Ti-doped fly ash‑aluminum‑iron based composite coagulant for efficient fluoride removal from mine drainage over a wide pH range

The concentration of F⁻ (2.67–16.2 mg/L) in mine drainage exceeding the discharge standard is significantly toxic to human health. Traditional coagulants are ineffective in achieving effective fluoride removal over a wide pH range. Therefore, studying novel coagulants suitable for treating fluoridated mine drainage is urgently required. First, we screened the active components of the coagulants, determined their contents, and investigated the effects of the process parameters and coexisting matter. Lastly, we explored the mechanisms of coagulation and fluoride removal using zeta potential, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), particle size analysis, and microscopy. The results indicated that fly ash could be used as a major component (50 % by mass) when the fluoride removal efficiency exceeded 92 % (residual F⁻ < 1.0 mg/L), synchronizing the reuse of solid waste. Ti-doped coagulants extended the applicable pH range (5.5–8.0). The optimal process parameters were a dosage of 0.8 g/L, pH 6.5, stirring intensity (200 and 40–60 rpm) and time (60 and 120 s) for the mixing and slow flocculation stages, respectively, and a settling time of 5 min. Suspended solids (SS), CO₃²⁻, SO₄²⁻, and HCO₃⁻ all reduced fluoride removal efficiency. F⁻ removal mechanisms during coagulation included coordination exchange and electrostatic adsorption. This study provides practical applications and theoretical references for removing F⁻ from mine drainage.