TiO2 composite

Composites: Part B 67 (2014) 262–269

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The stability of photoactive kaolinite/TiO2 composite Michaela Tokarcˇíková a,⇑, Jonáš Tokarsky´ a,b, Kristina Cˇabanová a, Vlastimil Mateˇjka a, Katerˇina Mamulová Kutláková a, Jana Seidlerová a,⇑ a b

Nanotechnology Centre, VŠB-Technical University of Ostrava, 17. listopadu 15/2172, 708 33 Ostrava–Poruba, Czech Republic CE IT4Innovations, VŠB-Technical University of Ostrava, 17. listopadu 15, 708 33 Ostrava, Czech Republic

a r t i c l e

i n f o

Article history: Received 31 March 2014 Received in revised form 13 June 2014 Accepted 8 July 2014 Available online 18 July 2014 Keywords: A. Nano-structures B. Chemical properties C. Computational modeling D. Non-destructive testing Aquatic toxicity test

a b s t r a c t Nanoparticles are increasingly used in many applications. However, not only their positive properties are important but also their potential impact on environment. This article deals with the stability and potential toxicity of photoactive composite kaolinite/TiO2 with verified photoactive properties. Clay mineral kaolinite, calcined kaolinite and dried or calcined composite containing of photoactive oxide TiO2 were studied by leaching test, aquatic toxicity tests and molecular modeling. Materials were leached in deionized water and acidic or basic extraction agents. The conductivity, pH, and leached elements were determined in leachates. Desmodesmus subspicatus or Chlorella vulgaris were used to the algal toxicity tests and the inhibition of growth was monitored. Changes in clay mineral and photoactive composite were evaluated on the basis of leached elements and results of aquatic toxicity tests. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Titanium dioxide is a mineral commonly occurring in three modifications: anatase, brookite and rutile. Titanium dioxide in the form of anatase has promising photoactive properties. The main application of anatase is in photocatalysis, where it is widely used for decomposition of organic substances (such as phenols, toluene or formaldehyde) and gases (such as CO2, NOx) in waters and air [1–4]. The rate of decomposition changes can be evaluated by using discoloration of dyes during the exposure to irradiation in the UV–VIS region [5]. Despite this advantage, the anatase form is more toxic than rutile form [6]. TiO2 nanoparticles (NPs) are widely used in various products such as cosmetics, paints and white pigments. The surface of TiO2 NPs has antibacterial, fungicidal and self-cleaning effects. They can be used for cleansing water or in many technological applications, such as photovoltaic solar cells, sensors and photocatalysis [7]. Since the usage of photoactive TiO2 NPs is rapidly increasing in many industries, there is an up-sloping risk both for the environment and all living organisms. A comprehensive toxicity assessment including modified acute (72 h) and chronic (21 days) toxicity tests, using Daphnia magna as a model organism were conducted in [8]. NPs TiO2 exerted minimal toxicity to D. magna within a traditional 48-h-exposure time. The prolonged exposure time to NPs up to 72 h had an adverse effect ⇑ Corresponding authors. Tel.: +420 597 321 552; fax: +420 597 321 640. E-mail address: [email protected] (M. Tokarcˇíková). http://dx.doi.org/10.1016/j.compositesb.2014.07.009 1359-8368/Ó 2014 Elsevier Ltd. All rights reserved.

on aquatic organisms and showed possible risks to aquatic ecosystem. A high concentration of TiO2 NPs owing to their antimicrobial properties [6], promote the growth of plant roots. Effects of nano-scale TiO2 and their bulk counterparts on zebrafish, acute toxicity, oxidative stress, and oxidative damage were studied by Xiong et al. [9]. The authors found out that although the size distribution of NPs in suspension was similar to that of the bulk particles, the acute toxicity of the TiO2 NPs to zebrafish was greater than that of the bulk TiO2, which was essentially non-toxic. TiO2 NPs were able to induce toxicity effects without entering the cells and the extracellular OH generated by TiO2 NPs could induce oxidative damage directly on the cell membranes of gill tissue. Other study on mice, as tested organisms, observed that after exposing mice to TiO2 NPs, they accumulate in their lungs and cause pulmonary inflammation and bleeding [10]. With regard to previously mentioned environmental risk of TiO2 NPs (as well as of all nano-particles) it is important to put emphasis on diminishing the possible release of NPs into environment. One of the ways is to anchor the TiO2 NPs on the suitable material, forming new nano-structured composites. Clay minerals can be considered suitable because clays are natural materials, inexpensive, commonly available, non-toxic and harmless to environment. Phyllosilicates can be used for example as sorbents of heavy metals cations or for fixing the metal particles or metal oxide NPs [11]. Kaolinite theoretical formula is Al2Si2O5(OH)4, other formulas are Al2O32SiO22H2O and Al2Si2O72H2O. Tetrahedral silicate sheets bonded to octahedral

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aluminum oxide/hydroxide sheets form the structure of kaolinite (Fig. 1). After the calcination of kaolinite at 400–650 °C, the process of kaolinite dehydroxylation results in formation of metakaolinite [12]:

Si2 Al2 O5 ðOHÞ4 ! Si2 Al2 O5 ðOHÞx O2x þ ð2  x=2ÞH2 O with a low value of x (about 10% of residual hydroxyl groups in metakaolinite). Disordered structure of metakaolinite possesses a huge reactive potential. With respect to application areas of photocatalysis, both their photoactive properties as well as their stability in different environment are of importance. As it was mentioned before, the TiO2 NPs may be potentially dangerous to environment and it is necessary to monitor their stability. There are several approaches for testing it. Molecular modeling is a theoretical method enabling to estimate whether or not the material is stable. Ecotoxicity tests are other methods performed on testing plants and organisms. The methods used for monitoring the leaching of wastes are also applicable to composite materials as they are considered as waste. There are many articles dealing with leaching tests on industrial sludge [13], construction and demolition wastes [14], soils [15], etc. The main objectives of this study are to assess (a) the mobility of selected elements from photoactive composite, and (b) their possible influence on the aquatic system. Experimental tests and molecular modeling were used to evaluate the stability of kaolinite/TiO2 photoactive composite. Algal ecotoxicity tests were conducted on the freshwater algae Desmodesmus subspicatus and green algae Chlorella vulgaris for 72 h. The leaching test was used to determine ions leached from composites to solution during 24 h.

2. Experimental details 2.1. Materials and methods Being dried for 3 h at 105 °C, kaolinite sample SAK47 (LB MINERALS s.r.o.) was used to prepare the kaolinite/TiO2 composite. Thermal hydrolysis of KA and TiOSO4 suspension were used to prepare photoactive composite. The KA was mixed with an appropriate volume of TiOSO4 in order to reach ideal theoretical yield of TiO2 60 wt.%. The resulting solid phase was separated by decantation and washed with distilled water until the conductivity of filtrate reached the value lower than 1  102 lS cm1 to remove sulphate. Composite was further dried at 105 °C (KATI16) or calcined at 600 °C (KATI66). Preparation and photoactivity study of kaolinite/TiO2 composites with different content of TiO2 NPs is described in detail in [5]. The TiO2 NPs size is 7 nm (KATI16) and 19 nm (KATI66). Selected composite showed photoactivity but

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the calcined kaolinite/TiO2 composite had better photoactive properties (KATI66) [5]. 2.2. X-ray fluorescence Chemical composition of the prepared samples was determined using energy dispersive fluorescence spectrometer (XRFS) SPECTRO XEPOS (SPECTRO Analytical Instruments GmbH) equipped with 50 W Pd X-ray tube. The samples were prepared in the form of pressed tablets (wax was used as binder) for this measurement. The chemical composition of dried kaolinite (KA), calcined kaolinite at 600 °C (KA6), photoactive composite KATI16 and KATI66 is shown in Table 1. 2.3. X-ray powder diffraction The X-ray powder diffraction (XRPD) patterns were recorded under Co Ka irradiation (k = 1.789 nm) using Bruker D8 Advance diffractometer (Bruker AXS, Germany) equipped with a fast position-sensitive detector VÅNTEC 1. Measurements were carried out in a reflection mode, powder samples were pressed in a rotational holder. Phase composition was evaluated by using database PDF 2 Release 2004 (International Centre for Diffraction Data). 2.4. Leaching test The leachates were prepared in accordance with European technical standard EN 12457-2. Materials KA, KA6, KATI16 and KATI66 were leached in deionised water (denoted as extraction agent DM) and in extraction agents with pH 2 ± 0.2 or pH 12 ± 0.2 (solid: liquid = 1:10) in continuous rotation container for 24 h. Extraction agent with pH 2 (denoted as extraction agent I) was modified by concentrated H2SO4 (96%, p.a. MACH CHEMIKÁLIE, s.r.o.) and basic extraction agent with pH 12 (denoted as extraction agent II) was modified by NaOH (solid, MACH CHEMIKÁLIE, s.r.o.). After mixing for 24 h the mixture was centrifuged (3000 rpm) for 30 min and then filtered through the filter paper with density of 84 g/m2 in order to separate the solid phase. Conductivity and pH in the final extracts were determined immediately after filtration. The conductivity was measured by inoLab Cond 730 and pH was measured by inoLab SenTix 41. Conductivity and pH in the initial extraction agents were also determined. After stabilization of the extracted solution with HNO3 (65%, p.a.) the concentration of Ti, Al, Si, Na, K and Mg was determined by AES–ICP SPECTRO CIROS VISION (determination of Al, Si, Mg, Ti) or flame AAS UNICAM 969 (determination of Na, K). Photodegradation activity of composite after leaching test was evaluated by procedure described in detail in article of Mamulová Kutláková et al. [5].

Table 1 Chemical composition of kaolinite (KA), kaolinite calcined at 600 °C (KA6), dried kaolinite with 51 wt.% of TiO2 (KATI16) and kaolinite with 58.3 wt.% of TiO2 calcined at 600 °C (KATI66).

Fig. 1. Structure of kaolinite.

Al2O3 SiO2 SO3 TiO2 Na2O K2O MgO CaO Fe2O3 LOI

KA (wt.%)

KA6 (wt.%)

KATI16 (wt.%)

KATI66 (wt.%)

35.8 ± 0.1 47.7 ± 1.0 0.001 ± 0.1 0.84 ± 0.1 0.097 ± 0.1 1.16 ± 0.1 0.06 ± 0.1 0.12 ± 0.1 0.57 ± 0.1 11.6 ± 0.1

42.2 ± 0.1 51.6 ± 1.0 0.021 ± 0.1 0.40 ± 0.1 0.098 ± 0.1 1.64 ± 0.3 0.27 ± 0.1 0.09 ± 0.1 0.527 ± 0.1 1.84 ± 0.4

11.9 ± 0.2 18.5 ± 0.4 0.941 ± 0.1 51.0 ± 1.0 0.46 ± 0.1 0.38 ± 0.1 0.07 ± 0.1 0.08 ± 0.1 0.214 ± 0.1 16.0 ± 0.3

13.5 ± 0.3 22.1 ± 0.4 0.314 ± 0.1 58.3 ± 1.2 0.38 ± 0.1 0.45 ± 0.1 0.05 ± 0.1 0.04 ± 0.1 0.24 ± 0.1 4.60 ± 0.1

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2.5. Aquatic toxicity tests

Ead ¼ Etot  ðEtot;TiO2 þ Etot;KA Þ

Algal toxicity tests are commonly used to test the toxic effects of samples on aquatic autotrophic organisms. In this study the toxicity assessment was performed in accordance with the OECD 201 methodology, i.e. monitoring the growth inhibition of organisms (D. subspicatus) after 72-h exposure to the tested material [16]. Results of experiments are compared with the control samples and are given as percentage of inhibition/stimulation. The increased concentration of salts in the studied samples can be assumed to significantly modify the toxic response of the organism. With D. subspicatus being sensitive to relatively low salt concentrations, the second detection organism, freshwater green algae C. vulgaris was used for its higher tolerance to increased salt content in the aqueous environment [17,18]. To determine the acute aquatic toxicity of the aqueous leachates and suspensions of the studied samples it was necessary to adjust pH value to be within the physiological range of 8.1 ± 0.2 for the given species. Concentration series were prepared both from the aqueous leachates and the control samples. The concentration series and control samples were inoculated with the same volume of the algae suspension to achieve the cell concentration of 10,000 cells per cm3. Thus the prepared test samples were cultivated ensuring constant temperature and light conditions for 72 h. Algal cell density was measured by using Olympus CX31 light microscope and Bürker counting chamber.

where Etot is the total energy of the nanocomposite (i.e., the TiO2 nanoparticle anchored to the kaolinite substrate). Etot,TiO2 is the total energy of the TiO2 nanoparticle, and Etot,KA is the total energy of the kaolinite substrate. The lowest Ead value had the strongest interaction between kaolinite and TiO2. These energies are given in kcal/mol.

2.6. Strategy of molecular modeling Molecular modeling with the universal force field (UFF) [19], as implemented in the Accelrys Materials Studio modeling environment, was used to study the adhesive forces and formation of bonds between the TiO2 nanoparticles and kaolinite surfaces. Under the binding conditions, kaolinite cell unit (KA) with crystallochemical formula Al4Si4O10(OH)8 and cell parameters a = 0.51489 nm, b = 0.8934 nm, c = 0.7384 nm, a = 91.93°, b = 105.042°, c = 89.791° [20] enabled us to prepare the following four models: tetrahedral kaolinite surface (denoted as KA(0 0 1)Si), octahedral kaolinite surface (denoted as KA(0 0 1)OH), unprotonated kaolinite edge (denoted as KA(1 0 0)) and protonated kaolinite edge (denoted as KA(1 0 0)-H). The surface and edge sizes were 8.2  9.6 nm and 8.5  9.3 nm, respectively. In order to create three dimensional periodic models of surface the vacuum slab with the height of 40.37 nm was added in c direction. Crystallochemical formula of both KA(0 0 1)Si and KA(0 0 1)OH was Al800 Si800 O2000 (OH)1600. Crystallochemical formula of KA(1 0 0) was Al960 Si960 O2400 (OH)1920 and KA(1 0 0)-H was Al720 Si960 O2040 (OH)1920. All four substrates had total zero charge of a layer. TiO2 (anatase structure [21]) was prepared in two forms of spherical nanoparticles, protonated (Ti728 O1515 H118) and unprotonated (Ti771 O1542) one, with the diameter of 0.38 nm and total zero charge. The QEq (charge equilibration) method [22] was used to calculate atomic charges. As implemented in the Accelrys Materials Studio modeling environment, molecular modeling with the universal force field (UFF) [19] was used to study the adhesion forces between the TiO2 nanoparticles and kaolinite surfaces. The Smart algorithm, with 50,000 iteration steps (i.e., a cascade of the steepest descents, conjugate gradient, and quasi-Newton optimization algorithms) was used. Convergence criteria were 1  104 kcal for the energy and 5  105 Å for the displacement. The interaction between TiO2 nanoparticles and kaolinite substrates has been quantified by using the adhesion energy (Ead) calculated from the equation

ð1Þ

3. Results and discussion 3.1. X-ray powder diffraction XRPD patterns of KA, KA6, KATI16 and KATI66 are shown in Fig. 2. The TiO2 in the anatase form is demonstrated in sample KATI16 and KATI66 (Fig. 2). The intensities are relative, which means that they do not reflect the quantity. XRPD patterns are described in [5] in more detail. 3.2. Leaching test 3.2.1. Conductivity of leachates Stability of composite materials can be deduced from results obtained by measuring conductivity in leachates. The conductivity of extraction agents was measured before and after leaching and the results are shown in Fig. 3. The conductivity was found to be the highest for leachate (after extraction in I, II and DM) obtained from KATI66, which is supposed to be the least stable in comparison to other samples. On the other hand, the conductivity values of extracts I and II show that the dissolved components interacted with the components of extraction agents, forming insoluble compounds. Consequently, the conductivity of extracts compared to conductivity of extraction agents was lower. Although calcination of native kaolinite does not affect solubility of some components, the results show that the calcination of kaolinite/TiO2 composite increases the solubility of kaolinite. KATI16 will be considerably more stable than KATI66. 3.2.2. The pH of leachates The pH of leachate gives important information about interaction between a solid and liquid phase. The acidic or the basic elements can dissolve to extraction solution and change the pH that can significantly influence chemical or biochemical processes in water and in plants because the pH value is an important factor for receiving nutrients from soils. Fig. 4 shows both pH of extraction agents (E.A.) and pH of leachates obtained from samples KA, KA6, KATI16 and KATI66. The pH of extract that was prepared by leaching the samples in deionised water shows that the presence of TiO2 changed their stability. On the other hand the leachates from composites KATI16 and KATI66 show the lowest values of pH, which can be attributed to the residual H2SO4 present in TiOSO4 used for preparing KATI16 composite. The dried KA samples are stable both in the extraction agent I (acid) and the extraction agent II (basic) because the change in pH of leachates in comparison with extraction solution is within an interval of uncertainty of pH measuring. When the extraction agent is acid, the pH of leachate changes slightly. The alkaline extraction agent promotes the release of acidic elements from the studied samples. The effects of extraction agent II – alkaline is the most obvious in the case of KATI66 composite (see Fig. 4). The pH of leachates depends on chemical equilibrium between photoactive composite and given extraction agent (I or II). The concentrations of dissolved elements are discussed below.

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Fig. 2. XRPD patterns of 1 – KA, 2 – KA6, 3 – KATI16 and 4 – KATI66. Legend: K – kaolinite, M – muscovite, Q – quartz, A – anatase.

where c1 is the concentration of an element in leachate [mg/l], mn is the sample weight [g], and VEA represents the volume of extraction agent [l]. Concentration c3 of element in the original sample [mg/g]:

c 3 ¼ x1 

Mr ðelem:Þ Mr ðoxideÞ

ð3Þ

where x1 is the quantity of element in oxide form in the original sample [mg/g], Mr(elem.) is the relative molecular mass of element [g/mol], and Mr(oxide) is the relative molecular mass of element in oxide form [g/mol]. The relative portion of elements x [%] released from composite into leachate: Fig. 3. Conductivities of extraction agents before leaching (E.A.) and conductivities of leachates.

x¼

c2  100 c3

ð4Þ

The shielding of aluminum cations by silicate network and presence of Al–O–Si bonds results in low solubility of kaolinite in acid [23]. It is likely that the character of Al bond changed after calcination because some elements from kaolinite were diluted in basic and acidic extraction agents. TiO2 content in the structure impacted the binding of Al, so Al was disengaging to demineralised water. The greatest amount of Al in all leachates was released from photoactive composite KATI66, especially in extraction agent I according to the chemical equation proposed in [23]:

Si2 Al2 O5 ðOHÞ4 þ 3H2 SO4 ¼ Al2 ðSO4 Þ3 þ 2SiO2 þ 5H2 O

ð5Þ

or in [24] where different equilibrium is considered:

Si2 Al2 O5 ðOHÞ4 þ 6Hþ ¼ 2Al Fig. 4. Initial pH of extraction agents (E.A.) and pH of extracts obtained by leaching.

3þ

c2 ¼

c1  V EA mn

ð2Þ

þ 2H4 SiO4 þ H2 O

ð6Þ

According to [25] the equilibrium constant of Eq. (6) can be expressed in terms of activities of H+, Al3+ and H4SiO4 as follows:

log Al 3.2.3. Leached elements The concentration of leached elements after material interaction with DM, extraction agent I and II are listed in Table 2. The content of selected elements leached from the tested samples (see Figs. 5–10) was estimated as follows. Measured concentration c2 of element per 1 gram of sample [mg/g]:

3þ

þ 3pH ¼ 2:73  log H4 SiO4

ð7Þ

Aluminum ion can undergo a number of hydrolysis in water solution to form several hydroxy-metal complexes. Also the H4SiO4 forms various ions in water solution [25] while complicated equilibrium between all of the species in leachates is established. The calcined kaolinite is assumed to dissolve in a similar way. The dissolution of kaolinite in alkaline solution can be expressed as [26]: 

Si2 Al2 O5 ðOHÞ4 þ 2OH þ 5H2 O ¼ 2½AlðOHÞ4  þ 2H4 SiO4

ð8Þ

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Table 2 Concentration of leached elements from the samples KA, KA6, KATI16 and KATI66 in extracts after interaction with extraction agents I or II and DM. ND – concentration was not determined, because the extraction agent was prepared from NaOH, DL – detection limit of determination. Extraction agent

Material

Al (mg/l)

Si (mg/l)

Na (mg/l)

K (mg/l)

Mg (mg/l)

Ti (mg/l)

DM

KA KA6 KATI16 KATI66

0.62 ± .1 0.16 ± 0.1 53.0 ± 4.2 208 ± 16

1.78 ± 0.2 10.7 ± 1.2 6.99 ± 0.8 8.14 ± 0.9

1.82 ± 0.1 0.53 ± 0.1 5.08 ± 0.1 8.91 ± 0.2

2.76 ± 0.1 6.48 ± 0.3 44.0 ± 1.3 103 ± 5.2

0.28 ± 0.1 0.90 ± 0.1 0.95 ± 0.1 0.43 ± 0.1

0.04 ± 0.1 0.02 ± 0.1 0.43 ± 0.1 0.08 ± 0.1

I

KA KA6 KATI16 KATI66

1.31 ± 0.1 102 ± 8.2 165 ± 13 245 ± 19

5.75 ± 0.6 21.2 ± 2.3 14.5 ± 1.6 10.0 ± 1.1

2.37 ± 0.2 1.12 ± 0.1 5.03 ± 0.4 8.80 ± 0.7

5.10 ± 0.3 22.3 ± 1.1 49.0 ± 2.5 86.0 ± 4.3

3.81 ± 0.2 2.65 ± 0.2 0.52 ± 0.1 0.42 ± 0.1

<0.1 (DL) 0.06 ± 0.1 0.69 ± 0.1 0.23 ± 0.1

II

KA KA6 KATI16 KATI66

9.99 ± 0.8 51.3 ± 4.1 0.94 ± 0.1 137 ± 11

16.7 ± 1.8 12.1 ± 1.3 2.10 ± 0.2 9.73 ± 1.0

ND ND ND ND

5.90 ± 0.3 2.94 ± 0.1 30.0 ± 1.5 97.0 ± 4.9

0.21 ± 0.1 <0.05 (DL) 0.13 ± 0.1 0.38 ± 0.2

0.22 ± 0.1 1.21 ± 0.1 <0.1 (DL) <0.1 (DL)

Fig. 5. The relative portion of aluminum leached from the samples (expressed in wt.%).

Fig. 7. The relative portion of sodium leached from the samples (expressed in wt.%).

Fig. 8. The relative portion of potassium leached from the samples (expressed in wt.%).

Fig. 6. The relative portion of silicium leached from the samples (expressed in wt.%).

Process of kaolinite dissolution was described by several authors [27–30]. Fig. 5 clearly demonstrates the effect of TiO2 content on Al dissolution. It is obvious that alkaline extraction agents affected the dissolution of Al less than acid extraction agent due to hydrolysis reactions. It means that the calcination disrupted binding of Al in structure of the composite KATI66. Higher concentration of Al released from KATI16 and KATI66 may entail some risks for environment. Unfortunately, Al is not generally evaluated as a potentially dangerous element, therefore, the concentration of Al cannot be compared with Al limit values commonly listed for waste materials.

Fig. 9. The relative portion of magnesium leached from the samples (expressed in wt.%).

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Silicon dissolution is described by Eqs. (6) and (8). The amount of Si which is released from studied samples is below 0.16 wt.% (see Fig. 6). Extraction agent I has the greatest influence on leaching Si, especially in the case of sample KATI16. Changed pH did not affect the composite KATI66, which gives a clear signal that calcination did not cause disruption of Si bond in KATI66 distinctly. The concentration of Na was determined only in acid extract and deionised water because the basic extraction agent was prepared from NaOH. The greatest amount of Na was leached from photoactive composite KATI16 and KATI66, both in water and in extraction agent I (Table 2). On the other hand, the relative portion of released Na was greatest in case of KA and KATI66 (Fig. 7). The amount of leached Na does not reach 3.5 wt.%. Na is accompanying element and, therefore, treatment of KA by TiO2 decreased the stability of Na in KATI16 and KATI66. Fig. 8 shows the effect of TiO2 content in composite on dissolution of K. The amount of K released from KA and KA6 is lower than 2 wt.%. Both photoactive composites released 25–28 wt.% of K. This is probably caused by the presence of TiO2 and calcination. K is not present in tetrahedral and octahedral sheets of kaolinite structure, and, therefore, it is easily released by extraction agent. The similar result (i.e. effect of TiO2 content on Mg leaching) was observed for Mg (see Fig. 9). The greatest amount of Mg as ion exchange element was released from KATI16 after leaching in DM. Therefore, treatment by TiO2 decreased the bound of Mg in KATI16. About 1.5 wt.% of Mg was dissolved after interaction of KATI66 with all extraction agents. Neither the calcination of kaolinite/TiO2 composite nor the pH of extraction agent had a significant influence on dissolution of Mg from KATI66 composite. As magnesium hydroxide is formed in leachates the amount of leached Mg in alkaline solution decreases. Fig. 10 shows the relative portion of Ti leached from the samples. In spite of the fact that the size of TiO2 NPs is lower than 20 nm and NPs can easily pass through the filter paper and although the energy used in ICP analysis is sufficiently high to dissociate the Ti–O bond, leached amount of Ti is lower than 0.5 wt.% in all samples. Taking into account that TiO2 content in photoactive composite materials (KATI16 and KATI66) is higher than 51 wt.%, it can be concluded that TiO2 NPs are very strongly anchored in both photoactive composites (KATI16 and KATI66). This fact was also confirmed by photoactivity test performed on composites after leaching process. The photoactivities exhibit no significant changes in comparison with results published by Mamulová Kutláková et al. [5]. The concentrations of leached elements which were observed in this study are not stated in leaching limit values for wastes. Therefore, the potential risk for environment was further studied by aquatic toxicity test. Nevertheless, the results show that TiO2 content in photoactive composite changed the ability of kaolinite to get dissolved, especially when the composite KATI16 was calcined. 3.3. Aquatic toxicity test The results of aquatic toxicity tests are summarized in Tables 3–7. Table 3 The results of acute aquatic toxicity using green algae Desmodesmus subspicatus on samples of aqueous leachate prepared from dried kaolinite (KA) and calcined kaolinite (KA6). Desmodesmus subticatus

Desmodesmus subticatus

KA – leachate

Effect

KA6 - leachate

Effect

Stimulation 0.3%

Solid to liquid ratio 1:10

Inhibition 6%

Solid to liquid ratio 1:10

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Table 4 The results of acute aquatic toxicity using green algae Desmodesmus subspicatus on samples of aqueous leachate prepared from KATI16 and KATI66. Desmodesmus subspicatus KATI16 – leachate Solid to liquid ratio 1:10

Desmodesmus subspicatus

Effect Inhibition 0.3%

KATI66 – leachate

Effect

Solid to liquid ratio 1:10 0.1 g/100 ml 1 g/100 ml 5 g/100 ml

Inhibition Inhibition Inhibition Inhibition

62% 52% 53% 65%

Table 5 The results of acute aquatic toxicity using green algae Desmodesmus subspicatus on samples of suspensions prepared from KATI16 and KATI66. Desmodesmus subspicatus

Desmodesmus subspicatus

KATI16 – suspension Effect 1 mg/100 ml Stimulation 0.1% 5 mg/100 ml Inhibition 5%

KATI66 – suspension Effect 1 mg/100 ml Inhibition 9%

10 mg/100 ml

10 mg/100 ml

Inhibition 5%

5 mg/100 ml

Inhibition 10% Inhibition 19%

Table 6 The results of acute aquatic toxicity using green algae Chlorella vulgaris on samples of aqueous leachates prepared from KATI16 and KATI66. Chlorella vulgaris

Chlorella vulgaris

KATI16 – leachate

Effect

Solid to liquid ratio 1:10 0.1 g/100 ml 1 g/100 ml 5 g/100 ml

Inhibition Inhibition Inhibition Inhibition

71% 37% 41% 48%

KATI66 – leachate

Effect

Solid to liquid ratio 1:10 0.1 g/100 ml 1 g/100 ml 5 g/100 ml

Inhibition Inhibition Inhibition Inhibition

67% 23% 35% 52%

Table 7 The results of acute aquatic toxicity using green algae Chlorella vulgaris on samples of suspensions prepared from KATI16 and KATI66. Chlorella vulgaris KATI16 – suspension 1 mg/100 ml 5 mg/100 ml 10 mg/100 ml

Chlorella vulgaris Effect Inhibition 1% Inhibition 15% Inhibition 33%

KATI66 – supension 1 mg/100 ml 5 mg/100 ml 10 mg/100 ml

Effect Stimulation 0.2% Inhibition 14% Inhibition 37%

The leachates prepared from dried kaolinite showed a slight stimulation effect. The leachate of calcined kaolinite showed very low toxicity effect (Table 3). KATI16 and KATI66 leachates caused growth inhibition of test strain D. subspicatus (Table 4). 50% growth inhibition (i.e. IC50 values) was caused by KATI16 and KATI66 leachates with concentrations of about 100 mg/l and 10 mg/l, respectively. Both KATI16 and KATI66 suspensions caused much lower inhibition on the strain D. subspicatus than the leachates of these samples (Table 5). Higher inhibition by leachates KATI16 and KATI66 could be caused by an increased concentration of Al in the aqueous leachates of these samples (Table 2), which was detected by AES–ICP, especially in the leachate of the KATI66 sample in DM where the concentration of Al in the final extract was 208 mg/l. The aluminum oxide was demonstrated to cause inhibition to both algal species, 100.4 mg/l for Desmodesmus sp. and 110.2 mg/l for Chlorella sp. [31]. We can assume that Ti (ions and/or compounds) does not directly contribute to the toxicity. Therefore, TiO2 NPs are likely to be tightly anchored to the surface of clay matrix. The KATI16 and KATI66 leachates tested on C. vulgaris caused 48% and 53% growth inhibition (Table 6), respectively, while the

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Table 8 Adhesion energies Ead between TiO2 nanoparticles and kaolinite substrates and the shortest observed distances d(X  Y), where X and Y are atoms (or hydroxyl groups) from kaolinite and TiO2, respectively. Model

Ead (kcal/mol)

d(Al  O) (nm)

d(O  Ti) (nm)

d(O  HO) (nm)

KA(0 0 1)Si/TiO2 KA(0 0 1)Si/TiO2–H KA(0 0 1)OH/TiO2 KA(0 0 1)OH/TiO2–H KA(1 0 0)/TiO2 KA(1 0 0)/TiO2–H KA(1 0 0)–H/TiO2 KA(1 0 0)–H/TiO2–H

7.3 11.2 0.4 9.9 1144.8 1420.0 3268.0 427.9

– – – – 0.2221 – – –

– – – – – – 0.1913 –

– – – – 0.2608 0.2740 0.1754 –

Fig. 11. Optimized KLT(1 0 0)-H/TiO2 model. Detailed view in the upper right corner shows the newly formed Ti–O bond. Fig. 10. The relative portion of titanium leached from the samples (expressed in wt.%).

suspensions of KATI16 and KATI66 caused much lower growth inhibition of the strain C. vulgaris (Table 7).

formed bond between Ti atom from TiO2 and O atom from kaolinite. This fact is well consistent with the strongest Ead value (see Table 8). The formation of covalent bonds between TiO2 and kaolinite explains almost no leaching of Ti from KATI composites (see Figs. 10 and 11).

3.4. Molecular modeling 4. Conclusion Adhesion energies Ead calculated using Eq. (1) are listed in Table 8. One can see a significant difference between the surface and edge of kaolinite. Our previous study [21] revealed that in real KATI composite the TiO2 forms ‘‘collars’’ encircling the edges of kaolinite platelets and current results confirmed this fact also for models of protonated form of the KATI composite. The molecular modeling was also used to study the formation of new bonds in KATI composites. Despite the fact that no new chemical bonds can be formed in systems treated by molecular mechanics, they are easily inferred to exist based on the distances between atoms. Ability of molecular modeling using UFF to predict the creation of new bonds by simple measurement of the distances between atoms was proved by Tokarsky et al. [32]. Therefore, also in this study the distances d(X...Y), where X and Y are single atoms (or hydroxyl groups) from kaolinite and TiO2, respectively, were measured. The shortest observed distances are listed in Table 8. Only distances shorter than 0.3 nm were taken into account. Absence of these short distances between atoms in models KA(0 0 1)Si/ TiO2, KA(0 0 1)Si/TiO2–H, KA(0 0 1)OH/TiO2, KA(0 0 1)OH/TiO2–H is in good agreement with negligible Ead values. The situation is similar in model KA(1 0 0)–H/TiO2–H, where the protonation of both TiO2 and kaolinite prevents stronger interaction. Taking the bond lengths in kaolinite (Si–O  E0.16 nm, Al–O  0.19 nm [20]) and TiO2 (Ti–O  0.19 nm [21]) into account, it can be stated that the KA(1 0 0)–H/TiO2 model contains newly

This study deals with the stability and potential impact of photoactive composite of kaolinite/TiO2 with verified photoactive properties on aquatic system. Leaching and aquatic toxicity tests were carried out on freshwater algae D. subspicatus and green algae C. vulgaris, comparing dried and calcined kaolinite/TiO2 composites with pure kaolinite (dried and calcined). Deionised water and extraction agents with both pH 2 and 12 were used in the leaching tests. The stability of the studied materials was also studied using molecular modeling. Photodegradation activity of composite material after leaching test was determined as well. Native dried kaolinite is the most stable after interaction with all extraction agents. However, the calcination of kaolinite causes cracks in structure and amounts of elements leached from calcined kaolinite sample were in most cases higher than from dried kaolinite. Results show that the presence of TiO2 in kaolinite leads to release of Al, Si, Na, K and Mg from composites to leachates. Calcination at 600 °C also resulted in decreased stability of Al, Si, K, Mg and Na. Leached amount of Ti from studied composites was lower than 0.05 wt.%, despite the fact that content of TiO2 NPs in composite was higher than 51wt.%. The photoactivity of composite before and after leaching test remained the same, i.e. no significant influence of leaching on the photoactivity of kaolinite/TiO2 composite was found. Leaching tests together with molecular modeling and photodegradation activity test proved strong anchoring of TiO2 NPs, very

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probably by covalent bonds. Higher inhibition of algal species by aqueous extracts of dried kaolinite/TiO2 and calcined kaolinite/ TiO2 composites could be caused by an increased concentration of Al. Therefore, it would be appropriate to find the compromise between high photoactivity and the stability of composite kaolinite/TiO2. Acknowledgments This research was supported by the Ministry of Education of the Czech Republic (SP2013/74 and SP2013/67). The study was supported by Ministry of Education, Youth and Sports of the Czech Republic within the project LH 12184 and by the European Regional Development Fund in the IT4Innovations Centre of Excellence project (CZ.1.05/1.1.00/02.0070). Authors also wish to thank Daniel Casten for correction and all colleagues, who participated in the measurements. References [1] Marcí G, Sclafani A, Augugliaro V, Palmisano L, Schiavello M. Influence of some aromatic and aliphatic compounds on the rate of photodegradation of phenol in aqueous suspensions of TiO2. J Photochem Photobiol A: Chemistry 1995;89:67–74. [2] Kocˇí K, Mateˇjka V, Kovárˇ P, Lacny´ Z, Obalová L. Comparison of the pure TiO2 and kaolinite/TiO2 composite as catalyst for CO2 photocatalytic reduction. Catal Today 2011;161:105–9. [3] Augugliaro V, Coluccia S, Loddo V, Marchese L, Martra G, Palmisano L, et al. Photocatalytic oxidation of gaseous toluene and anatase TiO2 catalyst: mechanistic aspects and FT-IR investigation. Appl Catal B 1999;20:15–27. [4] Liu T-X, Li F-B, Li X-Z. TiO2 hydrosols with activity for photocatalytic degradation of formaldehyde in a gaseous phase. J Hazard Mater 2008;152:347–55. [5] Mamulová Kutláková K, Tokarsky´ J, Kovárˇ P, Vojteˇšková S, Kovárˇová A, Smetana B, et al. Preparation and characterization of photoactive composite kaolinite/ TiO2. J Hazard Mater 2010;188:212–20. [6] Clément L, Hurel CH, Marmier N. Toxicity of TiO2 nanoparticles to cladocerans, algae, rotifers and plants – effects of size and crystalline structure. Chemosphere 2013;90:1083–90. [7] Fujishima A, Hashimoto K, Watanabe T. TiO2 photocatalysis fundamentals and applications. 1st ed. Tokyo: BKC’’; 1999. [8] Zhu X, Chang Y, Chen Y. Toxicity and bioaccumulation of TiO2 nanoparticle aggregates in Daphnia magna. Chemosphere 2010;78:209–15. [9] Xiong D, Fang T, Yu L, Sima X, Zhu W. Effects of nano-scale TiO2, ZnO and their bulk counterparts on zebrafish: acute toxicity, oxidative stress and oxidative damage. Sci Total Environ 2011;409:1444–52. [10] Suna Q, Tana D, Zea Y, Sanga X, Liub X, Guia S, et al. Pulmotoxicological effects caused by long-term titanium dioxide nanoparticles exposure in mice. J Hazard Mater 2012;235–236:47–53.

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