Phase Interactions

• Objective– to understand the chemical principles, significance and

application of Phase changes in Environmental Engineering.

Phase TypesLiquid/Gas Exchange

Gas SolubilitySolid/liquid Exchange

Solid SolubilitySorptionColloids

Phases

• Gas (Air)• Liquid (Water)• Solid (many)• Exchanges between phases important:

– biogeochemical and nutrient cycles• microbial metabolism

– pollutants• location affects stability• air stripping; land remediation

– effluent treatment processes• exploit beneficial phase changes• rate limiting step

Processes of Exchange

Getting species in and out of water� Water-Air solubility � Water surface interface � Sedimentation� Solubility/insolubility of solids� Sorption and desorption� Colloids

Water-Gas Exchanges E.g.

� O2 21% of atmosphere

� CO2 about 0.0350% of atmosphere

� H2S very low

� CHCl3 very low

� Pressure exerted by gas (partial pressure)

proportional to % v/v in gas (Dalton’s law)

Water- Gas exchanges

Solubility of gas in water

At any given temperature solubility depends on partial pressure of gas according to:

Henrys law

KH is Henrys law constant (mole/litre.atm), Px is the partial pressure of the gas (atm).

Takes no account of further reactions

or speed of transfer

(aq)(g) XX

xH(aq) PKX

OHNHOHNH aq 4)(3 2

E.g. Oxygen

Solubility gasses decreases with� increasing temp� altitude (lower partial pressure)

Supersaturation possible algal ponds

9.2mg/l 43.8 x 0.21O

0.21atmP

20Cat tm43.8mg/l.aK

(aq)

O

H

2

2

Solubility of Solids

Nothing is completely insoluble Solubility of solids expressed as solubility product Ksp

Different chemicals have different Solubility Products

CaCO Ca CO

KspCa CO

CaCO

Ksp = Ca CO

e.g. Ca(OH) Ksp

Mn(OH) Ksp

32+

32-

2+32-

3 (s)

2+32-

2 (s)

3 (s)

5 10 25

8 10 25

1 10 25

9

6

36

at C

at C

at C

Calculation of solubility

Solubility = S = Max. dissolved in water (moles/l) Conc. > S = super saturation

BaSO O

at C

S

moles litre

CaF then S S

S S

S moles litre

42+

42-

2+42-

2+42-

2+ -

2+ -

Ba S

Ksp = Ba SO

Ba SO

Ksp = S S

S = Ksp

Ca F

Ksp = Ca F

123 10 25

111 10

2

2 3 10

2 10

10

5

2

2 2 11

4

.

. /

( )( )

/

For

Eg Lime softening of water

Lime is Ca(OH)2

Phase change 1 Ca2+ added 2 Ca2+ removed

i.e. net removal of material

Carbonate Hardness Ca HCO

Add Lime

Ca HCO Ca OH CaCO H Os

23

23 2 3 2

2

2 2 2( ) ( )

Soda Ash softening of water

Soda-ash is How does soda ash remove Calcium?

phase change but a net addition of ions

NaClCaCOCONaClCa

ClCaHardnessCarbonateNon

s

AshSodaAdd

222

2

)(332

2

2

Water-solid interface

binding of solutes to solids� Surface sorption

Binding onto a solid surface� Adsorption

Partitioning of contaminant into solid� Absorption

Don't know� sorption

Adsorption to charged surface

Many naturally occurring solids Negatively charged� Readily bind metals, M+

� Anions may also bind????? � natural anion binding rare� Ion exchange

e.g. clays, humic acids, metal oxides and resinsone ion displaced by another

-

-

-M

M

M

Adsorption by physical forces

Chemicals more weakly bound E.g. use of activated carbon surface area of 1000m2/gram!!! Equations like Freundlich's Isotherms used measure

adsorbtion:

X = amount sorbed/g solid C = equil. conc. of contaminant in water phase K and n are constants

n can have values n<1, n=1, n>1When n=1 the relative distribution between solid and liquid

phases is the same at any concentration

X = KCn

Freundlich Isotherm

Sediments adsorb metal ions. Cation Exchange Capacity (CEC)is 20 - 30 meq/100g. Humic acid has CEC of 500 meq/100g.

Absorbtion into solids Very important for organic pollutants Dissolve in organic parts of solids

� Eg Humic substance, Sorbtion equilibria observed

S = conc in solid, C = conc in water Kp Partition Coefficient judged from

� affinity of pollutant for organic solvent measured as Kow, octanol water coefficient

� organic material in solid Absorption into solids is reversible.

Kp S

C

Colloids

• Large organic molecules (i.e. small particles)• Size 0.001 to 1 m• Surface Area - 600 m2 per gram• Hydrophobic or Hydrophilic• Charged - cation exchange capacity (CEC)

– bind metals, organics• Electrical Double Layer

– Stern Layer, Diffuse Layer– stabilizes the colloidal ‘Sol’

• Charge varies with pH– point of zero charge (isoelectric point)

Removal of Colloids

• Coagulation (destabilisation)– overcome charge repulsive forces– zeta potential - speed of movement in electrical field– Destabilise by:

• Double Layer compression (by increasing ionic strength)• Charge Neutralisation (add hydrophobic cations)• Interparticle Bridging (polymers, polyelectrolyte)• Entrapment in a Sweep Floc of Fe(OH)3 or Al(OH)3

– other methods• Heat - particle collision rate increases• Freeze - salt solution increases • isoelectric point