A novel flake-ball-like magnetic Fe3O4/γ-MnO2 meso-porous nano-composite: Adsorption of fluorinion and effect of water chemistry.

A novel flake-ball-like magnetic Fe3O4/γ-MnO2 meso-porous nano-composite was synthesized and characterized for defluoridation. Adsorption process, characters, and effects of solution chemistry on the adsorption of flourinion in Fe3O4/γ-MnO2 were evaluated. The results show that the adsorption of fluorinion in the Fe3O4/γ-MnO2 nano-composite is fitted with the Pseudo-first model and the Langmuir model, indicating that the adsorption process of fluorinion in the Fe3O4/γ-MnO2 nano-composite was a physical process and not only controlled by the film diffusion but also controlled by the intra-particle diffusion and surface adsorption. It shows that the adsorption of fluorinion sharply decrease with the increase of pH due to the negative changed surface of Fe3O4/γ-MnO2 in water and the competition of OH- for the active points. The competition from decreases the adsorption of fluoride in the order of Cl- < NO3- < SO42-, which relied on the ratio of charge towards radius (z/r) of the anions, and the negatively charged humic acid competed with fluorinion for the adsorption sites. Based on the adsorption results and the XPS analysis, the OMn bond in the raw adsorbent supported the active site (OMnOH) for fluoride adsorption by forming an OMnF bond on the surface of Fe3O4/γ-MnO2.