Humic Polymers Research Articles

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

Fungal laccase has aroused great concern in rapidly removing estrogens because of its ability to accelerate humification and oligomerization. Here, the effect of two humic acids (HAs) on the reaction kinetics and products distribution of 17α-ethynylestradiol (EE2) in laccase-initiated humification and coupling was systematically elucidated. Laccase from Trametes versicolor exhibited over 98.3% removal rate for EE2 at pH 5.0 within 120 min, while HAs invariably restrained EE2 transformation by competing with target-substrate for the enzymatic catalytic center. EE2 removal followed pseudo-first-order kinetics, and the rate constant was decreased markedly with increasing concentration of two HAs (0–60 mg L−1). Additionally, laccase heightened the aromaticity and humification degrees (A250 nm/A365 nm ratio) of HAs probably due to the formation of new humic polymers such as (HA)m and/or (HA)m-(EE2)n (m and n represent the number of HA and EE2 units, respectively). Three major EE2 oligomers were identified, in accordance with a mechanism involving the phenoxy radical-driven polymerization to yield a wide variety of self-coupling products. Notably, HAs diminished the extent of EE2 self-coupling but aggrandized the cross-coupling between EE2 and HAs, and the inhibition degree of EE2 self-coupling increased with the concentration of HAs. One major reason is EE2 could be covalently incorporated into humic molecules to produce (HA)m-(EE2)n cross-coupling products via radical-caused C–C, C–O–C, and/or C–O–C bonds, thereby reducing EE2 self-oligomerization. These findings highlight that HAs play a vital role in the fungal laccase-induced humification and oligomerization of EE2, which obviously alter the geochemical fate and transport of EE2 in natural aquatic ecosystems.

Temperature Induced Aggregation and Clouding in Humic Acid Solutions

Humic acids in aqueous solution demonstrate inverse temperature-solubility relationships when solution conditions are manipulated to reduce coulombic repulsion among the humic polyanions. These effects were followed by dynamic light scattering (DLS) measurements of the resulting aggregates, as well as the addition of a polarity sensitive fluorescent probe (pyrene). The humic solutions could be primed for temperature induced clouding by carefully lowering the pH to a point where hydration effects became dominant. The exact value of the cloud point (CP) was a function of both pH and humate concentration. The CPs mostly lay in the range 50–90°C, but DLS showed that temperature induced aggregation proceeded from approximately 30°C onward. Similar effects could be achieved by adding multivalent cations at concentrations below those which cause spontaneous precipitation. The declouding of clouded humate solutions could be affected by lowering the temperature combined with mechanical agitation to disentangle the humic polymers.

Effect and mechanism of manganate preoxidation for organics removal

The performance and mechanism of manganate preoxidation process for organics removal were investigated in the present paper. The results showed that manganate was a potentially powerful oxidizing agent and could make the natural organic matter (NOM) concentration of sample solution increase. The process of manganate in combination with ferrous sulphate (FeMnO) was effective for organics removal and with the highest removal rate of 89% when the FeMnO dose was 0.18 mmol/L. The fluorescence analysis showed that the fluorescence intensity values related to hydrophobic acids and model humic acid polymers were the highest and the relative position of the main peak fluorescence intensity was shifted towards lower emission wavelengths, which indicated the reduction in the degree of aromaticity of residual organic matter fraction.

Fe-loaded zeolites as catalysts in the formation of humic substance-like darkcoloured polymers in polycondensation reactions of humic precursors

AbstractTo enhance the catalytic activities of zeolites for the polycondensation reactions of humic precursors, Fe was loaded into a zeolite via an ion-exchange reaction and the resulting product was subjected to calcination at 773 K. Two types iron-loaded zeolites were prepared using one equivalent (Fe-Z-1) and 10-equivalents (Fe-Z-10) of Fe2+ to the cation-exchange capacity of a natural zeolite from Niki town (Hokkaido, Japan). X-ray diffraction (XRD) patterns and X-ray photoelectron spectroscopy (XPS) spectra showed that the Fe(II) that was originally loaded into the cation-exchange sites in the zeolite became oxidized to a Fe(III) ionic species during the preparation. The catalytic activities of each zeolite were evaluated, based on the degree of darkening for reaction mixtures containing catechol, glycine and glucose as model humic precursors. The catalytic activities of Fe-Z-1 and Fe-Z-10 were higher than that for an untreated zeolite, and increased with the amount of Fe in the zeolite.

Humic polymer complexes for purification of mineralized water

Humic polymer complexes for purification of mineralized water

Study of a Model Humic Acid-type Polymer by Fluorescence Spectroscopy and Atomic Force Microscopy

A model HA-type polymer of para-benzoquinone synthetic humic acid (SHA) and its complexes with copper, iron and manganese metal ions were studied using atomic force microscopy (AFM). Natural humic acids (HA) and synthetic humic acids (SHA) were examined by fluorescence spectroscopy, which indicated similarity of SHA and HA spectra. The AFM images of SHA and its complexes revealed variable morphologies, such as small spheres, aggregates and a sponge-like structure. The SHA complexes displayed morphologies similar to those of natural HA. The presence of copper, iron and manganese ions led to the formation of aggregate-type structures in an apparent arrangement of smaller SHA particles.

Evaluation of humic and fulvic acid extracts of compost, oilcake, and soils on complex formation with arsenic

Fulvic acid (FA) and humic acid (HA) were extracted from compost, oilcake, and surface soils collected from arsenic-contaminated and uncontaminated sites of West Bengal. These HA/FA samples were characterised by pH–potentiometric titrations, viscometric measurements, visible spectrophotometry, and surface tension determinations. The results were correlated with coiling–decoiling behaviour, as well as aliphatic/aromatic balance of HA/FAs. The stability constant (logK) of the complexes formed by the natural HA/FA fractions of the given soils were quite stable, and the HA/FA fractions of the organic manures with arsenate in aqueous phases suggested the dependence of such complexation on the nature and properties of the humic polymers, which, in turn, would affect the retention/release of arsenate in soil. The release potential of arsenic from the arsenate–HA/FA complexes by soluble sulfate and nitrate salts was also examined in terms of the appropriate exchange isotherms. In general, sulfate demonstrated a moderately greater degree of exchangeability with arsenate than did nitrate, at higher concentrations.

Polymerization of catechin catalyzed by Mn-, Fe- and Al-oxides

The role of short-range order (SRO) metal oxides, which are common in acid soils and associated environments, in influencing the abiotic transformations of catechin, which is common in the soil of tea plantations, still remains poorly understood. The aim of this study was to investigate the catalytic power of SRO Mn(IV)-, Fe(III)- and Al-oxides in influencing the abiotic transformations of catechin. At the end of a 90-h reaction period, the release of CO 2 in all the oxide-catechin systems is higher than that for the system with only catechin. Polymerization of catechin is catalyzed and enhanced by SRO-oxides, as is indicated by the absorbance values of the supernatants, which were obtained via visible adsorption spectroscopy, and the yields of humic polymers. The sequence of the oxides that increased the yield of total humic polymers in these systems under ambient atmosphere is: Fe(III)-oxide > Mn(IV)-oxide > Al-oxide ≫ no catalyst (catechin). The electron spin resonance (ESR) and Fourier transformation infrared absorption spectrometry (FT-IR) of humic polymers formed in the oxide-catechin systems were similar to the spectra obtained from the humic polymers extracted from the soil. The catalytic power of SRO-oxides in promoting the oxidative polymerization of catechin, the resultant formation of humic substances, and C turnover in acid soils thus merit attention.

Structural study of humic acids during composting of activated sludge-green waste: Elemental analysis, FTIR and 13C NMR

The humic acids extracted from a compost of activated sludge at different stages of maturity were characterized by various chemical techniques. Elemental analysis showed the reduction of H, and the H/C and C/N ratios and an increase in the proportion of N and S. At the end of composting C% and O% presented the same values as initially, although they increased in the intermediate stage. Based on the ratios of FTIR absorbance it was shown that the end product was enriched in etherified and peptidic compounds absorbing at 1384, 1034 and 1544 cm −1. The alkyl and other N-rich and oxidized recalcitrant structures compose the new humic polymers produced during composting. In principal components analysis, the first axis, PC1: 49.75% considers the variability between structures in decomposition from the other parameters that concern the stable new humic polymers formed after composting. PC2 (40.5%) shows a negative correlation between (aromatic carbon, FA level) and (aliphatic carbon, HA level) during composting.

Biomolecule-induced carbonate genesis in abiotic formation of humic substances in nature

Most soil C sequestration research has focused on organic C stabilization, while carbonate precipitation has received little attention. Mineral colloids can accelerate abiotic humification reactions of biomolecules such as amino acids, sugars, and polyphenols, derived from the breakdown of biological residues and metabolites. During these reactions CO2 is produced as a result of the oxidation of biomolecules. However, the biomolecule-induced formation of carbonate during abiotic humification remained to be uncovered. Here we demonstrate using X-ray diffraction, Fourier transform infrared spectroscopy and C K-edge and Mn L-edge near edge X-ray absorption fine structure spectroscopy that the Maillard reaction (glucose and glycine) and the integrated polyphenol-Maillard reaction pathway (catechol, glucose and glycine), in the presence of birnessite (δ-MnO2) produce MnCO3 (rhodochrosite). Increasing the molar ratio of catechol to glucose and glycine dramatically hampered carbonate formation, which is attributed to the enhanced formation of humic polymers, which increased proton generation and perturbed rhodochrosite crystallization through Mn(II)-humic complexation in the reaction systems. Thus, rhodochrosite formation was a competing reaction with humic substance formation. Our findings are of fundamental significance in understanding the vital role of the nature and relative abundance of biomolecules in abiotic carbonate formation, which merits close attention in understanding and regulating C sequestration in natural environments.Key words: Abiotic humification, polyphenol-Maillard reaction, rhodochrosite, birnessite, C K-edge and Mn L-edge NEXAFS

The influence of aggregation on the redox chemistry of humic substances

Environmental context. The ability of humic substances (decaying plant and animal matter) to partake in redox reactions in the environment depends on the extent to which the various humic polymers aggregate in solution to form larger particles. This aggregation, in turn, is predicated on the solution conditions, especially ionic strength, the pH, and the types of cations present. Abstract. Aggregation and conformation play an important role in the aqueous redox chemistry of humic substances (HS). The reduction potentials of dissolved humic and fulvic acids vary with pH, ionic strength, and type of humate used, and depending on the solution conditions, they can abiotically reduce various species. Changes in HS reduction potential ranged from 60 to 140 mV on addition of divalent cations, whereas no significant changes were observed with equivalent additions of monovalent cations. Dynamic light scattering measurements showed that this behaviour paralleled the size changes obtained with humic aggregates under the same conditions. The effect was more pronounced at higher pH, where divalent cations caused a significant decrease in the average hydrodynamic radius, whereas monovalent cations did not. At pH 4, neither mono- nor divalent cations substantially affected aggregate sizes. Quinoid moieties, which are known to play an important role in the redox chemistry of HS, displayed fluorescence excitation–emission matrices with features related to changes in the reduction potential of HS. An increase in the reduction potential (Eh) induced by the addition of Ca2+, for instance, caused a red shift in the excitation–emission matrix maximum.

Complexation of TbIII with size fractions of humic acid: evidence from luminescence sensitisation and anisotropy measurements

Environmental context. Organic ligands, especially those derived from humic acid (HA), play a major role in the fate and transport of metal ions in the environment. For the modelling of subsurface pollutant transport, it is important to understand which components of a heterogeneous humic material interact most strongly with multivalent cations. Abstract. The luminescence sensitisation and anisotropy characteristics of a series of TbIII complexes with a leonardite humic acid (LHA) were investigated in order to evaluate the interactions between the metal and different components of the humate. Ultrafiltration was used to separate LHA into six size fractions, which ranged from 500 Da to 0.2 μm, and were then used to form the TbIII complexes. Each fraction was first characterised by 13C NMR and UV-Vis spectroscopy, which showed that the smaller ones (<3 kDa) had a significantly lower aliphatic content than the larger ones. These smaller components were good energy donors, which could effectively sensitise TbIII luminescence. At the same time, the luminescence anisotropy of TbIII increased significantly when these LHA fractions were added, which indicated the formation of tightly bound complexes. In this sense, the smaller LHA fractions were comparable to ethylenediaminetetraacetate, although their effect was not as strong. In contrast, the larger LHA sizes had little or no influence on TbIII sensitisation or anisotropy. The results obtained suggest that the sizes and aliphatic content of humic polymers play a major role in their aqueous interactions with trivalent metal ions. Divalent metals are expected to behave in a similar way.

Stabilization of organic matter in temperate soils: mechanisms and their relevance under different soil conditions – a review

SummaryMechanisms for C stabilization in soils have received much interest recently due to their relevance in the global C cycle. Here we review the mechanisms that are currently, but often contradictorily or inconsistently, considered to contribute to organic matter (OM) protection against decomposition in temperate soils: (i) selective preservation due to recalcitrance of OM, including plant litter, rhizodeposits, microbial products, humic polymers, and charred OM; (ii) spatial inaccessibility of OM against decomposer organisms due to occlusion, intercalation, hydrophobicity and encapsulation; and (iii) stabilization by interaction with mineral surfaces (Fe‐, Al‐, Mn‐oxides, phyllosilicates) and metal ions. Our goal is to assess the relevance of these mechanisms to the formation of soil OM during different stages of decomposition and under different soil conditions. The view that OM stabilization is dominated by the selective preservation of recalcitrant organic components that accumulate in proportion to their chemical properties can no longer be accepted. In contrast, our analysis of mechanisms shows that: (i) the soil biotic community is able to disintegrate any OM of natural origin; (ii) molecular recalcitrance of OM is relative, rather than absolute; (iii) recalcitrance is only important during early decomposition and in active surface soils; while (iv) during late decomposition and in the subsoil, the relevance of spatial inaccessibility and organo‐mineral interactions for SOM stabilization increases. We conclude that major difficulties in the understanding and prediction of SOM dynamics originate from the simultaneous operation of several mechanisms. We discuss knowledge gaps and promising directions of future research.

Transformation of catechol in the presence of a laccase and birnessite

The transformation of naturally occurring phenols to humic polymers through oxidative coupling reactions may involve oxidoreductive enzymes and soil minerals as catalysts. There is limited information on the possible inhibitory or synergistic interactions between oxidoreductases and mineral catalysts as they participate in oxidative coupling of phenolic substrates. In this study, a ternary system was investigated, in which a fungal enzyme ( Trametes villosa laccase), birnessite (δ-MnO 2), and a naturally occurring phenolic compound (catechol) were reacted together to model soil processes. Binary systems (catechol/laccase and catechol/birnessite) were included for comparison. In the absence of the mineral, T. villosa laccase (950 katal ml −1) transformed 31% of catechol, whereas birnessite (1 mg ml −1) in the absence of the enzyme showed a 24% catechol transformation. The percentages of catechol transformation in the binary systems did not accumulate in the ternary system; instead, birnessite and laccase tested together transformed only 36% of catechol. This suggested that birnessite had an inhibitory effect on substrate transformation by laccase catalysis. Enzyme assays indicated that inhibition was a result of enzyme deactivation by humic-like polymers produced by birnessite, and by Mn 2+ ions released from the mineral. These observations underscore the importance of considering enzyme-soil mineral-organic matter interactions in studies of humus formation and contaminant removal.

Characterization of organic matter in rainfall, throughfall, stemflow, and streamwater from three subtropical forest ecosystems

Little is known about the characteristics of dissolved organic matter derived from precipitation, throughfall, stemflow, and streamwater in subtropical forest ecosystems. Water samples were, thus, collected from rainfall, throughfall and stemflow of a China-fir plantation, a secondary hardwood and a natural hardwood, and streams in forest ecosystems in the central part of subtropical Taiwan on March 31 and May 8, 1999. We examined pH, electrical conductivity, dissolved organic carbon, and acid-neutralizing capacity (ANC) in the samples. Furthermore, the humic polymers (MW>1000) and fulvic acids (FAs; MW>1000) were isolated from the water samples and characterized by elemental analyses and Fourier transform infrared (FTIR) spectroscopy. Results showed that the contents of carboxylic acid of both humic polymers (MW>1000) and FAs (MW>1000) contributed to the water sample ANC, ranging from 8.7 to 61.2% and 0.2 to 38.1%, respectively. FTIR and elemental analyses of isolated humic polymers (MW>1000) and FAs (MW>1000) from throughfall and stemflow had characteristics previously reported for humic substances. The study showed that throughfall and stemflow, not affected by soils, may be additional sources of humic substances.

Fluorescence Emitted During the Autooxidation of 2,3,4,6-tetrahydroxy-5H-benzocyclohepten-5-one

Data suggest that the purpurogallin (2,3,4,6-tetrahydroxy-5H-benzocyclohepten-5-one, hydroxybenzotropolone) and its analogues formed from the polyphenols can be precursors of the most complex natural compounds—humic acids and melanin-like polymers. Therefore, to confirm this possibility, the autooxidation of purpurogallin to humus-like substances has been performed. The oxidation process has been assayed by means of fluorescence and UV/VIS absorption spectroscopy. The obtained results indicate that purpurogallin undergoes a free-radical-mediated autooxidation via purpurogallinquinones to humus-like polymers. The scheme of the complex transformation of purpurogallin has been proposed.

Dynamic light scattering measurements of particle size development in aqueous humic materials.

Dynamic light scattering (DLS) has been used to monitor changes in aggregate sizes of aqueous humic materials as a function of solution properties. Humic and fulvic acids were dissolved at relatively low concentrations (15-30 mg L(-1)) in solutions of different temperature, cation and ethanol content, and pH. The results could be explained in terms of intramolecular contraction and intermolecular aggregation of humic polymers. The former were prevalent in soil humic acids, and less so in aquatic humic acids and fulvic acids. Increasing the temperature of humic solutions generally led to an increase in particle sizes, which was ascribed to an effect akin to surfactant clouding. The addition of cations led to either contraction or expansion, depending on the charge and concentration of the ion, and the nature of the humic material. Reducing the pH initially caused contraction, followed by growth and precipitation in more highly acidic media.

RING CLEAVAGE AND OXIDATIVE TRANSFORMATION OF PYROGALLOL CATALYZED BY Mn, Fe, Al, AND Si OXIDES

The nature of oxide in soil and associated environments in influencing the abiotic transformations of polyphenols still remains to be established. The objective of this study was to investigate the catalytic power of Mn(IV), Fe(III), Al, and Si oxides in the abiotic ring cleavage of pyrogallol and the polycondensation of the resulting fragments in air and N2 atmosphere. The catalytic power varied greatly with the nature of the oxides. The synergistic effects of oxygen molecules and oxides also played an important role in affecting the transformations of pyrogallol. The results indicate that the release of CO2 as a result of ring cleavage of pyrogallol enhanced the development of carboxylic group contents of humic polymers formed in the reaction systems. The abiotic ring cleavage of polyphenols catalyzed by soil inorganic components such as short-range ordered oxides of Al and especially Fe and Mn may account, in part, for the carboxylic group contents and the origin of the aliphaticity of humic substances in soils.

CHARACTERISTICS OF PYROGALLOL-DERIVED POLYMERS FORMED BY CATALYSIS OF OXIDES

The influence of the nature and properties of oxides in soil and sediment environments on the characteristics of polyphenol-derived humic polymers formed by abiotic catalysis is not fully understood. The objective of this study was to investigate the catalytic power of Mn(IV), Fe(III), Al, and Si oxides in the transformation of pyrogallol and the development of carboxylic and aliphatic groups in the resulting humic polymers. The metal oxides differed significantly in their effectiveness for catalyzing pyrogallol humification. The results indicate that the carboxylic group of humic polymers developed as a result of ring cleavage of pyrogallol catalyzed by the oxides. The ability of short-range ordered oxides of Al, Si, and especially Fe and Mn, in enhancing the formation of aliphatic and carboxylic groups in humic polymers derived from polyphenol humification may, in part, contribute to the origin of aliphaticity of humic substances and the variation of their characteristics, including carboxylic group contents with soils from different climatic zones.

Non-bonded organo-mineral interactions and sorption of organic compounds on soil surfaces: a model approach

Molecular mechanics and molecular dynamics calculations have been performed on organo-mineral composites that model the sorption of high-molecular-weight humic polymers on mineral surfaces and the sorption of low-molecular-weight organic contaminants on both mineral and organic surfaces in soil. Muscovite mica was chosen as a mineral model; an oxidized topological lignin-carbohydrate complex was chosen as a humic model; benzene, sodium benzoate, atrazine, and DDT represent different classes of contaminants. Sorption energies were estimated based on molecular mechanics calculations. Flexible linear polymers undergo drastic conformational changes when approaching the mineral surface, to ensure a gain in the interaction energy that outweighs a loss in the conformational energy proper. Therefore, the gas-phase conformation composi tion of environmental organic polymers is not directly related to their spatial organization in soil composites. Molecular dynamics simulation suggests high stability of the organic polymer coatings of mineral surfaces in the environment. Low-molecular-weight organic molecules demonstrate much less affinity for the mineral surface, which implies unhindered exchanges between the surface and its near environment. Ionizable compounds, e.g. salts of organic acids, are different, because they can form strong associations with a mineral surface through cation bridges. Sorption energies are compound-specific and depend on the sorbate-sorbent orientation. The calculations suggest some preference for the edges of a model muscovite sheet in comparison with the basal oxygen surface as a sorption site. Coating of mineral surfaces with organic polymers does not hinder the sorption of organic molecules except in the special case of organic ions.