Dye Adsorption on Expanding Three-Layer Clays - Green Chemistry for Dyes Removal from Wastewater - page 349

9.3.6 Adsorptionhermodynamics
The temperature dependence of adsorption reveals whether adsorption is
exothermic (decrease of adsorption capacity with increasing temperature)
or endothermic (increase of adsorption capacity with increasing tempera-
ture). The rate of diffusion of the adsorbate molecules across the exter-
nal boundary layer and into the internal pores of the adsorbent particles
increases with increasing temperature as a result of the reduced viscos-
ity of the solution [115]. The temperature dependence also gives valu-
able information about the enthalpy and entropy changes accompanying
adsorption [116].
Tahir and Rauf [88] used bentonite to remove cationic dye (Malahite
Green) from aqueous solution. The ΔH is positive (endothermic), as has
been found in most cases [18,83,117], and adsorption capacity increases
with temperature [118]. The ΔS is also positive, corresponding to an
increased degree of freedom in the system as a result of adsorption of the
dye molecules. Rytwo et al. [116] attributed this overall increase in entropy
to the release of hydrated inorganic cations from the clay. In addition,
Seki and Yurdakoç [118] explained the positive value of ΔS by structural
changes which take place as a result of interactions of dye molecules with
active groups on the clay surface. Tahir and Rauf [88] also found nega-
tive ΔG values in the −20 to 0 kJmol −1 range, corresponding to spontane-
ous physical processes. Table 9.7 summarizes studies on the temperature
dependence of adsorption of cationic dyes, including endothermic effects.
Eren and Afsin [27] found negative ΔH and ΔS for the adsorption of
cationic Crystalline Violet (CV) on bentonite pretreated with Ni, Co and
Zn. They ascribed the negative value of ΔH to an ion-exchange mecha-
nism that also includes the release of water molecules hydrated around the
exchangeable cations. They also showed large differences between natural
and pretreated bentonite and conclude that pretreatment created an irreg-
ular increase of the randomness at the bentonite solution interface during
adsorption.
Interestingly, cationic and anionic dye adsorption responds differently
to temperature. The adsorption of anionic dye adsorption on natural and
modified bentonites is illustrated in Table 9.8. Toor and Jin [25] studied
the adsorption of Congo Red (CR) by thermal activated (TA), acid acti-
vated (AA), and combined acid and thermal activated (ATA) bentonites.
They showed decreasing adsorption rates with increasing temperature
and described weakening sorptive forces both between dye-clay sites and
between adsorbed dye molecules [119]. Similar results have been reported
by Vimoneses et al. [39]. They showed a decrease in adsorption with