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 CO32–, SO42–, 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.