(X-Ray Treatment of Aqueous Solutions)
X-ray treatment of organically contaminated aqueous solutions is based on the in-depth deposition of ionizing radiation. X-rays collide with matter, generating a shower of lower energy secondary electrons within the contaminated waste material. The secondary electrons ionize and excite the atomic electrons, break up the complex contaminant molecules, and form highly reactive radicals. These radicals react with the volatile organic compounds (VOC) and semivolatile organic compounds (SVOC) to form nontoxic by-products such as water, carbon dioxide, and oxygen.
An efficient, high-power, high-energy, linear induction accelerator (LIA) plus X-ray converter generates the X-rays used in the treatment process. The LIA energy, which must be small enough to avoid nuclear activation and as large as possible to increase the bremsstrahlung conversion efficiency, will most likely be in the range of 8 to 10 million electron volts (MeV). A repetitive pulse of electrons 50 to 100 nanoseconds long is directed onto a cooled converter of a high atomic number metal to efficiently generate X-rays. The X-rays then penetrate the container and treat the waste materials contained within.
Based on coupled electron/photon Monte Carlo transport code calculations, the effective penetration depth of X-rays produced by converting 10-MeV electrons is 32 centimeters in water after passing through the side of a standard 55-gallon drum. Large contaminant volumes can be easily treated without absorbing a significant fraction of the ionizing radiation in the container walls. Either flowing waste or contaminated waste in stationary or rotating containers can be treated. No additives are required for the process, and in situ treatment is feasible. The cost of high throughput X-ray processing is estimated to be competitive with alternative processes which decompose the contaminants.
X-ray processing can treat a large number of organic contaminants in aqueous solutions (groundwater, liquids, leachates, or wastewater) without expensive waste extraction or preparation. The technology has successfully treated 17 organic contaminants, listed in the table on the next page. No hazardous by-products are predicted to form or have been observed in the experiments.
This technology was accepted into the SITE Emerging Technology Program in May 1991 and was completed in April 1994. A 1.2-MeV, 800-ampere, 55-nanosecond LIA gave a dose rate of 5 to 10 rads per second. Twelve different VOCs and SVOCs found in Superfund sites were irradiated in 21 aqueous matrices prepared with a neat solution of the contaminant in reagent grade water. The amount of X-ray dose (1 rad = 10-5 Joules per gram) required to decompose a particular contaminant was a function of its chemical bond structure and its reaction rate with the hydroxyl radical. When carbonate and bicarbonate ions (hydroxyl radical scavengers) were present in contaminated well water samples, approximately five times the X-ray dose was required to decompose contaminants that react strongly with the hydroxyl radical. The remediation rate of carbon tetrachloride, which does not react with hydroxyl radicals, was not affected.
An X-ray dose of 150 kilorads (krad) reduced the moderate contamination levels in a well water sample from a Superfund site at Lawrence Livermore National Laboratory (LLNL) to less than those set by the California Primary Drinking Water Standards. For a more highly contaminated LLNL well water sample, experimental data suggested a 500-krad dose was needed to reduce the contamination levels to drinking water standards.
In principle, the rate coefficients determined from the data can be used to estimate the dose level required to destroy mixtures of multiple VOC contaminants and OH- radical scavengers. However, these estimates should be applied judiciously. Only the experimentally determined destruction curves, based on the remediation of test samples of the actual mixture, can be used with confidence at the present. The table below summarizes the X-ray treatment results from the SITE evaluation.
| CONTAMINANT | MATRIX | INITIAL CONCENTRATION (ppb)* | FINAL CONCENTRATION (ppb) | CPDWS** (ppb) | X-RAY DOSE (krad) |
|---|---|---|---|---|---|
| TCE | Deionized Water |
9,780 |
<0.1 |
5 |
50.3 |
| PCE | 10,500 |
<0.1 |
5 |
69.8 |
|
| Chloroform | 2,000 |
4.4 |
-- |
178 |
|
| Methylene Chloride | 270 |
3.1 |
5 |
145.9 |
|
| Trans-1,2-Dichloroethene | 260 |
0.78 |
10 |
10.6 |
|
| Cis-1,2-Dichloroethene | 13 |
<0.5 |
6 |
10.6 |
|
| 1,1,1-Trichloroethane | 590 |
54 |
200 |
207.1 |
|
| Carbon Tetrachloride (CCl4) | 180 |
14 |
0.5 |
224 |
|
| Benzene | 240 |
<0.5 |
1 |
8.8 |
|
| Toluene | 150 |
<0.5 |
150 |
4.83 |
|
| Ethylbenzene | 890 |
3.6 |
680 |
20.4 |
|
| Xylene | 240 |
1.2 |
1,750 |
5.6 |
|
| Benzene/CCl4 | Contaminated Well Water |
262/400 |
<0.5/196 |
1/0.5 |
39.9/93.8 |
| Ethylbenzene/CCl4 | 1,000/430 |
<0.5/70.9 |
680/0.5 |
33.2/185 |
|
| Ortho-xylene/CCl4 | 221/430 |
<0.5/85 |
1,750/0.5 |
20.5/171 |
|
| TCE | LLNL Well Water Sample #1 |
3,400 |
<0.5 |
5 |
99.0 |
| PCE | 500 |
<0.5 |
5 |
99.0 |
|
| 1,1-Dichloroethane | <10 |
1 |
5 |
145.4 |
|
| 1,1-Dichloroethene | 25 |
<1 |
6 |
49.9 |
|
| 1,1,1-Trichloroethane | 13 |
2.0 |
200 |
145.4 |
|
| Cis-1,2-Dichloroethene | 14 |
<0.5 |
6 |
49.9 |
|
| TCE | LLNL Well Water Sample #2 |
5,000 |
<1.0 |
5 |
291 |
| PCE | 490 |
1.6 |
5 |
291 |
|
| Chloroform | 250 |
81 |
-- |
291 |
|
| CCl4 | 14 |
4 |
0.5 |
291 |
|
| 1,2-Dichloroethane | 38 |
17 |
5 |
291 |
|
| 1,1-Dichloroethane | 11 |
6.8 |
5 |
291 |
|
| Freon | 71 |
32 |
-- |
291 |
|
| * parts per billion | |||||
| **California Primary Drinking Water Standards | |||||
EPA PROJECT MANAGER:
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U.S. EPA
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