Kinetics and equilibria of the interactions between diethylhexyl phthalate and sediment particles in simulated estuarine systems

The sorption of diethylhexyl phthalate (DEHP) by sediment particles has been studied under simulated estuarine conditions, using freshwater and seawater samples collected from the Conwy Estuary, North Wales. The sorption process was rapid, reaching a sorption equilibrium within 6 h at a sediment concentration (SC) of 2.16 g l−1. With an increase in SC, it took longer for the sorption equilibrium to be reached. The sorption equilibrium can be adequately described by the linear partition model at low equilibrium aqueous concentrations. The sorption of DEHP was enhanced in the presence of salts, due to the salting out effect, and a salting constant of 1.156 l mol−1 was obtained. The sorption coefficient of DEHP decreased with increasing SC, from 1.28×104 ml g−1 at 0.1 g l−1 SC to 6.75×103 ml g−1 at 14.66 g l−1 SC, due to the presence of colloidal particles in suspension which were able to bind DEHP. This third phase was quantified by measuring its dry weight, which was found to increase with increasing SC. The effect of colloids on DEHP sorption was studied by correcting the observed partition coefficients based on the concentration of colloid particles, and the true partition coefficients obtained (1.61×104±1.29×103 ml g−1) were found to show much smaller variations than apparent partition coefficient (9.37×103±2.12×103 ml g−1) at different SCs. Furthermore, the true organic carbon-normalised partition coefficients for the two sediments with different organic carbon content became similar and relatively independent of SCs.