A novel UV-LED hydrogen peroxide electrochemical photoreactor for point-of-use organic contaminant degradation
Graphical abstract
Introduction
Advanced oxidation processes (AOPs) are a set of chemical, photochemical, and electrochemical processes that are mainly based on the in situ generation of highly reactive oxidants, such as hydroxyl radicals (·OH), which can unselectively oxidize organic compounds (Bolton et al., 1996). One of the main advantages of AOPs compared with other water treatment methods (e.g., membrane processes) is the degradation of recalcitrant compounds with no generation of a secondary waste stream (Dewil et al., 2017). AOPs include various processes for the generation of ·OH, such as those that are ozone-based and ultraviolet (UV)-based as well as electrochemical AOPs and photocatalysis. Among these processes, the UV-hydrogen peroxide (H2O2) process is well-established for the removal of refractory organic pollutants and is currently operated in drinking water treatment facilities at full-scale (Miklos et al., 2018). H2O2 is a versatile, environmentally friendly compound with no harmful residues, as its decomposition products are oxygen and water (Sheng et al., 2011). The treatment method is based on the photolyzing the H2O2 molecule by UV radiation with a wavelength of 200–280 nm (i.e., the UV-C region), which forms two ·OH due to the homolytic scission of the oxygen-oxygen bond of the H2O2 molecule as shown in reaction 1(Oturan and Aaron, 2014). Moreover, the UV irradiation employed in the UV/H2O2 process can effectively inactivate microorganisms and disinfect water, so that the UV/H2O2 process can be applied to both microbial inactivation and chemical contaminant degradation (von Gunten, 2018).
The UV/H2O2 process is one of the processes applied to the degradation of organic pollutants in large-scale municipal water treatment plants. However, it is not readily feasible to scale down this process for point-of-use (POU) applications due to the costs and hazards related to the transportation, storage, and handling of H2O2. Therefore, the on-site generation of H2O2 has gained attention in recent years. Among the different methods of production of H2O2, which are direct synthesis from hydrogen and oxygen, photocatalysis, and two-electron oxygen reduction reaction (2e-ORR), the latter method is considered the most promising choice since it can be done at moderate pressure and temperature using earth-abundant carbon-based materials as electrodes, as opposed to the traditional anthraquinone method, which requires high pressure and special operational safety considerations (Yi et al., 2016; Zhou et al., 2019). Carbonaceous cathodes, such as graphite and activated carbon, have several advantages over other materials, including low cost, non-toxicity, high stability, high electrical conductivity, high surface area, and chemical resistance (Barros et al., 2015; Yu et al., 2015).
The 2e-ORR process is the result of the electrochemical reaction of oxygen, proton, and electron on the surface of the cathode, as illustrated in reaction 2; which is a well-known process and the basis of the widely applied electro-Fenton process (Brillas et al., 2009; Olvera-Vargas et al., 2021). Carbon-based gas diffusion electrodes (GDEs) are a class of suitable candidates as electrocatalysts for this process since they provide a three-phase boundary to increase reaction rates by favoring the diffusion of oxygen to the reaction zone so that low solubility of oxygen in water can be easily overcome (Barros et al., 2015; Yu et al., 2015; Perry et al., 2019). This class of electrodes is composed of two layers: the gas diffusion layer (GDL), which is in contact with the ambient air or pure oxygen and has the role of supplying reactant oxygen to the reaction zone on a diffusion basis, and the active layer, which is in contact with the electrolyte and allows the oxygen passed through the GDL to encounter the other reactants (i.e., proton and electron) and thus produce H2O2 on the surface of the electrocatalyst (Sheng et al., 2011). On the other hand, on the surface of the anode, the oxygen evolution reaction (OER; reaction 3) takes place, which produces protons which are one of the reactants in reaction 2(Cornejo et al., 2020). The overall reaction, shown in reaction 4, clearly demonstrates that H2O2 is generated from oxygen and water using an electrochemical system.
Ultraviolet light-emitting diodes (UV-LEDs) are a new source of UV radiation based on semiconductors that can emit UV rays at a single, tunable wavelength by adjusting the composition of gallium nitride (GaN) and aluminum nitride (AlN) to obtain aluminum gallium nitride (AlXGa1–XN) alloy (Kneissl et al., 2019). In contrast to traditional UV lamps, which suffer from some disadvantages, such as having a cylindrical shape, long warm-up time, and high electrical power input, UV-LEDs have a small footprint and instant on-off ability, and they can be operated with low input power, so they are recommended for POU water treatment photoreactors (Kheyrandish et al., 2017; Keshavarzfathy and Taghipour, 2019). Indeed, UV-LEDs have been studied for both water disinfection and organic pollutant degradation with promising results (Song et al., 2016; Matafonova and Batoev, 2018).
2,4-Dichlorophenoxyacetic acid (2,4-D) is a chlorinated phenoxy compound that is used as an herbicide to control broadleaf weeds. Due to its widespread use (currently third most commonly used herbicide in the world) and high solubility in water, 2,4-D and its residues leach from the root zone and have been found in surface and ground waters (Maleki et al., 2005; Samir et al., 2015). It is considered a carcinogenic agent that affects the heart, liver, and central nervous system (Silva et al., 2007). Previously studied AOPs to degrade 2,4-D include photocatalytic oxidation (Abdennouri et al., 2015; Rezaei and Mohseni, 2017), ozonation (Jaafarzadeh et al., 2017; Hama Aziz et al., 2018), and UV/H2O2 (Chu, 2001; Adak et al., 2019). In the photocatalytic oxidation process, ·OH is generated by the reaction of water with the electron gap produced as a result of irradiating a photocatalytic surface with a UV source, while in the ozonation process, ·OH is produced via a chain of reactions initiated by reaction of ozone and hydroxyl ion (Beltran, 2003).
To tackle issues associated with the POU UV/H2O2 process mentioned above, this study reports the development of a novel water treatment system enabled by the in situ generation of H2O2 which is coupled with UV-LEDs in a single unit. The electrochemical, on-site generation of H2O2 was performed on the surface of an air-breathing cathode, which is a carbon-based GDE. Thus, no aerator was employed, which aided in energy savings. The electrochemical H2O2 generation was studied in a continuous flow-through, single-pass reactor. The degradation of 2,4-D, as a persistent model organic contaminant, was then performed at 277 nm in the electrochemical UV-LED photoreactor. The effect of several operating parameters, such as UV fluence rate, initial concentration of 2,4-D, and initial pH, was investigated. Moreover, a discussion on the electric power consumption of the system and the durability of the device was provided. To the best of our knowledge, this is the first study to integrate the in situ electrochemical generation of H2O2 with UV-LEDs as the radiation source in a single unit and thoroughly discuss the influencing factors. Moreover, the degradation kinetic of 2,4-D with the UV/H2O2 process at 277 nm is reported in this work for the first time. The application of UV-LED as the radiation source in an electrochemical cell offers much flexibility in terms of the design, system integration, and operating conditions. Moreover, more control of the radiation profile was achieved by employing UV-LEDs (as opposed to UV lamps), and an optimal radiation distribution could be obtained. Besides, fluid hydrodynamics in the system was controlled by designing and implementing flow channels that provided a uniform flow profile, as well as enhanced mixing of chemical species. The integrated system developed in our research showed promising results in fast chemical contaminant degradation, and it could be applied to both chemical and microbial water treatment at the POU.
Section snippets
Materials
All of the following materials, which were supplied by Sigma-Aldrich, were of American Chemical Society (ACS) reagent grade or higher and used with no further purification: 2,4-D (Cl2C6H3OCH2CO2H; 97%), sodium sulfate (Na2SO4; ≥99.0%), acetonitrile (CH3CN; ≥99.9%), acetic acid (CH3CO2H; ≥99.7%), hydrogen peroxide solution (H2O2, 30% w/w in H2O), potassium iodide (KI; ≥99.0%), potassium iodate (KIO3; 99.5%), ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O; 99.98%), sodium tetraborate
Results and discussion
The device performance was evaluated for both cathodic reduction of oxygen to H2O2 and organic contaminant degradation when coupled with UV-LEDs. For each set of experiments, the system was characterized according to operational conditions, such as current density and influent flow rate for H2O2 generation, and the effect of UV fluence, initial concentration of 2,4-D, and pH of the influent for 2,4-D degradation. In the end, the longevity of the system was assessed, and an electric power
Conclusions
In this study, a POU electrochemical device was developed with the aim of in situ generation of H2O2 using ambient oxygen and a dilute supporting electrolyte. This would help to produce H2O2 on demand in a safe, inexpensive, and green way. The electrochemical system was characterized based on the applied current density and fluid flow rate. A cathodic H2O2 generation rate of 7.94 mg cm−2 h−1 was obtained at a current density of 134 A m−2 and flow rate of 50 mL min−1. A high current efficiency
CRediT author statement
Seyyed Arman Hejazi: Methodology, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Visualization. Fariborz Taghipour: Conceptualization, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition.
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.
Acknowledgements
The authors would like to thank Innovate BC for its financial support. Our appreciation also extends to Kai Song for assistance with the fabrication of the electrochemical photoreactor.
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