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Sorption phenomena on soils
Lee, Young-Chul
Introduction
- Adsorption: accumulation of a substance or materials at an interface
between the solid surface and the bathing solution
- Surface precipitation or polymerization (Χ)
- Sorption: adsorption, surface precipitation, polymerization
- Adsorbate: The materials that accumulates at an interface
- Adsorbent: The solid surface on which the adsorbate accumulates
- Adsorptive: The molecule or ion in solution that has the potential of being adsorbed
- Affecting the electrostatic properties
- Coagulation and settling of suspended particles and colloids
- Physical forces
- Van der Waals forces (e.g. partitioning)
- Electrostatic outer-sphere complexes (e.g. ion exchange)
- Chemical forces
- Inner-sphere complexation: ligand exchange mechanism, covalent bonding, hydrogen
bonding
Surface functional groups
- “Chemically reactive molecular unit bound into the structure of a solid at its
periphery such that the reactive components of the unit can be bathed by a fluid”
- Organic (e.g., carboxyl, carbonyl, phenolic) or inorganic molecular units
- Lewis acids: metal oxides, Lewis bases: oxide anions
- Protonation or deprotonation by adsorption of H+ and OHS-OH + H+ ↔ S-OH2+
S-OH ↔ S-O- + H+
- Ligand exchange: The water molecule is unstable and can be exchanged for an
inorganic or organic anion (Lewis base or ligand) in solution
which then bonds to the metal cation
- A type: Amphoteric
- B- and C type: Unreactive
- Lewis acid type
- Chemisorption of a water molecule on a bare Fe(Ⅲ) ion
- Releasing a proton
Fig. Goethite(α-FeOOH) surface hydroxyls and Lewis acid site
Surface complex
-When the interaction of a surface functional group with an ion or molecules present
in the soil solution creates a stable molecular entity
Fig. Inner- and outer-sphere complexes formed
between metal cations and siloxane ditrigonal cavities
on 2:1 clay minerals
Inner-sphere
Outer-sphere
1. Involve electronic interaction
2. Rapid process (reversible)
3. Affected by ionic strength of the aqueous phase
4. Adsorption occurs (on opposite charge adsorbate)
5. Uncharged outer-sphere complex: ion pairs, ex) CaSO40
1. Covalent or ionic bonding
2. Slow than outer-sphere
3. Often not reversible and weakly affected
by ionic strength
4. Adsorption occurs
(regardless of the original charge)
Between inorganic ions and hydroxyl groups
of an oxide surface
Adsorption isotherms
- Relation between the activity or equilibrium concentration of the adsorptive and the
quantity of adsorbate on the surface at constant temperature and pressure
- S-type isotherm
At low conc., the surface has a low affinity
for the adsorptive which increases at higher conc.
- L-shaped (Langmuir) isotherm
At low conc., the adsorbent has a high affinity
for the adsorptive which decreases as conc. increases
- H-type (high affinity) isotherm
Strong adsorbate-adsorptive interactions
(e.g. inner-sphere complexes)
- C-type isotherm
Partitioning mechanism whereby adsorptive ions or
molecules are distributed or partitioned between
the interfacial phase and the bulk solution phase w/o
any specific bonding between the adsorbent and
adsorbate
Fig. The four general categories of adsorption isotherms, from Sposito (1984)
Adsorption experiment
Process & method for adsorption experiment
- Carried out by equilibrating (shaking, stirring)
- Adsorptive solution of a known composition and volume
- Known amount of adsorbent at constant temp.& press
- pH and ionic strength are also controlled
- Separation (centrifugation, settling, filtering)
- Precipitation & dissolution considering
- Good mixing is needed but not vigorous (modification)
Mass balance equation!
Q : amount of adsorption (mol/kg)
Cf, Co : final and initial adsorptive concentration (mol/liter)
Vf, Vo : final and initial adsorptive volumes (liters)
M : mass of adsorbent (kg)
Plotting Cf (x-axis) and q (y-axis) = adsorption graphing
Partitioning coefficients
- Partitioning mechanism is usually suggested from linear adsorption isotherm (C-type)
Kp: a measure of the ratio of the amount of a material that is
adsorbed to the amount that is in solution
C: equilibrium conc. of the adsorptive
foc: fraction of organic carbon in soil
Koc: organic carbon-water partition coefficient
Equilibrium-based adsorption models
-Freundlich Equation
- Describing gas phase adsorption and solute adsorption
- Empirical adsorption model
- Expressed q=KdC1/n ( Kd : distribution coefficient, n : correction factor)
- Disadvantage : it does not predict an adsorption maximum
Fig. Use of the Freundlich equation to describe zinc adsorption(x)/desorption(o) on soils
- Part 1  linear portion of the isotherm (initial Zn conc. <100 mgL-1)
- Part 2  nonlinear portion of the isotherm
Equilibrium-based adsorption models
-Langmuir Equation
- Describing the adsorption of gas molecules on a planar surface
- Heavily employed to describe sorption on colloidal surfaces
- Best at low sorptive concentration
- Well describe both adsorption and precipitation
- Expressed q=KCb/(1+KC), C/q=1/kb + C/b
(K : constant related to the binding strength,
b : maximum amount of adsorptive that can be adsorbed)
Fig. Zinc adsorption on the A and B2t horizons of a Cecil soil
-The plots were resolved into two linear portions
Assumption
1. Adsorption occurs on planar surfaces that have a fixed number of sites which are identical
and the sites can hold only one molecule.
2. Adsorption is reversible
3. No lateral movement of molecules on the surfaces
4. The adsorption energy is the same for all sites and independent of surface coverage (homogeneous)
5. No interactive between adsorbate molecules
bkC
bkC
q 1 1  2 2
1  k1C 1  k 2C
- Different binding sites or both adsorption and precipitation
- 1, 2: adsorption on high- and low-energy sites  sorption on soil
of different physiochemical and mineralogical properties
- Not multiple sites with different binding affinities exist