Liquid waste decomposition by long DC arc under atmospheric pressure

• Liquid waste of biphenyl solution was completely decomposed in long DC arc system.
• C6H5, CH3, and C2H2 are important intermediates to formation of liquid by-products.
• Decomposition by-products are suppressed by oxygen addition.
• High decomposition rates of 99.83% of biphenyl and 100% of acetone were achieved.

A model substance of biphenyl solution was used for the study of polychlorinated biphenyls (PCBs) decomposition. Biphenyl has the same chemical and molecular structure with the PCBs. Because of the severe toxicity of PCBs. the biphenyl was introduced instead of the PCBs. The biphenyl solution was directly injected to a long DC arc as a fine spray with argon carrier gas. The long DC arc system with a 350 mm arc length was operated under atmospheric pressure at a 20 kW DC power supply. The plasma gas of argon containing decomposition products was bubbled through ethanol to remove the decomposition products. The dissolved ingredients of decomposition products in ethanol were defined as liquid by-products, while the gas by-products are insoluble in ethanol as exhaust gas. The gas by-products of H2, CH4, and CO were identified with gas chromatography. The liquid by-products of benzene, toluene, phenylacetylene, styrene, indene, naphthalene, biphenyl, acenaphthylene, fluorine, phenanthrene and 4,5-methylenephenanthrene were qualitatively analyzed by a gas chromatography–mass spectrometry and were quantitatively measured by a high performance liquid chromatography. When introducing a sufficient oxygen into the arc, the high decomposition efficiencies of 99.83% biphenyl and 100% acetone were achieved and the by-products of gas and liquid are suppressed to convert to a desirable product of CO2. For the formation of liquid by-products, C6H5, CH3, and C2H2 are the important intermediates in low temperature region. Pyrolysis of biphenyl solution is initiated from the C6H5 to the polycyclic aromatic hydrocarbons (PAHs).