Biosorption of Organic Dyes: Research Opportunities and Challenges - Green Chemistry for Dyes Removal from Wastewater - page 309

ligands, redox reactions and precipitation of the dyes are strongly influ-
enced by pH [3,4,9].
In a relevant study, Chatterjee et al. [88] analyzed the chitosan-Congo
Red interactions as a function of pH. The main findings of their research
were: i) the high biosorption of the dye at lower pH (pH < 6.4) was probably
due to the increase in electrostatic attraction between negatively charged
dye molecule (-SO3− Na+) and positively charged amine group of chito-
san (-NH 3 + ); ii) at pH 6.4, where surface charge of chitosan was neutral,
biosorption of the dye was attributed to hydrogen bond formation between
some of the molecular components of Congo Red such as N, S, O, benzene
ring and CH 2 OH groups of the chitosan molecule; iii) at pH above 6.4, sur-
face charge of chitosan was negative and the chitosan-Congo Red interac-
tions were due to hydrogen bonding and van der Waals forces. In another
investigation, Weber et al. [89] studied the effect of pH (2.5-8.5) in the
biosorption of Direct Blue 38 onto Papaya seeds. They found that at low
pH values, high electrostatic attraction occurred between the positively
charged surface of papaya seeds and the negatively charged anionic dye. As
a consequence, pH 2.5 was selected as the more adequate. On the contrary,
Khataee et al. [48] obtained best biosorption capacities at pH 10.0 in the
biosorption of Basic Red 46 onto Spirogyra sp.
8.3.2 Temperature
Generally, the discharge temperature of DCEFs is in the range of 298-303 K
[8]. However, various textile dye effluents are discharged at relatively high
temperatures (323-333 K) [12]. Thus, temperature is an important design
parameter, which should be evaluated in biosorption studies. The tem-
perature can affect the biosorbent and the dyes from a negative or positive
viewpoint. Dotto et al. [19] studied the temperature effect on the bio-
sorption of food dyes by Spirulina platensis . They found that the tempera-
ture increase from 298 to 328 K caused a strong decrease in biosorption
capacity. They assumed that the temperature increase causes an increase
in the solubility of the dyes, so, the interaction forces between the dyes
and the solvent become stronger than those between dyes and biosorbent.
Furthermore, they suggested that at temperatures above 318 K the dam-
age of sites can occur on the surface of biosorbent and, consequently, a
decrease in the surface activity. This behavior was also observed by Piccin
et al. [59] and Dotto et al. [74] in the biosorption of SODs onto chitosan,
Barka et al. [90] in the biosorption of synthetic dyes onto dried prickly
pear cactus and Cengiz et al. [91] in the biosorption of Astrazon Red onto