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PubMed2003

Quantitation in the solid-state 13C NMR analysis of soil and organic soil fractions.

13C CP-MAS and DP-MAS spin-counting experiments have been carried out on an absolute basis for a specific whole soil and its humin, humic acid, and fulvic acid fractions, as well as a sample of the soil that was treated with 2% HF(aq). The results confirm previous conclusions that a substantial fraction of the carbon content indicated by classic elemental analysis is missed in some samples, especially whole soil and humin, by both CP-MAS and DP-MAS 13C NMR methods, and that the problem is more serious for CP-MAS than for DP-MAS. This study also confirms the fact that treatment of soil organic matter with 2% HF(aq) dramatically reduces this problem but may generate some structural uncertainties associated with significant structural alterations that accompany the HF(aq) treatment, as indicated by the 13C NMR data. The relationship between the "missing carbon" problem and the concentration of paramagnetic centers, especially Fe(III) centers, is explored in substantial detail.

Carbon Isotopes↗
PubMed2003

Limitations of electrospray ionization of fulvic and humic acids as visible from size exclusion chromatography with organic carbon and mass spectrometric detection.

A method was developed to link size exclusion chromatography electrospray ionization mass spectrometry (SEC-ESI-MS) analyses of fulvic and humic acids with SEC and organic carbon detection (SEC-OCD), the latter providing an absolute measure of the amount of organic matter eluting from the SEC column. This approach allows us to determine which molecular weight fraction of the complex polydisperse mixtures is detectable by ESI-MS. It could be shown that the cone voltage setting for the ESI interface has strong impact on ESI-MS detection. Using conventional settings for low molecular weight compounds, the high molecular weight (HMW) compounds are hardly amenable to ESI-MS. With increasing cone voltage, an increasing signal intensity is obtained for the HMW fraction that elutes at shorter retention times. However, mostly fragment ions are obtained under these conditions. Thus, the range of compounds amenable to ESI-MS analysis is restricted by the limited stability of the fulvic and humic acid molecules of higher molecular weight in the electrospray process rather than by the mass spectrometer used. Compounds above 1000 amu are hardly visible as intact ions. However, insight into structural characteristics of these compounds can be gained by investigating their fragment ions by SEC-ESI-MS. The use of SEC-OCD parallel to SEC-MS helps to assess and optimize the detection potential of ESI-MS for polydisperse mixtures.

Benzopyrans↗
PubMed2003

Graphical method for analysis of ultrahigh-resolution broadband mass spectra of natural organic matter, the van Krevelen diagram.

Electrospray ionization mass spectrometry (ESI-MS) is becoming an important tool in the study of natural organic matter (NOM) at the molecular level. Ultrahigh-resolution ESI-MS analyses of NOM often produce very complicated spectra; therefore, visual presentation and structural interpretations of the spectra are difficult. To meet this analytical challenge, we herein propose and demonstrate an approach using the van Krevelen diagram. With this approach, complicated mass spectra can be visualized in a way that allows for (1) possible reaction pathways to be identified and presented, and (2) qualitative analyses on major classes of compounds that comprise ultrahigh-resolution spectra. The qualitative analyses are in a good agreement with results obtained from analyses by other analytical techniques. Additionally, the van Krevelen diagram can be expanded to a 3D plot by using peak intensities or relative intensities as the z-axis. The 3D van Krevelen diagram allows for an evaluation of the relative significance of structurally related compounds. The 3D plot can also be a useful tool for compositional differentiation among samples.

Coal↗
PubMed2005

Construction and evaluation of a humic acid column: implication for pesticide risk assessment.

In this paper, humic acid (HA), known to play a large role in the binding and transport of pesticides in soil, was immobilized on a chromatographic support. Then, the association of some herbicides and rodenticides with the main soil component HA was examined using this novel chromatographic column. It appeared that HA has a lower affinity for neutral than for charged pesticides. Moreover, the influence of various parameters was investigated on the pesticide retention in order to providevaluable information about both the binding mechanism and the utilization conditions of the HA column. For all the pesticides studied, a change was clearly vizualized in the HA-pesticide association mechanism at a critical value of the Na+ concentration in the bulk solvent, x(c), equal to 0.6 M. Around this value, the HA structure balanced between a flexible linear conformation for x < x(c) and a random coil form for x > x(c). This work confirmed the conformation change on HA immobilized on silica. As well, only for the charged pesticides, it was clearly pointed out that below a Na+ concentration equal to 0.3 M, the pesticide binding to HA decreased when the salt concentration was enhanced due to an ion pair formation and a competition effect between the sodium cation and pesticide to bind to the HA molecule. Furthermore, it was established that the HA column was stable during an extended period of time, indicating that the HA column could soon become very attractive to determine the risk assessment of pesticides.

Herbicides↗
PubMed2005

Equilibrium sampling through membranes of freely dissolved chlorophenols in water samples with hollow fiber supported liquid membrane.

The freely dissolved concentration (C(free)) of pollutants is generally believed to be bioavailable and thus responsible for toxic effects. The C(free) of organic weak acids and bases consists of a dissociated and a nondissociated fraction. By using chlorophenols as model compounds, a negligible-depletion extraction technique, equilibrium sampling through membranes (ESTM), was developed for the measurement of the nondissociated part of the C(free). Polypropylene hollow fiber membranes (280-microm i.d., 50-microm wall thickness, 0.1-microm pore size, 15-cm length) were impregnated with undecane in the pores in the fiber wall as liquid membrane and filled with buffer solution in the lumen as acceptor. Then, the hollow fiber membranes were placed into the sample (donor) for an equilibrium extraction after sealing the two ends. The chlorophenol concentrations in the acceptor were then determined by direct injection into a HPLC system. Finally, the C(free) of the nondissociated and the dissociated species of a chlorophenol were calculated based on its measured concentration in the acceptor, its pK(a) value, and the measured pH in sample and acceptor. Theoretically calculated distribution coefficients (D = 8-970) agree well with the experimental enrichment factors (E(e(max)) = 6-1124), and the equilibration time was observed to increase with increasing distribution coefficients (hours to days). The freely dissolved concentration of five chlorophenols, with a wide range of pK(a) (4.9-9.2) and log K(ow) (2.35-5.24), were successfully determined in model solutions of humic acids and at low-ppb levels in river and leachate water.

Buffers↗
PubMed2006

A stepwise stoichiometric representation to confirm the dependence of pesticide/humic acid interactions on salt concentration and to test the performance of a silica bonded humic acid column.

In a previous paper (André et al., in press), a novel chromatographic column was developed in our laboratory for studying the binding of pesticides with humic acid (HA), the main organic component in soil. It was demonstrated that this column supported a low fraction of organic modifier in the aqueous mobile phase (<0.25 (v/v)). To overcome this limitation for a practical use, a column in which the stationary phase was based on silica gel with chemically bonded humic acid was created. It was shown that this novel HA column supported a higher methanol fraction (<0.55 (v/v)). As well, the dependence of pesticide/humic acid interactions on salt (sodium chloride) concentration has been expressed in terms of a stepwise stoichiometric representation, which leads to a specific equation for the partition of the added salt between the pesticide molecule, the HA, and the pesticide/HA complex. Based on this novel equation, the dependence of the pesticide/humic acid association on the salt concentration can be formulated via a relation similar to the one of Tanford. In addition, for the first time, the calculation of the affinity energy distribution for different values of the salt concentration in the mobile phase confirmed the existence of several types of binding sites on the HA macromolecule.

Chromatography, High Pressure Liquid↗
PubMed2001

Characterization and biocompatibility studies of novel humic acids based films as membrane material for an implantable glucose sensor.

Multilayered films of humic acids (HAs) (naturally occurring biopolymers) were investigated as a potential semipermeable membrane for implantable glucose sensors. These films were grown using a layer-by-layer self-assembly process of HAs and oppositely charged ferric ions. The growth of these assemblies exhibited strong dependence on the pH and ionic strength of HAs solutions, which correlated with the degree of ionization of the carboxyl groups and neutralization-induced surface spreading. Quartz crystal microbalance (QCM) and ellipsometric studies have shown repeatable, stepwise increase in mass (as high as 5.63 microg/cm(2)) and in film thickness (ca. 24.3 nm per layer) for these assemblies. The permeability of glucose through these membranes can be regulated by varying the number of self-assembled HAs/Fe(3+) layers. Moreover, a 200 nm thick HAs/Fe(3+) film (in its hydrated state) had a shear modulus of about 80 MPa, implying stability upon implantation. These films were determined to be biocompatible since in vivo studies indicated only mild tissue reaction along with some neovascularization.

Animals↗
PubMedDate not supplied

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.

Catalysis↗
PubMed2001

Americium binding to humic acid.

The binding of americium (Am) by peat humic acid (PHA) has been investigated at Am concentrations between 10(-1) and 10(-7) M at pH approximately 2.6 in the presence and absence of Cu as a competing ion. Cu-PHA binding was also investigated in order to derive independent binding constants for use in modeling the competitive binding studies. Humic ion-binding model VI was used to compare the acquired data with previously published binding data and to investigate the importance of high-affinity binding sites in metal-PHA binding. Am was not observed to bind to high-affinity, low-concentration binding sites. The model VI parameter deltaLK2 takes into accountthe small number of strong sites in PHA and was found to be important for Cu-PHA binding but not for Am-PHA binding, regardless of whether Cu was present. Analysis of the PHA sample revealed that it contained a considerable quantity of Fe not removed by the extraction procedure, much of which is believed to be present as Fe(III). Model VI was then used to investigate the possible importance of the presence of Fe(III) in the Am-PHA binding experiments. When Fe(III) was assumed to be present, improved descriptions of the data by model VI were obtained by assuming that all of the metals [Am, Cu, and Fe(III)] undergo strong binding. This highlights the importance of Fe(III) competition in metal-PHA binding studies and possible shortcomings in the extraction procedure used to extract PHA.

Americium↗
PubMed2001

Hydrophobic interactions between spin-label 5-SASL and humic acid as revealed by ESR spectroscopy.

The spin-label probe 5-SASL (stearic acid spin-label with nitroxide free radical in position 5 of hydrocarbon chain), detectable by electron spin resonance (ESR), was tested to evaluate pH and reaction time dependencies of hydrophobic interactions with humic acid (HA). Strong changes were observed in 5-SASL ESR spectra in the presence of HA suspensions below pH 5, with disappearance of the three isotropic narrow hyperfine lines of the nitroxide group (typical of free spin-label) and formation of "immobilized" 5-SASL spectra. These changes were interpreted as due to 5-SASL bonding with hydrophobic groups of HA, by van der Waals forces and/or hydrogen bonds, in very hydrophobic sites (probably water-protected) existent in HA below pH 5. However, such sites are absent above pH 5, as demonstrated by a specific experiment to check 5-SASL spectra reversibility. On the other hand, the HA suspension was more efficient in dissolving 5-SASL than water above pH 5. This fact also suggests the existence of "surface" hydrophobic sites, where the spin-label binds to HA while maintaining the nitroxide group in contact with water, as evidenced by the typical free spin-label spectrum and hyperfine interaction splitting (a0 = 1.574 mT). Also experiments checking 5-SASL reversibility bonding with HA were consistent with the supramolecular association model to HA.

Electron Spin Resonance Spectroscopy↗
PubMed2001

Degradation pathways of pentachlorophenol by photo-Fenton systems in the presence of iron(III), humic acid, and hydrogen peroxide.

The degradation characteristics and pathways of pentachlorophenol (PCP) by the photo-Fenton systems were studied in H2O2 aqueous solutions, which contained Fe(III) only [H2O2/Fe(III) system] and Fe(III) + humic acid (HA) [H2O2/Fe(III)/HA system] at pH 5.0. Although 40% of the PCP was degraded after 5 h of irradiation in the H2O2/Fe(III) system, more than 90% was degraded in the H2O2/Fe(III)/HA system. This shows that at pH 5.0 the degradation of PCP is clearly enhanced by the presence of HA in the photo-Fenton system. In the H2O2/Fe(III) system, the production of octachlorodibenzo-p-dioxin (OCDD) was detected, and 2-hydroxy nonachlorodiphenyl ether was also identified as a precursor of OCDD. However, no OCDD production was observed in the H2O2/Fe(III)/HA system. This indicates that the presence of HA represses the production of OCDD during the degradation of PCP by the photo-Fenton system. Such an effect by HA can be attributed to a reaction sequence wherein reaction intermediates derived from PCP, such as PCP., are incorporated into HA. This was verified by 13C NMR and pyrolysis-GC/MS studies.

Biodegradation, Environmental↗
PubMed2001

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.

Benzene↗
PubMed2001

Factors affecting humic-nickel complex mediated reduction of trichloroethene in homogeneous aqueous solution.

The kinetics of trichloroethene (TCE) reductive dechlorination mediated by humic-Ni complexes in homogeneous aqueous solution using titanium(III) citrate as the bulk reductant was examined under various environmental conditions (e.g., pH and ionic compositions). Using Ca2+, Zn2+, and Hg2+ ions to vary Aldrich humic acid (HA)-Ni complex concentrations, pseudo-first-order rate constants for TCE reduction were observed to be proportional to HA-Ni levels (as calculated by speciation modeling), confirming HA-Ni complexes as the probable active mediator species. TCE reduction by HA-Ni was observed to be strongly pH dependent and could be due to both the variations of HA-Ni concentration and Eh with pH. Evidence is presented which suggests that quinone moieties may not be crucial for the humic-Ni mediated reduction of TCE. A variety of natural soil and aqueous humic material and Ni systems were examined, and some showed reactivity toward TCE. Humic-metal complexes may be important electron-transfer mediators in natural systems.

Benzoquinones↗
PubMed2001

A distributed reactivity model for sorption by soils and sediments 13. Simulated diagenesis of natural sediment organic matter and its impact on sorption/desorption equilibria.

Subcritical water treatment was used to effect rapid compositional and functional changes to peat organic matter that mimic those of the natural diagenesis process. Elemental, solid state 13C NMR, FTIR, and calorimetry analyses all indicated that the organic matter of the artificially aged peat was chemically similar to that of geologically mature coal kerogens. This paper extends the work of the previous paper in this series, which investigated the effects of subcritical water treatment of humic topsoil on subsequent phenanthrene sorption and desorption equilibria. As opposed to the previous study, however, changes in sorptive reactivity herein were unequivocally related to changes in organic matter rather than other soil constituents, and organic matter functional changes due to the simulated diagenesis were more accurately characterized. Phenanthrene sorption capacity and isotherm nonlinearity both increased with increasing degrees of artificial aging, supporting the viewpoint that hydrophobic organic contaminant sorption equilibrium properties can be directly related to the degree of diagenesis of geosorbent organic matter. In addition, this work investigated effects of subcritical water treatment of a geologically mature, kerogen-containing shale sample. In contrast to the peat, the functional characteristics of the shale were unchanged by this treatment, and subsequent phenanthrene sorption equilibria were altered far less.

Adsorption↗
PubMed2001

A distributed reactivity model for sorption by soils and sediments. 14. Characterization and modeling of phenanthrene desorption rates.

Rates and extents of phenanthrene desorption were studied for more than 250 days as functions of sorbent type, initial loading level, and aging. Apparent first-order desorption rate constants for the slowly desorbing fraction were found to (i) range from 0.00086 to 0.148 days-1 for geosorbents that contain geologically mature kerogen and less rigid humic-type soil organic matter, respectively, (ii) decrease by as much as an order of magnitude with decreasing initial sorbed solid-phase phenanthrene concentration, (iii) decrease by a factor of 2 with increasing aging time for a humic topsoil but remain unaffected by aging time beyond 3 months for a shale, and (iv) be 1-2 orders of magnitude lower than rate constants for the rapidly desorbing phenanthrene fractions for any given contaminated sample. Six models were used to fit the desorption rate data. Biphasic diffusion and biphasic first-order models with three fitting parameters possess broad utility and are potentially useful in a variety of environmental applications. Disadvantages of a five-parameter triphasic first-order desorption model, a two-parameter gamma-function model, and a one- or two-parameter pore diffusion model are also discussed.

Adsorption↗
PubMed2001

Enhanced concentrations of PAHs in groundwater at a coal tar site.

Concentrations of polycyclic aromatic hydrocarbons (PAHs) in groundwater at a coal tar site were elevated by factors ranging from 3 (pyrene) to 50 (indeno[1,2,3-cd]pyrene) over purely dissolved concentrations. Air-groundwater surface tension measurements (70.6 +/- 3 dyn/cm) were not sufficiently different from air-pure water measures (72.2 +/- 0.1 dyn/cm) to ascribe the observed enrichments to either cosolvents or surfactants in the groundwater. Excess pyrene was associated with colloids that passed an ultrafilter at ambient pH but became ultrafilterable when the groundwater pH was lowered to 1. This suggested pyrene association with humic acids. Given the decrease in groundwater total organic carbon (TOC) of 4 mgc/L upon acidification and ultrafiltration, a partition coefficient of 10(5) L/kgc was estimated for this pyrene association. Use of the results for pyrene and scaling for the differences in PAH hydrophobicities enabled good predictions of the observed enrichments of less water-soluble PAHs in the groundwater. This is strong field evidence indicating colloid-facilitated transport of HOCs in groundwater. Assuming that humic-bound PAHs were as mobile as the dissolved PAHs, the fluxes of individual PAHs (e.g., benzo[a]pyrene) from the tar source were as much as 20 times greater than estimates based solely on tarwater partitioning predictions.

Coal↗