RETROSPECTIVE STUDY OF THE EXTENT OF HEAVY METAL EMISSIONS BY LICHEN BIOMONITORS EMPLOYING NUCLEAR TECHNIQUE IN CHINA
Zh. H. Zhang, Z. F. Chai*,
X. Y. Mao
Laboratory of
Nuclear Analytical Techniques, Institute of High Energy Physics, Chinese
Academy of Sciences, P. O. Box 2732, Beijing 100080, China
*E-mail: chaizf@ihepa.ac.cn
ABSTRACT
Concentrations
of 34 elements, Ag, As, Au, Ba, Ca, Ce, Co, Cr, Cs, Eu, Fe, Hf, K, La, Lu, Mo,
Na, Nd, Ni, Rb, Ru, Sb, Sc, Se, Sm, Sr, Ta, Tb, Th, Tm, U, W, Yb and Zn were
determined by instrumental neutron activation analysis (INAA) in the early
preserved epiphytic lichens (Parmotrema
recticulata) in a remote Southwestern Chinese area from 1960s to 1990s. The elemental
concentration levels obtained from the organisms indicate that the heavy metal
atmospheric deposition in the sampling sites has no significant change during
the past decades.
INTRODUCTION
Lichen
is one of the biomonitors with good accumulation property for determining the
heavy metal deposition in terrestrial ecosystems. This well-established
technique has been widely used in European countries for evaluating atmospheric
deposition status for a long history
[1].
This work is the first time in China to
apply this technique for evaluating the variation trend of atmospheric heavy
metal deposition level across nearly thirty years (1960s-1990s) in a remote
Southwestern Chinese area by using the early preserved lichen samples. The
purpose lies in providing a fundamental estimation of atmospheric background
depositions and understanding the long-term anthropogenic emission influences
in this area.
Species of large size epiphytic foliose
lichens (Parmotrema recticulata,
Parmotrema Austrosinensis, Parmotrema tinctorum, Parmotrema cristifera and Parmelia Subtinctoria) were offered by the State
Lichen Sample Bank in Institute of Microbiology, Chinese Academy of Sciences.
All the selected lichen samples have been well reserved in a specially
ultra-clean specimen bank. Total of 34 elements were determined by INAA
including rare-earth elements (REEs), thorium and uranium.
The significance of their concentrations
are discussed in this text on the bases of the environmental variation of metal
deposition during this long period.
METHODS
The
selected 23 epiphytic lichen samples (11 samples were P. recticulata) and 13 of the bark substrates were prepared
as the experimental materials.
About 100 to 300 mg samples dried at the room-temperature
were ashed by
ceramic fiber muffle furnace for 40 minutes at 540℃. Each ashed powder sample was packed with two
layers of aluminum foils. Samples were irradiated for 8 hours in a heavy water
nuclear reactor at the Chinese Institute of Atomic Energy (CIAE) at a thermal
neutron flux of 3.35×1013 n cm-2 s-1.
After decay for 5 and 20 days, the samples were counted twice for medium and
long-lived nuclides by an HPGe detector. Analytical quality was assured by
using a series of standard reference materials (SRMs): IAEA-336 Portuguese
Lichen , NIST SRM1646 Estuarine Sediment, GBW08505 Chinese Tea, GBW08501
Chinese Peach Leaf and GBW 07605 Chinese Tea (GSC-4).
RESULTS AND DISCUSSION
The
mean values and ranges for 34 elemental concentrations in P. recticulata across nearly 30 years collected from the Yungui
Plateau are presented in Table 1.
Table 1 Element
concentrations (μg/g ) in P. recticulata from the Yungui
Plateau, Southwestern China, collected from 1964 to 1994
|
1960’s Range |
( n = 2 ) Mean |
1980’s Range |
( n = 5 ) Mean |
1990’s Range |
Mean |
|
Ag |
0.177-0.307 |
0.242 |
0.0843-0.174 |
0.116 |
0.0593-0.107 |
0.0728 |
|
As |
1.10-11.3 |
6.20 |
0.479-2.63 |
1.24 |
0.213-0.904 |
0.468 |
|
Au |
0.00096-0.0010 |
0.0010 |
0.00084-0.0021 |
0.0015 |
0.00037-0.00076 |
0.00049 |
|
Ba |
90-114 |
102 |
39-111 |
66 |
16-237 |
96 |
|
Ca |
3478-5062 |
4270 |
2223-11579 |
6640 |
3775-6288 |
5533 |
|
Ce |
11.0-11.0 |
11.0 |
1.6-7.8 |
5.5 |
1.5-4.6 |
2.9 |
|
Co |
1.04-16.6 |
8.82 |
0.477-2.29 |
1.14 |
0.339-4.08 |
1.30 |
|
Cr |
6.33-50.8 |
28.6 |
2.65-12.3 |
6.05 |
1.45-1.92 |
1.68 |
|
Cs |
1.24-2.28 |
1.76 |
0.11-1.55 |
0.83 |
0.190-0.618 |
0.493 |
|
Eu |
0.150-0.164 |
0.158 |
0.0484-0.256 |
0.118 |
0.00529-0.0424 |
0.0285 |
|
Fe |
3290-10432 |
6861 |
1111-4935 |
2523 |
664-885 |
805 |
|
Hf |
0.383-0.601 |
0.492 |
0.175-1.08 |
0.453 |
0.0906-0.148 |
0.123 |
|
K |
3773-4289 |
4031 |
2458-6787 |
4055 |
2478-3642 |
3058 |
|
La |
3.50-4.39 |
3.94 |
1.66-9.21 |
3.82 |
0.889-2.23 |
1.32 |
|
Lu |
0.0268-0.128 |
0.0774 |
0.0261-0.138 |
0.0559 |
0.00441-0.0107 |
0.0620 |
|
Mo |
0.475-0.829 |
0.652 |
0.486-2.56 |
1.14 |
0.179-0359 |
0.235 |
|
Na |
297-350 |
324 |
124-889 |
453 |
52.7-95.7 |
72.7 |
|
Nd |
3.21-3.84 |
3.52 |
1.79-7.54 |
3.23 |
0.673-1.28 |
0.952 |
|
Ni |
3.00-24.2 |
13.6 |
1.77-4.50 |
2.92 |
0.892-3.75 |
2.06 |
|
Rb |
12.5-17.5 |
15.0 |
7.27-20.5 |
11.7 |
12.6-15.5 |
13.8 |
|
Ru |
1.30-1.48 |
1.39 |
0.527-1.39 |
0.929 |
0.235-4.36 |
1.73 |
|
Sb |
0.195-0.918 |
0.556 |
0.216-24.0 |
5.05 |
0.0658-0.499 |
0.283 |
|
Sc |
1.17-5.57 |
3.37 |
0.428-2.45 |
1.12 |
0.232-0.314 |
0.277 |
|
Se |
0.388-0.423 |
0.406 |
0.148-0.386 |
0.254 |
0.136-0.254 |
0.178 |
|
Sm |
0.683-0.695 |
0.689 |
0.253-1.24 |
0.544 |
0.173-0.262 |
0.212 |
|
Sr |
19.6-25.8 |
22.7 |
11.9-38.2 |
23.8 |
16.3-47.1 |
29.6 |
|
Ta |
0.0564-0.0863 |
0.0714 |
0.0219-0.170 |
0.0745 |
0.0158-0.026 |
0.0205 |
|
Tb |
0.123-0.124 |
0.124 |
0.0391-0.186 |
0.0736 |
0.0152-0.0261 |
0.0207 |
|
Th |
1.50-1.59 |
1.54 |
0.364-2.51 |
1.02 |
0.214-0.293 |
0.256 |
|
Tm |
0.0298-0.0440 |
0.0369 |
0.0172-0.0469 |
0.0266 |
0.0136-0.017 |
0.0156 |
|
U |
0.151-0.230 |
0.190 |
0.0635-0.176 |
0.104 |
0.0374-0.0502 |
0.0450 |
|
W |
0.221-0.299 |
0.260 |
0.204-0.432 |
0.300 |
0.0998-0.190 |
0.128 |
|
Yb |
0.312-0.314 |
0.313 |
0.0885-0.554 |
0.213 |
0.0435-0.0696 |
0.0600 |
|
Zn |
35.3-39.0 |
37.2 |
17.7-58.8 |
33.1 |
10.8-27.2 |
18.0 |
The accumulation capacity for trace elements in P. recticulata during the past decades
shows no substantial difference in this sampling place. Comparing the above
data with some literature values from different lichen species in European
countries, it can be concluded that the reported values of most elements here
are within the range of concentrations in the European lichens, which should be
regarded as a characteristics of lichen from clean areas [2, 3].
The REEs concentration distribution
patterns in P. recticulata at various
sampling time were proximately consistent with each other (see Fig.1).
Fig.1 REEs concentrations in P. recticulata from 1960s to 1990s
The uptake of the widely considered industrial pollution elements, chromium, iron and zinc, in the lichen P. recticulata and its bark substrate was compared at the same condition of sampling sites and ambient environment. It indicates the higher elemental burdens of the three elements in P. recticulata. In terms of the airborne heavy metal accumulation concentrations, P. recticulata seems to be a very efficient biomonitor (see Fig.2).
Fig.2 Elemental enrichment
capacity in P. recticulata vs bark substrate
A significant positive correlation between thorium
and uranium was found in the 23 epiphytic lichen samples and 13 bark substrate
samples. Th and U are the naturally occurring elements in the earth’s crust.
Higher plants possess the ability to absorb the soluble Th and U via roots [4].
The corresponding enrichment in epiphytic lichens favors the mechanism that
vascular plants first absorb U and Th from soil, which are subsequently
transferred to lichen by leaching from living or dead plant materials [5].
Fig.3 Concentration correlation of thorium vs uranium
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