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.

Mechanisms of TiO2 nanoparticle transport in porous media: role of solution chemistry, nanoparticle concentration, and flowrate.

The role of solution chemistry, nanoparticle concentration and hydrodynamic effects in the transport and deposition of TiO(2) nanoparticles through porous media has been systematically investigated. Two solution chemistry variables, pH and ionic strength (IS), showed a significant influence on the transport due to their involvement in the aggregation of the nanoparticles and interaction with quartz sand. An electrostatically unfavorable condition for deposition existed at pH 7, at which the greatest retention occurred in the column, likely due to aggregation (>1000 nm) and straining effects. Under electrostatically favorable conditions (pH 5) significant elution from the column was observed and attributed to smaller aggregate size (~300 nm) and blocking effects. Nanoparticle concentration was found to contribute to the increased breakthrough of nanoparticles at pH 5 due to blocking and subsequent particle-particle repulsion. Increased flowrate resulted in greater elution of nanoparticles due to hydrodynamic forces acting on aggregates and subsequently contributed to blocking. Overall, a combination of mechanisms including straining, blocking, and DLVO-type forces were involved over the range of solution chemistry and nanoparticle concentrations tested. Consideration of these mechanisms is necessary for improved removal of TiO(2) nanoparticles via filtration and reliable prediction of transport of these potentially problematic nanoparticles through the subsurface.

Transport of iron-based nanoparticles: role of magnetic properties.

The transport of magnetic nanoparticles in aquatic environments was studied using maghemite (gamma-Fe(2)O(3)) and gamma-Fe(2)O(3) based (Fe(x)Ni(1-x))(y)O(z) nanoparticles as a function of pH and particle iron content that induced a different magnetic property. Transport studies were conducted in packed bed columns (1 mM KCl, pH 6 and 9) and stability studies were done by dynamic light scattering and sedimentation measurements. Results showed that the stability and transport of these magnetic nanoparticles were influenced by a combination of electrostatic and magnetic interactions. Transport results showed that the less magnetic nanoparticles (possessing higher nickel content) eluted to a greater extent than the more magnetic particles at both pH 6 and 9. The stability in water at both pH 6 and 9 also increased, as nickel content in particles increased suggesting that magnetic interactions enhance aggregation. The nanoparticles eluted to a greater extent at pH 9, at which they were more negatively charged, than at pH 6. Complementary experiments were conducted with alpha-Fe(2)O(3), a nonmagnetic, highly negatively charged nanoparticle which was transported more than the other magnetic particles. The majority of particles were retained at the column inlet (1-2 cm) for all transport experiments, with the greatest amount of retention being that of the magnetic nanoparticles (gamma-Fe(2)O(3)), indicating that magnetically induced aggregation and subsequent straining resulted in greater retention.