Environ. Sci. Technol. 2010, 44, 3332–3338
Aggregation and Charge Behavior of Metallic and Nonmetallic Nanoparticles in the Presence of Competing Similarly-Charged Inorganic Ions B I P L A B M U K H E R J E E * ,† A N D JAMES W. WEAVER‡ National Research Council Research Associate, and National Exposure Research Laboratory, U.S. Environmental Protection Agency, Athens Georgia 30605
Received November 13, 2009. Revised manuscript received March 19, 2010. Accepted March 22, 2010.
The influence of competing, similarly charged, inorganic ions on the size and charge behavior of suspended titanium-dioxide (nTiO2), silver (nAg) and fullerene (nC60) nanoparticles (NPs) was investigated. Under pH and ionic conditions similar to natural water bodies, Ca2+ induced aggregation of nTiO2 and nAg NPs more strongly than K+ and Na+. Although K+ and Na+ had a similar effect on aggregation, K+ provided better screening of the particle surface charge presumably because of its small hydrated radius. These effects were decidedly more prominent for TiO2 than Ag. Anions (co-ions), SO42- and Cl-, affected the surface charge behavior of nTiO2 but not of nAg NPs. The zeta potential (ZP) of nTiO2 NPs was more negative at higherSO42-/Cl- ratiosthanlower.WhenMg2+ wasthecounterion, charge inversion and rapid aggregation of nC60 NPs occurred under alkaline conditions, with a more pronounced effect for Cl- than SO42-. Response dissimilarities suggest fundamental differences in the interfacial-interaction characteristics of these NPs in the aquatic environment with corresponding differences in transport of these particles. Our study also shows the important role played by the iso-electric point pH (pHiep) of the NPs in determining their aggregation kinetics in the environment.
Introduction The use of engineered NPs in commercial applications has increased dramatically over the past few years (1), thus increasing the likelihood of release of these particles into the environment (2). Studies indicate that nanoscale materials can have potential toxic (3), mutagenic (4), and antimicrobial (2) properties, leading to growing concern about the possible adverse effects of these particles on the ecosystem and public health (5). One of the pathways of human exposure to NPs is via drinking water (5) derived from ground or surface water (6). Therefore, understanding the behavior of NPs in the aquatic environment is critical for designing effective drinking-water treatment facilities, and for developing strategies to minimize adverse impacts from NP exposure. * Corresponding author phone: 706-355-8135; fax: 706-355-8160; e-mail:
[email protected]. † National Research Council Research Associate. ‡ National Exposure Research Laboratory. 3332
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Owing to their size, individual NPs can be transported over large distances (7). But, because of their high surface area, individual NPs form aggregates to reduce their free interfacial energy (3). Transport of aggregates can differ extremely from the primary constituent particles (1). Inorganic ions, which form an inherent part of natural water bodies, play a significant role in determining the aggregation and electro-kinetic behavior of NPs. Ions charged oppositely to the surface charge of NPs (counterions) can strongly attenuate the effect of electrostatic double layer (EDL) repulsion by neutralizing the particle surface charge. This results in enhanced particle-particle aggregation (8). Neutralization of surface charge is directly reflected in the electrophoretic mobility (UE) and the zeta potential (ZP) of the NPs. With an increase in the counterion concentration, the absolute values of these parameters (|UE|and|ZP|) approach zero indicating a favorable aggregation condition. Ions charged similarly to the surface charge of NPs (co-ions) can also play important role in affecting the charge (9) and hence on the aggregation behavior of NPs. Co-ions can impart surface charges by preferential adsorption (10). This reason has been debated as one possible explanation for the acquisition of negative surface charge by fullerene nC60 NPs in aqueous media (8, 11, 12). An increase of ionic strength (IS), a measure of the combined effect of counter- and coions, induces interparticle aggregation by compressing EDL thickness around NPs (13). Variation in environmental pH also affects electro-kinetic properties of NPs and hence plays a significant role in determining their behavior. The probability of aggregation gets higher as the pH approaches the iso-electric point pH (pHiep) of the NPs (13). At pHiep, which depends on the particle type, the energy barrier preventing aggregation disappears and is marked by |ZP| approaching zero. Conversely, the further away from pHiep, the greater the stability of the suspended NPs. In the majority of previous, impacts of inorganic ion on NPs, investigations, one electrolyte was used; so that only one type of cation and anion was present. While these studies provide useful information, a more comprehensive approach that was used in this paper is to assess the behavior of NPs in the presence of multiple cations and anions (closer to a realistic environmental scenario). Titanium dioxide (nTiO2), silver (nAg), and fullerene (nC60) NPs were selected for our study. These are the most commonly used NPs in commercial products and they have been prioritized for research as representative manufactured nanomaterials by the Organization for Economic Cooperation and Development (www. olis.oecd.org). There were two related objectives of this research. The first was to investigate the effects of competing similarly charged inorganic ions at typical environmental conditions on the aggregate formation and ZP of suspended metal (Ag) and metal oxide (TiO2) NPs. Experiments were performed with constant concentrations of a single anion paired with differing proportions of two cations. A second set of experiments reversed the roles of the cations and anions. The second objective was to examine the effects of competing anions on the behavior of nC60 NPs as a function of pH.
Material and Methods NP Suspension. A stock suspension of nTiO2 (rutile) NP was prepared by adding 25 mg of TiO2 nanopowder as received from the manufacturer (rutile, 98+%, 10 × 40 nm; needle shaped, specific surface area: 160 ( 30 m2/g; Nano-Structured & Amorphous Material, Inc.), to 50 mL double deionized (DDI) water. This was followed by 10 min of bath-sonication 10.1021/es903456e
2010 American Chemical Society
Published on Web 04/06/2010
TABLE 1. Composition of Electrolyte Solutions Used to Investigate the Effects of Cations and Anions on Aggregation and Charge Properties of NPsa treatments -
CaCl2 and NaCl type Ib
Cl Ca2+ Na+ Ca2+/Na+
IS κ-1
KCl and NaCl type II
b
mM nm ClK+ Na+ K+/Na+
IS κ-1
MgSO4 and MgCl2 type III
c
IS κ-1
mM
mM
mM nm Mg2+ SO42ClSO42-/Cl-
mM
mM nm
1
2
3
4
5
10.0 5.0 0.0 ∞
10.0 4.7 0.6 8.0
10.0 3.3 3.3 1.0
10.0 1.0 8.0 0.1
10.0 0.0 10.0 0
15.0 2.51
14.7 2.53
13.3 2.66
11.0 2.93
10.0 3.07
10.0 10.0 0.0 ∞
10.0 8.9 1.1 8.0
10.0 5.0 5.0 1.0
10.0 1.1 8.9 0.1
10.0 0.0 10.0 0
10.0 3.07
10.0 3.07
10.0 3.07
10.0 3.07
10.0 3.07
10.0 10.0 0.0 ∞
10.0 9.4 1.2 8.0
10.0 6.7 6.7 1.0
10.0 2.0 16.0 0.1
10.0 0.0 20.0 0
40.0 1.54
39.4 1.55
36.7 1.6
32.0 1.72
30.0 1.77
a IS is the ionic strength and κ-1 is the Debye length. b pH of the suspensions containing nTiO2 and nAg NPs was 6.08 ( 0.2 and 6.13 ( 0.1, respectively. c pH of the suspensions containing nTiO2 and nAg NPs was 6.7 ( 0.03 and 6.44 ( 0.05, respectively.
using 750 W-20 kHz Ultrasonic Processor (model GEX 750). A working standard of strength ∼2 µg/mL was then prepared by diluting a requisite volume of the stock suspension. Similarly, a working standard was prepared for nAg NPs using Ag nanopowder ( 0.5). Changes in D to IS changes of the solutions due to the variation in Ca2+/ Na+. With an increase in Ca2+/Na+, IS also increased causing increased EDL compression (Table 1) and enhanced aggregation. j H and ZP of nTiO2 and nAg Effects of KCl and NaCl on D NPs are shown in Figure 2. Because Cl- concentrations and IS in these treatments were the same (Table 1, type II) the differences in effects were induced by differences in K+/Na+. j PN (P < j N and Z While the NP type significantly affected D 0.01), the effects of the electrolyte composition and the NPelectrolyte interaction were not significant (P > 0.3). Though j Nbetween treatments containing different the differences in D NP type were statistically significant (P < 0.05), they were not j PN. However, |ZP| for nTiO2 were consistently lower than for Z nAg NPs and favored the formation of larger aggregates under j N or the same ionic conditions. No differences either for D j PN were found among treatments containing the same NP Z j Hof type (P > 0.05). Although not statistically significant, D + + j nTiO2 NPs were higher at higher K /Na ; DH for treatments containing only K+ (10 mM) was 260 nm and only Na+ (10 mM) was 200 nm. Such an effect might be due to the better screening of the surface charge by K+ than Na+. Owing to its small hydrated radius (Rh), K+ (Rh ≈ 0.33 nm) is able to approach closer to the NP surface and neutralize the surface
FIGURE 3. Effect of variation in SO42-/Cl-ratios, near neutral pH, j H and (B) zeta potential, ZP, on the (A) average aggregate size, D of nTiO2 and nAg NPs. The Mg2+ concentration in all treatments was kept constant at 10 mM. The errors bars represent one standard deviation of three independent replicates. In a panel, bars labeled with at least one letter in common are not significantly different at the 95% confidence level according to j H and ZP, in DI Tukey’s mean comparison test. Values of D water, are represented by (s) for nTiO2 and (- -) for nAg NPs. charge more effectively than Na+ (Rh ≈ 0.36 nm) (11). The corresponding ZP, under these two conditions, was -27 mV (K+ only) and -35 mV (Na+ only). In addition to the DLS measurements, transmission electron microscope (TEM) analysis (SI) indicated that nTiO2 compared to nAg NPs formed larger aggregates under the same electrolyte condition. In the presence of 10 mM CaCl2, the average aggregate size was approximately 1000 and 200 nm for nTiO2 and nAg NPs, respectively (SI Figure S1). Aggregation between particles is enhanced as the surrounding pH (pHsur) approaches pHiep (13). In our experiments, pHsur drifted toward pHiep for both nTiO2 and nAg NP suspensions upon addition of electrolytes. However, pHsur was closer to pHiep for treatments containing nTiO2 (pHsur 6.1, pHiep 5.2 (14)) than for nAg NPs (pHsur 6.2, pHiep 2.0 (15)). Preferential adsorption of cations on nTiO2, but not on nAg, might also contribute toward the formation of larger nTiO2 aggregates. Several alkali earth ions have shown adsorption propensity on nTiO2 surfaces in the order: Mg2+ < Ca2+ < Sr2+ < Ba2+ (16). The primary shapes of TiO2 NPs were needle-like and Ag NPs were spherical. The corresponding sizes of the NPs were smaller for TiO2 than for Ag. Therefore, for a fixed mass, the exposed surface area and the particle concentration were much greater for TiO2 than for Ag NPs. No information on the use of surface agents on TiO2 NP surfaces was given by the manufacture, in contrast to Ag NP surfaces which were modified. All these above reasons might help explain the observed differences in the size and the charge behavior of the two NP types. j H and Effects of Anions. Effects of MgSO4 and MgCl2 on D ZP of nTiO2 and nAg NPs are shown in Figure 3. The pH for treatments containing nTiO2 and nAg NPs was approximately 6.7 and 6.5, respectively. Because Mg2+ concentrations in these treatments were the same (Table 1, type III) the differences in effects were induced by differences in SO42-/
FIGURE 4. Effect of variation in SO42-/Cl- ratios on the (A) j H and (B) zeta potential, ZP, of average aggregate size, D fullerene nC60 NPs as a function of pH. (Inset) Variation of the electrostatic repulsion energy (VES) with separation distance (h) as a function of SO42-/Cl- ratio for nC60 NPs at pH of 3. The Mg2+ concentration in all treatments was kept constant at 10 mM. The errors bars represent one standard deviation of three j H and ZP, in DI water, are independent replicates. Values of D j H(ZP)2 ln[1+e-Kh], represented by (- -) for nC60 NPs. VES ) πεrεo D where, εr, εo, and K-1 are the dielectric constant, the permittivity of free space, and the Debye length, respectively (13). j N (P < 0.01). Cl-. Only the NP type significantly affected D j H∼ 450 nm) NP j H∼160 nm), nTiO2 (D Compared to nAg (D formed larger aggregates, and the likely reason was discussed j H among earlier. For a fixed NP type, the differences in D treatments were statistically indistinguishable (P < 0.05). For these treatments, surface charge neutralization was the same because the counterion concentration was the same, and EDL compression with IS variation was similar (Table 1). j PN was significantly affected by the NP type (P j N, Z Unlike D < 0.01), the electrolyte composition (P < 0.01), and their interaction (P < 0.07). The trend evident from Figure 3B is a proportional relationship between |ZP| for nTiO2 NPs and SO42-/Cl-. No changes occurred for nAg NPs (ZP ∼ -18 mV). Charge acquisition by preferential adsorption of co-ions is a likely phenomena for many particles in aqueous systems (17). The reason that the co-ions are able to approach and adsorb is due to their small hydrated radius compared to the counterions (10). Fernandez-Nieves and Nieves (18) had shown specific adsorption of co-ions such assSO42-, Cl-, and NO3-son negatively charged TiO2 particles. However, as each SO42- carries two negative charges, ZP was more negative in the presence of SO42- than when either Cl- or NO3- was present (18). This reason might explain our observed increase in |ZP| of nTiO2 NPs suspensions with increase in SO42-/Cl-. j H and Fullerene nC60. The effects of SO42- and Cl- on D ZP of nC60 NPs are shown in Figure 4 as a function of pH. The Mg2+ (counterion) concentrations in all treatments were kept constant at 10 mM (Table 1, type III). Therefore, the differences in effects shown in Figure 4 were due to the variation in SO42- /Cl- and the pH. VOL. 44, NO. 9, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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j H increased from 115 nm in DDI water to 140 At pH 3, D nm with the electrolytes indicating aggregation of nC60 NPs (Figure 4A). Because the concentrations of the counterions were the same, and it is the counterions which neutralize the j H was repulsive surface charge affect, the change in D significantly indifferent among treatments (P > 0.05). Howj H was consistently smaller at higher compared to at ever, D lower SO42-/Cl-, which was unexpected. With an increase in SO42-/Cl- from 0.1 to 8.0, the corresponding IS increased from 32.0 to 39.4 mM (Table 1). As the inverse of Debye length is proportional to IS, the potential energy of interparticle repulsion should decrease with an increase in IS and result in the formation of larger aggregates (13). The lower aggregation propensity might be due to the higher interparticle electrostatic repulsion at higher SO42-/Cl-; |ZP| decreased with decrease in SO42-/Cl- (Figure 4B). Estimation of the electrostatic repulsion energy (Figure 4, inset) confirmed that at high IS (SO42-/Cl- of 8) the interparticle repulsion was high compared to when the IS was low (SO42-/Cl- of 1 and 0.1). A similar observation was reported by Elimelech and Omelia (10) on the measured UE of hydrophobic polystyrene latex in the presence of SO42- and NO3- co-ions. These authors showed that below a critical salt concentration (Scrit) co-ions played an important role in imparting charges onto particle surfaces; whereas above Scrit, EDL compression and charge neutralization occurred due to the influence of counterions. Imparting of surface charge was more pronounced for SO42-; as each SO42- carries two negative charges compared to only one by monovalent anions (Cl-, NO3-) (10). Such a phenomenon of charge acquisition by hydrophobic and noniogenic nC60 NPs (11) might explain the observed variation of ZP with SO42-/Cl-. j Hand ZP with SO42-/Cl- at pH 5.8 was similar Variation of D to that at pH 3 qualitatively but differed quantitatively. j H was lower and |ZP| was higher under Compared to pH 3, D similar treatments (Figure 4). This difference might be due to the lower value of surface charge of nC60 NPs at lower pH; compare ZP of -38 mV at pH 3 to -45 mV at pH 5.8 in DDI water. Although acid-base functionality is known to be absent, the surface charge of nC60 NPs is a sensitive function of pH (12). The surface charge of nC60 NPs is negative, over a wide range of pH, and has a pHiep of 1. The absolute value of the surface charge of nC60 NPs decreases with decrease in pH, most likely due to higher proton activity at lower pH (12). Electrolytes reduce the repulsive effect of the surface charge (reduces|ZP|) and enhances interparticle aggregation. However, under the same electrolytic conditions nC60 NPs should form larger aggregates and have lower |ZP| at lower pH (pH 3) compared to higher (pH 5.8). At pH 10, nC60 NPs showed sign of rapid aggregation in j H increased from 115 nm in the presence of electrolytes (D DDI water to 1350 nm), which was unexpected (Figure 4A). At pH 10, |ZP| of nC60 NPs in DDI water was much higher compared to at pH 5.8 and 3 (SI Table S1). Therefore, nC60 NP suspensions should have been more stable at pH 10 than any lower pH (19). Under the same electrolytic conditions, j H and |ZP| of nC60 NPs should follow the order of (D j H)pH 3 D j H)pH 5.8 >(D j H)pH 10 and(|ZP|)pH 3
