Dioxins can be extremely toxic, and they present potential problems wherever plastics are incinerated. Current methods for decomposing dioxins into safe compounds are difficult and expensive, and in some cases the dioxins spontaneously reform. A solution to these problems, however, may be at hand.
Researchers at the Advanced Photon Research Center of the Japan Atomic Energy Research Institute (JAERI; Tokyo, Japan) have been studying the use of infrared (IR) lasers to decompose dioxins and other pollutants. "Dioxins are a big problem due to their environmental hostility," says team member Eisuke Minehara. "That's what makes decomposition of these chemicals such an important research theme." For the study, JAERI has developed relatively compact femtosecond free-electron lasers (FELs) operating at 22 µm and 25 µm. In addition, the team used the 10.6-µm output of a CO2 laser. "We wanted to test the possibility of photolysis with this laser," Minehara says. The group reported on their results at InterOpto (paper #MG1-2, Chiba, Japan; 1620 July).
The group began with two dioxins dissolved in tolueneoctachlorodibenzo-para-dioxin (OCDD) and octachlorodibenzofuran (OCDF). "We chose these dioxins because they have the lowest toxicity equivalency quantity, or TEQ," Minehara says. "We prepared three samples of each type. One was left in the laboratory. One was irradiated. And the third was not irradiated."
There are two ways to decompose dioxins with IR lasers. One is dechlorination, and the other is a combustion reaction that produces CO2. In the dechlorination decomposition reaction, absorption of the IR photons dissociates the chlorine so the dioxin becomes a free radical. According to Minehara, these free radicals then combine with atoms of chlorine, hydrogen, or hydroxyl radicals.
The researchers ran the FELs at 600-µs pulse rate and a repetition frequency of 10 Hz. A lens focused each beam into a 50-µm-diameter spot on the dioxin solution sample. The team also used a 10 W CO2 laser operating in long pulse (millisecond) to continuous-wave (CW) mode. Because the process is wavelength dependent, results varied.
The team irradiated the OCDD sample for 50 minutes with 4 W of power from the CO2 laser, achieving a degradation level of more than 90% as measured by gas chromatograph. The degradation rate was more than 90%. Irradiation of the same type of OCDD sample with the 22 µm and 25 µm output from the FELs, however, failed to show clear decomposition.
The FELs were effective for other dioxins, however, including polychloronatedbiphenyls (PCBs). Experiments showed that decomposition rates of tetrachlorodibenzodioxin (TCDD) were higher than for OCDD, and pentachlorodibenzofuran (PCDF) decomposition was easier than pentachlorodibenzodioxin (PCDD). The decomposition levels achieved with PCB samples exceeded 87%.
Through their experiments, Minehara and his team found that CO2 laser irradiation efficiently triggered decomposition of dioxins. Further, the resulting free radicals of dioxins stabilized and did not recombine into noxious dioxins. The team also found that the hydrogen atoms and hydroxyl radicals dissociated from water by the laser played an important role in the stabilization of the free radicals of dioxins.
"I believe IR lasers will be a powerful and universal way to decompose dioxins," Minehara says. "And we don't have to worry about them surviving or recombining once the decomposition process is complete. Where environmentally harmful chemicals exist in the real world, we can irradiate them and decompose them so quickly and directly." He says the team is now doing field tests with an eye toward commercializing the FEL technology and is hoping to develop a system the size of a small trailer.
This will be welcome, according to Shigenori Ono, manager of the Machinery & Systems Department at Sumitomo Corp. (Tokyo, Japan). "The conventional process is to heat gas containing dioxins to 1800 to 1900°C, then rapidly quench it to around 400°C. Compared to this process, JAERI's new method of using IR lasers to decompose dioxins holds great potential for drastic reductions in initial investment and running costs. And unlike conventional methods, there is no regeneration of the dioxins. When [these systems] become commercially available, we look forward to large treatment capacities with high energy efficiency from the new method."