Supercritical water oxidation of polychlorinated biphenyls based on the redox reactions promoted by nitrate and nitrite salts

Abstract

The decomposition efficiency of chlorinated hydrocarbons by supercritical water oxidation (SCWO) using sodium nitrate (NaNO 3) and nitrite (NaNO 2) salts as oxidants has been evaluated in the batch and flow reactor systems. The oxidation reactions are promoted by the reduction of nitrate and nitrite (i.e., N(+V) + 5e − → N(0) and N(+III) + 3e − → N(0)) proceeding via nitrogen monoxide (NO) and dioxide (NO 2) intermediates. The stoichiometric reactions of 4-dichlorobenzene (4-DCBz) with nitrate and nitrite were described on the basis of the component and distribution of the gaseous reaction products. However, { mCO + nH 2} methanations as competing reactions have caused a discrepancy from the chemical equivalent of the stoichiometric reactions. The highest decomposition (net decomposition > 99.95%) of 4-dichlorobenzene and polychlorinated biphenyls (PCBs) has been achieved at 450 °C and ρ w > 0.25 g/cm 3 for 30 min. Advantageously, the oxidation reactions proceed in parallel with the neutralization of reactor-corrosive hydrogen chloride (HCl) produced by the decomposition of chlorinated hydrocarbons. However, the resulting salts are responsible for the plugging of the flow reactor vessel, because of its low solubility for SCW and high melting point. This work has solved the reactor plugging by introducing removal steps of the precipitated salts into the operation process, whereby a semi-continuous processing of PCBs has been carried out under relatively low pressure condition ( P = ca. 39 MPa).