Abstract
This study focused on the investigation of titanate-based piezoelectric materials such as barium titanate (BaTiO3) and barium zirconate titanate (BaZrTiO3) as anodes for sonoelectrochemical degradation of common pharmaceutical pollutants namely ciprofloxacin, zidovudine, paracetamol, sulfamethoxazole and aspirin. These titanate-based piezoelectric materials were synthesized using the sol-gel method and characterized structurally and morphologically using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectrometry (EDS). The electrode fabricated from these materials were further characterized electrochemically and sonoelectrochemically using cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) techniques. These fabricated electrodes were later used for the sonoelectrochemical degradation of some selected pharmaceutical pollutants in wastewater.
The first report in this work investigated the sonolecetrochemical degradation of ciprofloxacin (CIP) using BaTiO3 immobilized on a titanium sheet as the working electrode (Ti/BaTiO3). The prepared BaTiO3 conformed to its tetragonal phase as confirmed by the slight split observed at the 2Ɵ ~ 45° obtained from the XRD results of the material. The globular-shaped BaTiO3 was immobilized on a 2 mm × 2 mm etched Ti sheet using a dry-drying method. Results obtained from the sonoelectrochemical degradation of the CIP using the Ti/BaTiO3 working electrode showed that a degradation efficiency of 68.42% was attained after 180 min at optimum operating conditions of 2.0 V applied potential, 40 W ultrasound power and 10 mm distance maintained between the Ti/BaTiO3 electrode and the ultrasound probe. This result showed that the activated in-built polarization of the Ti/BaTiO3 electrode was sustained by the continuous presence of ultrasound irradiation in the system, thereby aiding the generation of reactive species responsible for the degradation of ciprofloxacin in wastewater.
The sonoelectrochemical degradation ability of BaTiO3 was further improved by immobilizing a synthesized composite of BaTiO3/NiTiO3 on fluorine-doped tin oxide (FTO glass). The prepared FTO-BaTiO3/NiTiO3 electrode was applied for the simultaneous sonoelectrochemical degradation of zidovudine (ZVD) and paracetamol (PCM) in wastewater. At optimal conditions (2.0 V applied potential, 40 W ultrasound power and 10 mm electrode-probe distance), degradation efficiencies of 68.10% and 57.86% were obtained for ZVD and PCM, respectively after 180 min. The result showed that the presence of the ferroelectric NiTiO3 with
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a better electrocatalytic property (as obtained from the EIS) on the electrode improved the piezo-polarization effect of the FTO-BaTiO3/NiTiO3 electrode, thereby aiding an enhancement in the degradation efficiency of the process on recalcitrant organic pollutants in wastewater.
To simultaneously improve the piezoelectric property as well as the electrochemical activity of the titanate-based piezoelectric material, BaZrTiO3 was prepared by introducing zirconium ion (Zr4+) with larger ionic radii into the structure of BaTiO3. A tetragonal structure comparable to that of BaTiO3 particle was obtained with a slight 2Ɵ shift to the left observed for the synthesized BaZrTiO3 particle observed from the XRD results of the materials. The BaZrTiO3 particle was immobilized on FTO glass to produce FTO-BaZr0.1Ti0.9O3 electrode. Electrochemical impedance spectroscopy (EIS), (CV) and (CA), confirmed an improvement in the electrochemical and piezo-polarization activities, respectively of the BaZrTiO3 electrode with an increased presence of Zr in its structure. The prepared FTO-BaZr0.1Ti0.9O3 electrode was applied as the working anode for the (SEC) degradation of sulfamethoxazole (SMX) in wastewater. An improved degradation efficiency of 86.16% and 63.16% total organic carbon (TOC) removal was obtained with FTO-BaZr0.1Ti0.9O3 electrode during the process. This result further revealed that the positive influence of acoustic cavitation and streaming produced from the ultrasound source, coupled with the mechanical stability of the electrode was instrumental to the efficiency of the process, which will also be useful in real-life applications. The proposed degradation pathways further revealed that the mineralization of SMX proceeded through rapid oxidation of the isoxazole ring oxidation on the analyte.
Furthermore, the study showed that the mineralization of many organic pollutants can be improved through the combination of two electrochemical advanced oxidation processes. In this work, sonoelectrochemical oxidation and electro-Fenton oxidation were combined for the mineralization of aspirin in wastewater. This was achieved by using Ti/BaZrTiO3 electrode as the anode while carbon-felt was applied as the cathode was used. Iron (II) (Fe2+) was employed as the Fenton reagent during the sonoelectro-Fenton process. The hybrid system achieved a degradation efficiency of 96.36% within 120 min which is a far improvement from the degradation efficiencies of 69.45% and 78.84% obtained during sonoelectrochemical and electro-Fenton oxidation, respectively. This reveals that electro-Fenton oxidation can greatly improve the performance of sonoelectrochemical oxidation for water treatment purposes.
Finally, the results obtained from the investigation carried out by this study revealed that titanate-based piezoelectric materials are reliable electrode materials for sonoelectrochemical
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degradation of various pharmaceutical pollutants in wastewater and they are feasible for applications in conventional wastewater systems and processes.