3.2. Effect of time
The results of experiments conducted to determine the
equilibrium time are represented in Fig. 2 for SBB-A and SR-BA.
It is clear that adsorption rate was initially very rapid followed by
slow adsorption until equilibrium reached. At the equilibrium, the
qe values were found out to be 19.97 mg/g and 18.61 mg/g for SBB-A
and SR-BA, respectively. The equilibrium reached after 180 min and
240 min for SBB-A and SR-BA, respectively. The percentage
removal indicates that the adsorption of both dyes was faster in
beginning and decreased linearly with time. From this behaviour, it
can be concluded that adsorption occurred in two steps; initially
very fast (first phase), and then slowed down (second phase) until
equilibrium reached. Further increase in contact time did not show
any increase in adsorption and the slow adsorption rate at the
latter stage may be due to the difficulty faced by ions to occupy the
remaining vacant surface sites because of repulsive force between
the solute molecules of the solid and bulk phase. Secondly, slow
adsorption with the passage of time may also be due to
intraparticle diffusion process dominating over adsorption [22,39].
3.3. Effect of particle size
The adsorbent particle size determines the surface area and is a
key factor in adsorption process. Effect of the particle size on
adsorption of dyes onto EAS is shown in Fig. 3. The article size
0.255 mm showed higher adsorption efficiency for both dyes and
qe values were recorded to be 17.31 mg/g and 18.15 mg/g for SR-BA
and SBB-A, respectively. It was observed that smaller the particle
size greater was the adsorption. This trend of dyes adsorption is
attributed with higher surface area and availability of more
binding sites for dyes adsorption onto adsorbent having smaller
particle size [40,41]. Authors revealed that adsorbent particle size
has significant effect on adsorption and correlated it with large
surface areas becasue smaller particle size offers more surface area
and the availability of more adsorption sites for adsorption. For
larger particle size, the diffusion resistance to mass transfer might
be significant and the internal adsorption sites are not available for
binding of sorbate ions [42].
3.4. Effect of temperature
Investigation of temperature effect on dyes adsorption is very
important in the real application of adsorption process since
efflunets discharged from industries may have different tempera-
ture. The effect of dyes (SR-BA and SBB-A) adsorption onto EAB as a
function of temperature is shown in Fig. 4. It is clear that
adsorption of both dyes decreased by increasing the temperature,
which reveal the exothermic adsorption process of SR-BA and SBBA
onto EAB. The maximum adsorptions of SR-BA and SBB-A were
18.98 mg/g and 20.15 mg/g at 303 K, respectively. Percentage
removal data showed that by increasing the temperature, the
percentage removal also reduced linearly (Fig. 4). The decrease in
dyes adsorption at higher temperature might be due to the
weakening of supportive forces between active sites on the sorbent
and dyes and between adjacent dye molecules during sorption phase [43]. The biosorption decreased beyond 303 K was due to the
decreased surface activity at higher temperature, which indicates
that the adsorption process was exothermic and dye adsorption
onto EAB occured mainly by physical adsorption [44]. Previously,
palm kernel
fibre also showed similar adsorption for anionic dye as
a function of temperature [45]. Therefore, the dye molecules
desorbed at higher temperature due to deteriorating of adsorptive
forces between dye molecule and functional group onto adsorbent
surface.