Phase Interactions Objective –to understand the chemical principles, significance and application...

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Phase Interactions Objective – to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase Types Liquid/Gas Exchange Gas Solubility Solid/liquid Exchange Solid Solubility Sorption Colloids

Transcript of Phase Interactions Objective –to understand the chemical principles, significance and application...

Page 1: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

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

Page 2: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

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

Page 3: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

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

Page 4: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

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)

Page 5: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

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

Page 6: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

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

Page 7: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

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

Page 8: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

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

Page 9: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

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( ) ( )

Page 10: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

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

Page 11: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

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

Page 12: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

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

Page 13: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

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

Page 14: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

Freundlich Isotherm

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

Page 15: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

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

Page 16: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

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)

Page 17: Phase Interactions Objective –to understand the chemical principles, significance and application of Phase changes in Environmental Engineering. Phase.

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