Biosorption of Organic Dyes: Research Opportunities and Challenges - Green Chemistry for Dyes Removal from Wastewater - page 298

uptake of pollutants from aqueous solutions by the use of non-growing
or non-living microbial mass, thus allowing the recovery and/or environ-
mentally acceptable disposal of the pollutants. The term is used to indicate
a number of metabolism-independent processes (physical and chemical
adsorption, electrostatic interaction, ion exchange, complexation, chela-
tion, and microprecipitation) taking place essentially in the cell wall [12].
It is common sense in the literature that the biosorption process
involves a solid phase (sorbent, biosorbent or biological material) and a
liquid phase (solvent, normally water) containing a dissolved species to
be sorbed (adsorbate, organic dyes for example). Due to the high affinity
between the biosorbent and adsorbate, the latter is attracted and bound
there by different interaction mechanisms. The process continues till equi-
librium is established between the amount of solid-bound adsorbate spe-
cies and its portion remaining in the solution. The degree of biosorbent
affinity for the adsorbate determines its distribution between the solid and
liquid phases [12,13,15-17].
In the same way as other techniques for the treatment of DCEFs, bio-
sorption has its own advantages and drawbacks. The main advantages
of biosorption are: biosorbent materials can be found easily as wastes or
byproducts and at almost no cost; the biosorbents are cheap, eco-friendly
and renewable; the biosorbents have several functional groups on the sur-
face, capable of binding with many classes of dyes; selectivity for many dyes;
competitive performance (biosorption is capable of a performance compa-
rable to the most similar technique, ion exchange treatment) [12,16,18,19];
there is no need of costly growth media; the process is independent of
physiological constraints of living cells; the process is very rapid, as non-
living material behaves as an ion-exchange resin; the conditions of the
process are not limited by the living biomass; no aseptic conditions are
required; the process is reversible and the dye can be desorbed easily, thus
recycling of the materials is quite possible; chemical or biological sludge is
minimized [15-17,20]. On the other hand, the drawbacks are: the charac-
teristics of the biosorbents cannot be biologically controlled; centrifuga-
tion or filtration operations are necessary for the solid-liquid separation
after the biosorption process; mass loss after regeneration; difficulties for
scale-up [12,20].
From the operational viewpoint, biosorption is generally based on
two types of investigations: batch systems (discontinuous operation) and
dynamic systems (continuous operation) [9,12]. The first and fundamen-
tal investigations regarding biosorption are made in batch systems. These
operations are cheap, simple to operate and, consequently, are adequate for
small- and medium-size applications, using simple and readily available