Cation treatment and drying-temperature effects on nonylphenol and phenanthrene sorption to a sandy soil

The objective of this study was to investigate the effects of mono- and polyvalent cations on sorption of the two hydrophobic compounds nonylphenol (NP) and phenanthrene (Phe). To this end, exchange sites of a sandy soil were saturated with either Na+, Ca2+, or Al3+ and excess salts were removed by washing. The samples were then sterilized and either stored moist, dried at room temperature, or at 20 degrees C, 60 degrees C, or 105 degrees C in a vented oven. Saturation with Na+ led to an increase of dissolved organic C (DOC) concentration in the soil water extracts, whereas the polyvalent cations Ca2+ and Al3+ decreased it. The H-1-NMR relaxometry analyses showed that Al3+ restricted the mobility of water molecules that are confined within the SOM structure to a higher extent than Ca2+ or Na+. According to contact-angle (CA) analyses, cation treatment did not significantly change the wetting properties of the samples. Batch sorption-desorption experiments showed no clear salt-treatment effects on the sorption and desorption equilibria or kinetics of NP and Phe. Instead, the sorption coefficients and sorption hysteresis of NP and Phe increased in dry soil. With increasing drying temperature the CA of the soils and the sorption of both xenobiotics increased significantly. We conclude that structural modifications of SOM due to incorporation of polyvalent cations into the interphase structure do not modify the sorption characteristics of the soil for hydrophobic compounds. Instead, increasing hydrophobization of organic soil constituents due to heat treatment significantly increased the accessible sorption sites for nonpolar organic compounds in this soil.