Abstract:

Objective The pollution of dye wastewater to the environment seriously harms people's health and is also an important factor restricting the development of textile enterprises. The adsorption method is a promising one for dye wastewater treatment. Natural polymeric cellulose and chitosan are widely used as adsorption materials for wastewater treatment because of the advantages in large capacity for storage, good adsorption, non-toxicity and easy degradation. In this research, an cotton stalk microcrystalline cellulose/chitosan(MCC/CS) adsorption fiber was prepared for the adsorption of Congo Red.

Method Firstly, modified chitosan (DCS) was prepared by surface modification to CS with formaldehyde as crosslinking agent and 2,5-disulfide diurea as modifier, to improve their adsorption of Congo Red. The modified chitosan process was optimized by the orthogonal test method. Urea solvent method was used to dissolve MCC at low temperature as spinning matrix, DCS was added as insoluble dispersion adsorption material, composite fiber was prepared by wet spinning and freeze-drying, and the spinning process was optimized by the single factor method. Finally, the chemical structure of DCS was analyzed, the morphology and dye adsorption properties of MCC/DCS adsorbed fibers were tested and characterized.

Results The modification of CS was carried out successfully, where the new C=S characteristic absorption peaks at 1 209 cm^-1appear and the absorption peak of -NH₂ (2,5-dithiourea, DB) disappeared at 1 640 cm^-1 in DCS infrared spectrum. The combination of conditions obtained by orthogonal test for preparation of DCS was optimized, where acetic acid dosage was 55 mL, DB dosage was 0.60 g, CS dosage was 1.0 g, and formaldehyde dosage was 8 mL. The modified chitosan increased the contents of N and S elements, and the changes were obvious. The pore size of the MCC/DCS was larger than the MCC adsorbent fibers, and the modification CS was more beneficial to the adsorption of Congo Red by adsorbent fibers. The fracture strength of MCC/DCS adsorption fiber decreased gradually with the increase of the amount of DCS in the fiber, while the linear density increased gradually because DCS is insoluble in alkaline solution. With the addition of DCS, the viscosity of the spinning solution decreased gradually. When the concentration of dye solution was 60 mg/L, the adsorption capacity of unmodified CS for Congo Red was 13.85 mg/g, and the DCS under optimal conditions was 17.63 mg/g, the adsorption capacity of DCS was increased by 27.29% compared with CS. At the same dye solution concentration, the adsorption capacity of MCC/DCS fiber was 49.55 mg/g, which was 47.82% higher than that of MCC/CS fiber 33.52 mg/g. The analysis of the influencing factors of adsorption performance showed that the adsorption capacity of MCC/DCS adsorption fiber for Congo Red decreased gradually with the increase of adsorption temperature. The adsorption capacity was increased with the adsorption time of MCC/DCS fiber and the change of the initial concentration of dye solution, and the adsorption equilibrium was reached when the concentration of dye solution was 250 mg/L. Ho kinetic model was suitable to simulate the adsorption process of DCS adsorption fibers. The adsorption thermodynamic analysis showed that the Langmuir model was suitable to simulate the adsorption process of DCS adsorbed fibers.

Conclusion CS was successfully modified with formaldehyde as crosslinking agent and 2, 5-dithiourea as modifier. The modification process of CS was optimized by orthogonal test, and the optimum process was obtained. The MCC was dissolved by urea dissolution system at low temperature as a spinning matrix, coated with DCS powder prepared by the optimized process. Adsorption experiment test results are as follows: when the concentration of dye solution was 60 mg/L, the average removal rate of MCC/DCS fiber Congo Red was 82.58%, and the average removal rate of MCC/CS fiber for Congo Red was 55.87%. The removal rate of MCC/DCS fiber was 47.82% higher than that of MCC/CS fiber, and the expected effect was achieved.

Key words: modified chitosan, cotton stalk bark microcrystalline cellulose, adsorption material, Congo Red, adsorption kinetics