Facile synthesis of pH-sensitive carboxymethyl chitosan and its tunable adsorption property toward anionic and cationic dyes

摘要

N, O-carboxymethyl chitosan (CCSx) adsorbents with tunable adsorption and separation capacity were successfully prepared by chitosan (CS) and monochloroacetic acid (MCA) with various amounts. The structures and compositions were characterized by Fourier-transform infrared spectroscopy, scanning electron microscopy, X-ray powder diffraction, zeta potential, thermogravimetric analysis, and N-2 adsorption-desorption isotherm. Results showed that carboxymethylation of CS with MCA occurred mainly at C-6-OH and C-2-NH2 positions. The pH-sensitive and tunable adsorption capacities toward various dyes were studied and optimized. At the optimal pHs (pH = 5 for methylene blue (MB) and pH = 3 for methyl orange (MO)), the highest removal efficiencies of 94.5% and 90.6% for MB and MO, respectively, were observed for CCS 1. The adsorption isotherm data better fitted with Langmuir model (R-2 0.99), and the maximum adsorption capacity of 64.56 and 92.51 mg g(-1) were obtained for MB and MO, respectively. The kinetics of adsorption was described by the pseudo-second-order model. The regeneration study showed that the percentage uptake declined slightly for MO after five cycles. Moreover, MB or MO can be separated efficiently at pH = 5 or pH = 3 in the MB/MO binary system, indicating its selective adsorption and separation property, especially its potential application in practical wastewater treatment.