In situ photoelectrochemical activation of sulfite by MoS2 photoanode for enhanced removal of ammonium nitrogen from wastewater

https://doi.org/10.1016/j.apcatb.2018.11.061Get rights and content

Highlights

  • Oxysulfur radicals based AOPs for ammonia conversion was developed.

  • Oxysulfur radicals were produced by photoelectrochemical oxidation of sulfite on MoS2.

  • Selective and efficient conversion of ammonia to dinitrogen was exclusively achieved.

  • Oxysulfur radicals are more powerful than radical dotOH for ammonia oxidation.

  • Sulfite serves as both hole scavenger and precursor of oxysulfur radicals.

Abstract

The advanced oxidation processes (AOPs) based on oxysulfur radicals (SO3radical dot, SO4radical dot and SO5radical dot) has been receiving growing attention in wastewater treatment. In this study, we report the in situ photoelectrochemical activation of sulfite to produce oxysulfur radicals with MoS2 nanosheets as a wide spectrum absorptive photoanode. At alkaline condition, the selective and efficient conversion of ammonia to dinitrogen was exclusively achieved in the presence of sulfite electrolyte under visible light irradiation. The sulfite plays multiple roles such as working as hole scavenger for improving stability of MoS2 electrode by inhibiting photo-corrosion and serving as precursor of oxysulfur radicals in the meantime. The influences of radical scavenger, dissolved oxygen and electrolyte on the photoelectrochemical, electrochemical and photochemical conversion of ammonia verified that oxysulfur radicals are more powerful than hydroxyl radicals in terms of ammonia conversion. The proposed system appears to be applicable to in situ treatment of wastewater containing of ammonia and sulfite pollutants, such as wastewater from ammonia-absorption-desulfurization of combustion smoke. This work also provides a new protocol in the design of new AOPs, where oxysulfur radicals can work together with hydroxyl radicals for simultaneous pollutants degradation and detoxification.

Graphical abstract

Efficient conversion of ammonium to dinitrogen was achieved by oxysulfur radicals oxidation that generated by In situ PEC activation of sulfite.

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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 radical dotOH radicals and valence band holes generated from photoirradiation of semiconductor photocatalysts are assumed as the main oxidants. However, the radical dotOH 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, radical dotOH 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 radical dotOH 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 radical dotOH 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 radical dotOH 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 SO4radical dot with bismuth phosphate photocatalyst for enhanced degradation of 2,4-dichlorophenol in water has been reported [16]. Clradical dot and Cl2radical dot have been demonstrated to be more efficient than radical dotOH 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 (SO3radical dot, SO4radical dot and SO5radical dot) 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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