Nano Metal Fluorides for Wood Protection against Fungi

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Nano Metal Fluorides for Wood Protection Against Fungi Shirin Usmani, Ina Stephan, Thomas Hübert, and Erhard Kemnitz ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00144 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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ACS Applied Nano Materials

Nano Metal Fluorides for Wood Protection Against Fungi Shirin M. Usmani1,2, Ina Stephan2, Thomas Hübert2, Erhard Kemnitz1* 1

Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor Str. 2, 12489 Berlin,

Germany 2

BAM Federal Institute for Materials Research and Testing, Unter den Eichen 44-46, 12203

Berlin, Germany KEYWORDS: fluoride nanoparticles, fluorolytic sol-gel, brown-rot fungi, Rhodonia placenta, Coniophora puteana, wood protection, SEM wood characterization

ABSTRACT: Wood treated with nano metal fluorides is found to resist fungal decay. Sol-gel synthesis was used to synthesize MgF2 and CaF2 nanoparticles. Electron microscopy images confirmed the localization of MgF2 and CaF2 nanoparticles in wood. Efficacy of nano metal fluoride-treated wood was tested against brown-rot fungi; Coniophora puteana (Cp) and Rhodonia placenta (Rp). Untreated wood specimens had higher mass losses (~30%) compared to treated specimens which had average mass loss of 2% against Cp and 14% against Rp, respectively. Nano metal fluorides provide a viable alternative to current wood preservatives.

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The economic cost of timber decay was estimated to be $300 million per year in the United States.1 Decay in wood products can be caused by fungi and fire. In the northern hemisphere, fungal decay is associated with brown-rot fungi which predominantly attack conifer trees, a major source for timber used for construction.2 Brown-rot fungi such as Coniophora puteana (Cp) and Rhodonia placenta (Rp) attack wood by sequential decomposition of cellulose and hemicellulose.3 Research has found that Rp possesses only the endoglucanase enzyme while Cp has endoglucanase and exoglucanase enzymes.4 This allows Rp to penetrate deeper inside the wood cells and thus have a higher fungal activity compared to Cp.4 Besides the difference in their enzymatic apparatus, it was found that compared to Cp, Rp has greater iron-reducing capability which promotes Fenton-type reaction, involved in oxidative degradation of polysaccharides.5 Moreover, Rp is tolerant to copper, a widely used active ingredient in wood preservatives.6 Thus, research efforts on replacing copper as the active ingredient in protecting wood from copper-tolerant fungi is important. Significant changes have occurred in the synthesis of wood preservatives from coal tar oils in the nineteenth century to proliferation of copper-based preservatives such as chromated copper arsenate (CCA) in the twentieth century.7-8 But, environmental and health concerns related to chromium and arsenic have led to restrictions on use of CCA in wood protection. Subsequently, other fixatives such as quaternary ammonium salts, azoles, or amines have been introduced to replace chromium and arsenic.8-9 Although effective, these copper-organic preservatives have a higher leaching rate compared to CCA.6 Leaching of a wood preservative can reduce its function in long-term wood protection and be detrimental to aquatic life. Therefore, the current research on wood preservatives focuses on increasing the efficacy of chemicals against fungi and reducing their leaching into the environment. An ideal wood preservative would not only hinder


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growth of wood destroying organisms but also have low water solubility to reduce its leaching into the environment. More recently, nanoparticles have been tested for wood preservation as they can be effectively impregnated into wood. Non-leached wood specimens treated with copper and silver nanoparticles (with average diameters < 20 nm) tested against Rhodonia placenta had mass losses below 3%.10 However, leached specimens with the same treatment showed mass losses higher than 20%, thus their efficacy was lost after leaching.10 Besides copper, silica and titania sols have been reported to protect against attack from brownrot fungi.11,12 Unleached wood specimens treated with titanium alkoxide had mass losses up to 5% against Coniophora puteana.12 Another set of compounds known for their toxicity against insects and fungi are fluorides such as sodium fluoride and fluosilicates.13-14 Similar to copper, these water soluble compounds need to be combined with fixatives such as chromium to reduce their leaching into the environment as higher concentration of fluorides in water can be toxic in the environment.15 Instead of using fixatives, fluorides with low water solubility can be used which will be less susceptible to leaching. Incidentally, low water soluble fluorides were suggested as a potential alternative to fluosilicates in 1926.13 Although review of current literature of wood preservatives does not report results on low water soluble fluorides such as MgF2 and CaF2.6,9,15 A potential explanation would be the challenge in impregnating such inorganic fluorides into wood as they have low water solubility. However, our results show that if these fluorides are synthesized as nanoparticles they are able to penetrate into wood. Therefore, nano metal fluorides present an opportunity to investigate slightly soluble fluorides for their potential application in protection of wood products. These fluorides are developed using fluorolytic sol-gel synthesis.16 Fluorolytic sol-gel synthesis is a cost-efficient process to


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synthesize nano metal fluorides for various applications such as antireflective coatings and transparent inorganic-organic composite materials.16-17 The fundamental idea is to impregnate nano metal fluorides into wood that will create a “fluoride-reservoir” inside the wood cells from which there can be controlled release of F ̄ ions dependent on the solubility of the fluoride compounds. In this way, uncontrolled environmental pollution can be reduced to an extremely low level. Inorganic nanoparticles composed of MgF2 and CaF2 were synthesized using sol-gel technique (see Experimental Section).18 This process involves the reaction of a metal alkoxide (M-OR) or another suitable precursor with HF as described below in Equation (1). Research on the formation of these sols have found no unreacted metal ions or HF in the synthesized sols.19

M-OR + HF → M-F + ROH ---------------------- (1)

The nano metal fluorides of MgF2 and CaF2 were synthesized and characterized to confirm their composition and size by X-ray diffraction, dynamic light scattering (DLS), transmission electron microscopy (TEM), and NMR respectively. As observed in Figure 1, the xerogels of the powders matches the respective reference patterns of MgF2 and CaF2.


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Figure 1. X-ray diffraction patterns of MgF2 and CaF2 xerogels. The reference peaks are shown as vertical lines. The MgF2 (green) reflections could be assigned to the reference PDF 41-1443 and those of CaF2 (blue) matched reference PDF 35-816 respectively. With DLS, the diameter of MgF2 and CaF2 particles were determined to be 13.6 nm and 21.04 nm, respectively as shown in Figure 2.

Figure 2. Hydrodynamic diameter of MgF2 and CaF2 sols measured by dynamic light scattering


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In addition to DLS, Figure 3 shows TEM images of nearly homodispersed nanoparticles of MgF2 and CaF2. The mean particle size distribution of MgF2 is 5 nm while CaF2 are bigger with a mean particle size distribution of 20 nm (see Experimental Section).

Figure 3. TEM images of nano metal fluorides. (a) MgF2 and (b) CaF2.

Following characterization of the nanoparticles, the nanoparticles were impregnated into pine sapwood (Pinus sylvestris L.) specimens in accordance with European standard, EN 113 and then the specimens were leached in accordance with EN 84 (see Experimental Section).20-21 The weight per gain (WPG) and retention of nanoparticles before and after leaching is listed in Table 1. It can be observed that the WPG after impregnation was similar for MgF2 and CaF2 nanoparticles, and it is significant to note that the mass loss after leaching was below 3%, thus high concentration of nanoparticles were still inside the wood specimens after leaching without use of fixatives.


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Table 1. Nanoparticles Uptake in Wood Specimens Retention (kg/m3) Mass lost after Leached Average WPG (%) leaching (%) Unleached MgF2 11,12 (0,47) 2,31 (1,92) 5,02 (0,25) 3,88 (1,15) CaF2 12,14 (0,81) 2,41 (1,89) 6,22 (0,27) 4,80 (1,28) In addition to WPG and retention, the amount leached during leaching is listed in Table 2. As expected, initially high amount of fluoride was leached from the samples. Because of higher solubility of MgF2, the amount of fluoride leached from wood specimens treated with MgF2 was higher than those treated with CaF2. Additionally, on 7th and 9th day, MgF2 treated samples had very low fluoride released from the wood specimens. This could be because the nanoparticles were fixated to the wood cells and needed time to be released from the specimens.

Table 2. Amount of Elements Leached from Wood Specimens Leaching time (days)

Amount leached (mg/l) MgF₂

Mg F 15,85 BDL* 0,25 16,04 27,86 1 19,53 0,06 2 5,98 11,65 5 6,94 13,36 6 4,69 BDL* 7 4,17 8,42 8 10,34 BDL* 9 4,55 8,68 11 3,49 6,66 12 *BDL – Below detection limit

CaF₂ Ca 13,40 8,88 9,80 7,19 5,52 4,99 4,55 6,76 3,36 3,69

F 14,16 11,90 0,06 9,37 7,29 0,06 0,06 8,79 4,42 4,47


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The inclusion of nano metal fluorides in the wood specimens was confirmed with backscattered electron microscopy and energy dispersive spectroscopy (see Experimental Section). Back-scattered electron images showing distribution of metal fluorides in wood cells is presented in Figure 4. Aggregates of MgF2 and CaF2 nanoparticles appear as bright particles in backscattered images (Figure 4a,d). The corresponding EDX maps confirms the distribution of Mg, Ca, and F inside the wood cells, respectively (Figure 4b,c,e,f).

Figure 4. Back-scattered electron (BSE) images and elemental distribution maps of wood (crosscut section) specimens after impregnation. (a) BSE-image of MgF2 treated wood and corresponding EDX map (b) Mg – yellow and (c) F – red; (d) BSE-image of CaF2 treated wood and corresponding EDX map (e) Ca – blue and (f) F – red. Scale bar – 60 µm.

After characterization of the treated wood specimens, their resistance against decay by brownrot fungi (Coniophora puteana and Rhodonia placenta) was assessed in accordance with the European standard EN 113.20 The final mass of the wood sample in relation to its initial mass


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was the calculated mass loss (%). Differences in mass loss between treated and control specimens exposed to fungi are depicted in Figure 5. The mass loss caused by Cp in treated wood was below 3%, while the control specimens had a significantly higher mass loss of 38% as shown in Figure 5. For Rp the mass loss in treated wood was lower than 15%, while the control specimens showed a mass loss of 34% as depicted in Figure 5. The mass loss in CaF2 and MgF2 treated specimens do not differ significantly. For Cp, the mass loss of treated wood specimens was 1% (MgF2) and 2% (CaF2), respectively. Similarly, for Rp the mass loss of treated wood specimens was 12% (MgF2) and 14% (CaF2), respectively. These mass losses against Cp are comparable to those reported by, Temiz et al. for CCA treated specimens (incubation time 8 weeks).22 However, the mass loss of treated wood specimens against Rhodonia placenta was 12% (MgF2) and 14% (CaF2), respectively which were significantly higher than those reported by Teniz et al. for CCA treated specimens exposed to Rp.22

Figure 5. Average mass losses of pine sapwood blocks treated with nanoscopic MgF2 and CaF2 after exposure to Coniophora puteana (Cp) and Rhodonia placenta (Rp).


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The change in the appearance of the wood specimens after exposure to fungi, Cp and Rp is shown in Figure 6. It is evident that the control specimens are severely damaged, while the fluoride treated specimens had minimal damage for specimens exposed to Cp. Thus, the nano metal fluorides protected the wood specimens from extensive fungal deterioration. Even though, treated specimens exposed to Rp show cracks on their surfaces, their damage is less compared to the control samples which have shrunk remarkably because of cellulose degradation.

Figure 6. Wood specimens after exposure to fungi, narrow and wide longitudinal surface. (a) (b) MgF2 treated after exposure to Cp; (c) - (d) CaF2 treated after exposure to Cp; (e) - (f) MgF2 treated after exposure to Rp and (g) - (h) CaF2 treated after exposure to Rp.


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Application of nano metal fluorides for biological protection of timber presents a promising prospect for bio-inorganic composite materials. Our results show that nano metal fluorides exhibit high potential in wood preservation. Because fluorides of Mg and Ca have low water solubility their impregnation into wood cells by means of classically prepared (microcrystalline) compounds is impossible. A facile fluorolytic sol-gel synthesis was used to produce nano metal fluorides. The nano formulation enabled impregnation and subsequent distribution of fluoride particles within the wood cells as confirmed by the back-scattered electron images. When exposed to fungi lower mass loss was observed in treated wood specimens compared to control specimens. There were minor differences in mass loss between MgF2 and CaF2 treated specimens. The small differences between these nano metal fluorides are obviously due to the differences in their solubility; with a free fluoride concentration of 16 mg/L for CaF2 and 130 mg/L for MgF2, the latter provides a larger F- concentration.23 Hence, MgF2 is more active because of the slightly higher fluoride ion concentration in the wood specimens. Notwithstanding, the lower water solubility of CaF2 makes it attractive for long term treatment because it would remain longer within the wood cells. It is likely that the combination of specific nano formulation and low water solubility of nano metal fluorides is responsible for reduction in mass loss against fungi. Possibly, the small size of nano metal fluoride particles allows them to diffuse through the fungal cells and disrupt their metabolic process. Additionally, the low water solubility of nano metal fluoride particles makes them less susceptible to leaching. However, nano metal fluorides were found to be more effective against Cp than Rp. The mass loss of treated specimens exposed to Rp was more than 10% while in Cp was below 3%. The higher mass losses observed for Rhodonia placenta compared to Coniophora puteana can be attributed to the difference in non-enzymatic and


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enzymatic mechanisms for polysaccharide degradation. Future experiments will optimize the synthesis procedure to make the wood more resistant to fungal attack from Rhodonia placenta. More importantly, these nano metal fluoride particles have been tested without any fixatives, thus their effectivity against fungi is promising. Further research is under progress to understand how the nanoparticles are fixated inside the wood specimens and their protective mechanism against fungi. Experimental Section Chemicals Magnesium ethoxide (Mg(OC2H5), 99% - Solvay), magnesium chloride (MgCl2, 99% SigmaAldrich), calcium oxide (CaO, 99% - Sigma-Aldrich), HF (Solvay Fluor GmbH), ethylene glycol (Sigma-Aldrich), ethanol (99.8%, Roth) were used as described in the next section. Synthesis A 1.5 M CaF2 sol was prepared by suspending CaO (1188 mmol) and MgCl2 (62 mmol) in 850 ml of ethylene glycol. This suspension was fluorinated with aqueous HF (2500 mmol). A transparent sol was obtained after one day of stirring. A 1.5 M MgF2 sol was prepared by dissolving Mg(OC2H5) (1250 mmol) in 850 ml of ethylene glycol (Sigma-Aldrich). This suspension was fluorinated with aqueous HF (2500 mmol). A transparent sol was obtained after one day of stirring. Characterization The 19F NMR spectra of the sols was carried out using a Bruker AVANCE II 300 (liquid state NMR spectrometer with a Larmor frequency of 282.4 MHz).


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A Philips CM200 TEM operating at 200 kV with tungsten filament was used. For TEM studies, one drop of the nanoparticle dispersions in ethanol was deposited on copper TEM grids. For SEM, cross-section of wood specimens were cut with a sledge microtome and sputtered with carbon (15 nm) prior to image acquisition. The SEM images were acquired on Leo 1530 VP (Zeiss) at 15 kV and variable pressure (5 Pa – 10 Pa) with a working distance of 10 mm. Elemental distributions of magnesium, calcium, and fluorine on the wood surface were mapped by X-ray dispersive spectroscopy (EDX). Elemental mapping was performed with EDX Bruker Esprit at 15 kV and variable pressure (5 Pa – 10 Pa) with a working distance 10 mm, and acquisition time was 600 seconds. Leaching Calcium and magnesium were measured using ICP-OES (iCap 7400 Duo, Thermo Scientific, Dreieich, Germany according to DIN EN ISO 11885:2009-09).24 Fluoride was determined on an IC320 ion chromatograph according to DIN EN ISO 10304-1: 2009-01.25 Treatment of wood specimens with nano metal fluoride nanoparticles Wood specimens of dimensions 15 mm x 25 mm x 50 mm (radial x tangential x longitudinal) were prepared from pine sapwood (Pinus sylvestris L.) in order to perform decay resistance tests according to EN 113.18 The CaF2 and MgF2 sols were diluted with ethanol to prepare the 0.4 M impregnating solution. For better penetration of the impregnating solutions into the wood, the specimens were oven-dried and evacuated at 0.1 – 0.4 kPa for 1 h prior to impregnation. After 1 h, the solutions were introduced into the vacuum chamber and then the specimens were soaked for 2 h at ambient pressure and 20 – 23 °C. Specimens impregnated with absolute ethanol as a solvent were also tested. After impregnation, the specimens were weighed and placed at 20 – 23 °C and a relative humidity of 65 % to achieve moisture equilibration for 2 weeks. The specimens


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were then leached according to EN 84 and again conditioned before being exposed to fungi in accordance with EN 113. Five replicates were used per treatment. The nano metal fluoride impregnated wood specimens was tested against the selected brown-rot fungi, Coniophora puteana (BAM Ebw. 15) and Rhodonia placenta (FPRL 280). A nano metal fluoride-treated sample and an untreated sample were introduced in a Kolle flask and then exposed to the selected brown-rot fungi for 16 weeks. At the end of testing, the mycelium adherent to the surface of the test specimens was gently removed and all specimens were oven-dried (18 hrs at 103 °C) before their final mass was recorded.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We thank Dr. Alexander Rehmer and Dr. Thoralf Krahl for synthesis of CaF2 and MgF2 sol, Mr. Jörg Schlischka for support in sol-gel impregnation into wood, Mrs. Kerstin Klutzny for wood decay testing, Dr. Ute Kalbe and Mrs. Katja Nordhauß for leaching of wood specimens,


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Mr. Romeo Saliwan Neumann for characterization of wood specimens with electron microscopy, and Mr. Stefan Mahn for TEM images acquisition.

REFERENCES (1) Côté, W.A. Jr. Biological Deterioration of Wood. In Principles of Wood Science and Technology; Springer: Berlin, 1968; pp 97 -135. (2) Arantes, V.; Goodell, B. Current Understanding of Brown-rot Fungal Biodegradation Mechanisms: A Review. In Deterioration and Protection of Sustainable Biomaterials; Schultz, T.P., Goodell, B., Nicholas, D.D., Eds.; ACS Symposium Series 1158; American Chemical Society: Washington, DC, 2014; pp 3 – 21. (3) Schwarze, F.W.M.R. Wood Decay Under the Microscope. Fungal Biol Rev. 2007, 21, 133– 170. (4) Irbe, I.; Andersons, B.; Chirkova, J.; U. Kallavus, U.; Andersone, I.; Faix, O. On the Changes of Pinewood (Pinus sylvestris L.) Chemical Composition and Ultrastructure During the Attack by Brown-rot Fungi Postia Placenta and Coniophora Puteana. Int. Biodeterior. Biodegrad. 2006, 57, 99 – 106. (5) Goodell, B.; Daniel, G.; Jellison, J.; Qian, Y. Iron-reducing Capacity of Low-molecularweight Compounds Produced in Wood by Fungi. Holzforschung. 2006, 60, 630-636.


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(6) Freeman, M.H.; McIntyre, C.R. A Comprehensive Review of Copper-based Wood Preservatives. For. Prod. J. 2008, 53, 6 – 27. (7) Ibach, R.E. Wood Preservation. In Wood handbook: Wood as an Engineering Material; Forest Products Laboratory: Madison, 1999; pp 14-1:14-2. (8) Schultz, T.P.; Nicholas, D.D.; Preston, A.F. A Brief Review of the Past, Present, and Future of Wood Preservation. Pest Manage. Sci. 2007, 63, 784 – 788. (9) Freeman, M.H.; Nicholas, D.D.; Schultz, T.P. Non-Arsenical Wood Protection: Alternatives for CCA, Creosote, and Pentachlorophenol. In Environmental Impacts of Treated Wood; Townsend, T.G., Helena Solo-Gabriele, H., Eds.; CRC Press: Boca Raton, 2006; pp 19 – 37. (10) Pařil, P.; Baar, J.; Čermák, P.; Rademacher, P.; Prucek, R.; Sivera, M.; Panáček, A. Antifungal Effects of Copper and Silver Nanoparticles Against White and Brown-rot Fungi. J Mater Sci. 2017, 52, 2720-2729. (11) Hübert, T.; Shabir, M.M.; Sol-gel Wood Preservation. In Handbook of Sol-Gel Science and Technology; Klein, L., Aparicio, M., Jitianu, A., Eds.; Springer International Publishing AG: Cham, 2017; pp 1-52. (12) Shabir, M.M.; Hübert, T.; Stephan, I.; Militz, H.; Decay Protection of Wood Against Brown-rot Fungi by Titanium Alkoxide Impregnations. Int. Biodeterior. Biodegrad. 2013, 77, 56 – 62. (13) Roark, R.C. Fluorides vs. Fluosilicates as Insecticides. Science 1926, 53, 431 – 432. (14) Becker, J. Fluorine Compounds for Wood Preservation. J. Inst. Wood Sci. 1973, 6, 51-62.


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(15) Reinprecht, L. Fungicides for Wood Protection - World Viewpoint and Evaluation/Testing in Slovakia. In Fungicides; Odile, C., Ed., InTech: Rijeka, 2010, pp 95 – 122. (16) Kemnitz, E.; Noack, J. The Non-aqueous Fluorolytic Sol-gel Synthesis of Nanoscaled Metal Fluorides. Dalton Trans. 2015, 44, 19411-19431. (17) Rehmer, A.; Scheurell, K.; Kemnitz, E.; Formation of Nanoscopic CaF2 via a Fluorolytic Sol-gel Process for Antireflective Coatings. J. Mater. Chem. C 2015, 3, 1716 – 1723. (18) Kemnitz, E.; Wuttke, S.; Coman, S.M. Tailor-made MgF2–Based Catalysts by Sol-gel Synthesis. Eur. J. Inorg. Chem. 2011, 2011, 4773 – 4794. (19) Scholz, G.; Stosiek, C.; Noack, J.; Kemnitz, E.; Local Fluorine Environments in Nanoscopic Magnesium Hydr(oxide) Fluorides Studied by 19F MAS NMR. J. Fluorine Chem. 2011, 132, 1079 – 1085. (20) EN 113, Wood Preservatives Test Method for Determining the Protective Effectiveness Against Wood Destroying Basidiomycetes. Determination of Toxic Values; European Committee for Standardization: Brussels, 1997. (21) EN 84, Wood Preservatives Accelerated Ageing of Treated Wood Prior to Biological Testing Leaching procedure; European Committee for Standardization: Brussels, 1997.

(22) Temiz A.; Alfredsen G.; Yildiz U.C.; Gezer E.D.; Kose G.; Akbas S.; Yildiz S. Leaching and Decay Resistance of Alder and Pine Wood Treated With Cu Based Wood Preservatives. Maderas Ciencia Y Tecnología, 2014, 16, 63-76.

(23) Lide, D. R. CRC Handbook of Chemistry and Physics: a Ready-reference Book of Chemical and Physical Data; CRC Press: Boca Raton, 2003.


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(24) DIN EN ISO 11885, Water quality - Determination of selected elements by inductively coupled plasma optical emission spectrometry (ICP-OES); European Committee for Standardization: Brussels, 2009. (25) DIN ISO 10304-1, Water quality - Determination of dissolved anions by liquid chromatography of ions - Part 1: Determination of bromide, chloride, fluoride, nitrate, nitrite, phosphate and sulfate; European Committee for Standardization: Brussels, 2009.

Supporting Information 19

F NMR spectra of MgF2 and CaF2 sols and additional SEM images and EDX maps of treated

wood specimens

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Nano Metal Fluorides for Wood Protection against Fungi

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