336
The mechanism of biosorption appears to occur through a combination of mechanisms such as complexation, ion exchange, coordination, chelation, physical
adsorption, and microprecipitation. However, complexation and ion exchange are
the most commonly reported mechanisms for biosorption (Lü et al. 2010). Zou et al.
(2012) in their study found that at pH above 5.0, deprotonation of carboxylic groups
happens producing negatively charged carboxylate ligands (–COO–). The ligands
then bind to the uranium (IV) molecules which are positively charged, hence, confirming the ion-exchange mechanism in the biosorption of the uranium (IV) ion by
the cell wall of grapefruit waste.
13.4 Factors Affecting Metal Ion Biosorption
There are many factors that affect the biosorption process. Biosorbent dosage is
important as it can decrease the ratio of biosorbed pollutant to weight of biosorbent
and increase the efficiency of the removal of biosorbent when used optimally. Initial
pollutant concentration has the potential to increase the amount of biosorbent used
but if too much can decrease the efficiency of the biosorption. The overall pH of the
solution can affect the biosorption of metallic cations in a positive way and the
anionic metals in the negative way.
An increase in the temperature of the reaction can potentially increase the kinetic
energy and surface activity of the biosorbent. However, too high temperature can
destroy the active sites on the biosorbent. As the biosorption process requires agitation, the speed of agitation also can help to increase the adsorption rate as it can
increase the mass transfer rate. However, too high speed can destroy the structure of
the biosorbent.
Another important factor is the size of the biosorbent. Smaller biosorbent particles will have higher surface area helping in more effective biosorption. This is only
true for batch processes as column process may be disrupted due to clogging. Other
pollutant concentration in the solution may also affect the metal ion absorption as it
can compete with the metallic cations for the binding sites on the biosorbent (Alfarra
et al. 2014).
13.4.1 Effect of pH
The presence of heavy metals and its characteristics are heavily dependent on the
pH of the water source. The pH of the water will determine the speciation of heavy
metals in aqueous solution. Heavy metals normally exist in a more soluble, mobile,
and cationic state at neutral to low pH. Complexes with hydroxides and other anions
in the water will start forming if the pH rises. Apart from that, pH also influences the
surface charge of the adsorbent, the ionization state of the adsorbent, as well as the
concentration of ions on the functional groups of the adsorbent (Taşar et al. 2014).
S. Ganesan
The mechanism of biosorption appears to occur through a combination of mechanisms such as complexation, ion exchange, coordination, chelation, physical
adsorption, and microprecipitation. However, complexation and ion exchange are
the most commonly reported mechanisms for biosorption (Lü et al. 2010). Zou et al.
(2012) in their study found that at pH above 5.0, deprotonation of carboxylic groups
happens producing negatively charged carboxylate ligands (–COO–). The ligands
then bind to the uranium (IV) molecules which are positively charged, hence, confirming the ion-exchange mechanism in the biosorption of the uranium (IV) ion by
the cell wall of grapefruit waste.
13.4 Factors Affecting Metal Ion Biosorption
There are many factors that affect the biosorption process. Biosorbent dosage is
important as it can decrease the ratio of biosorbed pollutant to weight of biosorbent
and increase the efficiency of the removal of biosorbent when used optimally. Initial
pollutant concentration has the potential to increase the amount of biosorbent used
but if too much can decrease the efficiency of the biosorption. The overall pH of the
solution can affect the biosorption of metallic cations in a positive way and the
anionic metals in the negative way.
An increase in the temperature of the reaction can potentially increase the kinetic
energy and surface activity of the biosorbent. However, too high temperature can
destroy the active sites on the biosorbent. As the biosorption process requires agitation, the speed of agitation also can help to increase the adsorption rate as it can
increase the mass transfer rate. However, too high speed can destroy the structure of
the biosorbent.
Another important factor is the size of the biosorbent. Smaller biosorbent particles will have higher surface area helping in more effective biosorption. This is only
true for batch processes as column process may be disrupted due to clogging. Other
pollutant concentration in the solution may also affect the metal ion absorption as it
can compete with the metallic cations for the binding sites on the biosorbent (Alfarra
et al. 2014).
13.4.1 Effect of pH
The presence of heavy metals and its characteristics are heavily dependent on the
pH of the water source. The pH of the water will determine the speciation of heavy
metals in aqueous solution. Heavy metals normally exist in a more soluble, mobile,
and cationic state at neutral to low pH. Complexes with hydroxides and other anions
in the water will start forming if the pH rises. Apart from that, pH also influences the
surface charge of the adsorbent, the ionization state of the adsorbent, as well as the
concentration of ions on the functional groups of the adsorbent (Taşar et al. 2014).
S. Ganesan