capacity, but adversely declining the adsorption capacities. The number of adsorbates available limits further adsorption reaction; therefore adding more adsorbent is
wasting the resources. Another work on Pb(II) removal using fig sawdust also
showed a similar finding [59]. The increase in adsorbent dosage to 1.67 g/L resulted
in 95.3% removal capacity, and additional adsorbent dosage did not increase the
percentage removal capacity.
3.4 Effect of Contact Time
Generally, researchers carry out investigations on contact time for adsorption processes to determine the equilibrium time of the reaction. Once the equilibrium time is
obtained, the kinetics and equilibrium studies of adsorption process can be
established. The initial stage of adsorption is usually the fastest, at the time that
the adsorption sites are still unoccupied and the adsorbent surface is yet to be
covered by adsorbate molecules. The adsorption eventually becomes slower by
time in the act of the adsorption sites are gradually filled and competition to fill in
a site becomes stiffer. No further adsorption takes place at equilibrium condition;
thus, the adsorption capacity and percentage removal remain constant after the
equilibrium time. Table 9.5 lists the examples of equilibrium time for several
agricultural waste-derived adsorbents for heavy metal removal.
3.5 Effect of Temperature
The adsorption of heavy metals on agricultural waste-derived adsorbents is usually
temperature-dependent, and the effect of temperature on the heavy metal removal
very much depends on the nature of the adsorption process. The adsorption efficiency for exothermic adsorption increases with temperature, whereas the opposite
occurs for endothermic adsorption process. Commonly, the studies on the effect of
temperature for heavy metal removal using the agricultural waste-derived adsorbent
are conducted at near-ambient temperature because at higher temperature the structure of agricultural wastes can be easily unstabilized. The operation at near-ambient
temperature also optimizes energy consumption.
The adsorption process is mostly exothermic. The adsorption of U(VI) using
palm shell powder demonstrated a decline in the heavy metal uptake as the temperature increased from 30
C to 70
C [49]. Similarly, increasing the temperature from
20
C to 40
C for Pb(II) removal using peanut shell has a negative effect to the
heavy metal removal [66].
Only a few adsorption process studies using agricultural waste-derived adsorbent
were reported to be endothermic adsorption. In an adsorption study of Cu(II) ions
using pomegranate peel by Ben-Ali et al. [63], the adsorption efficiency increased
from 57.39% to 78.85% when the temperature changed from 303 K to 313 K.
380
S.-F. Lim et al.
wasting the resources. Another work on Pb(II) removal using fig sawdust also
showed a similar finding [59]. The increase in adsorbent dosage to 1.67 g/L resulted
in 95.3% removal capacity, and additional adsorbent dosage did not increase the
percentage removal capacity.
3.4 Effect of Contact Time
Generally, researchers carry out investigations on contact time for adsorption processes to determine the equilibrium time of the reaction. Once the equilibrium time is
obtained, the kinetics and equilibrium studies of adsorption process can be
established. The initial stage of adsorption is usually the fastest, at the time that
the adsorption sites are still unoccupied and the adsorbent surface is yet to be
covered by adsorbate molecules. The adsorption eventually becomes slower by
time in the act of the adsorption sites are gradually filled and competition to fill in
a site becomes stiffer. No further adsorption takes place at equilibrium condition;
thus, the adsorption capacity and percentage removal remain constant after the
equilibrium time. Table 9.5 lists the examples of equilibrium time for several
agricultural waste-derived adsorbents for heavy metal removal.
3.5 Effect of Temperature
The adsorption of heavy metals on agricultural waste-derived adsorbents is usually
temperature-dependent, and the effect of temperature on the heavy metal removal
very much depends on the nature of the adsorption process. The adsorption efficiency for exothermic adsorption increases with temperature, whereas the opposite
occurs for endothermic adsorption process. Commonly, the studies on the effect of
temperature for heavy metal removal using the agricultural waste-derived adsorbent
are conducted at near-ambient temperature because at higher temperature the structure of agricultural wastes can be easily unstabilized. The operation at near-ambient
temperature also optimizes energy consumption.
The adsorption process is mostly exothermic. The adsorption of U(VI) using
palm shell powder demonstrated a decline in the heavy metal uptake as the temperature increased from 30
C to 70
C [49]. Similarly, increasing the temperature from
20
C to 40
C for Pb(II) removal using peanut shell has a negative effect to the
heavy metal removal [66].
Only a few adsorption process studies using agricultural waste-derived adsorbent
were reported to be endothermic adsorption. In an adsorption study of Cu(II) ions
using pomegranate peel by Ben-Ali et al. [63], the adsorption efficiency increased
from 57.39% to 78.85% when the temperature changed from 303 K to 313 K.
380
S.-F. Lim et al.