Preparation of aluminium-hydroxide-modified diatomite and its fluoride adsorption mechanism

As the current excessive accumulation of fluoride (F^-) in the environment can be hazardous to human health, it is essential to remove fluoride from wastewater. In this study, diatomite (DA) was used as a raw material and modified using aluminum hydroxide (Al-DA) for use in the adsorption of F^- from water bodies. SEM, EDS, XRD, FTIR, and Zeta potential characterization analyses were carried out; adsorption tests and kinetic fitting were performed, and the effects of pH, dosing quantity, and presence of interfering ions on the adsorption of F^- by the materials were investigated. The results show that the Freundlich model effectively describes the adsorption process of F^- on DA, which therefore involves adsorption-complexation interactions; however, the Langmuir model effectively describes the adsorption process of F^- on Al-DA, corresponding to unimolecular layer adsorption mainly via ion-exchange interactions, that is, adsorption is dominated by chemisorption. Aluminum hydroxide was shown to be the main species involved in F^- adsorption. The efficiency of F^- removal by DA and Al-DA was over 91% and 97% for 2 h, and the adsorption kinetics were effectively fit by the quasi-secondary model, suggesting that chemical interactions between the absorbents and F^- control the adsorption process. The adsorption of F^- was highly dependent on the pH of the system, and the maximum adsorption performance was obtained at pH 6 and 4. The optimal dosage of DA and Al-DA was 4 g/L. Even in the presence of interfering ions, the removal of F^- on Al-DA reached 89%, showing good selectivity. XRD and FTIR studies showed that the mechanism of F^- adsorption on Al-DA involved ion exchange and the formation of F-Al bonds.