Significance of FeZr-modified natural zeolite networks on fluoride removal

Abstract

Three different zeolitic structures, clinoptilolite, mordenite and chabazite, were modified with iron, zirconium and iron-zirconium in order to compare their fluoride removal behaviors from aqueous solutions. Natural and modified materials were characterized by SEM-EDS, XRD, IR and XPS. The BET, pHPZC and particle size measurements were also considered. After iron and/or zirconium modifications, iron content (8–18wt.%) found in the samples was higher than zirconium (2–10wt.%). However, fluoride adsorption capacities of zirconium-modified materials (up to 2.5±0.12mg/g) were higher than the iron ones (up to 1.6±0.15mg/g). Iron-zirconium-modified zeolite-rich materials presented the highest adsorption capacities in the following order: ZM-FeZr (3.5±0.15mg/g)>ZCH-FeZr (2.6±0.02mg/g)>ZC-FeZr (1.8±0.10mg/g). Kinetic data were described by Lagergren and pseudo-second order models, while the isotherm results were best adjusted to the Freundlich model, and according to the KF values, modified mordenite and chabazite networks showed the highest affinities to fluoride. It was observed a slightly acid pH range (from 4.6 to 6.4) where the zeolitic materials exhibited the most significant fluoride adsorption. Thermodynamic parameters (Ea, Δ‡H°, Δ‡S° and Δ‡G°) were calculated for the iron-zirconium modified zeolitic tuffs, indicating an exothermic and physical defluoridation process. It was found that the release of Fe and Zr from the ZC-FeZr, ZM-FeZr and ZCH-FeZr after the defluoridation of water were insignificant. Finally, according to the results, a fluoride adsorption mechanism was proposed.