Compositional and source characterization of base progressively extracted humic acids using pyrolytic gas chromatography mass spectrometry |
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| Institution: | 1. State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640 Guangdong, PR China;2. State Key Laboratory of Marine Geology, Tongji University, 200092 Shanghai, PR China;3. Department of Environmental Sciences, Cook College, Rutgers University, New Brunswick, 08901-8551 NJ, USA;1. Université d’Orléans, ISTO, UMR 7327, 45071 Orléans, France;2. CNRS/INSU, ISTO, UMR 7327, 45071 Orléans, France;3. BRGM, ISTO, UMR 7327, BP 36009, 45060 Orléans, France;4. Sologne Nature Environnement, Parc de Beauvais – BP136, Romorantin-Lanthenay, France;1. Laboratory of Critical Zone Evolution, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;2. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China;1. Institute of Organic Contaminant Control and Soil Remediation, College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China;2. Department of Crop and Soil Sciences, University of Georgia, Griffin, GA 30223, USA;1. Texas State University-San Marcos, Department of Chemistry and Biochemistry, 601 University Dr., San Marcos, TX 78666, USA;2. Suez Canal University, Faculty of Science, Physics Department, Ismailia, Egypt;3. Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia |
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| Abstract: | Cyclic base extraction is a commonly used method for the isolation of humic acids from soils and sediments. However, every extract may differ in chemical composition due to the complex nature of humic acids. To better understand the chemical composition of each extract, the heterogeneous property of humic acids and their speciation in environmental samples, eight fractions of humic acids were obtained in the present work by progressive base-extraction of Pahokee peat, and their chemical composition was characterized using two complementary pyrolytic techniques, namely conventional pyrolysis and methylation pyrolysis (TMAH) GC/MS. These quick and effective procedures provide an insight into the structure of macromolecules. The work shows that the lignin-derived aromatic compounds are major components of pyrolysates in both pyrolytic techniques, while aliphatic compounds originating from microorganisms and plants are minor components. Other compounds derived from proteins and carbohydrates at lower concentrations were also detected. Fatty acids were found in the pyrolysis without methylation, indicating their association with humic acid in a free state. These compounds are different from those formed during pyrolysis with in situ methylation, where fatty acids are generally believed to be the cleavage products of carboxylic groups bound to humic acids. A relative decreasing abundance of aromatic components and increasing abundance of aliphatic components in the pyrolysates as the peat was progressively extracted was also observed in this work, suggesting that the extraction of more hydrophobic aliphatics may be delayed in comparison to the aromatic components. Speciation and origin differences may also be important particularly considering that the contribution from lignin organic matter decreased with extraction number, as the contribution of microbial organic matter increased. The observed change in chemical composition with the extracted fractions indicates again that the humic acid distribution and their speciation are complex, and complete extractions are necessary to obtain a representative humic acid sample. |
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