Laccase- and electrochemically mediated conversion of triclosan: Metabolite formation and influence on antibacterial activity

Critical review on the environmental behaviors and toxicity of triclosan and its removal technologies

As a highly effective broad-spectrum antibacterial agent, triclosan (TCS) is widely used in personal care and medical disinfection products, resulting in its widespread occurrence in aquatic and terrestrial environments, and even in the human body. Notably, the use of TCS surged during the COVID-19 outbreak, leading to increasing environmental TCS pollution pressure. From the perspective of environmental health, it is essential to systematically understand the environmental occurrence and behavior of TCS, its toxicological effects on biota and humans, and technologies to remove TCS from the environment. This review comprehensively summarizes the current knowledge regarding the sources and behavior of TCS in surface water, groundwater, and soil systems, focusing on its toxicological effects on aquatic and terrestrial organisms. Effluent from wastewater treatment plants is the primary source of TCS in aquatic systems, whereas sewage application and/or wastewater irrigation are the major sources of TCS in soil. Human exposure pathways to TCS and associated adverse outcomes were also analyzed. Skin and oral mucosal absorption, and dietary intake are important TCS exposure pathways. Reducing or completely degrading TCS in the environment is important for alleviating environmental pollution and protecting public health. Therefore, this paper reviews the removal mechanisms, including adsorption, biotic and abiotic redox reactions, and the influencing factors. In addition, the advantages and disadvantages of the different techniques are compared, and development prospects are proposed. These findings provide a basis for the management and risk assessment of TCS and are beneficial for the application of treatment technology in TCS removal.

Biodegradation strategies of veterinary medicines in the environment: Enzymatic degradation

One Health closely integrates healthy farming, human medicine, and environmental ecology. Due to the ecotoxicity and risk of transmission of drug resistance, veterinary medicines (VMs) are regarded as emerging environmental pollutants. To reduce or mitigate the environmental risk of VMs, developing friendly, safe, and effective removal technologies is an important means of environmental remediation for VMs. Many previous studies have proved that biodegradation has significant advantages in removing VMs, and biodegradation based on enzyme catalysis presents higher operability and specificity. This review focused on biodegradation strategies of environmental pollutants and reviewed the enzymatic degradation of VMs including antimicrobial drugs, insecticides, and disinfectants. We reviewed the sources and catalytic mechanisms of peroxidase, laccase, and organophosphorus hydrolases, and summarized the latest research status of immobilization methods and bioengineering techniques in improving the performance of degrading enzymes. The mechanism of enzymatic degradation for VMs was elucidated in the current research. Suggestions and prospects for researching and developing enzymatic degradation of VMs were also put forward. This review will offer new ideas for the biodegradation of VMs and have a guide significance for the risk mitigation and detoxification of VMs in the environment.

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.

Treatment of triclosan through enhanced microbial biodegradation

Triclosan (TCS) is extensively used in healthcare and personal care products as an antibacterial agent. Due to the persistent and toxic nature of TCS, it is not completely degraded in the biological wastewater treatment process. In this research work, identification of TCS degrading bacteria from municipal wastewater sludge and applying the same as bioaugmentation treatment for wastewater have been reported. Based on the 16S rRNA analysis of wastewater sludge, it was found that Providencia rettgeri MB-IIT strain was active and able to grow in higher TCS concentration. The identified bacterial strain was able to use TCS as carbon and energy source for its growth. The biodegradation experiment was optimized for the operational parameters viz. pH (5-10), inoculum size (1-5% (v/v)) and different initial concentration (2, 5, and 10 mg/L) of TCS. During the TCS degradation process, manganese peroxidase (MnP) and laccase (LAC) enzyme activity and specific growth rate of P. rettgeri strain were maximum at pH=7% and 2% (v/v) inoculum size, resulting in 98% of TCS removal efficiency. A total of six intermediate products were identified from the Liquid chromatography-high-resolution mass spectrometry (LC-HRMS) analysis, and the two mechanisms responsible for the degradation of TCS have been elucidated. The study highlights that P. rettgeri MB-IIT strain could be advantageously used to degrade triclosan present in the wastewater.

Insights into the electrochemical degradation of triclosan from human urine: Kinetics, mechanism and toxicity

Electrochemical degradation of triclosan in human urine was firstly studied by using Ti/SnO₂-Sb/PbO₂ anode doped with rare-earth elements. The results indicated that the Ti/SnO₂-Sb/Gd-PbO₂ anode demonstrated the best performance with the degradation rate constants being 0.095 min^-1 in fresh urine and 0.045 min^-1 in hydrolyzed urine at a current density of 10 mA cm^-2. The electrochemical degradation was improved in the presence of phosphate and chloride, while the degradation was obviously inhibited by urea, bicarbonate and ammonia. Degradation mechanism mainly involved ether-bond cleavage, hydroxylation, cyclization, dehydrogenation and carboxylation. Quantitative structure-activity relationship model showed that ecological risks of cyclization products to fish, daphnid and green algae was higher than the parent compound, implying that the potential risks to aquatic organism should not be ignored before triclosan mineralized completely. Energy consumption for 90% triclosan degradation ranged from 4.5 to 47.8 Wh L^-1, and the consumption increased along with the hydrolysis of urine. The results indicate that electrochemical oxidation is a feasible and energy-saving technique to effectively remove triclosan from human urine.

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.

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.

Characterisation of electron beam irradiation-immobilised laccase for application in wastewater treatment

Laccase from Phoma sp. UHH 5-1-03 was cross-linked to polyvinylidene fluoride membranes by electron beam irradiation. Immobilised laccase displayed a higher stability than the non-immobilised enzyme with respect to typical wastewater temperatures, and pH at a range of 5 to 9. Batch tests addressed the removal of pharmaceutically active compounds (PhACs; applied as a mixture of acetaminophen, bezafibrate, indometacin, ketoprofen, mefenamic acid, and naproxen) by both immobilised and non-immobilised laccase in municipal wastewater. High removal rates (>85%) of the most efficiently oxidised PhACs (acetaminophen and mefenamic acid) indicated a high efficiency of the immobilised laccase in wastewater. Continuous elimination of the aforementioned PhACs by the immobilised enzyme in a continuously operated diffusion basket reactor yielded a PhAC removal pattern qualitatively similar to those observed in batch tests. Clearly higher apparent V_max values and catalytic efficiencies (in terms of both V_max/S_0.5 as well as V_max/K_m values obtained from data fitting according to the Hill and the Michaelis-Menten model, respectively) observed for acetaminophen oxidation by the immobilised compared to the non-immobilised enzyme are in support of a considerably higher functional stability of the immobilised laccase especially in wastewater. The potential influence of acetaminophen on the removal of comparatively less laccase-oxidisable water pollutants such as the antimicrobial triclosan (TCS) was investigated. TCS was increasingly removed upon increasing the initial acetaminophen concentration in immobilised as well as non-immobilised laccase reaction systems until saturation became evident. Acetaminophen was consumed and not recycled during laccase reactions, which was accompanied by the formation of various acetaminophen-TCS cross-coupling products. Nevertheless, the simultaneous presence of acetaminophen (and potentially even more pollutant removal-enhancing laccase substrates) and more recalcitrant pollutants in wastewater represents an interesting option for the efficiency enhancement of enzyme-based wastewater treatment approaches.

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.

Emergent contaminants: Endocrine disruptors and their laccase-assisted degradation - A review

Herein, an effort has been made to highlight the trends of the state-of-the-art of laccase-assisted degradation of emerging contaminants at large and endocrine disruptors in particular. Since first described in the 19th century, laccase has received particular interest for inter- and multidisciplinary investigations due to its uniqueness and remarkable biotechnological applicability. There has always been a paramount concern over the widespread occurrences of various pollutant types, around the globe. Therefore, pollution free processes are gaining ground all over the world. With ever increasing scientific knowledge, socioeconomic awareness, human health-related issues and ecological apprehensions, people are more concerned about the widespread environmental pollutants. In this context, the occurrences of newly identified pollutants so-called "emerging contaminants - ECs" in our main water bodies is of continued and burning concern worldwide. Undoubtedly, various efforts have already been made to tackle this challenging ECs concern though using different approaches including physical and chemical, however, each has considerable limitations. In this review, we present information on how laccase-assisted approach can change this limited tendency of physical and chemical based approaches. A special focus has been given to the laccase-assisted systems including pristine laccase, laccase-mediator catalyzed system and immobilized-laccase catalyzed system that promotes the endocrine disruptors removal. Towards the end, a list of outstanding questions and research gaps are given that can pave the way for future studies.

Photochemically Induced Bound Residue Formation of Carbamazepine with Dissolved Organic Matter

More than 400 new nitrogen containing products were detected upon experimental sunlight photolysis of the pharmaceutical carbamazepine (CBZ) in the presence of dissolved organic matter (DOM) by Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). These products were presumably formed through covalent binding of CBZ phototransformation products with DOM molecules. About 50% of these newly formed bound residues contained one nitrogen atom and had a molecular mass between 375 and 525 Da, which was 150 to 200 Da higher than for an average DOM molecule. In addition, a previously unknown CBZ phototransformation product, 3-quinolinecarboxylic acid (3-QCA), was identified by liquid chromatography high resolution tandem mass spectrometry (LC-HRMS/MS). 3-QCA was likely formed through oxidative ring cleavage and subsequent decarboxylation of acridine, a well-known phototransformation product of CBZ. Collision induced dissociation experiments and Kendrick mass defect analyses corroborated that about 160 of the new products were formed via covalent binding of 3-QCA with DOM molecules of above-average O/C and H/C ratios. Experiments at lower CBZ concentration suggested that the importance of bound residue formation increases with increasing DOM/CBZ ratios. Photochemically induced bound residue formation of polar contaminants with DOM in the aqueous phase is thus a disregarded pathway along which contaminants can be transformed in the environment. The method presented here offers a new possibility to study the formation of bound residues, which may be of relevance also for other transformation processes in natural waters where radical intermediates are generated.