The environment as compartments: (Compartment = phase) The phase may be continuous (e.g.
water) or consist of a number of particles that are not in contact, but all of which reside in one phase
(e.g. atmospheric particles (aerosols) o biota in water). In some cases, the phases may be similar
chemically but different physically (e.g. the troposphere and the stratosphere).
Concentrations in soils, sediments and biota can be expressed on a dry o wet weight basis.
Occasionally, concentrations in biota are expressed on a lipid or fat content basis.
Steady- and Unsteady-state conditions: If conditions change relatively slowly with time, there is an
incentive to assume “steady-state” behavior, i.e., that properties are independent of time.
Steady state and equilibrium:
Equilibrium: implies that phases have concentrations (or Tº or pressures) such that they experience no
tendency for net transfer of mass.
Steady state: merely implies constancy with time.
Diffusive and no diffusive environmental transport processes:
Flux = transport rate
Diffusive transfer processes:
 Volatilization form soil to air
 Volatilization form water to air
 Absorption or adsorption by sediments from water
 Diffusive uptake from water by fish during respiration
For diffusion, the net rate of transfer or flux is written as the product of the departure from equilibrium
and a kinetic quantity, and the net flux becomes zero when the phases are in equilibrium.
Nondiffusive processes:
 Fallout of chemical from air to water or soil in dustfall, rain or snow
 Deposition of chemical from water to sediments in association with suspended matter which
deposits on the bed of sediment
 The reverse processes of resuspension
 Ingestion and egestion of food containing chemical by biota
For nondiffusive processes, the flux is the product of the volume of the phase transferred (e.g. quantity
of sediment or rain) and the concentration.
Residence times and persistence: Comparison of the residence time with a chemical reaction time
(e.g. half-life) indicates whether a chemical is removed from a lake predominantly by flow or by
reaction.
Identifying priority chemicals – factors:
 Quantity
 Persistence
 Bioaccumulation
 Toxicity
 Long-range transport
 Other effects:








Ability to influence atmospheric chemistry
Alteration in pH (e.g. oxides of sulfur and nitrogen causing acid rain)
Unusual chemical properties such as chelating capacity, which alters the availability of other
chemicals in the environment
Interference with visibility
Odor
Color
The ability to cause foaming rivers
Formation of toxic metabolites or degradation products
Key chemical properties and classes
It transpires that we can learn a great deal about how a chemical partitions in the environment from its
behavior in an air-water-octanol (strictly 1-octanol) system. There are three partition coefficients: KAW,
KOW and KOA, only two of which independent, since KOA = KOW/KAW. These can be measured directly
or estimated from vapor pressure, solubility in water, and solubility in octanol, but not all chemicals
have measurable solubilities because of miscibility. Octanol is an excellent surrogate for natural
organic matter in soils and sediments, lipids, or fats, and even plant waxes. It has approximately the
same C:H:O ratio as lipids. Correlations are thus developed between soil-water and octanol-water
partition coefficients, as discussed in more detail.
An important attribute of organic chemicals is the degree to which they are hydrophobic. This implies
that the chemical is sparingly soluble in, or “hates”, water and prefers to partition into lipid, organic, or
fat phases. A convenient descriptor of this hydrophobic tendency is KOW. A high value of perhaps one
million (as applies to DDT) implies that the chemical will achieve a concentration in an organic
medium approximately a million times that of water with which it is in contact. In reality, most organic
chemicals are approximately equally soluble in lipid or fat phases, but they vary greatly in their
solubility in water. Thus, differences in hydrophobicity are largely due to differences of behavior in, or
affinity for, the water phase, not differences in solubility in lipids.
The chemical’s tendency to evaporate or partition into the atmosphere is primarily controlled by its
vapor pressure, which is essentially the maximum pressure that a pure chemical can exert in the gas
phase or atmosphere, It can be viewed as the solubility of the chemical in the gas phase. Indeed:
vaporpressure( Pa)
 soluility (mol / m 3 )
3
R( Pa  m / mol  K )  T ( K )
Organic chemicals vary enormously in their vapor pressure and correspondingly in their boiling point.
Some that are present in gasolines are very volatile, whereas others have exceedingly low vapor
pressures.
Partitioning from a pure chemical phase to the atmosphere is controlled by vapor pressure. Partitioning
from aqueous solution to the atmosphere is controlled by KAW, a joint function of vapor pressure and
solubility in water. A substance may have a high KAW, because its solubility in water is low.
Partitioning from soils and other organic media to the atmosphere is controlled by KAO (air/octanol),
which is conventionally reported as its reciprocal, KOA. Partitioning from water to organic media,
including fish, is controlled by KOW. Substances that display a significant tendency to partition into the
air phase over other phases are termed ‘volatile organic chemicals’ or VOCs. They have high vapor
pressures.
Another important classification of organic chemicals is according to their dissociating tendencies in
water solution. Some organic acids, notably phenols, will form ionic species (phenolates) at high pH.
The tendency to ionize is characterized by the acid dissociation constant KA, often expressed as p KA,
its negative base ten logarithm.
In concert with partitioning characteristics, the other set of properties that deterime environmental
behavior is reactivity or persistence, usually expressed as a half-life. It is misleading to assign a single
number to a half-life, because it depends on the intrinsic properties of the chemical and on the nature of
the environment. Factors such as sunlight intensity, hydroxyl radical concentration, the nature of the
microbial community, as well as temperature vary considerably from place to place and time to time.
Here, we use semiquantitative classification of half-lives into classes, assuming that average
environmental conditions apply. Different classes are defined for air, water, soils, and sediments. The
half-lives are on a logarithmic scale with a factor of approximately 3 between adjacent classes. It is
probably misleading to divide the classes into finer groupings; indeed, a single chemical may
experience half-lives ranging over three classes, depending on environmental conditions such as
season.
The nature of environmental media
The fate of a chemical depends on two groups of properties: those of the chemical and those of the
environment in which it resides. It transpires that it is convenient to define two evaluative
environments. First is a simple four-compartment system that is easily understood and illustrates the
application of the general principles of environmental partitioning. Second is a more complex, eightcompartment system that is more representative of real environments.
1. The Atmosphere
1.1. Air: The layer of the atmosphere that is in most intimate contact with the surface of the Earth
is the troposphere, which extends to a height of about 6-10km. We can also assume uniform
density at a pressure of one atmosphere.
1.2. Aerosols: These particles may range in size and composition from water in the form of fog or
cloud droplets to dust particles from soil and smoke from combustion. We can assume that the
particles have a density of 1.5 g/cm3 and are present at a concentration of 30 μg/m3.
1.3. Deposition Processes: Aerosols particles have a very high surface area and thus absord or
adsorb many pollutants, especially those of very low vapor pressure. Chemicals associated
with aerosol particles are subject of two important deposition processes. First is dry deposition,
in which the aerosol particle falls under the influence of gravity to the Earth’s surface. This
deposition velocity is quite slow and depends on the turbulent condition of the atmosphere, the
size and properties of the aerosol particle, and the nature of the ground surface. Second, the
particles may be scavenged or swept out of the air by wet deposition with raindrops or snow.
2. The Hydrosphere or Water
2.1. Water: Only near-surface water is accessible to pollutants in the short term.
2.2. Particulate Matter: Particulate matter consists of a wide variety of materials. It contains
mineral matter, which may be clay or silica in nature. It also contains dead or detrital organic
matter, which is often referred to as ‘humin’, ‘humic acids’, and ‘fulvic acids’ or, more
vaguely, as ‘organic matter’. It is relatively easy to measure the total concentration of organic
carbon (OC) in water or particles by converting the carbon to carbon dioxide and measuring
the amount spectroscopically. Alternatively, the solids can be dried to remove water, then
heated to ignition temperatures to burn off organic matter. The loss is referred to as ‘loss on
ignition’ (LOI) or as ‘organic matter’ (OM). Thus, there are frequent reports of the amount of
dissolved organic carbon (DOC) or total organic carbon (TOC) in water. These humic and
fulvic acids are organic materials of variable composition that probably originate from the
ligneous material present in vegetation. They contain a variety of chemical structures including
substituted alkane, cycloalkane, and aromatic groups, and they have acidic properties imparted
by phenolic or carboxylic acids. They are, therefore, fairly soluble in alkaline solution in which
they are present in ionic form, but they may be precipitated under acidic conditions. The
operational difference between humic and fulvic acids is the pH at which precipitation occurs.
It is also important to discriminate between organic matter (OM) and organic carbon (OC).
Typically, we will assume that OM contains 50% OC and that the density of both OM and OC
is equal to that of the water.
2.3. Fish and aquatic biota: Fish tend to bioconcentrate or bioaccumulate metals and organic
chemicals from water. For illustrative purposes, we can assume that all biotic material in water
is fish; and it proves useful later to define a lipid or fat content of fish, a figure of 5% volume
being typical. In shallow or near-shore water, there may be a considerable quantity of aquatic
plants or macrophytes. These plants provide a substrate for a thriving microbial community,
and they posses inherent sorptive capacity. Their importance is usually underestimated.
Because of the present limited ability to quantify their sorptive properties, we ignore them
here.
2.4. Deposition Processes: The particle matter in water is important, because, like aerosols in the
atmosphere, it serves as a vehicle for the transport of chemical from the bulk of the water to the
bottom sediments. Hydrophobic substances tend to partition appreciably on the particles and
are thus subject to fairly rapid deposition. Some of the deposited particulate matter is
resuspended from the bottom sediment through the action of currents, storms, and the
disturbances caused by bottom-dwelling fish and invertebrates.
3. Bottom sediments
3.1. Sediment Solids: Inspection of the state of the bottom of lakes reveal that there is a fairly
fluffy or nepheloid active layer at the water-sediment interface. This layer typically consists of
95% water and 5% particles and is often highly organic in nature. It may consist of deposited
particles and fecal material from the water column. It is stirred by currents and by the action of
the various biota present in this benthic region. The sediment becomes more consolidated at
greater depths, and the water content tends to drop toward 50%. The top few centimeters of
sediment are occupied by burrowing organisms that feed on the organic matter (and on each
other) and generally turn over (bioturbate) this entire “active layer” of sediment. Deoending on
the condition of the water column above, this layer may be oxygenated (aerobic or oxic) or
depleted of oxygen (anaerobic or anoxic). This has profound implications for the fate of
inorganic substances such as metals and arsenic, but it is relatively unimportant for organic
chemicals except in that the oxygen status influences the nature of the microbial community,
which in turn influences the availability of metabolic pathways for chemical degradation.
Bottom sediments serve as the depositories for much of the toxic material discharged into
water.
3.2. Deposition, Resuspension and Burial: Chemicals present in sediments are primarily removed
by degradation, burial, or resuspension back to the water column. For illustrative purposes we
adopt a sediment depth of 3cm and suggest that it consists of 67% water and 33% solids, and
these solids consist of about 10% organic matter or 5% organic carbon. Living creatures are
included in this figure. Some of this deposited material is resuspended to the water column,
some of the organic matter is degraded (i.e. used as a source of energy by benthic or bottomliving organisms), and some is destined to be permanently buried. The low 5% organic carbon
figure for deeper sediments compared to high 17% for the depositing material implies that
about 75% of the organic carbon is degraded. It is now possible to assemble an approximate
mass balance for the sediment mineral matter (MM) and organic matter (OM) and thus organic
carbon (OC).
4. Soils
4.1. The Nature of Soil: Soil is a complex organic matrix consisting of air, water, mineral matter
(notably clay and silica), and organic matter. The surface soil is subject to diurnal and seasonal
temperature changes and to marked variations in water content, and thus in air content. The
organic matter in the soil plays a crucial role in controlling the retention of the water and thus
in ensuring the viability of plants. Soils vary enormously in their composition and texture and
consist of various layers, or horizons, of different properties. There is transport vertically and
horizontally by diffusion in air and in water, flow, or advection in water and, of course,
movement of water and nutrients into plant roots and thence into stems and foliage. Burrowing
animals such as worms can also play an important role in mixing and transporting chemicals in
soil.
4.2. Transport in Soils: In most areas, there is a net movement of water vertically from the surface
soil to greater depths into a pervious layer of rock or aquifer through which groundwater flows.
This water tends to move very slowly through the porous sub-surface strata. Chemicals
associated with groundwater generally move more slowly than the velocity of the groundwater.
They are retarded by sorption to the soil to an extent expressed as a ‘retardation factor’, which
is essentially the ratio of the amount of chemical that is sorbed to the solid matrix to the
amount that is in solution. Sorption of organic chemicals is usually accomplished preferentially
to organic matter; however, clays also have considerable sorptive capacity, especially when
dry. Polar and especially ionic substances may interact strongly with mineral matter.
4.3. Terrestrial Vegetation: We ignore terrestrial vegetation because of how difficult it is to
calculate the partitioning of chemicals into plants. However, plants play a critical role in
stabilizing soils and in inducing water movement from soil to the atmosphere, and they may
serve as collectors and recipients of toxic chemicals deposited or absorbed from the
atmosphere. They can also degrade certain chemicals and increase the level of microbial
activity in the root zone, thus increasing the degradation rate in the soil.
Phase Equilibrium
1. The Nature of Partitioning Phenomena: The aim is to answer the question, “Given a
concentration in one phase, what will be the concentration in another phase that has been in contact
with it long enough to achieve equilibrium?”. It transpires that two approaches can be used to
develop equations relating equilibrium concentration s to each other. The simplest and most widely
used is Nernst´s Distribution law, which postulates that the concentration ratio C1/C2 is relatively
constant and is equal to a partition or distribution coefficient K12. K12 presumably can be expressed
as a function of temperature and, if necessary, of concentration. Experimentally, mixtures are
equilibrated, and concentrations measured and plotted. Linear on nonlinear equations then can be
fitted to the data. The second approach involves the introduction of an intermediate quantity, a
criterion of equilibrium, which can be related separately to C1 and C2. Chemical potential, fugacity,
and activity are suitable criteria, with fugacity being preferred for most organic substances because
of the simplicity of the equations that relate fugacity to concentration. The advantage of the
equilibrium criterion approach is that properties of each phase are treated separately using a phasespecific equation. Treating phases in pairs, as is done with partition coefficients, can obscure the
nature of the underlying phenomena. We may detect a variability in K12 and not know from which
phase the variability is derived. Further complications arise if we have 10 phases to consider. There
are then 90 possible partition coefficients, of which only 9 are independent. Mistakes are less likely
using an equilibrium criterion and the 10 equations relating it to concentration, one for each phase.
It is useful to discriminate between partition coefficients and distribution coefficients. Although
usage varies, a partition coefficient is strictly the ratio of the concentrations of the same chemical
species in two phases. A distribution coefficient is a ratio of total concentrations of all species.
Thus, if a chemical ionizes, the partition coefficient may apply to the unionized species, while the
distribution coefficient applies to ionized and nonionized species in total.
2. Fugacity: The underlying principle of phase equilibrium thermodynamics is that, when a solute
acheives equilibrium between phases such as air, water, and fish, it seeks to stablish an equal
chemical potential in all phases. Unfortunately, chemical potentially is logarithmically related to
concentration. A further complication is that a chemical potential cannot be measured absolutely,
therefore it is necessary to establish some standard state at which it has a reference value. It was
then when a new equilibrium criterion was introduced, fugacity (fleeing or escaping tendency),
which has units of pressure and is assigned the symbol f. It is identical to partial pressure in ideal
gases and is logarithmically related to chemical potential; thus, it is linearly o nearly linearly related
to concentration. Absolute values can be established because, at low partial pressures under ideal
conditions, fugacity and partial pressure become equal. Thus, we can replace the equilibrium
criterion of chemical potential by that of fugacity. When the solute migrates between water and air,
it is seeking to establish an equal fugacity in both phases; its escaping tendency, or pressures, are
equal in both phases. Another useful quantity is the ratio of fugacity to some reference fugacity
such as the vapor pressure. This is a dimensionless quantity and is termed “activity”. Activity can
also be used as an equilibrium criterion.
3. Properties of Solutes in Solution: At low concentrations, a substance’s fugacity and concentration
are linearly related:
CZ f
C: concentration (mol/m3)
f: fugacity (Pa)
Z: fugacity capacity (proportionality constant) (mol/m3·Pa)
The aim is to deduce Z for the substance in air, water, and other phases.
Z depends on:
– the nature of the solute (i.e., the chemical)
– the nature of the medium or compartment
– temperature
– pressure (but the effect is usually negligible)
– concentration (but the effect is negligible at low concentrations)
3.1. Solution in the Gas Phase: The fugacity equation as presented in thermodynamics texts is:
y: mole fraction
f  y    PT
Ф: fugacity coefficient
PT: total (atmospheric) pressure
P = y . PT : partial pressure
Gas law: PT  V  n  R  T or P  V  y  n  R  T
n: total number of moles present
R: gas constant
V: volume (m3)
T: absolute temperature (K)
Now the contratation of the solute in the gas phase will be:
y  PT
yn
P
1
f
CA 



  ZA  f
V
R T
R T
R T 
Fortunately, fugacity coefficient Ф rarely deviates appreciably from unity under environmental
conditions. The exceptions occur at low temperatures, high pressures, or when the solute
molecules interact chemically with each other in the gas phase. Therefore,
1
R T
The fugacity is thus numerically equal to the partial pressure of the solute P or y.PT. This raises
the question as to why we use the term fugacity in preference to partial pressure. The answers
are that (1) under conditions when Ф is not unity, fugacity and partial pressure are not equal,
and (2) there is somre conceptual difficualty about refering to a “partial pressure of DDT in a
fish” when there is no vapor present for a pressure to be present in-even partially.
ZA 
3.2.Solution in Liquid Phases: The fugacity equation for solute i in solution is:
xi: mole fraction of solute
f i  xi   i  f R
γi: activity coefficient
fR: reference fugacity on a Raoult´s law basis
The reference fugacity fR is by definition the fugacity that the solute will have (or tend to) when
in the pure liquid state when xi is 1 and γi is also (by definition) 1. This, then, is the figacity or
vapor pressure of pure liquid solute at the temperature (and strictly the pressure) of the system.
Now, xi can be converted to concentration C (mol/m3) using molar volumes v (m3/mol),
amounts n (mol), and volumes V (m3) of solute (subscript i) and solution (subscript w for
water): xi  Ci  vW . Thus,
1
Ci 
 f i  ZW  f
vW   i  f R
Hence,
1
ZW 
vW   i  f R
ZW can also be expressed in terms of aqueos solubility, SW, and vapor pressure PS:
S
1
1
 For liquid solutes: S W 
and f R  P S  Z W  WS 
 I  vW
H
P
S F S
F
PS
1
 For solid solutes: S W 
and f R 
 Z W  W S  WS 
 I  vW
F
H
FP
P
3
H: Henry`s law constant (Pam /mol)
These equations are general and apply to a nonionizing chemical in solution in any liquid
solvent, including water and octanol. The solution molar volumesand the activity coefficients
vary from solvent to solvent. The Z value for a chemical in octanol is, by analogy,
1
ZO 
vO: molar volume of octanol
vO   i  f R
3.3.Solutions of Ionizing Substances: Certain substances, when present in solution, adopt an
equilibrium distribution between two or more chemical forms. Examples are acetic acid,
ammonia, and pentachlorophenol, which ionize by virtue of association with water releasing H+
(strictly H3O+) or OH- ions. Some substances dimerize or form hydrates. For ionizing
substances, the distribution is pH dependent, thus the solubility and activity are also pH
dependent. This could be accommodated by defining Z as being applicable to the total
concentration, but it then becomes pH dependent. A more rigorous approach is to define Z for
each chemical species, noting that, for ionic species, Z in air must be zero under normal
conditions, because ions as such do not evaporate. In any event, it is useful to know the relative
proportions of each species, because they will partition differently. This issue is critical for
metals in which only a small fraction may be in free ionic form.
For acids, an acid dissociation constant Ka is defined as:
H  A 
Ka 
H+: hydrogen ion concentration
HA
A-: dissociated anionic form
HA: parent undissociated acid
A  Ka
The ratio of ionic to nonionic forms I is thus, I 

 10 ( pH  pKa )
HA H 
where: pH   log H  and pK a   log K a 
This is the Henderson-Hasselbalch relationship. For acids, when pKa exceeds pH by 2 units or
more, ionization can be ignored. When considering substances which have the potential to
ionize it is essential to obtain pKa and determine the relative proportion of each form.
Dissociation can be regarded as causing an increase in the Z value of a substance in aqueous
solution. The total Z value is the sum of the nonionic and ionic contributions, which will have
1
I
respective fractions
and
. The Z value of the nonionic form ZW can be calculated
( I  1)
( I  1)
by measuring solubility, activity coefficient, or another property under conditions when I is
very small, i.e., pH<<pKa. The same ZW value applies to the nonionic form at all pH levels.
The additional contribution of the ionic form is then calculated at the pH of interest as IZW, and
the total effective Z value is ZW (I+1), which can be used to calculate the total concentration.
An inherent assumption here is that the presence of the ionic form does not affect ZW.
3.4. Solutions in Solids: It is not usual to regard substances as having solubilities in solids. If the
structure of the solid and the size of the solute molecule are such that the molecule can diffuse
into and out of the solid matrix in a reasonable time period, then a solubility can be defined and
measured. The organic matter discussed in 2.2 is solid. The sorption phenomenon can be
regarded as simply partitioning into solid solution in this organic matter matrix. Solubilities
and Z values can thus be calculated for solutions in solids.
4. Partition Coefficients
4.1. Fugacity and Solubility Relationships: If we have two immiscible phases or media (e.g. air
and water or octanol and water), we can conduct experiments by shaking volumes of both
phases with a small amount of solute such as benzene to achieve equilibrium, then measure the
concentrations and plot the results as was shown in “the nature of partitioning phenomena”. It
is preferable to use identical concentration units in each phase of amount per unit volume but,
when one phase is solid, it may be more convenient to express concentration in units such as
amounts per unit mass (e.g. μg/g) to avoid estimating phase densities. The plot of the
concentration data is often linear at low concentrations; therefore, assuming an equilibrium
condition, we can write:
C
Z f
Z
K 21  2  2 2  2
C1 Z 1  f 1 Z 1
The line may extend until some solubility limit or “saturation” is reached. In water, this is the
aqueous solubility, but, for some substances such as lower alcohols, there is no “solubility”,
because the solute is miscible with water. In air, the “solubility” is related to the vapor pressure
of the pure solute, which is the maximum partial pressure that the solute can achieve in the air
phase.
Partition coefficients are widely available and used for systems of air-water, aerosol-air,
octanol-water, lipid-water, etc. Applying the theory that was developed earlier and noting that,
at equilibrium, the solute fugacities will be equal in both phases, we can define partition
coefficients for all theses systems.

Air-Water Partitioning:
f 
f W i  f Ai
xi   i  f R  y i  PT  Pi
yi  i  f R

xi
PT
y i  CiA  v A
xi  C iW  vW
( fR  PS )
C iA  v A  i  P S

C iW  vW
PT
So we can write,
C
  P S vW  i  P S  vW S iA
K AW  iA  I



C iW
PT
vA
R T
S iW
V
Where:
(vA  )
PT  v A  R  T
n
And we can also write,
C
  f v
H
K AW  iA  I R W 
CiW
R T
R T
H
PS L
CSL
Hence,
K AW 
Z A S iA
H


Z W S iW R  T
The simplest method of estimating Henry’s law constant of organic solutes is as a ratio of vapor
pressure to water solubility. It must be recognized that this contains the inherent assumption
that water is not very soluble in the organic material, because the vapor pressure that is used is
that of the pure substance (normally the pure liquid) whereas, in the case of solubility of a liquid
such as benzene in water, the solubility is not actually that of pure benzene but is inevitably of
benzene saturated in water. When the solubility of water in a liquid exceeds a few percent, this
assumption may break down, and it is unwise to use this relationship. If a solute is miscible
with water (e.g. ethanol), it is preferable to determine the Henry’s law constant directly; that is,
by measuring air and water concentrations at equilibrium. A desirable strategy is to measure
vapor pressure PS, solubility CS (CS = molar mass x solubility), and H or KAW and perform an
internal consistency check that H is indeed PS/CS or close to it. KAW is, of course, ZA/ZW.Care
must be taken when calculating Henry’s law constants to ensure that the vapor pressures and
solubilities apply to the same temperature and to the same phase.

Octanol-Water Partitioning: liquid-liquid
f 
f W i  f Oi
And,
CiO
CiW
xiO   iO  f R  xiO   iO  f R
xiO  iW

xiW  iO
 v
S
 K OW  iW W  iO
 iO  vO S iW
If the solute is solid the same final equation applies because F, like fR, cancels.
Because vW and vO are relatively constant, the variation in KOW between solutes is a reflection

of variation in the ratio of activity coefficients iW . Hydrophobic substances have very large
 iO
values of  iW and low solubilities in water. The solubility in octanol is usually fairly constant
for organic solutes, thus KOW is approximately inversely proportional to SiW.
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/m3). The solubility of
water in octanol of 2300 mol/ m3, however, is quite large. The molar volumes of these phases

are 18x10-6 m3/mol and 120 x10-6 m3/mol, a ratio of 0.15, It follows that K OW  0.15  w
O
KOW is a measure of hydrophobicity, i.e., the tendency of a chemical to “hate” or partition out
of water. 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/ m3, thus variation in KOW between chemicals is primarily due to variation in water
Z
solubility. Viewed in terms of Z values, K OW  O . ZO is (relatively) constant for organic
ZA
chemicals; however, ZW varies greatly and is very small (relatively) for hydrophobic
substances. Because KOW varies over such a large range, from approximately 0.1 to 107, it is
common to express it as log KOW.
KOW is usually measured by equilibrating layers of water and octanol containing the solute of
interest at subsaturation conditions and analyzing both phases. If KOW is high, the concentration
in water in necessarily low, and even a small quantity of emulsified octanol on the aqueous
phase can significantly increase the apparent concentration.

Octanol-Air Partition Coefficients
The octanol-air partition coefficient can be shown to be:
S
R T
K OA  iO 
(where γi applies to the octanol phase)
S iA  i  vO  P S L
Z
1
K OA  O
where Z O 
ZA
 i  vO  P S L
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 KOW/KOW or measured directly, usually by flowing air through a column
containing a packing saturated with octanol with the solute in solution.
Advection and Reactions
Advection: It means the directed movement of chemical by virtue of its presence in a medium that
happens to be flowing. The rate of advection (N) is:
G: flowrate of the advecting medium (m3/h)
N  GC
C: concentration of chemical in that medium (mol/ m3)
Turning to the evaluative environment, it is apparent that the primary candidate advective phases are air
and water. Burial of bottom sediments can also be regarded as an advective loss, as can leaching of
water from soils to groundwater.
Advective Processes:
▪ inflow and outflow of air
▪ inflow and outflow of water
▪ inflow and outflow of aerosol particles present in air
▪ inflow and outflow of particles and biota present in water
▪ transport of air from the troposphere to the stratosphere
▪ sediment burial
▪ flow of water from surface soils to groundwater
D values: The group (G · Z), and other groups like it, appear so frequently in later calculations that is is
convenient to designate them as D values (mol/Pa·h):
D  GZ
Of which we can deduce:
N  D f
Theses D values are transport parameters. When multiplied by a fugacity, they give rates of transport.
They are thus similar in principle to rate constants, which, when multiplied by a mass of chemical, give
a rate reaction. Fast processes have large D values. We can write the fugacity equation for the
evaluative environment in more compact form, as shown below:
I
I
f 

DAA  DAW   DAi
where,
DAA  GA  Z A
DAW  GW  ZW
The first subscript A refers to advection.
Diffusive Processes: Here are some examples:
▪
▪
▪
▪
▪
▪
▪
evaporation of chemical from water to air and the reverse process of absorption (note that
we consider the chemical to be in solution in water and not present as a film or oil slik, or in
sorbed form)
sorption from water to suspended matter in the water column, and the reverse desorption
sorption from the atmosphere to aerosol particles, and the reverse desorption
sorption of chemical from water to bottom sediment, and the reverse desorption
diffusion within soils, and from soil to air
absorption of chemical by fish and other organisms by diffusion through the gills, following
the same route traveled by oxygen
transfer of chemical across other membranes in organisms, for example, from air through
lung surfaces to blood, or from blood to organs in the body