The application of 1H high-resolution magic-angle spinning NMR for the study of clay-organic associations in natural and synthetic complexes

The preferential sorption of model compounds to calcium-exchanged montmorillonite surfaces was investigated using 1H high-resolution magic-angle spinning (HR-MAS) and liquid-state NMR. Synthetic mixtures, representing the major structural categories abundant in natural organic matter (NOM), and two soil extracts were sorbed to montmorillonite. The NMR spectra indicate that, of the organic components observable by 1H HR-MAS NMR, aliphatic components preferentially sorb to the clay surface, while carbohydrates and amino acids mainly remain in the supernatant. These results may help explain the highly aliphatic nature of organic matter associated with clay fractions in natural soils and sediments. Investigations using the synthetic mixtures demonstrate a specific interaction between the clay surface and the polar region in 1-palmitoyl-3-stearoyl-rac-glycerol. Similar observations were obtained with natural soil extracts. The results presented have important implications for understanding the role of organoclay complexes in natural processes, and provides preliminary evidence that HR-MAS NMR is a powerful analytical technique for the investigation of organoclay complex structure and conformation.     

Effect of multivalent cations,temperature, and aging on SOM thermal properties

Multivalent cations are suggested to influence the supramolecular structure of soil organic matter (SOM) via inter- and intra-molecular interactions with SOM functional groups. In this study, we tested the combined effect of cations, temperature treatment, and isothermal aging on SOM matrix properties. Samples from a peat and a mineral soil were either enriched with Na, Ca, and Al or desalinated in batch experiments. After treatment at 25, 40, 60, and 105 °C and after different periods of aging at 19 °C and 31 % relative humidity, we investigated the physicochemical matrix stability and the thermal stability against combustion. We hypothesized that multivalent cations stabilize the SOM matrix, that these structures disrupt at elevated temperatures, and that aging leads to an increase in matrix stability. The results show that cation-specific effects on matrix rigidity started to evolve in the peat only after 8 weeks of aging and were significantly lower than the temperature effects. Temperature treatment above 40 °C caused a non (or not immediately) reversible loss of water molecule bridges (WaMB) and above 60 °C a partly reversible melting process probably of semi-crystalline poly(methylene). Thermal stability increased with increasing cation valence and degree of protonation and was much less affected by temperature. Generally, Na-treated and control samples revealed lower thermal stability and lower increase in matrix rigidity with aging than those treated with Ca, Al, and H. We conclude that drying at elevated temperatures (>40 °C) may irreversibly change SOM structure via disruption of labile cross-links and melting of semi-crystalline domains.     

Sequestered carbon on clay mineral probed by electron paramagnetic resonance and X-ray photoelectron spectroscopy

This paper describes the interaction among soil organic matter components with kaolinite, an important clay mineral present in tropical soils, especially in Brazil. XPS data show that the soil organic matter adsorbed on kaolinite has aromatic and aliphatic structures, with phenolic and/or alcoholic functions and carbonyl carbons (CO) of amide and/or carboxylic groups. The N1s spectrum of the kaolinite shows an asymmetric peak that is assigned to amide and protonated ammines probably from humin. The interaction between them is strong enough to resist chemical oxidative or reductive attack besides loose amide functionalities. EPR data show that reductive treatment reduces some Fe3+ of the kaolinite structure, loosing organic components. A schematic representation of the reduction of structural Fe3+ in the concentrated domains and consequently increased concentration of Fe3+ ions in diluted domains of the spectrum is presented. This reinforces the hypothesis that humin is a stable carbon sink in soils when adsorbed to clays.     

Application of scanning calorimetry to estimate soil organic matter loss after fires

The severe heating of soil during wildfires and prescribed burns may result in adverse effects on soil fertility due to organic matter loss. No rapid and reliable procedure exists to evaluate soil organic matter (SOM) losses due to heating. Enthalpy of SOM combustion correlates with organic matter content. Quartz is a ubiquitous mineral in soils and has a remarkably constant composition and reversible α–β phase transition at 575 °C. We suggest that SOM content in heated and unheated soils can be compared using the ratio of SOM combustion enthalpy on heating to the β–α quartz transition enthalpy measured on cooling of the same sample. This eliminates the need to dry and weigh the samples, making possible field applications of the proposed method. The feasibility of using the (ΔH _{comb SOM})/(ΔH _β–α Qz) ratio was established with experiments on soil samples heated in the laboratory and the method was then used for evaluation of SOM loss on two pile burn sites at UC Berkeley’s Blodgett Forest Research Station in Georgetown, California.     

The influences of solid-phase organic constituents on the partition of aliphatic and aromatic organic contaminants

The influence of natural organic matter (NOM) constituents on contaminant distribution coefficients was evaluated by determining the Koc values of aromatic and aliphatic organic compounds (solutes) with clays modified with both aromatic- and aliphatic-rich organic constituents. The studied compounds consisted of naphthalene, phenanthrene, n-pentane, and 2,3,4-trimethylmethane; the solid samples comprised two clays with little organic content, kaolinite and Ca-montmorillonite. Two aliphatic surfactants and three aromatic dyes, sorbed to the clays, served as reference NOM constituents. For solutes of comparable water solubilities, the organic-carbon normalized distribution coefficients (Koc) of the aliphatic solutes between sorbed aliphatic organic matter and aqueous solution slightly exceed those of the aromatic solutes. By contrast, the aromatic solutes exhibited higher Koc values than did the aliphatic compounds with sorbed aromatic-rich organic matter. The difference in Koc values could be attributed to either comparable solubility parameters or the difference in the chemical structure between nonionic organic solutes and specific components of the simulated NOM. The much higher Koc values observed for the aromatic solutes indicate that the NOM composition is a major factor determining the NOC environmental distribution.     

Thermal decomposition of kerogens: Mechanism and analytical application

Specific features of the chemical structures of organic matter (Lerogen) in oil shales caused by the nature of the starting materials for the formation of shales and the routes of their subsequent alteration are reflected in the composition of shale semicoking (retorting) oil. In order to establish the analytical possibilities of the thermal decomposition method in elucidating the kerogen structure and to obtain more data on the mechanism of the pyrolysis of kerogens, the effect of a series of factors (rate of heating, pressure, presence of carrier gases, water and mineral matter of shale, treatment with reagents) on the yield and composition of the pyrolysis products of oil shales was investigated.The yield of shale oil and the phenol content in the latter increase when semicoking is performed in a stream of hydrogen at atmospheric pressure. In shale pyrolysis in the presence of water under pressure, the yield of oil and, in particular, water-soluble organic compounds also increase, as well as the content of neutral heteroatomic compounds in oil. With increasing content of mineral substances in shale, the yield of the semicoking oil (kerogen basis) and the content of polar compounds in it diminish owing to an increasing influence of oil adsorption on the mineral matter and its additional decomposition as a result.In the initial stage of thermal decomposition, both longer aliphatic substituents and side-chains of iso-structures split off and alkenes with a double bond in the middle of the chain (probably the products of elimination and dehydration of the aliphatic substituents with a hydroxyl group) are formed. The formation of n-1-alkenes, particularly those of even carbon number, which originate from the side-chains of odd carbon number by the cleavage of carbon–carbon bonds in the β-position to the cyclic nucleus of kerogen, becomes more pronounced in the final stage of pyrolysis when, owing to significant aromatization of the cyclic part of kerogen, the selectivity of the β-cleavage increases.     

PowerSlicing to determine fluorescence lifetimes of water-soluble organic matter derived from soils,plant biomass,and animal manures

Time-resolved fluorescence spectroscopy was used to characterize water-soluble organic matter (WSOM) which plays an important role in soil ecosystem processes. WSOM was extracted from plant biomass, animal manures, and soils from controlled cropping systems studies with known histories of organic amendments. Lifetime constants were derived using the multi-way PowerSlicing method which provides a non-iterative, multi-exponential fitting of decay profiles. The lifetimes obtained by PowerSlicing were not significantly different from those obtained using the traditional discrete components analysis. The three components attributed to WSOM had lifetimes of 0.38 ± 0.14, 2.11 ± 0.72, and 7.08 ± 1.18 ns which are in agreement with previous lifetimes reported for humic substances. This study provides further support for the new paradigm for the structure of soil organic matter where the organic matter is composed of low-molecular-weight components held together by hydrogen bonding and hydrophobic interactions.     

氯化亚铬脱除黄铁矿对桦甸油页岩有机质结构的影响

采用元素分析、~(13)C NMR、XPS和TG-MS技术考察了氯化亚铬(CrCl_2)脱除黄铁矿对桦甸油页岩有机质结构的影响。结果表明,CrCl_2可有效脱除有机质中的黄铁矿,脱除率为96.19%。CrCl_2对有机质的碳骨架结构影响较小,脱除黄铁矿前后有机质中脂碳、芳碳和羧基/羰基碳的相对含量以及有机质的热解特征温度基本保持不变,但CrCl_2可破坏有机质中的C-O键,使C-O/C-OH和O=C-O的含量减少,造成0.98%的有机碳损失和12.54%的有机质损失。CrCl_2处理后,有机质的C含量显著增加,H含量稍有增加,O含量显著降低,使得H/C略微降低,O/C明显降低。另外,CrCl_2处理后,单位质量有机质中脂碳的含量增加了5.28%,使其热解过程中产生了更多的挥发分,残留的氧化铬对有机质的热分解可能也具有促进作用。     

Spectroscopic study of the water-soluble organic matter isolated from atmospheric aerosols collected under different atmospheric conditions

The composition of the water-soluble organic matter from fine aerosols collected in a rural location during two different meteorological conditions (summer and autumn) was investigated by UV-vis, synchronous fluorescence (with Δλ = 20 nm), FT-IR and CPMAS-¹³C NMR spectroscopies. A seasonal variation in the concentration of total carbon, organic carbon and water-soluble organic carbon was confirmed, with higher values during the autumn and lower values during the summer season. The chemical characterisation of the water-soluble organic matter showed that both samples are dominated by a high content of aliphatic structures, carboxyl groups and aliphatic carbons single bonded to one oxygen or nitrogen atom. However, the autumn sample exhibits a higher aromatic content than the summer sample, plus signals due to carbons of phenol, ketones and methoxyl groups. These signals were attributed to lignin breakdown products which are likely to be released during wood combustion processes. The obtained results put into evidence the major contribution of biomass burning processes in domestic fireplaces during low temperature conditions into both the concentration and the bulk chemical properties of the WSOC from fine aerosols.     

Changes induced in the thermal properties of Galizian soils by the heating in laboratory conditions

The soil properties can be strongly affected by wildfires, causing direct effects on ecosystem productivity and sustainability. These effects depend, among other things, on the soil type and on the temperature reached during the fire. The variations of thermal properties of several Galizian soils heated in an oven in laboratory conditions at different temperatures (200–500 °C) during 15 min have been examined in this study. The measured properties are heat of combustion of soil organic matter, ignition temperature, specific heat and mass loss, determined using DSC 2920 TA Instruments and a TGA 7 Perkin Elmer under dry air gas flow. In agreement with other authors, this study establishes three temperature intervals with different effects on the soil: up to 200 °C, low intensity heating, with no significant changes in thermal properties; between 200 and 350 °C, medium intensity heating, with losses of organic matter up to 50%; and high intensity heating to temperature higher than 350 °C, with harmful effects on the soil organic matter. On the other hand and taking into account that the sampled soils had been affected by forest fires, the variations of thermal soil properties with the laboratory heating temperatures allowed for an estimation of the temperature reached by the soil in the real fire.     

Use of 13C to monitor soil organic matter transformations caused by a simulated forest fire

Soil organic matter (SOM) transformations caused by heating were analyzed using the stable carbon isotope (13)C as a tracer to follow C mineralization dynamics and C transfers between different organic compartments. A (13)C-labelled soil, obtained by incorporation of (13)C-enriched Lolium perenne phytomass into a pine forest soil, was heated for 10 min at 385 degrees C to reproduce conditions typical of a forest fire and changes in total C content, potential C mineralization activity and C distribution between the different soil organic fractions were determined. Changes caused by heating on the potential soil C mineralization, determined by laboratory aerobic incubation, reveal alterations to the SOM biodegradability; some stabilized SOM showed an increase in biodegradability, whereas less stabilized SOM became more resistant to microorganisms. Chemical fractionations of SOM allowed us to monitor changes in its composition. As a consequence of heating, the less polymerized humic fractions were the most strongly affected, with the total disappearance of fulvic acids. A significant increase in the quantity and degree of polymerization of the humic acids at the expense of other more (13)C-enriched substances was also found. Finally, a large decrease in humin was observed, its solubilizable part disappearing completely, probably as a consequence of the incorporation of the byproducts into the free organic matter fraction.     

Application of DSC–TG and NMR to study the soil organic matter

The environmental concern on soil exploitation, linked to global warming by the Kyoto protocol, is responsible for increasing interest in the understanding of the role of the composition and structure of the soil organic matter (SOM) on soil carbon, C, dynamics. Thermal analysis and nuclear magnetic resonance (NMR) are applied to study the thermal properties, the structure and composition of the SOM of six samples with different C contents in order to improve the interpretation of results given by thermal analysis. Results showed that the direct integral of the combustion peaks obtained by DSC and the percentage of SOM given by TG were both directly related to the quantity of total soil C. Thus, soils with higher C content showed higher energy content too. The combustion temperatures of the curves given by DSC are those reported for labile OM. NMR results indicated the presence of aliphatic C, carbohydrates, and a weak signal in the aromatic C band in all the samples that was not detected in the DSC curves. Only two samples showed carboxyl/carbonyl C which was not detected by DSC also.     

Structure and stability of natural organic matter/soil complexes and related synthetic and mixed analogues

This review aimed at the presentation of recent, new approaches which have been developed to improve our knowledge of soil structure and some related characteristics. The investigations reviewed addressed a large number of solid/polymer systems, from strictly synthetic to fully natural systems. Only studies correlating adsorption and solid structuring were considered, so that a great number of adsorption studies were not taken into account.Despite the very complex chemical nature, composition and structure of natural organic matter and soils, some analogues appeared to be useful for determining the role of the natural substances in the stability of soils. Stability means that the soil was able to resist fragmentation and dispersion induced by successive drying and wetting processes. Actually, in the domain of low polymer content, the cohesion of soils was found to be correlated to the concentration of natural organic matter and established by a mechanism which is similar to that inducing the flocculation of suspended colloidal particles. The paper reviews some characteristics of synthetic, natural and mixed systems which display the correlation existing between the structuring of solid agglomerates and the adsorption of polymers.     

Use of carbon isotope analysis to understand semi-arid erosion dynamics and long-term semi-arid land degradation

Many semi-arid areas worldwide are becoming degraded, in the form of C(4) grasslands being replaced by C(3) shrublands, which causes an increase in surface runoff and erosion, and altered nutrient cycling, which may affect global biogeochemical cycling. The prevention or control of vegetation transitions is hindered by a lack of understanding of their temporal and spatial dynamics, particularly in terms of interactions between biotic and abiotic processes. This research investigates (1) the effects of soil erosion on the delta(13)C values of soil organic matter (SOM) throughout the soil profile and its implications for reconstructing vegetation change using carbon-isotope analysis and (2) the spatial properties of erosion over a grass-shrub transition to increase understanding of biotic-abiotic interactions by using delta(13)C signals of eroded material as a sediment tracer. Results demonstrate that the soils over grass-shrub transitions are not in steady state. A complex interplay of factors determines the input of SOM to the surface horizon of the soil and its subsequent retention and turnover through the soil profile. A positive correlation between event runoff and delta(13)C signatures of eroded sediment was found in all plots. This indicates that the delta(13)C signatures of eroded sediment may provide a means of distinguishing between changes in erosion dynamics over runoff events of different magnitudes and over different vegetation types. The development of this technique using delta(13)C signatures of eroded sediment provides a new means of furthering existing understanding of erosion dynamics over vegetation transitions. This is critical in terms of understanding biotic-abiotic feedbacks and the evolution of areas subject to vegetation change in semi-arid environments.     

酸洗对桦甸油页岩矿物质以及有机结构的影响

基于FT-IR和XRD技术研究了逐级酸洗对桦甸油页岩矿物质以及有机结构的影响。结果表明,采用HCl/HF酸洗方法可以有效去除黄铁矿以外的矿物质,但盐酸处理破坏油页岩中高岭石的立体框架结构。油页岩中有机质以脂肪族结构为主,存在形式为无序非晶态聚合体且变质程度较低。酸洗处理对油页岩有机大分子结构影响很小,但对有机结构产生了一定的影响。盐酸处理主要影响含氧官能团和苯环结构,会生成大量羧酸并破坏苯环的多环结构,但对脂肪族化合物的影响较小。氢氟酸处理主要对脂肪族化合物产生影响,破坏脂肪链的桥键结构,脂肪链断裂变短,进而使样品中脂肪族化合物含量降低。盐酸和氢氟酸处理均会破坏油页岩的羟基官能团,尤其对自缔合羟基氢键影响最大。     

Subcritical water extraction of organic matter from sedimentary rocks

Subcritical water extraction of organic matter containing sedimentary rocks at 300 °C and 1500 psi produces extracts comparable to conventional solvent extraction. Subcritical water extraction of previously solvent extracted samples confirms that high molecular weight organic matter (kerogen) degradation is not occurring and that only low molecular weight organic matter (free compounds) are being accessed in analogy to solvent extraction procedures. The sedimentary rocks chosen for extraction span the classic geochemical organic matter types. A type I organic matter-containing sedimentary rock produces n-alkanes and isoprenoidal hydrocarbons at 300 °C and 1500 psi that indicate an algal source for the organic matter. Extraction of a rock containing type II organic matter at the same temperature and pressure produces aliphatic hydrocarbons but also aromatic compounds reflecting the increased contributions from terrestrial organic matter in this sample. A type III organic matter-containing sample produces a range of non-polar and polar compounds including polycyclic aromatic hydrocarbons and oxygenated aromatic compounds at 300 °C and 1500 psi reflecting a dominantly terrestrial origin for the organic materials. Although extraction at 300 °C and 1500 psi produces extracts that are comparable to solvent extraction, lower temperature steps display differences related to organic solubility. The type I organic matter produces no products below 300 °C and 1500 psi, reflecting its dominantly aliphatic character, while type II and type III organic matter contribute some polar components to the lower temperature steps, reflecting the chemical heterogeneity of their organic inventory. The separation of polar and non-polar organic compounds by using different temperatures provides the potential for selective extraction that may obviate the need for subsequent preparative chromatography steps. Our results indicate that subcritical water extraction can act as a suitable replacement for conventional solvent extraction of sedimentary rocks, but can also be used for any organic matter containing mineral matrix, including soils and recent sediments, and has the added benefit of tailored extraction for analytes of specific polarities.     

Native surface structure of natural soil particles determined by combining atomic force microscopy and X-ray photoelectron spectroscopy

A procedure was developed for handling natural soil particles and probing their native surface structure by atomic force microscopy (AFM) under water. This procedure was used to investigate the nanometer scale organisation of organic matter at the surface of sand particles taken from three soil horizons. The latter were selected for the contrasted properties of their organic matter, namely Podzol E and Bh horizons and a Cambisol A–B horizon. The presence of an adsorbed layer was visualised at the surface of Podzol Bh and Cambisol particles in the form of aggregated structures that interacted with the AFM probe. Surface analysis by X-ray photoelectron spectrometry (XPS) confirmed the carbonaceous nature of this adsorbed layer. Displacement of organic matter by the scanning probe was directly evidenced for Podzol Bh sand particles. Such displacement was not observed for Cambisol particles. A dramatic effect of drying on the concentration, nanometer scale distribution and properties of the adsorbed organic matter was clearly demonstrated by combining AFM imaging and XPS analysis. The procedure developed here gives access to direct, nanoscale information of the surface structure of sand particles and offers promising prospects for the characterisation of other environmentally-relevant particles in native conditions.     

Analysis of phenoxyalkanoic acid herbicides and their phenolic conversion products in soil by microwave assisted solvent extraction and subsequent analysis of extracts by on-line solid-phase extraction-liquid chromatography

A multiresidue method for the determination of phenoxyalkanoic acid herbicides and their phenolic conversion products in soil was developed. The method was based on microwave-assisted solvent extraction (MASE) of soil samples by an aqueous methanolic mixture and subsequent analysis of extracts by automated solid-phase extraction followed by on-line high-performance liquid chromatography and diode array detection. MASE parameters (extraction temperature and time, composition of the extraction mixture and extraction volume) were optimized with respect to analyte recoveries. The method was validated with two types of soils containing 1.5 and 3.5% organic matter, respectively, both types containing fresh and aged residues of sought analytes. Under the selected analytical conditions when soils with fresh residues were analyzed all target analytes were recovered above 80% from the soil containing 1.5% organic matter, while limits of identification at the level of 20-40 ng/g were achieved. From the soil containing 3.5% organic matter the least polar phenolic analytes exhibited slightly reduced recoveries, while identification limits of 30-50 ng/g were achieved. Samples with aged residues exhibited reduced recoveries for some analytes, the reduction amounting up to 6-12% within 1 month of aging period depending on soil organic matter.     

Influence of heteroatom pre-selection on the molecular formula assignment of soil organic matter components determined by ultrahigh resolution mass spectrometry

Soil organic matter (SOM) is involved in many important ecosystem processes. Ultrahigh resolution mass spectrometry has become a powerful technique in the chemical characterization of SOM, allowing assignment of elemental formulae for thousands of peaks resolved in a typical mass spectrum. We investigated how the addition of N, S, and P heteroatoms in the formula calculation stage of the mass spectra processing workflow affected the formula assignments of mass spectra peaks. Dissolved organic matter extracted from plant biomass and soil as well as the soil humic acid fraction was studied. We show that the addition of S and P into the molecular formula calculation increased peak assignments on average by 17.3 % and 10.7 %, respectively, over the assignments based on the CHON elements frequently reported by SOM researchers using ultrahigh resolution mass spectrometry. The organic matter chemical characteristics as represented by van Krevelen diagrams were appreciably affected by differences in the heteroatom pre-selection for the three organic matter samples investigated, especially so for the wheat-derived dissolved organic matter. These results show that inclusion of both S and P heteroatoms into the formula calculation step, which is not routinely done, is important to obtain a more chemically complete interpretation of the ultrahigh resolution mass spectra of SOM.     

Structure and dynamics of a discotic liquid-crystalline charge-transfer complex.

The structure of the charge-transfer complex hexakis(n-hexyloxy)triphenylene-2,4,7-trinitro-9-fluorenone (HAT6-TNF) has been characterized by neutron scattering, X-ray diffraction (XRD), optical microscopy, and dielectric relaxation spectroscopy (DRS). On the basis of these data and the 1:1 stoichiometry, a consistent structure for the complex is proposed. This structure differs markedly from structures previously proposed for similar materials, because the TNF molecules are found to be situated between the discotic columns rather than sandwiched between the discotic molecules of a given column. The addition of TNF to HAT6 is found to stiffen the structure, and quasi-elastic neutron scattering shows that the local dynamics of the discotic molecules in HAT6-TNF is slowed by the presence of the TNF molecules. This scenario is consistent with the observation of two VFT-type (VFT=Vogel-Fulcher-Tamman) dielectric relaxation processes that relate to the columnar glass transition and a polyethylene-like hindered glass transition originating from the nano-phase-separated fraction of the aliphatic tails.