Biosorption of Organic Dyes: Research Opportunities and Challenges - Green Chemistry for Dyes Removal from Wastewater - page 313

the initial stages of biosorption, where the external mass transfer is the
controlling mechanism. However, the intraparticle diffusion is not influ-
enced [110]. As a consequence, the equilibrium time is affected, but the
equilibrium biosorption capacity is not a function of stirring rate [111].
The stirring rate increase causes an increase in impeller Reynolds number
[112], so, the energy dissipation and turbulence in the mixing zone is
increased. As a consequence, a decrease in film thickness occurs and the
boundary layer resistance is lower [68]. The above facts cause an increase
in biosorption capacity at the initial stages.
Dotto and Pinto [68] studied the stirring rate effect on the biosorption
of Acid Blue 9 and Food Yellow 3 onto chitosan. They observed that, at 40
min, the increase in stirring rate from 15 to 400 rpm caused a large increase
in Acid Blue 9 biosorption capacity, from 110 to 220 mg g -1 . In the same
way, for Food Yellow 3, an increase in stirring rate from 15 to 400 rpm
increased biosorption capacity from 220 to 350 mg g -1 . Furthermore, they
proved that the stirring rate increase caused a decrease in film diffusion
effect. Hanafiah et al. [113] studied the biosorption of Acid Blue 25 onto
Shorea dasyphylla sawdust. They verified that the stirring rate increase
from 100 to 200 rpm improved the biosorption. They explained that at a
higher stirring rate, a good degree of mixing is achieved, and the boundary
layer thickness around the biosorbent particles is reduced. Similar behav-
ior was found by Uzun and Guzel [114] in the biosorption of Orange II
and Crystal Violet onto chitosan. Dotto and Pinto [111] also corroborate
this effect in the biosorption of Acid Blue 9 and FD&C Red 40 by Spirulina
platensis nanoparticles.

8.4 Biosorption Isotherms, Thermodynamics

and Kinetics

8.4.1 Equilibrium Isotherms

The biosorption equilibrium is normally described by the biosorption
isotherm curves. These curves relate the amount of dye adsorbed on the
biosorbent at equilibrium (qe, mg g -1 ) and the equilibrium dye concentra-
tion remaining in liquid phase (Ce, mg L -1 ). From the isotherm curves it
is possible to infer how the SODs-biosorbent interactions occur, and so
these curves are crucial to improve the biosorbent performance [12,13].
Furthermore, the isotherm curves are a key tool for obtaining the maxi-
mum biosorption capacity of the biosorbents in a specific condition, as
well as for elucidating the possible biosorption mechanisms [115,116].