radiation Control Center. The source consists of 20 radioactive 60Co slugs, sealed in a cylindrical capsule 3.18 cm in diameter (Wang, 1993). The source is located at the center of a sample table marked with concentric circles of 10–100 cm. Due to the relatively short length of the capsule, the irradiator behaves as a point source. Most samples were irradiated at distances of 10–30 cm from the source, which resulted in dose rates ranging from 0.10 to 0.65 Gy s1. These dose rates are up to five orders of magnitude lower than those obtained using the electron beam. 2.2. Experimental design The data used to compare the removal of organic compounds using gamma irradiation and electron beam irradiation in this paper are taken from a series of largescale experiments designed to examine the effects of pH, the presence of clay, and initial concentration on the relationship between dose and removal of various organic compounds (benzene, toluene, phenol, PCE, TCE and chloroform) from simulated groundwater. These experiments were generally conducted at three target concentrations (100, 1000, and 10,000 mg l1), three pH values (5,7, and 9), and two clay concentrations (0% and 3% by weight) using a factorial design with partial replication. Influent and effluent samples for large-scale electron beam experiments at the EBRF were collected prior to and after irradiation in 47 ml PFTE lined screw cap glass vials. The vials were completely filled without headspace, immediately chilled on ice and returned to the laboratory for analysis. Samples for gamma experiments were taken from the influent stream at the same time as the electron beam samples. Gamma influent samples were collected in 500 ml screw cap bottles, put on ice, transported to the University of Miami, and refrigerated until processed for irradiation. The samples were subsequently placed in 47 ml PFTE lined screw cap vials, and labeled as to experimental condition and desired dose. In general, four vials were prepared from each sample (experimental condition) taken at the EBRF. One vial was used as the zero dose control and the others irradiated at three specified doses. The doses actually used to irradiate the electron beam samples were used as the target gamma doses. The necessary irradiation time and distances from the 60 Co source required to obtain the desired target doses were calculated from available calibration data and the vials were then transported to the gamma source for irradiation. The zero dose (control) samples were transported also but were not placed in the gamma irradiator. After irradiation, samples were transported
C.N. Kurucz et al. / Radiation Physics and Chemistry 65 (2002) 367–378
to the Florida International University Drinking Water Research Center for analysis. Details of the sampling procedures used for the electron beam experiments are described elsewhere (Nickelsen et al., 1992; Kurucz et al., 1995a). Concentrations of organics were measured by gas chromatography using liquid–liquid extraction and high-performance liquid chromatography as described in detail previously (Nickelsen et al., 1992, 1994; Cooper et al., 1993a, b). 2.3. Data reduction The ideal experiment to compare the results of large-scale electron beam irradiations to gamma (60Co) irradiations would prepare split samples and irradiate them with the same dose, handling procedures, etc. using each method. In this case a direct comparison of the removal percentages or fraction of compound remaining would be possible. However, inherent differences in the way the irradiations are conducted do not allow such direct comparisons. For example, actual doses were not always the same at the EBRF and 60 Co irradiator because of variation in flow rates at the EBRF and/or physical limitations on the placement of vials at the gamma source. Actual doses obtained were calculated adjusting for these variations as the first step in the data reduction process. Another consideration involves the use of the open weir system at the EBRF, which can result in some losses at zero dose due to volatilization and pipeline residuals. These losses do not occur in the closed vials used in the 60Co irradiations. In order to overcome these problems it was decided to compare the parameters of appropriate removal models for the gamma and electron beam irradiations. The general approach involved fitting models relating the compound removal and irradiation dose, to the data for each experimental condition. For gamma experiments the dependent variable was the ratio between the resulting concentration at a particular dose and the concentration at zero dose for a given experimental condition. Since the electron beam removals included losses due to the use of the weir system, the dose constants were estimated using only effluent concentration as the dependent variable. Because volatilization and mixing, if any, take place prior to the effluent sampling port, changes in effluent concentration at nonzero doses are due primarily to radiation effects. In both cases the independent variable was the estimated total absorbed dose. Various models relating removal and dose were evaluated using TableCurve (Jandel Scientific, 1991) and LOTUS (Lotus Development Corporation, 1986) software. Although many functions were fit to the data automatically by the software, the following models
369
almost always were among the best fits in terms of minimum squared error: the ‘‘first-order’’ model xD ¼ x0 ekD ee
ð1Þ
that assumes change in concentration with respect to change in dose is proportional to initial concentration, and the ‘‘second-order’’ model 1=xD ¼ 1=x0 þ kD þ e
ð2Þ
that assumes change in concentration is proportional to the square of initial concentration. In these models, D is the absorbed dose, xD the compound remaining (effluent concentration for electron beam data or concentration at dose D over concentration at zero dose for gamma data) at dose D; x0 the compound remaining at zero dose, k the dose destruction constant, e the error term (normally distributed with mean=0 and variance=s2 ). Note that the error term ee in Eq. (1) is multiplicative thus making the variation in xD proportional to the magnitude of xD : This is consistent with the manner in which the removal data behave and implies that xD follows the Lognormal probability distribution. As in earlier work using electron beam data (Nickelsen et al., 1992; Kurucz et al., 1995b), the first-order model (Eq. (1)) generally provided a better fit to the gamma data than the second-order model (Eq. (2)). The second-order model, however, occasionally provided a somewhat better fit at low concentrations. Although the differences were not great enough to offset the added complication of working with more than one model, both the gamma and electron beam data suggest that removal might be second order with respect to dose for contaminant concentrations under 100 mg l1. The dose constants, k; obtained by fitting Eq. (1) to the gamma data and the corresponding electron beam data are given in Table 1. Since target concentrations were not exactly the same for each compound due to solubility limits, initial concentrations are designated low, medium, and high in Table 1. Occasionally fewer than four data points were available at each experimental condition because concentrations were obtained that were below the minimum detection limits of our analytical procedures. In these cases, the concentrations were assumed to be equal to the non-detection limits of our analytical procedures for purposes of curve fitting, even though the true concentrations were not known. The analytical procedures utilized in this study, however, are extremely sensitive and non-detect values are considered to be very low. There are several more sophisticated methods of handling the non-detect problem (Helsel, 1990); however, these apply to situations in which more data are available under a given set of experimental conditions. Our approach is conservative in that it tends to somewhat understate removal at higher doses. If more than one below detection limit
0.204 0.071 0.181 0.042
0.050 0.062 0.030 0.018
0.006 0.017 0.009 0.008
TCE No s.s. 0.051 3% s.s. 0.067
PCE No s.s. 0.017 3% s.s. 0.023
Chloroform No s.s. 0.005 3% s.s. 0.005
Co
E-beam
pH 9
0.129
0.008 0.009 0.4
0.053 0.013 0.033
+ 0.050 0.080
0.241 0.159 +
+ +
0.231 0.086 59.8
60
Co
E-beam
pH 5
0.018 0.007 0.006
0.027 0.013 0.012
0.075 0.053 0.046
0.259 0.024 0.031
0.198 0.106 0.212
0.222 0.054 0.046
60
Co
E-beam
pH 7
0.073 0.107
0.008 0.007 0.013 0.008
0.025 0.015 59.3 0.016
0.107 0.053 0.216 0.043
0.016 0.028 0.013 0.027
+ +
0.132 0.107 0.195 0.046
60
E-beam
pH 9
0.015 0.010 0.006
0.027 0.016 0.023
+ 0.044 0.106
0.009 0.016 0.032
0.220 0.057 0.209
0.103 0.051 0.058
Co
60
Co
E-beam
pH 5
0.006 0.004 0.004
0.023 0.008 0.011
0.053 0.034 0.032
0.013 0.006 0.007
0.218 0.026 0.031
E-beam
pH 7
0.008 0.005 59.5 0.006
0.022 0.013 0.017 0.010
0.058 0.034 0.125 0.030
0.002 0.006 0.002 0.006
0.066 0.025 0.127 0.027
0.047 0.022 0.038 0.020
Co
60
High concentration
0.116 0.023 0.031
60
Co gamma experiments
60
Medium concentration
See text. =not planned experiment (see text), +=insufficient data for collection.
0.255 0.131 + 0.127
Phenol No s.s. 0.155 3% s.s. 0.126
a
0.167 0.209
+ +
E-beam
Toluene No s.s. 0.135 3% s.s. +
Co
pH 7
0.315 0.223 59.7 0.105
60
Benzene No s.s. + 3% s.s. 0.180
E-beam
pH 5
Low concentration
a
Table 1 Electron beam dose constants (krad1) and estimated dose constants for corresponding
E-beam
pH 9
0.009 0.007 0.009
0.026 0.012 0.019
0.067 0.029 59.7
0.001 0.003 0.002
0.061 0.028 0.068
0.042 0.021 0.033
Co
60
0.009
0.023
0.071
0.001
0.056
0.033
Co
60
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C.N. Kurucz et al. / Radiation Physics and Chemistry 65 (2002) 367–378
value was generated for a given set of data only the first value was used since the additional values did not provide information as to the shape of the removal curve. Note that even though there appear to be relatively few data points for curve fitting, both Eqs. (1) and (2) are single parameter models that leave at least two degrees of freedom for error estimation. Further, in our experience the consistency with which these equations model contaminant removal makes additional data points unnecessary even though they would be desirable. Since reliable estimation of the model parameters does require at least three data points, however, it was not possible to estimate the dose constant k for some experimental conditions. For example, the concentration of toluene was below detection limits at all doses utilized. As will be discussed later, the apparent improved contaminant removal for 60 Co radiation contributed to the generation of below detection values at the target doses. These cases are indicated by aþsymbol in Table 1. Note that no data were obtained for the pH 9 and 3% clay condition because the addition of clay reduced the pH of the groundwater to approximately 7 and therefore, this experimental condition was not run at the EBRF. 2.4. Comparison of dose constants In order to compare the gamma and electron beam dose constants it is necessary to estimate, in effect, the dose destruction constants that would have resulted if the gamma experiments and the electron beam experiments had been run under the same conditions. To this end several models which relate dose constant to initial (zero dose) concentration, pH, and the presence of suspended solids were fit to both the electron beam and gamma dose constant data to determine whether the dose constants were significantly affected by experimental conditions. Since target concentrations were not always achieved, the actual initial concentrations obtained during individual experiments were used in developing the models. Further, the models were used to determine whether there were practical and statistically significant differences in the dose constants and the effects of experimental conditions between electron beam and gamma irradiation. The following ln/ln model was found to provide the best fit to the dose constant data: lnðkÞ ¼ a0 ¼ a1 lnðCI Þ þ a2 pH þ a3 IC þ a4 RT þ a5 lnðCI ÞpH þ a6 lnðCI ÞIC þ a7 lnðCI ÞRT þ a8 pH IC þ a9 RT pH þ a10 RT IC þ a11 RT pH lnðCI Þ þ a12 RT IC lnðCI Þ þ e; ð3Þ where CI is the average zero dose electron beam influent concentration, or the concentration of the zero dose
371
control sample for gamma experiments, pH the negative log of [H+] for the zero dose influent samples, IC the indicator variable (0=no suspended solids, 1=3% suspended solids present), RT the radiation type (0-electron beam, 1=60Co). The inclusion of RT in Eq. (3) allows individual effects to differ by radiation type. For example, if the only non-zero parameter is a4 ; there is an overall fixed difference in lnðkÞ between radiation types. If any of the parameters associated with the other terms involving RT are different from zero, the effects of the corresponding variables differ by radiation type. The inverse of the expected value of Eq. (3) is a power function relating dose constant k and concentration. For example, restricting attention to electron beam irradiation (RT=0), we obtain k ¼ ðea0 þ a2 pH þ a3 IC þ a8 pH IC Þ ðCI a1 þ a5 pH þ a6 IC Þ ¼ aCIb :
ð4Þ
In Eq. (4) the multiplier and exponent of CI are assumed to be functions of the experimental conditions defined by pH and the presence of suspended solids. For example, the exponent of the multiplier base, e; is a0 þ a2 pH when no suspended solids are present (IC ¼ 0) and ða0 þ a3 Þ þ ða2 þ a8 ÞpH when they are present ðIC ¼ 1Þ: The cross product terms pH IC thus allows for the possibility that the effect of pH differs depending on the presence of suspended solids. Eq. (3) is a descriptive model and does not directly represent the physical and chemical processes involved. For example, pH is a logarithmic function of H+ concentration, [H+], which when used directly in the model causes scaling problems in statistical analysis routines. The effects of changing [H+] concentration are understood to be primarily due to the resulting changes in carbonate and bicarbonate alkalinity in our test water (alkalinity=50 mg l1, DO=3.5 mg l1, DOC=4.7 mg l1, NO3=0.4 mg l1) since the carbonate is a very effective OHd scavenger. However, it is possible that experimental differences (e.g., longer mixing times) resulting from performing pH adjustments on large 11,500–23,000 l (3000–6000 gal) batches could also have a small effect. In any case, Eq. (3) can be used to determine whether dose constants are affected by experimental conditions and to adjust for these differences as discussed below.
3. Results With the exception of phenol at high concentrations, the 60Co constants given in Table 1 are smaller (more negative) than those for electron beam irradiation indicating that removal is more efficient using gamma
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Table 2 Results of fitting model (Eq. (3)) comparing E-beam and
60
Co dose constants
Compound
Variable: Constant ln CI pH IC RT ln CI pH ln CI IC ln CI RT pH IC RT pH RT IC RT pH ln CI RT IC ln CI
Parameter: a0 a1 a2 a3 a4 a5 a6 a7 a8 a9 a10 a11 a12 R2 n pmax
Benzene
Toluene
Phenol
TCE
PCE
Chloroform
0.6620 0.5170 * * * * * 0.0816 * * * * * 82.2% 38 0.028
0.5527 0.3583 * 0.3236 0.9458 * * * * * * * * 79.8% 34 0.039
* 0.3616 0.5873 * 2.9195 0.0706 * 0.3960 * * * * * N/A 49 0.001
2.3111 0.1229 * * 5.5480 * * 0.6032 * 0.6629 1.3405 0.0809 -0.2459 87.5% 44 0.008
* 0.6745 0.4092 * * 0.0650 * 0.1020 * * * * * N/A 44 0.000
5.5612 0.0762 0.1570 * * * * 0.1829 * * * 0.0211 * 34.2% 45 0.054
*The null hypothesis ai ¼ 0 is not rejected at the a ¼ 0:05 significance level. R2 =% variation explained by model (not applicable (N/A) when constant is zero). n=number of data points. pmax =maximum significance probability (probability of getting estimated value when a is actually zero) for significant terms.
irradiation. The results of fitting Eq. (3) to these data are presented in Table 2 (Minitab, Inc., 1991). As can be seen, the dose constant k is significantly affected by initial concentration in all cases. Further, all compounds have at least one significant term involving RT, which indicates that radiation type significantly affects the dose constant relationships even after adjusting for differences in experimental conditions. The effect of initial concentration on dose constant depends on the type of radiation for all compounds except toluene as indicated by the significant values of the parameter a7. The individual dose constants are plotted in Figs. 1 and 2 along with curves of k versus concentration for both electron beam and gamma irradiations. Note that although these plotted curves are averaged over the other experimental variables (pH and % clay), the models specified by Eq. (3) and Table 2 adjust for all variables so the differences between the individual dose constants and model values are smaller than those apparent in the figures. The differences in removal efficiencies, as measured by dose constant k; between bench-scale gamma and largescale electron beam irradiation are definite and statistically significant. This indicates that bench-scale gamma results must be adjusted to estimate removal for largescale electron beam irradiations. The ratio ðkEB =kg Þ of electron beam to 60Co gamma dose constant models in Table 2 are presented for each compound in Table 3. These equations can be used to calculate the values by
which dose constants from bench-scale 60Co experiments should be multiplied in order to estimate the dose constants for large-scale electron beam irradiations. Example ratios for each compound are also included in Table 3 assuming an initial concentration of 1000 mg l1, water similar to our potable ground water at pH 7, and that no significant suspended solids are present. For these example ratios, the electron beam dose constants range from approximately 39% of the corresponding gamma dose constant in the case of Toluene to 83% for Phenol. These results are applicable in situations that generally match conditions used in our experiments and for dose rates similar to those generated by our 60Co source and electron accelerator at the EBRF. The above differences between electron beam and gamma dose constants can have a significant impact on the determination of the dose required to achieve a given contaminant removal objective in commercial electron beam process design. Given a target treatment concentration (xT ), the required dose based on a gamma dose constant (DReq-g) is given by DReqg ¼ ð1=kg Þln ðx0 =xD Þ:
ð5Þ
From Eq. (5) it can be seen that the required dose for a large-scale electron beam system (DReq-EB) is kg =kEB times DReq-g. Using the example ratios, the dose actually required using an electron beam system would be approximately 1.2 times that indicated by bench-scale
C.N. Kurucz et al. / Radiation Physics and Chemistry 65 (2002) 367–378
373
Fig. 1. Dose constant (kGy1) as a function of initial concentration of organic for an aqueous solution of (a) benzene and (b) toluene and (c) phenol.
gamma experiments for Phenol and 2.5 times the bench scale indicated dose for Toluene. Since required dose determines the flow capacity of a treatment system, the use of unadjusted bench-scale results can lead to a significant overestimate of the flow capacity of a particular electron beam system and a consequent underestimation of the cost of treatment.
4. Discussion The observed differences between 60Co and electron beam dose constants can be due to inherent differences in the way the irradiations are conducted, the logistics of conducting the experiments, or differences in the nature of gamma radiation and high-energy electron radiation (e.g., dose rate differences). Large-scale electron beam experiments are performed by injecting a subject
compound into a selected influent wastewater or dissolving it in tanker trucks filled with potable water. The water is then passed in front of the scanned beam of electrons as a sheet of water falling over a weir. Samples matched by transit time estimates are taken of the water before (influent) and after (effluent) beam irradiation. Samples are then taken directly from the EBRF to the laboratory for analysis. On the other hand, gamma experiments involve placing split samples in a 60Co irradiator at selected distances from the source and times of exposure. An unirradiated control sample is held out and the fraction remaining is calculated as the ratio of irradiated sample concentrations to the control sample concentration. The gamma samples first obtained at the EBRF are taken to the 60Co source for irradiation, then to the laboratory for analysis. The differences in handling and timing due to scheduling the 60 Co irradiation facility could lead to changes in
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374
Fig. 2. Dose constant (kGy1) as a function of initial concentration of organic for an aqueous solution of (a) TCE (b) PCE and (c) Chloroform.
Table 3 Ratio of estimated electron beam dose constants to gamma dose constants
60
Co
Compound
kEB =kg
Example ratioa
Benzene Toluene Phenol TCE PCE Chloroform
CI:0816 e.9458 e2:9195 CI0:3960
0.569 0.388 0.832 0.539 0.494 0.784
CI0:1020 CI0:1829þ:0211pH
a Assumes initial concentration CI ¼ 1000 mg l1, pH=7, and no suspended solids (IC ¼ 0).
concentrations of compounds for reasons other than radiation dose. For example, continuing reactions of sample constituents after irradiation and before analysis
can produce differences as sample vials are handled differently for the two types of irradiation. Loss of volatile compounds either before or after gamma irradiation is another source of possible change in concentration. An attempt was made to minimize these problems by quenching known reactions as appropriate, use of proper vials, and careful handling and storage of samples. The estimation of absorbed dose is also different for the two types of irradiations. Absorbed dose for the gamma irradiations was estimated using calibration data based on dosimetry provided by the U of Miami Radiation Control Center and liquid (Fricke) dosimetry in 47 ml vials similar to those used in our experiments. These methods are very different from that used at the EBRF where dose is estimated using the temperature rise of the water as it flows into and out of the
C.N. Kurucz et al. / Radiation Physics and Chemistry 65 (2002) 367–378
beam (Kurucz et al., 1995a). Since these dose estimates are used in determining dose destruction constants, differences in the accuracy of the methods could contribute to the observed differences in the gamma and electron beam results. It is believed, however, that the methods are accurate to within approximately 5% and this would not account for the large observed differences. A probable reason for the observed differences in gamma and electron beam results is the difference in dose rate between gamma and electron beam irradiations. Although the production of high-energy electrons in an irradiated material and the underlying radiation chemistry is thought to be similar for both (Spinks and Woods, 1990), the effect of differences in dose rates has been observed in a wide variety of situations. For example, variation in liquid and solid chemical dosimeter response with dose rate is commonly observed (McLaughlin et al., 1989). Differences in the removal efficiency of TCE in groundwater by bench-scale 60Co gamma irradiation and a prototype 500 keV, 50 mA electron beam system operating at approximately 16 l min1 have been reported (Gehringer et al, 1995). Dose constants were estimated from this reference and the ratio of kEB =kg was found to be approximately 0.46. This is comparable to the ratio of 0.54 presented in Table 3. However, when the tabled value is adjusted for the initial concentration (120 mg l1) using the equation for TCE in Table 3, the estimated ratio becomes 0.48, which is in very close agreement. Bench-scale studies of chlorobenzene and 1,2-Dichloroethane utilizing both 60Co and a 5 MeV LINAC with a 2.3 ms pulse have yielded similar differences (Kartasheva et al, 1996). The ratios of electron beam to gamma GðCl Þ values were approximately 0.68 and 0.51, respectively. The GðCl Þ values were also found to be dependent on initial concentration. Since G values are proportional to dose constants (Mincher and Curry, 2000) and GðCl Þ values are related to removal of the
375
parent compound, these results are analogous to ours. As in the TCE study mentioned above, dosimetry for both the gamma and electron beam irradiators was conducted with liquid chemical dosimetery, thus eliminating one of the possible causes for the differences observed in our data. We have also observed that gamma irradiation is somewhat better than large-scale electron beam irradiation for disinfecting various municipal wastewaters (Farooq et al., 1993). To explore the dose rate effect, the MAKSIMA kinetic modeling program (Carver et al., 1979) was used to examine the effect of different dose rates on removal of selected compounds. The calculated removals of TCE and benzene for a 0.25 kGy (25 krad) dose administered in 0.05–0.10 s (2.5–5.0 kGy s1), which approximates irradiations at the EBRF and approximately 1650 s (0.15 Gy s1) for our gamma source, are presented in Table 4. As can be seen, there are significant differences in the percent removal for different dose rates, with the lower dose rate of the gamma source resulting in better removal. These results are consistent with other modeling efforts intended to examine the removal efficiencies of high dose rate (15 to 123 107 kGy s1) pulsed accelerators as compared to continuous DC accelerators (Rososcha et al., 1992). In this work radical yields for e aq, OHd, and Hd were found to be significantly greater for lower dose rates, as were removals of TCE and carbon tetrachloride. The dependence of removal efficiency on initial concentration of these compounds was also observed. The advantage of lower dose rates appears to be due to non-linearities in the radiolytic breakdown of water which favor radical–radical recombination at high dose rates. The increased spacial density of tracks and spurs for high dose rates lead to higher probabilities of radical–radical reactions. In addition, lower dose rates also generate lower concentrations of radicals which results in a higher fraction of the radicals reacting with contaminants as opposed to other radicals. For example, Fig. 3 shows the calculated concentration of
Table 4 Estimated yields for selected compounds irradiated to 0.25 kGy (1.56 1021 eV l1) at various dose rates using kinetic modeling (MAKSIMA) Dose rate(eV l1 s1)
Time(s) 6
TCE (initial concentration: 7.69 10 0.05 0.10 1650.0
TCE yield (mol l1)
% Removal
1.286 107 8.939 108 5.309 109
98.3 98.8 99.9
8.910 108 6.952 108 4.930 1010
95.7 96.7 99.9
1
mol l ) 3.120 1022 1.560 1022 9.455 1017
Benzene (initial concentration: 2.08 106 mol l1) 0.05 3.120 1022 0.10 1.560 1022 1650.0 9.455 1017
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C.N. Kurucz et al. / Radiation Physics and Chemistry 65 (2002) 367–378
Fig. 3. Calculated (MAKSIMA) concentrations of TCE, OHd and eaq versus time for a 0.25 kGy dose using a 2.50 kGy s1 electron beam and a 0.15 Gy s1 gamma source.
TCE versus time for an electron beam dose rate of approximately 2.50 kGy s1 and a 60Co irradiation at 0.15 Gy s1. Calculations were performed using MAKSIMA and considered reactions which include the radiolysis of pure water plus additional reactions with TCE and naturally occurring compounds such as carbonate, bicarbonate, nitrate, and DOC. Fig. 3 also shows the concentration of the primary reactive species
over time for both the electron beam and gamma dose. The rate of e aq production for electron beam irradiation is such that the concentration of e aq is initially four orders of magnitude higher than for gamma irradiation. Similarly, the electron beam concentration of OHd is higher than that for gamma irradiation. Because of competition kinetics, these higher concentrations lead to increased reactions between e aq and OHd for a given
C.N. Kurucz et al. / Radiation Physics and Chemistry 65 (2002) 367–378
concentration of a contaminant. Although this example does not consider all the possible reactions, it illustrates the nature of the difference between the reaction kinetics of the two types of irradiations. Other researchers have obtained similar experimental results. Pulsed electron beam irradiation of 400 mg l1 solutions of benzene at dose rates between 14 105 and 108 kGy s1 resulted in better removal at the lower dose rates (Moran, 1994). Experiments involving both gamma and electron beam produced bremsstrahlung irradiation of various organic compounds with dose rates ranging from 4 to 5 105 kGy s1 also resulted in higher removal for lower dose rates for several compounds (Matthews et al., 1991). The results appeared to be the opposite, however, for carbon tetrachloride experiments. Another exception to the advantage of lower dose rates has been reported based on a series of electron beam experiments conducted at Sandia National Laboratories in which simulated radioactive waste was irradiated at dose rates of 2.5 103 and 2.7 109 kGy s1 (Neau, 1994). In these experiments better removal was obtained at the higher dose rate. The reasons for this are unclear at this time, however, it should be noted that 2.7 109 kGy s1 is a higher dose rate than the experiments mentioned previously and high enough to change the primary yields of reactive species available to react with contaminants (Spinks and Woods, 1990). Since the concentration of organics was also much higher, direct radiolysis of the contaminants plays a greater role in their removal. The chemistry at extremely high dose rates and/or contaminant concentrations thus may be more favorable to removal by high dose rates in some situations.
5. Conclusions The results of our experiments and others show a definite difference in removal efficiency between gamma and electron beam irradiation for most compounds investigated. In general, removal efficiencies are greater for gamma irradiation. Electron beam dose constants were found to be 39–83% of the corresponding gamma dose constants for the six compounds considered in this study under a set of uniform conditions. This means that extrapolating results from bench-scale 60Co studies to large-scale electron beam treatment of water must be done with care. For example, required doses for electron beam treatment range from 1.2 to 2.5 times those indicated by bench-scale gamma studies. Equations for adjusting 60Co results to predict large-scale electron beam results have been presented in Table 3. These equations will hold for electron beam treatment facilities similar to the Miami EBRF and over the range of experimental conditions used in our studies (e.g., Gehringer et al., 1995).
377
The most likely reason for the difference in removal efficiencies is the difference in the dose rates between the two types of irradiations. The lower dose rate of the 60 Co irradiations produces lower concentrations of the important reactive species OHd and e aq which results in a higher fraction of radical/contaminant reaction and thus better removal. Also, higher dose rates and the consequent greater density of tracks and spurs may contribute to greater overlap and increased chance of radical–radical recombination. There is some evidence that extremely high dose rates may be better at high contaminant concentrations. Further research is necessary to determine whether the increased role of direct radiolysis in high-concentration solutions favors removal using high dose rates.
References Carver, M.B., Hanley, D.V., Chaplin, K.R., 1979. MAKSIMACHEMIST A Program from Mass Action Kinetics Simulation Manipulation and Integration Using Stiff Technologies, Chalk River Nuclear Laboratories Report AECL-6413. Cooper, W.J., Nickelsen, M.G., Waite, T.D., Kurucz, C.N., 1992. High energy electron beam irradiation: an innovative process for the treatment of aqueous based organic hazardous waste. J. Environ. Sci. Health. A 27, 219. Cooper, W.J., Cadivid, E., Nickelsen, M.G., Lin, K., Kurucz, C.N., Waite, T.D., 1993a. Removal of trihalomethanes from drinking water using high energy electron beam irradiation. J. Am. Water Works Assoc. 85 (9), 106–112. Cooper, W.J., Meacham, D.E., Nickelsen, M.G., Lin, K., Ford, D.B., Kurucz, C.N., Waite, T.D., 1993b. An innovative treatment process for the removal of trichloroethylene (TCE) and tetrachloroethylene (PCE) from aqueous solution using high energy electrons. J. Air Waste Manag. Assoc. 48, 1358–1366. Farooq, S., Kurucz, C.N., Waite, T.D., Cooper, W.J., 1993. Disinfection of wastewaters: high energy electron vs. gamma irradiation. Water Res. 27, 1177–1184. Gehringer, P., Eschweiler, H., Fiedler, H., 1995. Ozone-electron beam treatment for groundwater remediation. Radiat. Phys. Chem. 46 (4-6), 1075–1078. Helsel, D.R., 1990. Less than obvious: statistical treatment of data below the detection limit. J. Environ. Sci. Technol. 24 (12), 1766–1774. Jandel Scientific, 1991. TableCurve Curve Fitting Software, Version 3.0, Corte Madera, CA. Kartasheva, L.I., Zhestkova, T.P., Chulkov, V.N., Didenko, O.A., Pikaev, A.K., 1996. The effect of dose rate on the radiolysis of dilute aqueous solutions of chlorinecontaining organic compounds. High Energy Chem. 30 (3), 207–208. Kurucz, C.N., Waite, T.D., Cooper, W.J., Nickelsen, M.G., 1991. High energy electron beam irradiation of water, wastewater and sludge. In: Advances in Nuclear Science and Technology, Vol. 22,1. Plenum Press, New York.
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Kurucz, C.N., Waite, T.D., Cooper, W.J., 1995a. The Miami electron beam research facility: a large scale wastewater treatment application. Radiat. Phys. Chem. 45, 299–308. Kurucz, C.N., Waite, T.D., Cooper, W.J., Nickelsen, M.G., 1995b. Empirical models for estimating the destruction of toxic organic compounds utilizing electron beam irradiation at full scale. Rad. Phys. Chem. 45 (5), 805–816. Lotus Development Corporation, 1986. Lotus Software, Version 2.01, Cambridge, MA. Matthews, S.M., Boegel, A.J., Eccles, S.F., Homan, S.G., Rice, D.W., Loftis, J.A., Jovanovich, M.C., Caufield, R.A., 1991. High energy irradiation of chlorinated hydrocarbons. American Nuclear Society International Topical Conference, Kona, Hawaii, April 21–27. McLaughlin, W.L., Boyd, A.W., Chadwick, K.H., McDonald, J.C., Miller, A., 1989. Dosimetry for Radiation Processing. Taylor & Francis, London, New York, Philadelphia, pp. 185–188. Mincher, B.J., Curry, R.D., 2000. Considerations for choice of a kinetic figure of merit in process radiation chemistry for waste treatment. Appl. Radiat. Isot. 52, 189–193. Minitab, Inc., 1991. Minitab Reference Manual, PC Version, Release 8, State College, PA. Moran, J.P., 1994. Cost-effective red water disposal by electron beam radiolysis. Draft Final Technical Report, SRL-10-F1994, Science Research Laboratory, Inc. Neau, E.L., 1994. Environmental and industrial applications of pulsed power systems. IEEE Trans. Plasma Sci. 22 (1), 1–10.
Nickelsen, M.G., Cooper, W.J., Kurucz, C.N., Waite, T.D., 1992. Removal of benzene and selected alkyl-substituted benzenes from aqueous solution utilizing continuous highenergy electron irradiation. J. Environ. Sci. Technol. 26, 144. Nickelsen, M.G., Cooper, W.J., Lin, K., Kurucz, C.N., Waite, T.D., 1994. High energy electron beam generation of oxidants for the treatment of benzene and toluene in the presence of radical scavengers. Water Res. 28, 1227. Nickelson, M.G., Cooper, W.J., O’Shea, K.E., Aguilar, M., Kalen, D.V., Kurucz, C.N., Waite, T.D., 1998. The elimination of methane phosphonic acid, dimethyl ester (DMMP) from aqueous solution using 60Co-g and electron beam induced radiolysis: a model compound for evaluating the effectiveness of the e-beam process in the destruction of organophosphorus chemical warfare agents. J. Adv. Oxid. Technol. 3 (1), 43–54. Rososcha, L.A., Secker, D.A., Smith, J.D., 1992. Kinetic modeling of TCE and carbontetrachloride removal from water by electron-beam irradiation. Proceedings of I & EC Special Symposium on Emerging Technologies in Hazardous Waste Management, ACS, Atlanta, GA. Spinks, J.W.T., Woods, R.J., 1990. An Introduction to Radiation Chemistry. Wiley, New York. Wang, T.Z., 1993. Theoretical and experimental aspects of dewatering effects on wastewater sludge induced by high energy electrons and gamma radiation. Ph.D. Dissertation, University of Miami.
A comparison of large-scale electron beam and bench-scale 60 Co irradiations of simulated aqueous waste streams * c, Charles N. Kurucza,*, Thomas D. Waiteb, Suzana E. Otano William J. Cooperd, Michael G. Nickelsend a
Department of Management Science, University of Miami, P. O. Box 248237, Coral Gables, FL 33124, USA b Department of Civil and Architectural Engineering, University of Miami, Coral Gables, FL 33124, USA c Department of Biomedical Engineering, University of Miami, Coral Gables, FL 33124, USA d Department of Chemistry, University of North Carolina at Wilmington, Wilmington, NC 28403, USA
Abstract The effectiveness of using high energy electron beam irradiation for the removal of toxic organic chemicals from water and wastewater has been demonstrated by commercial-scale experiments conducted at the Electron Beam Research Facility (EBRF) located in Miami, Florida and elsewhere. The EBRF treats various waste and water streams up to 450 l min1 (120 gal min1) with doses up to 8 kilogray (kGy). Many experiments have been conducted by injecting toxic organic compounds into various plant feed streams and measuring the concentrations of compound(s) before and after exposure to the electron beam at various doses. Extensive experimentation has also been performed by dissolving selected chemicals in 22,700 l (6000 gal) tank trucks of potable water to simulate contaminated groundwater, and pumping the resulting solutions through the electron beam. These large-scale experiments, although necessary to demonstrate the commercial viability of the process, require a great deal of time and effort. This paper compares the results of large-scale electron beam irradiations to those obtained from bench-scale irradiations using gamma rays generated by a 60Co source. Dose constants from exponential contaminant removal models are found to depend on the source of radiation and initial contaminant concentration. Possible reasons for observed differences such as a dose rate effect are discussed. Models for estimating electron beam dose constants from bench-scale gamma experiments are presented. Data used to compare the removal of organic compounds using gamma irradiation and electron beam irradiation are taken from the literature and a series of experiments designed to examine the effects of pH, the presence of turbidity, and initial concentration on the removal of various organic compounds (benzene, toluene, phenol, PCE, TCE and chloroform) from simulated groundwater. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Electron beam irradiation; Gamma irradi; Liquid waste treatment; Aqueous waste streams; Large-scale irradiation; Benchscale irradiation; Dose rate; Toxic organic chemical removal
1. Introduction The effectiveness of using high energy electron beam irradiation for the removal of toxic organic chemicals from water and wastewater has been demonstrated by large-scale experiments conducted at the Electron Beam Research Facility (EBRF) located at the Miami Dade *Corresponding author. Fax: +1-305-284-2321. E-mail address: [email protected] (C.N. Kurucz).
Water and Sewer Authority Central District Plant in Miami, Florida and elsewhere. The EBRF, described in detail elsewhere (Kurucz et al., 1991, 1995a, b), utilizes a 1.5 MV, 75 kW electron accelerator to treat various waste and water streams at 450 l min1 (120 gal min1) with doses up to 8 kGy. Many experiments have been conducted by injecting toxic organic compounds into various plant feed streams, such as chlorinated secondary municipal wastewater and raw wastewater, and measuring the concentrations of compound(s) before
0969-806X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 2 ) 0 0 3 3 7 - 7
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and after exposure to the electron beam at various doses (Nickelsen et al., 1992; Cooper et al., 1992). Extensive experimentation has also been performed by dissolving selected chemicals in 22,700 l (6000 gal) tank trucks of potable water to simulate contaminated groundwater, and pumping the resulting solutions through the electron beam (Cooper et al., 1993a, b; Nickelson et al., 1998). Large-scale experiments are required to demonstrate feasibility and to establish engineering parameters for full-scale commercial applications of the process. However, they require a great deal of time and effort. A typical experiment requires five to eight people to run and preparation time is often quite extensive. For example, tanker experiments require large quantities of chemicals to be added to the tanker after it is filled. In experiments designed to determine the effects of suspended solids on the removal efficiency of target compounds, as much as 340 Kg (750 lb) of kaolin clay are added. Because of these difficulties and the relative lack of large-scale electron beam testing facilities, much of the experimentation on the use of ionizing radiation utilizes bench-scale gamma irradiators. Bench-scale experiments are much easier to perform and can save a great deal of effort when attempting to determine the feasibility of using high-energy electrons to treat specific chemical wastes. This paper compares the results of electron beam and gamma experiments and discusses the feasibility of predicting the results of large-scale electron beam irradiations utilizing data from bench-scale gamma (60Co) tests. Possible reasons for observed differences, including the effects of differences in dose rates that result from performing electron beam irradiations versus 60 Co irradiations, are also presented.
2. Methods and materials 2.1. Irradiation facilities As mentioned previously, electron beam experiments were performed using a 1.5 MeV, 50 mA insulated core transformer (ICT) accelerator at the Miami EBRF. Aqueous streams of contaminated water are exposed to a scanned beam in a waterfall approximately 127 cm wide and 0.44 cm thick. Doses of 0–8 kGy at flow rates up to 450 l min1 (120 gal min1) are obtained by varying beam current between 0 and 50 mA. The vertical spread of the beam is approximately 15 cm, including scatter, after passing through primary and secondary titanium window foils. At this point the average speed of the falling water in front of the beam is approximately 1.5 m s1. This yields a residence time of approximately 0.1 s and a dose rate of up to 80 kGy s1.
Gamma irradiation studies were conducted using a 5000 Curie 60Co g-source (Gamma Cell 150-A) located at the University of Miami Radiation Control Center. The source consists of 20 radioactive 60Co slugs, sealed in a cylindrical capsule 3.18 cm in diameter (Wang, 1993). The source is located at the center of a sample table marked with concentric circles of 10–100 cm. Due to the relatively short length of the capsule, the irradiator behaves as a point source. Most samples were irradiated at distances of 10–30 cm from the source, which resulted in dose rates ranging from 0.10 to 0.65 Gy s1. These dose rates are up to five orders of magnitude lower than those obtained using the electron beam. 2.2. Experimental design The data used to compare the removal of organic compounds using gamma irradiation and electron beam irradiation in this paper are taken from a series of largescale experiments designed to examine the effects of pH, the presence of clay, and initial concentration on the relationship between dose and removal of various organic compounds (benzene, toluene, phenol, PCE, TCE and chloroform) from simulated groundwater. These experiments were generally conducted at three target concentrations (100, 1000, and 10,000 mg l1), three pH values (5,7, and 9), and two clay concentrations (0% and 3% by weight) using a factorial design with partial replication. Influent and effluent samples for large-scale electron beam experiments at the EBRF were collected prior to and after irradiation in 47 ml PFTE lined screw cap glass vials. The vials were completely filled without headspace, immediately chilled on ice and returned to the laboratory for analysis. Samples for gamma experiments were taken from the influent stream at the same time as the electron beam samples. Gamma influent samples were collected in 500 ml screw cap bottles, put on ice, transported to the University of Miami, and refrigerated until processed for irradiation. The samples were subsequently placed in 47 ml PFTE lined screw cap vials, and labeled as to experimental condition and desired dose. In general, four vials were prepared from each sample (experimental condition) taken at the EBRF. One vial was used as the zero dose control and the others irradiated at three specified doses. The doses actually used to irradiate the electron beam samples were used as the target gamma doses. The necessary irradiation time and distances from the 60 Co source required to obtain the desired target doses were calculated from available calibration data and the vials were then transported to the gamma source for irradiation. The zero dose (control) samples were transported also but were not placed in the gamma irradiator. After irradiation, samples were transported
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to the Florida International University Drinking Water Research Center for analysis. Details of the sampling procedures used for the electron beam experiments are described elsewhere (Nickelsen et al., 1992; Kurucz et al., 1995a). Concentrations of organics were measured by gas chromatography using liquid–liquid extraction and high-performance liquid chromatography as described in detail previously (Nickelsen et al., 1992, 1994; Cooper et al., 1993a, b). 2.3. Data reduction The ideal experiment to compare the results of large-scale electron beam irradiations to gamma (60Co) irradiations would prepare split samples and irradiate them with the same dose, handling procedures, etc. using each method. In this case a direct comparison of the removal percentages or fraction of compound remaining would be possible. However, inherent differences in the way the irradiations are conducted do not allow such direct comparisons. For example, actual doses were not always the same at the EBRF and 60 Co irradiator because of variation in flow rates at the EBRF and/or physical limitations on the placement of vials at the gamma source. Actual doses obtained were calculated adjusting for these variations as the first step in the data reduction process. Another consideration involves the use of the open weir system at the EBRF, which can result in some losses at zero dose due to volatilization and pipeline residuals. These losses do not occur in the closed vials used in the 60Co irradiations. In order to overcome these problems it was decided to compare the parameters of appropriate removal models for the gamma and electron beam irradiations. The general approach involved fitting models relating the compound removal and irradiation dose, to the data for each experimental condition. For gamma experiments the dependent variable was the ratio between the resulting concentration at a particular dose and the concentration at zero dose for a given experimental condition. Since the electron beam removals included losses due to the use of the weir system, the dose constants were estimated using only effluent concentration as the dependent variable. Because volatilization and mixing, if any, take place prior to the effluent sampling port, changes in effluent concentration at nonzero doses are due primarily to radiation effects. In both cases the independent variable was the estimated total absorbed dose. Various models relating removal and dose were evaluated using TableCurve (Jandel Scientific, 1991) and LOTUS (Lotus Development Corporation, 1986) software. Although many functions were fit to the data automatically by the software, the following models
369
almost always were among the best fits in terms of minimum squared error: the ‘‘first-order’’ model xD ¼ x0 ekD ee
ð1Þ
that assumes change in concentration with respect to change in dose is proportional to initial concentration, and the ‘‘second-order’’ model 1=xD ¼ 1=x0 þ kD þ e
ð2Þ
that assumes change in concentration is proportional to the square of initial concentration. In these models, D is the absorbed dose, xD the compound remaining (effluent concentration for electron beam data or concentration at dose D over concentration at zero dose for gamma data) at dose D; x0 the compound remaining at zero dose, k the dose destruction constant, e the error term (normally distributed with mean=0 and variance=s2 ). Note that the error term ee in Eq. (1) is multiplicative thus making the variation in xD proportional to the magnitude of xD : This is consistent with the manner in which the removal data behave and implies that xD follows the Lognormal probability distribution. As in earlier work using electron beam data (Nickelsen et al., 1992; Kurucz et al., 1995b), the first-order model (Eq. (1)) generally provided a better fit to the gamma data than the second-order model (Eq. (2)). The second-order model, however, occasionally provided a somewhat better fit at low concentrations. Although the differences were not great enough to offset the added complication of working with more than one model, both the gamma and electron beam data suggest that removal might be second order with respect to dose for contaminant concentrations under 100 mg l1. The dose constants, k; obtained by fitting Eq. (1) to the gamma data and the corresponding electron beam data are given in Table 1. Since target concentrations were not exactly the same for each compound due to solubility limits, initial concentrations are designated low, medium, and high in Table 1. Occasionally fewer than four data points were available at each experimental condition because concentrations were obtained that were below the minimum detection limits of our analytical procedures. In these cases, the concentrations were assumed to be equal to the non-detection limits of our analytical procedures for purposes of curve fitting, even though the true concentrations were not known. The analytical procedures utilized in this study, however, are extremely sensitive and non-detect values are considered to be very low. There are several more sophisticated methods of handling the non-detect problem (Helsel, 1990); however, these apply to situations in which more data are available under a given set of experimental conditions. Our approach is conservative in that it tends to somewhat understate removal at higher doses. If more than one below detection limit
0.204 0.071 0.181 0.042
0.050 0.062 0.030 0.018
0.006 0.017 0.009 0.008
TCE No s.s. 0.051 3% s.s. 0.067
PCE No s.s. 0.017 3% s.s. 0.023
Chloroform No s.s. 0.005 3% s.s. 0.005
Co
E-beam
pH 9
0.129
0.008 0.009 0.4
0.053 0.013 0.033
+ 0.050 0.080
0.241 0.159 +
+ +
0.231 0.086 59.8
60
Co
E-beam
pH 5
0.018 0.007 0.006
0.027 0.013 0.012
0.075 0.053 0.046
0.259 0.024 0.031
0.198 0.106 0.212
0.222 0.054 0.046
60
Co
E-beam
pH 7
0.073 0.107
0.008 0.007 0.013 0.008
0.025 0.015 59.3 0.016
0.107 0.053 0.216 0.043
0.016 0.028 0.013 0.027
+ +
0.132 0.107 0.195 0.046
60
E-beam
pH 9
0.015 0.010 0.006
0.027 0.016 0.023
+ 0.044 0.106
0.009 0.016 0.032
0.220 0.057 0.209
0.103 0.051 0.058
Co
60
Co
E-beam
pH 5
0.006 0.004 0.004
0.023 0.008 0.011
0.053 0.034 0.032
0.013 0.006 0.007
0.218 0.026 0.031
E-beam
pH 7
0.008 0.005 59.5 0.006
0.022 0.013 0.017 0.010
0.058 0.034 0.125 0.030
0.002 0.006 0.002 0.006
0.066 0.025 0.127 0.027
0.047 0.022 0.038 0.020
Co
60
High concentration
0.116 0.023 0.031
60
Co gamma experiments
60
Medium concentration
See text. =not planned experiment (see text), +=insufficient data for collection.
0.255 0.131 + 0.127
Phenol No s.s. 0.155 3% s.s. 0.126
a
0.167 0.209
+ +
E-beam
Toluene No s.s. 0.135 3% s.s. +
Co
pH 7
0.315 0.223 59.7 0.105
60
Benzene No s.s. + 3% s.s. 0.180
E-beam
pH 5
Low concentration
a
Table 1 Electron beam dose constants (krad1) and estimated dose constants for corresponding
E-beam
pH 9
0.009 0.007 0.009
0.026 0.012 0.019
0.067 0.029 59.7
0.001 0.003 0.002
0.061 0.028 0.068
0.042 0.021 0.033
Co
60
0.009
0.023
0.071
0.001
0.056
0.033
Co
60
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C.N. Kurucz et al. / Radiation Physics and Chemistry 65 (2002) 367–378
value was generated for a given set of data only the first value was used since the additional values did not provide information as to the shape of the removal curve. Note that even though there appear to be relatively few data points for curve fitting, both Eqs. (1) and (2) are single parameter models that leave at least two degrees of freedom for error estimation. Further, in our experience the consistency with which these equations model contaminant removal makes additional data points unnecessary even though they would be desirable. Since reliable estimation of the model parameters does require at least three data points, however, it was not possible to estimate the dose constant k for some experimental conditions. For example, the concentration of toluene was below detection limits at all doses utilized. As will be discussed later, the apparent improved contaminant removal for 60 Co radiation contributed to the generation of below detection values at the target doses. These cases are indicated by aþsymbol in Table 1. Note that no data were obtained for the pH 9 and 3% clay condition because the addition of clay reduced the pH of the groundwater to approximately 7 and therefore, this experimental condition was not run at the EBRF. 2.4. Comparison of dose constants In order to compare the gamma and electron beam dose constants it is necessary to estimate, in effect, the dose destruction constants that would have resulted if the gamma experiments and the electron beam experiments had been run under the same conditions. To this end several models which relate dose constant to initial (zero dose) concentration, pH, and the presence of suspended solids were fit to both the electron beam and gamma dose constant data to determine whether the dose constants were significantly affected by experimental conditions. Since target concentrations were not always achieved, the actual initial concentrations obtained during individual experiments were used in developing the models. Further, the models were used to determine whether there were practical and statistically significant differences in the dose constants and the effects of experimental conditions between electron beam and gamma irradiation. The following ln/ln model was found to provide the best fit to the dose constant data: lnðkÞ ¼ a0 ¼ a1 lnðCI Þ þ a2 pH þ a3 IC þ a4 RT þ a5 lnðCI ÞpH þ a6 lnðCI ÞIC þ a7 lnðCI ÞRT þ a8 pH IC þ a9 RT pH þ a10 RT IC þ a11 RT pH lnðCI Þ þ a12 RT IC lnðCI Þ þ e; ð3Þ where CI is the average zero dose electron beam influent concentration, or the concentration of the zero dose
371
control sample for gamma experiments, pH the negative log of [H+] for the zero dose influent samples, IC the indicator variable (0=no suspended solids, 1=3% suspended solids present), RT the radiation type (0-electron beam, 1=60Co). The inclusion of RT in Eq. (3) allows individual effects to differ by radiation type. For example, if the only non-zero parameter is a4 ; there is an overall fixed difference in lnðkÞ between radiation types. If any of the parameters associated with the other terms involving RT are different from zero, the effects of the corresponding variables differ by radiation type. The inverse of the expected value of Eq. (3) is a power function relating dose constant k and concentration. For example, restricting attention to electron beam irradiation (RT=0), we obtain k ¼ ðea0 þ a2 pH þ a3 IC þ a8 pH IC Þ ðCI a1 þ a5 pH þ a6 IC Þ ¼ aCIb :
ð4Þ
In Eq. (4) the multiplier and exponent of CI are assumed to be functions of the experimental conditions defined by pH and the presence of suspended solids. For example, the exponent of the multiplier base, e; is a0 þ a2 pH when no suspended solids are present (IC ¼ 0) and ða0 þ a3 Þ þ ða2 þ a8 ÞpH when they are present ðIC ¼ 1Þ: The cross product terms pH IC thus allows for the possibility that the effect of pH differs depending on the presence of suspended solids. Eq. (3) is a descriptive model and does not directly represent the physical and chemical processes involved. For example, pH is a logarithmic function of H+ concentration, [H+], which when used directly in the model causes scaling problems in statistical analysis routines. The effects of changing [H+] concentration are understood to be primarily due to the resulting changes in carbonate and bicarbonate alkalinity in our test water (alkalinity=50 mg l1, DO=3.5 mg l1, DOC=4.7 mg l1, NO3=0.4 mg l1) since the carbonate is a very effective OHd scavenger. However, it is possible that experimental differences (e.g., longer mixing times) resulting from performing pH adjustments on large 11,500–23,000 l (3000–6000 gal) batches could also have a small effect. In any case, Eq. (3) can be used to determine whether dose constants are affected by experimental conditions and to adjust for these differences as discussed below.
3. Results With the exception of phenol at high concentrations, the 60Co constants given in Table 1 are smaller (more negative) than those for electron beam irradiation indicating that removal is more efficient using gamma
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Table 2 Results of fitting model (Eq. (3)) comparing E-beam and
60
Co dose constants
Compound
Variable: Constant ln CI pH IC RT ln CI pH ln CI IC ln CI RT pH IC RT pH RT IC RT pH ln CI RT IC ln CI
Parameter: a0 a1 a2 a3 a4 a5 a6 a7 a8 a9 a10 a11 a12 R2 n pmax
Benzene
Toluene
Phenol
TCE
PCE
Chloroform
0.6620 0.5170 * * * * * 0.0816 * * * * * 82.2% 38 0.028
0.5527 0.3583 * 0.3236 0.9458 * * * * * * * * 79.8% 34 0.039
* 0.3616 0.5873 * 2.9195 0.0706 * 0.3960 * * * * * N/A 49 0.001
2.3111 0.1229 * * 5.5480 * * 0.6032 * 0.6629 1.3405 0.0809 -0.2459 87.5% 44 0.008
* 0.6745 0.4092 * * 0.0650 * 0.1020 * * * * * N/A 44 0.000
5.5612 0.0762 0.1570 * * * * 0.1829 * * * 0.0211 * 34.2% 45 0.054
*The null hypothesis ai ¼ 0 is not rejected at the a ¼ 0:05 significance level. R2 =% variation explained by model (not applicable (N/A) when constant is zero). n=number of data points. pmax =maximum significance probability (probability of getting estimated value when a is actually zero) for significant terms.
irradiation. The results of fitting Eq. (3) to these data are presented in Table 2 (Minitab, Inc., 1991). As can be seen, the dose constant k is significantly affected by initial concentration in all cases. Further, all compounds have at least one significant term involving RT, which indicates that radiation type significantly affects the dose constant relationships even after adjusting for differences in experimental conditions. The effect of initial concentration on dose constant depends on the type of radiation for all compounds except toluene as indicated by the significant values of the parameter a7. The individual dose constants are plotted in Figs. 1 and 2 along with curves of k versus concentration for both electron beam and gamma irradiations. Note that although these plotted curves are averaged over the other experimental variables (pH and % clay), the models specified by Eq. (3) and Table 2 adjust for all variables so the differences between the individual dose constants and model values are smaller than those apparent in the figures. The differences in removal efficiencies, as measured by dose constant k; between bench-scale gamma and largescale electron beam irradiation are definite and statistically significant. This indicates that bench-scale gamma results must be adjusted to estimate removal for largescale electron beam irradiations. The ratio ðkEB =kg Þ of electron beam to 60Co gamma dose constant models in Table 2 are presented for each compound in Table 3. These equations can be used to calculate the values by
which dose constants from bench-scale 60Co experiments should be multiplied in order to estimate the dose constants for large-scale electron beam irradiations. Example ratios for each compound are also included in Table 3 assuming an initial concentration of 1000 mg l1, water similar to our potable ground water at pH 7, and that no significant suspended solids are present. For these example ratios, the electron beam dose constants range from approximately 39% of the corresponding gamma dose constant in the case of Toluene to 83% for Phenol. These results are applicable in situations that generally match conditions used in our experiments and for dose rates similar to those generated by our 60Co source and electron accelerator at the EBRF. The above differences between electron beam and gamma dose constants can have a significant impact on the determination of the dose required to achieve a given contaminant removal objective in commercial electron beam process design. Given a target treatment concentration (xT ), the required dose based on a gamma dose constant (DReq-g) is given by DReqg ¼ ð1=kg Þln ðx0 =xD Þ:
ð5Þ
From Eq. (5) it can be seen that the required dose for a large-scale electron beam system (DReq-EB) is kg =kEB times DReq-g. Using the example ratios, the dose actually required using an electron beam system would be approximately 1.2 times that indicated by bench-scale
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Fig. 1. Dose constant (kGy1) as a function of initial concentration of organic for an aqueous solution of (a) benzene and (b) toluene and (c) phenol.
gamma experiments for Phenol and 2.5 times the bench scale indicated dose for Toluene. Since required dose determines the flow capacity of a treatment system, the use of unadjusted bench-scale results can lead to a significant overestimate of the flow capacity of a particular electron beam system and a consequent underestimation of the cost of treatment.
4. Discussion The observed differences between 60Co and electron beam dose constants can be due to inherent differences in the way the irradiations are conducted, the logistics of conducting the experiments, or differences in the nature of gamma radiation and high-energy electron radiation (e.g., dose rate differences). Large-scale electron beam experiments are performed by injecting a subject
compound into a selected influent wastewater or dissolving it in tanker trucks filled with potable water. The water is then passed in front of the scanned beam of electrons as a sheet of water falling over a weir. Samples matched by transit time estimates are taken of the water before (influent) and after (effluent) beam irradiation. Samples are then taken directly from the EBRF to the laboratory for analysis. On the other hand, gamma experiments involve placing split samples in a 60Co irradiator at selected distances from the source and times of exposure. An unirradiated control sample is held out and the fraction remaining is calculated as the ratio of irradiated sample concentrations to the control sample concentration. The gamma samples first obtained at the EBRF are taken to the 60Co source for irradiation, then to the laboratory for analysis. The differences in handling and timing due to scheduling the 60 Co irradiation facility could lead to changes in
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374
Fig. 2. Dose constant (kGy1) as a function of initial concentration of organic for an aqueous solution of (a) TCE (b) PCE and (c) Chloroform.
Table 3 Ratio of estimated electron beam dose constants to gamma dose constants
60
Co
Compound
kEB =kg
Example ratioa
Benzene Toluene Phenol TCE PCE Chloroform
CI:0816 e.9458 e2:9195 CI0:3960
0.569 0.388 0.832 0.539 0.494 0.784
CI0:1020 CI0:1829þ:0211pH
a Assumes initial concentration CI ¼ 1000 mg l1, pH=7, and no suspended solids (IC ¼ 0).
concentrations of compounds for reasons other than radiation dose. For example, continuing reactions of sample constituents after irradiation and before analysis
can produce differences as sample vials are handled differently for the two types of irradiation. Loss of volatile compounds either before or after gamma irradiation is another source of possible change in concentration. An attempt was made to minimize these problems by quenching known reactions as appropriate, use of proper vials, and careful handling and storage of samples. The estimation of absorbed dose is also different for the two types of irradiations. Absorbed dose for the gamma irradiations was estimated using calibration data based on dosimetry provided by the U of Miami Radiation Control Center and liquid (Fricke) dosimetry in 47 ml vials similar to those used in our experiments. These methods are very different from that used at the EBRF where dose is estimated using the temperature rise of the water as it flows into and out of the
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beam (Kurucz et al., 1995a). Since these dose estimates are used in determining dose destruction constants, differences in the accuracy of the methods could contribute to the observed differences in the gamma and electron beam results. It is believed, however, that the methods are accurate to within approximately 5% and this would not account for the large observed differences. A probable reason for the observed differences in gamma and electron beam results is the difference in dose rate between gamma and electron beam irradiations. Although the production of high-energy electrons in an irradiated material and the underlying radiation chemistry is thought to be similar for both (Spinks and Woods, 1990), the effect of differences in dose rates has been observed in a wide variety of situations. For example, variation in liquid and solid chemical dosimeter response with dose rate is commonly observed (McLaughlin et al., 1989). Differences in the removal efficiency of TCE in groundwater by bench-scale 60Co gamma irradiation and a prototype 500 keV, 50 mA electron beam system operating at approximately 16 l min1 have been reported (Gehringer et al, 1995). Dose constants were estimated from this reference and the ratio of kEB =kg was found to be approximately 0.46. This is comparable to the ratio of 0.54 presented in Table 3. However, when the tabled value is adjusted for the initial concentration (120 mg l1) using the equation for TCE in Table 3, the estimated ratio becomes 0.48, which is in very close agreement. Bench-scale studies of chlorobenzene and 1,2-Dichloroethane utilizing both 60Co and a 5 MeV LINAC with a 2.3 ms pulse have yielded similar differences (Kartasheva et al, 1996). The ratios of electron beam to gamma GðCl Þ values were approximately 0.68 and 0.51, respectively. The GðCl Þ values were also found to be dependent on initial concentration. Since G values are proportional to dose constants (Mincher and Curry, 2000) and GðCl Þ values are related to removal of the
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parent compound, these results are analogous to ours. As in the TCE study mentioned above, dosimetry for both the gamma and electron beam irradiators was conducted with liquid chemical dosimetery, thus eliminating one of the possible causes for the differences observed in our data. We have also observed that gamma irradiation is somewhat better than large-scale electron beam irradiation for disinfecting various municipal wastewaters (Farooq et al., 1993). To explore the dose rate effect, the MAKSIMA kinetic modeling program (Carver et al., 1979) was used to examine the effect of different dose rates on removal of selected compounds. The calculated removals of TCE and benzene for a 0.25 kGy (25 krad) dose administered in 0.05–0.10 s (2.5–5.0 kGy s1), which approximates irradiations at the EBRF and approximately 1650 s (0.15 Gy s1) for our gamma source, are presented in Table 4. As can be seen, there are significant differences in the percent removal for different dose rates, with the lower dose rate of the gamma source resulting in better removal. These results are consistent with other modeling efforts intended to examine the removal efficiencies of high dose rate (15 to 123 107 kGy s1) pulsed accelerators as compared to continuous DC accelerators (Rososcha et al., 1992). In this work radical yields for e aq, OHd, and Hd were found to be significantly greater for lower dose rates, as were removals of TCE and carbon tetrachloride. The dependence of removal efficiency on initial concentration of these compounds was also observed. The advantage of lower dose rates appears to be due to non-linearities in the radiolytic breakdown of water which favor radical–radical recombination at high dose rates. The increased spacial density of tracks and spurs for high dose rates lead to higher probabilities of radical–radical reactions. In addition, lower dose rates also generate lower concentrations of radicals which results in a higher fraction of the radicals reacting with contaminants as opposed to other radicals. For example, Fig. 3 shows the calculated concentration of
Table 4 Estimated yields for selected compounds irradiated to 0.25 kGy (1.56 1021 eV l1) at various dose rates using kinetic modeling (MAKSIMA) Dose rate(eV l1 s1)
Time(s) 6
TCE (initial concentration: 7.69 10 0.05 0.10 1650.0
TCE yield (mol l1)
% Removal
1.286 107 8.939 108 5.309 109
98.3 98.8 99.9
8.910 108 6.952 108 4.930 1010
95.7 96.7 99.9
1
mol l ) 3.120 1022 1.560 1022 9.455 1017
Benzene (initial concentration: 2.08 106 mol l1) 0.05 3.120 1022 0.10 1.560 1022 1650.0 9.455 1017
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Fig. 3. Calculated (MAKSIMA) concentrations of TCE, OHd and eaq versus time for a 0.25 kGy dose using a 2.50 kGy s1 electron beam and a 0.15 Gy s1 gamma source.
TCE versus time for an electron beam dose rate of approximately 2.50 kGy s1 and a 60Co irradiation at 0.15 Gy s1. Calculations were performed using MAKSIMA and considered reactions which include the radiolysis of pure water plus additional reactions with TCE and naturally occurring compounds such as carbonate, bicarbonate, nitrate, and DOC. Fig. 3 also shows the concentration of the primary reactive species
over time for both the electron beam and gamma dose. The rate of e aq production for electron beam irradiation is such that the concentration of e aq is initially four orders of magnitude higher than for gamma irradiation. Similarly, the electron beam concentration of OHd is higher than that for gamma irradiation. Because of competition kinetics, these higher concentrations lead to increased reactions between e aq and OHd for a given
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concentration of a contaminant. Although this example does not consider all the possible reactions, it illustrates the nature of the difference between the reaction kinetics of the two types of irradiations. Other researchers have obtained similar experimental results. Pulsed electron beam irradiation of 400 mg l1 solutions of benzene at dose rates between 14 105 and 108 kGy s1 resulted in better removal at the lower dose rates (Moran, 1994). Experiments involving both gamma and electron beam produced bremsstrahlung irradiation of various organic compounds with dose rates ranging from 4 to 5 105 kGy s1 also resulted in higher removal for lower dose rates for several compounds (Matthews et al., 1991). The results appeared to be the opposite, however, for carbon tetrachloride experiments. Another exception to the advantage of lower dose rates has been reported based on a series of electron beam experiments conducted at Sandia National Laboratories in which simulated radioactive waste was irradiated at dose rates of 2.5 103 and 2.7 109 kGy s1 (Neau, 1994). In these experiments better removal was obtained at the higher dose rate. The reasons for this are unclear at this time, however, it should be noted that 2.7 109 kGy s1 is a higher dose rate than the experiments mentioned previously and high enough to change the primary yields of reactive species available to react with contaminants (Spinks and Woods, 1990). Since the concentration of organics was also much higher, direct radiolysis of the contaminants plays a greater role in their removal. The chemistry at extremely high dose rates and/or contaminant concentrations thus may be more favorable to removal by high dose rates in some situations.
5. Conclusions The results of our experiments and others show a definite difference in removal efficiency between gamma and electron beam irradiation for most compounds investigated. In general, removal efficiencies are greater for gamma irradiation. Electron beam dose constants were found to be 39–83% of the corresponding gamma dose constants for the six compounds considered in this study under a set of uniform conditions. This means that extrapolating results from bench-scale 60Co studies to large-scale electron beam treatment of water must be done with care. For example, required doses for electron beam treatment range from 1.2 to 2.5 times those indicated by bench-scale gamma studies. Equations for adjusting 60Co results to predict large-scale electron beam results have been presented in Table 3. These equations will hold for electron beam treatment facilities similar to the Miami EBRF and over the range of experimental conditions used in our studies (e.g., Gehringer et al., 1995).
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The most likely reason for the difference in removal efficiencies is the difference in the dose rates between the two types of irradiations. The lower dose rate of the 60 Co irradiations produces lower concentrations of the important reactive species OHd and e aq which results in a higher fraction of radical/contaminant reaction and thus better removal. Also, higher dose rates and the consequent greater density of tracks and spurs may contribute to greater overlap and increased chance of radical–radical recombination. There is some evidence that extremely high dose rates may be better at high contaminant concentrations. Further research is necessary to determine whether the increased role of direct radiolysis in high-concentration solutions favors removal using high dose rates.
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