Polymerization and oxidation of phenols in supercritical water

Supercritical water oxidation of phenol and process enhancement with in situ formed Fe2O3 nano catalyst

During the past few decades, the treatment of hazardous waste and toxic phenolic compounds has become a major issue in the pharmaceutical, gas/oil, dying, and chemical industries. Considering polymerization and oxidation of phenolic compounds, supercritical water oxidation (SCWO) has gained special attention. The present study objective was to synthesize a novel in situ Fe₂O₃nano-catalyst in a counter-current mixing reactor by supercritical water oxidation (SCWO) method to evaluate the phenol oxidation and COD reduction at different operation conditions like oxidant ratios and concentrations. Synthesized nano-catalyst was characterized by powder X-ray diffraction (XRD) and transmission electron microscope (TEM). TEM results revealed the maximum average particle size of 26.18 and 16.20 nm for preheated and non-preheated oxidant configuration, respectively. XRD showed the clear peaks of hematite at a 2θ value of 24, 33, 35.5, 49.5, 54, 62, and 64 for both catalysts treated preheated and non-preheated oxidant configurations. The maximum COD reduction and phenol oxidation of about 93.5% and 99.9% were observed at an oxidant ratio of 1.5, 0.75 s, 25 MPa, and 380 °C with a non-preheated H₂O₂ oxidant, while in situ formed Fe₂O₃nano-catalyst showed the maximum phenol oxidation of 99.9% at 0.75 s, 1.5 oxidant ratio, 25 MPa, and 380 °C. Similarly, in situ formed Fe₂O₃ catalyst presented the highest COD reduction of 97.8% at 40 mM phenol concentration, 1.0 oxidant ratio, 0.75 s residence time, 380 °C, and 25 MPa. It is concluded and recommended that SCWO is a feasible and cost-effective alternative method for the destruction of contaminants in water which showed the complete conversion of phenol within less than 1 s and 1.5 oxidant ratio.

Review of the destruction of organic radioactive wastes by supercritical water oxidation

Organic materials, such as ion exchange resins, plastic, oils, and solvents, are widely used in the operation and decommission of nuclear facilities. The generated radioactive organic wastes are both radioactive and organic; therefore, the degradation of such wastes becomes more difficult. Due to delays in the disposal of radioactive organic wastes, potential safety risks are increasing. With the advantages of degrading refractory organics rapidly and thoroughly, supercritical water oxidation (SCWO) has become a potential alternative way to degrade radioactive organic wastes. This review focused on the degradation characteristics of different radioactive wastes from the perspective of potential practical applications. Some improved methods for facilitating the degradation of radioactive wastes by SCWO are considered and analyzed. Moreover, the kinetics and intermediate pathways of radioactive organic wastes are further analyzed. The distribution, migration and transformation of radionuclides during the SCWO reaction, as well as the further processing of radionuclides in gas-, liquid- and solid-phase products, were summarized and discussed. Furthermore, some fruitful areas for further work were reviewed for the highly efficient degradation of radioactive organic wastes. This review can provide useful information and guidance for the industrial applications of SCWO treatment for radioactive wastes.

Supercritical water oxidation of chlorinated waste from pulp and paper mill

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.

Partial oxidation of phenol in supercritical water with NaOH and H2O2: Hydrogen production and polymer formation

The catalytic supercritical water partial oxidation of phenol using H₂O₂ as oxidant in the presence of NaOH was explored to enhance hydrogen production and inhibit phenol polymerization. The results indicated that H₂ production was enhanced in the presence of NaOH when phenol supercritical water oxidation was controlled at a lower O/C ratio. Compared with the individual catalytic partial oxidation of phenol, the reaction with NaOH and H₂O₂ simultaneously enhanced H₂ production and inhibited polycyclic polymer generation at O/C ratios below 0.5. A peak hydrogen gasification efficiency value of 62.35% was observed at an O/C ratio of 0.3 with 1.0 wt% NaOH, and a phenol removal efficiency of nearly 75% was reported. Phenol polymerization was effectively inhibited for reaction times limited to 50 s. Moreover, other phenol reaction pathways reported in the literature were compared with the partial oxidation of phenol in supercritical water with NaOH and H₂O₂.

Analysis of chemical characteristics of lignite upgrading wastewater and its agricultural utilization

The lignite upgrading wastewater (LUW) produced in the drying and upgrading process of lignite cannot be discharged directly. Conventional wastewater treatment methods are usually costly and unable to achieve efficient utilization of water resources which are rich in activity components. In this study, the water quality analysis showed that LUW belonged to seriously polluted waters with low pH and very high total nitrogen content. Fifty-five compounds, mainly phenols and organic acids, were identified by gas chromatography-mass spectrometry (GC-MS) analysis. The study confirmed that the LUW, after being diluted to an appropriate concentration, could significantly promote the growth of wheat seedlings. The phenols and organic acids were the activity material basis of LUW, which promoted seed germination possibly through playing a role similar to plant hormones and simultaneously enhancing the utilization of nutrient elements. LUW had the natural advantages of directly developing high-end liquid fertilizers in terms of its physical form, chemical composition, biological activity, safety and economy. This study confirmed the feasibility of applying LUW to agricultural field as liquid fertilizer only through simple dilution without other treatments. Applying LUW as liquid fertilizer can not only supply a fertilizer product with low production cost and outstanding efficacy, but also provide an efficient and green way for the treatment of upgrading wastewater, which utilize the LUW as natural resources instead of purifying and discharging.