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Research Article
Arianit A. Reka*, Blagoj Pavlovski, Emira Fazlija, Avni Berisha, Musaj Pacarizi,
Maria Daghmehchi, Carmen Sacalis, Gligor Jovanovski, Petre Makreski, Ayhan Oral
Diatomaceous Earth: Characterization, thermal
modification, and application
https://doi.org/10.1515/chem-2020-0049
received March 26, 2020; accepted March 4, 2021
Abstract: The diatomaceous earth (DE), collected from
the Mariovo region in North Macedonia, was characteri-
zed and thermally modified. The material represents a
sedimentary rock of biogenic origin, soft solid that can be
easily disintegrated, with white to grayish color, with bulk
density of 0.51–0.55 g/cm
3
,totalporosityof61–63%, and
specific gravity of 2.25 g/cm
3
. The chemical composition is
as follows: SiO
2,
86.03; Al
2
O
3
,3.01;Fe
2
O
3
, 2.89; MnO, 0.06;
TiO
2,
0.20; CaO, 0.76; MgO, 0.28; K
2
O, 0.69; Na
2
O, 0.19;
P
2
O
5
, 0.15; and loss of ignition, 5.66 (wt%). The mineralogy
of the raw DE is characterized by the predominant presence
of amorphous phase, followed by crystalline quartz, mus-
covite, kaolinite, and feldspar. Significant changes in the
opal phase are observed in the 1,000–1,200°C temperature
region. At 1,100°C, the entire opal underwent solid–solid
transition to cristobalite. Further ramp of the temperature
(1,100–1,200°C)induced formation of mullite. Scanning
electron microscopy (SEM)and transmission electron micro-
scopy depict the presence of micro-and nanostructures with
pores varying from 260 to 650 nm. SEM analysis further
determined morphological changes in terms of the pore dia-
meters shrinkage to 120–250nmincomparisontothelarger
pores found in the initial material. The results from this
investigation improve the understanding of mechanism of
silica phase transition and the relevant phase alterations
that took place in DE upon calcination temperatures from
500 to 1,200°C.
Keywords: diatomaceous earth, calcination, thermal modi-
fication, natural nanomaterial
1 Introduction
Diatomaceous earth (DE, otherwise known as diatomite)
is a very important natural material used in industry com-
prising distinctive combinations of physical and chemical
properties. Typically, it is a soft, friable, fine-grained, weakly
cemented, porous, and light-weight sedimentary siliceous
rock. Other valuable characteristics of DE are low bulk den-
sity, low thermal conductivity, inert chemical reactivity with
most liquids and gases, and sparingly solubility in water.
These properties classify DE as a very attractive natural
material with distinctive properties, i.e., high permeability,
high porosity, and large surface area. Depending on the
amount of present impurities, its color varies from white to
yellowish gray, dark gray, and brownish-gray [1–5].DEis
considered to be a natural nanomaterial [6],composed
mainly of accumulated remains of skeletons [7].DEhasa
variety of uses and applications such as obtaining humidity
control materials [8],materialforfiltration [9], raw material
* Corresponding author: Arianit A. Reka, Department of Chemistry,
Faculty of Natural Sciences and Mathematics, University of Tetovo,
Bldv. Ilinden n.n., 1200 Tetovo, Republic of North Macedonia;
NanoAlb, Albanian Unit of Nanoscience and Nanotechnology,
Academy of Sciences of Albania, Fan Noli square, 1000 Tirana,
Albania, e-mail: arianit.reka@unite.edu.mk
Blagoj Pavlovski: Institute of Inorganic Technology, Faculty of
Technology and Metallurgy, Ss. Cyril and Methodius University,
Ruger Boskovic n.n., 1000 Skopje, Republic of North Macedonia
Emira Fazlija: Department of Chemistry, Faculty of Natural Sciences
and Mathematics, University of Tetovo, Bldv. Ilinden n.n.,
1200 Tetovo, Republic of North Macedonia
Avni Berisha, Musaj Pacarizi: NanoAlb, Albanian Unit of
Nanoscience and Nanotechnology, Academy of Sciences of Albania,
Fan Noli square, 1000 Tirana, Albania; Department of Chemistry,
Faculty of Natural Sciences and Mathematics, University of Pristina
“Hasan Prishtina”, 10000 Pristina, Republic of Kosovo
Maria Daghmehchi: Department of Archaeology, University of
Tehran, Tehran 6619-14155, Iran
Carmen Sacalis: Department of Chemistry, Faculty of Chemistry and
Chemical Engineering, Babes-Bolyai University, 11 Arany Janos St.,
400028 Cluj-Napoca, Romania
Gligor Jovanovski: Research Center for Environment and Materials,
Academy of Sciences and Arts of North Macedonia, MASA, Bul. Krste
Misirkov 2, 1000 Skopje, North Macedonia
Petre Makreski: Institute of Chemistry, Faculty of Natural Sciences
and Mathematics, Ss. Cyril and Methodius University, Arhimedova
5, 1000 Skopje, Republic of North Macedonia
Ayhan Oral: Department of Materials Science and Engineering,
Çanakkale Onsekiz Mart University, Campus of Terzioglu, 17100
Çanakkale, Turkey
Open Chemistry 2021; 19: 451–461
Open Access. © 2021 Arianit A. Reka et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0
International License.
for production of cement [10], initial material for production
of prolonged-release drug carriers [11],sorption,desorption
and industrial scale absorption material [12–14],production
of porous ceramics [15–17], glass industry [18,19], catalyst
support [20],filler in paints and plastics [21],purification of
industrial waters [22], pozzolanic material, pesticide holder,
and also as a material for improving the physical and che-
mical characteristics of certain soils, etc. [22–28].
The excavated DE from geological deposits may con-
tain diverse metal oxides and organic matter associated
with the dominant silica content (SiO
2
). The most abun-
dant association involves Al
2
O
3
,Fe
2
O
3
, CaO, MgO, K
2
O,
Na
2
O, and P
2
O
5
that might be even beneficial toward its
application properties [29]. One way to improve the prop-
erties of DE is through calcination process that facilitates
the removal of impurities from the frustules resulting in
improvement of the DE industrial quality [30–34].
The USA was the main producer of DE in 2019,
accounting for an approximate 34% of its world produc-
tion, accompanied by China and Denmark with 15% each,
Turkey with 6%, Republic of Korea 5%, Peru 4%, and
Mexico with 3%. Smaller quantities of DE were mined
in 23 other countries [35]. North Macedonia fits in this
group being rich in DE and other silica-based materials
(trepel, perlite, pumice, etc.)[36–42]with a wide scope
of potential utilization and application. The economic
benefit of using DE from North Macedonia is based on
its fine microstructure and, more importantly, because
of the presence of non-crystalline (amorphous)phase.
Such characteristics make this material distinctly reac-
tive. The determination of the mineralogical alterations
that arise during the thermal treatment process of DE is
of a great significance that will govern its further use
and application in various technological processes. Thus,
the aim of this research was to monitor the effect of the
calcinationonthesilicaphasetransformationalongwith
determining the temperature at which it remains amor-
phous. Despite the thermal behavior of the DE, particular
interestwasstressedtofollowupthechangesthatoccurred
in the frustules. The transition mechanisms of the occurring
phenomena were followed by the synergistic use of struc-
tural, thermal, microscopy, and spectroscopy techniques.
2 Materials and methods
The raw DE used in this study was collected from the
Mariovo area, North Macedonia [43].
The chemical composition of natural DE was deter-
mined using the classical chemical analysis (for silicates).
DE was fused in a mixture of carbonates, while the percen-
tages of other oxides found in the material were determined
by complexometric titration. The presence of Na
2
OandK
2
O
was determined by flame atomic emission spectroscopy.
The trace elements were determined using ICP-MS (Agilent
7500cx).
The determination of the mineralogical content of DE
was performed using X-ray powder diffraction (XRPD),
thermal analysis (thermogravimetric/differential thermal
analysis [TGA/DTA]), and scanning electron microscopy
(SEM-EDX). XRPD analysis was carried out on Rigaku
Ultima IV X-ray diffractometer set-up with D/teX high-
speed 1-dimensional detector, using CuKαradiation (λ=
1.54178 Å)in the range from 5 to 60°C. The accelerating
voltage was set at 40 kV, while the current power was set
at 40 mA.
The Perkin-Elmer FTIR system 2000 interferometer
was engaged to record the infrared spectra in the 4,000–
450 cm
–1
spectral region using the KBr pellet method. The
pellet was prepared by loading pressure (10 tons)onto a
solid mixture of 1 mg of sample with 250mg of dried KBr.
The thermal analyses (DTA/TGA)of the raw DE were
carried out in air conditions using a Universal V4.5A TA
Instrument, SDT Q600 V20.9 Build 20 apparatus, under
the following experimental set-up: temperature span of
20–1,200°C; rate of heating of 10°C/min and a thermal
treatment duration time of 108 min; and mass of sample
of 10 mg, with a ceramic pot as a carrier for the material.
SEM VEGA3 LMU coupled with energy-dispersive
X-ray spectroscopy (INCA Energy 250 Microanalysis System)
was used to quantitatively examine the material. The accel-
erating voltage of the SE detector was set to 20 kV.
Transmission electron microscopy (TEM)on the nat-
ural DE was performed using Hitachi H-7650 instrument
(120 kV automatic microscope).
For the purpose of the thermal investigations, raw DE
was calcined ramping the temperature from 500 to 1,200°C
with a duration of 1 h and 100°C between each step.
Ethical approval: The conducted research is not related to
either human or animal use.
3 Results and discussion
3.1 Physicomechanical properties of the DE
From the physicomechanical perspective, the tested DE
(Figure 1)represents a very light and soft (1–2 Mohs)
452 Arianit A. Reka et al.
sedimentary rock of biogenic origin white in color. It repre-
sents a fine-superfine grained structure, porous (61–63%),
with a shell-like fragility, and sticks to the tongue. No
obvious reaction with HCl was observed. The bulk density
of DE is 0.51–0.55 g/cm
3
,andthedensityis2.25g/cm
3
,
while the compressive strength in its natural state (raw)is
7.67 MPa.
3.2 Chemical analysis of the DE
The chemical composition of DE (Table 1)was determined
using the classical chemical analysis (for silicates). The
loss of ignition (LOI), determined during heating the nat-
ural DE at 1,000°C for a duration of 1 h, was 5.66%. The
results acquired from the chemical composition of DE
indicate that the tested material constitutes an acidic
rock with predominating percentage of SiO
2
(86.03%)
and relatively low content of Al
2
O
3
(3.01%)and Fe
2
O
3
(2.89%)with the content of each of the remaining oxides
remains below 1% (Table 1).
The content of the trace elements (Table 2)revealed
abundance of Cu, Cr, V, Rb, Sr, Cs, and Mo in the 30–100
ppm range, whereas the major presence was found for Ba
(165 ppm),U(249 ppm), and As (586 ppm). The signifi-
cant content of U is explained by the existence of the U-
bearing zones and minerals in the Mariovo region [44,45].
The relatively high content of As could be related to the
high presence of arsenic ores and the very abundant
arsenic mineralization particularly typical for the nearby
site of Allchar. Namely, the famous Allchar mine as a part
of the Кozhuf volcanic area and as the youngest volcanic area
lies only around 10 km east from the Mariovo-Kajmakčalan
volcanic area that stretches between the Nidže Mountain
with Kajmakčalan in the south and the village of Vitolište
in the north being in the vicinity of the examined DE
locality [43]. The content of the remaining trace elements
is very low, not exceeding 10 ppm.
3.3 XRPD analysis of DE
The X-ray diffractogram of DE (Figure 2)mainly repre-
sents the amorphous phase with a minor presence of
crystalline phases. The manifestation of the wide “bump”
positioned between 15° and 28° (2θ)is ascribed to the exis-
tence of opal in the sample.
The crystalline phases evident in the sample are as a
result of quartz, SiO
2
(peaks d4.25 at 20.88°; d3.34 at
26.66°; d2.27 at 39.51°; d1.81 at 50.16°; d1.54 Å at 59.58°
2θ); muscovite, KAl
2
(Si
3
Al)O
10
(OH,F)
2
(d9.91 at 8.91°;
d4.96 at 17.85°; d4.48 at 19.76°; d3.31 at 26.90°; d2.98
at 29.89°; d2.80 at 31.85°; d2.59 at 34.51°; d2.35 at 38.19°;
d2.14 at 42.15°; d1.99 at 45.59°; d1.97 at 45.94°; d1.74 at
52.54°; d1.65 Å at 55.53° 2θ); kaolinite, Al
2
Si
2
O
5
(OH)
4
(d3.58 at 24.79°; d7.18 at 12.36°; d1.65 at 55.44°; d2.33
at 38.84°; d2.55 Å at 35.05° 2θ); and plagioclase feld-
spars, NaAlSi
3
O
8
–CaAl
2
Si
2
O
8
(d3.25 at 27.42°; d3.77
at 23.55°; d4.02 at 22.10°; d3.19 at 27.86°; d3.60 Å at
24.68° 2θ)[46,47].
3.4 Infrared spectra analysis of DE
The FTIR spectrum (Figure 3)of the DE displays a very
strong absorption band at 1,100 cm
‒1
with an associated
shoulder at 1,250 cm
‒1
that are attributed to the antisym-
metric stretching Si‒O vibrations. The absorption band
at 800 cm
‒1
evolves from the corresponding symmetric
extension–compression vibration of Si‒O[48–50]. The
bands at 469, 532, and 695 cm
‒1
fingerprint the presence
of muscovite [51], whereas the weak absorption bands at
Figure 1: Natural (crude)DE from Mariovo.
Table 1: Chemical composition of the DE
Oxide SiO
2
Al
2
O
3
Fe
2
O
3
MnO TiO
2
CaO MgO K
2
ONa
2
OP
2
O
5
LOI Total
Mass% 86.03 3.01 2.89 0.06 0.20 0.76 0.28 0.69 0.19 0.15 5.66 99.92
Diatomaceous Earth: Characterization, modification, and application 453
913, 3,621, and 3,696 cm
‒1
originate from the present kao-
linite [51–54]. The broad band at ∼3,430 cm
–1
is because
of the H‒O‒H stretching vibrations of absorbed water,
while the band at 1,639 cm
−1
is attributed to the presence
of opal in the sample and is because of the H‒O‒H
bending vibrations from the absorbed water in opal.
3.5 Thermally induced study of DE
TGA along with DTA of the tested specimen were under-
taken because the response of the material upon heating
is of great importance to determine its technological
properties (Figure 4).
The results from the TG analysis inferred that weight
loss took place in three temperature intervals. The first
temperature span extends from room temperature to
265°C exhibiting weight loss of 8.07% that is attributed
to the elimination of adsorbed and absorbed water in DE.
The second temperature interval spans between 265 and
600°C are characterized by weight loss of 3.26% being
attributed to the dehydration process of the chemically
bonded water in opal structure and burning of the organic
matter existing in diatomite [53]. The third temperature
interval (from 600 to 1,100°C)followed by the minor
weight loss of 2% is ascribed to the dehydroxylation of
the clay constituents (muscovite and kaolinite)[54,55].
3.6 SEM of DE
The results from the SEM (Figure 5)revealed the biogenic
identity of the raw DE. Namely, various frustules and/or
entire skeletal structures of diatoms algae (most of the
time in the shape of sunflowers)ranging from 5 to 15 μm
were registered. SEM morphology of DE indicates the pre-
sence of preserved forms of the diatom frustules. The
existence of other shapes, which are in all probability
as a result of the clay constituent in the material, is
also evident. The size of the pores ranges between 200
and 460 nm in diameter.
The EDX spectrum facilitated into the quantitative
determination of the chemical content of the analyzed
Table 2: Content of trace elements found in DE
Element ppm Element ppm Element Ppm Element ppm
Cu 97 Cd 0.076 Sr 87.8 Pd 4.1
Cr 33.0 As 586.4 Cs 35.0 Ag 1.2
Ni 9.0 Se 1.1 Th 7.9 Ga 6.8
Co 3.2 Tl 6.88 U 248.7 Ge 0.6
Zn 1.22 Bi 0.41 Mo 42.4 Li 11.64
V 56.7 Ba 165.4 Sn 1.1 Be 1.2
Pb 8.9 Rb 45.3 Sb 0.4 B <10
Figure 2: XRPD pattern of the raw DE. The strongest peaks arising from muscovite (M), kaolinite (K), quartz (Q), and feldspars (F)are marked.
454 Arianit A. Reka et al.
sample (Figure 6a)and confirmed the purity of skeletons
being actually majorly composed of silica, SiO
2
(O: 70.03%
and Si: 29.97%). However, the surplus deviation of the
oxygen content from the ideal SiO
2
stoichiometry is incor-
porated in the calculation of the chemical formulae
(Figure 6b, O: 64.65%, Al: 3.16%, Si: 30.20%, K: 0.69%,
and Fe: 1.30%)of the other associated clay minerals that
evolve from the associated clayey minerals within the
sample (muscovite and kaolinite).
3.7 TEM of the natural diatomite
The results from the TEM (Figure 7)were complementary
with SEM analysis regarding the texture and the mor-
phology of the raw material. A heterogeneous population
constituted by morphologically size-different nanostruc-
tures was observed. The results from the TEM analysis of
the DE indicated mainly glassy features (Figure 7),resulting
from the amorphous phase in the material.
Figure 3: FTIR spectrum of the DE.
Figure 4: TG/DT analysis of DE.
Diatomaceous Earth: Characterization, modification, and application 455
The diatom shells in DE exhibit rich porous structure
and uncontaminated surface, while the impurities observed
inside the nanometric pores were, most likely, related to
the association of clayey minerals [57]. In comparison to
other nanocarriers, the nanometric proportions and mor-
phology of structures found in the DE positioned the
material as a suitable candidate for use in drug delivery
applications [58].
3.8 XRPD examination of the calcined DE
Results from the XRPD examination of the thermally
induced DE at 500, 600, 700, 800, 900, 1,000, 1,100,
and 1,200°C (Figure 8)revealed the presence of opal
in the temperature interval 500–1,000°C explained by
complex halo peak (typical for any amorphous phase)
between 19° and 25° (2θ)[47]. On the contrary, significant
alteration of the opal phase was observed in the tempera-
ture interval 1,000–1,100°C with the entire opal compo-
nent underwent solid–solid transition to cristobalite that
remained unaffected at 1,200°C.
The muscovite phase was registered in thermal steps
until 900°C, whereupon its complete disintegration was
observed [59].
The XRPD results also pointed out on the existence of
quartz phase up to 1,000°C whose presence is diminished
at higher temperatures evidenced by the severe intensity
reduction in the corresponding peaks at d-values of 3.34,
4.26, and 1.81 Å (2θangles at 26.66°, 20.83°, and 50.37°).
At 1,100°C, crystallization of opal into cristobalite occurs,
and at the same time transformation of quartz into cris-
tobalite also occurs. No presence of quartz was evidenced
at the highest calcined temperature of 1,200°C.
XRPD pattern of the calcined DE at 1,000°C showed
minimal intensity of peaks resulting from the mullite
phase, Al
6
Si
2
O
13
. Further temperature increase at 1,100°C
resulted in the appearance of all mullite characteristic
maxima at d-values of 5.40, 3.44, 3.40, 2.69, 2.55, and
2.22 Å (2θangles at 16.40°, 25.87°, 26.18°, 33.28°, 35.16°,
and 40.60°)that remained practically unchanged at the
highest ramped temperature of 1,200°C [46,47].
The kinetics and the mechanism of transformation
of opal in the DE mainly depend on the purity of the
Figure 5: SEM examination of raw DE. (a)Several valves of Tertiarius jurijlii and probably one frustule of T. mariovensis in right bottom
corner, (b)a whole valve of T. jurijlii,(c)T. jurijlii girdle view, and (d)Areolae in the central area of T. jurijlii [56].
456 Arianit A. Reka et al.
material itself. The DE with higher percent of SiO
2
(92.97 wt%)calcined in powder state at a temperature
of 1,100°C in an interval of 1 and 2 h remains amorphous.
The same DE during heating at 1,200°C in an interval of 1
and 2 h underwent partial crystallization of opal into
quartz and cristobalite. The sample calcined at 1,200°C
for 2 h shows increase in the cristobalite phase associated
with a decrease in the content of the quartz phase. This
result indicates that the crystallization of the amorphous
SiO
2
to cristobalite goes through the quartz phase [31].
This shows that the impurities in the analyzed DE act
toward lowering of the crystallization temperature of
opal into cristobalite for about 100°C.
3.9 SEM of the calcined DE
SEM examinations were also conducted in the calcined
regime (500–1,200°C),withnosignificant changes observed
until 900°C. However, further ramp of the temperature at
1,000, 1,100, and 1,200°C for an interval of 1 h (Figure 9)
Figure 6: SEM/EDX analysis of the skeletons (a)and the associate clayey minerals (b)found in DE.
Figure 7: TEM analysis of natural DE.
Diatomaceous Earth: Characterization, modification, and application 457
generated different morphology in the surface texture, pore
dimensions, and form. Namely, the average pore dimen-
sions of the calcined DE decreased while ramping the tem-
perature from 1,000 to 1,200°C (Figure 9).Theabrupt
shrinkage was evidenced at 1,200°C when some of the dia-
tomite pores completely annihilated and disappeared.
Such observation was mainly attributed to the impurities
(causing fusion during calcination)in the DE that pro-
mote the eutectics formation at lower temperatures. The
SEM observation is neatly supporting the XRPD results for
transformation of opal into cristobalite (1,000–1,200°C)in
terms of agglomeration of the particles resulting in the
decrease in the pore size that lowered the total porosity.
4 Conclusion
The physical–mechanical characteristics revealed that
the DE from Mariovo represents white, soft material
with a low bulk density and high porosity. The minera-
logical composition showed predominantly the amor-
phous phase with a small fraction of crystalline phases.
The amorphous phase is attributed to the amorphous
opal of biogenic origin (frustules), while the crystalline
phases mainly consist of quartz and clay minerals mostly
pronounced by dominant muscovite followed by kaoli-
nite. Microscopic SEM and TEM examinations demon-
strate an existence of micro-and nanostructures with
pores ranging from 250 to 650 nm. The conducted calci-
nation experiment helped to resolve and understand
the silica phase transition and the relevant phase altera-
tions that took place. During the calcination process
of the DE, the amorphous opal transformed into cristoba-
lite (not to tridymite)as a result of the aggregation of
Figure 8: XRPD analysis of the starting and the series of the calcined
DE in the 500–1,200°C range. The evolution of the peaks from
mullite (Mu)and cristobalite (C)at the 1,100 and 1,200°C diagrams
is marked. The peaks from muscovite (M), kaolinite (K), quartz (Q),
and feldspars (F)in the starting material are also denoted.
Figure 9: SEM analysis of calcined DE, calcined for 60 min: (a)at 1,000°C, the average pore size 380–400 nm; (b)at 1,100°C, average pore
size 260–280 nm; and (c)at 1,200°C, average pore size 120–250 nm.
458 Arianit A. Reka et al.
microcrystalline cristobalite in the amorphous opal.
Significant changes in the opal phase were observed in
the temperature interval 1,000–1,100°C. Based on the
debayegram of the calcined DE at 1,100°C, it can be
observed that the entire opal underwent total solid–solid
phase transformation to cristobalite. SEM examinations
revealed that during the transformation temperature
of opal, an agglomeration of the nano-sized opal and
particle growth took place, which was correlated with
the decrease in the porosity and specific surface of the
obtained cristobalite. The obtained cristobalite remained
stable at 1,200°C. The type and the amount of the impu-
rities (clay minerals and feldspars)contribute to the
lowering of the crystallization temperature of the opal
component into cristobalite for about 100°C. Moreover,
the temperature interval 1,100–1,200°C depicted the for-
mation of mullite. Having in mind the mullite affects both
physical and mechanical characteristics of ceramics by
increasing their shock resistance along with mechanical
strength, the studied DE from Mariovo stands as a mate-
rial suitable for the production of various ceramic mate-
rials. In addition, the presence of the diatom frustules
with nanometric structures and the water absorption cap-
ability makes this material favorable for further use as
absorption and filtration material.
Acknowledgment: The authors thank Professor Zlatko
Levkov, Institute of Biology, Faculty of Natural Sciences,
Ss. Cyril and Methodius University in Skopje, for his help
during the process of identification and characterization of
the frustules.
Funding information: The authors thank the financial
support from the Ministry of Education and Science of
North Macedonia and the Chemists Society of Turkey.
Author contributions: A. A. R.: conceptualization, inves-
tigation, methodology, writing –original draft, project
administration, resources, visualization, and supervision;
B. P.: conceptualization, project administration, writing–
review and editing, supervision,and resources; E.F., A. B.,
M. P,. M. D., and C. S.: formal analysis and investigation;
G. J.: investigation and writing –review and editing; P. M.:
methodology, formalanalysis, investigation, visualization,
writing –review and editing, resources, and supervision;
A. O.: formal analysis and resources.
Conflict of interest: The authors of this manuscript have
no conflicts of interest to declare.
Data availability statement: The datasets generated during
and/or analyzed during the current study are available
from the corresponding author on rational request.
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