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Particle size. Ions  molecular clusters  nanocrystals  colloids  bulk minerals Small particles can have a significant % of molecules at their surface Thermodynamics are different (surface free energy) Surface area per mass is huge and charged through interaction with water

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particle size
Particle size
  • Ions  molecular clusters  nanocrystals  colloids  bulk minerals
  • Small particles can have a significant % of molecules at their surface
    • Thermodynamics are different (surface free energy)
    • Surface area per mass is huge and charged through interaction with water
    • Sorption of ions to these surfaces can be critical part of contaminant mobility
surface area
Surface area
  • Selected mineral groups often occur as colloids / nanoparticles:
    • FeOOH  SA up to 500 m2/g, site density 2-20/nm2
    • Al(OH)3  SA up to 150 m2/g, site density 2-12/nm2
    • MnOOH  SA hundreds m2/g, site density 2-20/nm2
    • SiO2  SA 0.1 – 300 m2/g, site density 4-12/nm2
    • Clays  SA 10-1000 m2/g, site density 1-5/nm2
    • Organics  SA up 1300 m2/g, site density 2/nm2
definitions
DEFINITIONS
  • Sorption - removal of solutes from solution onto mineral surfaces.
  • Sorbate - the species removed from solution.
  • Sorbent - the solid onto which solution species are sorbed.
  • Three types of sorption:
    • Adsorption - solutes held at the mineral surface as a hydrated species.
    • Absorption - solute incorporated into the mineral structure at the surface.
    • Ion exchange - when an ion becomes sorbed to a surface by changing places with a similarly charged ion previously residing on the sorbent.
mineral surfaces
Mineral Surfaces
  • Minerals which are precipitated can also interact with other molecules and ions at the surface
  • Attraction between a particular mineral surface and an ion or molecule due to:
    • Electrostatic interaction (unlike charges attract)
    • Hydrophobic/hydrophilic interactions
    • Specific bonding reactions at the surface
inner sphere and outer sphere
Inner Sphere and Outer Sphere
  • Outer Sphere surface complex  ion remains bounded to the hydration shell so it does not bind directly to the surface, attraction is purely electrostatic
  • Inner Sphere surface complex  ion bonds to a specific site on the surface, this ignores overall electrostatic interaction with bulk surface (i.e. a cation could bind to a mineral below the mineral pHzpc)
charged surfaces
Charged Surfaces
  • Mineral surface has exposed ions that have an unsatisfied bond  in water, they bond to H2O, many of which rearrange and shed a H+
  • ≡S- + H2O  ≡S—H2O  ≡S-OH + H+

OH

OH

OH2

H+

OH

OH

OH

H+

OH

surfaces as acid base reactants
Surfaces as acid-base reactants
  • The surface ‘SITE’ acts as an amphoteric substance  it can take on an extra H+ or lose the one it has to develop charge
  • ≡S-O- + H+↔ ≡S-OH ↔ ≡S-OH2+
  • The # of sites on a surface that are +, -, or 0 charge is a function of pH
  • pHzpc is the pH where the + sites = - sites = 0 sites and the surface charge is nil

OH

OH2+

O-

OH

O-

OH

OH2+

ph zpc
pHzpc
  • Zero Point of Charge, A.k.a: Zero Point of Net Proton Charge (pHZPNPC) or the Isoelectric Point (IEP)
  • Measured by titration curves (pHzpc similar to pKa…) or electrophoretic mobility (tendency of the solids to migrate towards a positively charged plate)
  • Below pHzpc more sites are protonated  net + charge
  • Above pHzpc more sites are unprotonated  net - charge
ion exchange reactions
ION EXCHANGE REACTIONS
  • Ions adsorbed by outer-sphere complexation and diffuse-ion adsorption are readily exchangeable with similar ions in solution.
  • Cation exchange capacity: The concentration of ions, in meq/100 g soil, that can be displaced from the soil by ions in solution.
  • Also anion exchange capacity for positively charged surfaces
ion exchange reactions1
ION EXCHANGE REACTIONS
  • Exchange reactions involving common, major cations are treated as equilibrium processes.
  • The general form of a cation exchange reaction is:

nAm+ + mBX  mBn+ + nAX

  • The equilibrium constant for this reaction is given by:
sorption isotherms i
SORPTION ISOTHERMS - I
  • The capacity for a soil or mineral to adsorb a solute from solution can be determined by an experiment called a batch test.
  • In a batch test, a known mass of solid (Sm) is mixed and allowed to equilibrate with a known volume of solution (V) containing a known initial concentration of a solute (Ci). The solid and solution are then separated and the concentration (C) of the solute remaining is measured. The difference Ci - C is the concentration of solute adsorbed.
slide15
Kd
  • Descriptions of how solutes stick to the surface
  • What would the ‘real’ behavior be you think??

Kd

sorption isotherms ii
SORPTION ISOTHERMS - II
  • The mass of solute adsorbed per mass of dry solid is given by

where S m is the mass of the solid.

  • The test is repeated at constant temperature but varying values of Ci. A relationship between Cand S can be graphed. Such a graph is known as an isotherm and is usually non-linear.
  • Two common equations describing isotherms are the Freundlich and Langmuir isotherms.
freundlich isotherm
FREUNDLICH ISOTHERM

The Freundlich isotherm is described by

where K is the partition coefficient and n 1.

When n < 1, the plot is concave with respect to the C axis. When n = 1, the plot is linear. In this case, K is called the distribution coefficient (Kd ).

langmuir isotherm
LANGMUIR ISOTHERM

The Langmuir isotherm describes the situation where the number of sorption sites is limited, so a maximum sorptive capacity (S max) is reached.

The governing equation for Langmuir isotherms is:

sorption of organic contaminants
Sorption of organic contaminants
  • Organic contaminants in water are often sorbed to the solid organic fractions present in soils and sediments
  • Natural dissolved organics (primarily humic and fulvic acids) are ionic and have a Koc close to zero
  • Solubility is correlated to Koc for most organics
measuring organic sorption properties
Measuring organic sorption properties
  • Kow, the octanol-water partition coefficient is measured in batches with ½ water and ½ octanol – measures proportion of added organic which partitions to the hydrophobic organic material
  • Empirical relation back to Koc:

log Koc = 1.377 + 0.544 log Kow

adsorption of metal cations i
ADSORPTION OF METAL CATIONS - I
  • In a natural solution, many metal cations compete for the available sorption sites.
  • Experiments show some metals have greater adsorption affinities than others. What factors determine this selectivity?
  • Ionic potential: defined as the charge over the radius (Z/r).
  • Cations with low Z/r release their waters of hydration more easily and can form inner-sphere surface complexes.
adsorption of metal cations ii
ADSORPTION OF METAL CATIONS - II
  • Many isovalent series cations exhibit decreasing sorption affinity with decreasing ionic radius:

Cs+ > Rb+ > K+ > Na+ > Li+

Ba2+ > Sr2+ > Ca2+ > Mg2+

Hg2+ > Cd2+ > Zn2+

  • For transition metals, electron configuration becomes more important than ionic radius:

Cu2+ > Ni2+ > Co2+ > Fe2+ > Mn2+

adsorption of metal cations iii
ADSORPTION OF METAL CATIONS - III
  • For variable-charge sorbents, the fraction of cations sorbed increases with increasing pH.
  • For each individual ion, the degree of sorption increases rapidly over a narrow pH range (the adsorption edge).
exchange reaction and site competition
Exchange reaction and site competition
  • For a reaction: A + BX = B + AX
  • Plot of log[B]/[A] vs. log[BX]/[AX] yield n and K
  • When n and K=1  Donnan exchange, exhange only dependent on valence, bonding strictly electrostatic
  • When n=1 and K≠1  Simple ion exchange, dependent on valence AND size, bonding strictly electrostatic
  • When n≠1 and K≠1 Power exchange, no physical description (complicated beyond the model) and unbalanced stoichiometry
electrostatic models
Electrostatic models
  • Combining electrostatic interactions and specific complexation using mechanistic and atomic ideas about the surface yield models to describe specific sorption behavior