Peroxidase-mediated removal of a polychlorinated biphenyl using natural organic matter as the sole cosubstrate

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.

Removal of PFAS from water by aquatic plants

We have found that aquatic plants can reduce the content of perfluorinated alkyl substances (PFAS) within a short period of time. The aim of this study was to determine the variation in the uptake of PFAS from contaminated water by various wetland plant species, investigate the effect of biomass on PFAS removal, and determine whether laccases and peroxidases are involved in the removal and degradation of PFAS. Seventeen emergent and one submerged wetland plant species were screened for PFAS uptake from highly contaminated lake water. The screening showed that Eriophorum angustifolium, Carex rostrata, and Elodea canadensis accumulated the highest levels of all PFAS. These species were thereafter used to investigate the effect of biomass on PFAS removal from water and for the enzyme studies. The results showed that the greater the biomass per volume, the greater the PFAS removal effect. The plant-based removal of PFAS from water is mainly due to plant absorption, although degradation also occurs. In the beginning, most of the PFAS accumulated in the roots; over time, more was translocated to the shoots, resulting in a higher concentration in the shoots than in the roots. Most PFAS degradation occurred in the water; the metabolites were thereafter taken up by the plants and were accumulated in the roots and shoots. Both peroxidases and laccases were able to degrade PFAS. We conclude that wetland plants can be used for the purification of PFAS-contaminated water. For effective purification, a high biomass per volume of water is required.

Lignin precursors enhance exolaccase-started humification of bisphenol A to form functional polymers

Humification plays a significant role in converting phenolic pollutants and forming heterogeneous polymers, but few studies have been performed to investigate exolaccase-started humification (ESH). Herein, the influences of lignin precursors (LPs) on exolaccase-induced bisphenol A (BPA) removal and humification were explored. In particular, the architectural features and botanical effects of the formed humification products were also tested. ESH was extremely beneficial in boosting BPA removal in the presence of LPs. Compared with LP-free (58.49%), 100% of BPA was eliminated after the reaction with ESH for 72 h. Such a process was controlled by an exolaccase-caused random assembly of radicals, which generated a large number of hydrophobic polymers through nonspecific covalent binding of C-C and/or C-O. These humified polymers were extremely stable at pH 2.0-10.0 and -20 °C to 80 °C and displayed unique functions, i.e., scavenged 2,2-diphenyl-1-picrylhydrazyl/2,2'-azino-bis3-ethylbenzothiazoline-6-sulphonic acid radicals and exerted antioxidant capacities. More importantly, the functional polymers could act as auxin analogs to increase the germination index (>100%), plant biomass, and salt tolerance of radish seedlings. Our findings disclosed that ESH could not only be optimized to mitigate the ecological risks of phenolic pollutants and sequester organic carbon in environmental bioremediation, but the resulting abundant auxin analogs also contributed to agricultural productivity.

Trade-off effect of dissolved organic matter on degradation and transformation of micropollutants: A review in water decontamination

The degradation of micropollutants by various treatments is commonly affected by the ubiquitous dissolved organic matter (DOM) in the water environment. To optimize the operating conditions and decomposition efficiency, it is necessary to consider the impacts of DOM. DOM exhibits varied behaviors in diverse treatments, including permanganate oxidation, solar/ultraviolet photolysis, advanced oxidation processes, advanced reduction process, and enzyme biological treatments. Besides, the different sources (i.e., terrestrial and aquatic, etc) of DOM, and operational circumstances (i.e., concentration and pH) fluctuate different transformation efficiency of micropollutants in water. However, so far, systematic explanations and summaries of relevant research and mechanism are rare. This paper reviewed the "trade-off" performances and the corresponding mechanisms of DOM in the elimination of micropollutants, and summarized the similarities and differences for the dual roles of DOM in each of the aforementioned treatments. Inhibition mechanisms typically include radical scavenging, UV attenuation, competition effect, enzyme inactivation, reaction between DOM and micropollutants, and intermediates reduction. Facilitation mechanisms include the generation of reactive species, complexation/stabilization, cross-coupling with pollutants, and electron shuttle. Moreover, electron-drawing groups (i.e., quinones, ketones functional groups) and electron-supplying groups (i.e., phenols) in the DOM are the main contributors to its trade-off effect.

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.

Peroxidase enzymes as green catalysts for bioremediation and biotechnological applications: A review

The fast-growing consumer demand drives industrial process intensification, which subsequently creates a significant amount of waste. These products are discharged into the environment and can affect the quality of air, degrade water streams, and alter soil characteristics. Waste materials may contain polluting agents that are especially harmful to human health and the ecosystem, such as the synthetic dyes, phenolic agents, polycyclic aromatic hydrocarbons, volatile organic compounds, polychlorinated biphenyls, pesticides and drug substances. Peroxidases are a class oxidoreductases capable of performing a wide variety of oxidation reactions, ranging from reactions driven by radical mechanisms, to oxygen insertion into CH bonds, and two-electron substrate oxidation. This versatility in the mode of action presents peroxidases as an interesting alternative in cleaning the environment. Herein, an effort has been made to describe mechanisms governing biochemical process of peroxidase enzymes while referring to H₂O₂/substrate stoichiometry and metabolite products. Plant peroxidases including horseradish peroxidase (HRP), soybean peroxidase (SBP), turnip and bitter gourd peroxidases have revealed notable biocatalytic potentialities in the degradation of toxic products. On the other hand, an introduction on the role played by ligninolytic enzymes such as manganese peroxidase (MnP) and lignin peroxidase (LiP) in the valorization of lignocellulosic materials is addressed. Moreover, sensitivity and selectivity of peroxidase-based biosensors found use in the quantitation of constituents and the development of diagnostic kits. The general merits of peroxidases and some key prospective applications have been outlined as concluding remarks.

Development of a horseradish peroxidase-Fenton-like system for the degradation of sulfamethazine under weak acid condition

Both horseradish peroxidase (HRP) and ferrous ion (Fe²⁺) can degrade organic micropollutants (e.g., sulfamethazine) in the presence of hydrogen peroxide (H₂O₂), but they have their own disadvantages, such as low degradation efficiency and low pH condition, respectively. In order to overcome the above shortcomings, this study is to develop a HRP-Fenton-like system for sulfamethazine (SMR) efficient degradation by analyzing the optimal reaction conditions, degradation mechanisms, and effect of ions. Results show that HRP and Fe²⁺ achieve effective coupling by adding trace Fe²⁺ (≤ 10 μmol/mL) to the HRP system at pH = 5, and the degradation rate of SMR increased by 20.7-42% depending on Fe²⁺ concentration compared with single HRP treatment. Consumption of H₂O₂ and quenching of hydroxyl radicals confirmed that HRP dominated SMR removal in the HRP-Fenton-like system. HPLC-MS analysis shows that SMR was degraded by C-S bond breaking, N-S bond breaking, hydroxyl substitution, and rearrangement. Furthermore, Cl^-, HCO₃^-, and NO₃^- exhibit an acceptable negative effect, while SO₄^2- shows a positive effect on the degradation of SMR.

Polymerization of micropollutants in natural aquatic environments: A review

Micropollutants with high ecotoxicological risks are frequently detected in aquatic environments, which has aroused great concern in recent years. Humification is one of the most important natural detoxification processes of aquatic micropollutants, and the core reactions of this process are polymerization and coupling. During humification, micropollutants are incorporated into the macrostructures of humic substances and precipitated from aqueous systems into sediments. However, the similarities and differences among the polymerization/coupling pathways of micropollutants in different oxidative systems have not been systematically summarized in a review. This article reviews the current knowledge on the weak oxidation-induced spontaneous polymerization/coupling transformation of micropollutants. First, four typical weak oxidative conditions for the initiation of micropollutant polymerization reactions in aquatic environments are compared: enzymatic catalysis, biomimetic catalysis, metal oxide oxidation, and photo-initiated oxidation. Second, three major subsequent spontaneous transformation pathways of micropollutants are elucidated: radical polymerization, nucleophilic addition/substitution and cyclization. Different solution conditions are also summarized. Furthermore, the importance of toxicity evolution during the weak oxidation-induced coupling/polymerization of micropollutants is particularly emphasized. This review provides a new perspective for the transformation mechanism and pathways of micropollutants from aquatic systems into sediments and the atmosphere and offers theoretical support for developing micropollutant control technologies.

Peroxidases-assisted removal of environmentally-related hazardous pollutants with reference to the reaction mechanisms of industrial dyes

Environmental protection is one of the most important challenges for the humankind. Increasing number of emerging pollutants resulting from industrial/human-made activities represents a serious menace to the ecological and environmental equilibrium. Industrial dyes, endocrine disrupters, pesticides, phenols and halogenated phenols, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and other xenobiotics are among the top priority environmental pollutants. Some classical remediation approaches including physical, chemical and biological are being employed, but are ineffective in cleaning the environment. Enzyme-catalyzed transformation reactions are gearing accelerating attention in this context as potential alternatives to classical chemical methods. Peroxidases are catalysts able to decontaminate an array of toxic compounds by a free radical mechanism resulting in oxidized or depolymerized products along with a significant toxicity reduction. Admittedly, enzymatic catalysis offers the hallmark of high chemo-, regio-, and enantioselectivity and superior catalytic efficiency under given reaction environment. Moreover, enzymes are considered more benign, socially acceptable and greener production routes since derived from the renewable and sustainable feedstock. Regardless of their versatility and potential use in environmental processes, several limitations, such as heterologous production, catalytic stability, and redox potential should be overcome to implement peroxidases at large-scale transformation and bio-elimination of recalcitrant pollutants. In this article, a critical review of the transformation of different types of hazardous pollutants by peroxidases, with special reference to the proposed reaction mechanisms of several dyes is presented. Following that major challenges for industrial and environmental applications of peroxidases are also discussed. Towards the end, the information is also given on miscellaneous applications of peroxidases, concluding remarks and outlook.

Perfluorooctanesulfonate Degrades in a Laccase-Mediator System

Perfluorooctanesulfonate (PFOS) is a compound that has wide applications with extreme persistence in the environment and the potential to bioaccumulate, and could induce adverse effects to ecosystems. We investigated the degradation of PFOS by laccase-induced enzyme catalyzed oxidative humification reactions (ECOHRs) using 1-hydroxybenzotriazole (HBT) as a mediator. Approximately 59% of PFOS was transformed over 162 days of incubation, and the reaction appeared to follow a pseudo-first-order model with reaction rate constant of 0.0066/ d ( r² = 0.87) under one condition tested. Using differential absorption spectra and theoretical simulation, we elucidated the interaction between Cu²⁺/Mg²⁺ and PFOS, and proposed that Cu²⁺ and Mg²⁺ could serve as a bridge to bring the negatively charged PFOS and laccase to proximity, thus increasing the chance of radicals that are released from laccase to reach and react with PFOS. In addition, density functional theory modeling showed that PFOS complexation to the metal ions could unlock its helical configuration and decrease the C-C bond energy of PFOS. These changes allow the attack of PFOS C-C backbone by radicals to become easier. On the basis of products identification, we proposed that direct attack of PFOS by the HBT radical initiated the free radical chain reaction processes and led to the formation of fluoride and partially fluorinated compounds. These results suggest that ECOHR is a potential pathway by which PFOS could be degraded in the environment, and it may make a viable approach to remediate PFOS contamination via amendment of appropriate enzymes and mediators.

Influence of natural organic matter on horseradish peroxidase-mediated removal of 17α-ethinylestradiol: Role of molecular weight

Ubiquitous natural organic matter (NOM) plays a crucial role in the peroxidase-mediated transformation of phenolic pollutants in aquatic environment. As a poorly defined polydispersed mixture of assorted organic substances with wide molecular weight (MW) distribution, NOM has far prevented researchers from finding out the primarily responsible components for the specific effect. In this work, MW fractionated NOMs (Mf-NOMs) were used to investigate their roles on horseradish peroxidase (HRP)-mediated transformation of 17α-ethinylestradiol (EE2). The removal rate of EE2 was restrained in the presence of pristine or Mf-NOMs, and the inhibitory mechanism was MW-dependent. Low Mf-NOMs restrained the enzymatic reaction by acting as competitive substrates, while high Mf-NOMs retained freely dissolved EE2 which reduced its availability for enzymatic reaction. The contribution of these two processes to the inhibition induced by pristine NOM was further quantified and found to be relevant to the reaction conditions, especially EE2 concentration. The findings of this work reveal more complex influences of NOM on the enzymatic reaction than ever demonstrated, which aids in understanding the fate of EE2 and other congener contaminants in natural and municipal water.

Transformation of aqueous sulfonamides under horseradish peroxidase and characterization of sulfur dioxide extrusion products from sulfadiazine

The potential of horseradish peroxidase (HRP) to catalyze the removal of sulfonamides from water and the effects of different H₂O₂ and HRP concentrations were investigated. Six sulfonamides, each with a five- or six-membered heterocyclic group, including sulfamethoxazole (SMX), sulfathiazole (STZ), sulfapyridine (SPD), sulfadiazine (SDZ), sulfamerazine (SMR) and sulfamethoxypyridazine (SMP) were selected as target compounds. All sulfonamides exhibit a pseudo-first-order dependence of the concentration versus the reaction time. The decay rate (k, h^-1) of the six sulfonamides spiked individually exhibit a trend following the order of STZ > SMP, SPD > SMR > SDZ » SMX. When spiked together, the coexistent sulfonamides might act as mediators for the enhancement of SMX removal and as competitors for the decreased removal of most sulfonamides. Moreover, six transformation products of SDZ are identified by the Thermo Scientific LTQ Orbitrap Elite technique. SDZ transformation involves two steps: one is the Smiles re-arrangement of the structure, and the other is oxidation and sulfur dioxide extrusion. This study is the first to report the removal dynamics of sulfonamides in HRP-catalyzed reactions and the identified products of SDZ.

Multi-walled carbon nanotubes facilitated the removal of tetrabromobisphenol a mediated by horseradish peroxidase

In this study, we systematically investigated the effect of multiwall carbon nanotubes (MWCNTs) on the removal of tetrabromobisphenol A (TBBPA) mediated by horseradish peroxidase (HRP) at varying important conditions. The results suggested that the presence of MWCNTs significantly enhanced the removal of TBBPA mediated by HRP and the reaction rate constant was linear with the MWCNTs dosage. The enhancement of MWCNTs on the HRP-mediated reaction was attributed to two facts, one is that MWCNTs protected HRP from inactivation, the other is that the presence of MWCNTs made the homogeneous reaction of TBBPA be heterogeneous reaction by adsorbing TBBPA on its surface. Moreover, the influence of MWCNTs on TBBPA products distribution was further elucidated. We found that the species of reaction product had no difference between the HRP-mediated systems with and without the presence of MWCNTs. However, the presence of MWCNTs significantly decreased the yields of each product. These results give insight into the role of MWCNTs in HRP-mediated TBBPA reactions and provide theoretical foundation for potential development of novel enzymatic methods to control TBBPA contamination. MWCNTs enhanced the removal of TBBPA mediated by HRP/H₂O₂, because it protected HRP from inactivation and adsorbed TBBPA on its surface to form a heterogeneous reaction process.

Oxidative coupling of acetaminophen mediated by Fe3+-saturated montmorillonite

The wide usage of acetaminophen as human medicine has resulted in its ubiquitous occurrence in various environmental compartments. However, the information for the transformation of acetaminophen in soil is still limited. In this study, oxidative coupling of acetaminophen in bulk solution mediated by Fe³⁺-saturated montmorillonite was observed under different environmental conditions. In the absence of natural phenolic acids, acetaminophen could be fully eliminated from the solution within 72h at pH3.5, acetaminophen dimer was identified as the major reaction product. Reduction of montmorillonite associated Fe³⁺ coupled with the oxidation of acetaminophen was considered as the main mechanism for acetaminophen transformation on Fe³⁺-saturated montmorillonite. The clay associated Fe³⁺ showed higher reactivity than Fe³⁺ in solution due to the strong complexation between surface Fe³⁺ and acetaminophen. The cross-coupling reaction between acetaminophen and phenolic acids was also observed when phenolic acids were present in the system. While with the increase of phenolic acid concentration, the competition for the reactive sites between acetaminophen and phenolic acids significantly suppressed acetaminophen removal. These results demonstrated the importance of transition metal saturated clay minerals for the abiotic transformation of anthropogenic micropollutants.

Factors controlling the rate of perfluorooctanoic acid degradation in laccase-mediator systems: The impact of metal ions

This study investigated the factors that regulated the degradation of perfluorooctanoic acid (PFOA) in laccase-catalyzed oxidative humification reactions with 1-hydroxybenzotriazole (HBT) as a mediator. The reaction rates were examined under conditions with key factors varied, including initial PFOA concentrations, laccase and HBT dosages, and the ionic contents of the reaction solutions. The PFOA degradation followed pseudo-first order kinetics, and the rate constants (k) were similar for the high (100 μmol L^-1) and low (1.00 μmol L^-1) initial PFOA concentrations, respectively at 0.0040 day^-1 (r² = 0.98) and 0.0042 day^-1 (r² = 0.86) under an optimum reaction condition tested in this study. The metal ions contained in the reaction solution appeared to have a strong impact on PFOA degradation. Differential UV-Vis spectrometry revealed that Cu²⁺ can complex with PFOA, which plays an essential role to enable PFOA degradation, probably by bridging the negatively charged PFOA and laccase, so that the free radicals of HBT that are released from laccase can reach and react with PFOA. It was also found that Fe³⁺ plays a similar role as Cu²⁺ to enable PFOA degradation in the laccase-HBT reaction system. In contrast, Mg²⁺ and Mn²⁺ cannot complex with PFOA under the investigated conditions, and do not enable PFOA degradation in the laccase-HBT system. Fluoride and partially fluorinated compounds were detected as PFOA degradation products using ion chromatography and high resolution mass spectrometry. The structures of the products suggest the reaction pathways involving free-radical initiated decarboxylation, rearrangement, and cross-coupling.

The fate and transformation of tetrabromobisphenol A in natural waters, mediated by oxidoreductase enzymes

In this study, we examined the fate and transformation of tetrabromobisphenol A (TBBPA), mediated by the representative oxidoreductases (laccase and horseradish peroxidase (HRP)) in natural waters. Both enzymes could readily degrade TBBPA at environmentally relevant concentrations (e.g., 10 nmol L^-1) in natural waters. After 2 hour treatment, 0.5-25% and 35-65% of TBBPA were degraded in municipal wastewater and natural surface waters by a laccase or HRP-catalyzed reaction, respectively. Enzyme kinetics evaluations indicated that the k_CAT/K_M ratio of HRP (1.01 μM^-1 s^-1) was much higher than that of laccase (0.032 μM^-1 s^-1) for TBBPA degradation, suggesting that the catalytic performance of HRP towards TBBPA was more efficient than that of laccase. The effects of pH and organic matter on the enzymatic degradation efficiency were explored. Organic matter in the water inhibited the enzymatic degradation efficiency and the degree of inhibition was proportional to the UV₂₅₄ values of water. Product identification indicated that the product distribution of TBBPA at low concentration (10 nmol L^-1) was similar to that of TBBPA at high concentration (10 μmol L^-1). The degradation intermediates underwent further enzymatic reaction to yield higher molecular weight secondary products. Toxicity assessment showed that TBBPA toxicity was effectively eliminated by the oxidoreductase-catalyzed reaction.

The effect of dissolved organic matter on soybean peroxidase-mediated removal of triclosan in water

Dissolved organic matter (DOM) is ubiquitous in water and involved in numerous important chemical processes in aqueous systems, enabling it a unique challenge for a variety of water treatment processes. Soybean peroxidase (SBP)-based enzymatic process, as a promising treatment technique, has been successfully applied to remove pollutants in wastewaters such as coal-tar and refinery wastewater. In this study, the effect of DOM on the removal of polychlorinated aromatic antimicrobials triclosan (TCS) by SBP was investigated. Our results suggested that DOM significantly suppressed the catalytic performance of SBP to TCS, presumably resulting from the competition of the phenolic moiety in DOM structure as the active substrate of SBP via the analysis of excitation emission matrix (EEM) spectra of DOM. Although the product species of TCS in SBP-mediated system with DOM has no change compared with the system without DOM, the yields of self-coupling products relative to total transformed TCS were remarkably reduced in the presence of DOM, suggesting that DOM participated in the oxidative coupling reactions. Cross-coupling between TCS and DOM was also verified using guaiacol as a model DOM constituent. Moreover, the products including self-coupling products and co-polymers in SBP-mediated TCS reaction system with DOM were innocuous through growth inhibition assay of S. obliquus.

The potential ecological risk of multiwall carbon nanotubes was modified by the radicals resulted from peroxidase-mediated tetrabromobisphenol A reactions

Extensive studies have been conducted on the environmental degradation of multiwall carbon nanotubes (MWCNTs), but primarily focused on the extent and rate of MWCNTs mineralization. Few studies have explored possible structural changes that may occur to MWCNTs during natural or engineered processes. We systematically examined MWCNTs in oxidative coupling reactions in the presence of a common contaminant tetrabromobisphenol A (TBBPA). MWCNTs was modified by the radicals of TBBPA resulting from peroxidase-mediated coupling reaction. Interactions between TBBPA radicals and MWCNTs were definitely confirmed by analyzing the characteristic mass spectrometry response of bromine in TBBPA and the structures of MWCNTs. After reaction with TBBPA radicals for 60 min, the content of bromine contained in MWCNTs was 6.84(±0.12)%, a quantity equivalent to a 501.65(±2.19) mg loading of TBBPA per gram MWCNTs. Modified MWCNTs had better stability and smaller sizes than that of MWCNTs and TBBPA-adsorbed MWCNTs. Assessment using zebrafish embryos revealed that the modified MWCNTs passed through the chorion and entered the embryo inducing acute toxicity, while the MWCNTs/TBBPA-adsorbed MWCNTs was trapped by chorion. These findings indicated that MWCNTs was modified in peroxidase-mediated coupling reactions, and suggested that such modifications may have an influence on the ecological risks of MWCNTs.

Graphene Facilitated Removal of Labetalol in Laccase-ABTS System: Reaction Efficiency, Pathways and Mechanism

The widespread occurrence of the beta-blocker labetalol causes environmental health concern. Enzymatic reactions are highly efficient and specific offering biochemical transformation of trace contaminants with short reaction time and little to none energy consumption. Our experiments indicate that labetalol can be effectively transformed by laccase-catalyzed reaction using 2, 2-Azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) as a mediator, while no significant removal of labetalol can be achieved in the absence of ABTS. A total of three products were identified. It is interesting that the presence of graphene greatly increased the reaction rate while not changed the products. In the presence of 100 μg/L graphene, the pseudo-first-order reaction rate constant was increased ~50 times. We found that the enhancement of graphene is probably attributed to the formation and releasing of ABTS²⁺ which has a much greater reactivity towards labetalol when graphene is present. This study provides fundamental information for laccase-ABTS mediated labetalol reactions and the effect of graphene, which could eventually lead to development of novel methods to control beta-blocker contamination.

Removal of sulfadimethoxine in soil mediated by extracellular oxidoreductases

Sulfadimethoxine (SDM) is an antibiotic commonly used in concentrated animal feeding operations and released into the environment via manure application on agricultural lands. Transformation of antibiotics in soil impacts the likelihood of their entry to water bodies, uptake by plants, and thus their effect on terrestrial and aquatic organisms. We conducted experiments to incubate SDM in a sandy loam soil in the presence of humification enzymes commonly found in natural soil, laccase, horseradish peroxidase, and lignin peroxidase. Incubation with the enzymes led to significant reduction in the fraction of SDM extractable from soil, indicating the formation of bound residues. Such transformation was enhanced when the organic matter content in soil is increased or when certain chemical mediators were used along with laccase. The study provided a basis for understanding the environmental fate of sulfonamides and help with the development of remediation methods to mitigate the release of sulfonamides from soil to water.

A combination of electro-enzymatic catalysis and electrocoagulation for the removal of endocrine disrupting chemicals from water

We in this study investigated a novel electrochemical approach combining electro-enzyme and electrocoagulation to precipitate bisphenol A (BPA) from water containing humic acid (HA). Horseradish peroxidase was immobilized on the graphite felt of Ti electrode as HRP-GF/Ti cathode, with aluminum plate anode establishing a pair of working electrodes. BPA was 100% removed and the reduction of total organic carbon (TOC) reached 95.1% after 20-min sequencing treatment with the current density of 2.3 mA/cm(2). Real wastewater (TOC=28.76 mg/L, BPA=4.1 μg/L) also can achieve 94% BPA removal and 52% TOC reduction after sequencing treatment. Additionally, coupled electro-system with continuous flow only required energy of 0.016 kWh/m(3) to achieve simultaneous 90% BPA and 85% TOC removal. As indicated in the time-of-flight mass spectrometry and FTIR spectra, the electro-enzymatic process not only oxidized BPA into dimer and BPA-3,4-quinone, but also greatly altered the chemical and structural features of HA, where hydrophilic moieties (phenolic and alcohols) transformed into hydrophobic forms (ethers, quinone and aliphatic). These polymerized products were effectively separated from aquous solution during anodic electrocoagulation, leading to significant removal of BPA and TOC. Thus, the coupled process may provide a faster and less energy strategy to control certain emerging contaminants in water/wastewater treatment.

Manganese peroxidase degrades pristine but not surface-oxidized (carboxylated) single-walled carbon nanotubes

The transformation of engineered nanomaterials in the environment can significantly affect their transport, fate, bioavailability, and toxicity. Little is known about the biotransformation potential of single-walled carbon nanotubes (SWNTs). In this study, we compared the enzymatic transformation of SWNTs and oxidized (carboxylated) SWNTs (O-SWNTs) using three ligninolytic enzymes: lignin peroxidase, manganese peroxidase (MnP), and laccase. Only MnP was capable of transforming SWNTs, as determined by Raman spectroscopy, near-infrared spectroscopy, and transmission electron microscopy. Interestingly, MnP degraded SWNTs but not O-SWNTs. The recalcitrance of O-SWNTs to enzymatic transformation is likely attributable to the binding of Mn2+ by their surface carboxyl groups at the enzyme binding site, which inhibits critical steps in the MnP catalytic cycle (i.e., Mn2+ oxidation and Mn3+ dissociation from the enzyme). Our results suggest that oxygen-containing surface functionalities do not necessarily facilitate the biodegradation of carbonaceous nanomaterials, as is commonly assumed.

Ligninase-mediated transformation of 4,4'-dibromodiphenyl ether (BDE 15)

The structurally related hydroxylated polybrominated diphenyl ether (PBDE) like hydroxylated 4,4'-dibromodiphenyl ether widely occur in precipitation, surface water, and biotic media. The origins of hydroxylated PBDEs (OH-PBDEs) are of particular interest due to their greater toxic potencies than the corresponding PBDEs. We studied the transformation behavior and products of 4,4'-dibromodiphenyl ether (BDE 15) mediated by lignin peroxidase (LiP), an extracellular enzyme that is produced by certain white rot fungus and is widely present in the natural environment. We found that BDE 15 can be effectively transformed through the reaction mediated by LiP, and two different mono-OH-dibromodiphenyl ethers were identified by using gas chromatography-mass spectrometry (GC-MS) and GC-MS/MS. In particular, we compared the reaction behavior for systems variously containing natural organic matter (NOM) and/or veratryl alcohol (VA), a metabolite that certain fungus produces along with LiP in nature. It was found that the VA's enhancement effect on LiP performance was impaired by the presence of NOM. The findings in this study provide useful information for better understanding the origins of OH-PBDEs found in the environment.

A novel mechanism of bisphenol A removal during electro-enzymatic oxidative process: chain reactions from self-polymerization to cross-coupling oxidation

The catalyzed removal of bisphenol A (BPA) by a horseradish peroxidase (HRP) cathode in the presence of humic acid (HA) was investigated. At an optimal condition, the removal of BPA achieved 100% within 2min reaction. In the electro-enzymatic process, products were analyzed by high performance liquid chromatography with diode array detector (HPLC-DAD) and high performance size exclusion chromatography (HPSEC). HPLC-DAD results showed that BPA was oxidized into self-polymers and then self-polymers as important intermediate products decreased and disappeared. HPSEC results showed the order of molecular weight (MW): HA+BPA cross-coupling products>HA self-coupling products>initial HA. According to above results, a novel mechanism of BPA transformation in the presence of HA was proposed in electro-enzymatic process. In summary, under oxidation of in situ hydrogen peroxide on HRP electrode, the BPA first are polymerized into self-polymers, and then, the polymers may be incorporated into HA matrix and finally larger MW of BPAn-HA might be formed. The presence of HA can provide chain reactions from BPA self-polymerization to cross-coupling oxidation. Therefore, in the presence of HA, the electro-enzymatic oxidation is an effective way to improve BPA removal.

Simultaneous laccase production and color removal by culturing fungus Pycnoporus sp. SYBC-L3 in a textile wastewater effluent supplemented with a lignocellulosic waste Phragmites australis

We conducted experiments to culture Pycnoporus sp. SYBC-L3 in a medium comprising an industrial waste (dye-containing textile effluent) and a lignocellulosic waste (Phragmites australis) that achieved laccase production while having the color removed from the wastewater. Our experimental results showed that the fungus grew well in liquid submerged cultivation with the diluted textile effluent as the sole culture medium, but relatively low extracellular laccase activity (1.8 U/mL) was produced. Addition of the lignocellulosic biomass enhanced laccase production and color removal. The highest laccase activity was found to be 6.5 U/mL in the presence of Phragmites australis stem. Under this condition, 70 % color removal occurred in the culture medium. This study provided an alternative novel scheme to remove color in textile wastewater while having an economic value added by producing laccase.

Peroxidase-mediated removal of endocrine disrupting compound mixtures from water

Several classes of oxidative enzymes have shown promise for efficient removal of endocrine disrupting compounds (EDCs) that are resistant to conventional wastewater treatments. Although the kinetics of reactions between individual EDCs and selected oxidative enzymes are well documented in the literature, there has been little investigation of reactions with EDC mixtures. This makes it impossible to predict how enzyme-mediated treatment systems will perform since wastewater effluents generally contain multiple EDCs. This paper reports pseudo-first order rate constants for a model oxidative enzyme, horseradish peroxidase (HRP), during single-substrate (k1) and mixed-substrate (k1-MIX) reactions. Measured values are compared with literature values of three Michaelis-Menten parameters: half-saturation constant (KM), enzyme turnover number (kCAT), and the ratio kCAT/KM. Published reports had suggested that each of these could be correlated with HRP reactivity towards EDCs in mixtures, and empirical results from this study show that KM can be used to predict the sequence of EDC removal reactions within a particular mixture. We also observed that k1-MIX values were generally greater than k1 values and that compounds exhibiting greatest estrogenic toxicities reacted most rapidly in a given mixture. Finally, because KM may be tedious to measure for every EDC of interest, we have constructed a quantitative structure-activity relationship (QSAR) model to predict these values. This model predicts KM quite accurately (R2=89%) based on two molecular characteristics: molecular volume and hydration energy. Its accuracy makes this QSAR a useful tool for predicting which EDCs will be removed most efficiently during enzyme treatment of EDC mixtures.

QSAR-assisted design of an environmental catalyst for enhanced estrogen remediation

A quantitative structure-activity relationship (QSAR) was used to streamline re-design of a model environmental catalyst, horseradish peroxidase (HRP), for enhanced reactivity towards a target pollutant, steroid hormone 17β-estradiol. This QSAR, embodying relationship between reaction rate and intermolecular binding distance, was used in silico to screen for mutations improving enzyme reactivity. Eight mutations mediating significant reductions in binding distances were expressed in Saccharomyces cerevisiae, and resulting recombinant HRP strains were analyzed to determine Michaelis-Menten parameters during reaction with the target substrate. Enzyme turnover rate, ln(kCAT), exhibited inverse relationship with model-predicted binding distances (R2=0.81), consistent with the QSAR. Additional analysis of native substrate degradation by selected mutants yielded unexpected increases in ln(kCAT) that were also inversely correlated (R2=1.00) with model-predicted binding distances. This suggests that the mechanism of improvement comprises a nonspecific "opening up" of the active site such that it better accommodates environmental estrogens of any size. The novel QSAR-assisted approach described herein offers specific advantages compared to conventional design strategies, most notably targeting an entire class of pollutants at one time and a flexible hybridization of benefits associated with rational design and directed evolution. Thus, this approach is a promising tool for improving enzyme-mediated environmental remediation.

Ligninase-mediated removal of 17beta-estradiol from water in the presence of natural organic matter: efficiency and pathways

Lignin peroxidase (LiP) is excreted by certain lignin-degrading fungi, such as white rot fungus Phanerochaete chrysosporium, in natural environments and is thus widely present in the natural environment. We have found in our earlier studies that LiP mediates effective reactions of a few natural and synthetic estrogens to form oligomeric products via radical coupling. We in particular examined the identity and property of the products resulting from 17beta-estradiol (E2) in LiP-mediated oxidative coupling reactions, and the results suggest that such reactions hold great potential in water/wastewater treatment to remove E2 and estrogenicity. Herein, we report a further investigation to postulate possible reaction pathways of E2 with the assistance of ab initio molecular modeling and to more systematically examine the reaction behavior of E2 under sequenced reaction conditions and in systems containing natural organic matter (NOM) at different levels. Our molecular modeling suggested the coupling of E2 likely proceeded via covalent bonding between two E2 radicals at their unsubstituted carbons in phenolic rings. Results obtained from sequenced reagent feed experiments revealed that the coupling products tended to be consumed with increment enzyme treatments, suggesting that most E2 coupling products may still be LiP substrates that can undergo further coupling reactions under catalysis. Higher concentration of NOM present in the reaction system tended to reduce E2 transformation. NOM moieties seemed to couple to each other upon reaction with LiP, which was evidenced by the development of a characteristic absorbance band.

Ligninase-mediated removal of natural and synthetic estrogens from water: I. Reaction behaviors

Our experiments revealed that a few natural and synthetic estrogens can be effectively transformed through reactions that are mediated by lignin peroxidase (LiP), an extracellular enzyme that is produced by a white rot fungus Phanerochaete chrysosporium and is widely present in the natural environment We systematically assessed the reaction efficiencies at varying important conditions and identified the reaction products using mass spectrometry. In particular, we compared the reaction behaviors for systems variously containing natural organic matter and/or veratryl alcohol, a secondary metabolite that P. chrysosporium produces along with LiP in nature to play a role in mediating LiP activity. On the basis of the observed reaction behaviors and the molecular characteristics of the substrates and the enzyme, we postulate that the active binding site for estrogens is located within the LiP heme cavity, whereas that for veratryl alcohol is on the enzyme surface. Our study suggests that the processes mediated by LiP and other naturally occurring enzymes of similar nature may influence the environmental transformation and fate of estrogen contaminants. The findings in this study provide useful information for understanding LiP-mediated estrogen reactions and for potential development of novel enzymatic method to control estrogen contamination.