Insight into the ion exchange in the adsorptive removal of fluoride by doped polypyrrole from water

In this study, the polypyrrole (PPy) samples doped with Cl^- (PPy-Cl), SO₄^2- (PPy-SO4) and SO₄^2-+Cl^- (PPy-SO4+Cl) were synthesized by chemical polymerization for the adsorptive removal of fluoride ion from water. The structure and morphology of the as-prepared PPy samples were characterized by FT-IR, BET, SEM, XPS, and zeta potential. The adsorption experiments revealed that the PPy-Cl exhibited faster kinetics and higher adsorption capacity (13.98 mg/g), more than 4 times that of PPy-SO4 (3.08 mg/g) and PPy-SO4+Cl (3.17 mg/g). The kinetics of the adsorption followed the pseudo-second-order model and the adsorption isotherm data fitted well to the Langmuir model. FT-IR, EDX, and XPS tests for PPy samples before and after fluoride adsorption demonstrated that anion exchange between F^- and Cl^- or SO4^2- was the prior mechanism for fluoride ion removal from water. Cl^- was more favorable than SO₄^2- in the ion exchange with F^-. Meanwhile, the Cl^- or SO₄^2- exchanged with F^- was mainly bound to the active nitrogen that accounts for 6% of the total nitrogen in PPy molecular matrix. Further study of zeta potential and pH influence experiment demonstrated the electrostatic interaction is auxiliary interaction for the fluoride removal by doped PPy samples.