Adsorption of Pb (II) and Cd (II) from Aqueous Solution onto Sodium Tetraborate-modified Kaolinite Clay: Equilibrium and Thermodynamic Studies
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Date
2008
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Publisher
Elsevier
Abstract
Kaolinite clay sample obtained from Ubulu-Ukwu in Delta State of Nigeria was modified with sodium
tetraborate to obtain NTB-modified kaolinite clay. XRD measurements of NTB-modified kaolinite adsorbent
showed no observable change in the d-spacing of its crystal lattice. Also, the data of XRD confirmed that
this kaolinite clay sample is a mixture of kaolinite and Illite clay minerals. The SEM of modified and
unmodified samples showed irregular crystal structures. FTIR results proved the surface modification
of the kaolinite at –Al–O and –Si–O centers. The NTB-modified adsorbent presented with broader peaks of
inner –OH. Modification of kaolinite clay sample with sodium tetraborate decreased its PZC from pH 4.40 to
3.70 while its Specific Surface Area (SSA) was increased from 10.56 m2 g−1 to 15.84 m2 g−1
.
Modification with sodium tetraborate reagent increased the adsorption capacity of kaolinite clay from
16.16 mg/g to 42.92 mg/g for Pb (II) and 10.75 mg/g to 44.05 mg/g for Cd (II) at 298 K. Increasing temperature
was found to increase the adsorption of both metals onto both adsorbents suggesting an endothermic
adsorption reaction. The simultaneous presence of electrolyte in aqueous solution with Pb and Cd (II) was
found to decrease the adsorption capacity of NTB-modified adsorbent for Pb and Cd (II). Using the Pearson's
Hard and Soft Lewis Acid and Base (HSAB) theory the higher selectivity of unmodified kaolinite clay adsorbent
for Pb and NTB-modified kaolinite clay for Cd (II) was justified.
The thermodynamic calculations for the modified kaolinite clay sample indicated an endothermic nature of
adsorption (ΔHmean + 4.35 kJ mol−1 for Pb(II) and +3.79 kJ mol−1 for Cd (II)) and an increase in entropy as a
result of adsorption of Pb (II) and Cd (II) (ΔSmean −21.73 J mol−1 K for Pb (II) and −18.30J mol−1 K for Cd (II)). The
small positive values of free energy change (ΔGmean) indicated that the adsorption of Pb (II) and Cd (II) onto the
modified adsorbent may require some small amount of energy to make it more feasible.
Modeling equilibrium adsorption data obtained suggested that NTB-modified adsorbent sample has
homogeneous adsorption sites and fit very well with Langmuir adsorption model.
Regeneration studies suggest that ≈85% of the metals were desorbed from both adsorbents. On reuse of the
adsorbents only ≈80% of metals were adsorbed. NTB-modified kaolinite clay sample show some very good
potentials as a low-cost adsorbent for the adsorption of Pb (II) and Cd (II) from aqueous solutions
Description
Keywords
Adsorption, Desorption, Thermodynamics, Kaolinite–Illite, Selectivity, Wastewater treatment