Effect of salting out on the desorption-resistance of polycyclic aromatic hydrocarbons (PAHs) in coastal sediment

• Distribution coefficients of PAHs in coastal sediment increased with salinity.
• A 3-D model was proposed to describe the effect of solubility and log Kow on log Koc.
• A biphasic desorption model was used to fit sequential desorption data.
• Labile fraction was affected by the salinity in the sorption stage.
• Desorption-resistant fraction was affected by the salinity in the desorption stage.

The effects of “salting out” on the sorption and desorption of polycyclic aromatic hydrocarbons (PAHs) in coastal sediment were investigated. The sorption and desorption isotherms of three PAHs (naphthalene, phenanthrene and pyrene) in coastal sediment were linear. The distribution coefficients of sorption (Kd,s) and desorption (Kd,d) increased with salinity and hydrophobicity (log Kow). A three-dimensional model was proposed to explain the combined effects of solubility (S), log Kow and the organic-carbon normalized partition coefficient (log Koc). The sequential desorption of phenanthrene at different salinities of 0–30 ppt from sediment was conducted after sequential sorption at 0 and 30 ppt, respectively. The desorption data was fitted to a biphasic desorption model including linear labile fraction and non-linear desorption-resistant fraction. The linear distribution coefficient (Kdlab) for the labile desorption fraction was affected by the salinity in the previous sequential sorption stage, whereas the desorption-resistance parameters such as the maximum capacity of the desorption-resistant fraction (qmaxres, mg/kg) and the organic carbon-normalized distribution coefficient in the desorption-resistant fraction (Kocres, L/kg-OC), were affected by the salinity in the current desorption stage. In conclusion, the salinity increased the sorbed capacity of PAH as well as the desorption-resistance (as indicated by both qmaxresand Kocres) due to the “salting out effect”.