The behaviour of di-(2-ethylhexyl) phthalate in estuaries

Abstract

Because of their extensive use as non-reactive plasticisers, phthalate esters have become widespread contaminants of the aquatic environment. There is, however, little accurate information on the solubility and sorptive behaviour of phthalates in estuaries, where contaminants generally occur at elevated concentrations and are under the influence of a number of reaction-controlling variables. In this work we have investigated the relative solubility and particle–water interactions of di-(2-ethylhexyl) phthalate (DEHP), one of the most important and abundant phthalate esters, under simulated estuarine conditions using water and sediment samples from a small, organic-rich estuary (Beaulieu, southern England). Dissolved DEHP was salted out in sea water from distilled water, and a salting constant of about 1.2 l mol−1 was derived. Although the compound apparently interacts with dissolved organic matter in river water, it showed no evidence of enhanced solubility over the range in DEHP concentrations studied. Adsorption onto estuarine particles was defined by the Freundlich equation, and was significantly greater in sea water than river water suggesting that the particulate organic matter is subject to either salting out or salinity-induced structural modification which improves its solvency for DEHP. Distribution coefficients (KDs) exhibited a strong inverse relationship with particle concentration (SPM), an effect defined by the equations: KD=2.63×106SPM−1.15 and KD=2.64×106SPM−0.75; in river water and sea water, respectively, and only partly accounted for by experimental artefacts (e.g., adsorption of DEHP to container walls). That the gradient of the relationship was greater in river water than in sea water suggests the effect is caused, to some extent, by a particle–particle interaction mechanism (e.g., flocculation) which is inhibited at high salinities. A comparison of the results of this study with a compilation of data on DEHP distributions and partitioning in aquatic environments suggests that the estuarine behaviour of DEHP is a function of salinity, particle concentration, particulate organic carbon, and its degradation rate in the aqueous phase. A model for predicting the retention of DEHP in estuaries, incorporating these effects, is presented, and calculations indicate that, under certain hydrodynamic and chemical conditions, more than 50% DEHP discharged to a catchment may be retained by estuarine sediment, at least over a timescale equivalent to the estuarine particle residence time.