Applied Clay Science 48 (2010) 349–357 Contents lists available at ScienceDirect Applied Clay Science j o u r n a l h o m e p a g e : w w w e l s ev i e r c o m / l o c a t e / c l a y UV-protection characteristics of some clays T Hoang-Minh a,⁎, T.L Le b, J Kasbohm c, R Gieré d a Hanoi University of Science, Vietnam National University, Hanoi, Viet Nam Institute of Geological Sciences, Vietnamese Academy of Science and Technology, Viet Nam c GeoENcon Ltd & Institut für Geographie und Geologie, Ernst-Moritz-Arndt-Universität, Greifswald, Germany d Institut für Geowissenschaften, Albert-Ludwigs-Universität, Freiburg, Germany b a r t i c l e i n f o Article history: Received 12 August 2009 Received in revised form 19 January 2010 Accepted 24 January 2010 Available online February 2010 Keywords: Ultraviolet radiation Clay Fe2O3 a b s t r a c t The potential of various clays including kaolin, smectite, mixed-layer series-dominated clay and micadominated clay to protect against ultraviolet (UV) radiation in the range 250–400 nm was examined In order to understand the UV-protection abilities of the clays, properties of the clays were also characterized The clays blocked UV radiation and bulk Fe2O3 content played a key role in the UV-protection properties, whereby the higher the bulk Fe2O3 content, the lower the UV-transmission level However, UV-protection ability also depended on the expandability of the clay or the combination between clay mineral and mixed ointment © 2010 Elsevier B.V All rights reserved Introduction UV radiation, a type of electromagnetic radiation with wavelengths ranging from 10 to 400 nm, is well known for its harmful acute and chronic effects on the human skin and eye, e.g., sunburn, skin aging, and the extreme case of skin cancer (de Fabo et al., 1990; Baadsgaard, 1991; Longstreth et al., 1994) On the basis of wavelength, several types of UV radiation are distinguished, with the following being most important: long-wave UV-A (400–320 nm), UV-B (320–280 nm), and short-wave UV-C (280–100 nm) (Coblentz, 1932; Setlow, 1974; Baadsgaard, 1991; Young et al., 1998; Allen, 2001; Lim et al., 2005) UV-A is thought to cause skin aging and erythema or sunburn (Bissett et al., 1989; Diffey, 2002), whereas UV-B may cause DNA damage and skin cancer (Setlow, 1974; Young et al., 1998) UV-C is the highest-energy and most dangerous type of UV radiation, but it is generally absorbed by the ozone layer in the atmosphere (WHO, 2002) Diffey (2002) suggested that the contribution of UV-B to the harmful effects on the human skin is about 80%, with the remaining ∼20% caused by UV-A Because of these health effects of UV radiation, many types of skin creams have been designed specifically for the purpose of UV protection These creams contain various synthetic UV-protection compounds, including organic and inorganic materials However, the non-natural substances present in these creams as main UV-protection agents, e.g micronized TiO2, are known to cause an unexpected photo-catalytic effect, which may be a serious problem because it takes place on the skin (Clechet et al., 1979; Tan et al., 1996; Hidaka et al., 1997; Dunford et al., 1997) Therefore, ⁎ Corresponding author 334 Nguyen Trai Road, Thanh Xuan District, Hanoi, Viet Nam Tel.: + 84 38585097; fax: +84 38583061 E-mail address: hoangminhthao@vnu.edu.vn (T Hoang-Minh) 0169-1317/$ – see front matter © 2010 Elsevier B.V All rights reserved doi:10.1016/j.clay.2010.01.005 natural materials are being sought as replacements for the synthetic UVprotection agents Potential candidates as natural UV-protection agents in sunscreens include clays and clay minerals, which due to their many benefits for human health are already now utilized in various types of pharmaceutical and cosmetic products (Carretero, 2002) In previous studies, sepiolite and smectites have been investigated in terms of their ability to form complexes with organic compounds, which absorb UV radiation — but the function of the clays was considered only as carriers (Vicente et al., 1989; del Hoyo et al., 1998, 2001) Absorption properties in the UV and visible ranges of some clays and clay minerals suspended in water were also characterized (Chen et al., 1979; Babin and Stramski, 2004) To test the role of clay minerals as potential UVprotection agents in skin creams, it is therefore necessary to characterize a variety of clays systematically from this point of view The present study examines the physical and chemical properties of several types of clays and provides systematic data for the efficiency of the UVprotection capabilities in the UV-A and UB-B spectral ranges Our data provide the basis for a comparison of various mineralogical parameters and their potential influence on the UV-protection efficiencies of clays Materials and methods 2.1 Materials The studied clay samples include various types of kaolins (Caminau, Wolfka, Spergau, and Seilitz); bentonites (Garfield, Chambers, Wyoming, SHCa-1, STx-1, SAz-1, and SWy-2); mixed-layer series-dominated clay (Friedland); mica-dominated clays (Plessa, Gorrenberg, Teistungen, and Thierfeld); as well as two dithionite-treated clays (Teistungen–dithionite, from Teistungen; Thierfeld–dithionite, from Thierfeld) 350 T Hoang-Minh et al / Applied Clay Science 48 (2010) 349–357 Non-clay materials include a commercially available sun cream with a sun-protection factor (SPF) of 20 (Ladival® allerg 20 from STADA GmbH, Bad Vilbel, Germany); pharmaceutical substances (Ferrum Oxydatum Flavum [C.I.Nr.77492] and Ferrum Oxydatum Rubrum [C.I.Nr.77491], both from Caesar & Lorentz GmbH; Titandioxid [Eu Rho® Ph.Eur.4], from Euro OTC Pharma GmbH); and cream matrix and cream admixtures (wool– wax–alcohol ointment [SR 90; composed of wool–wax–alcohol, sorbitanum trioleinicum GOT and mainly Vaseline], from Bombastus Werke AG, Plantacare® 2000 (Decyl Glucoside); glycerol, from Merck) 2.2 X-ray diffraction (XRD) Randomly oriented powder samples were studied using a Siemens D5000 X-ray diffractometer equipped with a Cu tube (Kα1,2 radiation) and operated at a current of 30 mA and a voltage of 40 kV Some samples with high Fe contents were re-investigated using a Co tube with an Fe filter XRD analyses were also carried out on oriented mounts, including air-dried, ethylene-glycolated, and heated (to 550 C for h) specimens by means of a Freiberg Präzitronic diffractometer HZG 4A-2 equipped with a Seifert C3000 control unit (Co tube, Kα1,2 radiation, 30 kV, 30 mA) The WinFit program (Krumm, 1994) was used for analyzing the line-profile of clay minerals with broad and strongly overlapping XRD reflections and coherent scatter domains as approximation to the particle thickness distribution The AutoQuan program, which is based on the Rietveld method, was used for the quantification of kaolins (Bergmann et al., 1998; Kleeberg et al., 2005) 2.3 Transmission electron microscopy (TEM) TEM investigations were carried out on the b2 µm fraction by using a Jeol JEM-1210 microscope, operated at 120 kV and equipped with a LaB6-cathode Attached to the microscope are an ISIS LINK-OXFORD energy-dispersive X-ray (EDX) system and a GATAN MULTISCAN camera, which allowed to characterize morphology, crystal habit, and stack order (by electron diffraction) of the particles as well as to obtain element distribution images Selected particles were analyzed chemically (semiquantitative data) by EDX, which allowed for calculation of mineral formulae using the software toolkit of Kasbohm et al (2002) In this contribution, illite, recognized during the TEM investigation, is referred to as illite in the sense of Środón et al (1992), i.e conforming to the following structural formula of illite sensu stricto: FIX0:89 ðAl1:85 Fe0:05 Mg0:10 ÞðSi3:20 Al0:80 ÞO10 ðOHÞ2 where: FIX represents fixed K + Na cations in the interlayer Furthermore, K- and/or charge-deficient dioctahedral micas, with tetrahedral Si ranging from 2.8 to 3.3 per O10(OH)2, are referred to as dioctahedral vermiculite The acronyms “IS-ml” and “diVS-ml” refer to illite/smectite mixed-layer and dioctahedral vermiculite/smectite mixed-layer, respectively a non wetting agent and/or oxidizer Loss on ignition (LOI) was determined at about 1000 °C as an approximate measure of volatile H2O 2.5 Mössbauer spectroscopy Mössbauer spectra of selected samples were recorded at room temperature by an MS-1104Em spectrometer, using Doppler velocity and a 57Co isotope source at St.-Petersburg State University (Russia) The Univem MS program was applied to fit the obtained spectra in order to determine a series of sub-spectra 2.6 Dithionite treatment To remove free Fe phases, such as, oxides and/or hydroxides present in the samples, the dithionite method based on the procedure of Mehra and Jackson (1960) was applied 2.7 UV measurement UV measurements were performed to determine the UV-transmission characteristics of cream samples containing various types of clay Before the measurements of UV transmission could be carried out, the clay samples, ground to b63 µm, were first mixed with glycerol at a mass ratio of 1:2 This mixture was then mixed with a wool–wax– alcohol cream at a ratio of either 10 wt.% or 20 wt.% (clay:wool–wax– alcohol), and subsequently stirred to obtain a homogeneous cream Cream samples were kept 0.003 mm thick for the measurement The UV-transmission characteristics of all cream samples were determined using an AnalytikJenaAG SPECORD 50 photometer, equipped with a UV lamp as light source and operated at an electric potential of 483 V, a current of 0.3 A, and a frequency of 200 Hz The samples were kept at a distance of 10 cm from the light source The measurements were performed at room temperature in the range 250–400 nm For each clay-cream sample, the measurement was repeated 20 times to obtain an average value Some clay-cream samples were measured again after approximately six months, but no difference in UV transmission was noticed The data are expressed in terms of transmission according to the following equation: T = I ì 100ị = I0 %ị where: I0 is the intensity of the incident UV ray; I is intensity of the UV ray after passing through the sample The listed and plotted transmission values represent the value due to the presence of clay in the cream, i.e with the UV-transmission capability of the wool– wax–alcohol cream subtracted The results are displayed from 280 to 400 nm, i.e covering the entire UV-B and UV-A spectral ranges Mineralogical properties of the clays 2.4 X-ray fluorescence (XRF) spectroscopy 3.1 Kaolins Selected bulk samples were analyzed by XRF spectroscopy using a wavelength-dispersive X-ray spectrometer (Philips PW 2404) operated with a current of 10 mA and a voltage of 20 kV The analysis used The Caminau, Wolfka, Spergau, and Seilitz kaolins are known as German reference kaolins (ASMW, 1988) Our XRD results, obtained for randomly oriented powder mounts and for oriented specimens and by Table Mineralogical composition (in wt.%) of kaolins, as determined with the AutoQuan program (Rietveld method) Kaolins Kaolinite 2:1 clay mineral Quartz Caminau Wolfka Spergau Seilitz This study ASMW This study ASMW This study ASMW This study ASMW 83 15 84.9 14.3 1.4 86 12 81.7 2.9 15.4 72 14 13 71.6 12.8 15.0 34 45 21 34.1 45.0 19.4 Note: ASWM = data as certified by ASMW (1988) T Hoang-Minh et al / Applied Clay Science 48 (2010) 349–357 351 Table Mineral phases of selected bentonites and mixed-layer series-dominated clay, as determined by XRD and TEM–EDX measurements (for details, see Hoang-Minh, 2006) Garfield Chambers Wyoming SHCa-1 STx-1 SAz-1 SWy-2 Friedland Nontronite montm Quartz M++-montm M+-montm IS-ml series Kaolinite Calcite Dolomite Quartz M+-montm M++-montm Quartz Feldspar Hectorite M+-montm M++-montm IS-ml series diVS-ml series Halloysite Beidellite Opal-CT M++-Montm M+-Montm M++-montm IS-ml series diVS-ml series IS-ml series diVS-ml series diVerm montm Kaolinite Halloysite Beidellite Anatase/rutile Feldspar Notes: abundant clay minerals are presented in bold fonts montm = montmorillonite; M+ = Na and K; M++ = Ca and Mg; diVerm = dioctahedral vermiculite quantification using Rietveld refinement, were in very good agreement with the certified data for these kaolins (Table 1, see also Hoang-Minh, 2006) Kaolinite was the dominant phase (N72 wt.%) in three kaolins (Caminau, Wolfka, Spergau) The Seilitz kaolin, however, contained only 34 wt.% kaolinite, but showed the highest content of expandable 2:1 clay minerals Other, non-clay minerals identified in all these kaolins included minor amounts of quartz, pyrite, and anatase/rutile 3.2 Bentonites The Garfield, Chambers, and Wyoming bentonites are certified as clay mineral standards by the American Petroleum Institute (API), where they are labeled as H-33a, H-23, and H-25, respectively Mineral phases present in these bentonites were identified by XRD and TEM–EDX analysis (Tables 2, 3) The Garfield bentonite is an Fe- Table Mineral formulae (cations per [O10(OH)2]) of montmorillonite and mixed-layer series, based on TEM–EDX analyses Previously published data are given for comparison Interlayer Octahedral layer 2+ 3+ Al Fe 0.07 0.04 0.00 0.00 0.20 0.12 0.16 0.15 1.79 0.12 1.82 1.84 0.06 0.00 0.00 0.00 1.45 1.37 0.02 0.03 0.22 0.30 0.21 0.01 0.01 0.04 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.28 0.21 0.03 0.03 0.05 0.14 0.08 0.05 0.00 0.07 0.07 0.02 SAz-1: montmorillonite Average (this study) STDV CMSa 0.05 0.03 0.20 0.13 0.03 0.00 SWy-2: montmorillonite Average (this study) STDV CMSa 0.07 0.04 0.06 0.03 0.04 0.00 Tetra layer Mg Ti Si Al 0.00 0.00 0.01 0.00 0.00 0.01 0.02 0.02 0.01 0.02 0.00 0.00 3.60 0.06 3.61 3.46 0.40 0.07 0.39 0.54 (Al, Fe) 0.32 0.18 0.00 0.00 0.18 0.41 0.02 0.02 3.97 3.86 0.03 0.14 1.61 0.08 1.54 1.55 1.55 0.21 0.04 0.17 0.20 0.19 0.00 0.01 0.00 0.01 0.00 0.16 0.06 0.26 0.24 0.26 0.01 0.01 0.00 0.00 0.00 3.97 0.02 3.95 3.96 3.96 0.03 0.02 0.05 0.04 04 0.00 0.70 (Li+) 0.00 0.00 2.30 0.01 3.88 0.11 (Al, Fe) 0.04 0.02 0.01 0.00 0.00 0.00 1.63 0.07 1.57 0.05 0.05 0.05 0.00 0.00 0.00 0.31 0.05 0.36 0.01 0.01 0.02 3.97 0.02 4.00 0.03 0.02 0.00 0.00 0.00 0.18 0.01 0.01 0.01 0.00 0.00 0.00 1.53 0.05 1.36 0.07 0.04 0.06 0.00 0.00 0.00 0.38 0.04 0.56 0.01 0.02 0.02 4.00 0.00 4.00 000 0.00 0.00 0.01 0.01 0.16 0.01 0.01 0.03 0.00 0.00 0.00 1.60 0.05 1.51 0.17 0.02 0.21 0.00 0.00 0.00 0.21 0.04 0.27 0.01 0.01 0.01 3.97 0.02 3.99 0.03 0.02 0.01 1.26 0.15 0.42 0.14 0.00 0.00 0.29 0.06 0.01 0.01 3.76 0.03 0.24 0.03 1.73 0.18 0.17 0.14 0.00 0.00 0.10 0.06 0.02 0.01 3.40 0.02 0.60 0.02 Ca Mg Na K Fe Garfield: nontronite Average (this study) STDV Fialips et al (2002); by IR Besson et al (1983); by XRD, MB 0.02 0.02 0.00 0.00 0.03 0.03 0.00 0.00 0.07 0.05 0.40 0.00 0.02 0.03 0.00 0.57 Chambers: montmorillonite Average (this study) Cuardros (2002); by XRF 0.03 0.28 0.01 0.00 0.16 0.02 Wyoming: montmorillonite Average (this study) STDV Kasbohm et al (1998); by TEM Madsen (1998) Wold and Eriksen (2002); by ICP, CEC 0.02 0.02 0.07 0.00 0.03 0.05 0.03 0.00 0.00 0.01 SHCa-1: hectorite CMSa 0.00 STx-1: montmorillonite Average (this study) STDV CMSa Friedland Illite/smectite mixed-layer (illite = 40%; montmorillonite = 60%) Average (this study) 0.03 0.02 0.21 0.29 0.00 STDV 0.03 0.03 0.11 0.15 0.00 Dioctahedral vermiculite/smectite mixed-layer (diVerm = 80%; montmorillonite = 20%) Average (this study) 0.02 0.11 0.12 0.23 0.01 STDV 0.02 0.08 0.10 0.20 0.02 Fe 2+ Notes: MB: Mössbauer spectroscopy; IR: infrared spectroscopy; ICP: inductively coupled plasma spectroscopy; CEC: cation exchange capacity; STDV: standard deviation a According to CMS source (van Olphen and Fripiat, 1979) 352 T Hoang-Minh et al / Applied Clay Science 48 (2010) 349–357 Table Bulk chemical composition (main components only, in wt.%) of selected samples, as determined by XRF analysis Clay samples SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI H2O− Total Garfield Chambersa Wyoming Friedland Teistungen Teistungen–dithionite Thierfeld Thierfeld–dithionite Caminaub Wolfkab Spergaub Seilitzb 40.1 43.3 58.1 56.7 63.1 64.2 59.2 61.5 46.2 53.8 53.8 61.5 0.07 0.29 0.14 0.94 0.82 0.86 1.64 1.70 0.61 0.16 0.73 0.29 5.54 14.5 16.7 18.1 16.2 16.9 17.6 18.6 37.1 32.8 31.2 23.9 31.8 2.81 3.30 7.31 4.52 4.06 7.89 4.11 1.25 0.34 0.71 1.29 0.01 0.04 0.02 0.04 0.06 0.04 0.09 0.04 0.69 3.29 2.08 2.01 2.18 2.36 2.16 2.27 0.20 0.11 0.36 1.07 2.07 8.18 1.58 0.54 0.80 0.61 0.61 0.51 0.14 0.13 0.18 0.27 0.23 0.12 1.69 0.96 1.70 2.09 0.85 1.32 0.03 0.02 0.07 0.04 0.17 0.06 0.39 3.01 3.41 3.74 3.35 3.48 1.53 0.10 1.09 4.09 0.050 0.027 0.045 0.106 0.346 0.195 0.337 0.259 6.50 11.7 5.44 7.24 4.10 4.09 5.01 5.25 12.9 12.1 11.2 7.18 13.9 13.5 8.76 2.65 1.05 0.95 0.77 0.74 101.1 99.0 98.3 99.6 98.4 100.1 99.6 99.8 a b The total includes 1.1 wt.% of fluorine According to data as certified by ASMW (1988) rich (Table 4) clay dominated by nontronite, whereas the Chambers and Wyoming bentonites are Al-rich clays dominated by montmorillonite The Fe content of the Garfield bentonite was studied in more detail by Mössbauer spectroscopy, which revealed that it contained ∼ 22 wt.% structural Fe, which was slightly higher than that of nontronite (20 wt.%) calculated from the statistical data of Stucki (2006) No sub-spectrum of tetrahedral Fe3+ was observed (Table 3) The samples SHCa-1, STx-1, SAz-1, and SWy-2 are reference bentonites of the Clay Mineral Society (CMS) and were described in many previous studies (van Olphen and Fripiat, 1979; Chipera and Bish, 2001; Vogt et al., 2002) Here, we investigated these bentonites particle by particle using TEM–EDX With the exception of SHCa-1, which was dominated by hectorite, these reference bentonites were dominated by montmorillonite (Table 2) The average formulae of the dominant phases were calculated from the TEM–EDX analyses, and they were in good agreement in regard to the Fe-component in the octahedral sheet with those reported by other studies (Table 3) However, there were differences in tetrahedral and interlayer components, which may be caused by alteration processes as described by Kasbohm (2003) the report of Kranz et al (1990) During our investigation, non-clay minerals, including hematite, quartz, feldspar (plagioclase and orthoclase), and anatase/rutile were also observed Ammann (2003) calculated the proportion of illite in the bulk clay of the Plessa, Teistungen and Thierfeld samples, and obtained values of 52 wt.%, 77 wt.% and 80 wt.%, respectively To study in more detail the nature of the mica-like and smectite phases, TEM–EDX was performed — these clays contained dioctahedral vermiculite, diVS-ml series, IS-ml series, with their abundances and mineral formulae as shown in Tables and The Teistungen and Thierfeld clays are both relatively rich in Fe (Table 4) The Mössbauer spectra and TEM–EDX data revealed that Fe was present in these samples mainly as Fe3+ in hematite and in clay minerals (Table 6; see also Hoang-Minh, 2006) Hematite made up approximately 1.4 wt.% in the Teistungen and 4.7 wt.% in the Thierfeld samples (HoangMinh, 2006) Treatment of these samples by the dithionite method showed that the hematite removal from the Teistungen sample was much less pronounced than for the Thierfeld sample (Table 4) UV-protection properties of the clays 3.3 Mixed-layer series-dominated clay The Friedland clay was supplied by the Pilot Vegetable Oil Technology Magdeburg e.V (PPM e.V.) Its general characteristics were published by Henning and Kasbohm (1998) and Kasbohm (2003) Here, the Friedland clay was re-investigated by XRD, TEM– EDX, XRF, and Mössbauer spectroscopy in order to characterize in greater detail the clay mineral phases The mineralogical results of this re-investigation are shown in Tables and The bulk chemical composition of the Friedland sample, as determined by XRF spectroscopy, is presented in Table The Mössbauer spectra suggested that the Friedland sample contained the Fe3+-ion in cis- and trans-octahedral positions of mixed-layer series, the Fe2+-ion in dioctahedral vermiculite and trace amounts of the Fe2+-ion in chlorite (Hoang-Minh, 2006) However, chlorite could not be found by XRD and TEM because the amount of this mineral was too low to be detected by XRD and because its particle size may have been too large for TEM preparation The fine particles (b2 µm) of dioctahedral vermiculite, which are included in the mixed-layer series with the formula presented in Table 3, did not contain octahedral Fe2+ Therefore, the Fe2+ observed by Mössbauer spectroscopy in dioctahedral vermiculite is probably present in the coarse particles, which were not analyzed with the TEM–EXD method 3.4 Mica-dominated clays The selected mica-dominated clay samples (Plessa, Gorrenberg, Teistungen, Thierfeld) were studied by XRD, which revealed that they contained mica-like phases, kaolinite, and chlorite, in agreement with The measured UV-protection ability of the different clays in clay cream is expressed as UV transmission and plotted against the wavelength from 280 nm to 400 nm (Figs 1–3) Table shows the data for three selected wavelengths: 280 nm (short-wave end of the UV-B range), 300 nm (the most effective wavelength in terms of skin damage; see Young et al., 1998), and 400 nm (end of the UV-A range) When the term “average UV transmission” is used below, the value refers to the average UV transmission at these three wavelengths To survey the UV-protection ability of the well-characterized clays, some samples were selected for preparation of a cream with two different clay mass ratios: 10 wt.% and 20 wt.% For the creams containing the Caminau kaolin and the Chambers bentonite, the UVtransmission profiles obtained from the two mass ratios showed very similar trends (Fig 1) The creams with 20 wt.% clay, however, exhibited distinctly lower UV-transmission values than the creams containing 10 wt.% clay For example, the average UV-transmission value of the cream containing 20 wt.% Caminau clay was 28.8%, whereas the Table Proportions of identified mixed-layer series of mica-dominated clays, as determined by TEM–EDX measurements Clay mineral Plessa Gorrenberg Teistungen Thierfeld diVerm diVS-ml IS-ml Smec 31% 13% 48% 7% 24% 11% 53% 13% 37% 13% 46% 5% 22% 13% 62% 3% Notes: diVerm = dioctahedral vermiculite; Smec = smectite T Hoang-Minh et al / Applied Clay Science 48 (2010) 349–357 353 Table Mineral formulae (cations per [O10(OH)2]) of diVerm., diVS-ml, IS-ml and smectite from German clays, based on TEM–EDX analyses Interlayer Octahedral layer Tetra layer Ca Mg Na K Fe2+ Al Fe3+ Fe2+ Mg Ti Si Al 0.17 0.04 0.06 0.03 0.04 0.13 0.26 0.26 0.13 0.04 0.03 0.02 0.14 0.29 0.21 0.26 0.04 0.02 0.01 0.00 1.81 1.61 1.61 1.59 0.13 0.22 0.25 0.30 0.00 0.00 0.00 0.00 0.06 0.06 0.13 0.10 0.02 0.08 0.03 0.02 3.20 3.19 3.14 3.12 0.81 0.81 0.86 0.88 Dioctahedral vermiculite/smectite mixed-layer Plessa — 70/30 0.05 0.06 Gorrenberg — 60/40 0.02 0.13 Teistungen — 70/30 0.02 0.09 Thierfeld — 70/30 0.01 0.05 0.02 0.01 0.02 0.04 0.25 0.16 0.28 0.36 0.00 0.01 0.02 0.00 1.81 1.62 1.75 1.71 0.12 0.24 0.15 0.14 0.00 0.00 0.00 0.00 0.07 0.12 0.08 0.13 0.01 0.03 0.02 0.04 3.48 3.55 3.48 3.48 0.52 0.45 0.52 0.52 Illite/smectite mixed-layer Plessa — 50/50 Gorrenberg — 40/60 Teistungen — 50/50 Thierfeld — 50/50 0.07 0.02 0.00 0.02 0.03 0.12 0.06 0.05 0.09 0.00 0.05 0.02 0.33 0.26 0.43 0.40 0.00 0.00 0.00 0.00 1.57 1.50 1.72 1.51 0.14 0.27 0.12 0.25 0.00 0.00 0.03 0.04 0.24 0.18 0.04 0.11 0.01 0.00 0.00 0.03 3.71 3.79 3.73 3.69 0.29 0.21 0.27 0.31 Beidellite Plessa Gorrenberg Teistungen Thierfeld 0.12 0.03 0.03 0.01 0.03 0.04 0.05 0.03 0.03 0.02 0.01 0.13 0.04 0.05 0.08 0.08 0.00 0.05 0.03 0.12 1.88 1.78 1.87 1.93 0.16 0.25 0.17 0.13 0.00 0.01 0.00 0.00 0.05 0.00 0.01 0.00 0.00 0.04 0.02 0.02 3.38 3.46 3.49 3.45 0.62 0.54 0.51 0.55 Montmorillonite Plessa 0.00 0.00 0.20 0.20 0.00 1.32 0.35 0.00 0.32 0.00 3.96 0.04 Dioctahedral vermiculite Plessa Gorrenberg Teistungen Thierfeld Notes: the values are average; 70/30 (and all analogue ratios) means a mixed-layer series with 70% of the first member (dioctahedral vermiculite or illite) and 30% of the second member (smectite); mixed-layer series did not include end-members corresponding value of the cream with 10 wt.% was 43.7% (Fig 1a) Similar average UV-transmission values were also observed for the creams containing the two different concentrations of Chambers bentonite (34.5% and 40.6%, respectively) The creams containing the Chambers clay, however, displayed UV-transmission profiles that exhibited a distinct minimum around 289 nm (Fig 1b), a feature that was not observed in the creams containing the Caminau kaolin Increasing the amount of clay added to the cream clearly reduced the UV transmission considerably To compare the UV-protection ability of the various types of clays, the data for creams with a clay content of 20 wt.% are presented below The protection capacity of the Caminau, Wolfka, Spergau, and Seilitz kaolins ranged from approximately 60% to 80% of the incident UV rays, i.e their UV-transmission values ranged from 40% to 20% (Fig 2a) No major difference was observed between the creams containing the four types of kaolin, even though the Seilitz kaolin had a markedly different mineralogical composition compared to the other kaolins (Table 1) Conversely, the cream samples containing bentonite showed distinct UV-transmission behavior (Fig 2b) — whilst the Garfield bentonite, consisting primarily of nontronite (Table 2), showed the lowest transmission values across the entire spectral range, the STx-1 sample, which consists mostly of montmorillonite, showed the highest transmission values — the values being higher than those of the studied kaolins (Fig 2a) The cream containing the Garfield bentonite allowed for a passage of only approximately 9% to 28% of the incident rays (at 280 nm and 400 nm, respectively), much less than the creams containing the other bentonites The Chambers bentonite displayed a distinct UV-protection behavior, especially in the UV-B range The Wyoming bentonite, which is commonly used in cosmetics and pharmaceutical products, exhibited an intermediate UV-protection capacity for all Fig UV-transmission values for cream samples containing 10 wt.% and 20 wt.% of (a) Caminau kaolin, and (b) Chambers bentonite 354 T Hoang-Minh et al / Applied Clay Science 48 (2010) 349–357 Fig UV-transmission values for creams containing 20 wt.% of (a) kaolins, (b) bentonites, (c) mixed-layer series-dominated clay, and (d) mica-dominated clays wavelengths; the protection capacity was parallel to, but approximately 10% lower than that of the Garfield bentonite The mixed-layer series-dominated Friedland clay was characterized by a UV transmission that ranges approximately from 19% to 33% (Fig 2c) The values for the cream containing the Friedland clay were higher than those exhibited by the Garfield bentonite, but lower than those of other bentonites and kaolins All creams containing the mica-dominated clays exhibited low UVtransmission values and very similar profiles, all characterized by a distinct minimum at ∼298 nm (Fig 2d) They also showed a wavelengthdependent variation in UV transmission that is similar to that of the Chambers bentonite (Fig 2b), but the protection ability was more pronounced The cream containing the hematite-rich Thierfeld clay displayed particularly high UV-protection capacities across the entire Fig UV-transmission values in relation to Fe2O3 content of two mica-dominated clays: (a) Thierfeld, and (b) Teistungen, their dithionite-treated equivalents, and of two pharmaceutical substances (Fe2O3 and Fe(OH)3) T Hoang-Minh et al / Applied Clay Science 48 (2010) 349–357 355 Table UV transmission (in % of incident radiation at three selected wavelengths) of the cream samples containing 20 wt.% of the respective clays Samples Caminau Wolfka Spergau Seilitz Wyoming Chambers Garfield SHCa-1 STx-1 SAz-1 SWy-2 UV transmission (%) at Samples 280 nm 300 nm 400 nm 25.8 29.9 24.0 20.1 19.7 35.0 8.7 30.6 37.1 24.6 22.2 26.4 28.1 28.4 24.9 25.3 23.7 16.8 33.3 38.9 29.4 26.2 34.1 34.5 39.1 34.7 40.4 44.8 28.0 43.4 53.1 43.2 38.9 UV transmission (%) at Friedland Plessa Gorrenberg Teistungen Thierfeld Teistungen–dithionite Thierfeld–dithionite Ferrum Oxydatum Rubrum [Fe2O3] Ferrum Oxydatum Flavum [Fe(OH)3] Ladival® allerg 20 spectrum (Fig 2d), whereas the lowest values were observed for the Plessa clay At the short-wavelength (high-energy) end of the UV-B spectrum, the mica-dominated group transmitted between ∼23% (Plessa) and ∼8% (Thierfeld), and showed even lower transmission values at 298 nm (13% and 5%, respectively) At larger wavelengths, the UVtransmission capability increased gradually to ∼32% and ∼15%, respectively (Table 7) At the short-wavelength end of the UV-B spectrum, between 280 and 286 nm, the cream containing 20 wt.% of the Thierfeld clay exhibited lower transmission values than one of the commercially available sunscreens with an SPF of 20 (Ladival® allerg 20) In summary, the UV-transmission characteristics were very different for different types of clay, whereby some of the clays, e.g the Thierfeld mica-dominated clay, exhibited particularly low UV transmission at the high-energy end of the UV spectrum, which is especially harmful to the skin Discussion and conclusion Clays show potential for UV protection through absorption or reflection of UV radiation The studied samples of creams containing different clays exhibited different levels of UV transmission, and the transmission values varied across the UV-A and UV-B spectral ranges Several parameters, including grain size distribution and chemical composition, could play an important role in determining the UVprotection ability of clays and clay minerals However, Hoang-Minh (2006) did not observe an influence of grain size distributions on the 280 nm 300 nm 400 nm 18.8 23.3 19.1 14.8 8.3 14.7 13.3 0.6 0.3 15.0 22.4 13.5 11.0 9.5 4.6 10.2 13.3 0.2 b 0.1 1.6 32.9 31.5 25.5 22.1 15.1 24.7 25.2 0.2 b 0.1 0.7 UV-transmission value of the clay-cream samples Therefore, another factor must be responsible for the observed effect 5.1 Iron effect on UV-protection ability of clays and clay minerals The UV-transmission values for the nontronite-dominated Garfield bentonite and for the mica-dominated Thierfeld and Teistungen clays are low compared to all other samples (Fig 2), suggesting that the low values might be associated with the high Fe contents of these materials (Table 4) To test this hypothesis, we determined the UV transmission for the Thierfeld and Teistungen clays after dithionite treatment The results for the Thierfeld clay documented that the cream samples containing 20 wt.% of untreated material exhibited transmission values ranging from ∼5% to 15%, whereas the cream containing the dithionitetreated clay showed higher values, which range from ∼13% to 25% (Fig 3a, Table 7) For the Teistungen clay, the observed trend was similar, albeit much less pronounced (Fig 3b) Inspection of Fig reveals that the UV-transmission characteristics of the creams containing the dithionite-treated Thierfeld and Teistungen clays are very similar in terms of both shape and absolute values The two dithionitetreated clays also showed similar contents of Fe2O3 (Table 4) The main difference between the original and the dithionite-treated samples of the Thierfeld and Teistungen clays was the reduced hematite content The magnitude of the shift towards higher transmission values from original to dithionite-treated samples seems to be related to the extent of reduction in Fe2O3 content: the average UV-transmission value of the Fig Relationship between bulk Fe2O3 content of the studied clay samples and their average UV-transmission value (a) Expandable clays; (b) non-expandable clays UVtransmission values are for cream samples containing 20 wt.% clay Symbols: solid triangles: bentonites including Chambers, Wyoming, SHCa-1, STx-1, SAz-1, and SWy-2; empty triangle: Friedland sample; dashed curve: exponential fit of the bentonite and mixed-layer series-dominated clays; dark squares: kaolins including Caminau, Wolfka, Spergau, and Seilitz; dots: mica-dominated clays including Plessa, Gorrenberg, Teistungen, Teistungen–dithionite, Thierfeld, and Thierfeld–dithionite; solid line: fit of the kaolins and micadominated clays 356 T Hoang-Minh et al / Applied Clay Science 48 (2010) 349–357 Thierfeld samples was shifted by about 7.9%, whilst the Fe2O3 content was reduced by ∼3.8 wt.% (see Table 4) For the Teistungen samples, the corresponding values were much less pronounced, i.e a shift by 1.1% in the average UV transmission, which is associated with a reduction in Fe2O3 by 0.5 wt.% Fig further shows that the average UV-transmission values for the pharmaceutical substances Ferrum Oxydatum Rubrum (Fe2O3) and Ferrum Oxydatum Flavum are ∼0.3% and 0.1%, respectively (cf Table 7) In regard to the octahedral Fe in the structure of clay minerals, the Garfield bentonite, characterized as pure nontronite with 1.79 Fe3+ per [O10(OH)2], showed distinctly lower UV-transmission values than the Wyoming bentonite (Fig 2b), which was characterized as dominated by Al-rich smectite with only 0.21 Fe3+ per [O10(OH)2] These values demonstrate that the Fe content of clay minerals has a significant effect on the UV-protection behavior The Fe phases in kaolins were mostly non-clay minerals (pyrite, goethite, hematite, ilmenite, jarosite, limonite, magnetite, siderite, titanomagnetite (ASMW, 1988)), except for the Seilitz kaolin, which had the highest amount of 2:1 clay minerals (45%, mostly as IS-ml with 90% of illitic layers) Mica-dominated clays were composed mostly of dioctahedral vermiculite and diVS-ml Therefore, in addition to Fe in non-clay phases, the mica-dominated and dithionite-treated clays contained Fe primarily as Fe3+ in the octahedral sheets of micalike components With two notable exceptions, the studied bentonites contained no Fe-bearing phases other than clay minerals: SAz-1 contained magnetite, and Garfield contained traces amounts of maghemite The major part of the Fe present in the bentonites and the mixed-layer series-dominated clay occurred as Fe3+-ion in smectites or mixed-layer series To further test the hypothesis that the Fe2O3 content of the clay samples influences their UV-transmission behavior, the average UVtransmission values were plotted against the bulk Fe2O3 content of the clays (Fig 4) In this diagram, an exponential relationship is observed for the studied expandable clays, i.e bentonites and mixed-layer series-dominated clay (Fig 4a), whereas a linear relationship is obtained for the non-expandable clays, i.e kaolins, mica-dominated clays, and dithionite-treated clays (Fig 4b) Fig demonstrates that the relationship between bulk Fe2O3 content and UV transmission of non-expandable clays is different from that of expandable clays This difference could be caused by the arrangement or distribution of the non-expandable clay minerals and the expandable clay minerals in the cream samples Using XRD measurement for some selected cream samples, we found that whilst cream samples of Wolfka and Thierfeld showed the same reflection positions in comparison with their airdried specimens, the cream samples of Garfield, Wyoming and Friedland showed a clear shift of the expandable clay mineral reflections to lower º2Θ values (Fig 5) These shifts demonstrate that a part of the wool–wax–alcohol ointment (liquid Vaseline) can enter the interlayer space of the expandable clay minerals such as nontronite, montmorillonite and mixed-layer series Conversely, the ointment distributes in the voids between the mineral particles of the non-expandable clays These different behaviors also result in different textures of the cream samples (Hoang-Minh, 2006) An increase in the total Fe2O3 content of all studied clays yielded a lower UV transmission (Fig 4) The Fe-bearing clay minerals and Febearing non-clay minerals are responsible for this effect At a given Fe2O3 content, the protection abilities of bentonite and mixed-layer structures were lower than those of kaolin and mica-dominated clays 5.2 Role of Fe-electron configuration As discussed above, hematite was found to contribute to the UVtransmission behavior Hematite (Fe2O3) contains Fe3+ bonded to O2− The electron configuration of Fe3+ is characterized by an empty orbital (4s), so that inner electrons that absorb energy can move from the 3d orbital to the 4s orbital with a higher energy level (Hoang-Minh, 2006) This absorption takes place in the near-UV part of the electromagnetic spectrum, as has been demonstrated by experiments and in electronic spectra of Fe3+ oxides (including hematite; Sherman and Waite, 1985) When combining the theoretical and experimentally determined properties of hematite with our UV measurements, it can be concluded that hematite plays an important role in the UV-protection behavior of clay matter The octahedral-sheet Fe3+ in the structure of clay minerals also has an electron configuration with an empty orbital, and thus, it should theoretically absorb photons Indeed, Chen et al (1979) have shown experimentally that the absorption intensity in the UV-C range was directly correlated to structural octahedral contents of Fe3+ in smectites Similarly, our own experiments demonstrated that octahedral-sheet Fe3+ also had this effect in the case of the Garfield sample In summary, our results show that each clay cream had a different average UV-transmission value indicating that clays can absorb UV by themselves, without additives We conclude that the bulk Fe2O3 content of a clay played a key role in determining the UV-protection ability of Fig XRD patterns of selected clay samples and their cream samples, °2Θ CoKα position Notes: clay, AD: air-dried specimen of clay sample; clay cream: specimen of clay-cream sample containing 20 wt.% clay T Hoang-Minh et al / Applied Clay Science 48 (2010) 349–357 this material Our results further showed that the UV-protection ability also depended on whether the clay is expandable or non-expandable The average UV-transmission level obtained from samples containing 20 wt.% of clay in a wool–wax–alcohol cream showed a negative linear correlation with the total Fe2O3 content of non-expandable clays, but an exponential relationship for expandable phases Most of the UV-transmission values of the studied clay-cream samples were still higher than those of the commercial sun cream Ladival® allerg 20 Furthermore, the clays with high UV-protection potential, especially those characterized by high Fe-oxide and -hydroxide contents, are due to their color not well-suited for the production of sun-protection creams Therefore, the clays exhibiting a high UV-protection capability should be studied in more detail and from different perspectives to determine their potential as additives for sun-protection creams It is also suggested that other clays, or clay mixtures, be tested in view of their application as UV-protection agents Acknowledgements This study is part of the Ph.D project of the first author, which was carried out at the Ernst-Moritz-Arndt-University of Greifswald, Germany The research was supported by the Ministry of Education and Training of Viet Nam and was organized by the “Joint Educational Training Center Hanoi-Greifswald.” We thank the Institute of Pharmacy, University of Greifswald for access to the UV-photometer; Dr Sofia Lessovaia (St Petersburg State University, Russia) for Mössbauer spectroscopy analysis; and the TRIG A project of the Hanoi University of Science (Vietnam National University, Hanoi, Viet Nam) for the support to complete the manuscript at the University of Freiburg, Germany We are very grateful to two anonymous reviewers and to the editor, Prof Gerhard Lagaly, for their valuable comments and suggestions, which helped us to improve the final version of this manuscript References Allen, J., 2001 Ultraviolet Radiation: How It Affects Life on Earth — on NASA's Earth Observatory website URL: http://earthobservatory.nasa.gov/Library/UVB/printall php (accessed January 20, 2006) Ammann, L., 2003 Cation Exchange and Adsorption on Clays and Clay Minerals Dissertation — Christian-Albrechts-Universität Kiel ASMW, 1988 Katalog der vom ASMW geeichten Normalproben.- Amt für 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from that of expandable... transmission at these three wavelengths To survey the UV-protection ability of the well-characterized clays, some samples were selected for preparation of a cream with two different clay mass ratios:... diffractometer equipped with a Cu tube (Kα1,2 radiation) and operated at a current of 30 mA and a voltage of 40 kV Some samples with high Fe contents were re-investigated using a Co tube with an Fe filter