关键词: 沉积物有机碳, 傅里叶变换红外光谱, 有机碳稳定性, 有机-矿物相互作用, 光谱表征

Abstract:

Sediment organic carbon (SOC) plays a crucial role in the global carbon cycling and ecosystem health. This review systematically examines the advances in applying Fourier Transform Infrared (FT-IR) spectroscopy to assess the SOC composition and stability, highlighting its potential as an environmental marker. FT-IR spectroscopy enables the identification of the molecular structures and functional group characteristics of organic carbon in sediments through non-destructive analysis. The effectiveness of this technique lies in its ability to detect specific functional groups, such as aliphatic stretching bands (3010-2800 cm⁻¹), carboxyl and carbonyl groups (1750-1700 cm⁻¹), aromatic C=C stretching (1550-1500 cm⁻¹), and polysaccharide C−O stretching (1000-1090 cm⁻¹). These spectral features provide crucial information regarding the composition and stability of organic matter. The relative intensities of these peaks can reveal the degree of humification or degradation of organic matter and their stability in the sediments. For example, by analyzing the FT-IR spectra of aquatic sediments, researchers can identify the characteristic absorption peaks of different functional groups, thereby inferring the organic carbon composition and stability of the sediments. Furthermore, FT-IR technology can effectively characterize organic carbon components through specific spectral features, such as the methyl-to-methylene ratio (CH₂/CH₃ Ratio, R_CH2/CH3), aliphatic-to-oxygenated compound ratio (CH_al/Ox Ratio, R_CHal/Ox), siloxane-to-oxygenated compound ratio (SiO/Ox Ratio, R_SiO/Ox), humification index (HI), degree of decomposition index (DDI), and hydrophobicity index (HPI), providing researchers with quantitative tools to evaluate SOC functions and stability. The application of FT-IR in SOC analysis spans various aquatic environments from freshwater to marine systems. FTIR has been instrumental in tracking organic matter transformation processes and evaluating anthropogenic impacts in freshwater ecosystems. Studies have shown that FTIR can effectively monitor changes in organic matter composition during early diagenesis and assess the influence of human activities, such as agricultural runoff and aquaculture on sediment quality. For instance, FTIR can reveal the relationship between humification and methanogenesis pathways, providing important insights into how human activities alter the rates of methane production in sediments. Additionally, FT-IR spectroscopy has been widely used to evaluate the impact of human activities on freshwater systems, such as changes in organic matter composition due to agricultural runoff and organic matter migration caused by aquaculture activities. These studies provide critical data to understand the impact of human activities on freshwater ecosystems. In marine and coastal systems, FT-IR spectroscopy has revealed complex interactions between carbon, minerals, and pollutants across temporal and spatial scales, providing key insights into carbon sequestration mechanisms and environmental evolution. For example, in coastal ecosystems, such as mangrove forests, FT-IR can delineate the carbon components in extracellular polymers and their association with specific microbial groups, thereby revealing the mechanisms of carbon preservation. Furthermore, FT-IR has been used to assess the impacts of human activities on marine systems, such as the effects of microplastics and heavy metal contamination on sediment organic carbon. However, several factors influence the FT-IR analysis of SOC, including the sediment composition, sample preparation, instrumental parameters, and environmental conditions. The mineral matrix effect, particularly from clay minerals and iron oxides, can interfere with the organic matter signals. For instance, the preferential adsorption of organic carbon by the interlayer structure of montmorillonite can cause a redshift in the C=O stretching vibration signal (1640 cm⁻¹). Additionally, the co-precipitation of iron oxides with estuarine organic matter can lead to the selective attenuation of the C−H stretching vibration signal (2850−2920 cm⁻¹). Carbonate interference in the 1400−1450 cm⁻¹ region poses challenges for spectral interpretation. For example, when the calcium carbonate content was high, a prominent absorption peak appeared in the 1400−1450 cm⁻¹ region of the FTIR spectrum. Sample preparation, including drying methods and particle size control, significantly affected the accuracy of analysis. For instance, treatment is crucial for preserving the organic structure of sediments, and appropriate drying can improve the accuracy and reliability of FT-IR analysis. Furthermore, particle size control is a key factor in ensuring spectral consistency, as studies have shown that grinding samples to <125 μm can significantly improve the homogeneity and reduce the recovery rate differences caused by particle heterogeneity. These challenges necessitate standardized procedures and careful consideration of the analytical conditions. This review also discusses the advantages and limitations of FT-IR technology for SOC analysis. Although FT-IR offers rapid, non-destructive analysis and high sensitivity to molecular structural changes, it faces challenges in the quantitative analysis of complex sediment matrices and spectral peak overlap. To overcome these limitations, the integration of advanced technologies, optimization of data processing algorithms, and establishment of standardized analytical procedures are recommended. For example, the establishment of partial least-squares regression models can effectively improve the accuracy and reliability of data. In the future, several key developments are anticipated to enhance FT-IR applications in SOC research: 1) integration of machine learning algorithms to improve spectral resolution and quantitative accuracy, as machine learning algorithms can handle complex spectral data and enhance the resolution of overlapping absorption peaks, thereby improving the accuracy of quantitative analysis; 2) combination with synchrotron radiation techniques for nanoscale characterization of organic-mineral interfaces, such as FT-IR imaging revealing the electron transfer process between microbial nanowires and iron oxides (1540 cm⁻¹ for amide) and the increased carboxylate content (1420 cm⁻¹ and 1600 cm⁻¹) in the anoxic zone; 3) establishment of standardized operating procedures for aquatic sediment analysis, as standardized sample preparation and analysis procedures will greatly improve the comparability and scientific validity of research results; and 4) development of multi-modal coupling techniques and in-situ analysis solutions, which will significantly expand the application potential of FT-IR under complex environmental conditions, such as the coupling of FTIR and confocal laser scanning microscopy achieving 3D visualization of organic-mineral distribution with a spatial resolution of 2.3 μm. In summary, this review demonstrates that FT-IR spectroscopy is an invaluable tool for understanding SOC dynamics, particularly for revealing organic-mineral interactions and tracking environmental changes. Its non-destructive nature and high sensitivity make it particularly suitable for large-scale and multi-analytical studies. Although challenges remain in the quantitative analysis and complex matrix effects, continued technological advancement and methodological refinement will further expand its applications in carbon cycle research and environmental management. Through the integration of advanced technologies, optimization of data processing algorithms, and establishment of standardized analytical procedures, FT-IR technology will continue to play a crucial role in SOC research, providing important support for the scientific understanding and management of the global carbon cycle.

Key words: sediment organic carbon, fourier transform infrared spectroscopy, organic carbon stability, organic-mineral interaction, spectral characterization