Construction of model structure of humic substances toward the elucidation of their stability in the environment

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Toward an elucidation of immobilization mechanism of humic substances in th environment based on the establishment of their average chemical structure model, structural units especially condensed aromatic components and composition of functional groups in humic acids (HAs) were investigated with regard to the degree of humification and then ^<14>C age. The 11 band analysis of X-ray diffraction (XRD) profile succed in finding 0.24-1.68 nm C-layer planes corresponding to 4- to 3-ring condensed aromatic structure. Their mean size and contents increased with increasing degree of humification and relative abundance of aromatic C in HA molecules. Fourie transform-ion cyclotron mass spectrometry (FT-ICR/MS) detected 11-69 compounds of a series of linearly fused and net-like condensed aromatic acids with max 24 biphelyl-type compounds. The number and peak intensity of those components were also larger in the HAs with a greater degree of humification. Based on the relation to ^<14>C age, it was concluded that the relative enrichement of condensed aromatic structures with ≧5 rings contributes to the enhancement of recalcitrance of HAs in soil. Quantification of C functional groups using solide-state ^<13>C NMR spectroscopy was improved by introducing phase-adjusted spinning-sideband technique.