According to the adsorption exchange theory, for the solid–
liquid exchange adsorption, the adsorption process of solute
exchange from liquid to an interface of solid–liquid will lose
some degree of freedom (including translation and rotation).
So the entropy of process is reduced; but in the liquid phase,
the solute exchange adsorption tends to make the resin contraction
occur, making part of the water of the hydration in
the resin get released into the liquid phase, and the desorption
process of water molecule makes the entropy of process increased.
The value of entropy depends on the sum of those
phenomena.
Based on the values of both enthalpy and entropy, DGh is
obtained to be positive by Eqs. (9) and (10). The results account
for the reason that the entropy of adsorption plays a
leading role, namely the driving force of adsorption process
is more dominant than that of chemical bonding force. Therefore,
even if the adsorption process is exothermic, sometimes,
it is not conducive to an adsorption process. And the higher
the temperature is, the more difficult the adsorption is. But it
is still able to initiate the adsorption process, which is consistent
with experiments (Li and Li, 2009).
As the temperature increases, from the model of Langmuir,
it shows that qmax is increasing, but from Table 2, it is clear
that the adsorption process will become non-spontaneous by
the main driving force of chemical bonding force. It is important
to make a balance between qmax and spontaneity, so the
process of absorption benefits in the low temperature.
When DHh is in the range of (30–70) kJ mol1, the
adsorption belongs to chemisorption; i.e., a chemical bond is
formed between the adsorbate and the surface. In the present
system, the value of DHh demonstrates that neither fully physical
nor fully chemical and some complex mechanism dictated
the adsorption process, and it also reveals that chemical
adsorption is the dominant process of adsorption (Li et al.,
2011). DSh is also negative, which implies that the degree of
freedom decreased at the solid–liquid interface during the
adsorption process. It also suggests that there are some structural
changes in both the adsorbate and adsorbent.