Biosorption of Organic Dyes: Research Opportunities and Challenges - Green Chemistry for Dyes Removal from Wastewater - page 312

the first 45-60 min, and then the biosorption rate decreased gradually and
reached equilibrium in about 150 min. They assumed that at the begin-
ning, the biosorption rate was very fast because the dye was adsorbed on
the exterior surface, but, when the exterior surface was saturated, the dye
entered into the pores of the biosorbent particles and was adsorbed on the
interior surface.

8.3.6 Initial Dye Concentration

The initial dye concentration also affects the biosorption process. Initial
concentration provides an important driving force to overcome all mass
transfer resistances of the dye between the aqueous and solid phases [12].
The effect of the initial dye concentration factor depends on the immedi-
ate relation between the concentration of the dye and the available binding
sites on a biosorbent surface [3]. The amount of the dye adsorbed onto the
biosorbents increases with an increase in the initial concentration, if the
amount of biosorbent was kept unchanged. This is due to the increase in
the driving force of the concentration gradient with the higher initial dye
concentrations [105]. On the other hand, the percentage of dye removal
decreases with an increase in the initial dye concentration, which may be
due to the saturation of biosorption sites on the biosorbent surface [106].
At a low concentration there will be unoccupied active sites on the bio-
sorbent surface, and when the initial dye concentration increases, the
active sites required for biosorption of the dye molecules will be lacking
[107]. Zhang et al. [93] showed that in the biosorption of Rhodamine B
(RhB) and Basic Blue 9 onto sugarcane bagasse, the removal of both dyes
by bagasse decreased with increasing initial dye concentration. However,
the biosorption capacity increased with the initial dye concentration. In
the study of Vucurovic et al. [108], the equilibrium biosorption increased
from 1.9 to 4.8 mg g -1 when the initial Methylene Blue (MB) concentration
increased from 20 to 50 mg L -1 . Fan et al. [109] indicated that in the bio-
sorption of Azure Blue onto Cladosporium sp., the uptake of dye increased
up to 55 mg g -1 with the increasing initial dye concentration, but the per-
centage of dye removal decreased from 98% to 56% with the increase in the
initial dye concentration from 100 to 500 mg L -1 .

8.3.7 Stirring Rate

The stirring rate is an important parameter in the biosorption process,
since it influences the distribution of solute in the bulk solution and the
formation of the external boundary film [96,97]. This parameter influences