Released from Nano-TiO - American Chemical Society

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Behavior of TiO2 Released from Nano-TiO2‑Containing Paint and Comparison to Pristine Nano-TiO2 Ahmed Al-Kattan,†,‡ Adrian Wichser,† Stefano Zuin,§ Yadira Arroyo,∥ Luana Golanski,⊥ Andrea Ulrich,† and Bernd Nowack*,‡ †

Laboratory for Analytical Chemistry and ∥Electron Microscopy Center, EMPASwiss Federal Laboratories for Material Science and Technology, Ü berlandstrasse 129, 8600 Dübendorf, Switzerland ‡ Technology and Society Laboratory, EMPASwiss Federal Laboratories for Material Science and Technology, Lerchenfeldstrasse 5, CH-9014 St. Gallen, Switzerland § Venice Research Consortium, Via della Libertà 12, c/o VEGA Park, 30175 Venice, Italy ⊥ CEA Commissariat à l’Energie Atomique et aux Energies Alternatives, Rue des Martyrs 17, 38000 Grenoble, France S Supporting Information *

ABSTRACT: In the assessment of the fate and effects of engineered nanomaterials (ENM), the current focus is on studying the pristine, unaltered materials. However, ENM are incorporated into products and are released over the whole product life cycle, though mainly during the use and disposal phases. So far, released ENMs have only been characterized to a limited extent and almost nothing is known about the behavior of these materials under natural conditions. In this work we obtained material that was released from aged paint containing nano-TiO2, characterized the particulate materials, and studied their colloidal stability in media with different pH and ionic composition. A stable suspension was obtained from aged paint powder by gentle shaking in water, producing a dilute suspension of 580 μg/L TiO2 with an average particle size of 200− 300 nm. Most particles in this suspension were small pieces of paint matrix that also contained nano-TiO2. Some free nano-TiO2 particles were observed by electron microscopy, but the majority was enclosed by the organic paint binder. The pristine nano-TiO2 showed the expected colloidal behavior with increasing stability with increasing pH and strong agglomeration above the isoelectric point and settling in the presence of Ca. The released TiO2 showed very small variations in particle size, ζ potential, and colloidal stability, even in the presence of 3 mM Ca. The results show that the behavior of released ENM may not necessarily be predicted by studying the pristine materials. Additionally, effect studies need to focus more on the particles that are actually released as we can expect that the toxic effect will also be markedly different between pristine and product released materials.

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Ag, and nano-TiO2.11 Several studies have targeted the release of Ag from nano-Ag-containing textiles and other consumer products,12−16 the release of TiO2 from nanotextiles,17 or of several different nanomaterials from sprays.18,19 These studies have shown that dissolution and phase transformation reactions may occur and that most of the released particles are no longer the same as the original ENM incorporated into the product. Whereas the pristine materials may be released during production and manufacturing, the materials released during use and disposal may be significantly altered and/or transformed. Nowack et al. (2012) categorized these transformed materials as “product-modified ENM”, “product-weathered ENM”, and “environmentally-transformed ENM”.3 We have recently investigated the release of TiO2 from paints containing

INTRODUCTION The potential impacts and risks of engineered nanomaterials (ENMs) are the focus for a topic that currently receives a lot of attention.1,2 Almost all of the published studies with these materials worked with so-called pristine, as-produced materials.3 All of these studies have provided very useful information about the effects and the hazards of the pristine materials, both in nanotoxicology as well as in nanoecotoxicology.4 Furthermore, many studies about their fate in the environment have resulted in a relatively good understanding of their behavior under natural conditions.5,6However, the wealth of studies about fate and effects contrasts with only a few of studies about environmental exposure and release.7 Relatively few studies have quantified the release of materials from actual products and applications, and even fewer have characterized in detail the released materials. Some data are available concerning the release of nano-Ag from paint,8,9 the release of nanosized TiO2 from houses painted with conventional paint,10 and the leaching of materials from paints containing nano-SiO2, nano© 2014 American Chemical Society

Received: Revised: Accepted: Published: 6710

February 6, 2014 May 14, 2014 May 20, 2014 May 20, 2014 dx.doi.org/10.1021/es5006219 | Environ. Sci. Technol. 2014, 48, 6710−6718

Environmental Science & Technology

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TiO2 (anatase) stabilized with polyacrylate) and pigment TiO2 (RC823 by Cinkarna). Nano-TiO2 was added as a photocatalyst, while pigment TiO2 was added as white pigment. Figure S1 (Supporting Information) shows transmission electron microscopy (TEM) images of the two TiO2 particles before being added to the paint. The complete composition of the paint is given in Table S1 (Supporting Information). Preparation of Aged Paint. For the preparation of artificially aged paint, the paint was applied on plastic (poly(vinyl chloride)) sheets by a manual film applicator with a thickness of the paint film of 200 μm. After drying for 24 h in indoor conditions, the paint was scraped off the panels by a plastic spatula. The collected paint fragments were milled using a planetary mill (Fritsch Planetary Mill Pulverisette 4 classic line) in an yttria stabilized zirconia vial and milling balls with a ball:powder ratio of 2:1 for 40 min with a rotor speed of 320 rpm. The low ratio and the use of zirconia grinding media produced milled powders with little contamination from the milling balls.31 The milling media (i.e., vials and balls) were cleaned using 70% EtOH:water (v/v) solution (Milli-Q water). The milled powders were then exposed to UVA light in an accelerated weathering machine that consisted of a conditioned enclosure in which the lamps were housed, a rack on which the milled powders were irradiated, and a tray for collection of condensation water. The powders were wetted by condensation, which is ensured by cooling of the back of the tray with the exposed powders. A fluorescent 340 nm UVA lamp (PHILIPS TL20W/09N) was used for irradiation. This lamp corresponded to solar radiation in the photochemically important short-wavelength range, emitting radiation over the whole UVA part of the spectrum (315−400 nm), with 20 W/ m2. The paint powder was continuously shaken at low intensity for 500 h to expose all paint particles in the same way. The duration of UV exposure was based on an existing standard for aging of paint (ISO 11507:2007) and is representative of outdoor aging. Radiation, temperature, and humidity all contributed to the aging process of paints. An additional milling of the aged paint powder was performed using a PM 100 Planetary Ball Mill Retsch system to further reduce the size. The use of α-Al2O3 grinding balls avoided the contamination of the paint powder with the elements of interest, mainly Ti. The rotor speed was set at 380 rpm with reversing the mode every 10 min and a pause of 30 s between the motor reversals. The total grinding time was 40 min. This aged and milled paint was used for studying the extractability of TiO2 in batch experiments. To determine the size distribution of the milled paint, the aged paint powder was suspended in water and measured by laser granulometry. The Mastersizer allows the determination of particle number distribution in the range of 0.01−1000 μm. For total element analysis, samples of the aged paint powder of approximately 2 mm × 2 mm and milled powder pellets were analyzed using a X-ray fluorescence analysis (XRF). Samples were prepared as fused disks, using a mixture of 7.5 g of Merck A12 dilithium tetraborate/lithium metaborate (66:34) flux and 1.5 g of sample. Total Fe content is reported as Fe2O3. For total element analysis, 10 g of sample of the aged paint powder was pressed with a Herzog HTP 60 press at 20 ton for 10 s, to obtain a pellet of 40 mm in diameter. For smaller samples H3BO3 was added as a binder. The samples were then analyzed with a Pioneer S4 (Bruker AXS) X-ray spectrometer with a rhodium (Rh) source, operating at 4 kW. The SPECTRAplus software package was finally used to elaborate X-ray analysis

pigment TiO2 (added as white pigment) and nano-TiO2 (added as a photocatalyst).20 Panels covered with either paint containing pigment TiO2 and nano-TiO2, and, as a reference, paint with only pigment TiO2 were weathered in climate chambers by simulated sunlight and rain. A very low release of TiO2 close to the background of the irrigation water was observed, with less than 1.5 μg/L TiO2 released over 113 irrigation cycles. The leachates of the two paints were also studied in small-scale tests to understand the influence of solution composition on the release of TiO2. Between 0.5 and 14 μg/L TiO2 were released into solution, observing the highest concentrations after prolonged UV exposure. The total release of TiO2 over all weathering cycles was only 0.007% of the TiO2 contained in the paint, indicating that the TiO2 was strongly fixed in the paint. Extraction of UV-aged and milled paint resulted in about 100 times larger releases of TiO2 from the paint with nano-TiO2 compared to the paint with only pigment TiO2. Photocatalytic degradation of the organic paint matrix (observed by electron microscopy of the aged paint surface) or direct release of nano-TiO2 into suspension could be responsible for this increase in the release of TiO2. In another study with the small panels under static conditions, only minimal release of nano-TiO2 was observed.11 Studies about the stability of the ENM dispersion under realworld conditions have been proposed as an essential part of the general characterization of ENM.22 The detailed understanding of the behavior of ENM in the environment is a crucial prerequisite for the comprehensive assessment of their final fate, including determining the most important sinks and their possible risks.21 The distribution of ENM within the environment is mainly determined by the dispersion state and agglomeration behavior of particles, which directly affects the exposure of these materials to organisms. It has been shown that TiO2 agglomeration depends on the ionic strength of the solution, the pH of the solution, the presence of monovalent and divalent ions, and the presence of natural organic matter (NOM),22,23 all as a function of the isoelectric point. However, all of this work has been performed with pristine particles and only very few studies have evaluated the characteristics of the released and altered nanomaterials and they have shown that the altered materials may have a very different characterization compared to the pristine materials. Rose and co-workers, for example, have shown that sunscreen TiO2 composite nanomaterials are significantly altered by simple exposure to water and light,24−26 altering their physicochemical behavior, but also their effects on organisms.27−29 These studies show how important it is to evaluate the complete life cycle of the materials,30 as during different stages the materials may be imposed to very diverse chemical and physical conditions that can strongly affect their behavior and toxicity. The aim of this work was therefore to investigate the behavior of TiO2 released from paint and compare it to the behavior of the pristine nano-TiO2. For this, aged paint was extracted in water and the supernatant containing a stable suspension was exposed to different media. The experiments were also repeated with the pristine material. The materials in the suspensions were characterized using electron microscopy.

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MATERIALS AND METHODS Materials. A paint formulation with both nano-TiO2 were provided by industrial project two TiO2 forms added to the paint were (Hombikat UV 100 WP, an aqueous dispersion

pigment and partners. The a nano-TiO2 of 50% nano6711

dx.doi.org/10.1021/es5006219 | Environ. Sci. Technol. 2014, 48, 6710−6718


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Figure 1. Pigment TiO2 and nano-TiO2 in diluted paint (left) and enlargement of the nano-TiO2 agglomerate visible in the middle of the left image (right).

matter as humic acid (HA) (humic acid sodium salt, SigmaAldrich; 10 mg/L) and under two ionic strength conditions: 10 mM NaCl and 3 mM CaCl2, representative of Swiss freshwaters.32 To achieve the desired pH value, suprapur nitric acid (65%, Merck) and ammonia solution (Alfa Aesar) were added while stirring. Samples with 10 mL aliquots of the supernatants were used, so the added volume of the acid and the alkaline solutions represent at maximum 1% of the total volume of solution, which is neglected. The solutions were placed on a horizontal shaker for 24 h. The pH value was confirmed after the shaking step, and this final pH value is reported in the results. Immediately after shaking, the samples were filtered on a cellulose filter of 0.45 μm (Sartorius Stedim) and on a polycarbonate/polyester filter of 0.1 μm (Whatman). The filtrates were then analyzed by ICP-MS to determine the content of Ti. Initial experiments with 10 kDa filters showed that the dissolved Ti fraction was negligible, so all Ti measured in the 0.1 μm filters consisted of particulate TiO2. The hydrodynamic size measured by DLS and the ζ potential of the particles present in the supernatants was determined without pretreatments using a ZetaSizer Nano ZS from Malvern Instruments. Electron Microscopy. To identify released TiO2 particles, scanning transmission electron microscopy (STEM) was performed on a Jeol 2200FS TEM/STEM microscope equipped with an energy dispersive X-ray detector (EDX) for elemental analysis and operated at 200 kV. The preparation of the supernatant samples for electron microscopy analysis was performed by direct centrifugation of the sample onto TEM grids in a swing bucket centrifuge at 5000 rpm for 60 min. The pristine materials and diluted liquid paint were deposited on the grid by drop deposition.

(Version 1.7) and SEMIquant Software for semiquantitative analysis. The elements present in the paint, in weight percent, are represented in Table S2 (Supporting Information). The major elements detected by XRF analysis were calcium, titanium, and silica, as expected. Other minor elements, including aluminum, magnesium, potassium, and sodium were also found through XRF analysis. Production of Supernatant Solution from Aged Paint Powder Containing Nano-TiO2 Particles and Pigment TiO2. To produce an aqueous suspension of released TiO2 particles, the aged powder was used. A suspension of 20 g/L of aged paint was prepared in milli-Q water (>18 MΩ). The suspension was shaken for 3 days on a shaking machine at 40% of the maximal intensity. The solution was then kept in a separatory funnel for 24 h to allow sedimentation of large particles. The supernatant was then separated from the sedimented particles and characterized and used within 48 h. Characterization of the Supernatant. The total content of metals Ti, Si, Ca, K, Mn Mg, P, Na, Zn, and S of supernatants of the aged paint were quantified by inductively coupled plasma optical emission spectroscopy (ICP-OES) (Vista pro) and mass spectrometry (ICP-MS) (Elan 6000). The total organic matter (TOC) and the dissolved organic matter (DOC) of the supernatants were also measured. The concentrations of nitrate, phosphate, and sulfate were quantified by ion chromatography. The composition of the supernatant is given in Table S3 of the Supporting Information. The hydrodynamic size of the particles present in the supernatant was determined by dynamic light scattering (DLS) using a ZetaSizer Nano ZS from Malvern Instruments. The ICP-MS measured the concentration of Ti, but we converted all Ti concentrations into TiO2 and report only those in this work. This is justified because the dissolved Ti (i.e., fraction smaller than 10 kDa) made up only a very small percentage of the total Ti measured. Behavior of TiO2 in Water. Fate experiments were performed with the supernatant and with pristine nano-TiO2 suspension (at approximately the same Ti concentration as the supernatant). The pH value of the solutions was varied in the range of 4−9 in the absence of and in the presence of organic

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RESULTS Characterization of Paint and Extracted Particles. The paint contains two types of TiO2 particles, both visible in a wet paint sample diluted in water (Figure 1). The pigment TiO2 particles are elongated crystals with sizes of approximately 100−300 nm in length, and the nano-TiO2 particles consist of grainy agglomerates with very small primary particle sizes (only 6712



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from the aged paint was then used to study the behavior of the released TiO2 under natural water conditions and compared to pristine nano-TiO2. The stability of the extracted TiO2 as a function of pH in four different ionic media is shown in Figure 5. In these experiments the extracted particles were shaken in the ionic medium for 24 h and then let to sediment for 24 h. Up to 50% of the added pristine nano-TiO2 remained in suspension and were able to pass through a 0.45 μm filter in NaCl medium at high pH. At points below pH 5, TiO2 was almost undetectable in suspension, so either all particles had agglomerated and settled or were retained by the filter. The fraction able to pass through a 0.1 μm filter was at maximum 20% at high pH. The presence of HA did not significantly change particle stability. In the presence of 3 mM CaCl2, TiO2 was almost not found in both filtrates, so was either sedimented or agglomerated to particles larger than 0.45 μm. The TiO2 extracted from aged paint behaved very differently in all media. The pH dependence was much less pronounced in NaCl background, and especially at low pH the particles were much more stable than the pristine TiO2. Again, the presence of HA did not have a major effect on stability. About 20% of the initial TiO2 remained stable in suspension. In CaCl2 background between 10 and 20% of the TiO2 remained in suspension with a slight increase with increasing pH in the absence of HA and the opposite trend in the presence of HA. In all media the fraction passing through 0.1 μm was about 50% of the 0.45 μm fraction. The particle size measurement by DLS showed that the average diameter of the particles of pristine nano-TiO2 was between 300 and 500 nm in NaCl and up to 2000 nm in CaCl2 background (Figure 6). In NaCl the size decreased with increasing pH, corresponding to the greater stability at elevated pH. The influence of HA on particles size was not significant. In the aged TiO2, the particle size did vary greatly in the various media and at the different pH values and was between 200 and 300 nm. The complete particle size distribution measured by DLS of aged and pristine TiO2 is shown in Figure S2 of the Supporting Information. The ζ potential of the pristine TiO2 decreased with increasing pH from about −10 mV to about −40 mV, corresponding to the greater stability at higher pH (Figure 6). In the presence of Ca the ζ potential was much less negative with values between −10 and −15 mV. Again, the influence of HA on this parameter was not significant. The ζ potential of the aged particles was less negative and varied less as a function of pH. At higher pH values, the particles in NaCl were more negatively charged than in the CaCl2 matrix. In this matrix, the ζ potential values were mostly around −10 mV and again no influence of HA was observed.

a few nanometers). An enlargement of the nano-TiO2 agglomerate is also shown in Figure 1 (right panel). These particles therefore represent the pristine particles shown in Figure S1, Supporting Information. The aged paint powder was characterized by a particle size distribution in the range between 1 and 10 μm. This powder served as the source of released particles, extracted by gentle shaking in water. The TiO2 concentration in this extract was 580 μg/L, which corresponds to 0.02% of the total TiO2 added to the paint powder. The particle size of the released materials was in the range of 0.1−0.5 μm (Figure 2). Therefore, only a

Figure 2. Size distribution of aged and milled paint powder (solid line) and of the particles in the supernatant of the paint extract (dotted line).

small fraction of the released particles was in the nanorange. Both types of TiO2 particles were also observed in this paint extract: large crystals of pigment TiO2 (Figure 3a) and nanoTiO2 particles (Figure 3b). These particles were the same as the original pristine particles (Supporting Information Figure S1) and so were not distinguishable from the particles observed in the diluted paint before aging. However, the majority of the particles were large paint fragments containing paint components (Ca and TiO2 particles) embedded in a matrix (Figure 3c). The EDX spectra in Figure 3d show the analyses of different areas in Figure 3c, representing the three major components of the paint: Ca from CaCO3, large pigment TiO2 crystals, and small nano-TiO2 agglomerates. These particles were held together by a matrix, most likely the polyacrylate polymer, because the EDX spectra only showed C and O peaks alongside the Ca peak (representing the main component of the paint). While most of the observed particles were present in the form of agglomerates or large paint fragments, a few single nano-TiO2 particles were also observed in the leachate (Figure 4). Figure 4d shows EDX spectra from two different areas in Figure 4a. According to the EDX results, these areas were identified as nano-TiO2 (point 2) and the organic matrix (point 1). At point 1, the EDX analysis only revealed peaks for C, O, and Si. The Si likely originates from the silicon defoamer (Table S1, Supporting Information). In some cases the nanoTiO2 was present as single dispersed particles with sizes between 20 and 50 nm (Figure 4a,b). Other nano-TiO2 particles of the same size were also still embedded in an organic matrix (Figure 4c). The number of the single dispersed nanoparticles was relatively small, and the majority of the nanoTiO2 particles on the grid were present as part of larger paint fragments. Behavior of the Released Particles in Water of Different Composition. The stable suspension obtained

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DISCUSSION The importance of studying the fate and effects of nanomaterials which have been actually released from products has been emphasized in recent years.3,5,24 However, if the release studies are performed within a real product matrix, the aged and released nanomaterials are no longer available in pure form but are found together with parts of the matrix they were incorporated in. The percentage of the ENM within the released material is therefore only small; the major fractions are matrix particles. This problem is not restricted to paints but to most situations where nanoparticles are contained in a matrix, e.g., TiO2 in sunscreens26 or textiles,17 carbon nanotubes (CNTs) in polymers33 or nano-Ag in textiles.15 A separation of 6713



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Figure 3. TiO2 particles observed in the extract of aged paint: (a) pigment TiO2 with an agglomerate of nano-TiO2 attached on the right side; (b) magnified portion of panel a designated by the red square; (c) paint fragment with Ca and pigment TiO2 and nano-TiO2 (red circles refer to spots where EDX spectra were taken); (d) EDX spectra of the areas marked in panel c.

allowed us to collect a suspension with about 500 μg/L of released Ti, a concentration that is sufficient for testing the fate of the material. Because characterization of the materials released during the climate chamber experiments was not possible, we cannot verify that the material we worked with represents the actually released materials. However, when reviewing the TEM images from Kaegi et al., (2008) these images suggested that the TiO2 particles detected in runoff from conventional paint closely resemble the particles released in our experiments.10 In both cases, the main aging process was partial degradation of the organic paint matrix by UV light and release of fragments, especially in the presence of photocatalytic TiO2.34,35 We have to emphasize that the milled paint only serves as a source of released particles, and it was not the material that was studied. The gentle extraction method we used (shaking in water) was able to generate a dilute suspension that we propose is mimicking the released particles under natural conditions. The concentration of particles used in our experiments is much lower than those used in most agglomeration studies of

the ENM of interest from the rest of the particles is not possible. In most cases the released materials are present within a large volume of leachate, e.g., washing liquid or drainage water. We therefore not only have the problem that the ENMs are mixed with (and sometimes indistinguishable from) other particles but also that the concentrations are very low. This is also true for the paint we have used in this work: we have previously shown that during accelerated weathering in climate chambers or in leaching tests only a very low concentration of TiO2 is released (just a few micrograms per liter).20 With these low concentrations, which are close to the natural background in the water used for leaching, no further fate experiments could be performed. In this work we have therefore used an artificially aged paint that was crushed and milled in order to obtain sufficient exposed surface area. The artificial aging of milled paint under similar standard weathering conditions compared to the climate chamber experiments presented by Al-Kattan et al.20 provided a suitable aged paint powder from which particles could be released under gentle conditions of stirring. This method 6714



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Figure 4. Nano-TiO2 particles observed in extract of aged paint. (a and b) single nano-TiO2 agglomerates (red circles represent areas where EDX spectra were taken); (c) nano-TiO2 agglomerate embedded in the paint matrix; (d) EDX spectra from the marked areas in panel a.

a paint with the same composition but with only pigment TiO2 released much less TiO2, indicating that the majority of the released TiO2 is actually nano-TiO2, or at least the release is related to the photocatalytic activity of nano-TiO2 that results in weakening of the organic paint matrix.20 The stability experiments in water of different ionic composition and the characterization of the stable particle suspension showed that indeed the pristine nano-TiO2 and the released TiO2 behaved very differently. The nano-TiO2 showed the expected behavior of TiO2 agglomeration and settling in the presence of monovalent and divalent cations.22,23,39−41 With increasing pH above the isoelectric point when the ζ potential became more negative, the particle size decreased and the number of particles stabilized in suspension increased. The addition of 3 mM Ca reduced the ζ potential values to around −10 mV, where significant particle agglomeration can occur, greatly increasing particle size and thus reducing stability to almost zero. Contrary to other studies,42,43 the addition of humic acid did not greatly affect the size, surface charge, and/or stability of the released particles.43 However, von der Kammer et al. (2010) have already shown that different types of nano-

pristine ENMs and therefore environmentally more relevant. For example, one of the most comprehensive studies of nanoTiO2 stability in water used a concentration of 25 mg/L TiO2.22 Only a few studies have worked with concentrations that could be expected in natural waters or wastewater,36 e.g., Brunelli et al. (2013) who used concentrations as low as 10 μg/L.37 This study showed that at lower concentrations the particles were much more stable than at higher concentrations, which is to be expected based on agglomeration kinetics. The characterization of the released materials showed the type of particles observed in most release studies: the majority of the released ENM is contained within pieces of the matrix, and only few free nanoparticles are observed.7,38 In the case of the paint matrix these fragments contained both Ca and an organic matrix, characterized by the absence of EDX signals of heavy elements. Only a Si signal was detected in almost all paint fragments, but this most probably comes from siliconecontaining defoamer (the total SiO2 concentration was 6.6% in the aged paint). It is therefore clear that the fate of the released TiO2 in natural systems after release will be very different from that of pristine nano-TiO2. The released TiO2 in 6715



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Figure 5. Percent of TiO2 remaining in the 0.45 and 0.1 μm filtrate when the composition of the stable suspension from aged paint A1 was changed. The pH was varied between 4 and 9, and the composition was varied by adding 10 mM NaCl, 2 mM CaCl2, or 10 mg/L humic acid. The total TiO2 was 580 μg/L for A1 and 480 μg/L for nano-TiO2.

to make predictions of the fate of the released TiO2 particles under natural conditions. However, there is almost no published research on the fate of released materials, and the investigations on the characterization of these materials are just now beginning. For some materials, the released particles/ materials will be transformed into other forms, e.g., metallic nano-Ag into AgCl during washing;15 for others such as in our case there is no transformation but release of composite materials where the behavior is determined by the matrix and not by the enclosed ENM. In both cases it is important to first characterize the released particles and then use this material to further study the fate. Most release studies have observed the matrix-enclosed ENM, 7 and in all of these cases it is the matrix that determined the further fate of the ENM. Of equal importance are the implications of this work for effect studies. Studying the toxicity of pristine nano-TiO2 will be of only limited relevance for predicting the effects that the actually released materials may have on organisms. It is therefore very important that more studies are performed where the effects of pristine and aged/released materials are compared. Only a few studies with sunscreen TiO227−29 and CeO245 have been performed with aged materials so far. Our work has only targeted the short-term behavior of release and stability. The long-term fate and effects of the released TiO2 are governed by the fate of the matrix, which is in our case the styrene-acrylic copolymer binder. If this material is biologically, chemically, or photochemically degraded, then free nano-TiO2 could be finally released. The added nano-TiO2 is photochemically active, and under illuminated conditions further degradation of the paint matrix is likely, resulting in

TiO2 behave differently and their results with the same material that we have used also showed only a limited influence of NOM on surface charge and stability in suspension at much higher TiO2 concentration.39 The particle size of the released TiO2 changed neither as a function of pH nor in the different media, resulting in only small changes in stability of the materials in water. This clearly shows that the behavior of the released TiO2 was no longer determined by the properties of the TiO2 but by components that do not readily interact with cations, especially Ca, and that do not show significant pH-dependent properties. The most likely candidate for this component is the styrene-acrylic copolymer binder that forms the main part of the paint matrix. The TEM analysis of the extract revealed that most TiO2 particles were contained within small paint fragments or embedded in an organic matrix (identified by the absence of element signals other than C and O). The surface of these particles exposed to the medium is thus completely different from the surface of the pristine TiO2, which is governed by the acid−base and surface complexation reactions on the oxide surface. Although the particles have less negative surface charge than the TiO2, they do not agglomerate under the conditions of the experiments to larger particles that are removed by the filtration or that settle. Styrene-acrylic copolymer nanoparticles in water are quite stable; a critical coagulation concentration of 450 mM CaCl2 has been reported,44 much above the 3 mM that was used in our work. The results from this work have important implications for the assessment of the fate and the effects of ENM in the environment. The pristine nano-TiO2 particles cannot be used 6716



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Figure 6. DLS particle size (top) and ζ potential (bottom) of pristine and aged TiO2 in different background media.

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the release of the ENM. It is well-known that the organic paint matrix can be degraded in the presence of photocatalytic TiO2.34,35 During this process the organic binder is removed from the surface of the paint, exposing more of the photoactive particles on the surface of the paint, ultimately resulting in release of the ENM into suspension.

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(1) Klaine, S. J.; Koelmans, A. A.; Horne, N.; Carley, S.; Handy, R. D.; Kapustka, L.; Nowack, B.; von der Kammer, F. Paradigms to assess the environmental impact of manufactured nanomaterials. Environ. Toxicol. Chem. 2012, 31 (1), 3−14. (2) Hristozov, D. R.; Gottardo, S.; Critto, A.; Marcomini, A. Risk assessment of engineered nanomaterials: A review of available data and approaches from a regulatory perspective. Nanotoxicology 2012, 6 (8), 880−898. (3) Nowack, B.; Ranville, J. F.; Diamond, S.; Gallego-Urrea, J. A.; Metcalfe, C.; Rose, J.; Horne, N.; Koelmans, A. A.; Klaine, S. J. Potential scenarios for nanomaterial release and subsequent alteration in the environment. Environ. Toxicol. Chem. 2012, 31 (1), 50−59. (4) Krug, H. F.; Wick, P. Nanotoxicology: An interdisciplinary challenge. Angew. Chem., Int. Ed. 2011, 50 (6), 1260−1278. (5) Lowry, G. V.; Gregory, K. B.; Apte, S. C.; Lead, J. R. Transformations of nanomaterials in the environment. Environ. Sci. Technol. 2012, 46 (13), 6893−6899. (6) Farre, M.; Sanchis, J.; Barcelo, D. Analysis and assessment of the occurrence, the fate and the behavior of nanomaterials in the environment. TrAC, Trends Anal. Chem. 2011, 30 (3), 517−527. (7) Gottschalk, F.; Nowack, B. Release of engineered nanomaterials to the environment. J. Environ. Monit. 2011, 13, 1145−1155. (8) Kaegi, R.; Sinnet, B.; Zuleeg, S.; Hagendorfer, H.; Mueller, E.; Vonbank, R.; Boller, M.; Burkhardt, M. Release of silver nanoparticles from outdoor facades. Environ. Pollut. 2010, 158 (9), 2900−2905. (9) Künniger, T.; Gerecke, A. C.; Ulrich, A.; Huch, A.; Vonbank, R.; Heeb, M.; Wichser, A.; Haag, R.; Kunz, P.; Faller, M. Release and environmental impact of silver nanoparticles and conventional organic biocides from coated wooden façades. Environ. Pollut. 2014, 184, 464− 471.

ASSOCIATED CONTENT

S Supporting Information *

Figures showing TEM images of pristine pigment TiO2 and nano-TiO2 and size distribution of particles in the supernatant and tables listing paint composition details, XRF analysis of the paint powder, and composition of the supernantant. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: +41 (0)58 765 76 92; e mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was founded by the European Commission within the Seventh Framework Program (FP7; NanoHouse Project, Grant Agreement No. 247810). We thank Aron Christofel and Renato Figi for help with the analyses and Denise Mitrano for proof-reading the English. 6717



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