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

mechanism on the cell surface after the interaction between the metal and active
groups occurs. This type of mechanism is only valid for Ca, Mg, Cd, Zn, Cu, and Hg
(Vegliò 1997). Organic acids such as citric, oxalic, gluonic, fumaric, lactic and malic
acids can be be produced by micro-organisms, and chelation of toxic metals with
these acids forms metallo-organic molecules. Carboxyl groups found in microbial
polysaccahrides and other polymers may give complexation reaction with metals.
It is clear to be understanding that biosorption mechanisms can also take place
at the same time.
1.4.2. Factors Affecting Biosorption
In order to understand the quality of the biosorption process and to design the
system for the removal of contaminants, we need to evaluate the effect of factors
which influences the performance of biosorption. The following factors are thougth to
be responsible for biosorption:
1. When the temperature of the sorption media is in the range of 20-35 0 C the
biosorption process is not affected by temperature (Aksu, et al. 1992).
However, if the increment in the temperature reduces the sorption capacity of
biomass (Aksu 2001).
2. The major challenging factor for the biosorptive process is pH seems to be the
most important parameter in the biosorptive process. Since, it influences not
only the solution chemistry of the metals but also the activity of the
functional groups in the biomass and the competition of metallic ions. Table
1.2 shows the functional groups responsible for the binding of metal and their
acidity constants. These groups create a negatively charged surface at above
the isoelectronic point of the biomass and electrostatic attractions among
metals and the cell surface is involved in the biosorption process. At low pH
values below isoelectronic point, the functional groups are protonated surface
charge. This comes to a conclusion with lower metal uptake when the metal is
in the cationic form such as Pb 2+ , this is due to blocking of functional groups
responsible for the metal uptake. However, in the case of Cr(VI) which is
anionic in nature, it can bind at lower pH values , that is the surface charge of
the cell wall is positive in nature.
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