23
In order to obtain a good adsorption result, ∆G° must be negative. This shows
that the adsorption process is possible and spontaneous. A widely accepted rule follows that a decrease in the negative value of ∆G° with an increase in temperature is
an indication that the adsorption process responds well at high temperatures. This
may be due to increase movement of heavy metal ions in the solution with increase
in temperature, and the attraction of the heavy metal ions on the synthesized activated carbon is high at high temperatures. Contrariwise, an increase in the negative
value of ∆G° with an increase in temperature indicates that lower temperatures
predict an easy adsorption.
The exothermic and endothermic nature of the adsorption process can be verified
by the value of ∆H° obtained for any specific study. A negative value ∆H° indicates
that the adsorption process is exothermic, while a positive value ∆H° indicates that
the adsorption process is endothermic.
During the adsorption process, the heavy metal ions displaced more than one
water molecule thereby resulting in the endothermic nature of the process, hence
making ∆H° positive, whereas in an exothermic process, the energy released during
the adhesion of bonds between the heavy metal ions and the activated carbon is
greater than the energy absorbed in the breaking of bonds. Thus, this leads to the
release of surplus energy in the form of heat making the ∆H° to be negative (Saha
et al. 2010).
A positive value of ∆S° indicates the strong attraction of the synthesized activated carbon for the heavy metal ions. In addition, the positive value also points to
increased randomness at the solid-solution boundary with some morphological
alteration in the heavy metal ions and the activated carbon.
As shown in Table 1.9, most of the adsorption systems show that the Langmuir
model portrays an adequate fit for the adsorption system as the model signals that
the monolayer coverage is responsible for the adsorption of the metal ion onto the
surface of the activated carbon rather than the multilayer adsorption (Hadi
et al. 2014).
1.9 Conclusions and Future Investigation
Adsorption technology has been widely embraced for the removal of heavy metal
ions from contaminated media, especially in water systems. Based on existing
research works, this review extensively detailed the different areas of investigation
associated with the adsorption process. The utilization of low-cost adsorbents synthesized from agricultural wastes into activated carbon is recommended since they
are readily available and also renewable. The review also emphasized that the modification of these adsorbents helps increase their efficiency in heavy metals removal.
Optimization of the adsorption system is vital, and this was discussed based on the
isotherms and thermodynamics of previous studies carried out in batch systems. For
future investigation, we suggest that subsequent research should be carried out in
1 Synthesis of Activated Carbons for Heavy Metals Removal
In order to obtain a good adsorption result, ∆G° must be negative. This shows
that the adsorption process is possible and spontaneous. A widely accepted rule follows that a decrease in the negative value of ∆G° with an increase in temperature is
an indication that the adsorption process responds well at high temperatures. This
may be due to increase movement of heavy metal ions in the solution with increase
in temperature, and the attraction of the heavy metal ions on the synthesized activated carbon is high at high temperatures. Contrariwise, an increase in the negative
value of ∆G° with an increase in temperature indicates that lower temperatures
predict an easy adsorption.
The exothermic and endothermic nature of the adsorption process can be verified
by the value of ∆H° obtained for any specific study. A negative value ∆H° indicates
that the adsorption process is exothermic, while a positive value ∆H° indicates that
the adsorption process is endothermic.
During the adsorption process, the heavy metal ions displaced more than one
water molecule thereby resulting in the endothermic nature of the process, hence
making ∆H° positive, whereas in an exothermic process, the energy released during
the adhesion of bonds between the heavy metal ions and the activated carbon is
greater than the energy absorbed in the breaking of bonds. Thus, this leads to the
release of surplus energy in the form of heat making the ∆H° to be negative (Saha
et al. 2010).
A positive value of ∆S° indicates the strong attraction of the synthesized activated carbon for the heavy metal ions. In addition, the positive value also points to
increased randomness at the solid-solution boundary with some morphological
alteration in the heavy metal ions and the activated carbon.
As shown in Table 1.9, most of the adsorption systems show that the Langmuir
model portrays an adequate fit for the adsorption system as the model signals that
the monolayer coverage is responsible for the adsorption of the metal ion onto the
surface of the activated carbon rather than the multilayer adsorption (Hadi
et al. 2014).
1.9 Conclusions and Future Investigation
Adsorption technology has been widely embraced for the removal of heavy metal
ions from contaminated media, especially in water systems. Based on existing
research works, this review extensively detailed the different areas of investigation
associated with the adsorption process. The utilization of low-cost adsorbents synthesized from agricultural wastes into activated carbon is recommended since they
are readily available and also renewable. The review also emphasized that the modification of these adsorbents helps increase their efficiency in heavy metals removal.
Optimization of the adsorption system is vital, and this was discussed based on the
isotherms and thermodynamics of previous studies carried out in batch systems. For
future investigation, we suggest that subsequent research should be carried out in
1 Synthesis of Activated Carbons for Heavy Metals Removal