Structural characterization of humic acids isolated from typical soils in China and their adsorption characteristics to phenanthrene

Humic acids (HAs) were isolated from four typical soils, including red soil, fluvo-aquic soil, black soil and grey desert soil, in China. The HAs samples were characterized jointly with Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and cross-polarization (CP) with magic-angle spinning (MAS) 13C nuclear magnetic resonance (NMR) spectroscopy. The FTIR spectra show that HAs isolated from different soils have almost the same chemical groups but the content of these groups is varied, such as the content of aromatic CH of HAs from fluvo-aquic soil and grey desert soil is relatively high, while the content of CO of HAs from black soil and red soil is larger. The SEM images indicate that all HAs are sheet-like regular amorphous aggregates, the surface of HAs isolated from black soil is smooth, while those of HAs from the other soil samples are rough and many small particles can be observed on them. The CP-MAS 13C NMR spectra show that the content of aromatic carbon of the four HAs from red soil, fluvo-aquic soil, black soil and grey desert soil is 25.8, 39.9, 24.0 and 41.7%, respectively. The four HAs were used to adsorb phenanthrene from water solution, and the isotherms were regressed with the linear and Freundlich equation. The isotherms reveal that the adsorption conforms linearity a little more. The phenanthrene Koc values for HAs isolated from red soil, fluvo-aquic soil, black soil and grey desert soil samples are 13096, 14338, 6281 and 26156 mL/g, respectively. The Koc value is positively correlated with the content of aromatic carbons. The adsorption mechanism may be the host-guest interaction and the overlap of π-electrons in the aromatic structure of HAs and the planar aromatic ring of phenanthrene, so as to form π–π conjugation between the aromatic ring of HAs and phenanthrene.