Biosorption of Organic Dyes: Research Opportunities and Challenges - Green Chemistry for Dyes Removal from Wastewater - page 311

the relationship between the effective specific surface area of the biosorbent
particles and their sizes [9,12,96,97], the surface area usually increases when
the particle size is decreased [12,98]. As a consequence, more biosorption
sites are exposed to the dyes for the same amount of biosorbent, increas-
ing the biosorption capacity [99]. Furthermore, McKay et al. [100,101]
indicated that the equilibrium is attained faster with small particles, since
the intraparticle diffusion resistance is lower, and the accessibility of dye
molecules to internal sites is facilitated. Piccin et al. [98] evaluated the par-
ticle size effect on the biosorption of FD&C Red 40 by chitosan. They used
particles of 100, 180 and 260 μm, and obtained specific surface areas of 4.2,
3.4 and 1.6 m 2 g -1 , respectively. As a consequence, the biosorption capaci-
ties were 191.6, 157.5 and 104.9, respectively. Daneshvar et al. [42] studied
the particle size effect in the biosorption of Acid Black 1 onto Sargassum
glaucescens and Stoechospermum marginatum. Their results showed that
the biosorption capacity of the biosorbents increased with the decrease in
particle size. The same effect was observed by Prola et al. [102] in the bio-
sorption of Reactive Red 120 onto Jatropha curcas shells.
8.3.5 ContactTime
In a biosorption process, the contact time is one of the more important
parameters from the industrial viewpoint. A good biosorbent should not
only provide high biosorption capacities, but also furnish a fast process.
In this way, contact time is fundamental to select an adequate biosor-
bent. Generally, during the process, the biosorbent surface is progres-
sively blocked by the adsorbate molecules, becoming covered after some
time. When this happens, the biosorbent cannot adsorb any more dye
molecules [9,37,67]. This reflects that the biosorption capacity increases
with time and, at some point in time, reaches a constant value where no
more dye is removed from the solution [99]. At this point, the amount of
dye being adsorbed onto the material is in a state of dynamic equilibrium
with the amount of dye desorbed from the biosorbent [9,96,97]. Deniz
and Saygideger [29], using princess tree leaf as biosorbent, verified that
the removal of Basic Red 46 increased with time and attained saturation in
about 70 min. Furthermore, they found that, initially, the biosorption rate
of BR 46 was rapid, but it gradually decreased with time until it reached
equilibrium. This behavior was attributed to the aggregation of the dye
molecules around the biosorbent particles. Yang et al . [103] observed that
the biosorption rate of Acid Black 172 and Congo Red onto Penicillium
YW 01 was initially fast and then gradually decreased to reach equilib-
rium. In the same way, Kumar and Ahmad [104] found that in the biosorp-
tion of Crystal Violet onto ginger waste, the dye was rapidly adsorbed in