Enhanced phosphate and fluoride removal from aqueous solution by ferric-modified chromium (Ⅲ)-fibrous protein

Ferric modified chromium (III)-fibrous protein (Fe–CrFP) sorbent for phosphate and fluoride removal was prepared through crosslinking reaction. This adsorbent was characterized by elemental analysis, X-ray diffraction (XRD), Scanning electron microscope (SEM), Fourier Transform infrared spectroscopy (FTIR) and zeta potential. Results indicated that Fe–CrFP possessed good adsorption ability for both phosphate and fluoride due to its higher isoelectric point (pH = 9.2). Phosphate and fluoride had different pH dependence on solution pH: phosphate removal decreased rapidly beyond pH = 6.0, while a higher fluoride sorption amount was obtained in a pH range of 3.0–9.0. Adsorption isotherms for PO43− and F− were well described by the Langmuir equation. XRD analysis illustrated that Fe(III) on Fe–CrFP was amorphous, and hydrous ferric oxide formed on Fe–CrFP played an important role in adsorption. The main adsorption mechanisms of phosphate and fluoride were attributed to electrostatic attraction, additionally Fe–CrFP could also capture phosphate and fluoride through specific adsorption with formation of inner-sphere complex by surface hydroxyl groups (Fe–OH). Fluoride was easier to be influenced by coexisting anions in comparison to PO43−, and competitive sorption occurred in F− in presence of PO43−. Phosphate and fluoride adsorbed on Fe–CrFP could be totally desorbed by alkali solution, recycling experiment suggested that Fe–CrFP had a good utilization in treatment of phosphate and fluoride and provided a possibly valuable application for tanning leather waste.