Permanganate activation with Mn oxides at different oxidation states: Insight into the surface-promoted electron transfer mechanism

Highlights

• Mn oxides with high oxidation states exhibited higher activity for KMnO₄ activation.

• Mn(II) species in MnO reacted with KMnO₄ to produce cMnO₂.

• KMnO₄ can form stable complexes with the surface Mn(III/IV) species.

• The oxidation potential and ET reactivity of adsorbed KMnO₄ increased.

• Direct ET mechanism played a significant role in phenol degradation.

Abstract

The development of new strategies to improve the removal of organic pollutants with permanganate (KMnO₄) is a hot topic in water treatment. While Mn oxides have been extensively used in Advanced Oxidation Processes through an electron transfer mechanism, the field of KMnO₄ activation remains relatively unexplored. Interestingly, this study has discovered that Mn oxides with high oxidation states including γ-MnOOH, α-Mn₂O₃ and α-MnO₂, exhibited excellent performance to degrade phenols and antibiotics in the presence of KMnO₄. The MnO₄^- species initially formed stable complexes with the surface Mn(III/IV) species and showed an increased oxidation potential and electron transfer reactivity, caused by the electron-withdrawing capacity of the Mn species acting as Lewis acids. Conversely, for MnO and γ-Mn₃O₄ with Mn(II) species, they reacted with KMnO₄ to produce cMnO₂ with very low activity for phenol degradation. The direct electron transfer mechanism in α-MnO₂/KMnO₄ system was further confirmed through the inhibiting effect of acetonitrile and the galvanic oxidation process. Moreover, the adaptability and reusability of α-MnO₂ in complicated waters indicated its potential for application in water treatment. Overall, the findings shed light on the development of Mn-based catalysts for organic pollutants degradation via KMnO₄ activation and understanding of the surface-promoted mechanism.

Introduction

The discharge of persistent, toxic, and bioaccumulative contaminants such as industrial chemicals, personal care and pharmaceutical products into the environment poses a significant threat to both nature and humans [25]. Therefore, efficient techniques are required to address the occurrence of these pollutants in water. Recently, permanganate (KMnO₄) has gained increased attention in water treatment due to its relatively low cost, easy availability, environmental friendliness, and effectiveness over a wide pH range [17], [2], [34]. However, KMnO₄ is a selective oxidant and exhibits low reactivity with some refractory pollutants. As a result, reducing agents (bisulfite), ligands (e.g., humic acid (HA), pyrophosphate (PP)), silicate, photo and so forth, have been employed to increase the oxidation capacity of the oxidant [1], [15], [27], [38], [39], [40]. But the additional reagents and energy input might give rise to a high cost or some toxic effects. Accordingly, exploring more powerful techniques for KMnO₄ activation is in need.

Heterogeneous activation of KMnO₄ is an alternative method to enhance the elimination of pollutants and several attempts have been successfully made using Ru/TiO₂, Co₃O₄, and graphite as the catalysts [22], [31], [41]. The oxidation of low valent metal species in metal oxides by KMnO₄ produced high valent metal species such as Ru^VI, Ru^VII and Co^IV, which were suggested to contribute to the degradation of organic contaminants [31], [41]. In the presence of graphite, the degradation was mainly processed with an electron transfer mechanism with a high utilization efficiency of KMnO₄ [22]. Manganese oxides are non-toxic and inexpensive, and can be found widely in soils, sediments, and aquatic environments. It has also been reported that the reduction of KMnO₄ by organic pollutants or HA generated colloid MnO₂ (cMnO₂), to increase phenols oxidation as a catalyst, and a substance-dependent mechanism was proposed [14]. The enhanced chlorophenols degradation via KMnO₄ activation was also observed with the stabilized cMnO₂ by dissolved silicate [38]. Recent studies suggested that cMnO₂ only acted as an oxidant to degrade the pollutants under acid conditions [28], [33]. However, the interactions of KMnO₄ with oxides have not been considered until now, and the role the Mn oxides remains ambiguous.

Mn oxides such as MnO₂ have been widely used as catalysts in Advanced Oxidation Processes (AOPs) through interactions with other oxidants such as peroxymonosulfate (PMS), ferrate (Fe(VI)) and periodate (IO₄^-), and the surface-promoted mechanisms were proposed [11], [18], [29]. For example, the charge rearrangement of PMS after the formation of complexes with MnO₂ made it more reactive to degrade organic pollutants through the direct electron transfer mechanism [32]. In our previous studies, it was found that the rate of electron transfer during PMS activation with the hybrids between Mn oxides and graphite was further increased [12], [18]. A higher oxidation potential of adsorbed Fe(VI) on vacancies in cMnO₂ than that of alone Fe(VI) was observed in Fe(VI) activation system with cMnO₂ as the catalyst [19]. In MnO₂/IO₄^- system, iodate radicals and singlet oxygen were produced in the Mn(IV)-O-IO₃ complexes [6]. Different from MnO₂, Mn(VI) species and radicals in Mn₂O₃-PMS system were proposed as the main reactive species for pollutants degradation [16], [4]. For MnO@MnO_x microspheres with the lower valent Mn(II) species, sulfate radicals, hydroxyl radicals, and singlet oxygen also played an imperative role in PMS activation [30]. Thus, the mechanism for the activation of PMS and other oxidants is related to the valences of Mn oxides, and a systematic investigation of Mn oxides with different oxidation states for KMnO₄ activation should be performed to clarify the nature of the oxidation systems.

In this study, the feasibility of Mn oxides with various valences (MnO, γ-Mn₃O₄, γ-MnOOH, α-Mn₂O₃ and α-MnO₂) for KMnO₄ activation to degrade phenols and antibiotics, the typical pollutants presented in the aquatic environment, was studied. Several analyses were utilized to investigate the relationship among Mn oxides structure, Mn(VII) adsorption and pollutant degradation. The probes for electron transfer and oxygen atom transfer, as well as DFT calculation, were carried out to understand deeply the interactions of KMnO₄ with the catalyst. The influences of catalyst dosage, solution pH, as well as the common matters in waster on pollutant degradation were also studied. It is hoped to provide some research ideas for KMnO₄ activation with Mn-based catalysts for organic pollutants degradation.