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


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


• 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.


• Phase diagram and aggregate state


• 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


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


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

10

-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


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


12

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


H-bonding ability

13

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


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

14

Refractive index


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.

15

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 16

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:

17

21

11ln

1

2

TTR

H

p

p vap

T

T

bTpaTH iLvap )(log)( 1*

1


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

18

0.70)298(log80.8)298( * KpKH iLvap


• 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:

19

)()()()( TTTCTHTH bbpvapbvapvap

T

T

R

TC

T

T

R

TC

TTR

THp bbpvapbbpvap

b

bvap ln)(

1)(11)(

ln 0


• Substitution in previous equation gives:

• Generally:

20

)()( bvapbbvap TSTTH

T

T

R

TC

T

T

R

TC

R

TSp bbpvapbbpvapbvap ln

)(1

)()(ln 0

)(8.0)( bvapbpvap TSTC

molKJTS bvap /88


• 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

21

T

T

T

TTKp bb

bF ln8.018.1)ln4.4(ln 0 (bar)


Aqueous Solubility

• Equilibrium partitioning of a compound between its pure phase and water

• Will lead us to Kow and Kaw

22

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


Relationship between solubility and activity coefficient

• Organic liquid dissolving in water:

• At equilibrium:

24

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!


• 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

25

RT

G

RT

RTx

satEiw

satiwsat

iw

,lnln

satiw

satiwx

1

satiww

satiw

VC

1

or for liquids


• Solids

– additional energy is needed to melt the solid before it can be solubilized:

26

RTGsatiw

satiw

ifusesCLC /)()(

is

iLifus p

pRTG

*

ln

is

iLsatiw

satiw p

psCLC

*

)()(


• 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:

27

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)


• 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


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.


• 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


• 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

31

)( emixeSSS

emixS

SSTHSTHG

GGG


• 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


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:

33

)56.6)(())(( ijijiji naNV

dsizecLC satiw )()(ln

bsizeaLiw )()(ln


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


• 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


Factors Influencing Solubility in Water

• Temperature

• Salinity

• pH

• Dissolved organic matter (DOM)

• Co-solvents


• 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:

39

CT

H

RCsat

1ln


• Solids:

• Liquids:

• Gases:

40

CRT

HHsC

Eiwifussat

iw

)(ln

CRT

HLC

Eiwsat

iw )(ln

CRT

HHgC

Eiwivapsat

iw

)(ln


• 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)

41

totsi saltK

iwsaltiw][

, 10 typical seawater

[salt] = 0.5M

k

kssalti

sseawateri xKK k,,


• 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.


• 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.

45

)1(, DOMsatDOMsat KDOMCC


• 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


• 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


• 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


pollutants and environmental compartments

Documents