Models of natural organic matter and interactions with organic contaminants

Adsorption of organic contaminants onto soils, sediments and other particulates has the potential to be a major controlling factor in their bioavailability, fate and behavior in the environment. Models for estimating the amount and stability of sorbed organic contaminants based on the fraction of organic carbon in a soil or sediment can oversimplify the process of sorption in the environment. In order to help understand sorption of organic contaminants in soils and sediments, we modeled various components of natural organic matter (NOM) that are possible substrates for sorption. These substrates include soot particles, lignin, humic and fulvic acids. The molecular scale interactions of selected aromatic hydrocarbons with different substrates were also simulated. Results of the simulations include the 3-D structures of the NOM components, changes in structure with protonation state and solvation and the sorption energy between PAH and substrate. This last parameter is an indicator of the amount of contaminant that will sorb and the energy required to free the contaminant from the substrate. Although the simulation results presented in this paper represent a first-order examination of NOM and contaminant interactions, the findings highlight a number of essential features that should be included in future molecular models of NOM and contaminant sorption.     

Characterization of Bulgarian oil shale kerogen revealed by oxidative degradation

A 13-step alkaline permanganate degradation of Bulgarian oil shale kerogen concentrate at ambient temperature was carried out. A high yield of oxidation products (90.1%) and a low yield of gaseous products (2.79%) were obtained. IR and ¹H NMR spectroscopic studies have shown that two significantly different types of high molecular products are present in kerogen. Further oxidation of these structures leads to the formation of low molecular aliphatic and aromatic acids, proven by gas chromotography (GC) and gas chromatography-mass spectrometry (GC-MS). The data obtained at these mild conditions allow us to acquire detailed information about the aromatic structures and polymethylene chain lengths in kerogen.The 5-step oxidation of the kerogen at 90 °C provides information about stable aromatic structures. Soluble and insoluble polyfunctional acids in acid medium have close molecular masses and spectral characteristics. The amount of benzene and naphthalene carboxylic acids is 11.3% of the organic matter of the oil shale.     

Effect of N content and soil texture on the decomposition of organic matter in forest soils as revealed by solid-state CPMAS NMR spectroscopy

N has a controlling effect on litter biodegradation in the forest floor, while stabilization of organic matter in the mineral soil may be influenced by physical parameters related to soil texture. In this study, in order to understand the processes involved in soil organic matter (SOM) formation, the chemical composition of SOM was followed and evaluated with regards to N contents and soil texture. Samples were taken on sites covered with Norway spruce and displaying contrasting values of C/N ratios in the forest floor. The chemical structure of OM was characterized using solid-state CPMAS ¹³C and ¹⁵N nuclear magnetic resonance (NMR) spectroscopy, along with Proton Spin Relaxation Editing (PSRE) sequences. Four groups of sampling sites were defined based on the NMR spectra of Oh and A horizons. In each group displaying similar NMR characteristics, N content and soil texture could be highly different among sites. Some Oh horizons with similar NMR spectra had very different N contents. Highly humified OM in Oh horizons were observed mainly on sites with low N contents. Some A horizons with different soil texture displayed similar OM chemical structure. High contents of O-alkyl C in some A horizons could originate from higher fresh root material input.     

不同土壤有机质组分对憎水有机物的吸附机理研究

采用完全混合一步平衡反应实验方法，选择菲、萘、三氯苯和二甲苯作为憎水有机物(HOCs)的探针，研究了它们在土壤及其中的胡敏酸、胡敏素等有机质组分中的等温平衡吸附行为，探讨了非线性吸附的机制。结果表明，HOCs在水与土壤体系中表现为非线性吸附，其非线性的吸附行为主要受到土壤有机质(SOMs)的控制。SOMs具有高度不均匀的性质，HOCs表现出非线性吸附行为是其在不同的SOMs相扩散转移以及在相对聚结的胡敏素和“黑炭”的表面吸附的结果。     

Physical protection of lignin by organic matter and clay minerals from chemical oxidation

The role of organic matter (OM) concentration, structure and composition and how these relate to mineral protection is important for the understanding of long term soil OM dynamics. Various OM–clay complexes were constructed by sequential sorption of lignin and dodecanoic acid to montmorillonite. Humic acid–montmorillonite complexes were prepared at pH 4 and 7 to vary OM conformation prior to sorption. Results obtained with constructed OM–clay complexes were tested with isolated mineral fractions from two soils. Oxidation with an acidic NaClO₂ solution was used to chemically oxidize lignin in the OM–clay complexes, sand-, silt- and clay-size soil fractions to test whether or not it can be protected from chemical attack. Gas chromatography–mass spectrometry was used to analyze lignin-derived phenols, cutin OH–acid (after CuO oxidation), fatty acid and n-alkanol concentrations and composition. We found that carbon content was not solely responsible for lignin stability against chemical oxidation. Lignin was protected from chemical oxidation through coating with dodecanoic acid and sorption of humic acid to clay minerals in a stretched conformation at pH 7. Therefore, interactions between OM constituents as well as OM conformation are important factors that protect lignin from chemical oxidation. Lignin-derived phenol dimers in the Grassland-Forest Transition soil fractions were protected from chemical oxidation to a greater extent compared to those in Grassland soil fractions. Therefore, although lignin was protected from degradation through mineral association, the extent of this protection was also related to OM content and the specific stability of lignin components.     

Reduction of iron(III) minerals by natural organic matter in groundwater

Construction of the entrance tunnel to the Äspö Hard Rock Laboratory, a prototype repository in Sweden for research into the geological disposal of spent nuclear fuel, has resulted in increased transport of organic carbon from the surface into the groundwater. This increased input of organic matter has induced accelerated oxidation of organic carbon associated with reduction of iron(III) minerals as the terminal electron acceptor in microbial respiration. Hydrochemical modeling of major solute ions at the site indicates an apparent first-order decay constant for organic carbon of 3.7 ± 2.6/yr. This rapid turnover is not accompanied by an equivalent mobilization of ferrous iron. Thermodynamic calculation of iron mineral solubility suggests that ferrous clay minerals may form in hydraulically transmissive fractures. The conditional potentials for the oxidation–reduction of such phases coincide with measured redox potentials at the site. The calculated potential is sufficiently low so that such phases would provide reducing capacity against future intrusion of O₂ into the groundwater, thus buffering a repository against oxic corrosion of the engineered barriers.     

Spectral fingerprinting of soil organic matter composition

Large scale environmental monitoring schemes would benefit from accurate information on the composition of soil organic matter (SOM), but so far routine procedures for describing SOM composition remain a chimera. Here, we present the initial assessment of a two step strategy for expeditious determination of SOM composition that involves: (i) building infrared fingerprints from near and mid infrared spectroscopies, two rapid and cheap yet reliable technologies; and (ii) calibrating such infrared fingerprints with multivariate chemometrics from a molecular mixing model based on the more expensive and time consuming ¹³C nuclear magnetic resonance technique, which discriminates five biochemical components: carbohydrate, protein, lignin, lipid and black carbon. We show fair to excellent predictive ability of the calibrated infrared fingerprints for four out of these five biochemical components, with cross-validated ratios of performance to inter-quartile distance from 3.2 to 8.3, on a small set of 23 soil samples with a wide range of organic carbon content (12–500 g/kg). Multivariate calibration models were highly selective (<2% of infrared data were used for all models). However, the specificity to one particular biochemical component of the infrared wavebands automatically selected by each model was relatively low, except for lipid. Achieving direct predictions of SOM composition on unknown soil samples with infrared spectroscopy alone will require further independent validation and a larger number of samples. Overall, the implementation of our strategy at a broader scale, based on available ¹³C nuclear magnetic resonance soil libraries, could provide a cost effective solution for the routine assessment of SOM composition.     

Pyrolysis-combustion C dating of soil organic matter

Radiocarbon (¹⁴C) dating of total soil organic matter (SOM) often yields results inconsistent with the stratigraphic sequence. The onerous chemical extractions for SOM fractions do not always produce satisfactory ¹⁴C dates. In an effort to develop an alternative method, the pyrolysis-combustion technique was investigated to partition SOM into pyrolysis volatile (Py-V) and pyrolysis residue (Py-R) fractions. The Py-V fractions obtained from a thick glacigenic loess succession in Illinois yielded ¹⁴C dates much younger but more reasonable than the counterpart Py-R fractions for the soil residence time. Carbon isotopic composition (δ¹³C) was heavier in the Py-V fractions, suggesting a greater abundance of carbohydrate- and protein-related constituents, and δ¹³C was lighter in the Py-R fractions, suggesting more lignin- and lipid-related constituents. The combination of ¹⁴C dates and δ¹³C values indicates that the Py-V fractions are less biodegradation resistant and the Py-R fractions are more biodegradation resistant. The pyrolysis-combustion method provides a less cumbersome approach for ¹⁴C dating of SOM fractions. With further study, this method may become a useful tool for analyzing unlithified terrestrial sediments when macrofossils are absent.     

Diagenesis of organic matter and fine clay minerals: a comparative study

A geochemical study has been made of fine clay minerals and organic matter in subsurface shale samples from the Canadian Northwest Territories. The mixed layer clays comprise smectite-vermiculite-illite but are transformed during thermal diagenesis to a pseudo-quaternary system (smectitevermiculite-illite-chlorite) by incorporation of amorphous inorganic material. The first clay dehydration occurs prior to hydrocarbon generation and is accompanied by adsorption of K⁺ and substitution of Al³⁺ for Si⁴⁺ in the clay lattice. Vermiculite is an intermediary in the transformation of smectite to illite and in the presence of Ca²⁺ ions delays the second dehydration step to the zone where cracking of liquid hydrocarbons to gas occurs.Hydrocarbon generation commences at a vitrinite reflectance level of ? 0.5% R₀ maximum in both amorphous and woody-herbaceous organic matter but does not reach a significant level in the latter case until reflectance levels of 0.7% R₀ maximum. The pristane to phytane ratio and proportion of n-alkanes and acyclic isoprenoids increase during hydrocarbon generation from woody-herbaceous organic matter. Anomalous hydrocarbon yields from certain samples are attributed to high concentrations of plant resins. Destructions of liquid hydrocarbons by cracking commences at a reflectance level of 1.0% R₀ maximum and is essentially complete by a reflectance level of 1.4% R₀ maximum. The proportion of n-alkanes in the saturates decreases during cracking of the liquid hydrocarbons.     

模拟土样有机碳和矿物质对TCE吸附贡献的实验研究

为研究均一介质条件下有机碳含量及矿物质对TCE吸附行为的影响,简化了土壤环境的复杂性和异质性,以固定矿物质(高岭土∶石英砂=3∶7)作为土壤基体,添加不同质量分数的腐殖质(foc=0.16%~2.29%)配制成模拟土样,进行矿物质和模拟土样对TCE的吸附批实验。实验结果显示,TCE的吸附等温线呈非线性。随有机碳含量增加,表现为线性吸附增强,矿物质对TCE的吸附贡献随有机碳含量增加而减小。foc>0.82%时,矿物质对吸附作用的贡献率<5%;foc>1%时,可以基本忽略矿物质对吸附的影响。此外,TCE初始浓度也会影响有机碳和矿物质的吸附能力。用Freundlich模型分别拟合TCE吸附等温线的低浓度段和高浓度段(以Ce=500μg/L为浓度高低的分界),Freundlich指数n值呈现由大到小的趋势;TCE初始浓度越高,有机碳的吸附贡献率相对上升,而矿物质的贡献率则下降。TCE初始浓度在50~500μg/L之间、foc由0.16%增大到1%时,矿物质的吸附贡献率范围由28%~16%缩小到3%~1%之间,此时TCE初始浓度对有机碳和矿物质的吸附贡献率基本没有影响。     

Extensive degradation and fractionation of organic matter during subsurface weathering

Organic carbon, sulphur, ¹³C_org, iron, manganese and calcium have been measured across a subsurface-weathering front in Pliocene sediments in southern Sicily. The results show an almost quantitative removal of C_org and sulphur and an increase in iron and manganese oxides over the weathering front, accompanied with a significant shift of the ¹³C_org to lower values. These data are among the first to support the rapid, extensive weathering of sedimentary organic matter and sulphur, a basic assumption made in global biogeochemical models on a Phanerozoic timescale.     

纳米探矿——用地气携带的纳米物质勘查隐伏矿

矿体周围广泛赋存着纳米物质,其可在地球排气的作用下从深部向地表迁移。在地表或地表附近可俘获这些纳米物质,这些纳米物质与矿体发出的直接信息密切相关。通过对其成分或聚集态等进行分析,可勘查隐伏矿。本文介绍了纳米探矿的应用现状和一些实例,探讨了该技术所存在的问题,并对今后的研究工作进行了展望。     

Molecular fossils from chemical degradation of macromolecular organic matter

Hydrogenolysis with rhodium-on-charcoal was found to be an effective method for degrading different types of macromolecular material of geological origin. Between 6 and 70% of coals, kerogens and humic substances were transferred into low-molecular-weight soluble materials. The reaction products contained a series of monomeric and dimeric lignin-derived compounds which strongly suggest intact fossil lignin as their source. The substitutional patterns of the released phenols reflect the type and amount of terrestrial organic matter input and diagenetic alterations.In addition to the lignin constituents, a complete suite of hydrocarbons differing from those of the low-molecular-weight fraction was obtained after hydrogenolysis of the sample material. Hydrocarbons released by the hydrogenolytic degradation technique were attached to the polymeric matrix as monoor polyethers. Deuterium was used in the degradation experiment to evaluate the sites of chemical bonds by which 4-methylsteranes and hopanes are attached to the kerogen matrix. These experiments suggested a linkage of the released molecules with ring A and the side-chain, respectively.     

Interactions between organic matter and minerals in two bituminous coals of different rank

The association between specific mineral and organic constituents in two Asturian bituminous coals of different rank was studied. For this, raw coals were fractionated by density and the variation of a number of parameters was followed in parallel. Results of coal chemical analyses, including analyses for 22 elements, were used to establish the elemental association with coal organic matter. Petrographic analyses determined the distribution of macerals among densimetric fractions, vitrinite reflectance being at a minimum in the intermediate density fractions. Mineral species were identified by X-ray diffraction, FT-IR spectroscopy and Mössbauer spectroscopy. Comparison of trends for different parameters determined using this set of techniques allowed classification of the various minerals according to their association with organic matter. Carbonates seem to be specifically associated with the organic matter of the low-volatile bituminous coal whereas sulfides concentrate in the organic matter of the high-volatile bituminous coal. Vitrinite is the maceral exhibiting the most probable association with inorganic matter. The possibility of a merely physical association of fine-grained detrital minerals with organic matter cannot be excluded; nevertheless, one must bear in mind that even this type of interaction is important due to its effect on various coal preparation and utilization processes.     

不同有机酸对矿物溶解的动力学实验研究

用石英、微斜长石和方解石混合颗粒模拟碎屑组分分别与不同有机酸水溶液进行溶蚀实验,以比较储集层内不同矿物在含低分子量有机酸地层水中溶解的速率,并试图探讨矿物溶蚀的微观机理。结果表明：①所有矿物颗粒都发生了不同程度的溶解,表现为颗粒失重,溶液中SiO2和金属阳离子含量增加,pH值上升。②优先溶解的是方解石,其次是硅酸盐矿物。③温度增加,硅酸盐矿物在水中的溶解度明显增加,而方解石的溶解度基本不变。④由于不同有机酸与二氧化硅和金属阳离子生成的络合物稳定性不同,因此,在不同有机酸水溶液中,矿物的溶蚀速率有较大差别。多官能团有机酸的水溶液中,不同矿物有更大的溶解;但在含有丰富Ca2＋的草酸水溶液中,由于矿物颗粒表面难溶草酸钙的沉淀,方解石的溶解变得更加困难。⑤多官能团有机酸与SiO2形成的多环螯合物由于完全取代了硅氧四面体的氧原子,在水中具有高度稳定性,可能有助于(铝)硅酸盐的溶蚀和硅元素的迁移     

Kinetics of arsenopyrite oxidative dissolution by oxygen

We used a mixed flow reactor system to determine the rate and infer a mechanism for arsenopyrite (FeAsS) oxidation by dissolved oxygen (DO) at 25 °C and circumneutral pH. Results indicate that under circumneutral pH (6.3-6.7), the rate of arsenopyrite oxidation, 10^{−10.14±0.03} mol m⁻² s⁻¹, is essentially independent of DO over the geologically significant range of 0.3-17 mg L⁻¹. Arsenic and sulfur are released from arsenopyrite in an approximate 1:1 molar ratio, suggesting that oxidative dissolution by oxygen under circumneutral pH is congruent. Slower rates of iron release from the reactor indicate that some of the iron is lost from the effluent by oxidation to Fe(III) which subsequently hydrolyzes and precipitates. Using the electrochemical cell model for understanding sulfide oxidation, our results suggest that the rate-determining step in arsenopyrite oxidation is the reduction of water at the anodic site rather than the transfer of electrons from the cathodic site to oxygen as has been suggested for other sulfide minerals such as pyrite.     

Chloride retention in forest soil by microbial uptake and by natural chlorination of organic matter

Inorganic chlorine (i.e. chloride; Cl_in) is generally considered inert in soil and is often used as a tracer of soil and ground water movements. However, recent studies indicate that substantial retention or release of Cl_in can occur in soil, but the rates and processes responsible under different environmental conditions are largely unknown. We performed ³⁶Cl tracer experiments which indicated that short-term microbial uptake and release of Cl_in, in combination with more long-term natural formation of chlorinated organic matter (Cl_org), caused Cl_in imbalances in coniferous forest soil. Extensive microbial uptake and release of Cl_in occurred over short time scales, and were probably associated with changes in environmental conditions. Up to 24% of the initially available Cl_in within pore water was retained by microbial uptake within a week in our experiments, but most of this Cl_in was released to the pore water again within a month, probably associated with decreasing microbial populations. The natural formation of Cl_org resulted in a net immobilization of 4% of the initial pore water Cl_in over four months. If this rate is representative for the area where soil was collected, Cl_org formation would correspond to a conversion of 25% of the yearly wet deposition of Cl_in. The study illustrates the potential of two Cl_in retaining processes in addition to those previously addressed elsewhere (e.g. uptake of chloride by vegetation). Hence, several processes operating at different time scales and with different regulation mechanisms can cause Cl_in imbalances in soil. Altogether, the results of the present study (1) provide evidence that Cl_in cannot be assumed to be inert in soil, (2) show that microbial exchange can regulate pore water Cl_in concentrations and (3) confirm the controversial idea of substantial natural chlorination of soil organic matter.