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Radiological investigation of phosphate fertilizers: Leaching studies
Journal of Environmental Radioactivity 173 (2017) 34e43
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Radiological investigation of phosphate fertilizers: Leaching studies s Hegedu } s, Edit To th-Bodrogi, Szabolcs Ne meth, Ja nos Somlai, Tibor Kova cs* Miklo Institute of Radiochemistry and Radioecology, University of Pannonia, 10 Egyetem Str., H-8200, Veszpr em, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 August 2016 Received in revised form 4 October 2016 Accepted 6 October 2016 Available online 19 October 2016
The raw materials of the phosphate fertilizer industry are the various apatite minerals. Some of these have high levels of natural radionuclides, and thus phosphate fertilizers contain significant amounts of U238, K-40 and Ra-226. These can leach out of the fertilizers used in large quantities for resupplying essential nutrients in the soil and can then enter the food chain through plants, thereby increasing the internal dose of the affected population. In the current study, the radiological risk of eight commercially available phosphate fertilizers (superphosphate, NPK, PK) and their leaching behaviours were investigated using different techniques (gamma and alpha spectrometry), and the dose contributions of using these fertilizers were estimated. To characterize the leaching behaviour, two leaching procedures were applied and compared ethe MSZ 21470-50 (Hungarian standard) and the Tessier five-step sequential extraction method. Based on the evaluation of the gamma-spectra, it is found that the level of Th-232 in the samples was low (max.7 ± 6 Bq kg1), the average Ra-226 activity concentration was 309 ± 39 Bq kg1 (min. 10 ± 8 Bq kg1, max. 570 ± 46 Bq kg1), while the K-40 concentrations (average 3139 ± 188 Bq kg1, min. 51 ± 36 Bq kg1) could be as high as 7057 ± 427 Bq kg1. The high K-40 can be explained by reference to the composition of the investigated fertilizers (NPK, PK). U concentrations were between 15 and 361 Bq kg1, with the average of 254 Bq kg1, measured using alpha spectrometry. The good correlation between P2O5 content and radioactivity reported previously is not found in our data. The leaching studies reveal that the mobility of the fertilizer's uranium content is greatly influenced by the parameters of the leaching methods. The availability of U to water ranged between 3 and 28 m/m%, € solution mobilized between 10 and 100% of the U content. while the Lakanen-Ervio © 2016 Elsevier Ltd. All rights reserved.
Keywords: Fertilizer Leaching Gamma-spectrometry Alpha-spectrometry NORM
1. Introduction Given the ongoing requirement for increased food production, the use of phosphate fertilizers is necessary for today's intensive agriculture to achieve high yields. To maintain annual production in agriculture, fields must be cared for and provided with the necessary macro- and micronutrients needed for the particular crop we hope to harvest. To resupply the basic nutrients in soil, nitrogen (N), phosphate (P2O5) and potash (K2O) fertilizers are used in large quantities, sometimes (and especially in rapidly developing countries) leading to their overuse (Mueller et al., 2012). According to a report by the International Fertilizer Industry Association (IFA), the world's nutrient demand was expected to reach 188 Mt nutrients (N 114.3 Mt, P2O5 42.6 Mt, K2O 31.0 Mt) by
* Corresponding author. Institute of Radiochemistry and Radioecology, University m, Hungary. of Pannonia, P.O. Box 158, H-8201, Veszpre cs). E-mail address: [email protected] (T. Kova http://dx.doi.org/10.1016/j.jenvrad.2016.10.006 0265-931X/© 2016 Elsevier Ltd. All rights reserved.
2018/2019, with an average expected annual growth of 1.8% (Heffer and Prud'homme 2014). Before the industrial revolution, various materials naturally and renewably produced by agriculture and human habitation were used as fertilizers, such as animal manure, crushed animal bones, fish and their remains, human and bird wastes, city waste and ash. With increased food demand, however, these renewable resources proved to be insufficient and a more concentrated, higher quantity source was required. Thus, people turned to mining natural ore deposits and harnessing unique opportunities like the harvesting of guano (piled up bird or bat manure) (Cordell et al., 2009; Van Vuuren et al., 2010). While N compounds such as saltpetre (KNO3) were mined, better methods needed to be invented. Since the beginning of the 20th century, the Haber-Bosch synthesis has provided an N source for fertilizer production using a process of N2 fixation from air. Novel methods attempt either to improve the existing technology or to create a more efficient nitrogen fixation process (Cherkasov
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et al., 2015). There was a production potential of 152.77 million metric tonnes of N in 2014, and it is predicted that the increase in production capacity will remain ahead of consumption expansion in the near future (Heffer and Prud'homme 2014). K2O was traditionally provided by the ash remaining after burning wood, but with the increase of demand production turned to natural salt deposits, salt lakes and brines. Current estimates of known, high-quality reserves of potassium ore range from nine to 20 billion tonnes of K2O, which are expected to last at least 350 years at current consumption rates; other sources (150 billion tonnes) extend this estimation much further. Potassium chloride (muriate of potash, MOP) is the main chemical form (95%) of K in potassium fertilizers used in agriculture, due to its high K content (60%), low price, good availability and ease of integration into fertilizer production (Johnston, 2003). In the case of phosphorus, close to 46.71 million tonnes of P2O5 equivalent was mined in 2014, according to the IFA, with increasing yearly demand and slowly dwindling resources. However, recently, some new deposits have been emerging in Africa and Asia, increasing the expected phosphate rock supply to 258 million tonnes in 2018 (Heffer and Prud'homme 2014). Phosphate rock is the main source of phosphate for the fertilizer industry. The P2O5 content varies between 25% and 37%, the higher grades being more desirable. Amongst the existing sources, the leading role is undeniably taken by sedimentary marine deposits (approximately 75%), followed by igneous and weathered deposits (15e20%), while biogenic resources only account for 1e2% of production. Primary phosphate minerals include fluorapatite (Ca10(PO4)6F2), hydroxyapatite (Ca10(PO4)6(OH)2), carbonatehydroxyapatites (Ca10(PO4, CO3)6(OH)), and francolite (Ca10xyNaxMgy(PO4)6z(CO3)zF0$4zF22). The phosphate ores can contain various toxic elements such as fluor (F), cadmium (Cd), arsenic (As), mercury (Hg), chromium (Cr) and lead (Pb), along with radionuclides such as U-238, Ra-226, Th-232, Pb-210, Po-210 (Gupta et al., 2014). The various phosphate rocks are well known for their elevated levels of natural radionuclides. For example, the typical activities for sedimentary phosphate rock types are between 1500 and 1700 Bq kg1 for the U-238 decay chain, often in equilibrium (Casacuberta et al., 2011). During phosphoric acid production, some of these are removed with the phosphogypsum fraction (most of Po-210), some are evenly distributed (Ra-226), while others remain in the phosphoric acid (most of the uranium) and reach the fertilizer. The ratio can vary depending on the technology used in the production (Casacuberta et al., 2011; Olszewski et al., 2015). Since phosphate fertilizers contain excess amounts of radionuclides and heavy metals, their prolonged and extensive use can lead to gradual contamination (Schipper et al., 2011), using this kind of fertilizer could increase the internal and external radiation exposure of humans compared to the original situation. The leaching properties of the radionuclides are very important to the calculation of the ingestion dose rate as they determine the radionuclides' mobility and thus their availability to the food chain. Plants are able to obtain the necessary macro- (primary: N, P, K, secondary: Ca, S, Mg) and micronutrients (B, Cl, Mn, Fe, Zn, Cu, Mo, Ni, etc.) from soil, yet the quantity of said elements is of great importance. If there are not enough essential elements, such as Zn, Cu, B, Cl, Mo or even Fe, plant growth will suffer (White and Zasoski, 1999). On the other hand, excess amounts of essential and nonessential metals can have a toxic effect and inhibit growth. Radionuclides behave similarly to other elements in their group and are available to plants through similar mechanisms (Gupta et al., 2014). While some plants are capable of absorbing materials through their leaves and stem, the root zone plays a crucial role in metal
35
uptake. The metals bound to soil particles or in precipitated form can become available through contact with water, enzymes or organic acids produced by the roots and microbial activity in the soil. The uptake of a particular element is influenced by many factors, such as soil microorganisms, chelating agents and pH changes made by the plant, redox reactions, the transporter proteins of the plants, even the co-transportation or inhibition of other ions (Gupta et al., 2014). The European Union, despite encouraging efforts towards the standardization of methods, such as the LEAF protocols (Kosson et al., 2014) and the harmonization of protocols for waste evaluation, still does not offer commonly accepted methods for the testing of NORM materials' leaching characteristics. A great variety of tests are reportedly used for this purpose, ranging from single-step batch extractions (Lysandrou and Pashalidis, 2008) to column tests (Nisti et al., 2015) and sequential extraction procedures (Vandenhove et al., 2014), both for regulatory and scientific purposes (Tiwari } s et al., 2016). The availet al., 2015; Kosson et al., 2014; Hegedu ability of the leaching tests vary greatly, the standards used in the EU have to be bought, while the US EPA standards for leaching (Kosson et al., 2014) and the scientific experimental setups are freely available. The differences and similarities of the available tests have been scrutinized many times in the past and still hold interest (Rauret, 1998; Sahuquillo et al., 2003; Kosson et al., 2014; Vandenhove et al., 2014). Since there is no commonly accepted protocol for the leaching of natural radionuclides, the already existing national protocols for the leaching of heavy metals have been evaluated. The MSZ-21470-50:2006 Hungarian standard has been selected for this study due to its frequented use in agriculture and by the Hungarian authorities for environmental protection, while the Tessier five-step speciation method reveals information regarding which chemical forms the investigated radionuclides are bound to, and the environmental conditions under which they become mobile. In this study, uranium was investigated to evaluate the leaching tests, given that it has toxic effect as a heavy metal and its chemical toxicity are considered more harmful to the general population than its radiological hazard (Schipper et al., 2011). It has welldocumented toxicity and carcinogenesis in the lungs and kidneys, while recently absorption through the skin is also considered to be a major route of exposure. Additionally, new mechanisms for chemical carcinogenic effects have been proposed, while the UV activation of uranyl ions' DNA strand breaks and uranium-DNA adducts, and mutations can occur (George et al., 2011; Wilson et al., 2014a, 2014b). The dose assessment from ingestion has been included in UNSCEAR 2000 and 2008 reports, based on the International Atomic Energy Agency Safety Series No. 115 (1996). In addition to the ingestion dose, the assessment of the external dose rate could be reasonable in some cases (e.g., workers). Several approaches can be used during the external dose assessment of phosphate fertilizers. One of the most common methods is to calculate the absorbed dose rates due to gamma radiations in air at 1 m above the ground surface, assuming the uniform distribution of Ra-226, Th-232 and K-40, based on guidelines provided by UNSCEAR (Shahul Hameed et al., 2014; Uosif et al., 2014; Hassan et al., 2016). Using this method means overestimating the radiological risk for the majority of the populace; it may thereby provide an upper limit of the expected excess dose per year with the conservative assumptions made before. This study aims to assess the radiological characteristics of different types of commercially available phosphate fertilizers by using gamma-spectrometry, complemented by leaching studies of the availability of uranium species contained within them. The results are expected to be useful in the assessment of public doses
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and in the estimation of the effects of long-term application of phosphorous fertilizers. 2. Measurements and methods 2.1. Sampling and sample preparation In this study, eight different phosphate fertilizer samples commercially available in Hungary were obtained (Table 1). The samples were dried until they achieved a constant mass, before being pulverized and then filtered through a 0.63 mm mesh sieve. For the gamma-spectrometry measurements, each sample was weighed and sealed in a radon-tight metal container for 30 days to achieve the secular equilibrium between Ra-226 and its decay products (Sas et al., 2015). 2.2. Gamma spectrometry The samples were measured using an ORTEC GMX40-76 HPGe (relative efficiency 42%) detector attached to an ORTEC DSPEC LF 8196 MCA for 80,000 s. The Ra-226 activity concentration was calculated from the 295 keV line of Pb-214 and the 609 keV line of Bi-214; the Th-232 activity concentration was calculated from the 911 keV line of Ac-228 and the 2614 keV line of Tl-208; the K-40 was measured directly at 1460 keV (Shakhashiro et al., 2012). IAEA326 soil reference material was used for the calibration of the system. 2.3. Leachability experiments The uranium leaching features of phosphate fertilizers were investigated using a batch test method compliant with the MSZ21470-50:2006 Hungarian standard, along with the Tessier sequential extraction method. Both methods have been applied in the Institute of Radiochemistry and Radioecology at the University }s of Pannonia for the characterization of NORM materials (Hegedu et al., 2016). The MSZ-21470-50:2006 Hungarian standard has been selected for this study due to its frequented use in agriculture and by the Hungarian authorities for environmental protection. By comparison with other methods it requires less amount of sample and less time, and also boasts a low cost. It consists of two “total digestion methods”, one using aqua regia and the other using HNO3þH2O2, for determining the basis of comparison for these leaching tests. It is worth keeping in mind that these “total digestion methods” are not truly total, as there some residue remains e other reagents, such as HF and HClO4, would be required to achieve the true total dissolution of the sample. The two other studied methods are related to environmental availability e the distilled water method is used for testing the availability to rainwater (similarly to the EN 12457-2, DIN 38414-S4 or the ASTM D 3987-85 € solution is used for methods) and the leaching by Lakanen-Ervio simulating the availability to the root system of plants. It should be
Table 1 Types of obtained phosphate fertilizers. ID
N [wt%]
P2O5 [wt%]
K2O [wt%]
S [wt%]
NPK1 NPK2 NPK3 PK1 PK2 SP1 SP2 SP3
15 10 15 e e e e e
15 5 15 10 10 16.2 20,5 18
15 10 15 24 24 e e e
e 10.16 e 6 8 e e e
noted that, while batch tests are simple, fast and well-liked by authorities, they only offer predictions about future leaching behaviour, whereas actual leaching behaviour is influenced by many factors, such as soil and environmental parameters, and individual plant characteristics (Ruyters et al., 2011). The Tessier fivestep speciation method reveals information regarding which chemical forms the investigated radionuclides are bound to, and the environmental conditions under which they become mobile. The leachates were then separated from the samples; U was selectively extracted from the matrix by UTEVA resin extraction chromatography and was electrodeposited onto stainless steel disks with high Ni contents (Jobb agy et al., 2010). The prepared sources were measured using alpha-spectrometry. While other relevant radionuclides are present in phosphate fertilizers (such as Ra-226, Po-210 and Pb-210) the U-238 and U-234 were selected for analysis and evaluation of the leaching tests. 2.3.1. Batch test method compliant to the MSZ-21470-50:2006 Hungarian standard MSZ-21470-50:2006 is the Hungarian standard (Hungarian Standards Institution) for the measurement of toxic elements, heavy metals and chrome(VI) in soil for environmental protection purposes. Four separate single-step batch processes are included to investigate the “total” amount and to indicate the availability to } s et al., 2016), as can be seen in Fig. 1 rainwater and plants (Hegedu below. 2.3.2. Tessier sequential extraction The five-step Tessier sequential extraction is a speciation process, originally used for heavy metals in sediments, which has since been adapted for other materials as well. The original procedure was for 1 g samples but, for the leaching experiments for natural radionuclides, altering the process for 5 g samples was necessary } s et al., 2016). The due to the instruments' detection limits (Hegedu adapted process is shown in Fig. 2 below. 2.3.3. Alpha-spectrometry After separating the leachates from the solid phase, the samples were put through radiochemical separation by UTEVA resin } s et al., 2016). The extraction chromatography was per(Hegedu formed using NO3 form UTEVA resin and, after removing the other, undesired fractions, U was eluted by 0.02 M HCl from the column. The uranium fractions were subjected to electro-deposition with a Canberra Electro a with a platinum electrode, and the process was carried out at 1 A for 1 h on a stainless steel disk with a high Ni gy et al., 2010). The prepared alpha-sources were content (Jobba measured for 80,000 s with three alpha chambers equipped with PIPS detectors of Ortec Soloist, Canberra Model 7401 and Tennelec TC 256 types. The chambers' respective MDAs were 0.7, 1.4 and 2 Bq kg1, respectively. 2.3.4. Excess dose calculation To better compare the total gamma dose potential of the samples, the radium equivalent activity index (Raeq) was calculated according to equation (1) (Boukhenfouf and Boucenna, 2011). Raeq ¼ ARa þ ATh 1.43þAK 0.077,
(1)
where ARa, ATh and AK are the respective activity concentrations of Ra-226, Th-232 and K-40 in Bq kg1. The absorbed dose rates due to gamma radiations in air at 1 m above the ground surface were calculated according to equation (2) (Shahul Hameed et al., 2014; Uosif et al., 2014; Hassan et al., 2016). D ¼ 0.462 ARaþ0.604 AThþ0.0417 AK,
(2)
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Fig. 1. Flowchart of the four parallel single-step extractions compliant with MSZ-21470-50:2006.
where ARa, ATh and AK are the respective activity concentrations of Ra-226, Th-232 and K-40 in Bq kg1 and D is the dose rate in nGy h1. The annual estimated average effective dose equivalent was estimated using equation (3) (Uosif et al., 2014). AED ¼ D t FC,
(3)
where D is the calculated dose rate (in nGy h1), t is the number of work hours per year and FC is the conversion factor (0.7 Sv Gy1). A working period of 1820 h a year was used.
2.3.4.1. Internal dose calculation. To estimate the excess internal dose, a few assumptions must be made. In the considered scenario, the application rate of the fertilizers was taken to be 80 kg P2O5 ha1 per year (actual application rates may differ depending on the plant and soil properties), a homogenous distribution of radionuclides was assumed in the upper 20 cm of the soil, and the soil's apparent density was taken as 1.3 g cm3 (Saueia and Mazzilli, 2006). The soil's U concentration was taken as 32Bq kg1, assuming secular equilibrium with the Ra-226 value reported in UNSCEAR 2008 Annex B. The estimation of radionuclide concentrations in plants was performed using the geometric mean transfer factors calculated by Vandenhove et al., (2009) for each plant group. The annual estimated average effective dose equivalent from the ingestion of plants grown in the soil treated with phosphorus fertilizer was calculated using equation (4) (Saueia and Mazzilli, 2006). IAEDi ¼ Ci*Ui*EDC
(4)
where IAEDi is the annual estimated average effective dose equivalent from ingestion, Ci is the activity concentration of U in the specific plant, Ui is the consumption rate of the specific plant according to the European Food Safety Authority (EFSA) taken from the INRAN SCAI 2005-06 report, and EDC is the effective dose coefficient according to the UNSCEAR 2000 report (U-238 0.045, U234).
3. Results and discussion 3.1. Gamma spectrometry The natural radionuclide content of the investigated fertilizer samples varied significantly (Table 2). The values measured for the fertilizers commercially available in Hungary agree well with fertilizers used in other countries (Hassan et al., 2016; Shahul Hameed et al., 2014; Uosif et al., 2014). The average activity concentrations for the fertilizers used in the current study were 309 ± 39 Bq kg1 for Ra-226, 6 ± 5 Bq kg1 for Th232 and 3139 ± 188 Bq kg1 for K-40. These are not especially high concentrations, but they are well above the average activity concentrations for soil UNSCEAR 2008 Annex B (Ra-226: 32 Bq kg1; Th-232: 45 Bq kg1; K-40: 412 Bq kg1) and Radiation Protection 112 (Ra-226: 40 Bq kg1; Th-232: 40 Bq kg1; K-40: 400 Bq kg1). The application of these fertilizers would mean a very slow, gradual increase in the external gamma dose rate compared to the unaltered situation. For better comparison of the total gamma dose potential of the samples, the radium equivalent activity index (Raeq) of the samples is shown in Fig. 3. The values for Raeq also demonstrate that our samples certainly have the potential to lead to excess gamma doses as compared to the average soil, but the values are similar to those of other fertilizer samples reported in the literature (Boukhenfouf and Boucenna 2012; Shahul Hameed et al., 2014; Uosif et al., 2014). It has been reported in several cases that the activity of phosphate fertilizers correlates well with their phosphate content (Boukhenfouf and Boucenna, 2011; Hassan et al., 2016). The activity concentrations of the samples have been normalized for 1 kg phosphate content for better comparison. As can be seen in Fig. 4, the good correlation previously reported is not present in our case: while the superphosphate samples seem to agree with that trend, for the NPK fertilizers not only were low activity concentrations found, but also the values do not depend on the phosphate content. This is probably due to the different raw phosphate ore and production process employed. If the types of the fertilizers are disregarded, and the Ra-226 and U-238 activity concentrations are plotted against the P2O5 content (Fig. 5), it can be seen even more clearly that the correlation mentioned above is not true in our case.
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radiological perspective these seem to be the safest to use, if their composition is acceptable. 3.2. Leachability experiments
Fig. 2. Flowchart of the Tessier five-step sequential extraction, adapted for 5 g samples.
Table 2 Activity concentrations measured in the fertilizer samples.
NPK1 NPK2 NPK3 PK1 PK2 SP1 SP2 SP3
Ra-226 [Bq kg1]
Th-232 [Bq kg1]
K-40 [Bq kg1]
570 ± 46 10 ± 8 38 ± 17 310 ± 34 336 ± 48 351 ± 48 439 ± 55 414 ± 53
7±6 7±9 4±7 < MDA < MDA 6±9
4097 ± 148 3429 ± 297 3664 ± 307 6623 ± 188 7057 ± 427 111 ± 54 51 ± 36 76 ± 44
Comparing these results, it can be seen that (with the exception of NPK1, which showed significant excess of Ra-226, and NPK3, which showed significant excess of U species) the activity concentration of U-238, U-234 and Ra-226 are in or nearly in equilibrium. Amongst the investigated samples, NPK2 and NPK3 have relatively low levels of U-238 and Ra-226, not only on their own, but also when their P2O5 content is taken into account and the majority of their activity comes from their K-40 content. From a
The results of the leachability measurements can be seen in Fig. 6. The U-238 activity concentrations of the samples measured using microwave digestion with aqua regia ranged from 21 Bq kg1 to 361 Bq kg1, as shown in Fig. 6. The microwave digestion with HNO3 and H2O2 was not adequate for the measurement of the “total” U concentration, while in some cases (NPK2, NPK3, SP1) the same results as the aqua regia digestion were shown and in other cases (most distinctly NPK1 and PK1) a gross underestimation of the activity concentration was caused. For phosphate fertilizers, measurement of the “total” U concentration could be excluded from the measurement protocol, and possibly substituted with a parallel measurement of the aqua regia “total digestion” for approximately the same cost. The indicated availability to plants and rainwater differed greatly from sample to sample. The availability of U to water ranged between 3 and 28 m/m%, while the € solution mobilized between 10 and 100 m/m% of the Lakanen-Ervio U content. In the case of the NPK3 sample, virtually all U was indicated to be available for uptake by plants, while other samples, such as NPK1 and PK1, retained nearly all U. The widely varying values are suspected to be caused by the different raw materials and technological processes used at each plant. To better explain these differences, the Tessier sequential extraction was carried out to see which chemical form the U species are bound to. The speciation of U-238 revealed that not only did the activity concentrations vary greatly, but the availability of U had great variation even within the same type of fertilizers. As you can see on Fig. 7, the investigated fertilizers have significant fractions of U bound to species available to changes in red-ox potential and under oxidizing conditions. The ratios of the water available fraction are similar to the values obtained in the MSZ-21470-50:2006 test, and indicate leachability from negligible to one third of the total U content. The fraction that becomes available with pH poses a further 5 to 10 percent. NPK2, PK1 and SP1 have a significant portion (57, 47, and 31% respectively) of U in the remaining fraction that does not become available under environmental conditions. The fertilizers don't show a trend connected to the P2O5 content, or characteristics representing the fertilizer categories. NPK1 and NPK3 have similar major constituent composition and leaching characteristics, while PK1 and PK2, also having similar composition to each other, do not have the same leaching characteristics. As for environmental impact, the fractions that are readily available pose risk by their contribution to the internal dose through the food-chain, while the fractions that do not become available cause a slow contamination of the land. This last problem is twofold, there is a small increase in the external gamma-dose, and there is a small increase of the internal dose from the inhalation of dust in the area, gradually increasing annually by a small fraction, and is likely only to cause a problem in a few hundred years. 3.3. Dose assessment The calculated absorbed dose rate and the annual effective dose are shown in Table 3. The values presented in Table 3 agree with data on similar fertilizers (Hassan et al., 2016; Uosif et al., 2014). It should also be noted that the assumptions made in the annual effective dose calculations do not represent the general populace, but can be applied for workers in fertilizer factories and storage facilities.
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Fig. 3. The radium equivalent activity index (Raeq) of the phosphate fertilizers.
Fig. 4. Activity concentration normalized for 1 kg phosphate content.
Fig. 5. U-238 and Ra-226 activity concentration in relation to P2O5 content.
The general populace, however, is affected by phosphate fertilizer use through the consumption of agricultural products. The calculated excess annual effective dose from ingestion according to the considered scenario can be found in Figs. 8 and 9 for U-238 and U-234, respectively. It can be seen that the maximum excess dose was calculated for the PK1 fertilizer, 12.97 nSv year1 for U-238 and 14.12 nSv year1 for U-234. In the case of NPK2, the negative value indicates that the fertilizer has less activity concentration than the soil in the
considered scenario and thus plays a diluting effect, causing a decrease in the received dose. Compared to the doses calculated directly from soil (4.59 mSv year1 for U-238, and 4.99 mSv year1 for U-234) these values seem small, but after decades of agricultural activities the increase would become noticeable. The first four steps of the Tessier speciation method reveal the species that become available under various environmental conditions. The residue is not available under environmental conditions and, therefore, the excess annual effective dose should also be reduced accordingly. The calculated excess annual effective dose from the ingestion of plants according to the scenario whereby they are unable to take up the non-leachable U species (according to the Tessier method) can be found in Figs. 10 and 11. If Figs. 8 and 9 are compared to Figs. 10 and 11 the most notable changes are in the cases of samples PK1 and SP1, for which a high ratio of residue is combined with high activity concentration. In both scenarios, the highest values reach 10 nSv year1, compared to 0.12 mSv year1 for the ingestion of U and Th series (summing up the dose contribution of all daughter elements as well) presented in the UNSCEAR 2008 report. The excess doses calculated from the use of phosphate fertilizers according to the considered scenario are lower by five magnitudes. However if compared to the original dose caused by the soil's U content in the considered scenario (4.59 mSv year1 for U-238, and 4.99 mSv year1 for U-234), the increment would become comparable over a century.
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Fig. 6. Leaching of U-238 from phosphate fertilizers.
Fig. 7. Distribution of U-238 in the Tessier extraction process.
Table 3 The calculated absorbed dose rate (D) and the annual effective dose (AED) originated from the observed phosphate fertilizers.
NPK1 NPK2 NPK3 PK1 PK2 SP1 SP2 SP3
D [nGy h1]
AED [mSv year1]
438.4 151.8 172.7 419.4 449.5 170.4 204.9 194.4
0.56 0.19 0.22 0.53 0.57 0.22 0.26 0.25
4. Conclusion In this study, eight commercially available phosphate fertilizers were evaluated using gamma spectrometry, while their U leaching characteristics were tested using both the MSZ-21470-50:2006 standard and the Tessier speciation process. The selected leaching tests can be applied for the estimation of the uranium leaching characteristics of phosphate fertilizer samples, yet it should be noted that actual leaching will be different depending on actual field conditions e batch tests only offer an indication of future behaviour. Microwave digestion with HNO3 and H2O2 cannot be relied upon for measuring the “total” U content, since it not only showed significantly lower values for U activity concentration, but also failed to indicate the values that became available during the Tessier speciation method. This method could be excluded entirely from the measurement protocol for phosphate fertilizers, or
substituted by a parallel measurement with the aqua regia method for approximately the same cost and work hours. The fertilizers' activity concentrations and leaching characteristics show great variation, both between and within fertilizer types. The average activity concentrations for the fertilizers used in the current study were 309 ± 39 Bq kg1 for Ra-226, 6 ± 5 Bq kg1 for Th-232 and 3139 ± 188 Bq kg1 for K-40. The good correlation between P2O5 content and radioactivity reported previously is not found in our case e only the superphosphate samples seem to agree with that trend ethough, for the NPK fertilizers, not only were low activity concentrations found, but additionally the values did not depend on the phosphate content. This is probably due to a difference in the raw phosphate ore and production process employed. Those fertilizers that contain uranium in water-soluble or plantavailable species cause excess internal doses through the food chain, while the insoluble fractions remain in place, causing a gradually increasing low-level contamination of the fields, leading to a continually increasing excess external gamma dose. The annual effective gamma dose values calculated are similar to values reported for the same types of fertilizers (Hassan et al., 2016; Uosif et al., 2014). The speciation of U in the Tessier sequential extraction method revealed that, while the U content of some samples can become almost fully available under environmental conditions, some can retain up to 57% of their activity in the residue, unavailable under natural circumstances. This also has similarly significant effects on the internal dose calculated from the consumption of plants according to the considered scenario.
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Fig. 8. Excess dose caused byU-238 in plants according to the considered scenario.
Fig. 9. Excess dose caused by U-234 in plants according to the considered scenario.
Fig. 10. Excess dose caused by U-238 in plants reduced by the non-leachable amount according to the considered scenario.
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Fig. 11. Excess dose caused by U-234 in plants reduced by the non-leachable amount according to the considered scenario.
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Nisti, M.B., Saueia, C.R., Malheiro, L.H., Groppo, G.H., Mazzilli, B.P., 2015. Lixiviation of natural radionuclides and heavy metals in tropical soils amended with phosphogypsum. J. Environ. Radioact. 144, 120e126. http://dx.doi.org/10.1016/ j.jenvrad.2015.03.013). Olszewski, G., Boryło, A., Skwarzec, B., 2015. Uranium (234U, 235U and 238U) contamination of the environment surrounding phosphogypsum waste heap in Wislinka (northern Poland). J. Environ. Radioact. 146, 56e66. http://dx.doi.org/ 10.1016/j.jenvrad.2015.04.001. Rauret, G., 1998. Extraction procedures for the determination of heavy metals in contaminated soil and sediment. Talanta 46, 449e455. http://dx.doi.org/ 10.1016/S0039-9140(97)00406-2. Ruyters, S., Mertens, J., Vassilieva, E., Dehandschutter, B., Poffijn, A., Smolders, E., 2011. The red mud accident in ajka (Hungary): plant toxicity and trace metal bioavailability in red mud contaminated soil. Environ. Sci. Technol. 45, 1616e1622. http://dx.doi.org/10.1021/es104000m. Sahuquillo, A., Rigol, A., Rauret, G., 2003. Overview of the use of leaching/extraction tests for risk assessment of trace metals in contaminated soils and sediments. Trends Anal. Chem. 22, 152e159. http://dx.doi.org/10.1016/S0165-9936(03) 00303-0. nto , J., Kov Sas, Z., Sza acs, J., Somlai, J., Kov acs, T., 2015. Influencing effect of heattreatment on radon emanation and exhalation characteristic of red mud. J. Environ. Radioact. 148, 27e32. http://dx.doi.org/10.1016/ j.jenvrad.2015.06.002. Saueia, C.H.R., Mazzilli, B.P., 2006. Distribution of natural radionuclides in the production and use of phosphate fertilizers in Brazil. J. Environ. Radioact. 89, 229e239. http://dx.doi.org/10.1016/j.jenvrad.2006.05.009. Schipper, L.A., Sparling, G.P., Fisk, L.M., Dodd, M.B., Power, I.L., Littler, R.A., 2011. Rates of accumulation of cadmium and uranium in a New Zealand hill farm soil as a result of long-term use of phosphate fertilizer. Agric. Ecosyst. Environ. 144, 95e101. http://dx.doi.org/10.1016/j.agee.2011.08.002. Shahul Hameed, P., Sankaran Pillai, G., Mathiyarasu, R., 2014. A study on the impact of phosphate fertilizers on the radioactivity profile of cultivated soils in Srirangam (Tamil Nadu, India). J. Radiat. Res. Appl. Sci. 7, 463e471. http:// dx.doi.org/10.1016/j.jrras.2014.08.011. Shakhashiro, A., Tarjan, S., Ceccatelli, A., Kis-Benedek, G., Betti, M., 2012. IAEA-447: a new certified reference material for environmental radioactivity measurements. Appl. Radiat. Isot. 70, 1632e1643. http://dx.doi.org/10.1016/ j.apradiso.2012.03.024. Tiwari, M.K., Bajpai, S., Dewangan, U.K., Tamrakar, R.K., 2015. Suitability of leaching test methods for fly ash and slag: a review. J. Radiat. Res. Appl. Sci. 8, 523e537. http://dx.doi.org/10.1016/j.jrras.2015.06.003. United Nations Scientific Committee on the effects of Atomic Radiation, 2000. Sources and Effects of Ionizing Radiation, UNSCEAR 2000 Report to the General Assembly with Scientific Annexes (United Nations, New York). United Nations Scientific Committee on the effects of Atomic Radiation, 2010. Sources and Effects of Ionizing Radiation, UNSCEAR 2008 Report to the General Assembly with Scientific Annexes (United Nations, Vienna). Uosif, M.A.M., Mostafa, A.M.A., Elsaman, R., Moustafa, E., 2014. Natural radioactivity levels and radiological hazards indices of chemical fertilizers commonly used in Upper Egypt. J. Radiat. Res. Appl. Sci. 7, 430e437. http://dx.doi.org/10.1016/ j.jrras.2014.07.006. Van Vuuren, D.P., Bouwman, A.F., Beusen, A.H.W., 2010. Phosphorus demand for the 1970e2100 period: a scenario analysis of resource depletion. Glob. Environ. Chang. 20, 428e439. http://dx.doi.org/10.1016/j.gloenvcha.2010.04.004. Vandenhove, H., Olyslaegers, G., Sanzharova, N., Shubina, O., Reed, E., Shang, Z., Velasco, H., 2009. Proposal for new best estimates of the soil-to-plant transfer factor of U, Th, Ra, Pb and Po. J. Environ. Radioact. 100, 721e732. http://
} s et al. / Journal of Environmental Radioactivity 173 (2017) 34e43 M. Hegedu dx.doi.org/10.1016/j.jenvrad.2008.10.014. ne, L., Antunes, K., Wannijn, J., 2014. ComVandenhove, H., Vanhoudta, N., Duque parison of two sequential extraction procedures for uranium fractionation in contaminated soils. J. Environ. Radioact. 137, 1e9. http://dx.doi.org/10.1016/ j.jenvrad.2014.05.024. White, J.G., Zasoski, R.J., 1999. Mapping soil micronutrients. Field Crops Res. 60, 11e26. http://dx.doi.org/10.1016/S0378-4290(98)00130-0.
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Wilson, J., Young, A., Civitello, E.R., Stearns, D.M., 2014a. Analysis of heat-labile sites generated by reactions of depleted uranium and ascorbatein plasmid DNA. J. Biol. Inorg. Chem. 19, 45e57. http://dx.doi.org/10.1007/s00775-013-1057-6. Wilson, J., Zuniga, M.C., Yazzie, F., Stearns, D.M., 2014b. Synergistic cytotoxicity and DNA strand breaks in cells and plasmid DNA exposed to uranylacetate and ultraviolet radiation. J. Appl. Toxicol. http://dx.doi.org/10.1002/jat.3015. Published online in Wiley Online Library.
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Radiological investigation of phosphate fertilizers: Leaching studies s Hegedu } s, Edit To th-Bodrogi, Szabolcs Ne meth, Ja nos Somlai, Tibor Kova cs* Miklo Institute of Radiochemistry and Radioecology, University of Pannonia, 10 Egyetem Str., H-8200, Veszpr em, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 August 2016 Received in revised form 4 October 2016 Accepted 6 October 2016 Available online 19 October 2016
The raw materials of the phosphate fertilizer industry are the various apatite minerals. Some of these have high levels of natural radionuclides, and thus phosphate fertilizers contain significant amounts of U238, K-40 and Ra-226. These can leach out of the fertilizers used in large quantities for resupplying essential nutrients in the soil and can then enter the food chain through plants, thereby increasing the internal dose of the affected population. In the current study, the radiological risk of eight commercially available phosphate fertilizers (superphosphate, NPK, PK) and their leaching behaviours were investigated using different techniques (gamma and alpha spectrometry), and the dose contributions of using these fertilizers were estimated. To characterize the leaching behaviour, two leaching procedures were applied and compared ethe MSZ 21470-50 (Hungarian standard) and the Tessier five-step sequential extraction method. Based on the evaluation of the gamma-spectra, it is found that the level of Th-232 in the samples was low (max.7 ± 6 Bq kg1), the average Ra-226 activity concentration was 309 ± 39 Bq kg1 (min. 10 ± 8 Bq kg1, max. 570 ± 46 Bq kg1), while the K-40 concentrations (average 3139 ± 188 Bq kg1, min. 51 ± 36 Bq kg1) could be as high as 7057 ± 427 Bq kg1. The high K-40 can be explained by reference to the composition of the investigated fertilizers (NPK, PK). U concentrations were between 15 and 361 Bq kg1, with the average of 254 Bq kg1, measured using alpha spectrometry. The good correlation between P2O5 content and radioactivity reported previously is not found in our data. The leaching studies reveal that the mobility of the fertilizer's uranium content is greatly influenced by the parameters of the leaching methods. The availability of U to water ranged between 3 and 28 m/m%, € solution mobilized between 10 and 100% of the U content. while the Lakanen-Ervio © 2016 Elsevier Ltd. All rights reserved.
Keywords: Fertilizer Leaching Gamma-spectrometry Alpha-spectrometry NORM
1. Introduction Given the ongoing requirement for increased food production, the use of phosphate fertilizers is necessary for today's intensive agriculture to achieve high yields. To maintain annual production in agriculture, fields must be cared for and provided with the necessary macro- and micronutrients needed for the particular crop we hope to harvest. To resupply the basic nutrients in soil, nitrogen (N), phosphate (P2O5) and potash (K2O) fertilizers are used in large quantities, sometimes (and especially in rapidly developing countries) leading to their overuse (Mueller et al., 2012). According to a report by the International Fertilizer Industry Association (IFA), the world's nutrient demand was expected to reach 188 Mt nutrients (N 114.3 Mt, P2O5 42.6 Mt, K2O 31.0 Mt) by
* Corresponding author. Institute of Radiochemistry and Radioecology, University m, Hungary. of Pannonia, P.O. Box 158, H-8201, Veszpre cs). E-mail address: [email protected] (T. Kova http://dx.doi.org/10.1016/j.jenvrad.2016.10.006 0265-931X/© 2016 Elsevier Ltd. All rights reserved.
2018/2019, with an average expected annual growth of 1.8% (Heffer and Prud'homme 2014). Before the industrial revolution, various materials naturally and renewably produced by agriculture and human habitation were used as fertilizers, such as animal manure, crushed animal bones, fish and their remains, human and bird wastes, city waste and ash. With increased food demand, however, these renewable resources proved to be insufficient and a more concentrated, higher quantity source was required. Thus, people turned to mining natural ore deposits and harnessing unique opportunities like the harvesting of guano (piled up bird or bat manure) (Cordell et al., 2009; Van Vuuren et al., 2010). While N compounds such as saltpetre (KNO3) were mined, better methods needed to be invented. Since the beginning of the 20th century, the Haber-Bosch synthesis has provided an N source for fertilizer production using a process of N2 fixation from air. Novel methods attempt either to improve the existing technology or to create a more efficient nitrogen fixation process (Cherkasov
} s et al. / Journal of Environmental Radioactivity 173 (2017) 34e43 M. Hegedu
et al., 2015). There was a production potential of 152.77 million metric tonnes of N in 2014, and it is predicted that the increase in production capacity will remain ahead of consumption expansion in the near future (Heffer and Prud'homme 2014). K2O was traditionally provided by the ash remaining after burning wood, but with the increase of demand production turned to natural salt deposits, salt lakes and brines. Current estimates of known, high-quality reserves of potassium ore range from nine to 20 billion tonnes of K2O, which are expected to last at least 350 years at current consumption rates; other sources (150 billion tonnes) extend this estimation much further. Potassium chloride (muriate of potash, MOP) is the main chemical form (95%) of K in potassium fertilizers used in agriculture, due to its high K content (60%), low price, good availability and ease of integration into fertilizer production (Johnston, 2003). In the case of phosphorus, close to 46.71 million tonnes of P2O5 equivalent was mined in 2014, according to the IFA, with increasing yearly demand and slowly dwindling resources. However, recently, some new deposits have been emerging in Africa and Asia, increasing the expected phosphate rock supply to 258 million tonnes in 2018 (Heffer and Prud'homme 2014). Phosphate rock is the main source of phosphate for the fertilizer industry. The P2O5 content varies between 25% and 37%, the higher grades being more desirable. Amongst the existing sources, the leading role is undeniably taken by sedimentary marine deposits (approximately 75%), followed by igneous and weathered deposits (15e20%), while biogenic resources only account for 1e2% of production. Primary phosphate minerals include fluorapatite (Ca10(PO4)6F2), hydroxyapatite (Ca10(PO4)6(OH)2), carbonatehydroxyapatites (Ca10(PO4, CO3)6(OH)), and francolite (Ca10xyNaxMgy(PO4)6z(CO3)zF0$4zF22). The phosphate ores can contain various toxic elements such as fluor (F), cadmium (Cd), arsenic (As), mercury (Hg), chromium (Cr) and lead (Pb), along with radionuclides such as U-238, Ra-226, Th-232, Pb-210, Po-210 (Gupta et al., 2014). The various phosphate rocks are well known for their elevated levels of natural radionuclides. For example, the typical activities for sedimentary phosphate rock types are between 1500 and 1700 Bq kg1 for the U-238 decay chain, often in equilibrium (Casacuberta et al., 2011). During phosphoric acid production, some of these are removed with the phosphogypsum fraction (most of Po-210), some are evenly distributed (Ra-226), while others remain in the phosphoric acid (most of the uranium) and reach the fertilizer. The ratio can vary depending on the technology used in the production (Casacuberta et al., 2011; Olszewski et al., 2015). Since phosphate fertilizers contain excess amounts of radionuclides and heavy metals, their prolonged and extensive use can lead to gradual contamination (Schipper et al., 2011), using this kind of fertilizer could increase the internal and external radiation exposure of humans compared to the original situation. The leaching properties of the radionuclides are very important to the calculation of the ingestion dose rate as they determine the radionuclides' mobility and thus their availability to the food chain. Plants are able to obtain the necessary macro- (primary: N, P, K, secondary: Ca, S, Mg) and micronutrients (B, Cl, Mn, Fe, Zn, Cu, Mo, Ni, etc.) from soil, yet the quantity of said elements is of great importance. If there are not enough essential elements, such as Zn, Cu, B, Cl, Mo or even Fe, plant growth will suffer (White and Zasoski, 1999). On the other hand, excess amounts of essential and nonessential metals can have a toxic effect and inhibit growth. Radionuclides behave similarly to other elements in their group and are available to plants through similar mechanisms (Gupta et al., 2014). While some plants are capable of absorbing materials through their leaves and stem, the root zone plays a crucial role in metal
35
uptake. The metals bound to soil particles or in precipitated form can become available through contact with water, enzymes or organic acids produced by the roots and microbial activity in the soil. The uptake of a particular element is influenced by many factors, such as soil microorganisms, chelating agents and pH changes made by the plant, redox reactions, the transporter proteins of the plants, even the co-transportation or inhibition of other ions (Gupta et al., 2014). The European Union, despite encouraging efforts towards the standardization of methods, such as the LEAF protocols (Kosson et al., 2014) and the harmonization of protocols for waste evaluation, still does not offer commonly accepted methods for the testing of NORM materials' leaching characteristics. A great variety of tests are reportedly used for this purpose, ranging from single-step batch extractions (Lysandrou and Pashalidis, 2008) to column tests (Nisti et al., 2015) and sequential extraction procedures (Vandenhove et al., 2014), both for regulatory and scientific purposes (Tiwari } s et al., 2016). The availet al., 2015; Kosson et al., 2014; Hegedu ability of the leaching tests vary greatly, the standards used in the EU have to be bought, while the US EPA standards for leaching (Kosson et al., 2014) and the scientific experimental setups are freely available. The differences and similarities of the available tests have been scrutinized many times in the past and still hold interest (Rauret, 1998; Sahuquillo et al., 2003; Kosson et al., 2014; Vandenhove et al., 2014). Since there is no commonly accepted protocol for the leaching of natural radionuclides, the already existing national protocols for the leaching of heavy metals have been evaluated. The MSZ-21470-50:2006 Hungarian standard has been selected for this study due to its frequented use in agriculture and by the Hungarian authorities for environmental protection, while the Tessier five-step speciation method reveals information regarding which chemical forms the investigated radionuclides are bound to, and the environmental conditions under which they become mobile. In this study, uranium was investigated to evaluate the leaching tests, given that it has toxic effect as a heavy metal and its chemical toxicity are considered more harmful to the general population than its radiological hazard (Schipper et al., 2011). It has welldocumented toxicity and carcinogenesis in the lungs and kidneys, while recently absorption through the skin is also considered to be a major route of exposure. Additionally, new mechanisms for chemical carcinogenic effects have been proposed, while the UV activation of uranyl ions' DNA strand breaks and uranium-DNA adducts, and mutations can occur (George et al., 2011; Wilson et al., 2014a, 2014b). The dose assessment from ingestion has been included in UNSCEAR 2000 and 2008 reports, based on the International Atomic Energy Agency Safety Series No. 115 (1996). In addition to the ingestion dose, the assessment of the external dose rate could be reasonable in some cases (e.g., workers). Several approaches can be used during the external dose assessment of phosphate fertilizers. One of the most common methods is to calculate the absorbed dose rates due to gamma radiations in air at 1 m above the ground surface, assuming the uniform distribution of Ra-226, Th-232 and K-40, based on guidelines provided by UNSCEAR (Shahul Hameed et al., 2014; Uosif et al., 2014; Hassan et al., 2016). Using this method means overestimating the radiological risk for the majority of the populace; it may thereby provide an upper limit of the expected excess dose per year with the conservative assumptions made before. This study aims to assess the radiological characteristics of different types of commercially available phosphate fertilizers by using gamma-spectrometry, complemented by leaching studies of the availability of uranium species contained within them. The results are expected to be useful in the assessment of public doses
} s et al. / Journal of Environmental Radioactivity 173 (2017) 34e43 M. Hegedu
36
and in the estimation of the effects of long-term application of phosphorous fertilizers. 2. Measurements and methods 2.1. Sampling and sample preparation In this study, eight different phosphate fertilizer samples commercially available in Hungary were obtained (Table 1). The samples were dried until they achieved a constant mass, before being pulverized and then filtered through a 0.63 mm mesh sieve. For the gamma-spectrometry measurements, each sample was weighed and sealed in a radon-tight metal container for 30 days to achieve the secular equilibrium between Ra-226 and its decay products (Sas et al., 2015). 2.2. Gamma spectrometry The samples were measured using an ORTEC GMX40-76 HPGe (relative efficiency 42%) detector attached to an ORTEC DSPEC LF 8196 MCA for 80,000 s. The Ra-226 activity concentration was calculated from the 295 keV line of Pb-214 and the 609 keV line of Bi-214; the Th-232 activity concentration was calculated from the 911 keV line of Ac-228 and the 2614 keV line of Tl-208; the K-40 was measured directly at 1460 keV (Shakhashiro et al., 2012). IAEA326 soil reference material was used for the calibration of the system. 2.3. Leachability experiments The uranium leaching features of phosphate fertilizers were investigated using a batch test method compliant with the MSZ21470-50:2006 Hungarian standard, along with the Tessier sequential extraction method. Both methods have been applied in the Institute of Radiochemistry and Radioecology at the University }s of Pannonia for the characterization of NORM materials (Hegedu et al., 2016). The MSZ-21470-50:2006 Hungarian standard has been selected for this study due to its frequented use in agriculture and by the Hungarian authorities for environmental protection. By comparison with other methods it requires less amount of sample and less time, and also boasts a low cost. It consists of two “total digestion methods”, one using aqua regia and the other using HNO3þH2O2, for determining the basis of comparison for these leaching tests. It is worth keeping in mind that these “total digestion methods” are not truly total, as there some residue remains e other reagents, such as HF and HClO4, would be required to achieve the true total dissolution of the sample. The two other studied methods are related to environmental availability e the distilled water method is used for testing the availability to rainwater (similarly to the EN 12457-2, DIN 38414-S4 or the ASTM D 3987-85 € solution is used for methods) and the leaching by Lakanen-Ervio simulating the availability to the root system of plants. It should be
Table 1 Types of obtained phosphate fertilizers. ID
N [wt%]
P2O5 [wt%]
K2O [wt%]
S [wt%]
NPK1 NPK2 NPK3 PK1 PK2 SP1 SP2 SP3
15 10 15 e e e e e
15 5 15 10 10 16.2 20,5 18
15 10 15 24 24 e e e
e 10.16 e 6 8 e e e
noted that, while batch tests are simple, fast and well-liked by authorities, they only offer predictions about future leaching behaviour, whereas actual leaching behaviour is influenced by many factors, such as soil and environmental parameters, and individual plant characteristics (Ruyters et al., 2011). The Tessier fivestep speciation method reveals information regarding which chemical forms the investigated radionuclides are bound to, and the environmental conditions under which they become mobile. The leachates were then separated from the samples; U was selectively extracted from the matrix by UTEVA resin extraction chromatography and was electrodeposited onto stainless steel disks with high Ni contents (Jobb agy et al., 2010). The prepared sources were measured using alpha-spectrometry. While other relevant radionuclides are present in phosphate fertilizers (such as Ra-226, Po-210 and Pb-210) the U-238 and U-234 were selected for analysis and evaluation of the leaching tests. 2.3.1. Batch test method compliant to the MSZ-21470-50:2006 Hungarian standard MSZ-21470-50:2006 is the Hungarian standard (Hungarian Standards Institution) for the measurement of toxic elements, heavy metals and chrome(VI) in soil for environmental protection purposes. Four separate single-step batch processes are included to investigate the “total” amount and to indicate the availability to } s et al., 2016), as can be seen in Fig. 1 rainwater and plants (Hegedu below. 2.3.2. Tessier sequential extraction The five-step Tessier sequential extraction is a speciation process, originally used for heavy metals in sediments, which has since been adapted for other materials as well. The original procedure was for 1 g samples but, for the leaching experiments for natural radionuclides, altering the process for 5 g samples was necessary } s et al., 2016). The due to the instruments' detection limits (Hegedu adapted process is shown in Fig. 2 below. 2.3.3. Alpha-spectrometry After separating the leachates from the solid phase, the samples were put through radiochemical separation by UTEVA resin } s et al., 2016). The extraction chromatography was per(Hegedu formed using NO3 form UTEVA resin and, after removing the other, undesired fractions, U was eluted by 0.02 M HCl from the column. The uranium fractions were subjected to electro-deposition with a Canberra Electro a with a platinum electrode, and the process was carried out at 1 A for 1 h on a stainless steel disk with a high Ni gy et al., 2010). The prepared alpha-sources were content (Jobba measured for 80,000 s with three alpha chambers equipped with PIPS detectors of Ortec Soloist, Canberra Model 7401 and Tennelec TC 256 types. The chambers' respective MDAs were 0.7, 1.4 and 2 Bq kg1, respectively. 2.3.4. Excess dose calculation To better compare the total gamma dose potential of the samples, the radium equivalent activity index (Raeq) was calculated according to equation (1) (Boukhenfouf and Boucenna, 2011). Raeq ¼ ARa þ ATh 1.43þAK 0.077,
(1)
where ARa, ATh and AK are the respective activity concentrations of Ra-226, Th-232 and K-40 in Bq kg1. The absorbed dose rates due to gamma radiations in air at 1 m above the ground surface were calculated according to equation (2) (Shahul Hameed et al., 2014; Uosif et al., 2014; Hassan et al., 2016). D ¼ 0.462 ARaþ0.604 AThþ0.0417 AK,
(2)
} s et al. / Journal of Environmental Radioactivity 173 (2017) 34e43 M. Hegedu
37
Fig. 1. Flowchart of the four parallel single-step extractions compliant with MSZ-21470-50:2006.
where ARa, ATh and AK are the respective activity concentrations of Ra-226, Th-232 and K-40 in Bq kg1 and D is the dose rate in nGy h1. The annual estimated average effective dose equivalent was estimated using equation (3) (Uosif et al., 2014). AED ¼ D t FC,
(3)
where D is the calculated dose rate (in nGy h1), t is the number of work hours per year and FC is the conversion factor (0.7 Sv Gy1). A working period of 1820 h a year was used.
2.3.4.1. Internal dose calculation. To estimate the excess internal dose, a few assumptions must be made. In the considered scenario, the application rate of the fertilizers was taken to be 80 kg P2O5 ha1 per year (actual application rates may differ depending on the plant and soil properties), a homogenous distribution of radionuclides was assumed in the upper 20 cm of the soil, and the soil's apparent density was taken as 1.3 g cm3 (Saueia and Mazzilli, 2006). The soil's U concentration was taken as 32Bq kg1, assuming secular equilibrium with the Ra-226 value reported in UNSCEAR 2008 Annex B. The estimation of radionuclide concentrations in plants was performed using the geometric mean transfer factors calculated by Vandenhove et al., (2009) for each plant group. The annual estimated average effective dose equivalent from the ingestion of plants grown in the soil treated with phosphorus fertilizer was calculated using equation (4) (Saueia and Mazzilli, 2006). IAEDi ¼ Ci*Ui*EDC
(4)
where IAEDi is the annual estimated average effective dose equivalent from ingestion, Ci is the activity concentration of U in the specific plant, Ui is the consumption rate of the specific plant according to the European Food Safety Authority (EFSA) taken from the INRAN SCAI 2005-06 report, and EDC is the effective dose coefficient according to the UNSCEAR 2000 report (U-238 0.045, U234).
3. Results and discussion 3.1. Gamma spectrometry The natural radionuclide content of the investigated fertilizer samples varied significantly (Table 2). The values measured for the fertilizers commercially available in Hungary agree well with fertilizers used in other countries (Hassan et al., 2016; Shahul Hameed et al., 2014; Uosif et al., 2014). The average activity concentrations for the fertilizers used in the current study were 309 ± 39 Bq kg1 for Ra-226, 6 ± 5 Bq kg1 for Th232 and 3139 ± 188 Bq kg1 for K-40. These are not especially high concentrations, but they are well above the average activity concentrations for soil UNSCEAR 2008 Annex B (Ra-226: 32 Bq kg1; Th-232: 45 Bq kg1; K-40: 412 Bq kg1) and Radiation Protection 112 (Ra-226: 40 Bq kg1; Th-232: 40 Bq kg1; K-40: 400 Bq kg1). The application of these fertilizers would mean a very slow, gradual increase in the external gamma dose rate compared to the unaltered situation. For better comparison of the total gamma dose potential of the samples, the radium equivalent activity index (Raeq) of the samples is shown in Fig. 3. The values for Raeq also demonstrate that our samples certainly have the potential to lead to excess gamma doses as compared to the average soil, but the values are similar to those of other fertilizer samples reported in the literature (Boukhenfouf and Boucenna 2012; Shahul Hameed et al., 2014; Uosif et al., 2014). It has been reported in several cases that the activity of phosphate fertilizers correlates well with their phosphate content (Boukhenfouf and Boucenna, 2011; Hassan et al., 2016). The activity concentrations of the samples have been normalized for 1 kg phosphate content for better comparison. As can be seen in Fig. 4, the good correlation previously reported is not present in our case: while the superphosphate samples seem to agree with that trend, for the NPK fertilizers not only were low activity concentrations found, but also the values do not depend on the phosphate content. This is probably due to the different raw phosphate ore and production process employed. If the types of the fertilizers are disregarded, and the Ra-226 and U-238 activity concentrations are plotted against the P2O5 content (Fig. 5), it can be seen even more clearly that the correlation mentioned above is not true in our case.
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radiological perspective these seem to be the safest to use, if their composition is acceptable. 3.2. Leachability experiments
Fig. 2. Flowchart of the Tessier five-step sequential extraction, adapted for 5 g samples.
Table 2 Activity concentrations measured in the fertilizer samples.
NPK1 NPK2 NPK3 PK1 PK2 SP1 SP2 SP3
Ra-226 [Bq kg1]
Th-232 [Bq kg1]
K-40 [Bq kg1]
570 ± 46 10 ± 8 38 ± 17 310 ± 34 336 ± 48 351 ± 48 439 ± 55 414 ± 53
7±6 7±9 4±7 < MDA < MDA 6±9
4097 ± 148 3429 ± 297 3664 ± 307 6623 ± 188 7057 ± 427 111 ± 54 51 ± 36 76 ± 44
Comparing these results, it can be seen that (with the exception of NPK1, which showed significant excess of Ra-226, and NPK3, which showed significant excess of U species) the activity concentration of U-238, U-234 and Ra-226 are in or nearly in equilibrium. Amongst the investigated samples, NPK2 and NPK3 have relatively low levels of U-238 and Ra-226, not only on their own, but also when their P2O5 content is taken into account and the majority of their activity comes from their K-40 content. From a
The results of the leachability measurements can be seen in Fig. 6. The U-238 activity concentrations of the samples measured using microwave digestion with aqua regia ranged from 21 Bq kg1 to 361 Bq kg1, as shown in Fig. 6. The microwave digestion with HNO3 and H2O2 was not adequate for the measurement of the “total” U concentration, while in some cases (NPK2, NPK3, SP1) the same results as the aqua regia digestion were shown and in other cases (most distinctly NPK1 and PK1) a gross underestimation of the activity concentration was caused. For phosphate fertilizers, measurement of the “total” U concentration could be excluded from the measurement protocol, and possibly substituted with a parallel measurement of the aqua regia “total digestion” for approximately the same cost. The indicated availability to plants and rainwater differed greatly from sample to sample. The availability of U to water ranged between 3 and 28 m/m%, while the € solution mobilized between 10 and 100 m/m% of the Lakanen-Ervio U content. In the case of the NPK3 sample, virtually all U was indicated to be available for uptake by plants, while other samples, such as NPK1 and PK1, retained nearly all U. The widely varying values are suspected to be caused by the different raw materials and technological processes used at each plant. To better explain these differences, the Tessier sequential extraction was carried out to see which chemical form the U species are bound to. The speciation of U-238 revealed that not only did the activity concentrations vary greatly, but the availability of U had great variation even within the same type of fertilizers. As you can see on Fig. 7, the investigated fertilizers have significant fractions of U bound to species available to changes in red-ox potential and under oxidizing conditions. The ratios of the water available fraction are similar to the values obtained in the MSZ-21470-50:2006 test, and indicate leachability from negligible to one third of the total U content. The fraction that becomes available with pH poses a further 5 to 10 percent. NPK2, PK1 and SP1 have a significant portion (57, 47, and 31% respectively) of U in the remaining fraction that does not become available under environmental conditions. The fertilizers don't show a trend connected to the P2O5 content, or characteristics representing the fertilizer categories. NPK1 and NPK3 have similar major constituent composition and leaching characteristics, while PK1 and PK2, also having similar composition to each other, do not have the same leaching characteristics. As for environmental impact, the fractions that are readily available pose risk by their contribution to the internal dose through the food-chain, while the fractions that do not become available cause a slow contamination of the land. This last problem is twofold, there is a small increase in the external gamma-dose, and there is a small increase of the internal dose from the inhalation of dust in the area, gradually increasing annually by a small fraction, and is likely only to cause a problem in a few hundred years. 3.3. Dose assessment The calculated absorbed dose rate and the annual effective dose are shown in Table 3. The values presented in Table 3 agree with data on similar fertilizers (Hassan et al., 2016; Uosif et al., 2014). It should also be noted that the assumptions made in the annual effective dose calculations do not represent the general populace, but can be applied for workers in fertilizer factories and storage facilities.
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Fig. 3. The radium equivalent activity index (Raeq) of the phosphate fertilizers.
Fig. 4. Activity concentration normalized for 1 kg phosphate content.
Fig. 5. U-238 and Ra-226 activity concentration in relation to P2O5 content.
The general populace, however, is affected by phosphate fertilizer use through the consumption of agricultural products. The calculated excess annual effective dose from ingestion according to the considered scenario can be found in Figs. 8 and 9 for U-238 and U-234, respectively. It can be seen that the maximum excess dose was calculated for the PK1 fertilizer, 12.97 nSv year1 for U-238 and 14.12 nSv year1 for U-234. In the case of NPK2, the negative value indicates that the fertilizer has less activity concentration than the soil in the
considered scenario and thus plays a diluting effect, causing a decrease in the received dose. Compared to the doses calculated directly from soil (4.59 mSv year1 for U-238, and 4.99 mSv year1 for U-234) these values seem small, but after decades of agricultural activities the increase would become noticeable. The first four steps of the Tessier speciation method reveal the species that become available under various environmental conditions. The residue is not available under environmental conditions and, therefore, the excess annual effective dose should also be reduced accordingly. The calculated excess annual effective dose from the ingestion of plants according to the scenario whereby they are unable to take up the non-leachable U species (according to the Tessier method) can be found in Figs. 10 and 11. If Figs. 8 and 9 are compared to Figs. 10 and 11 the most notable changes are in the cases of samples PK1 and SP1, for which a high ratio of residue is combined with high activity concentration. In both scenarios, the highest values reach 10 nSv year1, compared to 0.12 mSv year1 for the ingestion of U and Th series (summing up the dose contribution of all daughter elements as well) presented in the UNSCEAR 2008 report. The excess doses calculated from the use of phosphate fertilizers according to the considered scenario are lower by five magnitudes. However if compared to the original dose caused by the soil's U content in the considered scenario (4.59 mSv year1 for U-238, and 4.99 mSv year1 for U-234), the increment would become comparable over a century.
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Fig. 6. Leaching of U-238 from phosphate fertilizers.
Fig. 7. Distribution of U-238 in the Tessier extraction process.
Table 3 The calculated absorbed dose rate (D) and the annual effective dose (AED) originated from the observed phosphate fertilizers.
NPK1 NPK2 NPK3 PK1 PK2 SP1 SP2 SP3
D [nGy h1]
AED [mSv year1]
438.4 151.8 172.7 419.4 449.5 170.4 204.9 194.4
0.56 0.19 0.22 0.53 0.57 0.22 0.26 0.25
4. Conclusion In this study, eight commercially available phosphate fertilizers were evaluated using gamma spectrometry, while their U leaching characteristics were tested using both the MSZ-21470-50:2006 standard and the Tessier speciation process. The selected leaching tests can be applied for the estimation of the uranium leaching characteristics of phosphate fertilizer samples, yet it should be noted that actual leaching will be different depending on actual field conditions e batch tests only offer an indication of future behaviour. Microwave digestion with HNO3 and H2O2 cannot be relied upon for measuring the “total” U content, since it not only showed significantly lower values for U activity concentration, but also failed to indicate the values that became available during the Tessier speciation method. This method could be excluded entirely from the measurement protocol for phosphate fertilizers, or
substituted by a parallel measurement with the aqua regia method for approximately the same cost and work hours. The fertilizers' activity concentrations and leaching characteristics show great variation, both between and within fertilizer types. The average activity concentrations for the fertilizers used in the current study were 309 ± 39 Bq kg1 for Ra-226, 6 ± 5 Bq kg1 for Th-232 and 3139 ± 188 Bq kg1 for K-40. The good correlation between P2O5 content and radioactivity reported previously is not found in our case e only the superphosphate samples seem to agree with that trend ethough, for the NPK fertilizers, not only were low activity concentrations found, but additionally the values did not depend on the phosphate content. This is probably due to a difference in the raw phosphate ore and production process employed. Those fertilizers that contain uranium in water-soluble or plantavailable species cause excess internal doses through the food chain, while the insoluble fractions remain in place, causing a gradually increasing low-level contamination of the fields, leading to a continually increasing excess external gamma dose. The annual effective gamma dose values calculated are similar to values reported for the same types of fertilizers (Hassan et al., 2016; Uosif et al., 2014). The speciation of U in the Tessier sequential extraction method revealed that, while the U content of some samples can become almost fully available under environmental conditions, some can retain up to 57% of their activity in the residue, unavailable under natural circumstances. This also has similarly significant effects on the internal dose calculated from the consumption of plants according to the considered scenario.
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Fig. 8. Excess dose caused byU-238 in plants according to the considered scenario.
Fig. 9. Excess dose caused by U-234 in plants according to the considered scenario.
Fig. 10. Excess dose caused by U-238 in plants reduced by the non-leachable amount according to the considered scenario.
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Fig. 11. Excess dose caused by U-234 in plants reduced by the non-leachable amount according to the considered scenario.
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