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

摘要

To investigate the possibility of removing titanium dioxide nanoparticles (TiO_2 NPs) from water by coagulation, as well as to find the optimal coagulant and experimental conditions for TiO_2 NP removal, four types of coagulant were adopted: polyferric sulfate (PFS), ferric chloride (FeCl _3), polyaluminum chloride (PACl), and alum (Al_2(SO _4)_3). It was found that the removal of TiO_2 NPs by coagulation was affected by ionic strength, alkalinity, as well as types and dosages of coagulants. PFS and FeCl_3 achieved much higher removal efficiency of TiO_2 NPs than PACl and Al_2(SO _4)_3 did. For 30 mg/L TiO_2 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 FeCl_3 to remove TiO_2 NPs. Natural organic matter, as represented by humic acid (HA) up to 11 mg/L, reduces the removal of TiO_2 NPs by coagulation. These findings indicate that coagulation is a good option for the removal of TiO_2 NPs from water, and more attention should be paid to the effects of water quality when using coagulation to remove TiO_2 NPs from aqueous matrices. This provides a possible solution to alleviate the potential hazard caused by TiO_2 NPs.