Experimental Determination of Hydroxyl Radical Reactivity in Supercritical Water Using Pulse Radiolysis

Complete mineralization of hazardous organic compounds to innocuous products can be achieved using supercritical water oxidation (SCWO). At typical SCWO process conditions (500 – 650 °C), free radical chemistry has been shown to be the dominant reaction path. Detailed Chemical Kinetic Mechanisms (DCKM), consisting of elementary free-radical reactions, have been used to model the oxidation of a number of organics in supercritical water, but with mixed success. Thus, there is the need for experimental verification of the rates of some of the elementary reactions that play a key role in the destruction of organic species in supercritical water. Using pulse radiolysis, we have measured the reactivity of hydroxyl radicals, which have been identified as one of the primary oxidizing species in supercritical water. In particular, we have measured the bimolecular rate constants (kbi) of the hydrogen abstraction reaction between hydroxyl radical and methanol. In DCKM modeling of methanol destruction in supercritical water, this reaction has been shown to be one of the most important elementary steps. We find that the hydroxyl abstraction reaction follows Arrhenius behavior from ambient to 390 °C, but the bimolecular rate constants, kbi , in the supercritical region are significantly greater than the values used in the DCKM modeling.