Photoelectrocatalytic degradation of microcystin-LR using Ag/AgCl/TiO2 nanotube arrays electrode under visible light irradiation
Introduction
The cyanobacterial toxins, which released from cyanobacteria, are identified as freshwater contaminating material [1], [2]. Microcystins (MC) are the most common toxigenic cyanobacteria found in lakes, rivers, ponds, potable water sources and mineral water [3]. Thus it becomes a severe poisoning compound to animals and humans who interacts with toxic blooms and contaminated water [4].
There are about 80 types of MC identified so far and in which the most common MCs are MC-LR, MC-LA, MC-YR, MC-RR, MC-LF and MC-LW. The MC-LR is most prevalently available among MCs, having around 46–99.8% of its total concentration is in natural blooms [5]. As shown in Fig. 1 MC-LR contains three d-amino acids (alanine (Ala), methylaspartic acid (MeAsp), and glutamic acid (Glu)), two unusual amino acids (N-methyldehydroalanine (Mdha) and 3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid (Adda)), and two l-amino acids (leucine (Leu) and arginine (Arg)). The common Adda side chain shared in MC types is mainly responsible for its toxicity, which irreversibly inhibits the eukaryotic serine/threonine protein phosphatases 1 (PP1) and 2A (PP2A) [6], [7]. Acute toxicity of MC-LR leads to health problems such as hepatotoxicity and neurotoxicity, kidney impairment, gastrointestinal disorders and decreased testosterone levels [5], [6]. It needs to be mentioned that the MCs are highly stable under sunlight and resist to get decomposed even under higher temperatures and UV irradiation due to their cyclic structures [8], [9]. Conventional treatment techniques such as chemical and biological degradation methods [10], [11] are found to be inefficient in removing the toxins from potable water system.
Advanced oxidation processes (AOPs) are proved to be promising techniques in degrading MCs. The earlier investigations on degradation of MC-LR in aqueous solution, by photocatalytic method using TiO2 [12], Fenton and Photo-Fenton techniques [13], and photochemical (UV/H2O2) method [14] was found to be effective, and among which the photocatalytic method using TiO2 was broadly studied. In TiO2 photocatalytic treatment system, the major reactive species responsible for degradation of MC-LR are often proposed to be hydroxyl radical (OH), which preferentially oxidizes the aromatic ring, i.e., the C4–C5/C6–C7 diene bond of the Adda chain [15], [16], the double bond of the Mdha chain, and the two free carboxylic groups in d-Glu and d-MeAsp [17]. However, TiO2 could only be activated under UV irradiations due to its large optical band gap (3.0–3.2 eV). To overcome this drawback, numerous studies have been performed to enhance the photocatalytic efficiency and to extend the absorption of TiO2 into the visible region (impurity doping, metallization and sensitization) [18], [19]. It has been reported that Ag/AgX/TiO2 (X = Cl, Br and I) is found to be an efficient photocatalyst driven by visible light [19], [20]. Wang et al. have fabricated a highly efficient and stable plasmonic photocatalytic Ag/AgCl material, suggesting that the electron-hole separation might occur smoothly in the presence of Ag0 species on the surface of AgCl under visible-light illumination [21]. Though silver halides have been extensively used as source materials for photographic films based on their photosensitivity, they have seldom been used as photocatalysts because of their instability in sunlight and high cost. According to the earlier work, the surface plasmon resonance (SPR) effect of Ag/AgX makes it feasible to synthesize a new type of active and stable photocatalyst by coupling the advantages of Ag/AgX nanoparticles with TiO2 [22]. However, the drawback of using powder-form catalyst, in which the separation and regeneration of the catalyst become quite difficult, restricts its practical application.
In this work, new visible-light-driven plasmonic photocatalyst Ag/AgCl/TiO2-NTs were prepared and its photoelectrocatalytic activity towards the degradation of MC-LR was studied. The reaction mechanism for MC-LR degradation by PEC is discussed in terms of the reactive species (h+, e−, OH, and H2O2) generated during the process. In addition, the main operating variables such as the initial pH, and the nature of anions were studied in detail to optimize the experimental conditions.
Section snippets
Preparation of Ag/AgCl/TiO2 nanotube arrays
The highly ordered TiO2 Nanotube Arrays (TiO2-NTs) was fabricated by anodic oxidation of Ti substrate in NH4F electrolyte, adopting a method described in the earlier work [23]. AgCl nanoparticles were deposited onto the ordered TiO2-NTs by electrodeposition method. A conventional three-electrode system was made by employing TiO2-NTs as cathode, a Pt sheet as anode and a saturated calomel electrode (SCE) as reference electrode. Initially, the foil of TiO2-NTs was subjected to an electrolysis run
Morphology and structure of NTs film
Fig. 2a and b show the top-views of the TiO2-NTs and the Ag/AgCl deposited TiO2-NTs, respectively. After the electrodeposition/electroreduction and photoassisted reduction process, Ag/AgCl nanoparticles distributed on the TiO2 nanotube arrays. The dimensional quality of highly ordered nanotubular structure of TiO2 was observed to be retained after the deposition of Ag/AgCl NPs, suggesting that the matrix of the ordered TiO2 was not disturbed by the deposition process. The inset shows a
Conclusions
In this work, The Ag/AgCl/TiO2-NTs electrode exhibits great potential for PEC degradation of MC-LR. The degradation of MC-LR is about 92%, and the decrease in the TOC of solution is about 77.8% after 5 h. Remarkable improvement of oxidizability for the Ag/AgCl/TiO2-NTs electrode probably benefits from enhanced visible-light harvesting and reduced recombination of photogenerated electron–hole pairs due to the synergistic effect of Ag/AgCl nanoparticles and TiO2-NTs. The mechanism studies show
Acknowledgements
This work was supported by International Science & Technology Cooperation Program of China (No. 2013DFG50150), Ministry of Science and Technology of China (No. 2010DFA22770) and the National Natural Science Foundation of China (No. 51079056/E090301). The authors thank the Analytical and Testing Center of HUST for the use of SEM, XRD and DRS equipments.
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