Fluoride adsorption on modified natural siderite: Optimization and performance

Natural siderite modified by aluminum sulfate and aluminum sol was synthesized and characterized for F− removal. Natural siderite powders, Al2(SO4)3, and AlOOH were mixed with different ratios, granulated by an extrusion method, and calcined at different temperatures for different time durations. The optimum modified natural siderite (OMNS) was obtained with the mass ratio of 50 g:0.3 g:10 g for natural siderite powders:Al2(SO4)3:AlOOH and calcinated at 450 °C for 3 h. Solution pH had no significant effect on F− removal between 3.5 and 10.0. Kinetic results showed that adsorption followed the pseudo-second-order kinetic equation. Both the external adsorption and the intra-particle diffusion contributed to adsorption processes, while pore diffusion controlled fluoride adsorption on OMNS according to Bahangam equation. Thermodynamic data showed that fluoride adsorption on OMNS was reasonably spontaneous and endothermic. Moreover, fluoride adsorption was obviously constrained by the coexistence of HCO3- and PO43-. Column studies revealed that breakthrough time and adsorption capacity were lower at higher concentration and/or higher flow rate. The XRD studies indicated the uptake of F− by the OMNS attributed to the coexistence of hematite and γ-Al2O3 in the material.