Special issue mini‐review Received: 20 November 2014
Revised: 13 December 2014
Accepted: 16 December 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/mrc.4209
Interactions between natural organic matter and organic pollutants as revealed by NMR spectroscopy Pierluigi Mazzei and Alessandro Piccolo* 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. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: NMR; NOM; interactions; organic pollutants; DOSY; relaxation times; saturation transfer difference
Introduction The prevention of environmental contamination by organic pollutants (OPs) and the cleanup of contaminated sites have become a worldwide priority, requiring an advanced knowledge as to both the pollutants reactivity and fate within environmental matrices. Such an in-depth understanding is essential to design efficient strategies for decontamination, reduction of pollutants toxicity and bioavailability, and prediction of their sequestration/ migration in the environment.[1–3] There are different types of organic compounds of both anthropogenic and natural origin that are potentially toxic to plants and animals, even at low concentrations.[4] Most of anthropogenic organic contaminants are industrial products, such as pesticides, pharmaceuticals, endocrine disrupting compounds, and flame retardants. Depending on their chemical structure and reactivity, they are typically classified as polycyclic aromatic hydrocarbons, polychlorinated biphenyls (PCBs), nitroaromatic compounds, phenols, and anilines.[4–6] Polycyclic aromatic hydrocarbons are a large group of organic compounds with two or more fused aromatic rings and include molecules such as naphthalene, anthracene, and phenanthrene. They result prevalently from pyrolytic processes and incomplete combustion of organic materials during industrial and other human activities, including the processing of coal and crude oil and the combustion of natural gas.[7] PCBs are persistent OPs differing in the number of chlorine atoms attached to their biphenyl rings. Mixtures of PCBs congeners have been widely manufactured from 1930 to the 1970s and used as coolants in transformer oil, dielectric fluids, and lubricants.[8] Although the usage of PCBs has been banned over 30 years ago in many countries, they still exist in old electrical equipment and environmental media to which humans can be exposed.[6] Nitroaromatic compounds, which are organic molecules made of at least one nitro group attached to an aromatic ring, are largely used in chemical manufacturing, oil refining, pesticides, bactericides, explosives, and intermediates in the synthesis of
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dyes and other high volume.[5] Phenols and anilines are toxic constituents of dyestuff wastewater, and pentachlorophenol, in particular, represents one of the most diffused phenols, as it is widely used as a wood preservative or biocide.[9] Because of the serious health and ecological risks deriving from the continuous and uncontrolled release of pollutants in the environment and their transformation products of even higher toxicity, more research has been progressively devoted to better understand the environmental reactivity of these persistent OPs.[10–13] In particular, interactions of pollutants with natural organic matter (NOM) have been increasingly studied. A relevant body of literature has shown that NOM associated with aquifers, soils, and sediments plays a principal role in regulating the transport and fate of contaminants in the environment.[14–20] NOM is the most abundant form of organic carbon on the earth’s surface, ubiquitous in terrestrial and aquatic environments,[21,22] and widely heterogeneous, depending on the climatic, morphological, and microbiological features of its development.[23–25] NOM significantly affects the quality of drinking water, the yield of crop production, and the environmental mobility of chemicals and heavy metals.[26–28] NOM is also an important factor in the carbon biogeochemical cycle, and its abiotic and biotic dynamics in soil is directly related to either carbon sequestration or CO2 emission in the atmosphere.[29–32] The multifunctionality of NOM is ascribed to its peculiar chemical nature. In fact, NOM is reckoned to be a supramolecular association of relatively small (10 ms) CP contact from 1H, without any significant magnetization loss due to relaxation and with a moderate duty cycle of the radio-frequency irradiation. This is followed by multiple 1 ms CP times, alternated with 1H spin–lattice relaxation periods to repolarize protons.[126] The quantitative reliability of this pulse sequence can be appreciated by the comparison (Fig. 6) of 13C spectra for a prairie soil HA acquired by either a direct polarization or a multiCP technique, with and without the application of a dipolar dephasing filter.[126] The advanced multiCP pulse technique was applied to characterize different NOM types, which were then investigated by twodimensional 1H–13C HETCOR correlation NMR spectroscopy after their interactions with a fully deuterium-exchanged, carbonyl-13C-labeled benzophenone.[127] The full deuteration for benzophenone had the objective to eliminate intramolecular 1 H–13C correlations, thus allowing the benzophenone 13C-carbonyl to interact exclusively with the nearby protons in NOM. The spin diffusion from benzophenone carbons to NOM protons was followed by HETCOR experiments, which employed three different mixing times (0.01, 0.3, and 10.25 ms). As for the interactions with a Pakoee peat, the three HETCOR experiments indicated the molecular domains with which benzophenone preferentially interacted (Fig. 7a–c). The signals related to benzophenone carbonyl, at 198 ppm, and aromatic, at 137 ppm, were further shown by 1H HETCOR vertical 1D slices, at different mixing times, in Fig. 7e
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Figure 6. Quantitative C-NMR spectra of a standard prairie soil humic acid, measured at 14 kHz MAS within 5 h each, after 11.5 ms (10 × 1.1 ms + 0.5 ms) multiCP with tz = 0.4 s (thick black line) and after direct polarization (DP) with 30 s recycle delays (dashed red line), scaled to match peak intensities. Thin continuous (green) and dashed (blue) lines: corresponding multiCP and DP spectra, respectively, of nonprotonated C and mobile segments selected by recoupled dipolar dephasing. (Reprinted with permission from Johnson and Schmidt-Rohr,[126] Copyright 2014 by Elsevier).
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Interactions between natural organic matter and organic pollutants
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Figure 7. Two-dimensional HETCOR NMR spectra of the Pahokee peat sorbed with benzophenone-( C¼O)-d10 with (a) 1 ms LG-CP (tm,e = 0.01 ms), (b) 1 ms 13 HH-CP (tm,e = 0.3 ms), and (c) 1 ms HH-CP with 10 ms spin diffusion (tm,e = 10.25 ms). (d) C-NMR-detected 1H inversion recovery of Pahokee peat sorbed 13 1 13 13 with benzophenone-( C¼O)-d10, after the indicated recovery delays (ms). The H spectra extracted at the C chemical shifts of C¼O (197 ppm, panel e) and nonprotonated aromatic C (137 ppm, panel f) at different mixing times. (g) Proton projection along 0–220 ppm at tm,e = 10.25 ms. (Reprinted with permission from Cao and coworkers,[127] Copyright 2014 by American Chemical Society).
and 7f, respectively, whereas the proton projection for the HETCOR experiment at the longest mixing time is reported in Fig. 7g. Interestingly, 13C-detected 1H inversion-recovery spectra were also acquired for the Pahokee peat sorbed with benzophenone (Fig. 7d), in order to detect the spatial proximity of NOM domains and predict their availability for interactions.[127] The 19F spectroscopy was also employed in the solid state to investigate the role of lipid composition in geosorbents, such as coastal sediments, humin, and HAs, on the adsorption of
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phenanthrene.[70] The authors used the 2,2,2-trifluoroethyl laurate as a probe to show the relevant contribution of the lipid components of the selected geosorbents. In fact, they observed that a significant change occurred in 19F spectra after adding phenanthrene to a humic matter that was previously sorbed with the fluoroprobe. The 19F → 1H solid-state CP-MAS spectroscopy was also applied to study the association between organofluorine xenobiotic compounds and an unmodified peat.[128] In particular, the 19F nuclei of organofluorine compounds were used to induce observable
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P. Mazzei and A. Piccolo transverse magnetization to the 1H nuclei in the peat molecular components, which were in direct interaction with the sorbed xenobiotics. This experiment was conducted with the recently conceived comprehensive multiphase MAS probe that involves a spectrometer lock. The latter device provides an enhanced line shape for 1H detection, as compared to a traditional solid-state hardware, and also includes gradients that allow to conduct NMR diffusion experiments in the solid state.[129,130]
HR-MAS NMR The versatility of NMR spectroscopy was further enlarged by the introduction of the high-resolution MAS (HR-MAS) technique that enables NMR investigation on semisolid or gel-phase samples.[131] The sample is placed in a locked-up rotor and added with a small amount of a deuterated solvent that helps to reduce dipolar interactions, which are further minimized by spinning the rotor at the magic angle. The HR-MAS probes are generally provided with a deuterium lock and a gradient system. The latter device permits experiments (originally designed for solution-state NMR), such as diffusion-based and diffusion-edited experiments, or those exploiting the selection of spoil gradients blocks for both 1D and multidimensional spectroscopy. A further singular advantage of HR-MAS is that it may distinguish between the free and bound ligand forms, because the technique detects the latter only by fully removing anisotropy through MAS rotation. Simpson and coworkers[28] early applied HR-MAS NMR to study samples of environmental interest. They showed that the structure of a whole soil at the solid–aqueous interface may be characterized by HR-MAS spectra, by combining one-dimensional and twodimensional solution-like experiments, including T2 filters when necessary. In addition, they observed variations in spectra induced by specific solvents, such as deuterated water, that produced broader signals, and dimethylsulphoxide-d6 that conversely led to an improved resolution, prevalently because of the solvent-induced displacement of H-bonds. When the herbicide trifluralin was put in contact with the soil, new signals appeared in the spectra and were attributed to the herbicide bound to the external soil matrix.[28] The role of the physical conformation of organic matter in the processes of pollutants sorption was also explored by HRMAS.[132] A number of organo-clay complexes were formed by coating montmorillonite and kaolinite with humic matter and phenanthrene in Na+-dominated or Ca2+-dominated solutions at different pH and ionic strength values. The resulting HR-MAS spectra suggested that the sorption of the pollutant onto the organo-clay complexes occurred as a function of the distribution of hydroxyl sites on the mineral surfaces. In fact, these sites determined the interfacial configuration of the humic coating through the bonds formed with the complementary carboxyl groups in humic molecule, thus varying the accessibility of sorptive sites to phenathrene.[132] The HR-MAS spectroscopy was also applied to study the interactions between toluene and HA.[133] This work showed that a correlation existed between the molecular motion of humic aliphatic components and the attenuation of self-diffusion for toluene through the humic matrix.
Perspectives The NMR spectroscopy has so far largely contributed to a molecular understanding of the interactions between OPs and NOM in the environment. However, much is left to be done in this sector,
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especially because of the complex properties of NOM, which may vary in different environmental compartments, as well as in different geographical and climatic conditions. Future efforts should be devoted to elucidate further not only the molecular content of NOM but also how the molecular components are stereochemically arranged in water and soil. To such an aim, a great contribution to the unraveling of the molecular composition of NOM can be again provided by NMR spectroscopy with its fast advances in novel and highly performing pulse sequences or additional hardware devices (powerful magnets, cryosystems, new probes, hyphenation with chromatographic separations, etc.). Most of the techniques described here will be made more efficient in the near future with the advance of NMR technologies. Nevertheless, coupling of NMR spectroscopy with highresolution and ultra-high-resolution liquid-chromatography/massspectrometry techniques should also enhance considerably the structural precision in identifying the humic molecules responsible for the binding and environmental transport of pollutants in different conditions. It is also believed that the large amount of refined data that can be obtained in the future by advanced NMR and mass-spectrometry instrumentation will have to undergo extensive elaboration by intelligent software and multivariate analyses in order to reduce statistical uncertainty in data interpretation. Finally, the stereochemical understanding that is required to predict the adsorption on and diffusion through NOM of pollutants of different polarity will be achieved by handling the vast amount of molecular data with computational chemical software. This computational support will become fundamental to increase the significance of collected NMR data, such as those acquired with DOSY or through-space correlations techniques, which can be used to feed the predictive models of pollutant behavior in the environment.
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Magn. Reson. Chem. (2015)
Revised: 13 December 2014
Accepted: 16 December 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/mrc.4209
Interactions between natural organic matter and organic pollutants as revealed by NMR spectroscopy Pierluigi Mazzei and Alessandro Piccolo* 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. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: NMR; NOM; interactions; organic pollutants; DOSY; relaxation times; saturation transfer difference
Introduction The prevention of environmental contamination by organic pollutants (OPs) and the cleanup of contaminated sites have become a worldwide priority, requiring an advanced knowledge as to both the pollutants reactivity and fate within environmental matrices. Such an in-depth understanding is essential to design efficient strategies for decontamination, reduction of pollutants toxicity and bioavailability, and prediction of their sequestration/ migration in the environment.[1–3] There are different types of organic compounds of both anthropogenic and natural origin that are potentially toxic to plants and animals, even at low concentrations.[4] Most of anthropogenic organic contaminants are industrial products, such as pesticides, pharmaceuticals, endocrine disrupting compounds, and flame retardants. Depending on their chemical structure and reactivity, they are typically classified as polycyclic aromatic hydrocarbons, polychlorinated biphenyls (PCBs), nitroaromatic compounds, phenols, and anilines.[4–6] Polycyclic aromatic hydrocarbons are a large group of organic compounds with two or more fused aromatic rings and include molecules such as naphthalene, anthracene, and phenanthrene. They result prevalently from pyrolytic processes and incomplete combustion of organic materials during industrial and other human activities, including the processing of coal and crude oil and the combustion of natural gas.[7] PCBs are persistent OPs differing in the number of chlorine atoms attached to their biphenyl rings. Mixtures of PCBs congeners have been widely manufactured from 1930 to the 1970s and used as coolants in transformer oil, dielectric fluids, and lubricants.[8] Although the usage of PCBs has been banned over 30 years ago in many countries, they still exist in old electrical equipment and environmental media to which humans can be exposed.[6] Nitroaromatic compounds, which are organic molecules made of at least one nitro group attached to an aromatic ring, are largely used in chemical manufacturing, oil refining, pesticides, bactericides, explosives, and intermediates in the synthesis of
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dyes and other high volume.[5] Phenols and anilines are toxic constituents of dyestuff wastewater, and pentachlorophenol, in particular, represents one of the most diffused phenols, as it is widely used as a wood preservative or biocide.[9] Because of the serious health and ecological risks deriving from the continuous and uncontrolled release of pollutants in the environment and their transformation products of even higher toxicity, more research has been progressively devoted to better understand the environmental reactivity of these persistent OPs.[10–13] In particular, interactions of pollutants with natural organic matter (NOM) have been increasingly studied. A relevant body of literature has shown that NOM associated with aquifers, soils, and sediments plays a principal role in regulating the transport and fate of contaminants in the environment.[14–20] NOM is the most abundant form of organic carbon on the earth’s surface, ubiquitous in terrestrial and aquatic environments,[21,22] and widely heterogeneous, depending on the climatic, morphological, and microbiological features of its development.[23–25] NOM significantly affects the quality of drinking water, the yield of crop production, and the environmental mobility of chemicals and heavy metals.[26–28] NOM is also an important factor in the carbon biogeochemical cycle, and its abiotic and biotic dynamics in soil is directly related to either carbon sequestration or CO2 emission in the atmosphere.[29–32] The multifunctionality of NOM is ascribed to its peculiar chemical nature. In fact, NOM is reckoned to be a supramolecular association of relatively small (10 ms) CP contact from 1H, without any significant magnetization loss due to relaxation and with a moderate duty cycle of the radio-frequency irradiation. This is followed by multiple 1 ms CP times, alternated with 1H spin–lattice relaxation periods to repolarize protons.[126] The quantitative reliability of this pulse sequence can be appreciated by the comparison (Fig. 6) of 13C spectra for a prairie soil HA acquired by either a direct polarization or a multiCP technique, with and without the application of a dipolar dephasing filter.[126] The advanced multiCP pulse technique was applied to characterize different NOM types, which were then investigated by twodimensional 1H–13C HETCOR correlation NMR spectroscopy after their interactions with a fully deuterium-exchanged, carbonyl-13C-labeled benzophenone.[127] The full deuteration for benzophenone had the objective to eliminate intramolecular 1 H–13C correlations, thus allowing the benzophenone 13C-carbonyl to interact exclusively with the nearby protons in NOM. The spin diffusion from benzophenone carbons to NOM protons was followed by HETCOR experiments, which employed three different mixing times (0.01, 0.3, and 10.25 ms). As for the interactions with a Pakoee peat, the three HETCOR experiments indicated the molecular domains with which benzophenone preferentially interacted (Fig. 7a–c). The signals related to benzophenone carbonyl, at 198 ppm, and aromatic, at 137 ppm, were further shown by 1H HETCOR vertical 1D slices, at different mixing times, in Fig. 7e
13
Figure 6. Quantitative C-NMR spectra of a standard prairie soil humic acid, measured at 14 kHz MAS within 5 h each, after 11.5 ms (10 × 1.1 ms + 0.5 ms) multiCP with tz = 0.4 s (thick black line) and after direct polarization (DP) with 30 s recycle delays (dashed red line), scaled to match peak intensities. Thin continuous (green) and dashed (blue) lines: corresponding multiCP and DP spectra, respectively, of nonprotonated C and mobile segments selected by recoupled dipolar dephasing. (Reprinted with permission from Johnson and Schmidt-Rohr,[126] Copyright 2014 by Elsevier).
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Interactions between natural organic matter and organic pollutants
13
Figure 7. Two-dimensional HETCOR NMR spectra of the Pahokee peat sorbed with benzophenone-( C¼O)-d10 with (a) 1 ms LG-CP (tm,e = 0.01 ms), (b) 1 ms 13 HH-CP (tm,e = 0.3 ms), and (c) 1 ms HH-CP with 10 ms spin diffusion (tm,e = 10.25 ms). (d) C-NMR-detected 1H inversion recovery of Pahokee peat sorbed 13 1 13 13 with benzophenone-( C¼O)-d10, after the indicated recovery delays (ms). The H spectra extracted at the C chemical shifts of C¼O (197 ppm, panel e) and nonprotonated aromatic C (137 ppm, panel f) at different mixing times. (g) Proton projection along 0–220 ppm at tm,e = 10.25 ms. (Reprinted with permission from Cao and coworkers,[127] Copyright 2014 by American Chemical Society).
and 7f, respectively, whereas the proton projection for the HETCOR experiment at the longest mixing time is reported in Fig. 7g. Interestingly, 13C-detected 1H inversion-recovery spectra were also acquired for the Pahokee peat sorbed with benzophenone (Fig. 7d), in order to detect the spatial proximity of NOM domains and predict their availability for interactions.[127] The 19F spectroscopy was also employed in the solid state to investigate the role of lipid composition in geosorbents, such as coastal sediments, humin, and HAs, on the adsorption of
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phenanthrene.[70] The authors used the 2,2,2-trifluoroethyl laurate as a probe to show the relevant contribution of the lipid components of the selected geosorbents. In fact, they observed that a significant change occurred in 19F spectra after adding phenanthrene to a humic matter that was previously sorbed with the fluoroprobe. The 19F → 1H solid-state CP-MAS spectroscopy was also applied to study the association between organofluorine xenobiotic compounds and an unmodified peat.[128] In particular, the 19F nuclei of organofluorine compounds were used to induce observable
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P. Mazzei and A. Piccolo transverse magnetization to the 1H nuclei in the peat molecular components, which were in direct interaction with the sorbed xenobiotics. This experiment was conducted with the recently conceived comprehensive multiphase MAS probe that involves a spectrometer lock. The latter device provides an enhanced line shape for 1H detection, as compared to a traditional solid-state hardware, and also includes gradients that allow to conduct NMR diffusion experiments in the solid state.[129,130]
HR-MAS NMR The versatility of NMR spectroscopy was further enlarged by the introduction of the high-resolution MAS (HR-MAS) technique that enables NMR investigation on semisolid or gel-phase samples.[131] The sample is placed in a locked-up rotor and added with a small amount of a deuterated solvent that helps to reduce dipolar interactions, which are further minimized by spinning the rotor at the magic angle. The HR-MAS probes are generally provided with a deuterium lock and a gradient system. The latter device permits experiments (originally designed for solution-state NMR), such as diffusion-based and diffusion-edited experiments, or those exploiting the selection of spoil gradients blocks for both 1D and multidimensional spectroscopy. A further singular advantage of HR-MAS is that it may distinguish between the free and bound ligand forms, because the technique detects the latter only by fully removing anisotropy through MAS rotation. Simpson and coworkers[28] early applied HR-MAS NMR to study samples of environmental interest. They showed that the structure of a whole soil at the solid–aqueous interface may be characterized by HR-MAS spectra, by combining one-dimensional and twodimensional solution-like experiments, including T2 filters when necessary. In addition, they observed variations in spectra induced by specific solvents, such as deuterated water, that produced broader signals, and dimethylsulphoxide-d6 that conversely led to an improved resolution, prevalently because of the solvent-induced displacement of H-bonds. When the herbicide trifluralin was put in contact with the soil, new signals appeared in the spectra and were attributed to the herbicide bound to the external soil matrix.[28] The role of the physical conformation of organic matter in the processes of pollutants sorption was also explored by HRMAS.[132] A number of organo-clay complexes were formed by coating montmorillonite and kaolinite with humic matter and phenanthrene in Na+-dominated or Ca2+-dominated solutions at different pH and ionic strength values. The resulting HR-MAS spectra suggested that the sorption of the pollutant onto the organo-clay complexes occurred as a function of the distribution of hydroxyl sites on the mineral surfaces. In fact, these sites determined the interfacial configuration of the humic coating through the bonds formed with the complementary carboxyl groups in humic molecule, thus varying the accessibility of sorptive sites to phenathrene.[132] The HR-MAS spectroscopy was also applied to study the interactions between toluene and HA.[133] This work showed that a correlation existed between the molecular motion of humic aliphatic components and the attenuation of self-diffusion for toluene through the humic matrix.
Perspectives The NMR spectroscopy has so far largely contributed to a molecular understanding of the interactions between OPs and NOM in the environment. However, much is left to be done in this sector,
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especially because of the complex properties of NOM, which may vary in different environmental compartments, as well as in different geographical and climatic conditions. Future efforts should be devoted to elucidate further not only the molecular content of NOM but also how the molecular components are stereochemically arranged in water and soil. To such an aim, a great contribution to the unraveling of the molecular composition of NOM can be again provided by NMR spectroscopy with its fast advances in novel and highly performing pulse sequences or additional hardware devices (powerful magnets, cryosystems, new probes, hyphenation with chromatographic separations, etc.). Most of the techniques described here will be made more efficient in the near future with the advance of NMR technologies. Nevertheless, coupling of NMR spectroscopy with highresolution and ultra-high-resolution liquid-chromatography/massspectrometry techniques should also enhance considerably the structural precision in identifying the humic molecules responsible for the binding and environmental transport of pollutants in different conditions. It is also believed that the large amount of refined data that can be obtained in the future by advanced NMR and mass-spectrometry instrumentation will have to undergo extensive elaboration by intelligent software and multivariate analyses in order to reduce statistical uncertainty in data interpretation. Finally, the stereochemical understanding that is required to predict the adsorption on and diffusion through NOM of pollutants of different polarity will be achieved by handling the vast amount of molecular data with computational chemical software. This computational support will become fundamental to increase the significance of collected NMR data, such as those acquired with DOSY or through-space correlations techniques, which can be used to feed the predictive models of pollutant behavior in the environment.
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