Nonradical activation of peroxymonosulfate by hematite for oxidation of organic compounds: A novel mechanism involving high-valent iron species

Highlights

• Hematite activates PMS to accelerate the oxidation of organic compounds.

• Evidence is presented against the generation of radicals and singlet oxygen.

• The electron-transfer mediation by hematite between organics and PMS does not occur.

• A unique nonradical mechanism involving high-valent iron species is proposed.

Abstract

Peroxymonosulfate (PMS) activated by Hematite (α-Fe₂O₃) was capable of degrading different organic compounds. Several lines of experimental evidence are presented which argue against the generation of radical species as well as singlet oxygen (¹O₂). Tests using radical scavengers and probes, and electron paramagnetic resonance (EPR) spectroscopy suggested that the PMS-activation by α-Fe₂O₃ does not involve the generation of SO₄^•−, ^•OH, and O₂^•−. The possibility of ¹O₂ generation was also excluded by EPR spectroscopy and kinetic experiments performed in deuterium oxide. The ternary reactive complex that mediates the electron-transfer between target organic compounds and PMS (frequently proposed as a nonradical mechanism in previous studies) was not likely formed in this system. Based on the electrochemical analyses as well as other experiments, a high-valent iron species (most likely ferryl species, Fe(IV)) is believed to be responsible for the oxidation of organic compounds by the α-Fe₂O₃/PMS system.

Introduction

In recent years, persulfates (i.e., peroxymonosulfate (PMS) and peroxydisulfate (PDS)) have been extensively studied as alternative oxidants for the oxidation of organic compounds in environmental media [1], [2], [3], [4], [5]. Applications using persulfates have mainly focused on the oxidative degradation of organic contaminants in water (including groundwater and wastewater), but other applications such as soil remediation [1], [6], [7], disinfection [8], [9], biofilm control [10], and waste sludge treatment [11], [12] have been also explored. Although persulfates themselves are strong oxidants (E°(HSO₅⁻/SO₄²⁻) = 1.75 V_NHE for PMS [13] and E°(S₂O₈²⁻/SO₄²⁻) = 1.96 V_NHE for PDS [14]), most of studies have pursued the activation of persulfates to generate more reactive forms of oxidizing species. Various methods using heat [15], [16], base [17], UV [18], [19], ultrasound [20], [21], and catalysts [22], [23], [24] have been reported for persulfate activation. Among them, catalytic activation of persulfates (particularly using heterogeneous catalysts) have been the most studied [25], [26], [27], [28], [29], [30].

Iron-based heterogeneous catalysts have been widely studied for persulfate activation because iron is environmentally benign, inexpensive, and of high catalytic activity [31]. Iron compounds such as iron oxides [32], [33], [34], [35], [36], zero-valent iron [37], [38], iron sulfide [39] and iron carbonate [40] as well as composite materials incorporating iron (e.g., spinel ferrite MFe₂O₄, Fe/montmorillonite, and Fe₃C@N-CNT [41], [42], [43], [44]) have been reported as persulfate activators. Most of these iron-based materials are suggested to decompose persulfates into reactive radical species such as sulfate and hydroxyl radicals (SO₄^•− and ^•OH) via the Fenton-like reactions [32], [33], [34], [35], [36], [37], [39], [40], [41], [42]. In a few cases, the generation of singlet oxygen (¹O₂) has also been reported [43], [44].

Hematite (α-Fe₂O₃) is the most stable iron oxide and widespread in rocks and soils [45], [46]. Hence, based upon its ability to activate persulfate, α-Fe₂O₃ can be a cost-effective material for use as a catalyst in engineered treatment systems, as well as an essential mineral component for in situ soil and groundwater remediation using persulfates. Several previous studies have demonstrated that α-Fe₂O₃ and its related materials can activate persulfates to accelerate the oxidation of organic compounds in water [32], [33], [34], [35], [36], [47], [48]; PMS was generally employed rather than PDS due to its greater performance (consistent with results in this study). In these studies, radical species such as SO₄^•− and ^•OH have been proposed as the main reactive oxidants responsible for the degradation of target organic compounds.

However, contrary to the aforementioned investigations, this study found no clear evidence for the generation of SO₄^•− and ^•OH from PMS activation by α-Fe₂O₃; and indeed all of the experimental observations disproved the involvement of radical species such as SO₄^•−, ^•OH, superoxide radical anion (O₂^•−), as well as ¹O₂. This study attempted to revisit the mechanism through which α-Fe₂O₃ activates PMS for the oxidation of organic compounds. Phenol was chosen as a main target compound because phenolic compounds are commonly used for persulfate studies (abundant reference data are available for comparison), and phenol is known to be degraded via both radical and nonradical mechanisms in different activated persulfate systems [27], [40], [42], [44], [5]. In addition to phenol, organic compounds including phenol derivatives and pharmaceuticals were also tested. Kinetics experiments for the oxidative degradation of organic compounds were performed under different conditions. In order to identify the major reactive species formed in the α-Fe₂O₃/PMS system, various approaches using oxidant scavengers and probes, electrochemical analyses, electron paramagnetic resonance (EPR) spectroscopy, and X-ray absorption near edge structure (XANES) spectroscopy, among others were conducted. Based on the results obtained, a novel nonradical mechanism for the PMS activation by α-Fe₂O₃ is proposed.