Dom-Induced Artifacts In Kd Measurement for Hydrophobic Contaminants.

Batch equilibration method is usually used for deriving K_d (and K_oc) of hydrophobic organic compounds (HOCs) in soils and sediments, where centrifugation is typically used to separate the solid and aqueous phases. However, centrifugation does not exclude DOM from the aqueous phase. Presence of DOM in the aqueous phase artificially increases the aqueous phase concentration (C_w) due to sorption of HOCs to the DOM phase, leading to an artificially depressed K_d. We have employed solid-phase microextraction (SPME) as a selective method to measure the freely dissolved aqueous concentration (C_free) to estimate the true K_d and K_oc for a wide range of HOCs. In each experiment, we compared the SPME measurement against the conventional approach for the same sediment or soil. Paired comparisons consistently showed that K_d for strongly hydrophobic compounds was grossly underestimated for the standard batch method, with underestimation often up to an order of magnitude. Therefore, many K_d or K_oc values of HOCs in the literature may be significantly underestimated and should be revised by correcting for the artifact due to DOM.