BIOSORPTION OF Pb2+, Cd2+, & Ni2+ FROM WATERS BY ...

cell metabolism. From the other point of view, precipitation may not be dependent on
the cells' metabolism when a chemical interaction happens between the metal and
cell surface.
Transport across
cell membrane
BIOSORPTION MECHANISMS
METABOLISM DEPENDENT NON METABOLISM DEPENDENT
Precipitation Physical
adsorption
Figure 1.1. Biosorption mechanisms
(Source:Vegliò 1997)
Ion exchange Complexation
Physico-chemical interaction between the metal and the functional groups
present on the microbial cell surface exists throughout non-metabolism dependent
biosorption. In this case, biosorption mechanism is relied on physical adsorption, ion
exchange and complexation that are independent of metabolism. Cell walls of
microbial biomass contains a great degree of polysaccharides, proteins and lipids, and
these biomolecules have binding groups such as carboxyl, hydroxyl, sulphate,
phosphate and amino groups (Christ, et al. 1981). The functional groups carries
negative charge to the cell surface. The binding process of this type of action is
mainly a physical nature, usually rapid and reversible,and it also requires minimum
energy for activation. Physical adsorption takes place owing to Waals' forces,
Kuyucak and Tsezos proved that biosorption of uranium and thorium by Rhizopus
arrhizus is physical adsorption in th cell-wall chitin structure. Copper biosorption by
alga Chlorella vulgaris and bacterium Zoogloea ramigera was studied and the metal
uptake occurs by electrostatic interactions (Aksu, et al. 1992). Polysaccharides are the
cell wall constituents of microorganisms and the counter ions (K + , Na + , Ca 2+ , and
Mg 2+ ) of the polysaccharides are exchanged with bivalent metal ions. For example,
marine algae contains alginate salts of K + , Na + , Ca 2+ , and Mg 2+ and the biosorption of
heavy metals takes place with the replacement of counter ions with the bivalent metal
ions (Kuyucak and Volesky 1988). Complex formation is another type of biosorption
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