Effects of soil organic matters on adsorption-desorption equilibria of phenanthrene

Sorption-desorption to natural soil is an underlying process affecting the transport, degradation, and biological activity of hydrophobic organic contaminants (HOCs) in the environment. Mineral fractions and organic matters in soil play important roles in determining the sorption and desorption patterns of hydrophobic organic contaminants. For these contaminants, the mineral fractions of soil play minor roles than organic matter. A study was carried out to determine the relationship between organic matter content and composition of soil and the sorption-desorption of phenanthrene. Hydrogen peroxide was used to reduce the content of soil organic matter (SOM). An examination of soil sample under scanning electron microscope suggested that the pore structure of soil was slightly destroyed and even 60% of SOM was oxidized. Elemental analysis and solid state ¹³C NMR indicated that the proportion of alkyl C increased, but he proportion of O-alkyl C decreased with reducing the content of SOM. Phenanthrene sorption capacity and isotherm nonlinearity both decreased with increasing the degrees of SOM oxidation, supporting the viewpoint that sorption equlibrium properties of hydrophobic organic contaminants can directly related to the content of SOM. The apparent sorption-desorption hysteresis was quantified for each sample. Sorption-desorption hysteresis index varies from sample to sample, and they also appear to correlate with the chemical characteristics of SOM. The more condensed the soil organic matter is, the more geologically old the soil organic matter is, and the more apparent the sorption-desorption hysteresis is. We suggest that the content of SOM is a major determinant of phenanthrene sorption capacity, isotherm nonlinearity and hysteresis.