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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.
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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
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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.
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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+H+
OH
OH
OH
OH
H+OH2
OH
OH
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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
OH2+
OH
OH
OH
O-
O-
OH2+
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GOUY-CHAPMAN DOUBLE-LAYER
MODEL
STERN-GRAHAME TRIPLE-LAYER
MODEL
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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-OH
+ M2+
≡S-O
≡S-O
M + 2 H+
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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
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POINT OF ZERO CHARGE CAUSED BY BINDING OR
DISSOCIATION OF PROTONS
Material pHpznpc Material pHpznpc Material pHpznpc
-Al2O3 9.1 -Fe2O3 8.5 ZrSiO4 5-Al(OH)3 5.0 Fe(OH)3 8.5 Feldspars 2-2.4-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 2-FeOOH 7.8 SiO2 2 Chrysotile >10
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From Stumm and Morgan, Aquatic Chemistry
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Anion-Cation sorption• Equilibrium description for sorption of:• ≡S-OH + M2+ ≡S-OM+ + H+
Where z 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
RT
zF
MOHS
HOMSK surfaceintr
M exp]][[
]][[2
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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)
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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.
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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+
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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).
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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.
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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 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.
m
i
S
VCCS
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FREUNDLICH ISOTHERM
The Freundlich isotherm is described by
where K is the partition coefficient and n 1.
nKCS
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 ).
C (mg L-1)
0 10 20 30 40
S (
mg
g-1
)
0
10
20
30
40
50
60
S = 1.5C1.0
S = 5.0C0.5
FREUNDLICH ISOTHERMS
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LANGMUIR ISOTHERMThe Langmuir isotherm describes the situation where
the number of sorption sites is limited, so a maximum sorptive capacity (S max) is reached.
C (mg L-1)
0 10 20 30 40
S (
mg
g-1
)
0
10
20
30
40 LANGMUIR ISOTHERMS
C
CS
1.01
1.030
C
CS
5.11
5.130
The governing equation for Langmuir isotherms is:
KC
KCSS
1max
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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.
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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:
mB
nA
nA
mB
N
N
a
aK
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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
solutionin g/ml
C organic solidadsorbed/g gKoc
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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