Dye Adsorption on Expanding Three-Layer Clays - Green Chemistry for Dyes Removal from Wastewater - page 352

increasing temperature and described relatively weak hydrogen bonds
and van der Waals forces. Toor and Jin [25] showed negative ΔS for CR
adsorption onto bentonite, indicative of a decrease in randomness at the
solid/solution interface. However, CR adsorption did not result in any
noticeable changes in the internal structure of the adsorbent. Results given
in Table 9.8 also support the findings of Toor and Jin [25]. In addition,
Table 9.8 shows that the adsorption of anionic dye onto montmorillonite is
an exothermic process, and a negative value of the Gibbs free energy, ΔG,
indicates spontaneity and thermodynamic stability. The negative change of
the entropy ΔS further suggests a decrease in the degree of freedom of the
adsorbed species [23,97].
On the other hand, Table 9.8 also shows how the adsorption of the same
dye with different bentonites may result in endothermic or exothermic
heats. For instance, Congo red uptake on Ca-bentonite increases with an
increase in temperature (positive enthalpy and endothermic) and also has
physical bonding [63]. Similar results have been reported for the adsorption
of Congo red onto both bentonite and surfactant-modified bentonite [37],
and for the adsorption of Supranol Yellow 4GL onto Al-pillared bentonite
[73]. It is common that increasing temperature may create a swelling influ-
ence inside the adsorbent structure, leading to additional penetration of
big dye molecules [120], so the nature of the adsorption of dyes is endo-
thermic. Endothermic adsorption has also been reported for modified
montmorillonite [72].
9.4 GeneralRemarks
The ready availability of clay makes it a viable candidate as an alternative
adsorbent. Various methods and adsorbents can potentially be used for
the removal of dyes from aqueous solutions, and each treatment process
has its own specific disadvantage. For instance, activated carbon and ion
exchange processes are expensive and sophisticated. Clay is low cost, has
great specific surface area, and has excellent physical and chemical stability.
The expanding three-layer clays have recently been used as an efficient dye
adsorbent. This chapter has attempted to highlight some important studies
of anionic and cationic dye adsorption on both natural and modified clays.
It has been shown that natural expanding three-layer clays are effective
adsorbents for cationic and anionic dyes, and when they are modified by
different methods and chemicals, the uptake of anionic and cationic dyes
is markedly increased. The most important adsorption parameters are pH,
ionic strength and modification process. Specific surface area has less of an