Removal of titanium dioxide nanoparticles by coagulation: effects of coagulants, typical ions, alkalinity and natural organic matters

To investigate the possibility of removing titanium dioxide nanoparticles (TiO₂ NPs) from water by coagulation, as well as to find the optimal coagulant and experimental conditions for TiO₂ NP removal, four types of coagulant were adopted: polyferric sulfate (PFS), ferric chloride (FeCl₃), polyaluminum chloride (PACl), and alum (Al₂(SO₄)₃). It was found that the removal of TiO₂ NPs by coagulation was affected by ionic strength, alkalinity, as well as types and dosages of coagulants. PFS and FeCl₃ achieved much higher removal efficiency of TiO₂ NPs than PACl and Al₂(SO₄)₃ did. For 30 mg/L TiO₂ NPs, a dosage of 0.3 mM PFS (as Fe) achieved 84% removal after coagulation followed by 30 min settlement. Optimal ionic strength (0.1 M NaCl or 0.03 M CaCl₂) is of vital importance for the performance of PFS. Na₂SO₄ is unfavorable for the performance of PFS. Optimal alkalinity (0.01–0.03 M NaHCO₃) is necessary for FeCl₃ to remove TiO₂ NPs. Natural organic matter, as represented by humic acid (HA) up to 11 mg/L, reduces the removal of TiO₂ NPs by coagulation. These findings indicate that coagulation is a good option for the removal of TiO₂ NPs from water, and more attention should be paid to the effects of water quality when using coagulation to remove TiO₂ NPs from aqueous matrices. This provides a possible solution to alleviate the potential hazard caused by TiO₂ NPs.