Partitioning of polycyclic aromatic hydrocarbons to dissolved organic matter from different soils

Dissolved organic matter (DOM) in the soil solution or groundwater can have a considerable effect on the dissipation of hydrophobic organic pollutants. We used the fluorescence quenching technique as well as the reversed-phase separation technique to determine the partition coefficient of 3- to 5-ring PAH to dissolved organic matter from agricultural soils and forest floor materials. Several techniques (centrifugation, pressure filtration, and percolation) are compared for obtaining DOM solution most representative of the DOM in the natural soil solution to use for the sorption experiments. Centrifugation was shown to be the most suitable procedure to obtain DOM solutions in large quantities with a composition most similar to the field soil solution. PAH binding to DOM from mineral soil was found to be considerably lower compared to DOM from acid forest floor materials. These differences can partly be explained by compositional differences with respect to hydrophobic and hydrophilic components of DOM. Binding capacity of DOM was only slightly affected by changes in pH and electrical conductivity of the soil solution. The partition coefficients for DOM from soils are rather different to the data given for model DOM solutions, such as humic acids, fulvic acids, or Aldrich humic acid. To obtain realistic estimates for the effect of DOM on the mobility of hydrophobic organic chemicals in soils, it is essential to use DOM solutions representative of the dissolved organic matter in the soil solution.