The determination of eight heavy metals was performed in 45 moss samples collected throughout the Kosovo territory (Fig. 2). To better present and understand the concentrations which resulted from the chemical analysis, the crude data were processed by statistical methods. In Table 3, descriptive statistics quantities are presented. In the table, high concentrations of Al and Fe stand out of all heavy metals studied in this work. High concentrations of these heavy metals are always expected as they are the prevailing heavy metals of the earth's crust. Zn and Pb also occur in high concentrations. The order of median concentration of the analysed metals is, Al > Fe > Zn > Pb > Cu > Cr > Ni > Cd. A striking difference between the maximum concentration and the 90th 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 90th 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 also found of Zn. The big difference between these two statistical quantities indicates different zones over the study area with the presence of particular heavy metals, which can also mean a presence of pollution. The high skewness and kurtosis values 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 fit more closely the lognormal distribution. Pb and Ni also have a high coefficient of variation, most probably arising from the artificial introduction of these two elements in the environment.
Table 3
Descriptive statistic of measurements for moss samples (in mg/kg)
Element
|
X
|
Md
|
Min
|
Max
|
P10
|
P90
|
S
|
CV
|
SX
|
MAD
|
A
|
E
|
Ala
|
1100
|
890
|
480
|
2700
|
600
|
1900
|
520
|
48
|
77
|
230
|
1.32
|
1.26
|
Cdb
|
0.48
|
0.36
|
0.16
|
2.1
|
0.23
|
0.80
|
0.38
|
78
|
0.056
|
0.11
|
2.59
|
7.69
|
Cra
|
3.1
|
2.6
|
0.93
|
10.1
|
1.5
|
5.5
|
1.8
|
58
|
0.27
|
1.0
|
1.64
|
3.90
|
Cua
|
3.9
|
3.9
|
2.6
|
8.1
|
2.9
|
4.6
|
0.94
|
24
|
0.14
|
0.52
|
2.14
|
8.00
|
Fea
|
920
|
820
|
430
|
2000
|
530
|
1400
|
390
|
42
|
58
|
200
|
1.09
|
0.47
|
Nib
|
4.4
|
1.7
|
0.46
|
79
|
0.82
|
6.1
|
12
|
270
|
1.8
|
0.75
|
5.98
|
37.83
|
Pbb
|
6.6
|
7.3
|
0.58
|
38
|
0.63
|
10
|
6.0
|
90
|
0.89
|
2.5
|
3.24
|
16.78
|
Zna
|
36
|
31
|
20
|
150
|
23
|
49
|
21
|
58
|
3.1
|
5.2
|
3.97
|
18.82
|
X - arithmetical mean, Md - median, Min - minimum, Max - maximum, P10–10 percentile, P90–90 percentile, S - standard deviation, SX - standard error of mean, CV - coefficient of variation, MAD—median absolute deviation, A -skewness, E - kurtosis, a - Determined by ICP-AES, b - Determined by ICP-MS. |
Spatial distribution
Spatial distribution maps of studied heavy metals are presented in Fig. 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. The maximum concentration of these two metals is found in Siboc 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 the soil, reaching mosses under wind and precipitation. However, the anthropogenic origin cannot be entirely excluded (Ötvös et al., 2003). The maximum Fe concentration in the present moss survey (2000 mg/kg) is lower than that of the 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 that the difference between the present heavy metals concentration in mosses and that of the 2010 survey cannot be taken as entirely valid as the sampling locations are not the same, and the number of sampling points is more significant 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 is primarily found in the South and the most North of the sampling area. Both metals are primarily present in the districts of Prishtina and Gjilani. Geologically they are mainly present in the areas of clastites of Neogene-Paleogene and flysch of Mesozoic, where they occur in fine soil particles owing to rock weathering under atmospheric conditions and geological processes, whereby transferred in moss by wind and precipitations.
Zinc, lead, and copper come next as the most concentrated heavy metals in mosses samples in the territory of Kosovo. All three are found around pollution sources, such as lead and zinc Trepça mines and ore processing units, ferronickel smelter in Drenas, the coal power plant in Obiliq, and the cement production plant Hani i Elezit. The elements in the just mentioned areas are clearly of anthropogenic origin. Zn is found at highest concentrations in the North of Kosovo at sampling points Zveqan 86 mg/kg and Stanterg 146 mg/kg, then in Çikatovë 64 mg/kg and Shalc 61 mg/kg. All those points in the district of Mitrovica where Zn concentration is the highest are shown in Fig. 3. The median of Zn is lower than in the 2010 survey; the maximum is around twice as high. The median and max of Pb are slightly lower than in 2010, whereas the median of Cu is higher and the maximum lower than in 2010. The median concentration of Zn and Pb are higher than in Albania, North Macedonia and Norway, except for Zn, which median is the same as in Norway (Steinnes et al. 2016; Lazo et al. 2018; Stafilov et al. 2018).
On the contrary, the median concentration of Cu was lower than in the countries mentioned above. The highest concentration of Zn corresponds with the geologic formation of the Magmatic rocks of Neogene-Paleogene, which are located around the same sampling sites. Lead is more present in the North, South-West, and the South-East, with concentrations lower than Zn and more heterogeneously distributed. Districts with the highest concentration of Pb are Mitrovica and Prizren, whereas according to geological composition, Pb reaches the highest concentrations in Magmatic rocks of Paleogene-Neogene. Pb shows a similar pattern to Zn because of the Lead/Zinc mines and smelter in Mitrovica, the primary source of these two heavy metals. Relatively high concentrations of Pb were found in the district of Prizren (7.9–10.08 mg/kg), although there is no industry involving Pb particularly. In this regard, it can be said that, apart from the traffic 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 of Mitrovica, Prishtina, Prizren, and Gjilan. Its highest concentration is found at the sampling site in Shalc, which is 8 mg/kg. Cu concentrations are lower than Zn and Pb, whereas they are more evenly spread than Pb. The districts with the highest concentrations lie in geologic formations where Cu concentration is also high. Because 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. Exceptionally high concentrations of these heavy metals in the North have also been reported for soil samples (Šajn et al. 2013; Kerolli-Mustafa et al. 2015b; Kastrati et al. 2021), which partly explains their concentration in mosses. Fine dust particles contaminated with heavy metals can become airborne during dry seasons and spread by the wind in larger areas. But emissions during the processing can also be spread and settle over a vast territory under gravity or by precipitations.
Chromium and nickel have a spatial distribution somewhat similar. Their highest concentrations appear in the west, centre and the east of the sampling area. The highest concentrations of Cr are those in sampling points Prapashticë 10 mg/kg, Reqan 6.6 mg/kg, and Lupç 5.9 mg/kg. Cr and Ni medians are only slightly smaller than the 2010 survey in Kosovo. In contrast, medians of these two heavy metals are lower than those of Albania and North Macedonia but higher than Norway (always surveys of 2015). Prishtina is the district where Cr is at the highest concentration, followed by Prizren, and Peja. Geological formations that contain mostly Cr are clastic sediments of Neogene-Paleogene, whereas it is found at the lowest concentrations in carbonate and metamorphic formations of Paleozoic. Ni represents an outstanding peak in sampling point Harilaq 79 mg/kg, then in Llapushnik 19.8 mg/kg, and Çikataovë 11.6 mg/kg. These high concentrations compared to other samples are expected as the three sampling points are located just close to the Ferro-Nickel facilities and are a result of direct pollution. Ni is most present in samples of the District of Prishtina, whereas Magmatic rocks of Mesozoic are the geologic areas where it is present mostly, with a very high difference compared to other geologic formations. The three sampling points with peak Ni concentration also fall in the areas of Mesozoic rocks, but outstandingly higher concentrations than in other Mesozoic rocks areas indicate the presence of pollution.
Cadmium is the heavy metal with the lowest concentration in mosses samples in the territory of Kosovo. Cd concentrations are the highest in sampling points Stanterg 2 mg/kg and Zveqan 1.7 mg/kg, Siboc 1.15 mg/kg, Te Kalaja 1 mg/kg, Kaçanik 0.8 mg/kg. All these sampling points correspond to sites where pollution is expected because of industrial activities. Cd median is greater than that of the 2010 survey in Kosovo, as well as it is greater than the medians found in Albania, North Macedonia, and Norway (2015 surveys). Districts with the highest concentrations of Cd are Mitrovica and Gjilani. In contrast, according to geological formations, it is Magmatic rocks of Paleogene-Neogene the formation with the highest concentration, and Carbonates of Paleozoic come second.
Multivariate analysis
Multivariate statistical analyses were carried out to identify the pollution or geogenic origin of heavy metals studied. In Table 4, Pearson correlation coefficients are given for each combination of elements, where six significant correlations can be seen, all with a p-value under 0.05. The strongest correlation for Al and Fe with a value of 0.936 was observed, then Zn and Cd with 0.858. Strong correlation coefficients showed the pairs Fe – Cr (0.751) and Al - Cr (0.656), whereas there was only one moderate correlation Pb – Cd (0.437). Other correlations were weak or very weak.
Table 4
Pearson correlation coefficients between element concentrations in mosses in Kosovo
Elements
|
Al
|
Cd
|
Cr
|
Cu
|
Fe
|
Ni
|
Pb
|
Zn
|
Al
|
1.000
|
|
|
|
|
|
|
|
Cd
|
-0.068
|
1.000
|
|
|
|
|
|
|
Cr
|
0.656
|
-0.146
|
1.000
|
|
|
|
|
|
Cu
|
0.035
|
0.173
|
0.027
|
1.000
|
|
|
|
|
Fe
|
0.936
|
-0.094
|
0.751
|
0.037
|
1.000
|
|
|
|
Ni
|
-0.059
|
-0.086
|
0.193
|
-0.155
|
0.203
|
1.000
|
|
|
Pb
|
0.069
|
0.437
|
-0.092
|
0.037
|
0.018
|
-0.174
|
1.000
|
|
Zn
|
-0.075
|
0.858
|
-0.069
|
0.213
|
-0.048
|
-0.035
|
0.351
|
1.000
|
Principal component analysis (PCA) was performed to identify possible patterns of heavy metals distribution in mosses samples and potentially polluted areas and pollution sources. The results obtained are shown in Fig. 4. The PCA analysis produced two principal components (PCs); the first PC accounted for 33.42% of the total variance and the second PC for 26.81% of it. Sampling sites could be visualised in the plot of scores of PC1 and PC2. In the PCs score plot can be easily noticed the two sampling sites, Zveqan and Stanterg, which score highly on PC2 and on PC1 almost just as well. Shalc score is also 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 and Stanerg in the PCs score plot.
Similarly, can be argued for Shalc. The angle between Zn, Pb, and Cd vectors is also tiny, particularly for Zn and Cd, which indicates a high correlation between these heavy metals. Cu vector is shorter but still considerably in a slight angle with the Zn, Cd, and Pb vectors, meaning that it is correlated with these metals and related with the sampling points Zveqan, Stanterg, and Shalc. A high correlation of these elements and the vectors' directions show that Stanterg and Zveqan have high scores primarily due to these elements' high presence in that area. This is a consequence of the Trepça mining and ore enrichment processes and mineral tailing dump just in the South of Mitrovica city (Šajn et al. 2013; Kerolli-Mustafa et al. 2015a). To a lesser extent sampling point, Vidishiq-Bare was also influenced by those four elements, but more significantly by Cd and Zn, as their vectors' directions are slightly closer to this point. The provenance of these heavy metals in this area is from Trepça's facilities, as it is located just a few kilometres in the North.
As the variables loading plot shows, all the sampling sites in the fourth quadrant of the sampling points PCA plot (Fig. 4) are mainly influenced by heavy metals of geogenic origin. Sampling point Siboc (28) scored highly in both PCs, and Prapashticë, Lupç, and Reqan scored highly on PC1. The high scores of these sampling sites can be explained by high values of the variables Al, Fe, and Cr. Although Al, Fe, and Cr are mostly of geogenic origin, their exceptionally high concentrations in the moss sample of Siboc reveal the atmospheric pollution effect of coal digging and burning in electricity plants in Obiliq. A high concentration of Pb was also found in Siboc (12.5 mg/kg), and this site is among those with the highest Cu concentration too (4.5 mg/kg). This can explain the fact that Siboc tends slightly to the left closer to Cu and Pb vectors, which reveals the contribution of these two elements in its score. Thus, an overlap of polluted air currents emerging from Mitrovica (Trepça plant) and Obiliq (electricity power plant) is expected in this sampling point and almost all along the Kosovo basin. Cr, not being a crustal element but present in this group of elements, also may be an indicator of the pollution effect of electricity plants.
Moreover, Cr was found at relatively high concentrations in the fly ash of lignite used to power the electricity plant (Kittner et al., 2018). However, Cr content in most of the moss samples is probably of geogenic origin. They are not close to industrialised sites, such as Prapashticë and Reqan, where the highest concentration is. Harilaq scores relatively high in PC1 and PC2 primarily due to the high presence of Ni, and it reflects the atmospheric deposition of dust emitted majorly by the ferronickel mine in Magure, which is located just nearby, and probably also the geochemical composition of the surface soil of the area. As indicated by the direction of Ni vector, Izbicë, Janjevë, Llapushnik are also influenced by Ni, which can be transported there by winds from the ferronickel plant in Drenas and mine in Golesh. As can be seen in the variables loading plot, Cu, and mostly Ni do not show a significant correlation between them or any of the two already discussed groups. The score plot of the sampling points also indicates 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 influenced by the variables under consideration.
Pollution indices
To further estimate the atmospheric pollution level, the values of CF and PLI were calculated and are presented in Table 5. According to Fernandez's categorisation of the CF values (Fernández and Carballeira 2001), most of the CF of Cu also do not exceed 1 as well as those for Zn, except for Zn in sampling points Çikatovë with a CF = 2.1 which corresponds to slightly contaminated, and Zveqan CF = 2.8 and Stanterg CF = 4.7 as moderately contaminated. Ni shows 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 and Çikatovë, whereas extreme CF resulted in Harilaq. In the case of Al and Fe all sampling sites lie in the 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 near industrial facilities discussed previously, except Kukljan where no industrial activities are performed. The CF value of this site is in the lower limit of the severe category (8.9). The highest CF appears to be for Pb. Most of the sampling sites fall in the extreme contamination factor with values over 100, and the rest of the samples correspond to the severe contamination category.
Table 5 Contamination factor (CF) values for heavy metals measured in 45 moss samples
(in mg/kg)
Sampling site
|
Al
|
Cd
|
Cr
|
Cu
|
Fe
|
Ni
|
Pb
|
Zn
|
1
|
1.5
|
8.9
|
3.1
|
1.1
|
1.9
|
1.1
|
158.0
|
0.9
|
2
|
1.7
|
3.9
|
3.3
|
1.1
|
2.1
|
1.0
|
179.0
|
0.9
|
3
|
1.0
|
4.5
|
2.2
|
0.9
|
1.4
|
0.7
|
180.7
|
1.1
|
4
|
1.2
|
8.9
|
1.3
|
1.0
|
1.5
|
0.4
|
11.9
|
0.9
|
5
|
3.1
|
7.7
|
4.0
|
0.8
|
3.5
|
1.0
|
206.6
|
0.7
|
6
|
2.5
|
10.0
|
4.0
|
1.0
|
3.0
|
0.8
|
145.9
|
0.9
|
7
|
1.7
|
5.1
|
5.1
|
0.6
|
2.3
|
4.9
|
201.7
|
1.1
|
8
|
3.3
|
3.4
|
6.1
|
1.0
|
3.5
|
1.5
|
196.2
|
0.9
|
9
|
4.0
|
4.9
|
9.4
|
1.1
|
4.6
|
2.8
|
25.5
|
0.9
|
10
|
2.8
|
4.0
|
3.2
|
1.1
|
3.6
|
1.0
|
173.7
|
1.1
|
11
|
1.3
|
2.9
|
1.8
|
0.8
|
1.9
|
0.7
|
139.2
|
0.8
|
12
|
2.6
|
5.9
|
2.1
|
0.8
|
2.9
|
3.3
|
58.6
|
0.9
|
13
|
1.3
|
2.8
|
7.3
|
0.7
|
2.2
|
7.2
|
11.8
|
0.7
|
14
|
1.8
|
2.9
|
4.9
|
0.7
|
2.7
|
5.5
|
192.6
|
0.8
|
15
|
1.5
|
3.6
|
2.6
|
0.9
|
1.9
|
1.0
|
170.7
|
0.9
|
16
|
1.6
|
5.1
|
2.6
|
0.7
|
2.1
|
1.0
|
185.1
|
0.7
|
17
|
3.2
|
3.3
|
5.0
|
0.9
|
4.4
|
0.8
|
137.1
|
0.8
|
18
|
2.0
|
4.5
|
4.8
|
0.8
|
2.8
|
3.4
|
147.6
|
0.8
|
19
|
2.1
|
5.6
|
3.8
|
0.9
|
2.5
|
2.3
|
12.5
|
0.9
|
20
|
1.6
|
3.7
|
2.9
|
0.7
|
2.0
|
1.5
|
166.1
|
1.0
|
21
|
1.5
|
4.6
|
6.8
|
1.1
|
2.6
|
18.0
|
33.6
|
1.0
|
22
|
2.0
|
4.0
|
5.7
|
0.7
|
4.5
|
72.2
|
36.3
|
1.0
|
23
|
2.6
|
3.9
|
5.1
|
1.0
|
3.2
|
1.8
|
75.6
|
1.3
|
24
|
4.1
|
4.8
|
6.2
|
0.9
|
5.2
|
1.8
|
181.4
|
1.2
|
25
|
1.3
|
12.3
|
1.7
|
1.0
|
1.7
|
0.5
|
45.8
|
1.3
|
26
|
1.9
|
5.9
|
3.0
|
0.9
|
2.7
|
2.7
|
54.3
|
1.2
|
27
|
2.4
|
9.6
|
8.5
|
1.1
|
4.0
|
10.5
|
53.0
|
2.1
|
28
|
6.0
|
14.4
|
7.9
|
1.1
|
6.5
|
2.1
|
249.5
|
1.6
|
29
|
1.5
|
2.0
|
2.8
|
0.9
|
2.2
|
1.1
|
11.6
|
0.8
|
30
|
4.5
|
5.2
|
14.4
|
0.8
|
5.6
|
3.6
|
150.0
|
1.1
|
31
|
1.9
|
2.2
|
2.8
|
0.9
|
2.6
|
1.1
|
12.0
|
0.8
|
32
|
1.9
|
3.2
|
2.6
|
0.8
|
2.1
|
0.8
|
12.9
|
0.8
|
33
|
3.6
|
2.8
|
6.7
|
0.8
|
3.6
|
1.7
|
111.2
|
0.7
|
34
|
1.9
|
10.0
|
5.4
|
1.9
|
2.9
|
2.2
|
87.5
|
2.0
|
35
|
4.5
|
3.1
|
4.8
|
0.8
|
4.7
|
1.3
|
183.5
|
0.9
|
36
|
2.3
|
3.4
|
2.3
|
0.8
|
2.8
|
0.7
|
138.3
|
0.8
|
37
|
4.1
|
2.1
|
8.4
|
0.8
|
5.8
|
2.6
|
147.4
|
1.0
|
38
|
3.9
|
3.0
|
4.8
|
1.1
|
4.6
|
1.7
|
152.9
|
1.2
|
39
|
1.7
|
8.8
|
2.5
|
0.9
|
2.2
|
1.1
|
27.2
|
1.4
|
40
|
1.2
|
3.2
|
2.1
|
0.8
|
1.6
|
2.9
|
161.6
|
1.1
|
41
|
1.4
|
20.9
|
2.1
|
1.0
|
2.0
|
0.9
|
760.3
|
2.8
|
42
|
1.6
|
25.7
|
2.3
|
0.8
|
2.0
|
1.1
|
177.8
|
4.7
|
43
|
1.2
|
6.0
|
2.2
|
1.4
|
1.7
|
0.9
|
245.5
|
1.3
|
44
|
1.8
|
3.4
|
5.3
|
0.6
|
2.4
|
2.5
|
141.4
|
1.0
|
45
|
1.8
|
4.9
|
3.2
|
1.0
|
2.4
|
1.4
|
31.8
|
1.2
|
The PLISite values for eight heavy metals throughout the sampling area, and then only for four of them, are shown in Fig. 5. According to Zhang (Zhang et al. 2011) PLI categorisation, none of the moss sampling sites falls in the unpolluted category site (Fig. 5a). Four samples fall in the unpolluted to moderately category, 13 are moderately polluted, 17 are moderate to highly contaminated, eight correspond with the highly polluted type, and very highly polluted are three of them. The PLIzone for the whole territory of Kosovo when all eight elements were included in the calculation was 3.4, which corresponds to the moderately to highly polluted category. However, when only Cd, Cr, Ni, and Pb were included in PLI calculation, three sampling sites were put in the moderately polluted category. Only one in moderate to highly polluted, three in highly polluted, and 38 samples correspond to the highly polluted type (Fig. 5b). The PLIzone of the entire study area now is 7.4, which is just within the threshold of the very highly polluted category.