255
Zeolites can provide adsorption sites for heavy metal ions by exchanging the
cations. Other than ion exchange and adsorption, many more mechanisms involved
in removal of heavy metals by zeolite are surface precipitation, destruction, or dissolution of natural zeolite structure. Zeolites have large surface areas in the range of
600–800 m
2
g
−1
, and because of its ion-exchange capability, it is an efficient adsorbent for heavy metal removal. The price of zeolite considered itself very cheap
depending on the quality of the zeolites. It was found that clinoptilolite gives high
selectivity and cation exchange capacity for metal ions like Pb
2+
, Cd
2+
, Zn
2+
, and
Cu
2+
which are greatly influenced by pretreatment procedure (Babel and Kurniawan
2003). The adsorption parameter such as pH plays important role on selection of
heavy metal. 4A zeolite synthesized from kaolin showed good result in Cu
+2
and
Zn
+2
removal at both neutral and high pH, while Cr
+6
was better adsorbed at low pH
(Barakat 2008). It was reported that zeolites in both forms clinoptilolite and chabazite, have significant removal efficiency for treating effluent contaminated with
Pb
+2
, Cd
+2
, Cu
+2
, Zn
+2
, Ni
+2
, and Co
+2
(Ouki and Kavanagh 1997). Therefore both the
ions have greater opportunity to exchange with Na ions of zeolite. Magnetically
modified synthetic zeolite with Fe 2 O 3 showed high Pb
+2
adsorption capacities for
and good chemical resistance at a wide pH range 5–11 (Nah et al. 2006). Overall we
can say that zeolite has a great potential to remove metal ions from various industrial effluent.
10.3.2 Clay
Clays as an adsorbent have many advantages compared to other adsorbents such as
its abundant availability, low cost, high specific surface area, excellent adsorption
ability, non-toxic nature, and large ion-exchange potential. Usually clay minerals
are very efficient and widely used to adsorb metal cations as they are negatively
charged by a number of adsorption mechanisms like ion exchange and metal cations
and with clay surface bonding. Clays are mainly mixture of clay minerals composed
of hydrous aluminum phyllosilicates with an average particle size of less than 2 μm,
crystals of other minerals, and metal oxides. Clays are classified into groups like
smectites such as montmorillonite and saponite; mica such as illite; kaolinite; vermiculite; serpentine; pyrophyllite such as talc; and sepiolite (Shichi and Takagi 2000).
Montmorillonite, a subgroup of smectites is considered to be most efficient for
adsorption due to its availability, adsorption capacity, high specific surface area, and
compatible cation exchange capacity among all of the clay families (Kennedy
1990). Copper and nickel removal using raw montmorillonite claimed that internal
solute pore diffusion at clay wall was more essential than external mass transfer for
the adsorption mechanism (Ijagbemi et al. 2009). The other study reported that
sodium-montmorillonite is potentially more effective for adsorption of Cu
+2
, Pb
+2
,
Zn
+2
, Cd
+2
, and Co
+2
ions than calcium–montmorillonite (Chen et al. 2015). Because
of hydrophilic nature of clay mineral, it cannot be used without certain modification. Organically modified montmorillonite appeared to be better adsorbent than the
10 Heavy Metal Removal by Low-Cost Adsorbents
Zeolites can provide adsorption sites for heavy metal ions by exchanging the
cations. Other than ion exchange and adsorption, many more mechanisms involved
in removal of heavy metals by zeolite are surface precipitation, destruction, or dissolution of natural zeolite structure. Zeolites have large surface areas in the range of
600–800 m
2
g
−1
, and because of its ion-exchange capability, it is an efficient adsorbent for heavy metal removal. The price of zeolite considered itself very cheap
depending on the quality of the zeolites. It was found that clinoptilolite gives high
selectivity and cation exchange capacity for metal ions like Pb
2+
, Cd
2+
, Zn
2+
, and
Cu
2+
which are greatly influenced by pretreatment procedure (Babel and Kurniawan
2003). The adsorption parameter such as pH plays important role on selection of
heavy metal. 4A zeolite synthesized from kaolin showed good result in Cu
+2
and
Zn
+2
removal at both neutral and high pH, while Cr
+6
was better adsorbed at low pH
(Barakat 2008). It was reported that zeolites in both forms clinoptilolite and chabazite, have significant removal efficiency for treating effluent contaminated with
Pb
+2
, Cd
+2
, Cu
+2
, Zn
+2
, Ni
+2
, and Co
+2
(Ouki and Kavanagh 1997). Therefore both the
ions have greater opportunity to exchange with Na ions of zeolite. Magnetically
modified synthetic zeolite with Fe 2 O 3 showed high Pb
+2
adsorption capacities for
and good chemical resistance at a wide pH range 5–11 (Nah et al. 2006). Overall we
can say that zeolite has a great potential to remove metal ions from various industrial effluent.
10.3.2 Clay
Clays as an adsorbent have many advantages compared to other adsorbents such as
its abundant availability, low cost, high specific surface area, excellent adsorption
ability, non-toxic nature, and large ion-exchange potential. Usually clay minerals
are very efficient and widely used to adsorb metal cations as they are negatively
charged by a number of adsorption mechanisms like ion exchange and metal cations
and with clay surface bonding. Clays are mainly mixture of clay minerals composed
of hydrous aluminum phyllosilicates with an average particle size of less than 2 μm,
crystals of other minerals, and metal oxides. Clays are classified into groups like
smectites such as montmorillonite and saponite; mica such as illite; kaolinite; vermiculite; serpentine; pyrophyllite such as talc; and sepiolite (Shichi and Takagi 2000).
Montmorillonite, a subgroup of smectites is considered to be most efficient for
adsorption due to its availability, adsorption capacity, high specific surface area, and
compatible cation exchange capacity among all of the clay families (Kennedy
1990). Copper and nickel removal using raw montmorillonite claimed that internal
solute pore diffusion at clay wall was more essential than external mass transfer for
the adsorption mechanism (Ijagbemi et al. 2009). The other study reported that
sodium-montmorillonite is potentially more effective for adsorption of Cu
+2
, Pb
+2
,
Zn
+2
, Cd
+2
, and Co
+2
ions than calcium–montmorillonite (Chen et al. 2015). Because
of hydrophilic nature of clay mineral, it cannot be used without certain modification. Organically modified montmorillonite appeared to be better adsorbent than the
10 Heavy Metal Removal by Low-Cost Adsorbents