Jessica
Englehart
,
Bonnie A.
Lyon
,
Matthew D.
Becker‡
,
Yonggang
Wang
,
Linda M.
Abriola
and
Kurt D.
Pennell
*
Department of Civil and Environmental Engineering, Tufts University, 200 College Avenue, Medford, Massachusetts 02155, USA. E-mail: kurt.pennell@tufts.edu; Fax: 617 627 3994; Tel: 617 627 3099
First published on 24th November 2015
Titanium dioxide nanoparticles (nTiO2) are utilized in an array of consumer products including paints, sunscreens, cosmetics, and food. These products typically contain stabilizing agents that may alter nTiO2 fate when released into the environment. The objective of this study was to investigate the effects of TEGO carbomer, a polymeric stabilizing agent used in sunscreen, on the transport and deposition behavior of nTiO2 in porous media. Aqueous nTiO2 suspensions at pH 5.0 or 7.5 ± 0.2 were introduced into water-saturated columns packed with Federal Fine Ottawa sand. In the absence of carbomer, nTiO2 was not detected in effluent samples at pH 5, which was below the estimated point of zero charge (PZC) of nTiO2 (pH 6.3), while greater than 80% elution of nTiO2 was observed at pH 7.5. The addition of 3 mg L−1 carbomer decreased the PZC from 6.3 to less than 5, and resulted in greater than 94% elution of nTiO2 at pH 5 and 7.5. A nanoparticle transport model that incorporates a first-order, maximum retention capacity term was able to capture column breakthrough and retention data. Model results indicate that the presence of carbomer reduced the average retention capacity of the solid phase from 3.40 to 1.10 μg TiO2 g−1 sand, irrespective of solution chemistry changes. These findings demonstrate the substantial impact that polymeric stabilizing agents can have on the fate of nTiO2 in porous media, potentially enhancing nTiO2 mobility in the environment and reducing the efficiency of filtration systems for nTiO2.
Nano impactTo evaluate the potential for human and ecological exposure to engineered nanomaterials, it is important to consider the impact of stabilizing agents on their environmental fate. This study coupled experimental work with mathematical modeling to investigate the influence of a polymeric sunscreen additive on the transport and deposition behavior of titanium dioxide nanoparticles (nTiO2) in porous media. The transport model was able to predict nanoparticle breakthrough behavior and total retained mass in column experiments, and could be applied in future studies to evaluate different nanomaterials and experimental conditions. The findings demonstrate that the addition of a polymeric sunscreen additive to nTiO2 suspensions could increase their mobility in the environment, increasing the potential for exposure and contamination of drinking water sources. |
The widespread use of nTiO2 in consumer products will inevitably lead to direct or indirect releases to the environment. In a study detailing the lifecycle of nTiO2 in sunscreen, Botta et al.5 found that a substantial amount of nTiO2 residue will disperse into aquatic environments as a result of sunscreen use (up to 30% of the total nTiO2 in the applied sunscreen). Nano-TiO2 contained in food additives or in cosmetic products and sunscreens that are washed off during bathing or cleaning are likely to enter wastewater treatment plants (WWTPs). A recent study by Weir and colleagues estimated a daily loading rate to sewage systems of 0.1 mg nTiO2 per person per day, based on ingestion of nTiO2-containing foods in the United States.2 Although partial removal of nTiO2 in WWTPs has been reported, concentrations of 5–15 μg L−1 as Ti have been measured in wastewater effluents,6,7 demonstrating that nTiO2 will persist and be discharged after treatment. Furthermore, if WWTP biosolids containing nTiO2 are land applied, additional nTiO2 could enter the environment. Release of part per billion (μg L−1) levels of nTiO2 from exterior building paint was shown by Kaegi and colleagues,8 indicating that urban runoff represents another route for nTiO2 to enter the environment.
TiO2 has been classified by the International Agency for Research on Cancer (IARC) as a possible human carcinogen, primarily based on health effects resulting from inhalation.9 nTiO2 ingestion has been linked to Crohn's disease,10 and prior studies have demonstrated nTiO2 toxicity in human and mammalian cells, as well as ecotoxicity including inhibition of algae growth following exposure to nTiO2.11,12 Many factors impact the toxicity of nTiO2,13–15 including crystalline structure (e.g., rutile vs. anatase),16 particle coating, and size, but comprehensive physicochemical characterization is often lacking in nanotoxicity studies making it difficult to correlate nTiO2 properties with observed effects.17 Hence, further research is needed to determine human health impacts and ecological risks associated with nTiO2 in the environment. One critical component of such assessments is a greater understanding of nTiO2 fate and transport under environmentally relevant conditions.
Particle stability and aggregation are important factors in the fate and transport of nTiO2. Jiang et al. found that increasing ionic strength (IS) from 1 to 100 mM NaCl resulted in a 50-fold increase in the hydrodynamic diameter of uncoated nTiO2 with a primary particle size of 15 nm.18 In the same study, the average nTiO2 size was approximately 90 nm when suspension pH was below 4.2 or above 8.2, at a constant IS of 1 mM. The point of zero charge (PZC) of nTiO2 is reported to range from pH 5.5–6.8,18–21 and falls within the pH range observed in the aquatic environment.22 Particle aggregation increases as the pH approaches the PZC, with maximum aggregation occurring at the PZC. In the environment, dissolved natural organic matter (NOM) can stabilize nTiO2 suspensions by reducing particle aggregation, presumably due to steric repulsion.23 Data presented by Domingos and colleagues suggests that, due to the presence of NOM, nTiO2 dispersion and mobility in the environment may occur to a greater extent than predicted based on prior laboratory experiments.23 In addition to naturally occurring stabilizing agents, artificial dispersants are frequently added to nanoparticle suspensions to increase stability and maintain or minimize aggregate size.24,25 Joo et al. observed improved nTiO2 suspension stability in the presence of carboxymethyl cellulose (CMC), where suspensions of uncoated nTiO2 exhibited a PZC of 5.6 while CMC-containing nTiO2 suspensions had a PZC of less than 2.25 An additional study, which evaluated the impact of polyacrylic acid on nTiO2 stability, reported a decrease in the PZC by 0.42–2.08 pH units, depending upon the molecular weight (2000–120000 g mol−1) and concentration (10–100 mg L−1) of added polyacryclic acid.26
Petosa and colleagues demonstrated that polymer-coated (partially cross-linked polyacrylic acid) nTiO2 particles had greater mobility in quartz sands compared to bare nTiO2 for IS ranging from 0.1–100 mM as NaNO3.27 In another study by Joo et al., nearly complete retention of uncoated anatase nTiO2 was reported in quartz sand columns, while the addition of CMC to nTiO2 suspensions (nTiO2:CMC ratio of 0.1:1) resulted in nanoparticle breakthrough after 1 pore volume (PV), which was attributed to electrosteric stabilization from the adsorption of CMC to the nanoparticles.25 With the exception of a few studies,24,25,27 the majority of previous nTiO2 transport experiments were performed with uncoated nTiO2 or in the absence of stabilizing agents, and did not consider the matrices utilized in most commercial products containing nTiO2. Sunscreens represent a major class of personal care products and potential route of entry for nTiO2 into the environment.5 Thus, in order to predict the fate of nTiO2, it will be important to understand the influence of specific sunscreen additives on stability and transport in the environment. The stabilizing agents considered in previous nTiO2 transport studies include non-ionic and anionic surfactants24 carboxymethyl cellulose,25 and clay particles,28 but the effect of a specific sunscreen additive has not been evaluated. Additionally, very few prior studies measured the amount of retained nTiO2 (i.e., the solid-phase concentration) in column transport experiments28–30 due the difficulties associated with accurately measuring deposited TiO2 concentrations. Measurement of retained mass, however, is critical to experimental mass balance closure and is also important for transport model validation.
The objective of this research was to investigate the effects of a polymeric stabilizing agent used in sunscreen formulations (carbomer) on the mobility of TiO2 nanoparticles in porous media. Batch studies were carried out to determine the effects of pH and IS on the aggregation, particle size distribution, and zeta potential of nTiO2 in the presence and absence of carbomer. Column experiments were conducted to evaluate the influence of the stabilizing agent on the transport and deposition of nTiO2 in water-saturated quartz sands at pH values of 5.0 and 7.5 ± 0.2. In addition to column effluent samples, nTiO2 solid phase concentrations were measured, allowing for retention profiles and total nTiO2 mass balance to be calculated. A nanoparticle transport model that incorporates a first-order attachment expression and a Langmuirian blocking function was implemented to provide further quantitative interpretation of the experimental results.
nTiO2 suspensions were prepared by mixing a pre-weighed mass of dry P25 powder with deionized (DI) water, generating a final suspension concentration of approximately 30 mg L−1. Suspension IS was adjusted to values ranging from 0.01–100 mM using 1 M NaCl. Samples were adjusted to pH 5 using 0.1 M HCl, while pH 7.5 samples were buffered with 1 mM HEPES buffer (Acros Organics, New Jersey). For samples containing stabilizing agent, carbomer was first added to DI water and mixed for ten minutes prior to the addition of nanoparticles, followed by IS and pH adjustment using the same procedure as the samples without stabilizing agent. The final suspensions (including nTiO2) were sonicated for 10 minutes using a Branson Sonifier 450 sonication probe (Branson Ultrasonics, Danbury, CT) with a microtip attachment.
Column ID | pH | Experimentally determined parameters | Mathematical model fitted parameters | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Suspension additives | C 0 (mg nTiO2 L−1) | Porosity (unitless) | v p (m d−1) | Mass breakthrough (%) | Mass balance (%) | Particle diameterc (nm) | Zeta potential (mV) | k att (1 s−1) | α (unitless) | S max (μg TiO2 g−1 sand) | ||
a Influent nTiO2 concentration. b Pore-water velocity. c Average of influent particle diameter at beginning and end of pulse injection. d Attachment rate. e Attachment efficiency. f Maximum retention capacity. | ||||||||||||
1 | 5.0 | None | 24 | 0.38 | 7.1 | 0 | 85 | 119 | 11 | 3.63 × 10−3 | 0.32 | n/a |
2 | 5.0 | None | 24 | 0.36 | 7.8 | 0 | 84 | 116 | 23 | 4.06 × 10−3 | 0.32 | n/a |
3 | 7.4 | 1 mM HEPES | 28 | 0.38 | 7.1 | 90 | 98 | 107 | −28 | 2.69 × 10−4 | 0.022 | 2.51 |
4 | 7.4 | 1 mM HEPES | 29 | 0.39 | 7.3 | 82 | 93 | 132 | −21 | 5.95 × 10−4 | 0.059 | 4.28 |
5 | 5.1 | 3 mg L−1 carbomer | 28 | 0.37 | 7.3 | 104 | 104 | 108 | −32 | 4.08 × 10−4 | 0.031 | 0.34 |
6 | 5.1 | 3 mg L−1 carbomer | 28 | 0.38 | 7.4 | 97 | 97 | 109 | −36 | 1.25 × 10−4 | 0.010 | 1.32 |
7 | 7.6 | 1 mM HEPES 3 mg L−1 carbomer | 32 | 0.38 | 7.1 | 100 | 103 | 112 | −27 | 3.61 × 10−4 | 0.031 | 0.78 |
8 | 7.6 | 1 mM HEPES 3 mg L−1 carbomer | 32 | 0.38 | 7.5 | 94 | 98 | 105 | −36 | 4.50 × 10−4 | 0.036 | 1.82 |
9 | 5.2 | 3 mg L−1 carbomer 3 mM NaCl | 24 | 0.38 | 7.2 | 98 | 98 | 129 | −50 | 1.19 × 10−4 | 0.012 | 0.96 |
10 | 5.2 | 3 mg L−1 carbomer 3 mM NaCl | 25 | 0.36 | 7.8 | 96 | 96 | 135 | −43 | 7.31 × 10−4 | 0.065 | 0.78 |
11 | 7.7 | 1 mM HEPES 3 mg L−1 carbomer 3 mM NaCl | 33 | 0.38 | 7.0 | 97 | 99 | 116 | −38 | 2.78 × 10−4 | 0.024 | 1.42 |
12 | 7.7 | 1 mM HEPES 3 mg L−1 carbomer 3 mM NaCl | 33 | 0.37 | 7.6 | 97 | 98 | 124 | −39 | 2.81 × 10−4 | 0.024 | 1.39 |
Following the tracer test, a three PV pulse of nTiO2 suspension was injected into the column using a PHD 2000 syringe pump (Harvard Apparatus, Holliston, MA) at a flow rate of 1 mL min−1, followed by at least two PVs of nTiO2-free background electrolyte solution, also at a flow rate of 1 mL min−1. The average Darcy velocity of background electrolyte and nTiO2 injections was 2.8 ± 0.06 m d−1, corresponding to an average pore-water velocity of 7.3 ± 0.3 m d−1. This value is similar to the seepage velocity used by Cai et al. (8 m d−1) in a previous nTiO2 transport study28 to represent flow through coarse aquifer sediments or engineered filtration systems. Column effluent samples were collected continuously (at least five samples per PV) using a Spectrum Labs Spectra/Chrom CF-2 Fraction Collector (Spectrum Laboratories, Inc., Rancho Dominguez, CA). Columns 1–8 were run with a DI water background to first evaluate the role of pH and carbomer addition, and then the effect of background electrolyte (3 mM NaCl) in the presence of carbomer was investigated (columns 9–12). After each experiment, the columns were sectioned into ten 1 cm increments and approximately 2 g of sand from each increment was analyzed to determine the amount of retained nTiO2.
Column effluent and solid samples were oven-dried at 90 °C and then digested in 18.7 M sulfuric acid (2 mL H2SO4 for aqueous samples, 5 mL for solid samples) with a CEM SP-D Discover Microwave Digester (CEM Corporation, Matthews, NC). Acid digestion was conducted at 200 °C for 45 minutes for aqueous samples and 200 °C for 60 minutes for solid samples. After digestion, the samples were diluted to 1 M H2SO4 using DI water and quantified using an Optima 7300 DV Inductively Coupled Plasma Optical Emission Spectrometer, ICP-OES (PerkinElmer, Waltham, MA). Standard curves were prepared from an Ultima Titanium stock standard (1000 mg L−1). The average background titanium concentration of the cleaned Federal Fine Ottawa Sand was 18 μg TiO2 g−1 sand. Titanium was quantified at a wavelength of 336.121 nm, which yielded a detection limit of 12 μg Ti L−1 (equivalent to 20 μg TiO2 L−1), based on the U.S. Environmental Protection Agency method for determining a lowest concentration minimum reporting level.36
(1) |
(2) |
(3) |
Eqn (1)–(3) were solved with an implicit-in-time and central-in-space finite difference scheme implemented in MATLAB R2010a (The Mathworks Inc., Natick, MA). Pore-water velocity, vp, and hydrodynamic dispersion, DH, terms were independently determined from column tracer data. katt and Smax were then determined by fitting the model to effluent breakthrough data using a non-linear least squares minimization algorithm.38 The transport model (with zero attachment) was also validated against the tracer data fit (that was performed using the CXTFIT program)35 to confirm that it appropriately captured hydrodynamic dispersion.
Fig. 1 Effect of pH on nTiO2 particle size and zeta potential in DI water. Error bars represent standard deviation of 3 replicate measurements. |
The influence of IS (0.01–10 mM) on the mean particle diameter and zeta potential of nTiO2 suspensions without stabilizing agent at pH 5 and 7.5 is shown in Fig. 2. As expected, the nanoparticles were highly sensitive to changes in solution chemistry in the absence of a stabilizing agent. At pH 5, an increase in nTiO2 particle diameter compared to that in DI water was observed at NaCl concentrations as low as 0.1 mM. When the IS was further increased to 1 mM NaCl, the particle diameter increased more than two-fold (288 ± 50 nm) compared to its value in DI water (112 ± 17 nm). At IS values ≥5 mM NaCl and pH 5, the nTiO2 suspension became unstable, resulting in sedimentation of larger nanoparticle aggregates, and, therefore, unstable size distribution and zeta potential readings. In a previous study using 100% anatase nTiO2 at pH 4.6, Jiang et al. observed a similar trend of increasing nTiO2 size with increasing IS, with substantial aggregation occurring at IS values of 5 mM NaCl and higher.18 In contrast to pH 5 samples, the zeta potential of nTiO2 at pH 7.5 remained relatively constant even when the IS was increased from 0.01 to 10 mM NaCl (mean zeta potential = −22 ± 2.1 mV) in the present study. Although the zeta potential remained stable in pH 7.5 suspensions, particle aggregation was observed and micron-sized aggregates began to form between IS values of 1 mM and 5 mM NaCl.
The addition of 3 mg L−1 carbomer to nTiO2 suspensions resulted in stable particle size and zeta potential for IS values ranging from 0.01 to 100 mM, effectively mitigating the pH effects observed in the absence of carbomer (Fig. 3). At a carbomer concentration of 3 mg L−1, the mean nTiO2 diameter over the entire range of IS was 124 ± 37 nm and 117 ± 37 nm at pH 5 and 7.5, respectively, and the average zeta potential was −42 ± 2 mV and −37 ± 4 mV at pH 5 and 7.5, respectively. The observed differences in mean nTiO2 diameters across IS values were not statistically significant (paired two-tailed t-test, ∝ = 0.05, p-value = 0.50), even though the mean zeta potential values were significantly different (p-value < 0.01). The enhanced stability of nTiO2 suspensions that was observed following the addition of carbomer was likely due to electrosteric repulsion resulting from adsorption of the polymer to the nTiO2 surface. A previous study by Liufu et al. demonstrated that adsorption of polyacrylic acid on nTiO2 occurs by hydrogen bonding and chemical interaction between the TiO2 surface and carboxyl groups of the polymer, which stabilizes nTiO2 suspensions through electrosteric repulsion.26
Fig. 3 Effect of IS on zeta potential and nTiO2 particle size in the presence of 3 mg L−1 carbomer at (a) pH 5 and (b) pH 7.5. Error bars represent standard deviation of 3 replicate measurements. |
Traditional DLVO theory does not account for enhanced stability of nanoparticles resulting from the presence of polymers such as carbomer. Therefore, extended DLVO (XDLVO) was used to compute interaction energy profiles for suspensions containing carbomer (Fig. 4). In XDLVO, the total interaction energy includes not only the traditional electronic double layer repulsive energy and van der Waals attractive energy, but also the osmotic and elastic–steric repulsion energies induced by the presence of a polymer layer coating a colloid or nanoparticle.39 The equations and parameters used for XDLVO calculations are provided in ESI† (Table S1). The repulsive energy barriers based on XDLVO calculations reached maximum values of >20000 kT at pH 5 and 7.5 for IS values of 0.01–100 mM (Fig. 4, inset). The large primary energy barrier exists within the region of the polymer layer surrounding the particle (d = 0–10 nm), and the repulsive force is primarily due to elastic–steric forces. At both pH 5 and pH 7.5, a secondary energy minimum is observed at IS values of 10 and 100 mM, indicating that, although there is large repulsive energy barrier, attractive forces do exist with higher IS solutions. In comparison, the primary energy barriers obtained using traditional DLVO theory for nTiO2 with carbomer were higher than those obtained for nTiO2 alone (e.g., 63 kT with carbomer versus 15 kT without carbomer at pH 5 and IS = 0.01 mM, ESI† Fig. S2 and S3), but were several orders of magnitude lower than those obtained using XDLVO theory.
Fig. 4 Interaction energy profiles obtained using XDLVO theory for two nTiO2 particles in the presence of carbomer at (a) pH 5 and (b) pH 7.5. Insets show the y-axis on a larger scale. |
To evaluate the impact of steric repulsion on interaction energy in the presence of a polymer, XDLVO theory was applied to the nanoparticle-quartz surface system.40,41 For this case, the steric interaction energy was calculated between a polymer-coated TiO2 nanoparticle and an uncoated sand grain and summed with the nanoparticle–sand grain electronic double layer repulsive energy and van der Waals attractive energy to obtain the total interaction energy (Fig. 5, see ESI† for equations). The inclusion of steric energy in the XDLVO calculation results in a large positive (repulsive) energy in the region of the polymer layer 0–10 nm from the particle surface, which is consistent with the stabilization of nTiO2 due to an adsorbed polymer coating. In contrast to the interaction energy profiles for nTiO2 and quartz sand in the absence of carbomer (ESI,† Fig. S4), repulsive forces dominate for both pH 5 and 7.5 at low IS (≤1 mM NaCl) when carbomer was present (Fig. 5). Dominant repulsive forces suggest that under these conditions, nTiO2 will not be deposited on porous media, and therefore, the mobility of pH 5 nTiO2 suspensions is expected to be enhanced by the presence of carbomer. For higher IS suspensions (≥10 mM NaCl), a secondary minimum attractive energy was obtained even though a large primary energy barrier existed due to steric repulsion. A similar trend was observed in XDLVO profiles presented by Wang et al. for polyacryclic acid-octylamine-coated quantum dots, with secondary energy minimums observed at IS values of 30 and 100 mM NaCl.42
Fig. 7 Comparison of experimentally measured and simulated (a) transport and (b) retention of nTiO2 alone at pH 7.4 in Federal Fine (30–140 mesh) Ottawa sand (duplicate columns). |
Fig. 9 Comparison of experimentally measured and simulated (a) transport and (b) retention of nTiO2 with 3 mg L−1 carbomer at pH 7.6 in Federal Fine (30–140 mesh) Ottawa sand (duplicate columns). |
When 3 mM NaCl was added to carbomer–nTiO2 suspensions at pH 5.2 and pH 7.7 (columns 9–12, Fig. S5 and S6†), there was minimal change in nTiO2 mobility (<8% change in mass breakthrough) compared to identical column studies performed without NaCl (columns 5–8, Fig. 8 and 9). This finding was consistent with the XDLVO theory, which indicated that nTiO2 particle–particle and particle–surface interaction energy profiles were not sensitive to low electrolyte concentrations in the presence of carbomer (Fig. 4 and 5). Similar to the trend observed for column experiments conducted in the presence of carbomer without NaCl (columns 5–8), the nTiO2 effluent BTC obtained at pH 7.7 with 3 mg L−1 carbomer and 3 mM NaCl (columns 11–12) rose more slowly than the nTiO2 BTC obtained at pH 5.2 under the same conditions (columns 9–10). No measurable nTiO2 retention was observed at pH 5.2, while the solid phase sample collected nearest the column inlet yielded concentrations of 4.8 and 3.0 μg TiO2 g−1 sand in replicate experiments conducted at pH 7.7. Zeta potential values obtained in the presence of carbomer were negative at pH 5–5.2, indicating that the PZC shifted to a pH value lower than 5. Joo et al. observed a similar decrease in the PZC (from pH 5.6 to <2) and enhanced nTiO2 mobility through quartz sand following addition of a different polymer stabilizing agent (carboxymethyl cellulose, MW = 90000 g mol−1) to nTiO2 suspensions.25
Collision efficiency (α) values were calculated from the fitted katt values using clean-bed filtration theory:44
(4) |
Although the breakthrough behavior was well captured in all cases, the model based upon eqn (1) and (2) with a single (average) Smax value was unable to accurately reproduce the hyper-exponential retention curves exhibited in columns 3–12 (see, for example, Fig. 9b). Although models for mechanical filtration (i.e., straining) of colloidal particles in porous media predict hyper-exponential retention behavior, the measured particle sizes observed in this study were approximately 120 nm, resulting in nTiO2 to porous media diameter (d50 = 350 μm) ratios ranging from 3.0 × 10−4 to 3.9 × 10−4. Typically, straining is considered to be significant when the ratio of particle diameter to grain diameter is greater than 0.05,46 although more recent studies have suggested that straining may be a factor at ratios of 0.002 or 0.008.47,48 Since the nanoparticle:grain size diameter ratios were an order of magnitude lower than the reported threshold values, it is unlikely that physical straining contributed to the observed nTiO2 retention. Although the shape of the retention curves was not fully captured, the single-Smax retention model was able to accurately capture the total amount of retained mass in all cases. This result suggests that the maximum capacity for nTiO2 retention varied along the domain, but could be represented by a single average retention parameter.
In the presence of 3 mg L−1 carbomer, greater than 94% effluent mass breakthrough occurred in nTiO2 suspensions, regardless of suspension pH or electrolyte content. The enhanced stability of nTiO2 suspensions following carbomer addition was likely due to increased electrosteric repulsion resulting from adsorption of the polymer to the nanoparticle surface. As a result, the effects of solution pH were overshadowed by the addition of a polymer to nTiO2 suspensions. Simulation of experimental BTCs and retention profiles using a nanoparticle transport model with a first-order kinetic attachment expression and a maximum attachment capacity term adequately described the breakthrough behavior as well as the total amount of retained mass measured in all experiments. However, the model was unable to capture the hyper-exponential nTiO2 retention observed near the column inlet, indicating that there is likely a spatially-variable retention capacity that was not accounted for by the model.
The findings of this study demonstrate the importance of considering the impacts of polymer stabilizing agents on the environmental fate of manufactured TiO2 nanomaterials that are released into the environment from sunscreen and other consumer products containing a dispersing agent in their formulation. More specifically, the addition of product-relevant concentrations of carbomer could alter the system retention capacity substantially, thereby allowing for increased nTiO2 mobility through porous media. Such enhanced transport through porous media or filtration systems could increase the potential for contamination of drinking water sources and risk of human and ecological exposure.
Footnotes |
† Electronic supplementary information (ESI) available: TEM images; DLVO/XDLVO calculations and parameters; additional transport model equations; DLVO interaction profiles for two nTiO2 particles and for nTiO2-sand grains; BTCs and retention profiles for nTiO2 in the presence of carbomer and 3 mM NaCl. See DOI: 10.1039/c5en00174a |
‡ Current address: John and Willie Leone Family Department of Energy and Mineral Engineering and EMS Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, USA. |
This journal is © The Royal Society of Chemistry 2016 |