5 Mechanism of Biosorption
Biosorption can be classified based on the metabolism dependence and on the
location of the adsorption. Biosorption takes place either extracellular or intracellular. Extracellular biosorption may be precipitation or cell surface adsorption with the
help of physical forces.
The cells produce compounds that can precipitate the metal pollutant and precipitate them around the cells or on the cell membrane. This process can be either
metabolism dependent or due to chemical interaction between the cell surface and
the adsorbate. Some bacteria are capable of secreting ligands, phosphates, acids and
extracellular polymeric substances (EPS) including proteins, polysaccharides,
nucleic acids, lipids and glycoproteins. These EPS have a large diversity in the
functional groups and are capable of sequestrating heavy metals and precipitating
them. Pseudomonas stutzeri, Pseudomonas aeruginosa, Paenibacillus jamilae and
Bacillus firmicus are capable of sequestrating and removing lead from environmental
samples [40]. Arthrobacter and Pseudomonas remove cadmium by detoxification
system that precipitates cadmium on cell surface [41].
Physical forces involved in the cell surface adsorption include van der Waal’s
forces, electrostatic interaction and ion exchange. As microbial cell wall contains a
large number of binding sites for metals in the form of carbonyl, phosphoryl,
sulfhydryl and carboxyl groups, the adsorption can also take place through the
chemical adsorption. The cell wall structure of fungi, algae and bacteria can adsorb
metals such as copper, zinc, cadmium, lead, cobalt and even uranium. Fungal cells
have chitin on their cell wall which can have electrostatic interaction with the
adsorbate and adsorbs them. The natural polysaccharides on the cell wall of these
biomasses have bivalent metal ions like K
+
, Na
+
, Ca
2+ and/or Mg
2+ which exchanges
counter ions such as Co
+
, Cd
+
, Cu
+ and Zn
+
. The polysaccharides on the cell wall are
also capable of forming complexes with metal ions and thus adsorbing them on their
cell surface. This process is called as complexation [42].
Intracellular biosorption requires the adsorbate to be transported into the cell.
Heavy metals are taken into the cell by the same mechanism involved in transport of
metabolically important ions. After transfer of the adsorbate into the cell, it can go
through three processes: (1) binding to inner membrane of cell wall, (2) uptake as
nutrient and (3) formation of complexes with adsorbate and precipitating them inside
the cell. This kind of biosorption involving transport across cell membrane requires
cells metabolism and takes place only on viable cells.
6 Modes of Operation
Biosorption for the removal of pollutants from water can be carried out in batch and
continuous modes.
250
K. J. Samuel P N et al.
Biosorption can be classified based on the metabolism dependence and on the
location of the adsorption. Biosorption takes place either extracellular or intracellular. Extracellular biosorption may be precipitation or cell surface adsorption with the
help of physical forces.
The cells produce compounds that can precipitate the metal pollutant and precipitate them around the cells or on the cell membrane. This process can be either
metabolism dependent or due to chemical interaction between the cell surface and
the adsorbate. Some bacteria are capable of secreting ligands, phosphates, acids and
extracellular polymeric substances (EPS) including proteins, polysaccharides,
nucleic acids, lipids and glycoproteins. These EPS have a large diversity in the
functional groups and are capable of sequestrating heavy metals and precipitating
them. Pseudomonas stutzeri, Pseudomonas aeruginosa, Paenibacillus jamilae and
Bacillus firmicus are capable of sequestrating and removing lead from environmental
samples [40]. Arthrobacter and Pseudomonas remove cadmium by detoxification
system that precipitates cadmium on cell surface [41].
Physical forces involved in the cell surface adsorption include van der Waal’s
forces, electrostatic interaction and ion exchange. As microbial cell wall contains a
large number of binding sites for metals in the form of carbonyl, phosphoryl,
sulfhydryl and carboxyl groups, the adsorption can also take place through the
chemical adsorption. The cell wall structure of fungi, algae and bacteria can adsorb
metals such as copper, zinc, cadmium, lead, cobalt and even uranium. Fungal cells
have chitin on their cell wall which can have electrostatic interaction with the
adsorbate and adsorbs them. The natural polysaccharides on the cell wall of these
biomasses have bivalent metal ions like K
+
, Na
+
, Ca
2+ and/or Mg
2+ which exchanges
counter ions such as Co
+
, Cd
+
, Cu
+ and Zn
+
. The polysaccharides on the cell wall are
also capable of forming complexes with metal ions and thus adsorbing them on their
cell surface. This process is called as complexation [42].
Intracellular biosorption requires the adsorbate to be transported into the cell.
Heavy metals are taken into the cell by the same mechanism involved in transport of
metabolically important ions. After transfer of the adsorbate into the cell, it can go
through three processes: (1) binding to inner membrane of cell wall, (2) uptake as
nutrient and (3) formation of complexes with adsorbate and precipitating them inside
the cell. This kind of biosorption involving transport across cell membrane requires
cells metabolism and takes place only on viable cells.
6 Modes of Operation
Biosorption for the removal of pollutants from water can be carried out in batch and
continuous modes.
250
K. J. Samuel P N et al.