Chemical destruction using supercritical water

Abstract

Water becomes a supercritical fluid above 374°C and 220 bar. Under these conditions, the hydrogen-bonded structure has largely broken down. It is therefore much less polar and can homogenise relatively large amounts of non-polar organic compounds making them available for chemical reaction. The loss of structure also means that the medium is more like a collection of light mobile molecules and diffusion rates are rapid. For this reason reactions which are diffusion-controlled are much faster. Hence supercritical water can be a very reactive medium for organics. Of particular interest is the use of supercritical water to destroy toxic waste in an environmentally friendly manner. Organic compounds can be rapidly and almost completely converted by molecular oxygen to benign small species: CO₂, H₂0, N₂, Cl⁻, SO₄ ⁻, etc. Conversion of pollutants into more benign species can also take place rapidly by hydrolysis in supercritical water without the presence of oxygen, with for example chlorinated organics becoming hydrocarbons and HCl. An alternative source of oxidation is hydrogen peroxide, which is being used in experiments and may prove economical in processes. This behaviour has given rise to research into commercial processes for the safe destruction of toxic materials and industrial effluent.