Catalyzed oxidative coupling reactions mediated by enzyme have been proposed as an effective remediation strategy to remove micropollutants, however, little is known about how the Mn(II) redox cycling affects the horseradish peroxidase (HRP)-mediated reactions in wastewater treatment . Here, we explored the removal of two pharmaceuticals and personal care products (PPCPs), methylparaben (MeP) and acetaminophen (AAP), in HRP-mediated reaction system with dissolved Mn (II). It was found that the conversion rate of AAP was about 284 times higher than that of MeP, and Mn (II) significantly inhibited HRP-catalyzed MeP removal but had little influence on that of AAP. X-ray photoelectron spectroscopy (XPS) and theoretical calculations demonstrated that HRP converted Mn(II) into Mn(III), and then generated MnO 2 colloid, which inhibited the removal of the substrates. Moreover, the results of theoretical calculations also showed that the binding energy between HRP and Mn was 27.68 kcal/mol, which was higher than that of MeP (25.24 kcal/mol) and lower than that of AAP (30.19 kcal/mol). Therefore, when MeP and Mn (II) coexisted in the reaction system, HRP preferentially reacted with Mn(II), which explained the different impacts of Mn (II) on the removal of MeP and AAP. Additionally, Mn (II) significantly altered the product distribution by decreasing the amount of polymerization products. Overall, our work here revealed the roles of Mn (II) in the removal of MeP and AAP mediated by HRP, having strong implications for an accurate assessment of the influence of Mn(II) redox cycling on the removal of PPCPs in wastewater treatment.
The abiotic oxidation of divalent manganese (Mn(II)) and the formation of Mn oxides are important geochemical processes, which control the mobility and availability of Mn as well as element cycling and pollutant behavior in soils. It was found that iron (oxyhydr)oxides can catalyze Mn(II) oxidation , but the effects of the coexisting dissolved organic matter (DOM) molecules on the catalysis of different iron (oxyhydr)oxides for Mn(II) oxidation are poorly understood. Herein, we investigated Mn(II) oxidation under the impacts of the interactions between iron (oxyhydr)oxides (i.e., ferrihydrite, goethite and hematite) and DOM molecules. Simultaneously, we elucidated the variations of DOM composition and properties. Our results indicated that the catalysis of iron (oxyhydr)oxides for Mn(II) oxidation was significantly inhibited by DOM. Moreover, DOM had less inhibiting effect on the catalysis of ferrihydrite for Mn(II) oxidation and the formation of Mn oxides (e.g., hausmannite and buserite) relative to goethite and hematite, which was partially because of the higher electron transfer capacities of ferrihydrite. Meanwhile, DOM molecules with high nominal oxidation state of carbon (NOSC), molecular weight, unsaturation and aromaticity were selectively adsorbed and oxidized by Mn oxides , including the oxygenated phenols and polyphenols. The newly formed molecules mainly belonged to phenols depleted of oxygen and aliphatics. Furthermore, NOSC was a key molecular characteristic for controlling DOM composition during DOM adsorption and oxidation by Mn oxides when iron minerals were present. Overall, our research contributes to understanding Mn(II) oxidation mechanisms under heterogeneous systems and behaviors of DOM molecules in the environment.
Xuecong Qian,Luwei Li,Jinhao Duan,Shilong Wang,Yun Liu,Da Wang,Haijun Cheng,Jun Ma
In aqueous systems, Mn(II) oxidation by chlorine involves both electron and proton transfer. However, the slow proton transfer due to water molecule’s limited proton acceptance capacity leads to low Mn(II) oxidation efficiency under near-neutral conditions. Pyridine (pKa = 5.25), a strong proton acceptor, was selected to investigate electron–proton transfer principles in Mn(II) oxidation. Kinetic studies show that pyridine enhances Mn(II) oxidation efficiency by chlorine under neutral to alkaline conditions (pH 7.0–9.5), with maximal enhancement observed when pH approaches pyridine’s pKa. A high kinetic isotope effect (up to 6.9) and an E1/2-pH slope near the Nernstian value in cyclic voltammetry experiments confirm that pyridine induces a concerted electron–proton transfer (CEPT) pathway. Density functional theory calculations reveal that the pyridine-induced CEPT pathway is thermodynamically more favorable than stepwise electron transfer followed by proton transfer (ETPT) and proton transfer followed by electron transfer (PTET) pathways, explaining the enhanced oxidation efficiency. Similarly, in pyridine-free systems, increasing pH from 7.0 to 9.5 promotes Mn(II) oxidation efficiency through an OH–-mediated pathway switching from ETPT to PTET and CEPT pathways. This study demonstrates the controlling role of proton transfer in the Mn(II) oxidation process, providing theoretical guidance for the rational selection or design of Mn-removal catalysts.
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