Decomposition mechanism of phenol in water plasmas by DC discharge at atmospheric pressure

The decomposition mechanism of high concentration of phenol solution in water plasmas at atmospheric pressure was investigated at different arc currents. The results showed that the removal efficiencies of phenol, total organic carbon, and chemical oxygen demand were increased with the increase in arc current. The concentration of phenol was reduced from 52.8 g L−1 down to 1.0 × 10−5 g L−1 at an arc current of 8 A with the energy yield of 8.12 g kWh−1. Major gaseous compounds were H2, CO2, CO, and CH4. However, at a low arc current, trace levels of benzene (C6H6), and cyclopentadiene (C5H6) were detected in effluent gas, and formic acid (HCOOH) and formaldehyde (HCHO) were observed in liquid effluent.By the analysis of reaction intermediates and a calculation of carbon balance, the main reaction pathways were proposed as follows: firstly, electron dissociation in arc region to generate phenoxy (C6H5O) radical; second, chemical oxidation or reduction in plasma flame region to form C6H5O and C6H6. After phenol decomposition, the generated intermediate species would undergo complex reactions to form stable compounds in plasma flame region. The most favorable mechanism is the formation of CO, which is conducted by the ring open step of C6H5O and C6H6 by thermal decomposition or the attachment of active radicals such as O, H, and OH with respect to CO generation. In downstream region, the generated intermediate species were easily recombined with H or oxidized by OH to form unwanted products, such as HCOOH, HCHO, and H2O2.