Persulfate enhanced photoelectrochemical oxidation of organic pollutants using self-doped TiO2nanotube arrays: Effect of operating parameters and water matrix
Graphical abstract
Introduction
Over the past decade, advanced oxidation processes (AOPs) assisted by persulfate (PS) have gained increasing attention as viable technology to degrade toxic and recalcitrant organic pollutants by utilizing highly oxidizing radicals, such as hydroxyl (·OH) and sulfate (SO4·−) radicals. In addition to·OH (E0(·OH /OH−) = +1.89 − +2.72 VNHE, half-life = 10−3−1 µs), SO4·− has also been investigated extensively for its unique characteristics, such as (i) high redox potential with low pH dependence (E0(SO4·−/SO42−) = +2.5 − +3.1 VNHE), (ii) prolonged half-life of 30–40 µs in water, and (iii) electron abstraction as a preferred reaction mode that can cause rapid oxidative destruction of a specific class of organic contaminants (Lee et al., 2020; Oh et al., 2016).
PS activation is performed by cleaving the peroxide bond in a homolytic or heterolytic fashion through energy and electron transfers using activators, such as heat, transition metals, electricity, and photo-irradiation (Yang et al., 2019). The photoelectrochemical (PEC) process is a synergetic combination of photocatalysis (PC) and electrocatalysis (EC) carried out by applying an external electric bias to a semiconductor-based photoanode, which facilitates the separation of photogenerated charge carriers. The enhancing effect of PS on PEC performance relates to its twofold actions observed in EC and PC, i.e., reductive transformation into SO4·− and inhibition of charge recombination (Li et al., 2019; Shao et al., 2020; Zeng et al., 2016). For instance, SO4·− serves as the dominant oxidant in bisphenol-A (BPA) degradation when peroxymonosulfate (PMS) is added to the PEC system that use (Co-)BiVO4 as photoanode (Bacha et al., 2020). A valence band (VB) hole has been suggested as a major oxidizing species in oxidative BPA treatment with an identical PEC system (Shao et al., 2020), implying that the overriding role of PS is the enhancement of charge separation. As demonstrated by Zeng et al. (2016), the dual roles of PS are involved simultaneously in kinetically enhanced decomposition of Cu-EDTA by TiO2-based PEC and operate through the augmented formation yields of both ·OH and SO4·−. However, despite PEC performance improvement upon addition of PS, the prevalent use of miscellaneous heavy metals as constituents of photoanode and cathode materials, e.g., Co (Bacha et al., 2020), CuFe2O4 (Guo et al., 2018), and MnFe2O4 (Zhang et al., 2020) would likely cause secondary environmental contamination (Oh and Lim, 2019; Wacławek et al., 2017). Such secondary environmental contamination is likely, as metal-based components undergo leaching when exposed to electrochemically or photocatalytically driven redox reactions.
TiO2 is recognized as a suitable material for PEC systems because of its non-toxicity, low cost, environmental friendliness, and high physical and chemical stability (Cho et al., 2019). Among various TiO2-based nanomaterials, TiO2 nanotube arrays (TNAs) are characterized by a highly ordered open channel structure (Marien et al., 2016) and are employed widely for environmental, energy production, and storage applications (Liu et al., 2008; Park et al., 2006; Wang et al., 2005). Further, TNAs undergo in situ Ti3+ self-doping through cathodic polarization (Kim et al., 2014) without requiring (toxic) metal dopant or components. This process transforms these nanotube arrays into dark-blue-colored TNAs (hereafter bl-TNAs), with highly improved electrocatalytic activity for anodic oxidation and enhanced photocatalytic activity in ultraviolet (UV) and visible light due to the change of the electronic state in the band gap (Liao et al., 2014). Several recent studies (Cho et al., 2019; Koo et al., 2017; Liao et al., 2014; Xu et al., 2020) explored bl-TNAs for photoelectrochemical degradation of specific recalcitrant compounds. The results showed that they outperformed pristine TNAs because of their remarkably enhanced oxidant generation ability. This improvement stems from the pronounced charge separation in bl-TNAs, and from the charge transfer rate at the electrode/electrolyte interface. There is a high likelihood that adding PS would also improve the performance of bl-TNA-based PEC systems, with two primary expected benefits, namely (i) enhanced oxidant generation capability through accelerated charge separation, and (ii) formation of SO4·−, activated mainly by UV light and conduction band (CB) electrons. However, the impact of PS on the kinetics and mechanisms of photoelectrochemical organic oxidation for bl-TNAs has not been elucidated yet.
In this study, we examined a PEC system that used bl-TNAs as an anode with the addition of peroxydisulfate (PEC/PDS) for the oxidative degradation of BPA as a model compound. PDS was selected as the radical precursor instead of PMS because of its cost effectiveness, high solubility, and chemical stability (Wacławek et al., 2017; Wang and Wang, 2018). In addition, the enhancing effect of PDS in various processes, such as chemical injection (PDS only) and UVA photolysis, EC, PC, and PEC was evaluated comparatively. The treatment performance of the PEC/PDS system was monitored by varying the operating parameters, including PDS concentration, applied potential bias, initial pH, and background inorganic and organic constituents. The role of PDS in promoting treatment efficiency was assessed based on electrochemical analysis and radical scavenger experiments. Finally, to assess its practicability, the PEC/PDS system was tested for oxidative treatment of multiple organic mixtures, such as BPA, carbamazepine (CBZ), sulfamethoxazole (SMX), and 4-chlorophenol (4-CP) using surface water sampled from a pond in the Korea Institute of Science and Technology (KIST, Republic of Korea).
Section snippets
Materials
All chemicals and solvents used in this study are listed in the supporting information (Text S1). All chemicals except acetone (technical grade, 95%) were of analytical grade and used as received without further purification. Unless otherwise stated, all experiments were conducted in deionized (DI) water.
Fabrication of self-doped TNAs
The bl-TNAs were prepared by sequential anodization using Ti mesh and Pt foil as the anode and cathode, respectively. Each electrode had an identical working area of 9.5 cm2. The distance
Characterization of bl-TNAs
Top and cross-sectional SEM images of bl-TNAs (Fig. 1a, b) visually confirmed the successful growth of highly ordered open-end tubes on the Ti mesh, as well as the production of vertically aligned individual structures. The average outer diameter, inner diameter, length, and wall thickness of these structures were approximately 110.8 nm, 84.8 nm, 11.9 µm, and 13.0 nm, respectively. Structural characteristics (i.e., large internal voids and 10 µm tubes) facilitated effective UV light absorption
Conclusion
The PEC/PDS system using bl-TNAs as a photoanode enhanced organic contaminant degradation efficiencies because of the following: (i) increase in the capability for charge separation on bl-TNAs, and (ii) assistance of SO4·− production via photo-activation and CB electrons. Among various energy input conditions, the PEC/PDS system was found to be the most efficient for oxidative treatment of BPA. Whereas ·OH was confirmed as a primary oxidant, SO4·− also contributed to organic oxidation, as
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
This work was supported by the Korean Ministry of Environment (MOE) as “Global Top Project” [grant number 2016002190003] and the Korea Environment Industry & Technology Institute (KEITI) through “Project of Developing Innovative Drinking Water and Wastewater Technologies”, funded by the Korea Ministry of Environment [grant number 2019002710010)].
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