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Natural radioactivity in phosphates, phosphogypsum and natural waters in Morocco
Journal of Environmental Radioactivity 54 (2001) 231–242
Natural radioactivity in phosphates, phosphogypsum and natural waters in Morocco M. Azouazia, Y. Ouahidia, S. Fakhia,*, Y. Andresb, J.Ch. Abbeb, M. Benmansourc a
Laboratoire de Radiochimie, Universite´ Hassan II-Mohammedia, Faculte´ des Sciences Ben M’Sik, De´partement de Physique, Sidi Othmane, Casablanca, BP 7955, Morocco b Laboratoire SUBATECH, UMR 6457, Ecole des Mines, BP 20722, 44307 Nantes Cedex 3, France c Centre National de l’Energie, des Sciences et des Techniques Nucle´aires, Rabat, Morocco Received 10 September 1999; received in revised form 1 July 2000; accepted 21 July 2000
Abstract The contents of natural radionuclides (uranium, actinium and thorium series) were measured in sedimentary phosphate rock samples using high-resolution gamma spectrometry. Data obtained for uranium content (ppm) were compared with the results obtained by a method based on the measurements using solid-state nuclear track detectors (SSNTD) in the same samples. The potential leaching of radionuclides from sedimentary phosphate rock during the industrial production of the phosphoric acid was studied. The process of leaching of the radioisotopes from phosphogypsum was discussed. A method for the direct alpha counting of 226Ra thin source, elaborated by the deposition of Ra from aqueous solutions on manganese oxides film deposited on polyvinyl support, have been developed and applied for the determination of 226Ra in natural water samples. The results show that only the water sample from the mine area reveals the presence of 226Ra at a level of about 0.2 Bq l ÿ 1. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Radioactivity; Phosphate rock; Phosphogypsum; Gamma-spectrometry; Alpha-spectrometry
*Corresponding author. Tel.: +212-2-704-672; fax: +212-2-704-675. E-mail address: [email protected] (S. Fakhi). 0265-931X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 5 - 9 3 1 X ( 0 0 ) 0 0 1 5 3 - 3
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M. Azouazi et al. / J. Environ. Radioactivity 54 (2001) 231–242
1. Introduction The earth contains numerous radioactive elements; their origin, for part of them, dates back to the formation of our world, while others are continuously produced through nuclear reactions in the universe. Among the former elements, the most abundant are potassium-40 and the radioisotopes of the natural series of uranium, actinium and thorium including the parent nuclei 235U, 238U and 232Th (Table 1) and the decay products from the successive alpha or beta decays. The most abundant of the cosmogenic origin nuclei are 14C, 10Be, 26Al. Uranium and its decay products are found in phosphate rocks of sedimentary origin (Roesseler, 1990). Now, these phosphates are largely used for the production of phosphoric acid and fertilizers. Their radioactivities result in health problems from radiation at the level of the industrial processes for the preparation of fertilizers as well as for the fertilizers themselves at the origin of radioactivity dispersion in the geo- and biospheres. On the other hand, leaching of the minerals and of the wastes is another source of dissemination and possible transfer to waters and finally to the living beings. The radiological impact of this radioactivity varies with the mineralogical nature of the phosphates (Table 2) (Scholten & Timmermans, 1996) and requires special care, especially in Morocco where there are important mines of phosphates and phosphogypsum. This country is contributing about two-thirds of the phosphates commercialised worldwide. Furthermore, the Atomic Energy Commission of the European Community (EURATOM) has recently edited new rules (Directive 96/29/ EURATOM, 1996) applicable in the European countries in 2003, which will severely
Table 1 Main radioelements in the earth’s crust with related half-lives Radionuclide
Half-life ( 109 years)
238
4.47 0.70 14.1 1.28
U U 232 Th 40 K 235
Table 2 Radionuclide activity concentrations in sedimentary phosphate rock of different origin Origin
Florida S.Carolina Morocco China
Activity (Bq kg ÿ 1) 238
U
232
226
1500 4800 1700 150
16 78 30 25
1600 4800 1700 150
Th
Ra
M. Azouazi et al. / J. Environ. Radioactivity 54 (2001) 231–242
233
restrict the activity levels for handling as well as for storage of ores and wastes which will require control measurements and, possibly, selective separations of these radionuclides (Scholten & Timmermans, 1996). Simultaneously, dedicated waste disposals will be required. It is within this context that the present study has been undertaken aimed both at the development of measuring techniques and the determination of the radioactivities of naturally occurring nuclides in various phosphate ores and in natural waters of different origin.
2. Experimental techniques Uranium can be measured using various physico-chemical techniques (Fakhi, Paulus & Bouhlassa, 1988a, b; Godinez, Iturbe, Ordonez & Solache-Rios, 1997) such as fluorimetry, colorimetry and ICP-MS. We have explored nuclear measuring techniques which give access to a significant number of nuclides in the natural radioactive series. 2.1. Radioactivity measurements Several techniques have been used and are listed below. 2.1.1. Gamma-spectrometry These measurements were performed using a HPGe detector (Canberra) with 40% efficiency and resolution of 1.8 keV at 60Co. This detector was carefully shielded with low background lead. The activities of a lot of elements can thus be determined with the restrictions that some elements do not have a significant gamma ray and also of possible interference. This is the case, in particular, for the line at 186 keV which can originate both from 235U (185.72 keV, intensity: 54%) and/or from 226Ra (186.21 keV, intensity: 3.2%). The uncertainty of the measurements was 10%. In the case of secular equilibrium, it is easy to resolve the ambiguity from knowledge of the respective decay constants; in other cases, it is difficult to conclude unambiguously that 235U is the only detectable nuclide through the line at 163.35 keV, but with a low intensity (4.7%), 226Ra having no other line. The spectra were processed by a data acquisition system1 and the IAEA GANAAS programs. Three types of samples have been measured: (1) Samples of natural phosphates, with different apatite concentrations, and phosphogypsum, originating from the Khouribga region as described in Table 3 (2) Solutions of phosphoric acid and phosphogypsum prepared from the phosphates: 500 g of raw phosphates K09 are added to the concentrated sulfuric acid. After filtering, the phosphogypsum is dried in an oven at 808C for about 20 1
Canberra Genie PC.
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Table 3 Identification of the different sedimentary phosphate rock samples Sample
Origin
Nature
Phosphogypsum K09 Phosphate C-HT Phosphate C-MT Phosphate C-BT
Khouribga Khouribga Khouribga Khouribga
Raw Layer C, high concentration in apatite Layer C, medium concentration in apatite Layer C, low concentration in apatite
days. The counting of radioactivity was performed on aliquots of the solid and of the solution. The measurements of radioactivity were aimed at understanding the radionuclide distribution between the phosphates and the derived products. (3) Leachates of phosphogypsum so as to obtain information on the stability of these derivatives which are especially important in the phosphate industry: 20 g of phosphogypsum are added to 50 ml distilled water; after pH adjustment with HCl, NaOH or NH4OH 0.1 M solutions, the flask is agitated for about 20 h. The solid and liquid phases are separated by filtering and both fractions are measured.
2.1.2. Determination of 226Ra in solution by alpha-spectrometry The technique used for liquids (natural waters or solutions from the acid digestion of solid samples) relies on the trapping of radium on manganese oxide films deposited on a polyvinyl support, followed by direct alpha counting. (a) Film preparation. The support is polyamide 6-6, as a disk of 1.5 cm in diameter and 1 mm thick, which is first cleaned in 0.1 N HCl solution and then dipped for 5 h in a 0.1 M KMnO4 solution at 708C. A black brown film of manganese oxide is fixed onto the disk; it is rinsed several times with distilled water. (b) Preparation of the solutions. To a 100 ml of the solution to be measured are added several drops of 133Ba, chemical congener of radium, as BaCl2, used as a tracer for evaluating the trapping yield. The pH of the solution is adjusted to about 8 with 0.1 M calcium carbonate solution. (c) Radium trapping and measurement. The disk is placed for about 36 h in the stirred solution. At the end of the experiment, the solution is measured by gammaspectrometry in order to determine the Ba trapping yield linked to that of radium. Radium itself is measured by alpha-spectrometry at 4.78 MeV (detector2 resolution: 80 kev, detector efficiency: 13%, counting time: 2 105 s, the uncertainty of the measurement: 20%). Fig. 1 shows as an example a typical spectrum obtained from 226 Ra solution (1 Bq l ÿ 1). The 226Ra contents of several kinds of water from Morocco were determined and the results are shown in Table 10.
2
Canberra.
M. Azouazi et al. / J. Environ. Radioactivity 54 (2001) 231–242
Fig. 1. Typical alpha spectrum from
235
Ra solution (1 Bq l ÿ 1, pH 8 and counting time 17 4000 s).
226
3. Results and discussions Fig. 2 shows a gamma-ray spectrum of phosphate C–HT. It clearly shows several lines which were calibrated in energy leading to the identification of the radionuclides using well documented nuclear data tables. The related peak areas are, among other parameters, proportional to the activities evaluated to within 1s. All the data obtained by gamma-spectrometry and related to the phosphate samples are presented in Tables 4a–c. The nuclides pertaining to each natural radioactive series have been classified independently. This technique does not allow the direct measurement of 238U; the related activity was derived from the decay products of 234Th; namely 234Pa and 226Ra. The activity associated with 234Th is somewhat lower than that associated with the other radionuclides, the underestimation being attributed to insufficient correction of the absorption coefficient for the low-energy gamma ray (93 kev) in the matrix. As a mean value the activity associated with 238U is around 1700 Bq kg ÿ 1; this is equivalent to 160 ppm, in agreement with the value reported by Nourredine et al. (1999). For 235U and its decay products, the activities are deduced as indicated in the measurement protocol; for 235U, the mean value is about 100 Bq kg ÿ 1. One observes that the ratio of the activities associated with 238U and 235U, about 22, is as expected from the natural isotopic abundance and the tabulated life-times of the nuclei. Comparing the various phosphate samples, shows no significant differences in the activities; the values obtained for uranium and thorium are in good agreement with those reported by other authors (Table 2) as well as those obtained independently using solid-state nuclear track detectors, SSNTD (Misdaq et al., 2000) (Table 5).
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Fig. 2. Gamma-ray spectrum of C-HT phosphate (Sample mass 98 g; Counting time 220 000 s).
Table 4 Activity concentrations of the radioelements (in Bq kg ÿ 1) found in the sedimentary phosphate rock samples Sample
235
227
223
211
211
100 10 130 30 110 20
150 30 90 6 95 6
92 6 90 10 96 10
110 10 } 90 30
110 10 } 90 30
235
234
226
214
214
1640 320 1750 340 1820 350
2100 160 2450 400 2150 400
2320 370 1850 650 2280 580
1730 50 1540 20 1700 30
2300 200 2100 100 2370 100
228
212
212
208
32 3 } }
30 2 23 2 21 2
14 10 } }
71 52 }
U
Th
Ra
Pb
Bi
235
(a) U series Phosphate C-MT Phosphate C-BT Phosphate C-HT Sample
Th
Pa
Ra
Pb
Bi
238
(b) U series Phosphate C-MT Phosphate C-BT Phosphate C-BT Sample (c) 232Th series Phosphate C-MT Phosphate C-BT Phosphate C-HT
Ac
Pb
Bi
Tl
The results for phosphogypsum and phosphoric acid samples are presented in Tables 6a–c. The most salient observation is that all the 226Ra and its decay products were concentrated in the phosphogypsum; the same for uranium at a level of 70%. This result agrees with those reported by Awadalla and Thabashi (1985). On the other hand, a relative enrichment in 211Pb and 211Bi towards 223Ra, as well as in 210 Pb towards 226Ra, was noted, as already quoted by Horton et al. (1988) and Becker (1989).
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M. Azouazi et al. / J. Environ. Radioactivity 54 (2001) 231–242 Table 5 Uranium content (in ppm) in different sedimentary phosphate rock samples of Morocco Sample
SSNTD method
Gamma-spectrometry
PO4MAR1 PO4MAR2 PO4MAR3 PO4MAR4 C-MT C-BT C-HT
159.32 186.82 78.87 156.57 } } }
176 9 182 9 77 9 156 12 152 17 180 35 164 20
Table 6 Sharing of the radioelements (in Bq kg ÿ 1) between phosphogypsum and phosphoric acid from the same phosphate rock Sample
235
227
223
211
211
60 10 13.5 4.1
60 10 }
60 10 }
120 40 }
120 40 }
234
234
226
214
214
700 140 215 43
} 430 110
1420 330 }
1100 30 }
820 20 }
228
212
212
208
} }
10 3 21
} }
42 }
U
Th
Ra
Pb
Bi
235
(a) U series Phosphogypsum K09 Phosphoric acid K09 Sample
Th
Pa
Ra
Pb
Bi
238
(b) U series Phosphogypsum K09 Phosphoric acid K09 Sample
Ac
Pb
Bi
Tl
232
(c) Th series Phosphogypsum K09 Phosphoric acid K09
The preliminary data related to the leaching (vide supra) of phosphogypsum by aqueous solutions at various pH values showed that calculating the leaching rate on the basis of a simple ratio of the activities of the solid residue and the original phosphogypsum may lead to non-significant values. In fact, a decrease in the mass of phosphogypsum was observed at the end of the chemical procedure, by about 60% as a mean, as quoted in the literature although significantly higher (42%, (Rutherford, Dudas & Arocena, 1995)). This can be attributed to the short delay, approximately 2 months, between preparation of the phosphogypsum and the leaching procedures (Paul & Pillai, 1990) resulting in an increase of the solubility probably related to some precipitation or coprecipitation with slow kinetics. To take this mass reduction into account in the overall balance of the radioactivity in the phosphogypsum/distilled water system, the decrease in radioactivity of the different nuclides in the solid fraction was calculated by the following equation: tð%Þ ¼
aPG ÿ aS m 100; v aPG
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Table 7 Leaching rates t (in %) of the radioelements in raw phosphogypsum as a function of the pH pH
Decrease of the phosphogypsum mass (in %)Decrease of the activity concentration (in %) 235
234
226
214
214
30.9 32.3 32.93 33.8 30.1 32.3 32.1
23.4 25.9 26.6 28.4 21.5 25.6 25.7
26.8 29.1 27.8 24.5 24.6 25.4 26.4
24.8 25.9 26.4 25.4 23.8 25.3 25.3
20.6 22.2 22.8 21.1 19.8 20.9 21.3
U
2.10 61.3 2.50 64.2 630 66.8 8.37 61.0 8.70 57.6 8.84 60.3 Average value60.8
Th
Ra
Pb
Bi
Fig. 3. Leaching rate t (in%) of the different radioelements as a function of pH.
where aPG and as are the activities in Bq of the radionuclide in phosphogypsum before and after leaching, respectively; v the volume of distilled water used for the leaching; m the initial mass of phosphogypsum (before leaching). The experimental results are presented in Table 7 and Fig. 3. These results show that the variations of the rates of leaching and of the relative mass of phosphogypsum, both expressed in %, as a function of pH are parallel. The radionuclides that were effectively leached seem to be associated with the elements adsorbed exclusively on the surface of the solid, independent of their chemical forms (Snodgrass, 1990), while those that were unleached must be absorbed within less soluble sites with structures of the CaSO4 type. This hypothesis is corroborated by
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Table 8 Leaching rate t (in %) of radioelements in calcinated and crushed phosphogypsum in various conditions Distilled water
Conditions
t (%) 234
226
214
214
10.5 6.3 6.2
6.0 3.7 3.4
3.2 2.0 0.4
3.7 2.7 0.7
Th
H2O
1 h at ambient temperature 1 h at boiling temperature 16 h at ambient temperature
Ra
Pb
Bi
Table 9 Leaching rate t (in %) of radioelements in calcinated and crushed phosphogypsum by acidified aqueous solutions (HCl or H3PO4) Aqueous solution
234
214
214
HCl (4 N) H3PO4 (22.1 N)
25.4 18.5
20.7 2.4
22.1 5.4
Table 10 Activity concentration of
Th
Pb
Bi
Ra (in Bq l ÿ 1) in various natural water samples
226
Sample
Specific activitie (Bq l ÿ 1)
Water from Casablanca general supply Mineral water: Sidi Ali Mineral water: Sidi Harazem Water from area close to a phosphate mine
5L.L.Da 5L.L.Da 5L.L.Da 0.20 0.03
a
LLD=0.025 Bq l ÿ 1.
the data reported by Snodgrass (Snodgrass, 1990) on the structure of phosphogypsum formed by acid (H2SO4) digestion of phosphates. In fact, phosphogypsum has a complex structure characterized by several active sites in which radioelements at the solid–liquid interface can precipitate or be adsorbed. The data obtained for the same phosphogypsum, calcinated at 8008C and crushed (Table 8), show that the leaching rate for 226Ra is lower than 6% while for 234Th it reaches 10% and about 3% for 214Pb and 214Bi, respectively. For the same phosphogypsum, calcinated and crushed before leaching by the acidified aqueous solutions (HCl or H3PO4), it appears that the radium is not leached while 234Th, 214Pb and 214Bi are only leached with solutions at normalities higher than 4 and 22, respectively (Table 9). By using the carbonated media and EDTA, only the 234Th is leached, with a rate of 10.3% for Na2CO3 and 10.5% for EDTA. With the aim of getting information on the geochemistry of the elements in phosphates and to the evaluate their distributions from phosphate ores, a water sample from an area close to the phosphate mine was analysed. As a complement,
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Table 11 Activity concentration of 0K (in Bq kg ÿ 1) in the phosphate and water samples Sample
Activity of
Phosphate C-MT Phosphate C-BT Phosphate C-HT Phosphogypsum K09 Phosphoric acid K09 Mineral water : Sidi Harazem Mineral water : Sidi Ali Water from Casablanca general supply Water from area close to a phosphate mine
} 100 30 } } 25 12 40 10 5 70 10 }
K (Bq kg ÿ 1)
40
mineral waters from different origins as well as water from the Casablanca general supply were measured. Only the water sample from the mine area reveals the presence of radium at a level of about 0.2 Bq l ÿ 1 (Table 10). The absence of any other radionuclide accounts for the specificity of the chemistry of radium. As such, the value obtained is below the safety value recommended for 226Ra in water, 1 Bq l ÿ 1 (Surbeck, 1995); it is lower than the values reported by Othman (Othman & Yassin, 1996; Sill, 1987) for drinking, geothermal and underground waters. On the other hand, the activity measured for 40K (Table 11) in the water samples is just over 70 Bq l ÿ 1 in fair agreement with previously reported values (Bayes, Gomez, Garcias, Casas & Cerda, 1996); the level of 100 Bq kg ÿ 1 in one of the phosphate samples, CBT, is close to that observed in mineral grounds and rocks (Bygaart & Portz, 1999; Alam et al., 1999).
4. Conclusion In the general framework of the protection of the environment and the use of nuclear techniques in evaluation of the impact of radioactive and non-radioactive wastes from source to humans, the possible rejection of phosphates, from the mines to the elaboration of fertilizers and to the waste storage has been studied. The results show that the uranium concentration is below 1700 Bq kg ÿ 1, that is, 160 ppm, which concentrates in the phosphogypsum during the production of phosphoric acid. Phosphogypsum and some fertilizers may cause radioactivity dispersion in the environment. The value of 0.2 Bq l ÿ 1 for 226Ra, one of the most radiotoxic decay products of uranium, observed in the underground water alone, confirms the particular solubilization of this element. Although the value observed is significantly lower than that in the underground and geothermal waters in other countries, this radioelement deserves special attention especially related to the storage of huge amounts of phosphogypsum and fertilizers before commercialisation. Ventilation of the industrial area so as to avoid radon accumulation should be carefully considered.
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The control of the industrial processes, especially of the physico-chemical forms of the radioelements, and knowledge of their dispersion mechanisms in the ecosystem, should allow a better prospect for the related pollution and also for possible improvement in the manufacture of the products. These aspects are presently being investigated.
Acknowledgements This work has been conducted within the framework of an International Cooperation network sponsored by the CNRS (Institut National de Physique Nucle´aire et de Physique des particules, IN2P3) and the Moroccan CNCPRST (Centre National de Coordination et de Planification de la Recherche Scientifique et Technique).
References Alam, M. N., Miah, M. M., Chowdhury, M. I., Kamal, M., Ghse, S., Mustapha, M. N., & Miah, M. S. R. (1999). Radiation dose estimation from the radioactivity and analysis of line and cement used in Bangladesh. Journal of Environmental Radioactivity, 42, 77–85. Awadalla, F., & Thabashi, F. (1985). The removal of radium during the production of nitrophosphate fertiliser. Radiochimica Acta, 38, 207–210. Bygaart, V., & Portz, A. J. (1999). Gamma radioactivity in podzolic soils of Northen Ontario, Canada. Journal of Environmental Radioactivity, 42, 51–64. Bayes, J. C., Gomez, E., Garcias, F., Casas, M., & Cerda, V. (1996). Radium determination in Mineral waters. Applications in Radiation Isotopy, 47, 849–853. Becker, P. (1989). Phosphates and phosphoric acid : raw materials, technology and economics of the wet process. Fert. Sci. Technol. Ser., vol. 6 (2nd ed.), (p. 752). New York: Marcel Dekker. Directive 96/ 29:EURATOM 13 MAI 1996. Fakhi, S., Paulus, J. M., & Bouhlassa, S. (1988a). Neutron activation analysis of uranium via 239Np. Journal of Radioanalytical Nuclear Chemistry, 121, 99–107. Fakhi, S., Paulus, J. M., & Bouhlassa, S. (1988b). Neutron activation analysis of thorium via 233Pa. J. Radioanal. Nucl. Chem, 21, 109–114. Godinez, M. C., Iturbe, J. L., Ordonez, G. E., & Solache-Rios, R. M. (1997). Detrmination of radium – 226 in phosphate fertilizers and gypsum by Gamma ray spectrometry. International Journal of Environment and Pollution, 8, 195–200. Horton, T. R., Blanchard, R. L., & Wondham, S. T. (1988). Along-term Study of radon and air borne particles at phophogypsum stocks in Cenral Florida. USEPA Report 520/5-88-021. Eastern Enviro. Radiation Facility, Montgomery, USA. Misdaq, M. A., Ouahidi, Y., Fakhi, S., Lamine, J., Bakhchi, A., Andres, Y., Azouazi, M., & Abbe, J. Ch. (2000). The influence of the calcination and water washing treatments on the uranium content in sedimentary phosphate samples using solide state nuclear track detectors and gamma rays spectrometry. Journal of Radioanalytical Nuclear Chemical, 246, 2. Nourredine, A., Azkour, K., Benjelloun, M., Boukhair, A., Fahad, M., & Pape, A. (1999). Monte Carlo detection probabilities for SSNTD. Application to uranium and thorium analysis in Moroccan phosphates. Journal of Environmental Radioactivity, 42, 101–107. Othman, I., & Yassin, T. (1996). Natural radioactivity of drinking water in the southern and middle parts of Syria. Environment International, 22, S355–S359.
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Paul, A. C., & Pillai, K. C. (1990). Leachability of radium fertiliser and monazite process wastes. The environmental behaviour of radium, vol. 2, Technical Reports Series No. 310, (pp. 83–95). Vienna, Austria: IAEA. Roesseler, C.E. (1990). Control of radium in phosphate mining, benefication and chemical processing. The environmental behaviour of radium, vol. 2, Technical reports Series No. 310, (p. 269). Vienna, Austria: IAEA. Rutherford, P. M., Dudas, M. J., & Arocena, J. M. (1995). Radium in phosphogypsum leachates. Journal of Environmental Quality, 24(2), 307–314. Scholten, L. C., & Timmermans, C. W. M. (1996). Natural radioactivity in phosphate fertilizers. Fertilizer Research. Research, 43, 103–107. Sill, C. W. (1987). Determination of radium-226 in ores, Nuclear waste and environmental samples by high resolution alpha spectrometry. Nuclear Chemical and Waste Management, 7, 239–256. Snodgrass, W. J. (1990). The chemistry of 226Ra in the uranium milling process. The Environmental Behaviour of Radium, vol. 2, Technical Report Series No. 310, (pp. 5–26). Vienna, Austria: IAEA. Surbeck, H. (1995). Determination of natural radionuclides in drinking water, Tentative protocol. The Science of the Total Environment, 173/174, 91–99.
Natural radioactivity in phosphates, phosphogypsum and natural waters in Morocco M. Azouazia, Y. Ouahidia, S. Fakhia,*, Y. Andresb, J.Ch. Abbeb, M. Benmansourc a
Laboratoire de Radiochimie, Universite´ Hassan II-Mohammedia, Faculte´ des Sciences Ben M’Sik, De´partement de Physique, Sidi Othmane, Casablanca, BP 7955, Morocco b Laboratoire SUBATECH, UMR 6457, Ecole des Mines, BP 20722, 44307 Nantes Cedex 3, France c Centre National de l’Energie, des Sciences et des Techniques Nucle´aires, Rabat, Morocco Received 10 September 1999; received in revised form 1 July 2000; accepted 21 July 2000
Abstract The contents of natural radionuclides (uranium, actinium and thorium series) were measured in sedimentary phosphate rock samples using high-resolution gamma spectrometry. Data obtained for uranium content (ppm) were compared with the results obtained by a method based on the measurements using solid-state nuclear track detectors (SSNTD) in the same samples. The potential leaching of radionuclides from sedimentary phosphate rock during the industrial production of the phosphoric acid was studied. The process of leaching of the radioisotopes from phosphogypsum was discussed. A method for the direct alpha counting of 226Ra thin source, elaborated by the deposition of Ra from aqueous solutions on manganese oxides film deposited on polyvinyl support, have been developed and applied for the determination of 226Ra in natural water samples. The results show that only the water sample from the mine area reveals the presence of 226Ra at a level of about 0.2 Bq l ÿ 1. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Radioactivity; Phosphate rock; Phosphogypsum; Gamma-spectrometry; Alpha-spectrometry
*Corresponding author. Tel.: +212-2-704-672; fax: +212-2-704-675. E-mail address: [email protected] (S. Fakhi). 0265-931X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 5 - 9 3 1 X ( 0 0 ) 0 0 1 5 3 - 3
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1. Introduction The earth contains numerous radioactive elements; their origin, for part of them, dates back to the formation of our world, while others are continuously produced through nuclear reactions in the universe. Among the former elements, the most abundant are potassium-40 and the radioisotopes of the natural series of uranium, actinium and thorium including the parent nuclei 235U, 238U and 232Th (Table 1) and the decay products from the successive alpha or beta decays. The most abundant of the cosmogenic origin nuclei are 14C, 10Be, 26Al. Uranium and its decay products are found in phosphate rocks of sedimentary origin (Roesseler, 1990). Now, these phosphates are largely used for the production of phosphoric acid and fertilizers. Their radioactivities result in health problems from radiation at the level of the industrial processes for the preparation of fertilizers as well as for the fertilizers themselves at the origin of radioactivity dispersion in the geo- and biospheres. On the other hand, leaching of the minerals and of the wastes is another source of dissemination and possible transfer to waters and finally to the living beings. The radiological impact of this radioactivity varies with the mineralogical nature of the phosphates (Table 2) (Scholten & Timmermans, 1996) and requires special care, especially in Morocco where there are important mines of phosphates and phosphogypsum. This country is contributing about two-thirds of the phosphates commercialised worldwide. Furthermore, the Atomic Energy Commission of the European Community (EURATOM) has recently edited new rules (Directive 96/29/ EURATOM, 1996) applicable in the European countries in 2003, which will severely
Table 1 Main radioelements in the earth’s crust with related half-lives Radionuclide
Half-life ( 109 years)
238
4.47 0.70 14.1 1.28
U U 232 Th 40 K 235
Table 2 Radionuclide activity concentrations in sedimentary phosphate rock of different origin Origin
Florida S.Carolina Morocco China
Activity (Bq kg ÿ 1) 238
U
232
226
1500 4800 1700 150
16 78 30 25
1600 4800 1700 150
Th
Ra
M. Azouazi et al. / J. Environ. Radioactivity 54 (2001) 231–242
233
restrict the activity levels for handling as well as for storage of ores and wastes which will require control measurements and, possibly, selective separations of these radionuclides (Scholten & Timmermans, 1996). Simultaneously, dedicated waste disposals will be required. It is within this context that the present study has been undertaken aimed both at the development of measuring techniques and the determination of the radioactivities of naturally occurring nuclides in various phosphate ores and in natural waters of different origin.
2. Experimental techniques Uranium can be measured using various physico-chemical techniques (Fakhi, Paulus & Bouhlassa, 1988a, b; Godinez, Iturbe, Ordonez & Solache-Rios, 1997) such as fluorimetry, colorimetry and ICP-MS. We have explored nuclear measuring techniques which give access to a significant number of nuclides in the natural radioactive series. 2.1. Radioactivity measurements Several techniques have been used and are listed below. 2.1.1. Gamma-spectrometry These measurements were performed using a HPGe detector (Canberra) with 40% efficiency and resolution of 1.8 keV at 60Co. This detector was carefully shielded with low background lead. The activities of a lot of elements can thus be determined with the restrictions that some elements do not have a significant gamma ray and also of possible interference. This is the case, in particular, for the line at 186 keV which can originate both from 235U (185.72 keV, intensity: 54%) and/or from 226Ra (186.21 keV, intensity: 3.2%). The uncertainty of the measurements was 10%. In the case of secular equilibrium, it is easy to resolve the ambiguity from knowledge of the respective decay constants; in other cases, it is difficult to conclude unambiguously that 235U is the only detectable nuclide through the line at 163.35 keV, but with a low intensity (4.7%), 226Ra having no other line. The spectra were processed by a data acquisition system1 and the IAEA GANAAS programs. Three types of samples have been measured: (1) Samples of natural phosphates, with different apatite concentrations, and phosphogypsum, originating from the Khouribga region as described in Table 3 (2) Solutions of phosphoric acid and phosphogypsum prepared from the phosphates: 500 g of raw phosphates K09 are added to the concentrated sulfuric acid. After filtering, the phosphogypsum is dried in an oven at 808C for about 20 1
Canberra Genie PC.
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Table 3 Identification of the different sedimentary phosphate rock samples Sample
Origin
Nature
Phosphogypsum K09 Phosphate C-HT Phosphate C-MT Phosphate C-BT
Khouribga Khouribga Khouribga Khouribga
Raw Layer C, high concentration in apatite Layer C, medium concentration in apatite Layer C, low concentration in apatite
days. The counting of radioactivity was performed on aliquots of the solid and of the solution. The measurements of radioactivity were aimed at understanding the radionuclide distribution between the phosphates and the derived products. (3) Leachates of phosphogypsum so as to obtain information on the stability of these derivatives which are especially important in the phosphate industry: 20 g of phosphogypsum are added to 50 ml distilled water; after pH adjustment with HCl, NaOH or NH4OH 0.1 M solutions, the flask is agitated for about 20 h. The solid and liquid phases are separated by filtering and both fractions are measured.
2.1.2. Determination of 226Ra in solution by alpha-spectrometry The technique used for liquids (natural waters or solutions from the acid digestion of solid samples) relies on the trapping of radium on manganese oxide films deposited on a polyvinyl support, followed by direct alpha counting. (a) Film preparation. The support is polyamide 6-6, as a disk of 1.5 cm in diameter and 1 mm thick, which is first cleaned in 0.1 N HCl solution and then dipped for 5 h in a 0.1 M KMnO4 solution at 708C. A black brown film of manganese oxide is fixed onto the disk; it is rinsed several times with distilled water. (b) Preparation of the solutions. To a 100 ml of the solution to be measured are added several drops of 133Ba, chemical congener of radium, as BaCl2, used as a tracer for evaluating the trapping yield. The pH of the solution is adjusted to about 8 with 0.1 M calcium carbonate solution. (c) Radium trapping and measurement. The disk is placed for about 36 h in the stirred solution. At the end of the experiment, the solution is measured by gammaspectrometry in order to determine the Ba trapping yield linked to that of radium. Radium itself is measured by alpha-spectrometry at 4.78 MeV (detector2 resolution: 80 kev, detector efficiency: 13%, counting time: 2 105 s, the uncertainty of the measurement: 20%). Fig. 1 shows as an example a typical spectrum obtained from 226 Ra solution (1 Bq l ÿ 1). The 226Ra contents of several kinds of water from Morocco were determined and the results are shown in Table 10.
2
Canberra.
M. Azouazi et al. / J. Environ. Radioactivity 54 (2001) 231–242
Fig. 1. Typical alpha spectrum from
235
Ra solution (1 Bq l ÿ 1, pH 8 and counting time 17 4000 s).
226
3. Results and discussions Fig. 2 shows a gamma-ray spectrum of phosphate C–HT. It clearly shows several lines which were calibrated in energy leading to the identification of the radionuclides using well documented nuclear data tables. The related peak areas are, among other parameters, proportional to the activities evaluated to within 1s. All the data obtained by gamma-spectrometry and related to the phosphate samples are presented in Tables 4a–c. The nuclides pertaining to each natural radioactive series have been classified independently. This technique does not allow the direct measurement of 238U; the related activity was derived from the decay products of 234Th; namely 234Pa and 226Ra. The activity associated with 234Th is somewhat lower than that associated with the other radionuclides, the underestimation being attributed to insufficient correction of the absorption coefficient for the low-energy gamma ray (93 kev) in the matrix. As a mean value the activity associated with 238U is around 1700 Bq kg ÿ 1; this is equivalent to 160 ppm, in agreement with the value reported by Nourredine et al. (1999). For 235U and its decay products, the activities are deduced as indicated in the measurement protocol; for 235U, the mean value is about 100 Bq kg ÿ 1. One observes that the ratio of the activities associated with 238U and 235U, about 22, is as expected from the natural isotopic abundance and the tabulated life-times of the nuclei. Comparing the various phosphate samples, shows no significant differences in the activities; the values obtained for uranium and thorium are in good agreement with those reported by other authors (Table 2) as well as those obtained independently using solid-state nuclear track detectors, SSNTD (Misdaq et al., 2000) (Table 5).
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Fig. 2. Gamma-ray spectrum of C-HT phosphate (Sample mass 98 g; Counting time 220 000 s).
Table 4 Activity concentrations of the radioelements (in Bq kg ÿ 1) found in the sedimentary phosphate rock samples Sample
235
227
223
211
211
100 10 130 30 110 20
150 30 90 6 95 6
92 6 90 10 96 10
110 10 } 90 30
110 10 } 90 30
235
234
226
214
214
1640 320 1750 340 1820 350
2100 160 2450 400 2150 400
2320 370 1850 650 2280 580
1730 50 1540 20 1700 30
2300 200 2100 100 2370 100
228
212
212
208
32 3 } }
30 2 23 2 21 2
14 10 } }
71 52 }
U
Th
Ra
Pb
Bi
235
(a) U series Phosphate C-MT Phosphate C-BT Phosphate C-HT Sample
Th
Pa
Ra
Pb
Bi
238
(b) U series Phosphate C-MT Phosphate C-BT Phosphate C-BT Sample (c) 232Th series Phosphate C-MT Phosphate C-BT Phosphate C-HT
Ac
Pb
Bi
Tl
The results for phosphogypsum and phosphoric acid samples are presented in Tables 6a–c. The most salient observation is that all the 226Ra and its decay products were concentrated in the phosphogypsum; the same for uranium at a level of 70%. This result agrees with those reported by Awadalla and Thabashi (1985). On the other hand, a relative enrichment in 211Pb and 211Bi towards 223Ra, as well as in 210 Pb towards 226Ra, was noted, as already quoted by Horton et al. (1988) and Becker (1989).
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M. Azouazi et al. / J. Environ. Radioactivity 54 (2001) 231–242 Table 5 Uranium content (in ppm) in different sedimentary phosphate rock samples of Morocco Sample
SSNTD method
Gamma-spectrometry
PO4MAR1 PO4MAR2 PO4MAR3 PO4MAR4 C-MT C-BT C-HT
159.32 186.82 78.87 156.57 } } }
176 9 182 9 77 9 156 12 152 17 180 35 164 20
Table 6 Sharing of the radioelements (in Bq kg ÿ 1) between phosphogypsum and phosphoric acid from the same phosphate rock Sample
235
227
223
211
211
60 10 13.5 4.1
60 10 }
60 10 }
120 40 }
120 40 }
234
234
226
214
214
700 140 215 43
} 430 110
1420 330 }
1100 30 }
820 20 }
228
212
212
208
} }
10 3 21
} }
42 }
U
Th
Ra
Pb
Bi
235
(a) U series Phosphogypsum K09 Phosphoric acid K09 Sample
Th
Pa
Ra
Pb
Bi
238
(b) U series Phosphogypsum K09 Phosphoric acid K09 Sample
Ac
Pb
Bi
Tl
232
(c) Th series Phosphogypsum K09 Phosphoric acid K09
The preliminary data related to the leaching (vide supra) of phosphogypsum by aqueous solutions at various pH values showed that calculating the leaching rate on the basis of a simple ratio of the activities of the solid residue and the original phosphogypsum may lead to non-significant values. In fact, a decrease in the mass of phosphogypsum was observed at the end of the chemical procedure, by about 60% as a mean, as quoted in the literature although significantly higher (42%, (Rutherford, Dudas & Arocena, 1995)). This can be attributed to the short delay, approximately 2 months, between preparation of the phosphogypsum and the leaching procedures (Paul & Pillai, 1990) resulting in an increase of the solubility probably related to some precipitation or coprecipitation with slow kinetics. To take this mass reduction into account in the overall balance of the radioactivity in the phosphogypsum/distilled water system, the decrease in radioactivity of the different nuclides in the solid fraction was calculated by the following equation: tð%Þ ¼
aPG ÿ aS m 100; v aPG
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Table 7 Leaching rates t (in %) of the radioelements in raw phosphogypsum as a function of the pH pH
Decrease of the phosphogypsum mass (in %)Decrease of the activity concentration (in %) 235
234
226
214
214
30.9 32.3 32.93 33.8 30.1 32.3 32.1
23.4 25.9 26.6 28.4 21.5 25.6 25.7
26.8 29.1 27.8 24.5 24.6 25.4 26.4
24.8 25.9 26.4 25.4 23.8 25.3 25.3
20.6 22.2 22.8 21.1 19.8 20.9 21.3
U
2.10 61.3 2.50 64.2 630 66.8 8.37 61.0 8.70 57.6 8.84 60.3 Average value60.8
Th
Ra
Pb
Bi
Fig. 3. Leaching rate t (in%) of the different radioelements as a function of pH.
where aPG and as are the activities in Bq of the radionuclide in phosphogypsum before and after leaching, respectively; v the volume of distilled water used for the leaching; m the initial mass of phosphogypsum (before leaching). The experimental results are presented in Table 7 and Fig. 3. These results show that the variations of the rates of leaching and of the relative mass of phosphogypsum, both expressed in %, as a function of pH are parallel. The radionuclides that were effectively leached seem to be associated with the elements adsorbed exclusively on the surface of the solid, independent of their chemical forms (Snodgrass, 1990), while those that were unleached must be absorbed within less soluble sites with structures of the CaSO4 type. This hypothesis is corroborated by
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Table 8 Leaching rate t (in %) of radioelements in calcinated and crushed phosphogypsum in various conditions Distilled water
Conditions
t (%) 234
226
214
214
10.5 6.3 6.2
6.0 3.7 3.4
3.2 2.0 0.4
3.7 2.7 0.7
Th
H2O
1 h at ambient temperature 1 h at boiling temperature 16 h at ambient temperature
Ra
Pb
Bi
Table 9 Leaching rate t (in %) of radioelements in calcinated and crushed phosphogypsum by acidified aqueous solutions (HCl or H3PO4) Aqueous solution
234
214
214
HCl (4 N) H3PO4 (22.1 N)
25.4 18.5
20.7 2.4
22.1 5.4
Table 10 Activity concentration of
Th
Pb
Bi
Ra (in Bq l ÿ 1) in various natural water samples
226
Sample
Specific activitie (Bq l ÿ 1)
Water from Casablanca general supply Mineral water: Sidi Ali Mineral water: Sidi Harazem Water from area close to a phosphate mine
5L.L.Da 5L.L.Da 5L.L.Da 0.20 0.03
a
LLD=0.025 Bq l ÿ 1.
the data reported by Snodgrass (Snodgrass, 1990) on the structure of phosphogypsum formed by acid (H2SO4) digestion of phosphates. In fact, phosphogypsum has a complex structure characterized by several active sites in which radioelements at the solid–liquid interface can precipitate or be adsorbed. The data obtained for the same phosphogypsum, calcinated at 8008C and crushed (Table 8), show that the leaching rate for 226Ra is lower than 6% while for 234Th it reaches 10% and about 3% for 214Pb and 214Bi, respectively. For the same phosphogypsum, calcinated and crushed before leaching by the acidified aqueous solutions (HCl or H3PO4), it appears that the radium is not leached while 234Th, 214Pb and 214Bi are only leached with solutions at normalities higher than 4 and 22, respectively (Table 9). By using the carbonated media and EDTA, only the 234Th is leached, with a rate of 10.3% for Na2CO3 and 10.5% for EDTA. With the aim of getting information on the geochemistry of the elements in phosphates and to the evaluate their distributions from phosphate ores, a water sample from an area close to the phosphate mine was analysed. As a complement,
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Table 11 Activity concentration of 0K (in Bq kg ÿ 1) in the phosphate and water samples Sample
Activity of
Phosphate C-MT Phosphate C-BT Phosphate C-HT Phosphogypsum K09 Phosphoric acid K09 Mineral water : Sidi Harazem Mineral water : Sidi Ali Water from Casablanca general supply Water from area close to a phosphate mine
} 100 30 } } 25 12 40 10 5 70 10 }
K (Bq kg ÿ 1)
40
mineral waters from different origins as well as water from the Casablanca general supply were measured. Only the water sample from the mine area reveals the presence of radium at a level of about 0.2 Bq l ÿ 1 (Table 10). The absence of any other radionuclide accounts for the specificity of the chemistry of radium. As such, the value obtained is below the safety value recommended for 226Ra in water, 1 Bq l ÿ 1 (Surbeck, 1995); it is lower than the values reported by Othman (Othman & Yassin, 1996; Sill, 1987) for drinking, geothermal and underground waters. On the other hand, the activity measured for 40K (Table 11) in the water samples is just over 70 Bq l ÿ 1 in fair agreement with previously reported values (Bayes, Gomez, Garcias, Casas & Cerda, 1996); the level of 100 Bq kg ÿ 1 in one of the phosphate samples, CBT, is close to that observed in mineral grounds and rocks (Bygaart & Portz, 1999; Alam et al., 1999).
4. Conclusion In the general framework of the protection of the environment and the use of nuclear techniques in evaluation of the impact of radioactive and non-radioactive wastes from source to humans, the possible rejection of phosphates, from the mines to the elaboration of fertilizers and to the waste storage has been studied. The results show that the uranium concentration is below 1700 Bq kg ÿ 1, that is, 160 ppm, which concentrates in the phosphogypsum during the production of phosphoric acid. Phosphogypsum and some fertilizers may cause radioactivity dispersion in the environment. The value of 0.2 Bq l ÿ 1 for 226Ra, one of the most radiotoxic decay products of uranium, observed in the underground water alone, confirms the particular solubilization of this element. Although the value observed is significantly lower than that in the underground and geothermal waters in other countries, this radioelement deserves special attention especially related to the storage of huge amounts of phosphogypsum and fertilizers before commercialisation. Ventilation of the industrial area so as to avoid radon accumulation should be carefully considered.
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The control of the industrial processes, especially of the physico-chemical forms of the radioelements, and knowledge of their dispersion mechanisms in the ecosystem, should allow a better prospect for the related pollution and also for possible improvement in the manufacture of the products. These aspects are presently being investigated.
Acknowledgements This work has been conducted within the framework of an International Cooperation network sponsored by the CNRS (Institut National de Physique Nucle´aire et de Physique des particules, IN2P3) and the Moroccan CNCPRST (Centre National de Coordination et de Planification de la Recherche Scientifique et Technique).
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