Download Sorption

IS SOLUBILITY THE ONLY CONTROL
ON SOLUTE CONCENTRATIONS?
• The answer is NO! Solubility often controls the
concentrations of major solutes such as Si, Ca,
and Mg, and some minor or trace solutes such as
Al and Fe.
• However, for many trace elements, sorption
processes maintain concentrations below
saturation with respect to minerals.
• In other words, sorption is a means to remove
solutes even when the solution is undersaturated
with any relevant solids.
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
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.
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
GOUY-CHAPMAN
DOUBLE-LAYER
MODEL
STERN-GRAHAME
TRIPLE-LAYER
MODEL
Sorption to ≡S-OH sites
• ≡S-OH + M2+  ≡S-OM+ + H+
• ≡S-OH + L2-  ≡S-L- + OH• In addition, can also have bi-dendate
sorption reactions:
≡S-OH
≡S-O
M
+ M2+
≡S-OH
≡S-O
+ 2 H+
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
Anion-Cation sorption
• Equilibrium description for sorption of:
• ≡S-OH + M2+  ≡S-OM+ + H+
  DzFsurface 
[ S  OM ][ H ]


exp 
2
[ S  OH ][ M ]
RT



K
intr
M

Where Dz is the stoichiometric net change in surface
charge due to the sorption reaction (+1 here), F is
Faraday’s constant (96485 Coulombs per mole), 
is the electrical potential at the surface, R is the
gas constant, and T is temperature in Kelvins, the
whole right term is called the coulombic term
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)
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).
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.
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
1.0
S = 1.5C
-1
S (mg g )
40
30
0.5
S = 5.0C
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
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.
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
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