Abstract
This work is focused on the electrochemical advanced oxidation of pharmaceutical organic pollutants in water on tungsten trioxide and sub-stoichiometric titanium oxide substrates: degradation, kinetics and intermediate products studies. Therefore, a photocatalytic degradation of antibiotics ciprofloxacin and sulfamethoxazole in wastewater using a nanocomposite of tungsten trioxide (WO3) and carbon nanodots (CNDs) was carried out. This material was prepared by doping tungsten trioxide (WO3) with carbon nanodots (CNDs) by hydrothermal technique. The structural, optical, morphological and photocatalytic properties of the CNDs-doped WO3 nanocomposite as well as an undoped WO3 nanorod were successfully characterised by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman and Fourier-transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) and ultraviolet-visible diffusive reflectance spectroscopy (UV-Vis DRS). The photocatalytic degradation performance, kinetics and patterns were followed by UV-Visible spectroscopy and ultra-performance liquid chromatography coupled with mass spectrometry (UPLC-MS). The percentage removal of ciprofloxacin and sulfamethoxazole was higher with the WO3-CNDs than the pristine WO3 photocatalyst indicating the positive effect of doping. The percentage removal of the antibiotics when monitored by UPLC-MS was found to be higher than that of UV-Visible spectroscopy. The intermediate products and pathway were different with the doped and the undoped WO3 photocatalysts. Then, the mineralisation of tetracycline by EAOPs as well as the study of the toxicity of its intermediates and degradation products are presented. Electro-Fenton (EF), anodic oxidation (AO) and electro-Fenton coupled with anodic oxidation (EF/AO) were used to degrade tetracycline on carbon felt (cathode) and Ti4O7 layer deposited on Ti (anode). As compared to EF and AO, the coupled EF/AO system resulted in the highest pollutant removal efficiency: total organic carbon removal was 69±1% and 68±1%, at 20 ppm and 50 ppm of initial concentration of tetracycline, respectively. The effect of electrolysis current on removal efficiency, mineralisation current efficiency, energy consumption, and solution toxicity of tetracycline mineralisation were investigated for 20 ppm and 50 ppm tetracycline. The EF/AO process using Ti4O7 anode and carbon felt (CF) cathode provides low energy and high removal efficiency of tetracycline caused by the production of hydroxyl radicals both at the surface of the non-active Ti4O7 electrode and in solution by the electro-Fenton process at the cathodic carbon felt. Complete removal of tetracycline was observed from HPLC data after 30 min at optimised conditions of 120 mA and 210 mA for 20 ppm and 50 ppm tetracycline concentration. Degradation products were elucidated and the toxicity of the products were measured with luminescence using Microtox® bacteria toxicity test. Also presented are the results of degradation of tetracycline in real domestic wastewater effluents which were initially pre-treated in a membrane bioreactor (MBR). Tetracycline was spiked in the real wastewater effluent and treated using a coupled system of electro-Fenton and anodic oxidation (EF/AO). Carbon felt was used as the cathode and Ti4O7 as anode, at a constant current density of 0.96 mA.cm-2 for the treatment of the raw secondary effluent and the effluent spiked with 10 and 20 ppm of tetracycline. High performance liquid chromatography- mass spectroscopy (HPLC-MS) analyses were conducted to monitor the amount of tetracycline degradation. After 30 min, no traces of tetracycline were detected for the raw effluent and for the 10 and 20 ppm tetracycline spiked effluents. The removal efficiency of 82.7% was obtained for the 20 ppm tetracycline effluent after 4 h of electrolysis with an energy consumption of 0.06 kWh m-3. Whereas, for the raw effluent 18.86% TOC removal was achieved with 0.82 kWh m-3 of energy consumption and 59.97% TOC removal was obtained for the 10 ppm effluent with an energy consumption of 0.1 kWh m-3 after 4 h of electrolysis. Analyses including acute toxicity tests (Microtox®) and three-dimensional fluorescence spectroscopy (3DEEM) were also performed in order to better understand the fate of dissolved organic matter during the treatment. The results from this study revealed that electro-Fenton coupled with anodic oxidation is an efficient way to remove organic micro-pollutants from domestic Wastewater Treatment Plant (WWTP) secondary effluent. Furthermore, the efficiency of the titanium oxide (Ti4O7) electrode for potential application in electrochemical oxidation of complex organic pollutants in water was investigated. A degradation of a pharmaceutical cocktail that consisted of sulfamethoxazole (SMX), ciprofloxacin (CIP) and tetracycline (TERT) organic pollutants was investigated using photo/electro-Fenton coupled with anodic oxidation. Photo/Electro-Fenton coupled to anodic oxidation (PEF/AO or EF/AO), one of the advanced oxidation processes (AOPs), has been established as one of the efficient techniques for the treatment of organic polluted water. Treatment of an organic pharmaceutical cocktail pollutant at 20 ppm was conducted in an undivided cylinder where carbon felt (CF) was used as the cathode and Ti4O7 as anode, at a constant current density of 0.96 mA.cm-2. The treatment of the pharmaceutical cocktail was monitored by UV-Vis, TOC, UPLC-MS analysis. Results from the UV-Vis spectroscopy showed that almost all the major peaks present in the initial spectrum disappeared when compared with the final spectrum for both EF/AO and photo EF/AO. A high removal TOC efficiency of above 80% of the pharmaceutical cocktail was recorded within 30 min for both processes. The concentrations of each of the pharmaceuticals present in the cocktail was detected using UPLC-MS where the concentration of SMX was no longer detected in the cocktail solution in 30 min, and CIP and TERT were detected in trace amounts. Reaction pathways were proposed based on the data of high-resolution spectrometry and the intermediate products detected were smaller than the parent molecule. This has proven that the titanium oxide (Ti4O7) as an anode and carbon felt as a cathode electrode can be used to treat complex organic pollutants efficiently by EF/AO and photo EF/AO processes. These studies have proven that EAOPs can treat organic pollutants in water by means of WO3-CNDs and sub-stoichiometric titanium oxide as electrodes.