IR Spectroscopy, Soil Analysis Applications
author: cily
2022-12-19
1.Introduction
Fourier transform infrared spectroscopy (FTIR) is a unique tool for the study of mineral and organic components of soil samples. FTIR spectroscopy offers sensitive characterization of minerals and soil organic matter (SOM), and mechanistic and kinetic aspects of mineral–SOM interactions that underlie biogeochemical processes. The molecular resolution of mineral and organic functional groups provided by FTIR has contributed significantly to understanding mineral and SOM structures, and ion and organic molecule sorption to mineral surfaces. The versatility of FTIR spectroscopy makes it a foundational tool for soil scientists, despite challenges in the acquisition and interpretation of soil spectra that stem from chemical heterogeneity. Recent advances in acquisition methods increasingly enable resolution of in situ compositional and temporal complexity of the soil milieu and chemical reaction occurring in this complex matrix.
2.FTIR Sampling Techniques for Soils
Soil samples can be analyzed by FTIR spectroscopy using a variety of methods, the most common of which are transmission, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and attenuated total reflectance (ATR). Different modes of acquiring FTIR spectra offer complementary methods for evaluating soil components and processes. Transmission spectroscopy was the earliest method used to collect FTIR spectra of soil mineral and organic components. Soil extracts (eg, NaOH-extractable SOM) or suspensions are dried onto infrared (IR) windows (eg, ZnSe, Ge) prior to analysis, and solid samples such as soils or SOM fractions are ground, mixed, and diluted with potassium bromide (KBr; 0.5–3% sample), pressed into pellets, and dessicated prior to analysis. Transmission provides a bulk IR measurement because the beam encounters all parts of the sample, which is in contrast to other collection methods such as ATR. Due to the labor of sample preparation and artifacts that can be introduced upon pellet desiccation, transmission spectroscopy is used less frequently today and less labor-intensive methods have gained favor. One such method is DRIFTS, which entails minimal sample preparation (eg, drying and grinding). Soil samples should be uniformly and finely ground.
3.Soil Mineral Analysis
FTIR spectroscopy is a versatile tool for characterizing soil mineral components, including mineral identification, structural assessment, and in situ monitoring of pedogenic processes (e.g., mineral formation). FTIR spectroscopy complements other analytical techniques, most notably X-ray diffraction (XRD) used for mineral identification. Specific absorption fingerprints are sufficiently sensitive to distinguish among shared bond types (e.g., SidO, AldO) by the local structural environment, thus enabling soil mineral identification and characterization. Phyllosilicates Phyllosilicates are the most common class of soil minerals. Also known as layer silicates, these minerals consist of (i) Al in octahedral coordination with O(H) that are bound to (ii) one (1:1) or two (2:1) sheets of Si in tetrahedral coordination with O. The sensitivity of IR absorbance to these bond types, their coordination, and other characteristics of the mineral structure (e.g., isomorphic substitution, interlayer cations, and crystallinity) enable identification of phyllosilicate structural class (1:1 vs. 2:1 layer silicates) and specific mineral types within each structural class (e.g., kaolinite vs. nacrite), as well as structural details (e.g., di- vs. trioctahedral) and compositional information (e.g., interlayer cations). Phyllosilicate identification and structural characterization is based on absorbances by mineral structural units, most notably hydroxyl, silicate, and interlayer and octahedral layer cations. Hydroxyl OdH stretching and bending occur at 3750–3400 cm1 and 950–600 cm1 , respectively (Fig. 2). Silicate SidO stretching occurs at 1200–700 cm1 and 700–400 cm1 , potentially overlapping with octahedral cation absorbances in the latter range. In contrast, interlayer cations produce absorbance bands outside the mid-IR range, in far-IR (150–70 cm1 ). The 2:1 layer silicates contain a single OH stretching band 3700–3620 cm1 , whereas 1:1 layer silicates exhibit two or more OH stretching bands in this region. The number and location of bands in this range are sufficiently sensitive to mineral structure to differentiate minerals within each layer silicates class. For 1:1 layer silicates, absorbance at 3630–3620 cm1 reflects internal OH groups between the tetrahedral and octahedral sheets, and absorbance at 3700 cm1 reflects internal H-bonding between octahedral surface OH and O from the underlying tetrahedral layer. In 2:1 layer silicates, isomorphic substitution disrupts crystalline order, causes the single band at 3700 cm1 to be broad, and its precise wavenumber will vary depending on the cation(s) to which tetrahedral layer OH are bonded. The SidO stretch and OH bend region is useful for identifying layer silicates class. The 1:1 layer silicates express a triumvirate of SidO absorbances at 1120–950 cm1 , whereas the 2:1 layer silicates exhibit a single broad absorbance peak at 1030–1010 cm1 . Furthermore, such SidO stretch bands occur at higher wavenumbers for trioctahedral minerals (1030–1020 cm1 ) than for dioctahedral minerals (1010 cm1 ). Absorbance bands from OH bending in diand trioctahedral 1:1 layer silicates occur at 950–800 cm1 and 700–600 cm1 , respectively. In 2:1 layer silicates, OH bending absorbances occur 950–915 cm1 and can reflect octahedral cation composition, such as MgAlOH at 840 cm1 in montmorillonite.
5.Mineral Weathering and Pedogenesis
The monitoring of soil weathering reactions and pedogenesis, such as chemical alteration of mineral surfaces and the formation of new mineral phases, is possible using FTIR. FTIR spectroscopy offers characterization of structural and kinetic aspects of biogeochemical processes that are not otherwise possible in mixed, complex systems, nor possible at the spatial or temporal scale of traditional field approaches employed in soil science. A number of studies have used FTIR for rapid data acquisition that can aid in explaining mineral weathering characterized by XRD. Advantages of FTIR over XRD include the greater speed to temporally resolve reactions and the identification of non-crystalline and amorphous minerals. Finally, the in situ capabilities of FTIR facilitate monitoring of mineral weathering and formation. For example, in in situ techniques such as flow-cell ATR-FTIR can be used to monitor biogenic Mn oxide formation on bacteria biofilms.
6.Summary
FTIR spectroscopy is a potent tool for the study of soil and soil processes. Since its initial application to study the mineral and organic components of soils in the mid-20th century, this technique offers characterization of minerals, organic matter, and processes such as mineral weathering, reduction– oxidation, and ion and organic compound binding to mineral surfaces. The high degree of experimental and analytic flexibility furnished by FTIR spectroscopy reflects the wide variety of methods for collecting spectra, and the suitability of soil components and processes to molecular analysis by infrared spectroscopy. The recent advent of ATR techniques has further advanced FTIR spectroscopy from identification and characterization of soil components and oxyanion sorption complexes to in situ, real-time analysis of soil processes occurring at liquid–solid interfaces. The molecular resolution and flexibility of experimental approaches offered by FTIR spectroscopy will likely continue to support and drive advances in soil science.
Fourier transform infrared spectroscopy (FTIR) is a unique tool for the study of mineral and organic components of soil samples. FTIR spectroscopy offers sensitive characterization of minerals and soil organic matter (SOM), and mechanistic and kinetic aspects of mineral–SOM interactions that underlie biogeochemical processes. The molecular resolution of mineral and organic functional groups provided by FTIR has contributed significantly to understanding mineral and SOM structures, and ion and organic molecule sorption to mineral surfaces. The versatility of FTIR spectroscopy makes it a foundational tool for soil scientists, despite challenges in the acquisition and interpretation of soil spectra that stem from chemical heterogeneity. Recent advances in acquisition methods increasingly enable resolution of in situ compositional and temporal complexity of the soil milieu and chemical reaction occurring in this complex matrix.
2.FTIR Sampling Techniques for Soils
Soil samples can be analyzed by FTIR spectroscopy using a variety of methods, the most common of which are transmission, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and attenuated total reflectance (ATR). Different modes of acquiring FTIR spectra offer complementary methods for evaluating soil components and processes. Transmission spectroscopy was the earliest method used to collect FTIR spectra of soil mineral and organic components. Soil extracts (eg, NaOH-extractable SOM) or suspensions are dried onto infrared (IR) windows (eg, ZnSe, Ge) prior to analysis, and solid samples such as soils or SOM fractions are ground, mixed, and diluted with potassium bromide (KBr; 0.5–3% sample), pressed into pellets, and dessicated prior to analysis. Transmission provides a bulk IR measurement because the beam encounters all parts of the sample, which is in contrast to other collection methods such as ATR. Due to the labor of sample preparation and artifacts that can be introduced upon pellet desiccation, transmission spectroscopy is used less frequently today and less labor-intensive methods have gained favor. One such method is DRIFTS, which entails minimal sample preparation (eg, drying and grinding). Soil samples should be uniformly and finely ground.
3.Soil Mineral Analysis
FTIR spectroscopy is a versatile tool for characterizing soil mineral components, including mineral identification, structural assessment, and in situ monitoring of pedogenic processes (e.g., mineral formation). FTIR spectroscopy complements other analytical techniques, most notably X-ray diffraction (XRD) used for mineral identification. Specific absorption fingerprints are sufficiently sensitive to distinguish among shared bond types (e.g., SidO, AldO) by the local structural environment, thus enabling soil mineral identification and characterization. Phyllosilicates Phyllosilicates are the most common class of soil minerals. Also known as layer silicates, these minerals consist of (i) Al in octahedral coordination with O(H) that are bound to (ii) one (1:1) or two (2:1) sheets of Si in tetrahedral coordination with O. The sensitivity of IR absorbance to these bond types, their coordination, and other characteristics of the mineral structure (e.g., isomorphic substitution, interlayer cations, and crystallinity) enable identification of phyllosilicate structural class (1:1 vs. 2:1 layer silicates) and specific mineral types within each structural class (e.g., kaolinite vs. nacrite), as well as structural details (e.g., di- vs. trioctahedral) and compositional information (e.g., interlayer cations). Phyllosilicate identification and structural characterization is based on absorbances by mineral structural units, most notably hydroxyl, silicate, and interlayer and octahedral layer cations. Hydroxyl OdH stretching and bending occur at 3750–3400 cm1 and 950–600 cm1 , respectively (Fig. 2). Silicate SidO stretching occurs at 1200–700 cm1 and 700–400 cm1 , potentially overlapping with octahedral cation absorbances in the latter range. In contrast, interlayer cations produce absorbance bands outside the mid-IR range, in far-IR (150–70 cm1 ). The 2:1 layer silicates contain a single OH stretching band 3700–3620 cm1 , whereas 1:1 layer silicates exhibit two or more OH stretching bands in this region. The number and location of bands in this range are sufficiently sensitive to mineral structure to differentiate minerals within each layer silicates class. For 1:1 layer silicates, absorbance at 3630–3620 cm1 reflects internal OH groups between the tetrahedral and octahedral sheets, and absorbance at 3700 cm1 reflects internal H-bonding between octahedral surface OH and O from the underlying tetrahedral layer. In 2:1 layer silicates, isomorphic substitution disrupts crystalline order, causes the single band at 3700 cm1 to be broad, and its precise wavenumber will vary depending on the cation(s) to which tetrahedral layer OH are bonded. The SidO stretch and OH bend region is useful for identifying layer silicates class. The 1:1 layer silicates express a triumvirate of SidO absorbances at 1120–950 cm1 , whereas the 2:1 layer silicates exhibit a single broad absorbance peak at 1030–1010 cm1 . Furthermore, such SidO stretch bands occur at higher wavenumbers for trioctahedral minerals (1030–1020 cm1 ) than for dioctahedral minerals (1010 cm1 ). Absorbance bands from OH bending in diand trioctahedral 1:1 layer silicates occur at 950–800 cm1 and 700–600 cm1 , respectively. In 2:1 layer silicates, OH bending absorbances occur 950–915 cm1 and can reflect octahedral cation composition, such as MgAlOH at 840 cm1 in montmorillonite.
5.Mineral Weathering and Pedogenesis
The monitoring of soil weathering reactions and pedogenesis, such as chemical alteration of mineral surfaces and the formation of new mineral phases, is possible using FTIR. FTIR spectroscopy offers characterization of structural and kinetic aspects of biogeochemical processes that are not otherwise possible in mixed, complex systems, nor possible at the spatial or temporal scale of traditional field approaches employed in soil science. A number of studies have used FTIR for rapid data acquisition that can aid in explaining mineral weathering characterized by XRD. Advantages of FTIR over XRD include the greater speed to temporally resolve reactions and the identification of non-crystalline and amorphous minerals. Finally, the in situ capabilities of FTIR facilitate monitoring of mineral weathering and formation. For example, in in situ techniques such as flow-cell ATR-FTIR can be used to monitor biogenic Mn oxide formation on bacteria biofilms.
6.Summary
FTIR spectroscopy is a potent tool for the study of soil and soil processes. Since its initial application to study the mineral and organic components of soils in the mid-20th century, this technique offers characterization of minerals, organic matter, and processes such as mineral weathering, reduction– oxidation, and ion and organic compound binding to mineral surfaces. The high degree of experimental and analytic flexibility furnished by FTIR spectroscopy reflects the wide variety of methods for collecting spectra, and the suitability of soil components and processes to molecular analysis by infrared spectroscopy. The recent advent of ATR techniques has further advanced FTIR spectroscopy from identification and characterization of soil components and oxyanion sorption complexes to in situ, real-time analysis of soil processes occurring at liquid–solid interfaces. The molecular resolution and flexibility of experimental approaches offered by FTIR spectroscopy will likely continue to support and drive advances in soil science.
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