Download - Pollutants and environmental compartments
Pollutants and environmental compartments
1(ii)Physico-chemical properties of pollutants and their influence on their behaviour in
the environment
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
2Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
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
3Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
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 van’t Hoff equation)
– In equilibrium the change in chemical potential in the two systems is equal :
7
21 dd
dpVdTSd
dpVdTSd
222
111
where S = molar entropy
and V = molar volume
02112 G
12
12
21
21
)(
)(
V
S
VV
SS
dT
dp
12
12
VT
H
dT
dp
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
8
• 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:
gTV
H
dT
dp 12
212
00 )(
RT
Hp
dT
dp
212
0ln
RT
H
dT
pd
The van’t Hoff equation
where H12 = Hvap (gas) or Hsub (solid) = energy required to convert one mole of liquid (or solid) to gas without an increase in T
012 p
RTVV gas
Liquid-vapor equlibrium
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
9
• Integration assuming Hvap is constant over a given temperature range leads to:
• If the temperature range is enlarged Hvap is not constant:
BT
Ap 0ln
aRT
Hp
vap
0ln
BCT
Ap
0ln Antoine equation
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
Solid-vapor equilibrium
• For sublimation:
Hsub = Hmelt (~25%) + Hvap (~75%)
• Still use liquid phase as reference:
– Hypothetical subcooled liquid = liquid cooled below melting point without crystallizing
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-log p
compound pºs < Pºl
1,4-dichlorobenzene 3.04 2.76
phenol 3.59 3.41
22’55’ PCB 7.60 6.64
22’455’ PCB 8.02 7.40
Important for solubility
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
Molecular interactions affecting vapor pressure
• Molecule:molecule interactions in condensed phase (l or s) have greatest affect on VP:
– strong interactions lead to large Hvap, low VP
– weak interactions lead to small Hvap, 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
5.14))((1.152
149.4ln
2
2
23/2*
iiDi
DiiLiL n
nVp
based on regression of lots of VP data, best fit gives:
pressure in Pa, where:
index refractive
y)(MW/densit memolar volu
Di
iL
n
V
sizepolarizability
H-bonding ability
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
H-bonding ability
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compound (class) (H-donor) (H-acceptor)alkanes 0 01-alkenes 0 0.07aliphatic ethers 0 0.45aliphatic aldehydes 0 0.45aliphatic alcohols 0.37 0.48carboxylic acids 0.60 0.45benzene 0 0.14phenol 0.6 0.31naphthalane 0 0.2fluorene 0 0.2pyrene 0 0.29DCM 0.1 0.05Water 0.82 0.35
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
• Refractive index (response to light) is a function of polarizability
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Refractive index
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
Trouton’s rule
• At their boiling points, most organic compounds have a similar entropy of vaporization:
– exception: strongly polar or H-bonding compounds
• Kistiakowsky’s 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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molKJTS bvap /9085
bFbvap TKTS ln31.86.36
vapbvap STHG 0
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
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Estimating vapor pressure at other T
• Important: Hvap is not constant.
• Especially if Tb is high (> 100ºC), the estimate of Hvap from Trouton/Kistiakowsky may not be valid.
• Empirically, Hvap is a function of the vapor pressure:
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21
11ln
1
2
TTR
H
p
p vap
T
T
bTpaTH iLvap )(log)( 1*
1
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
• From a data set of many compounds, Goss and Schwarzenbach (1999) get:
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0.70)298(log80.8)298( * KpKH iLvap
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
• Less empirically, assume Hvap 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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)()()()( TTTCTHTH bbpvapbvapvap
T
T
R
TC
T
T
R
TC
TTR
THp bbpvapbbpvap
b
bvap ln)(
1)(11)(
ln 0
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
• Substitution in previous equation gives:
• Generally:
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)()( bvapbbvap TSTTH
T
T
R
TC
T
T
R
TC
R
TSp bbpvapbbpvapbvap ln
)(1
)()(ln 0
)(8.0)( bvapbpvap TSTC
molKJTS bvap /88
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
• Inserting Kistiakowsky’s 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 < 100 ºC
• High MW compounds, need correction for intermolecular forces
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T
T
T
TTKp bb
bF ln8.018.1)ln4.4(ln 0 (bar)
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
Aqueous Solubility
• Equilibrium partitioning of a compound between its pure phase and water
• Will lead us to Kow and Kaw
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Air
Water
Octanol
A gas is a gas is a gasT, P
Fresh, salt, ground, poreT, salinity, cosolvents
NOM, biological lipids, other solvents T, chemical composition
Pure Phase(l) or (s)
Ideal behavior
PoL
Csatw
Csato
KH = PoL/Csat
w
KoaKH
Kow = Csato/Csat
w
Kow
Koa = Csato/Po
L
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
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Relationship between solubility and activity coefficient
• Organic liquid dissolving in water:
• At equilibrium:
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iLiLiLiL xRT ln* for the organic liquid phase
iwiwiLiw xRT ln* for the organic chemical in the aqueous phase
iLiLiwiwLiiw xRTxRT lnln0
RT
RTRT
x
x satiwiL
iL
satiw lnln
ln
At saturation!
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
• If we assume: xiL = 1 and iL = 1
• The relationship between solubility and activity coefficient is:
– The activity coefficient is the inverse of the mole fraction solubility
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RT
G
RT
RTx
satEiw
satiwsat
iw
,lnln
satiw
satiwx
1
satiww
satiw
VC
1
or for liquids
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
• Solids
– additional energy is needed to melt the solid before it can be solubilized:
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RTGsatiw
satiw
ifusesCLC /)()(
is
iLifus p
pRTG
*
ln
is
iLsatiw
satiw p
psCLC
*
)()(
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
• 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:
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i
iLpiw
satiw p
pCLC i
*
)( Actual partial pressure of the gas in the system
theoretical “partial” pressure of the gas at that T (i.e. > 1 atm)
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
• Concentration dependence of – at saturation at
infinite dilution
– However, for compounds with > 100 assume:
• at saturation = 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)
TSmix = 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 130
i
fmixmixS x
xRTSTGG ln
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
• 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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)( emixeSSS
emixS
SSTHSTHG
GGG
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
• 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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)56.6)(())(( ijijiji naNV
dsizecLC satiw )()(ln
bsizeaLiw )()(ln
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
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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:
36
49.90472.0)(1.11)(77.8
)(78.52
1572.0lnln
2
23/2*
ixii
iDi
DiixiLiw
V
n
nVp
Vapour pressure
molar volume describes vdW forces
refractive index describes polarity
additional polarizability term
H-bonding cavity term
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
37Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
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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CT
H
RCsat
1ln
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
• Solids:
• Liquids:
• Gases:
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CRT
HHsC
Eiwifussat
iw
)(ln
CRT
HLC
Eiwsat
iw )(ln
CRT
HHgC
Eiwivapsat
iw
)(ln
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
• 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)
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totsi saltK
iwsaltiw][
, 10 typical seawater
[salt] = 0.5M
k
kssalti
sseawateri xKK k,,
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
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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.
44Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
• 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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)1(, DOMsatDOMsat KDOMCC
Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
• 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 propanol>ethanol>methanol
47Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment
• 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
48Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment