CE 520 Environmental Engineering Chemistry
Solid-Solution Interface Reactions
Dr. S. K. Ong
 Previous discussions to date are for aqueous interactions such as acid-base interactions, complexations and redox
reactions. Solid-solution interactions were considered in precipitation.
 In this section, the phenomenon of chemical interactions at the solid-solution interface will be discussed.
 Every object has a surface area. The surface area available for interactions is estimated to be in trillions of square
kilometers – example:
inorganic surfaces
organic
biological
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 Interfacial reactions occur in both natural water and engineered systems such as softening by ion exchange, carbon
adsorption columns.
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Terms/definitions
Brief discussion of forces at the interface
Sorption isotherms and models
Interactions with inorganic surfaces
Interactions with organic surfaces
Ion Exchange
Definitions
____________
- intraphase distribution of a solute where a solute is dissolved or absorbed into the absorbing
phase
example, _____________________________________________________________________.
_____________________________________________________________________________
Note that the gasoline may be considered as dissolved in the absorbing phase (water), i.e., there is
a distribution between the two phases. This phenomenon is sometimes referred to as ___________
The extent of he solute dissolved in the absorbing phase is related to the solubility of the solute in
the absorbing phase. In other words, the character of the solute and its interaction with the
absorbing phase or solution play an important role in its distribution.
For example, xylene, an organic compound in gasoline will prefer to remain in the gasoline phase
since the hydrogen-carbon bonds of xylene and other organic compounds in gasoline are more
compatible as compared to the ionic structure of water.
Compounds therefore can be classified as:
_________________
- water liking compounds
__________________
- water hating compounds
Most nonpolar organic compounds are hydrophobic. Salts and ionic or polar compounds are
hydrophilic.
An important parameter that measures the extent of hydrophobicity of an organic compound is the
____________________________________ Kow.
Kow = Concentration of chemical in octanol phase/Concentration of chemical in water phase
Kow < 10 - relatively hydrophilic
Kow > 104 - highly hydrophobic
___________
- accumulation of solute at the interface between two phases (see diagram)
Examples:
- surface complexation reactions (surface hydrolysis, formation of coordinate bonds at the surface
with metals and ligands)
- electrostatic interactions at the surface
- hydrophobic expulsion of hydrophilic compounds, causing accumulation at the interface.
Example, oil on the surface of water, surfactants at the air-water interface.
Solute is usually called the ___________and the surface is called ________________
___________
- when both are occurring and cannot be distinguish
In short, sorbate and sorbent are used to describe the solute and surface.
Example, sorption of nonpolar compounds onto soil humic material. The structure of soil humic
material consists of many different aromatic and aliphatic organic compounds forming a mesh-like
structure. The nonpolar compounds may adsorbed on the surface of one of the many functional
groups or may reside or be absorbed/partitioned within the mesh-like structure.
____________
The most general definition is any replacement of an ion in a solid phase in contact with a solution
by another ion.
CaCO3(s) + Sr2+ < === > SrCO3(s) + Ca2+
Fe(OH)3 (s) + HPO42- + 2H+ < === > FePO4(s) + 3H2O
A more restrictive definition is the replacement of an adsorbed, readily exchangeable ion by
another . Since replacement takes place at the interface – may be classified as adsorption.
Forces at the Interface (see diagram)
 Various forces exist between solute molecules and the molecule of adsorbing surfaces
 Can divide them up into chemical, electrostatic and physical
Chemical Adsorption
 solute-sorbent interaction having the characteristics of a true chemical bond
- formation of a ___________________ bond by merging of electron clouds.
- formation of ________________________bonds
- forces extend over short distances
 characterized by large heat of interactions/sorption, in the range of _____________________ kJ/mole.
 termed as ______________________
 desorption if it occurs may result in a different compound
Electrostatic Adsorption
involved high energy forces as opposed to bonds in chemisorption
Forces can be described by Coulomb’s law and tend to extend over longer distances than chemical forces
F  1/X2
where X is the distance between point charges
Result in an ion-ion interaction as in ion exchange or in dipole-ion interactions
Examples – sorption of Ca2+ on clay surface
- NH3 have permanent dipoles or water with temporary dipoles, interactions with surface of soils.
energy level/heat of sorption may be as high as ____________________ KJ/mole.
Physical Adsorption
 Results from he action of dipole forces or the action of van der Waal forces comprising of London dispersive
forces (London dispersive forces are the result of interactions among rapidly fluctuating temporary dipoles and
quadrapoles movements associated with the motion of electrons within orbitals.
 ____________________ interactions are the more important of all interactions, vary inversely with the sixth
power of the distance between molecules
 Heat of adsorption is _______________________ kJ/mole.
Adsorption Isotherms and Models
Adsorption is often described in terms of isotherms. Isotherms show the relationship between the bulk equilibrium
aqueous phase activity (concentration) of the adsorbate and the amount adsorbed on the interface at constant
temperature. Isotherms are at equilibrium conditions.
Linear Model
where

KD
Ce
= mass of sorbate sorbed per unit mass of solid (mg/mg)
= distributive coefficient or partitioning coefficient (L/mg)
= aqueous sorbate concentration at equilibrium (mg/L)
Langmuir Model
Assumptions
(a) Adsorption energy at each site is constant and independent of surface coverage
(b) Adsorption occurs only on well-defined localized sites (with no interaction between adsorbate molecules)
(c) Adsorption is ultimately limited by formation of a single layer coverage of solute molecules on the surface
The following equation can be written:

 K ads [A]
1 
also can be expressed in the form of
where 
[SA ]
[S T ]
The various parameters in the Langmuir equation can be found using nonlinear regression or by taking the reciprocal
of the equation.
plot 1/ vs 1/[A]
Caution:
 Fit of experimental data to a Langmuir adsorption isotherms does not mean that adsorption satisfies the criteria or
assumptions of the model.
 In many cases,
- adsorption to a surface is followed by additional interactions at the surface, example, two-dimensional
distortion of the surface forces
- charged ions tend to repel each other within the adsorbed layer
- surface charges are not the same
 There are many modifications to the Langmuir model.
If there are two adsorbates, A and B
Sorbent with two different sites of different affinities
BET Equation (Brunauer, Emmett and Teller)
The Langmuir model was extended by Brunauer and co-workers in 1938 to include adsorption of multiple layers of
molecules.
Assumptions:
- discrete sites of adsorption
- first layer similar to Langmuir with heat of adsorption
equal for each side
- subsequent layers are treated as condensation reactions
and assumed to have the same heat of reaction
Equation:
where
[As] - saturation concentration (solubility limit) of the solute, (P o = saturated vapor pressure of sorbate)
max - maximum adsorption capacity
Kads - constant related to energy of adsorption or distribution coefficient,  exp(-Ha/RT)
Generally applicable for sorption of gases. BET theory is generally more accurate for surface coverage from 0.5 to
2 monolayers or a relative vapor pressure of 5% to 60%. Beyond 60% relative pressure, pore condensation begins.
The model is used extensively to determine the surface area of sorbents.
Gibbs Equation
surface
concentration
(moles/m2)
activity or
concentration of species i
surface tension or interfacial
tension (J/m2)
   

Equation implies that a substance sorbed at the interface will reduce the interfacial tension  
  0
   ln a i 




Equation not that useful as it is difficult to measure the interfacial tension
Freundlich Equation
Despite the theoretical basis for Langmuir, BET and Gibbs equations (note the limited assumptions of these
equations), these isotherms often fail to describe the experimental data adequately.
At the same time when Langmuir was developing his equation, Heinrich Freundlich proposed a general empirical
equation
where
 = mass sorbed per unit mass of sorbent
K = distribution coefficient
Ce = equilibrium constant
n = constant
To obtain the various parameters, nonlinear regression may be used.
Simplification of the above equation and plotting will also yield the constants.
Plot log  vs log Ce, slope = n, intercept is log K
Although the equation was originally derived as an empirical equation, researchers have shown that the Freundlich
equation is actually a generalization of the following equation:
where there is an infinite number of adsorption sites. By taking the integral of the modified Langmuir equation, the
solution will be that of the Freundlich equation.
Example: Determine the Langmuir equation that will describe the following sorption data. Using the Langmuir
constants, determine the surface area of the sorbent (assume monolayer coverage applies). Assume molecule has a
cross-sectional surface area of 1 x 10-20 m2. Compound has a molecular weight of 131.
Mass of sorbent
(gm)
0
0.084
0.213
0.386
0.682
Equilibrium Concentration
(mg/L)
7.8
7.8
6.1
4.2
1.8
Mass of sorbent
(gm)
0.79
1.01
1.25
1.54
1.74
2.20
Equilibrium Concentration
(mg/L)
1.20
0.84
0.62
0.43
0.31
0.20