[Irreversible sorption of phthalate acid esters to river sediments]

Irreversible sorption behavior of two phthalate acid esters (PAEs), Dimethyl phthalate (DMP) and Di-(2-ethyl-hexyl) phthalate (DEHP), on four natural sediment samples from the Yangtze River and the Yellow River has been studied by equilibrium sorption and multiple cycles of adsorption/desorption experiments. The equilibrium sorption experiment results showed that the organic carbon-normalized partition coefficients (lgKoc) of DMP and DEHP were higher than those reported in references. This means that the sorption of DMP and DEHP on natural sediments include other sorption mechanisms besides the linear partition on organic carbon. The multiple cycles of adsorption/desorption experiment results showed that the sorption of PAEs included linear reversible sorption and irreversible sorption. For the four sediment samples, the maximum of irreversible sorption capacities were 125.19-337.37 microg/g and 515.89-591.41 microg/g for DMP and DEHP, respectively, which were positive correlated to the surface areas, cation exchange capacity and black carbon content of the sediments. The OC-normalized partition constants for the reversible compartment (lgk(oc)(rev)) were 3.69-4.98 L/kg for DMP; they were higher than those (lgKoc) reported in references, suggesting other reversible sorption mechanisms exist besides the linear partition on organic carbon. The lgk(oc)(rev) were 4.12-5.31 L/kg for DEHP; they were close to those (lgKoc) reported in references, suggesting the linear partition on organic carbon is the main reversible sorption mechanism. Although the physiochemical properties of DMP and DEHP are different, the OC-normalized partition constant for the irreversible compartment on the four sediments is essentially constant with lgk(oc)(rev) = (6.46 +/- 0.38) L/kg. As irreversible sorption exists for PAEs, the maximum of irreversible sorption capacity should be considered when studying the sediment quality criteria.