Bentonite, which is consisting essentially of montmorillonite
group, is one of natural clays composed of two silica tetrahedral
sheets with an octahedral alumina sheet. Bentonite surface is
negatively charged due to isomorphous substitution of Al3+ for Si4+
in the tetrahedral layer and Mg2+ for Al3+ in the octahedral layer.
This negative charge is balanced by exchangeable cations such as
Na+ in the lattice structure. The layered structure of bentonite
allows swelling after wetting. Na+ and Ca2+ are hydrates in the
presence of water; therefore there is a hydrophilic environment at
clay surface and natural bentonite is not an effective adsorbent for
the nonpolar organic compounds in water. Surface properties of
bentonite can be greatly changed by using surfactants or modifying
agents. These modified bentonites have been widely used in
wastewater treatment processes [16,17]