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
• 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
• 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
• 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
• 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
OH
OH
OH2
H+
OH
OH
OH
OH
H+
• 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  ≡SOH + H+
Surfaces as acid-base reactants
OH
OH2+
O-
OH
OOH
OH2+
• 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
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
POINT OF ZERO CHARGE
CAUSED BY BINDING OR
DISSOCIATION OF PROTONS
Material
pHpznpc Material pHpznpc
Material
pHpznpc
-Al2O3
9.1
-Fe2O3
8.5
ZrSiO4
-Al(OH)3
5.0
Fe(OH)3
8.5
Feldspars
-AlOOH
8.2
MgO
12.4 Kaolinite
4.6
CuO
9.5
-MnO2
2.8
Montmorillonite
2.5
Fe3O4
6.5
-MnO2
7.2
Albite
-FeOOH
7.8
SiO2
2
Chrysotile
5
2-2.4
2
>10
From Stumm and Morgan, Aquatic Chemistry
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 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
m
n
is given by:
aB N A
K
n
A
a N
m
B
CATION EXCHANGE CAPACITIES
OF MINERALS AND SOILS
Mineral
Chlorite
Illite
Kaolinite
CEC
Mineral
(meq/100 g)
10-40
Soil organic
matter
10-40
Sand
3-15
CEC
(meq/100 g)
>200
2-7
Sandy loam
2-18
Montmorillonite
80-150
Loam
8-22
Vermiculite
100-150
Silt loam
9-27
Oxides and
hydroxides
2-6
Clay loam
4-32
Clay
5-60
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 (S m) is
mixed and allowed to equilibrate with a known
volume of solution (V ) containing a known initial
concentration of a solute (C i). The solid and
solution are then separated and the concentration
(C ) of the solute remaining is measured. The
difference C i - C is the concentration of solute
adsorbed.
Kd
• Descriptions of
how solutes stick
to the surface
• What would the
‘real’ behavior be
you think??
Kd
SORPTION ISOTHERMS - II
• The mass of solute adsorbed per mass of dry solid
is given by
Ci  C V
S
Sm
where S m is the mass of the solid.
• The test is repeated at constant temperature but
varying values of C i. A relationship between C
and 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
n
S

KC
The Freundlich isotherm is described by
where K is the partition coefficient and n  1.
60
FREUNDLICH ISOTHERMS
50
S = 1.5C1.0
-1
S (mg g )
40
30
S = 5.0C0.5
20
10
0
0
10
20
C (mg L-1)
30
40
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
The Langmuir isotherm describes the
situation where the number of sorption
sites is limited, so a maximum sorptive
capacity
(S
)
is
reached.
max
LANGMUIR ISOTHERMS
40
30
-1
S (mg g )
The governing
equation for Langmuir
isotherms is:
30  1.5C
S
1  1.5C
20
S
30  0.1C
1  0.1C
Smax KC
S
1  KC
10
0
0
10
20
C (mg L-1)
30
40
Sorption of organic contaminants
• Organic contaminants in water are often sorbed
to the solid organic fractions present in soils and
sediments
g adsorbed/g solid organic C
K oc 
g/ml in solution
• 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
• 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
• 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
• 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
• 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
• For a reaction: A + BX = B + AX
[ B]  [ AX ] 


K ex 
[ A]  [ BX ] 
n
 [ AX ] 
[ B]

log
 log K ex  n log 
[ A]
 [ BX ] 
• 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
• Combining electrostatic interactions and
specific complexation using mechanistic
and atomic ideas about the surface yield
models to describe specific sorption
behavior