Bioaccumulation of polonium ( 210 Po ) , uranium ( 234 U , 238 U ) isotopes and trace metals in mosses from Sobieszewo Island , northern Poland

The objective of this study was determination of the polonium (Po), uranium (U and U) radionuclides and trace metals (Pb, Fe, Zn, Cu, Ni, Cd, Hg) concentrations in mosses samples from Sobieszewo Island near the phosphogypsum waste dump in Wiślinka (northern Poland). The obtained results revealed that the concentrations of Po, U, and U in the two analyzed kinds of mosses: Pleurozium schreberi and Dicranum scoparium were similar. Among the analyzed trace metals the highest concentration in mosses was recorded for iron, while the lowest for nickel, cadmium and mercury. The obtained studies showed that the sources of polonium and uranium isotopes, as well as trace metals in analyzed mosses are air city contaminations transported from Gdańsk and from existing in the vicinity the phosphogypsum waste heap in Wiślinka (near Gdańsk).


Introduction
Sobieszewo Island is located between Gdańsk Bay and the delta of the Vistula river (northern Poland) and is the part of area of the city of Gdańsk (the southern Baltic Sea).Mosses are useful as bioindicators of environmental contamination for a variety of natural and artificial origin radionuclides, as well as trace metals (Delfanti et al., 1999).Trace metals may be defined as metals occuring at 1000 µg g -1 or less in the earth' s crust and may be classified as heavy or light with respect to density.Trace heavy metals have densities greater than 5 g cm -3  whereas light metals less than 5 g cm -3  (Osuji and Onjuke, 2004).Trace metals in the environment are a result of natural geochemical processes, as well as from the numerous anthropogenic sources and depending on dispersion according to wind direction, soil characteristics, and on the meteorological and climatic conditions of the site (Rosamilia et al., 2004).Uranium occurs naturally in the Earth's crust and is present in much higher concentrations (Skwarzec, 1995).The principal sources of uranium in natural environment are the wet and dry atmospheric and terrigenic fallout, as well as human activities particularly in agriculture (Skwarzec et al., 2002).Also higher uranium and polonium concentration were observed in the immediate vicinity of the area around the phosphogypsum waste dump (Boryło et al., 2009(Boryło et al., , 2012;;Boryło and Skwarzec, 2011;Skwarzec et al., 2010).Polonium 210 Po belongs to a natural uranium decay series starting from 238 U but its fate depends on further members of this series, e.g. 226Ra and most of all on 210 Pb.Radon 222 Rn escaping from the Earth's surface constitutes the source of atmospheric 210 Po (Skwarzec, 1995).The main source of 210 Po in environment is 210 Pb and 210 Po falling to the ground from atmosphere, but small amount of 210 Po is formed in situ as a result of the radioactive decay of uranium contained in seawater, a result of forest fires and volcanic eruptions (Skwarzec, 1995).

Materials and Methods
The samples of mosses (Dicranum scoparium and Pleurozium schreberi) were collected with five positions of Sobieszewo Island (northern Poland) (Fig. 1).Radionuclide analysis was performed only in the spring 2009 because the amount of research material collected in the autumn this year was relatively small.The concentrations of trace metals (Fe, Pb, Ni, Zn, Cu and Cd) were determined by two methods: AAS (atomic absorption spectrometry) and OES-ICP (atomic emission spectrometry with inductively coupled plasma).The detection limit for all analyzed trace metals was 0.005 mg l -1 .Mercury content was measured by cold vapor technique CV AAS (Nippon Instruments Corporation), the advantage of which is the high precision and accuracy of determination of mercury directly from the sample.The detection limit for mercury analysis was 0.002 ng mg -1 .After chemical analysis in the rest of samples the polonium and uranium concentrations were determined by used alpha spectrometry (Skwarzec, 1995;1997).The accuracy and precision of the radiochemical methods were within 10% based on an international laboratory comparison using International Atomic Energy Agency reference materials (IAEA-384, IAEA-385, IAEA-414).The results of 210 Po, 234 U, and 238 U concentrations in analyzed samples are given with standard deviation (SD) calculated for a 95% confidence interval (±2 σ).

Results and Discussion
The obtained results of trace metals, polonium and uranium determination in mosses samples from Sobieszewo Island are given in Tables 1-3.The lead contents in analyzed mosses vary from 0.01 µg g -1 d. wt. to 26.4 µg g -1 d. wt.In the case of iron the highest concentrations were measured in samples of mosses collected in site 1 for Pleurozium schreberi and Dicranum scoparium (539.9 µg g -1 d. wt. and 255.9 µg g - 1 d.wt. and 580.4 µg g -1 d. wt. and 478.3 µg g -1 d. wt.respectively).The values of zinc accumulation in analyzed mosses from Sobieszewo Island vary from 8.1 µg g -1 d. wt. to 66.4 µg g -1 d. wt.The values of copper concentrations in analyzed mosses were between 6.7-28.3µg g -1 d. wt.Nickel concentrations in mosses range from 0.2 µg g -1 to 4.2 µg g -1 d. wt.For all the analyzed mosses samples from Sobieszewo Island cadmium concentration was below the detection limit (b.d.l.), without of Dicranum scoparium samples taken in the autumn from site 2 (1.2 µg g -1 d. wt.).Mercury concentrations in analyzed mosses species from Sobieszewo Island were between 0.04-0.08µg g -1 d. wt.The presence of heavy metals in Sobieszewo Island may be connected with industrial development in the area around this region.In the immediate vicinity of Sobieszewo Island (5 km of the island) in the southeastern part of Gdańsk the petroleum refinery Lotos SA, Gdańsk Power Station and "Remontowa" Shipyard SA are also located (Fig. 1).The higher levels of heavy metals were characterized for the spring season, which is connected with the fact that pollutants absorbed by mosses, covered in winter with a thick layer of snow do not have the ability to migrate into the environment.
The values of polonium and uranium concentrations in the two analyzed mosses samples were presented in Table 3.The values of the activity ratios 234 U/ 238 U in analyzed mosses ranged from 0.97±0.03 to 1.00±0.07and indicate that uranium in analyzed species originates from the phosphogypsum waste heap in Wiślinka.In phosphoric rocks, which are used to production phosphoric acid, uranium is usually concentrated, in variable amounts.As a consequence, if dust particles, even very small ones, are captured by mosses, they can strongly increase the total uranium content of these organisms (Loppi et al., 2004).Any differences between the obtained concentrations of polonium and uranium measured for various sites of one species collection are connected with location and distance from potential sources of contamination.In the analyzed area the dominating winds were from western and southern directions (Fig. 2).The results of measurements from Rębiechowo Airport station (Fig. 2a) show that western winds are prevailing and blew in the direction of Sobieszewo Island through Gdańsk city, transporting pollution from this agglomeration.On the other hand the results from Świbno (Fig. 2b) station suggest that in this area southern winds are dominant and could transport phosphogypsum particles both with alpha emitters and trace metals directly to Sobieszewo Island.The values of trace metals, polonium and uranium concentrations in mosses from Sobieszewo Island were analyzed also by chemometric techniques: principal component analysis (PCA).On the basis of this technique we observed that sites 1 and 3 are more abundant in analyzed trace metals what can be explained by urban pollution transported by winds according to meteorological stations in Rębiechowo Airport.Probably trace metals are of the same origin (sites 1-4) but polonium and uranium is of phosphogypsum origin (site 5) (Fig. 3, 4).

Conclusion
The main aim of this study was determination of 210 Po, 234 U, 238 U and trace metals (Pb, Fe, Zn, Cu, Ni, Cd, Hg) concentrations in two kind of mosses from Sobieszewo Island (northern Poland).The value of the activity ratio 234 U/ 238 U of close to unity shows that the uranium in the analyzed samples is mainly natural, but there is a possible uranium contribution from fallout of dry and wet atmospheric.After resuspension of such particles dispersed in the air may be captured by mosses in accordance with prevailing wind directions.We also noticed that the concentrations of analyzed metals in mosses samples are higher in spring than in autumn and it can be explained by the fact that during winter mosses are covered with snow so migration of the absorbed elements is not possible.

Fig. 2 .
Fig. 2. Demonstrative wind roses for: Świbno (a), Rębiechowo Airport (b) (The winds and rains measurements data were obtained from Institute of Meteorology and Water Management, National Research Institute according to the agreement no WSP 190 G000 26 01 11 between Institute of Meteorology and Water Management and University of Gdańsk).

Table 1 .
Average trace metals concentration in mosses samples collected in spring 2009 from Sobieszewo Island b.d.l.-below the detection limit

Table 2 .
Average trace metals concentration in mosses samples collected in autumn 2009 from Sobieszewo Island

Table 3 .
Average 210 Po, 238 U, total uranium concentration and values of the activity ratio 234 U/ 238 U in mosses samples collected in spring 2009 from Sobieszewo Island