engleski

MODELLING OF THE PHOTOCATALYTIC AND SONOCHEMICAL PROCESS FOR THE WASTEWATER TREATMENT

Advanced oxidation technologies (AOTs) represent an attractive alternative for treating wastewater containing organic pollutants. Among AOTs, TiO2 photocatalysis (UV/TiO2) and sono-Fenton process (US/Fen+/oxidant) have been widely studied in the frame of this work. Along these lines, an application and the modelling of slurry UV/TiO2 and US/Fe2+/H2O2 in the advanced oxidation of model wastewaters has been reported. Synthetic azo dyes, C.I. Reactive Violet 2, C.I. Reactive Yellow 3 and C.I. Mordant Yellow, and carboxylic acids, formic and oxalic acid, were chosen as model pollutants. Obtained values of TOC removal and respective concentration of model pollutants or expected by-products indicated the partial mineralization achieved in the studied model wastewaters. The obvious difference between UV/TiO2 and US/Fe2+/H2O2 processes is the time necessary for the treatment. Photocatalytic processes lasted for 60 minutes, while the sono-Fenton processes only 15 minutes, respectively. All photocatalytic processes assumed pH values around 4. Sono-Fenton and corresponding Fenton processes were performed at initial pH 3. In the development of the operation mode for the heterogeneous photocatalysis, the slurry reactors and the reactor with the supported photocatalytic films have been investigated. Slurry UV/TiO2 has been detected as optimal at the given conditions. The photocatalytic experiments involved the employment of UV-A and/or UV-C light. Two different configurations were chosen for the detailed analysis ; (i) cylindrical photoreactors, operated in batch mode and, (ii) annular photoreactor, operated in total recirculation mode. The developed model for the radiation field in the reaction space utilized a six-flux absorption-scattering model (SFM). The SFM was included in the reactor modelling allowing an integral analytical estimation of the local volumetric rate of photon absorption (LVRPA) at each point of the reactor. LVRPA in the cylindrical reactors was averaged and treated as a lumped parameter, LumpVRPA, obtained by integrating the local values of LVRPA over the total volume of the cylindrical reactor and dividing by the same volume. The kinetic models based on the Langmuir-Hinshelwood mechanism for the dyes and formic acid and a complex kinetics of the oxalic acid photocatalytic oxidation via hole trapping (h+) and hydroxyl radical attack (•OH) have been successfully confirmed and the corresponding rate constants independent on the photon effects were determined. Thorough studies have also been devoted to US-enhanced Fenton process and the modification in the Fenton reagent. The reactors used for the experiments involving the sonication were generally divided in two different types ; (i) horn type reactor and (ii) ultrasonic bath. The investigation of the performance of the sonoreactors in respect to the chosen vessels assumed the utilization of reaction vessels with different geometry and volume. The results showed that use of a sonication for the enhancement of the Fenton reaction greatly improved mineralization rate and lead to better use of reagent, both Fe2+ and H2O2. The study showed the following ; choice of the vessels of the same volume but with different ratio of horn tip to vessel diameter (dhorn tip/D) did not influence the overall sonication intensity. For the convenient analysis of the sonoreactor, global parameter of the ultrasound exploitation (GPUE) was introduced in the kinetic model and expressed as coefficient eUS, representing the effectiveness factor. Coefficient eUS has been modelled as a function of the employed frequency, calorimetric power of the transducer, working temperature and the ratio of the volume of the active zone in the total working volume. The latter has been described as the dimensionless active volume, V_US. By implementing the GPUE in the kinetic model, corresponding kinetics of the sono-Fenton oxidation has been obtained by using only the kinetic data for the Fenton process, completely independent on the applied ultrasonic field. Finally, based on the failure analysis and comparative reliability studies of the chosen photo- and sonoreactor on a laboratory scale, a three-criterion analysis approach for the risk assessment was proposed. The set criteria ; (i) probability of failure, (ii) estimated cost for the 10 000 working hours and (iii) residual organic content in the system after the applied treatment, resulted in the calculated risk scores, Srisk that enlighten all the crucial aspects of the investigated system.

advanced oxidation technologies (processes); wastewater treatment; azo dyes; carboxylic acid; TiO2 photocatalysis; sono-Fenton process; radiation field modelling; sonoreactor modelling; mineralization kinetics; reliability; failure analysis; risk assessment

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