The adsorption of a cationic surfactant (cetyltrimethylammonium bromid
e, CTAB) from aqueous solution onto the lignite was investigated. The
samples used for the adsorption experiments were: fresh; demineralized
; demineralized then oxidized. The oxidation process was conducted by
dry method (in air at 150 and 200 degrees C). The UV spectrophotometri
c techniques were used for the analysis of CTAB in solution. The adsor
ption isotherms were measured experimentally at 21 and 45 degrees C, a
nd it was found that the adsorption capacities of all samples increase
d with rising temperature, and the adsorption of CTAB decreased by dem
ineralization and oxidation. The changes in the thermodynamic quantiti
es, enthalpy and entropy, related adsorption process that occurred in
some steps were calculated. Isosteric heats of adsorption and the corr
esponding changes in entropy have found to be negative for all of the
samples. From the abrupt difference between Delta H and Delta S for th
e demineralized sample and those of fresh sample, it was concluded tha
t the orientation of CTAB molecules on the surface of the demineralize
d sample differs than that of fresh sample, and also that the adsorpti
on of CTAB on the demineralized sample takes place with the hydrophobi
c interactions in addition to the electrostatic interactions, while in
the case of fresh sample, the predominant mechanism of adsorption is
through electrostatic interactions. Also, the lower values of BH for b
oth demineralized and oxidized samples in comparison with the deminera
lized sample indicates that the oxidation process diminishes the contr
ibution of the hydrophobic interactions to the adsorption of CTAB.