Rate enhancement by cations in supercritical water oxidation of 2- chlorophenol

Interactions between cations and Cl species in the supercritical water oxidation (SCWO) of 2.-chlorophenol (2CP) has been investigated in a high- pressure quartz-lined reactor at 673-773 K. Experimentally, we found that the S/D ratio (defined as the amount of 2CP converted to CO₂ and H₂O/disappearance of 2CP) for SCWO of 2CP was enhanced by cations (such as Li⁺, Na⁺, K⁺, Ca²⁺, Fe²⁺, or Fe³⁺). The S/D ratio increased as the charge density of cations increased (Fe³⁺> Fe²⁺ > Li⁺ > Ca²⁺ > Na⁺ > K⁺). Due to the extremely low solubility of metal salts in the supercritical water, abstraction of Cl in 2CP via an intermediate ((OH)PhCl(δ^-)---- M(δ⁺) was postulated. Formation of these metal chloride (such as KCl, CaCl₂, and FeCl₃) precipitates in the SCWO of 2CP were identified by X-ray diffraction (XRD) spectroscopy. Since the formation of toxic highly chlorinated phenols and heavy polycyclic aromatic hydrocarbons (PAHs) was notably reduced, abstraction of Cl of 2CP by cations may occur in the early stage of the SCWO process.