Non-conventional Adsorbents for Dye Removal - Green Chemistry for Dyes Removal from Wastewater - page 383

that attempts to overcome the selectivity disadvantage of conventional
adsorption processes. The use of dead rather than live biomass eliminates
the problems of waste toxicity and nutrient requirements. Biomass adsorp-
tion is effective when conditions are not always favorable for the growth and
maintenance of the microbial population. In spite of good adsorption prop-
erties and high selectivity some problems can occur. The adsorption process
is slow: in the case of biomass of Aspergillus niger equilibrium was reached
in 42 h. Another problem is that the initial pH of the dye solution strongly
influenced the biosorption [232]. Biosorption was also influenced by the
functional groups in the fungal biomass and its specific surface properties
[235]. Biosorption performance depends on some external factors such as
salts and ions in solution which may be in competition. Another limitation
of the technology include the fact that the method has only been tested for
limited practical applications since biomass is not appropriate for the treat-
ment of effluents using column systems, due to clogging effect. Because of
major limitations for their efficient utilization in a column reactor, there is
the need for their immobilization. This step forms a major cost factor of the
process. In general, decolorization by living and dead cells involves several
complex mechanisms such as surface adsorption, ion-exchange, complex-
ation (coordination), complexation-chelation and microprecipitation. Cell
walls consisting mainly of polysaccharides, proteins and lipids, offer many
functional groups. The dyes can interact with these active groups on the cell
surface in a different manner. The accumulation of dyes by biomass may
involve a combination of active, metabolism-dependent and passive trans-
port mechanisms starting with the diffusion of the adsorbed solute to the
surface of the microbial cell [236,228,237]. Once the dye has diffused to the
surface, it will bind to sites on the cell surface. The precise binding mecha-
nisms may range from physical (i.e., electrostatic or van der Waal forces) to
chemical binding (i.e., ionic and covalent). However, it is now recognized
that the efficiency and the selectivity of adsorption by biomass are due to
ion-exchange mechanisms. Biosorption processes are particularly suitable
for the treatment of solutions containing dilute (toxic) dye concentration.
They are a potential promising alternative to conventional processes for the
removal of dyes. However, these technologies are still being developed and
much more work is required.
10.11 Starch-BasedDerivatives
There is an increasing interest in the production of novel materials with
particular functionalities from renewable resources. Natural polymers are
replacing synthetic polymers in many applications partly because of their