McKay, Donald. "Front matter" Multimedia Environmental Models ...

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have very low solubilities in the water, i.e., they are very hydrophobic; thus, their low vapor pressure is offset by their very low water solubility, and they have relatively large Henry’s law constants. They may thus partition appreciably from water into the atmosphere through evaporation from rivers and lakes. The solubility and activity of a solute in water are affected by the presence of electrolytes and other co-solvents; thus, the Henry’s law constant is also affected. The magnitude of the effect is discussed later in Section 5.4.5. Worked Example 5.2 Deduce H and K AW for benzene, DDT, and phenol given the following data at 25°C: Molar Mass (g/mol) Solubility (g/m3 ) Vapor Pressure (Pa) benzene 78 1780 12700 DDT 354.5 0.0055 0.00002 phenol 94.1 88360 47 In each case, the solubility C S in mol/m 3 is the solubility in g/m 3 divided by the molar mass, e.g., 1780/78 or 22.8 mol/m 3 for benzene. H is then P S /C S or 556 Pa m 3 /mol for benzene. K AW is H/RT or 556/(8.314 ¥ 298) or 0.22. Note that these substances have very different H and K AW values because of their solubility and vapor pressure differences. The vapor pressure of DDT is about 600 million times less than that of benzene, but H is only 400 times less, because of DDT’s very low water solubility. Phenol has a much higher vapor pressure than DDT, but it has a much lower H and K AW. Benzene tends to evaporate appreciably from water into air, and DDT less so but still to a significant extent, while phenol does not evaporate significantly. Inherent in this calculation for phenol is the assumption that it does not ionize appreciably. 5.4.3 Octanol-Water Partitioning The dimensionless octanol-water partition coefficient (K OW) is one of the most important and frequently used descriptors of chemical behavior in the environment. In the pharmaceutical and biological literature, K OW is given the symbol P (for partition coefficient), which we reserve for pressure. The use of 1-octanol has been popularized by Hansch and Leo, who have tested its correlations with many biochemical phenomena and have compiled extensive databases. Various methods are available for calculating K OW from molecular structure, as reviewed by Lyman et al. ©2001 CRC Press LLC H K AW benzene 556 0.22 DDT 1.29 5.2 ¥ 10 –4 phenol 0.050 2 ¥ 10 –5
(1982), Baum (1997) and Leo (2000). Extensive databases are also available as reviewed by Baum (1997). Octanol was selected because it has a similar carbon to oxygen ratio as lipids, is readily available in pure form, and is only sparingly soluble in water (4.5 mol/m 3 ). The solubility of water in octanol of 2300 mol/m 3 , however, is quite large (Baum, 1997). The molar volumes of these phases are 18 ¥ 10 –6 m 3 /mol and 120 ¥ 10 –6 m 3 /mol, a ratio of 0.15. It follows that K OW is 0.15 g W/g O. K OW is a measure of hydrophobicity, i.e., the tendency of a chemical to “hate” or partition out of water. As was discussed earlier, it can be viewed as a ratio of solubilities in octanol and water but, in most cases of liquid chemicals, there is no real solubility, because octanol and the liquid are miscible. The “solubility” of organic chemicals in octanol tends to be fairly constant in the range 200 to 2000 mol/m 3 , thus variation in K OW between chemicals is primarily due to variation in water solubility. It is therefore misleading to assert that K OW describes lipophilicity or “love for lipids,” because most organic chemicals “love” lipids equally, but they “hate” water quite differently. Viewed in terms of Z values, K OW is Z O/Z W. Z O is (relatively) constant for organic chemicals; however, Z W varies greatly and is very small (relatively) for hydrophobic substances. Because K OW varies over such a large range, from approximately 0.1 to 10 7 , it is common to express it as log K OW. It is a disastrous mistake to use log K OW in a calculation when K OW should be used! K OW is usually measured by equilibrating layers of water and octanol containing the solute of interest at subsaturation conditions and analyzing both phases. If K OW is high, the concentration in water is necessarily low, and even a small quantity of emulsified octanol in the aqueous phase can significantly increase the apparent concentration. A “slow stirring” method is usually adopted to avoid emulsion formation. An alternative is to use a generator column in which water is flowed over a packing containing octanol and the dissolved chemical. 5.4.4 Octanol-Air Partition Coefficients This partition coefficient is invaluable for predicting the extent to which a substance partitions from the atmosphere to organic media including soils, vegetation, and aerosol particles. It can be estimated as K OW/K AW or measured directly, usually by flowing air through a column containing a packing saturated with octanol with the solute in solution. Values of K OA can be very large, i.e., up to 10 12 for substances of very low volatility such as DDT, and values are especially high at low temperatures. Harner et al. (2000) have reported data for this coefficient and cite other data and measurement methods. 5.4.5 Solubility in Water This property is of importance as a measure of the activity coefficient in aqueous solution, which in turn affects air-water and octanol-water partitioning. It can be regarded as a partition coefficient between the pure phase and water, but the ratio of concentrations is not calculated. A comprehensive discussion is given in the text by Yalkowsky and Banerjee (1992), and estimation methods are described by Mackay ©2001 CRC Press LLC
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McKay, Donald. "Front matter" Multi
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Multimedia Environmental Models The
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hensive, and reliable, and they hav
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Chapter 1 Introduction Chapter 2 So
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McKay, Donald. "Introduction" Multi
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It is now evident that our task is
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McKay, Donald. "Some Basic Concepts
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give the derived units directly wit
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Energy (joule, J) The joule, which
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particles (aerosols), or biota in w
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Worked Example 2.1 A three-phase sy
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(i) What is the concentration (C) i
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Answer ©2001 CRC Press LLC 5.1 kg,
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or or When t is zero, C is zero, th
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What is the absolute minimum piscic
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©2001 CRC Press LLC C 1 = C O/(1 +
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containing nonequilibrium quantitie
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1. Fallout of chemical from air to
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validated using real environments.
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©2001 CRC Press LLC CHAPTER 3 Envi
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of other, somewhat similar or homol
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Table 3.2 List of Chemicals Commonl
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elatively high concentrations. This
Page 47 and 48: exists between toxicologists and ch
Page 49 and 50: determine priority. This is a subje
Page 51 and 52: Another important classification of
Page 53 and 54: Figure 3.1 (continued) ©2001 CRC P
Page 55 and 56: Table 3.5 (continued) dichlorometha
Page 57 and 58: Table 3.5 (continued) total PCB 326
Page 59 and 60: Figure 3.2 Plot of log K AW vs. log
Page 61 and 62: chlorofluoro compounds are very sta
Page 63 and 64: 3.3.2.7 Arsenic Compounds Arsenic,
Page 65 and 66: McKay, Donald. "The Nature of Envir
Page 67 and 68: Figure 4.1 Evaluative environments.
Page 69 and 70: are subject to two important deposi
Page 71 and 72: metals and organic chemicals from w
Page 73 and 74: carbon figure for deeper sediments
Page 75 and 76: flows. The quality of this groundwa
Page 77 and 78: Table 4.2 Evaluative Environments A
Page 79 and 80: McKay, Donald. "Phase Equilibrium"
Page 81 and 82: Figure 5.1 Some principles and conc
Page 83 and 84: likely using an equilibrium criteri
Page 85 and 86: Another useful quantity is the rati
Page 87 and 88: experimentally, but not directly, u
Page 89 and 90: ©2001 CRC Press LLC 5.3 PROPERTIES
Page 91 and 92: 1.0. This, then, is the fugacity or
Page 93 and 94: where ©2001 CRC Press LLC pH is -l
Page 95 and 96: In terms of solubilities, S iA and
Page 97: The temperature coefficient is the
Page 101 and 102: Although it may appear environmenta
Page 103 and 104: Figure 5.4 Illustration of quantita
Page 105 and 106: 5.5 ENVIRONMENTAL PARTITION COEFFIC
Page 107 and 108: C S = K PC W benzene = 1.1 ¥ 0.001
Page 109 and 110: Mackay (1986), in an attempt to sim
Page 111 and 112: Compartment Definition of Fugacity
Page 113 and 114: high concentration in the fish, but
Page 115 and 116: heat capacity of 0.38 J/g°C requir
Page 117 and 118: Therefore, ©2001 CRC Press LLC f =
Page 119 and 120: ©2001 CRC Press LLC Z T = S(V i/V
Page 121 and 122: ©2001 CRC Press LLC air Z = P S /R
Page 123 and 124: Table 5.1 Table 5.1 Summary of Defi
Page 125 and 126: Figure 5.7 Fugacity form 1 for dedu
Page 127 and 128: Calculate the following physical-ch
Page 129 and 130: ©2001 CRC Press LLC CHAPTER 6 Adve
Page 131 and 132: that all water will spend 100 days
Page 133 and 134: This enables us to conceive of, and
Page 135 and 136: ©2001 CRC Press LLC f = 15/0.5 = 3
Page 137 and 138: D AS dominates. A residence time of
Page 139 and 140: and Therefore, ©2001 CRC Press LLC
Page 141 and 142: eaction situation in which there is
Page 143 and 144: The rate constants in each case are
Page 145 and 146: ©2001 CRC Press LLC 1/t O = SD Ai/
Page 147 and 148: e achieved in 10 days. Detractors o
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sulfate to sulfide or by dechlorina
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to Zepp and Cline (1977) for the or
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©2001 CRC Press LLC 6.7 LEVEL II C
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Figure 6.3 Fugacity Level II calcul
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McKay, Donald. "Intermedia Transpor
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Figure 7.1 Illustration of nonequil
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5. Diffusion within soils, and from
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sionally, the term flux rate is use
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©2001 CRC Press LLC N = ABDC/ Dy m
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no currents or eddies. In practice,
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diffusivity, and some unknown layer
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where ©2001 CRC Press LLC X = y/ U
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fraction of the total volume that i
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Figure 7.6 Mass transfer at the int
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partition coefficient. The signific
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and the series retains a similar ra
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ments of resuspension rates are par
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Figure 7.8 Movement of air phase (k
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ates can thus be used to estimate m
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Figure 7.9 Combination of D values
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Figure 7.10 Four-compartment Level
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Table 7.2 Order of Magnitude Values
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Unlike the Level II calculation, it
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Figure 7.12 Sample Level III output
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McKay, Donald. "Applications of Fug
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applications. Citations are given t
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Another approach is to employ Monte
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LEVEL1B A six-compartment Level I p
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Figure 8.1 Air-water exchange proce
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The total rates of transfer are thu
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Figure 8.2 Chemical transport and t
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mean. For chemical between depths o
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8.5.2 Process Description The water
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learn the benefits of using fugacit
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where Figure 8.5 QWASI model: stead
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are similar to those described by M
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C F ª Z Ff W ª 1.608 mol/m 3 ª 4
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y Mackay et al. (1983), and an appl
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accordingly, but the final solution
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Figure 8.10 Fish bioaccumulation pr
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The nature of the processes control
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iomagnification is more significant
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An alternative and more elegant met
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8.11.2 Model of a Chemical Evaporat
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Paterson et al. (1991), and Hung an
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8.14.1 Introduction ©2001 CRC Pres
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mated again in mg/day. Food, the ot
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models. Most interest is in LRT in
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available from the University of To
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McKay, Donald. "Appendix" Multimedi
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©2001 CRC Press LLC
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