Dye Adsorption on Expanding Three-Layer Clays - Green Chemistry for Dyes Removal from Wastewater - page 340

Surface charge is also important for organoclays. Acid and reactive
dyes are water-soluble anionic dyes with negative charge, so the sur-
face of natural clays has to be modified by a cationic surfactant for dye
removal. The adsorption of anionic dyes on natural bentonite is difficult.
Adsorption depends on pH due to the increasing negative charge with
increasing pH. The zeta potential variation of bentonite versus pH is given
in Figure 9.1 [50]. Although the zeta potential trend is the same for hexa-
decyltrimethylammonium bromide (HTAB)-modified bentonite [38] and
cationic-polymer/bentonite, these modified bentonites remain positive for
a much higher pH range [51].
Figure 9.1 illustrates an IEP of pH = 3 for natural bentonite and an IEP
of pH = 7.5 for HTAB-modified bentonite. Below the IEP the positively
charged surface attracts negative dye molecules. Baskaralingam et al. [83]
have shown that natural bentonite had virtually zero adsorption of Acid
Red 1515, while cetyldimethylbenzylammonium- and cetylpyridinium-
modified bentonite yield good removal of the dye in the acidic pH range
and decrease with increasing pH. The same trend was observed by Socias-
Viciana et al. [98], who used Na-bentonite and dodecyltrimethylammonium
bromide-modified bentonite (DTMA-bentonite) to remove Acid Blue dye.
This trend was also observed by Khenifi et al. [99], who studied the removal
of Supranol Yellow 4GL using organo bentonite. They described how sur-
factants adsorb to the external surfaces of bentonite via cation exchange.
Then, due to extensive hydrophobic bonding, tail-tail interactions between
surfactant and dye cause positive charge development on the surfaces that
ultimately leads to clay dispersion [51,80,100,101]. Baskaralingam et al.
[83] also showed that dye uptake decreases as pH approaches 10, and the
Figure 9.1 Effect of pH on zeta potential of bentonite and HTAB-modified bentonite [50].