Effect of soil pH on the sorption capacity of soil organic matter for polycyclic aromatic hydrocarbons in unsaturated soils

Sorption by soil organic matter (SOM) is considered the most important process affecting the bioavailability of hydrophobic organic chemicals (HOCs) in soil. The sorption capacity of SOM for HOCs is affected by many environmental factors. In this study, we investigated the effects of soil pH and water saturation level on HOC sorption capacity of SOM using batch sorption experiments. Values of soil organic carbon-water partition coefficient (K_OC) of six selected polycyclic aromatic hydrocarbons (PAHs) were measured in an artificial soil under various soil pH and water saturation conditions. Passive sampling was used to measure K_OC with polydimethylsiloxane as the sampling material. Regardless of soil pH, K_OC increased with increasing soil water saturation level for lower-molecular-weight PAHs. In contrast, K_OC decreased with increasing soil water saturation level for higher-molecular-weight PAHs. Despite some fluctuations, K_OC tended to decrease with increasing soil pH at all water saturation levels. This indicates that earlier studies on the effects of soil pH on K_OC under saturated conditions could be extended to unsaturated soils. These K_OC tendencies were reproduced in three different natural soils, suggesting that the effects of soil water saturation level and pH might be generalized, at least for PAHs. The PAH sorption capacity of SOM was found to be resilient under dynamic soil pH conditions, which can be used to adjust the effects of soil pH.