Experimental and thermodynamic comparison between a novel CO2/CH4 and an oxygen submerged DC thermal plasma for treatment of sebacic acid in basic aqueous solution

• The decomposition of sebacic acid in basic solution is feasible with both plasmas.
• Consecutive photo-oxidation with CO2 plasma is due to plasma intense UV radiation.
• Sequential oxidation with the oxygen plasma is due to high oxidant plasma species.
• The CO2 plasma in the presence of H2O2 is suitable for treatment the basic solutions.
• Thermodynamic study has well described the sebacic acid decomposition.

A novel submerged direct current (DC) thermal plasma torch operating with a mixture of carbon dioxide and methane has been compared with an oxygen DC submerged thermal plasma torch for treatment of a basic solution containing organic acid. Sebacic acid, a long-chain organic contaminant in Bayer liquor, was selected as a representative of organic acid in wastewater and contaminated industrial liquids. The effects of different operating conditions including treatment time, reactor pressure as well as the role of oxidizing agents, such as H2O2, were investigated on the decomposition rate of sebacic acid and the resulting degradation by-products. Intermediate products were quantified by IC/MS (ion chromatography/mass spectrometry) and a TOC (total organic carbon) analyzer. It was shown that both plasmas would decompose the sebacic acid however, oxygen plasma showed a higher conversion rate. The sebacic acid conversion rate was increased five times by adding H2O2 with the CO2/CH4 plasma. It was increased up to the same conversion rate with the oxygen plasma. However, adding H2O2 had no positive effect on the conversion rate with the oxygen plasma. Increasing the pressure also enhanced the conversion of sebacic acid in both plasmas. The decomposition mechanism of the CO2/CH4 plasma was mainly due to the UV radiation of the plasma while in the case of the oxygen plasma, it was more attributed to the plasma oxidizing species. This work, therefore, shows the possibility of using the CO2/CH4 plasma and oxygen plasma torches for treatment of the contaminated species in basic solution.