CE 529 Hazardous Waste Management
Properties of Hazardous Materials
Dr. S.K. Ong
1. Octanol-Partition Coefficient (Kow)
-
-
Kow is used as a measure of the hydrophobicity of an organic compound, i.e., it represents the tendency
of the chemical to partition itself between an organic phase (e.g., lipids (fat tissue) in fish, or humic
materials in soil) and an aqueous phase.
Chemicals with low Kow is relatively hydrophilic and chemicals with high Kow (> 104) are very
hydrophobic). Values of Kow ranged from 0.001 to 100,000,000.
the Kow was first developed in the pharmaceutical industry
has become a key parameter in studies of environmental fate of organic chemicals
found to be related to water solubility, soil/sediment sorption coefficients, bioconcentration factors
(BCF)
is defined as the ratio of a chemical’s concentration in the octanol phase to its concentration in the
aqueous phase
Kow =
concentration in water saturated n-octanol (mg/L)
concentration in n-octanol saturated water (mg/L)
Estimation Methods
Methods
Information required
Comments
Fragment constants
and structural factors
structure of compound or
Kow of structurally related
Compound
- Fairly accurate
- Wide range of applicability
- Widely used
Regression equations
Ksw (solvent/water partition
coefficients)
- Easy and rapid calculations
- Fairly accurate
S – solubility in water
- Less accurate
- Wide range of applicability
structure
- Calculations lengthy and difficult
- Limited applicability by functional groups
- Fairly accurate
Estimated Activity
Coefficients
References
Leo, A., C. Hansch and D. Elkins, Partition Coefficients and Their Uses, Chem. Rev., 71, 525-621 (1971)
Handbook of Chemical Property Estimation Methods, Lyman et al.
2. Solubility in Water
-
the maximum or saturation amount of the chemical that will dissolve in pure water at a specified
temperature. Above this concentration, two phases will exist if the chemical is a solid or liquid at the
system temperature: a saturated aqueous solution and a solid or liquid organic phase.
The organic phase is usually called non aqueous phase liquid (NAPL). For NAPLs that are lighter than
water, they are called Light NAPLs (LNAPLs) and if they are heavier than water they are called Dense
NAPLs or DNAPLs.
Inorganic Chemicals
-
Solubility is dependent on the temperature, pH, ionic strength, size and structure of the chemical and
the presence of other chemical species.
Generally, the solubility product, Ksp, is used to relate the solubility of the chemical (cations and
anions) in water. For example, the cadmium compound present controls Cd concentration in water:
CdSO4 < == > Cd2+ + SO42Ksp = [Cd2+][SO42-]
The concentration of Cd in water is influenced by the common ion effect and by the presence of other
chemical species that may form complexes with Cd in water, therefore increasing its concentration in
water. For example:
Cd2+ + CN- < == > Cd(CN)+ + CN- < == > Cd(CN)2o
Organic Compounds
Water solubility of a compound is related to various factors. Some of them are:
(i) structure and presence of functional groups such as – OH, -COOH, -CO, - NH2, – NO2, - Cl, - Br,
-CHO, - CN. Addition of hydrophilic groups such as OH, COOH will increase the solubility of the
compound
Examples
Hexane has a solubility of 13 mg/L, addition of a –OH group to the compound, i.e., hexanol, results in
a solubility of 5,900 mg/L.
Addition of halogens to the compound decreased the water solubility of the compound
(ii) size of the compound – increase in size, solubility decreased,
Example, Naphthalene (2 rings) solubility is 32 mg/L, pyrene (4 rings) solubility is 0.13 mg/L
Estimation of solubility of organic compounds
Methods
Information required
Comments
Regression Method
Kow, Tm
Kow easily available and can be estimated
Simple calculations
Method of Irmann
Structure of compound
for hydrocarbons and halocarbons
(chemicals with C, H, Cl, Br, I, F only)
Limited applications
Theoretical equations
Using estimated
activity coefficients
Structure, Hf, Tm
Allows calculations at any temperature
Calculations may be difficult
May be more accurate than the other
methods
Limited application
_____________________________________________________________________________________
Reference: Handbook of Chemical Property Estimation Methods, Lyman et al.
3. Vapor Pressure
- pressure exerted by a pure chemical on the atmosphere
- provides a measure of the volatility of the compound
- usually expressed in units of mm Hg or atmosphere or bars.
4. Henry’s Law Constant
-
In a closed aqueous system containing a dilute contaminant, an equilibrium exists between the
concentration in solution and the concentration in the overlying gaseous phase.
Henry’s law states that the concentration of a compound in the aqueous phase is directly proportional
to the partial pressure in the gaseous phase:
P = H CL
where
P = partial pressure (atm)
H = Henry’s law constant (atm-m3/mole)
CL = concentration of the compound in water (mole/m3)
May be written in a dimensionless form:
CG = H’ CL
Where
CG = concentration of the compound in air (moles/m3 or mg/L)
H = dimensionless Henry’s law constant
To convert from H to a dimensionless H’, use the following equation:
H’ = H/RT
where
R = 8.25 x 10-5 atm.m3/mole K
T = temperature K
Henry’s law constant is dependent on temperature. The empirical equation is:
H = e (A-B/T)
where A and B are regression coefficients.
5. Diffusion Coefficients
Diffusion is the movement of a contaminant in a medium under the influence of a concentration
gradient. Diffusion is modeled using Fick’s law
J = - D (C/x)
where
J = flux (mole/cm2sec)
D = diffusion coefficient (cm2/sec)
C = concentration (mole/cm3)
x = length (cm)
Wilke and Chang Equation to estimate diffusivity of solute in bulk liquid
D L ,AB  B
T
Where
DL,AB
µB
T
MB
Vb
=
=
=
=
=

7.48 x10 8 (  B M B )1 / 2
Vb0.6
(1)
liquid diffusivity of solute A in solvent B (cm2/s)
viscosity of solvent (centipoise, cP)
absolute temperature (K)
molecular weight of solvent B
molal volume of solute at normal boiling temperature (cm3/mole)
B
=
association parameter for solvent B
water = 2.6, methanol = 1.9, benzene = 1.0
Hirschfelder, Bird and Spotz equation to estimate diffusivity of solute in gas
(nonpolar system)
DG, AB 
0.001858T 3/ 2 ( M1 A 
)
(2)
P D
2
AB
DG,AB
T
MA
MB
P
 AB
ΩD

1 1/ 2
MB
=mass diffusivity of solute A in solvent B (cm2/s)
=absolute temperature (K)
=molecular weight of A
=molecular weight of B
=absolute pressure (atm)
=collision diameter (Å)
=collision integral for molecular diffusion (a measure of the intermolecular
potential field of molecule A and molecule B)
can be estimated using the following equations
  1.18Vb1 / 3  0.841Vc1 / 3  2.44( PC )1 / 3
T
(3)
C
where
Vb = molecular volume at normal boiling point (cm3/g-mole)
Vc = critical molecular volume
Tc = critical temperature (K)
Tb = normal boiling temperature (K)
Pc = critical pressure (atm)
ΩD is a function of
where
T
 AB
 is the Boltzmann constant (1.38 x 10-16 ergs/K)
 AB = energy of molecular interaction (ergs)
See Tables for values of ΩD as a function of
T
 AB
or estimated from the following regression eqn.
 D  1.442  0.6915 ln( T )  0.2536[ln( T )] 2  3.01x10 2 [ln( T )] 3  4.966 x10 3 [ln( T )] 4
AB
A / 
AB
AB
can be estimated using the following equation
 A /  = 0.77 Tc
= 1.15 Tb
for a binary system
 AB 
 AB
 A  B
2
  A B
 AB
A B


 
AB