Fluxes of the 238U-series isotopes in the industrial production of dicalcium phosphate and the radiological impact due to the incorporation to poultry diets

Fluxes of the 238U-Series Isotopes in the Industrial Production of Dicalcium Phosphate and the Radiological Impact due to the Incorporation to Poultry Diets N. Casacuberta1, P. Masqué1, J. Garcia-Orellana1, J.M. Bruach1, M. Anguita2, J. Gasa2, M. Villa3, S. Hurtado3, G. Manjón3 & R. Garcia-Tenorio3 1 Institut de Ciència i Tecnologia Ambientals – Departament de Física. Universitat Autònoma de Barcelona. 08193 Bellaterra. Spain. Nuria.C[email protected]at. 2 Departament de Ciència Animal i dels Aliments. Universitat Autònoma de Barcelona. 08193 Bellaterra. Spain. 3 Servicio de Radioisótopos. CITIUS. Universidad de Sevilla. Avda. Reina Mercedes s/n. 41012 Sevilla. Spain. INTRODUCTION Phosphate rock is used as a source for the production of feed supplements, such as in the form of dicalcium phosphate (DCP), which is a supply of calcium and phosphorus for domestic animals. Due to the replacement of calcium by uranium in the apatite structure during the formation of sedimentary phosphate rocks, elevated quantities of 238U and its decay chain daughters (>10 Bq·kg-1) are found in this geological formation (Guilbert and Park, 1986). This is the case of Morocco and Florida ores, both being used as raw materials in the phosphate industry (UNSCEAR, 1988). Phosphate rock is normally wet-acid digested with either sulphuric acid (H2SO4) or hydrochloric acid (HCl). The former one is by far the most widespread and leads to phosphate acid as the main product and phosphogypsum as a byproduct; whereas the later is used to directly obtain dicalcium phosphate as a final product (Gäfvert, 2002). It is well known that the various members of the 238U decay chain series, including 238U, 230Th, 226Ra, 210Pb and 210Po, are in approximate radioactive equilibrium in phosphate rock and no disruption occurs but if chemical partitioning (Menzel, 1968; Roessler et al., 1979; Metzger et al., 1980). However, during the wet acid process, the radioactive equilibrium between the Uranium-series radionuclides is broken and may be partitioned into various phases according to their solubility, thus as a function of other variables (i.e pH, temperature, red-ox conditions). Therefore, potential accumulation of some of them in the final product, by-products and/or wastes is to be expected, largely depending on the processes of industrial production. The presence of Naturally Occurring Radioactive Materials (NORM) in the products and by-products of the phosphate industry may somehow increase the potential for exposure to population through either the ingestion of chicken previously fed with food containing some dicalcium phosphate as an additive or the exposure to wastes highly enhanced with 238U decay chain radionuclides. Of major concern are the radionuclides 210Pb and 210Po, since they would contribute the most in radiological dose due to ingestion. At present time, implementation of regulation at national level is not yet in place. Therefore, we carried out a comprehensive study aiming to evaluate: i) the concentration of radionuclides of the 238U -series in DCP samples in Spain; ii) the fate of the radionuclides during the production process via wet acid digestion; and iii) the accumulation of radiologically relevant isotopes in chicken fed with certain quantities of DCP and the potential radiological impact to humans by ingestion. MATERIALS AND METHODS Samples Analysed material consisted on DCP samples and industrial products and by-products, as well as chicken tissues and excrements. Dicalcium phosphate 16 DCP samples for both animal and human consumption commercialized in Catalonia during the first semester of 2006 were analysed. Gathering of the samples was carried out by the Agència Catalana de Seguretat Alimentària. DCP production process 22 samples in the DCP production process where collected and analyzed. Samples include the raw material, the solid and liquid wastes produced during the treatment of the rock and the middle-step products. Chicken tissues 42 chickens gathered in three groups were fed with three different diets. Diet A included a 2.5% of DCP for human consumption (<10 Bq·kg-1 of 210Pb and 210Po); diets B and C included DCP for animal consumption (with ~2·103 Bq·kg-1 of 210Pb and 210Po) at 5% and 2.5% respectively. After 21 days chickens were slaughtered and representative samples of breast, thigh, kidneys, liver and bones were collected and ice-dried. Excrements were also periodically collected to evaluate the radionuclide output fluxes. Sample quantification Alpha and gamma spectrometry techniques were used to quantify non-organic samples. For organic samples, only 210Pb and 210Po were analyzed. Uranium and thorium isotopes and 210Pb and 210Po were quantified through alpha spectrometry after radiochemical purification (Martinez-Aguirre et al., 1997; Sanchez-Cabeza et al., 1998). For the pair 210Pb-210Po, polonium is the isotope that is actually measured, and concentrations of 210Pb at sampling time are calculated by using appropriate ingrowth and decay corrections. Concentrations of 226Ra and 40K were determined by using a coaxial HPGe detector (GMX. EG&G Ortec) equipped with an iron, copper and lead shielding. For radionuclide quantification in DCP samples, between 500 and 1000 g of dry homogenized sample were placed into a 500 cm3 Marinelli geometry. The rest of inorganic samples were placed in a 100 cm3 geometry. Geometries were sealed and stored for three weeks before its quantification to reach secular equilibrium between 226Ra and its short-lived daughters. Data analysis A hierarchical cluster analysis of the data was carried out using SPSS 12.0 for Windows. This procedure allows the identification of relatively homogeneous groups of cases (or variables) based on selected characteristics, using an algorithm that starts with each case (or variable) in a separate cluster. Dendrograms are used to assess the cohesiveness of the clusters formed and provide information about the appropriate number of clusters. RESULTS AND CONCLUSIONS Dicalcium phosphate A hierarchical cluster analysis (Figure 1) of the 238U and its decay chain daughter’s concentrations resulted in three different groups of DCP samples, named as group A, B and C. Variables considered for the sample clustering are specific concentrations of 238U (234Th), 226Ra, 210Pb and 210Po. Figure 1: Hierarchical cluster analysis of DCP samples for human and animal consumption. Variables considered are specific concentrations of 238U (234Th), 226Ra, 210Pb and 210Po. Group A (samples #1, #2, #3, #4, #9 and #10) corresponded to samples with high concentrations of 238U (~1000 Bq·kg-1), 226Ra (~100 Bq·kg-1), 210Pb (2000 Bq·kg-1) and 210Po (800 Bq·kg-1). Group B (samples #5, #6, #7, #8, #11 and #12) was characterized by high concentrations of 238U (1000 Bq·kg-1) and 230Th (~1000 Bq·kg-1) but rather low values of 210Pb and 210Po (< 30 Bq·kg-1). Finally, group C (samples #13, #14, #15 and #16) had very low values of all 238U decay chain daughters (<50 Bq·kg-1). Indeed, samples of Group C consisted of MCP for human consumption. Regarding group A and B, in all cases were DCP samples for animal consumption, and differences between both groups rely on the chemical partitioning of 238U elements due to different chemical behaviours when the phosphate rock is digested with hydrochloric acid or sulphuric acid. Whereas uranium radioisotopes are generally soluble (90-95%) in any acid, thorium, radium, lead and polonium might behave differently. Thorium is soluble both in hydrochloric acid and sulphuric acid but its high affinity for particulate and colloid surfaces leads to a thorium reduction in the DCP when rock is digested via hydrochloric acid. During this process thorium complexes with fluoride ions forming ThF4 and eventually co-precipitate with the CaF2 in the slag (Gäfvert et al., 2001). On the contrary, when using sulphuric acid, thorium might remain in the phosphoric acid and is finally precipitated to the final product, DCP (Saueia and Mazzilli, 2006). Radium might be less present in samples processed via sulphuric acid since it precipitates together with the phosphogypsum and is eliminated in a proportion of 90 to 100% (Rutherford et al., 1994). In samples processed via hydrochloric acid, 226Ra might precipitate in the sludge with one of the reaction products (CaF2) in the form of RaF2, whereas another fraction remains in solution in the form of RaCl2 (Gäfvert et al., 2001). However, still a little fraction of 226Ra might be present in DCP. Lead dissolves in concentrated HCl, forming a PbCl2-4 complex precipitating with DCP, but it precipitates in a 90-100% together with phosphogypsum if phosphate rock is digested with sulphuric acid (Hull and Burnett, 1996). Po-210 co-precipitates either with the Mg(OH)2 when the raw material is processed via hydrochloric acid, or with the phosphogypsum if treated with sulphuric acid. (Hull and Burnett, 1996). Therefore, most of the 210Po found in DCP is derived from the 210Pb decay, although not having yet reached the secular equilibrium at the time of analysis. This lead to the conclusion that Group A samples might correspond to samples produced through the hydrochloric acid digestion of the phosphate rock, whereas group B samples might be produced after digestion of the rock with sulphuric acid. Dicalcium Phosphate production process To more accurately quantify the radionuclide partitioning in a factory in Spain, where phosphate rock is wet acid digested with hydrochloric acid (type group A samples), 238U- series radionuclides are being studied during the DCP industrial process. These are of major concern in terms of radiological impact to population due to the relatively high contents of two of the radionuclides that might pose a relevant dose due to its ingestion (i.e. 210Pb and 210Po). Moreover, it is of our interest to quantify radionuclides content in by-products that eventually could be of radiological relevance in terms of external exposure to workers an/or population. The phosphate rock is imported from Morocco, initially accounting for 1625±32 Bq·kg-1 of 238U; 1511±14 Bq·kg-1 of 226Ra, 1714±156 Bq·kg-1 of 210Pb and 1748±56 Bq·kg-1 of 210Po. The secular equilibrium prevailing in the phosphate rock is already disrupted once the rock is digested with hydrochloric acid. In terms of radionuclide fluxes, in a normal daily production of 300 Tm of DCP, lead clearly follows the DCP production line, whereas thorium and polonium are eliminated together with the sludges. In terms of radionuclide specific concentrations, accumulation of noticeable levels of some 238U -series radionuclides is observed in several by-products such as muds (i.e. ~104 Bq·kg-1 dw of both 230Th and 210Po) as well as in the final product, DCP (1530±25 Bq·kg-1 of 238U and 1940±141 Bq·kg-1 of 210Pb). Results are comparable to those reported by Gäfvert et al. (2001), who found 99% of the 230Th that enters the process is found in the sludge due to the high affinity for particulate or colloid surfaces and more than 60% is of 238U is measured in the final product. On the contrary, their study reported that lead was more uniformly distributed between the various products and wastes and that 30% of the 210Po was coprecipitated with the DCP. This might be due slightly differences between both production processes. Future regulations foreseen to be mandatory by 2009 in Spain shall allow making educated decisions on the need and/or convenience of adapting the production process to minimize the levels of specific radionuclides in both liquid and solid wastes. At present, liquid wastes are discharged to a major river, while muds are stored in a solid waste facility. Accumulation in chicken tissues Given the radionuclide contents in DCP, concern refers to 210Po and, to a lesser extern, 210Pb as they are the radionuclides which mostly contribute to human dose through ingestion Therefore, specific concentrations of both radionuclides were determined in kidneys, liver, bones, thigh, breast and excrements of 21 days age farm chickens. Specific concentrations in dry weight (dw) in chicken fed with diet B were slightly higher in liver (1.2 ± 0.6 and 11.0 ± 2.9 Bq·kg-1 for 210Pb and 210Po, respectively) and kidneys (3.0 ± 0.3 and 28.3 ± 0.3 Bq·kg-1 for 210Pb and 210Po, respectively). Accumulation of 210Pb and 210Po in edible parts (breast and thigh) was very low (<0.5 Bq·kg-1 for 210Pb and ~1 Bq·kg-1 for 210Po). However, most (98- 99%) of the ingested radionuclides were found in chicken excrements, agreeing with Reid et al. (1977) which obtained a similar response in their study of uranium intake by poultry. It must be noted that specific concentrations of 210Pb and 210Po in chicken tissues were proportional to the intaken quantity depending on their diet. Whereas diet B lead to doubled specific concentrations in chicken tissues than diet C; animals fed with diet A hardly accumulated any (<1 Bq·kg-1 dw). Having determined radionuclide contents in chicken tissues, estimated doses to humans due to poultry meat ingestion are obtained through the following formula: where M is the chicken meat consumed annually by humans in dw, C the concentrations of 210Pb and 210Po in Bq·kg-1 dw, h(g) the committed effective dose per unit intake and f the fractional nuclide absorption to human body. Total dose due to ingestion of chicken meat is the sum of doses due to both, 210Pb and 210Po intake. Table 1: Estimation of the annual dose to an adult through the ingestion of poultry meat fed with different types of dicalcium phosphate. Concentrations of 210Pb and 210Po are the average values in breast and thigh. The annual chicken meat consumption is 25 kg per person (7.5 kg dw). Results shown in Table 1 underscore the higher contribution of 210Po (>90%) due to chicken meat ingestion other than 210Pb. It furthermore highlights that even in animals fed with 5% in weight of DCP with relatively high concentrations of 210Pb and 210Po in their diets (~2·103 Bq·kg-1 and 103 Bq·kg-1, respectively), the dose to human would not exceed 5.5 µSv·y-1. In a similar study, Izak-Biran et al. (1989) reported a dose per person of 0.04 mSv·y-1 assuming that 210Pb and 210Po were in equilibrium in chicken meat. To the contrary, the present study shows that assimilation of 210Pb and 210Po to the chicken tissues is different. A final conclusion is that although the calculated dose of 5.5 µSv·y-1 would be larger than when DCP is virtually free of radionuclides, it however points to the absence of actual radiological risk to man associated to the consumption of current DCP as an additive in food for animals. REFERENCES Gäfvert, T., Holm, E. and Roos, P., (2001). Radionuclide fluxes at a plant manufacturing dicalcium phosphate for domestic animals. Journal of Environmental Radioactivity, 61-73. Gäfvert, T. 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