Adsorption of fluoride on synthetic siderite from aqueous solution

The study has investigated the feasibility of using synthetic siderite for F− removal from aqueous solution. Batch experiments were performed to test effects of adsorbent dosage, contact time, initial F− concentration, temperature, solution pH, and coexisting anions on F− removal. Results show that the kinetic rate of F− adsorption was high in the first 2 h, and thereafter significantly decreased. The kinetic data was better fitted to the pseudo-second order kinetic model than the pseudo-first order kinetic model. In comparison with Langmuir isotherm, both Freundlich and Redlich–Peterson isotherms better described the adsorption process, which indicates that the multilayer adsorption should be involved in the process of F− removal. Thermodynamic study manifests that F− adsorption on synthetic siderite was spontaneous and exothermic in nature. The synthetic siderite had high adsorption capacity for F− removal, which was up to 1.775 mg/g in the batch with an adsorbent dosage of 5 g/L and an initial F− concentration of 20 mg/L at 25 °C. The adsorption was relatively independent on solution pH between 4 and 9. The presence of Cl− and NO3− had less impact on F− adsorption, while PO43− significantly affected F− removal from aqueous solution. Results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) suggest that the high adsorption capacity possibly arose from both coprecipitation of ferric hydroxide with F− and adsorption of F− on the fresh goethite.