94
Further investment about the adsorption properties of polymeric, metal oxide, and
biosorbent composites of montmorillonite was performed as regards possible interaction mechanism and the latest environmental application.
Keywords Adsorption · Montmorillonite · Nanocomposite · Heavy metal ·
Permanently negative charge · Edge sites · Adsorption isotherms · Heavy metal
removal · Surface acidity · Biosorbent
4.1 Introduction: Montmorillonite as a Clay
and an Adsorbent
The clay minerals occur as a result of chemical weathering of rocks and are differentiated from soil with crystallographic arrangements of Al
3+
, Si
4+
, and oxygen
atoms. Clay minerals form from octahedral and tetrahedral structures involving
oxygen or hydroxyl groups. Regarding the tetrahedral to octahedral ratio, two types
of clay mineral exist in nature: 1:1 structures and 2:1 structures. The 1:1 kaolin
structures consist of tetrahedral sites, which are occupied by silicon, and octahedral
sites are occupied by aluminum (Hızal and Apak 2006a, b). In 2:1 type of clays
named as smectite-type clays, octahedral alumina sheets are constructed between
two tetrahedral silica sheets.
The isomorphic substitution of Si(IV) by Al(III) between adjacent octahedral
and tetrahedral lattice results in accumulation of negative charge throughout tetrahedral surface. While the tetrahedral surface permanently carries negative charge,
which enables to interact with positively charged ions, the tetrahedral surface also
carries pH-dependent variable charge because of protonation/deprotonation of surface hydroxyl groups. On the other hand, alumina surface has pH-dependent sites
called edge sites.
In aqueous solution, silanols and aluminols convert to hydrated form to generate
donor and proton-releasable hydroxyl groups. These edge sites on alumina and silica surfaces have acidity constant sourced from acidic character of hydrated oxygen. This property is majorly responsible for the interaction between clay and
environmental contaminants as heavy metal ions.
The adsorption of positively charged ions mainly occurs on proton-releasable
functional groups having donor character via covalent binding. Additionally, heavy
metal ions may bind to the organic components of clay having many functional
groups such as carboxyl, phenol, or amine.
The primarily construction components of montmorillonite are SiO 2 and Al 2 O 3 .
Other oxides as Fe 2 O 3 , TiO 2 , and oxides of alkali and alkaline earth metals can exist
in montmorillonite with variable percentage. The predominant species in nature is
calcium montmorillonite (Uddin 2018). Erçağ et al. declared that the diameter of
50% of montmorillonite particles and a mean particle diameter of montmorillonite
are 9.24 and 12.30 μm, respectively.
J. Hızal and M. Yılmazoğlu
Further investment about the adsorption properties of polymeric, metal oxide, and
biosorbent composites of montmorillonite was performed as regards possible interaction mechanism and the latest environmental application.
Keywords Adsorption · Montmorillonite · Nanocomposite · Heavy metal ·
Permanently negative charge · Edge sites · Adsorption isotherms · Heavy metal
removal · Surface acidity · Biosorbent
4.1 Introduction: Montmorillonite as a Clay
and an Adsorbent
The clay minerals occur as a result of chemical weathering of rocks and are differentiated from soil with crystallographic arrangements of Al
3+
, Si
4+
, and oxygen
atoms. Clay minerals form from octahedral and tetrahedral structures involving
oxygen or hydroxyl groups. Regarding the tetrahedral to octahedral ratio, two types
of clay mineral exist in nature: 1:1 structures and 2:1 structures. The 1:1 kaolin
structures consist of tetrahedral sites, which are occupied by silicon, and octahedral
sites are occupied by aluminum (Hızal and Apak 2006a, b). In 2:1 type of clays
named as smectite-type clays, octahedral alumina sheets are constructed between
two tetrahedral silica sheets.
The isomorphic substitution of Si(IV) by Al(III) between adjacent octahedral
and tetrahedral lattice results in accumulation of negative charge throughout tetrahedral surface. While the tetrahedral surface permanently carries negative charge,
which enables to interact with positively charged ions, the tetrahedral surface also
carries pH-dependent variable charge because of protonation/deprotonation of surface hydroxyl groups. On the other hand, alumina surface has pH-dependent sites
called edge sites.
In aqueous solution, silanols and aluminols convert to hydrated form to generate
donor and proton-releasable hydroxyl groups. These edge sites on alumina and silica surfaces have acidity constant sourced from acidic character of hydrated oxygen. This property is majorly responsible for the interaction between clay and
environmental contaminants as heavy metal ions.
The adsorption of positively charged ions mainly occurs on proton-releasable
functional groups having donor character via covalent binding. Additionally, heavy
metal ions may bind to the organic components of clay having many functional
groups such as carboxyl, phenol, or amine.
The primarily construction components of montmorillonite are SiO 2 and Al 2 O 3 .
Other oxides as Fe 2 O 3 , TiO 2 , and oxides of alkali and alkaline earth metals can exist
in montmorillonite with variable percentage. The predominant species in nature is
calcium montmorillonite (Uddin 2018). Erçağ et al. declared that the diameter of
50% of montmorillonite particles and a mean particle diameter of montmorillonite
are 9.24 and 12.30 μm, respectively.
J. Hızal and M. Yılmazoğlu