Article pubs.acs.org/est
Chemical Interactions between Nano-ZnO and Nano-TiO2 in a Natural Aqueous Medium Tiezheng Tong,† Kaiqi Fang,† Sara A. Thomas,† John J. Kelly,‡ Kimberly A. Gray,*,† and Jean-François Gaillard*,† †
Department of Civil and Environmental Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ‡ Department of Biology, Loyola University Chicago, 1032 West Sheridan Road, Chicago, Illinois 60660, United States S Supporting Information *
ABSTRACT: The use of diverse engineered nanomaterials (ENMs) potentially leads to the release of multiple ENMs into the environment. However, previous efforts to understand the behavior and the risks associated with ENMs have focused on only one material at a time. In this study, the chemical interactions between two of the most highly used ENMs, nano-TiO2, and nano-ZnO, were examined in a natural water matrix. The fate of nanoZnO in Lake Michigan water was investigated in the presence of nano-TiO2. Our experiments demonstrate that the combined effects of ZnO dissolution and Zn adsorption onto nano-TiO2 control the concentration of dissolved zinc. X-ray absorption spectroscopy was used to determine the speciation of Zn in the particulate fraction. The spectra show that Zn partitions between nano-ZnO and Zn2+ adsorbed on nano-TiO2. A simple kinetic model is presented to explain the experimental data. It integrates the processes of nanoZnO dissolution with Zn adsorption onto nano-TiO2 and successfully predicts dissolved Zn concentration in solution. Overall, our results suggest that the fate and toxicity potential of soluble ENMs, such as nano-ZnO, are likely to be influenced by the presence of other stable ENMs, such as nano-TiO2.
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INTRODUCTION
Nano-TiO2 and nano-ZnO, two semiconductor metal oxides, are among the most extensively used ENMs in industry. According to studies by Muller et al.13 and Gottschalk et al.,3 the annual production volume and the predicted environmental concentrations of nano-TiO2 and nano-ZnO are much higher than those of other frequently studied ENMs (including nanoAg, carbon nanotubes and fullerenes). Therefore, coupled with their wide and similar applications in personal-care products and industry (e.g., both nanomaterials are used as active components in sunscreens14,15 and effective photocatalysts16), the interactions between these two ENMs are worth investigating. Although these two nanomaterials share similar band gap and band edge energies, 17 their respective mechanisms of toxicity differ. Nano-ZnO is unstable in solution and readily undergoes dissolution. Its toxicity is largely attributed to the release of dissolved ionic zinc,18−21 although phototoxicity of nano-ZnO has also been reported.22,23 Accordingly, the level of dissolved zinc released from nanoZnO into solution is pivotal in evaluating the environmental impacts of nano-ZnO. In contrast, nano-TiO2 is chemically stable and its toxic effects are mainly due to its photo-
The early 21st century is witnessing the transition of nanotechnology from discovery to commercialization. Engineered nanomaterials (ENMs) are incorporated into numerous commercial products ranging from pharmaceuticals and cosmetics to alternative energy and electronic devices, leading to a global market projected to reach $3 trillion in 2020.1 As a result, diverse ENMs (such as nano-TiO2, nano-ZnO, nano-Ag, carbon nanotubes, and fullerenes) will inevitably be released into natural environments,2−6 raising concerns about their fate and potential toxicity toward living organisms. Considerable effort has been exerted to understand the environmental behavior and risks associated with ENMs. Information about the ecotoxicological effects of ENMs has been gathered for almost all the major ENMs and a broad range of taxa (e.g., bacteria, algae, plants, invertebrate, and vertebrate).7−11 However, these previous studies are mostly limited in scope since they evaluate the fate or toxicity of a single nanomaterial at a time. Little attention, then, has been paid to the potential interactions between different ENMs, let alone the effects of these interactions on the ENMs toxicity potential. In addition, while contaminant interactions at ENM surfaces (e.g., Trojan horse effect12) have been studied, the influences of ENM surfaces on the transformation of other coexisting ENMs have yet to be thoroughly explored. © 2014 American Chemical Society
Received: Revised: Accepted: Published: 7924
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toxicity.24−29 In our previous work studying the effects of various nano-TiO2 from different commercial sources and with varied morphologies on bacteria,27−29 we showed that anatase led to greater toxicity under simulated solar irradiation than rutile but negligible toxicity was observed under dark conditions. Furthermore, nano-TiO2 is a commonly used model substrate for adsorption of metal contaminants, including arsenic,30 uranium,31 and zinc.32 In particular, the average local coordination environment of Zn(II) at the water−TiO2 interface has been recently characterized by X-ray absorption spectroscopy (XAS).32 XAS data and ab initio calculations suggest that Zn(II) is coordinated to either 4, 5, or 6 oxygen, while forming bidentate binuclear or bidentate mononuclear surface complexes.32 On the basis of these previous studies, we think that it is important to fully characterize these chemical interactions and propose a quantitative model that can account for the interplay between dissolution of nano-ZnO and Zn scavenging at the surface of nano-TiO2. In the current study, the chemical interactions between nanoTiO2 and nano-ZnO in aqueous solutions were investigated in Lake Michigan water (LMW), a proxy for surface freshwaters. The kinetics of nano-ZnO dissolution were monitored in the presence of nano-TiO2, and the Zn(II) species in the particulate fraction were characterized and quantified by XAS. Finally, a kinetic model was developed to explain the experimental results. It combines the processes of nano-ZnO dissolution with Zn(II) adsorption onto nano-TiO2 to predict the dissolved Zn concentrations observed. To the best of our knowledge, this study represents one of the first investigations on the interactions between different ENMs in a natural aqueous medium, and our results indicate the importance of considering the role of adsorbing surfaces when dealing with the environmental fate of ENMs that undergo significant dissolution.
summarized in Tables S1 and S2, both of which are available in the Supporting Information. All the experiments were conducted in Lake Michigan water (LMW) to assess how these ENMs would behave in a freshwater matrix. LMW was collected from the Evanston drinking water plant and immediately transported to our laboratory where it was stored at 4 °C after filtration through a 0.2 μm pore size PES membrane (NALGENE, catalog no. 5974520). The chemical composition of LMW was characterized and summarized in Supporting Information Table S3. In LMW, the solubility limit for Zn released from ZnO, zincite, is 0.67 mg/L of total [Zn]dis as calculated using PHREEQC33 and the thermodynamic database MINTEQ, version 4. Adsorption Experiments. Adsorption experiments were conducted at room temperature (21 ± 1 °C) by mixing nanoZnO with different concentrations of P25. Sampling was conducted at five time intervals: 10 min, 3 h, 6 h, 1 d, and 2 d after the onset of the experiments. The aqueous phase was separated using high-speed centrifugation at 15 700 g (13 000 rpm) for 30 min, followed by filtration through a 3KDa membrane (Nanosep, Pall Life Sciences). The zinc remaining in the filtrate was analyzed as the dissolved zinc by using flame atomic absorption spectrometry (FAAS, AAnalyst 100, PerkinElmer) with a detection limit of 13 μg/L Zn. Recovery efficiencies for the processing of 0.5 mg/L and 1 mg/L solutions of dissolved Zn yielded 101.0 ± 2.1% and 100.0 ± 0.9%, respectively, showing that no loss of the analyte occurred during the filtration step. Dissolved Zn concentration in LMW was below the detection limit of FAAS, and no significant loss of Zn was observed due to adsorption to experimental container walls. Stock solutions of ZnCl2 were also prepared to conduct zinc adsorption experiments in the presence of P25 suspended in LMW, as a means of comparison. At least three independent experiments were performed to assess the reproducibility of the results. X-ray Absorption Spectroscopy. X-ray absorption spectroscopy (XAS) was performed to determine the speciation of zinc in suspended particles. In detail, 20 mg/L of P25 suspended in LMW was amended with an aliquot of the ZnCl2 stock solution to obtain a total concentration of 1 mg/L Zn. Aliquots of suspended nano-ZnO were also added using the same approach to yield total concentration of 1, 2, and 3 mg/L ZnO. Samples (500 mL) were prepared in acid-washed Teflon bottles to obtain enough material for XAS analysis. After a oneday incubation with constant shaking (100 rpm) at room temperature, the solids remaining suspended in solution were collected on 0.2 μm membranes (polycarbonate, Millipore). The samples were then sandwiched between two strips of Kapton tape (Dupont). When Zn salt is added into LMW, the solubility limit of Zn is controlled by the precipitation of Zn(OH)2 rather than ZnO, since ZnO does not form at low temperature. This results in a total dissolved Zn concentration of 4.12 mg/L, based on PHREEQC33 thermodynamic calculations. Hence, when dissolved zinc was added to yield a total Zn concentration of 1 mg/L, all the Zn remained dissolved in LMW, as verified by conducting dissolved zinc measurements using FAAS. Thus, the zinc collected in the dissolved Zn/P25 mixture represents only zinc bound to P25. XAS spectra of powder standards including nano-ZnO, γZn(OH)2, and ZnCO3 (smithsonite) were used as references to assess the speciation of Zn. The γ-Zn(OH)2 powder was synthesized following the method described by Waychunas et al.34 In addition, 5 mg/L of nano-ZnO in the absence of P25
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MATERIALS AND METHODS Nanomaterial Characterization and Exposure Medium. AEROXIDE P25 (P25) is the representative nanoTiO2 used in this study and was kindly donated by Evonik Industries (Germany). Nano-ZnO was purchased from SigmaAldrich (catalog no. 544906). P25 and nano-ZnO stock solutions (both at 1 g/L) were prepared in Milli-Q water (Millipore, 18 MΩ·cm), and a 30 min-sonication in ultrasonic bath (Health-Sonics, 110W, 42 kHz) was employed before diluting the stock solutions to the desired concentrations for the following experiments. Dissolution of ZnO nanoparticles in the stock solution resulted in an equilibrium concentration of dissolved zinc ([Zn]dis) of 7 mg/L, limited by the Zn solubility in Milli-Q water at equilibrium with atmospheric CO2 (398 ppm). This dissolved Zn contribution in the nano-ZnO stock solution did not affect our experimental conclusions but was included in our theoretical model as described below. The primary sizes and morphologies of both P25 and nano-ZnO were visually observed by transmission electron microscopy (TEM, Hitachi HD-2300), and powder X-ray diffraction (XRD) was used to confirm their chemical composition and to estimate crystallite sizes using the Scherrer eq (Supporting Information Figure S1 and Table S1). XRD was performed on a Rigaku X-ray diffractometer by varying the diffraction angle from 20° to 70° (2θ).27 In addition, hydrodynamic diameters and zeta potentials were measured using a ZetaPALS analyzer (Brookhaven Instruments). The characterization results are 7925
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Figure 1. TEM micrographs of nano-TiO2 P25 (left) and nano-ZnO (right).
Figure 2. [Zn]dis remaining in solution when (A) 0.5 mg/L of dissolved Zn, prepared from a stock solution of ZnCl2, (B) 0.5 mg/L nano-ZnO, and (C) 1 mg/L nano-ZnO was mixed with P25 at different concentrations. Note that 0.5 and 1 mg/L of nano-ZnO represent fully and partially dissolved nano-ZnO in Lake Michigan water after a two-day reaction period, respectively.
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was prepared in LMW to compare the spectra of nano-ZnO as dry powders and in solution. XAS measurements were performed at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) beamline located in Sector 5 of the Advanced Photon Source at Argonne National Laboratory. A Si (111) double crystal monochromator was used to vary the X-ray energy from −200 to +1000 eV of the absorption K edge of Zn (9659 eV). The XAS spectra, with the exception of the powder standards, were collected in the fluorescence mode using a Vortex ME-4 silicon drift detector. Spectra of the powder standards were measured in the transmission mode using Oxford ionization chambers with 296 mm path lengths. For energy calibration, the X-ray absorption spectrum of a Zn reference foil was measured with each sample in either transmission or fluorescence mode. At least four scans were performed to obtain good counting statistics. Data processing and analysis of the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were performed with the program Athena (version 0.9.17).35 Spectra decompositions using linear combination fits of the references were performed on the XANES (fit range from −20 to +100 eV above absorption edge) and EXAFS (fit range from 1 to 10 Å−1) parts of the data to determine the percentages of zinc remaining as nano-ZnO and Zn adsorbed onto P25.
RESULTS AND DISCUSSION
Physicochemical Characterization of Nanomaterials. Transmission electron microscopy (TEM) was used to define the primary sizes and the morphologies of P25 and ZnO nanoparticles (Figure 1). P25 is a spherical nanomaterial with diameters of 15−25 nm (20.4 nm on average), which is in agreement with literature reports.36 On the other hand, nanoZnO exhibits contrasting morphologies, with sphere-shape (63.6 nm on average) and rod-shape (156.6 nm × 47.1 nm on average) nanoparticles coexisting together. Dynamic light scattering (DLS) results (Supporting Information Table S2) reveal that both P25 and ZnO nanoparticles form aggregates well beyond their original nanoscale in LMW. The aggregate sizes of P25, assessed as the hydrodynamic diameters, increase from 484 ± 31 to 1641 ± 192 nm, when the concentration increases from 10 to 100 mg/L, while those of nano-ZnO grow from 680 ± 70 nm at 1 mg/L to 1103 ± 40 nm at 50 mg/L. Zeta potential measurements indicate that both nanoparticles are negatively charged in LMW (Supporting Information Table S2), demonstrating the presence of electrostatic repulsion between P25 and nano-ZnO particles. However, this repulsion is likely surpassed by an increased frequency of collision in P25/nano-ZnO mixtures, as evidenced by the larger aggregate sizes for nano-ZnO/P25 mixtures than those of the individual nanoparticles alone (Supporting Information Table S2). 7926
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Effects of Nano-TiO2 on Nano-ZnO Dissolution. The effects of nano-TiO2 P25 on nano-ZnO dissolution were examined by preparing suspensions of 0.5 and 1 mg/L of nanoZnO in the presence of different P25 concentrations, up to 100 mg/L of TiO2. In parallel, solutions of 0.5 mg/L Zn2+ were prepared from the ZnCl2 stock solution to study the adsorption of dissolved Zn on P25. Figure 2A shows that dissolved Zn decreased continuously with increasing concentration of P25 ([P25]), which is in agreement with our established knowledge about nano-TiO2 as an effective metal adsorbent. Zinc adsorption onto P25 was fast and reached equilibrium within minutes. Accordingly, we only monitored dissolved Zn adsorption for up to 6 h of reaction time. Because of the dissolution of the ZnO nanoparticles, the [Zn]dis increased with time and became relatively stable after 1 day (Figure 2B), and thus the experiments were extended to 2 days. It is important to note that after a two-day reaction time in LMW, solutions containing 0.5 and 1 mg/L of nano-ZnO represent fully and partially dissolved cases, respectively. This was verified by conducting dissolved zinc measurements. For 0.5 mg/L nano-ZnO mixed with P25 (Figure 2B), we observed the same trend as in the case of dissolved Zn prepared from ZnCl2, in which [Zn]dis decreased with increasing [P25]. When the concentration of nano-ZnO was increased to 1 mg/L, however, the variation of [Zn]dis with respect to the concentration of P25 presented a distinct feature after 3 h of reaction time (Figure 2C). Rather than a steady decrease of [Zn]dis with increasing [P25], a set of plateaux was observed during which [Zn]dis was maintained at about the same level until [P25] exceeded 10 mg/L. The pH of the solutions of nano-ZnO/P25 mixtures remained at 8.4 ± 0.1 as the concentration of P25 increased to 50 mg/L (Supporting Information Figure S2), which excludes the contribution of pH in this phenomenon. In addition, similar results were observed for a broader range of nano-ZnO concentrations (e.g., 0.25−3 mg/L, Figure S3 in the Supporting Information). The presence of Zn in solution is controlled by two processes: (1) the dissolution of nano-ZnO and (2) the adsorption of Zn at the surface of nano-TiO2. This is described by the following reactions: ZnO(s) + 2H+(aq) ⇌ Zn 2 +(aq) + H 2O(l)
Figure 3. [Zn]dis remaining in solution when 1 mg/L of nano-ZnO was aged for 1 day prior to the addition of P25.
subsequently shifted upward and became more similar to what was observed in Figure 2C, demonstrating the further dissolution of remaining nano-ZnO induced by Zn adsorption to P25. Zn Partitioning. XAS was performed to characterize the speciation of zinc in the particulate fraction of the nano-ZnO/ P25 mixtures.37 The Zn K-edge XANES (from −20 to +100 eV of the absorption edge) and the EXAFS data (k range from 0 to 14 Å−1) of the nano-ZnO/P25, as well as dissolved Zn/P25 mixtures with reference compounds used for spectral decomposition,38 are shown in Figure 4A and B. When 1 mg/L nano-ZnO was mixed with 20 mg/L P25 in LMW, the XANES and the EXAFS spectra were nearly identical to those of the dissolved Zn/P25 mixture. We can conclude in this case that the Zn present in the particulate phase is primarily adsorbed to the surface of P25. When the nano-ZnO concentration increased to 2 and 3 mg/L, however, the spectra become similar to those of nano-ZnO. It should be noted that the spectrum of nano-ZnO equilibrated with LMW was identical to that of the nano-ZnO powder standard. Therefore, we did not detect the presence of other Zn phases, such as Zn(OH)2 or ZnCO3, in our experiments. Spectral decomposition by linear combination fitting (LCF) of both the XANES and the EXAFS was performed to determine the percentages of Zn species present in the nano-ZnO/P25 samples. The data and the results of the curve fitting with the residuals of the EXAFS LCF are presented in Figure 4C, while the LCF of the XANES is included in the Supporting Information (Figure S4). Only two reference spectra were needed to perform the analysis: the spectrum of nano-ZnO in LMW and the spectrum corresponding to Zn adsorbed at the surface of P25 (the addition of a third zinc species did not improve the fit). For 1 mg/L nano-ZnO reacting with 20 mg/L P25, the best EXAFS fit was obtained considering 95.5% of the sample as zinc adsorbed to P25 and only 4.5% of zinc as nanoZnO. Since only ∼60% of 1 mg/L nano-ZnO dissolves in LMW after 2 days without P25 (Figure 2C), this result is consistent with our hypothesis that the remaining nano-ZnO was further dissolved in the presence of P25. When 2 and 3 mg/L of nano-ZnO interacted with the same dose of P25, nano-ZnO contributes to 54.6% and 57.0% of the total zinc in the particulate fraction, respectively. The percentages of the particulate Zn species obtained from the LCF of the XANES
(1)
Zn 2 +(aq) + ≡ TiOH(s) ⇌ ≡ TiOZn+(s) + H+(aq) (2)
The presence of dissolved Zn plateaux during the interactions between ZnO and TiO2−P25 nanoparticles in solution results from similar and relatively fast kinetic rates for ZnO dissolution and Zn2+ adsorption. Reaction 2 decreases the dissolved Zn2+ concentration and shifts reaction 1 to the right. The remaining nano-ZnO behaves as a Zn2+ reservoir whose further dissolution compensates the loss of [Zn]dis because of its adsorption on nano-TiO2. When the number of surface sites available for Zn sorption becomes large compared to the overall pool of Zn available, that is, the concentration of P25 is ≥20 mg/L in our experiments, a decrease of [Zn]dis becomes visible. To further test our explanation above, a set of 1 mg/L nanoZnO solution was aged for 1 day prior to the addition of P25. Thus, about 0.5 mg/L of dissolved zinc had been already released into LMW at the onset of the adsorption experiments (Figure 3). In this case, the [Zn]dis decreased readily and steadily with increasing [P25] within a short time (i.e., 10 min) after the addition of P25. However, the [Zn]dis curve 7927
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Figure 4. Zn K-edge XANES (A) and EXAFS (B) spectra of the nano-ZnO/P25 and dissolved Zn/P25 mixtures, as well as the powder standards for qualitative comparison; (C) EXAFS spectra (solid lines) and linear combination fits (open dots) of the nano-ZnO/P25 mixtures with different nanoZnO concentrations. The P25 concentration is 20 mg/L. The residues of the fittings are shown below the spectral decompositions of the corresponding fittings.
spectra are identical to those presented above (Supporting Information Figure S4). Therefore, the XAS results provide quantitative evidence, at the molecular level, that Zn(II) species coexist as nano-ZnO and adsorbed Zn onto P25 in a system where partially dissolved nano-ZnO interacts with nano-TiO2. Modeling Nano-ZnO and Nano-TiO2 Interactions. To explain the results obtained from the nano-ZnO/nano-TiO2 interaction experiments, we developed a simple kinetic model that integrates the processes of nano-ZnO dissolution and adsorption of the released zinc onto nano-TiO2. The dissolution of nano-ZnO is fast and diffusion controlled as shown by David et al.,39 so it can be expressed using the Noyes-Whitney equation40 as d[ZnO] A = −D ([Zn]eq − [Zn]dis ) dt d
where [Zn]total is the total amount of zinc added, [Zn]ad is the amount of zinc adsorbed on nano-TiO2, [TiO2] is the concentration of nano-TiO2, and qe is the amount of zinc adsorbed on the nano-TiO2 per unit weight. Since zinc adsorption onto nano-TiO2 reaches equilibrium very rapidly (Figure 2A), the parameter qe is obtained from an isothermtype experiment in LMW and expressed as a function of [Zn]dis using the nonlinear Langmuir model (i.e., f([Zn]dis), see details in the Supporting Information). qe =
(5)
For given concentrations of [Zn]total and [TiO2], we can rewrite eq 4 as
(3)
[ZnO] = [Zn]total − [Zn]dis − [TiO2 ]f ([Zn]dis ) = g ([Zn]dis )
where [ZnO] is the concentration of nano-ZnO, [Zn]dis is the concentration of dissolved zinc, [Zn]eq is the [Zn]dis at equilibrium with nano-ZnO, A is the surface area of nanoZnO, D is the diffusion coefficient of Zn2+, and d is the diffusion layer thickness.40,41 The mass balance of zinc added into the system can be expressed as
(6)
which indicates that [ZnO] is also a function of [Zn]dis (i.e., g([Zn]dis)). When combined with eq 3, the differential of eq 6 leads to d[Zn]dis d[ZnO] d[ZnO] d[Zn]dis = = g ′([Zn]dis ) dt d[Zn]dis dt dt A = −D ([Zn]eq − [Zn]dis ) d
[Zn]total = [ZnO] + [Zn]dis + [Zn]ad = [ZnO] + [Zn]dis + [TiO2 ]qe
[Zn]dis = f ([Zn]dis ) 12.7 + 14.6[Zn]dis
(4)
(7)
and therefore 7928
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Figure 5. Experimental and modeling results of [Zn]dis from 0.5 (A) and 1 mg/L (B) of nano-ZnO mixed with different concentrations of P25. The modeling results of 1 (1 d) and 2 days (2 d) overlap each other and are thus not distinguishable.
d[Zn]dis A 1 = −D ([Zn]eq − [Zn]dis ) dt d g ′([Zn]dis )
d[Zn]dis = −K[g ([Zn]dis )]1/3 ([Zn]eq − [Zn]dis ) dt 1 = h([Zn]dis ) g ′([Zn]dis )
(8)
Although the TEM observations show that the ZnO nanoparticles are present under various morphologies (Figure 1), we used a spherical approximation of the nano-ZnO aggregates with the same initial radius r0 and surface area A0. In addition, we assume that nano-ZnO aggregates undergo dissolution independently, and thus the number of ZnO aggregates (N) remains constant. Similar assumptions were made in the recent study by David et al.39 After dissolution, r0 and A0 become r and A, respectively. Accordingly, we have the relationships ⎛ [ZnO] ⎞1/3 Nρπr 3 [ZnO] r3 r = = 3 ⇒ =⎜ ⎟ [ZnO]0 r0 Nρπr0 3 r0 ⎝ [ZnO]0 ⎠ ⎛ [ZnO] ⎞2/3 A 4πr 2 r2 = = = ⎜ ⎟ A0 4πr02 r02 ⎝ [ZnO]0 ⎠
Equation 14 is a differential equation that was solved numerically using Mathematica 9 (Wolfram Research). The constant K was determined by least-squares fitting of the experimental data corresponding to the dissolution of nanoZnO without any addition of P25. The original [Zn]dis (at t = 0) is equal to the dissolved zinc concentration coming from the nano-ZnO stock solution (i.e., a contribution of 3.5 μg/L for the 0.5 mg/L nano-ZnO case and 7 μg/L for the 1 mg/L nanoZnO case). A series of simulations were performed to calculate the concentrations of dissolved Zn during the dissolution/ adsorption experiments and the results are shown in Figures 5 and S6 (Supporting Information). The experimental and simulated results for t = 10 min are not included in Figure 5 since the separation time of the particles from the solution, including centrifugation followed by filtration, takes about 40 min, and therefore exceeds the reaction time. As shown in Figure 5, our model successfully reproduces the trend observed in the measured [Zn]dis curves for both fully (0.5 mg/L) and partially dissolved (1 mg/L) nano-ZnO mixed with P25. In particular, it explains the presence of the [Zn]dis plateaux as shown in Figure 2C. We notice that the simulated [Zn]dis curves for 1 mg/L nano-ZnO mixed with P25 (Figure 5B) reaches the plateaux later than what was observed in Figure 2C. It might be due to a decrease in the adsorption capacity of P25 (per unit weight) because of P25/nano-ZnO coaggregation, which is not considered herein. If one plots all the experimental data points (56 in total) against the simulation results a slope of 1.006 ± 0.015 is obtained by linear regression (Figure 6). Hence, this simple model provides a reasonable description of the chemical processes occurring during the interactions of these two ENMs in solution. Overall, our study shows that [Zn]dis released from nano-ZnO is synergistically controlled by both the dissolution of nano-ZnO and the adsorption of Zn onto nano-TiO2. Our modeling approach can be extended to predict the fate of ENMs undergoing dissolution and subsequent metal adsorption onto either other stable ENMs or environmental particles. To do so, the metal adsorption capability of those adsorbing surfaces (described as the parameter qe in the current model) needs to be determined.
(9)
(10)
where ρ is the density of ZnO (5.606 g/cm ) and [ZnO]0 is the initial concentration of nano-ZnO that is equal to [Zn]total. Furthermore, the change in the diffusion layer thickness (d) of nano-ZnO during the dissolution process is incorporated into our model as proportional to the size of nano-ZnO aggregate,42 3
⎛ [ZnO] ⎞1/3 d = αr = αr0⎜ ⎟ ⎝ [ZnO]0 ⎠
(11)
where α is a constant. From eq 8−11, we can write DA 0 d[Zn]dis =− [ZnO]1/3 ([Zn]eq − [Zn]dis ) 1/3 dt αr0[ZnO]0 1 g ′([Zn]dis ) (12)
We also define another constant K=−
DA 0 αr0[ZnO]01/3
(14)
(13)
Combined with eq 6, eq 12 is converted to 7929
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ZnO. This possibility is evidenced by Yang et al., who observed the diminished toxicity of Cd2+ to green alga Chlamydomonas reinhardtii in the presence of nano-TiO2.48 In contrast, when nano-ZnO is only partially dissolved, the bioavailability of zinc ions in solution may remain unchanged due to the continued dissolution of nano-ZnO. A “Trojan horse effect” may occur if the metal-carrying nanoparticles are able to cross biological membranes and be internalized into living organisms. For example, it has been found that nano-TiO2 increases the toxicity of Cu2+49 and facilitates the biouptake and retention of Cd and Zn in Daphnia magna,50 as a result of the metal ion scavenging by and ingestion of nano-TiO2 by Daphnia magna. Overall, when multiple nanomaterials are present in aqueous solutions, the environmental behavior and risks of a soluble ENM, such as nano-ZnO, are affected by the presence of other stable ENMs, such as nano-TiO2. These effects are currently ignored since most toxicity studies focus on the behavior of a single ENM.
Figure 6. Comparison of the experimental and modeling results for [Zn]dis when nano-ZnO and P25 interact in solution. The red solid line, green curves, and blue curves represent the best linear fit, the 95% confidence interval, and the 95% prediction interval of the fit, respectively.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Additional tables, figures, and text showing characteristics of nano-TiO2 and -ZnO, hydrodynamic diameters and ζ potentials, characteristics of LMW, XRD spectra, effects of P25 on the pH of Lake Michigan Water and on [Zn]dis, XANES spectra with corresponding LCF, determination of qe, and simulation results of modeling. This material is available free of charge via the Internet at http://pubs.acs.org.
Environmental Implications. A recent review article by Lowry et al.7 emphasizes that nanomaterials are subjected to various physical, chemical, and biological transformations in natural environments, which determine their fate, transport, and corresponding toxicity potential. As a variety of ENMs are commonly used in industry and consumer products, they will inevitably be discharged into the environment, creating mixtures of ENMs. In the current study, we focused on the interactions between nano-ZnO and nano-TiO2, two extensively used ENMs that are the most likely to be released into aquatic environments. We demonstrate that the presence of nano-TiO2 influences the concentration of dissolved zinc released from nano-ZnO in solution, which may consequently alter the exposure and bioavailability of nano-ZnO to aquatic organisms. Previous studies have shown that inorganic (e.g., sulfide43 and phosphate44) and organic (e.g., citric acid45) ligands participate in the environmental transformation of nano-ZnO by forming Zn−ligand complexes or precipitates. Our results, on the other hand, indicate that other stable ENMs or adsorbing surfaces, such as nano-TiO2, also contribute to determining the fate of nano-ZnO in natural waters. When the concentration of nano-ZnO is below its solubility limit, nanoZnO will completely dissolve within a few days. The zinc released will form surface complexes with either nano-TiO2 or other environmental particles, decreasing the level of dissolved zinc in water. When the concentration of nano-ZnO exceeds its solubility limit (which represents either a future scenario or a spill event), nano-ZnO particles/aggregates can exist; however the presence of other stable ENMs in solution and surface sites on background particles will enhance dissolution. Since dissolved zinc ions released from nano-ZnO are considered to be the primary cause for nano-ZnO toxicity,18−21 our results indicate that interactions between nano-ZnO and nano-TiO2 may play a mediating role in ENM toxicity. According to chemical equilibrium-based models such as the free-ion activity model (FIAM)46 and the biotic ligand model (BLM),47 the formation of Zn(II)-TiO2 surface complexes will decrease the level of the free Zn ion in solution, consequently reducing the bioavailability and toxicity attributed to nano-
Corresponding Authors
*E-mail:
[email protected]. Phone: (847)-467-4252. Fax: (847)-491-4011. *E-mail:
[email protected]. Phone: (847)-467-1376. Fax: (847)-491-4011. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Science Foundation (Grant No. CBET-1067751 to K.A.G. and J.-F.G. and Grant No. CBET-1067439 to J.J.K.) and the Institute for Sustainability and Energy (ISEN) at Northwestern University (a graduate research grant to K.F. and T.T.). We thank Dr. Qing Ma and Dr. Jinsong Wu for their technical assistance, and Prof. Teri W. Odom for the use of ZetaPALS analyzer. Portions of this work were performed at the DND-CAT Synchrotron Research Center located at Sector 5 of the APS. DND-CAT is supported by the E.I. DuPont de Nemours & Co., The Dow Chemical Company, the U.S. National Science Foundation through Grant DMR-9304725, and the State of Illinois through the Department of Commerce and the Board of Higher Education Grant IBHE HECA NWU 96. XRD and TEM were performed in the J. B. Cohen X-ray diffraction facility and NUANCE center at Northwestern University, respectively.
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