Noncovalent interactions between monoaromatic compounds and dissolved humic acids: a deuterium NMR T1 relaxation study

Deuterium nuclear magnetic resonance spectroscopy (2H NMR) spin-lattice relaxation (T1) experiments were used to examine solution-phase, noncovalent interactions between deuterated monoaromatic compounds (phenol-d5, pyridine-d5, benzene-d6) and Suwannee River, soil, and peat humic acids. Noncovalent interactions, in aqueous solution, were examined as a function of solution pH, monoaromatic hydrocarbon functional groups, and humic acid identity. Benzene interacted with dissolved humic acids at all pH values; however, these interactions increased with decreasing pH and generally were proportional with the humic acid percent aromaticity. Pyridine behaved similarly as benzene; however, two modes of interaction between pyridine and humic acids were detected as a function of pH and humic acid type: bonding with the lone pair of electrons of pyridine's nitrogen and pi-pi interactions between the aromatic ring of pyridine and aromatic components of humic acid. The latter interaction was favored by increasing humic acid percent aromaticity and decreasing solution pH. On the other hand, because of its strong capacity for hydrogen bonding, phenol interacted preferentially with water, except at pH values 5 or lower and with humic acids with 45% or greater aromaticity. Under these conditions, strong interactions between phenol and humic acids were observed. These results demonstrate that solution-phase, noncovalent interactions between monoaromatic compounds and humic acids are a function of solution pH, percent aromaticity, and the monoaromatic functional group.     

Elucidating differences in the sorption properties of 10 humic and fulvic acids for polar and nonpolar organic chemicals

In this work we present a dataset of more than 1000 natural organic matter (NOM)/air partition coefficients covering polar and nonpolar organic compounds measured in 10 different humic and fulvic acids (HAs/FAs) from terrestrial and aquatic origins. Differences of more than 1 order of magnitude in the sorption coefficients of a given compound measured in HAs and FAs from different origins were found. The terrestrial HAs exhibited substantially higher sorption coefficients compared to aquatic HAs and FAs. The difference between any two types of NOM is mainly reflected by a constant shift in the partition coefficients that applies to all compounds in the same way. This indicates that it is the number of available sorption sites per mass of sorbent rather than the types of intermolecular interactions between the sorbate and the sorbent that governs the major differences between the sorption properties of various types of NOM. The experimental partition coefficients measured in all HAs and FAs were successfully described by polyparameter linear free energy relationships (pp-LFERs) that explicitly account for van der Waals as well as H-donor/acceptor interactions between the sorbate and the sorbent. These pp-LFER equations provide for the first time a tool that allows including the variability of the sorption properties of NOM in environmental fate models.     

Monitoring the effect of three humic acids on a model membrane system using 31P NMR

The sorption of three humic acids to 1,2-dipalmitoyl-sn-glycero-3-phosphocholine multilamellar vesicle model membrane systems was studied by phosphorus nuclear magnetic resonance (31P NMR). The effects of temperature and pH were investigated. The gel --> bilayer transition did not appear to be affected by any of the humic acids at pH 7; however, all three humic acids induced a perturbation to this transition and to the bilayer structure at pH 4. On the basis of the findings from this and other work, a conceptual adsorption/absorption model for the sorption of humic acid (HA) to biomembranes has been put forward. The model requires an initial adsorption step initiated at an acidic pH by hydrogen bridging and electrostatic interactions between the functional groups of the HAs and the head groups of the phospholipids. Once the HA material is adsorbed, its hydrophobic domains can further seek a more thermodynamically favorable environment within the bilayer using hydrophobic interactions. These interactions lead to the HA being absorbed into the membrane, which subsequently induces the observed perturbation by disturbing the ordered packing of the phospholipid tail groups. This model is also related to other humic substances/biomembrane observations in the literature.     

Impact of de-ashing humic Acid and humin on organic matter structural properties and sorption mechanisms of phenanthrene

Organic matter-mineral interactions greatly affect the fate of hydrophobic organic compounds (HOCs) in the environment. In the present study, the impact of organic matter-mineral interaction on sorption of phenanthrene (PHE) by the original and de-ashed humic acids (HAs) and humin (HM) was examined. After de-ashing treatment, the overall polarity of organic matter in HAs and HM consistently decreased. Differently, the surface polarity of HAs increased but that of HM decreased. No correlation between K(oc) values of PHE by all tested sorbents and their bulk polarity was observed due to inaccessibility of a portion of interior sorption domains. The inaccessibility of interior sorption domains in HAs and HM was partly due to the crystalline structure in organic matter as indicated by differential scanning calorimetric (DSC) and 13C NMR data and the interference from minerals. A good correlation between surface polarity of the original and de-ashed HAs and HMs and their K(oc) values for PHE indicated its importance in HOC sorption. Dissimilar changes in surface polarity of HAs and HM after de-ashing treatment can be ascribed to the distinct interactions between organic matter and minerals. The solid-state 13C NMR, XPS, and elemental composition data of all tested sorbents revealed that a larger fraction of O atoms in HAs were involved in organic matter-mineral interaction as compared to HM. Results of this work highlight the importance of soil organic matter (SOM)-mineral interactions, surface polarity, and microscaled domain arrangement of SOM in HOC sorption.     

Aggregation and disaggregation of humic supramolecular assemblies by NMR diffusion ordered spectroscopy (DOSY-NMR)

Diffusion ordered nuclear magnetic resonance spectroscopy (DOSY-NMR) was applied to a number of fulvic (FA) and humic (HA) acids of different origin. Spectral separation achieved by DOSY based on diffusion coefficients (D), and correlated to molecular sizes by calibration standards, showed that carbohydrates had the largest molecular size in FA, whereas alkyl or aromatic components were the most slowly diffusing moieties in HA. At increasing concentrations, these components had invariably lower D values in DOSY spectra for all humic samples,thereby indicating an aggregation into apparently larger associations, whose increased hydrodynamic radius was confirmed by viscosity measurements. When humic solutions were broughtfrom alkaline to acidic pH (3.6), components diffusivity detected by DOSY increased significantly, suggesting a decrease of aggregation and molecular size. A general comparison of HA and FA molecular sizes was achieved by multivariate statistical analysis. While a larger extent of aggregation and disaggregation was observed for HA than for FA, no aggregation was detected, under similar conditions, for a true macropolymeric standard. Such difference in diffusion between a polymeric molecule and humic samples, is in line with the supramolecular nature of humic matter. The possible formation of humic micelles was also investigated by both changes of diffusivity in DOSY spectra and shift of 1H NMR signals. Except for HA of peat and soil origin, revealing a self-assembling in micelle-like structures at the 4 mg mL(-1) concentration, no other humic sample showed evidence of critical micelle concentration (cmc) up to 20 mg mL(-1). These results indicated that DOSY-NMR spectroscopy is a useful technique to evaluate components of different molecular size in natural humic superstructures.     

Immobilizing humic acid in a sol-gel matrix: a new tool to study humic-contaminants sorption interactions

Humic substances originated from aquatic, soil, or sediment environments are mixtures of humic compounds with various characteristics. Sorption interactions with isolated, well defined humic fractions can be studied either in an aqueous phase ("dissolved humic substances"), or in a solid-phase, by coating mineral particles with the humic materials, or simply by working with humic acid particles (powder) at low pH to minimize dissolution. Each attitude, by definition, can be studied by different experimental techniques and has a different meaning for understanding natural environmental processes. In this study, a new tool for studying sorption interactions is presented. Sol-gel was used as an inert matrix to immobilize (entrap) various humic acids (HAs), and then used to study the interactions of several polycyclic aromatic hydrocarbons (PAHs) with the entrapped HA. Linear and nonlinear sorption coefficients were highly correlated with contaminant hydrophobicity. Sorption of pyrene to immobilized HA was in the order of soil HA > Aldrich HA approximately = peat HA. It was concluded that the entrapped HAs retained their original properties in the gel matrix and were accessible to the external contaminant through the pore network. Additionally, binding coefficients of pyreneto dissolved humic substances and to dissolved organic matter (DOM) were determined from the reduction in pyrene sorption to immobilized HA in the presence of dissolved humic material or DOM in solution. Binding coefficients of pyrene were in the order of the following: dissolved Aldrich HA > dissolved peat fulvic acid (FA) > DOM derived from mature compost > DOM derived from fresh compost.     

Birnessite-induced binding of phenolic monomers to soil humic substances and nature of the bound residues

The nature of the abiotic birnessite (δ-MnO(2))-catalyzed transformation products of phenolic compounds in the presence of soil organic matter is crucial for understanding the fate and stability of ubiquitous phenolic carbon in the environment. (14)C-radioactive and (13)C-stable-isotope tracers were used to study the mineralization and transformation by δ-MnO(2) of two typical humus and lignin phenolic monomers-catechol and p-coumaric acid-in the presence and absence of agricultural and forest soil humic acids (HAs) at pH 5-8. Mineralization decreased with increasing solution pH, and catechol was markedly more mineralized than p-coumaric acid. In the presence of HAs, the mineralization was strongly reduced, and considerable amounts of phenolic residues were bound to the HAs, independent of the solution pH. The HA-bound residues were homogeneously distributed within the humic molecules, and most still contained the unchanged aromatic ring as revealed by (13)C NMR analysis, indicating that the residues were probably bound via ester or ether bonds. The study provides important information on δ-MnO(2) stimulation of phenolic carbon binding to humic substances and the molecular distribution and chemical structure of the bound residues, which is essential for understanding the environmental fates of both naturally occurring and anthropogenic phenolic compounds.     

Proton binding to humic acids from organic amendments and amended soils by the NICA-Donnan model

The acid-base properties of humic acids (HAs) are known to significantly affect the acid-base buffering capacity of soils, thus having a marked influence on the speciation of cations in the soil solid and liquid phases. Detailed information on the proton binding behavior of humic-like acids (HALs) from organic amendments and humic acids (HAs) from amended soils is, therefore, of intrinsic interest for the evaluation of the agronomic efficacy and environmental impact of soil amendment. In this work, the acid-base properties of HLAs isolated from sewage sludge (SS) and municipal solid waste compost (MSWC), and HAs isolated from soils amended with either SS or MSWC and the corresponding nonamended control soils were investigated by potentiometric titrations at various ionic strengths (0.01, 0.05, 0.1, and 0.3 M) over the pH range from 3.5 to 10.5. The nonideal competitive adsorption (NICA)-Donnan model that describes proton binding by two classes of binding sites with low and high proton affinity, i.e., carboxylic- and phenolic-type groups, was fit to titration data, and a set of fitting parameters was obtained for each HLA and HA sample. The NICA-Donnan model successfully described the shapes of the titration curves, and highlighted substantial differences in site density and proton-binding affinity between the HLAs and HAs examined. With respect to the nonamended control soil HAs, SS-HLA and MSWC-HLA were characterized by smaller carboxylic-type and phenolic-type group contents, larger affinities for proton binding by the carboxylic-type groups, and smaller affinities for proton binding by the phenolic-type groups. Amendment with SS and MSWC determined a number of modifications in soil HAs, including decrease of acidic functional group contents, slight increase of proton affinity of carboxylic-type groups, and slight decrease of the affinities for proton binding by phenolic-type groups. These effects were more evident in the HA fraction from the SS-amended soil than in the HA fraction from the MSWC-amended soil. Thus, both organic amendments examined can be a considered as a valuable source of organic matter for soil. However, MSWC appears to be an amendment of greater quality producing a smaller impact than SS on proton-binding behavior of soil HA.     

Identification and characterization of sorption domains in soil organic matter using strucuturally modified humic acids

The sorption of phenanthrene was examined in humic acids (HAs) from different sources: a compost, a peat soil, and a mineral soil. Sub-samples of each HA were subjected to bleaching or hydrolysis to remove predetermined chemical groups from their structures. Bleaching successfully removed a large percentage of rigid, aromatic moieties, whereas hydrolysis removed the mobile, carbohydrate components. Phenanthrene sorption by all HAs was nonlinear (N < 1). However, the phenanthrene isotherms of the bleached HAs were more linear than those of the untreated HAs, whereas the removal of the carbohydrate components by hydrolysis produced more nonlinear isotherms. The introduction of pyrene to the phenanthrene sorption system yielded more linear isotherms for all the HAs, indicative of competitive sorption. Proton spin-spin (1H T2) relaxation determined by nuclear magnetic resonance (NMR) was used to identify separate rigid (condensed) and flexible (expanded) 1H populations and to determine their distribution. These 1H domains were highly sensitive to temperature and correlated well with reported glass transition temperatures for HAs. In combination with the chemical treatments, sorption, and spectroscopic data, we were able to observe some significant relationships among chemical groups, sorption behavior, and structural characteristics.     

3-D structural modeling of humic acids through experimental characterization,computer assisted structure elucidation and atomistic simulations. 1. Chelsea soil humic acid

This paper describes an integrated experimental and computational framework for developing 3-D structural models for humic acids (HAs). This approach combines experimental characterization, computer assisted structure elucidation (CASE), and atomistic simulations to generate all 3-D structural models or a representative sample of these models consistent with the analytical data and bulk thermodynamic/structural properties of HAs. To illustrate this methodology, structural data derived from elemental analysis, diffuse reflectance FT-IR spectroscopy, 1-D/2-D 1H and 13C solution NMR spectroscopy, and electrospray ionization quadrupole time-of-flight mass spectrometry (ESI QqTOF MS) are employed as input to the CASE program SIGNATURE to generate all 3-D structural models for Chelsea soil humic acid (HA). These models are subsequently used as starting 3-D structures to carry out constant temperature-constant pressure molecular dynamics simulations to estimate their bulk densities and Hildebrand solubility parameters. Surprisingly, only a few model isomers are found to exhibit molecular compositions and bulk thermodynamic properties consistent with the experimental data. The simulated 13C NMR spectrum of an equimolar mixture of these model isomers compares favorably with the measured spectrum of Chelsea soil HA.     

Adsorption of Insecticidal Cry1Ab Protein to Humic Substances. 2. Influence of Humic and Fulvic Acid Charge and Polarity Characteristics

Assessing the fate and potential risks of transgenic Cry proteins in soils requires understanding of Cry protein adsorption to soil particles. The companion paper provided evidence that patch-controlled electrostatic attraction (PCEA) and the hydrophobic effect contributed to Cry1Ab protein adsorption to an apolar humic acid (HA). Here, we further assess the relative importance of these contributions by comparing Cry1Ab adsorption to seven humic substances varying in polarity and charge, at different solution pH and ionic strength, I. Cry1Ab adsorption to relatively apolar HAs at I = 50 mM exhibited rapid initial rates, was extensive, and was only partially reversible at pH 5-8, whereas adsorption to more polar fulvic acids was weak and reversible or absent at pH >6. The decrease in adsorption with increasing HS polarity at all tested pH strongly supports a large contribution from the hydrophobic effect to adsorption, particularly at I = 50 mM when PCEA was effectively screened. Using insect bioassays, we further show that Cry1Ab adsorbed to a selected HA retained full insecticidal activity. Our results highlight the need to consider adsorption to soil organic matter in models that assess the fate of Cry proteins in soils.     

Absence of mobile carbohydrate domains in dry humic substances proven by NMR, and implications for organic-contaminant sorption models

The mobility and domain structure of various standard humic substances have been investigated by 1H and 1H-13C solid-state nuclear magnetic resonance (NMR) experiments. In four dry humic acids, a fulvic acid, a natural organic matter sample, and a whole peat sample, segments that undergo fast, large-amplitude motions account for <9% of the sample. This disproves a previous suggestion, based on 1H NMR data, that flexible domains, presumably carbohydrates, make up >40% of various humic acids; these putative mobile domains were also linked to dual-mode sorption. In particular, neither the polar alkyl (carbohydrate) nor the aromatic components show any fast, large-amplitude mobility. A small fraction of mobile nonpolar alkyl segments identified by us before is the only component undergoing large-amplitude motions, apart from absorbed water that we observe in humic acids exposed to ambient air. 1H-13C wide-line separation NMR shows that, contrary to previous suggestions, the dipolar couplings in the aromatic regions are smaller than in the polar alkyl segments, most likely due to differences in local 1H densities. Series of 1H-13C heteronuclear correlation experiments with 1H spin diffusion reveal close proximity of aromatic and polar alkyl segments in several humic acids, precluding carbohydrate domains on a scale of > 1-nm diameter. In the standard peat humic acid, nonpolar aromatic segments also do not form sorption domains of significant size, while nonpolar aliphatic domains, which we had previously shown to correlate with sorption capacity, have been confirmed.     

Antioxidant properties of humic substances

Humic substances (HS) are heterogeneous, redox-active organic macromolecules. While electron transfer to and from HS under reducing conditions is well investigated, comparatively little is known on the electron donating (i.e., antioxidant) properties of HS under oxic conditions. In this work, the electron donating capacities (EDCs) of terrestrial and aquatic HS were quantified by mediated electrochemical oxidation over a wide range of pH values and applied redox potentials (E(h)) using 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) as an electron transfer mediator. Electrochemical oxidation of three model humic acids (HAs) was largely irreversible, and the EDCs of these HAs increased with increasing E(h) and pH. These results suggest that HS contain a wide variety of moieties that are oxidized at different potentials and that, upon oxidation, release protons and undergo irreversible follow-up reactions. At a given pH and E(h), the EDCs of the HS correlated well with their titrated phenol contents suggesting phenolic moieties as major electron donating groups in HS. Comparing the EDCs of 15 HS with their electron accepting capacities (EACs), aquatic HS had higher EDCs and lower EACs than terrestrial HS of comparable aromaticities. These results indicate that oxidative transformation of HS in the environment results in a depletion of electron donating phenolic moieties with antioxidant properties relative to the electron accepting quinone moieties.     

Impact of natural organic matter on uranium transport through saturated geologic materials: from molecular to column scale

The risk stemming from human exposure to actinides via the groundwater track has motivated numerous studies on the transport of radionuclides within geologic environments; however, the effects of waterborne organic matter on radionuclide mobility are still poorly understood. In this study, we compared the abilities of three humic acids (HAs) (obtained through sequential extraction of a peat soil) to cotransport hexavalent uranium (U) within water-saturated sand columns. Relative breakthrough concentrations of U measured upon elution of 18 pore volumes increased from undetectable levels (<0.001) in an experiment without HAs to 0.17 to 0.55 in experiments with HAs. The strength of the HA effect on U mobility was positively correlated with the hydrophobicity of organic matter and NMR-detected content of alkyl carbon, which indicates the possible importance of hydrophobic organic matter in facilitating U transport. Carbon and uranium elemental maps collected with a scanning transmission X-ray microscope (STXM) revealed uneven microscale distribution of U. Such molecular- and column-scale data provide evidence for a critical role of hydrophobic organic matter in the association and cotransport of U by HAs. Therefore, evaluations of radionuclide transport within subsurface environments should consider the chemical characteristics of waterborne organic substances, especially hydrophobic organic matter.     

Phenanthrene sorption to sequentially extracted soil humic acids and humins

Humic substances strongly influence the environmental fate of hydrophobic organic chemicals in soils and sediments. In this study, the sorption of phenanthrene by humic acids (HAs) and humins was examined. HAs were obtained from progressively extracting a soil, eight times with 0.1 M Na4P207 and two times with 0.1 M NaOH solution, and then the residue was separated into two humin fractions by their organic carbon contents. The chemical and structural heterogeneity of the HAs and humins were characterized by elemental analysis, ultraviolet-visible spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy, and solid-state 13C NMR. There were significant chemical and structural differences among the HA fractions and humins; the later extracted HAs had relatively high aliphatic carbons content. All sorption data were fitted to a Freundlich equation, S = K(F)C(N), where S and C are the sorbed and solution-phase concentrations, respectively, and K(F) and N are constants. All of the phenanthrene sorptions were nonlinear, and the nonlinearity decreased with further extractions from 0.90 (first extracted HA) to 0.96 (ninth HA) and was the lowest (0.88) for the higher organic carbon content humin. Phenanthrene sorption coefficient by HAs significantly increased with progressive extractions, being the highest for the humins. For HAs isotherms, a positive trend was observed between the sorption coefficient and the aliphaticity, but a negative relation was shown between the nonlinearity and the aliphaticity and between the sorption capacity and polarity of HAs. Phenanthrene sorption was greatly affected by chemical structure and composition of humic substances, even from a same soil. In addition, polarity of humic substances seems to mainly regulate the magnitude of phenanthrene sorption rather than structure.     

Application of saturation transfer double difference NMR to elucidate the mechanistic interactions of pesticides with humic acid

Elucidation of mechanistic interactions of anthropogenic chemicals is critical to understanding and eventually predicting their behavior in the environment Here, a recently developed technique, saturation transfer double difference (STDD) NMR spectroscopy is employed to determine the interactions of pesticides with humic acid (HA) at the molecular level. The degree of interaction at each NMR observable nucleus in the pesticide can be quantified in the form of an epitope map, which depicts the mechanism of the pesticide-HA interaction. Our results indicate that, at pH 7, halogen atoms (F and Cl) in water-soluble pesticides (diflufenzopyr, acifluorfen, and chlorsulfuron) play a dominant role in influencing binding to HA, whereas carboxyl groups likely play a secondary role when halogen atoms are also present in the molecule, as observed with diflufenzopyr and acifluorfen. However, when present on its own, the carboxyl group dominates in binding affinityto HA (e.g., imazapyr). Electronegativity and electron density appear to play a key role in the mechanism of binding and results suggest that polar bonds are the primary points of HA contact in the water soluble pesticides investigated. Likely interactions may include hydrogen bonding and dipole-dipole interactions.     

Involvement of Disulfide Bonds in Bovine β-Lactoglobulin B Gels Set Thermally at Various pH

To obtain information on forces important for maintenance of the structure of heat-set β-lactoglobulin gels, gels set from pure β-lactoglobulin at pH 3.0, 5.0, and 7.0 were solubilized in dissociating buffers, and solubilized material was characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and size exclusion high performance liquid chromatography. The gel formed at pH 7.0 was largely soluble in urea (or SDS), and this gel seemed to be built from covalently linked soluble aggregates, associating into a gel network mainly by strong noncovalent interactions. The gel set at pH 5.0 was solubilized only in the presence of a reducing agent, indicating that this gel structure was supported mainly by covalent disulfide bonds. The gel set at pH 3.0 was almost completely solubilized in SDS or urea without a reducing agent, showing the importance of noncovalent interactions in maintaining the gel structure.     

Studies of the compositions of humic acids from Amazonian Dark Earth soils

The compositions of humic acids (HAs) isolated from cultivated and forested "Terra Preta de Indio" or Amazonian Dark Earth soils (anthropogenic soils) were compared with those from adjacent non-anthropogenic soils (control soils) using elemental and thermogravimetric analyses, and a variety of solid-state nuclear magnetic resonance techniques. The thermogravimetric index, which indicates the molecular thermal resistance, was greater for the anthropogenic soils than for the control soils suggesting polycyclic aromatic components in the former. The cultivated anthropogenic soils were more enriched in C and depleted in H than the anthropogenic soils under forest, as the result of the selective degradation of aliphatic structures and the possible enrichment of H-deficient condensed aromatic structures. The combination of variable amplitude cross-polarization (VACP) and chemical shift anisotropy with total suppression of spinning sidebands experiments with composite pi pulses could be used to quantify the aromaticity of the HAs from the anthropogenic soils. From principal component analysis, using the VACP spectra, it was possible to separate the different constituents of the HAs, such as the carboxylated aromatic structures, from the anthropogenic soils and plant derived compounds. The data show that the HAs from anthropogenic soils have high contents of aryl and ionisable oxygenated functional groups, and the major functionalities from adjacent control soils are oxygenated functional groups from labile structures (carbohydrates, peptides, and with evidence for lignin structures). The anthropogenic soils HAs can be considered to be more recalcitrant, and with more stable reactive functional groups which may, in part, explain their more sustainable fertility due to the organic matter contribution to the soil cation exchange capacity.     

微滤膜/光催化反应器在水处理中的应用

用微滤膜，光催化反应器处理含腐殖酸的合成污水，讨论了染的情况．在间歇操作的过程中，光催化反应使溶液中腐殖酸的浓度（分别以溶液在254nm处的吸收值UV254和总有机碳含量TOC来表示）不断升高，经过一段时间的反应后再逐渐降低4h时腐殖酸的UV254和TOC的去除效率分别为99％和89％．在操作全过程中无明显的膜污染．