The diatomaceous earth (DE), collected from the Mariovo region in North Macedonia, was characterized 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/cm3, total porosity of 61–63%, and specific gravity of 2.25 g/cm3. The chemical composition is as follows: SiO2, 86.03; Al2O3, 3.01; Fe2O3, 2.89; MnO, 0.06; TiO2, 0.20; CaO, 0.76; MgO, 0.28; K2O, 0.69; Na2O, 0.19; P2O5, 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, muscovite, 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 microscopy 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 diameters shrinkage to 120–250 nm in comparison to the larger 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.
Diatomaceous earth (DE, otherwise known as diatomite) is a very important natural material used in industry comprising 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 density, 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 [1,2,3,4,5]. DE is considered to be a natural nanomaterial , composed mainly of accumulated remains of skeletons . DE has a variety of uses and applications such as obtaining humidity control materials , material for filtration , raw material for production of cement , initial material for production of prolonged-release drug carriers , sorption, desorption and industrial scale absorption material [12,13,14], production of porous ceramics [15,16,17], glass industry [18,19], catalyst support , filler in paints and plastics , purification of industrial waters , pozzolanic material, pesticide holder, and also as a material for improving the physical and chemical characteristics of certain soils, etc. [22,23,24,25,26,27,28].
The excavated DE from geological deposits may contain diverse metal oxides and organic matter associated with the dominant silica content (SiO2). The most abundant association involves Al2O3, Fe2O3, CaO, MgO, K2O, Na2O, and P2O5 that might be even beneficial toward its application properties . One way to improve the properties of DE is through calcination process that facilitates the removal of impurities from the frustules resulting in improvement of the DE industrial quality [30,31,32,33,34].
The USA was the main producer of DE in 2019, accounting for an approximate 34% of its world production, 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 . North Macedonia fits in this group being rich in DE and other silica-based materials (trepel, perlite, pumice, etc.) [36,37,38,39,40,41,42] 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 reactive. 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 calcination on the silica phase transformation along with determining the temperature at which it remains amorphous. Despite the thermal behavior of the DE, particular interest was stressed to follow up the changes that occurred in the frustules. The transition mechanisms of the occurring phenomena were followed by the synergistic use of structural, thermal, microscopy, and spectroscopy techniques.
2 Materials and methods
The raw DE used in this study was collected from the Mariovo area, North Macedonia .
The chemical composition of natural DE was determined using the classical chemical analysis (for silicates). DE was fused in a mixture of carbonates, while the percentages of other oxides found in the material were determined by complexometric titration. The presence of Na2O and K2O 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 CuKα radiation (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 250 mg 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 accelerating voltage of the SE detector was set to 20 kV.
Transmission electron microscopy (TEM) on the natural 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) sedimentary rock of biogenic origin white in color. It represents 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/cm3, and the density is 2.25 g/cm3, 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 natural 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 SiO2 (86.03%) and relatively low content of Al2O3 (3.01%) and Fe2O3 (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 significant 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 . 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 represents 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 existence of opal in the sample.
The crystalline phases evident in the sample are as a result of quartz, SiO2 (peaks d 4.25 at 20.88°; d 3.34 at 26.66°; d 2.27 at 39.51°; d 1.81 at 50.16°; d 1.54 Å at 59.58° 2θ); muscovite, KAl2(Si3Al)O10(OH,F)2 (d 9.91 at 8.91°; d 4.96 at 17.85°; d 4.48 at 19.76°; d 3.31 at 26.90°; d 2.98 at 29.89°; d 2.80 at 31.85°; d 2.59 at 34.51°; d 2.35 at 38.19°; d 2.14 at 42.15°; d 1.99 at 45.59°; d 1.97 at 45.94°; d 1.74 at 52.54°; d 1.65 Å at 55.53° 2θ); kaolinite, Al2Si2O5(OH)4 (d 3.58 at 24.79°; d 7.18 at 12.36°; d 1.65 at 55.44°; d 2.33 at 38.84°; d 2.55 Å at 35.05° 2θ); and plagioclase feldspars, NaAlSi3O8–CaAl2Si2O8 (d 3.25 at 27.42°; d 3.77 at 23.55°; d 4.02 at 22.10°; d 3.19 at 27.86°; d 3.60 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 antisymmetric stretching Si‒O vibrations. The absorption band at 800 cm‒1 evolves from the corresponding symmetric extension–compression vibration of Si‒O [48,49,50]. The bands at 469, 532, and 695 cm‒1 fingerprint the presence of muscovite , whereas the weak absorption bands at 913, 3,621, and 3,696 cm‒1 originate from the present kaolinite [51,52,53,54]. 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 undertaken 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 . 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 presence 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 sample (Figure 6a) and confirmed the purity of skeletons being actually majorly composed of silica, SiO2 (O: 70.03% and Si: 29.97%). However, the surplus deviation of the oxygen content from the ideal SiO2 stoichiometry is incorporated 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 morphology of the raw material. A heterogeneous population constituted by morphologically size-different nanostructures 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.
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 . In comparison to other nanocarriers, the nanometric proportions and morphology of structures found in the DE positioned the material as a suitable candidate for use in drug delivery applications .
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) . On the contrary, significant alteration of the opal phase was observed in the temperature interval 1,000–1,100°C with the entire opal component 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 .
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 (2θ angles 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 cristobalite 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, Al6Si2O13. 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 (2θ angles 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 material itself. The DE with higher percent of SiO2 (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 SiO2 to cristobalite goes through the quartz phase . 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), with no significant 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) generated different morphology in the surface texture, pore dimensions, and form. Namely, the average pore dimensions of the calcined DE decreased while ramping the temperature from 1,000 to 1,200°C (Figure 9). The abrupt shrinkage was evidenced at 1,200°C when some of the diatomite pores completely annihilated and disappeared. Such observation was mainly attributed to the impurities (causing fusion during calcination) in the DE that promote 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.
The physical–mechanical characteristics revealed that the DE from Mariovo represents white, soft material with a low bulk density and high porosity. The mineralogical composition showed predominantly the amorphous 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 kaolinite. Microscopic SEM and TEM examinations demonstrate an existence of micro- and nanostructures with pores ranging from 250 to 650 nm. The conducted calcination experiment helped to resolve and understand the silica phase transition and the relevant phase alterations that took place. During the calcination process of the DE, the amorphous opal transformed into cristobalite (not to tridymite) as a result of the aggregation of 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 impurities (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 formation 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 material suitable for the production of various ceramic materials. In addition, the presence of the diatom frustules with nanometric structures and the water absorption capability makes this material favorable for further use as absorption and filtration material.
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, investigation, 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, formal analysis, 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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