Accepted Manuscript Molecular mechanism of anionic dyes adsorption on cationized

ACCEPTED MANUSCRIPT
13 
3.3 Mechanisms of dye removal 
The point of zero charge (pH
PZC
) was measured by means of the previous 
method [32]. The effect of pH on the adsorption of Diamine Green B (DG-B), Acid 
Black 24 (AB-24), and Congo Red (CR) by CRHC was also determined.
The pH 
values were adjusted from 5 to 11 by adding a few drops of NaOH or HCl solution. 
All data used for the above analysis were the mean values from three replicate tests. 
The pH values affect not only the surface charge properties, the functional 
groups, and the degree of ionization of the adsorbent in the bath, but also the 
structural properties of the dye molecules. 
The effect of pH of the aqueous solution on the adsorption of Diamine Green B 
(DG-B), Acid Black 24 (AB-24) and Congo Red (CR) onto CRHC was investigated 
(Fig. 3b, Fig. 3c and Fig. 3d). The adsorption capacity increased as the pH rose from 
5.0 to 8.0, and decreased as the pH rose from 8.0 to 11.0. Since the pH
pzc
of CRHC is 
7.27 (Fig. 3a), a higher positive charge develops on its surface when the pH values are 
lower than pH
pzc
; this charge decreases when the pH values are higher than pH
pzc 
[33]. 
For the upon dye adsorption, at lower pH, –SO
3
−
groups in the dye molecule are 
protonated to –SO
3
H groups, resulting in the decreasing of the electrostatic attraction 
between –SO
3
− 
and –NH
3
+
groups of CRHC. Upon increase in pH, the amount of 
–SO
3
−
groups increases, thereby increasing the adsorption capacity [34]. Under 
alkaline conditions, the competition of OH
−
ions reduces the adsorption capacity. 
The maximum adsorption capacities of Diamine Green B (DG-B), Acid Black 24 
(AB-24) and Congo Red (CR) were up to 186.14, 167.24, 255.78 mg•g
−1
at pH=8, 
respectively, and followed the order of CR > DG-B > AB-24. This is due to the 
hydrogen bonding between the –NH
2
of dye and the OH of the CRHC, the OH of the 
dye and the OH of the CRHC favored the adsorption. Moreover, the hydrogen 
bonding between –NH
2
and –OH is stronger than that of –OH and –OH. The possible 
molecular mechanism of Diamine Green B (DG-B), Acid Black 24 (AB-24) and 
Congo Red (CR) adsorption onto CRHC is shown in Fig. 4. 

tải về 1.26 Mb.

Chia sẻ với bạn bè của bạn: