The degradation products of UV filters in aqueous and chlorinated aqueous solutions

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Review

The degradation products of UV filters in aqueous and chlorinated aqueous solutions A. Joel M. Santos a, Margarida S. Miranda b, Joaquim C.G. Esteves da Silva a,* a Centro de Investigac¸a˜o em Quı´mica (CIQ-UP), Department of Chemistry and Biochemistry, Faculty of Sciences, Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal b Centro de Geologia da Universidade do Porto, Faculty of Sciences, Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal

article info

abstract

Article history:

Ultraviolet (UV) filters are vital constituents of sunscreens and other personal care prod-

Received 19 December 2011

ucts since they absorb, reflect and/or scatter UV radiation, therefore protecting us from the

Received in revised form

sun’s deleterious UV radiation and its effects. However, they suffer degradation, mainly

2 February 2012

through exposure towards sunlight and from reactions with disinfectant products such as

Accepted 24 March 2012

chlorine. On the basis of their increasing production and use, UV filters and their degra-

Available online 2 April 2012

dation products have already been detected in the aquatic environment, especially in bathing waters. This paper presents a comprehensive review on the work done so far as to

Keywords:

identify and determine the by-products of UV filter photodegradation in aqueous solutions

UV filters

and those subsequent to disinfection-induced degradation in chlorinated aqueous solu-

Water

tions, namely swimming pools.

Sunlight

ª 2012 Elsevier Ltd. All rights reserved.

Chlorine Swimming pools Disinfection by-products

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . UV filters degradation processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Photodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Benzophenones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Dibenzoylmethanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3. Cinnamates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4. Camphor derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.5. Para-aminobenzoates (PABA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

* Corresponding author. Tel.: þ351 220 402 569; fax: þ351 220 402 659. E-mail address: [email protected] (J.C.G. Esteves da Silva). 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2012.03.057

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3.

1.

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2.1.6. Benzimidazoles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Reaction with chlorine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction

The increasing concern about the effects of ultraviolet (UV) radiation resulted in an increased production and use of UV filters throughout the last decades. UV filters are vital ingredients of sunscreens and other personal care products as they absorb, reflect and/or scatter UV radiation (320e400 nm for UVA and 290e320 for UVB), therefore protecting us from its harmful effects on human skin and health. UV filters can be inorganic compounds (also regarded as physical UV filters) which reflect and scatter UV radiation or organic compounds (also regarded as chemical UV filters) which absorb the UV radiation. Despite this distinction, there are only two inorganic UV filters known to exist, titanium dioxide (TiO2) and zinc oxide (ZnO). Organic UV filters comprise various classes of compounds, with the most common being the para-aminobenzoates, cinnamates, benzophenones, dibenzoylmethanes, camphor derivatives and benzimidazoles (Shaath, 2010; Giokas et al., 2007). In general these compounds possess one benzenic moiety (or several), conjugated with electron releasing and electron accepting groups in either ortho or para positions, therefore allowing an efficient electronic delocalization and rendering them a specific maximum absorbance wavelength. In our days, many commercial products are marketed, with varying compositions which afford protection against UVA and UVB radiations. However, many UV filters have shown to present toxic effects, thus maximum concentrations have been established with a compromise between adequate protection and minimum side effects for users. Several papers have reported reviews on the toxicological effects of organic UV filters (Dı´az-Cruz and Barcelo´, 2009; Fent et al., 2010; Kunz et al., 2006; Zucchi et al., 2011). A wide number of UV filters have been found to exhibit oestrogenic, antiestrogenic, androgenic and antiandrogenic activities. There are about 55 UV filters approved for use in sunscreen products worldwide (EU, USA, Australia/New Zealand, Canada, Japan, S. Africa) (Shaath, 2010) with only 10 uniformly approved: benzophenone-3 (BP3), butyl methoxydibenzoylmethane (BMDM), ethylhexyl dimethyl PABA (EHDPABA), ethylhexyl methoxycinnamate (EHMC), ethylhexyl salicylate (EHS), homosalate (HS), octocrylene (OCR), PABA, phenyl benzimidazole sulfonic acid (PBSA) and titanium dioxide (TiO2). Relevant information on all the UV filters approved worldwide is presented in Table 1. Table 2 presents the physicochemical properties for all the UV filters approved in the EU, which includes all the most important and popular filters approved worldwide. Depending on the intended degree of protection and UV protection zone, several organic UV filters are typically combined and used in sunscreens and personal care products in concentrations that in general do not exceed 10% in combination with also an inorganic UV filter.

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The main concern used to focus merely on the UV filters utility and efficiency in protecting human skin and health from the harmful effects of UV radiation. Only very recently, concern has been raised regarding their path and their fate in the environment. UV filters may enter the environment through direct and indirect sources (Giokas et al., 2007). The direct sources regard the washing off effect during bathing activities in the ocean, lakes, rivers and swimming pools as well as industrial waste water discharges. Indirect inputs are related to domestic waste water discharges (during showering, clothes washing and urine excretion) and via waste water treatment plants. The main environmental concerns regarding these compounds are related to their considerable octanolewater coefficients, bioconcentration factors and organic carbon coefficients, as is visible in Table 2, which means that these compounds are significantly lipophilic and have a particular tendency to concentrate and/or accumulate in the aquatic environment’s soils and sediments as well as in the food chain (Dı´az-Cruz et al., 2008; Dı´az-Cruz and Barcelo´, 2009; Giokas et al., 2007). The increased release of UV filters into the environment has prompted them to be considered a new class of pollutants. Dı´az-Cruz (Dı´az-Cruz et al., 2008) compiled data regarding UV filters levels in the aquatic environment. According to this review the reported concentrations varied depending on the sampling location and the intensity of the recreational activities. UV filters have been primarily detected in bathing waters (rivers, lakes, sea water) with concentrations up to 10 mg/L. The maximum concentrations have been measured during the warmest summer days, especially in noon hours when sunscreen application is also maximum, as a consequence of the increased sunlight irradiation intensity and exposure. UV filters have also been detected in sewage water (untreated and treated sewage effluents), sludge, sediments and soils with levels that reach mg/L values, and in fish from rivers and lakes used for bathing, with levels that reach several g/kg. More recently UV filters have also been detected in human breast milk (Schlumpf et al., 2010) and human urine (Leo´n et al., 2010). Data regarding the presence of UV filters in swimming-pool water are rather scarce. Lambropoulou (Lambropoulou et al., 2002) reported the determination of two UV filters, BP3 and EHDPABA, in swimming-pool waters and in showers waste waters near swimming pools, in concentrations from 2 to 10 mg/L. Giokas (Giokas et al., 2004) reported the determination of three UV filters [BP3, EHMC and 4-methylbenzylidene camphor (4MBC)] in swimming-pool waters and in shower waste waters near swimming pools. The study showed that UV filters were present in concentrations up to 10 ng/L. Vidal (Vidal et al., 2010) reported the determination of UV filters [BP3, isoamyl p-methoxycinnamate (IMC), 4MBC, OCR, EHDPABA and EHMC] on two swimming pools, one public and

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Table 1 e Complete list of UV filters regulated and approved worldwide along with their most relevant UV properties. Name

Acronym EU

Benzophenone Benzophenone-1 Benzophenone-2 Benzophenone-3 Benzophenone-4 Benzophenone-5 Benzophenone-6 Benzophenone-8 Benzophenone-9 3-Benzylidene camphor Benzylidene camphor sulfonic acid Beta-2-glucopyranoxy propyl hydroxy benzophenone Bis-ethylhexyloxyphenol methoxyphenyl triazine Butyl methoxydibenzoylmethane Camphor benzalkonium methosulfate Cinoxate DEA methoxycinnamate Diethylamino hydroxy benzoyl hexyl benzoate Diethylhexyl butamido triazone Digalloyl trioleate Diisopropyl methyl cinnamate Dimethoxyphenyl-[1-(3,4)]-4, 4-dimethyl 1,3-pentanedione Disodium phenyl dibenzylimidazole tetrasulfonate Drometrizole Drometrizole trisiloxane Ethyl dihydroxypropyl PABA Ethylhexyl dimethoxy benzylidene dioxoimidazoline propionate Ethylhexyl dimethyl PABA Ethylhexyl methoxycinnamate Ethylhexyl salicylate Ethylhexyl triazone Ferulic acid Glyceryl ethylhexanoate dimethoxycinnamate Glyceryl PABA Homosalate Isoamyl p-methoxycinnamate Isopentyl trimethoxycinnamate trisiloxane Isopropyl benzyl salicylate Isopropyl methoxycinnamate Lawsone þ dihydroxyacetone Menthyl anthranilate 4-Methylbenzylidene camphor Methylene bis-benzotriazolyl tetramethylbutylphenol Octocrylene PABA PEG-25 PABA Pentyl dimethyl PABA Phenyl benzimidazole sulfonic acid Polyacrylamido methylbenzylidene camphor Polysilicone-15 Salicylic acid TEA salicylate Terephthalylidene dicamphor sulfonic acid Titanium dioxide Zinc oxide

BP BP-1 BP-2 BP3 BP4 BP-5 BP-6 BP-8 BP-9 3BC BCS GPH

X X X

USA

Japan

X X

X X X X X X

X X X X

EMT BMDM CBM CX DEAMC DHH DBT DT DMC DDP

X X X

PDT DR DRT EDP EDDP

X

EHDPABA EHMC EHS EHT FA GED GPABA HS IMC ITT IBS IPM LDHA MA 4MBC MBT OCR PABA P25 PDP PBSA PBC P15 SA TS TDS TiO2 ZnO

UV region

lmax /nm

3/L mol1 cm1

X X X X X X X X X X X X

UVA/B UVA/B UVA/B UVA/B UVA/B UVA/B UVA/B UVA/B UVA/B UVB UVB UVA/B

284 291 287 286 286 286 284 284 284 289 294

10,300 12,265 13,700 14,380 13,400

21,360 27,600

X X X X X X X X X X

UVB/B UVA UVB UVB UVB UVA UVB UVB UVB UVA

310 357 284 308 290 354 311

46,800 34,140 24,500 20,650 24,930 35,900 111,700

X

UVA UVA/B UVA/B UVB UVB

335 300 303 312

51,940 16,200 27,000

UVB UVB UVB UVB UVB UVB UVB UVB UVB UVB UVB UVB UVB UVA UVB UVA/B UVB UVB UVB UVB UVB UVB UVB UVB UVB UVA UVA/B UVA/B

311 311 305 314

27,300 23,300 4130 119,500

297 306 308

18,700 4300 24,335

336 300 305 303 283 309 310 302 297 312 300 298 345

5230 23,655 26,600 12,290 15,300

Country/regulatory body where it is approved

X X

X

X

X

X

X X

X

X X

X X

X

X X X X X

X X X

X X

X

X X X X X X X X X

X X

X

X X X X X X X X X X X X X X X X X X X X

X X X

X X

X X X

Othersa

X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

13,500 13,270

26,060 19,700 108,000 3000 47,100

a Outside the three main regulatory bodies (EU; USA; and Japan) other countries that apply legislation regarding these compounds are Australia, New Zealand, Canada, South Africa, or South Korea. Adapted from Shaath (2010).

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Table 2 e Physicochemical properties of all the UV filters regulated and authorized in the EU, which includes all the most important filters used and uniformly approved worldwide. Structure

Name

Acronym

Molecular mass

Log KOWd

Log BCFe,f

Log KOCe,g

Solubility /g/Lh

lmax /nm

Benzophenones

Benzophenone-3 Benzophenone-4

BP3 BP4

228.24 308.31

3.79 0.88

1.38 e

3.10 e

0.21 0.65

PABA and derivatives

PABA PEG-25 PABA Ethylhexyl dimethyl PABA Homosalate Ethylhexyl salicylate Ethylhexyl methoxycinnamate Isoamyl p-methoxycinnamate Camphor benzalkonium methosulfate Terephtalydene dicamphor sulfonic acid Benzylidene camphor sulfonic acid Polyacrylamido methylbenzylidene camphord 4-Methylbenzylidene camphor 3-Benzylidene camphor Ethylhexyl triazone Diethylhexyl butamido triazone Bis-Ethylhexyloxyphenol methoxyphenyl triazine Drometrizole trisiloxane

PABA P25 EHDPABA HS EHS EHMC IMC CBM TDSA

137.14 277.41 277.40 262.35 250.34 290.40 248.32 409.55 562.69

0.83 e 6.15 6.16 5.77 5.80 4.06 0.28 1.35

e e 3.74 e e 5.80 e e e

e e 3.38 e e 4.10 e e e

915 e 2.1  103 0.02 0.028 0.15 0.06 e 0.014

290 240b; 288 282 310a 310 e 240a 306b e 288a 340a

BCS PBC

320.40

2.74 e

e e

e e

0.038 e

297a e

4MBC 3BC EHT DBT EMT

254.37 240.34 826.10 765.98 627.81

4.95 4.49 15.53 11.90 13.89

3.51

3.89

e e e

e e e

5.1  103 9.9  103 e 4.6  107 4.9  108

300b 292a 310b e 340b

DRT

225.25

9.79

e

e

1.3  105

Methylene bis-benzotriazolyl tetramethylbutylphenol Phenyl benzimidazole sulfonic acid Disodium phenyl dibenzimidazole tetrasulfonate Butyl methoxydibenzoylmethane Diethylamino hydroxy benzoyl hexyl benzoate Octocrylene Polysilicone-15

MBT

658.87

14.35

e

e

3.0  108

344; 303 340b

PBSA PDT

274.30 674.60

0.01 e

0.50 e

2.46 e

0.26 e

300b 250

BMDM DHH

310.39 397.51

2.41 6.93

4.51 e

3.23 e

0.037 9.5  104

358a 360b

OCR P15

361.49 e

7.35 e

e e

e e

2.0  104 e

300b 313c

Salicylates Cinnamates Camphor derivatives

Triazines

Benzotriazoles

Benzimidazole derivatives Dibenzoyl methane derivatives Others

UV filters presented in grey are the organic filters uniformly approved worldwide. a Rastogi, S., Jensen, G.H., 1998. J. Chromatogr. A 828 (1e2), 311e316. b De Orsi, D., Giannini, G., Gagliardi, L., et al., 2006. Chromatographia 64 (9e10), 509e515. c Philippe Maillan Formulation, R&D Cosmetics, DSM Nutritional Products; Measurement of UV Protection in Hair. d Octanolewater partition coefficient (KOW); represents the ratio of the concentration of a substance in octanol and the concentration of the same substance in equilibrium in water, at a certain temperature. e Giokas, D.L., Salvador, A., Chisvert, A., 2007. Trends in Analytical Chemistry 26 (5), 360e374. f Bioconcentration factor (BCF); represents the ratio between the concentration of a substance in a biological organism and the concentration of the same substance in the water solution in which the organism is inserted. g Organic carbon distribution coefficient (KOC); represents the ratio between the mass of a substance adsorbed in the soil (per unit of organic carbon in the soil) and the concentration of the same substance in equilibrium in solution. h In water, at 25  C. Adapted from Dı´az-Cruz et al. (2008) and Giokas et al. (2007).

other one private. Only the public swimming-pool water contained detectable quantities of two of the six UV filters studied, concretely IMC at 700  300 ng/L and 4MBC below its limit of quantification. Giokas (Giokas et al., 2007) and Peck (Peck, 2006) recently reviewed the methods used for the analysis of UV filters in the environment. Reported methods for the determination of UV filters in environmental samples, normally involve first, an extraction/preconcentration step of the target analytes and second, chromatographic and mass spectrometry analysis. For sample preparation, liquideliquid extraction (LLE), membrane assisted liquideliquid extraction (MALLE),

micelar-mediated extraction (MME), solid-phase extraction (SPE), solid-phase microextraction (SPME), stir bar sorptive extraction (SBME) and single drop microextraction (SDME) have been used. Analysis is often performed using liquid chromatography (LC) coupled to diode array detection (DAD) or mass spectrometry (MS), or seldom with gas chromatography (GC) coupled to mass spectrometry (MS), given the fact that the vast majority of these compounds have relatively high boiling points and therefore require derivatization prior to analysis (Chisvert and Salvador, 2007). Oliveira (Oliveira et al., 2010) recently developed a mesofluidic-bead injectionlab-on-valve system for on-line coupling of microSPE with LC

w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 1 6 7 e3 1 7 6

for determination of UV filters (BP3, BMDBM, EHMC and HMS) in swimming-pool waters. These techniques have also been applied recently in the investigation of the fate of UV filters in the aquatic environment and in the detection and quantification of their degradation products.

2.

UV filters degradation processes

UV filters should be stable on exposure to sunlight. A high screening efficiency can only be guaranteed if a UV filter is of high photostability. However, studies have reported that exposure of UV filters to natural and/or artificial sunlight may lead to photodegradation reactions that can compromise their physical properties (e.g. maximum absorption wavelength and absorbance coefficient) and lead to the formation of undesirable photoproducts that accumulate on human skin. UV filters are also expected to photodegradate after being discharged into the aquatic environment in the presence of sunlight. Some UV filters are also known to photogenerate reactive oxygen species and other free radicals upon irradiation in aqueous solutions (Serpone et al., 2002; Inbaraj et al., 2002; Zhang et al., 2010). The photodegradation of UV filters in the aquatic environment is also known to be influenced by water constituents namely dissolved organic matter (DOM), nitrates, chloride and bicarbonates. Regarding this aspect, Giokas and Vlessidis (2007) demonstrated, through the use of a multivariate experimental design and/or method that investigates several different variables and/or factors simultaneously, that the presence of photosensitizers (oxygen, hydroxyl and/or peroxy radicals) in water, formed from the photolysis of humic acids, sodium chloride, and nitrate in solution, induces the photodegradation process and therefore influences the photodegradation kinetics of UV filters. In addition UV filters can also suffer degradation through reaction with disinfectant agents, like chlorine used in swimming pools, giving origin to chlorinated by-products. Therefore some attention has been paid in the last years to the degradation of UV filters in aqueous solution. The identification of degradation products is important to determine the environmental and human health effects of increased use of UV filters, since its degradation products normally present, for instance the disinfection by-products (DBPs), are often more toxic than the parent UV filters (Dı´az-Cruz and Barcelo´, 2009; Giokas et al., 2007; Dı´az-Cruz et al., 2008; La Farre´ et al., 2008; Richardson et al., 2007; Richardson et al., 2010; Hrudey, 2009).

2.1.

Photodegradation

Up to now, very few articles have dealt with the formation of photodegradation products in aqueous solutions. Despite the

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existence of many UV filters, only a few have attracted interest in aquatic environmental studies due to their frequency of application in commercial sunscreen products. In the following sections we present a review on the most recent studies regarding the photostability of all the UV filters uniformly approved worldwide, performed in aqueous solutions under natural and/or artificial sunlight and only when photodegradation has been reported.

2.1.1.

Benzophenones

Rodil (Rodil et al., 2009) has recently reported significant stability of benzophenone-3 (BP3) in aqueous solution upon exposure to artificial sunlight. Samples were exposed to artificial solar light provided by a halogen lamp, with a wavelength spectrum between 290 and 800 nm, covering therefore the UVA and UVB ranges (320e400 and 290e320 nm, respectively) together with the photosynthetic active radiation (PAR) range (400e700 nm; wavelength range of solar radiation that photosynthetic organisms are able to use in the process of photosynthesis). The sample solutions were exposed to artificial light when inserted in a thermostatically controlled water bath (temperature kept at 25  C), and light intensity above the surface of the water was about 350 mmol photons/ m2. No photodegradation products were determined. Liu (Liu et al., 2011) has also reported significant photostability of BP3 under both UV and natural sunlight. Interaction of BP3 with another UV filter (benzotriazole), and with humic acids in solution, resulted in significant increase of the degradation percentage of BP3. One photoproduct of BP3, produced through the loss of hydroxyl and benzoyl functional groups, was successfully determined and identified as 2,4dimethylanisole.

2.1.2.

Dibenzoylmethanes

Dibenzoylmethane derivatives are widely used as UVA filters and the most common among the dibenzoylmethanes is 4-tertbutyl-40 -methoxydibenzoylmethane (BMDM). In sunscreen products BMDM exists predominantly in the enol form (see Fig. 1) which displays a strong absorption band in the UVA region around 350 nm (Shaath, 2010). Huong (Huong et al., 2008) has found that, under irradiation in aqueous solution, the enol form of BMDM tautomerizes to the keto form (see Fig. 1) and is also fully photodegraded. Irradiations were carried out with a solar light simulator with a xenon lamp. Diluted solutions were irradiated at 250 W/m2 (60 kJ/m2) in spectroscopic quartz cuvettes (of 1 cm) capped with Teflon stoppers, while the concentrated solutions were done so at 550 kW/m2 (495 kJ/m2) in polymethylmethacrylate (PMMA) plates, whereas irradiation of the sunscreen commercial products was carried out at 550 kW/m2 (495 kJ/m2) in plates of sun protection factor (SPF)

Fig. 1 e The tautomerism of butyl methoxydibenzoylmethane.

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determination. During irradiation, temperature was regulated and kept between 25 and 45  C, depending on the length of irradiation. The authors identified various photoproducts: substituted benzoic acids, benzils, dibenzoylmethanes and dibenzoyl ethanes. Some of these compounds were also proposed by Schwack and Rudolph in a previous study (Schwack and Rudolph, 1995). These authors argued that the formation of these photoproducts involves primary a-bond cleavages of carbonyl groups of the 1,3-diketo form, followed by either hydrogen abstraction, oxidation and/or radical recombination.

2.1.3.

Cinnamates

2-Ethylhexyl-4-methoxycinnamate (EHMC) is one of the most commonly used cinnamates as it presents a high absorption capacity in the UVB region. EHMC can exist as a cis (Z ) or trans (E ) isomer due to the carbon-carbon double bond (see Fig. 2). In sunscreen products EHMC is present as the E isomer. Serpone (Serpone et al., 2002) and Huong (Huong et al., 2007) reported that EHMC experiences photo-isomerization from its E to its Z form, in water and upon irradiation. Serpone (Serpone et al., 2002) studied the photostability of EHMC upon artificial sunlight irradiation provided by a xenon lamp, with the emitted radiation being filtered through a water filter in order to remove infrared radiation. Irradiations were carried out at wavelengths longer than 290 nm. In Huong’s work (Huong et al., 2007), irradiations were carried out with artificial sunlight also provided by a xenon lamp, with the temperature being regulated automatically and kept between 25 and 45  C, depending on the length of irradiation. Energies of irradiation were not measured but calculated from the power provided by the simulator. Diluted solutions were exposed to artificial sunlight for 10 min at 250 W/m2, whereas concentrated solutions were irradiated for 20 min at 250 W/m2. The Z isomer has a maximum wavelength similar to that of the E isomer but it has a substantially lower molar absorption coefficient, which results in a decrease in the efficiency of the EHMC as a UVB absorber. The authors also reported the occurrence of degradation of the cinnamate structure and the formation of degradation products, but have not identified them. Rodil (Rodil et al., 2009) recently reported the photo-isomerization from E to Z of EHMC and IMC, in aqueous solution upon artificial irradiation. Samples were exposed to artificial sunlight provided by a halogen lamp, with a wavelength spectrum between 290 and 800 nm, covering therefore the UVA and UVB ranges (320e400 and 290e320 nm, respectively) together with the photosynthetic active radiation (PAR) range (400e700 nm; wavelength range of solar radiation that photosynthetic organisms are able to use in the process of photosynthesis). Exposure to artificial light was carried out with the sample solutions inserted in a thermostatically controlled water bath (temperature kept at 25  C), and light intensity above the water surface was about

350 mmol photons/m2. Parallel to the E to Z photo-isomerization of EHMC and IMC, it was also reported the formation of cyclobutane dimers which probably result from a [2 þ 2] cycloaddition reaction. The formation of these dimmers, which exhibit less filtering capabilities than the original UV filters, had been previously reported for EHMC (Broadbent et al., 1996; Dondi et al., 2006). Very recently, other photodegradation products from artificial irradiation of EHMC in aqueous solutions were identified including 4-methoxybenzaldehyde, 2-ethylhexanol, cyclodimers, and a dimer hydrolysis product (MacManusSpencer et al., 2011). In this study, samples were kept in quartz tubes and exposed to a solar simulator at an angle of 30 from the horizontal level. Artificial sunlight had an intensity of 250 W/m2 and the temperature was controlled and kept at 25  2  C. A solar filter was also used in order to establish and maintain a natural solar spectrum (300e800 nm).

2.1.4.

Camphor derivatives

4-Methylbenzylidene camphor (4MBC) is an important organic UV filter used in many sunscreens. 4MBC is classified as an UVB filter since it absorbs most efficiently in the 290e320 nm range. 4MBC can exist as a cis (Z ) or trans (E ) isomer due to the exocyclic carbonecarbon double bond (see Fig. 3). In commercial sunscreens 4MBC was shown to be present in the E form (Poiger et al., 2004). However, it is known that, upon exposure to light, the E isomer is converted into the Z isomer (Poiger et al., 2004; Balmer et al., 2005). So, as it could be expected, both isomers have been detected in surface and waste waters consistent with exposure of 4MBC to sunlight (Poiger et al., 2004; Balmer et al., 2005; Buser et al., 2005). Rodil (Rodil et al., 2009) recently reported photoisomerization of 4MBC from E to Z in aqueous solution upon exposure to artificial sunlight. Artificial irradiation was achieved by exposure to a halogen lamp, with a wavelength spectrum between 290 and 800 nm, covering therefore the UVA and UVB ranges (320e400 and 290e320 nm, respectively) together with the photosynthetic active radiation (PAR) range (400e700 nm; wavelength range of solar radiation that photosynthetic organisms are able to use in the process of photosynthesis). Samples exposed to artificial light, where done so when inserted in a thermostatically controlled water bath (temperature kept at 25  C), and light intensity above the surface of the water was about 350 mmol photons/m2. Apart from the detection of the photo-isomerization reaction and subsequent by-product (the Z form of 4MBC), no additional photoproducts were detected.

2.1.5.

Para-aminobenzoates (PABA)

2-Ethylhexyl-p-dimethylaminobenzoate (EHDPABA) (see Fig. 4) is one of the most widely used PABA UV filters. Serpone (Serpone et al., 2002) found that EHDPABA photodegradates significantly in water upon artificial solar irradiation. Artificial

Fig. 2 e E and Z isomers of 2-ethylhexyl-4-methoxycinnamate.

w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 1 6 7 e3 1 7 6

Fig. 3 e E and Z isomers of 4-methylbenzylidene camphor.

sunlight was provided by a xenon lamp, with the emitted radiation being filtered through a water filter in order to remove infrared radiation. Irradiations were carried out at wavelengths longer than 290 nm. Sakkas (Sakkas et al., 2003) studied the photostability of EHDPABA in three aqueous matrices: distilled water, sea water and swimming-pool water, under artificial and natural sunlight conditions. There are no details on the conditions of natural irradiation, such as the incident light intensity measurements throughout irradiation time and average value as well as the wavelength range, the temperature measurements throughout irradiation time and average value, etc. Artificial sunlight was obtained with a xenon arc lamp with an average light intensity about 750 W/ m2, with special glass filters also applied restricting the transmission of wavelength below 290 nm. The average irradiation intensity was about 750 W/m2 with the light dose for one hour of irradiation being 2700 kJ/m2, and the temperature of the samples exposed never exceeded 25  C (details of irradiation conditions were not available in this current publication but were discriminated, and therefore remitted by the authors, to a previous work: Sakkas et al., 2001). The influence of dissolved organic matter (DOM) on the degradation kinetics was also studied by varying the concentration of humic acids. The reaction rates followed the order: distilled water > swimming-pool water > sea water, depending mainly on the presence of DOM that retarded the photodegradation reaction. It was found that the degradation products were strongly dependent on the constitution of the irradiated media. Up to four degradation products were detected in the three aqueous solutions under natural or artificial light conditions: 2-ethylhexyl p-methylaminobenzoate, 2-ethylhexyl p-aminobenzoate and 2-ethylhexyl p-dimethylaminohydroxybenzoate. These resulted from dealkylation and hydroxylation reactions of EHDPABA. Rodil (Rodil et al., 2009) recently reported the occurrence of photodegradation of EHDPABA in aqueous solution upon exposure to artificial sunlight. Artificial irradiation was obtained from exposure to a halogen lamp. The irradiation

Fig. 4 e 2-Ethylhexyl-p-dimethylaminobenzoate.

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wavelength spectrum was kept between 290 and 800 nm, covering therefore the UVA and UVB ranges (320e400 and 290e320 nm, respectively) together with the photosynthetic active radiation (PAR) range (400e700 nm; wavelength range of solar radiation that photosynthetic organisms are able to use in the process of photosynthesis). Samples exposed to artificial light, where done so when inserted in a thermostatically controlled water bath (temperature kept at 25  C), and light intensity above the surface of the water was about 350 mmol photons/m2. This study (Rodil et al., 2009) reported the formation of three degradation products. Two of which (2-ethylhexyl p-methylaminobenzoate and 2-ethylhexyl p-aminobenzoate) were also previously detected by Sakkas (Sakkas et al., 2003) and are the result of a dealkylation of EHDPABA corresponding to the loss of one and two of the methyl groups of the parent compound. The other degradation product (2-ethylhexyl-p-dimethylamino-methylbenzoate) was proposed as a methylated derivative of the parent UV filter EHDPABA.

2.1.6.

Benzimidazoles

The UV filter 2-phenyl benzimidazole-5-sulphonic acid (PBSA) (see Fig. 5) is widely used in sunscreen commercial products because of its strong absorption in the UVB region. Serpone (Serpone et al., 2002) reported that the photodegradation of PBSA is particularly fast and extensive in water. Artificial sunlight was achieved by exposure to a xenon lamp, and a water filter was also applied to the system setup in order to remove infrared radiation. Irradiations were carried out at wavelengths longer than 290 nm. Inbaraj (Inbaraj et al., 2002) found that the artificial solar irradiation of PBSA in aqueous solution generates a variety of free radicals and reactive oxygen species (ROS), which lead to its degradation in water and to the decrease of its protection ability against sunlight. The detection of reactive oxygen species was accomplished upon artificial irradiation of samples using a 500 W mercury lamp operating at 300 W, with an interference filter transmitting at 314 nm. In order to examine the photochemical and photophysical properties of PBSA, spectrometry was used, particularly electron paramagnetic resonance (EPR). In this case, the samples were irradiated directly inside the microwave cavity of the spectrometer with a xenon lamp of 1 kW, with the emitting light then passing through a cutoff filter transmitting above 300 nm. Zhang (Zhang et al., 2010) confirmed that PBSA photodegradates upon artificial irradiation of aqueous solutions. Samples were artificially irradiated with a 500 W high pressure mercury lamp, water refrigerated and equipped with 365 nm filters, in order to mimic the UVA and UVB portions of solar radiation. The author found that PBSA photodegradates in aqueous solution through the generation of ROS and that the photodegradation was pH-dependent and inhibited by fulvic acid. The authors also identified six photodegradation products in pure water:

Fig. 5 e 2-Phenyl benzimidazole-5-sulphonic acid.

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w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 1 6 7 e3 1 7 6

a desulfonated product (2-phenylbenzoimidazole), three products formed from the cleavage of the benzene ring adjacent to the imidazole ring (phenylimidazolecarboxylic acid derivatives) and two compounds produced from the opening of the imidazole ring (benzimidamide and benzamide).

2.2.

Reaction with chlorine

Swimming-pool water requires disinfection in order to protect swimmers from pathogenic microorganisms. However, disinfection has also unintended consequences. Chlorine based disinfectants, loosely referred simply by chlorine, represent the most commonly used disinfectants, and include chlorine gas, sodium or calcium hypochlorite, chlorinated isocyanurates, bromochlorodimethylhydantoin and chlorine dioxide (Lakind et al., 2010). Chlorine reacts with the organic matter (natural and synthetic) present in the pool water, producing a variety of chlorinated organic compounds known as disinfection by-products (DBPs), some of which have been associated with adverse health effects (Lakind et al., 2010). Additionally to the precursors present in the water that is used to fill the pool, bathers also introduce many compounds that come from urine, sweat, saliva, hair, skin cells and personal care products. The type and levels of DBPs formed have been suggested to depend on several factors such as the water source, DOM, the type and amount of disinfectant used, disinfectant dose and residual available in the water, contact time between reactants, temperature, pH, bromide concentration and nitrate concentration (Kim et al., 2002; Kanan and Karanfil, 2011). Currently, more than one hundred DBPs have been identified in swimming pools including trihalomethanes, haloacids, halonitriles, haloaldeydes, haloketones, halonitromethanes, haloamines, haloamides, haloalcohols and halogenated derivatives of UV filters (WHO, 2006; Zwiener et al., 2007; Richardson et al., 2010). In general, chlorinated by-products are more prominent than its brominated counterparts, unless the reactions take place in highly brominated waters, where the opposite naturally occurs. People attending swimming pools are exposed to chemicals used to disinfect the pool water as well as to DBPs. The three main routes of exposure are inhalation, as some compounds are volatile, dermal contact and absorption through skin during bathing/swimming and ingestion by swallowing water. Exposure to DBPs in swimming pools has been linked to adverse health effects, like increased risk of asthma and other respiratory diseases as well as bladder cancer (Richardson et al., 2010). Studies on the fate of UV filters in chlorinated waters such as swimming pools are rather scarce with only three reports being known to exist. The first paper that reports the reaction between UV filters and chlorine in water samples was done by Sakkas (Sakkas et al., 2003). As mentioned in Section 2.1.4, Sakkas (Sakkas et al., 2003) studied the degradation kinetics of the UV filter EHDPABA in three aqueous matrices: distilled water, sea water and swimming-pool water, under artificial and natural sunlight conditions. In the case of swimming-pool water five more chlorinated products were found due to the additional presence of chlorine: (2-ethylhexyl dichloro-p-dimethylaminobenzoate,

2-ethylhexyl chloro-p-methylaminobenzoate, 2-ethylhexyl dichloro- p-methylaminobenzoate, 2-ethylhexyl p-aminochlorobenzoate, 2-ethylhexyl p-amino-dichlorobenzoate). These correspond to chlorination reaction products of both the parent compound (EHDPABA) and the previously mentioned degradation products. Despite the fact that sea water also contains chloride, no chlorinated derivative could be identified, probably due to the low existing concentration of chloride in solution. This study does not provide data for neither the rates of EHDPABA halogenation reactions nor the stability of the generated chlorine by-products. In a more recent study carried out by Negreira (Negreira et al., 2008), the stability of three UV filters [ethylhexyl salicylate (ES), EHDPABA and BP3] was studied (through the kinetics of the degradation reaction) in chlorinated water samples at neutral pHs and in the presence of potassium bromide salt. The light effect was not taken into account. Identification of the corresponding halogenated by-products as well as the study of their stability was also assessed. The stability of the UV filters increased in the following order: BP3 < EHDPABA < ES. The extension of ES halogenation reactions was estimated as negligible in real life situations (where several organic species compete for the available chlorine) and this filter was therefore not studied further. On the other hand, EHDPABA and BP3 reacted with free chlorine at significant rates, showing a lower stability. Differences among reactivities highlight the effect of different organic groups on the activation or deactivation of the phenolic ring towards electrophilic substitution reactions. This stability was also found to be pHdependent. EHDPABA was more stable at pH 8.2 than at pHs 6.2 and 7.2 while BP3 was less stable at pH 8.2 than at pH 7.2 and 6.2. Addition of bromide reduced the stability of both UV filters. This effect was explained by the formation of bromine, which shows a strong tendency to react with aromatic compounds. For EHDPABA only mono-halogenated species were detected (Cl-EHDPABA and Br-EHDPABA with the same structure as the parent UV filter), whilst for BP3 both monoand disubstituted by-products were identified. For EHDPABA a relatively simple degradation pathway was established. It consisted of aromatic substitution of one atom of hydrogen per one atom of chlorine or bromine. The same reaction pattern was observed for BP3, leading in this case, to monoand dihalogenated by-products: Cl-BP3 (2 isomers), Cl2-BP3 (1 isomer), Br-BP3 (2 isomers), Br2-BP3 (1 isomer) and Br-Cl-BP3 (2 isomers). Positions where those replacements occurred were not confirmed experimentally. However, considering the structures of the parent species and the activation effects of hydroxyl and amino groups towards electrophilic substitution reactions, the most probable ones are the carbons in ortho to the amino moiety (EHDPABA), and those in ortho and para positions to the hydroxyl group (BP3). In addition, several halogenated forms of 3-methoxyphenol were identified which could be generated from cleavage of the carbonyl bond between the two aromatic rings in the molecule of BP3, followed by halogenation of the methoxyphenol fragment. Moreover, mono- and dihalogenated substitution by-products of BP3 might also break down rendering different halogenated methoxyphenols. By-products of EHDPABA, dihalogenated BP3 forms and the tri-halogenated methoxyphenols showed a considerable stability to further oxidation reactions.

w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 1 6 7 e3 1 7 6

Nakajima (Nakajima et al., 2009) also reported a study on the reaction kinetics of two UV filters (EHDPABA and EHMC) with chlorine, and under the conditions that simulate swimming-pool disinfection sites. In agreement with the study of Negreira (Negreira et al., 2008), EHDPABA reacted rapidly with free chlorine and the rate of chlorine consumption increased with the decrease of pH. In comparison with EHDPABA, EHMC reacted more slowly under the same conditions. As referred by the authors, the difference in the reactivity of EHDPABA and EHMC towards chlorine in water can be explained by the chemical structures of their substituents in the benzenic ring, the dimethylamino group in the case of EHDPABA and the methoxyl group in the case of EHMC. The dimethylamino group, being a stronger p-donnor group, activates the benzenic ring for additional chlorine attack, more extensively than the methoxyl group. In what concerns the chlorination by-products of EHDPABA, the authors detected the same compounds found in the previously reviewed work of Sakkas (Sakkas et al., 2003): Cl-EHDPABA, Cl-EHMPABA, Cl2-EHMPABA and Cl-EHPABA, Cl2-EHPABA and also the dichloro form Cl2-EHDPABA, not previously detected by Sakkas (Sakkas et al., 2003). Regarding EHMC, four monochloro-substituted EHMC isomers (Cl-EHMC), and two dichloro-substituted EHMC isomers (Cl2-EHMC), were found.

3.

Conclusions

Increased production and use has led to a redefinition of sunscreens and personal care products as a new class of environmental pollutants. There is an increasing number of papers on the detection and determination of UV filters in the aquatic environment, particularly in bathing waters and in closed system. Subsequently, reports on the environmental fate of these compounds are still rather scarce, which means that a higher level of sensibility is currently the main goal, as additional and more comprehensive data may help us understand the relevance and significance of UV filters and their by-products of degradation in the environment. UV filters are known to undergo photo-isomerization and photodegradation processes upon exposition to natural and/or artificial sunlight, therefore originating a series of by-products that do not have the same protecting capabilities nor the same toxicological profiles of the parent compounds. UV filters are also released by bathers into swimming-pool water and so react with disinfecting agents, like chlorine, giving origin to chlorinated by-products, a group of compounds that has also serious toxicological effects. In outdoor swimming-pool waters, the combined effect of solar radiation and disinfection agents, yields chlorinated compounds of both parent UV filters and photodegradation products. In conclusion, given the relative scarcity of reports on UV filter detection in the environment, as well as degradation studies in both the environment and in the laboratory, focus should therefore converge on both these aspects. Parallel to this, a wider range of the uniformly regulated and approved UV filters should also be approached and studied, since only a very small group of these compounds has ever been investigated in this or in any other relevant context.

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