where R is the universal gas constant (8.314 J/molÁK) and T is temperature in Kelvin.
The K DR can be used then to calculate the mean free energy, E (kJ/mol), by using the
formula:
E ¼
1
ffiffiffiffiffiffiffiffiffiffiffi
2K DR
p
ð9:13Þ
Table 9.7 shows the adsorption of heavy metals that fit to either one of these
isotherms. It can be seen that most heavy metal adsorption using agricultural wastederived adsorbents fits well with Langmuir isotherm, thereby indicating monolayer
adsorption.
Glossary
Adsorbent
A material which will allow a liquid, gas, or dissolved solid to adhere to its
surface.
Adsorption
A process by which a solid holds molecules of a gas or liquid or solute as a
thin film.
Table 9.7 Comparison of best fit isotherm models for adsorption of heavy metals on agricultural
waste-derived adsorbents
Agricultural
wastes
Heavy
metals
Isotherm constant
Isotherm
model
References
Bamboo
Cr(VI)
0.049 L/mg (Fe-modified)
Langmuir
[40]
Cr(VI)
0.082 L/mg (Fe-Comodified)
Langmuir
[40]
Ficus carica
Cr(VI)
0.034 L/mg
Langmuir
[41]
Pineapple peel
Cr(VI)
0.067 L/mg
Langmuir
[50]
Rice straw
Cd(II)
4.2 Â 10
3 Æ 0.1 mg
1–1/n
L
1/
n
g
À1
Freundlich
[81]
Cu(II)
2.7 Â 10
3 Æ 0.2 mg
1–1/n
L
1/
n
g
À1
Freundlich
[81]
Hg(II)
1.2 Â 10
3 Æ 0.4 mg
1–1/n
L
1/
n
g
À1
Freundlich
[81]
Zn(II)
2.3 Â 10
3 Æ 0.1 mg
1–1/n
L
1/
n
g
À1
Freundlich
[81]
U(vi)
0.53 L/mg
Langmuir
[82]
Spartina
alterniflora
Cu(II)
0.110 Æ 0.024 L/mg
Langmuir
[83]
Sugarcane bagasse As(III)
0.0958 L/mg
Langmuir
[60]
As(V)
0.038 L/mg
Langmuir
[60]
Wheat straw
Cu(II)
0.1609 L/mg
Langmuir
[55]
Cr(VI)
0.03831 L/mg
Langmuir
[55]
9 Agricultural Waste-Derived Adsorbents for Decontamination of Heavy Metals
385