3.1. Finest Fraction Washing Separation Experiment and pH Values
Separation of the finest particles was achieved by dispersing 10 g of the initial soil sample in 100 mL of bi-distilled water, followed by agitation for 30 min at 50 °C. Moderate heating of the dispersion aqueous environment ensures the proper detachment of the finest fractions from the coarse mineral particles. The agitated dispersion was left to rest for 10 min at 50 °C to ensure sedimentation of coarse fractions on the vial’s bottom, and the pH value was measured at this temperature,
Figure 3a. The aqueous dispersion of the finest fractions was transferred to another vial and allowed to cool naturally to 23 °C, at which point the pH was measured again,
Figure 3b. The finest fractions were further extracted from the aqueous dispersion by water vaporization at 60 °C until the powder was completely dried, and the amount recovery was calculated,
Figure 3c.
The washing experiment and pH measurements were carried out in triplicate to support the statistical requirement (sample size N = 3). Statistical analysis of the pH variation at 50 °C identified two distinct groups based on p-values compared to the 0.05 significance level. The first one is represented by acid pH, containing Samples A and C, and the second statistical group is formed by Sample B, which has alkaline characteristics. The pH of the first group shows a slight increase after the dispersion cools, indicating a reduction in the influence of acidic components. Advanced physicochemical analysis is required to identify the unstable soil components that influence pH values. In the alkaline group, Sample B, the pH value remains almost unchanged, indicating a strong chemical stability of the dispersed matter. The further investigation performed in this research aimed to discover the mineral compounds that ensure such stable alkaline pH.
An important feature of the first washing process is the recovery of the finest fractions from the aqueous supernatant, as presented in
Table 1. The recovery rate in
Figure 3c is given by the mass of the finest particles obtained by drying the supernatant reported to the initial sample mass.
Sample A has a sandy texture containing large amounts of coarse particles. Thus, the recovered finest fractions are about 5.77 wt.%. It forms a distinct statistical group, clearly evidenced in
Figure 3c. The smoother, clay-textured appearance of Samples B and C indicates the likely presence of significant amounts of fine particles. The recovery percentages range from about 18.60 to 20.69 wt.%, indicating that Samples B and C form another distinct statistical group. The increased recovery rate indicates the efficiency of the primary physical washing separation process. The mineral and elemental composition of the separated finest fraction brings important clues about their technological viability and is further investigated.
3.2. Mineral Distribution Assessment
Soil samples have strong crystalline characteristics, evidenced by the diffraction peaks. Their shape is narrow, having strong intensities diffracted by the strong and densely crystallized materials; some other peaks are less intense and broadened because of the nanostructural characteristics of the finest fractions,
Figure 4a.
The upper soil, having a dark red color and sandy texture, is dominated by quartz, which is the most representative mineral in this sample. It features narrow peaks with strong intensities. It is followed by a clay mixture composed of kaolinite and lepidolite. Kaolinite peaks have an intense tip and slightly broadened base, indicating a mixture of nano and small micro particles. On the other hand, the lepidolite peaks are less intense and broadened, indicating their nanostructural consistency. Iron hydroxide, crystallized as goethite, shows a few small peaks, indicating its low concentration in the upper sample. The lower dark red soil (Sample B) has a clayey texture. The relative intensity of the quartz peaks significantly decreases while lepidolite peaks strongly increase compared to the upper layer. Kaolinite peaks are weak and broadened, indicating the presence of a low amount of the finest nanostructured particles. Goethite is present only in weak traces, indicating a small amount of iron hydroxide particles dispersed within the kaolinite. The bottom layer (Sample C) is dominated by a significant presence of quartz, having intense and narrow peaks. It is followed by kaolinite, which also has well-developed peaks with a slightly broadened base, indicating a mixture of micro and nano fractions. Lepidolite disappears completely on this layer, but the intensity of goethite peaks grows significantly, indicating its relative increase regarding the other constituent minerals.
Sample B exhibits an interesting characteristic related to the diffraction peak observed at the position of 26.5°, which commonly fits the most intense peak of the quartz standard PDF 89-8936, but it overlaps with one of the most intense peaks of the manganese lepidolite standard PDF 73-0294. This is more clearly illustrated in
Figure 5a, where the quartz position is clearly observed at 26.53° followed by the lepidolite at 26.57°. Quartz still dominates lepidolite in the composition of Sample B.
The washing separation process disperses the finest clay particles into ultra-pure water while the coarse fraction settles at the bottom of the recipient. The aqueous dispersion containing the finest fraction is separated and subsequently dried. The XRD patterns resulting from the finest fractions indicate a strong decrease in the quartz peak intensities and a strong increase in the clay minerals peaks,
Figure 4b. Lepidolite and kaolinite amounts increase in Sample A, and a very intense increase in the lepidolite amount is observed in Sample B. Sample C reveals a strong decrease in the quartz peaks and a significant increase in the kaolinite peaks. The increased amount of lepidolite in Sample B is further supported by a notable rise in intensity at 26.57°, close to the quartz peak at 26.53°, as shown in
Figure 5b.
The amounts of individual minerals can be calculated through X-ray diffraction based on the specific peak intensities and the corundum factor. This method is referred to in the literature as the Relative Intensity Ratio (RIR), as described in our previous studies [
38,
39]. It was applied to the XRD patterns in
Figure 4 for both initial samples and samples resulting from the washing separation process, and the resulting amounts of identified minerals are collected in
Table 2.
Mineralogical optical microscopy (MOM) examines samples under cross-polarized light. Each mineral affects the oscillation angle of the polarized light beam, producing illumination peaks and color extinction specific to that mineral due to the birefringence phenomenon. Quartz has a green-gray nuance in thin sections and for small particles, but when bigger particles occur, they often feature a rainbow glitter with intense green spots. Larger and well-defined kaolinite particles have a white–pale blue nuance but turn yellow upon the compact massing of the finest particles, and goethite has a predominantly brown nuance with intense red spots in thin sections; all of these aspects were evidenced on etalon samples in our previous studies [
39,
40]. Lepidolite particles feature pink nuances under cross-polarized light, but manganese occurrence turns it into red-orange nuance [
41,
42]. The sample observation under cross-polarized light in
Figure 5 allows for establishing the size range for each mineral, a fact also featured in
Table 2.
Sample A has a sandy texture given by the abundant large quartz particles. These have a boulder-like aspect,
Figure 6a; their size ranges from 5 to about 18 µm. Quartz particles are accompanied by some large kaolinite slivers up to 15 µm having a preponderantly white aspect and by a few rounded boulders of goethite ranging from 2 to 25 µm, which appear to have a brown nuance. All these mineral particulate matter are surrounded by small particles of lepidolite of about 1 µm and below, with only a few of them reaching 10 µm. These finest lepidolite particles appear in
Figure 4a as a red-orange fog surrounding the other mineral particles. The separation process facilitates the dispersion of the finest particles while the coarser fraction settles to the bottom of the vial.
Figure 6d reveals the complete removal of all fractions larger than 10 µm. The remaining finest particles consist mainly of a lepidolite and kaolinite mixture, with traces of quartz particles measuring approximately 5–10 µm. The smaller goethite particles remain mixed up with the finest clay particles.
Sample B has a clayey texture very abundant in lepidolite and kaolinite fine particles with sizes predominantly about 1 µm (only a few being about 5–10 µm) that tightly surrounds the coarser fractions composed of quartz boulder-like particles and goethite,
Figure 6b. During the separation process, the coarser sandy fractions settle at the bottom of the vial while the clay mixture remains suspended. The obtained powder contains only the finest fractions dominated by lepidolite with moderate traces of kaolinite and small quartz particles,
Figure 6e. The washing separation process enriches the filtered sample in lepidolite fine particles organized in small microstructural clusters of about 3–15 µm. They have an intense and uniform red-orange color generated by the random disposal of the finest particles of lepidolite, giving a clue about their nanostructural organization (fact in good agreement with the XRD peaks broadening).
Sample C displays a dense kaolinite clay matrix closely enveloping the quartz particles, which have a boulder-like aspect and sizes of about 5–75 µm (smaller than observed in the sandy Sample A). The kaolinite aggregates have a yellow nuance caused by their random orientation of the finest particles,
Figure 6c. Some of these aggregates turn brown because of the goethite amount. The brown color intercalation with kaolinite yellow indicates a refined microstructural distribution of the iron hydroxide particles, which features only a few grainy particles up to 15 µm in diameter. The washing separation process successfully removes coarse particles, including quartz and goethite, from the aqueous dispersion. The resulting fine powder is primarily composed of kaolinite stained with goethite, as clearly indicated by the yellow clusters with brown spots shown in
Figure 6f.
Both lepidolite and kaolinite form a foggy embedding matrix of the coarser mineral particles, as observed by MOM images in
Figure 6, which are well separated through the separation process. It corresponds to the broadened aspect of the lepidolite and kaolinite diffraction peaks in
Figure 3b, indicating nanostructural organization. The crystallite size can be calculated using the Scherrer formula [
43]. It was applied to the XRD patterns in
Figure 3b, and the obtained values are displayed in
Table 3.
The organization of lepidolite and kaolinite fine particles into small microstructural clusters of approximately 1–2 µm, as observed after the washing separation process, along with the nanoscale size of their crystallites, suggests the potential presence of a significant quantity of clay nanoparticles. This fact requires advanced ultra-structural investigation, which will be properly assessed by atomic force microscopy in subchapter 3.4.
3.3. Microstructural Characterization
Scanning electron microscopy (SEM) allows an advanced microstructural investigation of the soil samples before and after the washing separation process. It presents the benefit of coupling with the elemental distribution maps that relate the samples’ elemental composition to the mineral particles. Thus, the microstructural aspect of the initial samples,
Figure 7, confirms the MOM observations. Quartz particles have boulder-like aspects with irregular margins caused by the blunting induced by prolonged friction with other sandy particles during the geological eras. Clay particles have a tabular–lamellar aspect, but it is difficult to distinguish which are lepidolite and which are kaolinite only by morphological observation. Furthermore, MOM observation reveals that goethite particles appear in both forms as rounded boulder microparticles of about 2–25 μm and fine microdispersion, which is difficult to distinguish among other fine details in
Figure 6.
Elemental mapping helps to detect each mineral by its specific composition. Thus, quartz, having the chemical formula SiO2, appears as a bright green color due to the overlapping of the O turquoise label and Si green label. Kaolinite has the chemical formula Al2Si2O5(OH)4, and it became very distinct as pale green due to the overlapping of the O and Si characteristic color with yellow labeled for Al. The difference between kaolinite and lepidolite is caused by the presence of manganese, labeled in deep blue. Iron (orange labeled) makes observing its distribution easier within the sample’s morphology.
The sandy texture of Sample A,
Figure 7a, is formed by large quartz particles associated with some bigger kaolinite particles. These coarse components are surrounded by a dense mixture of fine particles, mainly lepidolite and kaolinite. Iron particles are predominantly distributed within the finest fractions.
Sample B is formed predominantly by the finest fractions of lepidolite with traces of kaolinite mixed up with quartz particles,
Figure 7b. The blue areas within the elemental map evidence significant clustering of lepidolite particles, sustaining the MOM observations. Goethite particles appear to cluster near larger kaolinite particles, a phenomenon previously observed in ceramic slurries [
44]. This arrangement of iron hydroxide particles makes the sample more stable and could be useful for its removal through washing separation.
In Sample C,
Figure 7c, the finest kaolinite fractions dominate the microstructure, closely enveloping the quartz particles. Goethite particles are uniformly spread within the kaolinite, making it very difficult to separate them. However, there appear to be some concentrated clusters of goethite, which are prone to sedimentation during the primary washing separation process.
Elemental compositions of the initial soil samples are presented in
Table 4. The dominant elements are O, Si, Al, and K, related to quartz and clay particles. These have a significant participation in all initial samples.
Manganese has a significant presence of 1.9 at.% in Sample A because of lepidolite particles observed in the elemental map in
Figure 6a. It increases to about 2.1 at.% in sample B proportionally to the lepidolite particles observed in the elemental map and completely disappears in Sample C, which contains huge amounts of iron compared with A and B. Traces elements of Ca, Mg, and Ti are often found in the forest soils of the Apuseni Mountains [
45,
46].
One would expect elemental analysis to identify lithium within the lepidolite particles; however, SEM energy-dispersive spectroscopy (EDS) is unable to detect it because of lithium’s small atomic size. Rubidium is the other specific element within the lepidolite formula, which should have been evidenced by the EDS spectra; its energy peak occurs at 1.694 keV, which is very close to the Si peak at 1.739 keV. Since the Si peak is very strong, the detector cannot distinguish the smaller Rb peak, which is confounded with the Si peak base.
Thiery and Farhat investigated lithium ores by SEM-EDX, revealing EDS spectra for pure lepidolite platelets at different acceleration voltages [
47]. They revealed the dominant elements as O, Al, Si, and K, which are characteristics of all phyllosilicates. The lepidolite characteristics are spotted by a small energy peak corresponding to F occurring at 0.677 keV and by a stronger Mn peak occurring at 5.894 keV. It is noteworthy that the pure lepidolite ore evidence traces Na and an unassigned energy peak at 3.690 keV that belongs to Ca [
47]. These elemental characteristics were obtained at an acceleration of 15 kV, with the Mn peak disappearing at lower acceleration voltages like 5 kV. Thus, the F energy peak is a specific signature for the micas subclass mineral, and Mn represents the lepidolite signature.
Our EDS spectra were obtained at an acceleration voltage of 30 kV, ensuring optimal detection of both light and heavy elements. The lepidolite signature was evidenced in both Samples A and B, but the F energy peak was not detected. This phenomenon is explained by the presence of stronger energy peaks from other elements adjacent to its peak position of 0.677 keV, such as O (0.525 keV) and Fe (0.776 keV). It would be interesting to observe if the F energy can be found on the finest fractions separated by washing.
The primary washing separation procedure removes the coarser fractions, mainly quartz, increasing the finest clay particle amount in all investigated samples. Thus, Sample A still evidences a sandy morphology based on a few small quartz particles with a boulder-like aspect and sizes up to 10 μm surrounded by fine lamellar–tabular clay particles,
Figure 8a. Some of the clay particles have a planar size up to 5 μm, and their thickness is about 1.5–2 μm, but most of them appear as very fine fractions well distributed within the observed microstructure. The elemental map reveals a concentration of blue areas within the finest particles observed, indicating their segregation regarding quartz particles but still mixed with small kaolinite particles. These specific areas look like nanostructured matter clusters, which require an advanced surface investigation. Iron appears as small, concentrated orange-brown spots predominantly dispersed throughout the kaolinite mass, indicating contamination with fine traces of goethite.
Quartz particles are less prominent in Sample B,
Figure 8b, due to their extensive removal during the primary washing separation; however, a few boulder-like quartz particles, approximately 5 μm in size, are still present and appear bright green on the elemental map. Fine lamellar clay particles dominate the microstructure of Sample B following washing separation. Elemental mapping reveals extensive blue regions indicating the distribution of lepidolite alongside areas of yellow-green coloration corresponding to kaolinite particles. The clay particles’ planar size is situated up to 5 μm (in good agreement with MOM observation), but most of them are very fine, indicating a nanostructural distribution. Iron distribution is related mainly to the random contamination of kaolinite with goethite. The elemental map in
Figure 8b shows a distinct, dense orange spot approximately 2.5 μm in diameter in the upper left region, indicating a well-defined goethite particle deposited near the junction of a lepidolite and kaolinite cluster.
Sample C has a very fine texture, which allows easy removal of coarse quartz fractions through the separation process. The resulting powder is primarily composed of very fine kaolinite particles, as shown in
Figure 8c, densely surrounding a few coarser quartz and kaolinite particles measuring approximately 5 μm. The elemental map clearly reveals the distribution of fine particles covering quartz boulders. It indicates a nanostructural distribution within the kaolinite mass embedding large amounts of goethite. Iron distribution is finely dispersed within phyllosilicate particles, making them very hard to remove by simple washing.
Table 5 presents the elemental composition of the samples following separation. They are dominated by oxygen and silicon because of the significant presence of phyllosilicate particles and quartz small microstructural remains; overall, O and Si amounts are unchanged as a consequence of the separation process. It is only about elemental redistribution.
The distinct mineral signatures reflect the elemental redistribution. Clay amount enrichment is proved by a significant increase in the Al amount in Samples A and B, indicating quartz decrease. It certainly implies an increase in kaolinite, which sustains the MOM observation. The moderate increase in Mn in Sample A and its significant rise in Sample B supports the enrichment of lepidolite. This confirms the MOM observation that Sample B exhibits a significant increase in lepidolite content relative to the other minerals in the composition. Sample C has an almost unchanged silicate elemental composition but reveals a significant decrease in iron content. It sustains a partial elimination of goethite particles through the washing separation along with the coarse quartz particles.
3.4. Ultrastructure Assessement
X-ray diffraction results reveal that lepidolite and kaolinite have pronounced nanocrystalline states after separation, which deals with the finest fractions observed by SEM imagining. The samples’ ultrastructure is susceptible to containing nanoparticles. Therefore, the glass slides of the samples used for MOM microscopy were scanned with atomic force microscopy,
Figure 9.
Denser regions containing the finest fractions were selected for analysis, while quartz particles were avoided to prevent damage to the cantilever tip. The sandy aspect of Sample A after separation influences the ultrastructure and its subsequent topography,
Figure 9a. Two submicron quartz particles are observed, one situated on the left upper corner of the image and the second situated in the lower right corner, which are partly covered with small clay nanoparticles. Slightly elongated lamellar–tabular nanoparticles approximately 60 nm in size were observed intermixed with smaller particles measuring around 45 nm. The correlation with XRD observation indicates that bigger nanoparticles belong to lepidolite and the smaller ones are kaolinite,
Table 6.
After separation, Sample B exhibits a uniform topography characterized by lepidolite nanoparticles of about 60 nm, forming a dense layer that interlocks with finer kaolinite nanoparticles of about 40 nm. The lack of submicron quartz particles indicates the advanced refinement of Sample B through the washing separation dealing with the observed clayey texture.
Sample C, after separation, has a relatively complex ultrastructure affected by the kaolinite nanoparticles of about 40 nm. These have a high coalescence tendency—forming submicron clusters of about 600 nm in diameter formed around some rounded cores of about 200 nm. SEM-EDX analysis indicates that these cores correspond to the goethite (e.g., the finest orange-brown spots observed in the elemental maps). The incorporation of iron hydroxide into the kaolinite nanostructure makes it difficult to remove through simple washing.