The Handbook of Environmental Chemistry Vol. 3, Part Q (2003): 57– 84 DOI 10.1007/b11463
Physical-Chemical Properties and Evaluative Fate Modelling of Phthalate Esters Ian T. Cousins 1 · Donald Mackay 1 · Thomas F. Parkerton 2 1 2
Canadian Environmental Modelling Centre, Environmental and Resource Studies, Trent University, Peterborough, Ontario, K9J 7B8, Canada. E-mail:
[email protected] Exxon Mobil Biomedical Sci. Inc., Hermeslaan 2, 1831, Machelen, Belgium
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. Keywords. Phthalate ester, Structure, Physical-chemical property, Model, Fate
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Introduction
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Structure-Property Analysis of Physical-Chemical Properties
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Observations on Physical-Chemical Properties
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Physical State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Solubility in Water . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Vapour Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
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© Springer-Verlag Berlin Heidelberg 2003
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Air-Water Partition Coefficient . . Octanol-Water Partition Coefficient Octanol-Air Partition Coefficient . Data Gaps . . . . . . . . . . . . . .
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Degrading Reactions . . . . . . . . . . . . . . . . . . . . . . . . . 72
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Evaluative Fate Modelling with the EQC Model . . . . . . . . . . . 74
5.1 5.2 5.3 5.4
EQC Level I Modelling . . . . . . . . . . . . . . . . . . . . EQC Level II Modelling . . . . . . . . . . . . . . . . . . . . EQC Level III Modelling . . . . . . . . . . . . . . . . . . . Estimating Persistence and Long-Range Transport Potential with the TaPL3 Model . . . . . . . . . . . . . . . . . . . .
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Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
1 Introduction Phthalate esters are widely used as plasticizers, serving as important additives that impart flexibility to polymers including poly(vinyl chloride) (PVC), polyvinylacetates, cellulosics and polyurethanes [1]. The stability, fluidity and low volatility of high-molecular mass phthalate esters make them ideal for use as plasticizers. The variety of possible chemical structures of phthalate esters results in a wide range of physical-chemical properties and hence environmental partitioning behaviour for this class of compounds. This wide range of properties is principally a result of the variation in the length of the alkyl chains substituted on the diester groups. The names, molecular formulae, molar masses and melting points of 22 phthalate esters are listed in Table 1. The objectives of this chapter are to review the published physical-chemical and reactivity data for the phthalate esters, seek relationships between chemical structure and properties and determine how these properties will influence partitioning between abiotic media in the environment with the use of evaluative environmental fate models. The accumulation of phthalate esters in biotic media (i.e. food webs) is the focus of a separate chapter in this volume.
2 Structure-Property Analysis of Physical-Chemical Properties Physical-chemical properties which can be measured readily in the laboratory with a view to determining environmental partitioning include: solubility in water, vapour pressure, the Henry’s law constant (H), the octanol-water partition coefficient (KOW) and the octanol-air partition coefficient (KOA). There are few direct measurements of Henry’s law constants for the phthalate esters and no
Physical-Chemical Properties and Evaluative Fate Modelling of Phthalate Esters
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Table 1. List of phthalate esters studied and their associated molar masses, molar volumes and melting points
Phthalate ester
Abbreviation
Molar mass (g mol–1)
Le Bas molar volume (cm3 mol–1)
Dimethyl phthalate Diethyl phthalate Diallyl phthalate Dipropyl phthalate Di-n-butyl phthalate Disiobutyl phthalate Di-n-propyl phthalate Butylbenzyl phthalate Diisohexyl phthalate Di-n-heptyl phthalate Di-n-octyl phthalate Butyl 2-ethylhexyl phthalate Di(n-hexyl, n-octyl, n-decyl) phthalate a Di(2-ethylhexyl) phthalate Diisooctyl phthalate Di-n-nonyl phthalate Diisononyl phthalate Di-n-decyl phthalate Diisodecyl phthalate Di(heptyl, nonyl, undecyl) phthalate a Diundecyl phthalate Ditridecyl phthalate
DMP DEP DAP DPP DnBP DIBP DnPP BBP DHP DIHpP DnOP BOP 610P DEHP DIOP DnNP DINP DnDP DIDP D711P DUP DTDP
194.2 222.2 246.2 250.3 278.4 278.4 250.3 312.4 334.4 362.5 390.6 334.4 404.6 390.6 390.6 418.6 418.6 446.7 446.7 418.7 447.7 530.8
206.4 254.0 283.6 298.4 342.8 342.8 387.2 364.8 431.6 476.0 520.4 416.6 542.6 520.4 520.4 564.8 564.8 609.2 609.2 564.8 653.6 742.4
a
Melting point (°C) 5.5 –40 – – –35 –58 – –35 –27.5 – – –37 –4 –46 –46 – –48 – –46 100 days). BBP is a special case in that it does not contain two straight alkyl chains in its structure and thus the mechanism for primary degradation is likely to be different. Measured data for BBP from Staples et al. [4] suggest that its aerobic biodegradation half-lives in natural waters, soils and sediments are 0.4 day to 8 ¥ 104 days, 9.6 days and 1.6–2.2 days, respectively. The value of 8 ¥ 104 days seems to be erroneous because there are five other data points in the range 0.4–1.4 days. There is only one data point for aerobic biodegradation in soil for BBP, which is particularly disappointing because soil is the primary medium of accumulation for BBP. Degradation rates of phthalate esters in anaerobic media are slower, but the models used in this chapter only treat aerobic environmental media. Only surface soils (top 5 cm) and sediments (top 3 cm) are treated. The above analysis of measured biodegradation half-life data from Staples et al. [4] has been used to allocate approximate half-lives by using a semi-decade logarithmic scale for water, soil and sediment compartments in the EQC Level II and III simulations (Table 5). This approximate allocation takes account of the large uncertainty in measured biodegradation half-lives. We have taken a conservative approach in our allocation of half-lives and assigned half-lives that are near to the top of the range reported by Staples et al. [4]. This conservative approach results in estimated half-lives that are higher than those suggested in the chapter of this handbook focussing on environmental degradation rates of phthalates. However, it is believed that a conservative approach is appropriate for allocation of half-lives because degradation studies are often conducted at a constant 25 °C, whereas the environment is often at a lower temperature, some studies use inoculums and some allow the microbial population to become acclimated. Furthermore, some microcosm studies may not separate losses from degradation from losses due to partitioning to sediments and volatilisation.
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Table 5. Allocation of half-lives for use in Level II and III EQC simulations
Phthalate ester
DMP
DEP
DnBP
BBP
DEHP
Assumed reaction half-lives (h)
Class Mean Range Class Mean Range Class Mean Range Class Mean Range Class Mean Range
Air
Water
Soil
Sediment
5 550 300–1000 4 170 100–300 3 55 30–100 2 17 10–30 2 17 10–30
4 170 100–300 4 170 100–300 4 170 100–300 3 55 30–100 5 550 300–1000
5 550 300–1000 5 550 300–1000 6 1700 1000–3000 6 1700 1000–3000 7 5500 3000–10,000
6 1700 1000–3000 6 1700 1000–3000 7 5500 3000–10,000 6 1700 1000–3000 7 5500 3000–10,000
5 Evaluative Fate Modelling with the EQC Model Conducting evaluative assessments can provide invaluable insights into the characteristics of chemical behaviour in the environment. Because the environment considered is purely evaluative or hypothetical, there is no possibility of validation, but the equations used to describe partitioning, transport and transformation are identical to those used successfully in validated models of chemical fate in more defined environments. The aim is to establish the general features of chemical behaviour, namely, into which media the chemical will tend to partition, the primary loss mechanisms, the tendency for intermedia transport, the tendency to bioaccumulate, the tendency to undergo long-range transport and environmental persistence. Multimedia models of this type are widely used by the scientific community as useful tools for providing information on chemical fate and have also found acceptance in regulatory practice in a number of countries. The Equilibrium Criterion or EQC model, the model of choice here, has been described fully elsewhere [65]. Briefly, this model in the form of a computer program, deduces the fate of a chemical in Level I, II and III evaluative environments by using principles described by Mackay [36]. The EQC evaluative environment is an area of 100,000 km2 that is regarded as being representative of a jurisdictional region such as the US state of Ohio, or the country of Greece. EQC can simulate the chemical fate of a variety of different chemical class types, classified according to the data requirements to run a model simulation. Phthalate esters partition to all environmental media and are thus classified as type 1 chemicals
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Physical-Chemical Properties and Evaluative Fate Modelling of Phthalate Esters
for which all partition coefficients and Z values (fugacity capacities) must be defined [36]. 5.1 EQC Level I Modelling
EQC Level I modelling has been performed for the 22 phthalate esters listed in Table 1 for which physical-chemical properties have previously been estimated (Table 3). Level I EQC model results indicate that under equilibrium, steady state conditions, with no reaction, the vast majority of phthalates released will reside in soil, sediment or water with over 99% being distributed to these three media (Table 6). The low vapour pressures ensure that only small percentages partition to air. Phthalate esters with alkyl chains containing greater than five carbons partition almost exclusively to the organic carbon component of soil and sediment, whereas those with short alkyl chains (
