Flocculation performance of PCFA composite coagulant for removing nanoparticles

In this study, red mud was used as the main raw material to prepare polyaluminum-ferric chloride (PCFA) with high added value. The effects of the liquid–solid ratio, reaction temperature, reaction time, hydrochloric acid concentration during acid leaching, polymerization time, OH⁻/Fe molar ratio, and polymerization temperature on the preparation of PCFA were investigated and determined by conducting single-factor optimization experiments under optimal preparation conditions. Characterization results show that the surface of the PCFA flocculant has compact and dense porosity and a dense sheet-like structure, and PCFA contains hydroxyl-bridged iron–aluminum polymers. When the settling time was 30 min, the stirring intensity was 200 rpm; the initial concentration of Nano-titanium dioxide（TiO₂-NPs） was 30 mg·L⁻¹; the pH value was 7.0; the dosage of PCFA was 25 mg·L⁻¹, and the optimum removal rates of TiO₂-NPs and turbidity were 84.8% and 81.3%, respectively. The flocs produced by PCFA treatment of TiO₂-NPs were large and dense. Under optimal flocculation conditions, the average particle size of PCFA-TiO₂-NP flocs was 2.98 µm, and the fractal dimension was 1.66. Based on the abovementioned flocculation experimental data, a Grey correlation method was established to evaluate the degree of correlation among factors such as ionic strength, settling time, stirring strength, kaolin concentration, initial concentration, reaction pH value, and dosage and the performance of PCFA coagulation to remove TiO₂-NP-containing wastewater. In the reaction system of the PCFA coagulation of TiO₂-NPs, the dosage is the most important factor.