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

Humic Acids Isolated from Selected Soils from the Russian Arctic and Antarctic: Characterization by Two-Dimensional 1H-13C HETCOR and 13C CP/Mas NMR Spectroscopy

Here we describe the molecular composition and resistance to decomposition of humic acids isolated from selected soils of the Russian Arctic and Antarctic. The degree of soil organic matter stabilization was assessed using modern instrumental methods: nuclear magnetic resonance spectroscopy (cross peak magic-angle spinning (CP/MAS) 13C-NMR and 1H-13C heteronuclear-correlation (HETCOR)). Analysis of the humic acids showed that aromatic compounds prevail in the organic matter formed in cryoconites, located on the surfaces of the glaciers. The predominance of aliphatic fragments is revealed in the soils of the Yamal peninsula and Antarctica. This could be caused by sedimentation of fresh organic matter exhibiting low decomposition stage due to the severe climate and processes of hydrogenation in the humic acids, destruction of the C-C bonds, and formation of chains with high hydrogen content. These processes result in formation of aliphatic fragments in the humic acids.

Advanced solid-state NMR spectroscopy of natural organic matter

Solid-state NMR is essential for the characterization of natural organic matter (NOM) and is gaining importance in geosciences and environmental sciences. This review is intended to highlight advanced solid-state NMR techniques, especially a systematic approach to NOM characterization, and their applications to the study of NOM. We discuss some basics of how to acquire high-quality and quantitative solid-state ¹³C NMR spectra, and address some common technical mistakes that lead to unreliable spectra of NOM. The identification of specific functional groups in NOM, primarily based on ¹³C spectral-editing techniques, is described and the theoretical background of some recently-developed spectral-editing techniques is provided. Applications of solid-state NMR to investigating nitrogen (N) in NOM are described, focusing on limitations of the widely used ¹⁵N CP/MAS experiment and the potential of improved advanced NMR techniques for characterizing N forms in NOM. Then techniques used for identifying proximities, heterogeneities and domains are reviewed, and some examples provided. In addition, NMR techniques for studying segmental dynamics in NOM are reviewed. We also briefly discuss applications of solid-state NMR to NOM from various sources, including soil organic matter, aquatic organic matter, organic matter in atmospheric particulate matter, carbonaceous meteoritic organic matter, and fossil fuels. Finally, examples of NMR-based structural models and an outlook are provided.

Secondary Structures in a Freeze-Dried Lignite Humic Acid Fraction Caused by Hydrogen-Bonding of Acidic Protons with Aromatic Rings

A lignite humic acid (HA) was separated from inorganic and non-HA impurities (i.e., aluminosilicates, metals) and fractionated by a combination of dialysis and XAD-8 resin. Fractionation revealed a more homogeneous structure of lignite HA. New and more specific structural information on the main lignite HA fraction is obtained by solid-state nuclear magnetic resonance (NMR) spectroscopy. Quantitative (13)C multiple cross-polarization (multiCP) NMR indicated oxidized phenyl propane structures derived from lignin. MultiCP experiments, conducted on potassium HA salts titrated to pH 10 and pH 12, revealed shifts consistent with carboxylate and phenolate formation, but structural changes associated with enolate formation from aromatic beta keto acids were not detected. Two-dimensional (1)H-(13)C heteronuclear correlation (2D HETCOR) NMR indicated aryl-aliphatic ketones, aliphatic and aromatic carboxyl groups, phenol, and methoxy phenyl ethers. Acidic protons from carboxyl groups in both the lignite HA fraction and a synthetic HA-like polycondensate were found to be hydrogen-bonded with electron-rich aromatic rings. Our results coupled with published infrared spectra provide evidence for the preferential hydrogen bonding of acidic hydrogens with electron-rich aromatic rings rather than adjacent carbonyl groups. These hydrogen-bonding interactions likely result from stereochemical arrangements in primary structures and folding.

Interactions between natural organic matter and organic pollutants as revealed by NMR spectroscopy

Natural organic matter (NOM) plays a critical role in regulating the transport and the fate of organic contaminants in the environment. NMR spectroscopy is a powerful technique for the investigation of the sorption and binding mechanisms between NOM and pollutants, as well as their mutual chemical transformations. Despite NMR relatively low sensibility but due to its wide versatility to investigating samples in the liquid, gel, and solid phases, NMR application to environmental NOM-pollutants relations enables the achievement of specific and complementary molecular information. This report is a brief outline of the potentialities of the different NMR techniques and pulse sequences to elucidate the interactions between NOM and organic pollutants, with and without their labeling with nuclei that enhance NMR sensitivity.

Role of structure and microporosity in phenanthrene sorption by natural and engineered organic matter

Natural sorbents including one humic acid (HA), humins (HMs), nonhydrolyzable carbons (NHCs), and engineered sorbents (biochars) were subject to bleaching to selectively remove a fraction of aromatic C. The structural properties and sorption isotherm data of phenanthrene (Phen) by original and bleached sorbents were obtained. Significant correlations between Phen Koc values by all sorbents and their organic carbon (OC)-normalized CO2 cumulative surface area (CO2-SA/OC) suggested that nanopore-filling mechanism could dominate Phen sorption. After bleaching, natural sorbents still contained large amounts of aromatic C, which are resistant to bleaching, suggesting that they are derived from condensed or nonbiodegradable organic matter (OM). After eliminating the effect of aromatic C remaining in the bleached samples, a general trend of increasing CO2-SA/OC of natural sorbents with increasing aliphaticity was observed, suggesting that nanopores of natural sorbents are partially derived from their aliphatic moieties. Conversely, positive relationships between CO2-SA/OC or Phen logKoc of engineered sorbents and their aromaticity indicated the aromatic structures of engineered sorbents primarily contribute to their nanopores and dominate their sorption of HOCs. Therefore, this study clearly demonstrated that the role of structure and microporosity in Phen sorption is dependent on the sources of sorbents.

Sorption selectivity in natural organic matter probed with fully deuterium-exchanged and carbonyl-13C-labeled benzophenone and 1H-13C NMR spectroscopy

Specific functional-group or domain interactions of fully deuterium-exchanged, carbonyl-(13)C-labeled benzophenone and different types of natural organic matter (NOM) were investigated through two-dimensional (1)H-(13)C heteronuclear correlation NMR spectroscopy. The sorbents included Beulah-Zap lignite, type II kerogen (IL-6), Pahokee peat, Amherst humic acid, and a polystyrene-poly(vinylmethyl ether) (PS-PVME) blend. PS-PVME consists of PS and PVME chains that are mixed on a scale of <5 nm. The NOM sorbents all consist predominantly of a mixed aromatic-alkyl or aromatic-O-alkyl matrix that is homogeneous on the 3 nm scale, as evidenced by fast equilibration of aromatic and alkyl (1)H magnetization. In addition, Beulah lignite and IL-6 kerogen exhibit small fractions of distinct polymethylene (CH2)n domains, and Pahokee peat contains significant fractions of polar and nonpolar alkyl domains. Benzophenone-((13)C═O)-d10 shows proximity to both aromatic rings and alkyl segments in all samples but preferentially interacts with aromatic rings in PS-PVME and Beulah lignite, possibly due to π-π electron donor-acceptor interactions. The data for IL-6 kerogen are also compatible with preferential location of benzophenone near the alkyl-substituted edges of aromatic rings, while in Pahokee peat, clear signatures of benzophenone affinity to both aromatic-rich and nonpolar alkyl domains have been detected. Amherst humic acid shows evidence of some affinity to polar alkyl segments but which is weaker than that to aromatic rings. Our results indicate that specific interactions of the sorbate and the presence of domains in the sorbent influence the magnitude and selectivity of sorption.

Sorption selectivity in natural organic matter studied with nitroxyl paramagnetic relaxation probes

Sorption site selectivity and mechanism in natural organic matter (NOM) were addressed spectroscopically by the sorption of paramagnetic nitroxyl compounds (spin probes) of different polarity, TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and HTEMPO (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl). The sorbents were Pahokee peat, Beulah-Zap lignite, and a polystyrene-poly(vinyl methyl ether) (PS-PVME) polymer blend representing the mixed aliphatic-aromatic, polar-nonpolar character of NOM. Nuclear-electron spin interaction serves as an efficient relaxation pathway, resulting in attenuation of the (13)C-CP/TOSS NMR signal for (13)C nuclei in proximity to the N-O· group (r(-6) dependence). In the natural solids the spin probes sorbed more specifically (greater isotherm nonlinearity) and had lower rotational mobility (broader electron paramagnetic resonance signals) than in PS-PVME. Titration with spin probe indicated almost no selectivity for the different carbon functional groups of PS-PVME, and little to no selectivity for the different carbon moieties of Pahokee and Beulah, including aromatic, alkyl, O-alkyl, di-O-alkyl, and O-methyl. In any case, sorption site selectivity of spin probes to NOM was always weaker than partition selectivity found in model solvent-water (toluene, hexadecane, anisole, octanol) and cellulose-water systems. The results indicate little or no preferential sorption in NOM based on functional group chemistry or putative microdomain character, but rather are consistent with the filling of pores whose walls have an average chemical environment reflecting the bulk chemical composition of the solid. This work demonstrates for the first time the use of paramagnetic probes to study sorption specificity.

Advanced solid-state NMR characterization of marine dissolved organic matter isolated using the coupled reverse osmosis/electrodialysis method

Advanced (13)C solid-state techniques were employed to investigate the major structural characteristics of two surface-seawater dissolved organic matter (DOM) samples isolated using the novel coupled reverse osmosis/electrodialysis method. The NMR techniques included quantitative (13)C direct polarization/magic angle spinning (DP/MAS) and DP/MAS with recoupled dipolar dephasing, (13)C cross-polarization/total sideband suppression (CP/TOSS), (13)C chemical shift anisotropy filter, CH, CH(2), and CH(n) selection, two-dimensional (1)H-(13)C heteronuclear correlation NMR (2D HETCOR), 2D HETCOR combined with dipolar dephasing, and (15)N cross-polarization/magic angle spinning (CP/MAS). The two samples (Coastal and Marine DOM) were collected at the mouth of the Ogeechee River and in the Gulf Stream, respectively. The NMR results indicated that they were structurally distinct. Coastal DOM contained significantly more aromatic and carbonyl carbons whereas Marine DOM was markedly enriched in alkoxy carbon (e.g., carbohydrate-like moieties). Both samples contained significant amide N, but Coastal DOM had nitrogen bonded to aromatic carbons. Our dipolar-dephased spectra indicated that a large fraction of alkoxy carbons were not protonated. For Coastal DOM, our NMR results were consistent with the presence of the major structural units of (1) carbohydrate-like moieties, (2) lignin residues, (3) peptides or amino sugars, and (4) COO-bonded alkyls. For Marine DOM, they were (1) carbohydrate-like moieties, (2) peptides or amino sugars, and (3) COO-bonded alkyls. In addition, both samples contained significant amounts of nonpolar alkyl groups. The potential sources of the major structural units of DOM were discussed in detail. Nonprotonated O-alkyl carbon content was proposed as a possible index of humification.

15N and 13C{14N} NMR investigation of the major nitrogen-containing segment in an aquatic fulvic acid: evidence for a hydantoin derivative

A nitrogen-rich segment in a fulvic acid (FA) from Pony Lake, a coastal pond in Antarctica, was investigated by (15)N and (13)C{(14)N} solid-state NMR techniques. As reported previously, the (13)C{(14)N} spectrum of C bonded to N exhibits a peak at 157 ppm that is assigned to an sp(2)-hybridized carbon bonded to at least two nitrogen atoms. This segment contains 48% of all N in the sample. (15)N NMR shows distinct signals, 20 ppm upfield and downfield from the typical peptide resonance; dipolar dephasing confirmed that they are due to protonated N. The well-resolved downfield peak, which accounts for 1/4 of the spectral area, cannot be assigned to aromatic heterocycles, such as purines, because the fraction of aromatic C bonded to N in this sample is very small. Analysis of (15)N chemical-shift trends and (15)N NMR of model compounds, such as arginine and its derivatives, excludes assignment to a guanidinium ion or to substituted guanidino groups. Similarly, ureido groups, -NH-CO-NH-, that are not bonded to a second C = O do not match the observed (15)N peaks in the FA, since both N resonate upfield from the peptide resonance. On the other hand, all chemical shifts are matched within the observed range by the -C(alkyl)-NH-CO-NH-CO-C structure found in two nonaromatic heterocycles, hydantoin and dihydrouracil. The five-membered hydantoin ring, which is found in the purine metabolite allantoin, provides a better match than the six-membered dihydrouracil ring. Regular uracil or thymine fails to produce adequate agreement with observed chemical shifts.

Mobile aliphatic domains in humic substances and their impact on contaminant mobility within the matrix

Using a novel NMR option, magic angle spinning pulsed field gradient (MAS PFG) NMR, the mobility of aliphatic domains in humic substances in the presence of toluene (about 4.5 wt. %) has been monitored. Results show a strong correlation between the diffusivities of the mobile aliphatic chains and those of the adsorbed toluene molecules in the matrix as a function of temperature. Particularly, a strong influence of structural relaxation of the humic matrix on the diffusivity of toluene is observed. Our findings confirm that the aliphatic domains in humic substances play an important role in the mobility of sorbed contaminants within this matrix. These findings further confirm the potential of MAS PFG NMR method in monitoring diffusion processes in particulate humic substances.

Chemical structures of corn stover and its residue after dilute acid prehydrolysis and enzymatic hydrolysis: insight into factors limiting enzymatic hydrolysis

Advanced solid-state NMR techniques and wet chemical analyses were applied to investigate untreated corn stover (UCS) and its residues after dilute acid prehydrolysis (DAP) and enzymatic hydrolysis (RES) to provide evidence for the limitations to the effectiveness of enzyme hydrolysis. Advanced solid-state NMR spectral-editing techniques as well as 1H-13C two-dimensional heteronuclear correlation NMR (2D HETCOR) were employed. Our results indicated that dilute acid prehydrolysis selectively removed amorphous carbohydrates, increased aromatic CH/other protonated -C═C- and enriched alkyl CH and CH2 components. Cinnamic acids were increased, and proteinaceous materials and N-containing degradation or condensation compounds were absorbed or coprecipitated in RES. 2D HETCOR experiments indicated a close association between lignin and the residual carbohydrates. Ketones/aldehydes were not detected in the DAP, in contrast to a report in which an appreciable amount of ketones/aldehydes was generated from the acid pretreatment of a purified cellulose in the literature. This suggested that acid pretreatment may modify the structure of purified cellulose more than biomass and that biomass may be a better substrate than model biopolymers and compounds for assessing structural changes that occur with industrial processing. On the basis of NMR and wet chemical analyses, we found the following factors could cause the limitations to the effectiveness of enzymatic hydrolysis: (1) chemical modification of carbohydrates limited the biologically degradable carbohydrates available; (2) cinnamic acids in the residue accumulated; (3) accessibility was potentially limited due to the close association of carbohydrates with lignin; and (4) proteinaceous materials and N-containing degradation or condensation compounds were absorbed or coprecipitated.

New option for characterizing the mobility of organic compounds in humic acids

A new NMR option for monitoring the mobility of organic contaminants in SOM in the solid state has been successfully applied for the first time. This recently available noninvasive technique, magic angle spinning pulsed-field gradient (MAS PFG) NMR, combines both NMR spectroscopy and diffusometry to selectively monitor the diffusion of compounds sorbed in porous media or polymer matrices. Using this technique, the diffusion of toluene in humic acid particles has been studied. Measurements were performed under varying temperatures from 25 to 80 degrees C. The obtained diffusion coefficients were found to be in good agreement with those obtained from computer simulations reported elsewhere. Our results show a strong influence of the interaction of toluene with humic acid on its diffusion in the matrix even at elevated temperatures of up to 80 degrees C. The Arrhenius plot of the diffusivities shows a decrease in the activation energy of diffusion above 50 degrees C by a factor of 3. This change of activation energy is attributed to a structural change in the humic acid matrix that influences the mobility of toluene.

Differences in PAH desorption and sediment organic matter composition between non-vegetated and recently vegetated fuel-oiled sediments

We assessed the desorption behavior of pyrene, chrysene, phenanthrene, and tri-alkylated (C3) phenanthrene/anthracenes for non-vegetated and recently vegetated (< 2 yrs) fuel-oiled sediments collected from the Indiana Harbor Canal (IHC), Gary, IN. Bulk sediment and humin were analyzed for PAH concentrations, organic matter composition, and PAH desorption behavior. PAH desorption isotherms and kinetics were determined using batch aqueous extractions and a two compartment, first-order kinetic model Vegetated sediments contained more plant carbon and were more nonpolar and less oxidized than non-vegetated sediments. Desorption kinetics indicated that PAH desorption was primarily controlled by a slow PAH-desorbing fraction (F2) of IHC sediments. However, in vegetated sediments, particularly humin, PAH release from a faster PAH-desorbing fraction (F1) increased as did the rates (k2) of PAH desorption from the dominant slow PAH-desorbing fraction (F2). We propose that vegetation provides aliphatic, nonpolar carbon to IHC sediments that facilitates more rapid PAH desorption from bulk sediment and humin.

Radioiodination of humic substances via azocoupling with 3-[125I] iodoaniline

A new method is described for radiolabeling humic substances (HS) with iodine radioisotopes. The method radiolabels the electron-rich aromatic moieties of HS with the 3-[125I]iodobenzenediazonium ion via azocoupling. The method uses four steps: (i) 3-aminobenzenetrimethylstannane is synthesized and isolated by using a silica gel column, (ii) 3-[125I]iodoaniline is synthesized and isolated by HPLC, with radiochemical yields of up to 60%, (iii) 3-[125I]iodobenzenediazonium chloride is synthesized, and the reaction mixture from this step is used in step iv to radioiodinate HS with radiochemical yields of up to 95% (with reference to 3-[125I]iodoaniline). The advantage of this method is that it is selective radiolabeling, placing the radiolabel in a specific site (the 3-position of the phenyl ring) within HS molecules, which minimizes unwanted secondary chemical interactions. Investigations of the stability of the radiolabel and the effect of photoreductive dehalogenation showed that there was a negligible release of 125I. The production of radiolabeled HS using this method allows the sensitive detection of HS in laboratory and field studies. In addition, the method offers the possibility of using different iodine radioisotopes simultaneously in investigations using HS.

Chemical structures of manure from conventional and phytase transgenic pigs investigated by advanced solid-state NMR spectroscopy

Nonpoint phosphorus (P) pollution from animal manure is becoming a serious global problem. The current solution for the swine industry includes the enzyme phytase as a component in oil meal and cereal grain-based swine diets. A long-term approach is the production of transgenic phytase pigs that express phytase in the salivary glands and secrete it in the saliva. This study provides a detailed comparison of chemical structures of manure from conventional pigs and transgenic pigs that express phytase under growing and finishing phases using new solid-state NMR techniques. Spectral editing techniques and quantitative NMR techniques were used to identify and quantify specific functional groups. Two-dimensional (1)H- (13)C heteronuclear correlation NMR was used to detect their connectivity. Manure from conventional and transgenic pigs had similar peptide, carbohydrate, and fatty acid components, while those from transgenic pigs contained more carbohydrates and fewer nonpolar alkyls. There was no consistent effect from diets with or without supplemental phosphate or growth stages.