Accepted Manuscript Molecular mechanism of anionic dyes adsorption on cationized

ACCEPTED MANUSCRIPT
22 
further analyzed by the R
L
values. When R
L
=0, the adsorption is irreversible. When 
R
L
=0-1, the adsorption is a favorable process; When R
L
>1, the adsorption process is 
an un-favorable process. In this experiment, the R
L
values of adsorbent CRHC on 
Diamine Green B (DG-B), Acid Black 24 (AB-24) and Congo Red (CR) were all in 
the range of 0-1, indicating that it is a favorable adsorption process. 
Table 5 Isotherm parameters on the adsorption of Diamine Green B (DG-B), Acid 
Black 24 (AB-24) and Congo red (CR) onto CRHC.
Isotherms 
DG-B 
AB-24 
CR 
303.15 
K 
313.15 
K 
323.15 
K 
303.15 
K 
313.15 
K 
323.15 
K 
303.15 
K 
313.15 
K 
323.15 
K 
Langmuir 
model 
q
m
 
207.15 190.19 190.08 268.88 267.68 243.87 580.09 711.74 729.12 
K
a
 
0.058 
0.060 
0.042 
0.037 
0.036 
0.019 
0.083 
0.050 
0.048 
R
2
 
0.9731 0.9209 0.9657 0.9887 0.9919 0.9975 0.9988 0.9960 0.9939 
R
L
 
0.15 
0.14 
0.19 
0.21 
0.22 
0.34 
0.11 
0.17 
0.18 
Freundlich 
model 
1/n
f
 
0.29 
0.26 
0.31 
0.41 
0.39 
0.50 
0.61 
0.70 
0.69 
k
F
 
48.16 
49.15 
36.78 
33.32 
32.34 
18.42 
63.38 
48.07 
46.12 
R
2
 
0.8959 0.8085 0.8988 0.9816 0.9715 0.9823 0.9672 0.9861 0.9822 
Temkin 
model 
A
T
 
0.73 
0.88 
0.46 
0.31 
0.30 
0.15 
0.73 
0.53 
0.50 
B 
41.88 
36.62 
40.39 
62.63 
56.44 
65.17 
133.54 149.45 147.13 
R
2
 
0.9348 0.8563 0.9351 0.9882 0.9877 0.9964 0.9917 0.9993 0.9957 
Sips 
model 
q
m
 
181.65 162.67 163.92 322.41 245.01 235.35 461.68 464.98 415.92 
K
s
 
0.011 
0.0015 0.0071
0.049 
0.036 
0.013 
0.047 
0.051 
0.076 
1/n 
1.65 
2.29 
1.63 
0.80 
0.99 
1.17 
1.35 
1.36 
1.38 
R
2
 
0.9865 0.9728 0.9689 0.9821 0.9838 0.9971 0.9966 0.9999 0.9912 
q
e
 
171.75 157.75 152.08 200.60 184.47 172.88 308.39 308.63 311.42 
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23 
3.7 Thermodynamic evaluation 
Thermodynamic parameters were calculated from the equilibrium adsorption 
data at different temperatures. The different thermodynamic parameters can be used to 
determine the spontaneous, reversible, endothermic or exothermic nature of the 
adsorption process. The standard Gibbs free energy change (ΔG
0
), standard enthalpy 
change (ΔH
0
) and standard entropy change (ΔS
0
) can be calculated by using the 
following eauations [41]: 
ΔG
0
=−RTlnK
d
(4) 
K
d
=q
e
/C
e
(5) 
lnK
d
=−ΔH
0
/RT+ΔS
0
/R (6) 
where R is the gas constant (8.314 J K
-1
mol
-1
) and K
d
is the equilibrium constant for 
the adsorption at standard temperature and pressure. The values of ΔH
0 
and ΔS
0
can be 
calculated from the slope and intercept of plot between ln K
d
versus 1/T. All the data 
used for the above analysis are the mean values from three replicate tests.
Table 6 shows the thermodynamic parameters of Diamine Green B (DG-B), Acid 
Black 24 (AB-24) and Congo Red (CR) onto CRHC. The value of ΔG
0
is negative, 
and the increasing trend of its value is consistent with the increasing trend of 
temperature. This reveals that the adsorption is a spontaneous and feasible purification 
process; the values of ΔH
0
are all positive, indicating that the temperature is directly 
related to the adsorption behavior. The adsorption capacity should increase with the 
increasing of temperature. The values of ΔS
0
are all positive, revealing the high 
degree of confusion and randomness of the purification behavior. In addition, the 
physical or chemical properties of the adsorption can be analyzed by analyzing the 
ΔH
0
value. When ΔH
0 
<20 kJ mol
-1
, the physical purification process plays an 
important role with van der Waals interaction. When 20 kJ mol
-1
<ΔH
0 
<80 kJ mol
-1
, 
the electrostatic interaction plays an important role in the physical purification process. 
When ΔH
0
>80 kJ mol
-1
, the adsorption process is mainly chemical adsorption [42]. In 
this experiment, the CRHC for the purification of Diamine Green B (DG-B), Acid 
Black 24 (AB-24) and Congo Red (CR) have a ΔH
0
value of less than 80 kJ mol
-1
, 
indicating that the physical adsorption play an important role in the purification 

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