Characterization of Food-Grade Titanium Dioxide: Presence of Nano-Sized Particles

Characterization of Food-Grade Titanium Dioxide: The Presenceof Nanosized Particles

Yu Yang,*,† Kyle Doudrick,*,†,¶ Xiangyu Bi,† Kiril Hristovski,‡ Pierre Herckes,§ Paul Westerhoff,† and Ralf Kaegi∥

†School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, Arizona 85287-5306, United States¶Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Indiana 46556, United States‡College of Technology and Innovation, Arizona State University, Polytechnic Campus, Mesa, Arizona 85212, United States§Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, United States∥EAWAG, Swiss Federal Institute of Aquatic Science and Technology, Ueberlandstrasse 133, CH-8600 Duebendorf, Switzerland

*S Supporting Information

ABSTRACT: Titanium dioxide (TiO2) is widely used in food products, which will eventually enter wastewater treatment plantsand terrestrial or aquatic environments, yet little is known about the fraction of this TiO2 that is nanoscale, or the physical andchemical properties of TiO2 that influence its human and environmental fate or toxicity. Instead of analyzing TiO2 properties incomplex food or environmental samples, we procured samples of food-grade TiO2 obtained from global food suppliers and then,using spectroscopic and other analytical techniques, quantified several parameters (elemental composition, crystal structure, size,and surface composition) that are reported to influence environmental fate and toxicity. Another sample of nano-TiO2 thatis generally sold for catalytic applications (P25) and widely used in toxicity studies, was analyzed for comparison. Food-gradeand P25 TiO2 are engineered products, frequently synthesized from purified titanium precursors, and not milled from bulkscale minerals. Nanosized materials were present in all of the food-grade TiO2 samples, and transmission electron microscopyshowed that samples 1−5 contained 35, 23, 21, 17, and 19% of nanosized primary particles (<100 nm in diameter) by number,respectively (all primary P25 particles were <100 nm in diameter). Both types of TiO2 aggregated in water with an averagehydrodynamic diameter of >100 nm. Food-grade samples contained phosphorus (P), with concentrations ranging from 0.5 to1.8 mg of P/g of TiO2. The phosphorus content of P25 was below inductively coupled plasma mass spectrometry detectionlimits. Presumably because of a P-based coating detected by X-ray photoelectron spectroscopy, the ζ potential of the food-gradeTiO2 suspension in deionized water ranged from −10 to −45 mV around pH 7, and the iso-electric point for food-grade TiO2

(<pH 4) was significantly lower than that for P25. The presence of other elements in or on the TiO2 (Si content of 0.026−0.062% and Al content of 0.0006−0.810%) was also different from the case for P25 and would influence the environmental fateof TiO2. X-ray diffraction analysis confirmed the presence of anatase and/or rutile in the food-grade materials, and although thepresence of amorphous TiO2 could not be ruled out, it is unlikely on the basis of Raman analysis. The food-grade TiO2 was solarphotoactive. Cationic dyes adsorbed more readily to food-grade TiO2 than P25, indicating very different potentials for interactionwith organics in the environment. This research shows that food-grade TiO2 contains engineered nanomaterials with propertiesquite different from those of P25, which has previously been used in many ecotoxicity studies, and because food-grade TiO2 ismore likely than P25 to enter the environment (i.e., potentially higher exposure levels), there is a need to design environmental(and human) fate and toxicity studies comparing food-grade to catalytic TiO2.

Received: January 25, 2014Revised: April 10, 2014Accepted: April 22, 2014Published: April 22, 2014

Article

pubs.acs.org/est

© 2014 American Chemical Society 6391 dx.doi.org/10.1021/es500436x | Environ. Sci. Technol. 2014, 48, 6391−6400

■ INTRODUCTION

Titanium dioxide (TiO2), widely used as a white pigment inpaints (including lacquers and varnishes), paper, plastics, andfood,1,2 is also commonly used in catalytic applications.3 Thelevel of TiO2 applied in food and personal products varies indifferent countries. In the United States, TiO2 can be used asa color additive up to a maximal concentration of 1%.4 TheEuropean Union permits the use of TiO2 (E171) in foodsgenerally in quanum satis levels (i.e., as much as needed, but notmore). Currently, the titanium content in food and personalcare products ranges from <0.0001 to 0.014 μg of Ti/mg, but insunscreens, the levels may be as high as 10% by mass.5 Use of∼38000 tons/year of nano-TiO2 has been projected,6 althoughthe current annual consumption of nanosized TiO2 wasestimated to be 2780 tons in the United States,7 of which aportion is used in foods and personal products. Screening ofcommon food and personal care products has demonstratedthe presence of nanosized TiO2, which comprised up to 36%of total particles in a food-grade TiO2.

5Nano- and macrosizedTiO2 were identified in foods and treated municipalwastewater biosolids.5,8 The use and end-of-life disposal offood-grade TiO2 particles can lead to human exposure andentrance into the environment7,9−13and have led to growingconcern about its potential risk in terms of the environmentand human health.Because TiO2 is used in thousands of food products, testing

each product for the presence of nanosized TiO2 andcharacterization of their properties would be overly time-consuming. Fewer TiO2 powders used as food additives areavailable from global suppliers, and a representative portion canbe obtained for characterization work. As the impact of food-grade TiO2 on humans and the environment depends on its sizerange, element purity, surface property, and lattice structure,14,15

comprehensive characterization is initially needed to fully assessthe potential impact of food-grade TiO2. Advanced materialscharacterization of food containing TiO2, within these complexmaterials, will eventually be needed to validate findings workingwith bulk food-grade TiO2.The surface properties and/or coatings of TiO2 powders affect

the stability, mobility, and photoactivities of these particles.Surface coatings of TiO2 powders such as silica, alumina, andorganic compounds can change their surface charge (i.e.,ζ potential) and thereby influence their stability in the dispersionmedium.16,17 The presence of phosphate on a TiO2 surface isexpected as well, which is generally used to control the particlesize during the calcination process of TiO2 manufacturing.18

During this so-called “sulfate process” for the production ofTiO2, metatianic acid is calcinated to microcrystals in the pres-ence of phosphate, alkali ions, and/or rutile nuclei. Therefore, itis necessary to quantify the presence of phosphate and otherpotential elements, which can contribute to the stability andreactivity of food-grade TiO2 powders.Information about the physicochemical properties of nano-

sized TiO2 found in foods is limited even though the number offood and personal care products that include nanosized TiO2 israpidly increasing.19 Such information is critical because itwidens the discussion about nanoparticles and could clearly showthat a traditional, well-established material (i.e., TiO2) that isconsidered safe for human consumption may actually containengineered nanoparticles, and thus, human exposure to nano-particles is occurring. Furthermore, use of nanoscale TiO2 infoods would lead to human exposure, and through disposal offood and excretion in feces, TiO2 in foods will be present in

sewage, which enters wastewater treatment plants, and biosolidsthat are land applied, incinerated, or landfilled or treated liquideffluent that enters surface waters. It is extremely difficult toisolate and characterize by advanced analytical techniques TiO2

in complex food or environmental matrices, often at very lowconcentrations, despite extensive literature stating the impor-tance of nanomaterial size, crystal structure, surface properties, ζpotential, and other parameters. Therefore, an approach isneeded to characterize properties of actual nanoparticles used infood products, as opposed to only reference nanoparticles, toinform their environmental fate and toxicity.The objective of this study is to characterize TiO2 that is used

in bulk food additives so that we can better inform the design oftoxicity tests for environmental and human health applications.Many studies have examined the cytotoxicity of TiO2 based onphotocatalytic TiO2 materials (e.g., P25) rather than food-gradeTiO2,

20,21 although these photocatalysts are used globally in amuch smaller quantity than food-grade TiO2. In this study, fivetypes of food-grade TiO2 and P25 were characterized by variousanalyses to provide information about their chemical composi-tion [inductively coupled plasma mass spectrometry (ICP-MS)],surface functionality [X-ray photoelectron spectroscopy (XPS)],morphology [transmission electron microscopy (TEM)], crystalstructure [X-ray diffraction (XRD)], size and surface charge inwater (ZetaPALS), and photoactivity (diffuse reflectance; solaractivity toward organic dyes). Results presented here are usefulfor determining the potential impacts of food-grade TiO2 onhuman health and the environment.

■ MATERIALS AND METHODS

Sources of TiO2. Five samples of food-grade TiO2 wereselected because they were from different vendors, identifiedby the vendor as food-grade (i.e., E171), and were present aspowders (samples 1, 2, 4, and 5) or a paste (sample 3). Samples1−4 were purchased from Chinese vendors that ship to theUnited States and/or Europe. Sample 5 was obtained froma large commercial supplier in Italy (FiorioColori Spa) andpartially characterized elsewhere.5 E171 is a European Uniondesignation for a white food color additive (listed in Annex I ofDirective 94/36/EEC),22 and other designations includeCI77891 and Pigment White 6. For XPS, XRD, and photo-catalytic activity tests on sample 3, the paste was heated at 400 °Cfor 2 h to remove carbon constituents and isolate the TiO2. Thiswas the maximal temperature used to prevent the anatase crystalstructure (per XRD results given below) of sample 3 fromchanging to rutile. Unfortunately, although this treatment isrequired for XPS analysis of sample 3, this process will alsoremove any carbon and oxygen functionalities that may bepresent on the surface. P25 (Evonik, formerly Degussa, >99.5%)was obtained in powder form and used as received.

Electron Microscopy Analysis. A transmission electronmicroscope (TEM, HD-2700 Cs, Hitachi) operated at 200 kVcoupled to an energy dispersive X-ray analysis system (EDX,EDAX) was used to characterize the particle size. Approximately0.02 g of each sample was suspended in 5 mL of deionized water.After sonication in a water bath for 30 min, 1 mL of slurry wasdiluted with 25 mL of methanol. Finally, one or two drops of thissuspension were dripped on the surface of a copper grid (carbon-coated, Plano GmbH) for subsequent electron microscopy work.The imaging of P25 was conducted on another transmissionelectron microscope ((S)TEM Phillips CM-200), and sampleswere prepared on copper grids with carbon coating (TedPella, Inc.).23 The TiO2 particle sizes were based upon counting

Environmental Science & Technology Article

dx.doi.org/10.1021/es500436x | Environ. Sci. Technol. 2014, 48, 6391−64006392

at least 100 primary single particles (not aggregates) andanalyzed using ImageJ. For each sample, the particles wereselected randomly from 10 TEM images. The TiO2 powderswere well mixed, and random sampling for TEM analysis wasconducted to ensure that a representative sample was imaged,though the detection of the nanoparticle by TEM with a highmagnification limited the number analyzed.24 For an unknownsize sample, sampling of 100 particles has a confidence level of95% with confidence interval of 10% to represent the sourceparticle sizes, with an assumption that the size distribution ofsource particles follows a normal distribution.Spectroscopic Analysis of the TiO2 Structure.The crystal

structure of TiO2 powders was determined by XRD with aCu Kα source (Siemens D5000) and an aluminum holder. Eachsample was scanned at 2θ values from 20° to 60° to detectthe characteristic TiO2 peaks. Semiquantitative analysis of thecrystalline phases (e.g., the ratio of anatase to rutile) wascalculated for appropriate samples using PANalytical HighScorePlus software (based on the Rietveld method). Raman spec-troscopy was performed using a 532 nm laser on a custominstrument (Leroy Center for Solid State Science at ArizonaState University) with a maximal power of 100 mW, a spatialresolution of <1 μm, and a spectral resolution of ∼1 cm−1.Multiple TiO2 particles (or particle aggregates) were analyzed toobtain crystal structure information representative of the entiresample.XPS Analysis of TiO2 Powders. The surface elemental

composition and chemical state were analyzed using XPSperformed on an ESCALAB 220i-XL instrument (VacuumGenerators) with a monochromatic Al Kα source at hν =1486 eV, a base pressure of 7 × 10−10 mbar, and a spot analysissize of 500 μm. Samples were prepared by pressing the powderinto a disk on clean indium foil. Peak fitting was performedmanually using Casa XPS based on theoretical atomicpercentages calculated from the wide scan. The estimateddepth of analysis is approximately 2.87 nm as determined fromthe inelastic mean free path of electron scattering for TiO2.

26

Analysis of Phosphorus, Aluminum, and SiliconContents in TiO2 Powders. The water content of TiO2

powders was determined by drying the sample at 105 °C for24 h, and the mass loss after drying was assumed to be fromwater evaporation.27 The dissolved organic content (DOC) inthe filtrate was analyzed using a total organic carbon (TOC)analyzer (Shimadzu TOC-V). To determine the total amount ofphosphorus in food-grade TiO2 powders, approximately 0.03 gof each sample was digested in HNO3 and HF [4:1 (v/v)]using microwave digestion, according to Standard Method3030G.27Element concentrations of Ti, P, Si, Al, and other elementswere measured by ICP-MS (X-Series 2, Thermo Scientific).To determine the amount of soluble phosphorus, 0.2 g of

each sample was stirred in 200 mL of ultrapure water. After beingcontinuously mixed for 40 h, samples were centrifuged for 5 minat 3000 rpm (F = 978 g), the supernatant was passed through a0.2 μm membrane filter (Polypropylene, syringe filter, ThermoScientific), and an aliquot of 2 mL of filtrate was analyzed fororthophosphate P concentration, the sum of inorganicphosphate ions, using ion chromatography (IC). Because somenano-TiO2 may pass through the 0.2 μm membrane filter, addi-tional separations were made using centrifugal filters (AmiconUltra, cellulose, 30000 NMWL) operated at 5000×g for 30 min,which we have previously shown to remove nanoparticles.28

Phase Analysis Light Scattering and ζ PotentialMeasurement. Samples were prepared for phase analysis

light scattering (ZetaPALS, Brookhaven Instruments Corp.) todetermine the hydrodynamic diameters of particles bysuspending 0.02 g of each TiO2 sample in 1000 mL of nanopurewater and bath sonicating for 5 min. Eventually, a 5-fold dilutionof each suspension was used for light scattering analysis.Additionally, 0.03 g of each sample powder was introducedinto 50 mL of a 10 mM potassium nitrate solution (Sigma-Aldrich) for ζ potential measurement. The electrode wasstabilized in 10 mM potassium nitrate at room temperature for24 h. The electrolyte suspension of TiO2 powder was sonicatedfor 30 min prior to ζ potential measurements.

Ultraviolet−Visible Light Diffuse Reflectance (UV−visDR) Measurements. UV−vis DR spectra were obtained on aLambda 18 instrument (Perkin-Elmer) with a 150 mmintegrating sphere accessory. Powder samples were pressedinto disks, analyzed for reflectance, and then converted toabsorbance.

Photocatalytic Activity Measurement. The presence orabsence of photocatalytic activity of the samples was measuredusing a simulated solar reactor consisting of a 300 W xenon arclamp (Spectraphysics Oriel, 91160). The spectrum was filteredthrough a standard air mass filter (AM 1.5 Global) to simulatesunlight irradiance at ground level with the sun at a zenith angleof 48.2°. Actinometric measurements using 2-nitrobenzaldehydeyielded a flux of (6.7 ± 1.0) × 1015 photons cm−2 s−1. Acid green25 (C28H20N2Na2O8S2, Sigma-Aldrich catalog no. 214566), ananionic dye, was used as a probe compound to test thephotocatalytic activity of the TiO2 powders. The concentrationwas measured using UV−vis (Hach DR 5000) at an absorptionpeak of 625 nm. TiO2 (0.1 mg/L) and an acid green solution(60 mg/L, 96.4 μM) were added to a 160 mL double-walledreactor andmixed for 1 h in the dark prior to being tested.Minimaladsorption was observed for all samples (<5%); samples were notrun in replicate as only the presence or absence of photoactivitywas sought. Samples were tested for 1 h under simulated solarconditions. Samples were mixed using a magnetic stir bar, and thereactor temperature was maintained at 25 °C throughout the test.

■ RESULTS AND DISCUSSION

Size Distribution Determined by TEM and PALS.Number-based particle size distributions were derived fromTEM images (Figure 1). The diameters of samples 1−5 were106 ± 38, 122 ± 48, 132 ± 56, 124 ± 42, and 117 ± 41 nm,respectively (average ± one standard deviation). Samples 1−5contained 35, 23, 21, 17, and 19% nanosized particles,respectively, based on the size distributions with a confidencelevel of 95% (Figure 2). The average primary particle size of P25was 23 ± 10 nm as taken from TEM images, so 100% of theprimary TiO2 particles are <100 nm in diameter.The number-based distribution of the hydrodynamic diameter

for TiO2 particles suspended in ultrapure water was determinedby DLS and analyzed using built-in software assuming a multi-modal size distribution of particles. Upon addition to water, themean hydrodynamic sizes of samples 1−5 were 273, 405, 209,504, and 127 nm, respectively. The hydrodynamic diameterdistributions of samples 1−4 showed that all particle sizeswere larger than 100 nm, whereas only 29% of particles insample 5 were smaller than 100 nm. Following the same samplepreparation for P25 that was used for the food-grade samples, themean hydrodynamic mean diameter was observed to be 210 ±

62 nm, which is significantly larger than the primary TiO2 particlesizemeasured byTEM(23± 10 nm). Thismean hydrodynamic sizeis similar to those reported elsewhere, where effects of sonication



protocols and the duration of the analysis influenced the size ofaggregates produced upon addition of P25 to water.35 WhereasTEM quantifies the primary particle size distribution, thesignificantly larger sizes measured by phase analysis light scattering(PALS) indicates that some of the primary TiO2 particles aggregatein ultrapure water.5 In prior work, we observed that P25 aggregatedmore rapidly and into larger aggregates than food-grade TiO2.

5

Crystal Structure. XRD spectra for samples 1−5 are shownin Figure 3. Sample 4 contained both anatase and rutile, but onlyanatase was detected in the other four samples. The main anatasereflection for all samples is the (101) reflection at 2θ = 25.27°,and the main rutile reflection for sample 4 is the (110) reflectionat 2θ = 27.41°. The ratio of anatase to rutile in sample 4 wasdetermined to be 22:78. Sample 4 is similar to P25 in that it is amixed phase TiO2;

23 however, anatase is the dominant phase inP25, while rutile is the dominant phase in sample 4.Raman spectroscopy was used to determine the presence of

amorphous TiO2 and to complement the results of XRD(Figure SI6 of the Supporting Information). As seen via XRD, forsamples 1−3 and 5, only the anatase crystal phase was observed,and for sample 4, a mixture of anatase and rutile was observed.No amorphous TiO2 was found. Using Raman spectroscopy,individual particles or particle aggregates can be analyzed. Thisfeature was used to determine if sample 4 was simply a mixture ofanatase and rutile or if the two crystal phases existed as acomposite (spectra not shown). Both rutile and anatase werepresent as separate particle clusters, and the presence of rutile

and anatase particles clustered together was also observed butwas less common.

Chemical Composition. The concentrations of Ti, P, Al,and Si in food-grade TiO2 powders were derived from ICP-MSmeasurements. The TiO2 contents, normalized to the dry massof powder (calculated from the mass of Ti), for samples 1−5were (102 ± 0.88)%, (100 ± 4.90)%, (49 ± 2.14)%, (94 ±

14.35)%, and (106 ± 2.07)%, respectively (Table S1 of theSupporting Information), which indicated that all samplescontained mainly TiO2 except sample 3 with 51% non-TiO2

components. Sample 3, the pastelike slurry, contained 21%organic carbon and 12% water, which was challenging to removecompletely. The other samples had organic carbon concen-trations in the range of 3.6−5.4 mg/g of dry powder and watercontents of <1%. The concentration of Al was <0.05% in allsamples except sample 4, which had an Al content of 0.81%. TheSi content of samples 1−5 ranged from 0.026 to 0.062%. Otherelements were detected in acid-digested food-grade TiO2 atlevels of >0.01 mg/g of powder (K, Cu, Nb, V, Mg, Ca, Fe, andNa), while others were generally detectable at lower concen-trations (Ag, Pb, Sn, As, Cr, Mo, Ba, Au, Ni, and Cd) (seeFigure SI1 and Table SI1 of the Supporting Information forconcentrations).Figure 4 presents data on the release of phosphorus (P) into

water. The food-grade samples had P concentrations rangingfrom 0.5 to 1.8 mg of P/g of TiO2, while no phosphorus was

Figure 2. Size distributions of food-grade titanium dioxide particles andP25 derived from TEM images with ImageJ.

Figure 1. Bright-field TEM images of food-grade and P25 titaniumdioxide.



detected in solutions prepared using P25. Figure 4a shows thereleased P concentrations (measured by ICP-MS) in filtrateswith 0.2 μm filters. Phosphorus concentrations for samples 1−5were 0.7, 0.7, 0.3, 0.1, and 0.7 mg of P/g of dry TiO2, respectively,which correspond to 52, 51, 61, 4, and 45%, respectively, ofthe total phosphorus content. In these 0.2 μm membranefiltrates, between 30 and 70% of the P content was inorganicphosphate ion, except in sample 4, which contained nomeasurable phosphate ion P. The presence of phosphate ionindicates that inorganic P, rather than organic P that is present infood-grade surfactants, was attached to the TiO2 surface.On the basis of the turbidity measured by optical density at

700 nm and Ti measured by ICP-MS, some nanosized TiO2

particles clearly passed through the 0.2 μm filters (see Figure SI2and Tables SI2 and SI3 of the Supporting Information). Samples1−3 and 5 allowed approximately 0.8−2% of the TiO2 to passthrough the 0.2 μm filter, while no TiO2 particles were observedin the filtrates with sample 4 or P25. To determine the P con-centration not associated with TiO2 particles, 30 kDa centrifugalultrafilters were used instead of 0.2 μm filters. These ultrafiltersremoved the turbidity and decreased the Ti levels to belowdetection levels, yet inorganic phosphate ions were still detected,confirming the release of phosphate from the TiO2 surface intowater. Overall, these results show obvious chemical compositiondifferences between the food-grade TiO2 samples and P25.Surface Analysis. XPS was used to determine the chemical

composition of the surface and to determine the binding statesof the compounds. For samples 1−5, the O 1s spectra (532 eV)have shifted negatively, indicating the bonding of oxygen on thesurface. Closer inspection of the O 1s spectra (Figure 5) revealedthe presence of TiO2 (530.1 eV) and a phosphate compound(531.4 eV), as indicated by the binding energy of a PO bondat 531.3 eV.42 The phosphate was presumably added duringmanufacturing as a potassium salt, as indicated in the wide scanelemental analysis (Figure SI3 of the Supporting Information).The K 2p spectra (not shown) reveal that the K 2p3/2 line(294 eV) has shifted negatively to 293 eV, which is in agreementwith potassium phosphate (e.g., K3PO4 and K4P5O7) binding

energies.43 For sample 4, the O 1s spectra revealed numerouspeaks attributed to carbonyl groups (534.3 eV), Al2O3 (532.9 eV),SiO2 (532.0 eV), phosphate (531.3 eV), and TiO2 (530.1 eV).Inspection of the Al 2p and Si 2p spectra (not shown) confirmedthat Al and Si are in the Al2O3 and SiO2 states, respectively. In theTi 2p spectra, TiO2 was the only compound identified for allsamples except for sample 4, which had two peaks (Figure SI4 ofthe Supporting Information). The peaks are attributed to anatase(459 eV) and rutile (458.5 eV), and their peak area percentages of22 and 78%, respectively, are in good agreement with the XRDresults (i.e., 22% anatase and 78% rutile).44

XPS results verified the presence of phosphorus on the surfaceof particles in the range of 2.0−3.8% of total surface elements bymass, which was approximately 10 times higher than the averagephosphorus content of 0.049−0.16% determined by ICP-MS.XPS measures surface-bound phosphorus; thus, the enrich-ment of phosphorus on the surface indicated by XPS could beattributed to a phosphate coating, as confirmed by solublephosphate analysis (Figure 5). This phosphate coating results ina more negative surface charge on the food-grade TiO2 samples,and thus, it increases their stability in solution and during thepreparation and consumption of foods.

ζ Potential. Figure 6a shows the ζ potential measurements ofsamples 1−5 and P25. The isoelectric points (IEPs) of samples 1,3, and 4 were approximately 3.2, 3.5, and 4.0, respectively,whereas those of samples 2 and 5 were <2.5. P25 has an IEP of6.8, which is much higher than the IEPs of the other samples.Whereas P25 exhibits the typical pH dependence of the ζ

potential of metal oxides, the food-grade samples exhibit verylittle change in ζ potential as a function of pH, at least abovepH 4. This behavior is more typical of metal oxides with adsorbedanions such as phosphate P.To examine the impact of inorganic phosphate on ζ potential,

P25 was mixed with ortho-P. Phosphate anions in solution can beadsorbed to the particle surface, resulting in a decrease in ζ

potential.45 Phosphate at levels of 3.22 and 200 μg of P/g Ti (themaximal value was determined for sample 4) was added to theP25/TiO2 suspension (Figure 6b). Addition of 3.22 μg of P/g Ti

Figure 3. X-ray diffraction pattern of food-grade TiO2. Sample 4 is mixed phase, and the anatase (A) and rutile (R) phases are labeled accordingly.



led to an IEP similar to that of P25 alone, but it also made the ζpotential less pH-dependent below the IEP. Increasing the phos-phate loading to 200 μg of P/g Ti decreased the IEP significantly,to approximately 3.2, and the ζ potential stabilized above pH 6.Phosphate adsorbed to TiO2 surfaces can decrease the IEP or ζpotentials at a specific pH level. In a comparison of food-gradeTiO2with phosphorus on the surface versus uncoated TiO2 (e.g.,P25), the differing ζ potential profiles (Figure 6a) presumablyresult from the use of anionic food-grade surfactants that containpolyphosphates.UV−Vis Absorbance. The UV−Vis DR absorbance spectra

(Figure 7) show that the food-grade TiO2 powders mostlyabsorbed light below 400 nm (like P25),23 except sample 4,which also absorbed a small portion of the visible light spectrum.This additional absorption is due to the presence of rutile, whichhas a band gap (3.1 eV) smaller than that of anatase (3.2 eV). Thepresence of alumina and silica on the surface should have noeffect on the UV−Vis absorbance measured, as both are wide-gapinsulators. The food-grade samples had slightly greaterabsorbance than P25 at wavelengths of >300 nm.

Photocatalytic Activity. Photocatalytic testing with samples1−5 and P25 showed all had the potential to photocatalyticallyremove acid green 25 and specifically showed removal of 11, 20,26, 14, 14, and 57%, respectively. P25, which is manufacturedas a photocatalyst, had the greatest activity. All of the food-gradeTiO2 samples were similar, with sample 3 having the highestactivity, which was presumably due to increased crystallinity fromthe thermal method used to isolate the TiO2 from the paste. Evensample 4, which has a coating very similar to that of TiO2 used insunscreen (i.e., Al2O3) and is supposed to suppress oxidation orradical generation, showed photocatalytic activity.Methylene blue, a common model compound and cationic

dye, was also tested for photocatalytic activity. However, after theaddition of food-grade samples to the methylene blue solution,the dye rapidly adsorbed to the TiO2, as seen by a metachromicshift from blue to purple (pH 6.6). This resulted in unreliablephotocatalytic activity results. After the samples had been mixedin the dark for 24 h, the levels of adsorption of methylene blueto samples 1−5 and P25 were 93, 84, 93, 38, 82, and 0%, respec-tively. All the food-grade TiO2 samples had pHIEP values muchlower than that for P25 (Figure 6), and hence, the food-grade

Figure 4. Content and fraction of phosphorus in food-grade TiO2 powders (normalized to the total mass of dried TiO2 powder). The amount ofphosphorus released was determined by adding 0.2 g of TiO2 to 200mL of nanopure water. After being mixed, the contents of released P were measuredby ICP-MS in the filtrates passing through a 0.2 μm filter (a) and a 30K Da filter (b). The contents of phosphate ion P in both filtrates were determinedby IC. The total level of phosphorus in P25 is below the detection limit (0.001 mg/g). Error bars represent one standard deviation of triplicatemeasurements.



TiO2 had more anionic surface groups at the experimental pH(6.6) than P25 (pHIEP 6.8). That of sample 4 was lower thanthose of other samples because it also has alumina and silica onthe TiO2 surface. The important outcome of these experiments isthat the food-grade TiO2 samples all showed some photocatalyticactivity, which could have unknown ecotoxicological implica-tions.Implications for Human Toxicity and Ecotoxicity. Five

types of food-grade TiO2 and a catalyst-grade TiO2 were char-acterized with respect to size distribution, ζ potential, impurities,crystal structure, and photocatalytic activity. TEM analysis con-firmed the presence of nanosized TiO2 particles. XRD analysisshowed that all samples were composed of the anatase crystalphase, except sample 4, which was a mixture of anatase and rutile.Combinatorial XPS and ICP-MS analyses indicated thatphosphorus and, in some cases, Al and Si were present on thesurface of TiO2, which indicates an engineered coating rather

than an impurity. ζ potential measurements revealed that thesesurface moieties provided a net negative surface charge belowneutral pH. The presence of P, Al, Si, or undetected organicmolecules likely added to the increased stability of TiO2 in acolloid suspension.16,17,29 The SiO2 and Al2O3 impurities insamples 1−5 were ≤1.5 and 0.13% of the total dry weight,respectively, lower than the maximal limit of 2% each specified inreports by the U.S. Food and Drug Administration and the JointFAO/WHO Expert Committee on Food Additives.1,4 While allforms of TiO2 aggregated, after the samples had been mixed inwater to assess P release, it was clear from optical density andICP-MSmeasurements of Ti that the food-grade TiO2was stableenough in solution and dispersed enough not to be separated bylow-g separation and pass through a 0.2 μm filter, whereas P25did not pass through the filter. Photocatalytic activity testsrevealed that all food-grade and P25 TiO2 samples exhibitedphotoactivity for oxidation. The characterization of food-grade

Figure 5. XPSO 1s spectra of samples 1−5. The peaks are (1) phosphate, (2) titanium dioxide, (3) silica, (4) carbonyl, and (5) alumina. Table S2 of theSupporting Information lists atomic percentages of surface species for each sample.



TiO2 powders provides important information for assessing theenvironmental and human health impacts resulting from TiO2 infood and consumer products.Nanosized particles were detected in all five TiO2 food-grade

samples and could pose workplace exposure hazards. Researchhas shown that nanosized TiO2 often exhibits more photoactivityand thereby possesses more inhibitory activity than micrometer-sized TiO2 powders.

29 Furthermore, nano-TiO2 led to elevatedlevels of pro-inflammatory cytokines and had more inhibitoryeffects on dendritic cells than micrometer-sized TiO2.

30 Anataseparticles in the 10−20 nm size range can induce oxidative DNAdamage, hydrogen peroxide generation, and nitric oxideproduction in a human bronchial epithelial cell line withoutillumination.31 However, treatment with >200 nm anataseparticles did not lead to any adverse effects under the sameconditions.31

The presence of nano-TiO2 in food could lead to dermal andingestion exposure in humans.32 Because five samples containedat least 17% <100 nm particles (by number) determined by TEM(Figure 2), the use of these food-grade TiO2 powders in foodmay cause direct ingestion exposure of humans to nanosizedTiO2. Currently, limited information regarding the potentialhuman hazards from ingestion of nano-TiO2 exists, althoughinhalation of nano-TiO2 has been proven to be risky tohumans.33,34

Food-grade TiO2 will enter the environment in many ways,and the resulting exposure may pose an ecological hazard. Spillsand direct discharge of bulk food-grade TiO2 to the environmentare unlikely sources of exposure compared to usage by foodmanufacturers or postconsumer disposal. Although someproducts will be added to landfills, TiO2-containing food prod-ucts intended for human consumption will be primarily excretedin feces or disposed “down the drain”. These wastewaterscombine in sewers with industrial wastewater (e.g., from foodprocessors) and flow to municipal WWTPs. Biological processesin WWTPs sequester large amounts of nanoparticles intobacteria,8,11,46,47 which are removed and disposed as biosolids tolandfills or incinerators, or in the United States, they may be landapplied. Land-applied biosolids could release TiO2 into surfacerunoff, which eventually enters streams. A smaller fraction(perhaps 10%) of nanoparticles will leave WWTPs in liquideffluent discharged to streams, rivers, or lakes.48

The bioavailability and toxicity of TiO2 may depend upon thestructure and size of TiO2, and the type of toxicity tests. On onehand, there was no toxicity of either anatase or rutile in thetoxicological database evaluated by the Joint FAO/WHO ExpertCommittee on Food Additives,36 individual reports ofbioavailability on rats,37 or chronic toxicity tests.38 However,different polymorphs of TiO2 may lead to different impacts onmicroorganisms. With an equivalent dosing of mass, anatase was100 times more toxic than rutile to human dermal fibroblasts andlung epithelial cells, and it was more effective in generatingreactive oxygen species (ROS) under illumination.39 Nano-anatase TiO2 exhibits more inhibitory effects than nanorutileTiO2 during in vitro genotoxicity tests on human bronchialepithelial BEAS 2B cells.40 Sample 4, which consisted of 78%rutile particles, may thus be less toxic to human cells than samplescontaining only anatase, although the exact toxicity of TiO2 couldvary according to cell line.Once TiO2 enters surface waters, interaction with sunlight

that generates reactive oxygen species (ROS) may occur, thuscreating a toxic environment for the aquatic ecosystem. Studieshave shown that TiO2 has a negative effect on aquatic organismswhen they are exposed to sunlight.49−52 These studies weregenerally conducted with TiO2 that is not found in food orconsumer products (e.g., P25). Results presented herein showedthat food-grade TiO2 is photoactive, which could be a resultof direct oxidation at the photohole site or ROS production. Forexample, sample 4, which is similar to the TiO2 found insunscreens, will have a much weaker toxic effect than TiO2 used inprevious toxicity studies because of its Al/Si coating, which maysuppress ROS formation.17,53,54 We also showed differenttendencies for organic dyes to sorb to food-grade TiO2 comparedwith those of P25. If food-grade TiO2 represents the majority ofthe TiO2 found in surface waters, fate and toxicity studies similar tothose available for P25 should be repeated using food-grade TiO2.

Figure 6.Average ζ potential of (a) food-gradeTiO2 and P25 and (b) P25at three phosphate concentrations of 0, 3.22, and 200 μg of P/mg of Ti.

Figure 7. Ultraviolet−visible light diffuse reflectance (UV−vis DR)absorbance spectra of food-grade TiO2 samples.



■ ASSOCIATED CONTENT

*S Supporting Information

Three tables and six figures of additional data. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Authors

*School of Sustainable Engineering and the Built Environment,Arizona State University, Tempe, AZ 85287-5306. E-mail:[email protected]. Phone: (480) 727-2913.*E-mail: [email protected].

Author Contributions

Y.Y. and K.D. contributed equally to this work.

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

Funding was provided by the National Science Foundation(CBET 0847710).

■ REFERENCES

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Characterization of Food-Grade Titanium Dioxide: Presence of Nano-Sized Particles

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