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

Abstract

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