Determination And Statistical Analysis of Atmospheric Deposition of Heavy Metals In Kosovo. A Moss Survey


 Atmospheric deposition of heavy metals on the territory of Kosovo was studied using the already widely used technique of mosses as biomonitors. This method is very convenient as it uses natural samples and thus avoids many difficulties associated with artificial samplers. Eight heavy metals (Al, Cd, Cr, Cu, Fe, Ni, Pb, and Zn) were determined in 45 moss samples. Statistical analysis was performed to better present and explain the data. High concentrations of Pb, Zn, Cd, and Ni were found near industrial sites and in more densely populated areas. Principal component analysis (PCA) identified more polluted sites such as Zveqan, Stanterg, Prapashticë, Siboc, and Lupç. It was also found that Pb, Zn, Cd, Cr, and Ni are the heavy metals that affect these polluted sites the most. High contents of Cd were found in Kaçanik and Paldemicë, Te Kalaja, Çikatovë, and Shalc, all sampling points found around industrial sights. The contamination factor (CF) and the polluted load index (PLI) were calculated. CF showed that only Cu, and Zn, had no or almost no contamination levels over the range of moss samples, while Cd and mainly Pb gave extremely high values for CF, indicating extreme contamination levels. The pollution load index also showed that only a few samples were slightly polluted, while most samples showed considerable and very high pollution levels.


Introduction
Industrialization and development as necessary as they are, bring di culties along. One of the worst consequences of industrialisation is the production of a lot of waste materials, which very frequently are harmful to the environment. Chemical pollutants generated from many industrial processes and other human activities are spread in waters, soil, and air, depending on their chemical-physical properties and the nature of reports have emerged indicating that mining and ore processing industry contributes to the atmospheric pollution with heavy metals (Lantzy and Mackenzie 1979;Walsh et al. 1979;Thayer and Brinckman 1982;Romo-Kröger et al. 1994; Mansha et al. 2012). Other sources of anthropogenic pollution are tra c, waste incineration, oil, fuel, and coal combustion (Trindade et al. 1981; Pacyna et al. 2007; Xia and Gao 2011; Suvarapu and Baek 2017).
The monitoring of metals atmospheric deposition, traditionally is performed by collectors which are spread throughout the area which is intended to be monitored. Despite the advantages this approach has, there are also drawbacks to be considered. Collectors have to be built, transported and maintained. This makes it complicated to be operated and expensive in comparison to some naturally grounded methods, such as moss survey (Schröder and Nickel 2019). Mosses grow naturally and it is easier to cover more area with higher density of sampling points with less cost (Harmens et al. 2010). They are currently widely used in monitoring of atmospheric heavy metals deposition because of their characteristic structure. Their cuticle being very thin, enables good permeability towards atmospheric gaseous, metals, water, and other chemical species along This work aims to estimate the atmospheric deposition of most commonly expected heavy metals in Kosovo using mosses as bioindicators. It also intends to nd patterns of the data distribution in order to locate the main sources of pollution through statistical analysis. The moss collection in Kosovo for the purpose of heavy metals atmospheric deposition analysis was rst performed in 2010 (Maxhuni et al. 2016). Nine metals were investigated, namely: Cd, Cr, Cu, Fe, Hg, Ni, Mn, Pb, and Zn, in 25 sampling points. It was found that the concentrations of metals, reached pollution levels with variation throughout the area studied. In this survey 45 samples were collected over the whole territory of Kosovo, evenly spread to the possible maximum allowed by moss presence. The concentration of Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn, as more commonly expected heavy metals were determined in moss samples. The data obtained were processed by statistical analysis in order to identify polluted sites and pollution sources as well as to estimate the level of contamination.

Study area
Kosovo is situated in the centre of Balkan Peninsula with geographical coordinates between longitudes 41°5 0′ 58″ and 43° 15′ 42″ and between the latitudes 20° 01′ 30″ and 21° 48′ 02″. It has a surface area of 10,900 km 2 with an average altitude of about 800 m above sea level. It is characterized by a complex geologic setting and high mineral activity of Pb-Zn, Cu, Ni, and Cr throughout the territory. The total estimated land is at 858,063 ha, and about 63.3 % is agricultural land area, and 35 % is forest area. The geology of Kosovo is complex, with formations created at different intervals of geological timeline (Fig. 1). Metamorphic, ysch, carbonate rocks and clastic sediments appear to cover most of the area. Then, there are deluvial/proluvial sediments mostly in the west, south and to the east of the territory, magmatic rocks mostly are found in the north and the east and magmatic rocks mostly in the north and north/east. There are also alluvial sediments which lie along the river basins, such as Drin, Sitnica and Morava rivers.

Sampling
Moss samples were collected during the dry season of summer, August to September. The procedure of sampling was in accordance with Monitoring Manual of "International Cooperative Programme on Effects of Air Pollution on Natural Vegetation and Crops, 2020. Three types of mosses were collected: Homalothecium sericeum (Hedw.) Schimp. 1851, Hypnum cupressiforme Hedwig. 1801, and Pseudoscleropodium purum (Hedw.) M.Fleisch. Three to ten (mostly over ve) samples were collected at each sampling site, in an area of (50 x 50) m. The collected mosses were put in paper bags of 1 L which were stored for 5-7 days at ambient temperature (20-25°C) prior to cleaning. Moss samples were taken at least 100 m away from small roads and over 300 m away from main roads, villages and industries. All samples were collected from ground soil avoiding rocks, and only sometimes from rotten branches or tree trunks, in open elds with low shrubs or without shrubs. In case of forest areas, open gaps of at least 10 m diameter were chosen for mosses sampling and at least a 3 m distance from the nearest tree canopy drip was always respected.

Digestion
Interfering materials such as other herbs, leaves, twigs, and vegetation parts were carefully removed to leave a clean moss mass. After this mosses were grinded by hand using gloves and dried at 40°C for 48 h. Samples were digested in a microwave system by the wet digestion method. 0.5 g of sample were put in the Te on tube, where 7 ml of nitric acid and 2.5 ml of hydrogen peroxide were added. The mixture was left to react for 10 minutes in order to give some time reactions to take place and some gases to release, so that less pressure Inductively coupled plasma-atomic emission spectrometry (ICP-AES) was used for the determination of the Detector attenuation mode was used to cover wide range of linearity without of collision gas. For the problematic isotopes such as 60 Ni, He was used as a collision gas. 60  Where site is sampling site and n is the number of sampling sites. The obtained PLI values are categorized as follows (Zhang et al. 2011): PLI = 0 background concentration, 0 < PLI ≤ 1 unpolluted, 1 < PLI ≤ 2 moderately to unpolluted, 2 < PLI ≤ 3 moderately polluted, 3 < PLI ≤ 4 moderately to highly polluted, 4 < PLI ≤ 5 highly polluted, and PLI > 5 very highly polluted.

Results And Discussion
The determination of nine heavy metals was performed in 45 moss samples collected throughout the Kosovo territory. To better present and understand the concentrations which resulted from the chemical analysis, the crude data were processed by statistical methods. In table 1 descriptive statistics quantities are presented. In the table high concentrations of Al and Fe stand out, which are followed by Mn. High concentrations of these heavy metals are always expected as they are the prevailing heavy metals of earth's crust. Zn and Pb also occur in high concentrations but less than Mn. The order of median concentration of the analysed metals is, Al > Fe > Mn > Zn > Pb > Cu > Cr > Ni > Cd. A striking difference between the maximum concentration and the 90 th percentile can be observed for Ni and Pb. Although the maximum concentrations of these two elements are very high -79 mg/kg for Ni and 38 mg/kg for Pb -their 90 th percentiles are only 6.1 mg/kg and 10 mg/kg correspondingly. A big difference between the maximum concentration of 150 mg/kg and the P90 49 mg/kg was found also for Zn. The big difference between these two statistical quantities, indicates distinguishable zones over the study area with the presence of particular heavy metals, which can also mean presence of pollution. The high values of skewness and kurtosis, indicate that the distribution of concentrations of heavy metals in moss samples is not normal. Except for Pb and Ni the concentration distributions of all other heavy metals t more closely lognormal distribution. Pb and Ni have also high coe cient of variation, most probably arising from arti cial introduction of these two elements in the environment.

Spatial distribution
Spatial distribution maps of studied heavy metals are presented in gure 3. Al and Fe show very similar distribution patterns. Their highest concentrations are found in the east of Kosovo as well as around the centre. Maximum concentration of these two metals is found in Siboc (28) municipality of Obiliq, 2700 mg/kg for Al and 2000 mg/kg for Fe. High concentrations of Al and Fe are expected as they are the most abundant metals naturally present in soil, whereby under the effects of wind and precipitation they reach mosses.
However the anthropogenic origin cannot be fully excluded (Ötvös et al. 2003) . The maximum Fe concentration in the present moss survey (2000 mg/kg) is lower than that of 2010 survey which was 3082 mg/kg, but the median is higher, 820 mg/kg vs 288 mg/kg (Maxhuni et al. 2016). Noteworthy is the fact that the difference between the present heavy metals concentration in mosses and that of 2010 survey, cannot be taken as completely valid as none of the sampling locations is the same, and the number of sampling points is larger in the present study. This should be considered every time the 2010 survey is referred to in this work. As can be seen in the distribution map, the lowest concentration of both elements are mostly found in the south and in the most north of sampling area. Both metals are present mostly in the districts of Prishtina and Gjilani. Geologically they are mainly present in the areas of clastites of Neogene-Paleogene and ysch of Mesozoic, where they occur in ne soil particles owing to rock weathering under atmospheric conditions and geological processes, whereby transferred in moss by wind and precipitations.
Manganese concentrations are also high in all moss samples with a maximum of 360 mg/kg in Petrovë (12), then Reqan (9)  Pb shows a similar pattern to Zn because of the Lead/Zinc mines and smelter in Mitrovica which clearly is the main source of these two heavy metals. Relatively high concentrations of Pb were found in the district of Prizren (7.9 -10.08 mg/kg), despite the fact that there is no industry involving Pb particularly. In this regard it can be said that, apart from the tra c emissions coming from the roads and the highway, which pass just beside or join in the Prizren city, long range atmospheric transport may also contribute to the overall Pb concentration in moss samples (Steinnes et al. 1997a;Steinnes 2001). Cu is mostly present From the North to the South in the districts Mitrovica, Prishtina, Prizren, and Gjilan. Its highest concentration is found at sampling site Shalc (34), it is 8 mg/kg. Cu concentrations are lower than those of Zn and Pb, whereas they are more evenly spread than Pb. The districts with highest concentrations lie on the areas of geologic formations where Cu concentration is also high.
Considering the fact that geologic areas where Cu concentration is higher, do not always correspond with pollution sources, it may be thought that Cu occurs in mosses due to natural processes. Particularly high concentrations of these heavy metals in the north have been reported also for soil samples (Šajn et  obtained are shown in gure 4. The PCA analysis produced two main components (PCs), the rst PC accounted for 29.75% of the total variance and the second PC for 26.25% of it. Sampling sites could be visualized in the plot of scores of PC1 and PC2. In the PCs score plot can be easily noticed the two sampling sites, Zveqan (41) and Stanterg (42), which score highly on PC2 but also considerably on PC1 and Shalc (34) score is relatively high in PC2. This result can be explained by the loading plot of the variables shown just under the samples plot. It can be seen that Zn, Cd, and Pb also load highly on PC2 and to some value on PC1 also, and the direction of the vectors in PCs loading plot are similar as it is the placement of the sampling points Zveqan (41) and Stanerg (42) in the PCs score plot. Similarly can be argued for Shalc (34). The angel between Zn, Pb, and Cd vectors is also small, particularly for Zn and Cd, which indicates a high correlation between these heavy metals. A high correlation of these elements as well as the vectors' directions, show that Stanterg and Zveqan have high scores mostly as a result of these elements' high presence in that area. This is obviously as a consequence of the Trepça mining and ore enrichment processes, as well as mineral tailing dump just in the south of Mitrovica city (Šajn et al. 2013;Kerolli-Mustafa et al. 2015a).
Sampling point Siboc (28) scored highly in both PCs, and Prapashticë (30), Lupç (37), and Reqan (9) scored highly in PC1. The high scores of these sampling sites can be explained by high values of the variables Al, Fe, and Cr. Although Al and Fe are mostly of geogenic origin, their particularly high concentrations in the moss sample of Siboc (28) clearly reveal the atmospheric pollution effect of coal digging and burning in electricity plant in Obiliq. Cr not being a crustal element but however present in this group of elements, also may be an indicator of the pollution effect of electricity plant. Moreover Cr was found at relatively high concentrations in the y ash of lignite used to power the electricity plant (Kittner et al. 2018). Harilq (22) scores relatively high in PC1 and PC2 mostly due to high presence of Ni, and it re ects the atmospheric deposition of dust emitted majorly by ferro-nickel mine in Magure which is located just nearby, and probably also the geochemical composition of the surface soil of the area. As can be seen in variables loading plot, Cu, Mn, and Ni do not show a signi cant correlation between them or to any of the two already discussed groups. The score plot of the sampling points also shows that most of them score around the origin or in the upper left quadrant of the PCs axes, which means that most of the samples are not strongly in uenced by the variables under consideration. as moderately contaminated. Ni shows a variable CF values, among those four sampling sites correspond to moderate contamination, two severe and one has extreme contamination factor. Severe CF was found in Llapushnik (21) and Çikatovë (27), whereas extreme CF resulted Harilaq (22). In case of Al and Fe all sampling sites lie on moderate contamination category or lower. Cr has three sampling sites with severe CF and one with extreme contamination. Cd CF values are the highest for 10 moss samples and fall in the extreme contamination category. All moss samples with high CF are located nearby industrial facilities discussed previously, except Kukljan (1) where no industrial activities are performed but also the CF value of this site is in the lower limit of the sever category (8.9). The highest CF appear to be for Pb, the most of the sampling sites fall in the extreme contamination factor with values over 100, and the rest of samples corresponds to severe contamination category.
The PLI site values for nine heavy metals throughout the sampling area, and then only for four of them are shown in gure 6. When taking all the nine heavy metals in calculation -according to Zhang (Zhang et al. 2011) PLI categorization -none of the moss sampling sites falls in the category unpolluted site ( gure 6a). Ten samples fall in the unpolluted to moderately category, 24 of them are moderately polluted, nine are moderately to highly polluted, and two correspond with highly polluted category. The PLI zone for the whole territory of Kosovo when all nine elements were included in the calculation was 2.5, which corresponds to the moderately polluted. However when only Cd, Cr, Ni, and Pb were included in PLI calculation three sampling sites were put in category moderately polluted, only one moderate to highly polluted, three highly polluted, and 38 samples corresponded to very highly polluted category ( gure 6b). The PLI zone of the entire study area now is 5.1, which is just in the threshold of very highly polluted category.

Conclusion
The estimation of atmospheric deposition of heavy metals in the territory of Kosovo was performed using mosses as bioindicators. The concentrations measured in mosses samples vary greatly between heavy metals, as well as for the single metals in different sampling sites. Particularly high concentrations of Zn and Pb were found around the Trepça mining area around the city of Mitrovica. However both these heavy metals are present in considerably high concentrations in most of the studied area, including areas where no great pollution sources exist. For this reason more studies are required to well establish if long range transport from known pollution sources is the main contributor to the content of these heavy metals in mosses, or if the geochemical composition of the soil is more signi cant. Ni, Cr, and Cd also showed spikes of concentrations around industrial sites. Principal component analysis revealed particularly polluted sites such as: Zveqan (41) Stanterg (42) polluted with Zn, Cd, and Pb. Then Siboc (28), Reqan (9), Prapashticë (30), and Lupç (37), with high contents of Al, Fe, and Cr, and the rest of the samples which were more similarly affected by the metals und consideration. Harilaq (22) was found to be polluted majorly with Ni. Based on distribution maps and the PCA analysis the most signi cant sources of heavy metal pollution appear to be: Trepça mining and ore processing facilities in Mitrovica, Artana, and Kishnica, then ferronickel smelter in Drenas and mine in Golesh, thermoelectric power plant in Obilq, and cement production plant in Hani i Elezit. Urban areas such as Prishtina city also contribute to pollution from heavy tra c, combustion of coal for house heating and various local businesses.
According to CF values only Mn, Cu, and Zn in most of sampling sites are not present at concentration when contamination is to be considered. All other heavy metals are found at some contamination category. CF shows that Pb content in mosses not only is at very extreme contamination category, but extremely high compared to the begging limit of this category which is 27. PLI site values also in none of the samples correspond to unpolluted category when all the heavy metals are taken in calculation, and its values increase even more when only Cd, Cr, Ni, and Pb are included in calculation. 38 sites of the sampling area fall in the highest category of pollution for these four heavy metals.