4.1 Effect of single factor on Cr (VI) and Cu(II) removal
It can be seen from Figure 1 (a) that as the reaction time increases, the amount of adsorption of Cr(VI) by the fiber increases, and the adsorption rate is faster in the first 60 minutes. It can be clearly seen that the adsorption capacity reaches the maximum at 180 minutes, and the adsorption capacity tends to balance at 180 minutes.50mg/L 100mg/L and 150mg/L Cr(VI) solutions with different concentrations show the same adsorption tendency.
It can be seen from Figure 1(b) that as the pH value of the Cr(VI) solution increases, the adsorption capacity of the fiber to Cr(VI) increases first, and the adsorption capacity of the fiber is the largest when the pH is 3, so the pH value is 3 Determined as the best adsorption pH value of Cr(VI) solution. When the pH of the solution is higher than 7, the absorption of Cr(VI) by the fiber begins to decrease.
It can be seen from Figure 1(c) that as the temperature increases, the amount of adsorption of Cr(VI) by the fiber increases. The adsorption capacity slowed down after 35℃, and the adsorption capacity showed a constant trend at 55℃. 35℃can be determined as the best adsorption temperature, and heating is beneficial to the adsorption of fibers.
Figure 1(d) shows the experimental effect of the amount of chitosan fiber on the removal of Cr (VI). The figure shows that the removal rate increases as the amount of chitosan fiber increases. This is mainly because as the amount of chitosan fiber increases, the number of effective active sites that can react with Cr (VI) increases, so that there is more mutual reaction between chitosan fiber and Cr (VI).
The experiment on the effect of Cr (VI) concentration on Cr (VI) removal efficiency was explored. The results are shown in Figure 1(e). It can be clearly seen that the Cr (VI) removal efficiency decreases with the increase of Cr (VI) concentration. After the action time of chitosan fiber and Cr(VI) is 180 min, when the initial concentration of Cr(VI) is 25mg/L, the best removal rate is 42.8%. The experimental results can show that CS fiber is more suitable for reaction with low concentration of Cr(VI).
Fig.2 Indicates the adsorption performance of CS fiber on Cu(II) shows the same trend as that on Cr(VI), but the adsorption performance on Cu(II) is better. As CS fiber dosage is 0.05g, after 180min reaction, the optimal removal rate 98.7% was obtained with the initial concentration of Cu(II) at 200mg/L. It might be the functional group amino of CS fiber has better adsorption property for divalent metal ions.
4.2 Kinetic experiments
As showed in Fig.3 and Tab.2. According to the value of R2, R22>R12,it can be concluded that the adsorption of Cr(VI) is more in line with the second-order kinetics. Hence, pseudo-second-order kinetic equations are suitable for explaining adsorption profiles. The kinetic parameter R2 is less than 1, which indicates the favor adsorption profile.
It can see from Fig.4 and Tab.3, the kinetic equation of the CS fiber for the adsorption of Cu(II) has the same properties as that of Cr(VI). the kinetic simulation using pseudo-first-order is not good and the actual data deviates seriously from the fitted curve. At the same time, the correlation coefficient of kinetic parameters is low. While the pseudo-second-order kinetic plot, with high correlation coefficient of kinetic parameters.
Table 2.
Kinetic model parameters of Cr(VI)
C(mg/L)
|
Quasi-first order kinetic equation
|
Quasi-second-order kinetic equation
|
set
|
Linear regression equation
|
R12
|
Linear regression equation
|
R22
|
50
|
y=-0.0235*X+2.0467
|
0.9713
|
y=0.0373*X+0.1869
|
0.9974
|
100
|
y=-0.0429*X+3.7166
|
0.9344
|
y=0.0187*X+0.0409
|
0.9991
|
150
|
y=-0.0341*X+2.7682
|
0.9833
|
y=0.0127*X+0.0593
|
0.9988
|
Table 3
Kinetic model parameters of Cu(II)
C(mg/L)
|
Quasi-first order kinetic equation
|
Quasi-second-order kinetic equation
|
|
Linear regression equation
|
R12
|
Linear regression equation
|
R22
|
200
|
y=-0.09322*X+149.84
|
0.9662
|
y=0.0028*X+0.0217
|
0.9899
|
300
|
y=-1.2226*X+210.7589
|
0.9434
|
y=0.0019*X+0.0158
|
0.9877
|
4.3 Isotherm experiments
From the parameters R12<R22 in Figure 5 and Table 4, we can see that the adsorption of Cr(VI) by chitosan fibers is more in line with the Langmuir model, and the adsorption process is multi-layer adsorption. At a temperature of 288K, the maximum theoretical saturated adsorption capacity of chitosan fiber for Cr(VI) is 111mg/g. Since 0<RL<1, the adsorption process is favorable. According to Freundlieh model, parameter 1/n value is between 0 and 1, indicating that the experimental concentration range is beneficial for Cr (VI) adsorption onto CS fiber.
In addition, based on the analysis of Cr(VI), we can get Freundlich model was used to illustrate non-ideal adsorption for the non-uniform surfaces and multilayer adsorption. We can see from Fig.6 and table5. A high kf value indicates the high affinity of CS fiber for Cu (II) ions.
Table 4
Fitting parameters of adsorption isotherms of Cr(VI)
Langmuir
|
Freundlich
|
Qe/mg/g
|
Qm/mg/g
|
KL
|
RL
|
R12
|
Kf
|
1/n
|
R22
|
75
|
111
|
0.014
|
0.742
|
0.6062
|
15.5
|
0.863
|
0.9923
|
Table 5
Fitting parameters of adsorption isotherms of Cu(II)
Langmuir
|
Freundlich
|
Qe/mg.g-1
|
Qm/mg.g-1
|
KL
|
RL
|
R12
|
Kf
|
1/n
|
R22
|
539.6
|
588.24
|
0.196
|
0.048
|
0.9966
|
195
|
0.315
|
0.8815
|
4.4 Adsorption mechanism
4.4.1SEM analysis
Fig.7(a) shows the surface of the CS material before adsorption is relatively smooth, Compare the surface of the material after adsorption of chromium(Fig.7(b)) and copper(Fig.7(c)), and the surface of the CS material after adsorption is rough, which is conducive to the adsorption of copper and chromium ions in the next step, Compare the surface of the material after adsorption of copper and chromium, the surface of the CS material after adorption with Cu(II) is more rough. A large amount of copper ions are uniformly distributed on the adsorbed material, which shows that the CS material has high adsorption.
4.4.2 XRD results
AS show in Fig.8. CS fiber has two different crystal forms, both of which belong to the monoclinic crystal system, namely Form I (2θ in10˚) and Form Ⅱ (2θ is about 20˚). As can be seen from Figure 5, CS fiber has a broad crystallization peak at 2θ=10.9˚, which represents the hydrated crystals of chitosan. It is due to the fact that water molecules enter the chitosan. After reacting with chromium, the peak at about 10˚ disappears, It shows that after the adsorption of CS molecules with chromium, the separation is weakened.The regularity of the three-dimensional structure of the sub-chain reduces the intramolecular crystalline area.
In addition, After contact with copper ions, the peak at about 20˚ disappears.This also indicates that the functional groups of chitosan fibers have chelated and cross-linked with copper ions.
4.4.3 FTIR pattern
Fig.9 shows that the infrared spectra of chitosan fiber before and after the absorption of copper ions are roughly similar, and the position of the characteristic absorption peak is based on the original value remains unchanged, the chitosan fiber is more polar than the copper ion after adsorption. This is because after the -OH in the chitosan fiber forms a coordination bond with Cu(II), it breaks the hydrogen bond between -OH, and then The added Cu(II). steric hindrance effect indicates that Cu(II) has been complexed with chitosan fiber. Chromium ions show the same trend.