Fluoride Sorption by Metal Ion-Loaded Fibrous Protein

Abstract

A new metal ion-loaded (Zr(IV), Al(III), Fe(III)) fibrous protein (MFP) was synthesized and characterized by surface charge (pHpzc), surface morphology, and infrared spectrum. The pHpzc (point of zero charge) of MFP was located in alkaline zone, higher than that of fibrous protein (FP, 5.5–6.0) before impregnation. FTIR studies indicated that metal hydroxyl functional groups (M–OH, M representing surface metal ions of Zr(IV), Al(III), Fe(III)) played an important role in fluoride adsorption by MFP. The effects of pH, contact time, initial fluoride concentration, sorbent dosage and coexisting anions were investigated in order to clarify the adsorption properties of MFP for fluoride ion. The results suggested that MFP adsorbents exhibited reasonably significant fluoride removal over a wide range of pH values (4.0–9.0). Zr(IV)-loaded fibrous protein (ZrFP) had a maximum fluoride adsorption capacity up to 58.41 mg/g, and the maximum fluoride adsorption capacities were 37.51 and 15.24 mg/g for Al(III)-loaded fibrous protein (AlFP) and Fe(III)-loaded fibrous protein (FeFP), respectively, much higher than those of other common adsorbents. The sorption process was subjected to Langmuir and Freundlich isotherms, and Langmuir isotherm fit better than Freundlich. Thermodynamic parameters such as ΔG°, ΔH°, and ΔS° indicated that the nature of fluoride sorption is spontaneous and exothermic. Kinetic studies indicated that the equilibrium was fast and was approached within 150 min, and the pseudo-second-order model could describe sorption of fluoride on MFP well.