Biomedical Engineering Reference
In-Depth Information
amount of dye adsorbed declined by 889.9, 800.0 and 1531.7 mg/g [27]. This contrasting
behavior may be attributed to a less proportionate increase in dye adsorption following a rise
in γ-PGA dose, and reduction in the vicinity of active sites because of aggregation of γ-PGA
particles, which in turn may result in removal of some weakly bound dyes from the γ-PGA
surface [60,113]. The relationship between γ-PGA dose (m, g/L) and percentage removal (R)
fitted the mathematical equations (16)-(18) with r2 being 0.990, 0.988 and 0.978 for Au-O,
Rh-B and Sa-O dyes, respectively [27].
m
R
=
(16)
−
3
−
3
(3.39
×
10
)
+
(9.57
×
10
m)
m
R
=
(17)
−
3
−
3
(2.37
×
10
)
+
(9.30
×
10
m)
m
R
=
(18)
(1 .1 3
×
1 0
−
3
)
+
(9 .0 5
×
1 0
−
3
m )
4.5. Effect of pH
Solution pH is a critical parameter which dictates the uptake and release of solutes in any
adsorption process involving ion exchange mechanism. Following an increment in pH from
1-5 in a 100 mg/L dye solution, the removal of MB, MG, Au-O, Rh-B, Sa-O and BB-Y dyes
rose from 6.7-94.7, 4.0-91.8, 7.6-69.1, 14.5-79.0, 23.0-95.6 and 15.0-93.5%, respectively, and
remained constant thereafter [27,70,71]. The reduced uptake at low pH may be due to the
excessive hydrogen ions in solution competing for active sites in γ-PGA. However, at pH ≥
p
K
a
of γ-PGA (4.09), the carboxyl groups would be largely deprotonated, facilitating
maximum exchange of dye cations. A significant drop in solution pH substantiated the
concomitant release of hydrogen ions during dye adsorption. For example, the pH of Au-O,
Rh-B and Sa-O solutions declined to an average value of 3.86, 3.75 and 3.68, respectively,
when the dye solutions at pH 7, 6, 5 and 4 were equilibrated with γ-PGA [27].
4.6. Effect of Electrolytes
Incorporation of different electrolytes (NaCl, KCl, CaCl
2
or MgCl
2
) at varying
concentrations (0.001-1 M) showed a declining trend on Au-O, Rh-B and Sa-O dyes
adsorption by γ-PGA (Figure 9) [27]. The metal salts added may screen the electrostatic
interaction between anionic γ-PGA and cationic dye, and an increase in salt concentration
could reduce the amount of dye adsorbed on γ-PGA [114,115]. The degree of screening by
added metal salts followed the order: Ca
2+
> Mg
2+
> K
+
> Na
+
, which may be accounted for
by the size and valence of added metal ions [114,115]. Accordingly, the divalent ions Ca
2+
and Mg
2+
showed a more pronounced effect than the monovalent ions Na
+
and K
+
, with Ca
2+
ions exerting a larger effect than Mg
2+
, and K
+
ions than Na
+
. Apparently, the larger the ions,
