[Structural characteristics of humic acids from a long-term petroleum contaminated soil]

Petroleum contamination in soil decreases with the increase in the distance of soil to the drilling well. Accordingly, an abandoned petroleum well which had been exploited for about twenty years in Songyuan city of Jilin Province, China, was selected to investigate the structural characteristics of soil humic acids (HAs) under different petroleum contamination levels. Surface (0-20 cm) soil samples were collected at 0.5, 1.5, 3.5, 5.5 and 7.5 m deep from well head, and the petroleum contents were respectively 153.3, 148.4, 129.2, 50.5 and 5.62 g x kg(-1). HAs were extracted with 0.1 mol x L(-) NaOH and 0.1 mol x L(-1) Na4 P2O7 and were characterized with elemental analysis, Fourier transformed infrared (FTIR) spectroscopy and solid-state 13C cross polarization magic angle spinning nuclear magnetic resonance (13C CPMAS NMR) spectroscopy. Results showed that the atomic C/H, O/C and (N+O) /C ratios of HAs increased from 0.74, 0.41 and 0.45 for 7.5 m to 0.80, 0.83 and 0.88 for 0.5 m, respectively. The relative intensity of the peaks assigned to aliphatic carbon (2 921, 2 851 and 1 454 cm(-1)) in the FTIR spectra gradually decreased with increasing contamination levels, while that of the peak assigned to aromatic C(1 600 cm(-1)) increased, and the calculated absorption intensity ratio of 2 921 to 1 600 cm(-1) (2 921/1 600) declined from 0.22 for 7.5 m to 0.11 for 0.5 m. The solid-state 13C NMR data suggested that the relative content of alkyl C(0-50 ppm ) decreased from 49.9% for 7.5m to 30.9% for 0.5 m, while that of O-alkyl C(50-110 ppm), aromatic C(110-160 ppm) and carboxyl C(160-190 ppm) increased respectively from 20.1%, 13.1% and 14.3% to 28.0%, 18.8% and 19.3%. These results showed substantial chemical, structural, and molecular differences among these HAs. The aliphaticity and hydrophobicity of HAs decreased while aromaticity and polarity increased with the increase in petroleum content. Namely, HAs tended to become aged in molecular structure. Therefore, it is imperative to renew and activate the aged HAs by adopting appropriate measures for the remediation of petroleum contaminated soil.