The prediction of kinetics is necessary for the design of sorption
systems. Chemical kinetics explain how fast the rate of
chemical reaction occurs and also on the factors affecting the
reaction rate. The nature of sorption process depends on physicochemical
characteristics of the adsorbent and system conditions
like temperature, pressure, etc. as well. Measurement of sorption
rate constants is an important physico-chemical parameter
to evaluate the basic qualities of a good sorbent such as time
required for a sorbent to remove particular metals/compounds,
efficacy of the sorbents, etc. [21–23]. In batch adsorption processes
the adsorbate molecules diffuse into the interior of the
porous adsorbent. It was investigated that the adsorption of lead
ions from aqueous solution is a linear phase within a time period
about 120 min. This behavior can be attributed to the utilization
of available adsorbing sites on the surface of adsorbent. After
this phase, sorption of lead was almost ignorable. This might
be attributed to extremely slow diffusion of the metal ions from
the surface film into the micropores which are the least accessible
sites for adsorption [11]. In order to observe the sorption
process of lead ions, on unmodified and chemically modified
adsorbents, four kinetic models were implemented, including
Bangham, pseudo-first, second-order and intra-particle diffusion.