Elsevier

Water Research

Volume 189, 1 February 2021, 116667
Water Research

Review
Enhanced transformation of organic pollutants by mild oxidants in the presence of synthetic or natural redox mediators: A review

https://doi.org/10.1016/j.watres.2020.116667Get rights and content

Highlights

  • Organic pollutants transformed by mild oxidant-mediator systems were summarized.

  • Mild oxidants react with mediators forming oxidized species with higher reactivity.

  • ABTS2+ and HBT are more broadly reactive to organic pollutants than ABTS•+.

  • Phenoxyl radicals and quinone-type compounds cross-couple with organic pollutants.

  • Future challenges on mild oxidant-mediator systems in water treatment are discussed.

Abstract

Synthetic or natural mediators (Med) can enhance the transformation of different types of organic pollutants by mild oxidants, which has been extensively studied in literature. This enhancing effect is attributed to the following two steps: (i) mild oxidants react with Med forming Medox with higher reactivity, and then (ii) these organic pollutants are more readily transformed by Medox. The present work reviews the latest findings on the formation of Medox from the reactions of synthetic (i.e., 2,2′-azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS) and 1-hydroxybenzotriazole (HBT)) or natural mediators (i.e., syringaldehyde (SA), acetosyringone (AS), p-coumaric acid, and catechol) with mild oxidants such as laccase, manganese oxidants including permanganate (Mn(VII)) and MnO2, and ferrate (Fe(VI)), as well as the transformation of organic pollutants including phenols, amines, polycyclic aromatic hydrocarbons (PAHs), organic dyes, pulp, and perfluoroalkyl acids (PFAAs) by Medox. First, reaction kinetics and mechanisms of the oxidation of synthetic or natural mediators by these mild oxidants were summarized. Reactivity and pathways of synthetic Medox including ABTS·+, ABTS2+, HBT· or natural Medox including phenoxy radicals and quinone-type compounds reacting with different organic pollutants were then discussed. Finally, the possibilities of engineering applications and new perspectives were assessed on the combinations of different types of mild oxidants with synthetic or natural mediators for the treatment of various organic pollutants.

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:

  • ·

    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 (

References (106)

  • Y. Kang et al.

    Activation of persulfate by a novel Fe(II)-immobilized chitosan/alginate composite for bisphenol A degradation

    Chem Eng J

    (2018)
  • Y.S. Keum et al.

    Fungal laccase-catalyzed degradation of hydroxy polychlorinated biphenyls

    Chemosphere

    (2004)
  • S. Kurniawati et al.

    Efficacy of mediators for enhancing the laccase-catalyzed oxidation of aqueous phenol

    Enzyme Microb Tech

    (2007)
  • A. Majcherczyk et al.

    Oxidation of polycyclic aromatic hydrocarbons (PAH) by laccase of Trametes versicolor

    Enzyme Microb Tech

    (1998)
  • J. Margot et al.

    Sulfamethoxazole and isoproturon degradation and detoxification by a laccase-mediator system: Influence of treatment conditions and mechanistic aspects

    Biochem Eng J

    (2015)
  • K. Murugesan et al.

    Decolorization of reactive dyes by a thermostable laccase produced by Ganoderma lucidum in solid state culture

    Enzyme Microb Tech

    (2007)
  • K. Murugesan et al.

    Enhanced transformation of triclosan by laccase in the presence of redox mediators

    Water Res

    (2010)
  • G.S. Nyanhongo et al.

    Decolorization of textile dyes by laccases from a newly isolated strain of Trametes modesta

    Water Res

    (2002)
  • S. Pang et al.

    Oxidation kinetics of anilines by aqueous permanganate and effects of manganese products: Comparison to phenols

    Chemosphere

    (2019)
  • L. Pereira et al.

    Enzymatic biotransformation of the azo dye Sudan Orange G with bacterial CotA-laccase

    J. Biotechnol

    (2009)
  • S. Rodríguez Couto et al.

    Influence of redox mediators and metal ions on synthetic acid dye decolourization by crude laccase from Trametes hirsuta

    Chemosphere

    (2005)
  • E. Rodríguez et al.

    Oxidation of microcystins by permanganate: reaction kinetics and implications for water treatment

    Water Res

    (2007)
  • K. Sadowska et al.

    Derivatization of single-walled carbon nanotubes with redox mediator for biocatalytic oxygen electrodes

    Bioelectrochemistry

    (2010)
  • V.K. Sharma

    Potassium ferrate(VI): an environmentally friendly oxidant

    Advances in Environmental Research

    (2002)
  • V.K. Sharma

    Ferrate(VI) and ferrate(V) oxidation of organic compounds: Kinetics and mechanism

    Coordin Chem Rev

    (2013)
  • Y. Song et al.

    Enhanced transformation of sulfonamide antibiotics by manganese(IV) oxide in the presence of model humic constituents

    Water Res

    (2019)
  • U. von Gunten

    Ozonation of drinking water: part I

    Oxidation kinetics and product formation. Water Res

    (2003)
  • Y. Wong et al.

    Laccase-catalyzed decolorization of synthetic dyes

    Water Res

    (1999)
  • M. Xue et al.

    Mechanism investigation on the formation of high valent iron intermediate in Fe(VI) oxidation using ABTS as a probe: Effect of excess Fe(VI)

    Chem Eng J

    (2020)
  • E. Almansa et al.

    Influence of structure on dye degradation with laccase mediator systems

    Biocatal Biotransfor

    (2004)
  • H.M. Bialk et al.

    Cross-coupling of sulfonamide antimicrobial agents with model humic constituents

    Environ Sci Technol

    (2005)
  • R. Bourbonnais et al.

    Reactivities of various mediators and laccases with kraft pulp and lignin model compounds

    Appl Environ Microb

    (1997)
  • B. Branchi et al.

    Kinetics of oxidation of benzyl alcohols by the dication and radical cation of ABTS

    Comparison with laccase–ABTS oxidations: an apparent paradox. Org Biomol Chem

    (2005)
  • S. Camarero et al.

    p-Hydroxycinnamic acids as natural mediators for laccase oxidation of recalcitrant compounds

    Environ Sci Technol

    (2008)
  • S. Camarero et al.

    Lignin-derived compounds as efficient laccase mediators for decolorization of different types of recalcitrant dyes

    Appl Environ Microb

    (2005)
  • A.I. Cañas et al.

    Transformation of polycyclic aromatic hydrocarbons by laccase is strongly enhanced by phenolic compounds present in soil

    Environ Sci Technol

    (2007)
  • S. Cho et al.

    Oxidation of polycyclic aromatic hydrocarbons by laccase of Coriolus hirsutus

    Biotechnol Lett

    (2002)
  • H. Claus et al.

    Redox-mediated decolorization of synthetic dyes by fungal laccases

    Appl Microbiol Biot

    (2002)
  • J.J. Coen et al.

    New insights into mechanisms of dye degradation by one-electron oxidation processes

    Journal of the Chemical Society, Perkin Transactions

    (2001)
  • P.J. Collins et al.

    Oxidation of anthracene and benzo[a]pyrene by laccases from Trametes versicolor

    Appl Environ Microb

    (1996)
  • L.M. Colosi et al.

    Peroxidase-mediated degradation of perfluorooctanoic acid

    Environ Toxicol Chem

    (2009)
  • F. D'Acunzo et al.

    Mechanistic and steric issues in the oxidation of phenolic and non-phenolic compounds by laccase or laccase-mediator systems

    The case of bifunctional substrates. New J. Chem

    (2006)
  • W. Deng et al.

    ABTS-multiwalled carbon nanotubes nanocomposite/bi film electrode for sensitive determination of Cd and Pb by differential pulse stripping voltammetry

    Electroanal

    (2009)
  • J.S. Dordick

    Catalysis of Organic Reactions

    (1992)
  • H. Forootanfar et al.

    Studies on the laccase-mediated decolorization, kinetic, and microtoxicity of some synthetic azo dyes

    J. Environ. Health Sci.

    (2016)
  • J. Fossey et al.

    Free radicals in organic chemistry

    (1995)
  • L. Gianfreda et al.

    Laccases: A useful group of oxidoreductive enzymes

    Bioremediat J

    (1999)
  • A. Gutiérrez et al.

    Removal of lipophilic extractives from paper pulp by laccase and lignin-derived phenols as natural mediators

    Environ Sci Technol

    (2007)
  • R. Hilgers et al.

    Laccase/mediator systems: Their reactivity toward phenolic lignin structures

    Acs Sustain. Chem. Eng

    (2018)
  • H. Hori et al.

    Efficient decomposition of environmentally persistent perfluorocarboxylic acids by use of persulfate as a photochemical oxidant

    Environ Sci Technol

    (2005)
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