Marine Algae Biomass for Removal of Heavy Metal Ions 25.4 Influence of Experimental Parameters 625
Part D | 25.4
capacity of the biomass with 0:07 mmol=g in the case
of Pb(II), with 0:06 mmol=g in the case of Cd(II) and
with 0:03 mmol=g in the case of Co(II), respectively.
On the basis of these experimental results, it
is recommended that at large scale, the removal
of heavy metal ions from aqueous solutions by
biosorption on marine algae and marine algae waste
biomass to be performed at ambient temperature, and
this is dictated especially by economical considerations [25.116].
On the other hand, the change in temperature
causes a change of thermodynamic parameters, such
as free Gibbs energy change (G), enthalpy change
(H) or entropy change (S), which contribute to
help understand the biosorption mechanisms [25.117].
Temperature data are usually used to determine these
parameters, because they provide valuable information
about the biosorption process. The negative value of
free Gibbs energy change (G) shows that the process is feasible and spontaneous. The increase of the
G value, in the negative direction, with temperature is an indication of the increase of biosorption
process probability [25.24]. Also, the standard enthalpy change (H) shows the route of energy in
the system, and has an important contribution to deciding whether a certain biosorbent can be used for
the removal of heavy metal ions at elevated temperature or not. A positive value indicates an endothermic
process that will be favored by high temperatures,
while a negative value of this parameter shows an
exothermic process, which is more efficient at low
temperatures.
Table 25.5 summarizes the values of thermodynamic parameters for some removal processes of heavy
metal ions from aqueous media, using marine algae
biomass as biosorbents.
The positive value of the entropy change indicates
the increasing randomness at the solid/liquid interface
during the biosorption process, whereas the negative
value of this parameter suggests a decrease in the randomness at the solid/liquid interface during the biosorption process [25.76]. Thus, even if some biosorption
processes are endothermic, in some conditions these
can be spontaneous because of the positive entropy
change.
All these parameters should be considered in the
experimental studies of heavy metal ions removal by
biosorption onto marine algae biomasses, and the obtained results will allow us to establish the optimum
conditions, to ensure the maximum efficiency of the
biosorption process.
25.4.2 Influence of Experimental
Parameters in a Dynamic
Continuous-Flow Technique
The process of heavy metal ions biosorption in a dynamic continuous-flow technique is mainly influenced
by three key experimental parameters: bed height, flow
rate of aqueous solution, and initial concentration of
heavy metal ions.
As the heavy metal ions solution passes through
the column, the biosorption zone (where the bulk of
biosorption takes place) starts moving out of the column and the effluent concentrations start rising with
time. As a consequence, the variation of these parameters will change the characteristics of the breakthrough
curve (breakthrough point and saturation point), and
this will also affect the applicability of the heavy metal
ion removal process. For this reason, in the experimental studies, for each of these parameters the optimal
values that should be established to ensure the efficient
delivery of the biosorption process.
Effect of Bed Height
Accumulation of heavy metal ions in a packed-bed
column is largely dependent on the quantity of biosorbent (marine algae biomass) inside the column. This is
mainly due to the higher contact time between heavy
metal ion solution and available biomass in the column,
and also due to more number of binding sites and functional groups available for biosorption of heavy metal
ions [25.118].
Generally, the characteristics of the breakthrough
curves (breakthrough time (volume) and saturation time
(volume)) increases with the rise in the bed height, and
this can be easily observed from Fig. 25.7 chosen for
exemplification. This may be due to an increase in the
surface area of the biosorbent as the quantity of biomass
packed in the column increase.
However, the increase in the quantity of algae
biomass deposited in the column also results in a broadened mass transfer zone, which makes breakthrough
curves less steeper. The more gradual shape of the
curves as the bed height increased implies that the
biosorbent bed was more difficult to be completely saturated, and the decrease in the slope of the breakthrough
curves with the increase of this parameter clearly reflects this fact [25.66]. Even if a high biosorbent bed
height increases the usage time of the column, a too
high value of this parameter has several disadvantages,
the most important being the increase in the difficulty
of solution flow through the column and the quantity of
Part D | 25.4
capacity of the biomass with 0:07 mmol=g in the case
of Pb(II), with 0:06 mmol=g in the case of Cd(II) and
with 0:03 mmol=g in the case of Co(II), respectively.
On the basis of these experimental results, it
is recommended that at large scale, the removal
of heavy metal ions from aqueous solutions by
biosorption on marine algae and marine algae waste
biomass to be performed at ambient temperature, and
this is dictated especially by economical considerations [25.116].
On the other hand, the change in temperature
causes a change of thermodynamic parameters, such
as free Gibbs energy change (G), enthalpy change
(H) or entropy change (S), which contribute to
help understand the biosorption mechanisms [25.117].
Temperature data are usually used to determine these
parameters, because they provide valuable information
about the biosorption process. The negative value of
free Gibbs energy change (G) shows that the process is feasible and spontaneous. The increase of the
G value, in the negative direction, with temperature is an indication of the increase of biosorption
process probability [25.24]. Also, the standard enthalpy change (H) shows the route of energy in
the system, and has an important contribution to deciding whether a certain biosorbent can be used for
the removal of heavy metal ions at elevated temperature or not. A positive value indicates an endothermic
process that will be favored by high temperatures,
while a negative value of this parameter shows an
exothermic process, which is more efficient at low
temperatures.
Table 25.5 summarizes the values of thermodynamic parameters for some removal processes of heavy
metal ions from aqueous media, using marine algae
biomass as biosorbents.
The positive value of the entropy change indicates
the increasing randomness at the solid/liquid interface
during the biosorption process, whereas the negative
value of this parameter suggests a decrease in the randomness at the solid/liquid interface during the biosorption process [25.76]. Thus, even if some biosorption
processes are endothermic, in some conditions these
can be spontaneous because of the positive entropy
change.
All these parameters should be considered in the
experimental studies of heavy metal ions removal by
biosorption onto marine algae biomasses, and the obtained results will allow us to establish the optimum
conditions, to ensure the maximum efficiency of the
biosorption process.
25.4.2 Influence of Experimental
Parameters in a Dynamic
Continuous-Flow Technique
The process of heavy metal ions biosorption in a dynamic continuous-flow technique is mainly influenced
by three key experimental parameters: bed height, flow
rate of aqueous solution, and initial concentration of
heavy metal ions.
As the heavy metal ions solution passes through
the column, the biosorption zone (where the bulk of
biosorption takes place) starts moving out of the column and the effluent concentrations start rising with
time. As a consequence, the variation of these parameters will change the characteristics of the breakthrough
curve (breakthrough point and saturation point), and
this will also affect the applicability of the heavy metal
ion removal process. For this reason, in the experimental studies, for each of these parameters the optimal
values that should be established to ensure the efficient
delivery of the biosorption process.
Effect of Bed Height
Accumulation of heavy metal ions in a packed-bed
column is largely dependent on the quantity of biosorbent (marine algae biomass) inside the column. This is
mainly due to the higher contact time between heavy
metal ion solution and available biomass in the column,
and also due to more number of binding sites and functional groups available for biosorption of heavy metal
ions [25.118].
Generally, the characteristics of the breakthrough
curves (breakthrough time (volume) and saturation time
(volume)) increases with the rise in the bed height, and
this can be easily observed from Fig. 25.7 chosen for
exemplification. This may be due to an increase in the
surface area of the biosorbent as the quantity of biomass
packed in the column increase.
However, the increase in the quantity of algae
biomass deposited in the column also results in a broadened mass transfer zone, which makes breakthrough
curves less steeper. The more gradual shape of the
curves as the bed height increased implies that the
biosorbent bed was more difficult to be completely saturated, and the decrease in the slope of the breakthrough
curves with the increase of this parameter clearly reflects this fact [25.66]. Even if a high biosorbent bed
height increases the usage time of the column, a too
high value of this parameter has several disadvantages,
the most important being the increase in the difficulty
of solution flow through the column and the quantity of