Supercritical Water Oxidation of Pyridine and 3-Cyanopyridine: TOC Removal, Kinetics, and Degradation Pathway

Abstract

Supercritical water oxidation (SCWO) has emerged as a promising technique to eliminate refractory aromatic compounds, which are difficult to remove by conventional wastewater-treatment processes. Decompositions and degradation mechanisms of pyridine and 3-cyanopyridine (3-CP) were investigated using SCWO in this work. It was found that total organic carbon (TOC) removal efficiencies of pyridine and 3-CP were significantly improved as the oxidant dose ratio rose from 0 to 5, temperatures increased from 350°C to 550°C, and reaction time extended from 0.5 to 6 min. Based on the experimental results, degradation kinetics constants of pyridine and 3-CP were evaluated, which were 0.2525–0.7097 min−1 for pyridine and 0.2739–0.9590 min−1 for 3-CP. It is obvious that the degradation kinetics constants of 3-CP were higher than those of pyridine in general. Based on density functional theory (DFT) method, Fukui indices based on OH radical attack (F(0)) of pyridine and 3-CP were calculated. The results showed the greatest F(0) value of carbon atom in 3-CP was higher than that in pyridine, which indicated 3-CP would be more easily attacked by OH radicals than pyridine. According to these results, the conceivable degradation pathways of pyridine and 3-CP were proposed, which include hydroxylation, ring cleavage, and mineralization.