Chitosan Fiber as Green Material for Removing Cr(VI) and Cu(II) Contaminants:Adsorption Properties,Kinetics and Machanism

Chitosan (CS) ber is used as a new green material to remove Cu(II) and Cr(VI) in wastewater.Varying factors, including pH value, dosage of CS, reaction time and original Cr (VI) contents and Cu(II) were studied to investigate the Cr (VI) and Cu(II) removal eciency.The adsorption of two metal ions by chitosan ber conforms to the second-order kinetic equation, and can be tted with Langmuir isotherms. The adsorption process is a spontaneous thermal reaction with both physical adsorption and chemical adsorption, and copper ions reach adsorption equilibrium. It takes longer than chromium ions, but the adsorption effect of copper ions is better. The maximum actual adsorption capacity of copper ions is 539.6 mg/g, and the maximum adsorption capacity of chromium ions is 75 mg/g. SEM, FTIR and XRD were used to characterize the physicochemical properties of CS ber. The result shows that the complex process of the Cr (VI) and Cu(II) removal involves physical and chemical adsorption, CS ber have exerted signicant role in Cr (VI) and Cu(II) removal.


Introduction
With the rapid development of industrial economy, heavy metals such as copper and chromium contained in industrial wastewater will cause serious environmental pollution [1][2][3][4][5]. Heavy metals can cause serious harm to the human body, such as chromium: it can cause numbness of the limbs and mental disorders, and copper can cause arthritis and accelerate human aging [6][7][8][9][10]. The concept of "cleaner production" was put forward in the 1950s and 1960s. The concept was to quickly realize the impact of industrial development on the environment, thereby establishing a safe and friendly environmental concept. Now that the concept of environmental protection has been deeply rooted in the hearts of the people, economic development is gradually pursuing green [11][12][13][14][15][16][17].
At present, many treatment methods are applied to remove heavy metals, includingg membrane separation, chemical precipitation, solvent extraction, ion-exchange, reduction, reverse osmosis and adsorption [1,18], The adsorption method is widely used for its good results with the advantages of nonproduction of potential secondary pollutants, recyclable usage, easy accessibility and high e cient usage.
Chitosan is widely used in the elds of food, medicine and wastewater due to its excellent biocompatibility, biodegradability, non-toxicity, adsorption and antibacterial properties. Nowadays, In the study of the adsorption of heavy metal ions, chitosan derivatives have excellent performance except relatively expensive price. Chitosan ber(CS) is dissolved in acid solution, the used CS can be re-dissolved and produced to achieve recycling, meeting the requirements of sustainable development and saving resources. At present,there are few studies on the use of CS for heavy metal adsorption.
In the previous research on the removal of heavy metal ions in wastewater, chitosan and modi ed substances were mostly used as adsorbents, and the research and application of chitosan bers were very few. In this paper, chitosan ber is used as the experimental material to discuss the in uence of chitosan ber, metal chromium ion and copper ion on reaction time, copper ion concentration, temperature and pH value. The kinetic and thermodynamic analysis of the adsorption process were carried out, and the adsorption mechanism was characterized by SEM, FTIR and XRD. It is expected to provide a basis for the application and expansion of chitosan ber in water treatment elds such as water puri cation or sewage treatment.

Materials And Instruments
Materials:The CS was purchased from Tianjin Zhongsheng Co.,LTD.(china). The average length of CS is 40mm, and the linear density is 1.3D.

Experimental test methods
Performance test of the CS to remove Cr (VI) and Cu(II) was conducted within a ask with 250 mL capacity. The employed Cr (VI) and Cu(II) solution quantity was 100 mL in each experiment. The 0.1 mol/L HCL and NaOH were utilized for adjusting solution pH value. The speed and temperature of shaker kept at 90 rpm and 288 K, respectively. After reaction, the ltrate is collected for analyzing Cr (VI) and Cu(II) concentration. The experimental methods of Cr(VI are presented in Table 1.

Determination of Cr ( ) and Cu(II) concentration
The adsorption capacity and adsorption e ciency of the ber to Cr(VI) and Cu(II) are calculated by formulas (1), (2) and (3) respectively.
In the formula: C 0 represents the original Cr (VI) and Cu(II) content (mg/L), while C e represents Cr (VI) and Cu(II) content (mg/L) at time e, C t represents Cr (VI) and Cu(II) content (mg/L) at time t, V represents Solution volume (mL), M represents dry weight of ber (g).

Results And Discussion
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 ber increases, and the adsorption rate is faster in the rst 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 ber to Cr(VI) increases rst, and the adsorption capacity of the ber 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 ber 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 ber 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 bene cial to the adsorption of bers. show that CS ber is more suitable for reaction with low concentration of Cr(VI). Fig.2 Indicates the adsorption performance of CS ber on Cu(II) shows the same trend as that on Cr(VI), but the adsorption performance on Cu(II) is better. As CS ber 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 ber has better adsorption property for divalent metal ions.

Kinetic experiments
As showed in Fig.3 and Tab.2. According to the value of R 2 , R 2 2 >R 1 2 ,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 pro les. The kinetic parameter R 2 is less than 1, which indicates the favor adsorption pro le.
It can see from Fig.4 and Tab.3, the kinetic equation of the CS ber for the adsorption of Cu(II) has the same properties as that of Cr(VI). the kinetic simulation using pseudo-rst-order is not good and the actual data deviates seriously from the tted curve. At the same time, the correlation coe cient of kinetic parameters is low. While the pseudo-second-order kinetic plot, with high correlation coe cient of kinetic parameters.

Isotherm experiments
From the parameters R 1 2 <R 2 2 in Figure 5 and Table 4, we can see that the adsorption of Cr(VI) by chitosan bers 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 ber 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 bene cial for Cr (VI) adsorption onto CS ber.
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 k f value indicates the high a nity of CS ber for Cu (II) ions.  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.

XRD results
AS show in Fig.8. CS ber has two different crystal forms, both of which belong to the monoclinic crystal system, namely Form I (2θ in10˚) and Form (2θ is about 20˚). As can be seen from Figure 5, CS ber 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 10d isappears, 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 bers have chelated and cross-linked with copper ions. Fig.9 shows that the infrared spectra of chitosan ber 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 ber is more polar than the copper ion after adsorption. This is because after the -OH in the chitosan ber 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 ber. Chromium ions show the same trend.

Conclusion
Chitosan ber adsorbs Cu(II), and the adsorption process conforms to the second-order kinetic equation, and electrochemical adsorption plays a leading role. The adsorption isotherm is more in line with the Langmuir model, and the adsorption process is multi-layer adsorption, which belongs to the spontaneous thermal reaction with both physical adsorption and chemical adsorption, and the adsorption type is effective adsorption. The optimal adsorption reaction time is 90 minutes, the equilibrium adsorption time is 180 minutes, the optimal adsorption pH is 5, and the maximum saturated adsorption capacity can reach 248.4 mg/g. The adsorption of chromium ions and copper ions by chitosan bers shows the same trend, and the adsorption of low-valent ions by chitosan bers is better due to cross-linking.
According to the test results of SEM, it can be concluded that the surface of chitosan ber adsorbs copper ions and becomes rougher. According to the results of FTIR and XRD, it can be obtained that chitosan ber adsorbs heavy metal ions, mainly physical and chemical adsorption.
Chitosan ber can be dissolved in acetic acid, and the spinning solution of chitosan ber is under acidic conditions, which is bene cial to recovery. Spinning into chitosan yarn or other textiles, used in water puri cation and other water treatment elds, can remove harmful metal ions. It is an environmentally friendly material that can be recycled continuously and has broad application and development prospects.   Quasi-rst-order and quasi-second-order kinetic equations on adsorption of Cr(VI) Figure 4 Quasi-rst-order and quasi-second-order kinetic equations on adsorption of Cu(II)    XRD of CS before and after use for Cr (VI) and Cu(II) adsorption.

Figure 9
FTIR of CS before and after use for Cr (VI) and Cu(II) adsorption.