Enhanced adsorption of fluoride from aqueous solution onto nanosized hydroxyapatite by low-molecular-weight organic acids

Abstract

The effects of low molecular weight organic acids (LMWOAs) on the defluoridation capacity of nanosized hydrpxyapatite (nHAP) from aqueous solution were investigated. Defluoridation capacity of nHAP was enhanced in the presence of LMWOAs. The nHAP adsorbed LMWOAs on its surface, and the LMWOAs on nHAP were considered to be the newly formed active sites for fluoride adsorption. At low pH, the adsorbed LMWOAs were protonated, positively charged, and thus could attract fluoride anions by electrostatic attraction. Meanwhile, the protonated LMWOAs could also form hydrogen bonding with fluoride. At high pH, the LMWOAs on nHAP were deprotonated and existed as anions. These organic acid anions on nHAP could be exchanged with fluoride. It was evident that with increasing aqueous LMWOAs concentrations, the LMWOAs adsorbed onto nHAP increased, and subsequently the defluoridation capacity of the adsorbent increased. Results also indicated that the defluoridation capacity increased with increasing the contact time, while decreased with the increase of the adsorbent dose or solution pH. In the presence/absence of LMWOAs, the adsorption isotherms were well fitted by the Freundlich model (R2>0.98), and the sorption kinetics were well described by the pseudo-second-order model. Moreover, thermodynamic parameters indicated that the adsorption was spontaneous and endothermic.