Dye Adsorption on Expanding Three-Layer Clays - Green Chemistry for Dyes Removal from Wastewater - page 347

Figure 9.4 The second-order-pseudo kinetics for adsorption of Congo red by modified
bentonite [25].
phases at equilibrium. Adsorption isotherms are mathematical models
that describe distribution of the adsorbate species among liquid and
solid phases. A literature survey indicates that Langmuir and Freundlich
adsorption iosotherms have been used extensively to describe clay-dye
adsorption.
Table 9.6 shows adsorption isotherm parameters for both anionic and
cationic dye adsorbed onto natural and modified clays. The Langmuir
iostherm generally fits the experimental data well. However, it should be
noted that comparative evaluation is limited because the solid-to-liquid
ratio might differ from one case to another.
Li et al. [51] investigated the adsorption properties and mechanisms of
a cationic-polymer/bentonite complex (EPI-DMA/bentonite) for anionic
dyes (Reactive Violet K-3R and Acid Dark Blue 2G). Their results on anionic
dye/EPI-DMA/bentonite system showed that the Freundlich model is most
suitable. Other investigations also showed that the Freundlich isotherm
can be a good fit. Rarasponin-modified bentonite was used to remove
basic dyes (MB and MG) and follows both Langmuir and Freundlich mod-
els [49]. Adsorption of MB is higher than MG dye because small molecules
gain easier access to the internal pore network of the adsorbent. Similar
results are observed for montmorillonite [23].