Removal of fluoride from aqueous solutions through Fe(III) modified water treatment residues

The extensive presence of fluoride (F-) in ground and surface water poses a potential threat to human health. Water treatment residues (WTR) are potential absorbents that can remove F-, but this removal capacity needs to be enhanced further for its optimal utilization. In this study, we have proposed a method of Fe-modified WTR (Fe-WTR) to ameliorate removal efficiency and investigated the associated mechanism. The physicochemical properties of Fe-WTR were characterized using X-ray fluorescence spectrometry (XRF), scanning electron mi-croscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The maximum adsorption capacities of WTR and Fe-WTR for F- reached 6.09 and 16.09 mg/g under natural pH (pH = 6.5). The F- adsorption kinetics most suitable model was the pseudo-second-order model, indicating that the chemical adsorption may play a primary role in the F- removal process. A probable mechanism for F- adsorption by Fe-WTR was proposed, where the complexation reaction of F- with Al3+ and Fe3+ and ion exchange of F- with OH- were the main processes. The excellent resistance to interference from coexisting anions and the change in pH value clearly demonstrates that Fe-WTR can be efficiently applied for various environments. To conclude, owing to its high adsorption capacity, economy, and environmental friendliness, Fe-WTR is a potentially efficient adsorbent for F- removal from polluted waters. Moreover, this study presents a deeper understanding of the mechanism of F-removal by WTR, which may pave way for future appropriate modifications of WTR for future applications.