Adsorption performance of cellulose/chitosan microspheres for congo red

Cellulose is abundant in nature, and it has good biocompatibility and biodegradability. However, it has shortcomings such as insufficient ductility, poor solubility, and difficult in forming, which limits its further application in many fields, such as food and medicine. Cellulose/chitosan composite microspheres were prepared using carboxymethyl cellulose and chitosan as raw materials, anhydrous ethanol, and distilled water as coagulants by the crosslinking method. Fourier infrared spectroscopy (FT-IR), specific surface area and aperture test (BET), and scanning electron microscopy (SEM) were used to characterize the adsorption properties. The results showed that the adsorption rate of cellulose/chitosan was higher than that of a single substance. The maximum adsorption capacity of the cellulose/chitosan composite microsphere was 57.63 mg/g when the equilibrium was reached. Compared with single cellulose and chitosan, cellulose/chitosan composite microspheres had a better adsorption effect on Congo red. As a potential adsorption material, this material has shown good application value in the field of sewage treatment.


Introduction
Congo red dye is widely used in printing and dyeing, leather, food, and other industries.The residual Congo red solution in wastewater will cause some pollution to the environment.How to remove Congo red in wastewater has been a hot topic in recent years.Chitosan is a typical environmentally friendly material with the advantages of natural non-toxicity, good biocompatibility, easy biodegradation, and wide source [1].Chitosan acts as both coagulant and flocculant.It destabilizes into fine particles under the action of charge and suspended particles.Important progress has been made in its application research in many fields, such as cosmetics, food and nutrition, biomedicine, drinking water engineering, textile printing, and dyeing [2].Chitosan molecule contains hydroxyl group, acetylamino group amino group, etc., which has a good flocculation effect and low price and is one of the natural polymer adsorbents with great development prospects [3,4].Some researchers prepared chitosan adsorbent, but the chitosan molecule had defects such as low polymerization degree, poor molecular rigidity, poor pellet formation, and dispersion.Researchers have prepared millimeter solid and medium chitosan pellets by sol-gel and spray methods and applied them to the immobilization of A-amylase.
Cellulose itself also has a certain adsorption effect, and the glucose group of cellulose molecules has 3 free hydroxyl groups, which have the properties of ordinary alcohol groups, including 4 free hydroxyl groups and 1 potential aldehyde group [5,6].Due to the increase in molecular rigidity, the packing of molecular chains become more tightly packed, resulting in their dissolution in traditional solvents becoming more difficult.Therefore, cellulose has excellent water resistance and heat stability [7].The complex hydrogen bond system exists in the crystal structure of cellulose, which is caused by the hydroxyl group in cellulose macromolecules.In this project, chitosan and cellulose are blended to maintain their physical properties and reduce their acid solubility.In the experiment, cellulose/chitosan microspheres with high specific surface area and high porosity were prepared by crosslinking method and were applied to the adsorption experiment of Congo red.The effects of Congo red addition amount and ambient temperature on the adsorption behavior of cellulose/chitosan microspheres were investigated from the perspectives of kinetics and thermodynamics.

Preparation of materials
2.1.1.Cellulose/chitosan composite microspheres were prepared by the crosslinking method.A 500 mL beaker is prepared, and the cellulose-sodium hydroxide solution is poured into the beaker.With a glue dropper, the chitosan-acetic acid solution is absorbed and kept at a distance.The chitosan-acetic acid solution is added drop by drop to the cellulose-sodium hydroxide solution.The two different solutions are cross-linked, and an acceleration of about 2 drops/second is dropped.The surface of the cellulose-sodium hydroxide solution can form a transparent spherical solid and gradually turn white.It is left for about 30 minutes.We then wait for the microspheres to form.The pH test paper is used to measure the cleaned distilled water until the pH value of the measured distilled water is about 7.

Freeze drying of cellulose/chitosan composite microspheres.
After cleaning the prepared cellulose/chitosan composite microspheres until neutral, the microspheres are placed in the petri dish in a thin layer as far as possible, without overlapping places, and the water is drained in the petri dish.After the moisture is absorbed, it is frozen in the refrigerator and waited to dry.After completely frozen, it is put in the freeze dryer, turned on the power, filtered the vacuum, and waited for it to dry.After drying, the cellulose/chitosan composite microspheres are white, fluffy balls.The prepared composite microspheres were placed in the prepared beaker and sealed with plastic wrap to avoid water absorption and keep the composite microspheres dry.

Testing and characterization
2.2.1.Infrared spectrum.The infrared spectrometer was preheated for 30 min in advance.The cellulose/chitosan composite microspheres were dried in the freeze-dryer, and a small number of microspheres were put into the agate mortar and ground with dried KBr.

Scanning electron microscopy (SEM) .
The microspheres were cleaned, dehydrated, dried, glued, coated, etc., and then fixed on the SEM sample rack with conductive adhesive to observe their morphology.

BET.
The specific surface area and pore structure of cellulose/chitosan composite microspheres were determined by JW-BK222 static nitrogen adsorption instrument.

Congo Red solution adsorption experiment
2.3.1.Adsorption kinetics.0.0500 g cellulose/chitosan composite microspheres are accurately weighed in glass bottles containing 10 mL 300 mg/L Congo red solution.The glass bottles are oscillated in a 25℃ constant temperature oscillator.At regular intervals, the absorbance of Congo red solution was measured with a type 752 UV-visible spectrophotometer until the adsorption of composite materials reached equilibrium.

Adsorption isotherm.
In the adsorption isothermal test, three cellulose/chitosan composite microspheres with a mass of 0.0500 g were weighed and added into a 10 mL 300 mg/L Congo red solution.They were shaken in a 25℃ constant temperature oscillator for 1-2 days at a revolution of 150 r/min until the adsorption was saturated.The temperature is changed and oscillated in a 35℃ thermostatic oscillator at 150 r/min until adsorption saturation is reached.The temperature is set to 45℃, and the revolution is still 150 r/min until the adsorption saturation is reached.

Characterization of cellulose/chitosan composite microspheres
3.1.1.Scanning electron microscope analysis.It can be seen from the figure 1 that there are a large number of gullies and apertures of different depths inside the composite microsphere.According to the scanning electron microscope image of the cellulose/chitosan composite microsphere, there are a large number of gaps and pore sizes in both the interior and interior structures, which can provide many adsorption sites and the pore sizes of gullies are different.

BET test analysis.
The specific surface area and pore size of the microspheres are shown in Table 1.As can be seen, the average adsorption pore size is 145.2043nm, which is within the range of large pores.The specific surface area of the composite microspheres is 12.055 m 2 /g.Table1.Specific surface area and pore size of cellulose/chitosan composite microspheres.2 shows the infrared spectrum of cellulose/chitosan.It can be seen from Figure 2 that the absorption peak of cellulose/chitosan composite microsphere near 3446.69cm -1 is the stretching vibration absorption of the hydroxyl group, which is the characteristic spectral band of cellulose and chitosan.The absorption peak at 1638.46 cm -1 is attributed to the carbonyl stretching vibration peak.The deformation vibration of the hydroxyl group is at 669.83 cm -1 .The data showed that the hydroxyl group between cellulose and chitosan was cross-linked through hydrogen bonds during the forming process of composite microspheres, which resulted in the weakening of the hydroxyl group absorption peak.The above shows that during the molding process of cellulose/chitosan composite microspheres, on the one hand, the structure of cellulose and chitosan is retained to a large extent, and on the other hand, the chemical structure is more stable due to cross-linking.

Standard curve analysis
The standard curve of Congo red is shown in Figure 3. From the standard curve of Congo red absorbance changing with concentration, it can be seen that when the concentration of Congo red is 0-160 mg/L, there is a good linear relationship between the absorbance A and the concentration C, the correlation is 0.9978, and the regression equation is y=0.0105x+0.0401.

Adsorption kinetics analysis
Figure 4 shows the influence curve of time on the adsorption of Congo red by cellulose/chitosan composite microspheres.It can be seen that the adsorption of Congo red solution by cellulose/chitosan composite microspheres changed greatly within 30 minutes.The adsorption rate of Congo red solution by cellulose/chitosan composite microspheres decreased slowly during 30 min-60 min.After 60 min, the data showed that the adsorption capacity of Congo red solution on cellulose/chitosan composite microspheres tended to be flat with the change of time and reached equilibrium after 2 h.In order to study the kinetic mechanism of the adsorption reaction, the quasi-second-order kinetic model is usually used to simulate the adsorption reaction.According to the quasi-second-order kinetic equation, we have: e e 1 t t q kq q   (Equation 1) In Equation (1), By plotting t against t/qt, a straight line can be obtained, as shown in Figure 5. From the intercept and slope of this line, q e and k can be obtained.(Equation 2) From the intercept and slope of this line, q max can be calculated as 57.63 mg/g.The results of data fitting by the Langmuir model show that the correlation coefficient R2 is 0.9993, and the Langmuir model is consistent with the process of cellulose/chitosan adsorption of Congo red.

Adsorption isotherm analysis
Figure 7 shows that the adsorption of Congo red solution changed slowly when the temperature increased.After 60 min, the adsorption changes of cellulose/chitosan composite microspheres on Congo red solution tended to be flat.In summary, the adsorption capacity of composite microspheres is accelerated when the temperature is increased, and the time to reach the adsorption equilibrium is shorter than that when the temperature is low.

Conclusion
The experimental results show that the microspheres have good adsorption properties for Congo red.The initial concentration and contact time of Congo red have a great influence on the adsorption effect.The adsorption experiment data showed that the maximum absorption amount of Congo red by cellulose/chitosan composite reached 57.63 mg/g, and the adsorption process was endothermic.

Figure 5 .Figure 6
Figure 5.Effect of time on the adsorption of Congo red.