List of Tables
Chapter 2 Literature Review
C. Other trends observed in experimental results Metal ion-
3.5 Shapes of Isotherm
The isotherm shapes provide information on the strength by which the sorbate is held to the soil and an indication of bonding mechanism. A general classification of adsorption as reflected by different features of the adsorption isotherm, such as the initial slope, the presence or absence of a plateau, or the presence of a maximum, was proposed by Giles et al. (1960).
There are four different shapes of isotherm commonly observed such as - C (constant partitioning) isotherms, S, L (Langmuir) and H (high affinity).
3.5.1. The C type isotherm
The curve is a line of zero-origin and the ratio between the concentration of the compound remaining in solution and adsorbed on the solid is the same at any concentration (Fig.3.1a). This ratio is usually named ‘‘partition coefficient’’: Kd or Kp (L/Kg). This isotherm is often used for a narrow range of concentration or very low concentrations such as observed for trace pollutants. The use of this isotherm based on its simplicity could lead to erroneous conclusions. For example, if the solid has a limited quantity of retention sites, the isotherm could be nonlinear because of a possible saturation plateau.
3.5.2. The L type isotherm
The ratio between the concentration of the compound remaining in solution and adsorbed on the solid decreases when the solute concentration increases, providing a concave curve (Fig.3.1b). It suggests a progressive saturation of the solid. There are two sub-groups: (i) the curve reaches a strict asymptotic plateau (the solid has a limited sorption capacity), and (ii) the curve does not reach any plateau (the solid does not show clearly a limited sorption capacity).
However, it often appears practically difficult to know if an isotherm belongs to the first or to the second sub-group.
3.5.3. The H type isotherm
This is only a particular case of the ‘‘L’’ isotherm, where the initial slope is very high (Fig.3.1c). This indicates that the compound exhibits sometimes such a high affinity for the solid that the initial slope cannot be distinguished from infinity, even if it does not make sense from a thermodynamic point of view (Toth, 1995).
3.5.4. The S type isotherm
The curve is sigmoidal and thus has got a point of inflection (Fig. 3.1d). This type of isotherm is always the result of at least two opposite mechanisms. Non-polar organic compounds are a typical case: they have a low affinity with clays. However, as soon as a clay surface is covered by these compounds, other organic molecules are adsorbed more easily (e.g. Karimi- Lotfabad et al., 1996; Pignatello, 2000). This phenomenon is called ‘‘cooperative adsorption’’
(Hinz, 2001) and is also observed for surfactants (Smith and Galan, 1995; Groisman et al., 2004).
The presence of a soluble ligand can also provide a sigmoidal isotherm for metallic species. At low metal concentrations, the adsorption is limited by the presence of the ligand. The ligand must be saturated and then the adsorption occurs normally (Sposito, 1984, p. 116). The point of inflection illustrates the concentration for which the adsorption overcomes the complexation.
Fig 3.1 The four main types of isotherms (after Giles et al., 1974) 3.6 Instrument used for contaminant analysis - Ion Chromatography
Ion chromatography is a form of liquid chromatography that uses ion-exchange resins to separate atomic or molecular ions based on their interaction with the resin. Its greatest utility is for analysis of anions (such as fluoride, chloride, nitrite, nitrate, and sulfate) for which there are no other rapid analytical methods. It is also commonly used for cations and biochemical species such as amino acids and proteins. Most ion exchange separations are done with pumps and metal columns using conductivity detectors. The column packing for ion chromatography consists of ion-exchange resins bonded to inert polymeric particles (typically 10 µm diameter). For cation separation the cation-exchange resin is usually a sulfonic or carboxylic acid, and for anion separation the anion-exchange resin is usually a quaternary ammonium group.
For example, retention of a metal cation on a cation-exchange resin occurs by the following reaction:
-SO3- H+(s) + Mx+(aq) ® -SO3- Mx+(s) + H+(aq)
Where Mx+ is a cation of charge x, (s) indicates the solid or stationary phase, and (aq)indicates the aqueous or mobile phase. The equilibrium constant for this reaction is:
[ ] [ ]
[
3- +] [
s x+]
aqaq+ s + x - 3
eq -S0 H M
H M
S0
= - K
Different cations have different values of equilibrium constant, Keq and are therefore retained on the column for different lengths of time. The time at which a given cation elutes from the column can be controlled by adjusting the pH ([H+]aq). Ions in solution are detected by measuring the conductivity of the solution. In ion chromatography, the mobile phase contains ions that create a background conductivity. This problem is reduced by selectively removing the mobile phase ions after the analytical column and before the detector. This is done by converting the mobile phase ions to a neutral form or removing them with an eluent suppressor, which consists of an ion-exchange column or membrane. For cation analysis, the mobile phase is often HCl or HNO3, which can be neutralized by an eluent suppressor that supplies OH-. The Cl- or NO3- is either retained or removed by the suppressor column or membrane. The same principle holds for anion analysis. The mobile phase is often NaOH or NaHCO3, and the eluent suppressor supplies H+ to neutralize the anion and retain or remove the Na. Any ionic substance that produces a detector response and has a retention time coinciding with that of an analyte, or near enough to cause peak overlap, may interfere with the determination. Most samples require dilution for determination of major ions by ion chromatography, which can introduce additional errors.