Lanthanide (Ce, Nd, Eu) environments and leaching behavior in borosilicate glasses

Borosilicate glasses will be used to stabilize the high-level radioactive wastes for disposal in a geological repository. Understanding the effects of actinide addition to a borosilicate glass matrix is of great importance in view of waste immobilization. Lanthanides were considered as chemical surrogates for actinides. The local structures of Ce3+, Nd3+ and Eu3+ ions in borosilicate glass, have been investigated by synchrotron radiation based techniques. The atomic parameters, such as bond lengths and coordination environments derived from X-ray diffraction, in combined with Reverse Monte Carlo simulations show correlation with X-ray absorption fine structure data. The lanthanide ions are in the common network with the tetrahedral SiO4 and with the mixed trigonal BO3 and tetrahedral BO4 units. Second neighbor atomic pair correlations reveal that the Ce3+, Nd3+ and Eu3+ ions are accommodated in both Si and B sites, supporting that the lanthanide-ions are stabilized in the glass-matrix network. Microscopy and microanalysis provided information on the amorphous state and on the major elemental composition of the high lanthanide-concentration samples. The release of matrix components (Si, B, Na, Ba, Zr) is higher than that of lanthanides (Ce, Nd, Eu). Both types of elements show a decreasing release tendency with time.


Lanthanide environments with XRD and Reverse Monte Carlo simulations.
The resulting Lncontaining glasses are proved to be transparent and fully amorphous, no crystalline phase can be observed supporting that the synthesis by employing melt quench technique is suitable. Detailed sample preparation steps are described in our previous work 9 . A comparison of RMC fitted total structure factors with experimental XRD data are performed. The structure factors of Matrix-Ln10 and Matrix-Ln30 glasses, within the series, do not differ from each other. These similarities come from the very close weighting factors, w ij (defined in Eq. (1)) of the partial structure factors, S ij (Q) (defined in Eq. (2)): (1) S(Q) = k i,j w ij S ij (Q) where c i , c j , f i (Q), f j (Q) and k are the molar fractions, the X-ray scattering amplitudes, and the number of elements in the sample, respectively, and Q is the momentum transfer. The X-ray scattering amplitudes are Q-dependent 10 . Table 1  For the RMC starting model a disordered atomic configuration was built up with a simulation box containing 10,000 atoms using experimentally obtained density data (ρ 0 ). The measured densities were 0.065 atoms Å −3 (half-box value: 27.59 Å) for Matrix-Ce10, 0.068 atoms Å −3 (half-box value: 26.26 Å) for Matrix-Nd10, 0.072 atoms Å −3 (half-box value: 25.89 Å) for Matrix-Eu10, 0.069 atoms Å −3 (half-box value: 26.79 Å) for Matrix-Ce30, 0.071 atoms Å −3 (half-box value: 26.01 Å) for Matrix-Nd30 and 0.078 atoms Å −3 (half-box value: 25.21 Å) for Matrix-Eu30. In the course of the RMC fitting, one constraint was chosen for minimum atom-atom distances to avoid physically invalid overlapping. Characteristic values obtained in our previous works were used as initial cut-off distances for the RMC simulation procedure. These distances were reported for binary 70SiO 2 -30Na 2 O 11 and 75B 2 O 3 -25Na 2 O glasses 12 , for the 45SiO 2 -25Na 2 O-10B 2 O 3 -5BaO-5ZrO 2 glass 8 and for the neutron diffraction data of the present series 9 .
For each sample about 25 RMC simulation steps were generated with close to 1,300,000 accepted moves of each atom.
The structure factors calculated by the RMC technique (black line) provided an excellent fit of the experimental XRD ones (colour symbols) (Fig. 3). The shape and features of the structure factors show similarities within the Ln-series.
From the RMC simulation several first and second neighbor partial atomic pair-correlation functions, g ij (r) and coordination number distributions, CN ij have been revealed with a fairly good stability and statistics.
The atomic partial pair-correlation distances are summarized in Table 2. Structural data for the Matrix-glass sample 8 and the neutron diffraction data for the present series were reported earlier 9 .
For Si-O a covalent bond length at 1.60 ± 0.01 Å was revealed for all studied glasses, showing an excellent agreement within the uncertainties and consistent with data reported in the literature 9,[12][13][14] . From the RMC analysis it was found that Si atoms are tetrahedrally coordinated with oxygen atoms. The obtained coordination Figure 2. EDX spectra of the Matrix-Ce30 (blue), Matrix-Nd30 (yellow) and Matrix-Eu30 (red) samples.  3 and tetrahedral BO 4 units, therefore it is expected that the ratio of BO 3 /BO 4 plays an important role in the formation of the glassy network. Modifiers such as lanthanides 9,19,20 , initiate the structural transformation and establish the ability of the glass matrix to incorporate heavy ions (e.g. radionuclides). The changes in boron coordination numbers revealed that a structural transformation of BO 3 to BO 4 occurs in the glass network by an increase in CeO 2 , Nd 2 O 3 or Eu 2 O 3 concentration which supports our previous ND results 9 . Based on the RMC calculations the basic network structure was established as mixed [3] Si-O- [3] B and [3] Si-O- [4] B linkages 8,9,13,21 , which lie in excellent agreement with our neutron diffraction data, i.e. the network structure is stable for all studied samples.
One characteristic O-O correlation peak was identified at 2.35 ± 0.05 Å, which appears at the same distance for all compositions (cf. Table 2).
The lanthanide surroundings were obtained with a great stability from the RMC calculations, the Ln-environment was achieved with a good reproducibility due to their high weight in the XRD experiment. Figure 4a displays g Ce-O (r), where one well-resolved peak appears at 2.45 ± 0.05 Å, with a shoulder at 3.05 ± 0.05 Å. The   23 ). This is an indication that the Ce atoms correlate with the network former Si and B atoms, as shown in Fig. 4b,c. The atomic pair correlation function of Nd-O indicates a first neighbor distance at 2.35 ± 0.05 Å, which is shorter than that previously reported values 22,24 but longer than the distance determined from the ND measurements 9 (see Fig. 5a). The average Nd-O coordination number is found equal to 7.3 ± 0.1 and 7.8 ± 0.1 for samples Matrix-Nd10 and Matrix-Nd30 samples, respectively (cf. Fig. 7b), which is higher than the respective obtained earlier from the ND data. The Si-Nd and B-Nd second neighbour distances were obtained at 2.80 ± 0.05 Å and 2.65/3.15 ± 0.1 Å, for the Matrix-Nd10 and for the Matrix-Nd30 concentrations, respectively (cf. Fig. 5b,c). Figure 6 presents g Eu-O (r) correlation function which shows one intensive peak at 2.33/2.35 ± 0.05 Å which is longer than the results obtained from ND data 9 . The peaks are above the value of 2.22 Å on the SiO 2 -Eu 2 O 3 system deduced from a molecular dynamics simulation 25,26 . The Eu-O coordination numbers are equal to 7.2 ± 0.1   www.nature.com/scientificreports/ and 7.7 ± 0.1 for the Matrix-Eu10 and Matrix-Eu30 compositions, respectively (cf. Fig. 7c). Characteristic correlation functions were obtained between Si-Eu at 3.25/3.75 ± 0.05 Å and for B-Eu at 3.05/3.10 ± 0.1 Å for the Matrix-Eu10 and for the Matrix-Eu30 concentrations, respectively (cf. Fig. 6b,c). Thanks to the relatively high XRD weighting factor of the Ln based connections, i.e. Ln-O and Si/B-Ln, well defined distances and stable glassy configuration can be revealed. The analysis of the second neighbour distances reveal that the B-Ln distances are shorter than the Si-Ln connections, suggesting that the Ln-ions preferentially connect to a B atom through an oxygen atom. Both types of connections suggest that the Ce, Nd and Eu atoms are incorporated into the basic borosilicate glass structure, and they are bound to oxygen atoms at a relatively short distances.

Oxidation state of lanthanides by XANES.
Lanthanides were added to the borosilicate glass matrix components as oxides prior to preparation by melt-quenching, raw materials were melted at 1300 °C (Matrix-Eu10, Eu30) and 1450 °C (Matrix-Ce10, Ce30, Nd10, Nd30). Ce was initially added as Ce 4+ (CeO 2 ), which can be easily reduced to Ce 3+ in the final glass structure, but a significant fraction can remain in the Ce 4+ form 6 . For this reason, the oxidation state was investigated by XANES measurements at the Ce-L III edge and least-squares fitting (LCF) with model compounds of known oxidation states, i.e. CeO 2 for Ce 4+ and CeTiO 3 for Ce 3+ . XANES spectra of Ln-containing borosilicate glass samples and model compounds are presented in Fig. 8. LCF analysis revealed that 76% to 84% of Ce is present as Ce 3+ in the glass and the amount of remaining Ce 4+ ions is higher (24%) in the sample prepared with the addition of a higher concentration of CeO 2 (Table 3). These findings are in accordance with results published for Ce-containing borosilicate glasses. Lopez et al. 5 presented a depend-    28 investigated the Eu oxidation state in synthetic Eu-doped granitic and basaltic silicate glasses using XANES. The Eu 2+ /Eu total molar ratios in glasses prepared in air at 1250-1650 °C were found to be in the range of < 1% to 22% strongly dependent on the composition.
Lanthanide environments by EXAFS. The EXAFS region of the energy scans at the Ce-L III and Ce-L II edges was highly affected by oscillations near the Ba-L I absorption edge due to the high barium content of the borosilicate samples. Thus, in the case of the Ce-L III spectra the available k-range is very limited, i.e. up to 7.5 Å −1 , yielding very poor Fourier Transforms (FT) and unambiguous fitting results. For this reason, we focused on the Nd-and Eu-samples.
In the case of Nd-L II and Nd-L III edges, the fitting was performed simultaneously for the two glasses Matrix-Nd10 and Matrix-Nd30 (cf. Fig. 9a,b). The FT of the k 3 × χ(k) Nd-L III -and Nd-L II -EXAFS spectra (k-range 3.0 to 9.5 Å −1 and 3.0 to 10.5 Å −1 , respectively) of the studied glassy samples are shown in Fig. 9a,b, respectively. The spectra were fitted assuming a mixed bonding environment of Nd in the borosilicate glasses: (a) a fraction x of Nd atoms is tenfold coordinated surrounded by BO 3 trigonal and BO 4 tetrahedral units (the crystalline structure of Nd(BO 2 ) 3 was used) 29 , (b) the rest y = 1-x is eightfold coordinated and link to SiO 4 units 30 . The fitting parameters where the percentage of Nd in different sites and the distances of the nearest neighbor atoms; the Debye-Waller factors (σ 2 ) were free to vary for sample Matrix-Nd10 and remain constrained for sample Matrix-Nd30 and the analysis results for both edges are shown in the Tables 4 and 5.
It is revealed that the majority of Nd 3+ ions (~ 80 at%) is surrounded mainly by boron chains, a finding previously reported in alkali-free borosilicate glasses at low Nd-concentrations 31 , while only a small fraction links to SiO 4 silica units. This preference is not disturbed by the different chemical composition in the glasses. The environment of Nd is mainly borate (Nd-O and Nd-B at 2.33-2.35 Å and 3.16 Å, respectively); nevertheless, the Nd-O bond length is much shorter than the one reported in Ln-metaborate, Na-rich borate (2.44 and 2.48 Å) and Nd-doped borosilicate glasses 24 . To a much less extent, neodymium is present at a silicate site (Nd-O and Nd-Si distances equal to 2.18-2.23 Å and 2.45-3.48 Å, respectively); the Nd-O bond length is much shorter than 2.35 Å, a finding expected in a silicate yet alkali free glass, however Nd has a sixfold coordination 32 or in natural garnets with Nd in trace amounts 33 . Finally, the invariance of the Nd-O distances in both glasses can be attributed to the same bonding environment of Nd, since a shorter bond length is expected when the Nd coordination number increases in glasses with varying Nd-concentration 34 .
As far as the Eu-L III EXAFS spectra are concerned, the FT of the of the k 3 × χ(k) Eu-L III -EXAFS spectra (k-range 3.0 to 11.5 Å −1 ) of the studied glassy samples are shown in Fig. 10. The fitting model used in this case, assumes a mixed bonding environment of Eu in the borosilicate glasses, (a) a fraction x of the Eu atoms is tenfold coordinated with O atoms and link to borate chains made up from [B 6 O 12 ] n 6structural units (the crystalline structure of LaB 3 O 6 , was used where Eu atoms substitute for La in the crystalline model) 29,35 and (b) the rest y = 1-x form pentagonal bipyramids EuO 7 that connect via corners to SiO 4 units 36 . The fitting was again performed simultaneously for the two glasses Matrix-Eu10 and Matrix-Eu30 (cf. Fig. 10) and the fitting parameters where those reported previously for the Nd-L II and Nd-L III edges and the results are listed in the Table 6.
According to the EXAFS analysis results, the Eu 3+ ions are equally distributed between the two different bonding geometries. Independently of the chemical composition, approximately half of the Eu ions form EuO 7   34 .
In conclusion, EXAFS results at both Nd-L II ,L III and Eu-L III edges deliver direct evidence that the Ln ions are not isolated from the silicate network. This is demonstrated by the absence of Ln atoms clustering in the glasses, with the presence of quasi-molecular complexes (molecular entities that modify the geometrical arrangement of the vitreous matrix to accommodate their own bonding requirements) containing Ln-O polyhedra with Ln-O-B and Ln-O-Si type of linkages. Finally, the difference in the ionic radii between Nd and Eu ions, accounts for the shorter Nd-O bond length compared to Eu-O in both environments.
Structural characterization of Ln-doped borosilicate glasses were performed by the combination of synchrotron and simulation techniques. The XAFS technique has been successfully applied, the structural factor being in good agreement with the ones obtained by RMC simulation. The local structure around Ln is nearly the same in both experiments. The partial pair correlation functions g Nd-O (r) and g Eu-O (r) shows similar peak positions, however the second neighbors from EXAFS strongly interfere with the XRD/RMC values. The differences are observed in the Nd and Eu average coordination numbers, which are lower in case of XRD data calculated by Table 4. Structural parameters (coordination number, N; bond length, r; Debye-Waller factor, σ 2 ; fraction of Nd link to borate, x and fraction of Nd link to silicate, y) for glasses Matrix-Nd10 and Matrix-Nd30 glasses obtained from the analysis of the Nd L III -edge EXAFS spectra. The asterisk denotes values that were kept fixed in the fitting process. Leaching tests. Concerning the release rates of surrogates in the studied glass waste forms, leaching tests in aqueous solutions were conducted on Matrix-glass and on Matrix-Ln30 sample series containing cerium, neodymium and europium as actinide surrogates for plutonium, americium and curium, based on ASTM C1285-02 protocol 39,40 . The results of the Product Consistency Tests (PCT), PCT-A and PCT-B on the glasses are shown in Table 7. In general, the stability of the surrogated structure is inversely proportional to the leaching rates, lower values represent a more durable glass waste form. Our data indicate that the releases of Ce, Nd and Eu are of a similar order of magnitude. The obtained release rates show a decreasing tendency in function of time, in all sample series similarly to those reported in the literature 41,42 . The normalized leaching rate of the Matrix-Nd30 sample at 7 days shows the same or lower magnitude compared to a Nd surrogated pyrochlore-borosilicate sample at 28 days 41 and very similar rate values with calcium neodymium/cerium oxide silicate glass-ceramics 43 and with the same range for glass-ceramic compositions with neodymium/cerium 44 . The decreasing tendency can be also identified for the Matrix-Ce30 sample test series and is in good agreement with the results reported in Ref 45 . Although our samples have substantially higher CeO 2 content the order of magnitude of the normalized leaching rates are similar. Europium was used as a surrogate for curium and had the best results regarding the elemental release before the last sampling. On the tenth day the normalized leaching rates of Matrix-Nd30 and Matrix-Eu30 reached the same order of magnitude. Matrix elements can be classified into two distinct groups based on the differences obtained in their release characteristics. The Si, B and Na elements tend to be more stable in the matrix without the presence of lanthanides Table 5. Structural parameters (coordination number, N; bond length, r; Debye-Waller factor, σ 2 ; fraction of Nd link to borate, x and fraction of Nd link to silicate, y) for glasses Matrix-Nd10 and Matrix-Nd30 obtained from the analysis of the Nd L II -edge EXAFS spectra. The asterisk denotes values that were kept fixed in the fitting process.   Our data presented in Table 7 are consistent with the data of Backhouse et al. 49 received for the six-component alumino-borosilicate glass, where the dissolution rates were 1.31E−02 g/m 2 /d, 1.56E−02 g/m 2 /d and 1.45E−02 g/ m 2 /d for Si, B and Na, respectively. Miekina 50 studied leaching of B, Na and Si after 7 days for Ca-and Mgborosilicate glasses. In that work, leaching values of 1.8-4.8 g/m 2 and 0.3-0.4 g/m 2 for B/Na and Si, respectively, which are lower than our results, however no lanthanides were added to those glasses. Leaching results obtained for SiO 2 -B 2 O 3 -Li 2 O-Na 2 O-ZnO glasses were presented by Vance et al. 51 based on PCT-B and MCC-1 tests, where the normalized data were in the range of 0.5-25 g/m 2 /d for Si, 0.4-40 g/m 2 /day for B and 0.7-70 g/m 2 /d for Na, which are higher than our results. Based on the discussed values and recommended limits, our leaching results obtained for Si, B and Na elements, predict a stable glassy matrix in term of ASTM conditions. The Zr and Ba elements act independently and show normalized release rate of similar order of magnitude in each sample. The normalized release of Zr is lower than that obtained for glass ceramic samples 44 , but for Ba we obtained the same numbers as calculated in the NEUP Report 44 . In case of Ba (test A/Matrix-Ce30) some discrepancy was found that is possibly connected to impurities.
PCT-A and PCT-B tests at the same time-range (7 days) show comparable values for Ln-elements and also for matrix-glass components.

Conclusions
Borosilicate matrix glass (55SiO 2 ·10B 2 O 3 ·25Na 2 O·5BaO·5ZrO) with 10 wt% and 30 wt% addition of lanthanide oxides of CeO 2 , Nd 2 O 3 and Eu 2 O 3 , respectively were investigated by synchrotron techniques (XRD, XAFS) and Reverse Monte Carlo simulation. The SEM investigation revealed the homogeneity and amorphous nature of the glasses. EDX analysis confirms the chemical compositions of the glasses, all of Ln-elements were identified. XANES results reveals that in addition to Nd and Eu which were originally added in the trivalent form, also Ce is mostly present in the reduced Ce 3+ form with 16-24% of remaining Ce 4+ content.
The RMC simulation of the experimental XRD data and XAFS analysis show that the Ln's are stabilized in the silicate and borate network. The second neighbor connections between the network former Si/B-atoms and Ln-atoms supports the incorporation of the Ln's in the basic network structure which are built up by mixed [3] Si-O- [3] B and [3] Si-O- [4] B linkages. The leaching characteristics of matrix-glass components and lanthanides are different but similar within the sample series. The leached amount of Ce, Nd and Eu decreases over time, supposing that the studied compositions at long-term can be a good choice for stabilization of surrogates of the selected (Pu, Am, Cm) actinides.

Scanning electron microscopy investigations. Powerful complementary characterization techniques
capable of analyzing morphology and chemical composition at the surface and in the bulk of single nanoparticles are analytical electron microscopy, i.e. a combination of Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDX). Morphology and shape information are derived from the detection of Table 7. Normalized elemental leaching rates in PCT-A and PCT-B tests from 3 to 10 days. *Ce, Nd, Eu for the respective samples. www.nature.com/scientificreports/ secondary electrons in SEM and analytical information from characteristic X-rays (EDX). Bulk composition is delivered by EDX with electron excitation because the excitation volume is typically at the micrometer scale. EDX can be used to obtain elemental information about the area of interest.
Microstructural characterization experiments were conducted with carbon coated bulk samples a Thermo Scientific Scios 2 DualBeam Sytem SEM with focused ion beam capabilities linked with an Oxford Instruments X-Max EDX attachment. X-ray diffraction experiments. The X-ray diffraction (XRD) measurements were performed on the P07 diffractometer at the PETRA III storage ring of Deutsches Elektronen-Synchrotron (Hamburg, Germany) 52 . The energy of the radiation was 47 keV (λ 0 = 0.1263 Å). The raw data were corrected for detector dead time, background, absorption and variations in detector solid angle. The total structure factor, S(Q) was calculated by the PDFgetX3 software packages 53 .
Reverse Monte Carlo simulations. The Reverse Monte Carlo (RMC) simulation is a powerful technique to build large 3D structural models in accordance with experimental data, in particular total structure factors (S(Q)) obtained from diffraction experiments. The S(Q) data were simulated for XRD diffraction by the RMC 2+ code 54 . The RMC algorithm calculates the g ij (r) one-dimensional partial atomic pair correlation functions, and by inverse Fourier transformation, calculates the S ij (Q) partial structure factors as: where ρ 0 and r max are the atomic number density and the half-edge-length of the simulation box in the RMC calculation. The actual computer configuration is modified by moving the atoms randomly until the calculated S(Q) and experimental data agree within the experimental error (cf. Eqs. (1) and (2)).
X-ray absorption fine structure (XAFS) measurements. The measurements were performed at the XAFS beamline of Elettra synchrotron radiation facility (Trieste, Italy) 55 . Spectra covering both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions were collected using a Si (111) monochromator around L III and L II absorption edges of Ce, Nd and Eu. The concentration of the elements of interest in the Ln-containing glass samples was sufficiently high for collection of XAFS spectra in transmission mode. Glass samples were investigated as 13-mm diameter pressed pellets with 50-100 mg/cm 2 of lanthanide-containing glass and polyvinylpyrrolidone as a binder. Reference compounds CeO 2 , Ce 2 (SO 4 ) 3 , Nd 2 O 3 and Eu 2 O 3 were measured in the form of pressed pellets as well.
All spectra were collected in air at room temperature. The Si(111) monochromator was tuned using energy steps of 5 eV below the absorption edge, 0.2 eV in the near-edge region and a k-constant step of 0.03 Å −1 in the extended region. The intensity of the incoming and transmitted X-rays were monitored by ionization chambers, using a 2 s dwell time per energy step. For each sample, multiple spectra have been collected and merged in order to increase the signal to noise ratio. The energy scale was calibrated by reference metal foils having K absorption edge within the energy regions of interest (Cr-K for Ce-L III ,L II , Mn-K for Nd-L III ,L II and Fe-K for Eu-L III ).
The oxidation state of cerium and europium in borosilicate glass samples was determined using linear combination fitting (LCF) of the XANES region based on reference spectra for model compounds of known oxidation states, CeO 2 for Ce 4+ ; Ce 2 (SO 4 ) 3 and CeTiO 3 for Ce 3+ ; Eu 2 O 3 for Eu 3+ and EuSe for Eu 2+ . For CeTiO 3 and EuSe, published reference XANES spectra were used from Lytle database 56 and XAS Data Library 57 , respectively. Background removal, normalization of XAFS spectra as well as LCF were performed using the Athena software package 58 . For EXAFS, FEFF8.2 was used to calculate the theoretical phase and amplitude functions for the scattering paths 59 , while curve fitting was carried out in both R-and k-space using FEFFIT 60 .
Leaching tests. Glass chemical durability is a direct measure of the wasteforms capability to immobilize its radionuclide content. The chemical durability depends on numerous factors such as composition, waste loading, leachate composition, pH rate, redox potential, diffusion coefficients, transport properties, the formation of surface layers, crystallization of the waste glass, phase separation and radiolysis 61,62 .
Concerning the stability of Ln-glasses, leaching tests in aqueous solutions were evaluated based on ASTM C1285-02 protocol 39,40 on Matrix-Ln30 sample series. The ASTM's PCT (Product Consistency Test) is generally used to investigate the chemical stability of nuclear glass waste form and study the release rate and kinetic characteristics of elements of interest. The glass samples were crushed in a ball mill and the tests were performed with the − 100 to + 200 mesh-size fraction. This static method is carried out in 304L stainless steel cylindrical containers at a constant temperature of 90 ± 1 °C using a laboratory oven. This temperature is higher than that expected to occur during the nuclear glasses alteration by groundwater (50 °C), enabling to increase leaching kinetic. In order to reach the saturation phenomena faster, the tests are generally performed with elevated surface to volume ratios. Following the ASTM protocol test A and test B were performed with a ratio of 10 ml/g regarding the leachate volume to the sample mass. The test A is conducted in strictly defined conditions, while test B allows applying different experimental parameters (temperature, duration, leachate volume to sample mass ratio, mesh size). During the test A after 7 days 6 ml, while during the test B after 3, 7 and 10 days 5 ml of aliquots were collected. The leachates were filtered through a 0,45 µm syringe filter and acidified with 20 µl of ultrapure cc. HNO 3 . The concentrations of the released elements were determined via ICP-OES (Inductively coupled plasma atomic emission spectroscopy). Normalized leaching rate was calculated with the following equation 39 : License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.