can alter the oxidation number of HMs and change into less toxic forms. Microprecipitation of HM removal in the form of phosphates and sulfates by AAB is a
potential approach to remove HMs from wastewater (Ungureanu et al. 2015). In case
of PAB, drying and crushing should increase the metal-adsorbing capacity. The algal
cell wall made up of microfibrillar exo-polysaccharides has typical chemical composition and contains functional groups as shown in Table 3.2 such as –COOH
À ,
-OH
À , -PO 4
À3 , -RSH, SO 4
À2 , etc.
These functional groups produce anionic nature to the cell wall and microfibrils.
Since HM ion in wastewater is cationic in nature, they are adsorbed by the cell and
microfibril surface. Cyanobacterial cell wall consists of mainly peptidoglycan,
polymer of N-acetylglucosamine and β-1,4-N-acetylmuramic acid, which provides
mostly –COOH functional group for HM adsorption. Few cyanobacterial cell walls
bear capsule wall which is anionic in nature due to acid in nature and thus help in
metal adsorption. Eukaryotic algae cell wall contains heteropolysaccharides, which
offer -COOH and SO 4
À2 groups for HM adsorption (David et al. 2012). Plasma
membrane and membranes of organelles, consisting of lipopolysaccharides and
lipoprotein, contribute significantly to metal sorption by algae and cyanobacteria.
2.3 Ion-Exchange Concept
Microalgal cell wall plays significant role in an ion exchange as it is composed of
polysaccharides, proteins, and lipids. These constituents contribute to various functional groups (carboxyl, hydroxyl, phosphate, amino, sulfhydryl, amide, alkyl, and
aromatic compound) and hence possess an overall negative charge to the cell
surface. The cell surface of alga acts as a strong binding site for metal cations and
is involved in metal exchange through the ion-exchange mechanism (Monteiro et al.
2011). During the interaction between metal ion and protein on biological surface,
metal ions coordinated in formation of complex groups. However, in marine system,
a major part of active sites are bonded with protons at low pH or with alkaline earth
metals (Ca, Na, and Mg) at higher pH. In the presence of cations such as Cu
+2 , Mn
+2 ,
Zn
+2 , Ni
+2 , Cd
+2 , Fe
+3 , and Pb
+2 , the previously bind protons and metals are
Table 3.2 Functional groups involved in metal ion biosorption and adsorption
Binding group
Functional group Ligand atom Occurrence
Hydroxyl
-OH
O
Polysaccharides, uric acid, amino acid
Sulfhydryl (thiol) -SH
S
Amino acid
Sulfonate
O¼S¼O
O
Sulfated
Amine
-NH 2
N
Amino acid
Imine
-NH
N
Amino acid
Imidazole
-C-NH > CH
O
Amino acid
Phosphodiester
>P¼O-OH
O
Teichoic acid, lipo PS
Chowdhury et al. 2015; Zhang et al. 2015; He and Chen 2014
3 Phycoremediation: Algae as Eco-friendly Tools for the Removal of Heavy. . .
59
potential approach to remove HMs from wastewater (Ungureanu et al. 2015). In case
of PAB, drying and crushing should increase the metal-adsorbing capacity. The algal
cell wall made up of microfibrillar exo-polysaccharides has typical chemical composition and contains functional groups as shown in Table 3.2 such as –COOH
À ,
-OH
À , -PO 4
À3 , -RSH, SO 4
À2 , etc.
These functional groups produce anionic nature to the cell wall and microfibrils.
Since HM ion in wastewater is cationic in nature, they are adsorbed by the cell and
microfibril surface. Cyanobacterial cell wall consists of mainly peptidoglycan,
polymer of N-acetylglucosamine and β-1,4-N-acetylmuramic acid, which provides
mostly –COOH functional group for HM adsorption. Few cyanobacterial cell walls
bear capsule wall which is anionic in nature due to acid in nature and thus help in
metal adsorption. Eukaryotic algae cell wall contains heteropolysaccharides, which
offer -COOH and SO 4
À2 groups for HM adsorption (David et al. 2012). Plasma
membrane and membranes of organelles, consisting of lipopolysaccharides and
lipoprotein, contribute significantly to metal sorption by algae and cyanobacteria.
2.3 Ion-Exchange Concept
Microalgal cell wall plays significant role in an ion exchange as it is composed of
polysaccharides, proteins, and lipids. These constituents contribute to various functional groups (carboxyl, hydroxyl, phosphate, amino, sulfhydryl, amide, alkyl, and
aromatic compound) and hence possess an overall negative charge to the cell
surface. The cell surface of alga acts as a strong binding site for metal cations and
is involved in metal exchange through the ion-exchange mechanism (Monteiro et al.
2011). During the interaction between metal ion and protein on biological surface,
metal ions coordinated in formation of complex groups. However, in marine system,
a major part of active sites are bonded with protons at low pH or with alkaline earth
metals (Ca, Na, and Mg) at higher pH. In the presence of cations such as Cu
+2 , Mn
+2 ,
Zn
+2 , Ni
+2 , Cd
+2 , Fe
+3 , and Pb
+2 , the previously bind protons and metals are
Table 3.2 Functional groups involved in metal ion biosorption and adsorption
Binding group
Functional group Ligand atom Occurrence
Hydroxyl
-OH
O
Polysaccharides, uric acid, amino acid
Sulfhydryl (thiol) -SH
S
Amino acid
Sulfonate
O¼S¼O
O
Sulfated
Amine
-NH 2
N
Amino acid
Imine
-NH
N
Amino acid
Imidazole
-C-NH > CH
O
Amino acid
Phosphodiester
>P¼O-OH
O
Teichoic acid, lipo PS
Chowdhury et al. 2015; Zhang et al. 2015; He and Chen 2014
3 Phycoremediation: Algae as Eco-friendly Tools for the Removal of Heavy. . .
59