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JOURNAL OF MINING INSTITUTE
Zapiski Gornogo instituta Journal homepage: pmi.spmi.ru
Research article
Quantitative determination of sulfur forms in bottom sediments for rapid assessment of the industrial facilities impact on aquatic ecosystems
Ivan P. SverchkovH, Vladimir G. Povarov
Empress Catherine II Saint Petersburg Mining University, Saint Petersburg, Russia
How to cite this article: Sverchkov I.P., Povarov V.G. Quantitative determination of sulfur forms in bottom sediments for rapid assessment of the industrial facilities impact on aquatic ecosystems. Journal of Mining Institute. 2024. Vol. 267, p. 372-380.
Abstract. The article describes an X-ray fluorescence method for quantitative analysis of sulfate and total sulfur in bottom sediments of watercourses and reservoirs located in the area of industrial enterprises impact. The quantitative determination of sulfur forms was carried out by analyzing the characteristic curves SKai,2 and SKPi,3, as well as the satellite line SKP' on X-ray emission spectra measured by an X-ray fluorescence spectrometer with wavelength dispersion. The study shows that these characteristic curves allow not only to determine the predominant form of sulfur, but also to separately conduct quantitative analyses of sulfates and total sulfur after fitting peaks and to separately analyze overlapping spectral lines. The results of quantitative analysis of the chemical state of sulfur by the proposed X-ray fluorescence method were compared with the results of inductively coupled plasma atomic emission spectroscopy and elemental analysis, as well as certified standard samples of soils and sediments. The results are in good agreement with each other.
Keywords: bottom sediments; sulfur; sulfates; X-ray fluorescence analysis; fitting peaks; satellite lines; quantitative analysis
Acknowledgment. The work was carried out under the state assignment of the Ministry of Science and Higher Education of the Russian Federation (FSRW-2024-0005).
Received: 16.04.2024 Accepted: 03.06.2024 Online: 04.07.2024 Published: 04.07.2024
Introduction. Sulfur is one of the most important elements of various components of the natural environment. It is also a necessary element for soils and plants, since it is a component of amino acids and enzymes that are vital for the livelihood of organisms and for metabolism in soils. However, an excess of sulfur in the soil can contribute to its acidification, inflict damage on plants and slow down their growth [1-3]. Also, sulfur can serve as one of the indicators of environmental pollution resulting from discharges and emissions of industrial enterprises. Analysis of the content and forms of sulfur in sediments and soils allows us not only to assess the ecological state, but also to develop measures to reduce pollution [4-6]. At the sediment-water interface, sulfur in bottom sediments undergoes a complex geochemical process before it finally forms a stable compound. The study of the content and forms of sulfur in the bottom sediments of water bodies is of great ecological importance for the early diagnosis of their contamination with heavy metals and control of the chemical composition of water [7-9].
Data on the chemical state of sulfur is also necessary for geochemical studies in the analysis of ores. Information about the content of sulfides and sulfates is important in assessing the genesis of the deposit and determining its industrial value. The method that is usually used to determine the forms of sulfur is the laborious gravimetric method with the sequential dissolution of sulfur-containing compounds and further precipitation of sulfates in the form of BaSO4 [10-12].
XRF is one of the most universal analytical methods for studying the elemental composition of objects of different composition. The method does not require time-consuming and expensive stages of
sample preparation [13-15]. As noted in the studies [16-18], the position and shape of the SKa u SKp lines for sulfur can vary greatly depending on the degree of oxidation of this element. The presence of various types of satellite lines on the spectra is explained by various processes and effects: multiple ionization, exchange interaction, plasmon excitation, radioactive Auger effect and molecular orbitals [19]. Also, the structure of the characteristic X-ray spectrum is affected by the valence and the compound the atom is in. For sulfur, this effect is more significant in the area of the SKp line than in the area of the SKa, since the SKp lines arise as a result of transitions that involve 3p-orbital, which is more external than the 2p-orbital associated with the SKa line. The intensities and widths of the forbidden and satellite SKp lines depend on the concentration and degree of oxidation of sulfur atoms. These spectra differences can be used for qualitative and quantitative analysis of sulfur forms, which can significantly reduce the analysis time [20].
Methods. Chemical reagents. To study the spectra of pure sulfur compounds having different degrees of oxidation, we used high purity elemental sulfur (AO REAChem, Moscow, Russia), chemically pure anhydrous sodium sulfate (AO LenReactiv, Saint Petersburg, Russia) and 99.9 % iron disulfide (Sigma-Aldrich Co. LLC, Saint Louis, USA).
When studying the effect of the cation on the intensity of peaks SKP1,3 u SKP' samples of sulfates of various metals with a mass sulfur content of 2 % were prepared from chemically pure reagents: CaSO4, CuSO4, FeSO4-7H2O, KAl(SO4)2-12H2O, MgSO4, Na2SO4 (AO LenReactiv).
Sample preparation. To calibrate the wavelength dispersive X-ray fluorescence spectrometer, a set of 9 samples of artificial mixtures obtained by mixing sulfur-free rock and pure anhydrous calcium sulfate was prepared. The composition of the sulfur-free rock, on the basis of which artificial reference samples were made, wt.%: SiO2 - 58.9; AhOs - 16.8; K2O - 5.59; Fe2O3 - 5.05; Na2O - 4.49; CaO - 3.58; MgO - 3.23; TiO2 - 0.95; P2O5 - 0.25; MnO - 0.06; LOI < 0.01.
The samples of soil and calcium sulfate previously dried in a drying cabinet (ED 23, Binder, Tuttlingen, Germany) were weighed on electronic scales (MSE124S-1CE-DU, Sartorius, Gottingen, Germany) and thoroughly homogenized using a mixing device (Ultra Turrax, IKA, Staufen, Germany). The samples were prepared by pressing the samples with a binder into reusable steel rings with a diameter of 32 mm using an automatic press (PP 40, Retsch, Haan, Germany) with a maximum pressing force of 25 tons.
To check the calibration dependencies, 8 samples were taken, a standard sample of bottom sediments with a certified total sulfur value (SGHM-4, Vinogradov Institute of Geochemistry SB RAS, Irkutsk, Russia), a standard rock sample with an approximate sulfur content (SP89, Rocklabs, Dunedin, New Zealand), 4 samples of bottom sediments and 2 samples of copper ore enrichment waste. Sample preparation was carried out in the same way as for artificial mixtures.
Determination of sulfur spectra. A scanning wavelength dispersive X-ray fluorescence spectrometer (XRF-1800, Shimadzu, Kyoto, Japan) with a power up to 4 kW equipped with an X-ray tube with Rh anode was used to obtain the spectra of the prepared samples. The parameters of the spectrometer operation for the studies carried out in the work: X-ray fluorescence spectrometer -XRF-1800 (Shimadzu); material of the X-ray tube anode - rhodium (Rh); Voltage/current of the X-ray tube - 25 kV/100 mA; crystal analyzer - Ge (2d-6.532 A); scanning range - 98.5-112.5 degree (2.283-2.505 keV); scan step - 0.05 degree; total scan time - 840 s; detector - FPC; collimator diameter - 30 mm; atmosphere - vacuum.
Determination of total sulfur and sulfates. The total sulfur concentrations in all samples were determined using an elemental analyzer (628S, LECO, Michigan, USA) by analyzing the composition of gases formed after complete combustion of the sample at a temperature of 1450 °C. The measurements were carried out in accordance with ISO 15178:2000. Soil quality - Determination of total sulfur by dry combustion. To determine the content of sulfates, the samples were boiled in a 15 % hydrochloric acid solution. Then the solution was filtered, the filter and the precipitate were repeatedly washed with hydrochloric acid. The resulting solution was analyzed by inductively coupled plasma atomic emission spectroscopy (ICPE-9000, Shimadzu).
Results and discussion. Investigation of spectral lines of various types of sulfur. Sulfur has different degrees of oxidation, S-2, S-1, S0, S+1, S+2, S+4 u S+6. However, native (S0), sulfide (S-2) and sulfate (S+6) forms are most often found in nature [21, 22]. With the help of WDXRF, spectral lines of sulfur with different valence can be observed. As can be seen in Fig.1, the lines SKai,2 u SKP13 are observed for all types of sulfur, and the satellite line SKP' is characteristic of sulfates. Along with the additional spectral line SKP', a shift of the main peak SKa1,2 by about 2 eV, towards lower energies is also observed for sulfates [23]. Such a slight shift of the analytical line is difficult to interpret for determination of the type of sulfur.
Determination of total sulfur by analyzing the intensity of the SKa1,2 line is a routine operation [14, 15, 24], therefore, further attention will be paid to the quantitative determination of sulfates in samples containing various types of sulfur and will focus on the analysis of the characteristic lines SKP1,3 and SKP'. As can be seen from Fig.1, the peak of the SKP sulfate ion has two main components, SKP1,3 u SKP', in contrast to elemental and sulfide sulfur. This is due to the fact that the SKP region of the sulfur line significantly depends on the degree of oxidation. For pure sulfur, the main transition of SKP1,3 corresponds to a peak formed by two lines caused by molecular orbitals that bind different levels of the Ss molecule [25], and in oxides it is associated with the transition of electrons from the molecular orbital consisting of the atomic orbitals 3p of sulfur and 2p of oxygen to the 1s orbital of sulfur. Thus, the difference between the transitions SKP1,3 u SKP' is determined by the atomic orbital of the ligand involved. Therefore, the energy difference is approximately determined by the energy difference of the 2s and 2p orbitals of the ligand, which for oxygen is approximately 15 eV [18, 19, 26].
Along with the SKP13 and SKP', lines, the spectra in the region selected for analysis may contain the SKPx and SKP'' lines. However, SKP'' is not characteristic of sulfates, and SKPx is not observed on the spectra obtained using WDXRF due to insufficient resolution [27, 28]. For these reasons, the analysis of these characteristic lines was not carried out.
Fitting peaks. Overlapping peaks of SKP1,3 and SKP' are difficult to analyze without preprocessing (Fig.1). This is especially difficult for cases when the sample includes a mixture of various compounds containing sulfur, in which case the main peak of SKP1,3 can raise several times higher relative to SKP' and cover the satellite line even more. For this reason, peak fitting was further used for all spectral lines (OriginPro, OriginLab Corporation, Northampton, USA). An example of a decoded peak after the fitting is shown in Fig.2. The position of the spectral lines SKP1,3 and SKP' at 2,464 and 2,452 keV respectively, agrees well with the values obtained in other studies where devices with better resolution were used [18, 29, 30].
2.28
2.32
2.36
2.40
2.44
2.48
Energy, keV
Fig. 1. X-ray spectra of various sulfur compounds
2.2 2 1.8 1.6
m
a
ä 1.4
-£?
I 1.2
<u
1
0.8 0.6
Spectrum CaSÜ4 Cumulative fit peak Peak SKßi,3 Peak SKß'
0.4 — 2.24
T"
2.42
2.48
2.50
2.44 2.46
Energy, keV
Fig.2. X-ray spectrum after fitting for a sample with a sulfur sulfate content of 1 wt.%
Gaussian function was used to fit the peaks. The function has the following equation
-4ln(2)(x-x„ )2
y = yo +■
A • e w2
n
i 4ln ( 2)
where yo - the height of the baseline; xc is the position of the peak center; A - the area under the peak; w is the width of the peak at half of its height [31, 32].
Using the Gaussian function to fit the peaks provided a correlation between the initial and the total spectrum. The total spectrum was obtained after adding the peaks SKPi,3 and SKP', more than 0.995 for each of the experiments.
Effect of the cation in sulfates on the intensity ratio of SKfi'/SKfii3. The ratio of the intensity of the lines and the areas under the peaks SKP' and SKPi,3 carries information about the form in which sulfur is contained in the sample only in the form of sulfates or other compounds. As shown in the conducted studies [26, 33], the ratio of the intensities of the SKP' satellite line to the main SKPi,3 line in the sample correlates with the concentration of sulfates in the sample.
To confirm that the cation does not significantly affect the intensity ratios of the lines (areas under the peaks) SKP' u SKPi,3, samples were prepared (sulfates of various metals with a mass sulfur content of 2 %), and the intensity ratios (excluding background) and peak areas (excluding background) of the selected analytical lines were determined, SKP'/SKPi,3 (Table 1). The ratios of intensities and peak areas of SKP'/SKPi,3 for sulfates having different cations remain the same. The largest deviation from the average value for the intensity ratio was 2.83 % and for the areas ratio it was i.75 %, which means that the influence of the cation on the sulfur spectrum is insignificant. This is due to the fact that in the ions of polyatomic compounds, individual ionic groups, for example SO42-, are isolated anions and they are not significantly affected by cations [30, 34]. Thus, the influence of the cation on the intensity of the characteristic sulfur lines in the SKP region will be insignificant and will have no recognizable effect on the quantitative determination of sulfates in real samples.
Table 1
Intensity ratio of SKß'/SKßi,3
Salt Intensity Area
CaSÜ4 0.526 0.627
CuSÜ4 0.515 0.632
FeSÜ4-7H2Ü 0.500 0.616
KAl(SÜ4)2l2H2Ü 0.507 0.623
MgSÜ4 0.511 0.636
Na2SÜ4 0.510 0.628
SES
© Ivan P. Sverchkov, Vladimir G. Povarov, 2024
10 n 9 8 -7 -6 -
ft o M
.1? 5
M
4 -3 -
1 -
•0.1%
• 0.3 % 0.5 %
■ 0.7 % 1.0%
1 2.0 %
■ 3.0 %
■ 4.0 %
• 5.0 %
;2.44 2.46 Energy, keV
2
0
Fig.3. Spectra in the area of the SKP line
Calibration of the device. To construct the calibration curve, pure anhydrous calcium sulfate was used and mixed with sulfur-free rock dried to an absolutely dry state in the required ratio. A series of samples with different sulfate sulfur content ranging from 0.1 to 5 wt.%, was prepared for calibration. The obtained spectra are shown in Fig.3. For each of the spectra, peaks were selected according to which a calibration curve was built on the intensity of the analytical line and the area under the peak SKP'. The results of the calibration are shown (Fig.4). As can be seen from the graphs, when calibrating with artificial mixtures, the coefficient of determination is high for both types of calibration: in terms of the intensity of the analytical line and the area under the peak.
Checking the calibration characteristics. To check the calibration characteristics, real objects of various nature: rocks, soils, bottom sediments and mining waste were taken, two of which are standard samples. The content of total sulfur and sulfate sulfur in the selected samples is presented in Table 2. The total sulfur content in the samples varies from 0.10 to 3.44 % and the sulfate content varies from 0.10 to 1.16 % in absolutely dry weight
Concentration, % Concentration, %
Fig.4. Calibration curves: a - sulfate sulfur by the intensity of the SKP' line; b - sulfate sulfur by the area under the SKP' peak
Table 2
Samples with known sulfur content, wt.%
Sample Type Total sulfur Sulfate sulfur
SP89 Rock 3.44±0.34 0.18±0.02
SGHM-4 Soil 0.43±0.04 0.23±0.02
1 Mining waste 0.24±0.02 0.23±0.02
2 Mining waste 1.17±0.12 1. 16±0. 12
3 Bottom sediments 2.14±0.21 0.81±0.08
4 Bottom sediments 0.10±0.01 0.10±0.01
5 Bottom sediments 2.04±0.20 0.73±0.07
6 Bottom sediments 0.61±0.06 0.30±0.03
7 Bottom sediments 0.82±0.08 0.12±0.01
8 Bottom sediments 0.26±0.03 0.24±0.02
e
£
S3
The spectra of the SKP line of rocks, soils, sediments and mining waste after normalization, superimposed on the spectral line of elemental sulfur, are shown in Fig.5. The SKP' line characteristic of sulfates is observed in each of the samples. For samples i and 2, the intensity of the satellite line is the highest, since almost all of the sulfur in these samples is in the form of sulfates. The spectral line of the SP89 sample almost completely coincides with the line of elemental sulfur and only a small part of it in the SKP' region goes beyond the region of the line of elemental sulfur, since the proportion of sulfate sulfur in this sample is only 5.2 % of the total sulfur. Sample 4 contains the smallest amount of total sulfur, only 0. i %. For this reason the satellite line is not pronounced in it, despite the fact that all the sulfur in this sample is presented in the form of sulfates.
For all the obtained spectra, the peaks were adjusted (see Fig.2), their maximum intensities and the areas under the peaks were determined, which were later used to determine concentrations by calibration curves (see Fig.4). The results of the analysis are presented in Table 3.
As can be seen from Table 3, the total sulfur content determined by the intensity of the SKai,2 line of sulfate sulfur practically does not differ from the reference values established by the other method. The content of sulfate sulfur determined by the height of the peak and by the area under the peak SKP' differs significantly for samples with a high content of other forms of sulfur. If the sample contains sulfur only in the form of sulfates, then both methods show themselves to be effective. Calibration by the area under the peak shows better results with samples containing different forms of sulfur, since this method of calibration depends less on the intensity of the main peak SKPi,3. A similar pattern is observed on the curves of reference concentrations of sulfur from those measured by the X-ray fluorescence method (Fig.6).
1 1 1 1 1 1 SP89
1
2 ________...... " '
3
4
5
6 ^^^ Kß' . 1 . Kßl 3
. 1 . 1 . II 1 1 1
2.42 2.43 2.44
2.45 2.46 Energy, keV
2.47 2.48 2.49 2.50
Fig.5. Spectra of various samples superimposed on the SKP line of elemental sulfur
QES
© Ivan P. Sverchkov, Vladimir G. Povarov, 2024
Table 3
The content of total and sulfate sulfur determined by the intensity of the line and the area under the peak, wt.%
Name of the sample Total sulfur (according to the intensity of the line SKa1,2) Sulfate sulfur (according to the intensity of the SKP' line) Sulfate sulfur (by area under the peak SKP' line)
SP89 3.46 0.58 0.19
SGHM-4 0.42 0.25 0.23
1 0.24 0.27 0.23
2 1.19 1.22 1.19
3 2.14 1.16 1.05
4 0.11 0.11 0.11
5 1.99 1.93 0.82
6 0.64 0.42 0.37
7 0.84 0.22 0.14
8 0.27 0.32 0.28
0.4
0.8
1.2
b
a
0
Measurement concentration, %
As can be seen from the graphs of the determination of sulfate sulfur (Fig.6), the theoretical line along which the points of the analysis results must be located goes beyond the confidence interval both when determining by intensity and by area under the peak. Both ranges of confidence intervals go below that theoretical line. This is due to the fact that the separation of SKP' peaks from SKP1,3 in case of high non-sulfate sulfur contents is difficult and the measured results exceed the reference values. However. for sulfate sulfur, which was determined by the area under the peak 95 % confidence interval is much smaller, which means that the range of measured values is also smaller [35, 36]. Determination of sulfate sulfur by the area under the peak can be used for rapid assessment of the content of sulfur forms in samples of different nature.
Conclusion. In this study. a new approach to the quantitative assessment of sulfur forms was developed. Calibration curves for the determination of total sulfur were obtained from the intensity of the SKa1.2 line, and of sulfates - after processing the SKP1,3 lines and the SKP' satellite line on the X-ray emission spectra after fitting the peaks. With the help of the presented method, it is possible to determine the concentrations of total and sulfate sulfur in ores, mining waste and bottom sediments with a WDXRF spectrometer using calibration curves constructed on the basis of artificial mixtures. The concentrations of sulfate sulfur determined by the area under the peak SKP' are consistent with the concentrations measured by the classical method. Determination of sulfate sulfur by the area under the peak can be used for rapid assessment of the content of sulfur forms in samples of different nature.
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Authors: Ivan P. Sverchkov, Candidate of Engineering Sciences, Senior Researcher, Sverchkov_IP@pers.spmi.ru, https:// orcid. org/0000-0003-4725-0050 (Empress Catherine II Saint PetersburgMining University, Saint Petersburg, Russia), Vladimir G. Povarov, Doctor of Chemical Sciences, Head of Research Project, https://orcid.org/0000-0001-6710-0514 (Empress Catherine II Saint Petersburg Mining University, Saint Petersburg, Russia).
The authors declare no conflict of interests.
