In the Classroom
Use of Physicochemical Parameters To Assess the Environmental Fate of Organic Pollutants: The Fugacity Model Xavier Domènech,* José Antonio Ayllón, and José Peral Departamento de Química, Universidad Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain; *
[email protected] The behavior and fate of pollutants in the environment is a component of most environmental chemistry courses (1, 2). In our university a series of laboratories have been developed in which environmentally relevant physicochemical parameters of chemicals, such as a vapor pressure, solubility, partition constants (Henry constant, octanol–water partition constant, distribution coefficient, and so forth), and rate constants of different processes (hydrolysis, OH radical oxidation, photochemical, and biological) are defined and their significance discussed in relation to the behavior and the environmental fate of pollutants (3). For organic chemicals some of these physicochemical constants can be estimated using methods based on structural contributions. The underlying idea is that each fragment of the molecule (e.g., C⫺H bond) has a substantially constant effect on the physicochemical constant regardless of the substance in which it occurs. These estimation methods can aid the student to relate the molecular structure of the pollutant with its environmental fate (4). A part of these laboratories are dedicated to the search of physicochemical data by using different Internet databases. A recent article by Boethling et al. (5) reviews and summarizes currently available Internet resources for obtaining physicochemical data relevant to the assessment of the behavior and environmental fate of pollutants. In these laboratories, the students perform exercises to predict the environmental fate and behavior of different organic pollutants based on the qualitative analysis of thermodynamic and kinetic data. The Fugacity Model In addition to the qualitative prediction, a quantitative fate assessment by means of the use of the Mackay fugacity model is introduced to determine the partition of organic pollutants in multiphase environmental systems (6, 7). The fugacity, f, of a substance is defined as its tendency to escape from a given phase and has the units of pressure. Thus, the fugacity of a gas is closely related to the partial pressure and the fugacity of a pure solid or liquid is given by its equilibrium vapor pressure at specified temperature (8). A substance in a multiphase system is distributed among the different phases and the equilibrium is achieved when the fugacity of the substance is equal in all the phases where it is present (9). In dilute systems, as is usually the case for environmental contamination, the fugacity is linearly related to concentration, C (10),
the phase volume of the environmental system are the key parameters needed to determine the distribution of a substance into different phases at equilibrium. In a multiphase system, a chemical tends to concentrate at those phases with high Z. An environmental system is constituted by different phases, such as air, water, soil, sediment, suspended solids in a fluid (air or water), biota, and so forth. To estimate the distribution of a pollutant among the different phases, the first step is to evaluate Z for the pollutant in the different phases.
Air (A) In a very dilute system, the fugacity can be approximated as the partial pressure, f ≅ P. Using the ideal gas equation and the previous equation, C =
f P 1 = A = fA ZA ⇒ ZA = RT RT RT
Thus, in this case (ideal gas) ZA only depends on the temperature and not on the nature of the pollutant.
Water (W) In this case, ZW is obtained from the Henry constant, KH, of the substance at the specified temperature, ZW =
where CW is the concentration of the substance in water in equilibrium with the gas phase.
Solid Phases (S) The fugacity capacity of a substance in a solid phase (soil, sediment, or suspended solid), ZS, can be expressed in terms of the distribution coefficient, K d, (i.e., the ratio of a substance’s total equilibrium concentrations in the sorbed phase, CS, and in the solution, CW, at a given temperature). C S = Kd C W f S ZS = Kd f W Z W At equilibrium fS = fW and ZW = 1兾KH. Then, ZS =
C = fZ where Z is the fugacity capacity for the substance in a given phase with the units of concentration and reciprocal pressure (e.g., mol m᎑3 Pa᎑1). Z depends on the temperature, the properties of the substance, and the nature of the phase in which the substance is associated. The fugacity capacity and www.JCE.DivCHED.org
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CW 1 = fW KH
Kd KH
Biota (B) For biotic phases the bioconcentration factor, KB, is used to evaluate the biota fugacity capacity, ZB. KB is the ratio between the substance concentration in the biota, CB, and in the solution, CW, at equilibrium for a given temperature. In
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this case, similarly to ZS, the following expression for ZB is deduced:
ZB =
KB KH
Application of the Model To determine the distribution of a pollutant in a multiphase system, the nature of the phases that are present and their volume must be known. By a way of example, in the following the fugacity model is applied to determine the phase distribution of three common pollutants: pentachlorophenol (PCP), hexachlorobenzene (HCB), and the gamma isomer of hexachlorocyclohexane (HCH) that is commonly named lindane. These three compounds are extensively used in a variety of applications and are usually found as contaminants in the environment (11). PCP is used as herbicide,
pollutant
air
water sediment
algacide, and molluscide to inhibit fermentation in various materials and also as defoliant on selected crops. HCB is widely used as chemical intermediate in the manufacture of PCP, as raw material for synthetic rubber, for dye materials, and as an additive in pyrotechnic compositions; also, HCB is used as an additive in seed treatment. Finally, HCH is applied in baits and seed treatment for rodent control and as insecticide for control of a broad spectrum of soil-inhabiting insects and public-health pests. The environmental system considered could be a lacustrine system with five major phases: air (A), water (W), sediments (S), suspended solids (SS) and aquatic biota (B) (Figure 1). This model environment has an area of 1 km2 and 1 km of altitude, with an atmosphere (air) that occupies practically all the system volume, except at the bottom that is covered by 10 m of water, and beneath it a 1-cm layer of sediment. In the bulk of the water body, suspended solids and aquatic biota are present with volume fractions with respect to the water reservoir of 5 ⫻ 10᎑6 and 1 ⫻ 10᎑6, respectively. It is assumed that the solid suspended matter comes from sediment resuspension. The physicochemical parameters needed to estimate the fugacity capacity of the three pollutants in the different environmental phases are the Henry constant and the octanol– water partition constant (K OW ). The values of these parameters have been gathered from the TOXNET database (11), and are listed in Table 1. From KH it is possible to calculate ZW, while knowing KOW allows us to estimate Kd, assuming that the sediment (and, hence, the suspended solids) has a fraction of organic matter ( fOM) of 0.02 and that PCP, HCB, and HCH are sorbed totally on the sediment organic matter owing to their relative high hydrophobicity (logKOW > 3). Then, it can be assumed that (8), K d = K OM f OM
suspended solids
biota
Figure 1. Representation (not at scale) of the model environmental system. The atmosphere (air) occupies the major part of the volume system (1-km2 surface by 1-km high). The bottom 10 m of the system is occupied by a water body and beneath it a 1-cm layer of sediment is found. Owing to resuspension of the sediment, some solid particles are suspended in the water body (volume fraction: 5 ⫻ 10᎑6). Also, some fish are living in the water reservoir (volume fraction: 1 ⫻ 10᎑6). The organic matter fraction of the sediments and suspended solids is 0.02 and the lipid fraction of the biota is 0.05.
Table 1. Physicochemical Data for PCP, HCB, and HCH Pollutant PCP HCB HCH
K H/ (Pa m3 mol᎑1)
logKOW
Kd/103
0.0025
5.12
0.16
5.73
2.5
3.72
0.11
58.6 0.35
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log K OM = a log K OW + b where a and b are empirical constants that depend on the family type of organic compounds. The a and b parameters for aromatic hydrocarbons, chlorinated hydrocarbons, and chlorophenols are: 1.01 and ᎑0.72, 0.88 and ᎑0.27, 0.81 and ᎑0.25, respectively (12). From these data, the KOM values for the three pollutants have been estimated and the corresponding obtained Kd values are summarized in Table 1. KB data are also estimated from KOW values assuming that fat tissue is the relevant storage medium in living organisms and octanol is the chemical surrogate for fats. Then, KB can be obtained from the relation (12),
KB/103
K B = K OW f lip
6.6
where flip is the lipid fraction of the biota. In the present case, it is assumed that flip for the aquatic biota is 0.05. The estimated KB values for PCP, HCB, and HCH are listed in Table 1. From the values of KH, Kd, and KB, the fugacity capacities of PCP, HCB, and HCH in water, sediment, suspended
27 0.26
Note: Data at 25 ºC. For the calculations, the dimensionless values of Kd and KB must be used.
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where KOM is the organic matter partition coefficient (i.e., the ratio of the substance concentrations between organic matter and water at equilibrium). On the other hand, it has been shown that KOM is directly related to KOW by (12),
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solids, and biota are calculated. ZA is 4.04 × 10᎑4 mol m᎑3 Pa᎑1 for the three pollutants, obtained from 1兾RT and taking 25 ⬚C for the calculation. The fugacity capacities of the three organic pollutants in all the environmental phases considered are summarized in Table 2. In the present example it is considered that 10 mol of pollutant are added to the environmental model system and allowed to distribute among the different phases. Then, the total mass of pollutant, m, is shared in the different phases
Table 2. Fugacity Capacities of the Pollutants in the Different Phases Z/(mol m᎑3 Pa᎑1) Pollutant
Air/ 10᎑4
Water 400
Sediment/ Suspended 102 Solid/102 640.
640.
Biota/ 102
PCP
4.04
26000.
HCB
4.04
0.017
0.43
0.43
4.6
HCH
4.04
2.9
3.1
3.1
7.4
m = m A + m W + mS + mSS + mB For each phase i:
Table 3. Amount of Pollutant in the Different Phases
mi = C iVi = f i Z i Vi
Amount of Pollutant/mol Pollutant
and, m =
∑ f i Z i Vi
PCP
i
At equilibrium fA = fW = fS = fSS = fB = f, and consequently, the pollutant fugacity can be determined by f =
m ∑ Z i Vi
Air/ 10᎑2 0.086
Water
Sediment
Suspended Solid/10᎑2
Biota/ 10᎑1
8.6
1.38
0.69
0.56
HCB
404
1.7
4.3
2.1
0.46
HCH
12
9.0
0.93
0.46
0.023
NOTE: Amount is given after the phase equilibrium is attained. The volumes for the given phases are air: 109 m3, water: 107 m3, sediment: 104 m3, suspended solid: 50 m3, and biota: 10 m3.
i
The calculated fugacities for PCP, HCB, and HCH are: 2.16 × 10᎑9, 1.0 × 10᎑5, and 3.1 × 10᎑7 Pa, respectively. From these fugacity values, the mass of pollutant in each phase once the equilibrium is achieved, can be calculated (mi = fi ZiVi). The results for the three pollutants are listed in Table 3. As can be seen, the pollutant mass distribution in the different phases depends on the nature of the chemical. In this way, the majority of PCP and HCH partitions in the water phase, while HCB distributes almost equally between the air and the sediment phases. This results are in agreement with the observed trend of fugacities: the highest fugacity, HCB, has the more escaping tendency and corresponds to the most pollutant in the air. However, concerning the pollutant equilibrium concentrations the higher values are found in the solid phases (sediment, suspended solids, and biota) for all pollutants (Table 4), in agreement with the high values observed for the fugacity capacities of these phases (see Table 2). Particularly, this is true for the accumulation of pollutants into the aquatic biota, for which the higher concentration of pollutants is found, attaining equilibrium concentrations that are between a thousand and ten million higher in aquatic biota than in water and air. In fact, taking advantage of this bioconcentration phenomenon, it has been proposed to manipulate fish harvests to remove highly hydrophobic toxic substances from polluted waters (13). Summary This model allows the use of different partition constants in an easy way, to determine the distribution of a chemical between different phases in equilibrium of an environmental system. Also, by extending the calculations to different pollutants a discussion of the affinity of different chemicals
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Table 4. Concentration of Pollutant in the Different Phases Concentration of Pollutant/(mol m᎑3) Pollutant PCP
Air/ 10᎑11 0.086
Water/ Sediment/ Suspended 10᎑7 10᎑5 Solid/10᎑5
Biota/ 10᎑3
8.6
14
14
5.6
HCB
404
1.7
43
43
4.6
HCH
12
9.0
9.3
9.3
0.23
NOTE: Amount is given after the phase equilibrium is attained.
in the distinct phases can be carried out. It is the case of the present example, that allows us to assess, in terms of the structural composition, the different environmental behavior of substances that differ in some functional groups, for example, when a Cl atom (in HCB) is replaced by a OH group (in PCP), or when an aromatic chlorinated hydrocarbon (HCB) is compared with respect to an aliphatic hydrocarbon with the same number of Cl atoms in the molecule (HCH). More complex calculations can be performed using other levels of increasing complexity. The Institut für Umweltsystemforschung of the University of Osnabrück (Germany) and the Canadian Environmental Modelling Centre of the University of Trent (Canada) host Internet sites where the three levels of the fugacity model are made available free of charge (14, 15). In this article we presented the Level I of the fugacity model, which gives the distribution of a pollutant among phases and is useful to predict, in an approximated way, the environmental fate of a persistent organic pollutant. Level II considers flows in and out of the system owing to advective transport and chemical transformation. Besides partition constants, data of pollutant degradation rate constants in the different environmental phases and advective
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rate constants are needed to run the model. In this case, a constant input flow to the system and a steady state regime is assumed. Level III takes into account the transfer of pollutant among phases, giving rise a steady state, but the pollutant is not in equilibrium and different fugacities are obtained at the distinct phases. Literature Cited 1. Casey, R. E.; Pittman, F. A. J. Chem. Educ. 2005, 82, 260– 264. 2. Dunnivant, F. M.; Kettel, J. J. Chem. Educ. 2002, 79, 715– 717. 3. Goss, K-U.; Schwarzenbach R. P. J. Chem. Educ. 2003, 80, 450–455. 4. Allen, D. T.; Schonnard, D. R. Green Engineering; Prentice Hall: New York, 2002. 5. Boethling, R. S.; Howard, P. H.; Meylan, W. M. Env. Toxicol. Chem. 2004, 23, 2290–2308. 6. Mackay, D. Env. Sci. Technol. 1979, 13, 1218–1223.
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7. Mackay, D. Multimedia Environmental Models. The Fugacity Approach; Lewis Publishing–CRC Press: Boca Raton, FL, 1991. 8. Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. Environmental Organic Chemistry; John Wiley and Sons: New York, 1993. 9. Thibodeaux, L. J. Environmental Chemodynamics, 2nd ed.; John Wiley and Sons: New York, 1996. 10. Mackay, D.; Paterson, S. Env. Sci. Technol. 1981, 15, 1006– 1014. 11. TOXNET Home Page. http://www.toxnet.nlm.nih.gov/ (accessed Nov 2005). 12. Tinsley, I. J. Chemical Concepts in Pollutant Behaviour, 2nd ed.; Wiley Interscience: New Jersey, 2004. 13. Mackenzie, B. R.; Almesjo, L.; Hansson, S. Env. Sci. Technol. 2004, 38, 1970–1976. 14. Institut für Umweltsystemforschung. http://www.usf.uniosnabrueck.de/projects/elpos/ (accessed Nov 2005). 15. Canadian Environmental Modelling Centre. http:// www.trentu.ca/cemc/models/models.html (accessed Nov 2005).
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