Physical-Chemical Properties and Evaluative Fate Modelling of Phthalate Esters

Abstract

A review is presented of the physical-chemical properties and reactivity of the phthalate esters including a discussion of how these properties control their partitioning and fate in the environment. The air and water solubilities decrease by orders of magnitude from the short alkyl chain phthalates such as dimethyl phthalate (DMP) to the long alkyl chain phthalates such as di-2-ethylhexyl phthalate (DEHP). The octanol-water partition coefficient, which is a measure of hydrophobicity, increases by orders of magnitude with increasing alkyl chain length and this increase is mainly controlled by the reduction in water solubility rather than an increase in octanol solubility. This increase in hydrophobicity results in strong sorption of the higher molecular weight phthalates to organic matter. Air-water partition coefficients (or Henry's law constants) also increase with increasing alkyl chain length. However, the greater evaporative potential of higher molecular weight phthalate esters from water is offset by sorption to suspended matter in surface waters. Phthalates have high values of KOA suggesting that they will be appreciably sorbed to aerosol particles, soil and vegetation. From available data obtained under environmental conditions, half-lives of phthalates in environmental media are proposed. Systematic differences in reactivity or half-life are apparent, with the primary biodegradation half-life tending to increase with increasing alkyl chain length. In contrast, the opposite pattern is observed for the air oxidation half-life. A series of evaluative modelling calculations is described to illustrate how the physical-chemical properties result in differences in environmental partitioning behaviour, persistence and transport potential. In comparison to other organic chemical classes, model results indicate that phthalates are not environmentally persistent or subjected to significant long-range transport. Although the overall environmental persistence of the higher molecular weight phthalates tends to increase, KOA and thus the propensity to partition to aerosols, vegetation and soils also increases, thereby reducing the potential for long-range transport. Recommendations for future research on physical-chemical properties of phthalate esters for environmental fate assessment are discussed.