Sonoelectrochemical degradation of ciprofloxacin in water on a Ti/BaTiO3 electrode
Introduction
The presence and persistence of various organic pollutants in the environment, particularly in bodies of freshwater, has highlighted the need for efficient and sustainable methods of treatment or removal. The occurrence of pharmaceuticals (an important class of these organic pollutants) in the environment and various water bodies at trace levels has been widely discussed and published in the literature [1], [2], [3], [4]. Ciprofloxacin, a fluoroquinolone antibiotic, generally used for treatment of bacterial infection, is one of the emerging pharmaceutical pollutants generally detected in water [4], [5]. Increased occurrence of antibiotics as pollutants in the environment can lead to the development of antibiotics resistive microorganisms (or antimicrobial resistant organism (AMR) which are risk to human health [6], [7], [8]. A positive correlation between the presence of ciprofloxacin in drinking water and antibiotic resistant genes was reported by Wang et al. [9]. Furthermore, antibiotics such as ciprofloxacin can also be detrimental to plant life and may find their way back into the food chain [10]. Though the concentration of ciprofloxacin is drinking water is in a low concentration range of μg/L [11], higher concentrations are found in treatment plant effluents. For example, a concentration of 31 mg/L of ciprofloxacin has been detected in the effluent of a pharmaceutical wastewater treatment plant responsible for the treatment of wastewater received from 90 pharmaceutical industries in India [12]. The removal of antibiotics in water has become a concern in recent times. Since conventional wastewater treatment plants cannot efficiently remove some of these recalcitrant pharmaceutical organics [2], [13], the need for other methods thus arises.
Sonoelectrochemical (SEC) oxidation and other advanced oxidation methods have gained increasing interest as more efficient methods for the removal of these pollutants from water. SEC oxidation is an emerging advanced oxidation process (AOP) that involves the combination of ultrasound and electrochemical oxidation process [14], [15] for the mineralization of different organic pollutants in water. This method utilizes the bubble cavitation effect of ultrasound in water to produce highly reactive intermediates such as hydroxyl (OH.) and superoxide (O2.-) radicals [16]. These species are also generated in situ during SEC process [17], [18] and they are effective towards the breaking down of organic pollutants into harmless substances. Furthermore, SEC offers variety of advantages for water treatment such as: improved mass transfer of the analyte to the electrode surface, prevention of blockage of active site of the material by continuous cleaning of electrode surface, improved degassing rate both at the surface of the electrode and in the bulk solution and fast reaction rate [14], [15], [19], [20].
The search for suitable materials and the development of stable and effective electrodes for SEC oxidation of organic substances in water has gained increasing attention in recent times. Moreover, fabrication of electrodes that can withstand varying degrees of agitation from the ultrasound source is of serious concern in SEC oxidation. Many materials including those with piezoelectric properties have found catalytic [21], [22], [23] and electrocatalytic [24] applications in the degradation of organics in water. Among the various materials frequently employed for these applications, BaTiO3 has been an outstanding material for removing various organic pollutants in water. For instance, it has been applied as catalyst during photocatalysis [25], [26], piezocatalysis [27], [28], piezo-photocatalysis [29]. However, the use of this material for sonoelectrochemical degradation remains an aspect yet to be explored. In addition, BaTiO3 can be suitable catalyst for sonoelectrochemical degradation of pharmaceutical pollutants in water due to its environmental friendliness, thermal and chemical stability, piezoelectric and high dielectric properties [30], [31], [32], [33]. These properties have been found to aid its applications in both electrical devices, sensors, and catalysis [26]. The piezoelectric effect of BaTiO3 is usually generated when mechanical disturbance from an external source interacts with the crystal structure of the material [34] thereby causing localized polarization of charges on the catalyst surface to form a built-in electric field within its crystal structure. The built-in electric field helps to coordinate and maintain the separation of free electrons-holes charges [29]. Furthermore, these charges help in the formation of reactive species such as hydroxyl and superoxide radicals which are usually responsible for the decomposition of the pollutants in water.
In this work, a titanate based BaTiO3 electrode (Ti/BaTiO3) was fabricated by drop-drying a suspension of BaTiO3 on a titanium sheet. The Ti/BaTiO3 was used as a working electrode in a three-electrode system to investigate the efficiency of the catalyst for sonoelectrochemical degradation of ciprofloxacin in synthetic wastewater and real wastewater spiked with ciprofloxacin in a measure close to the obtainable concentration in the environment. Furthermore, the catalyst was characterized using X-ray diffractometer, scanning electron microscopy and energy-dispersive X-ray spectrometer. Cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy were employed to study the electrochemical characteristics of the fabricated electrode. Furthermore, the synergistic effect for determining the effectiveness of SEC degradation was also calculated.
Section snippets
Chemicals
All reagents were purchased from Sigma Aldrich (South Africa) and were used without further purification. Barium acetate (Ba (CH3COO)2) and tetra-n-butyl orthotitanate ((C4H9O)4Ti) (TNBT) were used as the main precursors, while acetic acid (CH3COOH), ethylene glycol and acetyl acetone were used as solvent. N-methyl-2-pyrrolidone (NMP) and polyvinylidene fluoride (PVDF) were used as binding agent for coating the BaTiO3 powder on the substrate. Titanium sheet (2 cm × 2 cm) was employed as the
Structural, morphology and elemental characterisation of the synthesized BaTiO3
The X-ray diffractogram of BaTiO3 is presented in Fig. 1a. In the XRD pattern, the main diffraction peaks at 2Ɵ ≈ 22.5°, 31.5°, 39.5°, 44.8°, 45°, 56.5°, 66.5° can be clearly indexed as (100), (110), (111), (002), (200), (210), (211) and (220) respectively and these peaks are typical of tetragonal crystal phase of BaTiO3 [27], [38]. The asymmetrical splitting of the peak observed at 2Ɵ ≈ 45° into (002) and (200) indicates the transformation of the BaTiO3 cuboid phase into tetragonal symmetry
Conclusion
Barium titanate was successfully synthesized and coated on titanium sheet substrate to form Ti/BaTiO3 electrode. The electrode responded to ultrasound irradiation as shown from the sonocurrent generated. The Ti/BaTiO3 electrode was applied for the sonoeletrochemical degradation 10 ppm ciprofloxacin in water. A percentage degradation of 68.42% and TOC removal of 49.63% were calculated at best performing conditions of 40 W ultrasound power, 10 mm electrode-probe distance, and 2.0 V bias
CRediT authorship contribution statement
Babatope Ojo: Conceptualization, Methodology, Investigation, Writing – original draft. Nonhlangabezo Mabuba: Conceptualization, Methodology, Writing – review & editing, Supervision, Funding acquisition, Resources. Omotayo A. Arotiba: Conceptualization, Methodology, Writing – review & editing, Supervision, Funding acquisition, Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Financial contributions toward the work received from these South Africa institutions are delightedly acknowledged: Faculty of Science, University of Johannesburg; Centre for Nanomaterials Science Research, University of Johannesburg; National Research Foundation.
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