Nano-MoO2 activates peroxymonosulfate for the degradation of PAH derivatives

doi:10.1016/j.watres.2021.116834

Water Research

Volume 192, 15 March 2021, 116834

https://doi.org/10.1016/j.watres.2021.116834 Get rights and content

Highlights

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The first study of PAH derivatives removal in nano-MoO₂/peroxymonosulfate system.
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Nano-MoO₂ is a novel activator with high catalytic activity on peroxymonosulfate.
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Nano-MoO₂ activation of peroxymonosulfate produces SO₄^•−, ^•OH, O₂^•− and ¹O₂.
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Mo(IV) can be regenerated from the reduction of Mo(VI) on the nano-MoO₂ surface.
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It is a negative correlation between electrostatic potential and degradation rate.

Abstract

The rapid and efficient degradation of polycyclic aromatic hydrocarbon (PAH) derivatives with toxicological properties remains a substantial challenge. In this study, a cost-effective and eco-friendly catalyst, nano-MoO₂ (0.05 g L⁻¹), exhibited excellent performance in activating 4.0 mmol L⁻¹ peroxymonosulfate (PMS) for the degradation of naphthalene derivatives with 1 mg L⁻¹ in aqueous systems; these derivatives include 1-methylnaphthalene, 1-nitronaphthalene, 1-chloronaphthalene, 1-naphthylamine and 1-naphthol, with high degradation rates of 87.52%, 86.23%, 97.87%, 99.74%, and 77.16%. Nano-MoO₂ acts as an electron donor by transferring an electron causing O-O bond of PMS to cleave producing SO₄^·−, and later ^·OH. Electron paramagnetic resonance (EPR) analysis combined with free radical quenching research indicated that SO₄^·− and ^·OH dominated the degradation of naphthalene derivatives, and O₂^·− and ¹O₂ participated in the processes. X-ray photoelectron spectroscopy (XPS) revealed the transformation of Mo(IV) to Mo(V) and Mo(VI), which suggested that the activation process proceeded via electron transfer from nano-MoO₂ to PMS. The applicability of the nano-MoO₂/PMS system in influencing parameters and stability was explored. The degradation pathways were primarily elucidated for each naphthalene derivative based on the intermediates identified in the systems. The -CH₃, -NO₂, -Cl, -OH substituents increased the positive electrostatic potential (ESP) on the molecular surface of 1-methylnaphthalene, 1-nitronaphthalene, 1-chloronaphthalene, and 1-naphthol, which reduced the electrophilic reaction and electron transfer between the reactive species and pollutants, leading to a lower degradation rate of naphthalene derivatives than the parent compound. However, the effect of -NH₂ substituents is the opposite. These findings suggest that nano-MoO₂ may aid as a novel catalyst in the future remediation of environments polluted with PAH derivatives.

Graphical Abstract

Introduction

Polycyclic aromatic hydrocarbon (PAH) derivatives (methyl-, nitrogen-, chlorine- and hydroxyl-containing PAHs) are compounds that incorporate substituted groups into the molecular structure of the parent PAHs. They arise from the incomplete combustion of carbon-containing compounds and by chemical and microbial transformation of parent PAHs (Kojima et al., 2010; Qiao et al., 2016). These pollutants are widely present and more persistent in the aquatic environment because of their high polarity. They are more carcinogenic, mutagenic and estrogenic and can accumulate in sediments, then migrate and transform in the water (Ranjbar Jafarabadi et al., 2019; Trine et al., 2019). The concentration of PAH derivatives in the aquatic environment range from not-detected to µg L^-1 (Kong et al., 2021). Alkylated PAHs have light-induced genotoxicity and metabolic toxicity in aquatic organisms (Kang et al., 2016). The concentration of nitro-PAHs in Taihu Lake can reach 212 ng L^-1, and the carcinogenic risk level ranged from 2.09 × 10⁻⁷ to 5.75 × 10⁻⁵ (Kong et al., 2021). The content of halogenated PAHs with high lipophilicity in the sediments of the Pearl River Estuary reached 1.4 ng g⁻¹ to 40.9 ng g⁻¹, and was also detected in drinking water (Yuan et al., 2020); thus, frequently exposing humans and wildlife. Naphthylamines have high toxicity and carcinogenicity and are widespread in wastewater. They pose risks to aquatic ecosystems and lower dissolved oxygen (Wang et al., 2019b). Hydroxy-PAHs are widely present in printing and dyeing wastewater and are linked to health complications such as cancer, nephritis, and hemolysis (Rebekah et al., 2020). In summary, previous studies on PAH derivatives have focused on their conversion, content, and toxicity on human beings and the environment. However, there is a need to explore the efficient and green remediation of PAH derivatives in polluted water.

The current chemical method for degrading PAH derivatives is mainly advanced oxidation processes (AOPs). It's advantageous because of high efficiency and limited amount of secondary pollution (Sun et al., 2019). AOPs usually include ozonation, chlorine oxidation, Fenton/Fenton-like oxidation, photocatalytic oxidation and persulfate (including peroxymonosulfate (PMS) and peroxydisulfate (PDS)) oxidation (Ao et al., 2021). However, most of AOPs have studied the degradation of a single PAH derivative (Das et al., 2020), the degradation of derivatives with different substituents on the same parent is still unclear. The oxidation processes in the aquatic environment are based on ^·OH because of high oxidation potential (E(^·OH)=2.8V) (Ghanbari and Moradi, 2017). However, in complex water substrates containing ^·OH scavengers (such as inorganic anions and natural organics), the efficiency of AOPs may be limited, and ^·OH (t_1/2=20 ns) have a short half-life. Therefore, in recent years, employing reactive substances with higher selectivity and wide half-life (such as SO₄^·−, E(SO₄^·−)=2.5-3.1V, t_1/2=30–40 μs) have received increased attention (Ghanbari and Moradi, 2017). PMS is preferentially selected as an oxidant because it is easier to activate because of its asymmetric structure (HO-O-SO₃⁻) and superoxide O-O bond (Io-o=1.326 Å) (Guo et al., 2020). And the activated final product of PMS is SO₄²⁻, which is harmless in water. Persulfate can be activated by heat, light, and transition metal Fe, Co, Cu, Ag etc. (Lee et al., 2020). Transition metals have the advantages of convenience and outstanding catalytic efficiency, as well as their copresence in some polluted waters. However, most of the current catalyst are limited to several common transition metal compounds, and nano-molybdenum oxides have not been explored in PMS-AOPs. Furthermore, it is the first time to study how to effectively remove various PAH derivatives in aqueous systems based on PMS-AOPs and explore its mechanism.

Mo-containing minerals in nature are mainly molybdenite, wulfenite, and powellite (Giussani and Nriagu, 2011). The Mo oxidation states range from Mo(-II) to Mo(VI), and the variable oxidation state allows Mo to participate in a large number of redox reactions. Moreover, compared to other transition metals, Mo is toxicologically harmless and does not have severe health impacts (Giussani and Nriagu, 2011). Mo is present in rocks and soils at levels between 1 and 10 mg kg⁻¹, and the concentration can exceed 100 mg kg⁻¹ due to wind erosion under strongly reducing conditions near the sea (Smedley and Kinniburgh, 2017). Moreover, the groundwater concentration can reach 25 mg L⁻¹ near Mo ore districts (Giussani and Nriagu, 2011). Mo oxides generally have more advantages than other Mo-containing compounds from an engineering perspective (Wei et al., 2020). For instance, nanomolybdenum dioxide (nano-MoO₂) particles have high stability and low solubility and toxicity, and are easy to synthesize (Anh Tran et al., 2014; Song et al., 2014). MoO₂ has been only used as a cocatalyst to promote Fe(III)/Fe(II) cycling in AOPs (Ji et al., 2020; Shen et al., 2019). However, to our knowledge, the use of MoO₂ as a catalyst for PMS activation in AOPs is still being explored.

Herein, naphthalene derivatives (1-methylnaphthalene, 1-nitronaphthalene, 1-chloronaphthalene, 1-naphthylamine, 1-naphthol) of environmental concern were selected as model contaminants, and their degradation patterns in aqueous solution were primarily investigated by the nano-MoO₂ (as an activator) and PMS (as an oxidant) system.

This work has the following objectives: (a) For the first time, we use nano-MoO₂ to activate PMS for PAH derivatives' catalytic oxidation. (b) Exploring the mechanism of catalysis and degradation. (c) Finding the commonality and difference between the degradation rate and pathways of derivative, and clarifying the relationship between substituents and degradation. Those results of this work will provide insights into a new technology for the degradation of PAH derivatives, as well as an understanding of nano-MoO₂ as a new catalyst for the removal of toxic organic pollutants in aqueous matrices.

Section snippets

Chemicals

Naphthalene and its derivatives (all ≥99.0%), Peroxymonosulfate (2KHSO₅·KHSO₄·K₂SO₄, hereinafter PMS), 5,5-Dimethyl-1-pyrroline N-oxide (DMPO, 98.0%) and 2,2,6,6-tetramethylpiperidine (TEMP, ≥99.0%) were obtained from Sigma-Aldrich (Saint Louis, MO, USA). Nanomolybdenum oxides (nano-MoO_2, 99.9%; nano-MoO_3, 99.9%) were purchased from Shanghai Maikun Chemical Corporation Ltd (Shanghai, China). Tert-butyl alcohol (TBA, ≥98%), ethanol (EtOH, ≥99.8%), and MoCl₅ (≥99.6%). All water used was ultrapure

Catalytic performance of nano-MoO₂

The degradation kinetics of PAH derivatives in the nano-MoO₂/PMS system are displayed in Fig. 1(a)-(f). The results show that most PAH derivatives were not oxidized by PMS alone, except for 1-naphthylamine and 1-naphthol, which showed 100% and 36% degradation, respectively, by PMS in 180 min due to the weak oxidation ability of PMS itself (Li et al., 2016). Moreover, the degradation of naphthalene derivatives was negligible by nano-MoO₂ alone. All PAH derivatives underwent a great concentration

Discussion

A very small amount of nano-MoO₂ can have a significant catalytic effect on PMS, and all PAH derivatives can be degraded efficiently. The catalytic effect on PAH derivatives with substituents was slightly weaker than that on the parent PAH except for 1-naphthylamine, which had a higher catalytic effect than the parent PAH. Degradation was generally favored by an increase in the nano-MoO₂ loading or PMS concentration due to enhanced production of ROS. An increased nano-MoO₂ loading (i.e.,

Conclusion

Nano-MoO₂ is a promising catalyst for the activation of PMS to degrade PAH derivatives, owing to its high catalytic efficiency and limited production of secondary pollution. Nano-MoO₂ activates PMS by electron transfer to produce SO₄^·− and later ^·OH upon the reaction of SO₄^·− with H₂O and/or OH⁻. Both SO₄^·− and ^·OH are mainly dominated the degradation of PAH derivatives. Spin-trapping experiments suggest that additional transient species (O₂^·− and ¹O₂) were formed in the system but were found

Declaration of Competing Interest

None.

Acknowledgments

This work was financially supported by the National Science Fund for Distinguished Young Scholars (41925029), the National Natural Science Foundation of China (41877125) and the National Natural Science Foundation of China (42007104).

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Nano-MoO2 activates peroxymonosulfate for the degradation of PAH derivatives

Highlights

Abstract

Graphical Abstract

Introduction

Section snippets

Chemicals

Catalytic performance of nano-MoO2

Discussion

Conclusion

Declaration of Competing Interest

Acknowledgments

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Nano-MoO₂ activates peroxymonosulfate for the degradation of PAH derivatives

Catalytic performance of nano-MoO₂