Chemical heterogeneity of organic soil colloids investigated by scanning transmission X-ray microscopy and C-1s NEXAFS microspectroscopy

Colloid release and deposition in soils and sorption of inorganic and organic pollutants to soil colloids are strongly influenced by the composition and chemical heterogeneity of colloidal soil particles. To investigate the chemical heterogeneity of organic soil colloids at the particle scale, we used synchrotron scanning transmission X-ray microscopy (STXM) and C-1s near-edge X-ray absorption fine structure (NEXAFS) spectroscopy on 49 individual particles isolated from the surface horizons of three forest soils. Stacks of 130 images of each particle were collected at different X-ray energies between 280 and 310 eV. From these image arrays, NEXAFS spectra were obtained for each pixel and analyzed by principle component analysis and cluster analysis (PCA-CA) to characterize the intraparticle heterogeneity of the organic components. The results demonstrate that the organic matter associated with water-dispersible soil colloids is chemically heterogeneous at the single-particle scale. PCA-CA identified at least two distinct regions within single particles. However, the spectral variations between these regions were much smaller than the variations of averaged NEXAFS spectra representing different particles from the same soil horizon, implying that interparticle heterogeneity is much larger than intraparticle heterogeneity. Especially the contents of aromatic and carboxyl carbon exhibited a large variability. Overall, the NEXAFS spectra of water-dispersible soil colloids were similar to the NEXAFS spectrum of the humic acid fraction, but differed clearly from the fulvic acid and dissolved organic matter fractions extracted from the same soil horizon using conventional techniques.     

Adsorptive fractionation of humic acid at air-water interfaces

By using a simple bubble column, the adsorption behavior of a commercial soil-humic acid (CHA) at air-water interfaces was investigated. At pH 4.0, the concentrations of the CHA exhibited clear gradients in the bubble column, and increased significantly along the column height; smaller concentration gradients were also observed at pH 6.0. These concentration profiles demonstrate the surface activity of humic acid and pH-dependent affinity toward air-water interfaces. Taking advantage of the bubble column method, we interestingly found that the adsorptive fractionation of the CHA at air-water interfaces did occur. The components with higher molecular weight and stronger UV absorptivity showed greater affinity toward air-water interfaces, despite that the fractionation pattern was reduced to a certain extent as solution pH increased. The organic carbon-normalized pyrene partition coefficient Koc values deviated from the corresponding values of original bulk solutions at both pH 4.0 and 6.0, and increased along the height of the column. Our results demonstrate the usefulness of the simple bubble column, and suggest that the adsorptive fractionation of humic acid at air-water interfaces might have implications for some natural environments and engineered systems where air-water interfaces exist extensively.     

Carbon speciation of diesel exhaust and urban particulate matter NIST standard reference materials with C(1s) NEXAFS spectroscopy

The U.S. National Institute of Standards and Technology (NIST) provides a number of particulate matter (PM) standard reference materials (SRM) for use in environmental and toxicological methodology and research. We present here the first analysis with respect to the molecular structure of the carbon in three such NIST SRM samples, i.e., diesel engine exhaust soot from heavy duty equipment engines (SRM 1650), diesel soot from a forklift engine (SRM 2975), and urban PM collected in St. Louis, MO (SRM 1648), with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The NEXAFS spectra of the two diesel soot samples appear quite similar, while they differ significantly from the urban PM spectrum, in agreement with X-ray diffraction data published recently. Such comparison is made in terms of aromatic and aliphatic carbon species, as well as by a general comparison with graphitic materials. Both diesel soot SRM samples contain basic graphitic structures, but the presence of exciton resonance and extended X-ray absorption fine structure oscillations in SRM 1650 and the lack therof in SRM 2975 suggest that SRM 1650 is the more graphitic material.The presence of polycyclic aromatic hydrocarbons, which have a characteristic NEXAFS resonance at the same position as graphite, can obscure the graphitic character of soot, unless an extraction of the organic matter is made. Our NEXAFS data do not suggest that the urban PM sample SRM 1648 contains a substantial amount of graphite-like material.     

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.     

Sorption of Aldrich humic acid onto hematite: insights into fractionation phenomena by electrospray ionization with quadrupole time-of-flight mass spectrometry

Sorption induced fractionation of purified Aldrich humic acid (PAHA) on hematite is studied through the modification of electrospray ionization (ESI) quadrupole time-of-flight (QToF) mass spectra of supernatants from retention experiments. The ESI mass spectra show an increase of the "mean molecular masses" of the molecules that constitutes humic aggregates. The low molecular weight fraction (LMWF; m/z < or = 600 Da) is preferentially sorbed compared to two other fractions. The resolution provided by ESI-QToF mass spectrometer in the low-mass range provided evidence of further fractionation induced by sorption within the LMWF. Among the two latter fractions, the high molecular weight fraction (HMWF; m/z approximately 1700 Da) seems to be more prone to sorption compared to the intermediate molecular weight fraction (IMWF; m/z approximately 900 Da). The IMWF seems to be more hydrophilic as it should be richer in O, N, and alkyl C from the proportion of even mass, and poorer in aromatic structures from mass defect analysis in ESI mass spectra.     

Mechanisms of humic acids degradation by white rot fungi explored using 1H NMR spectroscopy and FTICR mass spectrometry

Enzymatic activities involved in decay processes of natural aromatic macromolecules, such as humic acids (HA) and lignin by white rot fungi, have been widely investigated. However, the physical and chemical analysis of degradation products of these materials has not been intensively explored. Fourier transform cyclotron resonance mass spectrometry (FTICR MS) and 1H NMR as well as CHNOS and size exclusion chromatography were employed to study the mechanisms of HA degradation by Trametes sp. M23 and Phanerochaete sp. Y6. Size exclusion chromatography analyses demonstrate and provide evidence for HA breakdown into low MW compounds. The 1H NMR analysis revealed oxidation, a decrease in the aromatic content, and an indication of demethylation of the HA during biodegradation. Evidence for oxidation was also obtained using CHNOS. Analysis of FTICR MS results using a new software program developed by our group (David Mass Sort) revealed consecutive series of masses suggesting biochemical degradation trends such as oxidation, aromatic cleavage, and demethylation. These results are in agreement with the 1H NMR analysis and with the suggested role of the ligninolytic system leading to HA degradation.     

Quantitative evaluation of noncovalent interactions between glyphosate and dissolved humic substances by NMR spectroscopy

Interactions of glyphosate (N-phosphonomethylglycine) herbicide (GLY) with soluble fulvic acids (FAs) and humic acids (HAs) at pH 5.2 and 7 were studied by (1)H and (31)P NMR spectroscopy. Increasing concentrations of soluble humic matter determined broadening and chemical shift drifts of proton and phosphorus GLY signals, thereby indicating the occurrence of weak interactions between GLY and humic superstructures. Binding was larger for FAs and pH 5.2 than for HAs and pH 7, thus suggesting formation of hydrogen bonds between GLY carboxyl and phosphonate groups and protonated oxygen functions in humic matter. Changes in relaxation and correlation times of (1)H and (31)P signals and saturation transfer difference NMR experiments confirmed the noncovalent nature of GLY-humic interactions. Diffusion-ordered NMR spectra allowed calculation of the glyphosate fraction bound to humic superstructures and association constants (K(a)) and Gibbs free energies of transfer for GLY-humic complex formation at both pH values. These values showed that noncovalent interactions occurred most effectively with FAs and at pH 5.2. Our findings indicated that glyphosate may spontaneously and significantly bind to soluble humic matter by noncovalent interactions at slightly acidic pH and, thus, potentially pollute natural water bodies by moving through soil profiles in complexes with dissolved humus.     

酶法拆分共轭亚油酸异构体的响应面优化

利用响应面法优化了酶法酯化拆分共轭亚油酸异构体的工艺条件,最佳条件为：乙醇添加量0．16g/g（以CLA为基准）,pH6．5,温度26℃。在此条件下,反应12h后,酯化率可以达到45．8％,其中乙酯中c9,t11-CLA含量达到56．22％,t10,c12-CLA的含量达到21．63％。     

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.     

Catalysis of PAH biodegradation by humic acid shown in synchrotron infrared studies

The role of humic acid (HA) in the biodegradation of toxic polycyclic aromatic hydrocarbons (PAHs) has been the subject of controversy, particularly in unsaturated environments. By utilizing an infrared spectromicroscope and a very bright, nondestructive synchrotron photon source, we monitored in situ and, over time, the influence of HA on the progression of degradation of pyrene (a model PAH) by a bacterial colony on a magnetite surface. Our results indicate that HA dramatically shortens the onset time for PAH biodegradation from 168 to 2 h. In the absence of HA, it takes the bacteria about 168 h to produce sufficient glycolipids to solubilize pyrene and make it bioavailable for biodegradation. These results will have large implications for the bioremediation of contaminated soils.     

XANES spectroscopy studies of Cr(VI) reduction by thiols in organosulfur compounds and humic substances

The reduction of Cr(VI) by the thiol-containing compounds cysteine and glutathione and by reduced sulfur in humic substances was monitored with sulfur and chromium X-ray absorption near-edge structure (XANES) spectroscopy in chromium-contaminated soils. Reaction of humic acids with Cr(VI) resulted in a reduction of the peak area of thiols and an increase in the peak area of disulfides in the sulfur XANES spectra. Analysis of the sulfur XANES spectra in various systems indicates that the reduction of Cr(VI) by humic substances involves a thiol/disulfide redox couple analogous to that of the Cr(VI) reduction by the simple thiol-containing compounds cysteine and glutathione. A fraction of the hexavalent chromium present in industrially-contaminated soils was not reducible by thiols. Reduction of Cr(VI) to Cr(III) in soils by thiols has little effect on the pH of the system in contrast to the pH decrease resulting from reduction by Fe(II).     

尿素包合法提纯油茶籽油中油酸的工艺研究

用尿素包合法对油茶籽油中的油酸进行包合。采用浸出法、直接酸解法和浸出后酸解法分离包合物固相中的油酸。实验结果表明,选用直接酸解法效果较好。尿素包合条件为脂肪酸/尿素/甲醇(W/W/V)比例1∶4∶10,通过包合,油酸的纯度从包合前的66.99%提高到77.96%。     

Role of humic acid and ouinone model compounds in bromate reduction by zerovalent iron

Experiments were conducted to examine the role of humic acid and quinone model compounds in bromate reduction by Fe(0). The reactivity of Fe(0) toward bromate declined by a factor of 1.3-2.0 in the presence of humic acid. Evidence was obtained that the quick complexation of humic acid with iron species and its adsorption passivated the iron surface and decreased the rate of bromate reduction by Fe(0). On the other hand, in the long run, the reduced functional groups present in humic acid were observed to regenerate Fe(II) and reduce bromate abiotically. Compared with the case of humic acid only, the simultaneous presence of Fe(II) and humic acid significantly increased the bromate removal rate. Fe(III)/Fe(II) acted as a catalyst in the oxidation of humic acid by bromate. Anthraquinone-2,6-disulfonate (AQDS) and lawsone did not cause any significant effect on the bromate reduction rate by Fe(0). However, the redox reactivity of lawsone in the presence of Fe(III) was evident, while AQDS did not show any under the tested conditions. The difference was attributable to the presence/ absence of reducing functional groups in the model compounds. The electron spin resonance further demonstrated that the redox functional groups in humic acid are most likely quinone-phenol moieties. Although the bromate reduction rate by regenerated Fe(II) is a few times slower than that by Fe(0), the reactive Fe(II) can be, alternatively, reductively formed to maintain iron surface activation and bromate reduction to prolong the lifetime of the zerovalent iron.     

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.     

Measurement of humic and fulvic acid concentrations and dissolution properties by a rapid batch procedure

Although humic substances (HS) strongly facilitate the transport of metals and hydrophobic organic contaminants in environmental systems, their measurement is hampered by the time-consuming nature of currently available methods for their isolation and purification. We present and apply a new rapid batch method to measure humic (HA) and fulvic (FA) acid concentrations and dissolution properties in both solid and aqueous samples. The method is compared with the conventional procedures and is shown to substantially facilitate HS concentration measurements, particularly for applications such as geochemical modeling where HS purification is not required. The new method can be performed within 1.5-4 h per sample and multiple samples can be processed simultaneously, while the conventional procedures typically require approximately 40 h for a single sample. In addition, specific dissolution properties of HS are identified and are consistent with recent views on the molecular structure of HS that emphasize molecular interactions of smaller entities over distinct macromolecular components. Because the principles of the new method are essentially the same as those of generally accepted conventional procedures, the identified HA and FA properties are of general importance for the interpretation of the environmental occurrence and behavior of HS.     

On-line prediction of chemical composition of semi-frozen ground beef by non-invasive NIR spectroscopy

The chemical composition of industrial scale batches of frozen beef was measured on-line during grinding by near infrared (NIR) reflectance spectroscopy. The MM55E filter based non-contact NIR instrument was mounted at the outlet of a meat grinder, and the fat, moisture and protein contents determined from the average of each filter reading throughout the grinding of the batch. The filters were selected from full spectra measurements to be as insensitive to water crystallization as possible. For on-line calibration and prediction, 55 beef batches of 400–800 kg in the range of 7.66–22.91% fat, 59.36–71.48% moisture, and 17.04–20.76% protein, were ground through 4 or 13 mm hole plates. The regression results, presented as root mean square error of cross validation (RMSECV) were between 0.48 and 1.11% for fat, 0.43 and 0.97% for moisture and 0.41 and 0.47% for protein.     

Determination of magnesium in soil extracts by atomic absorption spectroscopy and chemical methods

Magnesium, extracted from 244 soils using a range of extractants, was determined by (i) atomic absorption spectroscopy and (ii) a number of standard chemical techniques. From an examination of the results, atomic absorption spectroscopy was shown to offer a rapid and reliable method for the determination of magnesium and to have definite advantages over the chemical methods in a number of respects. The atomic absorption technique was distinctly superior for estimating low magnesium levels in the extracts, which correspond to deficiency levels in soils as far as crops and livestock are concerned. Ammonium nitrate appeared to be the most consistent extractant over a wide range of soils. For calcareous soils, acidic extractants produced higher magnesium values than the neutral type, due to the solution of free carbonates. The narrow soil/extractant ratio and a short shaking time for the Morgan's II method however, overcame this disadvantage on calcareous soils.     

Evaluation of chemical parameters in soft mold-ripened cheese during ripening by mid-infrared spectroscopy

The suitability of mid-infrared spectroscopy (MIR) to follow the evolution throughout ripening of specific physicochemical parameters in Camembert-type cheeses was evaluated. The infrared spectra were obtained directly from raw cheese samples deposited on an attenuated total reflectance crystal. Significant correlations were observed between physicochemical data, pH, acid-soluble nitrogen, nonprotein nitrogen, ammonia (NH₄⁺), lactose, and lactic acid. Dry matter showed significant correlation only with lactose and nonprotein nitrogen. Principal components analysis factorial maps of physicochemical data showed a ripening evolution in 2 steps, from d 1 to d 7 and from d 8 to d 27, similar to that observed previously from infrared spectral data. Partial least squares regressions made it possible to obtain good prediction models for dry matter, acid-soluble nitrogen, nonprotein nitrogen, lactose, lactic acid, and NH₄⁺ values from spectral data of raw cheese. The values of 3 statistical parameters (coefficient of determination, root mean square error of cross validation, and ratio prediction deviation) are satisfactory. Less precise models were obtained for pH.