Polymerization of humic substances by an enzyme-catalyzed oxidative coupling

Unraveling Multiple Pathways of Electron Donation from Phenolic Moieties in Natural Organic Matter

Natural organic matter (NOM) exhibits a distinctive electron-donating capacity (EDC) that serves a pivotal role in the redox reactions of contaminants and minerals through the transformation of electron-donating phenolic moieties. However, the ambiguity of the molecular transformation pathways (MTPs) that engender the EDC during NOM oxidation remains a significant issue. Here, MTPs that contribute to EDC were investigated by identifying the oxidized products of phenolic model compounds and NOM samples in direct or mediated electrochemical oxidation (DEO or MEO, respectively) using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). It was found that the oxidation of newly formed phenolic-OH (ArOH) and the oxidative coupling reaction of the phenoxy radical are the main MTPs that directly contribute to EDC, in addition to the transformation of hydroquinones to quinones. Notably, the oxidative coupling reaction of ArOH contributed at least 22-42% to the EDC. Ferulic acid-like structures can also directly contribute to EDC by incorporating H2O into their acrylic substituents. Furthermore, the opening of C rings can indirectly attenuate the EDC through structural alterations in the electron-donating process of NOM. Decarboxylation can either weaken or enhance the EDC depending on the structure of the phenolic moieties in NOM. These findings suggest that the EDC of NOM is a comprehensive result of multiple NOM MTPs, involving not only ArOH oxidation but also the addition of H2O to olefinic bonds and bond-breaking reactions. Our work provides molecular evidence that aids in the comprehension of the multiple EDC-associated transformation pathways of NOM.

Prospects for humic acids treatment and recovery in wastewater: A review

Clean water shortages require the reuse of wastewater. The presence of organic substances such as humic acids in wastewater makes the water treatment process more difficult. Humic acids can significantly affect the removal of heavy metals and other such toxins. Humic acids is formed by the decomposition and transformation of animal and plant remains by microorganisms, and naturally exists in soil and water. It is necessary to degrade and remove humic acids from wastewater. As it seriously human health, effective technologies for removing humic acids from wastewater have attracted great interest over the past decades. This study compared existing techniques for removing humic acids from wastewater, as well as their limitations. Physicochemical treatments including filtration and oxidation are basic and key approaches to removing humic acids. Biological treatments including enzyme and fungi-mediated humic acids degradation are economically feasible but require some scalability. In conclusion, the integrated treatment processes are more significant options for the effective removal of humic acids from wastewater. In addition, humic acids have rich utilization values. It can improve the soil, increase crop yields, and promote the removal of pollutants.

Formation and nature of non-extractable residues of emerging organic contaminants in humic acids catalyzed by laccase

Formation of non-extractable residues (NERs) is the major fate of most environmental organic contaminants in soil, however, there is no direct evidence yet to support the assumed physical entrapment of NERs (i.e., type I NERs) inside soil humic substances. Here, we used ¹⁴C-radiotracer and silylation techniques to analyze NERs of six emerging and traditional organic contaminants formed in a suspension of humic acids (HA) under catalysis of the oxidative enzyme laccase. Laccase induced formation of both type I and covalently bound NERs (i.e., type II NERs) of bisphenol A, bisphenol F, and tetrabromobisphenol A to a large extent, and of bisphenol S (BPS) and sulfamethoxazole (SMX) to a less extent, while no induction for phenanthrene. The type I NERs were formed supposedly owing to laccase-induced alteration of primary (active groups) and secondary (conformation) structure of humic supramolecules, contributing surprisingly to large extents (23.5%-65.7%) to the total NERs, particularly for BPS and SMX, which both were otherwise not transformed by laccase catalysis. Electron-withdrawing sulfonyl group and bromine substitution significantly decreased amount and kinetics of NER formation, respectively. This study provides the first direct evidence for the formation of type I NERs in humic substances and implies a "Trojan horse" effect of such NERs in the environment.

Humic Substances as Microalgal Biostimulants—Implications for Microalgal Biotechnology

Humic substances (HS) act as biostimulants for terrestrial photosynthetic organisms. Their effects on plants are related to specific HS features: pH and redox buffering activities, (pseudo)emulsifying and surfactant characteristics, capacity to bind metallic ions and to encapsulate labile hydrophobic molecules, ability to adsorb to the wall structures of cells. The specific properties of HS result from the complexity of their supramolecular structure. This structure is more dynamic in aqueous solutions/suspensions than in soil, which enhances the specific characteristics of HS. Therefore, HS effects on microalgae are more pronounced than on terrestrial plants. The reported HS effects on microalgae include increased ionic nutrient availability, improved protection against abiotic stress, including against various chemical pollutants and ionic species of potentially toxic elements, higher accumulation of value-added ingredients, and enhanced bio-flocculation. These HS effects are similar to those on terrestrial plants and could be considered microalgal biostimulant effects. Such biostimulant effects are underutilized in current microalgal biotechnology. This review presents knowledge related to interactions between microalgae and humic substances and analyzes the potential of HS to enhance the productivity and profitability of microalgal biotechnology.

The Central Role of Soil Organic Matter in Soil Fertility and Carbon Storage

The aim of the paper is to give an overview on the chemistry of soil organic carbon (SOC) affecting nutrient availability, the emission of greenhouse gases and detoxifying harmful substances in soil. Humic substances represent the stable part of SOC, accounting for between 50 and more than 80% of organically bound carbon in soil. Humic substances strongly affect the soil solution concentration of several plant nutrients and may increase P-, Fe-, and Cu- solubility, thereby increasing their plant availability. Soil organic carbon, mainly humic substances, can detoxify monomeric Al in acid soils, can strongly bind toxic heavy metals, making them unavailable to the plant roots, and may strongly bind a vast variety of harmful organic pollutants. Increasing SOC is an important goal in agriculture. The inclusion of mixtures of semi-perennial plant species and cultivars may strongly increase SOC and humic substance content in soils. To increase SOC, farmyard manure and its rotted or composted forms are superior compared to the separate application of straw and slurry to soil. The storage of carbon, mainly in organic form, in soils is very important in the context of the emission of greenhouse gases. Worldwide, soils release about 10 times more greenhouse gases compared to fossil fuel combustion. Small increments in SOC worldwide will strongly affect the concentration of atmospheric CO2. The public discussion on soil fertility and greenhouse gas emissionshas been politically controlled in a way that leaves the important and positive contribution of soil organic carbon and mainly humic substances partly misinterpreted and partly underestimated.

Interactions between Humic Substances and Microorganisms and Their Implications for Nature-like Bioremediation Technologies

The state of the art of the reported data on interactions between microorganisms and HSs is presented herein. The properties of HSs are discussed in terms of microbial utilization, degradation, and transformation. The data on biologically active individual compounds found in HSs are summarized. Bacteria of the phylum Proteobacteria and fungi of the phyla Basidiomycota and Ascomycota were found to be the main HS degraders, while Proteobacteria, Actinobacteria, Bacteroidetes, and Firmicutes were found to be the predominant phyla in humic-reducing microorganisms (HRMs). Some promising aspects of interactions between microorganisms and HSs are discussed as a feasible basis for nature-like biotechnologies, including the production of enzymes capable of catalyzing the oxidative binding of organic pollutants to HSs, while electron shuttling through the utilization of HSs by HRMs as electron shuttles may be used for the enhancement of organic pollutant biodegradation or lowering bioavailability of some metals. Utilization of HSs by HRMs as terminal electron acceptors may suppress electron transfer to CO2, reducing the formation of CH4 in temporarily anoxic systems. The data reported so far are mostly related to the use of HSs as redox compounds. HSs are capable of altering the composition of the microbial community, and there are environmental conditions that determine the efficiency of HSs. To facilitate the development of HS-based technologies, complex studies addressing these factors are in demand.

Stable isotopes reveal the formation diversity of humic substances derived from different cotton straw-based materials

Humic substances (HS) are essential in environment processes and carbon (C) sequestration in soils. In this study, organic materials such as cotton straw and its derived compost and biochar were added to the soil on a C-equivalent basis and incubated for 30 and 180 days in order to investigate the different forms of plant biomass derived C sequestration in HS. The C distribution in humic acid (HA), fulvic acid (FA), and humin (Hu) derived from organic materials was investigated using the ¹³C isotope method, while the catalase, sucrose, and β-glucosidase activities were also determined. The results showed that C3 distribution of Hu derived from straw, compost and biochar increased from 40.94% to 67.12%, 74.47% and 80.75%, respectively. In addition, the increase of C3 distribution of HA or FA derived from straw, compost and biochar were 4.69%, 10.09% and 1.49%, respectively. There were significantly positive correlations between catalase, sucrase and β-glucosidase activities and C3 derived HA and FA. The principal component analysis showed that catalase, sucrase and β-glucosidase were explained mainly by the first principal component indicating a significant correlation. These findings suggest that straw, compost and biochar are mainly sequestrated in Hu. Comparatively, the straw and compost are more likely to contribute to the formation of HA and FA in soil, but biochar favors the Hu, which helps in soil C sequestration. The formation of HA and FA derived from organic materials was supported by catalase, sucrase and β-glucosidase activities.

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.

Effects of natural organic matters on bioavailability of petroleum hydrocarbons in soil-water environments

The bioremediation efficiency of petroleum hydrocarbons in natural soil-water systems is regulated by active microbial populations and other system parameters. Relevant factors include the transfer rate of petroleum contaminants from a medium into microorganisms, the partitioning behavior of contaminants from water into the soil organic matter (SOM), and the influence of the dissolved organic matter (DOM) on the contaminant level in water. The objectives of this study was aimed to determine the correlation among bioavailability of petroleum hydrocarbons, SOM content, and DOM level in soil-water systems. Heptadecane, pristane, and decylcyclohexane were selected as model hydrocarbon contaminants. The bioavailability of target contaminants in soil was examined using soils of different SOM contents (2% and 20%) in slurry bioreactors. In addition, the contaminant bioavailability as affected by various DOM levels (0-100 mgC/L) was also examined. The results showed that the SOM content affected the degrading rate of hydrocarbons significantly, where the rate constant was 4 times higher in 2% SOM microcosm than in the 20% SOM bioreactor for heptadecane degradation. Similarly, the pristane degrading efficiency after 240 h operation was 95% for the 2% SOM microcosm and only 38% for the 20% SOM microcosm. The hydrocarbon degradation rates in water phase were found to be enhanced by the added DOM level. A positive correlation existed between the contaminant bioavailability and the contaminant level in water as impacted by the SOM content in soil and the DOM level in water.

Kinetic aspects of humic substances derived from macrophyte detritus decomposition under different nutrient conditions

Autochthonous particulate organic carbon (POC) is an important precursor of humic substances (HS), and macrophytes represent the major source of POC in tropical aquatic ecosystems. Autochthonous HS influence the carbon supply, light regime, and primary production within freshwater systems. This study addresses the conversion of POC from two macrophyte species into HS and their mineralization under different nutrient conditions (oligotrophic to hypereutrophic). A first-order kinetic model was adopted to describe the conversion routes. The POC conversion rate to HS for detritus derived from Paspalum repens was similar under different nutrient conditions, but eutrophication decreased the k_R (global coefficient reaction) for detritus from Pistia stratiotes due to its high detritus quality (C:N:P ratio). Fulvic acids were the main fraction of HS in both plants. The mineralization of humic acids from P. stratiotes was inhibited at higher nutrient availability, while eutrophication increased the mineralization of fulvic acids from P. repens. The main route of POC cycling is humification through fulvic acid formation (up to 40% of POC). The intrinsic characteristics of the source detritus were the main forcing functions that stimulated the cycling of HS. In tropical aquatic ecosystems, the degradation of autochthonous carbon decreased due to eutrophication, thus contributing to the diagenetic process in the long term.

A review of landfill leachate induced ultraviolet quenching substances: Sources, characteristics, and treatment

Landfill leachate contains extremely diverse mixtures of pollutants and thus requires appropriate treatment before discharge. Co-treatment of landfill leachate with sewage in wastewater treatment plants is a common approach because of low cost and convenience. However, some recalcitrant organic compounds in leachate can escape biological treatment processes, lower the UV transmittance of waste streams due to their UV-quenching properties, and interfere with the associated disinfection efficacy. Thus, the leachate UV quenching substances (UVQS) must be removed or reduced to a level that UV disinfection is not strongly affected. UVQS consist of three major fractions, humic acids, fulvic acids and hydrophilics, each of which has distinct characteristics and behaviors during treatment. The purpose of this review is to provide a synthesis of the state of the science regarding UVQS and possible treatment approaches. In general, chemical, electrochemical, and physical treatments are more effective than biological treatments, but also costlier. Integration of multiple treatment methods to target the removal of different fractions of UVQS can aid in optimizing treatment. The importance of UVQS effects on wastewater treatment should be better recognized and understood with implemented regulations and improved research and treatment practice.

Laccase induced degradation of perfluorooctanoic acid in a soil slurry

Perfluorooctanoic acid (PFOA) can induce undesirable effects to humans and animals and has thus attracted much attention from the public and scientific communities in recent years. Extensive studies have been devoted to exploring PFOA degradation in aqueous phase, while information for that in soil is rather limited. Understanding the transformation of PFOA in soil is important in developing strategies to assess and manage its environmental fate. In this study, we have demonstrated that PFOA can be effectively degraded by enzyme catalyzed oxidative humification reactions (ECOHRs) with a natural organic material, soybean meal, as the mediator. In the presence of soybean meal and laccase, PFOA was degraded 24% in water after 36 days, 40% in soil slurry after 140 days. The water extract of soybean meal contained high concentrations of natural organic mediators and multivalent metal ions, both of which were essential to PFOA degradation by ECOHRs. The ECOHR degradation products of PFOA in soil were identified to be partially fluorinated organic compounds, and the molecular features of the products suggest that the degradation mechanism involves free radical chain reaction processes, which was initiated by direct free radical attacks on the CC bonds in perfluoroalkyl acids.

Removal of pharmaceutical compounds in water and wastewater using fungal oxidoreductase enzymes

Due to recalcitrance of some pharmaceutically active compounds (PhACs), conventional wastewater treatment is not able to remove them effectively. Therefore, their occurrence in surface water and potential environmental impact has raised serious global concern. Biological transformation of these contaminants using white-rot fungi (WRF) and their oxidoreductase enzymes has been proposed as a low cost and environmentally friendly solution for water treatment. The removal performance of PhACs by a fungal culture is dependent on several factors, such as fungal species, the secreted enzymes, molecular structure of target compounds, culture medium composition, etc. In recent 20 years, numerous researchers tried to elucidate the removal mechanisms and the effects of important operational parameters such as temperature and pH on the enzymatic treatment of PhACs. This review summarizes and analyzes the studies performed on PhACs removal from spiked pure water and real wastewaters using oxidoreductase enzymes and the data related to degradation efficiencies of the most studied compounds. The review also offers an insight into enzymes immobilization, fungal reactors, mediators, degradation mechanisms and transformation products (TPs) of PhACs. In brief, higher hydrophobicity and having electron-donating groups, such as amine and hydroxyl in molecular structure leads to more effective degradation of PhACs by fungal cultures. For recalcitrant compounds, using redox mediators, such as syringaldehyde increases the degradation efficiency, however they may cause toxicity in the effluent and deactivate the enzyme. Immobilization of enzymes on supports can enhance the performance of enzyme in terms of reusability and stability. However, the immobilization strategy should be carefully selected to reduce the cost and enable regeneration. Still, further studies are needed to elucidate the mechanisms involved in enzymatic degradation and the toxicity levels of TPs and also to optimize the whole treatment strategy to have economical and technical competitiveness.

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 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.

Transformation of triclosan by laccase catalyzed oxidation: The influence of humic acid-metal binding process

Laccase is a widely present extracellular phenoloxidase excreted by fungi, bacteria, and high plants. It is able to catalyze one-electron oxidation of phenolic compounds into radical intermediates that can subsequently couple to each other via covalent bonds. These reactions are believed to play an important role in humification process and the transformation of contaminants containing phenolic functionalities in the environment. In this study, we investigated the kinetics of triclosan transformation catalyzed by laccase. It was found that the rate of triclosan oxidation was first order to the concentrations of both substrate and enzyme. Humic acid (HA) could inhibit the reaction by quenching the radical intermediate of triclosan generated by laccase oxidation. Such inhibition was more significant in the presence of divalent metal cations. This is because that binding to metal ions neutralized the negative charge of HA molecules, thus making them more accessible to laccase molecule that is also negatively charged. Therefore, it has greater chance to quench the radical intermediate that is very unstable and can only diffuse a limited distance after being released from the enzyme catalytic center. Based on these understandings, a reaction model was developed by integration of metal-HA binding equilibriums and kinetic equations. This model precisely predicted the transformation rate of triclosan in the presence of HA and divalent metal ions including Ca²⁺, Mg²⁺, Cd²⁺, Co²⁺, Mn²⁺, Ba²⁺, and Zn²⁺. Overall, this work reveals important insights into laccase catalyzed oxidative coupling process.

Interactions between natural organic matter and organic pollutants as revealed by NMR spectroscopy

Natural organic matter (NOM) plays a critical role in regulating the transport and the fate of organic contaminants in the environment. NMR spectroscopy is a powerful technique for the investigation of the sorption and binding mechanisms between NOM and pollutants, as well as their mutual chemical transformations. Despite NMR relatively low sensibility but due to its wide versatility to investigating samples in the liquid, gel, and solid phases, NMR application to environmental NOM-pollutants relations enables the achievement of specific and complementary molecular information. This report is a brief outline of the potentialities of the different NMR techniques and pulse sequences to elucidate the interactions between NOM and organic pollutants, with and without their labeling with nuclei that enhance NMR sensitivity.

Formation of Halogenated Polyaromatic Compounds by Laccase Catalyzed Transformation of Halophenols

Laccases are a type of extracellular enzyme produced by fungi, bacteria, and plants. Laccase can catalyze one-electron oxidation of a variety of phenolic compounds using molecular oxygen as the electron acceptor. In this study, transformation of halophenols (XPs) in laccase-catalyzed oxidation processes was explored. We first examined the intrinsic reaction kinetics and found that the transformation of XPs appeared first order to the concentrations of both XPs and laccase. A numerical model was developed to describe the role of humic acid (HA) in this process. It was demonstrated that HA could reverse the oxidation of XPs by acting as the inner filtrator of XP radical intermediates formed upon reactions between the substrates and laccase. The extent of such reversion was proportional to HA concentration. MS analysis in combination with quantum chemistry computation suggested that coupling products were generated. XPs coupled to each via C-C or C-O-C pathways, generating hydroxyl polyhalogenated biphenyl ethers (OH-PCDEs) and hydroxyl polyhalogenated biphenyls, respectively. Many of these polyhalogenated products are known to be hazardous to the ecosystem and human health, but they are not synthetic chemicals. This study shed light on their genesis in the environmental media.

Monitoring transitory profiles of leachate humic substances in landfill aeration reactors in mesophilic and thermophilic conditions

The presence of humic substances (HS) in landfill leachate is of great interest because of their structural stability and potential toxicity. This study examined the effects of temperature and waste age on the transformation of HS during in situ aeration of bioreactor landfills. By establishing aerobic conditions, dissolved organic carbon (DOC) rapidly accumulated in the bioreactor leachate. Fractional analysis showed that the elevated concentration of humic acids (HAs) was primarily responsible for the increment of leachate strength. Further structural characterization indicated that the molecular weight (MW) and aromacity of HS were enhanced by aeration in conjunction with thermophilic temperature. Interestingly, elevation of HAs concentration was not observed in the aeration reactor with a prolonged waste age, as the mobility of HAs was lowered by the high MW derived from extended waste age. Based on these results, aeration may be more favorable in aged landfills, since dissolution of HAs could be minimized by the evolution to larger MW compared to young landfills. Moreover, increased operation temperature during aeration likely offers benefits for the rapid maturation of HS.

Laccase mediated transformation of 17β-estradiol in soil

It is known that 17β-estradiol (E2) can be transformed by reactions mediated by some oxidoreductases such as laccase in water. Whether or how such reactions can happen in soil is however unknown although they may significantly impact the environmental fate of E2 that is introduced to soil by land application of animal wastes. We herein studied the reaction of E2 in a model soil mediated by laccase, and found that the reaction behaviors differ significantly from those in water partly because of the dramatic difference in laccase stability. We also examined E2 transformation in soil using (14)C-labeling in combination with soil organic matter extraction and size exclusion chromatography, which indicated that applied (14)C radioactivity was preferably bound to humic acids. The study provides useful information for understanding the environmental fate of E2 and for developing a novel soil remediation strategy via enzyme-enhanced humification reactions.

Effects of biochar and the geophagous earthworm Metaphire guillelmi on fate of (14)C-catechol in an agricultural soil

Both biochar and earthworms can exert influence on behaviors of soil-borne monomeric phenols in soil; however, little was known about the combined effects of biochar and earthworm activities on fate of these chemicals in soil. Using (14)C-catechol as a representative, the mineralization, transformation and residue distribution of phenolic humus monomer in soil amended with different amounts of biochar (0%, 0.05%, 0.5%, and 5%) without/with the geophagous earthworm Metaphire guillelmi were investigated. The results showed biochar at amendment rate <0.5% did not affect (14)C-catechol mineralization, whereas 5% biochar amendment significantly inhibited the mineralization. Earthworms did not affect the mineralization of (14)C-catechol in soil amended with <0.5% biochar, but significantly enhanced the mineralization in 5% biochar amended soil when they were present in soil for 9 d. When earthworms were removed from the soil, the mineralization of (14)C-catechol was significantly lower than that of in earthworm-free soil indicating that (14)C-catecholic residues were stabilized during their passage through earthworm gut. The assimilation of (14)C by earthworms was low (1.2%), and was significantly enhanced by biochar amendment, which was attributed to the release of biochar-associated (14)C-catecholic residues during gut passage of earthworm.

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.

Organic matter produced by algae and cyanobacteria: quantitative and qualitative characterization

This work aims at characterizing organic matter produced by an alga Euglena gracilis and a cyanobacteria Microcystis aeruginosa and assessing the evolution of its characteristics during growth. A culture medium was optimized. The species growth phases were monitored using both visible spectrophotometry and flow cytometry cell counting. Organic matter fractionation according to hydrophobicity and specific UV absorbance (SUVA) index were used to specifically characterize the produced algal organic matter (AOM). The AOM characteristics were both growth phase and species dependent. However, a similar evolution was observed. The hydrophilic fraction (HPI) was the major fraction whatever the growth phases and was almost the only one produced during lag and exponential phases. It represented around 75% of AOM during exponential phase and then decreased when the stationary phase appeared. It represented 46% and 60% of the AOM during late decline phase for the cyanobacteria and the alga respectively. The hydrophobic (HPO) and transphilic (TPH) fractions started to appear from the beginning of the stationary phase with more hydrophobic compounds coming from intracellular organic material of dying cells. HPO and TPH percentages still increased during the decline phase probably because of two additional processes: photo-dissolution and leaching of particulate organic matter from cells fragments. A comparison of AOM during late decline phase and natural organic matter (NOM) from Glane River (France) underlined that AOM was more hydrophilic and presented a lower SUVA for each fractions than NOM. However, the difference between NOM and AOM hydrophobicity narrowed during decline phase.

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: II. Reactions of 17beta-estradiol

We have demonstrated in our earlier work that a few natural and synthetic estrogens can be effectively transformed through reactions mediated by lignin peroxidase (LiP). The behaviors of such reactions are variously influenced by the presence of natural organic matter (NOM) and/or veratryl alcohol (VA). Certain white rot fungi, e.g. Phanerochaete chrysosporium, produce VA as a secondary metabolite along with LiP in nature where NOM is ubiquitously present. Herein, we report a study on the products resulting from LiP-mediated oxidative coupling reactions of a representative estrogen, 17beta-estradiol (E2), and how the presence of NOM and/or VA impacts the formation and distribution of the products. A total of six products were found, and the major products appeared to be oligomers resulting from E2 coupling. Our experiments revealed that these products likely formed colloidal solids in water that can be removed via ultrafiltration or settled during ultracentrifugation. Such a colloidal nature of the products could have important implications in their treatability and environmental transport. In the presence of VA, the products tended to shift toward higher-degree of oligomers. When NOM was included in the reaction system, cross-coupling between E2 and NOM appeared to occur. Data obtained from E-SCREEN test confirmed that the estrogenicity of E2 can be effectively eliminated following sequential reactions mediated by LiP.

Removal of acetaminophen using enzyme-mediated oxidative coupling processes: II. Cross-coupling with natural organic matter

The influence of natural organic matter (NOM) on the transformation of acetaminophen in laccase-mediated oxidative coupling systems was investigated in this study. It was found that the removal of acetaminophen was enhanced while the self-coupling of acetaminophen was suppressed in the presence of dissolved NOM, likely resulting from cross-coupling between dissolved NOM and acetaminophen. In additionto cross-coupling with acetaminophen, NOM moieties could couple to each other upon reaction with laccase. This was evidenced by the development of a characteristic absorbance band centered at 472 nm. According to the rate of the absorbance change at 472 nm, the NOM coupling reactions in four different NOM solutions were evaluated. Apparently, the tendency of NOM coupling reactions correlates with the tendency of acetaminophen cross coupling with NOM in these solutions. Possible reaction pathways of cross-coupling were explored using guaiacol as a model NOM proxy, and the products were extracted and analyzed with mass spectrometry (MS). The results suggested that acetaminophen and guaiacol molecules were cross-coupled via the formation of C-O-C bonds.

Removal of acetaminophen using enzyme-mediated oxidative coupling processes: I. Reaction rates and pathways

We found that acetaminophen could be effectively transformed and removed from water by laccase-mediated oxidative coupling processes. The removal of acetaminophen followed second-order kinetics with first-order to the concentrations of both the substrate and the enzyme. Mass spectrum analysis demonstrated that polymerization through radical-radical coupling mechanism was the pathway leading to acetaminophen transformation. Coupling products thus formed are believed to be biologically inactive and more readily removable from water. Secondary mass spectra of the dimers in combination with molecular modeling analysis further elucidated that the coupling proceeded via covalent bonding between two molecules at their unsubstituted carbons in benzene rings. These findings demonstrated that laccase-mediated oxidative coupling can potentially serve as an alternative strategy to control certain micropollutants in water/wastewater treatment and reuse.

High-performance size-exclusion chromatography of humic substances on the hydroxyethyl methacrylate column

HEMA, the copolymer of 2-hydroxyethyl methacrylate and ethylene dimethacrylate, has proven to be a suitable stationary phase for the size-exclusion chromatographic (SEC) characterization of humic substances in a wide pH range. The SEC measurements were performed typically with alkaline borate-based mobile phase: 0.025 mol/l Na2B4O7, 0.025 mol/l NaCl, 0.001 mol/l Na4P2O7, pH 9.1. After calibrating with globular protein standards, apparent molecular weights of typical peat- and coal-derived humic acids were estimated. The average molecular weights determined with the aid of the HEMA column were quite comparable with the values obtained previously by the method of low-pressure gel-permeation chromatography on the Sephadex column.

Advanced TMAH and TMAAc thermochemolysis–pyrolysis techniques for molecular characterization of size-separated fractions from aquatic dissolved organic matter

The structural similarities and differences between the original DOM and the eight size fractions separated were studied in detail with the pyrolysis technique in combination with gas chromatography and mass spectrometry (Py-GC-MS) using two alkylating reagents: TMAH (tetramethylammonium hydroxide), to find both esterified and free carboxylic acids; and TMAAc (tetramethylammonium acetate), to specify only free carboxylic acids. A statistical analysis of the original multidimensional TMAH and TMAAc pyrograms disclosed that the overall structural compositions of the five most important size fractions, accounting for 84% of the original DOM, greatly resembled each other. The remaining three minor size fractions were not classified as homogeneous associations, but they also contained the same total, covalently bound and free carboxylic acid species as the other size fractions and the original DOM mixture, thus representing some kind of intermediate forms. This fundamental outcome strongly supports the opinion that the native dissolved humic-like macromolecules resemble supramolecular associations of smaller molecular size moieties with similar structural functionalities. The concentrations of free aliphatic and aromatic dicarboxylic acids in the DOM solution were so low that their effects on the potential formation of multiply charged ions in electrospray ionization-MS (ESI-MS) studies are likely insignificant.

Reduction of 2,4-dichlorophenol toxicity to Pseudomonas putida after oxidative incubation with humic substances and a biomimetic catalyst

The effect of a synthetic iron(III)-porphyrin meso-tetra(2,6-dichloro-3-sulfonatophenyl)porphyrinate as a biomimetic catalyst in the oxidative treatment of 2,4-dichlorophenol (2,4-DCP) with humic substances and H(2)O(2) was evaluated in factorial design experiments conducted at different concentrations of 2,4-DCP (0-25 ppm) and different incubation treatment times (0, 24, 96, or 120 h). In the absence of this treatment, bioassays with the bacterium Pseudomonas putida (ATCC11250) showed decreasing specific growth rates mu (used here to quantify 2,4-DCP toxicity) with increasing concentrations of 2,4-DCP. However, when 2,4-DCP was treated as mentioned above the toxicity of the resulting 2,4-DCP solution was reduced significantly. At low 2,4-DCP concentrations (up to 5 ppm) and long incubation periods (as long as 120 h), the specific growth rate mu was comparable to that of cultures grown in the absence of 2,4-DCP. The reduction in toxicity was directly correlated to a decrease in the concentration of 2,4-DCP in the treated solutions, as measured by high-performance liquid chromatography. The reduced concentrations of 2,4-DCP in the treated solutions could be correctly predicted based on the relationship between the specific growth rates and the 2,4-DCP concentrations in untreated solutions. These results indicate that the oxidative coupling of 2,4-DCP to humic substances catalyzed by the synthetic iron(III)-porphyrin catalyst in the presence of H(2)O(2) is responsible for the removal of 2,4-DCP from solutions. This approach appears to be a promising alternative treatment to reduce 2,4-DCP bioavailability and thus toxicity in the environment.

Rates of oxidative coupling of humic phenolic monomers catalyzed by a biomimetic iron-porphyrin

A synthetic water-soluble meso-tetra(2,6-dichloro-3-sulfonatophenyl)porphyrinate of iron(lll) chloride (FeP) was used as biomimetic catalyst in the oxidative coupling of three monomeric phenols (catechol, caffeic, and p-coumaric acids), which are common constituents of natural humic substances. The extent of oxidation induced by the FeP catalyst in solutions of phenolic monomers was followed in the presence of an oxygen donor such as hydrogen peroxide or dissolved oxygen under daylight radiation. Both UV- and fluorescence-detected liquid chromatograms indicated that primary oxidation products had a larger electronic conjugation and molecular mass than the original phenols, thereby suggesting that the biomimetic oxidative catalysis produced covalently linked phenylene and oxyphenylene oligomers. However, the polyphenolic products were further oxidized in the progress of the catalytic reaction to possible undetectable aliphatic acids or even to complete mineralization. Rate constants describing the initial reaction period were larger for the catalyzed oxidation with hydrogen peroxide than those for the noncatalyzed control solutions under autoxidation or hydrogen peroxide treatment. However, the rate constants measured for the phenol solutions treated with just the FeP catalyst showed that the presence of dissolved oxygen and the action of the daylight radiation were sufficient to significantly increase the reaction rate in respect to control solutions. These results confirmed previous findings, showing that humic materials may undergo oxidative coupling catalyzed by metal-porphyrins in the presence of either an oxygen donor or, simply, dissolved molecular oxygen under daylight. The increase of molecular mass of natural humic and polyphenolic substances by this biomimetic technology may have useful applications in environmental chemistry.

Reduction of disinfection byproduct formation by molecular reconfiguration of the fulvic constituents of natural background organic matter

Experiments were performed to assess the effects of treating the fulvic acid fractions of background natural organic matter (NOM) by catalyst-induced oxidative coupling reactions. Changes in the molecular characteristics of the fulvic acids and related disinfection byproduct formation potentials of these important NOM constituents were investigated. The coupling reactions were induced by addition of horseradish peroxidase (HRP) and hydrogen peroxide to aqueous solutions of the fulvic acids (FAs) in semicontinuous flow reactors. Subsequent removal of organic matter by ultrafiltration was found to be markedly enhanced for FA solutions subjected to oxidative coupling treatment. Uniform formation condition tests further indicated that the disinfection byproducts formed upon chlorination of FAs treated via induced oxidative coupling were reduced significantly on a unit carbon basis relative to those formed upon chlorination of their untreated counterparts. Spectroscopic examinations revealed thatthe FA molecules were effectively reconfigured in the oxidative coupling reactions. Substantial conversion of aromatic hydroxyl groups into ether-bonded moieties is evident, and a loss of primary amine groups, probably through conversion into secondary or tertiary amines, was also observed. These conversions apparently result in cross-linking of the natural FA moieties to form stable species of larger sizes, thus rendering them more readily removable by ultrafiltration and less reactive with chlorine. The results of the study may be interpreted as indicating that catalytically induced oxidative coupling reactions of the type conducted in this work can be combined with ultrafiltration to provide an effective scheme for removal of disinfection byproduct precursors.

Impact of vegetation on sedimentary organic matter composition and polycyclic aromatic hydrocarbon attenuation

Results from natural and engineered phytoremediation systems provide strong evidencethatvegetated soils mitigate polycyclic aromatic hydrocarbon (PAH) contamination. However, the mechanisms by which PAH mitigation occurs and the impact of plant organic matter on PAH attenuation remain unclear. This study assessed the impact of plant organic matter on PAH attenuation in labile and refractory sediments fractions from a petroleum distillate waste pit that has naturally revegetated. Samples were collected in distinct zones of barren and vegetated areas to assess changes to organic matter composition and PAH content as vegetation colonized and became established in the waste pit. Sediments were fractionated into bulk sediment and humin fractions and analyzed for organic matter composition by isotope ratio mass spectrometry (delta (13)C), 13C nuclear magnetic resonance (13C NMR), delta 14C AMS (accelerator mass spectrometry), and percent organic carbon (%TOC). Gas chromatography mass spectrometry (GC/ MS) of lipid extracts of SOM fractions provided data for PAH distribution histograms, compound weathering ratios, and alkylated and nonalkylated PAH concentrations. Inputs of biogenic plant carbon, PAH weathering, and declines in PAH concentrations are most evidentfor vegetated SOM fractions, particularly humin fractions. Sequestered PAH metabolites were also observed in vegetated humin. These results show that plant organic matter does impact PAH attenuation in both labile and refractory fractions of petroleum distillate waste.

Enhanced Molecular Dimension of a Humic Acid Induced by Photooxidation Catalyzed by Biomimetic Metalporphyrins

Synthetic water-soluble meso-tetra(2,6-dichloro-3-sulfonatophenyl)porphyrinates of Mn(III) chloride [Mn-(TDCPPS)Cl] and iron(III) chloride [Fe-(TDCPPS)Cl] were employed as biomimetic catalysts in the photooxidative coupling of a terrestrial humic acid (HA). The changes in molecular dimension of HA induced by irradiation with UV light for different periods were followed by high-performance size-exclusion chromatography (HPSEC), with both spectrophotometric and refractive index (RI) detectors. Enhancement of apparent weight-average molecular weight (M(wa)) of sodium humate solutions at both pH 7 and 3.5, occurred after irradiation with UV light after 5.5 and 13 h by catalysis with either manganese or iron porphyrin. A significant increase in M(wa) values was also found when samples were kept in the dark for 8 and 11 days after the end of irradiation, thereby suggesting a progression of free-radicals coupling with time course after photoinduction. The enhancement of absorptivity of humic matter with catalyzed photooxidation was confirmed by the significant increase in apparent molecular mass as shown by the RI detector. The latter also indicated a slightly larger effect by iron-porphyrin than by manganese-porphyrin in the photooxidative coupling of humic molecules. These findings suggest that photooxidation may represent a valid alternative to oxidizing agents, such as H(2)O(2), in the polymerization of natural organic matter catalyzed by biomimetic metalporphyrins.

Increased Conformational Rigidity of Humic Substances by Oxidative Biomimetic Catalysis

A synthetic water-soluble meso-tetra(2,6-dichloro-3-sulfonatophenyl)porphyrinate of iron(III) chloride, Fe(TDCPPS)Cl, was employed as a biomimetic catalyst in the oxidative coupling of terrestrial humic materials. High-performance size-exclusion chromatography (HPSEC), solid-state nuclear magnetic resonance (CPMAS-(13)C NMR), electron paramagnetic resonance (EPR), and diffuse reflectance infrared spectroscopy (DRIFT) were used to follow conformational and structural changes brought about in different humic materials by the oxidative coupling. Increase in apparent weight-average molecular weight (Mw(a)) occurred invariably for all humic substances with the oxidative polymerization catalyzed by Fe(TDCPPS)Cl. HPSEC further showed that the polymerization reaction turned the loosely bound humic supramolecular structures into more stable conformations which could no longer be disrupted by the disaggregating effect of acetic acid. DRIFT spectroscopy suggested the formation of new alkyl and aromatic ethers following the oxidative coupling with the biomimetic catalyst. CPMAS-(13)C NMR and EPR spectra suggested a reduced molecular mobility of humic components and enhanced stabilization of free radicals in larger oxidized fragments. All findings concur in indicating that the biomimetic catalysis by Fe(TDCPPS)Cl increased the molecular mass and chemical rigidity of humic materials by formation of intermolecular covalent bonds via a free-radical mechanism. The development of a technology based on oxidative polymerization by biomimetic catalysis may be of importance in controlling the properties and reactivity of humic matter for industrial and environmental applications.

Separation methods in the chemistry of humic substances

Separation methods are widely used to isolate humic substances (HSs), to fractionate them before further investigation, and to obtain information about their structure and properties. Among the chromatographic methods, techniques based on a size-exclusion effect appear to be most useful, as they allow us to relate elution data to the molecular mass distribution of HSs. The limitations of this approach are discussed in this review. Gas chromatography with mass spectrometric detection is typically used to identify the products of pyrolysis or thermochemolysis of HSs; this technique is considered most important in the structural investigation of HSs. Electrophoretic methods (especially capillary zone electrophoresis) provide detailed characterization of HSs, but it is very difficult to relate the electrophoretic data to any specific subfraction, structure or properties of HSs. The electrophoretic patterns are often called "fingerprints" and can potentially be used for the identification and classification of HSs. This is limited, however, by the great diversity of the procedures employed and by the low degree of harmonization--no data on reproducibility and between-laboratory comparability are available. The same holds true, to a certain degree, for most methods utilized for the characterization of HSs. Separation methods play an important role in the examination of the interactions of HSs with heavy metals and other chemical pollutants. They allow us to determine binding constants and other data necessary to predict the mobility of chemical pollutants in the environment.

Reduced heterogeneity of a lignite humic acid by preparative HPSEC following interaction with an organic acid. Characterization of size-separates by Pyr-GC-MS and 1H-NMR spectroscopy

Preparative high performance size exclusion chromatography (HPSEC) was used to size-fractionate a humic acid (HA) solution with 0.05 M ionic strength before and after having made the humic solution 0.5 x 10(-3) M in acetic acid (AcOH). Size-fractions were characterized by pyrolysis-gas-chromatography/mass spectrometry (Pyr-GC-MS) and 1H NMR spectroscopy. Pyr-GC-MS showed that the AcOH treatment altered the distribution of humic molecular components in the size-fractions. The unsaturated alkyl chains were moved from size-fractions of larger molecular-size into those of lower molecular-sizes. Most of the aromatic moieties, which were found in larger molecular-size fractions for the untreated HA, were spread into fractions of lower molecular size after AcOH addition to HA. Carbohydrates, which were undetectable in anyfraction of the untreated HA, appeared instead in the pyrogram of the lowest molecular-size and most hydrophilic fraction after treatment with AcOH. Our results suggested that AcOH disrupted the weakly bound association of humic supramolecularstructures and HPSEC elution separated size-fractions of different composition without losing humic matter by adsorption on the HPSEC column. The fractions with the largest apparent molecular size were the richest in alkyl chains, thereby suggesting that humic molecules were stabilized into supramolecular associations by multiple weak interactions among apolar groups such as alkyl chains and aromatic moieties. 1H NMR spectra of size-fractions were greatly simplified and more resolved after AcOH treatment. This was attributed to a less complex molecular association in the size separates which provided a larger solubility in the NMR solvent and more favorable relaxation times. Combination of the procedure used here for size-fractionation with NMR and Pyr-GC-MS methods appears to be promising to advance knowledge on the molecular composition of humic substances.