Dye Adsorption on Expanding Three-Layer Clays - Green Chemistry for Dyes Removal from Wastewater - page 344

and this is much like other organo-bentonites used in anionic dye adsorp-
tion. Examples of cationic dye removal by surfactant activation are given
in Table 9.5.

9.3.3 Effect of Ionic Strength on Uptake of Anionic

and Cationic Dyes

Li et al. [51] described how actual dye wastewaters contain various types of
salts and surfactants, among which NaCl and the phenol sodium dodecyl
benzenesulfonate (SDBS) are most common. Salt gives high ionic strength
and increases the efficiency of the adsorption process [109]. Recently,
Akl et al. [37] found that increasing ionic strength with NaCl, KCl, and
CaCl 2 compresses the diffuse double layer and improves the removal of
acid dye from aqueous solution by both bentonite and modified bentonite
(Figure 9.3). Theoretically, when the electrostatic forces are attractive, an
increase in ionic strength will decrease the adsorption capacity [51,110].
To the contrary, the studies done by Akl et al. [37] and Li et al. [51] showed
that the uptake of anionic dyes on positively charged modified bentonite
increases with ionic strength.
Eren and Afsin [27] studied the effect of ionic strength using NaCl on
bentonite adsorption of cationic Crystalline Violet. They explained that
increasing ionic strength entraps dye molecules in newly generated aggre-
gates [111]. These aggregates initially form by edge-to-edge type agglom-
eration, and then with a more concentrated solution, form by face-to-face
type agglomeration, giving more compact, irregular, multilayered aggre-
gates [112]. There is an overall increase in mean aggregate radius.
Li et al. [51] also investigated the effect of ionic strength using SDBS on
epichlorhydrin-dimethylamine (Epi-DMA)-modified bentonite for four
different dyes. Increasing ionic strength decreases dye removal because
SDBS reacts with EPI-DMA/bentonite much stronger than the dyes [113].

9.3.4 Adsorption Kinetics

Physical or chemical characteristics of the adsorbent system affect the
nature of the adsorption process. To explain and examine the controlling
mechanism of adsorption process, time-dependent experimental adsorp-
tion data are used and several kinetics models are available. Literature
generally shows that the equilibrium adsorption between the dye and ben-
tonite is attained within a very short time. In the initial stage of adsorption,