Effects of pH, ionic strength and humic acid on the removal of TiO2 nanoparticles from aqueous phase by coagulation

The increasing use of nanotechnology will most likely lead to the appearance of nanoparticles in aqueous phase. It is necessary to find effective methods to remove these nanoparticles from water. In this study, we found TiO2 nanoparticles (NPs) could be removed effectively by poly-ferric sulfate (PFS) and ferric chloride (FeCl3) coagulants. The effects of pH, ionic strength and humic acid (HA) on the performance of these two coagulants in this application were investigated. The effective pH range of FeCl3 is very narrow (9.5–10.6). For PFS, its effective pH range is broader (7–10.8). The behaviour of PFS is different from FeCl3 for the removal of TiO2 NPs. Liquid PFS contains many polymerization cations, which have the role of charge neutralization and bridging flocculation in the removal of TiO2 NPs. The main mechanism for removing TiO2 NPs by PFS is the formation of polymerization cations during the hydrolysis process. For both PFS and FeCl3 coagulants, the increase of ionic strength can significantly improve the coagulation effect, although this effect is influenced by pH. For both coagulants, higher HA concentration makes the TiO2 NPs suspension more stable, and the TiO2 NPs removal rate decreased significantly because of the steric hindrance effect of HA molecules.