The transformation of triclosan by laccase: Effect of humic acid on the reaction kinetics, products and pathway

Thiadiazol ligand-based laccase-like nanozymes with a high Cu+ ratio for efficient removal of tetracyclines through polymerization

A promising water treatment technology involves inducing the polymerization of organic pollutants to form corresponding polymers, enabling rapid, efficient, and low CO2 emission removal of these pollutants. However, there is currently limited research on utilizing polymerization treatment technology for removing tetracyclines from water. In this study, we synthesized a laccase-mimic nanozyme (Cu-ATZ) with a high Cu+ ratio using 2-amino-1,3,4-thiadiazole as a ligand inspired by natural laccase. The Cu-ATZ exhibited enhanced resistance to more severe application conditions and improved stability compared to natural laccase, thereby demonstrating a broader range of potential applications. The excellent catalytic properties of Cu-ATZ enabled the nanozyme to be used in the polymerization process to remove tetracyclines from water. In order to simulate actual antibiotic pollution of water bodies, tetracyclines were added to the water from sewage treatment plants. Following Cu-ATZ treatment of the water sample, the chemical oxygen demand (COD) content was found to have decreased by over 80 %. In conclusion, this study presented a novel approach for tetracycline elimination from water.

A systematic study on the characteristics and applications of laccases produced by fungi: insights on their potential for biotechnologies

Laccases are polyphenol oxidase enzymes and form the enzyme complex known for their role in wood decomposition and lignin degradation. The present study aimed to systematically review the state-of-the-art trends in scientific publications on laccase enzymes of the last 10 years. The main aspects checked included the laccase-producing fungal genera, the conditions of fungal growth and laccase production, the methods of immobilization, and potential applications of laccase. After applying the systematic search method 177 articles were selected to compound the final database. Although various fungi produce laccase, most studies were Trametes and Pleurotus genera. The submerged fermentation (SmF) has been the most used, however, the use of solid-state fermentation (SSF) appeared as a promising technique to produce laccase when using agro-industrial residues as substrates. Studies on laccase immobilization showed the covalent bonding and entrapment methods were the most used, showing greater efficiency of immobilization and a high number of enzyme reuses. The main use of the laccase was in bioremediation, especially in the discoloration of dyes from the textile industry and the degradation of pharmaceutical waste. Implications and consequences of all these findings in biotechnology and environment, as well as the trends and gaps of laccase research were discussed.

Inhibit or promote? Trade-off effect of dissolved organic matter on the laccase-mediator system

A biocatalytic system comprising fungal laccase and mediators can generate phenol radicals and efficiently eliminate various triarylmethane dyes. This study systematically explores the kinetic impact of dissolved organic matter (DOM), represented by humic substance (HS consisting of 90% fulvic acid, from lignite), on the decolorization of seven typical triarylmethane dyes by Trametes versicolor laccase and twenty natural mediators. Among these, 4-hydroxybenzyl alcohol (4-HA) and methyl violet (MV) undergo in-depth investigation regarding degradation products, pathways, and reaction mechanisms. In instances where HS hampers laccase-alone decolorization, such as malachite green, Coomassie brilliant blue, bromophenol blue, and acid magenta, this inhibition may persist despite mediator introduction. Conversely, in cases where HS facilitates decolorization, such as crystalline violet and ethyl violet, most laccase-mediator systems (LMSs) can still benefit. For MV decolorization by laccase and 4-HA, HS's kinetic effect is controlled by concentration and reaction time. A 5 mg/L HS increased the decolorization rate from 50% to 67% within the first hour, whereas 10 mg/L HS only achieved 45%. After 16 h of reaction, HS's impact on decolorization rate diminishes. Furthermore, the addition of HS enhances precipitation production, probably due to its involvement in polymerization with MV and mediator. Computational simulations and spectral monitoring reveal that low HS concentrations accelerate laccase-mediated demethylation by disrupting the chromophores bound to MV, thus promoting the decolorization of MV. Conversely, inhibition by high HS concentrations stems from the competitive binding of the enzyme pocket to the mediator, and the reduction of phenol free radicals in the system. Molecular docking and kinetic simulations revealed that laccase forms complexes with both the mediator and MV. Interestingly, the decolorization of MV occurred through a non-radical mechanism in the presence of HS. This work provided a reference for screening of high catalytic performance mediators to remove triarylmethane dyes in the actual water environment.

Degradation and humification of steroidal estrogens in the soil environment: A review

Emerging pollutants are toxic and harmful chemical substances characterized by environmental persistence, bioaccumulation and biotoxicity, which can harm the ecological environment and even threaten human health. There are four categories of emerging pollutants that are causing widespread concern, namely, persistent organic pollutants, endocrine disruptors, antibiotics, and microplastics. The distribution of emerging pollutants has spatial and temporal heterogeneity, which is influenced by factors such as geographical location, climatic conditions, population density, emission amount, etc. Steroidal estrogens (SEs) discussed in this paper belong to the category of endocrine disruptors. There are generally three types of fate for SEs in the soil environment: sorption, degradation and humification. Humification is a promising pathway for the removal of SEs, especially for those that are difficult to degrade. Through humification, these difficult-to-degrade SEs can be effectively transferred or fixed, thus reducing their impact on the environment and organisms. Contrary to the well-studied process of sorption and degradation, the role and promise of the humification process for the removal of SEs has been underestimated. Based on the existing research, this paper reviews the sources, classification, properties, hazards and environmental behaviors of SEs in soil, and focuses on the degradation and humification processes of SEs and the environmental factors affecting their processes, such as temperature, pH, etc. It aims to provide references for the follow-up research of SEs, and advocates further research on the humification of organic pollutants in future studies.

Impacts of humic-based products on the microbial community structure and functions toward sustainable agriculture

Humic-based products (HPs) are carbon-rich organic amendments in the forms of extracted humic substances from manure, compost, and raw and extracted forms of lignites, coals and peats. HPs are widely used in agriculture and have beneficial effects on plants. While the agronomic benefits of HPs have been widely reported, information on their impact on the soil microbial community composition and functions is lacking, despite claims made by companies of humic substances as biostimulants. In this review, we explored published research on microbial responses with HPs application in an agronomic context. Although research data are sparse, current results suggest indirect impacts of HPs on microbial community composition and activities. HPs application changes the physico-chemical properties of the soil and influence root exudation, which in turn impact the microbial structure and function of the soil and rhizosphere. Application of HPs to the soil as biostimulants seemed to favor plant/soil beneficial bacterial community composition. HPs impacts on microbial activities that influence soil biogeochemical functioning remain unclear; existing data are also inconsistent and contradictory. The structural properties of HPs caused inconsistencies in their reported impacts on soil properties and plants. The sources of HPs and forms (whether extracted or raw), soil type, geographic location, crop species, and management strategies, among others, affect microbial communities affecting HPs efficacy as biostimulants. A more holistic approach to research encompassing multiple influential factors and leveraging the next-generation sequencing technology is needed to unravel the impacts of HPs on the soil microbiome. Addressing these knowledge gaps facilitates sustainable and efficient use of HPs as organic agricultural amendments reducing the use of chemical fertilizers.

Laccase-assisted degradation of emerging recalcitrant compounds - A review

The main objective of this review is to provide up to date, brief, irrefutable, organized data on the conducted experiments on a range of emerging recalcitrant compounds such as Diclofenac (DCF), Chlorophenols (CPs), tetracycline (TCs), Triclosan (TCS), Bisphenol A (BPA) and Carbamazepine (CBZ). These compounds were selected from the categories of pharmaceutical contaminants (PCs), endocrine disruptors (EDs) and personal care products (PCPs) on the basis of their toxicity and concentration retained in the environment. In this context, detailed mechanism of laccase mediated degradation has been conversed that laccase assisted degradation occurs by one electron oxidation involving redox potential as underlying element of the process. Further, converging towards biotechnology, laccase immobilization increased removal efficiency, storage and reusability through various experimentally conducted studies. Laccase is being considered noteworthy as mediators facilitate laccase in oxidation of non-phenolic compounds and thereby increasing its substrate range which is being discussed in further in the review. The laccase assisted degradation mechanism of each compound has been elucidated but further studies to undercover proper degradation mechanisms needs to be performed.

New insights into humic acid-boosted conversion of bisphenol A by laccase-activated co-polyreaction: Kinetics, products, and phytotoxicity

How humic acid (HA) modifies bisphenol A (BPA) conversion in exoenzyme-activated polyreaction is poorly understood. Herein, the influencing mechanism of HA on laccase-induced BPA self-polymerization was investigated, and the phytotoxicity of the produced BPA self/co-polymers was assessed for the first time. HA prominently boosted BPA elimination, and the rate constants of BPA conversion augmented from 0.61 to 1.43 h^-1 as HA level raised from 0 to 50 mg·L^-1. It is because the generated BPA-HA co-polymers promptly lowered the yields of long-chain BPA self-oligomers, consequently maintaining laccase activity through opening enzymatic substrate-binding pockets. Notably, a few BPA monomers were re-released from the loosely bound self-polymers and co-polymers, and the releasing amounts respectively were 13.9 - 22.4% and 0.3 - 0.5% at pH 2 - 11. Formation of self/co-polymers was greatly conducive to avoiding BPA biotoxicity. Compared with BPA self-polymers, the phytotoxicity of BPA co-polymers to germinated radish (Raphanus sativus L.) seeds was lower due to these covalently bound products were more complex and stable. It follows that laccase-mediated co-polymerization played a significant role in BPA conversion, contaminant detoxification, and carbon sequestration. These findings are not only beneficial to clarifying exoenzyme-activated the generation mechanism of BPA co-polymers in water, but to reusing these supramolecular aggregates in crop growth.

A pilot study for enhanced transformation of a metabolite 3,5-dichloroaniline derived from dicarboximide fungicides through immobilized laccase mediator system

This pilot investigation aimed to evaluate the removal efficiency and the underlying biocatalytic pathways of immobilized fungal laccase during the oxidative biotransformation of a non-phenolic metabolite, 3,5-dichloroaniline (3,5-DCA) derived from dicarboximide fungicides. The maximum loading of laccase on the microporous support surfaces could reach 36.4 mg/g. The immobilized laccase on the microporous support surfaces exhibited excellent thermal stability, pH adaptability, storage stability, and reusability compared to free laccase. The ILMS assay indicated that the immobilized laccase efficiently removed studied 3,5-DCA (99-100%) in the aqueous medium, within 72 h in the presence of catechol. In this study, we identified three coupling reaction products during the removal of 3,5-DCA through an ILMS assay. Based on the identified coupling reaction products, we proposed the reaction pathway for the biotransformation of 3,5-DCA by immobilized laccase, which was shown to be potentially useful in the sustainable environmental remediation of aniline metabolite (i.e., 3,5-DCA) derived from dicarboximide fungicides.

Quantification of 2-chlorohydroquinone based on interaction between N-doped carbon quantum dots probe and photolysis products in fluorescence system

As a member of chlorophenolic compounds, 2-chlorohydroquinone (H₂QCl) has been widely used as intermediates in various chemical industries and leaded to serious threat on the environment. It is urgent to develop simple and robust analytical method for sensitive and selective determination of H₂QCl. Carbon quantum dots (CQDs), a promising photoluminescence nanomaterial, have gained sufficient concern as optical sensors owing to their outstanding photochemical properties. In this work, nitrogen doped carbon quantum dots (N-CQDs) were successfully synthesized by a simple secondary hydrothermal method and applied as a fluorescent probe for the quantitation of H₂QCl. A new fluorescence region centered at excitation wavelength of 310 nm and emission wavelength of 390 nm appeared after nitrogen doping. It was found that the N-CQDs exhibited a high selectivity towards H₂QCl with sensitive fluorescence response and the fluorescence quenching of N-CQDs was linear with the concentration of H₂QCl in the range of 30-90 μM (Y = 0.0049X + 0.1255, R² = 0.996). This is the first time that the dual role of excitation light was observed in the fluorescence detection system. The ultraviolet light acted as not only the excitation energy source for N-CQDs photoluminescence, but also the light source for photolysis of H₂QCl. In the detection process, H₂QCl was degraded to p-benzoquinone by light, and then the CQDs combined with p-benzoquinone through Michael addition reaction under the action of doped nitrogen. The electron transfer from N-CQDs to the linked p-benzoquinone caused the quenching of fluorescence originated from the edge state of N-CQDs. Furthermore, this established method can be applied for the quantitative determination of H₂QCl in environmental water samples with satisfactory recoveries between 94.31 and 105.51%.

Degradation or humification: rethinking strategies to attenuate organic pollutants

The fate of organic pollutants in environmental matrices can be determined by degradation and humification. The humification process represents a promising strategy to remove organic pollutants, particularly those resistant to degradation. In contrast to the well-studied degradation process, the contribution and application prospects of the humification process for organic pollutant removal has been underestimated. The recent progress in synthesizing artificial humic substances (HS) has made directed humification of recalcitrant organic pollutants possible. This review focuses on degradation and humification of organic matter, especially recalcitrant organic pollutants. Challenges in understanding the contribution, underlying mechanisms, and artificial synthesis of HS for removing organic pollutants are also critically discussed. We advocate further investigating the humification of organic pollutants in future studies.

Suppression mechanism of model humic constituents on laccase-enabled 17β-estradiol oxidation and oligomerization

Humic constituents (HCs) are ubiquitous in the aquatic ecosystems, and contain various functional groups that seriously impact the conversion of 17β-estradiol (17β-E2) by fungal laccase. The purpose of this study was to explore the influencing mechanism of HCs on Trametes versicolor laccase-enabled 17β-E2 oxidation and oligomerization. Herein, T. versicolor-secreted laccase could rapidly convert 99.2% of 17β-E2 (rate constant = 3.7 × 10^-2 min^-1, half-life = 18.7 min) into multifarious oligomers at 25 °C and pH 5.0, by phenolic radical-caused C-C and/or C-O self-linking routes, whereas HCs with O-phenolic hydroxyl groups (O-p-OH, i.e., catechol, pyrogallol, gallic acid, and caffeic acid) dramatically suppressed 17β-E2 oligomerization. Compared with HC-free, 17β-E2 rate constants weakened 6.3-15.8 fold in the presence of HCs containing O-p-OH. It is largely because the O-p-OH was preferentially oxidized by T. versicolor laccase to create the electrophilic O-quinone monomers/oligomers. These unstable reactive O-quinone intermediates strongly reversed 17β-E2 phenolic radicals to their monomeric molecules via two proton-transfer versus two electron-transfer channels, thus intercepting 17β-E2 oxidation and oligomerization. These findings highlight new insights into the effect of HCs containing O-p-OH on T. versicolor laccase-started 17β-E2 conversion, which is beneficial to re-understanding the fate and geochemical behavior of 17β-E2 in water.

Development of a method to screen and isolate xanthine oxidase inhibitors from black bean in a single step: Hyphenation of semipreparative liquid chromatography and stepwise flow rate countercurrent chromatography

Black bean, in which isoflavones are the main active constituent, also contains saponins and monoterpenes. Soybean isoflavone is a secondary metabolite that is formed during the growth of soybean; it exhibits antioxidant and cardiovascular activities and traces estrogen-like effects. In this study, black bean isoflavones were extracted with n-butanol, and ultrafiltration-liquid chromatography-mass spectrometry was used to screen their activity. Subsequently, the inhibitors were isolated and purified using semipreparative liquid chromatography and stepwise flow rate countercurrent chromatography. Thereafter, five active compounds were identified using mass spectrometry and nuclear magnetic resonance experiments. Finally, the inhibition types of the xanthine oxidase inhibitors were determined using enzymatic kinetic studies. The IC₅₀ values of daidzin, glycitein-7-O-glucoside, genistin, daidzein, and genistein were determined to be 35.08, 56.22, 30.76, 68.79, and 95.37 μg/mL, respectively. Daidzin, genistin, and daidzein exhibited reversible inhibition, whereas glycitein-7-O-glucoside and genistein presented irreversible inhibition. This novel approach, which was based on ultrafiltration-liquid chromatography-mass spectrometry and stepwise flow rate countercurrent chromatography, is a powerful method for screening and isolating xanthine oxidase inhibitors from complex matrices. The study of enzyme inhibition types is helpful for understanding the underlying inhibition mechanism. Therefore, a beneficial platform was developed for the large-scale production of bioactive and nutraceutical ingredients.

Fungal laccase-triggered 17β-estradiol humification kinetics and mechanisms in the presence of humic precursors

Naturally-occurring phenolic acids (PAs) act as humic precursors that participate in the conversion behaviors and coupling pathways of steroidal estrogens (SEs) during laccase-triggered humification processes (L-THPs). Herein, the influences and mechanisms of PAs on Trametes versicolor laccase-evoked 17β-estradiol (E2) conversion kinetics and humification routes were explored. Fungal laccase was fleet in converting > 99% of E2, and the calculated pseudo-first-order velocity constant and half-time values were respectively 0.039 min^-1 and 17.906 min. PAs containing an O-dihydroxy moiety such as gallic acid and caffeic acid evidently hampered E2 humification owning to the yielded highly reactive O-quinones reversed E2 radicals by hydrogen transfer mechanism, implying that the inhibition effect was enormously dependent upon the number and position of the phenolic -OH present in humic precursors. Oligomers and polymers with carbon-carbon/oxygen links were tentatively found as E2 main humified species resulting from laccase-evoked successive oxidative-coupling. Note that PAs participating in the humification also encountered oxydehydrogenation, self-polymerization, and cross-binding to E2. Interestingly, the -COOH and -OCH₃ groups of PAs could be deprived in radical-caused self/co-polymerization. The generation of humified products not only circumvented the environmental risks of parent compounds but accelerated global carbon sequestration. To our knowledge, this is the first in-depth revelation of the humification pathways and related mechanisms of SEs with humic precursors in aquatic ecosystems by L-THPs.

Comparative transcriptome combined with transgenic analysis reveal the involvement of salicylic acid pathway in the response of Nicotiana tabacum to triclosan stress

Triclosan (TCS) is a highly effective antibacterial agent, which is widely distributed in wastewater and sludge. The application of sludge containing high concentration TCS in agriculture will cause physiological damage to plants. Nevertheless, little is known about the physiological and molecular mechanism of TCS to plants. So firstly the physiological and biochemical indexes of tobacco with treatment of different concentrations of TCS were evaluated in this study. The results showed that tobacco plants with TCS treatment exhibited lower germination rate, root development, photosynthesis efficiency, and higher ROS accumulation in comparison with control group. The transcriptome analysis of tobacco plants was then performed to reveal the molecular mechanism in the response of tobacco to TCS. There were 3, 819 differentially expressed genes (DEGs) were identified between groups with or without TCS treatment. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis demonstrated that these DEGs were mainly enriched in groups of the plant hormone signal transduction pathway. To further investigate the role of plant hormone, transgenic tobacco overexpressing a homologous of salicylic acid (SA) binding protein gene was used to assess the SA-mediate TCS tolerance in plant. The results showed that transgenic plants exhibited enhanced activities of antioxidant enzymes and stronger TCS resistance than wild-type ones, which verify the important role of SA signal pathway in TCS response of tobacco plants. This study could be used to better understand the key roles of plant hormones in the TCS stress response of higher plants, and find key pathways and candidate genes for phytoremediation.

Effect of Pleurotus ostreatus and Trametes versicolor on triclosan biodegradation and activity of laccase and manganese peroxidase enzymes

INTRODUCTION

Triclosan (TCS) is an extensively used antibacterial agent which has been frequently detected in different environmental compartments. Because of TCS inhibition effect on vast majority of bacterial species, it is important to explore fungal species and their involved enzymes in TCS biodegradation. The aim of this study was to compare the potential of two white rot fungi Pleurotus ostreatus and Trametes versicolor for TCS biodegradation through the whole cell culture of fungi in an aqueous culture medium. Additionally, the changes in ligninolytic enzyme activities and possible correlations and contributions of degradative enzymes during TCS biodegradation process were monitored.

MATERIAL AND METHODS

This study was carried out using a factorial experiment with a completely randomized design in three replications. factorial design in The experimental factors included: two white rot fungi Pleurotus ostreatus and Trametes versicolor and uninoculated controls which were subjected to five levels of TCS concentrations (0, 5, 10, 20, 30 and 50 μg mL-1) to assess ligninolytic enzymatic activity during biodegradation of TCS. Samples were harvested periodically at three time intervals (4, 7 and 10 days). An AB SCIEX 3200 QTRAP LC-MS/MS system was used in order to analyze the biodegradation of TCS in liquid medium.

RESULTS

Results suggested that the two white rot fungi responded differently when exposed to the different concentrations of TCS. In general, P. ostreatus exhibited more potential and ligninolytic enzymatic activity compared to T. versicolor. LC-MS/MS analyses also showed that P. ostreatus degraded TCS with higher efficiency compared to T. versicolor. In addition, almost all P. ostreatus biodegradation activity was completed within the first day of sampling. Contrasting, less efficient degradation was observed by T. versicolor, reaching around 88% of TCS biodegradation at concentration of 20 μg mL-1after 10 days. At higher TCS concentrations (≥30 μg mL-1), the growth of T. versicolor severely inhibited and led to a drop in enzymatic activity and biodegradation. Furthermore, laccase and manganese peroxidase (MnP) were determined as more involved enzymes which significantly correlated to TCS biodegradation by T. versicolor and P. ostreatus, respectively.

CONCLUSION

P. ostreatus might be considered as efficient fungus in biodegradation of high amount of TCS in environmental matrices. The results of the present study might provide insights for future investigations on potential of fungi for applications in bioaugmentation-based strategies to remove TCS from wastewater and activated sludge.

Humic Acid Mitigates the Negative Effects of High Rates of Biochar Application on Microbial Activity

Objective: Biochar and a commercial humic acid-rich product, Humac (modified leonardite), represent soil amendments with the broad and beneficial effects on various soil properties. Their combination has been scarcely tested so far, although the positive impact of their interaction might be desirable. Materials and Methods: The dehydrogenase activity (DHA), microbial biomass carbon (Cmic), soil respiration (basal and substrate-induced), enzyme activities, total carbon (Ctot), and both shoot and root biomass yield were measured and compared in the short-term pot experiment with the lettuce seedlings. The following treatments were tested: the unamended soil (control), the Humac-amended soil (0.8 g·kg−1), the biochar-amended soil (low biochar 32 g·kg−1, high biochar 80 g·kg−1), and the soil-amended with biochar + Humac. Results: The effect of both amendments on the soil pH was insignificant. The highest average values of Ctot and Cmic were detected in high biochar treatment and the highest average values of basal and substrate-induced respiration (glucose, glucosamine, alanine) were detected in the low biochar treatment. The phosphatase activity and fresh and dry lettuce aboveground biomass were the highest in the low biochar + Humac treatment. Conclusions: Even though the combination of both biochar + Humac decreased the microbial activities in the amended soil (Cmic, DHA, enzymes, substrate-induced respiration) at the low biochar dose, they mitigated the detrimental effect of the high biochar dose on respiration (all the types) and the enzyme (phosphatase, arylsulphatase) activities. In contrast to the previously published research in this issue, the effects could not be attributed to the change of the soil pH.

Influence of humic acids on fungal laccase-initiated 17α-ethynylestradiol oligomerization: Transformation kinetics and products distribution

Fungal laccase has aroused great concern in rapidly removing estrogens because of its ability to accelerate humification and oligomerization. Here, the effect of two humic acids (HAs) on the reaction kinetics and products distribution of 17α-ethynylestradiol (EE2) in laccase-initiated humification and coupling was systematically elucidated. Laccase from Trametes versicolor exhibited over 98.3% removal rate for EE2 at pH 5.0 within 120 min, while HAs invariably restrained EE2 transformation by competing with target-substrate for the enzymatic catalytic center. EE2 removal followed pseudo-first-order kinetics, and the rate constant was decreased markedly with increasing concentration of two HAs (0-60 mg L^-1). Additionally, laccase heightened the aromaticity and humification degrees (A_{250 nm}/A_{365 nm} ratio) of HAs probably due to the formation of new humic polymers such as (HA)_m and/or (HA)_m-(EE2)_n (m and n represent the number of HA and EE2 units, respectively). Three major EE2 oligomers were identified, in accordance with a mechanism involving the phenoxy radical-driven polymerization to yield a wide variety of self-coupling products. Notably, HAs diminished the extent of EE2 self-coupling but aggrandized the cross-coupling between EE2 and HAs, and the inhibition degree of EE2 self-coupling increased with the concentration of HAs. One major reason is EE2 could be covalently incorporated into humic molecules to produce (HA)_m-(EE2)_n cross-coupling products via radical-caused C-C, C-O-C, and/or C-O-C bonds, thereby reducing EE2 self-oligomerization. These findings highlight that HAs play a vital role in the fungal laccase-induced humification and oligomerization of EE2, which obviously alter the geochemical fate and transport of EE2 in natural aquatic ecosystems.

Persistence, ecological risks, and oxidoreductases-assisted biocatalytic removal of triclosan from the aquatic environment

Triclosan (TCS) has been immensely employed in health care products and consumer items, as an active agent with fungicidal and bactericidal potentialities, such as soaps, sanitizers, tubes of toothpaste, deodorants, skin creams, and so on for over last five decades. The ultimate excretory route of TCS ends in our water matrices, thus has been frequently detected with ecological and human-health related matters and hazards. Bioactive residues of TCS reach into the key atmosphere compartment through numerous routes, such as (1) scarce or ineffective elimination or degradation throughout the treatment practices, (2) abandoned landfill leachates, (3) leakage from the discarded TCS-containing materials, and so on. Such persistence and occurrence of TCS or its degraded but bioactive residues have growing attentions. Its complete removal and/or effective prevention are still challenging tasks for safeguarding the environment. Owing to the highly effective catalytic and stability potential, enzyme-based bio-degradation approaches are considered an evocative substitute for TCS mitigation from environmental matrices. As compared to enzymes in their pristine form, immobilized enzymes, with unique catalytic, stability, selectivity, and reusability profile, are of supreme and strategic interest in environmental biotechnology. Herein, an effort has been made to signify the novel bio-catalytic and bio-degradation potentialities of various oxidoreductases, including laccases, and peroxidases including soybean peroxidase, versatile manganese peroxidase, and horseradish peroxidase with suitable examples. Following a brief introduction, the focus is given to the presence of TCS in the key atmosphere compartments. Potential sources, acquaintance, and hazardous influence of TCS are also discussed with recent and relevant examples. The second half shows the TCS removal/degradation potentialities of soluble enzyme-based catalytic systems and immobilized-enzyme-based catalytic systems. Finally, the concluding remarks, along with possible future directions are given in this significant research arena.

MnO2 nanozyme-driven polymerization and decomposition mechanisms of 17β-estradiol: Influence of humic acid

Nanozymes, which display the bifunctional properties of nanomaterials and natural enzymes, are useful tools for environmental remediation. In this research, nano-MnO₂ was selected for its intrinsic enzyme-like activity to remove 17β-estradiol (E2). Results indicated that nano-MnO₂ exhibited laccase-like activity (7.22 U·mg^-1) and removed 97.3 % of E2 at pH 6. Humic acid (HA) impeded E2 removal (only 72.4 %) by competing with E2 for the catalytic sites of the MnO₂ nanozyme surface, and there was a good linear correlation between the kinetic constants and HA concentrations (R² = 0.9489). Notably, the phenolic -OH of E2 interacted with HA to yield various polymeric products via radical-driven covalent coupling, resulting in ablation of phenolic -OH but increase of ether groups in the polymeric structure. Intermediate products, including estrone, E2 homo-/hetero-oligomers, E2 hydroxylated and quinone-like products, as well as aromatic ring-opening species, were identified. Interestingly, HA hindered the extent of E2 oxidation, homo-coupling, and decomposition but accelerated E2 and HA hetero-coupling. A reasonable catalytic pathway of E2 and HA involving MnO₂ nanozyme was proposed. These findings provide novel insights into the influence of HA on MnO₂ nanozyme-driven E2 radical polymerization and decomposition, consequently favoring the ecological water restoration and the global carbon cycle.

Enhanced degradation of triclosan by cobalt manganese spinel-type oxide activated peroxymonosulfate oxidation process via sulfate radicals and singlet oxygen: Mechanisms and intermediates identification

Spinel is a kind of desirable catalyst to activate peroxymonosulfate (PMS) for chemical oxidation of organic contaminants in wastewater treatment. However, apart from classic sulfate radical based AOPs (SR-AOPs), the generation and oxidative pathways of singlet oxygen (¹O₂) by Co/Mn spinels have been little explored in PMS catalysis. In this study, spinel-type oxide Co₂Mn₁O₄ was successfully synthesized, and used as highly effective catalyst in PMS activation for heterogeneous degradation of TCS (up to 96.4% within 30 min) at initial pH of 6.8, which was also slightly impacted by coexisting ions. Based on radical scavengers and electron paramagnetic resonance (EPR) experiments, sulfate radicals and singlet oxygen (¹O₂) were unveiled to be the dominant reactive oxygen species (ROS) in Co₂Mn₁O₄/PMS system. Co₂Mn₁O₄ catalyst exhibited reversible redox properties based on the results of cyclic voltammetry (CV). More importantly, the generation of ¹O₂ might not only promote the TCS removal rate directly, but also facilitate the metal redox cycle in spinel structure in Co₂Mn₁O₄/PMS system. Finally, degradation pathways of TCS in Co₂Mn₁O₄/PMS system were proposed, which involved the breakage of ether bond and cycloaddition reaction.

Enhanced Transformation of Emerging Contaminants by Permanganate in the Presence of Redox Mediators

In this study, a permanganate/redox mediator system for enhanced transformation of a series of emerging contaminants was evaluated. The presence of various redox mediators (i.e., 1-hydroxybenzotriazole, N-hydroxyphthalimide, violuric acid, syringaldehyde, vanillin, 4-hydroxycoumarin, and p-coumaric acid) accelerated the degradation of bisphenol A (BPA) by Mn(VII). Since 1-hydroxybenzotriazole (HBT) exhibited the highest reactive ability, it was selected to further investigate the reaction mechanisms and quantify the effects of important reaction parameters on Mn(VII)/redox-mediator reactions with BPA and bisphenol AF (BPAF). Interestingly, not only HBT accelerated the degradation of BPA, but also BPA enhanced the decay of HBT. Evidence for the in situ formation of HBT^· radicals as the active oxidant responsible for accelerated BPA and BPAF degradation was obtained by radical scavenging experiments and ³¹P NMR spin trapping techniques. The routes for HBT^· radical formation involving Mn(VII) and the electron-transfer pathway from BPA/BPAF to HBT^· radicals demonstrate that the Mn(VII)/HBT system was driven by the electron-transfer mechanism. Compared to Mn(VII) alone, the presence of HBT totally inhibited self-coupling of BPA and BPAF and promoted β-scission, hydroxylation, ring opening, and decarboxylation reactions. Moreover, Mn(VII)/HBT is also effective in real waters with the order of river water > wastewater treatment plant (WWTP) effluent > deionized water.

Ecofriendly laccases treatment to challenge micropollutants issue in municipal wastewaters

In this study, a multidisciplinary approach investigated the enzymatic degradation of micropollutants in real, not modified, municipal wastewaters of a plant located in Italy. Stir Bar Sorptive Extraction combined to Gas Chromatography-Mass Spectrometric detection (SBSE-GC-MS) was applied to profile targeted pollutants in wastewaters collected after the primary sedimentation (W1) and the final effluent (W2). Fifteen compounds were detected at ng/L - μg/L, including pesticides, personal care products (PCPs) and drugs. The most abundant micropollutants were bis(2-ethylhexyl) phthalate, diethyl phthalate and ketoprofen. Laccases of Trametes pubescens MUT 2400 were very active against all the target micropollutants: except few cases, their concentration was reduced more than 60%. Chemical analysis and environmental risk do not always come together. To verify whether the treated wastewaters can represent a stressor for the aquatic ecosystem, toxicity was also evaluated. Raphidocelis subcapitata and Lepidium sativum tests showed a clear ecotoxicity reduction, even though they did not evenly respond. Two in vitro tests (E-screen test and MELN assay) were used to evaluate the estrogenic activity. Treatments already operating in the plant (e.g. activated sludge) partially reduced the estradiol equivalent concentration, and it was almost negligible after the laccases treatment. The results of this study suggest that laccases of T. pubescens are promising biocatalysts for the micropollutants transformation in wastewaters and surface waters.

Organic Contaminant Biodegradation by Oxidoreductase Enzymes in Wastewater Treatment

Organic contaminants (OCs), such as pharmaceuticals, personal care products, flame retardants, and plasticisers, are societally ubiquitous, environmentally hazardous, and structurally diverse chemical compounds whose recalcitrance to conventional wastewater treatment necessitates the development of more effective remedial alternatives. The engineered application of ligninolytic oxidoreductase fungal enzymes, principally white-rot laccase, lignin peroxidase, and manganese peroxidase, has been identified as a particularly promising approach for OC remediation due to their strong oxidative power, broad substrate specificity, low energy consumption, environmental benignity, and cultivability from lignocellulosic waste. By applying an understanding of the mechanisms by which substrate properties influence enzyme activity, a set of semi-quantitative physicochemical criteria (redox potential, hydrophobicity, steric bulk and pKa) was formulated, against which the oxidoreductase degradation susceptibility of twenty-five representative OCs was assessed. Ionisable, compact, and electron donating group (EDG) rich pharmaceuticals and antibiotics were judged the most susceptible, whilst hydrophilic, bulky, and electron withdrawing group (EWG) rich polyhalogenated compounds were judged the least susceptible. OC susceptibility scores were in general agreement with the removal rates reported for experimental oxidoreductase treatments (R2 = 0.60). Based on this fundamental knowledge, and recent developments in enzyme immobilisation techniques, microbiological enzymic treatment strategies are proposed to formulate a new generation of biological wastewater treatment processes for the biodegradation of environmentally challenging OC compounds.

Fungal laccase-mediated humification of estrogens in aquatic ecosystems

Estrogens are a category of non-degradable organic pollutants prevalent in aquatic environments with reported health risks in human and wildlife reproduction. A biotechnological approach is proposed for utilizing fungal laccase-mediated humification reactions (L-MHRs) to remove estrogens from water. Through a reactive radical-mediated C-C, C-O-C, or C-N-C covalent coupling mechanism, multifarious complex polymeric structures are generated having limited solubilities, which significantly reduces their estrogenic activity and ecotoxicity. This review highlights the available literature associated with the self/cross-coupling mechanism of fungal L-MHRs in catalyzing the single-electron oxidation of estrogens and humic acid (HA). Advances in identifying unknown estrogen-HA cross-coupling products using high-resolution mass spectrometry combined with ¹³C-isotope labeling and ¹³C NMR may provide key research directions beneficial to aquatic ecological restoration measures.

Immobilization of laccase onto meso-MIL-53(Al) via physical adsorption for the catalytic conversion of triclosan

Due to the abundant binding sites and high stability, a synthesized meso-MIL-53(Al) was selected as the backbone and used for immobilizing laccase (Lac-MIL-53(Al)) to catalytically degrade of TCS. XRD, BET and FTIR analyses proved that the carboxyl groups on PTA of meso-MIL-53(Al) could provide sufficient adsorption sites for physically immobilizing laccase through hydrogen bonds and electrostatic interactions. Although the catalytic efficiency of V_max/K_m slightly decreased from 785 to 607 min^-1 due to the mass transfer limitation upon immobilized, Lac-MIL-53(Al) showed high activity recovery (93.8%) and stability. The conformational analysis indicated the laccase could partially enter into the MOF by conformational changes without impairing laccase, although the laccase molecular (6.5 nm × 5.5 nm × 4.5 nm) was larger than the mesopore sizes of the MOF (4 nm). The kinetics indicated that Lac-MIL-53(Al) could remove 99.24% of TCS within 120 min due to the synergy effect of the adsorption of meso-MIL-53(Al) and catalytic degradation of laccase. Meanwhile, Lac-MIL-53(Al) could remain approximately 60% of activity for up to 8 times reuse without desorption. The GC/MS and LC/MS/MS analyses further confirmed that TCS could be transformed to 2, 4-DCP by laccase via the breakage of the ether bond, or to passivated dimers, trimers and tetramers by the self-coupling and oxidization of the phenoxyl radicals, and finally removed by precipitation. In summary, enzyme-MOF composite might be a potential strategy to control the micropollutants in the wastewater.

Metal and metal-oxide nanozymes: bioenzymatic characteristics, catalytic mechanism, and eco-environmental applications

Phenolic contaminants (R-OH) are a category of highly toxic organic compounds that are widespread in aquatic ecosystems and can induce carcinogenic risk to wildlife and humans; natural enzymes as green catalysts are capable of step-polymerizing these compounds to produce diverse macromolecular self-coupling products via radical-mediated C-C and C-O-C bonding at either the ortho- or para-carbon position, thereby evading the bioavailability and ecotoxicity of these compounds. Intriguingly, certain artificial metal and metal-oxide nanomaterials are known as nanozymes. They not only possess the unique properties of nanomaterials but also display intrinsic enzyme-mimicking activities. These artificial nanozymes are expected to surmount the shortcomings, such as low stability, easy inactivation, difficult recycling, and high cost, of natural enzymes, thus contributing to eco-environmental restoration. This review highlights the available studies on the enzymatic characteristics and catalytic mechanisms of natural enzymes and artificial metal and metal-oxide nanozymes in the removal and transformation of R-OH. These advances will provide key research directions beneficial to the multifunctional applications of artificial nanozymes in aquatic ecosystems.

Cu2+-assisted laccase from Trametes versicolor enhanced self-polyreaction of triclosan

Laccase-mediated humification processes (L-MHPs) can be used to polymerize and transform phenolic pollutants in water. However, the mechanism on Cu²⁺ impacts the self-polymerization of multi-purpose antimicrobial agent triclosan during L-MHPs is less understood. Here, the influence of divalent metal ions (DMIs) on Trametes versicolor laccase activity was investigated. Particularly, the performance of Cu²⁺-assisted laccase in polymerizing and transforming triclosan was systematically characterized. Compared with DMI-free, the activity of laccase was obviously accelerated with Cu²⁺ present due to copper is a vital component of laccase catalytic center. It was found that Cu²⁺-assisted laccase was effective in transforming triclosan, and the enzymatic reaction kinetic constants increased from 0.28 to 0.73 h^-1 as the Cu²⁺ concentration increased (0-3.0 mM). Identification of intermediate products revealed that laccase oxidation predominantly generated triclosan dimers, trimers, and tetramers. The presence of Cu²⁺ reinforced self-polymerization of triclosan via forming more triclosan oligomers relative to the Cu²⁺-free, which likely attributed to the enhancement of laccase activity and stability with Cu²⁺ present in L-MHPs. A possible transformation mechanism was proposed as follows: Laccase initially catalyzed the oxidation of triclosan to generate phenoxy radical intermediates, which self-coupled to each other subsequently by radical-mediated CC and COC covalent binding, forming oligomers and polymers. The growth inhibitory assays of freshwater microalgae (Chlamydomonas reinhardtii and Scenedesmus obliquus) demonstrated that the self-polymerized triclosan by L-MHPs had lower toxicity than the parent compound. These findings implied that Cu²⁺-assisted laccase was an effective strategy for rapidly self-polyreaction and detoxication of triclosan from Cu²⁺-triclosan combined polluted wastewater.

Insights into the interactions between triclosan (TCS) and extracellular polymeric substance (EPS) of activated sludge

Triclosan (TCS) contaminant has aroused wide concerns due to the high risk of converting into toxic dioxin in aquatic environments. During the wastewater treatment process, considerable amounts of TCS are accumulated in activated sludge but the mechanisms are still unclear. Especially, roles of extracellular polymeric substances (EPS), the main components of activated sludge, in TCS removal have never been addressed. In this work, the binding properties of loosely-bound EPS (LB-EPS) and tightly-bound EPS (TB-EPS) of activated sludge to TCS are investigated by fluorescence quenching approach. The influences of aquatic conditions including solution pH, ionic strength and temperature on the interactions between EPS and TCS are explored. Possible interaction mechanisms are discussed as well as the corresponding environmental implication. Results indicate that binding processes of EPS to TCS are exothermic mainly driven by the enthalpy changes. The proteins components in EPS dominate the interactions between EPS and TCS by hydrogen bond and hydrophobic interaction. The binding strength could be improved under the condition of weak alkaline and relative high ionic strength. Generally, LB-EPS exhibit stronger binding ability to TCS than TB-EPS under neutral environment, playing more crucial roles in the binding process. This work highlights the important contributions of EPS to TCS removal, that is beneficial to comprehensively understand the migration of TCS in activated sludge system.

Degradation of triclosan by environmental microbial consortia and by axenic cultures of microorganisms with concerns to wastewater treatment

Triclosan is an antimicrobial agent, which is widely used in personal care products including toothpaste, soaps, deodorants, plastics, and cosmetics. Widespread use of triclosan has resulted in its release into wastewater, surface water, and soils and has received considerable attention in the recent years. It has been reported that triclosan is detected in various environmental compartments. Toxicity studies have suggested its potential environmental impacts, especially to aquatic ecosystems. To date, removal of triclosan has attracted rising attention and biodegradation of triclosan in different systems, such as axenic cultures of microorganisms, full-scale WWTPs, activated sludge, sludge treatment systems, sludge-amended soils, and sediments has been described. In this study, an extensive literature survey was undertaken, to present the current knowledge of the biodegradation behavior of triclosan and highlights the removal and transformation processes to help understand and predict the environmental fate of triclosan. Experiments at from lab-scale to full-scale field studies are shown and discussed.