Evaluation of stacking nanostructure in soil humic acids by analysis of the 002 band of their X-ray diffraction profiles

Humic substances are ubiquitous in the environment. The development of models for their chemical structure can be expected to improve our understanding of their environmental behavior. In this study, we used X-ray diffraction (XRD) to develop models for the stacking nanostructure of 12 humic acids (HAs) extracted from Japanese Andisols, Inceptisols and Entisols. In the XRD profiles of the HAs, the γ band, which was attributed to aliphatic side chains, and the 002 band were separated by means of curve fitting with the Voigt function. The interlayer spacing of the carbon (C) planes (d₀₀₂) and the mean thickness of the stacking nanostructure along the c axis (L_c) were calculated from the position of the peak for the 002 band and the full width at half maximum by means of Bragg’s and Scherrer’s equations, respectively. The average number of C planes in the stacking nanostructure (N_c) was estimated from d₀₀₂ and L_c. The L_c and N_c values ranged from 1.04 to 1.84 nm and from 3.01 to 5.40, respectively. The mean L_c and N_c values of the Andisol HAs were larger than those of the Inceptisol and Entisol HAs (P 002 value of the HAs was approximately 0.34 nm, which was slightly larger than that of graphite (0.335 nm). The L_c value positively correlated with aromatic C content (r = 0.938, P 13C) nuclear magnetic resonance spectra. Furthermore, the degree of darkness of the HAs positively correlated with L_c (r = 0.961, P