Kinetics, isotherm, thermodynamic and bioperformance of defluoridation of water using praseodymium-modified chitosan

Macroscopic granular chitosan-praseodymium (CHN-Pr) composites were prepared using precipitation method and perforated aluminum mold, and were utilized as adsorbents to remove fluoride from aqueous solution. The adsorption characteristic and capability of fluoride on CHN-Pr composites were evaluated using a fixed-bed column, where the effects of contact time, initial fluoride concentration, CHN-Pr adsorbent dosage, and the presence of other anions in the solution were investigated. The mixture of 0.25 g CHN-Pr composite containing 6.07 wt% Pr³⁺ with 20 mg L^-1 fluoride in aqueous solution was demonstrated to have a removal efficiency up to 92.8 % with the maximum amount of fluoride adsorbed being approximately 7.41 mg g^-1 at the contact time of 60 min. With its adsorption capacity, the CHN-Pr can suppress the concentration of fluoride to be within an acceptable level. The adsorption of fluoride by CHN-Pr composite follows the Temkin and Dubinin-Radushkevich isotherm models, suggesting that adsorption of fluoride occurs on multilayer and heterogeneous adsorbent surfaces, while the adsorption kinetics is pseudo-second order and it is governed by two types of diffusion. The adsorption efficiency of fluoride was reduced in the presence of other anions, such as CO₃^2-, SO₄^2-, NO₃-, and Cl-. The spent CHN-Pr adsorbent can be regenerated for several adsorption-desorption cycles by washing it using 0.1 M NaOH and agitation at 50 °C. Overall results suggested that CHN-Pr composite synthesized by precipitation method is promising to be utilized as absorbent for fluoride removal to fulfill the recommended level of fluoride concentration in drinking water.