II International Symposium on Carbon for Catalysis ABSTRACTS

PP-I-46
zeroth moment expression, which corresponds to the area under the decay curve of
dimensionless concentration within the adsorber were used in the evaluation of effective
diffusivities.
μ =
o
R
2
o
⎛
⎜
ws
15DA
1+
⎝ ρ
p
( ε + ρ K )
a
⎞
( ε + ρ K ) ⎟
a
p
p
⎠
(1)
μ =
o
R
2
o
⎛ ⎞
⎜
w +
s
15D
⎟
A
1
⎝ ρ
p ⎠
Here, D A , R o and w s are the effective diffusivity, particle radius and the mass of adsorbent
charged per unit volume of the adsorber, respectively. The apparent adsorption equilibrium
constant ρ p
K was evaluated from the gradient of the adsorption isotherm at the equilibrium
adsorbate concentration.
Sorption results with Methylene Blue showed that, both the sorption capacity and the
sorption rate constant values obtained with powdered activated carbon were higher than the
corresponding values obtained with granular activated carbon. This was concluded to be due
to the much higher pore diffusion resistance in the granular activated carbon. Dissociative
chemisorption of the dyes near the pore mouths caused partial closure of the pores. This
caused a significant resistance for further sorption at the inner parts of the granular activated
carbon. The first order adsorption rate constant of methylene blue obtained with powdered
activated carbon was about twice the corresponding value obtained with the granular
activated carbon. For granular activated carbon, the sorption rate constant was found to be
about 3.2 10 -4 min -1 for w s values between 0.07 and 0.2 g/l. The diffusion models proposed
here gave good prediction of the sorption rate in the granular activated carbon. Especially for
Model <strong>II</strong> (Eq. 2) effective diffusivity was found to be not dependent on concentration. The
values of D A were found to be in the orders of magnitude of 10 -7 and 10 -9 cm 2 /s for Models I
and <strong>II</strong>, respectively. During the sorption process, pH of the solution did not significantly
change. However, the pH was found to be an important parameter on the sorption rate and on
the sorption capacity. Among the different adsorption isotherms, Freundlich, Radke-Prausnitz
and especially the Toth isotherms gave good fit to the sorption data obtained with both
granular and powdered activated carbons. Comparison of the sorption behavior of Methylene
Blue and the Basic Yellow 28 showed that both the sorption rate and the sorption capacity
values obtained with Basic Yellow 28 were higher than the corresponding values obtained
with Methylene Blue.
(2)
References
1 Yener, J. Kopac, T., Dogu, G., Dogu, T. “Adsorption of Basic Yellow 28 from Aqueous
Solutions with Clinoptilolite and Amberlite” J. Colloid and Interface Science, 2005 (in press).
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