Chitosan hydrogel beads for fulvic acid adsorption: Behaviors and mechanisms

Chitosan hydrogel beads were studied for the adsorption of fulvic acid (FA) from aqueous solutions to examine the adsorption behaviors and mechanisms. Kinetic and isotherm studies were carried out by considering the effects of various parameters, such as initial concentration (5–50 mg/L), temperature (15, 30, 45 °C), pH (4–12) and ionic strength (0–0.5 M NaCl). The results indicate that the adsorption of FA is strongly dependent on pH and ionic strength under our experimental conditions. The adsorption kinetics and isotherms showed that the sorption processes were fitted better by intra-particle model and the Freundlich equation, respectively. Point of zero charge (pHPZC) study indicated that the surface of chitosan hydrogel beads was positively charged at pH <9.9 and negatively charged at pH >9.9. Fourier transform infrared spectroscopy suggested that FA adsorption was mainly through interactions with the N atoms in chitosan hydrogel beads and further X-ray photoelectron spectroscopy (XPS) revealed the formation of organic complex between the protonated amino groups and FA after adsorption.