https://doi.org/10.1016/j.chemosphere.2021.131239

Abstract

The article presents the research results of the oxidation of watered toxic waste from the pulp and paper industry (sludge-lignin, the empirical formula of organic matter CH_1.51N_0.05S_0.03Cl_0.01O_0.54) in supercritical water-oxygen (SCW/O₂) fluid. The experiments were carried out using a flow tube reactor at a pressure of 25 MPa, temperature gradient along its vertical axis (from top to bottom: 390–600 °C), sludge-lignin flow rate of 9.5–14.5 g/min, oxygen ratio OR = 0.73–2.52, using NaOH (1.6 wt%) as a catalyst. Employing gas chromatography – mass spectrometry, polychlorophenols were identified in the composition of sludge-lignin, in which 2,4,6-trichlorophenol was the main component. The total yield of extracted phenols and chlorophenols per sludge-lignin organic matter was 20.82 and 2.88 μg/g, respectively. It is revealed that the conversion rate of sludge-lignin in SCW/O₂ fluid is limited by heterogeneous oxidation of the carbonized residue, and is determined by the O₂ content in the reaction mixture. At OR ≥ 1.16, only CO₂, CO, N₂, and N₂O were detected in the volatile oxidation products. An increase in OR from 0.73 to 2.52 leads to a decrease in the total content of phenols (from 45540.1 to 129.3 μg/dm³) and chlorophenols (from 51.4 to 2.2 μg/dm³) in the water collected at the reactor outlet. It is shown that 2,6-dichlorophenol and 2-chlorophenol are the most resistant to oxidation. From the analysis of the initial sludge-lignin and mineral residues, it follows that the bulk of the chlorine contained in its organic matter is converted into NaCl in the course of oxidation.

Keywords: Waste disposal, Phenols, Chlorophenols, Supercritical water oxidation, Alkali catalyst