In situ photoelectrochemical activation of sulfite by MoS2 photoanode for enhanced removal of ammonium nitrogen from wastewater
Graphical abstract
Efficient conversion of ammonium to dinitrogen was achieved by oxysulfur radicals oxidation that generated by In situ PEC activation of sulfite.
Introduction
The ammonium nitrogen (NH3 –N) species in wastewater poses a great threat to the environment and subsequently to human health. Therefore, a variety of traditional techniques such as biological treatment, membrane separation, ion exchange, chemical oxidation, physical adsorption and breakpoint chlorination [[1], [2], [3]], and new remediation techniques such as electrochemical oxidation and stripping [4,5], have been studied for the removal of NH3–N from wastewater. However, effective, low-cost and environmental-friendly approach for removal of NH3–N from wastewater has yet to be found. As one of the advanced oxidation processes (AOPs) for potential environmental remediation, photochemical or photoelectrochemical (PEC) degradation of different types of water pollutants has been intensively studied, mainly because of their advantages such as utilization of solar energy as driving force and without input of additional chemicals by in situ generation of active oxidation species (radical species, valence band hole, etc.). For example, photocatalytic removal of ammonium nitrogen has been attempted recently [[6], [7], [8], [9], [10], [11], [12], [13], [14], [15]]. In these work, both OH radicals and valence band holes generated from photoirradiation of semiconductor photocatalysts are assumed as the main oxidants. However, the OH radicals based AOPs may not suitable for removal of ammonia species regardless of their strong oxidation power (2.80 V vs. SHE). From structural point of view, OH with strong electrophilic character has similar electronic structure with NH3 and H2O molecules, thus NH3 in H2O can not be easily and selectively attacked by OH radicals. Normally, larger organic pollutant molecules (dyes, pesticides, aromatic phenols etc.) bearing with electron-withdrawing functional groups are often targeted as model substrates for evaluating oxidation performance of OH radicals. Furthermore, complete removal of ammonia nitrogen from water cannot be achieved, since most of ammonia species are converted into nitrite and nitrate ions by the OH radicals mediated oxidation [[6], [7], [8], [9], [10], [11], [12], [13], [14], [15]].
With a rapid development of AOPs, other radical species including carbonate, sulfate, phosphate, chloride and sulfite radicals draw much attention because they can also be utilized as oxidants and extend the applications of AOPs for environmental remediation [[16], [17], [18], [19], [20]]. For example, in situ photochemical activation of sulfate to generate SO4− with bismuth phosphate photocatalyst for enhanced degradation of 2,4-dichlorophenol in water has been reported [16]. Cl and Cl2− have been demonstrated to be more efficient than OH in terms of converting ammonia species in electrochemical or photoelectrochemical systems. Hoffman’s group specifically investigated electrochemical removal of ammonia in latrine wastewater via in situ generated chlorine species utilizing chloride anion in wastewater [[17], [18], [19]]. Recently, Zhou’s group reported a novel solar-driven photoelectrocatalytic-chlorine radical reactions system based on WO3 electrode for highly selective transformation of ammonia nitrogen to N2 [20]. Compared to photochemical removal of pollutants operated in slurry system, PEC system enables easy recovery of photocatalyst and facilitates photo-generated charge separation, which has more application potential in real wastewater treatment [21,22]. In order to apply the PEC remediation technique practically, some key issues are needed to be solved, especially the development of low-cost and stable electrode materials that can efficiently utilize solar light irradiation is crucial and thus intense research has been focused on this topic [23,24].
MoS2, emerging as a flagship two-dimensional material, has semiconducting behavior with an indirect-direct band gap transition between bulk (ca 1.1–1.3 eV) and monolayer (ca 1.7–1.9 eV) form. Both theoretical calculation and experimental studies suggest that the monolayer and few-layer MoS2 have suitable band edge positions for water splitting [25]. Therefore, large number of research papers relevant to MoS2 focus on solar energy conversion related applications, such as solar cells [26], photocatalysis [27], and PEC water splitting [28]. A variety of nanostructured MoS2 materials (eg. nanoflakes, nanoflowers, quantum dots and nanosheets) have been developed as visible light sensitizers of other semiconductors in both photocatalytic and photoelectrochemical systems [[29], [30], [31]]. Although nanostructured MoS2 has been frequently investigated as visible light sensitizer of other semiconductors, the intrinsic photochemical or PEC properties of MoS2 itself have been much less investigated. Particularly, the PEC degradation of pollutants utilizing MoS2 photoanode has been rarely studied and thus it is necessary to explore the PEC degradation behavior of nanostructured MoS2.
Herein, we report our work with trying to realize in situ PEC activation of sulfite in water, in order to generate oxysulfur (SO3−, SO4− and SO5−) radicals for efficient removal of ammonia nitrogen by employing few-layered MoS2 photoanode under visible light irradiation. Firstly, the physicochemical properties of chemically exfoliated MoS2 nanosheets and the microstructure of MoS2 photoanode was specifically investigated to optimize photocurrents generation by the MoS2 photoanode under visible light irradiation. Then, visible light driven PEC activation of sulfite for removal of NH3 by the optimized MoS2 photoanode was evaluated, and the effects of different operation parameters (e.g., pH, anodic potential and oxygen concentration), degradation products and mechanism were specifically investigated. The results show that the conversion of NH3 into N2 can be markedly improved by the proposed PEC system. We hope that the proposed PEC system could be further developed as an economical, sustainable, and efficient means of ammonia nitrogen polluted wastewater treatment technology.
Section snippets
Preparation of MoS2 nanosheets
All chemical reagents used were analytical reagent grade. The MoS2 nanosheets were obtained by liquid-exfoliation method. Bulk MoS2 (100 mg, Aladdin Reagent Inc.) powder was added to beaker filling with 100 mL of ethanol/water (45 : 55 vol%) as solvent. The mixed solution was sonicated for 4 h in the ultrasonic probe processor (400 W) with simultaneous cooling by ice water recirculation. Then, the dispersion was centrifuged at 3000 rpm for 1.0 h. Next, the top 3/4 portions of the supernatants
Characterization of exfoliated MoS2 nanosheets and photoanode
Liquid phase exfoliation of bulk MoS2 has been achieved by varying the composition of liquid solvents like N-methyl-2-pyrrolidone (NMP), pure water or water/ethanol mixture, controlling the exfoliation temperature or engineering the edges of MoS2 crystals [[32], [33], [34]]. However, in order to obtain monolayer or few-layer MoS2 nanosheets, repeated long-time sonication and centrifugation processes are normally required. In this work, we tried to optimize the exfoliation parameters by varying
Conclusions
In summary, this study offers a proof-in-concept demonstration of MoS2 based PEC activation of sulfite for producing oxysulfur radicals that can efficiently convert ammonia nitrogen to dinitrogen, which is considered as an ideal conversion path in terms of complete removal of ammonia nitrogen. The advantages of the present MoS2-based PEC activation system include: (i) MoS2 is composed by earth-abundant elements and the fabrication of nanostructured MoS2 photoanode is quite simple and
Notes
The authors declare no competing financial interest
Acknowledgements
The authors thank the financial supports from the Shenzhen Technology Innovation Support (Grant No. KQJSCX20170327162043431, JSGG20170413152540284), and the NSFC (Grant No. 51708153).
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