Investigating particle concentration effects of polycyclic aromatic hydrocarbon (PAH) sorption on sediment considering the freely dissolved concentrations of PAHs

Suspended particulate sediment (SPS) concentration has a great impact on the sediment to water partitioning coefficients (K (p)) of hydrophobic organic compounds (HOCs), which is called the particle concentration effect (PCE). However, the mechanisms regarding the PCE are not yet well understood, and there is little direct experimental evidence for these mechanisms. The aim of this study was to investigate the PCE of polycyclic aromatic hydrocarbon (PAH) sorption on sediment by analyzing the freely dissolved concentrations of PAHs. Sediments were collected from the Yellow River and the Haihe River in China. Pyrene was selected as a model PAH to investigate the sorption of PAH on sediments. In addition to the total dissolved concentration (C (TW)) of pyrene, the freely dissolved concentration (C (FW)) measured by polyethylene devices was used to investigate the PCE of pyrene in the presence and absence of phenanthrene and chrysene. For both the Haihe River and Yellow River sediments, in the presence and absence of other PAHs, the K (p) value of pyrene with C (FW) as the equilibrium concentration in the water phase was approximately two times higher than that with C (TW) as the equilibrium concentration. With either C (TW) or C (FW) as the equilibrium concentration in the water phase, the K (p) value of pyrene decreased with increasing SPS concentration as a power function. In addition, the K (p) value with C (TW) as the equilibrium concentration decreased faster than that with C (FW). This inferred that, apart from a third phase including dissolved organic carbon (DOC) and colloids, particle-particle, or particle-DOC interactions were important for the PCE. The contribution of the third phase to the PCE for the Haihe River sediment (72.5 +/- 26.4 %) was greater than the contribution for the Yellow River sediment (48.4 +/- 16.2 %), which had a larger particle size and lower total organic carbon and black carbon contents. The PCE of PAH sorption onto sediments was attributed to both the third phase and to particle-particle or particle-DOC interactions. The contribution of the third phase to the PCE depended on both the TOC content and the particle size of sediment. As high SPS and DOC concentrations exist in many rivers, their effects on the sorption of HOCs should be considered when conducting bioavailability and ecological risk assessment.