Abstract
Removal of TiO2 nanoparticles (NPs) from water by coagulation and microfiltration (MF) was investigated at various ionic species and strengths. Appropriate coagulant doses were determined based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, and coagulation with ferric chloride (FeCl3) at 0.1–0.2 mM or poly-aluminum chloride (PACl; [Al2(OH)nCl6-n]m) at 0.2–0.4 mM improved removal of TiO2 NPs to more than 90%, and, simultaneously, turbidity was significantly reduced. Using 0.2 mM FeCl3 as a coagulant, the mean particle size of suspended TiO2 NPs decreased from 145 to 43 nm after coagulation and sedimentation as a result of effective floc formation. Microfiltration (nominal pore size: 0.45 μm and 0.10 μm) of uncoagulated TiO2 NP suspensions produced lower NP concentrations in the filtrate than that in the filtrate of pre-coagulated water. This is because pre-coagulation and settling removed large NPs in the feed water that reduce the number of smaller particles than the pore sizes by clogging or blocking membrane pores. Results suggest that removal of NPs by coagulation/sedimentation followed by membrane filtration may be less efficient than direct membrane filtration. Addition of salts (NaCl 0.3–15 mM or CaCl2 0.1–5 mM) to TiO2 NP suspensions before coagulation led to significant reduction (0.28–0.47 mg/L) of TiO2 concentrations in supernatant after sedimentation. Phosphate addition (0.01, 0.1, and 1.0 mM) increased TiO2 NP concentration after FeCl3 or PACl coagulation and settling, but TiO2 NP removal rate by MF was improved by phosphate addition except for filtering TiO2 NP suspension with 1.0 mM phosphate by a 0.45 μm membrane.
