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Commercial Titanium Dioxide Nanoparticles in Both Natural and Synthetic Water: Comprehensive Multidimensional Testing and Prediction of Aggregation Behavior Stephanie Ottofuelling, Frank Von Der Kammer,* and Thilo Hofmann* Department of Environmental Geosciences, University of Vienna, Althanstrasse 14, Vienna 1090, Austria
bS Supporting Information ABSTRACT: Engineered nanoparticles (ENPs) from industrial applications and consumer products are already being released into the environment. Their distribution within the environment is, among other factors, determined by the dispersion state and aggregation behavior of the nanoparticles and, in turn, directly affects the exposure of aquatic organisms to EPNs. The aggregation behavior (or colloidal stability) of these particles is controlled by the water chemistry and, to a large extent, by the surface chemistry of the particles. This paper presents results from extensive colloidal stability tests on commercially relevant titanium dioxide nanoparticles (Evonik P25) in well-controlled synthetic waters covering a wide range of pH values and water chemistries, and also in standard synthetic (EPA) waters and natural waters. The results demonstrate in detail the dependency of TiO2 aggregation on the ionic strength of the solution, the presence of relevant monovalent and divalent ions, the presence and copresence of natural organic matter (NOM), and of course the pH of the solution. Specific interactions of both NOM and divalent ions with the TiO2 surfaces modify the chemistry of these surfaces resulting in unexpected behavior. Results from matrix testing in well-controlled batch systems allow predictions to be made on the behavior in the broader natural environment. Our study provides the basis for a testing scheme and data treatment technique to extrapolate and eventually predict nanoparticle behavior in a wide variety of natural waters.
’ INTRODUCTION Engineered nanomaterials and nanoparticles (ENPs) are already being emitted and can be found in the natural environment.1,2 The continuous release of ENPs into the environment raises concerns about their distribution, the adverse effects that they may have on either individual organisms or entire ecosystems,3 and the cotransport of classical contaminants4 in surface water and groundwater.5 Some TiO2 ENPs have already been shown to have adverse effects on organisms in aquatic environments.6 8 The most important distribution pathway for TiO2 ENPs into the environment is thought to be through wastewater treatment plants (WWTPs).9 Such emissions will then impact the aquatic environment, except for the fraction that is retained in sludge which, if later used in agriculture, may impact on agricultural soil systems.2,10,11 It is generally accepted that a detailed understanding of the behavior on ENPs in aquatic environments is crucial if a comprehensive assessment is to be made of their final distribution, the most important sinks, and the associated risks.12 We therefore propose that studies on the dispersion stability of ENPs under conditions that mimic real-world aquatic chemistries should be an essential part of their general characterization. The results of such studies would enable the transport and fate of ENPs within natural aquatic systems to be predicted and the results of toxicological tests to be accurately evaluated and compared.12 The possible effects resulting from the behavior of nanoparticles within the environment could thus be identified r 2011 American Chemical Society
and potentially minimized by modifying the ENPs before they are able to cause any harm to humans or to the environment. Recent studies have made use of various analytical techniques and experiments to describe and understand the behavior of ENPs in synthetic growth media,13 natural waters,7 wastewater,14 and well-controlled synthetic waters.15 Results have indicated that the behavior, transport, and fate of nanoparticles in aqueous systems are controlled both by their surface properties and by the chemistry of the aqueous system,8,16 18 but also that the investigated systems are highly complex and not yet understood in detail. Factors such as electrolyte concentration, the valence of ions countering the surface charge, the pH of the system, and the presence of organic matter will cause either aggregation or stabilization of ENPs.16 For example, the presence of Ca2+ could diminish or even cancel out the stabilizing effects of natural organic matter.19 Water constituents such as NOM or polyvalent ions can enhance, decrease, neutralize, or even reverse the surface charge of certain nanoparticles through specific interactions (e.g., adsorption).18 Expanding or compressing the electrostatic double layer by decreasing or increasing the ionic strength (IS) will also alter the stability.20 In addition, specific adsorption of ions (e.g., Ca2+, SO42 ) may induce nanoparticle aggregation;15,21 divalent Received: August 1, 2011 Accepted: October 20, 2011 Revised: October 18, 2011 Published: October 20, 2011 10045
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Environmental Science & Technology ions screen the surface charge more efficiently than monovalent ions,22 which is reflected in much lower critical coagulation concentrations (roughly 100 times lower) for divalent ions than for monovalent ions.17 The surface charge of oxide nanoparticles is controlled by the protonation or deprotonation of surface hydroxyl groups and the specific adsorption of, for example, carbonate, sulfate, Ca2+, or NOM. It is, for example, well-known that the surface charge of TiO2 varies with the pH, and the point of zero net proton charge can range between 4.8 and 5.9.23 Aggregation of particles is promoted at the pznpc24 due to the absence of electrostatic repulsion. To understand the behavior and characteristics of ENPs in aquatic systems researchers have conducted experiments in both complex natural7,14 and synthetic waters.25,26 Although results from natural waters may be more realistic they are often unable to provide information on the processes involved due to the complexity of the water chemistry. Results from synthetic waters, with their reduced complexity, can help in understanding the basic principles but are not necessarily representative of natural systems.27 In addition, quantitative descriptors can be deduced from dynamic studies28,29 that, depending on the experimental approach used, determine doublet formation, particle growth rate constants, or sedimentation rates.14,24 The objective of this study was to overcome the current limitations by investigating the effects that typical surface water constituents have on the stability of TiO2 nanoparticles, and to link these synthetic results to the real-world behavior of the particles. We performed static stability tests and selected NaCl, CaCl2, Na2SO4, and CaSO4 as typical, environmentally relevant, 1:1, 2:1, 1:2 ,and 2:2 electrolytes. The effect of NOM and the effect of CaCl2 in the presence of small but realistic concentrations of NOM were also investigated. Concentrations ranged from 10 μmol L 1 to 500 mmol L 1 and pH values ranged from 4 to 8. We hypothesized that the contour-type stability plots obtained would reveal zones of TiO2 nanoparticle stability and instability, which would then enable the colloidal stability of TiO2 under real conditions to be estimated. This was investigated by first testing TiO2 stability in some typical surface waters, in technical waters/effluents, and in synthetic test media with known hydrochemical compositions. By measuring their water chemistries we were then able to relate the real waters to an appropriate set of synthetic waters. Comparing the results from the real settings with those from the synthetic batch experiments revealed a good general agreement. The results may be used in exposure assessment and modeling.30
’ MATERIALS AND METHODS Nanoparticles. The TiO2 nanoparticles were obtained from a batch of Evonik P25 (produced by Evonik Industries, Germany) for the Organization for Economic Cooperation and Development (OECD) testing program. A detailed description of the TiO2 material is provided in the Supporting Information (Table S1). Stability Tests. We chose 9 different pH values and 7 different electrolyte concentrations per single test matrix. To produce each test matrix we suspended TiO2 in Milli-Q-water, added the required amounts of NaCl, CaCl2, Na2SO4, or NOM, and titrated with HCl or NaHCO3 to the chosen pH values. We followed the titration and sampling protocol described in v.d. Kammer et al.15 For tests with the natural water and the EPA water a stock suspension of 1250 mg L 1 TiO2 was first
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ultrasonicated for 30 min. One mL of this suspension was then added to 49 mL of natural or EPA water to achieve a TiO2 concentration of 25 mg L 1. Natural and Synthetic Water Samples. Seven natural water samples were collected from a variety of sources: groundwater from Hoersching (HOE), lake water from Lunz (LUNZ), tap water from a household tap in Vienna (TAP), water from a peat bog at Tanner Moor (TAN), wastewater inflow and outflow from a wastewater treatment plant (WWTP) in Vienna, (WWI and WWO), all of which are in Austria, and seawater from Normandy, France (FRA). Synthetic test water was prepared following the U.S. Environmental Protection Agency protocol (EPA821-R-02-013: see Supporting Information, Table S2) and is referred to in this paper as “EPA water”. All water samples were filtered through a 0.2-μm cellulose acetate membrane (Whatman, Austria) and stored at 4 °C in the dark. The water chemistry and the procedure for nanoparticle analyses are described in the Supporting Information.
’ RESULTS AND DISCUSSION Behavior and Characteristics of TiO2 in the Test Matrices. In our investigations we stabilized the pH by adding bicarbonate (as NaHCO3) to establish a pH-buffered environment typical of most natural systems The influence that NaHCO3 on its own has on the stability of TiO2 nanoparticles has been previously tested (Figure S2). At a pH either above or below the IEP (pH 5.0) TiO2 particles (residual concentration 13 mg L 1 or 52% of the initial concentration, which was 25 mg L 1 in all experiments) had a diameter of ∼300 nm. At the IEP the particles remaining in the supernatant (maximum 20% of the initial concentration) were large aggregates greater than 2.5 μm in diameter (Figure S3). As can be deduced from the residual TiO2 concentrations in the supernatant, the stability of TiO2 in the presence of sodium chloride is influenced in different ways by the NaCl concentration and the pH (Figure 1a and b). At a NaCl concentration below 5 mM the system is predominantly regulated by the pH and the magnitude of the resulting zeta potential. The TiO2 ENPs are relatively stable, with TiO2 concentrations of more than 20% of the initial concentration at pH values above and below the IEP (Figure 1a). As expected, at the IEP the system is destabilized and particles aggregated and were lost from the supernatant. At NaCl concentrations above about 5 mM the influence of pH is diminished, particles are aggregated to form entities larger than 1.5 μm, and the concentration in the supernatant is reduced (C/C0 10%). The zeta potential plot reveals that the IEP shifts from pH 5 to pH 6.5 with increasing NaCl (from 0.1 to 500 mmol L 1), indicating that the NaCl does not act as a completely indifferent electrolyte but that a weak interaction of the Na+ with the TiO2 surface is shifting the zeta potential to more positive values at elevated Na+ concentrations (Figure 1b). This interaction could be the result of adsorption on the mineral surface or of neutralization of negative patches, which produce elevated positive potentials at intermediate NaCl concentrations (5 100 mM). This is accompanied by an increased TiO2 stability at 10 mM NaCl concentration; it is evident from concentration and zeta potential data that this stability increase originates from Na+ interaction and deviates from the expected plot of a noninteracting electrolyte. The effects of a positively charged divalent cation and a monovalent anion on TiO2 stability were investigated using calcium chloride (Figure 1c and d). In this test matrix we used 10046
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Figure 1. Contour plots of particle stability expressed as the residual concentration in supernatant after a 15 h aggregation and sedimentation period, as a function of pH and electrolyte concentration (upper row). The particle zeta potential as a function of pH and electrolyte concentration is shown in the lower row.
concentrations ranging from 0.01 to 10 mmol L 1 due to an expected lower critical coagulation concentration (CCC) for the divalent cation. Typical calcium concentrations in the natural environment range from 1.0 mg L 1 in rainwater and 40 mg L 1 in surface water, up to 430 mg L 1 in seawater (Table S3). The marked shift of the IEP to a higher pH with increasing CaCl2 concentration indicates a specific interaction (adsorption) of Ca2 + ions to the titania surface. Increasing the CaCl2 concentration resulted in a charge reversal at pH values above 6.5 (Figure 1d). At the lowest CaCl2 concentration the stability follows a trend with pH similar to that followed by NaCl. With greater than 0.1 mM CaCl2 and a pH above 5 the TiO2 particles are generally aggregated (TiO2 particles >1000 nm, Figure S4). However an extended zone of relative stability can be identified at a pH below and above 5 with up to 40% of stable TiO2 in the supernatant (Figure 1c and d). At a pH below 5 and for Ca2+ concentrations ranging from 0.05 to 5 mmol L 1 the residual TiO2 concentrations are greater than 20% (Figure 1c) and the particle diameters are less than 1000 nm (Figure S4). However, at concentrations greater than 5 mM the particles again aggregate (>1000 nm diameter) and there is an accompanying decrease in TiO2 in the supernatant ( 50%) (Figure 2), and particle diameters of about 200 nm (Figure S4). So far we illustrated the effects of weakly and strongly interacting, as well as of surface potential determining, inorganic ions, and the stabilizing effect of even small amounts of NOM. We further aimed to investigate the effect of a combination of both NOM and electrolytes. Because Ca2+ interacts with NOM and TiO2 and acts as a strong coagulant we chose CaCl2 as the additional reagent in multicomponent testing.25,26 We set the background NOM concentration to a realistic surface water concentration of 1 mg L 1 DOC and repeated the CaCl2 matrix with electrolyte concentrations ranging from 0.01 to 10 mmol L 1 over a pH range from 4 to 8 (Figure 2c and d). In this system the relatively low NOM concentration of 1 mg L 1 DOC radically changes the behavior of the TiO2. At CaCl2 concentrations below 1 mmol L 1 elevated concentrations of colloidally stable TiO2 were observed (g50% of the initial concentrations) (Figure 2c). These match the concentrations found in the NOM matrix at 1 mg L 1 DOC (Figure 2a) and are higher than those of any pure 10048
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colloidal stable fraction mg L
The particle diameter (z-average), zeta potential, and concentration, as averages of 3 replicates (n = 3) and (1 SD, were analyzed in the supernatant after 15 h of sedimentation. All 0.2 μm filtered. a
10 ((0.2)
1 ((0.4) 2 ( 0.1) 8 ((0.7) 9 ((1.5) 15 ((2.2) 1 ((0.1) 24 ((1.2) 2 ((0.2) 2 ((0.1)
1520 ((96) nm
mV
particle diameter
zeta potential
1
2 ((0.6)
16 ((0.6)
1480 ((90) 1190 ((165)
20 ((1.6) 14 ((0.8)
1210 ((91.5) 750 ((55.1)
17 ((1.18) +2 ((2.2)
1340 ((92.4) 219 ((10)
26 ((0.2) 9 ((0.3)
1170 ((118) 1180 ((54)
15 ((0.6) 1 ((0.6)
1160 ((154)
VH MH VS WWO WWI FRA TAN TAP LUNZ HOE unit
waste water inflow sea water peat bogwater tap water lake water ground water
Table 1. Aggregation State of TiO2 Nanoparticles in Natural Water and Synthetic EPA Watera
waste water outflow
EPA very soft
EPA moderately hard
EPA very hard
Environmental Science & Technology
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inorganic matrix in this test (Figure 1a to h). Apart from this remarkable effect of 1 mg L 1 DOC at CaCl2 concentrations below 1 mmol L 1, it is the CaCl2 concentration rather than the pH that further controls the stability of TiO2 under these conditions. In contrast to the pure NOM matrix (Figure 2b), it is evident that the magnitude of the negative zeta potential is reduced when there is CaCl2 present (Figure 2d). The corresponding reduction in colloidal stability can be related to the reduction in electrostatic stabilization, or to compression of the NOM molecules due to complexation of the Ca2+ ions by the carboxylic groups of the NOM and associated loss of repulsive negative charges. Here it is not possible to distinguish between the effects of compression of the double layer thickness and the reduction of NOM charge density through complexation of Ca2+ by the NOM carboxyl groups. No indications of steric stabilization by the NOM have been observed. However, at CaCl2 concentrations greater than 1 mmol L 1 a slight decrease in zeta potential is observed with increasing pH, together with an even more marked increase in aggregate size (Figure 2d and Figure S4). This is remarkable since protonation of the NOM, accompanied by reduction in the negative surface potential of the surface adsorbed NOM, had been expected at low pH values. The observed behavior may be caused by the complexation of Ca2+ by carboxyl functional groups within the NOM structure,33 or by the combined effect of a reduction in NOM adsorption to the TiO2 followed by adsorption of Ca2+ to negatively charged surface functional groups. Although the nanoparticles were coated with NOM the aggregation was enhanced in the presence of more than 1 mmol L 1 CaCl2, and only 20% to almost 60% of the initial TiO2 concentration remained colloidally stable (Figure 2c). Above 0.5 mM CaCl2 we observed an increase in particle diameter from 260 to 900 nm for pH less than 6, and strong aggregation occurred where the pH was greater than 6 with particle diameters of 1.7 μm (Figure S4) concomitant with a slightly lower negative zeta potential (Figure 2d). Behavior and Characteristics of TiO2 in Natural and EPA Waters. By testing the behavior of P25 TiO2 nanoparticles under various hydrochemical conditions we have been able to demonstrate the usefulness of this generic approach to understand the basics of the material's behavior in natural waters. To facilitate a comparison between the stability of nanoparticles in natural water and the synthetic results of the matrix testing, a range of different, naturally occurring water types were selected, including marine, fresh, and wastewater. The chemical characteristics of the chosen waters are summarized in the Supporting Information (Table S4). The pH did not vary much among sources except for the slightly acidic peat bogwater, which occurred over granite and had a pH of 5.2. The ionic strength (IS: 0.5 to 146 meq L 1) and the DOC content (
