[Concentration and spectrum characteristic of the NaOH extracted humic substances in three size fractions of sediments from the Yellow River].

The concentrations of total organic carbon (TOC) and the NaOH extracted humic substances were analyzed in three size fractions (I : 100 - 300 microm, II : 63 - 100 microm and III : < 63 microm) of sediments sampled in the middle Yellow River. Moreover, UV-visible absorption spectra, fourier transform infrared spectra (FTIR) and three-dimensional excitation emission matrix fluorescence spectra (3DEEM) were used to characterize the chemical structures of the NaOH extracted humic substances in three sediment size fractions. The results show that sediment organic matter in all the size fractions of the collected sediments is dominated by humic substances absorbed on the clay minerals. The proportions of the NaOH extracted humic substances to TOC decrease with decreasing sediment grain size ( I > II > III). This may result from the stronger interaction of the humic substances with clay minerals in finer size fractions of the colleted sediments. Similar UV-visible spectra with different absorbance intensity are observed for the NaOH extracted humic substances in three sediment size fractions. The infrared spectra of the NaOH extracted humic substances show five strong peaks in all fractions of the samples. The area ratios of FTIR peaks suggest that the contents of the phenolic, alcoholic and carboxylic groups account for more than 75% in the NaOH extracted humic substances. Three characteristic excitation-emission peaks present in the 3DEEM of the NaOH extracted humic substances: peak A (UV humic-like compounds), peak C (visible humic-like compounds) and peak T'(phenolic compounds or protein like compounds). UV-visible, FTIR and 3DEEM results imply that the aromatic degree of the NaOH extracted humic substances is higher in fractions I and II than in fraction III. The results also indicate that the aliphatic and aromatic contents are higher in fractions I and II, whilst the phenolic, alcoholic and carboxylic contents are higher in fraction III.