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7Статья поступила в редакцию 08.02.10. Ред. рег. № 717
The article has entered in publishing office 08.02.10. Ed. reg. No. 717
УДК 624.131:551.3
МОНИТОРИНГ ЗАГРЯЗНЕНИЯ ВОЗДУХА В ГОРОДЕ АЛЖИР С ПОМОЩЬЮ РЕАКТОРА НЕЙТРОННО-АКТИВАЦИОННОГО
АНАЛИЗА
М. Беламри, К. Бенрачеди
Лаборатория технологии пищевых продуктов. Университет Бумердес, Алжир E-mail: benrachedik@yahoo.fr
Заключение совета рецензентов: 26.02.10 Заключение совета экспертов: 08.03.10 Принято к публикации: 15.03.10
Referred: 26.02.10 Expertise: 08.03.10 Accepted: 15.03.10
В городской зоне воздух должен быть чистым для обеспечения здоровья людей. Для достижения этой цели были взяты пробы в разных частях города Алжир. Такие токсичны элементы, как Na, Mg, Cl, Sc, Cr, Ti, V, Fe, Co, Cu, Zn, Se, Br, Ag, Sb, Ce, La, Hf, Ta и Hg, были замерены в фильтрах с помощью технологии нейтронно-активационного анализа. Облучение отфильтрованных образцов и эталонов проводилось в реакторе Эс-Салем. В данной работе обсуждается проведение экспериментов и результаты. В результате работы было установлено, что значения верхних границ взвешенной пыли и высокие концентрации некоторых обнаруженных токсичных элементов достигаются в результате погодных условий и интенсивного движения транспорта вокруг зон отбора образцов.
Ключевые слова: взвешенная пыль, токсичные тяжелые элементы, стандарты, нейтронно-активационный анализ, воздух.
MONITORING OF ALGIERS AIR POLLUTION USING REACTOR NEUTRON ACTIVATION ANALYSIS
M. Belamri, K. Benrachedi
Laboratory of Food Technology. University of Boumerds, Algeria E-mail: benrachedik@yahoo.fr
The urban zone needs clean air to assure a public health. To achieve this goal several filter samples were collected in different sites in Algiers city. Toxic elements such as: Na, Mg, Cl, Sc, Cr, Ti, V, Fe, Co, Cu, Zn, Se, Br, Ag, Sb, Ce, La, Hf, Ta and Hg have been measured in the filters using neutron activation analysis technique. Irradiation of filter samples and standards were carried out in Es-Salem reactor. The experimental procedure and the results are discussed. We noted during this work that the upper limit values for suspended dusts and the high concentrations for some toxic elements found are due to the weather conditions and intense road traffic around collecting sites.
Keywords: suspended dust, toxic heavy element, standards, neutron activation analysis, air.
Introduction
The intense human activity leads to the generation of aerosols which are composed of particles of very varied sizes, in particular, fine dust which constitutes a significant factor of pollution involving a deterioration of the air quality [1-3]. Dusts arrive in the atmosphere by different ways. The urban zone needs clean air to assure a public health. To achieve this goal, we need to identify the pollution sources, their respective contributions and the impact of the atmospheric pollutants.
This work is based on following steps:
- Air sampling in selected sites.
- Preparation of the samples.
. - Irradiation of the samples using a reactor facility.
- Sample analysis.
Among elemental analysis techniques, INAA present the advantage to be highly sensitive, accurate, nondestructive and fast when elements are determined using short-lived isotopes.
Es-Salem 15MW reactor is equipped with irradiation channels for short period with pneumatic system and also for long period elements analyzing. IAEA physical standards and chemical standards prepared in the laboratory were used.
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Materials and methods
Area of study Algiers is a big town with 5 millions inhabitants. The city knows a high growth of its daily traffic automobile park of any kind. More than one million cars come to Algiers in addition to 1.2 million which circulate daily in the city. Agriculture and industry which know actually appreciable growth rates paradoxically, contribute to the environmental pollution and degradation of the air quality. We collect the samples on several sites in urban center of Algiers. An estimated number of 100,000 vehicles per day are circulating near the vicinity of this site in a very limited area. The sampling system is placed near the road as shown in Fig. 1.
Рис. 1. Гистограмма общего числа взвешенных твердых частиц в городе Алжир с октября 2006 по январь 2007 в мкг/м3 Fig. 1. Histogram of TSP variations levels in Algiers site from October 2006 to January 2007 in |jg/m3
Sampling program
In sampling program, we followed the example of the many cities in the world, where intensive programs of air quality monitoring are launched [1-3]. Sampling of total suspended particles is conducted in several sites in Algiers city. The sampling program is established for a year. Sample filters were collected during the period October 2006 till January 2007 using a pump with flow rate of 17 l/mn. This flow rate corresponds to the range of an LVS sampler (low sampler volume). The collection time is 40 hours. The aim of this experiment is to test the effectiveness of load and the capacity to measure the concentration of elements directly related to the air pollution.
Using 0.80 ^m porosity filter, the total suspended particulate (TSP) variations were measured and are shown in Fig. 2. They range from 37.2 ^g/m3 to a maximum 115 ^g/m3 with mean value of 69.0 ± 25.5 ^g/m3. The Fig. 3 represents the comparison between two periods of sampling.
Рис. 2. Трехмерное изображение общего числа взвешенных твердых частиц в городе Алжир
с октября 2006 по январь 2007 в мкг/м3 Fig. 2. 3D of TSP variations levels in Algiers site from October 2006 to January 2007 in jg/m3
35
30
■b 25
с
О ■
^ 20
О
15 10 5"
ij
ша
Cr Sc Zn Ta Co Ce Se Hg Sb Ag Analysing Elements
Рис. 3. Накопительная концентрация некоторых анализируемых элементов Fig. 3. Stack concentration of some analysing elements
About 67% of these values don't exceed the limit value 80 ^g/m3. So 33% of all TSP levels are ranging belong the limit values. The concentration level of TSP collected in the period October-November are higher than in the period for January. We deduce that this can be allotted to the meteorological conditions mainly the rain falls.
Sample preparation We follow a conventional preparation procedure [4]. Once the collection carried out, the sample is deposited in Petri boxes and stored in a desiccator after weighing. The weighing is done using an analytical balance. Each filter is cut into pieces from approximately 1 cm2 (1x1)
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which are piled up one on the other and wrapped in high purity aluminium foil. For short life irradiation the filters are put in polyethylene vials to ovoid aluminium high dose and interference.
Irradiation
We carried out three irradiations with the aim to quantify a large number of elements with INAA technique in air pollution field [5]. For short life irradiation, we used a horizontal channel and the pneumatic system to send and receive the samples. The irradiation is done for 5 minutes at neutron flux of 2-1013 n/cm2 s.
For middle and long life irradiation the samples and standards are irradiated in the vertical channel under a thermal neutron flux of 2-1013 n/cm2 s during 2 hours and 6 hours.
The irradiation channel used presents a high ratio of thermal neutrons / fast neutrons. This makes possible to neglect the effect of fast neutron nuclear reactions of type (n, n), (n, 2n), (n, p) or (n, 6). At the end of the irradiation the irradiated capsule, containing the samples and the standards are put in a cell of transfer to let decrease the radioactivity up to an acceptable level. Cooling time is fixed for 08 days. The analysis of the empty envelope allows the correction of the background noise compared to the spectrum emitted by the sample or the standard.
necessary to identify and quantify the radio elements generated by neutron activation [6].
The concentration of the element in the sample is given by the following expression:
Ix _ Cx (e^*'")x (1 - e~K,<)x
is ~ cie71^!-?1^);'
x and s: refer respectively to the sample and the standard;
Cx and Cs: are respectively the concentrations of the element in the sample and in the standard;
Ix and Is: total count under the photoelectric peak of the element in the sample and in the standard.
It is significant to note that in certain cases the errors on the determination of the concentrations can be relatively high. The sources of errors are numerous one quotes [7]:
- Low precision of the concentrations of certain elements in the standard.
- In multi elements analysis, it is not always possible to reach the sufficient counting due to the low levels of concentrations in the sample.
- Position of the photoelectric peak of absorption on the spectrum contributes in certain cases to increase errors.
Results and discussion
Spectrum collection and calculation
Collection of gamma spectra was carried out in low background lead chamber and choosing adequate cooling time in order to improve the sensitivity. The energy range considered was from 0 to 2048 KeV. This energy interval is sufficient to detect the totality of gammas
Twenty one elements have been identified and quantified in the filters. The elements have been identified according the half life and energy depicted in the Table 1. The amounts of pollutants are ranging from the ng/m3 to the ^g/m3.
Энергия и время полураспада элементов, используемых в инструментальном нейтронно-активационном анализе
Energy and half life of elements used in INAA analysis
Таблица 1
Table 1
Element Al28 V52 Ti51 Mg27 Cl38 Mn56 Cu64 Na24 Br82 La140 Sb122
Energy (keV) 1778,9 1434,4 320 844 1642 846,9 511 1368,4 554,3 486,8 564
T1/2 2.31min 3.76min 5.79min 9.45min 37.9min 2.58 hr 12.8hr 15hr 35.87hr 40.27hr 2.75d
Element Cr51 Ce141 Hf181 Fe59 Hg203 Sc46 Ta182 Se75 Zn65 Ag110m Co60
Energy (keV) 320 145,4 482,2 1098,6 279,1 889,4 1121,2 264,6 1115,4 657,8 1332,5
T1/2 27.8d 32.5d 44.6d 45.1d 46.9d 83.9d 115.1d 121d 245d 253d 5.24yr
We can notice three levels for elemental concentrations which are depicted in Fig. 3-5; the high level for Na and Cl (2 ^g/m3 - 15 ^g/m3), the middle level (30-500 ng/m3) and the low level (0.3-10 ng/m3) indicating effective trace metal air pollution in the sampled area.
Among the polluting or toxic elements shown in Fig. 6, iron presents high content; lower levels are found for cupper, magnesium, manganese, bromine, zinc, vanadium, chromium, antimony, silver, cobalt, mercury and tantalum. This amount of concentration indicates relatively intense pollution in this area with high traffic and high population density.
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Рис. 4. Накопительная концентрация некоторых анализируемых элементов Fig. 4. Stack concentration of some analysing elements
Рис. 5. Накопительная концентрация некоторых анализируемых элементов Fig. 5. Stack concentration of some analysing elements
1000 •
10
0,1
" ...
I 1 I 1 I 1 I 1 I 1 I 1
Fe Cu Mg Mn Br Zn
I 1 I 1 I 1 I 1 I 1 I 1 I 1 I
V Cr Sb Ag Co Hg Ta Quantified elements
Рис. 6. Среднее значение загрязнителей и токсичных элементов, анализируемых в образцах воздушного фильтра Fig. 6. Median value of pollutants and toxic element analyzed in air filter samples
Table 2 shows a comparison of trace element levels between Algiers site and other sites like for example Hong Kong [8] and some urban sites in Spain [9]. One should mention that the conditions and times of samplings are not identical. We observe that the concentration levels are much higher for Spain urban sites than in Algiers or Hong Kong. The level of pollution seems to be relatively the same between Algiers and Hong Kong.
Таблица 2
Уровни следовых элементов (нг/м3) в городской зоне города Алжир,в *Гонконге, Китай [8] и в городских зонах *Испании [9]
Table 2
Trace element levels (ng/m3) in urban city of Algiers, in *Hong Kong China[8] and in *Spain urban sites [9]
Element Algiers Max level Algiers Mean level *Spain urban sites Max level *Hong Kong site Mean level
Hg 2 1.05 ± 0.7
Cd 15 1.61±1.86
Ni 143
As 23
Cr 17 8 ± 6 184 15.3 ± 11.8
V 53 32 ± 18 254 14.3 ± 16.4
Sb 4 2 ± 1.5 161
Cu 480 142 ± 168.5 1645 70.8 ± 88.2
Ag 2 2 ± 0.4
Co 2 1.35 ± 0.5 17
Mg 250 127 ± 76 546 ± 322
Mn 240 118 ± 69 338 48.3 ± 47.5
Br 130 75 ± 50.5
La 1 12
Ta 1 0.5 ± 0.2
Fe 4000 1858±1097 1480±2190
Zn 90 33 ± 40 10741 298±214
The trace elemental concentrations measured of some pollutants (Cu, V, Mg, and Mn) show a tendency of increasing. Further sampling is necessary in the same area in order to check the temporal variations.
We check if there is a correlation between (V, Mn) and (V, Cu). The correlation factors found are respectively R2 = 0.672, R2 = 0.577, which are fairly positive relationship between theses elements. Before to make a decision, we have to test the significance of the correlations found or if there is a probability that the
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observed correlation occurred by chance. In the case of our studies, we choose a two-tailed test and a significance level alpha = 0.05. Only R2 = 0.672 for (V, Mn) is a quite big higher than the critical value of correlation coefficient table. We can conclude that the correlation between manganese and vanadium is statistically significant.
We try also to apply PCFA (principal component factor analysis) which is a method of study of the proximities between variables with metric Euclidean (correlation) and differences between individuals (inertia) with another metric Euclidean [10].
One seeks for example to represent the proximities of the N individuals from X compared to P variables. The PCFA is divided into stages:
- Standardization of the data to be independent of the units of the P parameters.
- Calculation of a matrix of similarity C (very often the correlation).
- Research of the eingenvalue of C, which give the principal axes.
- Representation of the individuals in new space (by considering only eingenvalue explaining a sufficient cumulated variance).
PCFA technique leads to the calculation of the matrix R of the correlations between measured heavy metals. This matrix calculation starting from the following rule: R = DVD.
D is a diagonal matrix of the inverse of the standard deviation, and V the matrix of the variance covariance. The matrix R is calculated using software Matlab. On this matrix we notice strong positive values which mean the presence of a metal is dependent on the presence on another, for example (Ce, Br), ( Se, Br), (Hg, Cr), (Ag, Hg), (V, Ta), (V, Mn), (Ta, Hg) where r are found higher than 0.9. We also found negative values; it means that the growth of the content of a metal implies the reduction in the other and vice versa for example (Cu, Ce), (V, Co), (V, Sc), (Co, Mn).
Conclusion
The level of average charge filters varies from 40 ^g/m3 to 115 ^g/m3. These data assure us the presence of heavy metals in the air. It remains to measure the intensity of this pollution by more systematic controls in
a large scale area. The use of instrumental neutron activation analysis technique INAA in ESSALEM reactor enabled us to analyse more than twenty elements. This highlights the high performances of this technique of analysis in air pollution field studies. The analysis was carried out for three different irradiation times 5 minutes, 2 hours and 6 hours. The high sensitivity of the technique allowed us to reach levels of concentration around the ng/m3. This partly explains the great number of identified and measured elements. The most concentration levels found in this work are below the limit values for the specified elements. For the most elements identified and quantified we don't have a guide value in order to evaluate if the concentrations measured are in acceptable range or not. The most obvious way people are exposed to air toxics is by breathing air that contains toxic air pollutants. In order to enhance the monitoring of the air quality we need to analysis source profiles, potential source contribution and look for specific elemental ratios according the season.The results of this analysis do not claim to represent the average composition of the airborne dusts and pollutants in general but they were given to show the variety of the measurable elements and the differences in concentrations according to the nature of the elements.
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