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Chemical Engineering Journal 191 (2012) 95– 103

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

j ourna l ho mepage: www.elsev ier .com/ locate /ce j

epletion of the protective aluminum hydroxide coating in TiO2-basedunscreens by swimming pool water ingredients

urate Virkutytea, Souhail R. Al-Abedb,∗, Dionysios D. Dionysiouc

Pegasus Technical Services Inc, 46 East Hollister Street, Cincinnati, OH 45221, USANational Risk Management Research Laboratory, Environmental Protection Agency, 26 West Martin Luther King Drive, Cincinnati, OH 45268, USAEnvironmental Engineering and Science Program, University of Cincinnati, Cincinnati, OH 45221-0012, USA

r t i c l e i n f o

rticle history:eceived 10 January 2012eceived in revised form 23 February 2012ccepted 24 February 2012

eywords:unscreen lotionanosized TiO2

wimming pool waterl(OH)3 layer

a b s t r a c t

In sunscreen lotion (SSL) formulations, titanium dioxide (nTiO2) nanoparticles are coated with an Al(OH)3

layer to shield against the harmful effects of hydroxyl radicals (•OH), superoxide anion radicals (O2−•),

and other reactive oxygen species (ROS) (e.g. H2O2) generated when TiO2 nanoparticles are exposed toUV radiation. Therefore, it is crucial to ensure their structural stability in the environment where theprotective layer may be compromised and adverse health and environmental effects can occur. The mainfocal point of our work was to research the stability of the Al(OH)3 layer in swimming pool water. Thus2 g L−1 of SSL was subjected to treatment with swimming pool water (SPW) containing from 0.2 to 7 ppmchlorine (HOCl/OCl−) concentrations. The changes in the protective coating of TiO2 nanoparticles wereanalyzed using several X-ray based microscopic techniques in addition to Fourier transform infrared

spectroscopy (FTIR) and Zeta potential measurements. Results indicated that an increase in chlorineconcentration in SPW significantly affected the integrity of the Al(OH)3 protective layer and increasedzeta potential from −64 mV to nearly −8 mV, rendering rather unstable TiO2 nanoparticles. The highestredistribution of Al (At%) from ∼4 to as high as 15.6 was achieved when SSL was subjected to 3.5 and7 ppm of chlorine in SPW. Results strongly suggest that water chemistry influences the characteristics ofTiO2 in sunscreen environment.

. Introduction

Titania-based materials are widely used in water treatment, airreatment, clean energy production, and fabrication of self-cleaningurfaces due to their unique photovoltaic and photocatalytic prop-rties [1–3]. Furthermore, in the past few years, transparenticrofine titanium dioxide (TiO2) nanoparticles gained popular-

ty as inorganic sunscreen due to their efficacy and their increasedosmetic appeal compared to their counterparts with larger particleize [4–6].

It is reported that within the United States alone, more than.3 million new cases of squamous cell and basal cell cancer areeported every year, and more than 90% of these cases are causedy overexposure to UV radiation [7]. Therefore, the American Can-er Society recommends the use of a protective means (sunscreens)very two hours as a proactive measure to prevent skin cancer,

urn, and other skin damage caused by the exposure to UV radiationrom sunlight. However, to effectively protect the skin, sunscreens

ust possess several attributes [3] such as (i) photo-stability, (ii)

∗ Corresponding author. Tel.: +1 513 569 7849.E-mail address: al-abed.souhail@epa.gov (S.R. Al-Abed).

385-8947/$ – see front matter. Published by Elsevier B.V.oi:10.1016/j.cej.2012.02.074

Published by Elsevier B.V.

ability to dissipate the absorbed energy through photo-physicaland photochemical pathways, (iii) incapacity to penetrate the skin,and (iv) aptitude to minimize the extent of UVB (280–315 nm) andUVA (315–400 nm) radiation that might reach DNA in cell nuclei.The light absorption properties of two crystalline forms of titaniumdioxide, anatase, and rutile are excellent for application as sun-screens since their absorption onsets at ∼400 nm conveniently fallsbetween the visible and UV regions of the solar spectrum [8]. Fur-thermore, TiO2 protects the skin from UV radiation by scattering,reflecting, and absorbing most of the UVB and UVA radiation fromthe sun and also does not undergo any chemical decomposition [9].

However, titanium dioxide is also an efficient photocatalyst thatis able to produce hydroxyl radicals and superoxide anion radicalsupon ultraband gap photoexcitation in aqueous media [10]. Thus,when TiO2 particles are exposed to UV radiation, highly oxidiz-ing hydroxyl radicals (•OH), superoxide anion radicals (O2

−•), andother reactive oxygen species (ROS) such as H2O2 are generatedon the surface of TiO2 [11,12]. Notably, ROS may be responsiblefor the oxidation of organic components in sunscreens and for the

aging as well as the destruction of skin [8]. The hydroxyl radicalsare shown to induce damage to DNA plasmids in vitro and to wholehuman skin cells in cultures [13]. Hydroxyl radicals can also reactand damage the organic ingredients in the sunscreen. Incomplete

9 gineering Journal 191 (2012) 95– 103

pssD

iipiomaiotgt

A(otclou

2

2

tlsptEt(aathaAcTtb((

2

[hTpthoic

Table 1Experimental conditions – SPW composition with varying ppm of OCl− (pH 6).

Chemicals Quantity

DI water (L) 4CaSO4·2H2O (g) 1.53NaHCO3 (g) 0.6

6 J. Virkutyte et al. / Chemical En

rotection of a TiO2 formulation against UV radiation may causeome degree of TiO2 penetration and free-radical production in thekin, which in turn, may completely destroy supercoiled skin cellNA even in the absence of exposure to UV [14–17].

Therefore, to reduce the possibility of any photocatalytic activ-ty on the living tissues, TiO2 particles must be coated with a thick,nert, and uniform protective layer prior to use in any cosmeticreparations [18,19]. For instance, a layer of aluminum hydrox-

de (Al(OH)3) coating can increase the number of OH groupsn the particle surface providing more active sites for variousodifications and improving dispersability of the particles in the

queous solutions and suspensions [20,21]. Also, such Al(OH)3 layernsulates the TiO2 particles, inhibiting harmful reactions such asxidation and potential decomposition [4,19]. Unfortunately, whenhe surface layer is removed, the photo-active TiO2 are able toenerate photocatalytic reaction, which in turn, may lead to geno-oxicity and/or phototoxicity effects [12,22,23].

Herein we present new findings on the stability of a protectivel(OH)3 coating of SSL when in contact with swimming pool water

SPW). The main goals of the research were: (1) to assess the effectf various chlorine (HOCl/OCl−; pKa = 7.5) concentrations in SPW onhe integrity of redistribution of Al(OH)3 coating; (2) to examine theomposition, chemical state, and chemical bonding of a protectiveayer on the TiO2 surface, and (3) to identify the functional groupsf SSL and recognize chemical changes, which may have occurredpon treatment with SPW constituents.

. Materials and methods

.1. Commercially available sunscreen lotion and nanoparticles

Commercially available sunscreen lotion (Neutrogena, SPF 30)hat consisted of TiO2 (9.1% w/w of Ti) coated with an Al(OH)3ayer was tested in this study. Apart from coated TiO2, it also con-isted of various organic compounds, including potassium cetylhosphate (as surfactant and emulsifying agent), glycerin andrimethylpropane triethylhexanoate (as moisturizers), disodiumDTA (as moisturizer and chelating agent to sequester and decreasehe reducibility of metal ions), calcium pantothenate and panthenalas conditioning agents) and carbomer (as suspension agent) thatre soluble in water, thus may be relatively easy removed or dam-ged. Disodium EDTA is of particular interest, because it is usedo prevent particles from agglomeration due to steric effect, toelp uniform dispersion of the nanoparticles on skin, and to acts a thickening agent. Rutile TiO2 nanoparticles (5–30 nm) withoutl(OH)3 coating layer for zeta potential measurements were pur-hased from Nanostructured and Amorphous Materials (Houston,X, USA). To mimic the main constituents of the SSL, TiO2 nanopar-icles with Al(OH)3 coating (MT 500 SA) were generously donatedy Presperse Inc. (Presperse, Somerset, NJ). Para-chlorobenzoic acidp-CBA) and terephthalic acid were purchased from Sigma–AldrichSt. Louis, MO, USA).

.2. Experimental design

2 g L−1 of SSL was dissolved in swimming pool water (SPW)24] with varying concentrations (0.2, 0.4, 0.7, 3.5 and 7 ppm) ofypochloric acid, which was an active ingredient in SPW (Table 1).he control experiment was performed with 2 g L−1 of SSL sus-ended in 1 L DI water. All the experiments were performed inhe presence of fluorescent light and were not covered as it was

ypothesized that the effect of light is negligible on the damagef a protective Al(OH)3 layer in the presence of swimming poolngredients and chlorine. Prior to the treatment with chlorine andollecting samples, all the stock suspensions were magnetically

5.8% NaClO (�L) 17–3884

stirred for 12 h at room temperature (22 ± 1 ◦C). Clorox bleach (TheClorox Company, USA) containing 5.8% as Cl2 was used as the chlo-rine source. Potassium bi-sulfate (KHSO4) was used to maintainpH at 6. Despite the actual pH in swimming pools is 7.8, slightlyacidic conditions were adopted to maximize the amount of FreeAvailable Chlorine (FAC) available in the SPW for more accurateresearch [24]. After 45 min treatment with chlorine, suspensionswere centrifuged (10 000 × g) for 1 h, supernatant removed, 250 mLof DI water added, samples manually shaken for 30 seconds, andagain centrifuged for 1 h. “Washing” step was repeated 3 times toensure that all the inactive ingredients were removed from samples[24].

2.3. Characterization of the sunscreen

The stock solutions were analyzed using FEI XL30 Environ-mental Scanning Electron Microscope equipped with an EDAXEnergy Dispersive Spectroscope (EDS) operating at 15–30 kV. TheX-ray diffraction patterns of SSL were recorded on a PANnalyt-ical X’Pert Pro X-ray Diffractometer in the range of 2� = 20–90◦

using Cu K� radiation as the X-ray source. The crystal phaseswere identified with the aid of the International Center for Diffrac-tion Data (ICDD). The morphology and structure of coated TiO2nanoparticles were evaluated with a high resolution-transmissionelectron microscope (HR-TEM, Jeol 2010F) equipped with an OxfordINCA EDS system operating at an accelerated voltage of 200 keV.The zeta-potential of the particles was measured using a zeta-potential analyzer (Malvern Instruments Ltd., Zeta Sizer NanoSeries, Westborough, MA). X-ray photoelectron spectroscopy (XPS)was performed using Surface Science Labs SSX-100 XPS. Curve fit-ting as well as spectral/image analysis was performed with CasaXPSand Shirley background subtraction. Fourier transform IR (FTIR)spectra were analyzed with single bounce ATR-FTIR spectroscopy.Measurements were conducted with a Thermo Nicolet 5600 MagnaSpectrophotometer, equipped with a Pike Technologies (Waltham,MA) GladiATR single bounce Diamond ATR cell (Madison, WI). Spec-tra were corrected for the presence of water and anatase TiO2 usingthe Thermo Nicolet Omnic 3.1 computer program. More details canbe found in Supporting Information.

2.4. Quantification of hydroxyl (*OH) radicals and photocatalyticactivity

The quantification of *OH radicals were performed accordingly:to prepare a stock solution, 1 mM of terephthalic acid (TPA) wasdissolved in 2 mM NaOH. Prior to the TPA addition, nanoparti-cles were sonicated for 10 min to achieve nearly homogeneousdistribution and break potentially formed aggregates and agglom-erates. Then, 10 mL of freshly prepared suspension (0.4 mM TPA and0.667 g L−1 nanoparticles (MT 500 SA)) was irradiated under theartificial solar light (light intensity of 45 mW cm−2) for 0, 5, 10, 20,40, 60 and 90 min. Particles were filtered through a 0.1 �m filter and

fluorescent intensity was measured using Fluorimeter (RF-1501)(Shimadzu). The 2-HTPA concentration was determined accord-ing to the calibration curve of the standard. Photocatalytic activitywas evaluated by preparing a suspension containing 1 �M p-CBA

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J. Virkutyte et al. / Chemical Eng

nd 0.667 g L−1 nanoparticles. 15 mL of a suspension was irradiatednder the artificial solar light (light intensity of 45 mW cm−2) for, 0.5, 1, 2, 4 and 8 h. Particles were filtered through a 0.2 �m filternd fluorescent intensity was measured using HPLC (Agilent 1100eries quaternary LC) equipped with a photodiode array detectorPDA). The column was a C18 Discovery HS (Supelco, USA) column4.6 mm × 150 mm, 2.1 �m particle size). The analysis was con-ucted with 0.05% (v/v) trifluoroacetic acid (TFA) in acetonitrileolution and 0.05% (v/v) TFA in MilliQ water, at 40:60 ratio. Theow rate was 0.2 mL min−1 and the injection volume was 20 �L.he temperature of the column was set at 40 ◦C.

. Results and discussion

.1. Characterization of sunscreen lotion

The XRD pattern of nanoparticles in SSL, confirming the pres-nce of rutile TiO2 as the predominant polymorph in SSL, isresented in Supporting Information (Fig. SI 1). The morphologyf SSL control and SSL subjected to various chlorine concentrationsontaining SPW was characterized by SEM. Predominantly agglom-rates were observed in the SSL control sample, while the additionf chlorine from 0.2 to 7 ppm in SPW reduced agglomeration andore porous aggregates were formed (Fig. 1). Such change in mor-

hology may be attributed to the removal of some of the ingredientsriginally present in the SSL.

Fig. SI 2 shows low and high resolution TEM images and EDSnalysis of SSL samples, average Al composition (expressed astomic (At) surface %) and Al(OH)3 coating thickness. A high resolu-ion image of the SSL control sample suggested that nanoparticlesere elliptical in shape, surrounded by an amorphous matrix com-osed of a of the SSL organics and the aluminum hydroxide coatingn the nanoparticle (Fig. SI 2a) [4].

Further observation with TEM revealed the uniform distributionf Al (At%) of 4.07 and average thickness of a continuous Al(OH)3ayer of ∼1.5 nm (Fig. SI 2b). Furthermore, to visually assess anduantify the effect of chlorine on the integrity of the Al(OH)3 protec-ive layer, SSL was subjected to 0.2–7 ppm chlorine and the resultsan be seen in Fig. 2.

Clearly, the addition of chlorine significantly affected the uni-ormity of the coating layer. Notably, SEM-EDS analysis of the SSLontrol samples suggested average Al atomic percent concentra-ion of 2.7 At% (Fig. SI 3a), which was significantly lower than that4.07 At%) observed during TEM-EDS measurements (Fig. SI 3b). Theariation in At% (Fig. SI 3c) could be attributed to the differencen energy of the probing electron beam. For instance, during SEM

easurements, a maximum of 30 keV of energy was used for EDSnalysis, while in TEM the energy of the probing electron beam was00 keV. The penetration depth of the probing electrons in the sam-le and thus, the depth of the sample from which detectable X-rayignals were generated for the EDS analysis were directly relatedo the energy of the probing electron beam [25]. Therefore, in SEM,he EDS results represented the concentration of Al in the top fewayers, while in TEM, the EDS results showed concentration fromeeper layers. Despite the discrepancies in measurements, bothEM and TEM-EDS revealed that no significant effect of chlorineas observed up to 0.4 ppm and the highest Al (At%) redistribu-

ion was found in the presence of 3.5–7 ppm chlorine, commononcentration in the SPW [24].

Relevant to the above findings, we also tested the integrity ofhe Al(OH)3 layer on nanoparticles after SSL was subjected to cen-

rifugation to remove inactive ingredients that may interfere withhe sole effect of chlorine. In Fig. 3, the TEM scan shows that Alistribution in SSL nanoparticle control sample is nearly uniform,anging from 3.86 to 4.35 At%.

ng Journal 191 (2012) 95– 103 97

However, sample treatment with 3.5 ppm chlorine in SWPproduced rather inhomogeneous and concentrated in patches Aldistribution from 4.02 to 15.6 At%, which was in agreement withour aforementioned observations (Fig. SI 3).

3.2. XPS analysis

To further confirm the composition and chemical state of aprotective layer on nanoparticle surface, XPS measurements wereperformed for MT 500 SA nanoparticles (for comparison) andthe SSL was subjected to 7 ppm of OCl−, relative amounts ofCaSO4·2H2O, Ca2+, and SO4

2− ions (main SPW constituents) usingGaussian–Lorentzian fitting. The high resolution spectra of eachregion are shown in Fig. 4. The full scale patterns of MT 500SA and SSL with and without chlorine treatment are given inFig. SI 4 and SI 5. The atom% composition of MT 500 SA andSSL including standard deviations is summarized in Table SI 1.Undoubtedly, there was a difference in O (O Al and O Ti in particu-lar) and Ti atom% content in both MT 500 SA and SSL in the presenceof 7 ppm chlorine in SPW. For instance, O Ti content decreasedfrom 28.8 to 26.1% and from 10.3 to 9.8%, whereas O Al increasedfrom 28.2 to 35.6% and from 12.5 to 14.4% in the presence of chlo-rine in MT 500 SA and SSL samples, respectively. Such changes inatom% surface composition may indicate the redistribution or evena damage of a protective layer as supported by the TEM and SEMobservations (Figs. 3, SI 2, SI 3).

In addition to the chemical composition near the sample surface,XPS also provides valuable information on the chemical bonding.Thus, it is reported that the binding energy of O 1s peak for pureTiO2 is 529.9 eV [26]. Detailed electronic environment studies per-formed on MT 500 SA nanoparticles revealed the presence of twobroad and asymmetric peaks at ∼530.3 eV and 532.3 eV after thecharge shift correction (Fig. 4a), indicative of the presence of twospecies.

The stronger peak at 530.3 eV was attributed to TiO2 (Ti–O–Ti)and to O Al [27] and the weaker peak at 532.3 eV was attributedto the surface hydroxyl groups (Ti OH). According to Wu and col-leagues, the coating of hydrous alumina contains numerous OHgroups, thus the surface of the TiO2 particles mainly contains OHgroups that result from protonation of the reactive amphoteric sites[19]. Furthermore, we assumed that Al was strongly associatedwith a TiO2 surface forming a Ti O Al bond [28], which was fur-ther confirmed by FTIR measurements (Fig. 5). When nanoparticleswere subjected to the SPW (7 ppm chlorine), the binding energyincreased to 530.6 eV and 532.3 eV for TiO2 and Ti OH, indicatingthe deteriorating effect of the chlorine species in SPW. Moreover,treatment of SSL with 7 ppm chlorine in SPW resulted in the shifttoward lower binding energy (529.3 eV) for TiO2 and a nearly com-plete disappearance of a Ti OH peak.

Also, a subsequent structural analysis (Fig. 4b) showed peaks atthe binding energies of 74.3 eV for MT 500 SA nanoparticles in thepresence and absence of SPW and 73.5 eV for SSL in SPW, attributingto Al(OH)3 (i.e. Al in the oxidation state of 3+) [27,29]. Importantto note is that a slight increase in the Al 2p binding energy mayindicate that alumina is strongly interacting with the TiO2 surface.Surface atom% composition shown in Table SI 1 shows an increasein Al/Ti (%) content in SSL from an initial 26.3 to 53.5% provingthat chemical re-distribution of the Al(OH)3 layer took place in thepresence of 7 ppm chlorine in SPW. When MT 500SA nanoparticleswere subjected to the same treatment, Al/Ti (%) ratio also increasedfrom 41% to 43%, indicating that our hypothesis was correct andSPW ingredients indeed contributed to the redistribution of a coat-

ing on the nanomaterial surface. Interestingly, when nanoparticleswere subjected to SO4

2−, Ca2+ and CaSO4·2H2O, Al/Ti (%) ratio wasnearly the same, i.e. 40%, 39.5% and 39.4%, respectively signifyingthe effect of chlorine onto the coating integrity.

98 J. Virkutyte et al. / Chemical Engineering Journal 191 (2012) 95– 103

F centra

waSwio

F(

ig. 1. SEM micrographs of control and SSL in SPW in the presence of different connd (e) 7 ppm chlorine.

In addition, the spin-orbit components of Ti 2p (2p3/2 and 2p1/2)ere deconvoluted by two curves (Fig. 4c) at 458.1 eV, 458.8 eV,

nd 458.5 eV as well as 463.6 eV, 464.0 eV, and 463.7 eV for SSL inPW, MT500SA, and MT500SA nanoparticles in SPW, respectively,

ith a split of 5.2 eV for MT 500 SA nanoparticles and 5.5 eV for SSL

n SPW between the doublets, indicating that Ti existed in the formf Ti4+ [30].

ig. 2. The depletion of Al(OH)3 coating in the presence of (a) SSL control (no chlorine), (bsquares indicate areas without Al(OH)3 coating). (Scale is from 2 nm (a), to 5 nm (b, c, e)

ations of (a) SSL control (no chlorine species), (b) 0.2 ppm, (c) 0.4 ppm, (d) 3.5 ppm

3.3. FTIR analysis

FTIR analysis was performed to identify the functional groupsof MT 500 SA nanoparticles and SSL and recognize chemical

changes, which occurred upon their treatment with chlorine andCaSO4·2H2O. Thus, the FTIR spectra of SSL and MT 500SA nanopar-ticles in DI and SPW in the presence of 7 ppm chlorine and

) 0.2 ppm, (c) 0.4 ppm, (d) 3.5 ppm and (e) 7 ppm of chlorine species concentrationand 10 nm (d).)

J. Virkutyte et al. / Chemical Engineering Journal 191 (2012) 95– 103 99

after c

Cawvrwit

t3r[sOcAbp3hbamn

(mqatv

3r

dacfi

Fig. 3. TEM images of control and SSL nanoparticles

aSO4·2H2O are shown in Fig. 5. The peaks centered at 1550 cm−1

nd 1450 cm−1 in the SSL sample dispersed in DI water (Fig. 5a)ere attributed to the asymmetric and symmetric stretching

ibrations of the deprotonated carboxyl groups (COO− in EDTA),espectively [31]. However, when the SSL was subjected to SPWith 7 ppm of chlorine, the peaks were no longer observed indicat-

ng that SPW ingredients facilitated the removal of organics in SSL,herefore exposing the protective Al(OH)3 coating.

The absorption band observed at 631 cm−1 was ascribed tohe stretching vibration of the Ti O bond [32]. The regions100–4000 cm−1 and 900–1800 cm−1 are hydroxyl stretchingegion and characteristics vibration of Al O stretching, respectively33,34]. Thus, 3290–3430 cm−1 peaks were associated with thetretching vibrations of the Al atoms bridged by an O atom and the

H bonds, whereas 1040–1110 cm−1 were attributed to poorlyrystallized modification of aluminum hydroxide (AlOOH) andl O stretching vibration of alumna [33,35,36] as also confirmedy XPS measurements (Fig. SI 4). Relatively low wave-numberosition of the hydroxyl stretching vibrations of 3292 cm−1 and430 cm−1 for SSL in DI and SPW, respectively, indicated significantydrogen bonding between the neighboring hydroxyls, whereasroad bands (Fig. 5a) suggested a high level of heterogeneitynd the presence of largely distributed hydroxyls [37]. Further-ore, there was no O H stretching region observed in MT 500SA

anoparticles (Fig. 5b).According to Zaharaki et al. [38], all bands near 1000 cm−1

950–1200 cm−1) were a major “fingerprint” of the inorganicatrix, defining an extent of aluminum incorporation. Conse-

uently, the band at 1080–1040 cm−1 in the presence of DI waternd 7 ppm chlorine in SPW for MT 500 SA nanoparticles, respec-ively (Fig. 5b), might be due to Ti O Al asymmetric stretchingibration [39].

.4. Plausible mechanism for the SSL protective Al(OH)3 coatingemoval

To explain the quality of Al(OH)3 coating and a mechanism of itsisruption, zeta potential measurements of TiO2 nanoparticles with

n Al(OH)3 layer (MT 500SA) and pure rutile TiO2 to mimic the mainonstituents of SSL but without the organic layer were performedor comparison (Fig. SI 6). In addition, the effect of various SPWngredients on zeta potential of SSL at the original pH and pH 6 is

entrifugation immersed in 3.5 ppm chlorine in SPW.

given in Table SI 2. Moreover, zeta potential of rutile TiO2 and MT500SA nanoparticles subjected to Ca2+, SO4

2−, and CaSO4 × 2H2Oare given in Table SI 3.

According to Petryshin and colleagues, the analysis of theacid–base interactions of water molecules with alkaline and acidicsurface functional groups close to point of zero charge (PZC) is asfollows [40]:

Ti OH2+

(S) ↔ Ti OH(S) + H+ (1)

Ti OH(S) ↔ Ti O−(S) + H+ (2)

where Ti OH+2(S) is the surface group due to the adsorption of H+

ions from a bulk solution; Ti OH(S) is the neutral surface hydroxylgroup; and Ti O−

(S) is the surface group formed due to the dis-sociation of a proton and the transfer of H+ ions back to the bulksolution.

As can be seen in Table SI 2, the zeta potential of the SSL signifi-cantly increased from ∼−63 mV to nearly −8 mV in the presence ofcalcium sulfate dihydrate (CaSO4·2H2O), which is an active ingre-dient in the SPW (in addition to chlorine), decreasing the stabilityof nanoparticles due to the double-layer compression and possi-ble specific adsorption of calcium ions [41]. Complementing thezeta potential measurements with XPS results, the nearly posi-tive charge of SSL could be attributed to adsorbed Ca2+ groups onthe surface. However, other ingredients such as sodium bicarbon-ate (NaHCO3) and potassium bisulfate (KHSO4) did not alter thezeta potential of nanoparticles present in SSL at the same extent.Moreover, the changes in pH did not significantly influence thezeta potential because the original and adjusted pH were higherthan the isoelectric point of TiO2 nanoparticles used in the cur-rent study (pH 5.8), and therefore they carried a negative chargeaccording to the Derjaguin–Landau–Verwey and Overbeek (DLVO)theory.

Evidently, the lower absolute value of zeta potential of TiO2in the calcium solution resulted from the specific adsorption ofcalcium ions onto the negatively charged surface. In the pres-ence of a protective Al(OH)3 layer, OH Al and H+ ions were themain surface species with the negligible amount of negativelycharged SO4

2−, positively charged Ca2+ ions from SWP ingredi-

ents, and OH−. When OH Al, H+, and/or other species interactedwith OH groups that existed on TiO2 surface, the negative chargewas neutralized [19] and zeta potential increased from −63.9 mVto ∼−9 mV (Fig. SI 6). Contrary to our hypothesis, various chlorine

100 J. Virkutyte et al. / Chemical Engineering Journal 191 (2012) 95– 103

opart

cmiioda

ii

Fig. 4. XPS high resolution spectra of MT 500 SA nan

oncentrations did not significantly influence zeta potential, whichight be attributed to the concurrent adsorption of ions present

n the SPW onto the surface of nanoparticles. Furthermore, whenons were present in the solution, the ionic strength in the aque-us phase increased and the electric double layer was compressed,ecreasing the Coulombic repulsive interactions between OH Al

nd TiO2 surface [20].

Furthermore, SO42− ions can interact with the TiO2 surface only

f the pH is lower than 3.5–3, thus as the pH of SPW was close to 6,t was hypothesized that Ca2+ ions were predominantly attached to

icles and SSL in SPW (a) O 1s, (b) Al 2p and (c) Ti 2p.

Ti atoms. The adsorption of Ca2+ onto the solid TiO2 surface couldbe explained accordingly:

Ti O−(S) + Ca2+ ↔ Ti O−· · · Ca2+

(S) (3)

where Ca2+(S) were positively charged surface-attached groups

that resulted from the dissociation of selected chemicals in water.

Positively Ca2+

(S) were attached to negatively charged Ti O−(S)

surface groups, resulting in the aforementioned zeta potentialshifts. When nanoparticles were separated by centrifuging, thezeta potential of the control was very close to that of the rutile

J. Virkutyte et al. / Chemical Engineering Journal 191 (2012) 95– 103 101

MT 50

(etvlo

trCI[d

Fig. 5. FTIR spectra of (a) SSL in DI and SPW (7 ppm chlorine) water and (b)

−32.7 mV) indicating the very weak adsorption of SPW ingredi-nts on the surface of nanoparticles and possible dissolution ofhe organic layer. Also, when the nanoparticles were subjected toarious concentrations of chlorine, the zeta potential increased, fol-owing the same trend of the SSL dispersion in SPW in the presencef chlorine species.

Thus, based on the zeta potential, surface morphology and struc-ural results, we proposed the following route for the Al(OH)3 layeremoval as illustrated in Fig. SI 7. When added to water (e.g. SPW),

aSO4·2H2O underwent dissociation to Ca2+ and SO4

2− (Fig. SI 7).t is well documented that Ca2+, has a strong affinity for EDTA4−

42,43], which is one of the water more washable and soluble ingre-ients in the SSL formulation. Grande and co-authors reported that

0SA nanoparticles in DI and SPW water (7 ppm chlorine) and CaSO4·2H2O.

EDTA4− may also react with chlorine, which could lead to chelatingagent degradation [44]. Therefore, when in contact with the SSL,Ca2+ and chlorine species might strip the organic layer and leaveAl(OH)3 exposed, which might become susceptible to the attack byother SPW ingredients, which in turn, might cause it to leach fromthe TiO2 surface as demonstrated by aforementioned observationsfrom HR-TEM images, XPS and zeta potential measurements.

The removal of Al(OH)3 coating could also be affected by thepresence of phosphate (PO4

3−) ions. The EDS spectra showed ele-

vated amounts of phosphorus from Potassium Cetyl Phosphate(C16H34KO4P, PCP), which is one of the ingredients in SSL, whenSSL was subjected to high chlorine concentrations in SPW (Fig. SI 8).High oxidizing properties of chlorine-induced degradation of PCP

102 J. Virkutyte et al. / Chemical Engineer

F5

apc

3

atTtwmoprhao(t[t

eew

ig. 6. (a) OH radical production and (b) degradation of p-CBA in the presence of ppm, 7 days aged MT 500 SA nanoparticles in SPW.

nd the subsequent release of PO43− ions, which could form AlPO4

recipitates [45–47], thus accelerating the removal of the Al(OH)3oating from nanoparticles.

.5. Hydroxyl radical formation and photocatalytic activity

According to the aforementioned results, the distribution of protective Al(OH)3 layer and therefore damage occurred inhe presence of SPW constituents, and chlorine in particular.herefore, to strengthen the research and hypothesis, quantifica-ion of hydroxyl radicals and degradation of a model compoundas performed in the presence of MT 500 SA nanoparticles toimic the core compounds found in SSL without the layer of

rganics. Generally, the analysis of OH radical formation can beerformed by fluorescence technique using TPA, which readilyeacts with OH radical to produce highly fluorescent product, 2-ydroxyterephthalic (2-HTPA) acid [48]. The intensity of the peakttributed to 2-HTPA is assumed to be proportional to the amountf OH radicals formed [49]. Moreover, para-chlorobenzoic acidp-CBA) is typically used as a probe compound to indirectly quan-ify hydroxyl radicals formed during advanced oxidation processes50]. Thus, p-CBA was selected as a model compound to evaluatehe potential photoactivity of compromised TiO2 nanoparticles.

Fig. 6a shows the formation of OH radicals in 7 days in the pres-nce of 5 ppm of chlorine in SPW. Seven days were selected tovaluate the harsh aging conditions for the nanoparticles and 5 ppmere selected as an intermediate amount of chlorine between the

ing Journal 191 (2012) 95– 103

most (7 ppm) and less (3.5 ppm) effective chlorine dosage foundin SPW. If assumed that radical capture efficiency was ca. 35%,the radical production in the current study was ca. 1.34 nM min−1

and 54.7 nM min−1 for original and 5 ppm damaged nanoparticles,respectively (Fig. 6a). Results clearly indicated that SPW con-stituents induced the structural changes on the TiO2 surface, whichwas confirmed by the significantly increased radical production.Furthermore, photocatalytic degradation of p-CBA is presented inFig. 6b. Clearly, original MT 500 SA nanoparticles did not exhibitphotocatalytic activity in comparison to 5 ppm, 7 days treatedMT 500 SA nanoparticles in SPW, when nearly 35% of p-CBA wasdegraded in ca. 8 h.

4. Conclusions

The present study demonstrated that the protective Al(OH)3layer present in the SSL formulation may be compromised andleached when in contact with SPW ingredients, especially Ca2+

and OCl− ions, leaving TiO2 nanoparticles un-coated. This couldlead to un-wanted and even dangerous photocatalytic activities onhuman tissues and degradation of chemical compounds (e.g. 35%degradation of p-CBA) and in the environment through free rad-ical formation (e.g. formation rate of 54.7 nM min−1 observed ina current study). However, more detailed analysis of radical pro-duction and degradation of target compounds is warranted in thepresence of commercial SSL. The findings presented here will pro-vide noteworthy information to the public whether swimming poolwater ingredients can compromise the protective layer and exposephotoactive nanoparticles, assist in designing SPW-resistant con-stituents in sunscreens, and promote further research on TiO2nanoparticles due to the potential mutagenic effects.

Acknowledgments

This research was funded and conducted by the National RiskManagement Research Laboratory of U.S. Environmental ProtectionAgency (EPA), Cincinnati, OH. This paper has not been subjectedto internal policy review of the U.S. EPA. Therefore, the researchresults do not necessarily reflect the views of the agency or its pol-icy. Mention of trade names and commercial products does notconstitute endorsement or recommendation for use. The authorsare thankful to Dr. Ratandeep Kukreja for HR-TEM and SEM mea-surements, and Ms. Geshan Zhang for providing radical productionanalysis.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.cej.2012.02.074.

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