Research PaperActivation of molecular oxygen by tenorite and ascorbic acid: Generation of high-valent copper species for organic compound oxidation
Graphical Abstract
Introduction
Although molecular oxygen (O2), constituting 21 vol% of the Earth’s atmosphere, is the most eco-friendly and cost-effective oxidant, it is not well suited for the direct oxidation of organic pollutants in view of the spin-forbidden nature of the involved reactions (Hou et al., 2016, Zhao et al., 2013). Therefore, this oxidation is commonly achieved indirectly through the use of advanced oxidation processes (AOPs), which rely on the in situ generation of hydrogen peroxide (H2O2) or reactive oxygen species (ROS) via the activation of O2 by electrocatalysis, photocatalysis, metal catalysis, and other methods (Cao et al., 2020, Chen et al., 2012, Davison and Seed, 1983, Dey et al., 2017, Feng et al., 2022, Li et al., 2020a, Li and Li, 2021, Long et al., 2020, Mammeri et al., 2020, Mu et al., 2017, Zhou et al., 2018). Among these methods, metal ions in their reduced states (e.g., Fe(II) or Cu(I)) often promote the spontaneous reduction of O2 into ROS under mild conditions without requiring an external energy input (Davison and Seed, 1983, Hou et al., 2016, Long et al., 2020, Mammeri et al., 2020, Xu et al., 2020, Zhou et al., 2018). In particular, Cu(I) exhibits a high second-order rate constant for the reaction with O2 (4.6 × 105 M−1 s−1), which is five orders of magnitude higher than that of Fe(II) (3.9 M−1 s−1, pH 7.0) (Shen et al., 2021). Numerous studies have employed homogenous Cu(I) catalysts for O2 activation to remediate pollutants (Álvarez et al., 2020, Feng et al., 2017); however, such homogenous systems suffer from the difficulty of Cu(I) separation and reuse.
The above drawbacks can be overcome through the use of heterogeneous copper-containing materials, e.g., tenorite (CuO), which is one of the most stable and naturally abundant copper-containing minerals (Gutscher et al., 1989, Marani et al., 1995) and is widely used in AOPs (Cho et al., 2020, Fang et al., 2019, Wang et al., 2020). However, the ability of CuO to activate O2 is limited by the absence of Cu(I) species (Cu(I)) on its surface. This problem may be solved by introducing a reducing agent to convert Cu(II) into Cu(I) and thus trigger the activation of O2. Ascorbic acid (AA) is an environmentally friendly reducing agent that is found in fresh fruits and vegetables (Arrigoni and De Tullio, 2002, Hou et al., 2017a, Smirnoff and Wheeler, 2000) and features enol hydroxyl groups capable of complexing metals on the mineral surface to cause their dissolution and redox reactions (Cesario et al., 2017, Huang et al., 2017b). Consequently, AA is employed in Fenton-like systems to accelerate the Fe(III)/Fe(II) and Cu(II)/Cu(I) redox cycles on iron-/copper-based minerals and thus promote H2O2 decomposition (He et al., 2020, Huang et al., 2017a, Xiao et al., 2020). For example, Huang et al. reported that whereas alachlor was hardly degraded in a hematite/H2O2 system, a degradation efficiency of ~90% was achieved by the corresponding system in the presence of AA (Huang et al., 2017a). Xiao et al. found that the introduction of AA into the CuO/H2O2 system promotes the decomposition of H2O2 into •OH and thus accelerates rhodamine B degradation 3.4-fold (Xiao et al., 2020). Despite these studies, the feasibility and performance of AA-triggered O2 activation by CuO to in situ generate H2O2 and degrade organic contaminants have never been evaluated.
Meanwhile, the identity of the dominant reactive oxidant generated in copper-based AOPs remains controversial (Feng et al., 2017, Lee et al., 2016, Urbański and Beresewicz, 2000). For homogeneous systems, Urbañski and Berêsewicz reported that the dominant reactive oxidant generated during the activation of O2 by Cu(I) at neutral pH is •OH (Urbański and Beresewicz, 2000), as evidenced by the formation of dihydroxybenzoic acid isomers from salicylate, whereas Feng et al. claimed that Cu(III) is the dominant reactive oxidant generated in the Cu(I)/O2 system in a wide pH range (2.0–10.0), as supported by electron paramagnetic resonance (EPR) spectroscopy and experiments using radical scavengers and probes (Feng et al., 2017). For heterogeneous systems, most previous studies claim •OH to be the dominant reactive oxidant based on EPR and radical-scavenging experiments, e.g., Fang et al. suggested that CuO nanosheets activate H2O2 to generate •OH for the degradation of phenol at neutral pH (Fang et al., 2019), and Xiao et al. reported the generation of •OH in an AA-assisted CuO/H2O2 Fenton-like system for rhodamine B degradation (Xiao et al., 2020). In contrast, studies claiming Cu(III) to be the dominant reactive oxidant in heterogeneous copper systems for O2 or H2O2 activation are few (He et al., 2021). These controversies, which might arise from the biased identification methods and the similarity between the oxidation behaviors of Cu(III) and •OH, prevent us from understanding the catalytic mechanism of copper-based AOPs in detail. Therefore, systematically identifying •OH vs Cu(III), and revealing their oxidation properties are crucial to breaking these obstacles.
Herein, we investigated the activation of O2 by CuO and AA (CuO/AA) for the oxidative degradation of organic contaminants and used bisphenol A (BPA) as the main target compound. The dominant reactive oxidant generated in the above system was systematically identified on the basis of oxidant scavenging tests, EPR analyses, molecular probing, X-ray photoelectron spectroscopy (XPS) measurements, and X-ray absorption near-edge structure (XANES) characterizations. The properties of the reactive oxidant were further explored by analyzing the oxidation behavior of different target compounds and the oxidation products of BPA. Moreover, the practical applicability of the CuO/AA system was evaluated by examining pH and water constituent effects as well as reusability and optimum performance.
Section snippets
Chemicals
All reagents were used as received without further purification. CuO, Cu2O, AA, H2SO4, NaOH, CuSO4, NaCl, potassium phthalate, 4-hydroxyphenylacetic acid, horseradish peroxidase, catalase from bovine liver, p-benzoquinone (p-BQ), tert-butanol (t-BuOH), Na2HPO4, NaH2PO4, sodium thiosulfate, phenyl methyl sulfoxide (PMSO), phenyl methyl sulfone (PMSO2), hydroxylamine solution, BPA, BA, phenol (PhOH), 4-chlorophenol (4-CP), atrazine (ATR), caffeine (CAF), nitrobenzene (NB), 2-hydroxybenzoic acid
AA-induced Cu(II) reduction
ATR-FTIR spectroscopy was used to investigate the interfacial interaction between AA and the CuO surface as the first step of Cu(II) reduction (reaction (1)). As shown in Fig. 1a, the spectrum of aqueous AA mainly displayed four peaks. The signals at 1759 and 1688 cm−1 were assigned to the coupling of the C2 =C3 vibration and the carbonyl stretch (C1 =O) (Fig. 1b), while the peak at 1347 cm−1 was assigned to the vibration of C2 =C3 and C1 −O bonds, and the multiple peaks at ~1147 cm−1 were
Conclusions
This study presents a novel and promising strategy for the activation of O2 using CuO and AA for the oxidative degradation of organic contaminants. Results of experiments and DFT calculations revealed the reaction mechanism of the CuO/AA system as follows. AA formed inner-sphere complexes with Cu(II), reducing Cu(II) to Cu(I) along with its being oxidized to ascorbyl radicals. Then, AA complexed with the generated Cu(I) and notably favored the Cu(I) to activate O2 to produce H2O2. Meanwhile,
CRediT authorship Contribution statement
Na Chen: Conceptualization, Investigation, Writing – original draft. Donghyun Lee: Methodology, Validation. Min Sik Kim: Methodology. Huan Shang: Methodology. Shiyu Cao: Formal analysis. Erwin Jongwoo Park: Formal analysis. Meiqi Li: Formal analysis. Lizhi Zhang: Conceptualization, Formal analysis. Changha Lee: Conceptualization, Writing – original draft, Writing – review & editing, Supervision.
Declaration of Competing Interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Changha Lee reports financial support was provided by Korea Environmental Industry and Technology Institute.
Acknowledgement
This work was supported by the Korea Environmental Industry and Technology Institute (KEITI) through the Developing Innovative Drinking Water and Wastewater Technologies Project [grant numbers 2022002710001].
Environmental Implication
This study is the first report that evaluates the feasibility and performance of AA-triggered O2 activation by CuO. Contrary to many previous studies on copper-based heterogeneous Fenton-like reactions, which claimed hydroxyl radical to be the dominant reactive oxidant, our work employed
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