A novel approach to efficient degradation of indole using co-immobilized horseradish peroxidase-syringaldehyde as biocatalyst

Visible-light-driven photo-peroxidase catalysis: high-efficiency degradation of indole in water

The demand for H2O2 restricts the wider application of horseradish peroxidase (HRP) in degradation. In this work, a novel photoenzyme synergistic catalytic system was developed for high-efficiency degrading of indole in water by HRP without extra H2O2. The HRP was immobilized on CN-ZIF prepared by the combination of g-C3N4 and ZIF-8 to achieve photo-peroxidase catalyst HRP/Zn-CN-ZIF. Under visible light, photogenerated electrons and H2O2 from HRP/Zn-CN-ZIF participated in the biocatalytic cycle of HRP directly. As a result, the indole at 20 mg L-1 in water was degraded completely in 2 h by the HRP/Zn-CN-ZIF photoenzyme synergistic catalytic system without the addition of H2O2. Furthermore, HRP/Zn-CN-ZIF exhibited superior visible light absorption and charge transfer ability compared to g-C3N4. The results of the mechanism studies suggest that ·OH would play the most significant role from the HRP/Zn-CN-ZIF in indole degradation. This research provides an efficient approach for the removal of indole from water environments.

Recent Progress on Peroxidase Modification and Application

Peroxdiase is one of the member of oxireductase super family, which has a broad substrate range and a variety of reaction types, including hydroxylation, epoxidation or halogenation of unactivated C-H bonds, and aromatic group or biophenol compounds. Here, we summarized the recently discovered enzymes with peroxidation activity, and focused on the special structures, sites, and corresponding strategies that can change the peroxidase catalytic activity, stability, and substrate range. The comparison of the structural differences between these natural enzymes and the mimic enzymes of binding nanomaterials and polymer materials is helpful to expand the application of peroxidase in industry. In addition, we also reviewed the catalytic application of peroxidase in the synthesis of important organic molecules and the degradation of pollutants.

Bio-mitigation of organic pollutants using horseradish peroxidase as a promising biocatalytic platform for environmental sustainability

A wide array of environmental pollutants is often generated and released into the ecosystem from industrial and human activities. Antibiotics, phenolic compounds, hydroquinone, industrial dyes, and Endocrine-Disrupting Chemicals (EDCs) are prevalent pollutants in water matrices. To promote environmental sustainability and minimize the impact of these pollutants, it is essential to eliminate such contaminants. Although there are multiple methods for pollutants removal, many of them are inefficient and environmentally unfriendly. Horseradish peroxidase (HRP) has been widely explored for its ability to oxidize the aforementioned pollutants, both alone and in combination with other peroxidases, and in an immobilized way. Numerous positive attributes make HRP an excellent biocatalyst in the biodegradation of diverse environmentally hazardous pollutants. In the present review, we underlined the major advancements in the HRP for environmental research. Numerous immobilization and combinational studies have been reviewed and summarized to comprehend the degradability, fate, and biotransformation of pollutants. In addition, a possible deployment of emerging computational methodologies for improved catalysis has been highlighted, along with future outlook and concluding remarks.

Mechanism of norfloxacin transformation by horseradish peroxidase and various redox mediated by humic acid and microplastics

The catalysis of HRP coupling with redox mediator was a feasible technology for the transformation of antibiotics. This work screened three effective redox mediators syringaldehyde (SYR), acetosyringone (AS) and p-coumaric acid (PCA) for the norfloxacin (NOR) transformation in HRP/redox mediator system. Then, compared their transformation characteristics under varying conditions. The molecular docking results indicated HRP catalytic mediator was spontaneous, and the absolute value order of free energy between three redox mediators and HRP was consistent with the order of NOR removal in experiment. The presence of humic acid (HA) and polystyrene (PS) microplastics could block the removal of NOR, and the inhibition effect was proportional to the level of HA and PS particles. Seven and six possible intermediate products were identified by using SYR/AS and PCA as redox mediators, respectively, and potential NOR transformation pathways were proposed. SYR and AS treatment had the same transformation products and pathways due to their similar structure, including defluorination, oxidation, cross-coupled with mediator, demethylation and dehydrogenation. While for the PCA group, NOR not only performed the above action (except defluorination), but also underwent decarbonylation. These findings may expand our knowledge of the conversion and fate of fluoroquinolones through HRP coupled with redox mediator in the environment.

Recent advances in g-C3N4-based photo-enzyme catalysts for degrading organic pollutants

In recent years, photocatalytic reactions have shown great potential in degrading organic pollutants because of their simple operation and no secondary pollution. Graphitic carbon nitride (g-C₃N₄) is one of the most frequently used photocatalyst materials in the field of photocatalysis because it is a form of photocatalytic material with facile synthesis, no metal, visible light response, and strong stability. Enzyme-catalyzed degradation has received extensive attention due to its broad selectivity, high efficiency, and environmental friendliness. Horseradish peroxidase (HRP), one of several oxidoreductases utilized for pollutant degradation, has a wide range of applications due to its mild reaction conditions and high stability. Exploring efficient platforms for immobilizing g-C₃N₄ and HRP to develop photo-enzyme-coupled catalysis is an attractive practical topic. The coupling effect of g-C₃N₄ and HRP improves the carrier separation efficiency and generates more active species, which finally realize the solar-driven non-selective destruction of organic pollutants. We describe the alteration of g-C₃N₄ and the immobilization of HRP in detail in this study, and we outline recent developments in the photo-enzyme coupling of g-C₃N₄ and HRP.

Microfluidic fabrication of tunable alginate-based microfibers for the stable immobilization of enzymes

Immobilized enzymes have drawn extensive attention due to their enhanced stability, easy separation from reaction mixture, and prominent recyclability. Nevertheless, it is still an ongoing challenge to develop potent immobilization techniques which are capable of stable enzyme encapsulation, minimal loss of activity, and modulability for various enzymes and applications. Here, microfibers with tunable size and composition were fabricated using a home-made microfluidic device. These microfibers were able to efficiently encapsulate bovine serum albumin (BSA), glucose oxidase (GOx), and horseradish peroxidase (HRP). But the physically adsorbed enzymes readily diffused into the catalytic reaction system. The leakage of enzymes could be substantially inhibited by conjugating to polyacrylic acid (PAA) and incorporating into alginate-based microfibers, enabling stable immobilization, improved recyclability, and enhanced thermostability. In addition, GOx and HRP-loaded microfibers were fabricated under the optimized conditions for the visual detection of glucose using the cascade reaction of these enzymes, showing sensitive color change to glucose with concentration range of 0-2 mM. Due to the tunability and versatility, this microfluidic-based microfiber platform may provide a valuable approach to the enzyme immobilization for the cascade catalysis and diagnoses with multiple clinical markers. This article is protected by copyright. All rights reserved.

Homologous amino acids promoted co-immobilization of laccase and mediator onto geopolymer microspheres for enhancing degradation of dyes in water

A new strategy for co-immobilization of laccase (Lac) and mediator 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) on geopolymer microspheres (GM) was reported in this work, which was promoted by pre-embedded homologous amino acids, i.e., histidine (His) and cysteine (Cys). The GM-H₂C₁ produced with a His/Cys ratio of 2:1 was highly efficient in co-immobilizing Lac and ABTS. The as-prepared composite biocatalyst (Lac-ABTS@GM-H₂C₁) exhibited the highest degradation rate (94.78%) to the model pollutant (Congo Red, CR), which was superior to free Lac-ABTS (79.23%) and Lac@GM-H₂C₁ (53.82%). The enhanced degradation efficiency of CR by the Lac-ABTS@GM-H₂C₁ was due to the promoted electron transfer and shortened mass transfer distance. Moreover, Lac-ABTS@GM-H₂C₁ demonstrated good pH resistance, competitive storage stability, and good reusability during ten cycles of CR degradation.

The performance and pathway of indole degradation by ionizing radiation

Indole is a typical recalcitrant aromatic nitrogen heterocyclic compound, which usually exists in coal chemical wastewater, and cannot be effectively removed by conventional wastewater treatment process. In this study, ionizing radiation was applied for the degradation of indole in aqueous solution. The effect of absorbed dose (1, 2, 3 and 5 kGy), initial concentration of indole (10, 20, 40 and 100 mg/L) and pH (3, 5, 7 and 9) on the degradation of indole was investigated. The results showed that the removal efficiency of indole was 99.2% at its initial concentration of 10 mg/L, absorbed dose of 2 kGy, and pH of 5. In addition, quenching experiments confirmed that three reactive species, including hydroxyl radical, hydrated electron and hydrogen radical, contributed to indole degradation. Five intermediate products were identified during indole degradation, including 3-methylindole, 3-methylinodle radicals, hydroxylation inodole, anilinoethanol and isatoic acid. The possible pathway of indole degradation was proposed. The acute toxicity and chronic toxicity of intermediate products of indole degradation were significantly reduced, except for 3-methylindole. In summary, ionizing radiation is alternative technology for the degradation of indole in coal chemical wastewater.

Studies on the degradation of trace phenol and indole odorants by chlorine and permanganate in drinking water treatment

The frequent detection of phenols and indoles in source water gives rise to concern about the taste and odor problems mainly caused by some chemicals. Exploration for the efficient removal of trace amounts of phenols and indoles in source water is imperative. This study investigated the removals and oxidation kinetics of 3-methylphenol (3-MP), 2,6-dichlorophenol (2,6-DCP), indole and 3-methylindole (3-MI) by NaClO and KMnO₄. The results showed that the selected chemical odorants could be removed by NaClO and KMnO₄. Meanwhile, the oxidation processes could be well described by the second-order kinetics model, in which kinetics constants of chemical odorants were from 1.44 × 10⁴ to 1.45 × 10⁶ L·mol^-1·min^-1 and followed the order 3-MI > indole> 3-MP> 2,6-DCP by NaClO. However, the kinetics constants for the selected chemical odorants were also determined from 1.10 × 10³ to 2.25 × 10⁴ L·mol^-1·min^-1 and in the order 2,6-DCP> 3-MI> 3-MP > indole by KMnO₄. The phenols degradation mechanisms by NaClO are chlorine substitution, and the products generated are 3,4,6-trichloro-2-methylphenol, 2,4,6-trichlorophenol, etc. And that of indoles are chlorine substitution and hydroxylation by NaClO, which generated 6-chloroindole, 2,6-dichloroaniline, etc. The phenols degradation pathways are oxidative coupling reactions by KMnO₄, and that of indoles are hydroxylation reactions by KMnO₄. This study provides a further basis for NaClO and KMnO₄ oxidation to remove trace phenols and indoles in drinking water pre-treatments.

Co-immobilized bienzyme of horseradish peroxidase and glucose oxidase on dopamine-modified cellulose–chitosan composite beads as a high-efficiency biocatalyst for degradation of acridine

Co-immobilized bienzyme biocatalysts are attracting increasing interest in the field of wastewater treatment due to their widespread application.

ABTS as an electron shuttle to accelerate the degradation of diclofenac with horseradish peroxidase-catalyzed hydrogen peroxide oxidation

Horseradish peroxidase (HRP)-catalyzed hydrogen peroxide (H₂O₂) oxidation could degrade a variety of organic pollutants, but the intrinsic drawback of slow degradation rate limited its widespread application. In this study, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) was introduced into HRP/H₂O₂ system as an electron shuttle to enhance diclofenac degradation under neutral pH conditions. The green-colored ABTS radical (ABTS^•+), generated by the oxidation of ABTS with HRP-catalyzed H₂O₂ oxidation, was proved to be the main reactive species for the rapid degradation of diclofenac in HRP/H₂O₂/ABTS system. There was no destruction of ABTS/ABTS^•+ in HRP/H₂O₂/ABTS system, and ABTS was verified as an ideal electron shuttle. The reaction conditions including solution pH (4.5-10.5), HRP concentration (0-8 units mL^-1) and H₂O₂ concentration (0-500 μM) would impact the formation of ABTS^•+, and affect the degradation of diclofenac in HRP/H₂O₂/ABTS system. Moreover, compared with Fenton and hydroxylamine/Fenton systems, HRP/H₂O₂/ABTS system had better diclofenac degradation efficiency, higher H₂O₂ utilization efficiency and stronger anti-interference capacity in actual waters. Overall, the present study provided a meaningful and promising way to enhance the degradation of organic pollutants in water with HRP-catalyzed H₂O₂ oxidation.