Ruthenium Tetroxide Destroys Dioxin THE OXIDATIVE CONTROL OF AROMATIC POLLUTANTS By David C. Ayres Westfield College, University of London Ruthenium tetroxide is a powerful oxidising be used with advantage. The rate of these agent and has been widely used in laboratories oxidations is approximately doubled for every for small scale oxidations as it reacts rapidly IOOC rise in temperature. with most oxidisable organic functional groups Ruthenium tetroxide is toxic, but less so than at ambient temperature (I). In such conditions a osmium tetroxide; its solutions may be safely solution of ruthenium tetroxide in carbon handled in closed systems or in fume cupboards. tetrachloride is often employed, the organic A 2 per cent aqueous solution can be safely solvent taking up the ruthenium tetroxide as it obtained from warm ruthenium is generated by the oxidation of ruthenium dioxide/hypochlorite by its entrainment in a trichloride with aqueous hypochlorite. Initially stream of air followed by collection in a cooled the ruthenium tetrachloride solution is yellow, trap. In water there is considerable rate and a clear indication that the oxidative reac- enhancement compared to reactions in carbon tion is taking place is given by the precipitation tetrachloride. of black insoluble ruthenium dioxide. As the hypochlorite is capable of continuously Potential Industrial Applications regenerating the ruthenium tetroxide from the From a practical point of view, two specific hydrated dioxide, only catalytic quantities of examples of aqueous oxidations indicate the ruthenium compounds are required. industrial possibilities. Potassium permanganate is used com- Mono- and dichlorophenols are oxidised mercially for the oxidative control of air extremely rapidly. The destruction of pollutants by wet scrubbing; however a solution pentachlorophenol is best accomplished by the of ruthenium tetroxide-being more potent-is action of the tetroxide on the phenoxide. This an attractive alternative oxidant for persistent reaction requires treatment for 18 hours at aromatic pollutants. An illustration of its 3ooC, when complete fragmentation occurs effectiveness is provided by the reported giving carbon dioxide and chloride ions as sole destruction of polychlorodibenzodioxins (2), products (3). including the now notorious TCDD that was A group of thiophene odorants at concentra- accidentally released at Seveso in Italy in 1976. tions ofabout I or3grams/litrerequires treatment for hundreds of minutes with saturated per- manganate at 22T. When ruthenium tetroxide is used on comparable concentrations the disap- 2,3,7,8-Tetrachlorodibenzo-pdioxin (TCDD) pearance time is reduced to a few minutes (4). The electrondonating oxygen atoms of the References ruthenium compound enhance the oxidation of I U. G. Lee and M. van den Engh in “Oxidation in this structure: the simple phenols and their Organic Chemistry 5B”, ed. W. S. Tranhanovsky, ethers all react extremely rapidly, but Academic Press, New York, I 973, p. I 77 chlorosubstituents have a stabilising effect. z D. C. Avres, Nature, 1981,290,(5804), 323 3 D. C. Ayes and D. P. Levy, to be published Clearly the solvent employed must be consider- 4 D. C. Ayres and C. M. Scott, Environ. Sci. ably inactivated; nitromethane can sometimes Technol., 1979, 13, (II), 1383 Platinum Metals Rev., 1981, 25, (41, 160-160 160 .