Removal of high-concentration fluoride wastewater by calcined hydrated calcium aluminate with a two-step process

The hydrated calcium aluminate for fluoride removal of high concentration fluoride-containing wastewater was prepared by thermal activation modification. X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), Fourier transform infrared spectrometer (FT-IR) and isothermal adsorption model were used to study the adsorption process. The results showed that the main component of the modified calcium aluminate cement (CAC) were 12CaO·7Al2O3 (C12A7), and a large amount of calcium ion and aluminum ion released during the fluoride removal process. The calcination of hydrated calcium aluminate improved the removal rate of fluoride. After a two-step treatment, adsorption and pH value adjustment, the fluoride-containing solution with an initial concentration of 300 mg/L reduced to less than 1 mg/L. The removal of fluoride mainly depends on the adsorption action in low concentration but chemical precipitation at high concentration. The fluoride removal process involves three mechanisms: chemical precipitation, ion exchange and electrostatic interaction.