抄録
Decomposition of toxic substances such as dioxins and PCBs is an urgent environmental issue. Dioxins are present in envionmental matrices such as incinerator fly ash and contaminated soil. Since PCBs were used in large quantities as insulation oil in transformers and other applications, a large number of equipment and devices containing PCBs still exist. Solid materials containg dioxins and PCBs must be properly ptreated in additon to liquid materials. Supercritical fluid technology has focused on the treatment of these toxic chlorinated compounds. In this paper, recent developments of supercritical fluid technology for the decomposition of toxic materials are reviewed. A national project, starting in 2000, has developed two processes for the destruction of solid wastes containing dioxins or PCBs. One is a combined process of extraction/adsorption/SCWO (Ryotech Process) and the other is a hybrid SCWO process (Organo Process). In the former process, dioxins are extracted with supercritical carbon dioxide from a solid matrix and concentrated on an adsorbent. The dioxin-containing adsorbent is then destroyed by SCWO. In the latter process, solid materials containing PCBs are dechlorinated and neutralized in subcritical water. Then, the liquified materials are decomposed completely with SCWO. Mitsubishi Heavy Industries, Ltd. has also developed a SCWO process assisted by sodium carbonate. Solid organic materials containing PCBs are milled into a slurry and fed into a reactor with sodium hydroxide. These feed materials are completely destroyed by dechlorination and oxidation in the presence of sodium carbonate.
