Inorganic Surfaces
 To understand the phenomenon of adsorption, it is important to understand and characterize the sorbent surfaces.
The main focus here is on hydrous oxide surfaces which form a large fraction of solid phases in natural water,
sediments and soils.
 In the presence of water, the surfaces of these oxides are generally covered with surface hydroxyl groups (see
Figure 9.6). Surface density is about 5 hydroxyls per nm2 of an oxide mineral.
The various surface hydroxyls formed may not be fully structurally and chemically equivalent but is generally
represented as:
 These hydroxyl groups contain the same donor atoms properties as the surfaces of carbonates and silicates. The
more important adsorption equilibria (surface complex formation) are as follows:
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Following criteria are characteristics of all surface complexation models
1. Sorption takes place at specific surface coordination sites
2. Sorption reactions can be described by mass law equations
3. Surface charge results from the sorption reaction (surface complex formation) itself
4. The effect of surface charge on sorption (extent of complex formation) can be taken into account by applying a
correction factor derived from the electric double layer theory to the mass law constant for surface reactions.
Acid Base Equilibria
 Uptake and release of protons from the surface can be described by the acidity constants (by assuming a solution
of constant ionic strength and the activity coefficient of the surface species are equal)
S - OH + H+ < == > SOH2+
Note that {SOH} and {SOH2+}can be converted to moles/m2 by multiplying with 1/S where S is the specific surface
area of the solid (m2/kg), i.e., <SOH> = S-1{SOH}. < > denotes moles/m2.
 Oxides have a unique property called the points of zero charge (________). __________ are pH values where
the net surface charge is zero. It is also known as the point of zero net proton charge (_______________) which is a
proper definition since the charge itself is established solely by proton exchange H +.
 The PZC of a simple oxide is related to the appropriate cationic charge and the radius of the central ion. For a
composite oxide, the PZC is approximately the weighted average of the values of its component.
 The PZC of a salt type mineral depends on the pH and the activities of all potential determining ions on the
surface. Thus is the case of calcite, HCO3-, CO3-, Ca2+ will have an effect in addition to OH- and H+.
 Effect of pH on the surface charge is best illustrated by Figure 9.9. Note that SiO2 and clays (kaolinite) are typical
colloidal materials in surface waters and groundwater and are negatively charges.
Surface Complexation with Metal Ions
 Surface complex formation of cations by hydrous oxides involves the coordination of metal ions with oxygen
donor atoms and the release of protons from the surface. For example:
 The binding of a metal ion by surface ligands is strongly pH dependent (see Figure 9.12).
 Adsorption is competitive, i.e., metal ions compete amongst each other and with H+ ions.
 Usually a narrow pH interval (1 - 2 pH units) where the extent of sorption rises from zero to almost 100%.
 A cation can associate with a surface as an ______________________________________. (see Figure 9.10)
An _________________________________ is formed when a chemical bond (i.e., largely covalent) between the
metal and the electrons donating electron ions is formed.
 If the cation of opposite charge approaches the surface groups to a critical distance, i.e., the cation and the base are
separated by one or more water molecules, or the ions are in the diffuse swarm of the double layer, then the complex
is referred to as an __________________________________.
The inner sphere adsorption usually has different properties than the outer sphere adsorption.
Adsorption of Ligand (anions and weak acids)
 Main mechanism of ligand adsorption is ligand exchange where the surface hydroxyl is exchanged by another
ligand. Adsorption is competitive.
Example: Oxalate and Phosphate
Organic Surfaces
 Sorption of hydrophobic substances on organic bearing particles
 Focus will be on organic matter in the natural environment such as colloidal materials, soil organic matter
- organic matter – an accumulation of dead plants matter, partially decayed and partially synthesized
plant and animal residue including soil microorganism
- soil humus – generally defined as organic material that has been transformed into relatively stable form
by soil microorganism
 Organic matter content
Most surface mineral soils
0.5 to 10% organic matter
desert soil
< 1 % organic matter
poorly drained soils (swamp peat)
> 10% to as much as 100%
Praire grassland soil (top 15 cm)
5 to 10% organic matter
(to express organic matter in terms of %C divide the organic matter by a factor of 1.72 – 2.0.
 Importance of soil organic matter
- reservoir of essential elements for plant, a source of cation exchange capacity
- soil pH buffering
- reservoir for hydrophobic and xenobiotic compounds
 Characterization
- attempts made to identify individual compounds present have not been successful
- structure is still very much in speculation (see figure), consists of different functional groups such as
aromatic, carboxylic, hydroxyl and groups with hydrogen bonding O – H.
- It is important to note that the organic matter seems to form a mesh or cage-like structure. Therefore,
organic matter can serve as both as surface adsorption and partitioning into the cage-like structure
(absorption).
 Sorption of xenobiotic compounds (many of which are hydrophobic compounds, example, pesticide, PCBs,
petroleum products and solvents) in the soil environment tends to accumulate in the soil organic matter while there
are some that are adsorbed on to the mineral surface. Many researchers have proposed that the uptake of organic
compounds is by partitioning. The evidence presented are as follows:
- uptake of compounds has a linear isotherm at concentration close to the solubility limit of the
compound – indicating distribution between the two phases
- when the sorption of a compound is conducted with different soils and sediments with different
organic matter content, the Kd values obtained when divided by the organic carbon content (fraction)
of the soil or sediment tend to yield a consistent constant value.
Kd / OC (fraction) =
-
Koc
This is generally used by researchers to show that hydrophobic compounds tend to be sorbed to
the organic matter.
the low exothermic heat of sorption points to a partitioning effect rather than a surface effect
when one or more hydrophobic compounds are sorbed, there is a lack of competition, indicating
partitioning
There is a inverse linear relationship between partition coefficients K oc and the solute water solubility
Sw. This means that the xenobiotic compounds (hydrophobic compounds) prefer to be in the organic
phase rather than in water. Relationships may be developed between K oc with Sw and Kow (see Table
4-1). Kow is the octanol-water partition coefficient. Kow can be estimated easily in the laboratory.
Kow = Concentration of compound in octanol /Concentration of compound in water
If the compound is hydrophobic - Kow  and vice versa.
-
With that the Koc can be estimated and the Kd for the soil estimated.
The above equations are useful for OC% less than 0.01%. Note that the equations must be used with
caution as different areas have different organic matter (sediments as compared to soils).