The effect of shungite pretreatment on quantitative extraction of metals

Abstract

One of the most interesting natural materials with a rich composition of various elements, including rare earth elements, is shungite. Inductively coupled plasma mass spectrometry (ICP-MS) was successfully used to detect metals in various ores, which has high sensitivity for determining elements even in trace amounts. ICP-MS analysis revealed 53 elements in shungite, the total content of which in the original sample did not exceed 3 wt%. The effect of preliminary treatment with various solvents (extraction) on the isolation and determination of the maximum content of elements in shungite was considered. The effect of parameters such as the nature of the extractant, sample mass, extraction time, and preliminary heat treatment on the total content of elements in the original shungite sample was studied. It was found that extraction with water, methanol, potassium hydroxide, or hydrochloric acid significantly increased the total detected content of elements in shungite; the total content of elements can reach 20 wt%. Data analysis showed that the detectable content of elements in a 0.2 g sample was higher than in a 0.5 g sample, and an increase in extraction time led to an increase in the detectable amounts of metals. Recommendations were given on sample preparation to achieve effective extraction and determination of metals by the ICP-MS method. The most comprehensive information on the content of elements in shungite can be obtained by extracting the sample with methanol.

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1. Introduction

Shungites are specific natural rocks in which the carbon component is closely associated with aluminosilicates and other minerals. Shungite differs from graphite by the absence of a crystal lattice and from coals and bitumen by its two-dimensional structure and low content of volatile components. Many shungite rocks, due to their porous structure, contain a large number of valuable metals in the composition of minerals such as sericite (KAl₂(AlSi₃O₁₀)(OH)₂), calcite (CaCO₃), chlorite (A₅₋₆T₄Z₁₈, where A = Al, Fe, Li, Mg, Mn or Ni; T = Al, Fe, Si or a combination thereof; Z = O and/or OH), feldspar (metal silicates), vanadium carbides and others.^1,2

The applications of shungite are diverse, including the production of fullerenes and carbon nanoclusters,³ shungite fillers improve the strength and technological properties of rubbers, partially replacing carbon black,⁴ and cement composites with shungite have good electrical conductivity and can be used for monitoring stress and temperature.⁵ To understand the application of a particular type of shungite, it is necessary to know its full composition, including trace elements, which may contain transition metal, rare metal, and rare earth elements. The content of such elements in shungite can be up to 10⁻⁸ mass%.¹ However, the content of some metals, such as tungsten, can reach tens of grams per ton.⁶ Moreover, any deposit must be studied for the content of valuable elements before development.

The metal content in shungite can be determined using several methods. To determine the major components of natural minerals, it is convenient to use the X-ray spectroscopy method, which studies the fine structure of X-ray absorption spectra (XAFS). The method has a low sensitivity – at the level of whole percent, which is not suitable for studying trace amounts of elements in shungite.⁷ The most common and currently used approaches for determining the content of metals in natural minerals are inductively coupled plasma mass spectrometry (ICP-MS),^8,9 inductively coupled plasma optical emission spectrometry (ICP-OES),^10,11 inductively coupled plasma atomic emission spectrometry (ICP-AES),^12,13 energy dispersive X-ray fluorescence (EDXRF),^14–16 total reflection X-ray fluorescence (TXRF),¹⁷ and atomic scanning electron microscopy (ASEM).¹⁵ EDXRF allows direct analysis of solid samples without sample preparation but with a low detection limit for trace elements. In addition, EDXRF requires appropriate reference material matrices for quantitative analysis, which limits its widespread use. TXRF is an improved variation of EDXRF. Quantification is usually achieved by introducing an internal standard into the sample, which compensates for potential errors due to variations in the sample volume, particle size, or incident angle, ensuring the accuracy and reliability of the analysis.¹⁶

Techniques such as Laser-Induced Breakdown Spectroscopy (LIBS) offer rapid, minimally destructive, in situ analysis, which is particularly advantageous for field screening and large-scale sample surveys.¹⁸ Recent developments in calibration-free LIBS (CF-LIBS) have further enhanced its quantitative capabilities, enabling accurate measurements in diverse samples like industrial soils and agricultural products without the need for matrix-matched standards.¹⁹

ICP-MS methods have high sensitivity, reaching detection limits in the range of ppb fractions, and relative universality, which allows detecting even trace amounts of elements. For example, when detecting Mn, Fe, Ni, and Cu in seawater, their detection limits were about hundredths or thousandths of a nmol per kg, and when detecting metals such as Zn, Co, Cd, and Pb – about whole, tenths or hundredths of a picomole per kg.²⁰ However, these methods require careful sample preparation when analyzing solid samples, except for the case of laser ablation (LA-ICP-MS). LA-ICP-MS remains a benchmark technique, providing detailed elemental maps and precise quantification in solid samples.²¹ Nevertheless, it is necessary to note the difficulties in carrying out calibration of equipment using solid standard samples. The disadvantage of the ICP method in combination with laser ablation is the impossibility of studying the volume content of components, and the use only for point analysis of the surface of materials.^22,23 Separately, it is necessary to note the difficulties in carrying out calibration of equipment using solid standard samples. The review²⁴ shows that calibration often takes a lot of time and requires a certain amount of experience of the researcher to obtain the required reproducibility of results.

An accurate and reproducible method for assessing deposits for the content of valuable elements in natural materials is the classic ICP-MS with solution analysis, which allows you to estimate the average concentration of elements in a sample up to several tens of grams, which reduces the number of necessary measurements. Furthermore, it achieves improved sensitivity and reproducibility, as the analysis is conducted in a homogeneous solution. The only limitation of this method is the complexity of sample preparation, which depends on the type of sample and the elements being determined. At the same time, there is no universal method for extracting elements. Today, several methods for extracting transition and rare metals into an aqueous solution are known, which are used in production.²⁵ When analyzing shungite using ICP, samples are most often pre-treated with a leaching process, which can be alkaline, neutral, or acidic.^26,27 For acid leaching, HF is particularly effective due to its complex composition with high silica and carbon content.¹⁴ The concentration of elements in the resulting solution directly depends on the extraction time. Ponomaryov et al.⁸ showed that when carrying out water extraction from shungite for 30 min, the amount of extracted metals, such as Gd, Sm, Nd, Ce, La, Y and others, turned out to be 4–6 times higher than when carrying out extraction for 15 min, and tens of times higher compared to extraction for 5 min. Leaching can also be carried out with heating to increase the efficiency of the process. For example, the technological scheme for processing pyrochlore–monazite–goethite ores of the Chuktukon deposit includes two types of leaching: agitation, carried out at a temperature of 90–100 °C, and autoclaving, carried out by heating to 190–210 °C.²⁵ Sometimes microwave decomposition in autoclaves with temperature and pressure control is used to speed up and increase the completeness of decomposition.²⁸ In this case, microwave exposure may be preceded by treatment with an acid mixture, for example, treatment with a mixture of HF/HClO₄ followed by the addition of aqua regia (HCl/HNO₃) to completely dissolve the substance being studied.²⁹ Ultrasound can also be used in the preparation of minerals and ores for ICP-MS analysis. For example, a process for ultrasonic treatment of mineral suspensions (e.g. eudialyte) has been described, where ultrasound facilitates the effective removal of silicate gel from the surface of minerals and the destruction of grains, which increases the extraction of rare earth elements and zirconium by 29–44%.³⁰

Our work aims to optimize the pretreatment of shungite material to ensure the most complete and representative analysis of the entire sample volume using the ICP-MS method.

2. Experimental part

2.1 Materials

Shungite raw material of type III of the Zazhoginsky deposit (Karbon-Shungit PLC, Petrozavodsk, The Republic of Karelia, Russian Federation) was used as the initial shungite material. The main parameters of the type III shungite material used from the Zazhoginsky deposit were determined in the work in ref. 31: the carbon content is 39.5%, the specific surface area is 2–3 m² g⁻¹, and the total pore volume is 0.045–0.05 cm³ g⁻¹.

Concentrated nitric acid (HNO₃; 65%, Trace element grade, Fluka, Germany), concentrated hydrochloric acid (HCl; Special purity grade, Sigma Tek, Russia), and methanol (CH₃OH; >98%, EvaScience, Russia) were employed in the study. Freshly prepared 45% potassium hydroxide (KOH) solution was prepared from the solid (Reagent grade). Milli-Q water (18 MΩ) was obtained using the Aqualab water purification unit (Mediana-Filter, Russia).

2.2 Processing of shungite material

Shungite raw material of type III of the Zazhoginsky deposit (Karbon-Shungit PLC, Petrozavodsk, The Republic of Karelia, Russian Federation) was used as the initial shungite material. A laboratory vibratory mill (2 kW) was used to grind the solid initial shungite. The crushed substance was successively sifted through laboratory sieves (Vibrotekhnik, St. Petersburg, Russia). The medium-sized sample was selected for further work (0.4–0.8 mm).

In the work, two types of shungite were used: unprocessed (original) and after heat treatment.

A preliminary study using thermogravimetric analysis (SDT Q600 thermal analyzer; heating rate, 10 °C min⁻¹ up to 1000 °C) showed that when shungite was heated in air, active mass loss – burnout of various forms of shungite carbon³² – occurred at temperatures from 500 to 700 °C. Constancy of mass was achieved at 710–730 °C. Then, for complete annealing of shungite, a temperature of 750 °C was chosen with a small reserve.

Annealing was carried out in a muffle furnace in an open porcelain crucible for an hour. With complete annealing of shungite, a loss of 33% of mass occurred, which corresponded to the carbon content in type III. The color of the powder changed from black to beige.

2.3 Extraction of elements from shungite material

To extract metals from shungite material, 0.2 or 0.5 g of sample was placed in test tubes with various extractant solutions. H₂O, CH₃OH, KOH and HCl were used as extractants. The volume of the extractant was V = 10 mL. The resulting mixtures were hermetically sealed and periodically shaken. The extraction time was 2 or 24 h.

Then, the resulting solution was filtered on a paper filter. Shungite on the filter was washed with a small amount of water. The volumes of the resulting solutions of extracts and washings were recorded.

The shungite remaining after extraction was dried at room temperature and weighed.

Samples of shungite after extraction and solutions of extracts and washings were analyzed for the content of elements using the ICP-MS method.

2.4 Sample preparation process for ICP-MS analysis

The analysis of the obtained samples (shungite dried after extraction and solutions of extracts and washes) to determine the content of elements was carried out using the ICP-MS method.

A weighed portion of shungite sample (m = 0.025–0.030 g) was placed in nitric acid (0.5 mL) and heated at T = 80 °C for 40 min until the solution turned yellow. Then, the resulting solution was filtered on a paper filter from unreacted, undissolved shungite. The filtrate was diluted 100 times. The resulting solution was analyzed for the content of elements using the ICP-MS method.

The resulting alkaline and acid extracts were diluted with water 10 times for analysis. 0.5 mL of nitric acid was added to 0.5 mL of the alcohol extract and heated for 40 min at T = 80 °C. Then, they were diluted with water 200 times from the original extract. 0.1 mL of nitric acid was added to 10 mL of the aqueous extract.

Washings were prepared similarly to extracts.

Thus, the detected amount of one element in the studied sample of shungite consists of the amount found during the analysis of shungite after extraction, in the extract and in the solution washed from shungite after interaction with the solvent.

Blank samples of acids were prepared by mixing with water: HNO₃ : H₂O = 0.15 : 14.85 mL and HCl : H₂O = 1.5 : 8.5 mL.

2.5 Equipment

Mass spectrometry coupled with inductively coupled plasma (ICP-MS) studies were carried out on an Agilent 7500ce instrument (Agilent Technologies, USA). The main parameters of the experiment were: nebulizer temperature, 20 °C; peristaltic pump speed, 0.12 rpm; plasma-forming argon flow, 15 L min⁻¹; carrier gas, 0.68 L min⁻¹; makeup gas, 0.15 L min⁻¹; cooling and nebulization argon flows, 1 L min⁻¹; plasma generation power of 1500 W with a reflected power of no more than 1 W. Detector parameters: discriminator, 8 mV; analog HV, 2060 V; pulse HV, 1380 V. Calibration for quantitative analysis was carried out using standard calibration solutions: multielement standard solution 1 and multielement standard solution 2 for ICP (Sigma-Aldrich, Germany). Calibration was carried out in the concentration range from 100 ng L⁻¹ to 2 mg L⁻¹. Mass spectra were recorded in scanning mode for 56 elements. The experimental data were processed using Agilent ChemStation software (Agilent Technologies, USA). The peak profile of each element was approximated by 6 points in the range of ±0.5 Da. The measurements were repeated three times.

3. Results and discussion

3.1 Content of elements in the original shungite

In this study, the following factors affecting the selectivity and completeness of extraction were considered: type of solvent or extractant, sample weight, extraction time, and preliminary heat treatment of the sample. Extractants or solvents were selected based on two criteria: firstly, they were easily accessible and frequently used, and secondly they were different in nature. Thus, water (H₂O), methyl alcohol (CH₃OH), potassium hydroxide (KOH) and hydrochloric acid (HCl) were used.

A preliminary study of the elemental content of shungite was conducted without prior extraction using the method described in Section 2.4. Data on the detection of each individual element are given in the SI section (Table S1). 53 elements were detected using the ICP-MS method. Table 1 presents data on the total detected content (W, wt%) of elements in shungite.

Table 1 Total content of elements in shungite W (wt%)

Condition	0.5 g	0.2 g	0.2 g burnt
W, wt%	2.59	1.58	1.54

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According to the obtained results, with an increase in the sample mass by 2.5 times, the total content of the detected elements increases by approximately 1.6 times. The average total content of detected elements (CDE) in the original shungite does not exceed 3% of the sample weight.

3.2 Contribution of the extract to the total content of elements in shungite

As stated in Section 2.4, the resulting data represent the total content of the detected elements in the extract and in the solid shungite sample after extraction, expressed as a percentage of the obtained amount to the initial mass of the sample W. Table 2 summarizes the CDE results in the studied sample, as well as the percentage contribution of the CDE from the extract x.

Table 2 Total content of detected elements W (wt%) in the shungite sample and the contribution of the extract x (%)

Conditions	0.5 g and 24 h		0.2 g and 24 h		0.2 g and 2 h		0.2 g burnt and 24 h
Solvent	W, wt%	x, %	W, wt%	x, %	W, wt%	x, %	W, wt%	x, %
H2O	8.43	0.17	16.16	0.31	9.28	0.39	12.85	0.21
CH3OH	13.09	1.21	16.99	5.68	8.76	0.94	13.32	1.32
KOH	6.03	24.83	5.53	2.80	8.21	0.38	10.22	0.36
HCl	8.18	8.27	5.76	31.68	5.36	5.62	7.80	6.32

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Analysis of the data in Tables 1 and 2 shows that the use of any of the solvents used as an extractant leads to a significant increase in the CDE in shungite. It is not possible to identify a general trend in the influence of the nature of the solvent or the conditions of extraction; therefore, the influence of individual factors on the determination of the content of elements in the sample will be considered below.

It has been noted that increasing the extraction time leads to an increase in the CDE. It can be assumed that this results in a more complete opening of shungite and more elements pass from the bound state into the solution.

According to the results of Table 2, the elements of the acid extract make the largest contribution to CDE, while those of the water extract make the smallest contribution. It should be noted that the small contribution of the water extract to the CDE can be explained by the strong binding of elements in shungite. When using water as an extractant, most elements remain in the same poorly soluble forms and practically do not pass into solution, for example, CuO, FeO, CaCO₃, Al₂O₃, TiO₂, MgO, and Mg(OH)₂. However, relatively high W values may indicate the ability of water to promote the “appearance” of elements on the surface of shungite or the opening (destruction) of layers during further processing of the solid sample.

3.3 Effect of sample mass

Selecting the optimal sample size is an important step in metal extraction. When considering the effect of shungite sample weight on the total content of detected elements, experiments were conducted with 0.2 and 0.5 g. The extraction time was 24 h. The main results are presented in Fig. 1, and Tables S2 and S3 provide data for each element.

Fig. 1 Total content of detected elements (wt%) in shungite after extraction with different solvents and without them for 24 h. Initial mass of shungite: blue – 0.2 g; red – 0.5 g.

It is assumed that a larger sample weight should lead to more complete extraction due to increased contact between the solid sample and the solvent. However, our results were ambiguous. Safonov et al.³³ also failed to trace the absence of a clear pattern between the sample mass and the yield of metals when selecting the conditions for concentrating rare earth metals from shungite. The highest content of the detected elements was found in aqueous and methanol extracts. A detailed study of Tables S2 and S3 shows that the greatest contribution to the concentration is made by alkaline and alkaline earth metals, as well as iron, whose concentrations differ from those of acid and alkaline extracts by several times. The main reason for the absence of a specific dependence may be associated with the heterogeneity of the distribution of metals in the sample under study. Analysis of the obtained results (Fig. 1) shows that with an increase in the mass of shungite by 2.5 times, the total content of the detected elements can decrease by more than 1.3 times. Such an effect was observed when using water and alcohol as extractants. It is noteworthy that for these solvents, with an increase in mass, a decrease in the concentration of elements in the extract is observed: for H₂O – 2 times; for CH₃OH – 5 times (Table 2).

It should be noted that the use of acid or alkaline extraction increased the CDE with an increase in the weight of the shungite sample by only 2–4%. At the same time, for a shungite sample that had not previously been in contact with any extractant, the effect of changing the weight of the sample was not observed.

3.4 Effect of extraction time

The study of the effect of sample weight showed that a small sample weight of shungite (0.2 g) is sufficient to obtain a high CDE value. To examine the effect of solvent contact time on the CDE value, extraction was performed for 2 and 24 h. Data on the detected elemental content are given in the SI section in Tables S2 and S4. The effect of extraction time for different solvents is illustrated in Fig. 2.

Fig. 2 Total detected content of elements (wt%) in shungite (0.2 g) after extraction with different solvents and without them. Extraction time: blue – 2 h; red – 24 h.

Analysis of the obtained results showed that for most solvents, increasing the extraction time led to greater release of elements from the initial solid sample. The greatest effect of extraction time was observed for methyl alcohol, and the increase in the CDE was almost 94%.

Conversely, increasing the extraction time with an alkaline solution led to a decrease in the CDE by 32.8%. This effect may be associated with the ambiguous solubility of metal compounds. Often in saturated solutions, the formation of strong hydroxocomplexes was observed, which can be represented by the following equilibrium equation:

mMe^z+ + nOH⁻ ↔ Me_m(OH)_n^±(mz−n).

In this case, the charge of the formed hydroxocomplex can be positive, neutral or negative.^34–36 When alkalis act on metal salt solutions, hydrated hydroxide precipitates are usually formed. They are in a metastable state and undergo physical and chemical transformations during storage. The nature of these transformations varies and depends on the nature of the metal. In some cases, this is accompanied by a change in the composition of the substance, and in others by phase transitions. For example, the Cu(OH)₂ precipitate, when stored in solution and even with weak heating, turns into CuO.³⁷ Hydroxides of Ga(III), Sc(III) and some other metals over time transform into compounds of type GaO(OH), ScO(OH), and so on.^36,38,39 The composition of freshly precipitated amorphous Be(II) hydroxide can be expressed by the formula Be(OH)₂·xH₂O. Over time, dehydration occurs, and this precipitate first passes into the metastable α-form and then into the stable β-form.^40,41

It should be noted that a significant decrease in the detected amount was also observed for iron. Iron is one of the main components of shungite. When shungite material was treated with an alkaline solution, iron can also form poorly soluble hydroxides. Furcas et al.^42,43 systematically studied the formation of various forms of solid iron hydroxides and their complexes. The study⁴² shows that in a highly alkaline solution, rapid precipitation of iron(III) hydroxide occurs. Over time, poorly soluble goethite FeO(OH) forms.

Thus, in concentrated alkaline environments with long exposure times, the formation of poorly soluble and insoluble forms of metals is possible, which prevent the detection of the true content of the element in the sample by the ICP-MS method.

3.5 Effect of heat treatment

One of the possible ways to concentrate the metal in the sample is preliminary heat treatment of the sample.^44–46 Therefore, the influence of this factor on the CDE value was considered. The data on the detection of 53 elements in the shungite sample are given in Tables S1 and S5, and Fig. 3 illustrates the effect of heat treatment for different extractants on the value of the total number of elements detected.

Fig. 3 Total content (wt%) of detected elements in the original (blue) and heat-treated (red) shungite (0.2 g) after extraction with different solvents and without them for 24 h.

It is assumed that when fired in an atmospheric environment, shungite will partially “lose” its organic components – carbon, hydrogen, oxygen, and sulfur, while unstable metal compounds will transform into more durable ones.^46–49 In this case, the heat treatment process will promote the concentration of elements in the form of oxides on the released surface of silicate formations (oxides and carbides), but the resulting bonds will not be strong. Thus, formations on the surface can be removed by leaching or an acid solution. Krylov et al.⁴⁹ came to similar conclusions after studying the chemical composition of the washed-off surface layer during water leaching of thermally modified shungite. The method of X-ray spectral fluorescence analysis showed that in the surface layer of shungite rock grains during thermal modification, concentration of metals was observed.

It should be noted that the main contribution to the CDE value of calcined shungite was made by Fe (H₂O – 6.2 wt%; KOH – 4.9 wt%), Al (H₂O – 3.0 wt%; KOH – 2.0 wt%) and Mg (H₂O – 1.6 wt%; KOH – 1.3 wt%).

3.6 Change in shungite mass after extraction

According to the extraction method described above (Section 2.3), the remaining shungite was dried and weighed. Table 3 presents the average values of the mass loss of shungite from the initial.

Table 3 Average values (%) of mass loss of shungite after extraction

Solvent	Processing conditions
	0.5 g and 24 h	0.2 g and 24 h	0.2 g and 2 h	0.2 g burnt and 24 h
H2O	5.5	12.0	21.5	24.9
CH3OH	7.1	16.0	15.9	20.6
KOH	12.5	32.3	22.0	23.3
HCl	11.0	26.5	24.7	21.2

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The analysis of the data in Table 3 shows that for heat-treated shungite, the sample mass loss differs slightly when moving from one solvent to another and is in the range of 20–25%. It can be assumed that for the modified shungite material, further extraction leads to a more complete extraction of elements. In addition, the relatively stable value can be associated with a more ordered structure of shungite due to the minimal content of elements such as C, H, O, N, and S. In other words, a decrease in the number of elements leads to more stable results of metal extraction.

For the initial shungite, the decrease in the sample quantity after extraction depends largely on the nature of the extractant. The maximum deviation of the mass from the initial value was observed for the KOH solution, up to 33%. It should be noted that with a larger sample weight, the decrease in the total mass was not so significant: the maximum loss value only exceeded the experimental error (5%) by a few percent.

3.7 Recommendations for sample preparation to determine the most accurate content of elements

Preliminary processing of shungite revealed the detection of 53 elements, including rare earth elements (Sc and Y) and highly valuable metals (Au, Ag, Ru, Rh, Pd, Os, and Ir). The experiments conducted and the analysis of the obtained data allowed us to draw a conclusion about the most favorable conditions for detecting and calculating the most accurate value of the content of one or another element. Table 4 shows general recommendations for selecting the optimal treatment to quantify specific elements.

Table 4 Recommendations for pretreatment of shungite to detect the maximum content of the element

Processing	Element
Without pretreatment	None
H2O	None
CH3OH	Sc, Ti, Se, Zr, Pd, Te, Cs, Re, Os, Ir, Au, and Tl
KOH	B, Nb, Ag, Sb, Ta, and W
HCl	Li, V, Cr, As, and Br
Burnt	Be, B, Mg, Al, Ga, Sc, V, Cr, Ni, Ca, As, Br, Rb, Y, Nb, Ru, Rh, Pd, Cd, Sn, Sb, Te, I, Cs, Ba, Ta, W, Re, Ir, Au, Tl, Bi, Th, and U

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Not all 53 elements detected are presented in Table 4: only those elements are indicated for which, under different conditions (weight of sample and time), this solvent gives at least three times the maximum value. Thus, for a more accurate analysis, it is better to use preliminary processing of the shungite material in the form of extraction with alcohol, alkali or acid. It was not possible to trace patterns between the extractant used and the nature of the elements.

It is noteworthy that acid leaching is often used for the extraction and regeneration of ores (iron, magnesium, and nickel). In the case of shungite, hydrochloric acid proved more effective than other solvents for only five metals.

Despite the high CDE values for the alkaline solution, when considering individual elements, the maximum content values were most often observed for alcohol. Extraction with methanol, as a sample preparation, can be recommended for 12 elements, including Ti, Zr, Pd, Re, and Au. At the same time, a number of studies also report the success of using methanol for extracting metals from objects of various nature.^50–52

It should be noted that preliminary heat treatment gave positive results for only 35 elements.

4. Conclusions

Thus, this study examined extraction with various solvents as a sample preparation method for ICP-MS analysis of shungite to determine trace elements. Analysis of shungite sample revealed 53 elements. It was established that the use of any of the used solvents as an extractant led to a significant increase in the detected content of elements in shungite. The average content of all detected elements in the initial shungite did not exceed 3 wt%, and after extraction it reached 20 wt%.

A study of various extraction parameters showed that with an initial sample mass of m = 0.2 g, the value of the total content of the detected elements was greater than the sample with m = 0.5 g. Increasing the extraction time for most solvents led to an increase in the CDE value. When using an alkaline solution as an extractant, it is necessary to carefully select the extraction time, since with prolonged exposure, poorly soluble metal compounds may form, the subsequent detection of which by ICP-MS may be difficult.

Pre-heat treatment of shungite at T = 750 °C with a total mass loss of approximately 30% significantly increased the CDE value upon subsequent exposure to an alkali or acid solution.

In conclusion, this study demonstrates that the selection of a pretreatment protocol is critical for accurate quantitative analysis of elements in shungite. Our results provide a set of recommendations, highlighting the overall effectiveness of methanol extraction and outlining specific conditions for maximizing the recovery of various elements. These optimized protocols are directly applicable to geochemists, mining specialists, and materials scientists, seeking to reliably assess the resource potential or environmental impact of shungite deposits, ensuring complete and accurate characterization.

Author contributions

Irina V. Minenkova: formal analysis, investigation, methodology, writing – original draft. Victoria V. Voronkova: formal analysis, investigation, visualization, writing – original draft. Daniil I. Yarykin: formal analysis, investigation, methodology. Ivan S. Pytskii: formal analysis, investigation, methodology, data curation, writing – original draft. Alexey K. Buryak: data curation, project administration.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5ay01310k.

Acknowledgements

The work was supported by the Ministry of Science and Higher Education of the Russian Federation (grant agreement no. 075-15-2024-534).

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