Removal of TiO2 Nanoparticles During Primary Water Treatment: Role of Coagulant Type, Dose, and Nanoparticle Concentration.

Nanomaterials from consumer products (i.e., paints, sunscreens, toothpastes, and food grade titanium dioxide [TiO2]) have the capacity to end up in groundwater and surface water, which is of concern because the effectiveness of removing them via traditional treatment is uncertain. Although aggregation and transport of nanomaterials have been investigated, studies on their removal from suspension are limited. Hence, this study involves the development of scaled-down jar tests to determine the mechanisms involved in the removal of a model metal oxide nanoparticle (NP), TiO2, in artificial groundwater (AGW), and artificial surface water (ASW) at the primary stages of treatment: coagulation, flocculation, and sedimentation. Total removal was quantified at the end of each treatment stage by spectroscopy. Three different coagulants-iron chloride (FeCl3), iron sulfate (FeSO4), and alum [Al2(SO4)3]-destabilized the TiO2 NPs in both source waters. Overall, greater than one-log removal was seen in groundwater for all coagulants at a constant dose of 50 mg/L and across the range of particle concentrations (10, 25, 50, and 100 mg/L). In surface water, greater than 90% removal was seen with FeSO4 and Al2(SO4)3, but less than 60% when using FeCl3. Additionally, removal was most effective at higher NP concentrations (50 and 100 mg/L) in AGW when compared with ASW. Zeta potential was measured and compared between AGW and ASW with the presence of all three coagulants at the same treatment stage times as in the removal studies. These electrokinetic trends confirm that the greatest total removal of NPs occurred when the magnitude of charge was smallest (<10 mV) and conversely, higher zeta potential values (>35 mV) measured were under conditions with poor removal (<90%). These results are anticipated to be of considerable interest to practitioners for the assessment of traditional treatment processes' capacity to remove nanomaterials prior to subsequent filtration and distribution to domestic water supplies.