Adsorption of fluoride on clay minerals and their mechanisms using X-ray photoelectron spectroscopy

This research investigates the adsorption mechanisms of fluoride (F) on four clay minerals (kaolinite, montmorillonite, chlorite, and illite) under different F− concentrations and reaction times by probing their fluoride superficial layer binding energies and element compositions using X-ray photoelectron spectroscopy (XPS). At high F− concentrations (C0 = 5–1000 mg·L−1), the amount of F− adsorbed (QF), amount of hydroxide released by clay minerals, solution F− concentration, and the pH increase with increasing C0. The increases are remarkable at C0>50 mg·L−1. The QF increases significantly by continuously modifying the pH level. At C0 100 mg·L−1, new minerals precipitate and the product depends on the critical Al3+ concentration. At [Al3+]>10−11.94 mol·L−1, cryolite forms, while at [Al3+]<10−11.94 mol·L−1, AlF3 is formed. At low C0 (0.3–1.5 mg·L−1), proton transfer occurs, and the F− adsorption capabilities of the clay minerals increase with time.