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

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 v 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 n E.g. – O 2 21% of atmosphere – CO 2 about % of atmosphere – H 2 S very low – CHCl 3 very low – Pressure exerted by gas (partial pressure) proportional to % v/v in gas (Dalton’s law)

Water- Gas exchanges n Solubility of gas in water n At any given temperature solubility depends on partial pressure of gas according to: n Henrys law K H is Henrys law constant (mole/litre.atm), P x is the partial pressure of the gas (atm). n Takes no account of further reactions n or speed of transfer

E.g. Oxygen n Solubility gasses decreases with – increasing temp – altitude (lower partial pressure) n Supersaturation possible n algal ponds

Solubility of Solids n Nothing is completely insoluble n Solubility of solids expressed as solubility product K sp n Different chemicals have different Solubility Products

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

Eg Lime softening of water n Lime is Ca(OH) 2 n Phase change n 1 Ca 2+ added 2 Ca 2+ removed i.e. net removal of material

Soda Ash softening of water n Soda-ash is n How does soda ash remove Calcium? n phase change n but a net addition of ions

Water-solid interface n binding of solutes to solids – Surface sorption n Binding onto a solid surface – Adsorption n Partitioning of contaminant into solid – Absorption n Don't know – sorption

Adsorption to charged surface n 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 resins one ion displaced by another M M M

Adsorption by physical forces n Chemicals more weakly bound n E.g. use of activated carbon n surface area of 1000m 2 /gram!!! n Equations like Freundlich's Isotherms used measure adsorbtion: n X = amount sorbed/g solid n C = equil. conc. of contaminant in water phase n K and n are constants n can have values n 1 When n=1 the relative distribution between solid and liquid phases is the same at any concentration

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

Absorbtion into solids n Very important for organic pollutants n Dissolve in organic parts of solids – Eg Humic substance, n Sorbtion equilibria observed n S = conc in solid, C = conc in water n K p Partition Coefficient judged from – affinity of pollutant for organic solvent l measured as K ow, octanol water coefficient – organic material in solid n Absorption into solids is reversible.

Colloids Large organic molecules (i.e. small particles) Size to 1  m Surface Area m 2 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