Multielemental composition of some Slovenian coals determined with k0 -INAA method and comparison with ICP-MS method

Abstract

In this multi-elemental study, 34 elements (Ag, As, Au, Ba, Br, Ca, Cd, Ce, Co, Cr, Cs, Eu, Fe, Ga, Hg, Hf, K, La, Mo, Na, Nd, Rb, Sb, Sc, Se, Sm, Sr, Ta, Tb, Th, U, Yb, Zn and Zr) were analysed in Slovenian coals from operative (Velenje) and non-operative (Kanižarica and Senovo) coal mines and an imported Indonesia coal using k0-Instrumental Neutron Activation Analysis (k0-INAA) and compared to inductively coupled plasma-mass spectroscopy (ICP-MS). Weaker regressions between both methods ICP-MS and k0-INAA are obtained for following elements: Cs, Co, Eu, Se, Sm and Tb with low concentration (below 1 mg/kg). The k0-INAA data are comparable to the ICP-MS data for the majority of elements. The levels of major elements measured with k0-INAA are as follows: Ca>Fe>K>Na>Sr>Ba. Minor and trace elements, as well as rare earth elements (REEs), are comparable with coal values worldwide. Data of trace elements in coal are important since they are related to air emissions. According to our data obtained with both methods (ICP-MS and k0-INAA) we can conclude that concentrations of trace elements, which impact to human health and are combusted (Indonesian and Velenje coal) in Slovenia are comparable to world averages coal. Izvleček V tej raziskavi smo izmerili s k0-INAA (instrumentalno nevtronsko aktivacijsko analizo) metodo nekaj izbranih slovenskih premogov iz velenjskega premogovnika in ne operativnih premogovnikov: Kanižarica in Senovo. Prav tako smo s to metodo analizirali vzorec iz Indonezije (uvožen premog) in ga primerjali z že objavljenimi rezultati pridobljenimi z ICP – MS (masna spektrometrija z induktivno sklopljeno plazmo) metodo. S k0-INAA metodo smo določili naslednje elemente: Ag, As, Au, Ba, Br, Ca, Cd, Ce, Co, Cr, Cs, Eu, Fe, Ga, Hg, Hf, K, La, Mo, Na, Nd, Rb, Sb , Sc, Se, Sm, Sr, Ta, Tb, Th, U, Yb, Zn in Zr. Rezultati meritev pridobljeni s k0-INAA metodo so za večino elementov, obravnavanih v tej raziskavi, primerljivi z rezultati meritev pridobljenih z ICP-MS metodo. Slabše regresije med metodami ICP-MS in k0-INAA dobimo le pri nekaterih elementih (Cs, Co, Eu, Se, Sm and Tb) za katere so značilne nizke koncentracije (pod 1 mg/kg). Koncentracije glavnih elementov merjenih s k0-INAA metodo v premogu se znižujejo kot sledi: Ca> Fe> K> Na> Sr> Ba. Elementi z nizkimi koncentracijami in elementi redkih zemelj (REE) so primerljivi z vrednostmi premoga po vsem svetu. Podatki slednih elementov v premogu so pomembni, ker so povezani z emisijami v zraku. Glede na naše podatke pridobljene z obema metodama (ICPMS, k0-INAA) lahko zaključimo, da so koncentracije slednih elementov, ki vplivajo na človekovo zdravje in jih sežigamo (premog iz Velenja in Indonezije) v Sloveniji primerljivi s povprečnimi vrednostmi svetovnih premogov. GEOLOGIJA 62/2, 219-236, Ljubljana 2019 https://doi.org/10.5474/geologija.2019.010 220 Tjaša KANDUČ, Timotej VERBOVŠEK, Rok NOVAK & Radojko JAĆIMOVIĆ Introduction The chemical analysis of coal includes, as well as, proximate (Khandelwal and Singh, 2010, Yi et al., 2017) (moisture, volatile compounds, ash content, fixed carbon) and ultimate analyses (carbon, hydrogen, sulphur, oxygen, and nitrogen), the analysis of major, minor and trace elements. Usually, these elements are measured using inductively coupled plasma-mass spectrometry (ICP-MS) (Finkelman et al., 2018) and instrumental neutron activation analysis (k0-INAA) (Wagner and Matiane, 2018, Lin et al., 2018) methods. Other methods for determining trace elements include inductively coupled plasma optical emission spectrometry (ICP-OES) (Finkelman et al., 2018), hydride generation atomic absorption spectrometry (HAAS) (Chen et al., 2011) and X-Ray Fluorescence spectrometry (XRF) (Chen et al., 2011). It is widely known that these trace elements can occur in a wide variety of chemical forms or modes of occurrence, which determines the environmental, economic, technological impact, which in some cases can be significant (Finkelman, 1995, 2018). Twenty-five potential harmful trace elements (PHTEs) are typically present in coal in inorganic and organic forms (Radenovič, 2006). Among them As, Be, Cd, Cr, Co, Hg, Mn, Ni, Pb, Se, Sb and U are all potential air pollutants (Gürdal, 2008). Ketris and Yudovich (2009) include rare earth elements, yttrium, and scandium (REY + Sc) in the table of coal Clarke values, which has been a highly useful tool for making geochemical comparisons of coals globally. Indonesian coals are generally low in ash and sulphur, but have high content of volatile matter. They are classified as low rank coals with low caloric value. The sulphur content varies from 0.1 to 1 % (Internet 1). Elemental composition (wt %, dry basis) of TOT S varied for Velenje samples from this study from 1.4 to 3.9 %, Kanižarica from 1.6 to 2.2 % and Senovo 1.9 % (Burnik Šturm et al., 2009). The geological composition of the Velenje basin is described in detail in Brezigar et al. (1987). The origin of the Velenje basin is related to the transtention between Šoštanj and Smrekovec faults. In the pre-Pliocene basement of the basin, Triassic carbonates and dolomites prevail on the northeastern side of the Velenje fault, while Oligocene to Miocene clastic strata, consisting predominantly of marls, sandstones and volcanoclastics are dominant on the south-western side of the fault. The alkaline, calcium-rich environment during formation of Velenje basin also caused a relatively high degree of gelification, which is significantly higher than the degree of gelification observed in other lignites (Markič & Sachsehofer, 1997; Šlejkovec & Kanduč, 2005; Markič & Sachsehofer, 2010) as well as coals investigated in our study. A well known relation between alkalinity and gelification was clearly ascertained in the case of the Velenje lignite. Lignite samples with the highest calcium contents were also the samples with the strongest gelification (Markič & Sachsenhofer, 1997). The macroscopic description of the lignite samples, in term of lithotypes, was determined following the lithotype classification criteria for brown coals (lignites) provided by the International Committee for Coal Petrology (ICCP, 1993) and are described by Burnik Šturm et al. (2009). All of the samples from the Velenje excavation field -50/C in this study are classified as gelified detrital lignite (Kanduč et al., 2018). The lithological columns for Senovo, Kanižarica and Trbovlje are also presented in Burnik Šturm et al. (2009) and references therein (Brezigar, 1987; Kuščer, 1967; Markič et al., 1991). The macroscopic description of the lignite samples in terms of previous petrological (Markič & Sachsenhofer, 1997), geochemical and isotopic studies of light elements C, H, O, N, S (Bechtel et al., 2003; Kanduč et al, 2005; Burnik Šturm et al., 2009; Kanduč & Šlejkovec, 2005; Kanduč et al., 2012; Kanduč et al., 2018; 2019, Liu et al., 2019) were performed in the frame of various research projects. For example, three different lithotypes (xylitic, gelified and matrix) of Pliocene lignite for the Velenje basin, Slovenia, were investigated to establish the variations of biomarker compositions in solvent extracts and stable isotope composition of carbon and nitrogen in bulk material (Liu et al., 2019). All of these studies were focused on the Velenje basin since it is currently the only actively mined basin in Slovenia and is one of the biggest underground coal mines in Europe. All three of the Velenje lithotypes reflect the composition of the original plant material in the paleomire (Markič & Sachsenhofer, 1997). Arsenic speciation studies and the different forms of calcite present in the coal suggest that bacterial activity was a significant factor during sedimentation of the basin (Kanduč & Šlejkovec, 2005; Kanduč et al., 2018; Kanduč et al., 2019a). The analysis of other geological matrixes such as coalbed gas (Kanduč & Pezdič, 2005; Kanduč et al., 2012, Sedlar et al., 2014) and groundwater (Kanduč et al., 2014; Kanduč et al., 2019b) reveal more evidence of bacterial activity during sedimentation of the basin. 221 Multielemental composition of some Slovenian coals determined with k0-INAA method and comparison with ICP-MS method In the study of Kanduč et al. (2019a) organic and inorganic coal samples from -50/C excavation field of Velenje basin were measured using ICP-MS and revealed that the concentrations of the majority of the analysed elements were either equal to or below the global average for coal. Exceptions were Mo (7.76 ± 4.76 μg/g, 3.5 times higher) and U (5.24 ± 3.23 μg/g, 1.8 times higher) in organic-rich samples. It was found that higher than normal are concentrations of U (5-15 ppm – in comparison to 0.5-10 ppm concentrations in world coals), and of Mo (5-20 ppm – in comparison to 0.1-10 ppm in in world coals). Both elements are presumed to be organically bound (Markič & Sachsenhofer, 2010). This study aims to present results of major, minor and trace elements measured using k0INAA method in coal samples collected from operative (Velenje) and non-operative (Kanižarica and Senovo) Slovenia coal mines. The study also analysed an Indonesia coal supplied by the thermal power plant Moste. Additionally, one of the objectives was to compare k0-INAA and ICPMS methods used to analyse the same coal samples (Kanduč et al., 2019a, Supplementary material) from Velenje coal mine and perform a statistical analysis (PCA-Principal Component Analysis) of all data (Velenje, Senovo, Kanižarica, Indonesia coals) measured with k0-INAA method. Methods Sampling locations were taken from a local borehole database in the local coordinate system from the Velenje coal mine. Coordinates were then transformed to Gauss-Krüger D48 Slovenian national coordinate system and indicated on a hill-shaded relief map generated using the ESRI ArcGIS mapping software (Fig. 1). Figure 1A was produced using data from the Shuttle Radar Topography Mission SRTM data at 90 m spatial resolution. A more detailed map (Fig. 1B),