Pollutants and environmental compartments

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Pollutants and environmental compartments

• 1(ii)
• Physico-chemical properties of pollutants and
  their influence on their behaviour in the
  environment

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Aims

• To provide overview of molecular properties of
  pollutants in the environment
• Vapour pressure theoretical background,
  molecular interactions governing vapour pressure,
  availability of experimental vapour pressure data
  and estimation methods
• Activity coefficient and solubility in water
  thermodynamic consideration, effect of
  temperature and solution composition on aqueous
  solubility and activity coefficients,
  availability of experimental data and estimation
  methods

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Outcomes

• Students will be able to
• estimate relevant physico-chemical properties of
  pollutants from their structure
• predict reactivity of pollutants and possible
  environmental behavior of pollutants

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Vapour pressure

• Definition
• Pressure of a substance in equilibrium with its
  pure condensed (liquid or solid) phase pº
• Why is it important?
• Air/water partitioning
• Air/solid partitioning
• When is it important?
• Spills
• Pesticide application

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• Ranges of pº (atm)
• PCBs 10-5 to 10-9
• n-alkanes 100.2 to 10-16
• n-C10H22 10-2.5
• n-C20H42 10-9
• benzene 10-0.9
• toluene 10-1.42
• ethylbenzene 10-1.90
• propylbenzene 10-2.35
• carbon tetrachloride 10-0.85
• methane 102.44
• Even though VP is low, gas phase may still be
  important.

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• Phase diagram and aggregate state

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• Thermodynamic considerations(deriving the vant
  Hoff equation)
• In equilibrium the change in chemical potential
  in the two systems is equal

where S molar entropy and V molar volume
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Liquid-vapor equlibrium

• For a liquid vaporizing, the volume change can be
  assumed to be equal to the volume of gas
  produced, since the volume of the solid or liquid
  is negligible

where ?H12 ?Hvap (gas) or ?Hsub (solid)
energy required to convert one mole of liquid (or
solid) to gas without an increase in T
The vant Hoff equation
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• Integration assuming DHvap is constant over a
  given temperature range leads to
• If the temperature range is enlarged DHvap is not
  constant

Antoine equation
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Solid-vapor equilibrium

• For sublimation
• DHsub DHmelt (25) DHvap (75)
• Still use liquid phase as reference
• Hypothetical subcooled liquid liquid cooled
  below melting point without crystallizing

-log p -log p
compound pºs lt Pºl
1,4-dichlorobenzene 3.04 2.76
phenol 3.59 3.41
2255 PCB 7.60 6.64
22455 PCB 8.02 7.40
Important for solubility
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Molecular interactions affecting vapor pressure

• Moleculemolecule interactions in condensed phase
  (l or s) have greatest affect on VP
• strong interactions lead to large DHvap, low VP
• weak interactions lead to small DHvap, high VP
• Intermolecular interactions can be classified
  into three types
• van der Waals forces (nonpolar)
• Polar forces
• Hydrogen bonding

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Vapor Pressure Estimation Technique
based on regression of lots of VP data, best fit
gives
H-bonding ability
size
polarizability
pressure in Pa, where
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H-bonding ability
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Refractive index

• Refractive index (response to light) is a
  function of polarizability

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Troutons rule

• At their boiling points, most organic compounds
  have a similar entropy of vaporization
• exception strongly polar or H-bonding compounds
• Kistiakowskys expression gives slightly more
  accurate predictions
• KF 1 for apolar and many monopolar compounds
• For weakly bipolar compounds (e.g., esters,
  ketones, nitriles), KF 1.04
• Primary amines KF 1.10, phenols KF 1.15,
  aliphatic alcohols KF 1.30
• At Tb
• So, if we know Tb, we can estimate ?Hvap (at the
  boiling point) fairly accurately.

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Estimating vapor pressure at other T

• Important DHvap is not constant.
• Especially if Tb is high (gt 100ºC), the estimate
  of DHvap from Trouton/Kistiakowsky may not be
  valid.
• Empirically, DHvap is a function of the vapor
  pressure

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• From a data set of many compounds, Goss and
  Schwarzenbach (1999) get

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• Less empirically, assume DHvap is linearly
  proportional to T (i.e. assume that the heat
  capacity, ?vapCp is constant)
• Substitution into the Clausius-Clapeyron equation
  and integration from Tb to T gives

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• Substitution
  in previous equation gives
• Generally

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• Inserting Kistiakowskys expression, the
  following equation is obtained
• KF is the Fishtine factor, usually 1, but
  sometimes as high as 1.3
• OK for liquids with Tb lt 100 ºC
• High MW compounds, need correction for
  intermolecular forces

(bar)
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Aqueous Solubility

• Equilibrium partitioning of a compound between
  its pure phase and water
• Will lead us to Kow and Kaw

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Relationship between solubility and activity
coefficient

• Organic liquid dissolving in water
• At equilibrium

for the organic liquid phase
for the organic chemical in the aqueous phase
At saturation!
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• If we assume xiL 1 and giL 1
• The relationship between solubility and activity
  coefficient is
• The activity coefficient is the inverse of the
  mole fraction solubility

for liquids
or
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• Solids
• additional energy is needed to melt the solid
  before it can be solubilized

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• Gases
• solubility commonly reported at 1 bar or 1 atm (1
  atm 1.013 bar)
• O2 is an exception
• the solubility of the hypothetical superheated
  liquid (which you might get from an estimation
  technique) may be calculated as

theoretical partial pressure of the gas at that
T (i.e. gt 1 atm)
Actual partial pressure of the gas in the system
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• Concentration dependence of g
• g at saturation ? g at infinite dilution
• However, for compounds with g gt 100 assume
• g at saturation g at infinite dilution, i.e.
  solute molecules do not interact, even at
  saturation

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Molecular picture of the dissolution process

• The two most important driving forces in
  determining the extent of dissolution of a
  substance in any liquid solvent are
• an increase in entropy of the system
• compatibility of intermolecular forces.

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• Ideal liquids
• For ideal liquids in dilute solution in water,
  the intermolecular attractive forces are
  identical, and ?Hmix 0. The molar free energy
  of solution is
• ?Gs ,?Gmix Gibbs molar free energy of solution,
  mixing (kJ/mol)
• T?Smix Temperature ? Entropy of mixing
  (kJ/mol)
• R gas law constant (8.414 J/mol-K)
• T temperature (K)
• Xf, Xi solute mole fraction concentration
  final, initial
• for dilute solutions mole fraction of solvent ? 1

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• Nonideal liquids
• The intermolecular attractive forces are not
  normally equal in magnitude between organics and
  water
• ?Ge Excess Gibbs free energy (kJ/mol)
• ?He, ?Se Excess enthalpy and excess entropy
  (kJ/mol)
• ?He intermolecular attractive forces cavity
  formation (solvation)
• ?Se cavity formation (size) solvent
  restructuring mixing

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• For small molecules, enthalpy term is small ( 10
  kJ/mol)
• Only for large molecules is enthalpy significant
  (positive)
• Entropy term is generally unfavorable
• Water forms a flickering crystal around the
  compound, which fixes both the orientation of the
  water and of the organic molecule

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Solubility estimation techniques

• Activity coefficients and water solubilities can
  be estimated a priori using molecular size,
  through molar volume (V, cm3/mol). Molar volumes
  can be approximated
• Ni number of atoms of type i in j-th molecule
• ai atomic volume of i-th atom in jth molecule
  (cm3/mol)
• nj number of bonds in j-th molecule (all types)
• Solubility can approximated using a LFER of the
  type

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• This type of LFER is only applicable within a
  group of similar compounds

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• Another estimation technique universal valid
  for all compounds/classes/types

molar volume describes vdW forces
refractive index describes polarity
Vapour pressure
additional polarizability term
cavity term
H-bonding
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Factors Influencing Solubility in Water

• Temperature
• Salinity
• pH
• Dissolved organic matter (DOM)
• Co-solvents

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• Temperature effects on solubility
• Generally
• as T ?, solubility ? for solids.
• as T ?, solubility can ? or ? for liquids and
  gases.
• BUT For some organic compounds, the sign of ?Hs
  changes therefore, opposite temperature effects
  exist for the same compound!
• The influence of temperature on water solubility
  can be quantitatively described by the van't Hoff
  equation as

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• Solids
• Liquids
• Gases

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• The effect of salinity
• As salinity increases, the solubility of neutral
  organic compounds decreases (activity coefficient
  increases)
• Ks Setschenow salt constant (depends on the
  compound and the salt)

typical seawater salt 0.5M
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• The effect of pH
• pH effect depends on the structure of the solute.
  
• If the solute is subject to acid/base reactions
  then pH is vital in determining water solubility.
  
• The ionized form has much higher solubility than
  the neutral form.
• The apparent solubility is higher because it
  comprises both the ionized and neutral forms.
• The intrinsic solubility of the neutral form is
  not affected.

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• The effect of DOM
• DOM increases the apparent water solubility for
  hydrophobic compounds.
• DOM serves as a site where organic compounds can
  partition, thereby enhancing water solubility.
• Solubility in water in the presence of DOM is
  given by the relation
• DOM concentration of DOM in water, kg/L
• KDOM DOM/water partition coefficient
• Again, the intrinsic solubility of the compound
  is not affected.

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• The effect of cosolvents
• the presence of a co-solvent can increase the
  solubility of hydrophobic organic chemicals
• co-solvents can completely change the solvation
  properties of water
• examples
• industrial wastewaters
• gasohol
• engineered systems for soil or groundwater
  remediation
• HPLC

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• Solubility increases exponentially as cosolvent
  fraction increases.
• Need 5-10 volume of cosolvent to see an effect.
• Extent of solubility enhancement depends on type
  of cosolvent and solute
• effect is greatest for large, nonpolar solutes
• more organic cosolvents have greater effect
  propanolgtethanolgtmethanol

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• Bigger, more non-polar compounds are more
  affected by co-solvents
• Different co-solvents behave differently,
  behavior is not always linear
• We can develop linear relationships to describe
  the affect of co-solvents on solubility. These
  relationships depend on the type and size of the
  solute