1. Mass transfer of reactants to the surface of the metal particles
2. Adsorption of the reactant onto the surface
3. Reaction onto the surface
4. Desorption of the products
5. Mass transfer of products to the fluid
The overall reaction rate is determined by the slowest step, as the ratedetermining step. Generally, diffusion processes require less energy than chemical
reactions. Frequent values of activation energy for mass transfer are in the range
15–30 kJ mol
À1 (Lasaga 1981; Pilling and Seakins 1995).
Adsorption is a process where atoms or molecules of a liquid or gas phase are
deposited on the surface of a solid. The process is governed by interactions of
attraction between the solid (also called adsorbent) and molecules (also called
adsorbate). Two types of adsorption are possible:
• Physical adsorption involving weak electrostatic forces such as van der Waals
forces or hydrogen bond. This is usually a reversible phenomenon.
• Chemical adsorption involving strong bonds such as covalent bonds (Montgomery 1985), resulting in an electron transfer and a significant change in the structure
of the adsorbed molecule. Thus, contrary to the physical adsorption, chemisorption is a less reversible phenomenon.
Different models are used to characterize adsorption of COCs onto iron particles
or soils, like the linear model, the Freundlich isotherm and the Langmuir isotherm
(Burris et al. 1995, 1998; Dries et al. 2004, 2005). It is however important to note
that these adsorption isotherms are only representative for a given oxide shell, and do
not reflect the dynamic evolution of the oxide shell until Fe
0 is depleted (Noubactep
2010a).
Linear Model
The linear model assumes a linear relationship between the concentration of
dissolved solute C (g L
À1 ) and the solute concentration adsorbed on the solid C
Ã
(g kg
À1 ):
C
Ã
¼ K Ã C
ð6:43Þ
where K is the partition coefficient (L kg
À1 ).
Freundlich Isotherm
Freundlich isotherm is a nonlinear empirical relationship between the amount of
solute on the surface of the adsorbent and the amount of solute in the solution in
contact with the adsorbent (Eq. 6.44).
320
R. Rodrigues et al.
2. Adsorption of the reactant onto the surface
3. Reaction onto the surface
4. Desorption of the products
5. Mass transfer of products to the fluid
The overall reaction rate is determined by the slowest step, as the ratedetermining step. Generally, diffusion processes require less energy than chemical
reactions. Frequent values of activation energy for mass transfer are in the range
15–30 kJ mol
À1 (Lasaga 1981; Pilling and Seakins 1995).
Adsorption is a process where atoms or molecules of a liquid or gas phase are
deposited on the surface of a solid. The process is governed by interactions of
attraction between the solid (also called adsorbent) and molecules (also called
adsorbate). Two types of adsorption are possible:
• Physical adsorption involving weak electrostatic forces such as van der Waals
forces or hydrogen bond. This is usually a reversible phenomenon.
• Chemical adsorption involving strong bonds such as covalent bonds (Montgomery 1985), resulting in an electron transfer and a significant change in the structure
of the adsorbed molecule. Thus, contrary to the physical adsorption, chemisorption is a less reversible phenomenon.
Different models are used to characterize adsorption of COCs onto iron particles
or soils, like the linear model, the Freundlich isotherm and the Langmuir isotherm
(Burris et al. 1995, 1998; Dries et al. 2004, 2005). It is however important to note
that these adsorption isotherms are only representative for a given oxide shell, and do
not reflect the dynamic evolution of the oxide shell until Fe
0 is depleted (Noubactep
2010a).
Linear Model
The linear model assumes a linear relationship between the concentration of
dissolved solute C (g L
À1 ) and the solute concentration adsorbed on the solid C
Ã
(g kg
À1 ):
C
Ã
¼ K Ã C
ð6:43Þ
where K is the partition coefficient (L kg
À1 ).
Freundlich Isotherm
Freundlich isotherm is a nonlinear empirical relationship between the amount of
solute on the surface of the adsorbent and the amount of solute in the solution in
contact with the adsorbent (Eq. 6.44).
320
R. Rodrigues et al.