ReviewEnhanced transformation of organic pollutants by mild oxidants in the presence of synthetic or natural redox mediators: A review
Graphical abstract
Introduction
Mild oxidants such as organic laccase and inorganic permanganate (Mn(VII)), manganese oxide (MnO2), and ferrate (Fe(VI)) have been widely studied for the treatment of different types of organic pollutants in contaminated waters and soils, which were similar selective oxidants. Laccases are copper-containing oxidases which can oxidize various aromatic compounds with electron-donating groups such as phenols and anilines (Gianfreda et al., 1999; Lee et al., 2002). The organic pollutant (RH) donates one electron in the oxidation by laccase, and forms a free radical (R·), followed by the formation of smaller molecule and monomers (Claus, 2004; Dwivedi et al., 2011; Strong and Claus, 2011). Mn(VII) has been already widely used for in situ chemical oxidation of soil and groundwater as well as for water treatment, due to its wide reactivity with electron-rich organic moieties such as phenols, anilines, thiols and olefins (Waldemer and Tratnyek, 2006; Rodríguez et al., 2007; Hu et al., 2009; Jiang et al., 2009, 2010; Jiang et al., 2010; Hu et al., 2011, 2012, 2014; Pang et al., 2014). MnO2 is an abundant natural oxidant in sediments and soils, and it can react with various types of organic pollutants, such as phenols, anilines, fluoroquinolone, and tetracyclines especially at acidic pH (Stone, 1987; Laha and Luthy, 1990; Ukrainczyk and McBride, 1993; Wang et al., 1999; Zhang and Huang, 2005; Zhang et al., 2008; Forrez et al., 2011). For the oxidation of phenols by Mn(VII)/MnO2, an intermediate between Mn(VII)/MnO2 and phenols was initially formed, and the resulting intermediate decomposed to products (Stone, 1987; Laha and Luthy, 1990; Jiang et al., 2014; Pang et al., 2019). Fe(VI) is also a mild water treatment chemical with increasing interest, and it oxidizes phenols, amines, sulfur-containing organic compounds and alcohols via one/two-electron transfer (Sharma, 2002, 2013). Fe(VI) reacting with phenols first generated phenoxy radical then formed coupling products followed by their further oxidation (Rush et al., 1995; Huang et al., 2001).
Natural mediators such as syringaldehyde (SA) acetosyringone (AS), p-coumaric acid, violuric acid, 4-hydroxybenzoic acid (4-HBA), or catechol present in the environments can greatly enhance the oxidative transformation of different types of organic pollutants by mild oxidants such as laccase and MnO2 (Park et al., 1999; Johannes and Majcherczyk, 2000; Thorn and Kennedy, 2002; Camarero et al., 2007; Gutiérrez et al., 2007; Morozova et al., 2007; Camarero et al., 2008; Song et al., 2019). Synthetic mediators 2,2′-azino-bis(3-ethylbenzothiazoline)−6-sulfonate (ABTS) and 1-hydroxybenzotriazole (HBT) are also found to enhance the oxidation of a wide range of organic pollutants including phenols, sulfonamide antibiotics, organic dyes, pulp, polycyclic aromatic hydrocarbons (PAHs), and perfluoroalkyl acids (PFAAs) by the laccase-mediator systems (LMS) (Fig. 1) (Collins et al., 1996; Majcherczyk et al., 1998; Sealey and Ragauskas, 1998; Böhmer et al., 1998; Pickard et al., 1999; Keum and Li, 2004; Camarero et al., 2005; Murugesan et al., 2006, 2007; Jeon et al., 2008; Luo et al., 2015, 2018). Recent studies have found that synthetic mediators (i.e., ABTS and HBT) could accelerate the oxidation by Mn(VII) and Fe(VI) significantly (Song et al., 2015; Dong et al., 2017; Shi et al., 2019).
Mild oxidants including laccase and Mn/Fe oxidants were widely used for the treatment of contaminated soil and groundwater as well as drinking water and wastewater. Some natural mediators from waters and soils and synthetic mediators could enhance the oxidation by these mild oxidants. Currently, several reviews on the LMS process have been published (Morozova et al., 2007; Strong and Claus, 2011). However, the review involving the oxidation by other mild oxidants such as chemical ones in the presence of mediators is very limited. Although the laccase versus Mn/Fe oxidants systems are quite different, they are similar selective oxidants and exhibit similar reaction mechanism in the oxidation of synthetic mediators (e.g., ABTS and HBT), natural mediators (e.g., SA and AS), and organic pollutants. So, it is important and necessary to provide a comprehensive assessment on the mechanism and applicability of the transformation of organic pollutants by laccase and Mn/Fe based oxidants together with natural or synthetic mediators. This review provides an assessment of the role of synthetic (ABTS and HBT) and natural mediators (SA, AS,4-HBA, and catechol) during mild oxidation processes by laccase, Mn(VII), MnO2, and Fe(VI), with a focus on the following two aspects: (i) kinetics and mechanisms of reactive species (Medox) produced from the oxidation of mediators by mild oxidation processes and (ii) reactivities and pathways of Medox reacting with different types of organic pollutants including phenols, sulfonamide antibiotics, organic dyes, pulp, PAHs, and PFAAs. This information may also help to better understand the basic principles for organic pollutants transformations by mild oxidants combined with mediators. Finally, combinations of different types of mild oxidants with synthetic or natural mediators were assessed to address the possibilities of potential applications for organic pollutants treatment.
Section snippets
Synthetic mediators: ABTS and HBT
The most commonly used synthetic mediators for enhancing organic pollutants transformations are ABTS and HBT (Morozova et al., 2007). In general, ABTS could be oxidized into a stable radical (i.e., ABTS·+) by laccase, Mn(VII), Fe(VI) and MnO2 (at acidic pH), which was further transformed into ABTS2+ by laccase and Fe(VI), and HBT could be oxidized into HBT· by laccase, Mn(VII) and Fe(VI) (Fig. 2) (Song et al., 2015; Margot et al., 2015a, 2019; Xue et al., 2020).
Organic pollutants transformations by oxidized mediators
Previous literatures have reported the oxidative transformations of organic pollutants by stable reactive species of oxidized mediators (e.g., ABTS·+, ABTS2+, and quinone-type compounds) produced from the oxidation of synthetic and natural mediators (Thorn et al., 1996; Thorn and Kennedy, 2002; Bialk et al., 2005; Tian and Schaich, 2013; Song et al., 2015; Margot et al., 2015a). Since other oxidized mediators such as HBT· and phenoxy radicals were unstable, there were little research on the
Application of mild oxidants-mediators-organic pollutants (OMP)
The efficiency of synthetic or natural mediator enhancing the oxidative transformations of organic pollutants by mild oxidants depended on the of apparent rate constants of reactions of oxidant with mediator (k1), oxidized mediator with organic pollutants (k2), and mild oxidant with organic pollutants (k3). The enhancing effect in OMP system was due to the following two conditions: (i) k1 with high enough value, (ii) much higher k2 value than k3, and relatively stable oxidized mediator form.
Future challenges
Despite the widely studied transformations of organic pollutants in the oxidation by mild oxidants with synthetic and natural mediators, there are some knowledge gaps needed to be filled:
Firstly, previous studies showed that ABTS was an effective synthetic mediator for the transformation of some organic pollutants by mild oxidants due to the high reactivity of the oxidized ABTS forms including ABTS·+ or ABTS2+ (Song et al., 2015; Hilgers et al., 2018; Xue et al., 2020). Fig. 2 shows that ABTS·+
Conclusions
Some synthetic (ABTS and HBT) or natural mediators (SA, AS) could accelerate the transformation of organic pollutants by mild oxidants including laccase, Mn(VII), MnO2, and Fe(VI) (Fig. 9). An evaluation of kinetic and mechanistic information allows the following conclusions:
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ABTS·+ could be rapidly formed in the oxidation by Mn(VII) and Fe(VI) at a wide pH range, laccase at acidic-neutral pH, and MnO2 at acidic pH. ABTS2+ could be produced in the oxidation of ABTS·+ by laccase and Fe(VI).
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was supported by the National Natural Science Foundation of China (51908143), the Young Innovative Talent Project in Higher Education of Guangdong Province (2018KQNCX194), Guangdong Key R&D Program (2019B110205004), the National Natural Science Foundation of China (51979044), Guangdong Natural Science Foundation - Outstanding Youth Program (2019B151502023), and Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (
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