Photoelectrocatalytic degradation of microcystin-LR using Ag/AgCl/TiO₂ nanotube arrays electrode under visible light irradiation

Highlights

• Ag/AgCl/TiO₂ is effective for the photoelectrochemical degradation of microcystin-LR (MC-LR).

• The photogenerated h⁺, OH and ${\text{O}}_2^{-}$ are the major ROS for the degradation of MC-LR.

• The degradation rate of MC-LR increases with the decrease of initial pH.

• Cl⁻ can improve the degradation of MC-LR.

Abstract

The fabrication of Ag/AgCl/TiO₂ nanotube arrays electrode was achieved by a two-step method involving an electrochemical anodization process and an electrodeposition process. The Ag/AgCl/TiO₂ nanotube arrays electrode was employed for the photoelectrocatalytic (PEC) degradation of microcystin-LR (MC-LR) and found to be an effective visible-light-driven (VLD) PEC material. The reactive oxidative species (ROS) are observed to be generated from the hole (h⁺) and electron (e⁻) in the PEC process. The mechanism for PEC degradation of the MC-LR is investigated using different scavengers and found that the major reactive species generated in situ are h⁺, OH and ${\text{O}}_2^{-}$, which are actually responsible for the degradation of MC-LR. The effect of operating parameters, such as initial pH of MC-LR solution and the type of anions, are studied. And the results show that the degradation of MC-LR was highly promoted in NaCl electrolyte, and the degradation was enhanced in an acid medium.

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, ${\text{O}}_2^{-}$ 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.