engleski

Process intensification in phenolic wastewater treatment

The increasing complexity of wastewater streams as well as the unfavourable public opinion about some conventional waste management techniques, e.g. incineration and adsorption is forcing the development of cost-competitive and environmentally acceptable waste treatments. To overcome the inconveniences of conventional treatment methods various chemical oxidation techniques have emerged in the past last decades. The Catalytic Wet Air Oxidation (CWAO) process is one of the most promising technologies for the remediation of concentrated and/or biotoxic water pollutants, when a stable and active catalyst can be provided. In order to lower CWAO operation conditions, thus limiting the investment costs, hydrogen peroxide can be used as powerful source of highly reactive hydroxyl radicals which act as promoters in a process so-called Peroxide Promoted Catalytic Wet Air Oxidation (PP-CWAO). To study the potentialities of this newly developed oxidation process, the catalytic activity and stability of Cu/13X catalyst was tested in the PP-CWAO of phenolic aqueous solutions. Phenol was chosen as model compound due to its widespread discharge in the environment from pharmaceutical, chemical and petrochemical processes, where is used either as reagent, intermediate substance or solvent. Its high toxicity and suspected mutagenic and carcinogenic properties prohibit the release of untreated phenolic wastewater into the natural recipients. In this work phenolic water was treated in a batch reactor at given operating conditions: pO2=2-20 bar, T=313-353 K, cH2O2=0.01-0.14 mol dm-3, mcat.= 0.1-0.5 g dm-3 and constant initial phenol concentration (0.01 mol dm-3). The catalyst samples were prepared by ion exchange method of the protonic form of commercial 13X zeolite and were calcined in air to increase their stability. The experiments were carried out in the absence of mass transfer limitations. The obtained experimental data was tested to a proposed kinetic model for phenol oxidation rPh = kPh'cPhcHP and hydrogen peroxide decomposition rHP = kHP'cHP + kPH'cPHcHP. The validation of the model was successfully done with the available experimental data from the laboratory batch reactor and thus provided a reliable tool for the scale up study of the PP- CWAO process. The values of the activation energies for kinetically controlled PP-CWAO of phenol over Cu/13X was 49.2 kJ mol-1 for phenol oxidation and 43.7 kJ mol-1 for hydrogen peroxide decomposition what were slightly lower or close to the values given in the literature. The difference in the results may arise from the reaction conditions and the type of catalyst used. On the basis of the obtained results of catalysts characterization and performed catalytic tests the following can be concluded. The structure of calcined and noncalcined Cu/13X was confirmed with powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and AAS elemental analysis, while the adsorption technique was used for the measurement of the specific surface area. Their catalytic performance was monitored in terms of phenol and total organic carbon (TOC) conversions, by-products distribution and degree of copper leached into the aqueous solution. The experimental profiles for phenol and TOC conversions using hydrogen peroxide as promoter in CWAO have been compared to those obtained in the CWAO and CWPO process. The results showed that the addition of hydrogen peroxide to the CWAO process increases pollutant removal but also leads to higher mineralization of the remaining oxidation products than when only molecular oxygen was used. Analyses done at the 180th minute of reaction reflected that major products, when using fresh Cu/13X catalyst are fumaric and acetic acid although its concentration highly depends on the reaction conditions used in the process. If phenol oxidation is carried out at temperature of 353 K, oxygen pressure of 20 bars and stoichiometric ammount of hydrogen peroxide, fumaric acid is being formed already in the 5th minute of the reaction while catechol almost completely dissapears in 50th minute of the reaction. The stability measurements showed that thermal treatment stabilizes the catalyst since the leaching of copper was significantly lower for calcined than for noncalcined catalyst. To sum up, it can be concluded that this newly developed PP-CWAO process, if appropriately selected and operated, can be an efficient tool for enhancing the efficiency of the treatment of biorefractory wastewater.

catalytic wastewater purification; phenol oxidation; oxygen; hydrogen peroxide; zeolite; kinetic analysis

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