Photocatalytic Processes for the Removal of Dye - Green Chemistry for Dyes Removal from Wastewater - page 156

sorptions of Acid Blue 25 and Basic Blue 69 are favored in lower and higher
temperatures, respectively. They attributed this finding to the fact that at
higher temperatures, an increase in the free volume occurs due to the
increased mobility of the solute in the former case, whereas in the case of
acidic dye, an elevation in temperature increases the escaping tendency of
the dye from the interface. A very low surface area peat was employed by Ip
et al. for the removal of Reactive Black 5 from an aqueous solution [66]. By
measuring the acidic and basic surface groups as well as the pH PZC , it was
revealed that the peat surface is predominantly acidic due to the presence
of carboxylic groups of humic acid and fulvic groups. This surface acidity
would exert a repulsive force for the acidic dye molecules, resulting in a
low uptake capacity of the peat. It was noted that although the adsorption
capacity of the peat for this dye is quite low (7 mg.g -1 ), this capacity was
considerably increased when a salt was introduced into the solution (see
Table 5.6). This phenomenon was assigned to the reduced repulsive forces
between the dye molecules and the surface of the peat. Fernandes et al.
[67] obtained reasonably high uptake capacities by peat for methylene blue
(303 mg.g -1 ), as seen in Figure 5.9. Such high adsorption capability coupled
with fast equilibrium time was attributed to the cation exchange between
the H + ion of the carboxylic group associated with the humic substances
and the cationic groups of the dye. Also, an increase in the solution tem-
perature resulted in a slight improvement of the adsorption capacity. Two
opposing factors are created when increasing the temperature; this may
lead to an increase in the tendency of the adsorbed molecules to escape
from the pores and it can also enhance the intraparticle diffusion rate of
the dye molecules. Depending on the net effect, the removal capability of
the adsorbent will either be enhanced or decreased. Allen et al. [68] used
five different isotherm models together with several error analysis func-
tions to it the adsorption of three dyes onto the peat. It was shown that the

Table 5.6 The effect of salts on Reactive Black dye adsorption by peat [66].
Initial Dye
Concentration (ppm)

Type of Salt Adsorption Capacity

(mg.g -1 )

400 N/A 3.16

NaCl 24.28

Na 2 PO 4 51.45

1000 N/A 5.44

NaCl 29.82

Na 2 PO 4 55.27