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Applications of solid-state NMR spectroscopy in environmental science

https://doi.org/10.1016/j.ssnmr.2020.101698Get rights and content

Highlights

  • Solid-state NMR probes the interactions between pollutants and minerals/organic matter on environmental interfaces.

  • Solid-state NMR provides atomic-level speciation in environmental samples.

  • Application of solid-state NMR in contemporary environmental topics.

  • Future perspectives are provided.

Abstract

Environmental science is an interdisciplinary field, which integrates chemical, physical, and biological sciences to study environmental problems and human impact on the environment. This article highlights the use of solid-state NMR spectroscopy (SSNMR) in studies of environmental processes and remediation with examples from both laboratory studies and samples collected in the field. The contemporary topics presented include soil chemistry, environmental remediation (e.g., heavy metals and radionuclides removal, carbon dioxide mineralization), and phosphorus recovery. SSNMR is a powerful technique, which provides atomic-level information about speciation in complex environmental samples as well as the interactions between pollutants and minerals/organic matter on different environmental interfaces. The challenges in the application of SSNMR in environmental science (e.g., measurement of paramagnetic nuclei and low-gamma nuclei) are also discussed, and perspectives are provided for the future research efforts.

Introduction

Environmental science is an interdisciplinary field spanning chemistry, geoscience, biology, and engineering which studies environmental problems and the human impact on nature. The rapid industrialization and urbanization have resulted in global environmental challenges including pollution, climate change, resource crisis, food security, etc. Increasingly, research activities are driven to understand these environmental issues, and mitigation measures such as environmental remediation, carbon dioxide (CO2) sequestration, and resource recovery for a “Circular Economy” have become the focus of environmental studies. Environmental samples such as soil, sludge, and sediment are among the most complex and heterogeneous [1]. Furthermore, their analyses are often complicated due to the presence of multiple phases (e.g., liquids, solids, and gases) with components of both organic and inorganic origin. In addition, many environmental chemical processes proceed on interfaces, e.g., the binding of aqueous pollutants on mineral surfaces, a solid-water interface [1]. Thus, several characterization techniques, often in combination with analytical or computational models, are needed to fully understand these complex samples and processes, which usually cover multiple length scales from Ångstrøm (Å) to centimeter [2].

To obtain atomic-level information (i.e., ≈ Å) in an environmental sample or process, NMR spectroscopy is a powerful technique, as it is isotope (element) selective and can provide detailed insight into the chemical speciation (via the isotropic chemical shift, δiso) as well as allow for quantification of the different species in complex, multiphase environmental samples. Furthermore, NMR spectroscopy can probe the intermolecular interactions between pollutants and minerals/organic matter via correlation experiments, and provide insight into the transport and transformation of these substances in the environment [3,4]. More than a decade ago, the application of NMR spectroscopy in environmental science was reviewed with focus on the characterization of soil organic matter and sorption by soil as well as environmental contaminants and their transformation [5]. The reader is referred to the excellent reviews by Simpson et al. [1,2], in which the different NMR techniques applicable for structural characterization of natural organic matter (NOM) and its interaction with contaminants are discussed in detail. Liquid-state NMR spectroscopy provides high-resolution spectra, from which structural information of the targeted nucleus can be obtained. However, only the soluble fractions in the samples can be probed [1]. For environmental samples like soil, sediments, and sludge, which are mainly insoluble, extraction with solvents can alter the intermolecular interactions and the chemical composition of the targeted molecules/ions inside, thereby providing an incomplete insight into the actual sample composition and the relevant environmental processes. Solid-state NMR (SSNMR) can provide an overview of a bulk sample in a noninvasive manner with less sample handling. It is generally applied for the characterization of solid samples with emphasis on chemical speciation such as identification of mineral phases [6]. Moreover, SSNMR studies of minerals have been central for the benchmarking of density functional theory (DFT)-calculations of NMR parameters for inorganic materials [7]. It is also extensively used to probe the interactions between pollutants and solids, e.g., to identify binding sites, on the atomic level [4,8]. Local structural information such as the oxidation state and coordination number of a particular element can also be obtained from X-ray absorption spectroscopy (XAS) which covers both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine-structure spectroscopy (EXAFS) [9]. However, it is very difficult to employ XAS on light elements such as H, C, and N (i.e., the first two rows of the periodic table of the elements), which are generally accessible by SSNMR. Thus, SSNMR and XAS are complementary techniques and represent a powerful combination in environmental studies. Moreover, the use of XAS relies on the access to a synchrotron, where several beamlines at international synchrotron facilities are optimized for environmental science studies. However, instrument time is generally granted via peer-reviewed proposals with three to nine months from proposal submission to beam time. In contrast, SSNMR can often be performed in-house at moderate magnetic fields on routine NMR spectrometers equipped with a double resonance 1HX (H is proton and X denotes any other NMR nucleus) magic-angle spinning (MAS) NMR probe and standard-size rotors (3.2–7.0 ​mm). Although SSNMR is less sensitive than liquid-state NMR and is limited to isotopes present in reasonable quantities (i.e., the major components present), isotope enrichment may alleviate this issue in laboratory studies [10]. Table 1 summarizes the properties of selected NMR isotopes of relevance in environmental science [11], and nuclei with high natural abundance (e.g., 27Al, 31P, and 19F) in environmental samples are usually easily approachable by SSNMR. A second challenge is the presence of elements like iron, manganese, lanthanides, and actinides, which often contain unpaired electrons, can lead to substantially broadened and “invisible” NMR signals due to the paramagnetic properties, as will be discussed in Section 6.1.

This review focuses on the applications of SSNMR on several contemporary topics in environmental science. First, recent SSNMR studies on soil, whose characteristics are significantly related to many environmental processes given its interfaces with the lithosphere, biosphere, hydrosphere, and atmosphere, are reviewed and discussed. Second, the applications of SSNMR in environmental remediation, including sequestration of contaminants (e.g., heavy metals and radionuclides) and CO2 mineralization for mitigating climate change, are reviewed. Third, recent SSNMR studies on resource recovery, e.g., removal and recovery of phosphorus from wastewater to achieve “Circular Economy” [12], are summarized. Finally, the challenges in SSNMR studies of environmental samples and processes are discussed, and future perspectives are provided.

Section snippets

Practical and theoretical aspects of SSNMR studies in environmental science

Detailed knowledge about quantum mechanics beyond the basic undergraduate chemistry and physics courses is needed for a detailed understanding of NMR theory especially analyses of advanced pulse sequences and spin systems, but not needed for application of SSNMR as an analytical method. This render SSNMR a valuable characterization technique in environmental science. Below are some key aspects of SSNMR theory summarized with emphasis on application for non-NMR experts with limited knowledge of

Soil

The solid phase of soil is divided into the organic and the inorganic fractions, whose relative concentrations are quite variable as they depend on the geological and chemical conditions. The organic fraction, i.e., soil organic matter, consists of about 15% of living organisms and 85% of dead biomass [31]. Humic substances are generated as an important organic component in soil by the decay of plants and animals debris at the soil surface by microbes and other detritus feeders [32]. The

Sequestration and binding of heavy metals and radionuclides

Heavy metals are prevalent in everyday life, as they are (or have been) widely used in paints, textile, plastics, leather, batteries, pesticides, mining, electroplating, and other industry given their technological importance [61,62]. Moreover, there is a sharp increase in the industrial use of radionuclides, e.g., lanthanides and actinides, due to their nuclear, electrical, optical, and biochemical properties [63]. As a result, considerable quantities of heavy metals and radionuclides

Resource recovery: Phosphorus (P)

P is an essential element for living organisms. However, it is a non-renewable resource and the global P rock may be exhausted in 50–100 years [95]. On the other hand, P release from anthropogenic activities, e.g., agriculture runoff, wastewater treatment plants, and industrial streams, results in eutrophication in receiving water bodies. Hence, the speciation of P in environmental samples such as sewage sludge, lake sediments, soil, and adsorbents has drawn attentions from researchers

Paramagnetic effects in environmental samples

Paramagnetic effects originate from the unpaired electrons that are an intrinsic feature of many transition metal ions such as Fe, Mn, and Ni, which are often present in environmental samples. A paramagnetic ion interacts with the surrounding nuclear spins and alters the shape of their NMR spectra by increasing their relaxation rates and changing their chemical shifts as well as introduce shift anisotropy due to the significant electron-nucleus dipolar interaction [109]. SSNMR is a powerful

Summary

SSNMR spectroscopy is a powerful characterization technique for studies of environmental samples and processes, as it provides detailed insight into the speciation and quantification of different species. Environmental samples are often a mixture of different amorphous and crystalline mineral phases along with organic fractions. SSNMR can characterize these solid phases as well as their interactions with pollutants, which are adsorbed on the surface or sequestered, in diamagnetic and to some

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank the two anonymous reviewers for insightful comments, which improved the final version of the manuscript. UGN acknowledges funding from The Danish Research Council - Technology and Production Science (DFF–7017-00262) and The Danish Council for Independent Research Science and Universe (DFF-7014-00198).

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