Efficient peroxymonosulfate (PMS) activation by visible-light-driven formation of polymorphic amorphous manganese oxides

Highlights

• Amorphous MnOx was obtained through visible-light-driven Mn(II) oxidation.

• Amorphous MnOx-PMS can remove pollutants in a wide pH range.

• Surface Mn(III)-PMS complex is essential for PMS activation.

• Electron-transfer was regarded as the dominated pathway for AO7 degradation.

Abstract

Heterogeneous sulfate radical-based advanced oxidation processes (SR-AOPs) have been widely reported over the last decade as a promising technology for pollutant removal from wastewater. In this study, a novel peroxymonosulfate (PMS) activator was obtained by visible-light-driven Mn(II) oxidation in the presence of nitrate. The photochemically synthesized manganese oxides (PC-MnO_x) were polymorphic amorphous nanoparticles and nanorods, with an average oxidation state of approximately 3.0. It possesses effective PMS activation capacity and can remove 20 mg L⁻¹ acid organic II (AO7) within 30 min. The AO7 removal performance of PC-MnO_x was slightly decreased in natural waterbodies and in the presence of CO₃^2-, while it showed an anti-interference capacity for Cl^-, NO₃^- and humic acid. Chemical quenching, reactive oxygen species (ROS) trapping, X-ray photoelectric spectroscopy (XPS), in-situ Raman spectroscopy, and electrochemical experiments supported a nonradical mechanism, i.e., electron transfer from AO7 to the metastable PC-MnO_x-PMS complex, which was responsible for AO7 oxidation. The PC-MnO_x-PMS system also showed substrate preferences based on their redox potentials. Moreover, PC-MnO_x could activate periodate (PI) but not peroxydisulfate (PDS) or H₂O₂. Overall, this study provides a new catalyst for PMS activation through a mild and green synthesis approach.

Introduction

The dye manufacturing and textile industry often discharges spent dyes into effluents, with typical concentrations ranging from 600 to 800 mg L⁻¹ (Li et al., 2020). These dyes are usually bio-refractory, especially azo dyes that contain azo bonds (-NN-). Meanwhile, the production amount of azo dyes accounts for more than 50% of the total dye production, which makes their efficient removal an important issue. Advanced oxidation processes (AOPs), which take advantage of the strong oxidation capacities of various reactive oxygen species (ROS), have been widely used in azo dye wastewater treatment (Tufail et al., 2020). Compared with the standard Fenton process (·OH-based AOPs), peroxydisulfate (PDS) and peroxymonosulfate (PMS) are usually more favorable due to their better chemical stability and longer half-life during application (Giannakis et al., 2021). Compared with PDS, PMS is generally easily activated due to its unsymmetrical structure (Shen et al., 2020). Various homogeneous and heterogeneous catalytic PMS activations have been reported, and heterogeneous catalysts are attracting more interest due to their recoverability (Zhao et al., 2018, Chen et al., 2019a, Chen et al., 2019b, Yu et al., 2020b, Qian et al., 2021).

In recent decades, many heterogeneous PMS activators have been found and characterized (Ghanbari and Moradi, 2017). Carbocatalysts and transition metal-based catalysts are among the most investigated PMS activators (Kohantorabi et al., 2021). Carbocatalysts, including carbon nanotubes, biochar, graphene oxides, nanodiamonds, and carbon quantum dots, are usually effective PMS activators, but the high energy cost during the preparation process limits their feasible application (Chen et al., 2018). On the other hand, transition metal-based catalysts have the advantages of a large storage amount of their precursors and highly effective PMS activation. However, the potential metal leaching-caused toxicity (such as Co, Ni and Cu) is worth considering (Xiao et al., 2018). Manganese oxides (MnO_x), a type of transition metal oxide, have been demonstrated to be promising PMS activators due to their variability of valence states, high abundances, and low toxicity (Huang and Zhang, 2019a). The PMS activation capacity of MnO_x depends on both their valence states, morphology, and crystal phase (Wang et al., 2021b, Wang et al., 2021a). A systematic study on PMS activation by various MnO_x using phenol as substrate showed that Mn₂O₃ possesses the greatest activity, followed by MnO, Mn₃O₄, and MnO₂, which is related to their redox potential (Saputra et al., 2013a). Another study focused on bisphenol A degradation by PMS under acidic conditions by different structures of MnO₂ indicated that three crystalline MnO₂ have better activity than less crystalline δ-MnO₂, and α-MnO₂ has the highest PMS activation performance (Huang et al., 2019). Different morphologies with the same crystalline phase of α-MnO₂ were also reported to affect PMS activation, and phenol degradation followed the order of nanowires > nanorods > nanotubes (Wang et al., 2015). In view of active species, direct oxidation by Mn(III)/Mn(IV), radicals (sulfate radicals, hydroxyl radicals), and nonradical mechanisms (singlet oxygen or electron transfer) were all reported as dominant or coexisting pathways for MnO_x-PMS driven organic compound oxidation (Wang et al., 2015, Huang and Zhang, 2019b, Huang and Zhang, 2019c, Zhu et al., 2019, Liu et al., 2020). However, the detailed mechanism for a specific MnO_x system depends on the substrate, structure, and composition of MnO_x, and environmental conditions, which need to be unveiled case by case.

Compared with the well-crystallized phase MnO_x, reports involving amorphous MnO_x (AMO)-mediated PMS activation are limited, and the underlying activation mechanism by AMO is still debatable. Recently. Natural MnO₂ formation by simple photoirradiation in the presence of nitrate and Mn(II) was reported (Jung et al., 2017a). The characterization of photochemical formation and transformation of amorphous δ-MnO₂ were further investigated, but no attempt was made to activate PMS by this type of MnO_x (Zhang et al., 2018, Zhang et al., 2021). In this study, we facilitated the synthesis of AMO under visible light irradiation (termed PC-MnO_x as photochemically synthesized) within 2 h. A commonly used azo dye acid orange 7 (AO7) was used as the target pollutant to evaluate the PMS activation performance of PC-MnO_x. PC-MnO_x exhibited effective PMS activation performance over a wide range of pH values and excellent anti-interference capacities to various coexisting water matrices. The activation mechanism was investigated in detail by various techniques, and the nonradical pathways were confirmed. Overall, this study provides novel amorphous MnO_x materials that can be simply obtained and have potential applications in wastewater treatment.